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.     

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. II. Intracluster vibrational energy redistribution of the OH stretching vibration of hydrogen-bonded clusters

A picosecond time-resolved IR-UV pump-probe spectroscopic study has been carried out for investigating the intracluster vibrational energy redistribution (IVR) and subsequent dissociation of hydrogen-bonded clusters of phenol (C6H5OH) and partially deuterated phenol (C6D5OH, phenol-d5) with various solvent molecules. The H-bonded OH stretching vibration was pumped by a picosecond IR pulse, and the transient S1-S0 UV spectra from the pumped level as well as the redistributed levels were observed with a picosecond UV laser. Two types of hydrogen-bonded clusters were investigated with respect to the effect of the H-bonding strength on the energy flow process: the first is of a strong "sigma-type H-bond" such as phenol-(dimethyl ether)(n=1) and phenol dimer, and the second is phenol-(ethylene)(n=1) having a weak "pi-type H-bond." It was found that the population of the IR-pumped OH level exhibits a single-exponential decay, whose rate increases with the H-bond strength. On the other hand, the transient UV spectrum due to the redistributed levels showed a different time evolutions at different monitoring UV frequency. From an analysis of the time profiles of the transient UV spectra, the following three-step scheme has been proposed for describing the energy flow starting from the IVR of the initially excited H-bonded OH stretching level to the dissociation of the H bond. (1) The intramolecular vibrational energy redistribution takes place within the phenolic site, preparing a hot phenol. (2) The energy flows from the hot phenol to the intermolecular vibrational modes of the cluster. (3) Finally, the hydrogen bond dissociates. Among the three steps, the rate constant of the first step was strongly dependent on the H-bond strength, while the rate constants of the other two steps were almost independent of the H-bond strength. For the dissociation of the hydrogen bond, the observed rate constants were compared with those calculated by the Rice, Ramsperger, Kassel, and Marcus model. The result suggests that dissociation of the hydrogen bond takes place much faster than complete energy randomization within the clusters.     

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.     

Picosecond IR-UV pump-probe study on the vibrational relaxation of phenol-ethylene hydrogen-bonded cluster: difference of relaxation route/rate between the donor and the acceptor site excitations

Picosecond time-resolved IR-UV pump-probe spectroscopy has been performed to study intracluster vibrational energy redistribution (ICVR) and vibrational predissociation (VP) for the OH and CH stretch vibrations of phenol-ethylene hydrogen-bonded cluster. The transient UV spectra after the picosecond IR pulse excitation of these modes were observed by 1+1 REMPI via S(1) with a picosecond UV pulse. We have focused on the difference of the energy flow routes and their rates between the donor (phenol) and the acceptor (ethylene) site. Though the transient UV spectra showed a similar broad feature for all the vibrations examined, the time profiles exhibited a remarkable site dependence, as well as substantial mode dependence. Especially, we found a large difference in the early stage of the IVR evolution and the rates, whereas the VP rates were very similar.     

Vibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy

Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz(2)) and trimer (Bz(3)) has been investigated by picosecond (ps) time-resolved IR-UV pump-probe spectroscopy in a supersonic beam. The vibrational bands in the 3000-3100 cm(-1) region were excited by a ps IR pulse and the time evolutions at the pumped and redistributed (bath) levels were probed by resonance enhanced multiphoton ionization with a ps UV pulse. For Bz(2), a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C(6)H(6) and d = C(6)D(6), and its result was compared with that of hh homodimer. In the hd heterodimer, the two isomers, h(stem)d(top) and h(top)d(stem), show remarkable site-dependence of the lifetime of intracluster vibrational energy redistribution (IVR); the lifetime of the Stem site [h(stem)d(top), 140-170 ps] is ~2.5 times shorter than that of the Top site [h(top)d(stem), 370-400 ps]. In the transient UV spectra, a broad electronic transition due to the bath modes emerges and gradually decays with a nanosecond time scale. The broad transition shows different time profile depending on UV frequency monitored. These time profiles are described by a three-step VER model involving IVR and vibrational predissociation: initial → bath1(intramolecular) → bath2(intermolecular) → fragments. This model also describes well the observed time profile of the Bz fragment. The hh homodimer shows the stepwise VER process with time constants similar to those of the hd dimer, suggesting that the excitation-exchange coupling of the vibrations between the two sites is very weak. Bz(3) also exhibited the stepwise VER process, though each step is faster than Bz(2).     

The determination of vibrational anharmonicity in molecules from spectroscopic observations

This review article considers the origin of vibrational anharmonicity in molecules, and the effects that vibrational resonances have on the anharmonicity constants which may be extracted from spectroscopic observations. The importance of the effects of Darling—Dennison resonances, which increase with increasing excitation, as well as Fermi resonances, are considered. The local mode approach to X—Y stretching vibrations is dealt with, as a means of simultaneously accounting for Darling—Dennison resonances and of inter-relating normal mode stretching anharmonicity constants, thus reducing the number of parameters to be determined. The inclusion of Fermi resonances, as necessary, into the calculation is next considered, and the joint local mode-normal mode analysis explained.Applications to ethylenic and methyl group molecules are made. The success of the analyses is demonstrated through complete sets of physically representative anharmonicity constants which reproduce vibrational observations into the visible (16 500 cm⁻¹), and which are mutually self-consistent over molecules containing the same functional groups.Extensions of the simple local mode model are considered, as means of achieving anharmonicity parameters which should describe more closely the molecular potential energy surface, and hence the concomitant physical and chemical processes which it controls.     

