Knowledge Bank

The high resolution spectrum of the $A¯A\prime -X¯2A\prime$ 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 $Ka\prime =4$ and $Ka\prime \prime =5.$ Altogether over 3000 rotational lines have been measured and fitted with an A-reduced Hamiltonian. The $A¯2A$ state has rotational constants $A=9.693322(46)cm-1,B=0.1418647(33)cm-1$, and $C=0.1395810(33)cm-1$ The $A¯2A\prime$ state has a band origin at $15380.2847(2)cm-1$, and effective values for the rotational constants $A=9.090808(78)cm-1,B=0.1474598(34)cm-1$, and $C=0.1448042(34)cm-1$. The $A¯2A\prime$ state is the lower energy Renner-Teller component of the $A¯2\Pi$ state of the hypothetical linear CaSH molecule, and consequently was found to have a relatively large positive value for the spin-rotation parameter $\epsilon \ast \ast$ at $3.44569(26)cm-1$. 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 $A¯2A\prime$ 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 $|\epsilon \ast \ast +\epsilon \ast \ast |/2$ element of the spin rotation tensor, allowing us to determine its value at $0.065915(46)cm-1.$