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A high-resolution absorption spectrum of the S1–S0 transition of free-base phthalocyanine was observed and analyzed with improved reliability. The spectrum, with a partially resolved rotational structure, was obtained by using the buffer-gas cooling technique and a single-mode tunable laser. Our new analysis reveals that the S 1 ← S 0 0 0 0 band belongs to the a-type transition, where the electronic transition moment aligns parallel to the NH–HN direction, allowing the assignment of the S1 state to 1B3u. These results agree with a prior study using supersonic expansion and are well supported by theoretical calculations. Interestingly, the rotational constant B in the S1 state, which is often smaller than that in the ground state for typical molecules, was found to be slightly larger than that in the S01Ag state. This suggests a change in the character of π bonds with the electronic excitation. [ABSTRACT FROM AUTHOR]

The absorption spectra of thin films (CsCl)1–x(CuCl)x were studied at T = 90 K in the spectral range 2–6 eV. From the analysis of the spectra, the formation of CsCu2Cl3 and Cs2CuCl3 compounds in the CsCl–CuCl system was created. The Cs2CuCl3 compound is stable only in a vacuum; in air, it apparently decomposes into CsCu2Cl3 with a disordered crystal lattice and CsCl. The exciton spectrum of both compounds is interpreted on the basis of transitions in the Cu+ ion. The one-dimensional character of excitons in CsCu2Cl3 and the two-dimensional character in Cs2CuCl3 were established. [ABSTRACT FROM AUTHOR]

We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. [ABSTRACT FROM AUTHOR]

The local vibrational dynamics of hematite (α-Fe2O3) has been investigated by temperature-dependent extended x-ray absorption fine structure spectroscopy and molecular dynamics simulations. The local dynamics of both the short and long nearest-neighbor Fe–O distances has been singled out, i.e., their local thermal expansion and the parallel and perpendicular mean-square relative atomic displacements have been determined, obtaining a partial agreement with molecular dynamics. No evidence of the Morin transition has been observed. More importantly, the strong anisotropy of relative thermal vibrations found for the short Fe–O distance has been related to its negative thermal expansion. The differences between the local dynamics of short and long Fe–O distances are discussed in terms of projection and correlation of atomic motion. As a result, we can conclude that the short Fe–O bond is stiffer to stretching and softer to bending than the long Fe–O bond. [ABSTRACT FROM AUTHOR]

Poly(vinylidene fluoride)/graphene oxide (PVDF/GO) nanocomposites are synthesized and their structural, ferroelectric, and pyroelectric properties are investigated. The dielectric spectrum analysis and P-E loop tests indicate that the nanocomposites exhibit enhanced ferroelectric and pyroelectric properties compared with those of poly(vinylidene fluoride) samples. The isothermal crystallization kinetics of PVDF/GO nanocomposites quantitatively determined by differential scanning calorimetry demonstrates that GOs facilitate the crystallization of the PVDF. Dynamic mechanical analyses on the PVDF/GO reveal that the amorphous and crystalline phases of PVDF are modified by the addition of GO sheets. The GO-enhanced formation of crystalline β phase in PVDF could result from the strong interaction between the -C=O groups in GO and the -CF2 groups in PVDF, and the GO-induced ordering of the microstructures of amorphous and crystalline phases. The results suggest that PVDF/GO nanocomposites could be promising dielectric materials used in sensors, transducers, and actuators. [ABSTRACT FROM AUTHOR]

We have measured thermoelectric power in two series of polar InN:Mg samples with wide range of Mg content having In- as well as N-growth polarities. We have observed essential differences between both polarities: In the "p-type window" centered at about 1 × 1019 cm-3 of [Mg], reported recently, the thermoelectric power changed its sign from n to p-type, only for In-growth polarity samples. These results have been confirmed by the so-called mobility spectrum analysis. It strongly supports the suggestion that In-growth polarity is more propitious to p-type conduction in InN:Mg than the N one. [ABSTRACT FROM AUTHOR]

Room temperature absorption spectra of various transitions of pure CO2 have been measured in a broad pressure range using a tunable diode-laser and a cavity ring-down spectrometer, respectively, in the 1.6 μm and 0.8 μm regions. Their spectral shapes have been calculated by requantized classical molecular dynamics simulations. From the time-dependent auto-correlation function of the molecular dipole, including Doppler and collisional effects, spectral shapes are directly computed without the use of any adjusted parameter. Analysis of the spectra calculated using three different anisotropic intermolecular potentials shows that the shapes of pure CO2 lines, in terms of both the Lorentz widths and non-Voigt effects, slightly depend on the used potential. Comparisons between these ab initio calculations and the measured spectra show satisfactory agreement for all considered transitions (from J = 6 to J = 46). They also show that non-Voigt effects on the shape of CO2 transitions are almost independent of the rotational quantum number of the considered lines. [ABSTRACT FROM AUTHOR]

