Search

The photodissociation of O3 at 266 nm has been studied using velocity mapped ion imaging. We report temperature-dependent vector correlations for the O2(a1Δg, v = 0, j = 18–20) fragments at molecular beam temperatures of 70 K, 115 K, and 170 K. Both the fragment spatial anisotropy and the v-j correlations are found to be increasingly depolarized with increasing beam temperature. At all temperatures, the v-j correlations for the j = 19 state were shown to be reduced compared to those of j = 18 and 20, while no such odd/even rotational state difference was observed for the spatial anisotropy, consistent with previous measurements. We find that temperature-dependent differences in the populations and v-j correlations between the odd and even rotational states can be explained by a Λ-doublet propensity model. Although symmetry conservation should lead to formation of only the A′ Λ-doublet component, and only even rotational states, out-of-plane rotation of the parent molecule breaks the planar symmetry and permits the formation of the A″ Λ-doublet component and odd rotational states. A simple classical model to treat the effect of parent rotation on the v-j correlation and the odd/even rotational population alternation reproduces both the current measurements and previously reported rotational distributions, suggesting that the "odd" behavior originates from a Λ-doublet propensity, and not from a mass independent curve crossing effect, as previously proposed. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of acetone (CH3)2CO, cooled in a molecular beam, have been explored over the wavelength range 266-312 nm. Nascent CH3 fragments were detected by resonance-enhanced multiphoton ionization, followed by mass-selected ion imaging. For photolysis at λ = 306 nm, the image shows a sharp ring, which, when converted to a translational energy distribution, reveals a narrow Gaussian peak with a maximum at 90% of the available energy. As the photolysis energy is increased, the distribution slowly broadens and shifts to higher recoil translational energy. The fraction of available energy in translation energy decreases in favour of internal energy of the CH3CO fragment. These observations are consistent with a dynamical model in which the energy of the exit channel barrier on the T1 surface evolves mostly into relative translational energy. Energy in excess of the barrier is partitioned statistically into all degrees of freedom. No evidence was found for any other dynamical pathway producing CH3 fragments, including reaction on S0 or S1, for dissociation between 306 and 266 nm. For λ > 306 nm, a diffuse, slow recoil component to the image appears. The translational energy distribution for this component is fit well by a statistical prior distribution of energy. We attribute this component to dissociation on the S0, ground state surface; to our knowledge, this is the first direct observation of this channel. The appearance of S0 dynamics and the disappearance of the T1 component are consistent with previously inferred barrier height on T1 for the production of CH3CO + CH3. The possible atmospheric implications of our findings are discussed. [ABSTRACT FROM AUTHOR]

We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to easily resolve photo-electron recoil in (2 + 1) resonant enhanced multiphoton ionization of N2 photoproducts from N2O or from molecular beam cooled N2. Second, rapid changes between spatial imaging, velocity mapping, and slice imaging are straightforward. We demonstrate this technique's utility in a re-investigation of the photodissociation of N2O. Using a hot nozzle, we observe slice images that strongly depend on nozzle temperature. Our data indicate that in our hot nozzle expansion, only pure bending vibrations - (0, v2,0) - are populated, as vibrational excitation in pure stretching or bend-stretch combination modes are quenched via collisional near-resonant V-V energy transfer to the nearly degenerate bending states. We derive vibrationally state resolved absolute absorption cross-sections for (0, v2 < 7, 0). These results agree well with previous work at lower values of v2, both experimental and theoretical. The dissociation energy of N2O with respect to the O(¹D) + N2¹Σg+ asymptote was determined to be 3.65 ± 0.02 eV. [ABSTRACT FROM AUTHOR]

