Studies Of Vibrational Energy Transfer Research Articles

A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Abstract Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol −1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m A = 20 u, m B = 1 u, m C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.

Trajectory studies of vibrational energy transfer in gas–surface collisions

We have applied the stochastic trajectory method to the scattering of NO from the (001) surface of an LiF crystal. The surface was represented by a 32 atom primary zone and the effects of the rest of the surface atoms were included using the Generalized Langevin Equation method. The forces between the surface atoms were treated using a shell model and the gas–surface interactions included short-range repulsions, attractive dispersion forces, and the interaction of the dipole moment of NO and the surface ions. The dependence of the dipole moment and the dependence of the molecular polarizability on the internuclear separation of NO were also included. The calculated vibrational energy accommodation coefficient was less than 1% when trapping of the gas diatom was negligible. By modifying the vibrational frequency of the scattered diatom we have examined the dependence of vibrational energy transfer on frequency. An exponential dependence on the ratio of the molecular frequency to the surface Debye frequency was found, similar to behavior observed for vibrational relaxation in condensed phases. We have also examined the relative efficiency of the coupling of the vibrational mode to the translational and rotational modes of the gas diatom and to the phonon modes of the surface. We found that the vibrational mode is most strongly coupled to the surface phonons with a much weaker coupling to the rotational and translational modes.

Study of vibrational energy transfer at a surface by a time-of-flight method

A single collision, time-of-flight extension of the VEM method for the study of molecule-surface vibrational energy transfer is introduced. The technique helps election between possible alternative trial analytic forms of the collisional transition probability function. A Gaussian form is preferred over a Boltzmann-exponential form for cyclobutene isomerization to 1.3-butadiene energized by collisions at a silica surface at 800 K.

Temperature-dependent studies of vibrational energy transfer in sulfur dioxide (oxygen-18) and sulfur dioxide (S18O2)/rare gas matrixes

Temperature-dependent studies of vibrational energy transfer in sulfur dioxide (oxygen-18) and sulfur dioxide (S18O2)/rare gas matrixes

Level-to-level vibrational energy transfer studies: energy dependence and observation of product species for azulenes(S 0, Evib) + CO 2

V—V energy transfer from a large molecule excited to vibrational energies of chemical interest has been demonstrated by detection of ≈ 1.5% yield of CO 2(001) due to energy transfer from azulene ( E vib ≈ 30600 cm −1. Also, the average enery lost per collision by azulene was measured as a function of E vib, and the rate constant for CO 2(001) deactivation by azulene was determined.

Mode specific vibrational energy transfer in CH3F–N2O mixtures

Fluorescence signals have been observed from the symmetric stretching and bending fundamentals of N2O in CH3F–N2O mixtures following excitation of CH3F by a Q-switched CO2 laser. The rise rates of the N2O fluorescence signals show that energy is transferred selectively from CH3F to the symmetric stretch of N2O despite the fact that overtones of the N2O bending mode are more nearly in resonance with CH3F states expected to be involved in the energy transfer process. Vibrational energy transfer studies of the same two modes in pure N2O, in which the symmetric stretch is depleted by N2O laser radiation, provide independent measurements of rate constants necessary for a kinetic analysis of the CH3F–N2O results. In pure N2O a rate of 28±1.5 msec−1 Torr−1 is assigned to equilibration of the symmetric stretching fundamental with a component of the overtone of the bending mode. A mechanism for energy transfer in CH3F–N2O mixtures is proposed which explains the important features of the experimental observations in this system and is consistent with the results in pure N2O. This model allows an upper limit to be placed on the rate constant for direct energy transfer from CH3F to the bending mode manifold of N2O, and therefore a lower limit for the actual specificity of intermolecular energy transfer is determined.

A laser induced fluorescence study of vibrational energy transfer in dideuteromethane

V—V and V—T/R rates of ν 1, ν 2, ν 3, ν 6, ν 8, and ν 9 of CH 2D 2 were measured and are reported. The deactivation efficiencies of rare gas collision partners were measured and calculated by SSH theory, the results are reported and discussed. Possible V—V pathways are presented and discussed.

