Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v₆ and v₄ vibrational manifolds of D₂CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels     

The intramolecular vibrational energy redistribution threshold in S(1) deuterated p-difluorobenzene

The intramolecular vibrational energy redistribution (IVR) in S(1) deuterated p-difluorobenzene (pDFB-d(4) or -d(4)) has been studied to determine the IVR threshold. For this, the S(1) <-- S(0) fluorescence excitation (FE) spectrum of jet-cooled d(4) was investigated in the 2000-3250 cm(-1) vibronic energy range of the S(1) electronic state, and single vibronic level fluorescence (SVLF) spectra have been acquired by exciting selected levels lying between 750 and 2850 cm(-1) in vibrational energy in the S(1) excited state. Congestion of the dispersed fluorescence in this molecule first appears as the vibrational level energy climbs above 2000 cm(-1). By comparing the SVLF spectra of pDFB-d(4) with those of p-difluorobenzene (pDFB or -h(4)), it is obvious that IVR threshold in -d(4) is localized with a few hundreds cm(-1) lower than that in pDFB. This decrease is entirely due to the increase in vibrational state density due to deuteration.     

A normal vibrational analysis of benzene

The normal modes of benzene are recalculated to include the data from ¹³C-isotopes. An improved valence force field has been determined by inclusion of all CC stretch CCH bending interactions. The addition of these force constants is justified on theoretical grounds and by the data.     

Molecular vibrational energy flow and dilution factors in an anharmonic state space

A fourth-order resonance Hamiltonian is derived from the experimental normal-mode Hamiltonian of SCCl2. The anharmonic vibrational state space constructed from the effective Hamiltonian provides a realistic model for vibrational energy flow from bright states accessible by pulsed laser excitation. We study the experimentally derived distribution PE(sigma) of dilution factors sigma as a function of energy. This distribution characterizes the dynamics in the long-time limit. State space models predict that PE(sigma) should be bimodal, with some states undergoing facile intramolecular vibrational energy redistribution (small sigma), while others at the same total energy remain "protected" (sigma approximately 1). The bimodal distribution is in qualitative agreement with analytical and numerical local density of states models. However, there are fewer states protected from energy flow, and the protected states begin to fragment at higher energy, shifting from sigma approximately 1 to sigma approximately 0.5. We also examine how dilution factors are distributed in the vibrational state space of SCCl2 and how the power law specifying the survival probability of harmonic initial states correlates with the dilution factor distribution of anharmonic initial states.     

Raman study of vibrational relaxation of cyclohexane in benzene solutions

The Raman profiles of the ν₅ mode (802 cm^?1) of cyclohexane, ν₅ (723 cm^?1) of cyclohexane-_d12 and ν₂ (992 cm^?1) of benzene and its deuterated analogs have been measured as a function of concentration in the benzene—cyclohexane liquid system. The vibrational time correlation functions of cyclohexane in benzene solutions have been calculated by Fourier inversion of isotropic band contours. The concentration dependence of the experimental vibrational correlation times computed from the correlation functions and from the half width at half height have been compared with that predicted theoretically for various mechanisms of band broadening. We have tested the Fischer—Laubereau dephasing model and the Knapp—Fischer concentration-fluctuation model. We have found that the latter model reproduces well experimental data only for the ν₂ mode of benzene in solution.     

Picosecond IR-UV pump-probe spectroscopic study on the vibrational energy flow in isolated molecules and clusters

Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) from the XH stretching vibrations, where X refers to O or C atom, of aromatic molecules and their hydrogen(H)-bonded clusters are investigated by picosecond time-resolved IR-UV pump probe spectroscopy in a supersonic beam. For bare molecules, we mainly focus on IVR of the OH stretch of phenol. We describe the IVR of the OH stretch by a two-step tier model and examine the effect of the anharmonic coupling strength and the density of states on IVR rate and mechanism by using isotope substitution. In the H-bonded clusters of phenol, we show that the relaxation of the OH stretching vibration can be described by a stepwise process and then discuss which process is sensitive to the H-bonding strength. We discuss the difference/similarity of IVR/VP between the "donor" and the "acceptor" sites in phenol-ethylene cluster by exciting the CH stretch vibrations. Finally, we study the vibrational energy transfer in the isolated molecules having the alkyl chain, namely phenylalcanol (PA). In this system, we measure the rate constant of the vibrational energy transfer between the OH stretch and the vibrations of benzene ring which are connected at the both ends of the alkyl chain. This energy transfer can be called "through-bond IVR". We investigate the three factors which are thought to control the energy transfer rate; (1) "OH <--> next CH(2)" coupling, (2) chain length and (3) conformation. We discuss the energy transfer mechanism in PAs by examining these factors.     