Photodissociation dynamics of benzyl alcohol, C6H5CH2OH and C6H5CD2OH, in a molecular beam was investigated at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed, including OH elimination and H2O elimination from the ground electronic state, H atom elimination (from OH functional group), and CH2OH elimination from the triplet state. The dissociation rate on the ground state was found to be 7.7 × 106 s-1. Comparison to the potential energy surfaces from ab initio calculations, dissociation rate, and branching ratio from Rice-Ramsperger-Kassel-Marcus calculations were made. [ABSTRACT FROM AUTHOR]

We investigated the dynamics of photodissociation of propenal (acrolein, CH2CHCHO) at 157 nm in a molecular beam and of migration and elimination of hydrogen atoms in systems C3H4O and C3H3O using quantum-chemical calculations. Compared with the previous results of photodissociation of propenal at 193 nm, the major difference is that the C3H3O fragment present at the 193-nm photolysis disappears at the 157-nm photolysis whereas the C3H2O fragment absent at 193 nm appears at 157 nm. Optimized structures and harmonic vibrational frequencies of molecular species with gross formula C3H2-4O were computed at the level of B3LYP/6-311G(d,p) and total energies of those molecules at optimized structures were computed at the level of CCSD(T)/6-311+G(3df,2p). Based on the calculated potential-energy surfaces, we deduce that the C3H3O fragment observed in the photolysis of propenal at 193 nm is probably CHCCHOH (2A″) and/or CH2CCOH (2A″) produced from an intermediate hydroxyl propadiene (CH2CCHOH) following isomerization. Adiabatic and vertical ionization potentials of eight isomers of C3H3O and two isomers of C3H2O were calculated; CHCCHOH (2A″) and CH2CCOH (2A″) have ionization potentials in good agreement with the experimental value of ∼7.4 eV. We also deduce that all the nascent C3H3O fragments from the photolysis of propenal at 157 nm spontaneously decompose mainly to C2H3 + CO and C3H2O + H because of the large excitation energy. This work provides profound insight into the dynamics of migration and elimination of hydrogen atoms of propenal optically excited in the vacuum-ultraviolet region. [ABSTRACT FROM AUTHOR]

We use a combination of crossed laser-molecular beam scattering experiments and velocity map imaging experiments to investigate the three primary photodissociation channels of chloroacetone at 193 nm: C-Cl bond photofission yielding CH3C(O)CH2 radicals, C-C bond photofission yielding CH3CO and CH2Cl products, and C-CH3 bond photofission resulting in CH3 and C(O)CH2Cl products. Improved analysis of data previously reported by our group quantitatively identifies the contribution of this latter photodissociation channel. We introduce a forward convolution procedure to identify the portion of the signal, derived from the methyl image, which results from a two-step process in which C-Cl bond photofission is followed by the dissociation of the vibrationally excited CH3C(O)CH2 radicals to CH3 + COCH2. Subtracting this from the total methyl signal identifies the methyl photofragments that result from the CH3 + C(O)CH2Cl photofission channel. We find that about 89% of the chloroacetone molecules undergo C-Cl bond photofission to yield CH3C(O)CH2 and Cl products; approximately 8% result in C-C bond photofission to yield CH3CO and CH2Cl products, and the remaining 2.6% undergo C-CH3 bond photofission to yield CH3 and C(O)CH2Cl products. [ABSTRACT FROM AUTHOR]

