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Nonlinear, four-wave mixing vibrational spectroscopies are commonly used to probe electron-vibration coupling in isotropic media. Most of these methods rely on infrared and/or Raman transitions, but methods involving hyper-Raman transitions are also possible. Hyper difference frequency generation (HDFG) spectroscopy is an underdeveloped four-wave mixing vibrational spectroscopy based upon both infrared absorption and hyper-Raman scattering transitions. Despite several experimental reports on HDFG, its spectroscopic properties have not been fully explored. To this end, we investigate the selection rules and behavior of HDFG spectroscopy as an upconverted infrared spectroscopy and as a probe of vibronic coupling in molecular systems. We discuss the similarities between HDFG, a four-wave mixing technique, and vibrational sum frequency generation (vSFG) spectroscopy, a three-wave mixing technique. vSFG and HDFG appear to provide similar output intensities, making HDFG feasible for vSFG practitioners. HDFG is shown to be a sensitive probe of vibronic coupling in bulk systems and provides an alternative method to investigate electronic-nuclear coordinate correlations., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

2D hybrid organic-inorganic halide perovskites emerge as a new class of 2D semiconductors with the potential to combine excellent optoelectronic properties with symmetry-enabled properties such as ferroelectricity. Although many lead-based ferroelectric 2D halide perovskites are reported, there is yet to be a conclusive report of ferroelectricity in tin-based 2D perovskites. Here, the structures and properties of a new series of 2D Dion-Jacobson (DJ) Sn perovskites: (4AMP)SnI 4 , (4AMP)(MA)Sn 2 I 7 , and (4AMP)(FA)Sn 2 I 7 (4AMP = 4-(aminomethyl)piperidinium, MA = methylammonium, and FA = formamidinium), are reported. Structural characterization reveals that (4AMP)SnI 4 is polar with in-plane spontaneous polarization whereas (4AMP)(MA)Sn 2 I 7 and (4AMP)(FA)Sn 2 I 7 are centrosymmetric. Further, (4AMP)SnI 4 displays second harmonic generation (SHG) and polarization-electric field hysteresis measurements confirm it is ferroelectric with a spontaneous polarization of 10.0 µC cm -2 at room temperature. (4AMP)SnI 4 transitions into a centrosymmetric structure above 367 K. As the first direct experimental observation of the spontaneous ferroelectric polarization of a Sn-based 2D hybrid perovskite, this work opens up environmentally friendly 2D tin halide perovskites for ferroelectricity and other physical property studies., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)

The Paternò-Büchi reaction is the [2 + 2] photocycloaddition of a carbonyl with an alkene to afford an oxetane. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity toward alternate photoproducts. We show herein that a triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò-Büchi reaction, catalyzed by a novel hydrogen-bonding chiral Ir photocatalyst.

Vibrational fingerprints and combination bands are a direct measure of couplings that control molecular properties. However, most combination bands possess small transition dipoles. Here we use multiple, ultrafast coherent infrared pulses to resolve vibrational coupling between CH 3 CN fingerprint modes at 918 and 1039 cm -1 and combination bands in the 2750-6100 cm -1 region via doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS). This approach provides a direct probe of vibrational coupling between fingerprint modes and near-infrared combination bands of large and small transition dipoles in a molecular system over a large frequency range.

Floquet state spectroscopy is an optical analogue of multiple quantum coherence nuclear magnetic resonance (MQC-NMR). Tunable ultrafast excitation pulses resonantly excite multiple states in a sample to form the Floquet state. The Floquet state emits multiple coherent beams at frequencies and in directions that conserve energy and momenta. The different output beams differ in the time ordering and coherences created by the excitation beams. They correspond to the different methodologies in the NMR family. Isolating a specific beam and monitoring the output intensity as a function of excitation frequencies creates multidimensional spectra containing cross-peaks between coupled states. The frequency range of the multidimensional spectra is limited by phase matching constraints. This paper presents a new, to the best of our knowledge, active phase matching strategy that increases the versatility of multidimensional Floquet state spectroscopy through both longer sample path lengths and larger spectral ranges.

The compositional tunability of 2D metal halide perovskites enables exploration of diverse semiconducting materials with different structural features. However, rationally tuning the 2D perovskite structures to target physical properties for specific applications remains challenging, especially for lead-free perovskites. Here, we study the effect of the interplay of the B-site (Ge, Sn, and Pb), A-site (cesium, methylammonium, and formamidinium), and spacer cations on the structure and optical properties of a new series of 2D Ruddlesden-Popper perovskites using the previously unreported spacer cation 4-bromo-2-fluorobenzylammonium (4Br2FBZ). We report eight new crystal structures and study the consequence of varying the B-site (Pb, Sn, Ge) and dimension ( n = 1, 2, vs 3D). Dimension strongly influences local distortion and structural symmetry, and the increased octahedral tilting and lone pair effects in Ge perovskites lead to a polar n = 2 perovskite that exhibits second harmonic generation, (4Br2FBZ) 2 (Cs)Ge 2 I 7 . In contrast, the analogous Sn and Pb perovskites remain centrosymmetric, but the B-site metal influences the photoluminescence properties. The Pb perovskites exhibit broad, defect-mediated emission at low temperature, whereas the Sn perovskites show purely excitonic emission over the entire temperature range, but the carrier recombination dynamics depend on dimensionality and dark excitonic states. Wholistic understanding of these differences that arise based on cations and dimensionality can guide the rational materials design of 2D perovskites for targeting physical properties for optoelectronic applications based on the interplay of cations and the connectivity of the inorganic framework.

Lateral heterostructures of two-dimensional (2D) transition metal dichalcogenides offer promise as platforms for a wide variety of applications from exotic physics to environmental control. Further development and study of these heterostructures require characterization methods that assess the quality of the heterostructures. Here, we extend current characterization strategies to create photoluminescence (PL), Raman, reflection contrast, and second harmonic generation (SHG) maps of individual monolayer core-shell WS 2 -MoS 2 lateral heterostructures that were synthesized via water vapor assisted chemical vapor transport. Together, these methods provide the correlations required to resolve the effects of excitons, trions, lattice defects, strain, and alloying. The comparisons show substantial differences, especially in the regions near and at the narrow heterointerface. Comparisons between the different spectral maps show the importance of metal alloying for understanding the electronic and spatial structures of heterostructures. The results are compared to previous work on similar lateral heterostructures created by different methods.

