Radio-frequency spectroscopy inside a laser cavity; "pure" nuclear quadrupole resonance of gaseous CH3I

The highly sensitive method of infrared-radio-frequency double resonance inside a laser cavity has been applied to the observations of "pure" nuclear quadrupole resonances of gaseous C${\mathrm{H}}_3$I corresponding to the transitions $\Delta J=0\mathrm{}$ and $\Delta F=\pm 1$. Radio-frequency signals with large signal-to-noise ratios (∼ ${10}^4$) have been observed for many C${\mathrm{C}}_2$ and ${\mathrm{N}}_2$O laser lines. The characteristic pattern of the radio-frequency spectrum specific to each rovibrational level has enabled us to assign the rotational levels, the infrared coincidences, and the associated far-infrared laser transitions unambiguously. From the radio-frequency spectrum the accurate vibrational and rotational dependences of the nuclear-quadrupole coupling constant and the values of the spin-rotation coupling constants have been determined. Because of the high sensitivity of the method, it was possible to observe collision-induced radio-frequency resonances as satellites to main line. This effect not only increases the number of observable quadrupole resonances, but also provides information on the detailed mechanism of rotational energy transfer. This method of observing "pure" quadrupole spectra of gases is applicable to any polar symmetric-top molecule with hyperfine structure which has rotational levels with double parity.