We investigate the calculation of absorption spectra based on the mixed quantum classical Liouville equation (MQCL) methods. It has been shown previously that, for a single excited state, the averaged classical dynamics approach to calculate the linear and nonlinear spectroscopy can be derived using the MQCL formalism. This work focuses on problems involving multiple coupled excited state surfaces, such as in molecular aggregates and in the cases of coupled electronic states. A new equation of motion to calculate the dipole-dipole correlation functions within the MQCL formalism is first presented. Two approximate methods are then proposed to solve the resulted equations of motion. The first approximation results in a mean field approach, where the nuclear dynamics is governed by averaged forces depending on the instantaneous electronic states. A modification to the mean field approach based on first order moment expansion is also proposed. Numerical examples including calculation of the absorption spectra of Frenkel exciton models of molecular aggregates, and the pyrazine molecule are presented. [ABSTRACT FROM AUTHOR]

Stick-spectrum expressions for electronic two-dimensional (2D) photon-echo (PE) signal of a generic multi-level system are presented and employed to interrelate oscillations in individual peaks of 2D PE signal and the underlying properties (eigenstates and coherent dynamics) of excitonic or vibronic systems. When focusing on the identification of the origin of oscillations in the rephasing part of 2D PE it is found, in particular, that multiple frequencies in the evolution of the individual peaks do not necessarily directly reflect the underlying system dynamics. They may originate from the excited-state absorption contribution to the signal, or arise due to multi-level vibrational structure of the electronic ground state, and represent a superposition of system frequencies, while the latter may evolve independently. The analytical stick-spectrum predictions are verified and illustrated by numerical calculations of 2D PE signals of an excitonic trimer and of a displaced harmonic oscillator with unequal vibrational frequencies in the two electronic states. The excitonic trimer is the smallest excitonic oligomer where excited-state absorption may represent a superposition of excited-state coherences and significantly influence the phase of the observed oscillations. The displaced oscillator is used to distinguish between the frequencies of the ground-state and of the excited-state manifolds, and to demonstrate how the location of a cross peak in 2D pattern of the PE signal "predetermines" its oscillatory behavior. Although the considered models are kept as simple as possible for clarity, the stick-spectrum analysis provides a solid general basis for interpretation of oscillatory signatures in electronic 2D PE signals of much more complex systems with multi-level character of the electronic states. [ABSTRACT FROM AUTHOR]

The geometry and infrared absorption spectrum of (RbCs)2 have been studied by full dimensional quantum dynamics simulations. For this purpose, the potential energy and dipole moment surfaces are generated by means of a cluster expansion with all two and three mode correlations, and fitted to analytical expressions with negligible deviations. Accordingly, the ground state (RbCs)2 has a diamond geometry with D2h symmetry. The infrared spectrum with frequencies up to 120 cm-1, exhibits rich details of the fundamentals, overtones, and combination bands; the highest fundamental frequency of (RbCs)2 is only 40.26 cm-1. The present study unravels important details of the interactions between the widely investigated ultracold RbCs molecules. [ABSTRACT FROM AUTHOR]

Time-resolved optical techniques of photoluminescence (PL), light-induced transient grating (LITG), and differential transmission spectroscopy were used to investigate carrier dynamics in a single 50-nm thick In0.13Ga0.97N epilayer at high photoexcitation levels. Data in wide spectral, temporal, excitation, and temperature ranges revealed novel features in spectral distribution of recombination rates as follows: at low injection levels, an inverse correlation of carrier lifetime increasing with temperature and diffusivity decreasing with temperature confirmed a mechanism of diffusion-limited nonradiative recombination at extended defects. Carrier dynamics in the spectral region below the absorption edge but ∼70 meV above the PL band revealed a recombination rate that increased with excitation, while recombination rate in PL emission band (420-430 nm) decreased after saturation of trapping centers. Monitoring of spectrally integrated carrier dynamics by LITG technique allowed us to ascribe the enhanced recombination rate to bimolecular recombination and determine its coefficient B = 7 × 10-11 cm3/s. Complementary measurements unveiled the cause of PL efficiency saturation at injection levels above 5 × 1018 cm-3, attributable to bandgap renormalization in the extended states above the PL emission band, which encumbers carrier transfer from high-to-low energy states. As the degree of localization, and therefore, the total number of band tail states is expected to increase with In content, their impact to dependence of PL efficiency on excitation density could even be stronger for higher In compositions. These results provided insight that spectrally resolved carrier generation-recombination rates are excitation-dependent and would play a critical role in saturation of internal quantum efficiency in InGaN alloys used in light emitters, such as light emitting diodes. [ABSTRACT FROM AUTHOR]

A re-discovered member of the layered material family, Titanium Trisulfide (TiS3), has attracted intense research interest recently for the possibility of realizing an exciton insulator in the monolayer limit. However, due to their quasi-one-dimensional nature, thin TiS3 flakes are typically in the form of nanoribbons that are challenging to characterize by optical absorption spectra, due to their small cross section. Here, we employ a sensitive photocurrent spectroscopy technique to probe the absorption of a thin TiS3 nanoribbon in a field-effect transistor configuration. We have found a significant modification of the band structure of the thin TiS3 nanoribbon, compared to its bulk counterpart. In addition, the polarization-dependent photocurrent spectra of thin TiS3 nanoribbons exhibit greatly enhanced anisotropy compared to that of a thick TiS3 device. The atomically thin TiS3 flake, thus, not only provides an exciting platform for investigating many-body physics but also enables anisotropic quantum optoelectronics. [ABSTRACT FROM AUTHOR]

