Intramolecular vibrational energy redistribution from a high frequency mode in the presence of an internal rotor: classical thick-layer diffusion and quantum localization

We study the effect of an internal rotor on the classical and quantum intramolecular vibrational energy redistribution (IVR) dynamics of a model system with three degrees of freedom. The system is based on a Hamiltonian proposed by Martens and Reinhardt [J. Chem. Phys. 93, 5621 (1990)] to study IVR in the excited electronic state of para-fluorotoluene. We explicitly construct the state space and show, confirming the mechanism proposed by Martens and Reinhardt, that an excited high frequency mode relaxes via diffusion along a thick layer of chaos created by the low frequency-rotor interactions. However, the corresponding quantum dynamics exhibits no appreciable relaxation of the high frequency mode. We attribute the quantum suppression of the classical thick-layer diffusion to the rotor selection rules and, possibly, dynamical localization effects.     

Dynamical consequences of symmetry breaking in benzene and difluorobenzene

The systems benzene/benzene-d(1) and o-/m-/p-difluorobenzene were studied in the dense gas phase with ultrafast transient absorption spectroscopy to investigate the effect of symmetry reduction through monodeuteration and constitutional isomerism on the timescales of intramolecular vibrational energy redistribution (IVR). In both systems IVR proceeds faster in the molecules of lower symmetry. In addition the dynamics were simulated in vibrational quantum number space using a simple model based on scaling state-to-state interactions by coupling order and the energy gap law. These simulations (semi-) quantitatively reproduce the experimental data for benzene and benzene-d(1) without incorporating further molecular symmetry restrictions. The relative impact of molecular symmetry and vibrational state space structure on IVR is discussed.     

Progress in understanding the intramolecular vibrational redistribution dynamics in the S(1) state of para-fluorotoluene

We employ zero-kinetic-energy (ZEKE) photoelectron spectroscopy with nanosecond laser pulses to study intramolecular vibrational redistribution (IVR) in S(1) para-fluorotoluene. The frequency resolution of the probe step is superior to that obtained in any studies on this molecule to date. We focus on the behavior of the 13(1) (C-CH(3) stretch) and 7a(1) (C-F stretch) vibrational states whose dynamics have previously received significant attention, but with contradictory results. We show conclusively that, under our experimental conditions, the 7a(1) vibrational state undergoes significantly more efficient IVR than does the 13(1) state. Indeed, under the experimental conditions used here, the 13(1) state undergoes very little IVR. These two states are especially interesting because their energies are only 36 cm(-1) apart, and the two vibrational modes have the same symmetry. We discuss the role of experimental conditions in observations of IVR in some detail, and thereby suggest explanations for the discrepancies reported to date.     

Intramolecular vibrational redistribution in the vibrational predissociation dynamics of He-I2

The intramolecular vibrational redistribution (IVR) process is investigated in wave packet simulations of the vibrational predissociation dynamics of He-I(2)(B,upsilon') in the region of high upsilon' levels, upsilon' = 35-65. The simulations indicate that for upsilon' < or = 45 the dynamics is dominated by direct predissociation, whereas for higher upsilon' levels the onset of IVR appears and becomes increasingly important. The IVR process occurs via coupling of the initial state in the upsilon' manifold to intermediate long-lived resonances belonging to the lower upsilon < upsilon' vibrational manifolds. The IVR dynamics manifests itself in multiexponential behavior and oscillations in the time-dependent population curves associated with the He-I(2)(B,upsilon') initial state, the He-I(2)(B,upsilon < upsilon') intermediate complexes, and the final product states. The population curves corresponding to the upsilon'- 1 intermediate resonances located below the He + I(2)(B,upsilon'-1,j=0) dissociation limit are analyzed. It is found that initial population is transferred to all the intermediate resonance states considered, including those more separated in energy from the initial one. The results obtained for population transfer between the initial and the intermediate states can be explained by the intensity of the matrix elements coupling the initial and the intermediate resonances, in combination with the Rabi's formula for population exchange between two coupled states.     

