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Monitoring of Nonadiabatic Effects in Individual Chromophores by Femtosecond Double-Pump Single-Molecule Spectroscopy: A Model Study
摘要： We explore, by theoretical modeling and computer simulations, how nonadiabatic couplings of excited electronic states of a polyatomic chromophore manifest themselves in single-molecule signals on femtosecond timescales. The chromophore is modeled as a system with three electronic states (the ground state and two non-adiabatically coupled excited states) and a Condon-active vibrational mode which, in turn, is coupled to a harmonic oscillator heat bath. For this system, we simulate double-pump single-molecule signals with fluorescence detection for different system-field interaction strengths, from the weak-coupling regime to the strong-coupling regime. While the signals are determined by the coherence of the electronic density matrix in the weak-coupling regime, they are determined by the populations of the electronic density matrix in the strong-coupling regime. As a consequence, the signals in the strong coupling regime allow the monitoring of nonadiabatic electronic population dynamics and are robust with respect to temporal inhomogeneity of the optical gap, while signals in the weak-coupling regime are sensitive to fluctuations of the optical gap and do not contain information on the electronic population dynamics.
关键词： nonadiabatic dynamics,strong-field regime,weak-field regime,single-molecule spectroscopy
更新于2025-09-19 17:15:36
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Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr <sub/>3</sub> Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy
摘要： We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 nm to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ~ 10 ps) of ‘warm’ electrons in the vicinity (within ~ 0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1 and increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intra-band transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.
关键词： electron-phonon coupling,hot carriers,nonadiabatic molecular dynamics,Charge-carrier cooling,lead-halide perovskite nanocrystal,excited-states dynamics
更新于2025-09-19 17:13:59
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, and HD
摘要： We calculate the nonadiabatic relativistic correction to rovibrational energy levels of H2, D2, and HD molecules using the nonadiabatic perturbation theory. This approach allows one to obtain nonadiabatic corrections to all the molecular levels with the help of a single effective potential. The obtained results are in very good agreement with the previous direct calculation of nonadiabatic relativistic effects for dissociation energies and resolve the reported discrepancies of theoretical predictions with recent experimental results.
关键词： nonadiabatic perturbation theory,nonadiabatic relativistic correction,HD,D2,H2,rovibrational energy levels
更新于2025-09-10 09:29:36
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Let Digons be Bygones: The Fate of Excitons in Curved π-Systems
摘要： We explore the diverse origins of unpolarized absorption and emission of molecular polygons, consisting of π-conjugated oligomer chains held in a bent geometry by strain controlled at the vertex units. For this purpose, we make use of atomistic nonadiabatic excited-state molecular dynamics simulations of a bichromophore molecular polygon (digon) with bent chromophore chains. Both structural and photoexcited dynamics were found to affect polarization features. Bending strain induces exciton localization on individual chromophore units of the conjugated chains. The latter display different transition dipole moment orientations, a feature not present in the linear oligomer counterparts. In addition, bending makes exciton localization very sensitive to molecular distortions induced by thermal fluctuations. The excited-state dynamics reveals an ultrafast intramolecular energy redistribution that spreads the exciton equally among spatially separated chromophore fragments within the molecular system. As a result, digons become virtually unpolarized absorbers and emitters, in agreement with recent experimental studies on the single-molecule level.
关键词： transition dipole moment,unpolarized absorption and emission,π-conjugated oligomer chains,molecular polygons,nonadiabatic excited-state molecular dynamics,exciton localization
更新于2025-09-10 09:29:36
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Performance evaluation of the symmetrical quasi-classical dynamics method based on Meyer-Miller mapping Hamiltonian in the treatment of site-exciton models
摘要： The symmetrical quasi-classical dynamics method based on the Meyer-Miller mapping Hamiltonian (MM-SQC) shows the great potential in the treatment of the nonadiabatic dynamics of complex systems. We performed the comprehensive benchmark calculations to evaluate the performance of the MM-SQC method in various site-exciton models with respect to the accurate results of quantum dynamics method multilayer multiconfigurational time-dependent Hartree (ML-MCTDH). The parameters of the site-exciton models are chosen to represent a few of prototypes used in the description of photoinduced excitonic dynamics processes in photoharvesting systems and organic solar cells, which include the rather broad situations with the fast or slow bath and different system-bath couplings. When the characteristic frequency of the bath is low, the MM-SQC method performs extremely well, and it gives almost the identical results to those of ML-MCTDH. When the fast bath is considered, the deviations exist between the MM-SQC and ML-MCTDH results if the high-frequency bath modes are improperly treated by the classical manner. When the so-called adiabatic renormalization was employed to construct the reduced Hamiltonian by freezing high-frequency modes, the MM-SQC dynamics can give the results comparable to the ML-MCTDH ones. Thus, the MM-SQC method itself provides reasonable results in all test site-exciton models, while the proper treatments of the bath modes must be employed. The possible dependence of the MM-SQC dynamics on the different initial sampling methods for the nuclear degrees of freedom is also discussed.
