Effects of rotation on vibrational relaxation in diatomic molecules

An approximate method is presented for allowing for the effects of rotation on the rate of vibrational relaxation in a diatomic molecule; this is based on local separation of the variables in the interaction region. An interpretation is given for experiments on the vibrational relaxation of HI in a shock wave.     

The role of van der Waals molecules in vibrational relaxation kinetics

The relaxation kinetics of N₂O and CO₂ vibrationally excited molecules (VEM) in two-phase gas-cluster systems was investigated under conditions of supersonic expansion with condensation. The catalytic effect of clusters on the vibrational relaxation rate was revealed. The relaxation rate of clustered VEM, R_c, and the probability of relaxation of VEM per collision with a cluster, $\text{P}$, as functions of the average number of molecules in a cluster, $\text{N}$, were obtained. Values of R_c and $\text{P}$ increase rapidly with increasing $\text{N}$, and at $\text{N}$ = constant they decrease with decreasing cluster temperature.     

Comprehensive investigation of vibrational relaxation of non-hydrogen-bonded water molecules in liquids

The vibrational dynamics of isolated water molecules dissolved in the nonpolar organic liquids 1,2-dichloroethane (C(2)H(4)Cl(2)) and d-chloroform (CDCl(3)) have been studied using an IR pump-probe experiment with approximately 2 ps time resolution. Analyzing transient, time, and spectrally resolved data in both the OH bending and the OH stretching region, the anharmonic constants of the bending overtone (v=2) and the bend-stretch combination modes were obtained. Based on this knowledge, the relaxation pathways of single water molecules were disentangled comprehensively, proving that the vibrational energy of H(2)O molecules is relaxing following the scheme OH stretch-->OH bend overtone-->OH bend-->ground state. A lifetime of 4.8+/-0.4 ps is determined for the OH bending mode of H(2)O in 1,2-dichloroethane. For H(2)O in CDCl(3) a numerical analysis based on rate equations suggests a bending overtone lifetime of tau(020)=13+/-5 ps. The work also shows that full 2-dimensional (pump-probe) spectral resolution with access to all vibrational modes of a molecule is required for the comprehensive analysis of vibrational energy relaxation in liquids.     

Solvent dependence of OH bend vibrational relaxation of monomeric water molecules in liquids

The vibrational relaxation rates of the OH bending mode of monomeric H(2)O molecules diluted in various liquid halogenated methane and ethane derivates have been determined by a picosecond infrared pump-probe study. Relaxation time constants between 4.8 and 40.5 ps have been obtained. The discussion of the general solvent dependence suggests that in all cases the solvent fundamental with the smallest energy mismatch is favorably populated by this intermolecular energy transfer process.     

Femtosecond vibrational relaxation of large organic molecules

The intramolecular relaxation time of highly excited states of the vibronic manifold of the first excited singlet state of large organic molecules such as Nile Blue, Rhodamine 640, DODC iodide, Cresyl Violet, and Oxazine 725 in solution is shown to be extremely fast, less than 30 fs. We believe that this is a characteristic time of the relaxation due to anharmonic coupling among the many degrees of freedom of the molecule.     

Suprathermal vibrational excitation of hydrogen molecules by heated rhenium

Hydrogen molecules (H₂, HD, and D₂) with suprathermal vibration excitation energies have been observed accompanying the atomization of hydrogen     

On the role of nonbonded interactions in vibrational energy relaxation of cyanide in water

The vibrationally excited cyanide ion (CN(-)) in H2O or D2O relaxes back to the ground state within several tens of picoseconds. Pump-probe infrared spectroscopy has determined relaxation times of T1 = 28 ± 7 and 71 ± 3 ps in H2O and D2O, respectively. Atomistic simulations of this process using nonequilibrium molecular dynamics simulations allow determination of whether it is possible at all to describe such a process, what level of accuracy in the force fields is required, and whether the information can be used to understand the molecular mechanisms underlying vibrational relaxation. It is found that, by using the best electrostatic models investigated, absolute relaxation times can be described rather more qualitatively (T1(H2O) = 19 ps and T1(D2O) = 34 ps) whereas the relative change in going from water to deuterated water is more quantitatively captured (factor of 2 vs 2.5 from experiment). However, moderate adjustment of the van der Waals ranges by less than 20% (for NVT) and 7.5% (for NVE), respectively, leads to almost quantitative agreement with experiment. Analysis of the energy redistribution establishes that the major pathway for CN(-) relaxation in H2O or D2O proceeds through coupling to the water-bending plus libration mode.     

