HIGH-RESOLUTION LASER-RADIOFREQUENCY DOUBLE-RESONANCE MOLECULAR-SPECTROSCOPY

A primary difficulty in obtaining radiofrequency spectra is that the s ize of the quantum is usually much smaller than kT, so that upper and lower states of a transition have practically equal populations and an y resulting absorption must be weak; absorption and stimulated emissio n rates will be very similar. This difficulty is circumvented by using a laser to depopulate one of the states, while a further gain is obta ined by detecting a laser quantum (nu >> kT/h) following absorption of a radiofrequency quantum (nu much less than kT/h). A carbon dioxide l aser is used to saturate vibration-rotation transitions in the 10 mum region and, by using an expanded laser beam of diameter 35 mm and an i nteraction length of some 10 m, radiofrequency spectra are obtained at linewidths below 20 kHz. This approaches the limit implied by the tra nsit time for a molecule traversing the laser beam, and contrasts stro ngly with earlier work using a radiofrequency cell within the laser ca vity. A rate-equation model of the experiment is explored. The new res olution and precision available are applied to hyperfine transitions i n the ground and nu6 = 1 states of CH3I. It is shown that, although cu rrent hamiltonians represent most of the hyperfine structure well, a n ew term to represent vibration-nuclear magnetic coupling must be intro duced. Finally, a new interpretation is put on the hyperfine spectrum of CH3Cl.