Knowledge Bank

A glow discharge, slit supersonic expansion in conjunction with direct infrared laser absorption have been utilized to record high resolution vibration-rotation spectra of the $C3H5$ allyl radical. The slit supersonic expansion results in efficient rotational cooling $(Trot\le 20K)$, thereby facilitating assignment. Specifically, approximately 50 transitions are assigned for both the in-phase $(\nu 1)$ and out-of-phase $(\nu 13)CH2$ antisymmetric vibrations. Least squares fits of the transition frequencies to an asymmetric top Hamiltonian provide both ground and excited state rotational constants. While the overall quality of the fits $(\sigma \approx 5\times 10-4cm-1)$ are good, residuals do indicate perturbations in the vibrationally excited state. Due to the high instrumental resolution (slit supersonic Doppler width $\approx 70$ MHz) spin-rotation broadening is observed in several low J transitions. Small step size (4 MHz) scans over selected transitions coupled with a detailed lineshape analysis indicate a spin-rotation constant $\epsilon aa$ of -47(4) MHz.