Based on absorption spectroscopy, a spectrum measurement method utilizing light-controlled tunable grating (LTG) is presented for detecting combustible and explosive gases. When the broadband light with gas spectral absorption information is incident on the LTG, the pitch of the LTG is changed continuously under the illumination of controlled driving light. By analyzing diffraction light from the LTG, the information of the absorption spectrum can be obtained, thereby achieving the detection of the target gas. Owing to the usage of optical drive grating, a wavelength scanning over a wide range can be realized without any involvement of electronic components. For practical application, our proposal measurement method was utilized for methane detection, and the corresponding results demonstrate the effectiveness of our method. [ABSTRACT FROM AUTHOR]

Eleven very weak electric quadrupole transitions Q(2), Q(1), S(0)-S(8) of the first overtone band of D2 have been measured by very high sensitivity CW-cavity ring down spectroscopy (CRDS) between 5850 and 6720 cm-1. The noise equivalent absorption of the recordings is on the order of αmin ≈ 3 × 10-11 cm-1. By averaging a high number of spectra, the noise level was lowered to αmin ≈ 4 × 10-12 cm-1 in order to detect the S(8) transition which is among the weakest transitions ever detected in laboratory experiments (line intensity on the order of 1.8 × 10-31 cm/molecule at 296 K). A Galatry profile was used to reproduce the measured line shape and derive the line strengths. The pressure shift and position at zero pressure limit were determined from recordings with pressures ranging between 10 and 750 Torr. A highly accurate theoretical line list was constructed for pure D2 at 296 K. The intensity threshold was fixed to a value of 1 × 10-34 cm/molecule at 296 K. The obtained line list is provided as supplementary material. It extends up to 24 000 cm-1 and includes 201 transitions belonging to ten v-0 cold bands (v = 0-9) and three v-1 hot bands (v = 1-3). The energy levels include the relativistic and quantum electrodynamic corrections as well as the effects of the finite nuclear mass. The quadrupole transition moments are calculated using highly accurate adiabatic wave functions. The CRDS line positions and intensities of the first overtone band are compared to the corresponding calculated values and to previous measurements of the S(0)-S(3) lines. The agreement between the CRDS and theoretical results is found within the claimed experimental uncertainties (on the order of 1 × 10-3 cm-1 and 2% for the positions and intensities, respectively) while the previous S(0)-S(3) measurements showed important deviations for the line intensities. [ABSTRACT FROM AUTHOR]

Effect of Ce3+ ions introduction on 2 μm emission properties and energy transfer mechanism in Yb3+/Ho3+-doped fluorophosphate glass has been investigated. From the measured fluorescence spectra, strong emission near 2 μm is demonstrated due to the sensitization of Yb3+ and Ce3+. Based on the absorption spectra, the Judd-Ofelt parameters and radiative properties were calculated and compared with those of other glass hosts. In addition, the energy transfer coefficient from Yb3+ to Ho3+ is 9.52 × 10-40 cm6/s in Yb3+/Ho3+/Ce3+ triply doped sample and is three times larger than that in undoped Ce3+ sample. These results suggest that this Yb3+/Ho3+/Ce3+ fluorophosphate glass with excellent thermal stability and efficient energy transfer from Yb3+ to Ho3+ is a good candidate for 2 μm laser. [ABSTRACT FROM AUTHOR]

The high-frequency region was used to record the absorption spectrum of water in nanoscale pores during vacuum pumping or injection of water. The wide spectral range, which included the vibration overtones, allowed to resolve the structure of the absorption bands with variation of water concentration in the pores of SiO2. The absorption bands of water clusters in the 4570-5400 cm-1 range consist of well-resolved sub-bands with interpeak intervals of up to 580 cm-1. When the pore diameter is decreased from 11.8 to 2.6 nm, the absorption bands of clusters in this frequency range are shifted by 530 cm-1 in the direction of the water monomer which indicates an increase of hydrogen bond strength in confined water with an increase of the pore diameter. The spectrum recorded during water pumping is extremely variable in time, and the cluster dynamics in large pores (11.8 nm) differs greatly from the dynamics in small pores (2.6 nm). While all types of water clusters are removed from small pores uniformly, in the case of large pores, the water clusters relating to strong hydrogen bonds are removed from the sample at the beginning of the vacuum pumping and the loosely coupled clusters are removed later. The rate of this process is not steady and varies throughout pumping. [ABSTRACT FROM AUTHOR]

We theoretically discuss the nonlinear intersubband response of multiple-quantum-well structures embedded in microcavities with plane-wave approximation. We employ a semiclassical approach based on the transfer matrix formalism and the so-called sheet model. The linear absorptive losses are taken into account. It is shown that in the strong-coupling-regime the saturation effect leads to the evolution of the absorption spectra from a pair of simple harmonic oscillators to highly anharmonic ones. The usefulness of the simplified analytical approach, based on a standard multibeam interference analysis and the “mean-field” approximation is also demonstrated. [ABSTRACT FROM AUTHOR]

