Knowledge Bank

The nuclear spin modifications of molecular wavefunctions are hardly changed in radiactive or collisional processes. In the present study we have found for the lower rotational levels of $NH3$ experimental evidence for the nuclear spin intercombination by collision during the supersonic jet expansion; an over-population in the $(J,K)=(I,l)$ state has been observed in the jet spectra. The populations were determined by measuring the line intensities of the $v2$ band of $NH3$ by a diode laser spectrometer. The rotational temperature reached 9 K for 5 % $NH3$ seeded in Ar, 16 K for 10 %, and 53 K for pure $NH3$. The ratio of ortho and para concentration, which is 1:1 in room temperature, was found in the case of 10 % $NH3$ to be 0.65:1 after the cooling process by supersonic expansion.