Nonperturbative vibrational energy relaxation effects on vibrational line shapes

A general formulation of nonperturbative quantum dynamics of solutes in a condensed phase is proposed to calculate linear and nonlinear vibrational line shapes. In the weak solute-solvent interaction limit, the temporal absorption profile can be approximately factorized into the population relaxation profile from the off-diagonal coupling and the pure-dephasing profile from the diagonal coupling. The strength of dissipation and the anharmonicity-induced dephasing rate are derived in Appendix A. The vibrational energy relaxation (VER) rate is negligible for slow solvent fluctuations, yet it does not justify the Markovian treatment of off-diagonal contributions to vibrational line shapes. Non-Markovian VER effects are manifested as asymmetric envelops in the temporal absorption profile, or equivalently as side bands in the frequency domain absorption spectrum. The side bands are solvent-induced multiple-photon effects which are absent in the Markovian VER treatment. Exact path integral calculations yield non-Lorentzian central peaks in absorption spectrum resulting from couplings between population relaxations of different vibrational states. These predictions cannot be reproduced by the perturbative or the Markovian approximations. For anharmonic potentials, the absorption spectrum shows asymmetric central peaks and the asymmetry increases with anharmonicity. At large anharmonicities, all the approximation schemes break down and a full nonperturbative path integral calculation that explicitly accounts for the exact VER effects is needed. A numerical analysis of the O-H stretch of HOD in D(2)O solvent reveals that the non-Markovian VER effects generate a small recurrence of the echo peak shift around 200 fs, which cannot be reproduced with a Markovian VER rate. In general, the nonperturbative and non-Markovian VER contributions have a stronger effect on nonlinear vibrational line shapes than on linear absorption.     

Liquid phase vibrational spectra of 13C-isotopes of benzene

The Raman and i.r. spectra of liquid benzene—(μl)—¹³C (¹³C₆H₆) and benzene—1—¹³C are reported. The samples consist of a mixture of isotopes, but the lines and bands of the majority of the modes were separately resolvable. Because of the high resolution of the spectra the separation of bands down to 1 cm⁻¹ has been achieved. Band assignments are discussed in detail.     

Anomalous vibrational energy diffusion in carbon nanotubes

We study the vibrational energy diffusion in single-walled carbon nanotubes by using the molecular-dynamics method. It is found that energy transports ballistically at low temperature and superdiffusively at room temperature. The velocity of energy transport along the axis in carbon nanotube at room temperature is about 0.10 A/fs. It is also found that energy transport in carbon nanotube is different from that one in one-dimensional carbon lattice with the same interaction potential.     

Electronic, vibrational, and rotational structures in the S0 1A1 and S1 1A1 states of phenanthrene

Electronic and vibrational structures in the S(0) (1)A(1) and S(1) (1)A(1) states of jet-cooled phenanthrene-h(10) and phenanthrene-d(10) were analyzed by high-resolution spectroscopy using a tunable nanosecond pulsed laser. The normal vibrational energies and molecular structures were estimated by ab initio calculations with geometry optimization in order to carry out a normal-mode analysis of observed vibronic bands. The rotational structure was analyzed by ultrahigh-resolution spectroscopy using a continuous-wave single-mode laser. It has been demonstrated that the stable geometrical structure is markedly changed upon the S(1) ← S(0) electronic excitation. Nonradiative internal conversion in the S(1) state is expected to be enhanced by this structural change. The observed fluorescence lifetime has been found to be much shorter than the calculated radiative lifetime, indicating that the fluorescence quantum yield is low. The lifetime of phenanthrene-d(10) is longer than that of phenanthrene-h(10) (normal deuterium effect). This fact is in contrast with anthracene, which is a structural isomer of phenanthrene. The lifetime at the S(1) zero-vibrational level of anthracene-d(10) is much shorter than that of anthracene-h(10) (inverse deuterium effect). In phenanthrene, the lifetime becomes monotonically shorter as the vibrational energy increases for both isotopical molecules without marked vibrational dependence. The vibrational structure of the S(0) state is considered to be homogeneous and quasi-continuous (statistical limit) in the S(1) energy region.     

