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Multiple applied studies of slow nonadiabatic processes in nanoscale and condensed matter systems have adopted the "repetition" approximation in which long trajectories for such simulations are obtained by concatenating shorter trajectories, directly available from ab initio calculations, many times. Here, we comprehensively assess this approximation using model Hamiltonians with parameters covering a wide range of regimes. We find that state transition time scales may strongly depend on the length of the repeated data, although the convergence is not monotonic and may be slow. The repetition approach may under- or overestimate the time scales by a factor of ≤7-8, does not directly depend on the dispersion of energy gap and nonadiabatic coupling (NAC) frequencies, but may depend on the magnitude of the NACs. We suggest that the repetition-based nonadiabatic dynamics may be inaccurate in simulations with very small NACs, where intrinsic transition times are on the order of ≥100 ps.

Hot electrons play a crucial role in enhancing the efficiency of photon-to-current conversion or photocatalytic reactions. In semiconductor nanocrystals, energetic hot electrons capable of photoemission can be generated via the upconversion process involving the dopant-originated intermediate state, currently known only in Mn-doped cadmium chalcogenide quantum dots. Here, we report that Mn-doped CsPbBr

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

We present a new state tracking algorithm based on a stochastic state reassignment that reflects the quantum mechanical interpretation of the state time-overlaps. We assess the new method with a range of model Hamiltonians and demonstrate that it yields the results generally consistent with the deterministic min-cost algorithm. However, the stochastic state tracking algorithm reduces magnitudes of the state population fluctuations as the quantum system evolves toward its equilibrium. The new algorithm facilitates the thermalization of quantum state populations and suppresses the population revivals and oscillations near the equilibrium in many-state systems. The new stochastic algorithm has a favorable computational scaling, is easy to implement due to its conceptual transparency, and treats various types of state identity changes (trivial or avoided crossings and any intermediate cases) on equal footing.

Synergy of nonadiabatic molecular dynamics with real-time time-dependent density functional theory has led to significant progress in modeling excited-state dynamics in nanoscale and condensed matter systems over the past decade. Nonadiabatic coupling (NAC) is the central quantity in such simulations, and its accurate and efficient evaluation is an enduring challenge in time-dependent Kohn-Sham theory, particularly in conjunction with planewave basis sets and projector augmented-wave (PAW) pseudopotentials because of the complexity of the PAW "all-electron" wave function. We report a method for rigorous evaluation of the NAC with PAW wave functions and demonstrate an efficient approximation to the rigorous NAC that gives comparable accuracy. As a validation, we intensely examine the NAC matrix elements calculated using both pseudo- and all-electron wave functions under the PAW formalism in six representative systems. The approximate NAC obtained with pseudowave functions is close to the exact all-electron NAC, with the largest deviations observed when subshell d-electrons are involved in the transitions. The developed approach provides a rigorous and convenient methodology for the numerical computation of NAC in the Kohn-Sham theory framework.

The results of growing multilayered nitrogen-doped nanostructures that have the form of periodically arranged nanometer-thick nitrogen-containing layers in a single-crystal CVD diamond are presented. The possibility of creating nitrogen-doped diamond layers with extremely sharp boundaries (less than 1 nm) is demonstrated. The photoluminescence study have shown that multilayered structures make it possible to obtain a higher radiation intensity of practically important NV – sites at a spin coherence time close to that for uniformly doped layers with the same concentration of nitrogen.

We investigated the magnetic effect of Mn

The dynamics of electron–hole recombination in pristine and defect-containing monolayer black phosphorus (ML-BP) has been studied computationally by several groups relying on the one-particle description of electronic excited states. Our recent developments enabled a more sophisticated and accurate treatment of excited states dynamics in systems with pronounced excitonic effects, including 2D materials such as ML-BP. In this work, I present a comprehensive characterization of optoelectronic properties and nonadiabatic dynamics of the ground state recovery in pristine and divacancy-containing ML-BP, relying on the linear-response time-dependent density functional theory description of excited states combined with several trajectory surface hopping methodologies and decoherence correction schemes. This work presents a revision and new implementation of the decoherence-induced surface hopping methodology. Several popular algorithms for nonadiabatic dynamics algorithms are assessed. The kinetics of nonradiative relaxation of lower-lying excited states in ML-BP systems is revised considering the new methodological developments. A general mechanism that explains the sensitivity of the nonradiative dynamics to the presence of divacancy defect in ML-BP is proposed. According to this mechanism, the excited states’ relaxation may be inhibited by the presence of energetically close higher-energy states if electronic decoherence is present in the system.

