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

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.