This article proposes an improved version of recently developed multiconfigurational Ehrenfest approach to quantum dynamics. The idea of the approach is to use frozen Gaussians (FG) guided by Ehrenfest trajectories as a basis set for fully quantum propagation. The method is applied to simulation of nonadiabatic dynamics of pyrazine and shows that nonadiabatic dynamics on two coupled electronic states S2 and S1, which determines pyrazine absorption spectrum, can be simulated with the help of a basis comprised of very small number of trajectory guided basis functions. For the 24 dimensional (24D) model, good results were obtained with the basis of only 250 trajectories guided FG per electronic state. The efficiency of the method makes it particularly suitable for future application together with direct dynamics, calculating potentials on the fly. [ABSTRACT FROM AUTHOR]

We present ultrafast transient absorption spectra of two oligofluorene derivatives in dilute solution. These spectra display a photoinduced absorption band with clear vibronic structure, which we analyze rigorously using a time-dependent formalism of absorption to extract the principal excited-state vibrational normal-mode frequencies that couple to the electronic transition, the configurational displacement of the higher-lying excited state, and the reorganization energies. We can model the excited-state absorption spectrum using two totally symmetric vibrational modes with frequencies 450 (dimer) or 400 cm-1 (trimer), and 1666 cm-1. The reorganization energy of the ground-state absorption is rather insensitive to the oligomer length at 230 meV. However, that of the excited-state absorption evolves from 58 to 166 meV between the oligofluorene dimer and trimer. Based on previous theoretical work [A. Shukla et al., Phys. Rev. B 67, 245203 (2003)], we assign the absorption spectra to a transition from the 1Bu excited state to a higher-lying mAg state, and find that the energy of the excited-state transition with respect to the ground-state transition energy is in excellent agreement with the theoretical predictions for both oligomers studied here. These results and analysis permit profound understanding of the nature of excited-state absorption in π-conjugated polymers, which are the subject of general interest as organic semiconductors in the solid state. [ABSTRACT FROM AUTHOR]

The absorption spectra and ionization potentials of silver clusters Agn (n=4–20) are examined in the framework of density-functional theory (DFT) using several different methods of representing the exchange-correlation functional. Three different types of exchange-correlation functionals are used: those including gradient corrections to the density in the generalized gradient approximation, global hybrid functionals mixing in a portion of the Hartree–Fock exchange, and long-range-corrected (LC-) functionals. Comparison of ionization potentials calculated using DFT with those derived from experiments demonstrates that LC-functionals more accurately represent the electronic structure of the silver clusters studied. Absorption spectra are compared with both experimental spectra and those derived using higher level theoretical calculations showing that the LC-functionals appear to correctly describe the optical transitions in the gas phase, particularly when a small redshift in the experimental spectrum is accounted for due to matrix effects. It is also demonstrated that the LC-hybrid functionals significantly reduce the occurrence of spurious states in the optical absorbance spectrum while maintaining the intensity of plasmon like features of the spectra for larger silver clusters. [ABSTRACT FROM AUTHOR]

We investigated the magnitude and temperature dependence (T dependence) of the dimer absorption in the region of 0–600 cm-1 and the collision-induced absorption (CIA) in the region of 0–1150 cm-1. Together with our previous study of the self water-vapor continuum contributions resulting from far-wing line shapes of the allowed H2O lines in the infrared window between 800 and 1150 cm-1, we find that the three mechanisms have completely different T dependence behaviors. The dimer absorption has the strongest negative T dependence and the continuum absorption from far wings of the allowed lines has a moderately strong negative one. Meanwhile, the CIA exhibits a mild T dependence. In addition, their T dependence patterns are quite different. The T dependence of the far-wing theory varies significantly as the frequency of interest ω varies. For CIA, in general, its T dependence is mildly negative, but becomes slightly positive in the window region between the H2O bands. In contrast, the T dependence of the dimer absorption varies slightly as ω varies. In the microwave and submillimeter region, its T dependence becomes uniform. Concerning the relative importance for each of these three mechanisms, we find that in the infrared widow, the far-wing contributions are the dominant source of the self-continuum. Within the band, its contributions are definitely responsible for the measured continuum data. But, it is impossible to draw quantitatively conclusions on its relative importance unless one is able to improve the accuracy of the local line calculations significantly. On the other hand, within the pure rotational band, the dimer absorptions are a minor contributor to the self-continuum measurements, and its role becomes more important in the microwave and submillimeter regions. Finally, based on our study we conclude that contributions to the self-continuum from CIA in the frequency region of 0–1150 cm-1 are negligible. [ABSTRACT FROM AUTHOR]

Due to growth temperature differences during deposition of GaN heterostructures utilizing InAlN barriers, an inadvertent parasitic GaN layer can form in the InAlN barrier layer. In structures utilizing AlN spacer layers, this parasitic layer acts as a second conduction channel with a carrier density dependent upon polarization charges and lattice strain as well as the surface potential. The effect of an additional GaN spacer layer in InAlN/AlN/GaN structures is assessed using simulations, electron-microscopy observations, magnetoconductivity measurements with gated Hall bar samples, and with quantitative mobility spectrum analysis. We propose a possible formation mechanism for the parasitic layer, and note that although the additional unintended layer may have beneficial aspects, we discuss a strategy to prevent its occurrence. [ABSTRACT FROM AUTHOR]

