Measurement of the K_a = 1 (v_co = 1)stretching mode in Ar—CO using a Herriott multipass cell

Abstract

A Herriott multipass cell has provided a significant increase in the sensitivity of our computer controlled diode laser spectrometer for the investigation of van der Waals clusters in a continuous slit jet expansion. The assignment was then possible of 57 new lines to the perpendicular transition from the (v _co, j, K _a,n) = (0,2,2,0) ground state of the Ar—CO van der Waals complex to the combination band of the CO fundamental vibration (v _co = 1) and the K _a = 1 (j = 1) stretching mode (1, 1, 1, 2). The band origin of 2159·88 cm^-1 locates the energy of the (1,1,1,2) stretching mode at 26 ·19cm^-1. This is the highest van der Waals mode observed in Ar—CO so far. An analysis of the rotational constants resulted in a level ordering of this state relative to the nearby (1, 3, 2, 1) state, which is in contradiction to existing ab initio calculations. The experimental results demonstrate that even these high lying van der Waals modes still show relatively regular behaviour, and therefore will provide a sensitive test for future ab initio and semiempirical potential energy surfaces.