Vibrational energy transfer studies in CO2–hydrocarbon mixtures

Rate constants for the transfer of energy from CO2(00°1) to ethane and n-butane were measured using the laser induced fluorescence technique. The large magnitudes and the negative temperature dependence of the energy transfer rates indicate that near-resonant energy transfer processes caused by long range forces are responsible for the deactivation of the asymmetric stretching mode of CO2. It is shown that the combination bands in the alkanes are likely to receive the energy from CO2(00°1). Using Sharma–Brau theory with Tam’s modification and our experimental results, the square of the transition dipole matrix elements of ν3+ν6 and ν9+ν11 bands of ethane are estimated. At any given temperature in the range 300–730 °K, the energy transfer cross section was found to increase linearly with n of CnH2n+2. It is suggested that the absorption intensity of the combination bands of the alkanes also increases linearly with n in the spectral range 2300–2400 cm−1.

Contemporary perspectives in physical chemistry

Abstract

Vibrational Energy Transfer Studies of Matrix Isolated C2−

Vibrational Energy Transfer Studies of Matrix Isolated C₂⁻

On self-consistent and stochastic treatments of V—T energy transfer in collinear atom-diatom collisions

The use of stochastic reduction approximations in studies of vibrational energy transfer in collinear atom plus diatomic molecule (harmonic oscillator) collisions is investigated. We begin by deriving exact coupled Liouville equations for the time evolution of vibrational and translational degrees of freedom in these collisions. Neglect of terms involving T—V correlation leads directly to the well known self-consistent approximations (SCA), and with further simplification to impulsive approximations. A self-consistent stochastic (SCS) method is then introduced, and it is found to provide an intermediate level of approximation between SCA and impulsive. For the simple hamiltonian considered, the SCS approximation leads to closed form expressions for the translational equations of motion. A special partitioning of translation and vibration first developed by Rosen is then introduced via two canonical transformations and is shown to lead to an improved description of translation—vibration correlations. Application of the SCS approximation within the framework of this partitioning leads to an energy conserving closed form trajectory which reduces to the refined impulse approximation of Matahan in the appropriate limit. Consideration of collision ensembles corresponding to both forward and reverse trajectories leads to a velocity symmetrized method analogous to that used in the ITFITS method. Numerical applications of the refined SCS method to He + H 2 and (N 2) + N 2 indicates that this method is more accurate than ITFITS (with only a small increase in complexity) in predicting low order moments of the energy transfer, at least at low collision energies ( E T/ñω < 3–4). Transition probabilities for single quantum jump transitions are also accurately predicted at these energies (in comparing with exact quantum results) but probabilities of multiple quantum jump transitions, and of single quantum jump transitions at high collision energies are often less quantitative, primarily due to inaccuracies in the treatment of vibrational motion by master equation methods.

Laser studies of vibrational energy transfer and relaxation of CO trapped in solid neon and argon

The infrared emission of CO trapped in solid Ne and Ar is observed at low temperature. The first vibrational level of 12C 16O is excited by a Q-switched frequency doubled CO 2 laser. The emission spectrum consists of several lines arising from upper vibrational levels of 12C 16O and also of 13C 16O and 12C 18O which are present in natural abundance. An interpretation is proposed which is based on the assumption that long range dipole—dipole interaction is the main physical process involved in these experiments. Resonance energy transfer produces an energy migration among 12C 16O molecules without any change in vibrational populations. Phonon assisted energy transfer takes place between vibrational levels of the various isotopic species present in the solution. In order to satisfy the resonance condition a phonon is emitted or absorbed whose energy compensates for the energy mismatch between the transitions in each interacting molecule due to isotopic effect and or vibrational anharmonicity. The range of this process is greatly extended by energy migration. At the low phonon bath temperature phonon emission is much more probable than phonon absorption. So a strong excitation of upper vibrational levels with in some cases population inversions is observed. Molecular impurities act as efficient quenching centers even at very low concentration. When highly purified samples are used, the fluorescence decay time is found to be 20.6 ms in Ne and 14.5 ms in Ar and does not significantly depend upon concentration and temperature. It is concluded that radiationless relaxation is unimportant.