The Ã-X electronic absorption spectrum of vinoxy radical has been investigated using room temperature cavity ringdown spectroscopy. Analysis of the observed bands on the basis of computed vibrational frequencies and rotational envelopes reveals that two distinct types of features are present with comparable intensities. The first type corresponds to “normal” allowed electronic transitions to the origin and symmetric vibrations in the à state. The second type is interpreted in terms of excitations to asymmetric à state vibrations, which are only vibronically allowed by Herzberg–Teller coupling to the B state. Results of electronic structure calculations indicate that the magnitude of the Herzberg–Teller coupling is appropriate to produce vibronically induced transitions with intensities comparable to those of the normal bands. [ABSTRACT FROM AUTHOR]

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of CH3CO and O2; IR absorption spectra of CH3C(O)OO and α-lactone were observed. Absorption bands with origins at 1851±1, 1372±2, 1169±6, and 1102±3 cm-1 are attributed to t-CH3C(O)OO, and those at 1862±3, 1142±4, and 1078±6 cm-1 are assigned to c-CH3C(O)OO. A weak band near 1960 cm-1 is assigned to α-lactone, cyc-CH2C(==O)O, a coproduct of OH. These observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters and dipole derivatives, and observed vibrational wavenumbers agree with harmonic vibrational wavenumbers predicted with B3LYP/aug-cc-pVDZ density-functional theory. The observed relative intensities indicate that t-CH3C(O)OO is more stable than c-CH3C(O)OO by 3±2 kJ mol-1. Based on these observations, the branching ratio for the OH+α-lactone channel of the CH3CO+O2 reaction is estimated to be 0.04±0.01 under 100 Torr of O2 at 298 K. A simple kinetic model is employed to account for the decay of CH3C(O)OO. [ABSTRACT FROM AUTHOR]

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of CH3CO and O2; IR absorption spectra of CH3C(O)OO and α-lactone were observed. Absorption bands with origins at 1851±1, 1372±2, 1169±6, and 1102±3 cm-1 are attributed to t-CH3C(O)OO, and those at 1862±3, 1142±4, and 1078±6 cm-1 are assigned to c-CH3C(O)OO. A weak band near 1960 cm-1 is assigned to α-lactone, cyc-CH2C(==O)O, a coproduct of OH. These observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters and dipole derivatives, and observed vibrational wavenumbers agree with harmonic vibrational wavenumbers predicted with B3LYP/aug-cc-pVDZ density-functional theory. The observed relative intensities indicate that t-CH3C(O)OO is more stable than c-CH3C(O)OO by 3±2 kJ mol-1. Based on these observations, the branching ratio for the OH+α-lactone channel of the CH3CO+O2 reaction is estimated to be 0.04±0.01 under 100 Torr of O2 at 298 K. A simple kinetic model is employed to account for the decay of CH3C(O)OO. [ABSTRACT FROM AUTHOR]

SnO2:Eu is a well-known luminescent material, emitting red and orange lines. The intensity ratio of red to orange emission, being sensitive to the deviation of Eu3+ ions from symmetric location, finds wide application as sensor. The luminescence intensity of such lanthanide-doped sensors is generally optimized by high temperature annealing. However, for the present system (SnO2:Eu) it had been found that the red emission suddenly disappears while annealing beyond 900 °C, which can however be recovered by dispersing the system in a secondary host matrix of Y2O3. Understanding the mechanism of this recovery has important implication for designing of phosphor. In this work, we structurally explain this spectral evolution, by employing x-ray absorption fine structure technique. The initial disappearance of the red line is realized to be due to the formation of Eu2Sn2O7 and the recovery, to the intercalation of the Eu3+ ions from the SnO2 surface into Y2O3. Oxygen vacancy in Y2O3 creates the asymmetric environment required for red line emission. The design implications of these findings are discussed. [ABSTRACT FROM AUTHOR]

The C 1B2-X 1A1 photoexcitation of SO2 was studied to investigate excited-state dynamics and the effects of the initial vibrational state. Ultraviolet photoabsorption cross sections (σ's) of seven isotopologues (32S 16O2, 33S 16O2, 34S 16O2, 36S 16O2, 32S16O17O, 32S16O18O, 34S16O18O) were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces of the X and C states, which were calculated using the ab initio molecular orbital configuration interaction method. Numerous wave packet simulations were carried out under the adiabatic approximation and used to calculate the σ’s of the seven isotopologues at 298 K; we concluded that the absorption spectrum of SO2 can be reliably modeled within the adiabatic framework based on the analysis of the time evolution of the wave packet. The calculated σ’s are in reasonable agreement with the recent experiment in the 190–228 nm region, and the isotope shifts of the peaks for 33S 16O2 and 34S 16O2 relative to the corresponding peaks for 32S 16O2 are in good agreement with the observed data. Relative to the σ of 32S 16O2, isotopic substitution shows a significant increment for those of 34S 16O2 and 36S 16O2 in the 190–228 nm region. This trend is consistent with the observed data. [ABSTRACT FROM AUTHOR]

