Optothermal-detected microwave-sideband CO2-laser spectroscopy of Ar–NH3

Abstract

A microwave‐sideband CO₂‐laser optothermal spectrometer with a resolution better than 1 MHz has been used to record the infrared spectrum of Ar–NH₃ in the vicinity of the aR(0,0) line of the ν₂ vibration of free NH₃. A Π←∑ type band is observed, giving a positive l‐type doubling constant q, of 90.9 MHz for the upper state. The positive q indicates that the j=1, k=0, ∑ state is above the j=1, k=0, Π state in the v₂=1 excited state, where j and k specify the correlation of the internal‐rotor state of the Ar–NH₃ complex to the NH₃ monomer rotational state j,k. The ν₂ vibrationally excited complex is found to predissociate in less than the 0.9 ms transit time between the bolometer detector and laser‐excitation region. A lower limit to the upper‐state lifetime can be obtained from the observed linewidths, which range from 1.5 to 3 MHz (FWHM). The present results agree with and extend the previous free‐jet diode‐laser absorption measurements on this band.