Chandra Savage

Sub-Doppler infrared spectroscopy of CH2D radical in a slit supersonic jet: Isotopic symmetry breaking in the CH stretching manifold

First high-resolution infrared absorption spectra in the fundamental symmetric/asymmetric CH stretching region of isotopically substituted methyl radical, CH(2)D, are reported and analyzed. These studies become feasible in the difference frequency spectrometer due to (i) high density radical generation via dissociative electron attachment to CH(2)DI in a discharge, (ii) low rotational temperatures (23 K) from supersonic cooling in a slit expansion, (iii) long absorption path length (64 cm) along the slit axes, and (iv) near shot noise limited absorption sensitivity (5 × 10(-7)/√(Hz)). The spectra are fully rovibrationally resolved and fit to an asymmetric top rotational Hamiltonian to yield rotational/centrifugal constants and vibrational band origins. In addition, the slit expansion collisionally quenches the transverse velocity distribution along the laser probe direction, yielding sub-Doppler resolution of spin-rotation structure and even partial resolution of nuclear hyperfine structure for each rovibrational line. Global least-squares fits to the line shapes provide additional information on spin-rotation and nuclear hyperfine constants, which complement and clarify previous FTIR studies [K. Kawaguchi, Can. J. Phys. 79, 449 (2001)] of CH(2)D in the out-of-plane bending region. Finally, analysis of the spectral data from the full isotopomeric CH(m)D(3-m) series based on harmonically coupled Morse oscillators establishes a predictive framework for describing the manifold of planar stretching vibrations in this fundamental combustion radical.

Spectroscopy in slit supersonic jet discharges: fine and hyperfine structure calculations for asymmetric top radicals with multiple nuclear spins

The combination of shot-noise-limited direct absorption spectroscopy with long-pathlength slit supersonic discharges has proven a powerful general method for obtaining infrared spectra of jet-cooled radicals at 10–20-fold sub-Doppler resolution. In addition to rovibrational characterization, such spectral resolution also provides a novel infrared window into fine (spin rotation) and hyperfine (Fermi contact, dipole–dipole) structure in doublet asymmetric top radicals. This necessitates a detailed treatment of the rovibrational Hamiltonian to account for the inhomogeneous line contours generated by the manifold of transitions only partially resolved even under slit supersonic jet conditions. The present work describes least squares fitting these lineshapes with a detailed Hamiltonian predictor program that reflects contributions from (i) asymmetric top end over end tumbling (NK a K c), (ii) electron spin rotation interactions (Σϵα N α S α), and (iii) a suite of hyperfine coupling terms (dipole–dipole, Fermi contact and electric quadrupole) between multiple nuclei with non-zero spin. Independent code development and comparison efforts between our results and the Brown group provide crucial verification of the underlying fine/hyperfine predictor program.

High-resolution spectroscopy of jet-cooled CH5+: Progress

Protonated methane (CH5+) is thought to be a highly abundant molecular ion in interstellar medium, as well as a potentially bright μwave- mm wave emitter that could serve as a tracer for methane. This paper describes progress and first successful efforts to obtain a high resolution, supersonically cooled spectrum of CH5+ in the 2900-3100 cm−1 region, formed in a slit supersonic discharge at low jet temperatures and with sub-Doppler resolution. Short term precision in frequency measurement (< 5 MHz on an hour time scale) is obtained from a thermally controlled optical transfer cavity servoloop locked onto a frequency stabilized HeNe laser. Long term precision (< 20 MHz day-to-day) due to pressure, temperature and humidity dependent index of refraction effects in the optical transfer cavity is also present and discussed.