关键词： nonadiabatic dynamics,ML-MCTDH,site-exciton models,symmetrical quasi-classical dynamics,Meyer-Miller mapping Hamiltonian
更新于2025-09-10 09:29:36
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Power-law decay in the nonadiabatic photodissociation dynamics of alkali halides due to quantum wavepacket interference
摘要： The nonadiabatic photodissociation dynamics of alkali halide molecules excited by a femtosecond laser pulse in the gas phase are investigated theoretically, and it is shown that the population of the photoexcited molecules exhibits power-law decay with exponent ?1/2, in contrast to exponential decay, which is often assumed in femtosecond spectroscopy and unimolecular reaction theory. To elucidate the mechanism of the power-law decay, a diagrammatic method that visualizes the structure of the nonadiabatic reaction dynamics as a pattern of occurrence of dynamical events, such as wavepacket bifurcation, turning, and dissociation, is developed. Using this diagrammatic method, an analytical formula for the power-law decay is derived, and the theoretical decay curve is compared with the corresponding numerical decay curve computed by a wavepacket dynamics simulation in the case of lithium ?uoride. This study reveals that the cause of the power-law decay is the quantum interference arising from the wavepacket bifurcation and merging due to nonadiabatic transitions.
关键词： quantum wavepacket interference,nonadiabatic photodissociation,alkali halides,power-law decay,femtosecond laser pulse
更新于2025-09-10 09:29:36
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Potential energy surfaces and nonadiabatic transitions in the asymptotic regions of ICN photodissociation to study the interference effects in the <i>F</i> <sub/>1</sub> and <i>F</i> <sub/>2</sub> spin-rotation levels of the CN products
摘要： One of the most spectacular yet unsolved problems for the ICN ~A-band photodissociation is the non-statistical spin-rotation F1 = N + 1/2 and F2 = N ? 1/2 populations for each rotation level N of the CN fragment. The F1/F2 population difference function f(N) exhibits strong N and λ dependences with an oscillatory behavior. Such details were found to critically depend on the number of open-channel product states, namely, whether both I (2P3/2) and I (2P1/2) are energetically available or not as the dissociation partner. First, the exchange and dipole-quadrupole inter-fragment interactions were studied in detail. Then, as the diabatic basis, we took the appropriate symmetry adapted products of the electronic and rotational wavefunctions for the F1 and F2 levels at the dissociation limits. We found that the adiabatic Hamiltonian exhibits Rosen–Zener–Demkov type nonadiabatic transitions reflecting the switch between the exchange interaction and the small but finite spin-rotation interaction within CN at the asymptotic region. This non-crossing type nonadiabatic transition occurs with the probability 1/2, that is, at the diabatic limit through a sudden switch of the quantization axis for CN spin S from the dissociation axis to the CN rotation axis N. We have derived semiclassical formulae for f(N) and the orientation parameters including the 3A0 and 4A0 electronic states, and with a two-state model including the 3A0 through 6A0 electronic states. These two kinds of interfering models explain general features of the F1 and F2 level populations observed by Zare’s group and Hall’s group, respectively.
