Optical–optical double resonance (stimulated emission pumping) spectroscopy of HCF

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Abstract
Optical–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 the Ã1 A‘(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 the Ã1 A‘(000) state. The signals are observed at sub‐Doppler resolution (about 0.002 cm−1 FWHM), but their wave numbers are determined only to the precision 0.003 cm−1 of the dye laser wave number measurement. Molecular constants in the (010) and (020) states are determined precisely by a conventional least‐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 ν1 band origin thus determined for the first time is unusually low, i.e., 2643.0393 (26) cm−1 with one standard deviation in parentheses.