Direct Photodissociation Research Articles

Surface aligned photochemistry: Photodissociation of Cl2 and Cl2⋯Cl adsorbed on LiF(001)

Photodissociation of chlorine adsorbed on a LiF(001) surface at 25–70 K has been investigated by means of angularly resolved resonantly enhanced multiphoton ionization spectroscopy (REMPI). The translational-energy distributions and angular distributions for forming Cl(g) photofragments were determined. Photolysis was performed employing 351 nm radiation, with laser pulse energies of 0.3–1.2 mJ/cm2. A peak in the translational energy of Cl(g) at about 0.4 eV was identified as being due to the direct photodissociation of the Cl2(ad) molecule by 3.5 eV photons. Particular interest attached to the observation of a further channel (termed “A”) for photodissociation leading to Cl(g) with translational energy peaking at ∼1 eV and extending to 1.5 eV. The available photon energy renders it highly unlikely that this “high-energy” Cl(g) originates in Cl2(ad). Channel A had the same linear dependence of Cl-atom flux on laser pulse-energy as did the lower energy (0.4 eV) channel, termed “B,” but differed from it in exhibiting a slow approach to steady state. It appears that channel A requires the prior build-up of Cl(ad) concentration due to the photodissociation of Cl2. It is proposed that this leads to the formation of a steady-state concentration of Cl2⋯Cl which when photolyzed yields high-energy Cl(g) via channel A. Channel A exhibits a distinctive angular distribution at low coverage and a characteristic Cl*/Cl ratio, as compared with channel B. The suggested mechanism for channel A is Cl2⋯Cl+hν→Cl3*→Cl2•Cl→Cl2+Cl (where* is an electronically excited state and • represents repulsion in the lower electronic state to which Cl3* reverts). This mechanism is interpreted in terms of an extensive diatomics-in-molecules (DIM) model for the trichlorine radical, shown to be in agreement with high level ab initio multireference internally contracted configuration interaction (MRCI) calculations, and consistent with the observations.

Photolysis of CF3Cl adsorbed on Si(111)(7×7) surface by monochromatic synchrotron radiation

We present an analysis of the photolysis of an adsorbed molecule by monochromatic synchrotron radiation. The system studied was CF3Cl adsorbed on Si(111)(7×7) at 30 K, and the techniques employed were photoemission spectroscopy (PES) and photon-stimulated desorption (PSD). The valence-level photoemission spectra, under various photon exposures, show that the photolysis effect induced by the incident monochromatic synchrotron radiation photons (hν=98 or 110 eV) occurs on this adsorbed gas–solid system. Variations of the F− and F+ PSD ion yields were measured as a function of monochromatic (110 eV) photon exposure at three adsorbate coverages (the lowest dose=0.3×1015, the medium dose=0.8×1015, and the highest dose=2.2×1015 mol/cm2). For the lowest CF3Cl-dosed surface, the photon-exposure dependencies of the F− and F+ yields show the characteristics: (a) at early stages of photolysis, the desorption of F− yields is mainly due to dissociative attachment (DA) and dipolar dissociation (DD) of the adsorbed CF3Cl molecules induced by the photoelectrons emitting from the silicon substrate, while at high photon exposure the F− formation by electron capture of the F+ ion is the dominant mechanism; (b) the F+ ion desorption is associated with the bond breaking of the surface SiF which is formed by reaction of the surface Si atom with the neutral fluorine atom or F− ion produced by scission of the C–F bond of CF3Cl, CF2Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of the lowest CF3Cl-dosed surface. Based on this model and the variation rates of the F−/F+ signals during 110 eV photon bombardment, a photolysis cross section of ∼1.9×10−17 cm2 was determined. The photolysis cross section as a function of incident photon energy in the range 98–110 eV, near Si(2p), was also obtained, which shows a threshold at around 100 eV, corresponding to the same rise as the total electron yield (TEY) spectrum of the clean silicon substrate. This edge-jump indicates an enhancement of the photolysis efficiency by indirect excitation of the substrate core-level [Si(2p)] electrons, and confirms the substrate-mediated excitation as the photolysis mechanism for the lowest-dosed surface. However, for the medium- and the highest-dosed surfaces both the direct photodissociation and the substrate-mediated dissociation of the adsorbed CF3Cl occur at early stages of photolysis, while only the substrate-mediated dissociation was observed at higher photon exposure. For photolysis of all three surfaces the SiF is the only fluorosilyl product observed on the surface.

