The branching ratios into the O H and H 2 0 producing channels in the reaction of NH2 with N O have been measured at four temperatures ranging from room temperature to 900 O C . The reaction was initiated by production of NH2 by excimer laser photolysis of ammonia at 193 nm in the presence of nitric oxide and was probed using a color center laser in conjunction with fast IR detectors. Upon photolysis, the NH3 infrared absorption decreases with consequent appearance of NH2 infrared absorption lines. The magnitude of the decrease in the NH3 infrared absorption is compared with infrared absorbance of O H and H 2 0 produced by the reaction. Measurement of room temperature peak absorption cross sections for NH3, NH2, and H 2 0 combined with the literature value of the OH cross section permits the determination of the branching ratios.For elevated temperatures, the appropriate Boltzmann factors and partition functions were used to calculate the required cross sections from the room temperature values. The branching ratio into the O H channel was observed to increase from 10% a t room temperature to 17% at 900 OC. However, the total contribution of the two channels decreased from -94% a t 26 OC to -70% at 900 OC, possibly indicating the onset of an additional reaction channel. The possibility that the O H signal might arise from an artifact source was investigated.
The rate constants for the reactions of the ethynyl radical (C2H) with H2,02, C2H2, and NO have been measured by following the time decay of an C2H infrared transient absorption line originating from the ground vibronic state using color center laser spectroscopy. For the H2, 02, and NO reactions, the C2H was produced by excimer laser flash photolysis (ArF, 193 nm) of CF3C2H. In the case of the C2H2 reaction C2H was produced by flash photolysis of acetylene again using the 193-nm ArF excimer line. Excited states of C2H, which are abundant with 193-nm photolysis, were relaxed by buffering the photolysis cell with ~20 Torr of He buffer and ~160 mTorr of SF6. Rate constants of 4.2 X 10_11, 4.8 X 10"13, 1.5 X 10"10, and 3.5 X 10~u cm3 molecule"1 s"1 were obtained for the reactions of C2H with 02, H2, C2H2, and NO, respectively.
Ihfrared kinetic spectroscopy using excimer laser flash photolysis and color center laser probing has been used to study the NH, + NO reaction. The amidogen radical, NH2, was produced by ArF photolysis of NH3. Infrared absorptions of OH and H 2 0 were measured to determine the absolute contributions of the OH and HzO product channels. It was found that the OH channel accounts for 13 * 2% of the reaction. Using two different pairs of NH, and H 2 0 lines, we measured values of 0.85 * 0.09 and 0.66 h 0.03 for the ratio of H 2 0 formed to NH3 photolyzed. All of the H 2 0 signals exhibit a pronounced induction period suggesting that H 2 0 is produced in very high vibrational states. The time evolution of low-lying vibrationally excited and ground vibrational state H 2 0 lines is adequately simulated by a model in which a stepwise sequential loss of vibrational energy occurs with quenching cross sections for each step proportional to excess energy.
formation must be greater than any muting effects due to solvation. Potential solvation effects include steric exclusion and energetic effects. The latter can arise from the necessity of stripping the MeCN (possibly from both reactants) and the alteration of the dielectric media as the charged reactants approach each other. Surprisingly, the reactions are little affected by the greatly different reaction media and the species present. Reasons for this are discussed shortly.There are certainly changes in the excited-state properties caused by solvation effects. In water, shifts of 60 and 90 nm are obtained for the bimolecular and termolecular exciplexes, respectively. The emission spectra of the exciplexes are much less red-shifted with increasing [MeCN] relative to their spectra in pure water.2 For Ru(bpy)32+ in 3.8 M MeCN, the shifts are 10 and 50 nm, respectively. There are barely detectible shifts present in the spectrum of Ru(bpy)32+ in 8.9 M MeCN as a function of concentration of Ag+. Therefore, the energies of the exciplex states in the MeCN media are higher than those observed in pure water and closer to that of the parent complex.Similar to the water system, *D|Ag22+ has a lower state energy than *D|Ag+. Finally, the yields and r's of the exciplexes increase with increasing [MeCN], Indeed, for the exciplexes of the Ru-(4,7-Me2phen)32+, the emission yields are higher than for the parent complex in the same media for the 8.9 M MeCN. The primary effect in the higher quantum yields and lifetimes of the exciplexes in the mixed media appears to involve the energy gap law:36 the lower the state energy the more efficient is radiationless deactivation. Further, our data show that there is nothing inherently detrimental to the emission process on exciplex formation. Indeed, the mono-Ag+ exciplex at higher [MeCN] emits more efficiently than the parent. The Ag+ in this case, coupled perhaps with an associated MeCN solvation sphere, may protect the excited state. Whitten et al.37 have observed a much more efficient emission from *Ru(bpy)2(CN)2|Ag+ than the (36) Caspar,
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