Intramolecular Vibrational Energy Redistribution Dynamics Research Articles

IVR Dynamics of Vibrational Levels of the ν1 Mode in (CF3)2C═C═O Molecules Excited by Resonant IR Femtosecond Radiation.

The intramolecular dynamics of vibrational levels (up to v = 5) of the ν1 mode in the (CF3)2CCO molecule that is induced by a multiphoton selective excitation of this mode by resonant femtosecond IR radiation has been studied. The times of intramolecular vibrational energy redistribution (IVR) from each vibrational level to remaining molecular modes have been determined. In accordance with theoretical predictions, a decrease in the IVR time with increasing quantum number v has been observed for the first time. A sharp decrease in the IVR time (down to 1.5 ps) at a wavelength of 2129 cm-1, corresponding to the v = 3 → v = 4 vibrational transition, is revealed. It has been shown that, with a negative chirp of a femtosecond radiation pulse, the population of high-lying vibrational levels of the ν1 mode increases significantly.

Relevance of the Resonance Junctions on the Arnold Web to Dynamical Tunneling and Eigenstate Delocalization.

We study the competition and correspondence between the classical and quantum routes to intramolecular vibrational energy redistribution (IVR) in a three degrees of freedom model effective Hamiltonian. Specifically, we focus on the classical and the quantum dynamics near the resonance junctions on the Arnold web that are formed by an intersection of independent resonances. The regime of interest models the IVR dynamics from highly excited initial states near dissociation thresholds of molecular systems wherein both classical and purely quantum, involving dynamical tunneling, routes to IVR coexist. In the vicinity of a resonance junction, classical chaos is inevitably present, and hence one expects the quantum IVR pathways to have a strong classical component as well. We show that with increasing resonant coupling strengths the classical component of IVR leads to a transition from coherent dynamical tunneling to incoherent dynamical tunneling. Furthermore, we establish that the quantum IVR dynamics can be predicted based on the structures on the classical Arnold web. In addition, we investigate the nature of the highly excited eigenstates to identify the quantum signatures of the multiplicity-2 junctions. For the parameter regimes studies herein, by projecting the eigenstates onto the Arnold web, we find that eigenstates in the vicinity of the junctions are primarily delocalized due to dynamical tunneling.

Real-time visualization of the vibrational wavepacket dynamics in electronically excited pyrimidine via femtosecond time-resolved photoelectron imaging.

The vibrational wavepacket dynamics at the very early stages of the S1-T1 intersystem crossing in photoexcited pyrimidine is visualized in real time by femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. A coherent superposition of the vibrational states is prepared by the femtosecond pump pulse at 315.3 nm, resulting in a vibrational wavepacket. The composition of the prepared wavepacket is directly identified by a sustained quantum beat superimposed on the parent-ion transient, possessing a frequency in accord with the energy separation between the 6a1 and 6b2 states. The dephasing time of the vibrational wavepacket is determined to be 82 ps. More importantly, the variable Franck-Condon factors between the wavepacket components and the dispersed cation vibrational levels are experimentally illustrated to identify the dark state and follow the energy-flow dynamics on the femtosecond time scale. The time-dependent intensities of the photoelectron peaks originated from the 6a1 vibrational state exhibit a clear quantum beating pattern with similar periodicity but a phase shift of π rad with respect to those from the 6b2 state, offering an unambiguous picture of the restricted intramolecular vibrational energy redistribution dynamics in the 6a1/6b2 Fermi resonance.

Complex and Sustained Quantum Beating Patterns in a Classic IVR System: The 3(1)5(1) Level in S1 p-Difluorobenzene.

Using picosecond time-resolved photoelectron imaging, we have studied the intramolecular vibrational energy redistribution (IVR) dynamics that occur following the excitation of the 3(1)5(1) level, which lies 2068 cm(-1) above the S1 origin in p-difluorobenzene. Our technique, which has superior time resolution to that of earlier studies but retains sufficient energy resolution to identify the behavior of individual vibrational states, enables us to determine six distinct beating periods in photoelectron intensity, only one of which has been observed previously. Analysis shows that the IVR dynamics are restricted among only a handful of vibrational levels, despite the relatively high excitation energy. This is deduced to be a consequence of the high symmetry and rigid structure of p-difluorobenzene.

