Infrared spectra of matrix isolated ammonia at liquid helium temperature, behavior during irradiation with a CO2 laser beam

Infrared spectra of NH3 trapped in an argon matrix are studied in the nu(2) vibrational region. Under irradiation with a CO2 laser at 975 cm(-1), the spectrum is reversibly changed and, in particular, two nets lines appear. This phenomenon has probably a thermal origin; however a weak resonance effect is possible. It can only be understood with the assumption of molecular rotation of NH3 in the matrix. It confirms the proposed assignment of the rotational lines.