OBSERVATION OF VIBRATIONALLY EXCITED STATES OF SiC₂ BY STIMULATED EMISSION PUMPING (SEP) SPECTROSCOPY

Reilly, Neil J.

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Title

OBSERVATION OF VIBRATIONALLY EXCITED STATES OF SiC₂ BY STIMULATED EMISSION PUMPING (SEP) SPECTROSCOPY

Author(s)

Reilly, Neil J.

Contributor(s)

• McCarthy, Michael C
• Flores, Jonathan
• Lee, Kelvin
• Ross, Sederra D.

Issue Date

2023-06-23

Keyword(s)

Astronomy

Abstract

Recent 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 a reasonable supposition that some fraction of the observed U lines arise from vibrationally excited levels of SiC₂ populated at elevated temperatures. At present, the laboratory rotational data that would permit testing of this hypothesis are largely absent: ab initio prediction of relevant spectroscopic constants has proved particularly challenging for SiC₂, and its excited vibrational levels are not efficiently populated in supersonic jet sources. However, the electronic transition responsible for the well-known blue-green Merrill-Sanford bands of SiC₂ admits Franck-Condon access to vibrational levels at least 4000 K above ground, inviting the application of SEP spectroscopy for the observation of vibrationally excited states. SiC₂ has been generated in our laboratory 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 rovibrational levels for all three modes in the X˜-state. For known rotational levels of 1ν₃ and 2ν₃ (the pinwheel mode), the rotational energies derived from SEP spectra 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. The 1ν₂ level is notably perturbed, which likely accounts for its as-yet non-observation in the laboratory by rotational spectroscopy. A Fermi resonance with 6ν₃ depresses the 1ν₂ B and C constants significantly below the predictions of high-level theory. Vibrationally averaged rotational constants calculated using Fermi resonance mixing coefficients obtained from A˜-state zero-point dispersed fluorescence are broadly consistent with this interpretation.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

Text

Language

eng

Handle URL

https://hdl.handle.net/2142/122687

DOI

https://doi.org/10.15278/isms.2023.7228

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