Quantum confinement in two-dimensional (2D) Ruddlesden-Popper (RP) perovskites leads to the formation of stable quasi-particles, including excitons and biexcitons, the latter of which may enable lasing in these materials. Due to their hybrid organic-inorganic structures and the solution phase synthesis, microcrystals of 2D RP perovskites can be quite heterogeneous, with variations in excitonic and biexcitonic properties between crystals from the same synthesis and even within individual crystals. Here, we employ one- and two-quantum two-dimensional white-light microscopy to systematically study the spatial variations of excitons and biexcitons in microcrystals of a series of 2D RP perovskites BA 2 MA n -1 Pb n I 3 n +1 ( n = 2-4, BA= butylammonium, MA = methylammonium). We find that the average biexciton binding energy of around 60 meV is essentially independent of the perovskite layer thickness ( n ). We also resolve spatial variations of the exciton and biexciton energies on micron length scales within individual crystals. By comparing the one-quantum and two-quantum spectra at each pixel, we conclude that biexcitons are more sensitive to their environments than excitons. These results shed new light on the ways disorder can modify the energetic landscape of excitons and biexcitons in RP perovskites and how biexcitons can be used as a sensitive probe of the microscopic environment of a semiconductor.

There is great interest in developing fully coherent multidimensional X-ray/extreme ultraviolet (XUV) spectroscopic techniques because of their capability for achieving atomic spectral selectivity. Current proposals rest on using sequentially and coherently driven core excitations with multiple X-ray/XUV excitation pulses and measuring the output using time domain Fourier transform methods. In this paper, we propose an alternative method that creates an entanglement of core and optical transitions to form a Floquet state that creates directional and coherent output beams. Multidimensional spectra are obtained by measuring the intensity of output beams while tuning the optical frequencies across resonances. This approach expands on previous optical pump-XUV probe spectroscopy of MoTe 2 by theoretically demonstrating its multidimensional capabilities. Both parametric and non-parametric pathways are proposed to optimize the resolution of inhomogeneous broadening and k -selective features.

Modern instrumentation development often involves the incorporation of many dissimilar hardware peripherals into a single unified instrument. The increasing availability of modular hardware has brought greater instrument complexity to small research groups. This complexity stretches the capability of traditional, monolithic orchestration software. In many cases, a lack of software flexibility leads creative researchers to feel frustrated, unable to perform experiments they envision. Herein, we describe Yet Another acQuisition (yaq), a software project defining a new standardized way of communicating with diverse hardware peripherals. yaq encourages a highly modular approach to experimental software development that is well suited to address the experimental flexibility needs of complex instruments. yaq is designed to overcome hardware communication barriers that challenge typical experimental software. A large number of hardware peripherals are already supported, with tooling available to expand support. The yaq standard enables collaboration among multiple research groups, increasing code quality while lowering development effort., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Two-dimensional-on-three-dimensional (2D/3D) halide perovskite heterostructures have been extensively utilized in optoelectronic devices. However, the labile nature of halide perovskites makes it difficult to form such heterostructures with well-defined compositions, orientations, and interfaces, which inhibits understanding of the carrier transfer properties across these heterostructures. Here, we report solution growth of both horizontally and vertically aligned 2D perovskite (PEA) 2 PbBr 4 (PEA = phenylethylammonium) microplates onto 3D CsPbBr 3 single crystal thin films, with well-defined heterojunctions. Time-resolved photoluminescence (TRPL) transients of the heterostructures exhibit the monomolecular and bimolecular dynamics expected from exciton annihilation, dissociation, and recombination, as well as evidence for carrier transfer in these heterostructures. Two kinetic models based on Type-I and Type-II band alignments at the interface of horizontal 2D/3D heterostructures are applied to reveal a shift in balance between carrier transfer and recombination: Type-I band alignment better describes the behaviors of heterostructures with thin 2D perovskite microplates but Type-II band alignment better describes those with thick 2D microplates (>150 nm). TRPL of vertically aligned 2D microplates is dominated by directly excited PL and is independent of the height above the 3D film. Electrical measurements reveal current rectification behaviors in both heterostructures with vertical heterostructures showing better electrical transport. As the first systematic study on comparing models of 2D/3D perovskite heterostructures with controlled orientations and compositions, this work provides insights on the charge transfer mechanisms in these perovskite heterostructures and guidelines for designing better optoelectronic devices.

Heterostructures of three-dimensional (3D) halide perovskites are unstable because of facile anion interdiffusion at halide interfaces. Two-dimensional (2D) Ruddlesden-Popper halide perovskites (RPPs) show suppressed and anisotropic ion diffusion that could enable stable RPP heterostructures, yet the direct and general growth of lateral RPP heterostructures remains challenging. Here, we show that halide miscibility in RPPs decreases with perovskite layer thickness ( n ), enabling the formation of sharp halide lateral heterostructures from n = 1 and 2 RP lead iodide microplates via anion exchange with hydrogen bromide vapor. In contrast, RPPs with n ≥ 3 form more diffuse lateral heterojunctions, more similar to those in 3D perovskites. The anion exchange behaviors are further modulated by the spacer and A-site cations in the RPP structures. These new insights, and kinetic studies of the exchange reactions, enable the preparation of lateral heterostructures from various n = 2 RPPs that are more stable against anion interdiffusion and degradation for potential optoelectronic device applications.

Ruddlesden-Popper lead halide perovskites have emerged as a new class of two-dimensional semiconductors with tunable optoelectronic properties, potentially offering unlimited heterostructure configurations for exploration. However, the practical realization of such heterostructures is challenging because of the difficulty in achieving controllable direct synthesis or van der Waals integration of halide perovskites due to their mobile and fragile crystal lattices. Here we report direct growth of large-area nanosheets of diverse phase-pure Ruddlesden-Popper perovskites with thicknesses down to one monolayer at the solution-air interface and a reliable approach for gently transferring and stacking these nanosheets. These advances enable the deterministic fabrication of arbitrary vertical heterostructures and multi-heterostructures of Ruddlesden-Popper perovskites with greater structural degrees of freedom that define the electronic structures of the heterojunctions. Such rationally designed heterostructures exhibit interesting interlayer properties, such as interlayer carrier transfer and reduction of the photoluminescence linewidth, and could enable the exploration of exciton physics and optoelectronic applications.