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 use a combination of crossed laser-molecular beam experiments and velocity map imaging experiments to investigate the primary photofission channels of chloroacetone at 193 nm; we also probe the dissociation dynamics of the nascent CH3C(O)CH2 radicals formed from C-Cl bond fission. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence another photodissociation channel of the precursor, C-C bond fission to produce CH3CO and CH2Cl. The CH3C(O)CH2 radical formed from C-Cl bond fission is one of the intermediates in the OH + allene reaction en route to CH3 + ketene. The 193 nm photodissociation laser allows us to produce these CH3C(O)CH2 radicals with enough internal energy to span the dissociation barrier leading to the CH3 + ketene asymptote. Therefore, some of the vibrationally excited CH3C(O)CH2 radicals undergo subsequent dissociation to CH3 + ketene products; we are able to measure the velocities of these products using both the imaging and scattering apparatuses. The results rule out the presence of a significant contribution from a C-C bond photofission channel that produces CH3 and COCH2Cl fragments. The CH3C(O)CH2 radicals are formed with a considerable amount of energy partitioned into rotation; we use an impulsive model to explicitly characterize the internal energy distribution. The data are better fit by using the C-Cl bond fission transition state on the S1 surface of chloroacetone as the geometry at which the impulsive force acts, not the Franck-Condon geometry. Our data suggest that, even under atmospheric conditions, the reaction of OH with allene could produce a small branching to CH3 + ketene products, rather than solely producing inelastically stabilized adducts. This additional channel offers a different pathway for the OH-initiated oxidation of such unsaturated volatile organic compounds, those containing a C=C=C moiety, than is currently included in atmospheric models. [ABSTRACT FROM AUTHOR]

Singly and doubly charged scandium-water ion-molecule complexes are produced in a supersonic molecular beam by laser vaporization. These ions are mass analyzed and size selected in a specially designed reflectron time-of-flight spectrometer. To probe their structure, vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared laser photodissociation and the method of rare gas atom predissociation, also known as 'tagging.' The O-H stretches in these systems are shifted to lower frequency than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. These effects are more prominent for the doubly charged ions. Partially resolved rotational structure for the Sc+(H2O)Ar complex shows that the H-O-H bond angle is larger than it is in the free water molecule. Fragmentation and spectral patterns indicate that the coordination of the Sc2+ ion is filled with six ligands (one water and five argons). [ABSTRACT FROM AUTHOR]

In order to measure the state selective double differential cross section of a reactive collision, the preparation of the reactants with defined initial velocities and quantum states in number densities high enough to achieve an acceptable count rate is most important. At the same time, secondary collisions have to be prevented in order to ensure that the nascent products are not thermalized. Usually, the best way to control the initial conditions is to use crossed molecular beams, but the number density decreases quadratically with the distance from the nozzle orifice which can be a problem, especially if a molecular product with a large number of populated states is to be analyzed state specifically by REMPI spectroscopy. In this contribution we would like to present a method for measuring the quantum state selective differential cross section of a photoinitiated reaction that combines the advantages of the PHOTOLOC technique (high reactant densities) and the parallel beams technique used by the groups of Kitsopoulos, Orr-Ewing, and Suits (defined relative velocity of the reactants). Moreover, an algorithm based on a Bayesian backward reconstruction developed by W. H. Richardson [J. Opt. Soc. Am. 62, 55 (1972)] has been derived. Both, one reactant and the precursor of the other reactant, are present in the same molecular beam and the center of mass velocity is selected by shifting the dissociation and the detection laser in time and space. Like in comparable methods, this produces a bias in the measured velocity distribution due to the fact that the reaction takes place in the whole volume surrounding the laser beams. This has been also reported by Toomes et al. in the case of the parallel beams technique and presents a general problem of probing reaction products by REMPI spectroscopy. To account for this, we develop a general approach that can be easily adapted to other conditions. The bias is removed in addition to deconvolution from the spread in reactant velocities. Using the benchmark system O(1D)+D2 with N2O as the precursor, we demonstrate that the technique is also applicable in a very general sense (i.e., also with a large spread in reactant velocities, products much faster than reactants) and therefore can be used also if such unfortunate conditions cannot be avoided. Since the resulting distribution of velocities in the laboratory frame is not cylindrically symmetric, three dimensional velocity mapping is the method of choice for the detection of the ionized products. For the reconstruction, the distance between the two laser beams is an important parameter. We have measured this distance using the photodissociation of HBr at 193 nm, detecting the H atoms near 243 nm. The collision energy resulting from the 193 nm photodissociation of N2O is 5.2±1.9 kcal/mol. Our results show a preference for backward scattered D atoms with the OH partner fragment in the high vibrational states (v=4–6), in accord with previously published results claiming the growing importance of a linear abstraction mechanism for collision energies higher than 2.4 kcal/mol. [ABSTRACT FROM AUTHOR]

