A crucial foundation of astrochemistry lies in the observation and identification of highly reactive chemical intermediates in the interstellar medium. Optimal spectroscopic search requires precision predictions of transitions in infrared/micro/radio-wave regions of the spectrum, which can be greatly facilitated by high resolution infrared spectroscopy under terrestrial laboratory conditions. This talk will address two recent/ongoing examples. 1) Highly reactive benzyl radicals are generated by electron dissociative attachment to benzyl chloride and cooled to Trot = 15K in a high density, supersonic slit discharge expansion environment. The sub-Doppler spectra are fit to an asymmetric-top rotational Hamiltonian, thereby yielding spectroscopic constants for the ground (v = 0) and first excited (v3, v4) vibrational levels of the ground electronic state. To assist detection of benzyl radical in the interstellar medium, we have significantly improved the precision of the ground state rotational constants through combined analysis of the ground state IR and LIF combination differences. Of dynamical interest, there is no evidence in the sub-Doppler spectra for tunneling splittings due to internal rotation of the CH2 subunit, which implies a rotational barrier consistent with partial double bond character in the CC bond. The high-resolution infrared spectra are in excellent agreement with simple physical organic chemistry pictures of a strongly resonance-stabilized benzyl radical with a nearly rigid planar structure due to electron delocalization around the aromatic ring. 2) Sub-Doppler infrared spectroscopy in the symmetric (v1) and antisymmetric (v6) NH stretch modes of the ND2H2+ ammonium cation are reported, formed by isotopic exchange of ND3/H2O and protonation in the slit-jet discharge expansion environment. Rotational assignment for ND2H2+ is confirmed rigorously by four line ground state combination differences, which agree to within sub-Doppler precision in the slit jet (~9 MHz). Observation of both b-type (v1) and c-type (v6) bands enable high precision determination of the ground and vibrationally excited state rotational constants. The study predicts precise microwave/mm-wave transitions to facilitate spectroscopic search for ND2H2+ in interstellar clouds and star-forming regions of the interstellar medium.

Terrestrial progress toward infrared spectroscopy/detection of hydrocarbon radicals and molecular ions in the interstellar medium

256th National Meeting (2018)

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