Vibrational relaxation of CH3I in the gas phase and in solution

Transient electronic absorption measurements reveal the vibrational relaxation dynamics of CH(3)I following excitation of the C-H stretch overtone in the gas phase and in liquid solutions. The isolated molecule relaxes through two stages of intramolecular vibrational relaxation (IVR), a fast component that occurs in a few picoseconds and a slow component that takes place in about 400 ps. In contrast, a single 5-7 ps component of IVR precedes intermolecular energy transfer (IET) to the solvent, which dissipates energy from the molecule in 50 ps, 44 ps, and 16 ps for 1 M solutions of CH(3)I in CCl(4), CDCl(3), and (CD(3))(2)CO, respectively. The vibrational state structure suggests a model for the relaxation dynamics in which a fast component of IVR populates the states that are most strongly coupled to the initially excited C-H stretch overtone, regardless of the environment, and the remaining, weakly coupled states result in a secondary relaxation only in the absence of IET.     

Quantum dynamics study of fulvene double bond photoisomerization: the role of intramolecular vibrational energy redistribution and excitation energy

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies.     

Effect of ring torsion on intramolecular vibrational redistribution dynamics of 1,1'-binaphthyl and 2,2'-binaphthyl

The role of ring torsion in the enhancement of intramolecular vibrational energy redistribution (IVR) in aromatic molecules was investigated by conducting excitation and dispersed fluorescence spectroscopy of 1,1'-binaphthyl (1,1'-BN) and 2,2'-BN. The dispersed fluorescence spectra of 1,1'-BN in the origin region of S(1)-S(0) were well resolved, which presented 25-27 cm(-1) gaps of torsional mode in the ground state. The overall profile of the dispersed spectra of 1,1'-BN is similar to that of naphthalene. In contrast, the spectra of 2,2'-BN were not resolved due to the multitude of the active torsional modes. In both cases, dissipative IVR was observed to take place with a relatively small excess vibrational energy: 237.5 cm(-1) for 1,1'-BN and 658 cm(-1) for 2,2'-BN, which clearly shows that ring torsion efficiently enhances the IVR rate. Ab initio and density functional theory calculations with medium-sized basis sets showed that the torsional potential of 1,1'-BN has a very flat minimum over the range of torsional angles from ca. 60° to 120°, whereas that of 2,2'-BN showed two well-defined potential minima at ca. 40° and 140°, in resemblance to the case of biphenyl. In this work, we propose that aromatic molecules be classified into "strong" and "weak" torsional hindrance cases: molecules with strong hindrance case show shorter torsional progressions and more effective IVR dynamics than do those with weak hindrance.     

Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states

Picosecond time-resolved photoelectron spectroscopy is used to investigate intramolecular vibrational redistribution (IVR) following excitation of S(1) 18a(1) in p-fluorotoluene (pFT) at an internal energy of 845 cm(-1), where ν(18a) is a ring bending vibrational mode. Characteristic oscillations with periods of 8 ps and 5 ps are observed in the photoelectron signal and attributed to coupling between the initially excited zero-order bright state and two doorway states. Values for the coupling coefficients connecting these three vibrational states have been determined. In addition, an exponential change in photoelectron signal with a lifetime of 17 ps is attributed to weaker couplings with a bath of dark states that play a more significant role during the latter stages of IVR. A tier model has been used to assign the most strongly coupled doorway state to S(1) 17a(1) 6a(2)('), where ν(17a) is a CH out-of-plane vibrational mode and 6a(2)(') is a methyl torsional level. This assignment signifies that a torsion-vibration coupling mechanism mediates the observed dynamics, thus demonstrating the important role played by the methyl torsional mode in accelerating IVR.     

Intramolecular vibrational energy redistribution in aromatic molecules of type C₆H₅X (X = H, D, F, Cl, CH₃, CF₃)

Femtosecond IR pump UV probe spectroscopy was employed in the gas phase to study intramolecular vibrational energy redistribution (IVR) in benzene and five monosubstituted derivatives thereof. After selective excitation of the first overtone of the ring CH-stretch vibration, all molecules showed the same two-step redistribution dynamics characteristic for nonstatistical IVR. The nature of the substituent influences mainly the second, slower IVR component. The presence of an internal rotor does not alter the redistribution rate or pathway compared to that of a monatomic substituent of equal mass. Coupling order model calculations reflect the experimental trends well if the polyatomic substituents are regarded as decoupled from the intra-ring dynamics and modeled as point masses.     

