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.

Total collision cross sections (Q) for the interaction of atomic beams of K and Cs with a number of molecules were measured wi~h an apparatus of 30" angular resolution. Although absolute determinations of Q are difficult, relative values are readily obtained (±3%). Results are reported as the ratio (Q*) of the cross section for a given molecule to that of argon for the same beam atom. Seventy-seven molecules (of varied complexity and reactivity) were studied with K and 16 with Cs beams. Q* ranged from 0.29 to 2.8. The data were correlated using the Massey-Mohr theory, assuming an attractive intermolecular potential V(r) =-Cjr. For this case Q=b(Cjv r)2/5, where Vr is the relative velocity and b a known constant. C was estimated from standard formulas for the London dispersion and dipole-induced dipole forces, using known refraction and dipole moment data. The theoretical values of Q differ by a nearly constant factor from the experimental results; thus values of Q* are predicted with good accuracy. The deviation between Qcalc * and Qob, * was < ±3% for 57% (and < ± 10% for 87%) of the molecules. Most of the large deviations occurred for the light gases.

Many molecular processes, such as chemical reactions, inelastic collisions, photodissociation, and surface scattering, yield selective populations of Λ-doublet states. The interpretation of such experiments has been difficult, so that little quantitative information could be extracted. Based on the results of our experiment on the photodissociation of H2O, and upon calculations presented here, this situation becomes much better. We discuss processes which yield product 2Π diatomics XY in which the Λ doublets are a manifestation of two different orientations of an unpaired pπ electron orbital. Chemical dynamics which produce such selective populations indicate a stereochemical effect: for example, the unpaired pπ orbital in the XY may point in the direction of the previous bond between XY and the transition complex, and, in an energetic breakup, would have a spatial relationship to J, the total angular momentum of XY. We describe here: (a) the directions of the orbitals at high J, (b) the effect of J, and of the particular XY studied, upon the analysis, so that J-dependent chemical dynamics can be separated from those intrinsic to the molecule, (c) the effect of rotation of the reaction complex, and (d) the close relationship to polarization experiments, both LIF and chemiluminescence. We also calculate the J-dependent degree of alignment of electron density, and its dependence upon the electronic parity.

Tunable excimer lasers are being used to produce species-, space-, and time-resolved images of complex gaseous media. These media may be analyzed for composition, density, temperature, or flow velocities. The techniques are, in general, highly selective, sensitive, and nonintrusive and are being made possible by recent technological developments in these UV lasers and in intensified cameras, imaging spectrographs, and fast digital image processing. We describe the needs for laser diagnostics in combustion, the physical mechanisms, the relevant spectroscopy, typical experimental setups, and equipment considerations. Precision and accuracy are discussed on the basis of some simple, but realistic, calculations intended to guide the experimentalist in design considerations and to reveal potential sources of errors in the often difficult conversion of raw data to values for such quantitative parameters as densities or temperatures. Finally we present an overview of previous results, select some examples that show the power of tunable excimer laser diagnostics in combustion, and present some suggestions for future directions.