Flow-tube studies of vibrational energy transfer in HF(v)+HF, DF(v)+HF, and DF(v)+D2 systems

A medium-pressure (1 Torr), large-diameter (10 cm) flow tube has been used to measure rate coefficients at 298 °K for (a) total relaxation (sum of vibrational–vibrational and vibrational–rotational, translational processes) of HF(v=1, 2, 3, 4, and 5) by HF, (b) relaxation of DF(1, 2, 3, and 4) by HF, and (c) over-all relaxation of DF(v=1, 2, 3, and 4) by D2. The chemically produced vibrationally excited HF or DF species have been studied by monitoring their vibrational–rotational emission in a fast-flow system. The rate coefficient for the relaxation of HF(1) by HF is 5.3×104 sec−1⋅Torr−1. The measured rate coefficients for the deexcitation of HF(v=2, 3, 4, and 5) by HF are 5.3×105, 8.5×105, 8.8×105, and 2.8×105 sec−1⋅Torr−1, respectively. The rate coefficients for the vibrational–rotational, translational deexcitation from the upper vibrational levels of DF(v) by HF are found to have a nonlinear vibrational dependence. The rate coefficient for the relaxation of DF(v=1) by D2 is 1.35×104 sec−1⋅Torr−1.

Fully quantum study of vibrational energy transfer between H2 and D2

We present here the results of a detailed study of the collisional transfer of vibrational energy between H2 and D2. Following our work on D2–D2 collisions [J. Chem. Phys. 59, 6254 (1973)], we carry out a fully quantum treatment of the collision dynamics, assuming a collinear geometry. We have used one fully ab initio and four different model interaction potentials. For both the 10→01 and 11→02 processes, we find that the transition probabilities are extremely sensitive to the choice of potential surface. Another result is that nearest-atom model potentials yield inelastic probabilities which are too large. We further show that long-range forces play a far less important role in the case of these off-resonant processes than for the D2–D2 near-resonant collisions. Also of interest is the result that the calculated V–V transition probabilities are largely insensitive to the choice of a harmonic or Morse oscillator description of the colliding molecules.

Classical dynamics: The study of vibrational and rotational excitation in Li+, H2

A classical study of translational, rotational, and vibrational energy exchange in the systen Li+, H2 is presented. The potential surface used was the ab initio Lester surface appropriate for rotational and vibrational energy transfer studies. The effect of initial energy configuration and orientation variables on the energy transfer processes was studied. A central force calculation of the deflection function was made for comparison with that obtained from an anisotropic potential. The primary mode of energy transfer to the internal degrees of freedom is from translational energy.

Laser fluorescence study of vibrational energy transfer in CH3Cl

A laser induced infrared fluorescence technique has been used to measure both vibration to vibration and vibration to translation/rotation energy transfer rates in CH3Cl. The V-T/R rates were measured using H2, D2, CH4, and rare gases as collision partners. Qualitative comparison of these data with V → T and V → R theories indicates that the V → T mechanism dominates for collisions with small reduced masses while the V → R mechanism dominates for collisions with large reduced masses. The V–V rate equilibrating the ν6 and ν3 modes was measured as a function of CH3Cl pressure and was found to be 76 msec−1· torr−1. A lower limit was also obtained for the rate of V–V energy transfer from states near 1000 cm−1 to high lying states near 3000 cm−1.

MA1 - Laser-excited vibrational energy transfer studies of HF, CO, and NO

An electrically initiated pulsed HF chemical laser has been used to measure vibrational relaxation of HF ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ), CO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ), and NO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) at 295 ± 2 K. The self-relaxation rate of HF highly dilute in argon is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(8.7 \pm 0.1) \times 10^{4} s^{-1}</tex> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and is independent of rotational level excitation over laser transitions <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P_{1 \rightarrow 0}(2) - P_{1 \rightarrow 0}(9)</tex> . The rates at which O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , CO, and NO quench HF( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) have been found to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">350 \pm 25, (2.5 \pm 0.2) \times 10^{3}</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(6.2 \pm 0.3) \times 10^{3}</tex> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively. There is indirect evidence of transfer from HF( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) to the ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 2</tex> ) levels of NO and O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> . Through the use of HF ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) as a collisional pumping source we have measured the deactivation rates of CO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) by HF (V → R, T) and N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (V → V) to be 480 ± 25 s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 130 ± 15 s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Similarly, HF ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) has been used to pump NO ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\upsilon = 1</tex> ) and the NO self-relaxation rate has been measured to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2.7 \pm 0.2) \times 10^{3}</tex> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> torr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ; this rate is about a factor of 5 faster than would be predicted by the kinetic spectroscopy results of Billingsley and Callear [43].

Laser studies of vibrational energy transfer

Laser studies of vibrational energy transfer