关键词： nonadiabatic transitions,dipole-quadrupole interactions,Rosen-Zener-Demkov nonadiabatic transitions,quantum interference effect,ICN photodissociation,F1 and F2 spin-rotation levels,fine structure splitting
更新于2025-09-09 09:28:46
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Nonadiabatic transition probabilities in a time-dependent Gaussian pulse or plateau pulse: Toward experimental tests of the differences from Dirac’s transition probabilities
摘要： For a quantum system subject to a time-dependent perturbing field, Dirac’s analysis gives the probability of transition to an excited state |k? in terms of the norm square of the entire excited-state coefficient ck(t) in the wave function. By integrating by parts in Dirac’s equation for ck(t) at first order, Landau and Lifshitz separated ck(t) into an adiabatic term ak(t) that characterizes the gradual adjustment of the ground state to the perturbation without transitions and a nonadiabatic term bk(t) that depends explicitly on the time derivative of the perturbation at times t′ ≤ t. Landau and Lifshitz stated that the probability of transition in a pulsed perturbation is given by |bk(t)|2, rather than by |ck(t)|2. We use the term “transition probability” to refer to the probability that a true excited-state component is present in the time-evolved wave function, as opposed to a smooth modification of the initial state. In recent work, we have examined the differences between |bk(t)|2 and |ck(t)|2 when a system is perturbed by a harmonic wave in a Gaussian envelope. We showed that significant differences exist when the frequency of the harmonic wave is off-resonance with the transition frequency. In this paper, we consider Gaussian perturbations and pulses that rise via a half Gaussian shoulder to a level plateau and later return to zero via a down-going half Gaussian. While the perturbation is constant, the transition probability |bk(t)|2 does not change. By contrast, |ck(t)|2 continues to oscillate while the perturbation is constant, and its time averaged value differs from |bk(t)|2. We suggest a general type of experiment to prove that the transition probability is given by |bk(t)|2, not |ck(t)|2. We propose a ratio test that does not require accurate knowledge of transition matrix elements or absolute field intensities.
关键词： time-dependent Gaussian pulse,Nonadiabatic transition probabilities,experimental tests,Dirac’s transition probabilities,plateau pulse
更新于2025-09-09 09:28:46
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Ultrafast carbon monoxide photolysis and heme spin-crossover in myoglobin via nonadiabatic quantum dynamics
摘要： Light absorption of myoglobin triggers diatomic ligand photolysis and a spin crossover transition of iron(II) that initiate protein conformational change. The photolysis and spin crossover reactions happen concurrently on a femtosecond timescale. The microscopic origin of these reactions remains controversial. Here, we apply quantum wavepacket dynamics to elucidate the ultrafast photochemical mechanism for a heme–carbon monoxide (heme–CO) complex. We observe coherent oscillations of the Fe–CO bond distance with a period of 42 fs and an amplitude of ～1 ?. These nuclear motions induce pronounced geometric reorganization, which makes the CO dissociation irreversible. The reaction is initially dominated by symmetry breaking vibrations inducing an electron transfer from porphyrin to iron. Subsequently, the wavepacket relaxes to the triplet manifold in ～75 fs and to the quintet manifold in ～430 fs. Our results highlight the central role of nuclear vibrations at the origin of the ultrafast photodynamics of organometallic complexes.
关键词： Ultrafast carbon monoxide photolysis,nonadiabatic quantum dynamics,myoglobin,heme spin-crossover
更新于2025-09-09 09:28:46
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Solvent-assisted excited state proton transfer and photoacidity of 2-hydroxypyridine. A nonadiabatic dynamics study
摘要： Reactions involving proton transfer, especially those taking place in excited states (ESPT), received considerable attention. These reactions underlie many biological and biochemical processes that are vital to life. DNA mutations excited state funnelling and transport phenomena are just examples. Among many molecules undergoing this type of photoreactions, phenols show a unique property, the so-called photoacidity. In the present communication, solvent (methanol and ammonia) assisted excited state proton transfer and photo acidity of 2-hydroxypyridine (2HP) are investigated at the DFT M06-2X / def2pvv level of theory. Excited states of 2HP and its complexes with methanol and ammonia were examined at two levels. First, sampling a Wigner distribution of 300 points, the photoabsorption spectra were simulated within the nuclear ensemble approximation. Second, simulation of the dynamics of the excited states with the surface hopping method. Several separate spectral windows with three hundred trajectories each, were considered. Lifetime of excited states and photochemical channels observed were discussed.
关键词： nonadiabatic dynamics,photoacidity,Excited state proton transfer (ESPT),photoabsorption spectrum,2-hydroxypyridine
更新于2025-09-09 09:28:46