COLLISIONAL NUCLEAR-SPIN INTERCOMBINATION IN NH3 OBSERVED IN SUPERSONIC JET EXPANSION BY DIODE-LASER SPECTROSCOPY

Nuclear spin modifications of molecular wavefunctions are hardly changed in radiative or collisional processes. In the present study we have found for the lower rotational levels of NH3 experimental evidence for nuclear spin intercombination by collision during the supersonic jet expansion: An overpopulation in the (J, K) = (1, 1) state has been observed. The populations were determined by measuring the line intensities of the ν2 band of NH3 with a diode laser spectrometer. The rotational temperature reached 9 K for 5% NH3 seeded in Ar, 16 K for 10% NH3, and 53 K for pure NH3. The ratio of the ortho to para concentration, which is 1:1 at room temperature, was found in the case of 10% NH3 in Ar to be 0.65:1 after cooling by supersonic expansion. © 1992.