Vibrational energy relaxation in liquid oxygen (revisited) and in liquid nitrogen

The lifetimes of the first excited vibrational states in liquid oxygen and nitrogen are very long: 2.5 ms and 56 s, respectively. The theoretical calculation of these long lifetimes is challenging, ultimately because in each case the vibrational energy gap is much higher both than kT and than characteristic translational and rotational frequencies of the molecules in the liquid. A few years ago we presented a theoretical calculation for the vibrational lifetime in liquid oxygen based on a semiclassical implementation of Fermi's golden rule. In this paper we improve upon that work, primarily by using an improved intermolecular potential, and what we believe is a more reliable quantum correction factor. Our theoretical result for the lifetime at 77 K is in reasonable agreement with experiment. We also present a calculation for liquid nitrogen at 77 K. In this case our result for the (nonradiative) lifetime is about a factor of 35 longer than the experimental lifetime, lending support to the belief that in liquid nitrogen the vibrational lifetime is in fact radiative. (C) 2002 American Institute of Physics.