Collisional nuclear spin intercombination in NH 3 observed in supersonic jet expansion by diode laser spectroscopy

Abstract

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 NH 3 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 ν 2 band of NH 3 with a diode laser spectrometer. The rotational temperature reached 9 K for 5% NH 3 seeded in Ar, 16 K for 10% NH 3, and 53 K for pure NH 3. The ratio of the ortho to para concentration, which is 1:1 at room temperature, was found in the case of 10% NH 3 in Ar to be 0.65:1 after cooling by supersonic expansion.