A vibrational spectroscopic study on 1,1,2-tribromethane

On the basis of new data on the Raman spectra of solid and gaseous 1,1,2-tribromoethane a revised vibrational assignment is suggested.     

The effect of collisions on vibrational energy distribution in polyatomic molecules

We approach the problem of the effect of collisions on infrared absorption by an experimental and modeling study of a well-understood system, absorption of CO₂ laser radiation by cold CF₄. The spectroscopy, collisional energy transfer rates, and laser characteristics are all known. We conclude that the dominant collisional processes that influence the vibrational energy distribution during infrared laser absorption are rotational energy transfer-rotational relaxation and pressure broadening.     

Threshold energy dependence of intramolecular vibrational redistribution (IVR) rates of isolated polyatomic molecules

A thermodynamic Green's function technique is successfully applied to the calculation of IVR rates of polyatomic molecules. The energy dependence of IVR rates has interesting threshold features: rates are zero below some critical energy E_c, and non-zero for higher energies. This dependence can be understood as a transition from regular to chaotic intramolecular motion.     

Influence of vibrational energy flow on isomerization of flexible molecules: incorporating non-Rice-Ramsperger-Kassel-Marcus kinetics in the simulation of dipeptide isomerization

The conformational isomerization of a dipeptide, N-acetyl-tryptophan methyl amide (NATMA), is studied computationally by including important dynamical corrections to Rice-Ramsperger-Kassel-Marcus (RRKM) theory for the transition rate between pairs of isomers. The dynamical corrections arise from incomplete or sluggish vibrational energy flow in the dipeptide, a property suggested by the mode-selective chemistry that has been observed by Dian et al. [J. Chem. Phys. 120, 133 (2004)]. We compute the extent and rate of vibrational energy flow in NATMA quantum mechanically using local random matrix theory, which we then use to correct the RRKM theory rates. The latter rates are then introduced into a master equation to study the population dynamics of the dipeptide. Incomplete or slow vibrational energy flow is found to enhance the conformational selectivity of NATMA over RRKM estimates.     

A vibrational spectroscopic study of salicylaldoxime in the solid state

The infrared and Raman spectra of solid salicylaldoxime have been measured both for normal and deuterated substance; the polarized infrared spectra of oriented samples have been also obtained. Assignments have been proposed on the basis of i.r. dichroism, Raman data and isotopic effects. The possible occurrence of Fermi resonance effects in some regions of the i.r. spectra has been also investigated.     

Mode-to-mode vibrational energy flow in S1 benzene

Resolved fluorescence spectra from low pressures of benzene with nine added gases have been used to follow mode-to-mode vibrational relaxation in the S₁ state of benzene under “single-collision” conditions. Cw pumping of the S₁ fundamental 6¹ (ν″₆ = 522 cm^?1) allows study of collisional vibrational energy flow into each of four channels. Two channels consist of flow into single levels, and the others represent flow into unresolved pairs of levels. The mode-to-mode cross sections are much larger than those usually observed in ground electronic states, being near gas kinetic even for partners transferring energy by VT, R processes alone. The mode-to-mode transfer has highly specific patterns, with roughly seventy percent of the transfer going into the four channels in spite of many other nearby levels. The largest cross sections are always to a level 237 cm^?1 above the initial level rather than to a level nearly resonant (ΔE = 7 cm^?1) with the initia l level. A common pattern of flow occurs for the four gases transferring energy by VT, R processes alone, and another common pattern is established for the five gases which can also use VV transfers. With the exception of one channel, VV resonances with vibrationally complex partners increase cross sections by less than a factor of two over that provided by the VT, R path. VV transfers have a similarly small effect on the overall vibrational relaxation rate out of the initial level 6¹. Both the flow patterns and the VV versus VT, R competitions are accounted for with an extremely simple and general set of propensity rules taken directly from SSH calculations made by others for vibrational relaxation in ground electronic states. The rules are based on the degeneracies of the final levels, the number of vibrational quantum changes, and the amount of energy exchanged between vibrational and translational/rotational degrees of freedom. The rules seem general to relaxation in both ground and excited electronic states, whereas large cross sections seem a special property of the excited state. The cross sections for collision partners SF₆ and perfluorohexane are small relative to those for other partners with similar vibrational complexity and mass.     

Semiclassical three-dimensional model for vibrational energy transfer in diatomic molecules

A semiclassical model for calculation of rate constants for vibrational excitation in diatomic gases at low temperatures (below 1000 K) is suggested. The model has been tested by its ability to predict the relaxation times of hydrogen (τ_H1 in the temperature region 40–1000 K. The agreement with experimental values is excellent. The isotopic ratio τ_D2/τ_H2 as a function of temperature is predicted.     