Overlapping resonances in the control of intramolecular vibrational redistribution

Coherent control of bound state processes via the interfering overlapping resonance scenario [Christopher et al., J. Chem. Phys. 123, 064313 (2006)] is developed to control intramolecular vibrational redistribution (IVR). The approach is applied to the flow of population between bonds in a model of chaotic OCS vibrational dynamics, showing the ability to significantly alter the extent and rate of IVR by varying quantum interference contributions.     

Femtosecond time-resolved geometry relaxation and ultrafast intramolecular energy redistribution in Ag2Au.

The ultrafast dynamics of the bimetallic cluster Ag2Au is investigated by pump-probe negative ion-to-neutral-to-positive ion (NeNePo) spectroscopy. Preparation of the neutral cluster in a highly nonequilibrium state by electron detachment from the mass-selected anion, and subsequent probing of the neutral nuclear dynamics through two-photon ionization to the cationic state, leads to strongly probe-energy-dependent transient cation-abundance signals. The origin of this pronounced time and wavelength dependence of the ionization probability on the femtosecond scale is revealed by ab initio theoretical simulations of the transient spectra. Based on the analysis of underlying dynamics, two fundamental processes involving geometry relaxation from linear to triangular structure followed by ultrafast intramolecular vibrational energy redistribution (IVR) have been identified and for the first time experimentally observed in the frame of NeNePo spectroscopy under conditions close to zero electron kinetic energy.     

E−Z Isomerization in Guanidine: Second-order Saddle Dynamics,Non-statisticality,and Time-frequency Analysis

Our recent work on the E−Z isomerization reaction of guanidine using ab initio chemical dynamics simulations [Rashmi et al., Regul. Chaotic Dyn. 2021 , 26, 119] emphasized the role of second-order saddle ( SOS ) in the isomerization reaction; however, we could not unequivocally establish the non-statistical nature of the dynamics followed in the reaction. In the present study, we performed thousands of on-the-fly trajectories using forces computed at the MNDO level to investigate the influence of second-order saddle in the E−Z isomerization reaction of guanidine and the role of intramolecular vibrational energy redistribution (IVR) on the reaction dynamics. The simulations reveal that while majority of the trajectories follow the traditional transition state pathways, 15 % of the trajectories follow the SOS path. The dynamics was found to be highly non-statistical with the survival probabilities of the reactants showing large deviations from those obtained within the RRKM assumptions. In addition, a detailed analysis of the dynamics using time-dependent frequencies and the frequency ratio spaces reveal the existence of multiple resonance junctions that indicate the existence of regular dynamics and long-lived quasi-periodic trajectories in the phase space associated with non-RRKM behavior.     

Intramolecular vibrational energy redistribution in bridged azulene-anthracene compounds: ballistic energy transport through molecular chains

Intramolecular vibrational energy flow in excited bridged azulene-anthracene compounds is investigated by time-resolved pump-probe laser spectroscopy. The bridges consist of molecular chains and are of the type (CH(2))(m) with m up to 6 as well as (CH(2)OCH(2))(n) (n=1,2) and CH(2)SCH(2). After light absorption into the azulene S(1) band and subsequent fast internal conversion, excited molecules are formed where the vibrational energy is localized at the azulene side. The vibrational energy transfer through the molecular bridge to the anthracene side and, finally, to the surrounding medium is followed by probing the red edge of the azulene S(3) absorption band at 300 nm and/or the anthracene S(1) absorption band at 400 nm. In order to separate the time scales for intramolecular and intermolecular energy transfer, most of the experiments were performed in supercritical xenon where vibrational energy transfer to the bath is comparably slow. The intramolecular equilibration proceeds in two steps. About 15%-20% of the excitation energy leaves the azulene side within a short period of 300 fs. This component accompanies the intramolecular vibrational energy redistribution (IVR) within the azulene chromophore and it is caused by dephasing of normal modes contributing to the initial local excitation of the azulene side and extending over large parts of the molecule. Later, IVR in the whole molecule takes place transferring vibrational energy from the azulene through the bridge to the anthracene side and thereby leading to microcanonical equilibrium. The corresponding time constants tau(IVR) for short bridges increase with the chain length. For longer bridges consisting of more than three elements, however, tau(IVR) is constant at around 4-5 ps. Comparison with molecular dynamics simulations suggests that the coupling of these chains to the two chromophores limits the rate of intramolecular vibrational energy transfer. Inside the bridges the energy transport is essentially ballistic and, therefore, tau(IVR) is independent on the length.     

