Experimental and Theoretical Study of the Kinetics of Dimerization of Ammonia at Low Temperatures

Abstract

The kinetics of the dimerization of NH3 in helium and nitrogen bath gas within the supersonic flow of a Laval nozzle were investigated at very low temperatures. Experimentally, the fraction of the NH3 monomer, fmonomer, remaining in the flow at 91 K in N2 and at 35 K in He for a total bath gas density [M]∼5 × 1016 molecules cm–3 was monitored using fluorescence from the electronically excited NH2 photofragment formed following NH3 photolysis at 213 nm. No dimerization was observed up to [NH3] = 1 × 1015 molecules cm–3 for 160 mm downstream of the 91 K N2 nozzle, nor up to [NH3] = 5 × 1014 molecules cm–3 for 150 mm downstream of the 35 K He nozzle. Dimerization was observed at higher [NH3], being more pronounced at lower temperatures. For the Cs and C2h conformers of the NH3 dimer, calculations at the CCSD(T)/aug-cc-pVTZ level gave a zero-point vibrational-energy corrected binding energy of −7.52 and −7.33 kJ mol–1, respectively. Energy-grained master equation calculations based on statistical rate theory using the open-source MESMER package were used to calculate rate coefficients for dimerization, kdimer, over the temperature range T = 25–300 K and [M] = 1013–1022 molecules cm–3 for He and N2. kdimer displayed a negative T dependence and a positive [M] dependence and was found to be sensitive to changes in the low-lying vibrational frequencies of the NH3 dimer, for example, the inclusion of a hindered rotor potential for the internal twisting mode, which alters the density of states. Using the axial profiles of T, [M], and velocity for the Laval nozzles, the calculated values of kdimer were used to calculate fmonomer in the flow for comparison with the experiment. At higher [NH3], when dimerization was observed, the calculations significantly underestimated the degree of dimerization taking place in the flow, with a significant increase in the calculated value of kdimer required to match the experiment. The reasons for the discrepancy are discussed, for example, errors in the calculation of the density of states for the NH3 dimer and the average energy removed per collision by the bath gas at very low temperatures.