Photolysis of CH3I on Cu(110) at 337 nm: Direct and charge-transfer photodissociation

The photochemistry of methyl iodide adsorbed on Cu(110) surfaces has been studied using time-of-flight mass spectrometry, temperature programmed desorption, and retarding potential spectroscopy. The λ=337 nm photodissociation of CH3I adsorbed on clean and iodided Cu(110) is found to have an increased cross section (by ∼2 orders of magnitude) and altered dynamics from that of the gas phase at the same wavelength. On the clean Cu(110) surface both enhanced neutral photodissociation and charge-transfer photodissociation processes are observed, the latter being ascribed to hot photoelectrons generated in the substrate. On the Cu(110)–I surface, only enhanced neutral photodissociation is observed and the CH3I molecules are found to be orientationally ordered with a 20° tilt in the [11̄0] azimuth. Further evidence of altered neutral photodissociation dynamics is found in the observed I/I* branching ratio. In contrast to earlier studies of adsorbed CH3I, the branching ratio is found to favor the higher energy I* dissociation pathway as compared to the gas phase. The altered neutral photodissociation dynamics are ascribed to changes in the CH3I excited state potential energy surfaces, caused by interactions with the copper–iodine interface.

A test of the accuracy of the partially-separable time-dependent self-consistent-field approach

The accuracy of the time-dependent self-consistent-field (TDSCF) approach assuming partial factorization of the total wave packet is tested against an exact treatment, when applied to calculate asymptotic properties. The test is carried out in the framework of a three-dimensional simulation of the Ar–HCl UV photodissociation dynamics. All the partially-separable TDSCF ansatzs possible for this problem are investigated. The quality of the TDSCF results is found to be strongly dependent on the specific partially-separable ansatzs applied. In general, the TDSCF predictions are in very good (even quantitative) agreement with the exact ones for magnitudes associated with direct photodissociation dynamics, and are qualitative in the case of indirect photodissociation. The deviation of the TDSCF results from the exact dynamics is interpreted in terms of an error operator defined as the difference between the exact and the TDSCF Hamiltonians. The analysis of this operator also explains the different accuracy of the partially-separable ansatzs investigated. Based on this analysis, a simple procedure is suggested to estimate the relative average quality of the different TDSCF ansatzs.

RESONANCE RAMAN INTENSITY ANALYSIS OF DIRECTLY PHOTODISSOCIATING HALOALKANE AND DIHALOALKANE MOLECULES

We show some applications of resonance Raman intensity analysis to examine direct photodissociation reactions in polyatomic haloalkane molecules. These investigations indicate that direct C–I bond cleavage in iodoalkane and haloalkane molecules have distinctly multidimensional reaction coordinates that depend on the structure of the parent molecule. The Franck–Condon region photodissociation dynamics of the molecules studied seem to be qualitatively consistent with an impulsive ‘semi-rigid’ radical description of the photodissociation dynamics. The initial photodissociation dynamics also show intriguing correlation with the energy partitioning of the photoproducts as measured by time-of-flight transitional spectroscopy experiments. Gas and solution phase resonance Raman spectra and associated short-time photodissociation dynamics are usually very similar to one another except in cases where significant solvent symmetry breaking takes place (as in diiodomethane). Bond selective electronic excitation was investigated in several different dihaloalkane molecules and their short-time photodissociation dynamics exhibit some correlation with the amount of interaction between the two carbon-halogen chromophores. We discuss the ramifications of the initial photodissociation dynamics for energy partitioning and bond selective photochemistry.

Photodissociation of Ar–HCl: An energy-resolved study of the dynamics of total fragmentation into H+Ar+Cl

UV photolysis of Ar–HCl is simulated by means of an exact wave packet treatment in three dimensions. The focus of the work is on the mechanism of indirect dissociation of the hydrogen atom, which leads to total fragmentation of Ar–HCl into H, Ar, and Cl. The results predict for this photodissociation path a probability of about 13% of the photolysis process. The remaining probability would be associated with direct photodissociation of the H fragment. Kinetic-energy distributions of the hydrogen fragments produced by indirect photodissociation are calculated for different excitation energies of Ar–HCl. The distributions reflect a pronounced structure of peaks associated with broad and overlapping resonances of the system. The resonance structure is present in the whole energy range covered by the absorption spectrum. Hydrogen atoms initially populating the resonances can dissociate from the cluster extensively cooled down, after several collisions with Ar and Cl. A mechanism is suggested for the fragmentation process due to indirect photodissociation, which involves successive jumps of the hydrogen to lower-energy resonances, induced by the collisions. A classical collisional model is proposed to rationalize qualitatively the fragmentation dynamics.

