Fourier Transform Infrared Studies of Ammonia Photochemistry in Solid Parahydrogen

摘要

We present 193 nm in situ photochemical studies of NH_3 isolated in solid parahydrogen (pH_2) 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-NH_2(X~2B_1) and NH(X~3Σ~?). Spectroscopic analysis shows that these two radical species rotate freely in solid pH_2 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-NH_2 photoproduct is rapidly cooled within the pH_2 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-NH_3 in a defect site. Once the ortho-NH_3 is produced in this defect site it slowly converts (kconv = 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 pH_2 solids that are relatively stable at low temperature. However, the ortho-NH_2 + pH_2 → ortho-NH_3 + H back reaction substantially limits the sequential two-photon conversion of NH_3 to NH. These studies also reveal that extended photolysis of the NH_3/pH_2 system results in the generation of high concentrations of orthohydrogen that must result from repeated cycles of photodissociation and NH_2 back reaction within the pH_2 host.