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 energy redistribution as state space diffusion: Classical-quantum correspondence

We study the intramolecular vibrational energy redistribution (IVR) dynamics of an effective spectroscopic Hamiltonian describing the four coupled high frequency modes of CDBrClF. The IVR dynamics ensuing from nearly isoenergetic zeroth-order states, an edge (overtone) and an interior (combination) state, is studied from a state space diffusion perspective. A wavelet based time-frequency analysis reveals an inhomogeneous phase space due to the trapping of classical trajectories. Consequently the interior state has a smaller effective IVR dimension as compared to the edge state.

Real-time observation of the dynamics of vibrational-energy redistribution within an isolated polyatomic molecule by spontaneous raman spectroscopy

The dynamics of intramolecular vibrational-energy redistribution from an initially excited mode ν1 (the acetylene-type H-C bond) to the other modes of the H-C≡C-Si(CH3)3 molecule was studied experimentally. Probing was performed by spontaneous anti-Stokes Raman scattering. The measured deexcitation time of ν1 was 128±10 ps.

Vibrational Dynamics of Terminal Acetylenes: II. Pathway for Vibrational Relaxation in Gas and Solution

The pathway for vibrational-energy flow following the excitation of the first excited state of the acetylenic C−H stretch is investigated for a series of 10 terminal acetylenes in room-temperature gases and dilute solutions using transient absorption picosecond infrared spectroscopy. The transient absorption infrared spectra are obtained at three different probe frequencies. These experiments separately detect the population of the excited C−H stretch state, the population of vibrational states with 2 quanta of acetylenic C−H bend excitation, and the population of all other vibrational states with C−H stretch absorption frequencies within the laser bandwidth (25 cm-1) of the C−H stretch fundamental frequency. These measurements show that the initial redistribution event for the isolated molecule involves population transfer to vibrational states with bend overtone excitation. The secondary intramolecular vibrational-energy redistribution (IVR) process, which involves population transfer to the remaining near-resonant vibrational states, occurs on a time scale that is about 5 times slower than the initial redistribution event. The same relaxation pathway is observed in dilute solution. The total relaxation rate in solution for the slower process can be quantitatively described using a simple model where IVR and solvent-induced vibrational-energy relaxation (VER) proceed independently. The main effects of the solvent are to increase the extent of population relaxation for the first stage of IVR and to cool vibrational excitation rapidly in the low-frequency acetylene wag normal-mode vibrations produced by the IVR dynamics.

Molecular rotation in the presence of intramolecular vibrational energy redistribution

At high energy, the vibrational dynamics of a polyatomic molecule are qualitatively different from the separable normal-mode dynamics that characterize the low energy region of the spectrum. Once the total rovibrational state density exceeds 10-100 states cm-1, the effects of intramolecular vibrational energy redistribution (IVR) are readily observed in the frequency-domain spectrum. In an energy region where IVR occurs, the time scale for the flow of vibrational energy is comparable to the time scale for molecular rotation. The jostling of nuclear positions caused by the IVR dynamics leads to a time-dependent moment of inertia for the molecular rotation. The time-dependent modulation of the moment of inertia, in turn, affects the appearance of the rotational spectrum of the molecule. These effects can be described by the motional narrowing formalism first developed for nuclear magnetic resonance spectroscopy. We present a basic description of the rotational problem for the case where the molecule has a single energetically accessible nuclear geometry and the case where the total energy of the molecule is above the barrier to isomerization. In the latter case, the microcanonical isomerization rate can be obtained from the overall line shape of the rotational spectrum. An example of using rotational spectroscopy to measure the isomerization rate of 4-chlorobut-1-yne at 3330 cm-1 is presented.

