The origin of vibrational dephasing of polyatomic molecules in condensed phases

Abstract

The vibrational dephasing of polyatomic moleucles in condensed phases by intermolecular vibrational energy exchange is treated theoretically. In the exchange model, dephasing arises from random modulation of the vibrational frequency caused by intramoleuclar anharmonic coupling to low frequency modes which are undergoing intermolecular energy exchange with the bath. The exchange rates are temperature dependent and as a consequence manifest themselves experimentally as a temperature dependent broadening and shift of the Raman spectral lineshape. Using a reduced density matrix technique within the constraints of a Markoff approximation, the theory allows the time dependence introduced by the exchange process to be propertly accounted for, and allows explicit expressions for the vibrational correlation '. u J. ' 'J ..) 0 J J } This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of The Regents of the University of California, the Lawrence Berkeley Laboratory or the Department of Energy.

Vibrational Dephasing Mechanisms in Hydrogen-Bonded Systems

“…The anharmonic coupling term V anh between n s (X-H) and n s ͑XH · · · Y) is linear in the coordinate r s ͑t͒ of the n s ͑XH · · · Y) mode: V anh~rs ͑t͒. This description is in sharp contrast with that of the second theoretical model by Shelby and Harris [7], where the coupling depends on the coordinate of the low-frequency mode r s to second or higher order [7] V anh~r 2 s ͑t͒. In that model the modulation of the n s (X-H) transition frequency occurs through anharmonic coupling to a low-frequency mode, which undergoes energy exchange with the bath.…”

“…Recent developments in timeresolved spectroscopy have allowed for, e.g., the measurement of homogeneous vibrational line shapes with IR transient hole-burning techniques [2,3] and the timeresolved observation of vibrational dephasing by means of a photon-echo experiment [4]. A great number of sophisticated models have been developed to describe vibrational line shapes [5][6][7][8][9][10], some specifically addressing the effect of hydrogen bonds [8][9][10]. The spectral bandwidth of a hydrogen-bonded complex is typically an order of magnitude larger than that of the unassociated molecule or oscillator, indicating that the vibrational dephasing rate is strongly influenced by hydrogen-bond formation.…”

A critical test of vibrational dephasing theories in solids using spontaneous Raman scattering in isotopically mixed crystals

“…The important points to note are: 1) Both the width and shift (3) display the same apparent ''activation energy", E., which is equal to the…”

Raman investigations of collective vibrational motions in van der Waals liquids

“…All these processes are thermally activated and vanish at low temperatures. Similar problems were investigated by Harris, Shelby and Cornelius (70,71) and by Abbot and Oxtoby (50). However, the resonant interaction between molecules was not explicitly included in their calculations.…”