Features in the vibrational relaxation of laser excited Freon-22 molecules

The vibration-translation (V-T) of laser excited Freon-22 (CF₂HC1) molecules has been studied. An interferometric technique allows simultaneous measurement of the initial energy 〈E〉 stored in the molecules, and of the V-T relaxation time. Consequently, the V-T relaxation time and the energy released per collision can be determined as a function of the energy absorbed from the laser field. Three distinct regions have been observed for these dependences. This behaviour observed and reported by us for Freon-22 confirms the role played in the relaxation process by the initial distribution of the vibrational energy, and agrees qualitatively with the experimental results for other polyatomic molecules.     

Tunable raman excitation and vibrational relaxation in diatomic molecules

A generalization of the Raman vibrational excitation technique is presented. It is applied here to the room temperature study of V→T transfer in pure N₂ and O₂ or in mixtures with H₂ and He. The results on N₂ are the first obtained at room temperature, those on O₂ are in good agreement with existing data and provide a check of validity of the method.     

On the vibrational relaxation of diatomic molecules at intermediate excitation

The theory of quasistationary vibrational distributions of diatomic molecules at low gas temperatures is developed for the case of intermediate excitation temperatures when the flow of quanta is nondiffusional in Treanor's part of the distribution. In certain conditions the distribution contains a region of inversion which is followed by a plateau.     

Direct observation of vibrational relaxation of dye molecules in solution

The characteristic time for vibrational relaxation in an optically excited molecular electronic state has been measured with picosecond resolution for the molecules rhodamine B and rhodamine 6G in solution using a new experimental technique.     

Ultrafast vibrational energy relaxation of the water bridge

We report the energy relaxation of the OH stretch vibration of HDO molecules contained in an HDO:D(2)O water bridge using femtosecond mid-infrared pump-probe spectroscopy. We found that the vibrational lifetime is shorter (~630 ± 50 fs) than for HDO molecules in bulk HDO:D(2)O (~740 ± 40 fs). In contrast, the thermalization dynamics following the vibrational relaxation are much slower (~1.5 ± 0.4 ps) than in bulk HDO:D(2)O (~250 ± 90 fs). These differences in energy relaxation dynamics strongly indicate that the water bridge and bulk water differ on a molecular scale.     

Simultaneous vibrational relaxation and thermal dissociation of diatomic molecules

A model of a truncated harmonic oscillator with equidistant levels and one-quantum transitions is applied. The balance equations for O₂ have been solved numerically with 26 levels, with the transition probabilities taken as P _k,k +1k exp (k) P ₁₀, in which k is the number of the level and is an adjustable parameter that takes account of the anharmonicity. A Boltzmann distribution is obtained up to level 19 with =0, but for 0 there are deviations from that distribution in lower levels. Levels with k=1, 2, 3 are populated with relaxation times substantially less than the relaxation time for the vibrational energy. Dissociation depletes levels with k 19.     

Temperature dependence of the optical rotation in six bicyclic organic molecules calculated by vibrational averaging.

The vibrational corrections and the temperature dependence of the specific rotation of six rigid organic molecules (alpha-pinene, beta-pinene, cis-pinane, camphene, camphor, and fenchone) were calculated at three wavelengths using hybrid time-dependent density functional theory (TDDFT). A technique for calculating the temperature dependence of the vibrational average of a molecular property has been applied to obtain the specific rotation of the molecules as a function of temperature. For cases in which accurate equilibrium optical rotations can be obtained as a "base value," and for which there is little effect from solvation, accurate predictions of the trends in the temperature-dependence of the specific rotations can be calculated. For other cases, the method can be used to extract purely vibrational contributions to the overall temperature dependence of optical rotation.     

