Laser-induced fluorescence spectroscopy of the Ga-N-2 cluster

The first spectroscopic observation of the Ga-N-2 van der Waals cluster is reported. This was formed by laser ablation of a GaAs or GaP target followe d by pulsed supersonic expansion in pure nitrogen. Laser-induced fluorescen ce spectra have revealed two strong band systems above 30 000 cm(-1). The f irst, which has an onset at 33 468 cm(-1), is composed of eight observable members of a progression in the van der Waals stretching vibration. This ha s been assigned to the (2)Delta-(X) over tilde(2)Pi(3/2) transition correla ting with the Ga 4s(2)4d(1 2)D <-- 4s(2)4p(1) P-2(3/2) transition. A Birge- Sponer extrapolation gives a lower limit of 1270 cm(-1) for the van der Waa ls binding energy in the excited state. At higher wavenumbers another promi nent vibrational progression is observed which is attributed to a spin-forb idden transition correlating with the 4s(1)4p(2) P-4 <-- 4s(2)4p(1) P-2 ato mic asymptote, the cluster excited state most likely having (4)Sigma(-) sym metry. The excited state undergoes relatively slow spin-orbit-induced predi ssociation onto the repulsive (B) over tilde (2)Sigma(+) potential surface followed by rapid emission from the Ga 4s(2)5s(1) S-2 dissociation product.