Photoelectron spectroscopy of S1 toluene: I. Photoionization propensities of selected vibrational levels in S1 toluene

Laser photoelectron spectra have been obtained following the preparation of eight vibrational states in S(1) toluene. For four of the vibrational states (up to approximately 550 cm(-1) excess energy) excitation and ionization with nanosecond laser pulses give rise to photoelectron spectra with well-resolved vibrational peaks. For the other states (>750 cm(-1) excess energy) the photoelectron spectra show a loss of structure when nanosecond pulses are used, as a result of intramolecular dynamics [see Whiteside et al., J. Chem. Phys. 123, 204317 (2005), following paper]. A number of vibrational peaks in the photoelectron spectra are assigned, and we find that the common series of ion vibrational peaks observed following the ionization of p-fluorotoluene in various S(1) vibrational states is not reproduced in toluene.     

Spatially resolved vibrational energy transfer in molecular monolayers

We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.     

Converged vibrational energy levels and quantum mechanical vibrational partition function of ethane

The vibrational partition function of ethane is calculated in the temperature range of 200-600 K using well-converged energy levels that were calculated by vibrational configuration interaction, and the results are compared to the harmonic oscillator partition function. This provides the first test of the harmonic oscillator approximation for a molecule with more than five atoms. The absolute free energies computed by the harmonic oscillator approximation are in error by 0.59-0.62 kcal/mol over the 200-600 K temperature range.     

The role of orbiting collisions in vibrational energy transfer

Using a simple model of molecular collisions under a spherically symmetric interaction, it is shown that orbiting collisions can make very large contributions to the inelastic cross sections of non-resonant processes. Calculations for the system HX + CO₂(001) → HX(υ=1) + CO₂(000), where X = F, Cl, I show good agreement with experimental results.     

Temperature dependence of vibrational energy relaxation in liquid oxygen

The energy relaxation of the lowest vibrational level (υ = 1) of liquid oxygen in the electronic ground state was investigated within a wide temperature range (53.4 K ? T ? 96 K). The relaxation time exhibits a peak value of τ′ ≈ 3.1 ms around 65 K and is shorter at lower and higher temperatures. The observed temperature behavior is discussed in view of theoretical models of energy relaxation in liquids.     

Isomer effects on vibrational energy relaxation in perylene-argon complexes

Fluorescence spectroscopic measurements have been carried out on jet-cooled complexes of perylene with a number of guest species ranging from argon to small hydrocarbons. In the case of argon, a sequence of red-shifted absorption bands is assigned to a group of aggregates involving up to five guest atoms. Calculations using empirical atom-atom pair potentials have allowed unambiguous assignment of eight different perylene-argon complexes. Spectrally dispersed fluorescence measurements have studied the effect of internal energy on the IVR and predissociation processes. In particular, the two perylene diargon isomers are shown to exhibit different rates for IVR.It has been shown that arson forms a series of organised, readily identifiable complexes with perylene in a supersonic jet. On excitation of the different isomeric forms of the 2:1 complex, different rates for vibrational energy redistribution have been found. The greatest difference was observed at low vibrational energies of the parent, which confirmed that low-frequency van der Waals modes were responsible. Although more detailed calculations will appear elsewhere, the most obvious difference between the two isomeric forms is the exchange of two low-frequency x-y plane vibrations (≈5 cm^?1) for a hindered rotation and a substantially higher-frequency (15–30 cm^?1) argon-argon stretching mode. Photodissociation is also readily observed, confirming the computed values for the argon-perylene binding energies. Finally, strong resonance fluorescence is observed, accompanied by (relaxed) emission from the singly dissociated species when either the 1:1 or either 2:1 isomer is excited with a (parent) vibrational energy of 705 cm^?1. Thus, in spite of the presence of the dissociative pathway, all observable emission from the undissociated species appears to originate from a state in the “small molecule” limit, which survives for about 4 ns.     

