Matrix Isolation Spectroscopy and Nuclear Spin Conversion of NH3 and ND3 in Solid Parahydrogen

We present matrix isolation infrared absorption spectra of NH3 and ND3 trapped in solid parahydrogen (pH(2)) at temperatures around 1.8 K. We used the relatively slow nuclear spin conversion (NSC). of NH3 and ND3 in freshly deposited pH(2) samples as a tool to assign the sparse vibration inversion-rotation (VIR): spectra of NH3 in the regions of the nu(2), nu(4), 2 nu(4), nu(1) and nu(3) bands and ND3 in the regions of the nu(2), nu(4), nu(1), and nu(3) fundamentals. Partial assignments are also presented for various combination bands of NH3 Detailed analysis of the nu(2) bands of NH3 and ND3 indicates that both isotopomers are nearly free rotors; that the vibrational energy is blue shifted by 1-2%; and that the rotational constants and inversion tunneling splitting are 91-94% and 67-75%, respectively, of the gas phase values. The line shapes of the VIR absorptions are narrow (0.27-0.4 cm(-1)) for upper states, that cannot rotationally relax and broad (>1 cm(-1)) for upper states that can rotationally relax. We report and assign a number of NH3-induced infrared absorption features of the pH(2) host near 4150 cm(-1), along With a cooperative transition that involves simultaneous vibrational excitation of a pH(2) molecule and rotation-inversion excitation of,NH3. The NSCs of NH3 and ND3 were found to follow first order kinetics With rate constants at 1.8 K of k = 1.88(16) X 10(-3) s(-1) and k = 1.08(8) X 10(-3) s(-1), respectively. These measured rite constants are compared to previous measurements for NH3 in an Ar matrix and with the rate constants measured for Other dopant molecules isolated in solid pH(2).