Calculation of reduced partial cross sections of molecules photodesorbing from a cold crystal surface with internal vibrations: Inclusion of curve-crossing effects

A Gaussian wave packet/path integral (GWD/PI) method is used to compute final internal state distributions for a molecule photodesorbing from the surface of a zero-temperature crystal with internal vibrations in the situation where nonadiabatic coupling between two excited state potential surfaces is significant. The internal state distributions of the desorbed molecule are influenced by vast numbers of internal vibrational state transitions in the crystal which are not resolved in the calculation (or in experiment). A correlation function technique, introduced previously for the case of direct photodissociation on a single excited potential surface, is generalized to systems where two or more excited potential surfaces are nonadiabatically coupled. The accuracy of the method is successfully tested on a two-dimensional model for which numerically exact results can be computed. The method is then applied to a collinear model of a diatomic molecule photodesorbing from a chain of atoms coupled by Hooke’s law springs. While exact results cannot be obtained in this case, sum rule checks suggest that the results of the GWD/PI are of acceptable accuracy (fractional error of several percent). It is found that for the class of problems under study, which feature nonadiabatic coupling that decays to zero along the photodesorption coordinate, only a few paths through the electronic state space have significant weight. This suggests that the method can be utilized to treat more complicated problems.

Dewar pyridine studied by matrix isolation infrared spectroscopy and DFT calculation

Infrared spectra of Dewar pyridine, produced from pyridine in an argon matrix by UV irradiation, have been measured by matrix-isolation infrared spectroscopy. Vibrational assignments have been performed with an aid of the DFT calculation, where the 6-31++G ** basis set was used to optimize the geometrical structures. It has also been found that cyclobutadiene and HCN are produced by further photolysis of Dewar pyridine as well as by direct photodissociation of pyridine. The relative rate constants are estimated from the infrared intensity changes upon UV irradiation.

Photodissociation of a Surface-Active Species at a Liquid Surface: A Study by Time-of-Flight Spectroscopy

The photochemistry at a gas−liquid interface was investigated by time-of-flight/quadrupole mass spectroscopy (TOF/QMS). A thin liquid film of 4-iodobenzoic acid (IBA), dissolved in glycerol, was irradiated with nanosecond laser pulses at 275 nm. Atomic and molecular iodine were the only photoproducts leaving the liquid after a low-fluence laser pulse (<5 mJ/cm2). The amount of released I atoms was 2 orders of magnitude larger than the amount of desorbed I2. Model calculations were carried out that take into account laser photolysis of IBA, diffusion, and surface evaporation of I and I2, and the condensed-phase kinetics of radical reactions. Ejection of hyperthermal I atoms by direct photodissociation of surface layer molecules is also considered. The quantitative analysis is restricted to low laser fluence conditions at which laser-induced heating of the illuminated liquid is negligible. The results of the model calculations were compared to previously obtained TOF data of an alkyl iodide (C18H37I) dissolved in the apolar liquid squalane (C30H62). The velocity distribution of the iodine atoms from the alkyl iodide solution corresponds to the temperature of the liquid (278 K). The contribution of I atoms from depths greater than 1 nm is large (>99%). In contrast, I atoms desorbing from IBA/glycerol are hyperthermal (Ttrans = 480 K) and originate almost exclusively from the topmost molecular layer (1 nm). TOF measurements with a fast chopper wheel in front of the surface provide the time-dependent desorption flux from the surface and confirm that the contribution from deeper layers in the alkyl iodide solution is much larger than in the aryl iodide solution. Model calculations predict the behavior of the two solutions correctly if differences in caging of the geminate pair, diffusion coefficients of the free radicals, and the set of bulk radical reactions in the two solutions are taken into account. The hyperthermal energies of the ejected I atoms from the IBA solution are discussed in terms of the surface orientation of excited IBA molecules. The dependence of the TOF spectra on laser power and IBA concentration is interpreted by a surface excess of IBA. The result is compared to temperature-dependent surface tension measurements of IBA solutions in glycerol and water. The response of the surface tension to an accumulation of IBA at the surface is very weak.

