O(¹D) + N₂O Reaction: NO Vibrational and Rotational Distributions

The O(1D) + N2O → 2NO(X 2Π) reaction has been studied in a molecular beam experiment in which O3 and N2O were coexpanded. The precursor O(1D) was prepared by O3 photodissociation at 266 nm, and the NO(X 2Π) molecules born from the reaction as the O(1D) recoiled out of the beam were detected by 1+1 REMPI over the 220−246 nm probe laser wavelength range. The resulting spectrum was simulated to extract rotational and vibrational distributions of the NO(X 2Π) molecules. The product rotational distribution is found to be characterized by a constant rotational temperature of ≈4500 K for all observed bands, v = 0−9. An inverted vibrational distribution is observed. A consistent explanation of this and previous experimental results is possible if there are two channels for the reaction, one producing a nearly statistical vibrational distribution for low O(1D)−N2O relative velocity collisions and a second producing the inverted distribution observed here for high relative velocity collisions. The former might correspond to an insertion/complex-formation reaction, while the latter might correspond to a stripping reaction. Velocity relaxation of the O(1D) is argued to compete strongly with reaction in most bulb studies, so that these studies see predominantly the nearly statistical distribution. In contrast, the beam experiments do not detect the part of the vibrational distribution produced in low relative velocity reactions because the O(1D) is not relaxed from its initial velocity before it either reacts or leaves the beam

Photodissociation of Ozone from 321 to 329 nm: The Relative Yields of O(³P₂) with O₂(<i>X</i> ³Σ_g^–), O₂(<i>a</i> ¹Δ_g) and O₂(<i>b</i> ¹Σ_g⁺)

Product imaging of O­(³P₂) following dissociation of ozone has been used to determine the relative yields of the product channels O­(³P₂) + O₂(<i>X</i> ³Σ_g^–) of ozone. All three channels are prominent at all wavelengths investigated. O₂ vibrational distributions for each channel and each wavelength are also estimated assuming Boltzmann rotational distributions. Averaged over wavelength in the measured range, the yields of the O­(³P₂) + O₂(<i>X</i> ³Σ_g^–), O­(³P₂) + O₂(<i>a</i> ¹Δ_g), and O­(³P₂) + O₂(<i>b</i> ¹Σ_g⁺) channels are 0.36, 0.31,and 0.34, respectively. Photofragment distributions in the spin-allowed channel O­(³P) + O₂(<i>X</i> ³Σ_g^–) are compared with the results of quantum mechanical calculations on the vibronically coupled PESs of the singlet states B (optically bright) and R (repulsive). The experiments suggest that considerably more vibrational excitation and less rotational excitation occur than predicted by the quantum calculations. The rotational distributions, adjusted to fit the experimental images, suggest that the dissociation takes place from a more linear configuration than the Franck–Condon bending angle of 117°. The dissociation at most wavelengths results in a positive value of the anisotropy parameter, β, both in the experiment and in the calculations. Calculations indicate that both nonadiabatic transitions and intersystem crossings substantially reduce β below the nominal value of 2