D. J. Donaldson

Ultraviolet absorption spectroscopy of dissociating molecules: Effects of cluster formation on the photodissociation of CH3I

The ultraviolet absorption spectra of jet‐cooled CH3I, (CH3I)2, and (CH3I)n, n>2 are measured in the valence A state and the Rydberg B state. A significant blue shift of the valence state is observed upon cluster formation. The magnitude of this shift yields a dimer bond strength of at least 500 cm−1 which implies extensive dimer formation at room temperature and moderate pressures. The methyl iodide B‐state spectrum consists of a single progression in the ν2 mode. Dimerization retards the predissociation and therefore increases the excited state lifetime; consequently many new modes are observed which in the monomer are coupled to the dissociation. The implications of this result to the photodissociation dynamics are discussed. The Rydberg spectra of (CH3I)n, n>2 show asymmetric line shapes which may be due to interferences between the B state and an underlying continuum. The nature of these interferences and the possible indentity of this continuum are discussed.

Time-resolved FTIR photofragment emission spectroscopy: HCl vibrational distributions from the 193 nm photolysis of chloroethylenes

Abstract With minor modifications to commercial instrumentation, time-resolved FTIR emission spectroscopy is demonstrated for the determination of photofragment internal state distributions. Vinyl chloride and dichloroethylene photolysis at 193 nm serve as test cases and exhibit high signal-to-noise. The vibrational distributions in the first four levels of the HCl photo-product of vinyl chloride and 1,2-trans-dichloroethylene are in good agreement with previous determinations.

Photofragmentation dynamics of acetone of 193 nm: State distributions of the CH3 and CO fragments by time- and wavelength-resolved infrared emission

The photolysis of acetone at 193 nm is known to produce two methyl radicals and CO following excitation of a 1(n,3s) Rydberg transition. Vibrational excitation is detected in both products immediately following the dissociating laser pulse by observing the resulting infrared emission. Vibrational distributions are obtained for CH3(ν3) and for CO. These are, for CH3(ν3): v=1/2/3=0.73±0.05/0.14±0.05/0.13±0.05 and for CO: v=1/2/3=0.75±0.05/0.16±0.05/0.09±0.05. An approximate rotational temperature of 1500 K can be used to fit the CH3(ν3) emission spectrum. The CO is formed with very high, non‐Boltzmann rotational excitation. This result strongly suggests that the three‐body dissociation occurs via a two‐step mechanism, rather than a rigorously concerted process. The high rotational excitation is most likely imparted by the kinematics in the breakup of a bent acetyl fragment.

A two-laser pulse-and-probe study of T-R, V energy transfer collisions of H+NO at 0.95 and 2.2 eV

Vibrational, rotational, and spin‐orbit state distributions are obtained for inelastic collisions of H+NO at 2.2 and 0.95 eV. The H atoms are generated by excimer laser photolysis of H2S at 193 and 248 nm, respectively, and the excited states of the NO molecules are probed by laser‐induced fluorescence using a tunable dye laser. The rotational state distribution accompanying the T‐V excitation of v=1–3 at 2.2 eV is approximately characterized by a Boltzmann distribution at 1275 K, and is essentially independent of the vibrational level excited. At 0.95 eV, the rotational populations are approximately characterized by a 1050 K distribution. In each case, the temperatures of the spin‐orbit state populations and the rotational states are the same. No selective population of Λ‐doublet states is observed. The results are discussed in terms of chemical interactions between these two open‐shell species on the HNO potential energy surfaces.

Surface crossings and predissociation dynamics of methyl iodide Rydberg states

We report the results of a calculation of the adiabatic surface crossings between the B and C Rydberg states of CH3I and the dissociative A state. The bound potentials are derived empirically; the dissociative states are based on recent empirical and ab initio results. The surface crossings predissociate the B state at an energy below that of v=1 in the C–I stretching mode, and near the classical turning point for v=2 of the methyl umbrella mode. Our model predicts that at least two dissociative potentials are of importance in the B state predissociation. These results are in agreement with experimental predictions. Classical trajectory results suggest that the predissociation might exhibit mode specificity. The formation of methyl iodide dimers shifts the surface crossings to higher energy, which allows the absorption to v=1 of the C–I stretch and v=2 of the umbrella mode to be observed. This result is also in agreement with experiment.

Spontaneous fission of atmospheric aerosol particles

We show that spontaneous division of atmospheric aerosol particles is thermodynamically possible, in spite of the corresponding increase in surface area, when there is a highly compressed organic surfactant coating on the particle. This class of atmospheric aerosol is thought to be present (and potentially important) in both marine and urban atmospheres. Fully-coated particles have been suggested to have played a role in the origin of cell-based life. The possibility of their spontaneous division has important implications for atmospheric chemistry, radiative transfer and the evolution of life on Earth.

Fourier transform UV/VIS emission spectroscopy of jet-cooled CN(B 2Σ+)

CN(B 2Σ+) is produced in a corona-excited supersonic expansion and its fluorescence spectrum is measured with a Fourier transform UV/VIS spectrometer. Rotational temperatures as low as 75 K and vibrational temperatures on the order of 2200 K are achieved in the expansion. Under some expansion conditions, rotational levels as high as K′ = 66 are observed. We report new D0 and γ0 values for the B-state based on an analysis of P-branch line positions and spin-rotation splittings. These values are: D0 = 0.6626 × 10−5 cm−1 and γ0 = 1.553 × 10−2 cm−1. The power and sensitivity of this technique for the study of the electronic spectroscopy of highly reactive molecules is discussed.

Photooxidation of CS2 in the near‐ultraviolet and its atmospheric implications

Sequential two photon photolysis of CS2 is discussed. Experiments in our lab and others have shown that photolysis of CS2 and O2 mixtures with light of energies below the SC-S bond dissociation energy produces OCS and SO2. We explain these results by the two photon sequential absorption and subsequent dissociation of CS2 to CS+S. The implications of this mechanism for the oxidation of CS2 in the atmosphere are discussed. Comparison to the currently accepted OH-initiated oxidation mechanism suggests that the sequential two-photon mechanism is too slow to be important in the atmosphere.

Photodissociation of Methyl Iodide Clusters

The dissociation of methyl iodide molecules “solvated” in clusters has been investigated using both direct absorption spectroscopy and multiphoton ionization methods. Clusters of both neat methyl iodide and of methyl iodide with rare gases were studied in a molecular jet. It was found that dimerization slows the predissociation rate from the Rydberg states of CH3I, whereas in large clusters the direct dissociation from the valence state is slowed. We present a model which explains the effect of CH3I dimer formation on the pre-dissociation dynamics. Evidence is also presented for electron delocalization in higher clusters after excitation into the Rydberg states.