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

Abstract

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.

Iterative Diagonalization in the Multiconfigurational Time-Dependent Hartree Approach: Ro-vibrational Eigenstates

“…Here, experimental results for J values up to three are available. 30 The findings are very similar to the ones discussed for the out-of-plane bend excitation. The differences between ΔE exp rot and ΔE calc rot do not exceed 0.4 cm −1 .…”

“…The corresponding bands in the infrared absorption spectrum of CH 2 D have recently been studied experimentally. 29,30 A. Convergence of the J = 0 and J > 0 Calculations. To study the iterative approach introduced in sections II B and II C, the convergence of the calculated ro-vibrational energies with the number of iterations and the size of the basis sets employed is investigated.…”

“…The vibrational ground state, the out-of-plane bending fundamental mode, and the symmetric and antisymmetric stretch fundamental modes, respectively, are investigated. The corresponding bands in the infrared absorption spectrum of CH 2 D have recently been studied experimentally. , …”

“…The convergence of the approach will be tested by comparison with reference results and by internal checks which do not require a reference solution. Finally, the computed ro-vibrational levels are compared to recent experimental results, , and the accuracy of the ab initio potential energy surface employed is discussed.…”

Sub-Doppler infrared spectroscopy of CH2OH radical in a slit supersonic jet: Vibration-rotation-tunneling dynamics in the symmetric CH stretch manifold

“…Although such effects are typically outside the realm of Doppler limited infrared spectroscopy, the current sub-Doppler resolution of these slit jet discharge methods often makes such a fine (and sometimes even hyperfine) structure experimentally accessible. 51,[56][57][58][59] Indeed, closer inspection of low N transitions in the K a = 1 ← 1 band reveals quite clear spin-rotation structure, due to splitting of N K a K c states into J = N ± 1 2 levels. By way of example, Fig.…”