Study of collisional relaxation in NH3 by steady-state, infrared–infrared double resonance

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Abstract
Steady‐state double‐resonance technique is applied to the study of vibrational and rotational relaxation in NH3. A tunable diode laser is used to probe the population change caused by CO2 laser irradiation. Signals are observed for the mixture of 15NH3 and 14NH3, while the 15NH3 ν2 asQ(5,4) transition is pumped by the 13CO2 10 μm R(18) laser line. A theory based on rate‐equation approximation is developed to elucidate the contributions from thermal effects and intramolecular relaxation in steady‐state double resonance. The signals observed for 14NH3 and ortho (K=3n) 15NH3 are due to thermal effect induced by intermolecular energy transfer. The increase of vibrational temperature by laser irradiation is found to be an order of magnitude larger than increase of rotational temperature. The branching ratio of intramolecular rotational relaxation, which includes collision‐induced transitions from the pumped levels and subsequent cascading processes, is determined for para (K≠3n) 15NH3 in the ground and vibrationally excited states by correcting the thermal effect.