Metastability and cooling timescales of molecular complexes

Abstract

There is a variety of spectroscopic techniques that achieve low effective temperatures. The majority of these rely upon metastability in some coordinates of the total system. The simplest analysis frequently assumes for convenience as single temperature for the description of the state distributions of the subsystem which carries the spectrum. We explore the kinetic behaviour of systems that are being examined spectroscopically by this variety of ostensibly cryogenic methods to understand the timescales of relaxation of different coordinates. Of particular interest is the comparison of inert-gas matrix-isolation methods with beams seeded in inert gases.

This paper examines the rotation and rotation-vibration spectra of species in supersonic jets. Some considerations of the consequences of lack of nuclear spin transitions in adiabatic expansions are discussed. In general the number of states of a jet-cooled species differ from those occurring in a cryogenic matrix. This difference becomes accentuated in molecular clusters if these are not rigid. Particular attention is given to the ν₂ band of NH₃ in its monomeric and dimeric forms.