Accurate, high spatial resolution thermal measurements have provided fundamental insights into many fields of study; however, existing thermometer technology often suffers from one or more limitati...

Creation of localized NV center ensembles, produced by irradiation of CVD diamond with nitrogen doped delta-layer with 200 keV electron beam and the subsequent annealing is demonstrated. Results of optical measurements of activated areas at different irradiation doses are presented. Issues concerning defect formation in diamond lattice during the electron beam impact are discussed.

The fine structure of the band edge exciton and the dark exciton photoluminescence (PL) are topics of significant interest in the research of semiconducting metal halide perovskite nanocrystals, with several conflicting reports on the level ordering of the bright and dark states and the accessibility of the emitting dark states. Recently, we observed the intense dark exciton PL in strongly confined CsPbBr3 nanocrystals at cryogenic temperatures, in contrast to weakly confined nanocrystals lacking dark exciton PL, which was explained by the confinement enhanced bright-dark exciton splitting. In this work, we investigated the size-dependence of the dark exciton photoluminescence properties in CsPbBr3 and CsPbI3 quantum dots in the strongly confined regime, showing the clear role of confinement in determining the bright-dark energy splitting (ΔEBD) and the dark exciton lifetime (τD). We observe the increase in both ΔEBD and τD with increasing quantum confinement in CsPbBr3 and CsPbI3 QDs, consistent with the earlier predictions on the size-dependence of ΔEBD and τD. Our results show that quantum confinement plays a crucial role in determining the accessibility to the dark exciton PL and its characteristics in metal halide perovskite nanocrystals.

Recent time-resolved transient absorption studies demonstrated that the rate of photoinduced interfacial charge transfer (CT) from Zn-phthalocyanine (ZnPc) to single-layer graphene (SLG) is faster than to double-layer graphene (DLG), in contrast to the expectation from Fermi's golden rule. We present the first time-domain non-adiabatic molecular dynamics (NA-MD) study of the electron injection process from photoexcited ZnPc molecules into SLG and DLG substrates. Our calculations suggest that CT occurs faster in the ZnPc/SLG system than in the ZnPc/DLG system, with 580 fs and 810 fs being the fastest components of the observed CT timescales, respectively. The computed timescales are in close agreement with those reported in the experiment. The computed CT timescales are determined largely by the magnitudes of the non-adiabatic couplings (NAC), which we find to be 4 meV and 2 meV, for the ZnPc/SLG and ZnPc/DLG systems, respectively. The transitions are driven mainly by the ZnPc out-of-plane bending mode at 1100 cm−1 and an overtone of fundamental modes in graphene at 2450 cm−1. We find that dephasing occurs on the timescale of 20 fs and is similar in both systems, so decoherence does not notably change the qualitative trends in the CT timescales. We highlight the importance of proper energy level alignment for capturing the qualitative trends in the CT dynamics observed in experiment. In addition, we illustrate several methodological points that are important for accurately modeling nonadiabatic dynamics in the ZnPc/FLG systems, such as the choice of surface hopping methodology, the use of phase corrections, NAC scaling, and the inclusion of Hubbard terms in the density functional and molecular dynamics calculations.

NV centers in diamond recommend themselves as good sensors of environmental fields as well as detectors of diamond impurities. In particular, C-centers, often also called ${{p}_{1}}$-centers, can be detected via double electron-electron resonance. This resonance can be used to measure the C-center concentration. Here, we measured the concentration of C-centers in several diamond plates and investigated the influence of the free precession time of the NV center on the observed contrast in the measured double electron-electron resonance spectrum. The dependence of the resonance amplitudes and widths on the concentration of C-centers as well as the length of the combined C-center driving and NV-center $\pi$-pulse is also discussed. The optimal contrast-free precession time was determined for each C-center concentration, showing a strong correlation with both the concentration of C-centers and the NV-center ${{T}_{2}}$ time.

