MICROWAVE AND CO2-SIDEBAND - LASER OPTOTHERMAL SPECTROSCOPY OF $Ar-NH_{3}$

$^{1}$ D.D. Nelson, Jr., G.T. Fraser, K.I. Peterson, K. Zhao, W. Klemperer, F.J. Lovas, and R.D. Suenram, J. Chem. Phys. 85. 5512 (1986). $^{2}$ G.T. Fraser, D.D. Nelson, Jr., A. Charo, and W. Klemperer, J. Chem. Phys. 82, 2535 (1985). B.J. Howard, private communication.Author Institution: Department of Chemistry, Harvard University; National Institute of Standards and Technology, Molecular Physics Division; Abteilung Fur Physikalisch Chemie, Universitat Ulm, Oberer Eselsberg; Aerodyne Research,, Universitat Ulm, Oberer EselsbergMicrowave spectra of $Ar-NH_{3}$ states correlating to $j=1, k=1 NH_{3}$ have been recorded using electric-resonance and pulsed-nozzle Fourier-transform spectrometers. Hyperfine analysis and microwave-microwave double resonance are combined to obtain a definitive rotational assignment for these states, extending the earlier $j=0, k=0 Ar-NH_{3}$ results of Nelson et al1. As noted $previously^{1}$, the $j=1, k=1$ spectra display complications from the NH3 inversion and internal rotation, and are not well characterized using standard spectroscopic analyses. Here, we are attempting to model the spectra using various anisotropic potentials to couple the rotational angular momentum of the NH3 to the orbital angular momentum of the complex. The results of this approach will be discussed. A microwave-sideband $CO_{2}$ laser optothermal spectrometer with a resolution better than 1 MHz has been used to record the infrared spectrum of $Ar-NH_{3}$ in the vicinity of the R(0) line of the $\nu_{2}$ vibration of free $NH_{3}$. A $\Pi-\Sigma$ type band is observed giving a positive 1-type doubling constant of 90.9 MHz for the upper state, consistent with the $\Sigma, j=1, k=0$ state being above the $\Pi, j=1, k=0$ state. The vibrationally excited complex is found to predissociate in less then the - 1 ms transit time between the bolometer detector and laser excitation region. A lower limit to the upper-state lifetime is obtained from the observed linewidths, which are less than 3 MHz. The present results are in excellent agreement with the previous diode-laser measurements of Howard $et al.^{2.}