Intramolecular relaxation of excess vibrational energy in jet-cooled perylene

The intramolecular redistribution of excess vibrational energy (IVR) in electronically excited perylene is being studied by fluorescence techniques. Analysis has shown, in agreement with the literature, little evidence of relaxation of fundamental modes up to ? 1100 cm^?1. However, it is also shown, contrary to literature assertions, that combination states from 700 to 1100 cm^?1 do not relax significantly on the time-scale of molecular fluorescence. The picture is simplified by reassignment of several key combination bands in the spectrum. Excitation at higher energies reveals differences in behaviour between combination bands involving high-frequency fundamentals and those only using fundamentals < 800 cm^?1. In the latter case, the persistence of narrow-line emission indicates substantially slower relaxation rates. As an example, the 1600 cm^?1 fundamental state appears to relax substantially faster than the 1603 cm^?1 satellite state, which is assigned to 353³550¹. This kind of disparity has been observed up to 2000 cm^?1. These data provide evidence for the importance of anharmonic interactions in determining the relative rates of IVR over short energy ranges.     

On the preference for vibrational energy in diatomic dissociation

Recent measurements of dissociation and relaxation rates, and of dissociation induction times, are shown to indicate very serious depletion of vibrational state populations during thermal dissociation. The large measured dissociation rates are then only compatible with dissociation from low vibrational states, i.e., there can be at most a very weak bias favoring vibrational excitation in thermal dissociation. It is suggested that dissociation from low vibrational states is assisted by rotational excitation.     

Cis-trans isomerization in the S1 state of acetylene: identification of cis-well vibrational levels

A systematic analysis of the S(1)-trans (A?(1)A(u)) state of acetylene, using IR-UV double resonance along with one-photon fluorescence excitation spectra, has allowed assignment of at least part of every single vibrational state or polyad up to a vibrational energy of 4200 cm(-1). Four observed vibrational levels remain unassigned, for which no place can be found in the level structure of the trans-well. The most prominent of these lies at 46?175 cm(-1). Its (13)C isotope shift, exceptionally long radiative lifetime, unexpected rotational selection rules, and lack of significant Zeeman effect, combined with the fact that no other singlet electronic states are expected at this energy, indicate that it is a vibrational level of the S(1)-cis isomer (A?(1)A(2)). Guided by ab initio calculations [J. H. Baraban, A. R. Beck, A. H. Steeves, J. F. Stanton, and R. W. Field, J. Chem. Phys. 134, 244311 (2011)] of the cis-well vibrational frequencies, the vibrational assignments of these four levels can be established from their vibrational symmetries together with the (13)C isotope shift of the 46?175 cm(-1) level (assigned here as cis-3(1)6(1)). The S(1)-cis zero-point level is deduced to lie near 44?900 cm(-1), and the ν(6) vibrational frequency of the S(1)-cis well is found to be roughly 565 cm(-1); these values are in remarkably good agreement with the results of recent ab initio calculations. The 46?175 cm(-1) vibrational level is found to have a 3.9 cm(-1) staggering of its K-rotational structure as a result of quantum mechanical tunneling through the isomerization barrier. Such tunneling does not give rise to ammonia-type inversion doubling, because the cis and trans isomers are not equivalent; instead the odd-K rotational levels of a given vibrational level are systematically shifted relative to the even-K rotational levels, leading to a staggering of the K-structure. These various observations represent the first definite assignment of an isomer of acetylene that was previously thought to be unobservable, as well as the first high resolution spectroscopic results describing cis-trans isomerization.