Euclidean geometry is the fundamental mathematical framework of classical crystallography. Traditionally, layered materials are grown on flat substrates; growing Euclidean crystals on non-Euclidean surfaces has rarely been studied. We present a general model describing the growth of layered materials with screw-dislocation spirals on non-Euclidean surfaces and show that it leads to continuously twisted multilayer superstructures. This model is experimentally demonstrated by growing supertwisted spirals of tungsten disulfide (WS 2 ) and tungsten diselenide (WSe 2 ) draped over nanoparticles near the centers of spirals. Microscopic structural analysis shows that the crystal lattice twist is consistent with the geometric twist of the layers, leading to moiré superlattices between the atomic layers., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Layered two-dimensional Ruddlesden-Popper (RP) halide perovskites are an intriguing class of semiconductors being explored for their linear and nonlinear optical and ferroelectric properties. Second harmonic generation (SHG) is commonly used to screen for noncentrosymmetric and ferroelectric materials. However, SHG measurements of perovskites can be obscured by their intense multiphoton photoluminescence (mPL). Here, we apply multidimensional harmonic generation as a method to eliminate the complications from mPL. By scanning and correlating both excitation and emission frequencies, we unambiguously assess whether a material supports SHG by examining if an emission feature scales as twice the excitation frequency. Measurements of a series of n = 2, 3 RP perovskites reveal that, contrary to previous belief, n -butylammonium (BA) RP perovskites are not SHG-active and thus centrosymmetric, but RP perovskites with phenylethylammonium (PEA) and 2-thiophenemethylammonium (TPMA) spacer cations display SHG. This work establishes multidimensional harmonic generation as a definitive method to measure SHG in halide perovskites.

The invention of the laser generated great excitement, because its ability to create quantum state coherences could form a new family of coherent spectroscopies that were the optical analogue of multidimensional nuclear magnetic resonance (NMR). The full realization of this promise has not yet been realized, but the pathway forward is clear. The path involves the use of multiple, tunable lasers that create a Schrödinger cat state, where the system is simultaneously in a mixture of vibrational and/or electronic states. The multiplicity of these states confers many advantages for analytical methods: high selectivity from the multiple spectral dimensions, line-narrowing, isolation of spectral features where quantum states are coupled, and spectral decongestion. Now that the feasibility of Schrödinger cat spectroscopy has been demonstrated, the future is open for the development of a new frontier in analytical chemistry that creates a new set of tools for studying the complex systems that form the heart of analytical chemistry.

We report new features of recently developed ultrafast coherent multidimensional spectroscopy (CMDS), an optical analogue to multidimensional NMR. By using both frequency- and time-domain nonlinear four wave mixing methods, CMDS is able to directly observe coherence transfer (CT), the coherent quantum mechanical analogue of population relaxation. Using a mixture of acetonitrile and magnesium perchlorate (1.0 M) as a model system, we demonstrated that this one color- and population-involving CT process makes CMDS capable of measuring samples with features that mimic excitation and emission spectral measurements in fluorescence spectroscopy. With the new capabilities, one might develop CMDS into a versatile vibrational tool for revealing the role of coherence as a design element in realizing a function. Furthermore, CT-based vibrational resonance energy transfer (VRET) methods may be developed for label-free biosensing and imaging, such as those demonstrated by fluorescence resonance energy transfer (FRET).

Triply resonant sum frequency (TRSF) spectroscopy is a fully coherent mixed vibrational-electronic spectroscopic technique that is ideally suited for probing the vibrational-electronic couplings that become important in driving reactions. We have used cyanocobalamin (CNCbl) and deuterated aquacobalamin (D 2 OCbl + ) as model systems for demonstrating the feasibility of using the selectivity of coherent multidimensional spectroscopy to resolve electronic states within the broad absorption spectra of transition metal complexes and identify the nature of the vibrational and electronic state couplings. We resolve three short and long axis vibrational modes in the vibrationally congested 1400-1750 cm -1 region that are individually coupled to different electronic states in the 18 000-21 000 cm -1 region but have minimal coupling to each other. Double resonance with the individual vibrational fundamentals and their overtones selectively enhances the corresponding electronic resonances and resolves features within the broad absorption spectrum. This work demonstrates the feasibility of identifying coupling between different pairs of vibrational states with different electronic states that together form the reaction coordinate surface of transition metal enzymes.

Fabrication of heterostructures using two-dimensional (2D) materials with different bandgaps creates opportunities for exploring new properties and device applications. Ruddlesden-Popper (RP) layered halide perovskites have recently emerged as a new class of solution-processable 2D materials that demonstrate exotic optoelectronic properties. However, heterostructures using 2D halide perovskites have not been achieved. Here, we report a simple solution growth method for making vertically stacked double heterostructures and complex multilayer heterostructures of 2D lead iodide perovskites [(PEA) 2 (MA) n-1 Pb n I 3 n+1 , PEA = C 6 H 5 (CH 2 ) 2 NH 3 + , MA = CH 3 NH 3 + ] via van der Waals epitaxy. These heterostructures present atomically sharp interfaces and display distinct photoluminescence that allow fingerprinting the RP phases. Time-resolved photoluminescence measurements reveal internal energy transfer from higher energy bandgap (lower n value) perovskite layers to lower energy bandgap (higher n value) perovskite layers on the time scale of hundreds of picoseconds due to natural type I band alignments. These results offer new strategies to fabricate perovskite-perovskite heterojunctions by taking advantage of surface-bound ligands as spatial barriers to prevent ion migration across the junctions. These heterostructures capable of multicolor emission with high spectral purity are promising for light-emitting applications.

Triple sum-frequency (TSF) spectroscopy is a recently developed methodology that enables collection of multidimensional spectra by resonantly exciting multiple quantum coherences of vibrational and electronic states. This work reports the first application of TSF to the electronic states of semiconductors. Two independently tunable ultrafast pulses excite the A, B, and C features of a MoS 2 thin film. The measured TSF spectrum differs markedly from absorption and second harmonic generation spectra. The differences arise because of the relative importance of transition moments and the joint density of states (JDOS). We develop a simple model and globally fit the absorption and harmonic generation spectra to extract the JDOS and the transition moments from these spectra. Our results validate previous assignments of the C feature to a large JDOS created by band nesting.

Pedot: PSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.

