SPIN-ROTATION AND HYPERFINE SPECTROSCOPY IN THE NEAR-IR: A STUDY OF THE METHYL RADICAL

Author Institution: JILA, National Institute of Standards and Technology and Department of Chemistry and Biochemistry, University of ColoradoA glow discharge, slit supersonic expansion in conjunction with direct infrared laser absorption have been utilized to record high resolution vibration-rotation spectra of the $CH_{3}$ methyl radical. The slit supersonic expansion results in effective rotational cooling ($T_{101} \leq 25 K$), collapsing the radical population into the lowest rotational levels for the two nuclear spin states (I=3/2 and 1/2). All spectroscopically allowed vibration-rotation transitions from the lowest rotational level for each of the two nuclear spin states are observed and analyzed in the $v_{3}=1\leftarrow 0$ vibrational manifold. Due to the high instrumental resolution (slit supersonic Doppler width $-75$ MHz), fully resolved spin-rotation splittings and partially resolved hyperfine splittings are observed. The magnitude of the hyperfine coupling constant probes the spin density at each hydrogen nucleus, and the negative sign of the Fermi contact paramater confirms the hydrogen nuclear spins are coupled to the unpaired radical via spin polarization across the CH bond