Molecular beam infrared–radiofrequency double-resonance spectroscopy of [35Cl]chloromethane: hyperfine structure of the ν6 vibrational state

Abstract

Molecular beam infrared–radiofrequency double-resonance spectroscopy of [³⁵Cl]chloromethane has been demonstrated, using laser Stark spectroscopy to tune rovibrational levels of this molecule into resonance with fixed-frequency CW CO₂ laser radiation. Radiofrequency transitions in the 1–10 MHz region are observed with linewidths of ca. 20 kHz, close to the transit time limit of the apparatus. 18 ground state and 12 ν₆= 1 vibrational state transitions have been assigned and measured. The upper-state transitions are fitted to the energy expressions in the total Stark–quadrupole Hamiltonian matrix, to obtain estimates of the ν₆= 1 spin–rotation interaction constants for [³⁵Cl]chloromethane. The results from this work have been combined with a previous study by Man and Butcher to produce values of C_N=–6.1 (±4.8) kHz and C_k= 11.6 (±8.3) kHz. Also, estimates for the quadrupole centrifugal distortion coefficients have been obtained, including the previously undetermined Hougen distortion term, χ_D. The values obtained are χ_J=–0.55 (±0.67) kHz, χ_K=–0.98 (±2.93) kHz and χ_D= 2.9 (±2.2) kHz.