A HIGH-RESOLUTION ANALYSIS OF THE (A)OVER-TILDE(2)A'-(X)OVER-TILDE(2)A' TRANSITION OF CASH BY LASER EXCITATION SPECTROSCOPY

The high resolution spectrum of the A2A'-X2A' transition of CaSH has b een recorded near 650 nm using laser excitation spectroscopy with sele cted fluorescence detection. While both a-type and b-type rotational t ransitions have been observed, extensive measurements have been made f or the b-type transitions up to K(a)' = 4 and K(a)'' = 5. Altogether o ver 300() rotational lines have been measured and fitted with an A-red uced Hamiltonian. The X2A' state has rotational constants A = 9.693 32 2 (46) cm-1, B = 0. 141 864 7 (33) cm-1, and C = 0.139 581 0(33) cm-1. The A2A' state has a band origin at 15 380.284 7 (2) cm 1, and effect ive values for the rotational constants A = 9.090 808 (78) cm-1, B = 0 . 147 459 8 (34) cm-1, and C = 0. 144 804 2 (34) cm-1. An approximate ro structure for CaSH is discussed. The A2A' state is the lower energy Renner-Teller component of the ''A2PI'' state of the hypothetical lin ear CaSH molecule, and consequently was found to have a relatively lar ge positive value for the spin-rotation parameter espilon(aa), at 3.44 5 69(26) cm-1. The upper asymmetry component of the F1 spin component of the K(a) = 1 stack and the F2 spin component of the K(a) = 0 stack in the A2A' state perturb each other with an avoided crossing between J = 3 7.5 and J= 3 8.5. These two spin components interact through the off-diagonal \epsilon(ab) + epsilon(ba) \ /2 element of the spin-rota tion tensor. For CaSH, the A2A' state has \epsilon(ab) + epsilon(ba)\ /2 = 0.065 915(46) cm-1.