Photoproducts ejected from liquid surfaces: The importance of photochemical, diffusional, kinetic, and surface structural effects

Gas-phase products of liquid surface photochemistry in high vacuum were analyzed by time-of-flight/quadrupole mass spectroscopy. A thin liquid film of a long-chain alkyl iodide, C18H37I, dissolved in squalane (C30H62) was irradiated with nanosecond laser pulses at 275 nm. The photoproducts leaving the liquid after a low-fluence laser pulse (0.5 mJ/cm2) were I, HI, and I2. Since these species may desorb at times delayed by diffusion in the liquid phase, time-of-flight profiles were also recorded using a chopper wheel in front of the surface. This allowed a reconstruction of the time-dependent flux from the surface. The flight time distributions were compared to model calculations which take into account laser photolysis of C18H37I, diffusion and surface evaporation of I, HI, and I2, and the condensed-phase kinetics of radical reactions, and allow for a component of direct photodissociation of surface layer molecules, leading to hyperthermal I atoms. Simulations based on a comprehensive kinetic scheme are in good agreement with our measurements, with no specific surface processes other than evaporation of thermalized species formed at, or diffusing to the surface. However, compared to results previously found for liquid C2H5I, the caging of the geminate pair is much stronger in our system, and the fraction of I atoms promptly reacting to HI is smaller. The absence of prompt hyperthermal I fragments is interpreted by a preferential orientation of the C18H37I molecules in the topmost liquid layer with the I atom pointing into the liquid. The sensitivity of the method is discussed with respect to surface-specific processes, as well as primary and secondary radical reactions occurring in the bulk liquid.

Raman investigation of submicro-grained Si films obtained by incoherent UV photo-CVD of silicon hydrides

Silicon-based thin films have been obtained by the direct photodissociation of mono- or disilane gases in a windowless CVD reactor using the vacuum ultraviolet (VUV) argon emission generated by electrical discharges. Atomic force microscopy, Raman scattering and elastic recoil detection analysis techniques were applied to characterize the surface morphology, the structural properties, and the hydrogen content of the deposits, respectively, as a function of the silicon hydride concentration in the feed gas and, to a lesser extent, of the substrate temperature. Independent of the hydride concentration, it is found that the layers have a low hydrogen content of the order of 5% to 10%. On the other hand, their surface roughness is limited to a few nanometers providing that the partial hydride gas pressure in the reactor does not exceed 10 Pa. Raman investigations show that two different types of material can be obtained: either a purely amorphous one for hydride concentrations not exceeding 10% or a mixture of Si submicrocrystals embedded in an amorphous medium for the higher concentrations. Given the conditions of the growth technique used and the photodissociation mechanisms involved, it is concluded that a homogeneous nucleation process occurs already within the gas phase, leading to the formation of nanosized Si grains, that may agglomerate before reaching the substrate where a further growing step eventually takes place.

Study of the total and partial fragmentation dynamics of Ar–HCl after uv photodissociation

The uv photolysis of the Ar–HCl cluster is studied applying an exact time-dependent wave packet method in three dimensions, assuming zero-total angular momentum. The photodissociation process is found to occur via two different fragmentation mechanisms, depending on the initial excitation energy of the cluster. One mechanism leads to total dissociation of the complex, producing three fragments, Ar–HCl+hν→H+Ar+Cl. The fragmentation dynamics in this case is governed by resonance states at relatively low energies of the cluster, in which the H atom collides a number of times with Ar and Cl before dissociating. Manifestations of these collisions are found in the final kinetic energy distribution of the photofragments, which is redshifted in the case of the H fragment, and blueshifted in the Ar and Cl cases. The second type of mechanism consists of a fast and direct photodissociation of the hydrogen, leading to a partial fragmentation of Ar–HCl into hot H fragments and bound Ar–Cl radical molecules. This mechanism dominates at higher energies, which are those mostly populated by the wave packet initially prepared in the present calculations. The experimental implications of the results are discussed.