The facile chemical transformation of metal halide perovskites via ion exchange has been attributed to their "soft" crystal lattices that enable fast ion migration. Kinetic studies of such processes could provide mechanistic insights on the ion migration dynamics. Herein, by using aligned single-crystal nanowires of cesium lead bromide (CsPbBr 3 ) perovskite on epitaxial substrates as platforms, we visualize and investigate the cation or anion interdiffusion kinetics via spatially resolved photoluminescence measurement on heterostructures fabricated by stacking CsPbCl 3 , MAPbI 3 , or MAPbBr 3 microplates on top of CsPbBr 3 nanowires. Time-dependent confocal photoluminescence microscopy and energy-dispersive X-ray spectroscopy showed the solid-state anion interdiffusion readily occurs to result in halide concentration gradients along CsPbBr 3-3 x Cl 3 x ( x = 0-1) nanowires. Quantitative analysis of such composition profiles using Fick's law allowed us, for the first time, to extract interdiffusion coefficients of the chloride-bromide couple and an activation energy of 0.44 ± 0.02 eV for ion diffusion from temperature-dependent studies. In contrast, iodide-bromide interdiffusion is limited, likely due to the complex phase behaviors of mixed alloys of CsPb(Br,I) 3 . In contrast to the relatively mobile anions, A-site cation interdiffusion across the MAPbBr 3 /CsPbBr 3 junctions was barely observed at room temperature. Our results present a general method to investigate the kinetics of the solid-state ion migration, and the gained insights on ion diffusion can provide guidelines for rationally designing perovskite heterostructures that could lead to new properties for fundamental studies and technological applications.

Triply resonant sum frequency (TRSF) and doubly vibrationally enhanced (DOVE) spectroscopies are examples of a recently developed family of coherent multidimensional spectroscopies (CMDS) that are analogous to multidimensional NMR and current analytical spectroscopies. CMDS methods are particularly promising for analytical applications because their inherent selectivity makes them applicable to complex samples. Like NMR, they are based on creating quantum mechanical superposition states that are fully coherent and lack intermediate quantum state populations that cause quenching or other relaxation effects. Instead of the nuclear spin states of NMR, their multidimensional spectral fingerprints result from creating quantum mechanical mixtures of vibrational and electronic states. Vibrational states provide spectral selectivity, and electronic states provide large signal enhancements. This paper presents the first electronically resonant DOVE spectra and demonstrates the capabilities for analytical chemistry applications by comparing electronically resonant TRSF and DOVE spectra with each other and with infrared absorption and resonance Raman spectra using a Styryl 9 M dye as a model system. The methods each use two infrared absorption transitions and a resonant Raman transition to create a coherent output beam, but they differ in how they access the vibrational and electronic states and the frequency of their output signal. Just as FTIR, UV-vis, Raman, and resonance Raman are complementary methods, TRSF and DOVE methods are complementary to coherent Raman methods such as coherent anti-Stokes Raman spectroscopy (CARS).

We report novel two-dimensional lead halide perovskite structures templated by a unique conjugated aromatic dication, N,N-dimethylphenylene-p-diammonium (DPDA). The asymmetrically substituted primary and tertiary ammoniums in DPDA facilitate the formation of two-dimensional network (2DN) perovskite structures incorporating a conjugated dication between the PbX 4 2- (X = Br, I) layers. These 2DN structures of (DPDA)PbI 4 and (DPDA)PbBr 4 were characterized by single-crystal X-ray diffraction, showing uniquely low distortions in the Pb-X-Pb bond angle for 2D perovskites. The Pb-I-Pb bond angle is very close to ideal (180°) for a 2DN lead iodide perovskite, which can be attributed to the ability of the rigid diammonium DPDA to insert into the PbX 6 2- octahedral pockets. Optical characterization of (DPDA)PbI 4 shows an excitonic absorption peak at 2.29 eV (541 nm), which is red-shifted in comparison to similar 2DN lead iodide structures. Temperature-dependent photoluminescence of both compounds reveals both a self-trapped exciton and free exciton emission feature. The reduced exciton absorption energy and emission properties are attributed to the dication-induced structural order of the inorganic PbX 4 2- layers. DFT calculation results suggest mixing of the conjugated organic orbital component in the valence band of these 2DN perovskites. These results demonstrate a rational new strategy to incorporate conjugated organic dications into hybrid perovskites and will spur spectroscopic investigations of these compounds as well as optoelectronic applications.

High-quality metal halide perovskite single crystals have low defect densities and excellent photophysical properties, yet thin films are the most sought after material geometry for optoelectronic devices. Perovskite single-crystal thin films (SCTFs) would be highly desirable for high-performance devices, but their growth remains challenging, particularly for inorganic metal halide perovskites. Herein, we report the facile vapor-phase epitaxial growth of cesium lead bromide perovskite (CsPbBr 3 ) continuous SCTFs with controllable micrometer thickness, as well as nanoplate arrays, on traditional oxide perovskite SrTiO 3 (100) substrates. Heteroepitaxial single-crystal growth is enabled by the serendipitous incommensurate lattice match between these two perovskites, and overcoming the limitation of island-forming Volmer-Weber crystal growth is critical for growing large-area continuous thin films. Time-resolved photoluminescence, transient reflection spectroscopy, and electrical transport measurements show that the CsPbBr 3 epitaxial thin film has a slow charge carrier recombination rate, low surface recombination velocity (10 4 cm s -1 ), and low defect density of 10 12 cm -3 , which are comparable to those of CsPbBr 3 single crystals. This work suggests a general approach using oxide perovskites as substrates for heteroepitaxial growth of halide perovskites. The high-quality halide perovskite SCTFs epitaxially integrated with multifunctional oxide perovskites could open up opportunities for a variety of high-performance optoelectronics devices.

Ultrafast spectroscopy is often collected in the mixed frequency/time domain, where pulse durations are similar to system dephasing times. In these experiments, expectations derived from the familiar driven and impulsive limits are not valid. This work simulates the mixed-domain four-wave mixing response of a model system to develop expectations for this more complex field-matter interaction. We explore frequency and delay axes. We show that these line shapes are exquisitely sensitive to excitation pulse widths and delays. Near pulse overlap, the excitation pulses induce correlations that resemble signatures of dynamic inhomogeneity. We describe these line shapes using an intuitive picture that connects to familiar field-matter expressions. We develop strategies for distinguishing pulse-induced correlations from true system inhomogeneity. These simulations provide a foundation for interpretation of ultrafast experiments in the mixed domain.

