Intracavity triple resonance in the CH3OH far infrared laser

Abstract

A very interesting aspect of FIR laser spectroscopy is its contribution to the physical insight of the laser active molecules themselves. In fact, a molecule can display several FIR laser emissions only if its spectrum is dense and extended enough so as to have high coincidence probabilities with the available pump lines (usually CO 2 ). This usually means serious difficulties in the spectrum assignments, since experimental and computational inaccuracies lead to very frequent ambiguous cases. On the other hand, new line assignments are a continuous challenge to further improvements in the molecular model. The use of optical pumpimg for the excitation of the FIR lasers allows the direct application of external CW and oscillating fields to the active medium, thus making, for instance, Stark effect and triple resonance (IR-FIR-RF) experiments possible. The study of the Strak behavior of a FIR laser emission can give valuable hints for the J and K quantum numbers involved in the laser cycle [1]. Triple resonance experiments have proved to be very important for the study of the A symmetry states of CH 3 OH [2, 3], the richest laser active molecule known up to now. Here RF transitions are induced between the components of K-splitting doublets involved in the laser cycle. According to the position of the doublet in the cycle, an RF resonance is detected as a power decrease (common upper level of the pump and laser transitions) or increase (lower level either of the pump or of the laser transition) in the FIR emission, see Fig. 1. Our experimental apparatus has been described elsewhere [2]. It consists essentially of a Pyrex waveguide CO 2 pump laser, two waveguide FIR lasers, a sweeper oscillator connected to the FIR lasers via an RF amplifier, a frequency meter, Golay detectors for the FIR output and a lock-in amplifier. Comparison of the resonance RF frequency to the expression