Translational energy distributions of the products of the 193 and 157 nm photodissociation of chloroethylenes

The 193 and 157 nm photodissociations of three isomers of dichloroethylene (DCE) and trichloroethylene (TCE) were investigated using a technique of photofragmentation translational spectroscopy. The photofragmentation mechanisms were constructed by analyzing the time-of-flight spectra of C2H2+, Cl+, HCl+, C2HCl+, and C2Cl2+ produced by electron impact of neutral photofragments. In the 193 nm photodissociation, both the HCl elimination and the C–Cl bond rupture were important for all the compounds examined. It was concluded that secondary dissociation of the vibrationally excited chlorinated vinyl radical produced by the C–Cl bond rupture was important even at 193 nm. In the 157 nm photodissociation, the mechanisms were similar to those at 193 nm for cis-DCE, 1,1-DCE, and TCE, while only the C–Cl bond rupture occurred for trans-DCE. This result suggests that the 157 nm photodissociation of trans-DCE proceeds via the direct photodissociation following the photoexcitation to the repulsive nσ*1 state. A minor C–H bond rupture was also found in the 157 nm photodissociations of cis-DCE and TCE. On the basis of the present mechanisms, the translational energy distributions and the branching ratios were estimated for all the possible processes.

Quantum mechanical analysis of photofragment alignment near asymmetric resonances

Quantum mechanical analysis is presented for the alignment of the oxygen atoms produced from the photodissociation of OH. The alignment parameters are predicted to be independent of energy across the isolated Lorentzian resonances, when only one channel contributes to indirect dissociation. When more than one channel interferes with one another, they may change very slowly. Across the asymmetric resonances, the alignments exhibit rapid variations due to the quantum interference between the indirect and the direct dissociation pathways. The alignments of O(3P2) and O(3P1) exhibit different variations, both of which are asymmetric across the asymmetric resonances. It is also shown that photoexcitation to repulsive states, coupled with bound electronic states, can give asymmetric resonances and sharp variations of the alignment, suggesting that the analysis of the dynamics of direct photodissociation by measurements of vector properties could be complicated by the effects of quantum interference.

State resolved probe of an energetic surface reaction: Phosgene on silver

State resolved characterization of nascent gas phase products is used as probe for the dynamics of an energetic surface reaction. This is achieved in the photodissociation of monolayer phosgene adsorbed on Ag(110). Irradiation of adsorbed Cl2CO in a broad photon energy range (hν=1.9–6.4 eV) leads to dissociation, with Cl retained on the surface and CO desorbing into the gas phase. The translational energy of product CO (g), 〈Etrans〉=0.26 eV, is independent of hν, even at the threshold photon energy (1.9 eV). This result establishes a dissociative electron attachment mechanism involving a reactive intermediate, ClCO, whose prompt dissociation serves as a probe to surface dissociation dynamics. Consistent with translation, internal state distribution of product CO (g) also shows an energetic origin: The rotational distribution, with an overall flux-weighted mean rotational energy of 〈Erot〉=0.17 eV, can be approximated by a bimodal Boltzmann distribution with rotational temperatures of 700 K at low J(s) and 7000 K at high J(s); the relative vibrational population is Nν=1/Nν=0=0.30. Contrary to common expectation based on quenching rates, both translational and rotational energies of CO (g) from monolayer photodissociation are much higher than those from the direct photodissociation in multilayers. This is taken as evidence for concerted reaction dynamics on the surface: The high exothermicity in the Cl–Ag bond formation on the surface exerts part of the energy to the Cl–CO coordinate, leading to higher energies in CO (g).

A Wavepacket−Path Integral Method for Curve-Crossing Dynamics

A combined wavepacket−path integral method is proposed for computing the time evolution of nuclear coordinate wavepackets on several nonradiatively coupled potential energy surfaces. In order to make the technique useful for multidimensional problems, the method uses the Gaussian wavepacket dynamics (GWD) algorithm (Heller, E. J.; J. Chem. Phys. 1975, 62, 1544) to perform nuclear coordinate wavepacket evolution on successive individual surfaces as prescribed by the path integral formalism. Since the GWD algorithm is not exact for anharmonically coupled potential energy surfaces, this introduces some error into the calculation. However, as long as the GWD algorithm is sufficiently accurate to treat single-surface propagation on all of the relevant potential energy surfaces, the proposed method is expected to yield useful results. A simple illustration is given using a one-dimensional model of a direct photodissociation process involving two nonradiatively coupled electronic states. Good agreement with numerically exact results is obtained, even in the case of strong, and position-dependent, nonradiative coupling.