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. I. Intramolecular vibrational energy redistribution of the OH and CH stretching vibrations of bare phenol

The intramolecular vibrational energy redistribution (IVR) of the OH stretching vibration of jet-cooled phenol-h6 (C6H8OH) and phenol-d8 (C6D8OH) in the electronic ground state has been investigated by picosecond time-resolved IR-UV pump-probe spectroscopy. The OH stretching vibration of phenol was excited with a picosecond IR laser pulse, and the subsequent temporal evolutions of the initially excited level and the redistributed ones due to the IVR were observed by multiphoton ionization detection with a picosecond UV pulse. The IVR lifetime for the OH stretch vibration of phenol-h6 was determined to be 14 ps, while that of the OH stretch for phenol-d8 was found to be 80 ps. This remarkable change of the IVR rate constant upon the dueteration of the CH groups strongly suggests that the "doorway states" for the IVR from the OH level would be the vibrational states involving the CH stretching modes. We also investigated the IVR rate of the CH stretching vibration for phenol-h6. It was found that the IVR lifetime of the CH stretch is less than 5 ps. The fast IVR is described by the strong anharmonic resonance of the CH stretch with many other combinations or overtone bands.     

Iridium metal complexes as an unambiguous probe of intramolecular vibrational redistribution

Ultrafast luminescence spectroscopy has been undertaken on three iridium cored phosphorescent complexes, with the Ir(ppy)3 molecule being compared with two Ir(ppy)3 cored dendrimers. Energy dissipation by intramolecular vibrational redistribution (IVR) and cooling shows as a luminescence decay because it decreases the admixture of singlet character to the emitting triplet state. A larger amount of vibrational energy dissipates by IVR in dendrimer complexes. We have therefore found a methodology of obtaining unambiguous information on the IVR process and show its potential to study IVR rates as a function of vibrational energy.     

Modeling the photophysics and photochromic potential of 1,2-dihydronaphthalene (DHN): a combined CASPT2//CASSCF-topological and MMVB-dynamical investigation

The photochemical ring opening of 1,2-dihydronaphthalene (DHN) was investigated using two complementary computational approaches. CASPT2//CASSCF minimum energy paths were characterized for reaction channels on the three lowest-energy singlet excited states, describing initial evolution of the spectroscopic bright (ionic) state and its subsequent decay to dark (covalent) states of benzene-like and hexatriene-like character. Although the benzene-like state is unreactive and can radiate, the hexatriene-like state has indirect access to a low-energy conical intersection seam, at which radiationless decay to the ground state and subsequent product formation can take place. An MMVB molecular dynamics simulation was carried out on the reactive hexatriene-like excited state, suggesting that intramolecular vibrational energy redistribution (IVR) controls the radiationless decay and the photoproduct distribution (which is qualitatively reproduced).     

Picosecond IR-UV pump-probe spectroscopic study on the intramolecular vibrational energy redistribution of NH2 and CH stretching vibrations of jet-cooled aniline

Intramolecular vibrational energy redistribution (IVR) of the NH2 symmetric and asymmetric stretching vibrations of jet-cooled aniline has been investigated by picosecond time-resolved IR-UV pump-probe spectroscopy. A picosecond IR laser pulse excited the NH2 symmetric or asymmetric stretching vibration of aniline in the electronic ground state and the subsequent time evolutions of the excited level as well as redistributed levels were observed by a picosecond UV pulse. The IVR lifetimes for symmetric and asymmetric stretches were obtained to be 18 and 34 ps, respectively. In addition, we obtained the direct evidence that IVR proceeds via two-step bath states; that is, the NH2 stretch energy first flows into the doorway state and the energy is further dissipated into dense bath states. The rate constants of the second step were estimated to be comparable to or slower than those of the first step IVR. The relaxation behavior was compared with that of IVR of the OH stretching vibration of phenol [Y. Yamada, T. Ebata, M. Kayano, and M. Mikami J. Chem. Phys. 120, 7400 (2004)]. We found that the second step IVR process of aniline is much slower than that of phenol, suggesting a large difference of the "doorway state increasing the dense bath states" anharmonic coupling strength between the two molecules. We also observed IVR of the CH stretching vibrations, which showed much faster IVR behavior than that of the NH2 stretches. The fast relaxation is described by the interference effect, which is caused by the coherent excitation of the quasistationary states.     

