Spin–orbit effects in the formation of the Na–He excimer on the surface of He clusters

Abstract

In this paper we describe an application of reversed time-correlated single photon counting to the time-resolved spectroscopy of impurity atoms and molecules bound to large quantum clusters. The photo-induced dynamics of Na atoms on the surface of He and H2 clusters have been studied by following the time dependence of their emission at selected excitation and emission wavelengths. Collection of atomic (16980±145 cm-1) fluorescence arising from Na atoms excited on the cluster surface and immediately desorbed from it yields a finite (ca. 70 ps) rise time and a decay time of 16.3±0.1 ns, equal to the known lifetime of the 3P→3S transition of atomic Na. The frequency distribution of the red emission due to atoms that do not leave the cluster immediately after excitation is shown to be due to a desorbed Na*–He exciplex by obtaining quantitative agreement with predictions derived from available abinitio Na–He potentials. Formation of this excimer can occur along either the 2Π1/2 or 2Π3/2 excited state surface. ‘Slow’ (700 ps) and ‘fast’ (ca. 70 ps) components of the rise time of the red emission (15800±125 cm-1) are assigned, respectively, to the two formation channels. Introducing spin–orbit coupling effects into the long range abinitio pair potential for an isolated Na*–He generates a small barrier on the 2Π1/2 potential curve, which is linked to the observed slow exciplex formation time.