The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCl in the vOH=6 region, probed by infrared-visible double resonance overtone excitation

We use infrared-visible double resonance overtone excitation to promote HOCl molecules to single, well-characterized rotational levels of high OH stretching states just above the HOCl→HO+Cl dissociation threshold on the ground potential energy surface. Double resonance spectra are monitored by laser induced fluorescence detection of the OH dissociation products. We present here the results obtained in the 6ν1 region of HO35Cl where we have studied states with J ranging from 4 to 25, Ka from 0 to 5 and energy up to 300 cm−1 above the dissociation threshold. In the spectra for Ka=0–3 states, the zeroth-order (nOH,nθ,nOCl)=(6,0,0) level is split by mixing with a nearby dark state. Because the two states have very different A rotational constants, their separation increases with Ka, but the effects of the mixing remain observable in the spectrum up to Ka=3. Comparison with preliminary results from HO37Cl, together with analysis of the rotational constants, allows us to identify the perturbing state as (4,4,2). The lack of further strong perturbations compared to the average density of states allows us to infer that most of the matrix elements for couplings between the (6,0,0) bright state and other dark states are less than ∼0.1 cm−1. The average intramolecular vibrational energy redistribution (IVR) rate implied by these matrix elements (2.5×109 s) is two orders of magnitude longer than the predictions of statistical rate theory, indicating that IVR is likely to be the rate limiting step in the unimolecular dissociation process from (6,0,0). The present work provides the spectroscopic foundation for direct time-resolved studies of the unimolecular dissociation dynamics presented in a forthcoming paper.

A quantum mechanical study of the role of out-of-plane vibrations in benzene in the intramolecular redistribution of vibrational energy

In this work we have carried out a theoretical investigation on the role of out-of-plane vibrational modes in vibrational level mixing and intramolecular vibrational energy redistribution (IVR) in benzene. A fully symmetrized vibrational basis set, based on a combined local mode (LM)+normal modes (NM) formalism was employed in the study. The Hamiltonian formalism for description of out-of-plane vibrations has been developed in due detail. Model calculations on the absorption spectrum and IVR dynamics at the second overtone state CH(v=3) at ∼8800 cm−1 have been carried out, supplementing previous studies and demonstrating the importance of out-of-plane modes in benzene.

Intramolecular vibrational energy redistribution in the acetylenic C–H and hydroxyl stretches of propynol

The high-resolution infrared spectra of the acetylenic C–H and O–H stretches of propynol have been measured using an electric-resonance optothermal molecular beam spectrometer (EROS). Both spectra display extensive fragmentation of the hydride-stretch oscillator strength characteristic of the intramolecular vibrational energy redistribution (IVR) process. The IVR lifetime is strongly mode-specific. The IVR lifetime of the acetylenic C–H stretch is approximately 400 ps, with a slight increase in the lifetime with increasing values of the Ka quantum number. The lifetime of the O–H stretch is 60 ps and is independent of the rotational quantum numbers. The experimental upper limit for the anharmonic state densities are 30 and 40 states/cm−1 for the acetylenic C–H and O–H stretches, respectively. These values are in good agreement with the values obtained by a direct state count (19 and 32 states/cm−1, respectively). The measured density of states increases with an approximate (2J+1)-dependence. These results indicate that all energetically accessible states are involved in the IVR dynamics. However, neither the acetylenic C–H nor the O–H stretch shows a decrease in lifetime as the total angular momentum (J) increases. This result shows that Coriolis coupling of these two hydride stretches to the near-resonant bath states is much weaker than the anharmonic coupling. For the O–H stretch, we are able to obtain the root-mean-squared (rms) matrix element for the Coriolis coupling prefactor, 0.0015(5) cm−1. The rms anharmonic coupling matrix element is 0.03 cm−1. For the low J values measured in the O–H spectrum, the Coriolis-induced IVR rate is much slower than the initial redistribution rate resulting from the stronger anharmonic interactions leading to an IVR process with two distinct time scales.