This study utilized a mass-resolved detection of ClOOCl to determine its photodissociation cross section, which is the product of the absorption cross section and dissociation quantum yield. An effusive molecular beam of ClOOCl was generated and its photodissociation probability was determined through measuring the decrease in the ClOOCl beam intensity upon laser irradiation. By comparing with a reference molecule, the absolute cross sections of ClOOCl were obtained without knowing its absolute concentration. The determined cross section of ClOOCl at 248.4 nm is (8.85±0.42)×10-18 cm2 at 200 K, significantly larger than previously reported values. The temperature dependence of the cross section was investigated at 248.4 nm in the range of 160–260 K; only a very small and negative temperature effect was observed. Because 248.4 nm is very close to the peak of the UV absorption band of ClOOCl, this work provides a new calibration point for normalizing relative absorption spectra of ClOOCl. In this work, the photodissociation cross section at 266 nm and 200 K was also reported to be (4.13±0.21)×10-18 cm2. [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 of 3-(methylthio)propylamine and cysteamine, the chromophores of S atom containing amino acid methionine and cysteine, respectively, was studied separately in a molecular beam at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed for 3-(methylthio)propylamine, including (1) CH3SCH2CH2CH2NH2→CH3SCH2CH2CH2NH+H, (2) CH3SCH2CH2CH2NH2→CH3+SCH2CH2CH2NH2, (3) CH3SCH2CH2CH2NH2→CH3S+CH2CH2CH2NH2, and (4) CH3SCH2CH2CH2NH2→CH3SCH2+CH2CH2NH2. Two dissociation channels were observed from cysteamine, including (5) HSCH2CH2NH2→HS+CH2CH2NH2 and (6) HSCH2CH2NH2→HSCH2+CH2NH2. The photofragment translational energy distributions suggest that reaction (1) and parts of the reactions (2), (3), (5) occur on the repulsive excited states. However, reaction (4), (6) occur only after the internal conversion to the electronic ground state. Since the dissociation from an excited state with a repulsive potential energy surface is very fast, it would not be quenched completely even in the condensed phase. Our results indicate that reactions following dissociation may play an important role in the UV photochemistry of S atom containing amino acid chromophores in the condensed phase. A comparison with the potential energy surface from ab initio calculations and branching ratios from RRKM calculations was made. [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]

Using hexapole quantum state-selection of OCS (v2=0,1,2|JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v2=0,1,2|JlM)+hν→CO(J)+S(1D2) was measured for bending states up to v2=2. The population density of the state-selected OCS (v2=0,1,2|JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60–67. Integrating over all J states the authors find σ(v2=0):σ(v2=1):σ(v2=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle. [ABSTRACT FROM AUTHOR]

Using hexapole quantum state-selection of OCS (v2=0,1,2|JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v2=0,1,2|JlM)+hν→CO(J)+S(1D2) was measured for bending states up to v2=2. The population density of the state-selected OCS (v2=0,1,2|JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60–67. Integrating over all J states the authors find σ(v2=0):σ(v2=1):σ(v2=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle. [ABSTRACT FROM AUTHOR]

Photodissociation of water clusters doped with HX(X=Br,Cl), molecules has been studied in a molecular beam experiment. The HX(H2O)n clusters are dissociated with 193 nm laser pulses, and the H fragments are ionized at 243.07 nm and their time-of-flight distributions are measured. Experiments with deuterated species DBr(H2O)n and HBr(D2O)n suggest that the photodissociation signal originates from the presence of the HX molecule on the water cluster, but does not come directly from a photolysis of the HX molecule. The H fragment is proposed to originate from the hydronium molecule H3O. Possible mechanisms of the H3O production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the H3O+ ion remains rather immobile. [ABSTRACT FROM AUTHOR]

Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and co-workers [J. Chem. Phys. 95, 3848 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH2C(O)Cl was prepared in its ground electronic state (S0) and excited with a laser at 248 nm to its first excited singlet state (S1). The two main ensuing photoreactions are the ruptures of the C–Cl bond and of the C–Br bond. A nonadiabatic model was proposed in which the C–Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S1 and S2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C–Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S0 and S1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C–Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C–Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations. [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]

The dynamics of the 193 nm photodissociation of the CCl2 molecule have been investigated in a molecular beam experiment. The CCl2 parent molecule was generated in a molecular beam by pyrolysis of CHCl3, and both CCl2 and the CCl photofragment were detected by laser fluorescence excitation. The 193 nm attenuation cross section was estimated from the reduction of the CCl2 signal as a function of the photolysis laser fluence. The internal state distribution of the CCl photofragment was derived from analysis of laser fluorescence excitation spectra in the A 2Δ-X 2Π band system. Most of the energy available to the CCl(X 2Π)+Cl fragments appears as translational energy. The CCl fragment rotational energy is much less than predicted in an impulsive model. The excited electronic state appears to dissociate indirectly, through coupling with a repulsive state arising from the ground-state CCl(X 2Π)+Cl asymptote. The identity of the initially excited electronic state is discussed on the basis of what is known about the CCl2 electronic states. [ABSTRACT FROM AUTHOR]

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2×1014 W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n=3→2), CH (A 2Δ, B 2Σ-, and C 2Σ+→X 2Π), or C2 (d 3Πg→a 3Πu) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A→X) emission of methane on a log-log scale has a slope of 10±1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation. [ABSTRACT FROM AUTHOR]

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278–281.5 nm) and below (355 nm) the A 2Π3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O(1D2) threshold the contribution of the direct excitation to states other than the A 2Π3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O(1D2) threshold provides an accurate and direct determination of the A 2Π3/2 state dissociation threshold of 35418±35 cm-1, leading to a ground state bond energy of D00(BrO)=55.9±0.1 kcal/mol. [ABSTRACT FROM AUTHOR]

We report the photodissociation of laboratory oriented OCS molecules. A molecular beam of OCS molecules is hexapole state-selected and spatially oriented in the electric field of a velocity map imaging lens. The oriented OCS molecules are dissociated at 230 nm with the linear polarization set at 45° to the orientation direction of the OCS molecules. The CO(ν=0,J) photofragments are quantum state-selectively ionized by the same 230 nm pulse and the angular distribution is measured using the velocity map imaging technique. The observed CO(ν=0,J) images are strongly asymmetric and the degree of asymmetry varies with the CO rotational state J. From the observed asymmetry in the laboratory frame we can directly extract the molecular frame angles between the final photofragment recoil velocity and the permanent dipole moment and the transition dipole moment. The data for CO fragments with high rotational excitation reveal that the dissociation dynamics is highly nonaxial, even though conventional wisdom suggests that the nearly limiting β parameter results from fast axial recoil dynamics. From our data we can extract the relative contribution of parallel and perpendicular transitions at 230 nm excitation. © 2004 American Institute of Physics. [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]

