Collisional energy transfer in non-reactive systems

Abstract

If the internal energy of target molecules highly dispersed in a heat bath is monitored as a function of time, their one-photon laser excitation and subsequent bulk collisional relaxation yields information only about energy transfer in the bulk system (a macroscopic time-dependent property). This paper discusses three special cases when [Formula: see text], the average energy transferred in a collision (a microscopic property) can be deduced from bulk relaxation data without knowledge of the collisional transition probability: (i) initial excitation is a δ function; (ii) relaxation of bulk average energy follows simple exponential law; or (iii) it is linear in time. The implications of exponential relaxation is that [Formula: see text], which is the first moment of the collisional transition probability, is a linear function of internal energy. These conclusions are illustrated using available laser relaxation data on azulene, benzene, and hexafluorobenzene, and are compared with similar data on toluene and two cycloheptatrienes. Some inconsistencies are noted, the probable origin of which is discussed.