MEASUREMENT OF THE K-ALPHA=1 (NU(CO)=1) STRETCHING MODE IN AR-CO USING A HERRIOTT MULTIPASS CELL

A Herriott multipass cell has provided a significant increase in the s ensitivity of our computer controlled diode laser spectrometer for the investigation of van der Waals clusters in a continuous slit jet expa nsion. The assignment was then possible of 57 new lines to the perpend icular transition from the (nu(CO),j,K-a,n) = (0, 2, 2, 0) ground stat e of the Ar-CO van der Waals complex to the combination band of the CO fundamental vibration (nu(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 ener gy of the (I, 1, 1,2) stretching mode at 26.19 cm(-1). This is the hig hest van der Waals mode observed in Ar-CO so far. An analysis of the r otational constants resulted in a level ordering of this state relativ e to the nearby (1, 3, 2, 1) state, which is in contradiction to exist ing ab initio calculations. The experimental results demonstrate that even these high lying van der Waals modes still show relatively regula r behaviour, and therefore will provide a sensitive test for future ab initio and semiempirical potential energy surfaces.