The silicon dicarbide molecule, SiC2, has a celebrated presence in molecular astronomy. It is the carrier of the blue-green Merrill-Sanford (M-S) optical bands that have been used in star classification, and is a key molecular tracer of dust formation in carbon stars. Recent radio observations of the evolved carbon star IRC+10216 with unprecedented high angular resolution have revealed a plethora of unassigned (U) rovibrational lines associated with the dust formation zone. Because SiC2 is a known, abundant molecular constituent of this region, it is reasonable to posit that some fraction of the observed U lines arises from vibrationally excited levels of SiC2 that are populated at elevated temperatures. However, laboratory spectroscopic data that would permit testing of this hypothesis are absent, and ab initio prediction of relevant spectroscopic parameters for this system has proved challenging. To help constrain laboratory searches for vibrationally excited SiC2 in the rotational band, we have been taking advantage of the strong M-S bands to observe rotationally resolved vibronic spectra of SiC2 using stimulated emission pumping (SEP) spectroscopy. SiC2 has been efficiently generated in a jet-cooled discharge of silane and acetylene, optically pumped via the M-S bands, and fluorescence depletion SEP spectra observed for dump transitions terminating in a variety of excited vibrational levels of the ground electronic state. For known transitions in 1v3 and 2v3, our measurements are in generally excellent agreement (a factor of at least 5 smaller than the dump laser linewidth) with previous observations, giving us good faith in our experimental procedure; rotational analyses of previously unobserved transitions of all three vibrational modes are on-going at the time of writing, and will be discussed.

Stimulated emission pumping spectroscopy of SiC2: Key molecular progenitor of dust in carbon stars

2019 Fall National Meeting

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