Vacuum-ultraviolet photoionization molecular-beam mass spectrometry is a means of identifying primary photodissociation products and determining their recoil energies. At several photolysis wavelengths between 220 and 320 nm, we have observed three primary photodissociation pathways for nitrobenzene. Two of the pathways are C6H5NO2 →C6H5+NO2 and C6H5NO2→C6H5NO+O. The third pathway produces NO by one or both of the processes C6H5NO2→C6H5O+NO and C6H5NO2→C5H5+CO+NO. The relative yield of the pathways producing NO2 and NO varies strongly with the photolysis wavelength. The production of NO2 exceeds that of NO by about 50% for the 280 nm photolysis, but increases to almost a sixfold excess in 222 nm dissociation. The second pathway has a threshold energy that is about 0.50 eV greater than the thermodynamic limit for the formation of nitrosobenzene (C6H5NO) and an oxygen atom from nitrobenzene, probably reflecting the energy required to produce triplet nitrosobenzene and, perhaps, a barrier to dissociation on the triplet surface. The distribution in arrival times for a fragment provides an estimate of the recoil energy at each photolysis wavelength in these experiments. The channel producing nitric oxide (NO) radicals releases a relatively large amount of kinetic energy. Assuming the channel producing nitric oxide (NO) also produces phenoxy (C6H5O), we calculate a linear increase in kinetic energy from 0.29 eV at 320 nm to 1.1 eV at 220 nm. By contrast, the other two channels release only a small amount of kinetic energy (≊0.1 eV) at all wavelengths. An impulsive model does not describe the observed kinetic energy release for these low energy channels, suggesting that the energy release is more nearly statistical. The recoil energy predicted by an impulsive model for the channel producing nitric oxide and phenoxy radicals is closer to the observed kinetic energy release.

Laser induced fluorescence probing of the nitric oxide fragment determines the distribution of rotational and vibrational energies of NO produced in the 226 and 280 nm photolysis of nitrobenzene. Combining these results with kinetic energy measurements using vacuum ultraviolet photoionization to detect the fragment gives a detailed view of the energy release in the photolysis. Boltzmann distributions describe the rotational state populations at both photolysis wavelengths. The rotational temperature of NO from the 226 nm photolysis is (3700±350) K, corresponding to an average rotational energy of (0.32±0.03) eV, and that of NO from the 280 nm photolysis is (2400±200) K, corresponding to an average rotational energy of (0.20±0.03) eV. We observe no vibrationally excited NO and place an upper limit of 10% on the fraction of nitric oxide produced in any one vibrationally excited state. Two different limiting models, impulsive energy release and statistical energy redistribution, both correctly predict much more rotational than vibrational excitation, but neither completely describes the observed internal and kinetic energies. The impulsive model finds more NO rotational and translational energy, but much less phenoxy fragment internal energy than we observe. The statistical model does better for the NO rotation and phenoxy fragment internal energy, but underestimates the translational energy substantially. A combination of these two types of behavior provides a physical picture that qualitatively explains our observations. It is likely that statistical energy redistribution occurs during the approach to the transition state for isomerization of nitrobenzene to phenyl nitrite and impulsive energy release dominates during the subsequent rupture of the CO–NO bond.

We present analytical expressions relating the bipolar moment β(Q)(K)(k(1)k(2)) parameters of Dixon to the measured anisotropy parameters of different pump/probe geometry sliced ion images. In the semi-classical limit, when there is no significant coherent contribution from multiple excited states to fragment angular momentum polarization, the anisotropy of the images alone is sufficient to extract the β(Q)(K)(k(1)k(2)) parameters with no need to reference relative image intensities. The analysis of sliced images is advantageous since the anisotropy can be directly obtained from the image at any radius without the need for 3D-deconvolution, which is not applicable for most pump/probe geometries. This method is therefore ideally suited for systems which result in a broad distribution of fragment velocities. The bipolar moment parameters are obtained for NO(2) dissociation at 355 nm using these equations, and are compared to the bipolar moment parameters obtained from a proven iterative fitting technique for crushed ion images. Additionally, the utility of these equations in extracting speed-dependent bipolar moments is demonstrated on the recently investigated NO(3) system.

Consider irreversible cooperative filling of sites on an infinite lattice where the filling rates ki depend on the number, i, of occupied sites adjacent to the site(s) being filled. If clustering is significantly enhanced relative to nucleation (k1/k0≡ρ≫1), then the process is thought of as a competition between nucleation, growth, and (possible) coalescence of clusters. These could be Eden clusters with or without permanent voids, Eden trees, or have modified but compact structure (depending on the ki, i≥1). Detailed analysis of the master equations in hierarchial form (exploiting an empty-site shielding property) produces results which are exact (approximate) in one (two or more) dimensions. For linear, square, and (hyper)cubic lattices, we consider the behavior of the average length of linear strings of filled sites, lav=J∞s=1 sls/J∞s=1 ls, where ls is the probability of a string of length s [lav=(1−CTHETA)−1 for random filling, at coverage CTHETA]. In one dimension, ls=ns gives the cluster size distribution, and we write lav=nav. We consider the scaling lav∼A(CTHETA)ρω as ρ→∞ (with CTHETA fixed), which is elucidated by the introduction of simpler models neglecting fluctuations in cluster growth or cluster interference. For an initially seeded lattice, there exists an upper bounding curve lav+ for lav (as a function of CTHETA), which is naturally obtained by switching off nucleation (setting k0=0). We consider scaling of lav+ as the initial seed coverage ε vanishes. The divergence, lav∼C(1−CTHETA)−1 as CTHETA→1, is also considered, focusing on the cooperativity dependence of C. Other results concerning single-cluster densities and ls behavior are discussed. Disciplines