RADIOFREQUENCY ``PURE'' NUCLEAR QUADRUPOLE SPECTROSCOPY OF METHYL IODIDE INSIDE THE INFRARED LASER CAVITY

Abstract

The highly sensitive method of infrared radiofrequency double resonance inside the laser cavity has been applied to observation of ``pure” quadrupole transitions $(\Delta1 = \Delta J = \Delta K = 0, \Delta F = \pm1)$, Radiofrequency signals with large signal-to-noise ratio have been observed for the $127_{I}$ nucleus (spin 5/2) of $CH_{3}I$ in many vibrational (ground state, $\nu_{3}, \nu_{6}, 2\nu_{3} 3\nu _{3}, \nu_{3} + \nu_{6},2\nu_{6},\nu_{2}+\nu_{6}$ and $2\nu_{3} + \nu_{6}$ and rotational levels $(2 \leq J \leq 53)$. The characteristic pattern of the RF spectrum, specific to each rotation-vibration level has enabled us to assign the infrared coincidence and the associated far infrared laser transition unambiguously. Many indirect double resonance signals caused by velocity-conserving collisions have also been observed. From the observed RF spectra, accurate vibrational and rotational dependence of the nuclear quadrupole coupling constant eqQ, including Hougen’s $terms,^{1}$ the spin rotation constants $C_{N}$ and $C_{k}$, and $\ell$ -type doubling constants have been determined, this method of observing ``pure” quadrupole spectrum in gases is applicable to any polar molecule having rotational levels with double parity.