A new family of vibrational and electronic spectroscopies has emerged, comprising the coherent analogs of traditional analytical methods. These methods are also analogs of coherent multidimensional nuclear magnetic resonance (NMR) spectroscopy. This new family is based on creating the same quantum mechanical superposition states called multiple quantum coherences (MQCs). NMR MQCs are mixtures of nuclear spin states that retain their quantum mechanical phase information for milliseconds. The MQCs in this new family are mixtures of vibrational and electronic states that retain their phases for picoseconds or shorter times. Ultrafast, high-intensity coherent beams rapidly excite multiple states. The excited MQCs then emit bright beams while they retain their phases. Time-domain methods measure the frequencies of the MQCs by resolving their phase oscillations, whereas frequency-domain methods measure the resonance enhancements of the output beam while scanning the excitation frequencies. The resulting spectra provide multidimensional spectral signatures that increase the spectroscopic selectivity required for analyzing complex samples.

Molybdenum disulfide (MoS 2 ) is a two-dimensional material promising for electronic, optical, and catalytic applications. To fully harness its potential, functionalization is essential to controlling its properties. However, MoS 2 functionalization has been mostly limited to either 1T-phase MoS 2 or the edges of 2H-phase MoS 2 , and the chemistry of covalent functionalization on the basal plane of 2H-MoS 2 is poorly understood. Here, we report a facile approach to covalently functionalize chemical vapor deposition (CVD) grown 2H-MoS 2 monolayers (MLs), as well as mechanically exfoliated MoS 2 , via thiol conjugation at sulfur vacancies on the basal plane. Thorough characterization confirmed the functionalization by thiol molecules on MoS 2 MLs, and we experimentally proved that sulfur vacancies in MoS 2 MLs play a key role in the functionalization of basal planes. By the controlling of the amount of sulfur vacancies via sulfur annealing, the degree of MoS 2 functionalization was effectively tuned. Because thiol conjugation partially repairs or passivates sulfur vacancies, enhanced photoluminescence response and decreased active sites for hydrogen evolution catalysis were observed for functionalized MoS 2 . Moreover, such functionalization can be utilized for making MoS 2 -based heterostructures, an example of which was demonstrated using a dithiol molecule to link MoS 2 layers and PbSe quantum dots. These results provide new understanding and insights on the surface chemistry of MoS 2 and open up more opportunities for MoS 2 MLs with well-controlled properties and broader applications.

The interesting and tunable properties of layered metal dichalcogenides heavily depend on their phase and layer stacking. Here, we show and explain how the layer stacking and physical properties of WSe 2 are influenced by screw dislocations. A one-to-one correlation of atomic force microscopy and high- and low-frequency Raman spectroscopy of many dislocated WSe 2 nanoplates reveals variations in the number and shapes of dislocation spirals and different layer stackings that are determined by the number, rotation, and location of the dislocations. Plates with triangular dislocation spirals form noncentrosymmetric stacking that gives rise to strong second-harmonic generation and enhanced photoluminescence, plates with hexagonal dislocation spirals form the bulk 2H layer stacking commonly observed, and plates containing mixed dislocation shapes have intermediate noncentrosymmetric stackings with mixed properties. Multiple dislocation cores and other complexities can lead to more complex stackings and properties. These previously unobserved properties and layer stackings in WSe 2 will be interesting for spintronics and valleytronics.

We report the first coherent multidimensional spectroscopy study of a MoS2 film. A four-layer sample of MoS2 was synthesized on a silica substrate by a simplified sulfidation reaction and characterized by absorption and Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. State-selective coherent multidimensional spectroscopy (CMDS) on the as-prepared MoS2 film resolved the dynamics of a series of diagonal and cross-peak features involving the spin-orbit split A and B excitonic states and continuum states. The spectra are characterized by striped features that are similar to those observed in CMDS studies of quantum wells where the continuum states contribute strongly to the initial excitation of both the diagonal and cross-peak features, while the A and B excitonic states contributed strongly to the final output signal. The strong contribution from the continuum states to the initial excitation shows that the continuum states are coupled to the A and B excitonic states and that fast intraband relaxation is occurring on a sub-70 fs time scale. A comparison of the CMDS excitation signal and the absorption spectrum shows that the relative importance of the continuum states is determined primarily by their absorption strength. Diagonal and cross-peak features decay with a 680 fs time constant characteristic of exciton recombination and/or trapping. The short time dynamics are complicated by coherent and partially coherent pathways that become important when the excitation pulses are temporally overlapped. In this region, the coherent dynamics create diagonal features involving both the excitonic states and continuum states, while the partially coherent pathways contribute to cross-peak features.

Understanding crystal growth and improving material quality is important for improving semiconductors for electronic, optoelectronic, and photovoltaic applications. Amidst the surging interest in solar cells based on hybrid organic-inorganic lead halide perovskites and the exciting progress in device performance, improved understanding and better control of the crystal growth of these perovskites could further boost their optoelectronic and photovoltaic performance. Here, we report new insights on the crystal growth of the perovskite materials, especially crystalline nanostructures. Specifically, single crystal nanowires, nanorods, and nanoplates of methylammonium lead halide perovskites (CH3NH3PbI3 and CH3NH3PbBr3) are successfully grown via a dissolution-recrystallization pathway in a solution synthesis from lead iodide (or lead acetate) films coated on substrates. These single crystal nanostructures display strong room-temperature photoluminescence and long carrier lifetime. We also report that a solid-liquid interfacial conversion reaction can create a highly crystalline, nanostructured MAPbI3 film with micrometer grain size and high surface coverage that enables photovoltaic devices with a power conversion efficiency of 10.6%. These results suggest that single-crystal perovskite nanostructures provide improved photophysical properties that are important for fundamental studies and future applications in nanoscale optoelectronic and photonic devices.

