Matrix isolation spectroscopy and nuclear spin conversion of NH_3 and ND_3 in solid parahydrogen

摘要

We present matrix isolation infrared absorption spectra of NH_3 and ND_3 trapped in solid parahydrogen (pH_2) at temperatures around 1.8 K. We used the relatively slow nuclear spin conversion (NSC) of NH_3 and ND_3 in freshly deposited pH_2 samples as a tool to assign the sparse vibration-inversion-rotation (VIR) spectra of NH_3 in the regions of the ν_2, ν_4, 2ν_4, ν_1, and ν_3 bands and ND3 in the regions of the ν_2, ν_4, ν_1, and ν_3 fundamentals. Partial assignments are also presented for various combination bands of NH_3. Detailed analysis of the ν_2 bands of NH_3 and ND _3 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 (0.2-0.4 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 NH_3-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 NH_3. The NSCs of NH_3 and ND_3 were found to follow first-order kinetics with rate constants at 1.8 K of k = 1.88(16) × 10~(-3) s~(-1) and k = 1.08(8) × 10~(-3) s~(-1), respectively. These measured rate constants are compared to previous measurements for NH_3 in an Ar matrix and with the rate constants measured for other dopant molecules isolated in solid pH_2.