The ab initio/Rice–Ramsperger–Kassel–Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of benzene at various wavelengths upon absorption of one or two UV photons followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G2M level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for C6H5+H, C6H4+H2, C4H4+C2H2, C4H2+C2H4, C3H3+C3H3, C5H3+CH3, and C4H3+C2H3 have been calculated subsequently using both numerical integration of kinetic master equations and the steady-state approach. The results show that upon absorption of a 248 nm photon dissociation is too slow to be observable in molecular beam experiments. In photodissociation at 193 nm, the dominant dissociation channel is H atom elimination (99.6%) and the minor reaction channel is H2 elimination, with the branching ratio of only 0.4%. The calculated lifetime of benzene at 193 nm is about 11 μs, in excellent agreement with the experimental value of 10 μs. At 157 nm, the H loss remains the dominant channel but its branching ratio decreases to 97.5%, while that for H2 elimination increases to 2.1%. The other channels leading to C3H3+C3H3, C5H3+CH3, C4H4+C2H2, and C4H3+C2H3 play insignificant role but might be observed. For photodissociation upon absorption of two UV photons occurring through the neutral “hot” benzene mechanism excluding dissociative ionization, we predict that the C6H5+H channel should be less dominant, while the contribution of C6H4+H2 and the C3H3+C3H3, CH3+C5H3, and C4H3+C2H3 radical channels should significantly increase. © 2004 American Institute of Physics. [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 correlated angular and product rotational state distributions obtained in the 221.67 nm photodissociation of (NO)[sub 2] yielding NO(A [sup 2]Σ[sup +])+NO(X [sup 2]Π) have been examined in the molecular beam using the velocity map ion imaging technique. The translational energy and angular distributions of selected rotational states of NO(A [sup 2]Σ[sup +]) products in N=0, 5, 6 for which the maximum energies available to the NO(X [sup 2]Π) products are 202.5, 142.5, and 118.5 cm-1, respectively, have been measured. The recoil anisotropy parameter of the photofragments, β[sub eff], is 1.2±0.1, less than that previously measured at 213 nm (1.36±0.05). The correlated product state distributions near dissociation threshold agree with the predictions of phase space theory. These experimental results, as well as those obtained previously at 213 nm, are compared to statistical calculations, including v·J correlations. Application of the β-E[sub T] correlation model to the 213 nm results indicates that [NO(A,N),NO(X,J)] pairs with high NO(X,J) rotational levels are produced preferentially via planar dissociation, in contrast to the statistical expectation of the v·J correlation, which reveals no preference for planar dissociation. A mechanism involving vibrational predissociation with restricted intramolecular vibrational energy redistribution can explain both the observed scalar and vector properties. Specifically, the low frequency torsional (out-of-plane) mode does not couple efficiently to the other modes, especially at higher excess energies when the dissociation is rapid. On the other hand, the long-range attraction between NO(A) and NO(X), which is revealed both in the photodissociation dynamics of the dimer and in the quenching of NO(A) by NO(X), encourages long-range mode couplings and can explain the largely statistical rotational state distributions observed near threshold. From images obtained near threshold, the bond energy of the NO dimer in the ground state is determined to be 710±10 cm-1, in good agreement with previous results. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photodissociation of azulene at 193 nm was studied in a molecular beam using multimass ion imaging techniques. Most of the azulene molecules excited by 193 nm photon quickly relax to the ground electronic state through internal conversion, then isomerize to naphthalene, and eventually dissociate through the H atom elimination channel with a rate of 5.1 × 10[sup 4] s[sup -1]. A small amount of azulene entering different isomerization channels was found. The effect of dissociation in the energy transfer experiments using azulene as a vibrationally highly excited molecule and the existence of azulene in an interstellar medium is discussed. [ABSTRACT FROM AUTHOR]

Ultraviolet photolysis of highly vibrationally excited NH[sub 3](&Xtilde;[sup 1]A[sup ', sub 1]) in the 5 ν[sub N-H] band has been studied using a crossed laser and molecular beams method. Relative cross sections of N-H bond dissociation via the à ← &Xtilde; transition have been determined by the measurement of the action spectrum, utilizing a (2 + 1) resonance enhanced multiphoton ionization scheme of product H atoms. The obtained cross section for the photolysis of the 4 ν[sub 1] + ν[sub 3] state (4 quanta in the symmetric stretching and 1 quantum in the antisymmetric stretching) was 1.23±0.06 times as large as that of the 5 ν[sub 1] state. To interpret the ratio of the obtained cross sections, we have evaluated the FranckCondon factors for these vibrational states, by utilizing time-dependent wave-packet calculations on the potential energy surfaces constructed by an ab initio molecular orbital procedure. [ABSTRACT FROM AUTHOR]