High-resolution IR studies of hydrogen bonded clusters: large amplitude dynamics in (HCl)n

Structural and dynamical information on small hydrogen-bonded systems is revealed by high-resolution IR spectroscopy of HCl dimer, trimer and tetramer. In (HCl)2, four combination bands tentatively assigned to the Van der Waals stretch nu 4 and geared band nu 5 vibrations are observed. The study focuses on two unexpected results: (i) all of the observed bands are built only on the bound HCl stretch nu 2, and (ii) the bands predominantly originate from the 9-fold less populated upper tunneling level of the ground state. Model 3D quantum calculations are presented to show that both these surprising trends originate from the large amplitude tunneling dynamics in the dimer. The (HCl)3 spectra are assigned and analyzed for multiple isotopomeric contributions. The spectral fit reveals large homogeneous line broadening indicating the excited state lifetime of approximately 1.6 ns and tentatively associated with dynamics of intramolecular vibrational energy distribution (IVR) induced trimer ring opening. Finally, first high-resolution data on the HCl stretch fundamental spectrum of (HCl)4 are presented.     

Quasiclassical trajectory simulations of OH(v) + NO2 --> HONO2* --> OH(v') + NO2: capture and vibrational deactivation rate constants

Quasiclassical trajectory calculations are used to investigate the dynamics of the OH(v) + NO(2) --> HONO(2) --> OH(v') + NO(2) recombination/dissociation reaction on an analytic potential energy surface (PES) that gives good agreement with the known structure and vibrational frequencies of nitric acid. The calculated recombination rate constants depend only weakly on temperature and on the initial vibrational energy level of OH(v). The magnitude of the recombination rate constant is sensitive to the potential function describing the newly formed bond and to the switching functions in the PES that attenuate inter-mode interactions at long range. The lifetime of the nascent excited HONO(2) depends strongly not only on its internal energy but also on the identity of the initial state, in disagreement with statistical theory. This disagreement is probably due to the effects of slow intramolecular vibrational energy redistribution (IVR) from the initially excited OH stretching mode. The vibrational energy distribution of product OH(v') radicals is different from statistical distributions, a result consistent with the effects of slow IVR. Nonetheless, the trajectory results predict that vibrational deactivation of OH(v) via the HONO(2) transient complex is approximately 90% efficient, almost independent of initial OH(v) vibrational level, in qualitative agreement with recent experiments. Tests are also carried out using the HONO(2) PES, but assuming the weaker O-O bond strength found in HOONO (peroxynitrous acid). In this case, the predicted vibrational deactivation efficiencies are significantly lower and depend strongly on the initial vibrational state of OH(v), in disagreement with experiments. This disagreement suggests that the actual HOONO PES may contain more inter-mode coupling than found in the present model PES, which is based on HONO(2). For nitric acid, the measured vibrational deactivation rate constant is a useful proxy for the recombination rate, but IVR randomization of energy is not complete, suggesting that the efficacy of the proxy method must be evaluated on a case-by-case basis.     

Structural Dynamics upon Photoexcitation in a Spin Crossover Crystal Probed with Femtosecond Electron Diffraction

Photoexcitation of spin crossover (SCO) complexes can trigger extensive electronic spin transitions and transformation of molecular structure. However, the precise nature of the associated ultrafast structural dynamics remains elusive, especially in the solid state. Here, we studied a single‐crystal SCO material with femtosecond electron diffraction (FED). The unique capability of FED allows us to directly probe atomic motions and to track ultrafast structural changes within a crystal lattice. By monitoring the time‐dependent changes of the Bragg reflections, we observed the formation of a photoinduced structure similar to the thermally induced high‐spin state. The data and refinement calculations indicate the global structural reorganization within 2.3 ps, as the metal–ligand bond distribution narrows during intramolecular vibrational energy redistribution (IVR) driving the intermolecular rearrangement. Three independent dynamical group are identified to model the structural dynamics upon photoinduced SCO.