Iron pyrite (FeS2) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress, the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (VOC). Here we report a comprehensive investigation on {100}-faceted n-type iron pyrite single crystals to understand its puzzling low VOC. We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals. These insights were used to develop a circuit model analysis for the electrochemical impedance spectroscopy that allowed a complete characterization of the bulk and surface defect states and the construction of a detailed energy band diagram for iron pyrite crystals. A holistic evaluation revealed that the high-density of intrinsic surface states cannot satisfactorily explain the low photovoltage; instead, the ionization of high-density bulk deep donor states, likely resulting from bulk sulfur vacancies, creates a nonconstant charge distribution and a very narrow surface space charge region that limits the total barrier height, thus satisfactorily explaining the limited photovoltage and poor photoconversion efficiency of iron pyrite single crystals. These findings lead to suggestions to improve single crystal pyrite and nanocrystalline or polycrystalline pyrite films for successful solar applications.

This work demonstrates the use of triply resonant sum frequency (TRSF) spectroscopy as a "resonance IR" analogue to resonance Raman spectroscopy. TRSF is a four-wave-mixing process where three lasers with independent frequencies interact coherently with a sample to generate an output at their triple summation frequency. The first two lasers are in the infrared and result in two vibrational excitations, while the third laser is visible and induces a two-quantum anti-Stokes resonance Raman transition. The signal intensity grows when the laser frequencies are all in resonance with coupled vibrational and electronic states. The method therefore provides electronic enhancement of IR-active vibrational modes. These modes may be buried beneath solvent in the IR spectrum and also be Raman-inactive and therefore inaccessible by other techniques. The method is presented on the centrosymmetric complex copper phthalocyanine tetrasulfonate. In this study, the two vibrational frequencies were scanned across ring-breathing modes, while the visible frequency was left in resonance with the copper phthalocyanine tetrasulfonate Q band, resulting in a two-dimensional infrared plot that also reveals coupling between vibrational states. TRSF has the potential to be a very useful probe of structurally similar biological motifs such as hemes, as well as synthetic transition-metal complexes.

This article describes the new multidimensional spectroscopy technique triply resonant sum frequency spectroscopy, a four-wave mixing technique sharing advantages of both 2D-IR and resonance Raman experiments. In this technique, lasers with three independent frequencies interact coherently within a sample and generate an output frequency at their triple summation. The output intensity depends on coupled electronic and vibrational resonances in the sample. We use an organic dye as a model system to demonstrate fully resonant, fully coherent multidimensional spectroscopy using two independently tunable mid-infrared vibrational interactions and one visible electronic interaction. When the pulses are time ordered, the method has a single coherence pathway, eliminating interference between pathways. Fundamental vibrational transitions appear on one axis and overtones and combinations bands on the other, allowing anharmonicities of the modes to be determined easily and conveying molecular coupling information. The experiments demonstrate coupling between seven vibrational ring modes and an electronic state, the resolution of a Fermi resonance, detection of low concentrations, elimination of excitation pulse scattering and fluorescence, background suppression of solvent and co-solutes, and observation of coherence dephasing dynamics. The electronic resonance enhancements used in this methodology are similar to the enhancements responsible for resonance Raman spectroscopy and can be considered resonance 2D-IR spectroscopy.

In this paper we present a new multiresonant coherent multidimensional spectroscopy (CMDS) technique employing a pathway that is both fully coherent and necessarily unique. This technique is based on a Triple Sum Frequency (TSF) coherence pathway with three excitation pulses having frequencies ω1, ω2, and ω3 and the phase matching condition k→1 + k→2 + k→3. Two-dimensional spectra are created by independently tuning the ω1 and ω2 pulses across vibrational resonances while monitoring the intensity of a visible output beam created by a Raman transition induced by the ω3 pulse. Two-dimensional plots of the coherent dynamics are created by independently scanning the τ21 and τ31 delay times between the different frequency excitation pulses over all time orderings. TSF CMDS separates fundamental and overtone/combination band states uniquely onto the ω1 and ω2 axes when τ21 ≠ 0. TSF is valuable in its ability to probe states of complementary parity to those seen in Doubly Vibrationally Enhanced Four-Wave Mixing (DOVE-FWM), the other fully coherent mixed electronic/vibrational CMDS method. This capability is demonstrated through the use of neat benzene as a model system, where the center of inversion imposes strict parity selection rules.

Quantum dot nanoscale semiconductor heterostructures (QDHs) are a class of materials potentially useful for integration into solar energy conversion devices. However, realizing the potential of these heterostructured systems requires the ability to identify and synthesize heterostructures with suitably designed materials, controlled size and morphology of each component, and structural control over their shared interface. In this review, we will present the case for the utility and advantages of chemically synthesized QDHs for solar energy conversion, beginning with an overview of various methods of heterostructured material synthesis and a survey of heretofore reported materials systems. The fundamental charge transfer properties of the resulting materials combinations and their basic design principles will be outlined. Finally, we will discuss representative solar photovoltaic and photoelectrochemical devices employing QDHs (including quantum dot sensitized solar cells, or QDSSCs) and examine how QDH synthesis and design impacts their performance.

Recent work spectrally isolated and measured the quantum states associated with ultrafast relaxation from an initially excited 1S excitonic state to a lower energy state that is present in an inadequately capped PbSe quantum dot sample. The relaxed state was attributed to a surface-trapped exciton (STE). This letter reports the line-narrowed, multiresonant, two-dimensional spectrum of this sample. The multidimensional spectrum is unusual because diagonal peaks are absent, but there is a strong cross-peak between the 1S and STE transitions. Theoretical modeling provided values for the coherent and incoherent dynamics, the relative exciton and biexciton transition moments, the Coulombic coupling, and the homogeneous and inhomogeneous broadening. This work demonstrates the feasibility of using multiresonant methods to probe the quantum state dynamics of interface states in nanostructures.

Focused electron-beam-induced deposition using bulk liquid precursors (LP-EBID) is a new nanofabrication technique developed in the last two years as an alternative to conventional EBID, which utilizes cumbersome gaseous precursors. Furthermore, LP-EBID using dilute aqueous precursors has been demonstrated to yield platinum (Pt) nanostructures with as-deposited metal content that is substantially higher than the purity achieved by EBID with currently available gaseous precursors. This advantage of LP-EBID--along with the ease of use, low cost, and relative innocuousness of the liquid precursors--holds promise for its practical applicability in areas such as rapid device prototyping and lithographic mask repair. One of the feasibility benchmarks for the LP-EBID method is the ability to deposit high-fidelity nanostructures on various substrate materials. In this study, we report the first observations of performing LP-EBID on bare and metal-coated silicon-nitride membranes, and compare the resulting Pt deposits to those obtained by LP-EBID on polyimide membranes in terms of nucleation, morphology, size dependence on electron dose, and purity.

