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 ²Π_1/2 or ²Π_3/2 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 ²Π_1/2 potential curve, which is linked to the observed slow exciplex formation time.