Very Low Energy Collision Induced Vibrational Relaxation: An Overview

Rice, Stuart A.; Cerjan, C.

doi:10.1155/lc.2.137

Cited by 11 publications

(2 citation statements)

References 8 publications

(14 reference statements)

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“…While the extraction of absolute cross sections involves the uncertainties associated with a collisional environment that is hard to quantify, all workers agreed that the cross sections for relaxation of I 2 * with He, primarily by Δυ‘ = −1 transitions, are on the order of 100 Å 2 as opposed to a geometric cross section of about 20 Å 2 . Argument ensued concerning the extent to which a special mechanisms such as orbiting resonances that lengthens the I 2 *−He collision duration contributed to the large cross sections. ,− Others have proposed that the data may be rationalized by ordinary impulsive collisions . A specific revisitation of the I 2 −He system over a temperature range of 2−12 K rather convincingly supported this interpretation …”

Section: State-to-state Vibrational Energy Transfer Within Excited El...mentioning

confidence: 98%

Vibrational Energy Transfer

1996

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Many aspects of the collision dynamics of vibrational energy transfer are presented. Special emphasis is placed on three broad areas within this field: (1) vibrational energy transfer in large molecules (>10 modes) at low excitation, (2) vibrational energy transfer in large molecules at high vibrational excitation, and (3) vibrational energy transfer of highly excited small molecules. Advances in laser methods have revolutionized experimental investigations of all of these areas. Recent results are presented, and directions for the future are discussed.

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Section: State-to-state Vibrational Energy Transfer Within Excited El...mentioning

confidence: 98%

Vibrational Energy Transfer

1996

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“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Here the molecule under study is seeded in the collisional partner, generally a rare gas. The supersonic jet expansion provides an environment in which the collisional energy decreases as a function of distance from the nozzle.…”

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confidence: 99%

Vibrational energy transfer in S1 p-difluorobenzene: a comparison of low and room temperature collisions

Pursell¹,

Parmenter²

1993

J. Phys. Chem.

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Vibrational energy transfer in SI p-difluorobenzene has been studied at low temperature within a supersonic free jet. Quantitative relative cross sections for the state-to-state vibrational energy transfer channels (Le. flow patterns) from the 8* vibrational level (evib = 346 cm-') were obtained for low energy collisions (T = 20-35 K) with He and Ar. They are qualitatively similar to analogous flow patterns obtained earlier from the 00 level for room temperature (300 K) collisions with He and Ar. Both the 300 K and the low temperature flow patterns show that the vibrational energy transfer is selective among the possible channels and that the competition among vibrational channels is distinctive for each collisional partner. The low and room te&perature flow patterns differ, however, in quantitative detail. A treatment of the standard SSH-T vibrational energy transfer model describes semiquantitatively the flow patterns at both low and room temperature and for collisions with H e and Ar. The success of a single model suggests that vibrational energy transfer in SI p-difluorobenzene is governed by the same mechanism over the entire span of collision energies associated with temperatures from -20 to 300 K. IabodllctionMuch discussion has been given to the mechanism responsible for vibrational energy transfer (VET) at the low collisional energies occuring within a supersonic jet expansion. The debate has centered particularly on whether low and room temperature VET involve different mechanisms. As discussed below, several experimental studies with both diatomic and polyatomic molecules have been designed to probe this issue. In this paper we offer further experimental exploration of the question. We have obtained quantitative relative cross sections for the state-to-state VET channels that occur from the g2 vibrational level (ev,b = 346 cm-I) in SI p-difluorobenzene (pDFB) for low temperature (T = 2&35 K) collisions with He and Ar. The unique appeal of pDFB is that analogous sets of cross sections, or flow patterns, are available for room temperature (300 K) VET with He and Are1 We compare the low and room temperature pDFB results by modelling, adapting first a standard VET model to describe semiquantitatively our new low temperature flow patterns for collisions with He and Ar. We then show that the same model fits the room temperature flow patterns with equal fidelity. We infer from thiscorrespondence that the same collisional mechanism operates over the span of collision energies associated with temperatures from -20 to 300 K.One of the most productive techniques for the study of singlecollision VET in diatomics and polyatomics has been the optical pumpdispersed fluorescence techniquee2 A molecule is optically pumped to a specific vibrational level in the first excited electronic state, and SI -SO dispersed fluorescence is then used to monitor quantitatively the VET to nearby vibrational levels. Under singlecollision conditions, this technique yields relative state-to-state cross sections or, in some experiments, ...

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Theories of intramolecular vibrational energy transfer

1991

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T. Uzer, Theories of intramolecular vibrational energy transfer 75 PK(t) =~'~~' U~1~b1(0)j+~UkJUlbJ(0)b~(0).(2.8)The second sum vanishes if the terms in it have uncorrelated phases, leaving the coarse-grained level population asIf only the coarse-grained level populations P~, =~P1 are measurable, the same "random phase" argument as before allowswhere N~is the number of states in one level. This matrix Y, under fairly general conditions, can be expressed [Quack 1979a] as an exponential involving a time-independent matrix K [Quack 1978], Y(t) = exp[K(tta)], (2.11) 80 T. Uzer. Theories of intramolecular vibrational energy transfer and p(t) = Y(t)p(0). This can be reduced [Quack 1978, 1981a] to the differential form of the Pauhi master equation [Pauli 1928], Pz~Kp. (2.12)In contrast to the oscillatory solutions of eqs. (2.1)-(2.4), these equations, (2.11) and (2.12), have decaying solutions, denoting relaxation. For sufficiently short times the exponential function can be expanded and one obtains [Quack 1978]where WKJI2 stands for the average square coupling element between the states in levels K and J, and K is the average frequency separation of states in level K (1 1~K needs to be replaced by the density Pk in the case of the continuum). The preceding development makes clear the factors that need to be considered carefully when studying intramolecular relaxation: the definition of what is being observed (e.g., what is relaxing into what), the time scale over which the observation is taking place, which Hamiltonian and which basis set is being used for the description. Moreover, the preceding development is valuable in connecting Golden-Rule type of first-order perturbation theory prescriptions, which are widely used in IVR, to a fundamental view of the process [Voth 1987].Of course, intramolecular relaxation is a more general phenomenon than the somewhat restricted Pauhi master equation would suggest, i.e., there are selections of course-graining which make oscillatory behavior possible [Quack 1981a], which are, however, the exception rather than the rule. We will see examples of such exponential behavior as well as coherences in the following sections. The Pauhi master equation is merely one (and the earliest) attempt to describe relaxation phenomena in a quantummechanical setting [Van Hove 1962]; it proceeds by reducing the full density-matrix equation to a system of coupled equations for the diagonal density-matrix elements alone. The derivation, as well as the set of companion equations for the off-diagonal matrix elements, the "coherences", have been critically studied by Fox [1978,1989]. The effect of these coherences proves valuable, e.g., in the theory of spectral lineshapes [Faid and Fox 1988]. General molecular model for intramolecular vibrational energy redistribution ([VR)

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Very Low Energy Collision Induced Vibrational Relaxation: An Overview

Cited by 11 publications

References 8 publications

Vibrational Energy Transfer

Vibrational Energy Transfer

Vibrational energy transfer in S1 p-difluorobenzene: a comparison of low and room temperature collisions

Theories of intramolecular vibrational energy transfer

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