No abstract

The thermal dissociation of N2O in argon was investigated by monitoring the formation of O(3P) atoms in the reflected shock regime using atomic resonance absorption spectrophotometry (ARAS). The total density and [N2O] ranges were, (2.6 × 1018)−(5.4 × 1018) molecules cm-3 and (3.3 × 1012)−(7.9 × 1015) molecules cm-3, respectively. Values for the bimolecular rate constant (131 points), derived under low-pressure limit conditions are given by the Arrhenius expression:  k 1(T) = (1.18 ± 0.16) × 10-9 exp[(−57820 ± 460 cal mol-1)/RT] cm3 molecule-1 s-1 for the temperature range, 1195 ≤ T ≤ 2384 K. These results extend the low-temperature range of ARAS measurements of k 1 by about 200 °C which is very significant in 1/T; and the value of the rate constant was extended by more than an order of magnitude. The present data were combined with previously published ARAS data to form a composite data set with a total of 278 points. Although systematic differences between the data of the various groups were discernible, all the data are well represented by the following Arrhenius equation:  k 1(T) = (9.52 ± 1.07) × 10-10 exp[(−57570 ± 390 cal mol-1)/RT] cm3 molecule-1 s-1 for the temperature range, 1195 ≤ T ≤ 2494 K). Uncertainties in the Arrhenius expression are given at the one standard deviation level and the mean deviation of the experimental data from that predicted by the expression is ±26%. These results are compared to those from previous experimental studies. The rate of the reaction of O(3P) with N2O was investigated experimentally and by kinetic modeling, but only over a limited temperature range, 1200 ≤ T ≤ 1400 K. Upper limit values of the overall rate constant for the O(3P) + N2O reaction were estimated by a statistical technique. These values were about a factor of 10 lower (with an overall uncertainty of about a factor of three) than those calculated from the recommended Arrhenius expressions of Baulch et al. (1973), Hanson and Salimian (1984), and Tsang and Herron (1991).

No abstract

Photoionization efficiency (PIE) spectra of HOI were measured over the wavelength range A = 115-130 nm and in the ionization threshold region, A = 123-129 nm, using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. HOI was generated, in situ but in varying amounts, by three separate reactions: OH + I,; OH + CF3I; O(3P) + C2HJ The PIE spectra displayed steplike behavior near threshold, and the HO-I stretching frequency in the cation was determined to be 702 f 60 cm-I. A value of (9.811 & 0.020) eV was obtained for the adiabatic ionization energy (IE) of HOI from photoion thresholds, corresponding to the HOI+(X2A") -HOI(X' A') transition. Even though the present result appears to be the first reported determination of IE(HOI), the experimental value is compared to an estimated value previously derived via a trend analysis and it is considered in terms of trends in the series IE(HOX), where X = F, C1, Br, and I. The branching ratio for [HOI]/[IO] in the O(3P) + CzHJ reaction was estimated to be about 8/1 at T = 298 K if we assume that the photoionization efficiencies for HOI and IO are the same at 10.3 eV (A = 120 nm). Also, based on the value for IE(HO1) derived in the present study, a value for IE(I0) % 9.66 & 0.10 eV has been predicted.

The photoion efficiency spectrum of the nitrate radical (NO3) was measured over the region , I = 90-104 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. NO3 was generated by the reaction of fluorine atoms with nitric acid. A value of 12.57 f 0.03 eV was obtained for the adiabatic ionization energy (IE) of NO3 from photoion thresholds, corresponding to the N03+('A1') -N03(2A~') transition. These direct ionization measurements are the first to be reported for the NO3 radical. Relative energetics and optimized geometries for ground state NO3 and several states of NO3+ were determined in both D3h and CzV symmetries using multiconfiguration self-consistent-field calculations. The results of the present study strongly suggest that the neutral ground state of NO3 has D3h symmetry. This conclusion is based on the following observations: (1) the experimental photoion threshold exhibits a large, steep initial step with no evidence of significant structure, indicating that the neutral and cation must share the same symmetry, and (2) the theoretical evidence is unambiguous that the cation symmetry is D3h. A value of AfH0298(N03+) and the proton affinity of NO3 are also derived. A brief comparison is made of the ionization energies of NO, NO2, and NO3, and some new results on the dissociative ionization of HN03 are discussed.