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The dissociation dynamics of methoxysulfinyl radicals generated from the photodissociation of CH3OS(O)Cl at 248 nm is investigated using both a crossed laser - molecular beam scattering apparatus and a velocity map imaging apparatus. There is evidence of only a single photodissociation channel of the precursor: S-Cl fission to produce Cl atoms and CH3OSO radicals. Some of the vibrationally excited CH3OSO radicals undergo subsequent dissociation to CH3 + SO2. The velocities of the detected CH3 and SO2 products show that the dissociation occurs via a transition state having a substantial barrier beyond the endoergicity; appropriately, the distribution of velocities imparted to these momentum-matched products is fit by a broad recoil kinetic energy distribution extending out to 24 kcal/mol in translational energy. Using 200 eV electron bombardment detection, we also detect the CH3OSO radicals that have too little internal energy to dissociate. These radicals are observed both at the parent CH3OSO+ ion as well as at the CH3+ and SO2+ daughter ions; they are distinguished by virtue of the velocity imparted in the original photolytic step. The detected velocities of the stable radicals are roughly consistent with the calculated barriers (both at the CCSD(T) and G3B3 levels of theory) for the dissociation of CH3OSO to CH3 + SO2 when we account for the partitioning of internal energy between rotation and vibration as the CH3OSOCl precursor dissociates. [ABSTRACT FROM AUTHOR]

ICl photolysis in the ultraviolet region of the spectrum (235-265 nm) is studied using the Slice Imaging technique. The Cl*(2P1/2)/Cl(2P3/2) and the I*(2P1/2)/I(2P3/2) branching ratio between the I(2P3/2) + Cl(2P3/2)/Cl*(2P1/2) and I*(2P1/2) + Cl(2P3/2)/Cl*(2P1/2) channels is extracted from the respective iodine and chlorine photofragment images. We find that ground state chlorine atoms (Cl(2P3/2)) are formed nearly exclusively with excited state iodine atoms (I*(2P1/2)), while excited spin-orbit chlorine atoms (Cl*(2P1/2)) are concurrently produced only with ground state iodine atoms (I(2P3/2)). We conclude that photolysis of ICl in this UV region is a relatively 'clean' source of spin-orbit excited chlorine atoms that can be used in crossed molecular beam experiments. [ABSTRACT FROM AUTHOR]

We report on the first observation of the organoxenon HXeCCH molecule in the gas phase. This molecule has been prepared in a molecular beam experiment by 193 nm photolysis of an acetylene molecule on Xen clusters (n≈390). Subsequently the molecule has been oriented via the pseudo-first-order Stark effect in a strong electric field of the polarized laser light combined with the weak electrostatic field in the extraction region of a time-of-flight spectrometer. The experimental evidence for the oriented molecule has been provided by measurements of its photodissociation. For comparison, photolysis of C2H2 on Arn clusters (n≈280) has been measured. Here the analogous rare gas molecule HArCCH could not be generated. The interpretation of our experimental findings has been supported by ab initio calculations. In addition, the experiment together with the calculations reveals information on the photochemistry of the HXeCCH molecule. The 193 nm radiation excites the molecule predominantly into the 2 1Σ+ state, which cannot dissociate the Xe–H bond directly, but the system evolves along the Xe–C coordinate to a conical intersection of a slightly nonlinear configuration with the dissociative 1 1Π state, which then dissociates the Xe–H bond. [ABSTRACT FROM AUTHOR]

The CH fragment from the 193 nm photodissociation of CHCl is observed in a molecular beam experiment. This fragment is formed in the higher-energy dissociation pathway, the lower pathway involving formation of CCl. Both the CHCl parent molecule and the CH fragment were detected by laser-induced fluorescence. The 193 nm CHCl absorption cross section was estimated from the reduction of the CHCl signal as a function of the photolysis laser fluence. The CH internal state distribution was derived from the analysis of laser-induced fluorescence spectra of the A-X Δv=0 sequence. A modest degree of rotational excitation was found in the CH fragment; the most probable rotational level is N=1, but the distribution has a tail extending to N>25. Also observed is a slight preference for formation of Λ-doublets of A″ symmetry, which appears to increase with increasing rotational angular momentum N. Vibrationally excited CH was observed, and the degree of vibrational excitation was found to be low. The energy available to the photofragments is predominantly released as translational excitation. The preferential formation of A″ Λ-doublets suggests that dissociation occurs through a nonlinear excited state. [ABSTRACT FROM AUTHOR]

Photodissociation dynamics for various tryptophan chromophores was studied at 193 or 248 nm using multimass ion imaging techniques. The competition between internal conversion to the ground electronic state and dissociation from the repulsive excited state reveals size-dependent photostability for these amino acid chromophores. As the size of chromophore increases, internal conversion to the ground state becomes the major nonradiative process. For tryptophan and larger chromophores, dissociation directly from the repulsive state is completely quenched. [ABSTRACT FROM AUTHOR]