Mode-selective O-H stretching relaxation in a hydrogen bond studied by ultrafast vibrational spectroscopy

The ultrafast relaxation of the excited O-H stretching vibration is studied by ultrafast infrared-pump/infrared-probe and infrared-pump/Raman-probe spectroscopy. We demonstrate a 200 fs lifetime of the hydrogen-bonded O-H stretching mode in 2-(2'-hydroxy-5'-methyl-phenyl)benzotriazole (TINUVIN P). O-H stretching relaxation occurs through a few major channels that all involve combination and overtone bands of modes with considerable in-plane O-H bending character. In particular, the mode, which contains the largest O-H bending contribution, plays a prominent role for primary processes of intramolecular vibrational energy redistribution. Theoretical calculations of vibrational energy transfer rates based on a Fermi golden rule approach account for the experimental findings.     

Rotational quenching of monodeuterated water by hydrogen molecules

Cross sections and rate coefficients for low lying rotational transitions in HDO induced by para and ortho-H(2) collisions are presented for the first time. Calculations have been performed at the close-coupling and coupled-states levels with the deuterated variant of the H(2)O-H(2) interaction potential of Valiron et al. [J. Chem. Phys., 2008, 129, 134306]. Rate coefficients are presented for temperatures between 5 and 100 K and are compared to the corresponding rates for H(2)O and D(2)O. Significant differences caused by the isotopic substitution, in particular the C(2v) symmetry breaking, are observed. Finally, our rates are found to be significantly larger (by up to three orders of magnitude at 50 K) than the corresponding HDO-He rates and should lead to a thorough re-estimation of the abundance of interstellar HDO.     

Prediction of the vibrational spectra of interacting water molecules

A system of computer programs has been developed to predict and interpret the infrared spectra of molecules, ions, radicals and of chemically interacting species. The frequency parameters (or force constants) and the intensity parameters (or atomic polar tensors (APTs)) are calculated using the Gaussian-76 program for $\text{ab}$$\text{initio}$ quantum mechanical calculations with a 4–31G basis set. The resultant wavenumbers and intensities have been used to predict the fundamental infrared spectra of water molecules interacting with different molecular species. The corresponding wavenumber shifts and intensity changes from the spectrum of the non-interacting water molecule are dependent upon both the geometrical orientation and the electron-donating properties of the interacting species. The intensity parameters can be further analyzed by using the charge-charge flux-overlap (CCFO) interpretation of the APTs. This interpretation is helpful in pinpointing the exact source of the perturbation that produces the intensity change. The significance is discussed of each of these CCFO terms in predicting the infrared intensities of molecules undergoing intermolecular interaction.     

Classical vs quantum vibrational energy relaxation pathways in solvated polyatomic molecules

Vibrational energy relaxation (VER) of solvated polyatomic molecules can occur via different pathways. In this paper, we address the question of whether treating VER classically or quantum-mechanically can lead to different predictions with regard to the preferred pathway. To this end, we consider the relaxation of the singly excited asymmetric stretch of a rigid, symmetrical, and linear triatomic molecule (A-B-A) in a monatomic liquid. In this case, VER can occur either directly to the ground state or indirectly via intramolecular vibrational relaxation (IVR) to the symmetric stretch. We have calculated the rates of these two different VER pathways via classical mechanics and the linearized semiclassical (LSC) method. When the mass of the terminal A atoms is significantly larger than that of the central B atom, we find that LSC points to intermolecular VER as the preferred pathway, whereas the classical treatment points to IVR. The origin of this trend reversal appears to be purely quantum-mechanical and can be traced back to the significantly weaker quantum enhancement of solvent-assisted IVR in comparison to that of intermolecular VER.