Stimulated Emission Pumping Spectra and Intramolecular Vibrational Dynamics of DFCO(S0) from 9000 to 20 000 cm-1

Dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectra of DFCO from 9000 to 20 000 cm -1 on S0 are reported. Groups of features in the 2 1 5 1 6 2 DF spectrum are assigned as strongly coupled members of 266 [O2(CO stretch) T 2O6(out-of-plane bend)] polyads of vibrational states. The FranckCondon active states in these polyads are primarily the zero-order states 2 n6m with values of n ranging from 0 to 4 and values of m ranging from 13 to 19. The assignment of the Franck-Condon active states is consistent with the HFCO experimental results and with Franck-Condon factor calculations. Multiple intramolecular vibrational energy redistribution (IVR) time scales are evident from the dilution of the Franck -Condon factors observed in the spectra examined at different resolutions. The fastest IVR results from the low-order ( ¢ V) 3) 266 resonance that couples O2 and O6. This initial IVR occurs in 25 fs. Evidence of anharmonic coupling within the vicinity of single DF features signifies subsequent IVR on the 0.5-4.0 ps time scale. This second time scale can be understood in terms of further coupling to O3 (HCO bend) and O5 (FCO bend) resulting from a 3566 Darling-Dennison resonance. Clumps of vibrational lines within individual SEP bands indicate further anharmonic and weak A-type Coriolis coupling to background levels at times between 3 and 150 ps. Comparison of the observed and calculated vibrational-state densities indicates that, for many states, IVR may be nearly complete on the time scale of the measured dissociation rates. The IVR dynamics of DFCO are in marked contrast to the mode-specific IVR dynamics of HFCO. The quasistability of extreme-motion out-of-plane vibrations (V6 g 14), in HFCO is destroyed by the strong coupling of O2 and 2O6 in DFCO. Thus, the two isotopomers display qualitatively different IVR dynamics.

Femtosecond Real-Time Probing of Reactions. 20. Dynamics of Twisting, Alignment, and IVR in the trans-Stilbene Isomerization Reaction

The femtosecond real-time dynamics of the isomerization reaction of trans-stilbene under collisionless conditions are studied using (2+1) resonance-enhanced multiphoton ionization (REMPI) and femtosecond depletion spectroscopy (FDS) in a pump−probe scheme. The observed transients reflect the macroscopic sample anisotropy decay (rotational coherence) and intramolecular vibrational energy redistribution (IVR) as well as the ethylenic twisting isomerization reaction. Polarization-resolved measurements are performed to isolate the influence of rotational dynamics, and the measured anisotropy decay is compared with theoretical calculations of rotational coherence at room temperature. The IVR and nonradiative (isomerization) processes of trans-stilbene are studied as a function of S1 excess vibrational energy up to ∼6500 cm-1. These results are compared with previous measurements of trans-stilbene under collisionless conditions and with predictions of RRK and RRKM theories.

A quantum dynamical study of CH overtones in fluoroform. I. A nine-dimensional ab initio surface, vibrational spectra and dynamics

In this series, the characteristics of intramolecular vibrational-energy redistribution (IVR) present in the CH overtones of CHF3 are investigated. Particular attention is given to the multiple time scales and thus mechanisms present in the IVR dynamics. In Part I, a 9-dimensional ab initio potential energy surface is developed to adequately account for the vibrational couplings of all modes. Furthermore, all-mode vibrational state calculations, of large primitive space dimension, are performed using a recently developed wave operator sorting algorithm in tandem with the recursive residue generation method. All fundamentals, first overtones, and bimodal combination states with up to 3 quanta are presented. Also, the A1 and E-symmetry CH polyads are determined through the second overtone. Equilibrium geometry, rotational constants, and vibrational properties agree quantitatively with experiment in most cases. The error is systematic in origin and largely due to the error in the ab initio harmonic frequencies. New vibrational constants and resonance interactions are reported for the background modes. In contrast to the prominent CH stretch–bend Fermi resonance structure, responsible for ultrafast (t<50 fs) energy transfer, the CH polyads also exhibit vibrational fine structure of order 1 to 10 cm−1 due to background-mode coupling. This secondary coupling results in IVR on the picosecond time scale.

Femtosecond real-time probing of reactions. IX. Hydrogen-atom transfer

The real-time dynamics of hydrogen-atom-transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ‘‘proton’’ transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl-group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen-transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm−1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low-frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational-energy redistribution dynamics at longer times are important to the hydrogen-bond dissociation and to the nonradiative decay of the hydrogen-transferred form.