A theoretical and experimental study of isolated nanoparticles of II-VI semiconductor materials has been done. Using the framework of the effective mass model, the optical absorption spectrum of distributions of spherical quantum dots, freestanding, and under compressive or tensile stress, has been examined theoretically. The theoretical results allow one to foresee the absorption spectra of quantum dots made of a series of materials and having any size. The syntheses of colloidal quantum dots of CdS and CdSe has also been performed through wet chemical routes and characterized by means of optical techniques. The values of the strains in the synthesized quantum dots were inferred from a correlation established between the theoretical and the experimental results. [ABSTRACT FROM AUTHOR]

We present the first femtosecond soft x-ray spectroscopy in liquids, enabling the observation of changes in hydrogen bond structures in water via core-hole excitation. The oxygen K-edge of vibrationally excited water is probed with femtosecond soft x-ray pulses, exploiting the relation between different water structures and distinct x-ray spectral features. After excitation of the intramolecular OH stretching vibration, characteristic x-ray absorption changes monitor the conversion of strongly hydrogen-bonded water structures to more disordered structures with weaker hydrogen-bonding described by a single subpicosecond time constant. The latter describes the thermalization time of vibrational excitations and defines the characteristic maximum rate with which nonequilibrium populations of more strongly hydrogen-bonded water structures convert to less-bonded ones. On short time scales, the relaxation of vibrational excitations leads to a transient high-pressure state and a transient absorption spectrum different from that of statically heated water. [ABSTRACT FROM AUTHOR]

Plasmonic structures are commonly used to both confine and enhance surface electromagnetic fields. In the past ten years, their peculiar optical properties have given rise to many promising applications ranging from high density data storage to surface optical trapping. In this context, we investigated both far-field and near-field optical response of a collection of densely packed silver nanocolumns embedded in amorphous aluminum oxide using the discrete dipole approximation. In the far field, a good fit of the calculated to the experimental absorption spectra can only be achieved when in addition to interaction between neighboring nanocolumns, a nanorod shape with periodic shrinks mimicking the experimental morphology of the nanocolumns is used. In the near field, modulated field intensities following the nanocolumns distribution and tunable with the incident wavelength are predicted outside the region occupied by the nanocolumns. This plasmonic image transfer has a resolution of approximately 1.8D where D is the diameter of the nanocolumns that in our case is 2.4 nm. [ABSTRACT FROM AUTHOR]

The local structure of a Zn, Sn codoped In2O3 thin film grown on c⁁-plane sapphire by pulsed-laser deposition was examined by polarization-dependent x-ray absorption spectroscopy. The bixbyite film structure is both out-of-plane and in-plane oriented, and the structural results show that both Zn and Sn dopants occupy In sites. The In–O bond length is comparable to that in powder In2O3. However, both Sn–O and Zn–O bonds have two distinct distances in the first shell. Some of the Zn dopants are undercoordinated and, accordingly, some isovalent Sn dopants are overcoordinated for charge balance. In addition, the results suggest that the aliovalent Sn dopants form Frank–Köstlin clusters, (2SnIn•Oi″)x, which provide enough charge carriers to explain the Hall measurements. [ABSTRACT FROM AUTHOR]

The terahertz spectra of four kinds of vitamins are presented. The refractive index and absorption spectra of these vitamins are obtained by the terahertz time-domain spectroscopy. The full-geometry optimizations and frequency calculations using the density functional theory are applied to obtain the structure and vibration frequencies of these vitamin molecules. The calculated vibration frequencies are compared with the experimental data. The results show that there are terahertz fingerprint absorptions for all of four kinds of vitamins. The terahertz absorbance spectra of vitamins result from not only the intramolecular vibration modes, but also the intermolecular interaction or phonon modes. [ABSTRACT FROM AUTHOR]

The absorption line shapes of model molecular aggregates are investigated using the recently developed Liouville space hierarchical equations of motion (HEOM) method. The exact results are further exploited for the assessment of several approximation schemes, including the high temperature approximation of HEOM, the stochastic Liouville equation approach, and the perturbative time-local and time-nonlocal quantum master equations (QMEs). The calculations on dimers, larger ring-shaped aggregates, and a model of the B850 ring in the LH2 of purple bacteria show that while the other approximate methods can give reasonable absorption line shapes over a wide range of parameter regimes, the second-order time-nonlocal QME is generally inaccurate and may give spurious peaks in the absorption spectra. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H2P) and its phenyl derivative, free-base tetraphenylporphyrin (H2TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe–Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values. [ABSTRACT FROM AUTHOR]

We theoretically investigate the absorption spectra, dipole polarizabilities, and first-order hyperpolarizabilities of SiCn and SinC (n=2–6) clusters using the density functional response approach. Similar to other semiconductor clusters such as Si and gallium arsenide (GaAs) clusters, the absorption spectra of the SiCn and SinC clusters show long absorption tails in the low-transition-energy region and strong absorption peaks in the high-transition-energy region (>4.0 eV). For the same n, the absorption spectrum of the SinC cluster is blueshifted with respect to that of the SiCn cluster, which may be related to the larger highest occupied molecular orbital-lowest unoccupied molecular orbital gap in the former. The isotropic (<α>) dipole polarizabilities of the SiCn and SinC clusters are larger than the bulk polarizability of 3C-SiC and lie between the dipole polarizabilities of Si and C. The SiCn clusters have lower dipole polarizabilities and larger first-order hyperpolarizabilities than the SinC clusters. The size dependence of the first-order hyperpolarizabilities of the SiCn clusters, which have approximate Si-terminated linear chain geometry, is similar to that observed in π-conjugated organic molecules. [ABSTRACT FROM AUTHOR]