We have implemented a Python-based software package within the Libra software for performing nonadiabatic molecular dynamics (NA-MD) within the neglect of back reaction approximation (NBRA). Available in the software are a wide variety of proposed hop acceptance (PHA) and decoherence methodologies. Using Libra, a comparative analysis of PHA schemes and decoherence methods is performed to examine thermal equilibrium in NA-MD simulations within the NBRA. The analysis is performed using 3 model systems, each of which highlights the effects of the different decoherence methods and PHA schemes on NA transitions. We find that the interplay between decoherence and PHA schemes is important for achieving detailed balance in the NBRA and discuss the conditions by which the detailed balance is achieved for each model. We discuss the qualitative features of NA dynamics computed using various combinations of decoherence and PHA schemes for a wide range of model and condition parameters such as temperature, energy gap magnitude, and dephasing times. Furthermore, we extend the analysis to include the Boltzmann corrected Ehrenfest methodology of Bastida and co-workers and compare the dynamics produced with it with that obtained using the surface hopping-based approach.

Silicon-doped diamond nanopillars have been produced by a microwave plasma chemical vapor deposition (CVD) on a single crystal diamond substrate through holes in a Si mask perforated with a focused ion beam. Arrays of 400 nm diameter pillars with aspect ratio up to 2.8 are grown epitaxially being confined by channels in the mask, the latter serving also as the Si doping source. Strong photoluminescent (PL) emission of the SiV centers at 738.7 nm wavelength, localized within the pillars, has been detected and imaged with a fluorescence microscope. The SiV PL decay time of 1.1 ns has been deduced from PL kinetics measurements. An increase of specific PL intensity (intensity per unit volume of the pillar) with the aspect ratio is noted.

The "methodology discovery" library for quantum and classical dynamics simulations is presented. One of the major foci of the code is on nonadiabatic molecular dynamics simulations with model and atomistic Hamiltonians treated on the same footing. The essential aspects of the methodology, design philosophy, and implementation are discussed. The code capabilities are demonstrated on a number of model and atomistic test cases. It is demonstrated how the library can be used to study methodologies for quantum and classical dynamics, as well as a tool for performing detailed atomistic studies of nonadiabatic processes in molecular systems. The source code and additional information are available on the Web at http://www.acsu.buffalo.edu/~alexeyak/libra/index.html. © 2016 Wiley Periodicals, Inc.

We report a study on the non-adiabatic molecular dynamics (NA-MD) of the charge transfer (CT) process in the boron subphtalocyanine chloride (SubPc)/fullerene (C60) interface using our newly implemented Libra-X software package, which is based on an interface of the Libra NA-MD library and the GAMESS electronic structure software. In particular, we address the following aspects of the simulation protocol: (a) the choice of the potential used to treat interatomic interactions and its effect on the structures of the complex and CT rates; (b) the choice of the electronic structure methodology used; and (c) the choice of the trajectory surface hopping (TSH) methodology used. From our analysis of the electronic structure, we suggest that the distortion of the SubPc conical structure affects orbital localization and that the "breathing" motion of SubPc drives the CT process in SubPc/C60. This study illustrates that the choice of the TSH methodology and electronic decoherence are crucial for the CT simulation. We extend our analysis of CT in SubPc/(C60)n models by increasing the number of C60 molecules up to n = 4. We find that the details of the interfacial SubPc/(C60)n geometry determine the CT rate. Finally, we find the computed CT timescale to be in the range of 2.2-5.0 ps, which is in agreement with the experimentally determined timescale in the order of magnitude of ∼10 ps. The developed open-source Libra-X package is freely available on the Internet at https://github.com/Quantum-Dynamics-Hub/Libra-X.

Hybrid metal-organic halide perovskites have recently attracted a great deal of attention because of their interesting electronic, optical and transport properties, which make them promising materials for high-performance, low-cost solar cells. Fundamental understanding of the formation mechanisms and dynamics of photoinduced charge carriers is essential for improving the performance of perovskite solar cell devices. For example, a significant amount of absorbed solar energy is lost as a result of carrier thermalization. This energy could be harnessed by extracting hot carriers before they cool down to the band edges. Although such hot carrier collection is experimentally challenging, theoretical investigations based on time-dependent methods can guide future experimental research by providing insights into the thermalization process. Here, we perform ab initio nonadiabatic molecular dynamics simulations to study non-radiative relaxation dynamics of charge carriers in hybrid halide perovskites. We find that the carrier relaxation time can be considerably increased by mixing halogen atoms in the perovskite materials. These findings show that simple approaches could be adopted to slow down the thermalization process of hot carriers in perovskite materials.

