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The photodissociation of water in the first absorption band, H20(X) + ftoi -* H20(Á'B1) -H(1 2S) + (2 ), is a prototype of fast and direct bond rupture in an excited electronic state. It has been investigated from several perspectives-absorption spectrum, final state distributions of the products, dissociation of vibrationally excited states, isotope effects, and emission spectroscopy. The availability of a calculated potential energy surface for the Á state, including all three internal degrees of freedom, allows comparison of all experimental data with the results of rigorous quantum mechanical calculations without any fitting parameters or simplifying model assumptions. As the result of the confluence of ab initio electronic structure theory, dynamical theory, and experiment, water is probably the best studied and best understood polyatomic photodissociation system. In this article we review the joint experimental and theoretical advances which make water a unique system for studying molecular dynamics in excited electronic states. We focus our attention especially on the interrelation between the various perspectives and the correlation with the characteristic features of the upper-state potential energy surface.

The chemical dynamics of the reactions of O(3 P) with saturated hydrocarbons. II. Theoretical model Molecular beam-laser induced fluorescence experiments have probed the nascent internal state distributions and excitation functions of OH formed in the technologically important reactions OCP) + RH---+OH + R·. RH is a saturated hydrocarbon and R· is an alkyl radical. A variety of RH have been investigated corresponding to abstraction of primary, secondary, and tertiary hydrogens. The OH rotational state distribution is nearly identical for all RH and decrease rapidly from its peak at the lowest rotational level. This demonstrates that reaction occurs when OCP) is collinear to a C-H bond in the hydrocarbons. The vibrational state distribution of OH depends markedly on the type of hydrogen abstracted, with vibrational excitation increasing dramatically across the series primary to secondary to tertiary. This is interpreted as a shift from a repulsive towards a more attractive surface across the series. Partitioning into the OH spin doublets shows that these reactions are midway between the adiabatic and diabatic limits with respect to the spin-orbit surfaces. Excitation functions measure the dynamic thresholds, and are in good agreement with activation energies obtained from thermal rate constants. The excitation functions for v = I OH exhibit a sharp decrease at energies considerably above threshold. This suggests that excitation of internal modes of R· occurs only at high collisions energies. All of these results are interpreted qualitatively in terms of a simple, but general, triatomic model for the interaction of Oep) with RH, i.e., where R· can be considered a structureless particle.

SummaryIn this article we present the development of a multibeam two-photon laser scanning microscope. A new type of beam splitter to create the multitude of laser beams is described. This type of beam splitter has higher transmission and generates more uniform beams than can be achieved with the microlens approach used by other groups. No crosstalk exists between the different foci due to small temporal delays between the individual beams. The importance of dispersion compensation to obtain maximum efficiency of the microscope is discussed. With optimum compensation the fluorescence signal was raised by a factor of 14. Different modes of detecting the fluorescence signals and their effect on imaging speed and resolution are discussed.

The photodissociation of water in its first absorption band is studied by photolyzing H2O at 157 nm with an excimer laser. This dissociation proceeds directly to produce the electronic ground states of H and OH. Both nascent internal state distributions and alignment of the product OH (2Π) are probed by laser induced fluorescence. This is done with both warm (300 K) and cold (∼10 K) water. About 88% of the excess energy is translation, 10% vibration, about 2% rotation. The first three vibrational levels 0, 1, 2 have population ratios 1:1:0.15, respectively. The rotational distributions depend strongly upon the H2O temperature and are very different for the upper and lower energy components of the Λ doublets, which are measured via Q and P, R lines, respectively. For Q lines, the distributions can be described by rotational temperatures which are 930 K for warm and 475 K for cold water, a surprising difference. For P,R lines strong deviations from Boltzmann behavior are found for cold H2O. The spin distribution is almost statistical. A strong J dependent Λ-doublet population inversion is found from cold H2O, but there is no inversion from warm H2O. The inversion provides a possible pump mechanism for the astronomical OH maser and is simply explained by approximate symmetry conservation. The orientation of the unpaired pπ lobe in OH in the upper Λ-doublet state is measured to be perpendicular to the OH rotation plane. The J dependence of the inversion is explained by Λ-doublet mixing in OH and quantitatively described in terms of the singly occupied pπ-lobe in the excited water and the orientation of the corresponding singly occupied pπ-lobe in OH. The alignment of OH is measured by polarizing both lasers. The large polarization effects are strongly dependent upon J and also upon the temperature of H2O. It is shown that the dependence is related both to Λ-doublet mixing and hyperfine structure of OH. For the cold H2O the data indicate, despite the strong J dependence of both polarization and Λ-doublet inversion, a completely planar dissociation process. It is shown that due to Λ-doublet mixing the transition moment of Π molecules has a J dependent angle relative to the OH rotation plane which approaches the high J limit at the same rate that the molecule shifts from Hund’s case (a) to case (b). The model for the J dependence of the Λ-doublet population and the polarization is important for chemical reactions, surface scattering and other processes where Π molecules are analyzed with LIF.

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A new method for instantaneous temperature field measurements based on LIF studies of OH, O(2), and H(2)O in an open atmospheric flame with a tunable excimer laser is suggested. In this method the crucial problem of quenching at higher pressures is almost completely eliminated by excitation to a fast predissociating state. The various possible excitation and fluorescence processes that can be induced in the narrow tuning range of the KrF laser are characterized experimentally by excitation and dispersion spectra for the three molecules OH, O(2), and H(2)O. Of particular importance is the large power of the KrF laser, which allows efficient excitation of even weak transitions. The fast predissociation of these molecules in connection with the powerful excitation laser suggests that instantaneous temperature field measurements should be possible at higher pressures.

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