Melanie A. Roberts

High resolution slit-jet infrared spectroscopy of ethynyl radical: 2Π–2Σ+ vibronic bands with sub-Doppler resolution

High resolution infrared spectra for four (2)Π-(2)Σ(+) bands of jet-cooled ethynyl radical (i.e., C(2)H) in the gas phase are reported. The combination of (i) slit-jet cooling (T(rot) ≈ 12 K) and (ii) sub-Doppler resolution (≈ 60 MHz) permits satellite branches in each (2)Π-(2)Σ(+) band to be observed and resolved for the first time as well as help clarify a systematic parity misassignment from previous studies. The observed lines in each band are least squares fit to a Hamiltonian model containing rotational, spin-rotational, spin-orbit, and lambda-doubling contributions for the (2)Π state, from which we report revised excited state constants and band origins for the observed bands. Three of the four bands fit extremely well within a conventional (2)Π model (i.e., σ < 20 MHz), while one band exhibits a local perturbation due to an avoided crossing with a near resonant dark state. Vibronic assignments are given for the observed bands, with the dark state clearly identified as a highly excited stretch and bending overtone level X̃ (1,2(2),0) by comparison with high level ab initio efforts.

High-Resolution Direct-Absorption Spectroscopy of Hydroxymethyl Radical in the CH Symmetric Stretching Region

High-resolution, fully rotationally resolved direct absorption spectra of hydroxymethyl radical, CH2OH, are presented in the infrared CH stretching region. As a result of low rotational temperatures and sub-Doppler linewidths obtained in the slit supersonic expansion, the Ka = 0 ← 0 band of the symmetric CH stretch for CH2OH has been unambiguously identified and analyzed. By way of chemical confirmation, hydroxymethyl radical is generated via two different slit jet discharge syntheses: (i) direct dissociation of CH3OH to form CH2OH and (ii) dissociation of Cl2 followed by the radical H atom extraction reaction Cl + CH3OH → HCl + CH2OH. The identified transitions are fit to a Watson A-reduced symmetric top Hamiltonian to yield first precision experimental values for the ground state rotational constants as well as improved values for the symmetric stretch rotational constants and vibrational band origin. The results both complement and substantially improve upon spectral efforts via previous double resonance ionization detected infrared methods by Feng et al. [J. Phys. Chem. A, 2004, 108, 7093], as well as offer high-resolution predictions for laboratory and astronomical detection of hydroxymethyl radical in the millimeter-wave region.

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.

Hyperfine structure in the electronic spectrum of ReO

Hyperfine structure in the spectrum of the molecule rhenium monoxide (ReO) has been observed for the first time by recording the [14.0] 7/2–X 5/2 (0, 0) band near 712 nm at sub-Doppler resolution using intermodulated fluorescence spectroscopy. Hyperfine splittings due to both the 185Re and 187Re isotopes (I = 5/2) have been resolved and assigned. A least-squares fit to the observed transitions has produced accurate values for the rotational, magnetic hyperfine and electric quadrupole constants for both electronic states. The hyperfine constants are discussed in relation to the electronic configurations of the two states. In particular, the ground state magnetic hyperfine constant is found to be consistent with a 1σ22σ21π 41δ33σ2 (X 2Δ i ) electronic configuration, where the 1δ orbital containing the unpaired electron is a Re 5dδ non-bonding orbital.