We construct the Kohn–Sham density response function χ0 in a previously described basis of the space of orbital products. The calculational complexity of our construction is O(N2Nω) for a molecule of N atoms and in a spectroscopic window of Nω frequency points. As a first application, we use χ0 to calculate the molecular spectra from the Petersilka–Gossmann–Gross equation. With χ0 as input, we obtain the correct spectra with an extra computational effort that grows also as O(N2Nω) and, therefore, less steeply in N than the O(N3) complexity of solving Casida’s equations. Our construction should be useful for the study of excitons in molecular physics and in related areas where χ0 is a crucial ingredient. [ABSTRACT FROM AUTHOR]

The laser-induced fluorescence spectra (both fluorescence excitation and dispersed fluorescence) of jet-cooled 1,3-benzodioxan along with its ultraviolet absorption spectra have been recorded and analyzed in order to determine the vibrational quantum levels in both the ground and S1(π,π*) electronic excited states. A detailed energy map of the vibrational levels involving the six lowest frequency vibrations was established and utilized to better understand the structural and conformational differences between the ground and excited electronic states. The energies of more than a dozen vibrational excited states involving the out-of-plane ring twisting (ν47) and the out-of-plane ring bending (ν48) modes were determined for both S0 and S1 electronic states. Ab initio and density functional theory (DFT) calculations were also carried out to complement the experimental work. The data allowed one-dimensional potential energy functions in terms of the ring-twisting coordinate to be calculated. These show the molecule to have a twisting angle of 33° and a barrier to planarity of 4300±500 cm-1 for the S0 ground state and an angle of 24° and a barrier of 1500±200 cm-1 for the S1(π,π*) excited state. [ABSTRACT FROM AUTHOR]

Laser-ablated tantalum and niobium atoms react with CO and N2 mixtures in excess neon to produce carbonyl metal dinitrogen complexes, NNMCO (M=Ta, Nb), (NN)2TaCO, and NNTa(CO)2, as well as metal carbonyls and dinitrogen complexes. These carbonylmetal dinitrogen complexes are characterized using infrared spectroscopy on the basis of the results of the isotopic substitution and mixed isotopic splitting patterns. Density functional theory calculations have been performed on these novel species. The good agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these species from the matrix infrared spectra. Natural bond orbital analysis and plausible reaction mechanisms for the formation of the products are discussed. [ABSTRACT FROM AUTHOR]

A general formalism for introducing nuclear quantum effects in the expression of the quantum time correlation function of an operator in a multilevel electronic system is presented in the adiabatic limit. The final formula includes the nuclear quantum time correlation functions of the operator matrix elements, of the energy gap, and their cross terms. These quantities can be inferred and evaluated from their classical analogs obtained by mixed quantum-classical molecular dynamics simulations. The formalism is applied to the absorption spectrum of a hydrated electron, expressed in terms of the time correlation function of the dipole operator in the ground electronic state. We find that both static and dynamic nuclear quantum effects distinctly influence the shape of the absorption spectrum, especially its high energy tail related to transitions to delocalized electron states. Their inclusion does improve significantly the agreement between theory and experiment for both the low and high frequency edges of the spectrum. It does not appear sufficient, however, to resolve persistent deviations in the slow Lorentzian-like decay part of the spectrum in the intermediate 2–3 eV region. [ABSTRACT FROM AUTHOR]

We performed configuration interaction ab initio calculations on the valence and 5s, 5pa1, and 5pe Rydberg bands of the CH3Br molecule as a function of the methyl-bromide distance for frozen C3v geometries. The valence state potential energy curves are repulsive, the Rydberg state ones are similar to the one of the CH3Br+ ion with a minimum at short distance. One state emerging from the 5pe band has valence and ion-pair characters as distance increases and the corresponding potential curve has a second minimum at large distance. This state has a very strong parallel electric dipole transition moment with the ground state and plays a central role in UV photon absorption spectra. It is also responsible for the parallel character of the anisotropy parameters measured in ion-pair production experiments. In each band, there is a single state, which has a non-negligible transition moment with the ground state, corresponding to a transition perpendicular to the molecular axis of symmetry, except for the 5pe band where it is parallel. The perpendicular transition moments between ground and valence states increase sharply as methyl-bromide distance decreases due to a mixing between valence and 5s Rydberg band at short distance. In each band, spin orbit interaction produces a pair of states, which have significant transition moments with the ground one. In the valence band, the mixing between singlet and triplet states is weak and the perpendicular transition to the 1Q1 state is dominant. In each Rydberg band, however, spin-orbit interaction is larger than the exchange interaction and the two significant transition moments with the ground state have comparable strengths. The valence band has an additional state (1Q0) with significant parallel transition moment induced by spin-orbit interaction with the ground state at large distance. [ABSTRACT FROM AUTHOR]

