Infrared–microwave double resonance studies of collison-induced transitions and energy transfer processes between vibration–rotation–inversion levels of NH3

Abstract

When the 14NH3 molecules in the upper inversion level of the J=8, K=7 (para) rotational state are pumped by using the N2O P (13) laser line, many microwave inversion lines change their intensities because of collision-induced populational transfer. Pressure dependence of this effect has been studied for several tens of inversion transitions in the ground vibrational state. The non-Boltzmann distribution introduced into the pumped levels is transfered to other levels by two mechanisms: (i) collision-induced transitions into or out of the pumped levels and succeeding cascading processes and (ii) selective energy transfer mechanisms to collision partners. In the former mechanism the transfer of populational change is caused by the transition of pumped molecules whereas in the latter it is caused by collision partners. Although the first mechanism is more efficient than the second, the second mechanism is not negligible and causes the populational change in also ortho rotational state. We used mixtures of 15NH3 and 14NH3 and monitored the 15NH3 inversion lines to discriminate between the two mechanisms. Since 15NH3 is not pumped by the laser line, the double resonance signal is caused only by the second process. The result on 15NH3 is similar to that for the ortho-14NH3, which clearly indicates that the signal previously observed for the ortho-14NH3 is not due to the collision-induced ortho–para conversion but due to the second mechanism. Double resonance signals for para-14NH3 show different pressure dependence from those of ortho-14NH3 and 15NH3. The results are analyzed consistently by taking account of various collision mechanisms. In the course of this analysis it was found that the V–V energy transfer becomes much more inefficient when the energy discrepancy becomes several tens of wavenumbers.