Angular distributions of selected rotational states of NO(A ²Σ[sup +], ν= 0) products obtained in the 213 nm photodissociation of (NO)[sub 2] have been determined in a molecular beam by using the photofragment ion imaging technique. Specifically, images of NO(A, ν=0) products in N=0, 11, and 19 have been recorded, for which the maximum energies available to the NO(X²H) products are 2038, 1774, and 1278 cm[sup -1], respectively. The recoil anisotropy parameter of the photofragments, β[sub eff], decreases significantly at low center-of-mass translational energies from its maximum value of 1.36±0.05, and depends strongly on the rotational angular momentum of the photoproducts. This behavior is described well by a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. For each of the observed NO(A) N states, highly rotating NO(X) levels are produced via planar dissociation, and the angular momenta are established at an interfragment separation of about 2.6 Å. For most of the center-of-mass translational energy range, both corotating and counterrotating fragments are produced, but at the lowest energies, only the latter are allowed. The correlated rotational energy distributions exhibit deviations from the behavior predicted by phase space theory, suggesting that exit-channel dynamics beyond the transition state influences the product state distributions. In this study, a new method for image reconstruction is employed, which gives accurate angular distributions throughout the image plane. [ABSTRACT FROM AUTHOR]

Photodissociation dynamics of cyclopropane at 157 nm has been investigated using a new molecular beam apparatus based on vacuum ultraviolet ionization by synchrotron radiation. Four distinct dissociation pathways have been observed. The observed channels are the H formation process, the H[sub 2] formation process, the CH[sub 2] + C[sub 2]H[sub 4] formation process, and the CH[sub 3] + C[sub 2]H[sub 3] formation process. Experimental results indicate that the H atom products are possibly produced via a synchronous, concerted 2H elimination process, while other channels are all binary dissociation processes. Product kinetic energy distribution of each dissociation channel has been determined from simulating the experimental time of flight spectra. Relative branching ratios for all observed dissociation channels were also estimated based on all detected products. [ABSTRACT FROM AUTHOR]

Reports on crossed laser-molecular beam scattering experiments measuring the photofragment velocities and product branching in the photodissociation of trimethylamine at 193 nanometer. Observation of two primary N-CH[sub 3] bond fission channels; Contribution of a third minor channel to the dissociation dynamics; Calibration of mass spectrometry sensitivity.

Investigates the photodissociation dynamics of propyne and allene in two molecular beam/photodissociation instruments. Use of electron impact ionization and tunable vacuum ultraviolet light to photoionize the photoproducts; Primary dissociation channels for both reactants; Results of the measurement of the photoionization efficiency curves on the instruments.

Studies the photodissociation of the vinyl radical at 243.2 nm in a molecular beam using the technique of velocity map imaging. Primary product using the momentum conservation; Need for an out-of-plane motion of the third hydrogen atom to bring the excited states; Derivation of the heat of formation of vinylidene.

Photofragment angle-velocity distributions are measured in a molecular beam using a new two-dimensional imaging technique. We report on the detection of weak perpendicular transitions for CH3I A state photodissociation at 266 and 304 nm. The ratio of cross sections and the 3Q0-1Q crossing probability are σ⊥/σ||=0.00-0.10 and p=0.22-0.27 at 266 nm and σ⊥/σ||=0.20-0.30 and p=0.43-0.48 at 304 nm. [ABSTRACT FROM AUTHOR]