The formation of neutral I2 by the photodissociation of the methyl iodide dimer, (CH3I)2, excited within the A band at 249.5 nm is evaluated using velocity map imaging. In previous work [J. Chem. Phys. 122, 204301 (2005)], we showed that the formation of I2+ from photodissociation of the methyl iodide dimer takes place via ionic channels (through the formation of (CH3I)2+). It is thus not possible to detect neutral I2 by monitoring I2+. Neutral I2 is detected in this study by monitoring I atoms arising from the photodissociation of I2. Iodine atoms from I2 photodissociation have a characteristic kinetic energy and angular anisotropy, which is registered using velocity map imaging. We use a two-color probe scheme involving the photodissociation of nascent I2 at 499 nm, which gives rise to I atoms that are ionized by (2+1) resonance enhanced multiphoton ionization at 304.67 nm. Our estimate of the yield of nascent I2 is based on the comparison with the signal from I2 at a known concentration. Using molecular beams with a small fraction of CH3I (1% in the expanded mixture) where smaller clusters should prevail, the production of I2 was found to be negligible. An upper estimate for the quantum yield of I2 from (CH3I)2 dimers was found to be less than 0.4%. Experiments with a higher fraction of CH3I (4% in the expanded mixture), which favor the formation of larger clusters, revealed an observable formation of I2, with an estimated translational temperature of about 820 K. We suggest that this observed I2 signal arises from the photodissociation of several CH3I molecules in the larger cluster by the same UV pulse, followed by recombination of two nascent iodine atoms is responsible for neutral I2 production. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of CBr4 at 267 nm has been studied using time of flight (TOF) mass spectrometry and ion velocity imaging techniques. The photochemical products are detected with resonance enhanced multiphoton ionization (REMPI) as well as single-photon vacuum ultraviolet ionization at 118 nm. REMPI at 266.65 and 266.71 nm was used to detect the ground Br(2P3/2) and spin-orbit excited Br(2P1/2) atoms, respectively. The translational energy and angular distributions are consistent with direct dissociation from an excited triplet state and indirect dissociation from high vibrational levels on the singlet ground state surface. Br2+ ions are also observed in the TOF spectra with a focused 267 nm laser. The counter fragment, CBr2+, is observed when this photolysis laser is unfocused, and photons at 118 nm are used to ionize the radical products. The translational energy distributions of the CBr2+ and Br2+ products can be momentum matched, which indicates that molecular Br2 elimination is one of the primary dissociation channels. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of N2O in the wavelength region of 203–205 nm was studied by velocity map ion imaging. A speed resolution of 0.8% was obtained using standard projection imaging and subpixel centroiding calculations. To investigate N2O dissociation starting from the excited bending levels in the ground electronic state, a supersonic molecular beam and an effusive beam were used. The photoabsorption transition probability from the first excited bending level in the wavelength region of 203–205 nm was estimated to be seven times greater than that from the ground vibrational level. [ABSTRACT FROM AUTHOR]

We investigated the photodissociation of ethylene and its isotopomers at 157 nm in a molecular-beam apparatus using photofragment translational spectroscopy combined with synchrotron-based photoionization. The time-of-flight TOF spectra of all photofragments H, H2, C2H2, C2H3, and their deuterium isotopic variants were recorded, from which kinetic-energy distributions P(Et) and branching ratios were obtained. Most C2H3 spontaneously dissociates to C2H2+H and only C2H3 with small internal energy survives. The C2H2 fragment due to H2 elimination is observed leading the C2H2 fragment due to 2H elimination in TOF distribution because the former process has more kinetic-energy release. An analogous result is observed for C2D4 photolysis. That elimination of molecular hydrogen is site-specific and is revealed from photolysis of three dideuterated ethylene isotopomers, in which an isotopic effect plays a significant role. Observations of C2D2+2H and C2H2+2D product channels in the photolysis of 1,1-CH2CD2 provide evidence for migrations of H and D atoms. A comparison with previous experimental and theoretical results is made. [ABSTRACT FROM AUTHOR]

Ultraviolet photodissociation of NHD[sub 2] excited to the fourth overtone state of the NH stretching mode (5ν[sub NH]) and NH[sub 2]D excited to that of the ND stretching mode (5ν[sub ND]) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD[sub 2] in the 5ν[sub NH] state, the NH dissociation cross section is 5.1±1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH[sub 2]D in the 5ν[sub ND] state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68±0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH[sub 2]D and NHD[sub 2] [Koda and Back, Can. J. Chem. 55, 1380 (1977)], the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3–4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The interaction of free electrons with the potent greenhouse molecule SF[sub 5]CF[sub 3] is studied under different degrees of aggregation: single molecules at collision free conditions, clusters within a supersonic molecular beam and condensed molecules. Electron collisions with single molecules are dominated by SF[sub 5][sup -] formation produced via dissociative electron attachment (DEA) within a resonance located below 2 eV. In clusters, undissociated parent anions SF[sub 5]CF[sub 3][sup -] (and larger complexes containing undissociated anions) are observed in addition to the fragment ions. This indicates that (i) SF[sub 5]CF[sub 3] possesses a positive adiabatic electron affinity and (b) low energy attachment is partly channeled into nondissociative processes when the molecule is coupled to an environment. Electron impact to condensed phase SF[sub 5]CF[sub 3] exhibits a remarkably strong F[sup -] desorption signal appearing from a pronounced resonance located at 11 eV while in the gas phase at 11 eV only a weak DEA signal is observed. Electron induced desorption from sub-monolayers of SF[sub 5]CF[sub 3] on an amorphous H[sub 2]O ice surface is found to be more efficient compared to desorption of SF[sub 5]CF[sub 3] from a Xe surface. The implications of these results for the heterogeneous photochemistry of SF[sub 5]CF[sub 3] adsorbed on ice or dust particles in the Earth’s atmosphere are discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser-molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin-orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin-orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes. [ABSTRACT FROM AUTHOR]