The multiresonant coherent multidimensional spectroscopy study of D(2)O in AOT micelles reveals two unexpected features in addition to those expected for D(2)O. These features appear when the excitation pulse time ordering defines either fully coherent Liouville pathways or partially coherent pathways where there are intermediate populations. The features shift as the excitation pulse ordering changes between the two sets of pathways. They are attributed to the asymmetric stretch mode of CO(2) that is formed by the infrared excited decarboxylation of AOT. The cross-peaks' intensity is enhanced when the excitation pulse is resonant with the CO(2) P and R branches for the fully coherent pathways, but they are depressed at the P and R branch resonances for the partially coherent pathways. The enhancement is attributed to the simultaneous production of CO(2) during the first excitation pulse when it is resonant with both the CO(2) and the D(2)O. The depression is attributed to the absorption of the last excitation pulse by the CO(2) produced by the previous two excitation pulses. We believe that the decarboxylation occurs because of energy transfer from multiply excited D(2)O molecules. It may cause a Norrish type I α-cleavage in the AOT that is followed by a Hunsdiecker reaction β-scission of the radical to form the CO(2). This experiment demonstrates the marriage of infrared induced chemistry and multiresonant coherent multidimensional spectroscopy into a single nonlinear process.

Multiresonant four wave mixing has been used to measure the coherent multidimensional spectroscopy (CMDS) of representative aromatic ring modes using pyridine as a model system. This work identifies the cross-peaks that appear between several modes and measures their coherent and incoherent dynamics. The work also explores the consequences of using multiresonant CMDS for molecules with transition moments that are typical of most vibrational modes. Typically, CMDS experiments rely on using transitions with exceptionally large transition moments. To observe cross-peaks, the pyridine concentration was raised until absorption effects became very important. These effects interfere with the parametric CMDS coherence pathways, but they do not make important contributions to the nonparametric pathways.

Multiresonant coherent multidimensional spectroscopy is a frequency-domain method that uses tunable excitation pulses to excite multiple quantum coherences (MQCs) and/or state populations using fully coherent or partially coherent excitation pathways. Pairs of states that are coupled by intra- and intermolecular interactions re-emit light at their frequency differences. The MQCs are coherent and interfere constructively to create phase-matched output beams. Scanning the excitation frequencies with fixed-excitation-pulse time delays creates multidimensional spectra, whereas scanning the time delays with fixed excitation frequencies measures the MQCs' coherent and incoherent dynamics. Multiresonant methods can excite any combination of vibrational and/or electronic states and use any coherence pathway. Cross-peaks occur between states when the excitation of one perturbs the other. This requirement for coupling acts to eliminate spectral congestion. Spectral resolution is increased because multiresonant methods narrow inhomogeneous broadening, enhance peaks from specific components, and spread the resolution over multiple dimensions.

Nuclear magnetic resonance spectroscopy relies on using multiple excitation pulses to create multiple quantum coherences that provide great specificity for chemical measurements. Coherent multidimensional spectroscopy (CMDS) is the optical analogue of NMR. Current CMDS methods use three excitation pulses and phase matching to create zero, single, and double quantum coherences. In order to create higher order multiple quantum coherences, the number of interactions must be increased by raising the excitation intensities high enough to create Rabi frequencies that are comparable to the dephasing rates of vibrational coherences. The higher Rabi frequencies create multiple, odd-order coherence pathways. The coherence pathways that involve intermediate populations are partially coherent and are sensitive to population relaxation effects. Pathways that are fully coherent involve only coherences and measure the direct coupling between excited quantum states. The fully coherent pathways are related to the multiple quantum coherences created in multiple pulse NMR methods such as heteronuclear multiple quantum coherence (HMQC) spectroscopy with the important difference that HMQC NMR methods have a defined number of interactions and avoid dynamic Stark effects whereas the multiply enhanced odd-order wave-mixing pathways do not. The difference arises because CMDS methods use phase matching to define the interactions and at high intensities, multiple pathways obey the same phase matching conditions. The multiple pathways correspond to the pathways created by dynamic Stark effects. This paper uses rhodium dicarbonyl chelate (RDC) as a model to demonstrate the characteristics of multiply enhanced odd-order wave-mixing (MEOW) methods. Dynamic Stark effects excite vibrational ladders on the symmetric and asymmetric CO stretch modes and create a series of multiple quantum coherences and populations using partially and fully coherent pathways. Vibrational quantum states up to v = 6 are excited. A series of spectra provides different two-dimensional cross sections through the multidimensional parameter space involving two excitation frequencies, the frequency of the output coherence, and the excitation pulse time delays. The spectra allow the identification of 18 different overtone and combination band states. Comparison with a local mode model with two anharmonic Morse oscillators with interbond coupling shows excellent agreement.

Extending current coherent multidimensional spectroscopy (CMDS) methods to higher order multiwave mixing requires excitation intensities where dynamic Stark effects become important. This paper examines the dynamic Stark effects that occur in mixed frequency/time domain CMDS methods at high excitation intensities in a model system with an isolated vibrational state. The phase-matching restrictions in CMDS define the excitation beams that interact by nonlinear mixing while the dynamic Stark effects create vibrational ladders of increasingly more energetic overtone and combination band states. The excited quantum states form coherences that reemit the output beams. This paper uses the phase-matching conditions k(out) = k(1) - k(2) + k(2') and k(out) =- k(1) + k(2) + k(2'), where the subscripts denote the excitation frequencies of each excitation pulse and the output pulse. The phase-matching condition constrains each pulse to have an odd number of interactions so the overall mixing process that creates the output coherence must also involve an odd number of interactions. Tuning the excitation frequencies and spectrally resolving the output intensity creates three-dimensional spectra that resolve the individual overtone states. Changing the excitation pulse time delays measures the dynamics of the coherences and populations created by the multiple excitations. The multidimensional spectra probe the highly excited states of a molecular potential energy surface. This paper uses tungsten hexacarbonyl (W(CO)(6)) as a model for observing how dynamic Stark effects change the multidimensional spectra of a simple system. The simplicity of the W(CO)(6) system provides the experimental data required to develop the nonperturbative theoretical methods that will be necessary to model this new approach to CMDS.