We report on the improved measurement of the hyperfine structure of $4f^{12}(^3 H_6)5d_{5/2}6s^2$ $(J=9/2)$ excited state in Tm-169 which is involved in the second-stage laser cooling of Tm. To measure the absolute value of the hyperfine splitting interval we used Doppler-free frequency modulation saturated absorption spectroscopy of Tm atoms in a vapor cell. The sign of the hyperfine constant was determined independently by spectroscopy of laser cooled Tm atoms. The hyperfine constant of the level equals $A_J=-422.112(32)$ MHz that corresponds to the energy difference between two hyperfine sublevels of $-2110.56(16)$~MHz. In relation to the saturated absorption measurement we quantitatively treat contributions of various mechanisms into the line broadening and shift. We consider power broadening in the case when Zeeman sublevels of atomic levels are taken into account. We also discuss the line broadening due to frequency modulation and relative intensities of transitions in saturated-absorption experiments.

Scalar and tensor polarizabilities of 5D 3/2 and 5D 5/2 levels of rubidium were measured in a cloud of cold atoms. The measurements were carried out by using the shift of spectral lines corresponding to 5P–5D transitions in a constant external electric field. The following values of polarizabilities were obtained: αS(5D 3/2) = 18 400 (75), αT(5D 3/2) = –750 (30), α(5D 5/2) = 18 600 (76), and αT(5D 5/2) = –1440 (60).

Trapped and Coupled Trapped single atoms are ideal for storing and manipulating quantum information. Thompson et al. (p. 1202 , published online 25 April; see the Perspective by Keller ) were able to control single atoms interacting coherently with a field mode of a photonic crystal cavity. An optical tweezer was used to trap the single atom, which enabled positioning of the atom in close proximity to the photonic crystal waveguide, coupling the atom to the optical mode of the cavity. Such coupling should prove useful in quantum measurement, sensing, and information processing.

Review: emphasis on processes that are essential in terms of photovoltaic and photocatlytic cells; 655 refs.

We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 106 thulium atoms were trapped in the MOT at temperatures down to 25(5) μK which is approximately 10 times lower than the Doppler limit. The lifetime of atoms in the MOT varied between 0.3–1.5 s and was restricted mostly by optical leaks from the upper cooling level. The lower limit for the leaking rate was estimated to be 22(6) s−1. Due to a big magnetic moment of Tm atoms, a part of them were trapped in a magnetic trap from the quadrupole field of the MOT. We observed about 3 × 104 purely magnetically trapped atoms at temperature of 25 μK with a lifetime in the trap of 0.5 s. Also we set up a “dark” MOT consisting of six crossed hollow beams which increased the number of trapped atoms by a factor of 5 leading to 1.5 × 107 atoms at the expense of higher temperature.

The possibility of the use of the F = 2↔F = 1 transition of the D1 absorption line of the 87Rb atom for creating of a single-photon quantum filter based on coherent population trapping (CPT) has been analyzed. It has been shown that the external magnetic field is necessary for ensuring the creation of the quantum filter on boson isotopes of alkali atoms. The field strength should be enough for the manifestation of the splitting of the Zeeman CPT resonances that is much larger than their spectral widths. The splittings of the CPT resonances, which characterize the nonlinearity of the Zeeman effect, have been measured for the 87Rb atom and the possibility of the use of this system for the quantum filter is concluded.

We experimentally studied the narrow resonances of coherent population trapping (CPT) in a rubidium vapor cell with the help of a phase-coherent bichromatic light source based on a mode-locked femtosecond laser. We investigated the dependence of the contrast and the width of CPT resonance on the light power, as well as the sensitivity of the shape of the resonance to a magnetic field. Prospects of using the novel bichromatic source for phase-coherent Raman spectroscopy of CPT resonances in atomic systems with a large energy separation of the lower levels are discussed. Copyright © 2006 John Wiley & Sons, Ltd.

We propose a new approach to the study of coherent population trapping (CPT) resonances using a phase-coherent bichromatic light source based on a mode-locked femtosecond laser as an intermediate frequency coupler. The source consists of two diode lasers tuned to the rubidium D2 line (780 nm) which are phase locked to different longitudinal modes of a femtosecond laser MIRA-900F with 75 MHz repetition rate. Using this bichromatic source with a stable differential frequency, we excited and investigated the CPT resonances in the three-level system 5s S1/2 (F = 2) ↔ 5s P3/2 (F = 2) ↔ 5s S1/2 (F = 3) of the 85Rb atom. Potentially, this method opens up the possibility of coherently exciting Raman transitions with differential frequencies up to hundreds of terahertz. Copyright © 2005 John Wiley & Sons, Ltd.