Temperature dependences (4–300 K) of photoluminescence (PL) and absorption spectra of 16-mer oligothiophene (16 T) extremely diluted in polypropylene (PP) have been investigated in order to clarify temperature effects on quasi-isolated conjugated polymers. The PL and absorption spectra are found to blueshift with increasing temperature. The reason for the blueshift is discussed by comparing models based on the refractive index of the solvent (PP) and on the thermal conformational change of 16 T. The blueshift is concluded to result from the thermal conformational change. Time-resolved PL spectra show a redshift of PL band following photoexcitation (spectral migration). The amount of the migration is shown to increase with increasing temperature. The increased migration is concluded to be due to the thermal conformational change. The temperature dependence of the effective conjugation length (ECL) of 16 T is calculated for the absorption and PL transitions. The calculation suggests that ECL is reduced at room temperature to two-thirds of the intrinsic chain length. The activation energy of the conformational change is estimated to be 22.4 meV from the temperature dependence of ECL. We demonstrate that the steady-state PL spectra are well reproduced by simple Franck–Condon analyses using a single Huang–Ryes factor over a wide temperature range. The analyses reveal features of temperature dependence in important spectral parameters such as the Stokes shift, linewidth, and Huang–Ryes factor. [ABSTRACT FROM AUTHOR]

A method for the analysis of time-resolved x-ray absorption near edge structure (XANES) spectra is proposed. It combines principal component analysis of the series of experimental spectra, multidimensional interpolation of theoretical XANES as a function of structural parameters, and ab initio XANES calculations. It allows to determine the values of structural parameters for intermediates of chemical reactions and the concentrations of different states as a function of time. This approach is tested using numerically generated data and its possibilities and limitations are discussed. The application of this method to a reaction with methylrhenium trioxide catalyst in solution, for which experimental data were measured using stopped-flow energy-dispersive x-ray absorption spectroscopy technique, is demonstrated. Possibilities and limitations of this experimental technique are also discussed. [ABSTRACT FROM AUTHOR]

Pyrylium dyes having the same basic chemical structure, differing only to a specific substituent, have been used as novel materials for multiphoton three-dimensional data storage. Electronic absorption spectra, two-photon and three-photon absorption properties of this class of pyrylium dyes, have been studied theoretically and compared to experimental results. The effects of the counteranion, the surrounding solvent, and electron releasing and electron withdrawing groups in specific positions of the basic structure have been explored in detail. It is argued that on grounds of the quality of experimental spectroscopic agreement, the computed two- and three-photon data may be used in pulse propagation simulations of three-dimensional recording in optical memories. [ABSTRACT FROM AUTHOR]

General theory of the excitation energy transfer (EET) in the case of donor-mediator-acceptor system was constructed by using generalized master equation (GME). In this theory, we consider the direct and indirect transitions in the EET consistently. Hence, our theory includes the quantum mechanical interference between the direct and indirect transitions automatically. Memory functions in the GME were expressed by the overlap integrals among the time-dependent emission spectrum of the donor, the absorption spectrum of the mediator, the time-dependent emission spectrum of the mediator, and the absorption spectrum of the acceptor. In the Markov limit of the memory functions, we obtained the rate of EET which consists of three terms due to the direct transition, the indirect transition, and the interference between them. We found that the interference works effectively in the limit of slow thermalization at the intermediate state. The formula of EET rate in this limit was expressed by the convolution of the EET interaction and optical spectra. The interference effect strongly depends on the width of the absorption spectrum of mediator molecule and the energy gap between the donor and the mediator molecules. [ABSTRACT FROM AUTHOR]

The anharmonic S0→S1 vibronic absorption spectrum of the formaldehyde molecule is computed on the fly using semiclassical dynamics. This first example of an on-the-fly semiclassical computation of a vibronic spectrum was achieved using a unit prefactor modified frozen Gaussian semiclassical propagator for the excited state. A sample of 6000 trajectories sufficed for obtaining a converged spectrum, which is in reasonable agreement with experiment. Similar agreement is not obtained when using a harmonic approximation for the spectrum, demonstrating the need for a full anharmonic computation. This first example provides a resolution of ∼100 cm-1. Potential ways of improving the methodology and obtaining higher resolution and accuracy are discussed. [ABSTRACT FROM AUTHOR]

Dilute-nitride GaNSb bulk materials with up to 1.4% nitrogen were grown by molecular beam epitaxy on GaSb substrates. Hall measurements indicate residual hole concentrations of nearly 1019 cm-3 at room temperature, but a decrease to below 1016 cm-3 and a hole mobility of 1300 cm2/V s at 4.5 K for a sample with 0.6% nitrogen. Photoluminescence (PL) and optical absorption measurements demonstrate a bandgap reduction by up to 300 meV with increasing nitrogen incorporation. The experimental absorption spectra are well fit by a functional dependence corresponding to direct allowed optical transitions, and the PL spectra are also consistent with that interpretation. Room temperature carrier relaxation times in the picosecond range are measured using an ultrafast PL upconversion technique. [ABSTRACT FROM AUTHOR]