Coherent multidimensional spectroscopy (CMDS) is now the optical analogue of nuclear magnetic resonance (NMR). Just as NMR heteronuclear multiple-quantum coherence (HMQC) methods rely on multiple quantum coherences, achieving widespread application requires that CMDS also excites multiple quantum coherences over a wide range of quantum state energies. This Account focuses on frequency-domain CMDS because these methods tune the excitation frequencies to resonance with the desired quantum states and can form multiple quantum coherences between states with very different energies. CMDS methods use multiple excitation pulses to excite multiple quantum states within their dephasing time, so their quantum mechanical phase is maintained. Coherences formed from pairs of the excited states emit coherent beams of light. The temporal ordering of the excitation pulses defines a sequence of coherences that can result in zero, single, double, or higher order coherences as required for multiple quantum coherence CMDS. Defining the temporal ordering and the excitation frequencies and spectrally resolving the output frequency also defines a particular temporal pathway for the coherences, just as an NMR pulse sequence defines an NMR method. Two dimensional contour plots through this multidimensional parameter space allow visualization of the state energies and dynamics. This Account uses nickel and rhodium chelates as models for understanding mixed frequency-/time-domain CMDS. Mixed frequency-/time-domain methods use excitation pulse widths that are comparable to the dephasing times, so multidimensional spectra are obtained by scanning the excitation frequencies, while the coherence and population dynamics are obtained by scanning the time delays. Changing the time delays changes the peaks in the 2D excitation spectra depending upon whether the pulse sequence excites zero, single, or double quantum coherences. In addition, peaks split as a result of the frequency-domain manifestation of quantum beating. Similarly, changing the excitation and monochromator frequencies changes the dependence on the excitation delay times depending upon whether the frequencies match the resonances involved in the different time-ordered pathways. Contour plots that change a time delay and frequency visualize the temporal changes of specific spectral features. Frequency-domain methods are resonant with specific states, so the sequence of coherences and populations is defined. Coherence transfer, however, can cause output beams at unexpected frequencies. Coherence transfer occurs when the thermal bath induces a coherence between two states (a and g) to evolve to a new coherence (b and g). Since the two coherences have different frequencies and since there are different time orderings for the occurrence of coherence transfer, the delay time dependence develops modulations that depend on the coherences' frequency difference. Higher order coherences can also be generated by raising the excitation intensities. New features appear in the 2D spectra and dynamic Stark splittings occur. These effects will form the basis for the higher order multiple quantum coherence methods and also provide a method for probing molecular potential energy surfaces.

Mixed frequency/time domain, two color triply vibrationally enhanced (TRIVE) four wave mixing (FWM) spectroscopy is used to study the methyl and methylene modes in octane and dotriacontane. The experiments involve scanning different combinations of the two excitation frequencies, the monochromator frequency, and the two time delays between the three excitation pulses while the remaining variables are fixed. Two dimensional spectra of the methyl and methylene stretching region have weak, asymmetrical diagonal- and cross-peaks when the excitation pulses are temporally overlapped. As the time delays change, the spectra change as new peaks appear and their peak intensity and position change. Combined two-dimensional scans of the excitation frequency and time delay show the changes are caused by relaxation of the initially excited populations to other states that are coupled to the methyl and methylene stretching modes. Two dimensional time delay scans show that the coherence dephasing rates are very fast so fully coherent TRIVE FWM pathways involving multiple quantum coherences are not possible without shorter excitation pulses. Similar experiments involving the methyl and methylene bend and stretching modes identify cross-peaks between these modes and population transfer processes that create cross-peaks. The asymmetric methylene stretch/Fermi resonance band is observed to contain unresolved states that couple differently with the symmetric methylene stretching and scissor modes as well as with lower lying quantum states that are fed by population transfer. The TRIVE FWM data show that the multidimensional spectra are dominated by rapid population transfer within the methyl and methylene stretching modes and to lower quantum states that are coupled to the stretching modes.

Triply vibrationally enhanced four-wave mixing spectroscopy is employed to observe vibrational coherence transfer between the asymmetric and symmetric CO-stretching modes of rhodium(I) dicarbonyl acetylacetonate (RDC). Coherence transfer is a nonradiative transition of a coherent superposition of quantum states to a different coherent superposition due to coupling of the vibrational modes through the bath. All three excitation pulses in the experiment are resonant with a single quantum coherence, but coherence transfer results in new coherences with different frequencies. The new output frequency is observed with a monochromator that resolves it from the stronger peak at the original excitation frequency. This technique spectrally resolves pathways that include coherence transfer, discriminates against spectral features created solely by radiative transitions, and temporally resolves modulations created by interference between different coherence transfer pathways. Redfield theory simulates the temporal modulations in the impulsive limit, but it is also clear that coherence transfer violates the secular approximation invoked in most Redfield theories. Instead, it requires non-Markovian and bath memory effects. RDC may provide a simple model for the development of theories that incorporate these effects.

Objective: To compare antibody responses, feedlot morbidity and mortality rates, feedlot performance, and carcass value for calves vaccinated with 1 of 2 vaccination strategies and for unvaccinated control calves., Design: Randomized controlled clinical trial., Animals: 451 beef steers and heifers., Procedures: Calves were vaccinated with a modified-live infectious bovine rhinotracheitis virus (IBRV), bovine viral diarrhea virus types 1 (BVDV1) and 2 (BVDV2), parainfluenza type 3 virus, and bovine respiratory syncytial virus vaccine and Mannheimia haemolytica and Pasteurella multocida bacterin-toxoid at approximately 67 and 190 days of age (group 1; n = 151) or at approximately 167 and 190 days of age (group 2; 150) or were not vaccinated (control; 150). Serum antibody titers were measured at approximately 2, 67, 167, 190, and 232 days of age. Morbidity and mortality rates, feedlot performance, and carcass value were recorded for 361 calves shipped to feedlots., Results: Percentages of calves seroconverting to IBRV, BVDV1, and BVDV2 were significantly higher for groups 1 and 2 than for the control group. Mean treatment costs were significantly lower for vaccinated than for control calves, and mean mortality rate was significantly higher for control calves than for group 1 calves. Feedlot performance and carcass value did not vary significantly among groups., Conclusions and Clinical Relevance: Results suggested that vaccination of beef calves with a 5-antigen modified-live virus vaccine at 67 and 190 days of age was as effective in terms of immunologic responses as was vaccination at 167 and 190 days of age.