Photochemical growing of complex organics in planetary atmospheres

UV induced syntheses of organic compounds from the main atmospheric constituents can be a very important source of organics in a given planetary environment provided the atmosphere is in a reduced state. The evolution of a CO 2 rich medium only produces very low yields of formaldehyde and related oxygenated compounds. Considering a CO rich atmosphere, the photochemical yield of O-organics formation is much higher, when the synthesis of N-organics remains difficult. The most favourable atmosphere as far as photochemical organic synthesis is concerned is a CH 4 rich milieu.. The photochemical evolution of such a CH 4 atmosphere under UV irradiation leads to a chain of various organics, the complexity of which increases together with the number of pathways involved in their formation. Their complexity also closely correlates with their UV photoabsorption spectrum: the more complex they are, the more shifted is their UV spectrum toward the visible range. Direct photodissociation of methane requires UV photon of wavelengths shorter than about 145 nm. It mainly produces ethane which absorbs UV at wavelengths shorter than about 160 nm, and acetylene, that presents an absorption spectrum extending up to 200 nm. This shift still continuously increases with further increase in number of C atoms. Unsaturated hydrocarbons with 4 and more C atoms have UV absorption characteristics including noticeable band structures in the 250–300 nm range. This trend has very important implication in the photochemical behaviour of a CH 4-rich planetary atmosphere, as it induces many catalytic processes. The occurrence of such processes is closely related to vertical atmospheric and energy deposition profiles. Titan provides a very good example of such a UV-directed organic atmospheric chemistry.

Quantum dynamics within the multiconfiguration time-dependent Hartree approximation

A multiconfiguration version of the time-dependent Hartree approximation has been applied to various molecular systems, including direct photodissociation, vibrational predissociation and photodissociation on a solid surface. It is shown that the omission of dynamic correlations by the simple time-dependent Hartree approximation can be corrected by the multiconfiguration formalism and the results converge to the exact solution. Because of a different scaling law with the dimensionality of the problem, this method may find applications in large dimensional systems which are not accessible by conventional exact methods.

Photoinduced charge transfer reactions at surfaces: CF3I on Ag(111)

The surface photochemistry of submonolayer to multilayer amounts of CF3I, adsorbed on Ag(111) at 95 K, has been studied using 248 and 193 nm pulsed laser excitation. For low doses, there is some thermally activated dissociation, limited to 30% of the first monolayer, to form adsorbed CF3 and I. The remaining CF3I adsorbs molecularly. Neither CF3 nor I is photoactive, but adsorbed CF3I is photodissociated, by C–I bond cleavage, at both 248 and 193 nm. A fraction of the resulting CF3 and I desorbs during photolysis; the remainder is retained as chemisorbed CF3 and I. The former processes were probed using time-of-flight and Fourier transform mass spectrometry. The retained products were detected by post-irradiation temperature programmed desorption and Auger electron spectroscopy. The photochemistry varied with wavelength and coverage. Regarding the mechanism, for both 193 and 248 nm, there is good evidence that both submonolayer and multilayer CF3I molecules absorb photons and dissociate into CF3 and I, i.e., direct photodissociation. There is evidence, based on time-of-flight distributions of CF3 photofragments, that I(2p1/2), electronically excited I, is produced at both wavelengths, while ground state atomic iodine is produced only at 248 nm. At both 193 and 248 nm, and for coverages up to three monolayers, there is also evidence for a charge transfer process involving hot electrons produced by photon absorption in Ag(111), i.e., substrate mediated photodissociation. These hot carriers attach to CF3I, and the resulting anion dissociates into CF3 and I−. The latter is detected by Fourier transform mass spectrometry and the former as a low-velocity component in time-of-flight mass spectrometry. At 193, but not 248 nm, there is evidence for a second, substrate independent, charge transfer process also leading to CF3 and I−. For coverages exceeding ten monolayers, approximately 80% of the reaction was through this channel, the remaining 20% occurring through the direct photodissociation channel. To account for this second charge transfer channel, photoinduced intermolecular charge transfer is proposed.

Rydberg State Reactions of Atomic and Molecular Hydrogen

Various aspects of the atomic and molecular Rydberg state reactions that result in H[sub 3][sup +] formation by associative ionization and by chemiionization are presented. For associative ionization, the n = 2 Rydberg state was prepared both by direct photodissociation of H[sub 2] and D[sub 2] in the continuum both above the first dissociation limit and by photodissociation of H[sub 2] Rydberg states. Factors that combine to determine the onset of the high-n regime are presented. 87 refs., 10 figs., 3 tabs.