Opticalndash;optical double resonance or stimulated emission pumping spectroscopy has been applied to the HCF molecule to obtain precise molecular constants in excited vibrational states associated with the ground electronic state. A cw dye laser excites HCF molecules to a single rotational level of theAtilde;1Alsquo;(000) state, and transitions induced by a second cw dye laser downward to rotational levels in excited vibrational states of the ground electronic state are observed as dips of the fluorescence from theAtilde;1Alsquo;(000) state. The signals are observed at subhyphen;Doppler resolution (about 0.002 cmminus;1FWHM), but their wave numbers are determined only to the precision 0.003 cmminus;1of the dye laser wave number measurement. Molecular constants in the (010) and (020) states are determined precisely by a conventional leasthyphen;squares analysis of the observed signals. The (100) state is found to interact with the (011) state through Coriolis coupling, and the spectral data obtained for the two states are analyzed simultaneously. The ngr;1band origin thus determined for the first time is unusually low, i.e., 2643.0393 (26) cmminus;1with one standard deviation in parentheses.
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