A high-resolution analysis of the à 2A′–X̃ 2A′ transition of CaSH by laser excitation spectroscopy

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Abstract
The high resolution spectrum of the à 2A′–X̃ 2A′ transition of CaSH has been recorded near 650 nm using laser excitation spectroscopy with selected fluorescence detection. While both a-type and b-type rotational transitions have been observed, extensive measurements have been made for the b-type transitions up to K′a = 4 and K″a = 5. Altogether over 3000 rotational lines have been measured and fitted with an A-reduced Hamiltonian. The X̃ 2A′ state has rotational constants A=9.693 322(46) cm−1, B=0.141 864 7(33) cm−1, and C=0.139 581 0(33) cm−1. The à 2A′ state has a band origin at 15 380.284 7(2) cm−1, and effective 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 r0 structure for CaSH is discussed. The à 2A′ state is the lower energy Renner–Teller component of the ‘‘à 2Π’’ state of the hypothetical linear CaSH molecule, and consequently was found to have a relatively large positive value for the spin–rotation parameter εaa at 3.445 69(26) cm−1. The upper asymmetry component of the F1 spin component of the Ka=1 stack and the F2 spin component of the Ka=0 stack in the à 2A′ state perturb each other with an avoided crossing between J=37.5 and J=38.5. These two spin components interact through the off-diagonal ‖εab+εba‖/2 element of the spin–rotation tensor. For CaSH, the à 2A′ state has ‖εab+εba‖/2=0.065 915(46) cm−1.