Fourier Transform Infrared Studies of Ammonia Photochemistry in Solid Parahydrogen

Abstract

We present 193 nm in situ photochemical studies of NH3 isolated in solid parahydrogen (pH2) at 1.8 K using Fourier Transform Infrared (FTIR) spectroscopy. By recording FTIR spectra during and after irradiation we are able to identify and assign a number of rovibrational transitions to ortho-NH2(X(2)B1) and NH(X(3)Σ(-)). Spectroscopic analysis shows that these two radical species rotate freely in solid pH2 and that effects of the unpaired electron spin remain essentially unchanged from the gas phase. We provide detailed mechanistic studies that show the nascent ortho-NH2 photoproduct is rapidly cooled within the pH2 matrix to the ground vibrational and rotational state before (1) subsequent photodissociation or (2) tunneling-driven reaction (k(tun) = 1.88(17) min(-1)) with the pH2 host to produce ortho-NH3 in a defect site. Once the ortho-NH3 is produced in this defect site it slowly converts (k(conv) = 7.72(51) × 10(-3) min(-1)) back to a single substitution site even at 1.8 K. We demonstrate the in situ photolysis of NH3 can be utilized to generate NH doped pH2 solids that are relatively stable at low temperature. However, the ortho-NH2 + pH2 → ortho-NH3 + H back reaction substantially limits the sequential two-photon conversion of NH3 to NH. These studies also reveal that extended photolysis of the NH3/pH2 system results in the generation of high concentrations of orthohydrogen that must result from repeated cycles of photodissociation and NH2 back reaction within the pH2 host.