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Notes: A method is presented for computing the effective center-of-mass velocity distribution for photofragments produced by sequential bond ruptures using Fourier transforms. The method has the advantage that energy and linear momentum conservation are correctly accounted for while remaining computationally feasible. To illustrate the method, the one- and two-photon photodissociation of Fe(CO)5 at 248 nm has been experimentally investigated using the crossed laser-molecular beam method and the measured velocity distributions compared to the prediction of various statistical models for the photodissociation process calculated by the Fourier transform method. The strength of the Fourier transform method is illustrated by the two-photon channel which involves five sequential bond ruptures. The main conclusion regarding the photodissociation mechanism is that a modified form of the separate statistical ensemble theory developed by Wittig and co-workers can quantitatively explain the observed velocity distributions for the one-photon process. Fluxional interchange of the CO ligands is shown not to be important on the CO elimination time scale. The two-photon products have a faster than statistical translational energy distribution which is rationalized by both a dynamical constraint on CO rotational excitation and on the change in orbital– and spin–coupling configuration of the Fe atom as the last two CO ligands are removed.

Notes: Photodissociation of the group VIB (Cr,Mo,W) hexacarbonyls has been studied at 248 nm using molecular beam photofragment spectroscopy. One, two and three photon processes have been observed. Analysis of the product velocity distributions shows that the photodissociation mechanism consists of sequential CO eliminations with the nth photon channel best described as the single photon photodissociation of the stable products of the n-1st photon channel. The product translational energy distribution for the first CO elimination step is quantitatively similar for all three hexacarbonyls and characteristic of a repulsive translational energy release. The product translational energy distributions of all subsequent CO elimination steps are accurately described by a simple, microcanonical model. Qualitative molecular orbital considerations suggest that the large product translational energy observed in the first CO elimination step results from a repulsive σ interaction between the closed shell CO ligand and an excited molecular orbital which has a significant admixture of metal (n+1)pz, (n+1)s and ndz2 orbitals. This repulsive interaction is absent in the remaining CO elimination steps because there are vacancies in the coordination shell along the z axis.

Notes: A molecular beam of zinc–diethyl (ZnEt2) is photodissociated at 248 and 193 nm and the velocity distributions of the photofragments are measured by time-of-flight techniques. One and two photon processes are observed. The dominant one photon process at both wavelengths is the dissociation of (ZnEt2)2 to form two ZnEt2 monomers. The absence of secondary dissociation of the ZnEt2 photofragments at both excitation wavelengths and the small fraction of the available energy partitioned to product translation implicates dissociation to an excited electronic potential energy surface correlating to one electronically excited ZnEt@B|2 monomer. The mass spectrum of the ZnEt2 photofragments is the same as measured for "cold'' ZnEt2 monomers in the molecular beam, suggesting that the electronically excited ZnEt@B|2 monomers have fluoresced prior to ionization in the mass spectrometer. A small photodissociation signal of uncomplexed ZnEt2 is observed only at low expansion pressures. The sensitive dependence of this monomeric photodissociation signal to the Ar pressure of the adiabatic expansion suggests that ground state vibrational excitation is required for monomeric photodissociation at 248 nm.In contrast to the dimer single photon photodissocation channel, when ZnEt2 monomers are photodissociated, a significant fraction of the available energy appears as product translation. A qualitative molecular orbital analysis can explain the observed fast photoproduct velocity if dissociation occurs via a repulsive triplet state which correlates to electronic ground state products. The two photon process observed is assigned to single photon photodissociation of the electronically excited ZnEt@B|2 monomers produced in the dimer photodissociation step. The photofragment velocity distributions for the two photon channel can be quantitatively modeled by sequential ethyl eliminations on the ground state ZnEt2 and ZnEt potential energy surfaces. The product velocity distributions are consistent with a microcanonical energy distribution for both ethyl eliminations. Approximately 50% of the ethyl photofragments are created with sufficient vibrational energy to break the weak ethyl C–H bond (36 kcal/mol) forming ethylene. Implications of the ZnEt2 photodissociation mechanism for Zn film deposition using 248 and 193 nm excimer radiation are discussed.

Notes: A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 A(ring)2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193 nm can be quantitatively described as a statistical, unimolecular decomposition. At 248 nm, the measured product velocity distributions are qualitatively consistent with the mechanism deduced from the 193 nm results, but the energy available for translation at this wavelength is too small to extract quantitative producttranslational energy distributions which are required to independently test the applicability of the statistical dissociation model.

Notes: A crossed laser-molecular beam study of the photodissociation mechanism of Fe(CO)5 has been performed at 193 nm where time-of-flight measurements of the primary iron containing photofragments have been recorded under collision free conditions. The center-of-mass velocity distributions derived from the TOF data by the method of moments show that Fe(CO)2 accounts for 〉99% of all photoproducts formed after absorption of one photon. The only mechanism which quantitatively reproduces the measured velocity distributions is a sequence of three, uncorrelated, statistical CO eliminations. At high photon flux, a second photon can be absorbed by the Fe(CO)2 photofragment which decomposes by an uncorrelated sequential elimination of the remaining two CO ligands.

Notes: A molecular beam study of the Cl2+GaAs(s) reaction has been performed for surface temperatures in the range of 300–550 K. The gas phase neutral reaction products are identified by mass spectroscopy using electron bombardment ionization. Detailed analysis of the surface temperature dependence of the mass spectrum of the observed reaction products indicates that only three neutral reaction products are formed in this temperature range: GaCl3, AsCl3, and As4. At low (high) surface temperatures, only AsCl3 (As4) is observed. The ratio of the etching rates of Ga and As is independent of the surface temperature and within the range expected for stoichiometric etching. The change in the mode of As removal with surface temperature for the incident Cl2 flux implies that surface diffusion is important at surface temperatures above 400 K.