Manifestation of intramolecular vibrational energy redistribution on electronic relaxation in large molecules

Some universal characteristics are discussed of the decay lifetimes and fluorescence quantum yields from the S₁ manifold of large molecules, which originate from the coupling between intrastate vibrational energy redistribution and interstate electronic relaxation. The time-resolved total fluorescence decay from the S₁ state of jet-cooled 9-cyanoanthracene exhibits non-exponential decay in the energy range E_v= 1200–1740 cm⁻¹ above the S₁ origin, which does not originate from dephasing but rather manifests the effects of intrastate intermediate level structure for vibrational energy redistribution on intersystem crossing.     

Display of the flow of energy in molecules

In this article, we develop a method to graphically display the flow of energy within molecules. An energy continuity equation is derived leading to a molecular energy flux vector field. Computation of the flux calls for the intramolecular potential, any external interactions, and the phase space trajectories of the molecular motion. The flux provides a means to display energy flow in still frames and as a tool to visualize hitherto undiscovered dynamic pathways in molecules. Examples are presented that show energy flow in three molecular systems and illustrate the point that depiction of energy flux patterns has increasing utility and meaning as one moves to larger molecules. Simple extensions to this work would also allow visualization of the flux of such quantities as linear and angular momentum. © 1994 by John Wiley & Sons, Inc.     

Potential energy profiles along the doubly degenerate vibrational modes in conjugated molecules

In order to predict the molecular symmetries and the geometrical structures of conjugated molecules having the doubly degenerate first-order Jahn-Teller active modes (Q ₁ and Q ₂) or the doubly denegerate modes through which the second-order vibronic couplings occur (Q₁ and Q₂), the potential energy curves along these modes are expressed as the power series, including up to the third power. It is shown that although there are cases in which we cannot practically differentiate between the potential energy profiles along Q ₁, and Q ₂ or Q₁ and Q₂, in so far as we can differentiate between them, a potential energy minimum should always be located along Q ₁ or Q₁ that distorts a molecule in a more symmetrical way. This is in agreement with the available experimental facts. Finally on the basis of the perturbation theory, the coefficients of various powers (up to the third power) in the expansion of the electronic part of potential energy in the power series of the relevant mode are expressed in terms of the zeroth-order electronic wavefunctions and energies.     

Theoretical study on the potential energy surface and vibrational energy levels of HXeI

The potential energy surface for the electronic ground state of the HXeI molecule is constructed by using the internally contracted multi-reference configuration interaction with the Davidson correction（icMRCI＋Q）method and large basis sets. The stabilities and dissociation barriers are identified from the potential energy surfaces.The three-body dissociation channel is found to be the dominate dissociation channel for HXeI.Based on the obtained potentials,vibrational energy levels of HXeI are calculated using the Lanczos algorithm.Our theoretical results are in excellent agreement with the available observed values.     

Spin relaxation in isolated molecules and clusters: the interpretation of Stern-Gerlach experiments

Intramolecular spin relaxation may occur in isolated molecules or clusters provided that the density of rovibrational eigenstates is sufficiently high to serve as an energy bath and angular momentum is conserved. In the coupled, zero-field limit, total angular momentum (J) is the sum of spin (S) and rotational (N) momenta such that J and M(J) are good angular momentum quantum numbers. In the coupled limit, transitions between Zeeman levels (Delta M(J)++0) cannot occur in the absence of an external torque. However, in the high-field limit, J and M(J) are no longer good quantum numbers, as N and S are decoupled and only their projections on the z axis defined by the external field are invariant. In this case M(N) and M(S) remain as good quantum numbers so that angular momentum conserving transitions can occur subject to the selection rule Delta M(N)=-Delta M(S). Determination of the magnetic moments of isolated molecules and clusters via a thermodynamics-based analysis requires that their magnetizations are measured at sufficiently large fields that spin-rotation effects become negligible and the Zeeman level structure approaches the free-spin case.     

Probing isomer interconversion in anionic water clusters using an Ar-mediated pump-probe approach: Combining vibrational predissociation and velocity-map photoelectron imaging spectroscopies

We present the first results from an experiment designed to explore barriers for interconversion between isomers of cluster anions using an Ar-cluster mediated pump-probe technique. In this approach, anions are generated with many Ar atoms attached, and one of the isomers present is selectively excited by tuning an infrared laser to one of the isomer's characteristic vibrational resonances. The excited cluster is then cooled by evaporation of Ar atoms, and the isomer distribution in the lighter daughter ions is measured after secondary mass selection by recording their photoelectron spectra using velocity-map imaging. We apply the method to the water hexamer anion, (H(2)O)(6) (-), which is known to occur in two isomeric forms with different electron-binding energies. We find that conversion of the high-binding (type I) form to the low-binding (type II) isomer is not efficiently driven in (H(2)O)(6) (-) with excitation energies in the 0.4 eV range even though it is possible to create both isomers in abundance in the ion source. This observation is discussed in the context of the competition between isomerization and electron autodetachment, which depends on the relative positions of the neutral and ionic potential surfaces along the isomerization pathway. Application of the method to the more complex heptamer ion, however, does reveal that interconversion is available among the highest binding isomer classes (I and I(')).