Probing the dynamics of weakly bound complexes using high-resolution laser spectroscopy

Abstract

A combination of high-resolution infrared spectroscopy and photofragment detection methods has been used to study the vibrational predissociation of a number of weakly bound complexes at the state-to-state level. The goal of this research is to provide detailed information on the dissociation of these highly non-statistical systems that can be used to make comparisons with the results of emerging theoretical methods and to gain new insights into the nature of bond rupture. In this paper we discuss results for several systems which illustrate some of the new methods we have developed. In the case of the HF dimer, the photofragment angular distributions provide sufficient information to assign the internal state distributions of the fragments. The experimental results are compared with recently reported theoretical calculations. Preliminary experimental results are also reported for the HF–DF and DF–HF complexes which, when combined with previously reported theoretical calculations and the HF dimer results, reveal deficiencies in the existing potential-energy surfaces that are not evident from the available spectroscopy.

For systems where the fragments have smaller rotational constants or accessible excited vibrational states, the angular distributions no longer provide a unique assignment of the internal state quantum numbers. Without such, the detail with which the photofragmentation process can be studied is greatly reduced. For this reason we have developed two new methods that greatly aid in determining such information. The first involves the use of a second infrared laser to probe the fragments spectroscopically, while the second method involves orienting the parent molecular complex using a large dc electric field prior to dissociating it. In this way the two fragments can be detected separately, reducing the congestion in the angular distribution. These methods will be illustrated with several examples.