Vibrational and rotational relaxation of NH3 studied in a molecular jet by infrared-infrared double resonance

Infrared-infrared double resonance is applied to an expanding jet of NH₃. Molecules in the a(3,3) rotational level of the vibrational ground state are excited with a CO₂-laser into the s(4,3) level of the υ₂ vibrational state, 2s(4,3). Rotational distributions in the ground and υ₂ state are probed by the infrared absorption of color-center-laser radiation producing transitions to the υ₁ and υ₁ + υ₂ states. The time delay between pumping and probing is determined by the distance between the CO₂ laser focus and the color-center-laser focus, along the jet axis. The results indicate an average relaxation time of vibration to rotation/translation of 280 (50) ns Torr* over the temperature range 50–110 K. In that temperature range the population difference between ortho and para species comes into equilibrium within 425(190) ns Torr*, and the relaxation between (3,3) and other ortho ground state levels occurs within 56(20) ns Torr* and between the 2s(4,3) and 2s(3,3) levels within 20(8) ns Torr*. The inversion relaxation time between a(3,3) and s(3,3) is determined to 7.6 (20) ns Torr*, at 50 K. (Here 1 Torr* indicates a density, 1 Torr* = 3.27 × 10²² molecules/m³).