Two-photon dissociation of carbon monoxide near 193 nm with a tunable ArF* laser to yield excited C(2 1D) atoms is reported. The atoms are detected by subsequent absorption of a third laser photon to C(3 1P°) followed by emission of 248 nm fluorescence to C(2 1S). The process is found to be isotopically sensitive; the C(2 1D) yield from 13C 16O and 12C 18O is a factor of 6 greater than from 12C 16O. The isotope effect is attributable to an enhancement in the two-photon matrix element due to the shift in the near resonant a 2Π, v=2 intermediate state. C2 Swan band emission is also observed, arising from association of the free carbon atoms, and exhibits a corresponding isotope effect. Finally, collisional processes are observed involving excited carbon atoms and electrons, using two-photon ionization of a small quantity of added xenon atoms as a new technique for producing a controlled density of free electrons.
Photolytic studies performed at 193 nm demonstrate that NO in the highly excited D(v = 1,5) and E(v = 0) states is generated from N2O during irradiation in three sequential steps involving photodissociation, chemical reaction, and photoexcitation. The resulting NO fluorescence (160–230 nm) was analyzed with a system of rate equations, and the temporal behavior, intensity dependence, and pressure dependence were found to be consistent with a simple kinetic model. The quenching coefficient of NO by N2, Ar, and N2O were determined in this analysis to be qN2 = (2.7±0.8)×10−11 cm3 sec−1, qAr = (6.6±1.4)×10−11 cm3 sec−1, and qN2O = (1.5±0.4)×10−10 cm3 sec−1. Finally, dramatic changes in the spectral distribution of the ultraviolet NO fluorescence due to collisions with He were observed, which contrasts with the absence of spectral redistribution in collisions involving N2, Ar, and N2O.
The observation of two-photon excitation of selected 6p levels in krypton atoms using tunable Arp* laser radiation at 193 nm is reported. Using time-resolved detection of visible fluorescence lines in the vicinity of 430 nm arising from 6p-Ss transitions, radiative lifetimes for the 6p[1/2]0 and 6pP/2], states of 123+5 and 115+5 nsec, and collisional self-quenching rates of (4.1+0.4) X 10 ' cm'/sec and (6.7+0.7) X 10 cm'/sec, respectively, have been determined. By carefully measuring the visible fluorescence intensity as a function of incident laser intensity, the photoionization cross section of the 6p[3/2], state has been established as (3.2+2.0) X 10 ' cm', . The two-photon transition rates for both states have been theoretically calculated, and good agreement is found with measurements of the relative excitation rates for the two transitions.
polymer has photophysical properties very similar to those of the parent complex and that the structure of the polymer must remain sufficiently open so that the accessibility to quenchers is not greatly reduced. While the charge on the polymer does affect the quenching by MV2+ and the back-reaction between the Ru(II1) center and MV", it is also apparent that interactions between the heteroaromatic moieties of the polymer and viologen quenchers also play a significant role. It is clear therefore that if the charge of the polymer is to be used to control the yield of electron transfer (e.g., of MV"), then it will be necessary to use a polymer where the polymer is less hydrophobic than PVPEt'. Acknowledgment.We are grateful to Johnson-Matthey Ltd. for a loan of ruthenium and to Trinity College, Dublin, Dublin County Council, and the Department of Education for support (to P.M.E.).We report the clean and efficient conversion of silane to disilane by C 0 2 laser irradiation. The direct irradiation of pure silane at high pressures (from 75 to 1700 Torr) converts silane to disilane with high selectivity and with efficient use of the absorbed laser radiation. Hydrogen is the only other major volatile product, and the production of solid products is minimal. The proposed mechanism of the photochemical reaction includes (1) collisionally enhanced absorption of the laser radiation by silane, (2) collisional deexcitation of the vibrationally excited silane, (3) concurrent decomposition to SiH2 and H2, (4) production of vibrationally excited disilane by SiHz insertion into a silane Si-H bond, ( 5 ) collisional quenching of the excited disilane, and (6) rapid cooling of the irradiated gas by thermal expansion. We support the proposed mechanism by additional experiments and model calculations. IntroductionThe rapidly developing technology of amorphous silicon and hydrogenated amorphous silicon thin films require high-quality films that can be deposited a t high rate. The most developed technique for growing silicon films is chemical vapor deposition (CVD) by radio-frequency (rf) glow discharge. The deposition rate is high with this technique, and the efficiency exceeds 10%. However, the electric field accelerates charged particles that bombard the surface and damage the films. The use of disilane
Pressure-induced three-photon ionization of Kr atoms at 248 nm has been examined in mixtures involving Ar and Ne. This density-dependent effect is attributed to the role of a two-photon near resonance, which, although forbidden by parity in the free atom, acquires an allowed character in the two-center molecular system. The response of Ar/Kr mixtures was greater than that observed in pure krypton. It was found that these effects reveal both the detailed structure of the near-resonant molecular states and the variation of the participating matrix elements with internuclear separation. It was also determined that the three-body recombination of Kr* to Kr*2 when assisted by Ar, is governed by a rate constant whose value is (3.0±0.7) ×10−32 cm6/sec.
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