Multiphoton ionization vibrational state selection of H2O, D2O and HDO

We present a study of single color (2 + 1) resonance-enhanced multiphoton ionization (REMPI) of H₂O, D₂O, and HDO via several Rydberg states lying in the energy range from 80 000 to 86 000 cm⁻¹. Photoelectron spectra (PES) show that the corresponding cations can be vibrationally state-selected for most vibrational states. The exception is the bend of H₂O⁺ and HDO⁺, where mixing in the REMPI intermediate level results in weak ion intensity and only 50% state purity.     

Single vibronic level photochemistry of NO2 in the 2491 Å system

A photochemical decomposition study of NO₂(B?²B₂) in specified vibronic states is reported. The O(¹D)-atom yields below and above the dissociation limit (40973 cm^?1) have been measured. The role played by the rotational excitation energy and the vibrational excitation modes in promoting the predissociation is discussed.     

Retrieval of pressure and temperature of a gas sample by analysis of its absorption spectrum

A nonlinear least squares method of analyzing the i.r. spectrum of a gas sample to determine simultaneously the amounts of absorbing gases present and their temperature and pressure is described. The temperatures and pressures found for an air sample, whose spectrum between 2180 and 2210 cm^-1 was analyzed, agree with the observed values and the CO and N₂O abundances agree with the values obtained in a previous investigation.     

The pressure dependence of the rate constant for the reaction: HO + CO → H + CO2

Relative rate measurements of the reactions of the HO-radical with CO [HO + CO → H + CO₂ (1)] and with isobutane [HO + iso-C₄H₁₀ → H₂O + t-(or iso-)C₄H₉ (3)] have been made through the photolysis of dilute mixtures of HONO with CO, iso-C₄H₁₀, NO₂, and NO in simulated air at 700 and 100 torr pressure and 25 ± 2°C. In situ, long path, FT-IR analysis of the reactants and products provided essentially continuous monitoring of the reactions during the course of the experiments. The kinetic analysis of the data coupled with Greiner's estimate of k₃ give new estimates of k₁ = 439 ± 24 ppm^?1 min^?1 in air at 700 torr and k₁ = 203 ± 29 ppm^?1 in air at 100 torr. The results confirm the recent conclusions of Cox and Sie and their co-workers that the rate constant for reaction (1) is pressure dependent. Modeliers of the chemical changes which occur in the troposphere should adopt a new value for the rate constant k₁ which is about a factor of two larger than that in current use by most groups.     

Vibrational mode and collision energy effects on reaction of H2CO+ with CO2

The effects of collision energy (Ecol) and five different modes of H2CO+ vibration on the title reaction have been studied over the center-of-mass Ecol range from 0.1 to 3.2 eV, including measurements of product ion recoil velocity distributions. Electronic structure and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important along the reaction coordinate. Two product channels are observed, corresponding to Hydrogen Transfer (HT) and Proton Transfer (PT). Both channels are endothermic with similar onset energies of approximately 0.9 eV; however, HT dominates over the entire Ecol range and accounts for 70-85% of the total reaction cross section. Both HT and PT occur by direct mechanisms over the entire Ecol range, and have similar dependence on reactant vibrational and collision energy. Despite these similarities, and the fact that the two channels are nearly isoenergetic and differ only in which product moiety carries the charge, their dynamics appear quite different. PT occurs primarily in large impact parameter stripping collisions, where most of the available energy is partitioned to product recoil. HT, in contrast, results in internally hot products with little recoil energy and a more forward-backward symmetric product velocity distribution. Vibration is found to affect the reaction differently in different collision energy regimes. The appearance thresholds are found to depend only on total energy, i.e., all modes of vibration are equivalent to Ecol. With increasing Ecol, vibrational energy becomes increasingly effective, relative to Ecol, at driving reaction. For HT, this transition occurs just above threshold, while for PT it begins at roughly twice the threshold energy.     

State-selective preparation of NO2+ and the effects of NO2+ vibrational mode on charge transfer with NO

Two color resonance-enhanced multiphoton ionization (REMPI) scheme of NO(2) through the E (2)Sigma(u)(+) (3psigma) Rydberg state was used to prepare NO(2)(+) in its ground and (100), (010), (02(0)0), (02(2)0), and (001) vibrational states. Photoelectron spectroscopy was used to verify >96% state selection purity, in good agreement with results of Bell et al. for a similar REMPI scheme. The effects of NO(2)(+) vibrational excitation on charge transfer with NO have been studied over the center-of-mass collision energy (E(col)) range from 0.07 to 2.15 eV. Charge transfer is strongly suppressed by collision energy at E(col) < approximately 0.25 eV but is independent of E(col) at higher energies. Mode-specific vibrational effects are observed for both the integral and differential cross-sections. The NO(2)(+) bending vibration strongly enhances charge transfer, with enhancement proportional to the bending quantum number, and is not dependent on the bending angular momentum. The enhancement results from increased charge transfer probability in large impact parameter collisions that lead to small deflection angles. The symmetric stretch also enhances reaction at low collision energies, albeit less efficiently than the bend. The asymmetric stretch has virtually no effect, despite being the highest-energy mode. A model is proposed to account for both the collision energy and the vibrational state dependence.     

The kinetics and mechanisms of the HO2-NO2 reactions; the significance of peroxynitric acid formation in photochemical smog

Peroxynitric acid has been identified by Fourier transform, infrared, long-path spectroscopy as a product of the UV-irradiated dilute mixtures of HONO, CO, No_x in synthetic air at 24 ± 2°C. The characteristic peaks of this compound are identical to those observed by Niki, in irradiated Cl₂, H₂, NO₂, air mixtures and Hanst, in irradiated Cl₂, CH₂O, NO₂, air mixtures. The concentration of the reactants and products of the HONO, CO, NO_x, air system have been determined as a function of irradiation time. The changes observed were computer simulated using a combination of thirty elementary reactions. The data suggests that both reactions (1) and (2) occur as a result of HO₂-NO₂ interactions in the gas phase: HO₂ + NO₂ å HONO + O₂ (1); HO₂ + NO₂(+M) å HO₂NO₂(+M)(2). The data give the preliminary estimates: k₁/k₂ ≈0.7 ±0.4; k₁/k₃ ≈ 0.043 ± 0.02; k₂/k₃ ≈ 0.058 ± 0.02, where reaction (3) is: HO₂ + NO å HO + NO₂. These data and computer simulations of sunlight-irradiated, NO_x, hydrocarbon, aldehyde-polluted atmospheres suggest that peroxynitric acid may be formed in urban atmospheres at rates which are comparable to those observed for peroxyacetyl nitrate (PAN), and it is concluded that HO₂NO₂ may contribute to the “oxidant” found in these atmopheres.