A technique for creating the source of a bichromatic laser field with a stable mean value of the difference frequency is realized. The spectral characteristics of a femtosecond passive mode-locked laser are used in this technique. Such a source operating in the region of 780 nm is created. The possibility of using the phenomenon of coherent population trapping (CPT) in three-level systems of the rare-earth atoms for the development of secondary frequency standards in the optical range is considered. The contributions of the processes affecting the width and contrast of the CPT resonances and defining their attainable Q factors are studied. The prospects of using the experimental technique developed for the spectroscopy of the CPT resonances in samarium and thulium vapors are discussed.

A universal theory for calculating coherent population trapping resonances in multilevel atoms is suggested. The theory allows arbitrary schemes of multilevel atoms and their excitations to be calculated tak- ing into account the influence of relaxation effects in atoms, applied magnetic field, and the Doppler effect. The experimental data obtained by high-precision diode spectroscopy of coherent dark resonances in samar- ium vapor are systematically analyzed using the suggested theory. In the absence of a magnetic field, the model of samarium is based on consideration of a degenerate Λ system of the 4 f 6 6 s 2 ( 7 F 0 ) 4 f 6 ( 7 F )6 s 6 p ( 3 P 0 ) 9 4 f 6 6 s 2 ( 7 F 1 ) active transitions. If the fourth 4 f 6 6 s 2 ( 7 F 2 ) level is taken into account, this Λ system becomes open. Numerical simulation of coherent population trapping resonances shows that the open character of the system decreases the contrast of resonance curves in absorption spectra without changing resonance widths. The system under applied external longitudinal and transverse magnetic fields is correctly described by 7- and 12-level models of atomic transitions, respectively. © 2003 MAIK "Nauka/Interperiodica".

Spectra of saturated absorption of Sm atomic vapor from the ground state 7 F 0 and the first even metastable level 7 F 1, e′=292.58 cm−1 of the 4f 66s 2 configuration to the odd level 4f 6(7 F)6s6p(3 P o)9 F 1 o , e,=14863.85 cm−1 were recorded. The lines of the isotopic series were identified, and the hyperfine structure of lines in the spectra of isotopes with a nonzero nuclear spin was determined. The relative isotopic shifts and the hyperfine splitting of the even level 4f 66s 2(7 F 1) were determined.

We propose a numerical algorithm for calculation of quantized directed motion of a stochastic system of interacting particles induced by periodic changes of control parameters on the graph of microstates. As a main application, we consider models of catenane molecular motors, which demonstrated the possibility of a similar control of directed motion of molecular components. We show that our algorithm allows one to calculate the motion of a system in the space of its microstates even when the considered phase space is combinatorially large (~1 × 10(6) microscopic states). Several general observations are made about the structure of the phase diagram of the systems studied, which may be used for rational design and efficient control of new generations of molecular motors.

We propose a numerical approach to quantify the control of a nonautonomous molecular rotor motion. Unlike straightforward molecular dynamics simulations in an explicitly time-dependent framework, our method is based on the theory of geometric phases. This theory allows us to define a sensitivity field (SF) in control parameter space that characterizes average motion of a molecule induced by a cyclic perturbation. We show that the SF can be obtained using only equilibrium free energy sampling techniques. A density plot of the SF quantifies response of a molecule to an arbitrary cyclic adiabatic evolution of parameters. For demonstration, we numerically find the SFs for two surface mounted molecular rotor molecules that can be driven, in practice, by strong time-dependent electric fields of a STM tip.

We present a formulation of quantized Hamiltonian dynamics (QHD) using variables that arise naturally from the Heisenberg equation of motion. The QHD equations are obtained and solved either directly in terms of these generalized variables, or by employing a wavefunction ansatz. The approach avoids a Taylor expansion and other approximations to the potential, leading to more stable dynamics and a higher precision of the calculated quantities. The proposed formulation is also amenable to for analytic and numerical implementations, thus facilitating its use in molecular dynamics simulation.