By combining molecular beam photodissociation and photoionization measurements with ab initio Gaussian-2 (G2) calculations on the CH3S2 and CH3S+2 systems, we have shown that CH3SS is the dominant isomer formed in the photodissociation process, CH3SSCH3+hν(193 nm) →CH3S2+CH3. The experimental ionization energy for CH3SS (8.97±0.02 eV) and the heat of formation at 0 K for CH3SS+ (217.7±1.2 kcal/mol) are in excellent agreement with the G2 results. The photoionization efficiency spectrum observed for CH3SS is also consistent with the theoretical prediction that the Franck–Condon factor for the photoionization process, CH3SS+hν →CH3SS++e-, is not favorable. Based on the statistical modeling of experimental rates obtained previously for HS loss in the unimolecular decomposition of CH3SSCH+3 and the comparison with G2 ab initio predictions, we conclude that CH2SSH+ is most likely the isomer structure formed near the experimental appearance energy (11.07 eV) observed for the photodissociative ionization process, CH3SSCH3+hν→CH3S+2+CH3+e-. [ABSTRACT FROM AUTHOR]

We observe the photodissociation of SiH4 in a pulsed molecular beam after excitation with 125.1 nm vacuum ultraviolet light generated via resonant four-wave mixing in mercury vapor. Ultraviolet radiation from a Nd:YAG/dye laser combination ionizes the neutral photodissociation fragments and a time-of-flight mass spectrometer detects the ions. The photodissociation signal consists entirely of silicon atoms and silylidyne (SiH) in its first electronically excited state. We see no silylene (SiH2) or silyl radicals (SiH3). Thus, the photodissociation cleaves almost all of the silicon–hydrogen bonds, but the energetics of the dissociation require the production of at least one hydrogen molecule per dissociation event. These results imply that the high energy content of the initially excited Rydberg state prevents formation of the silylene and silyl radicals in stable vibronic states and that dissociation pathways exist that connect the Rydberg state directly to the corresponding silicon atom and silylidyne asymptotes. These pathways are likely to exist because of Jahn–Teller distortion from the initial Td symmetry. Very little of the available energy appears as kinetic energy of the fragments but rather as electronic excitation of the products. Our results differ from those of earlier studies that concluded silylene and silyl are the principle products. [ABSTRACT FROM AUTHOR]

The photodissociation of azomethane following absorption of a single 351 nm photon was studied using the method of molecular beam photofragment translational spectroscopy. The dissociation was observed to proceed via cleavage of both C–N bonds to yield N2 and two methyl radicals. The measured time-of-flight spectra show evidence that the two methyl radicals possess unequal velocities in the azomethane center of mass suggesting that the dissociation is not symmetric. The angles between the asymptotic center-of-mass velocities for all three fragments are strongly correlated, implying that the methyldiazenyl radical (CH3N2) intermediate decomposes within a fraction of its rotational period. We conclude, therefore, that the dissociation is concerted, not stepwise as was inferred from recent time-resolved experiments. The overall translational energy distributions for all the photofragments in the azomethane center of mass reveal that an average of 60% of the total available energy appears as translation. A possible mechanism, consistent with the experimental findings, will be proposed and discussed. [ABSTRACT FROM AUTHOR]

The nascent rotational distributions of HCl (v=0, 1, and 2) generated in the photodissociation of three isomers of dichloroethenes (DCE) at 214 and 220 nm were measured under molecular beam conditions. HCl molecules were probed by a (2+1) resonantly enhanced multiphoton ionization technique combined with time-of-flight mass spectrometry. The rotational distributions of vibrationally excited HCl (v=1 and 2) molecules were Boltzmann-type, while those of HCl (v=0) could not be represented by a Boltzmann distribution and consisted of two components. These results suggest that there are more than two processes in the photodissociation of DCE. Cl(2P3/2) and Cl*(2P1/2) could also be detected when DCE were photodissociated. The branching ratios of Cl*(2P1/2) to Cl(2P3/2) obtained in the present work were much larger than those obtained at 193 nm. [ABSTRACT FROM AUTHOR]

This work measures the change in branching between the CN+I(2P3/2) and the CN+I(2P1/2) product channels when one photodissociates vibrationally excited rather than cold ICN at 248.5 nm. The crossed-laser molecular beam experiment tests a model for the dependence of branching at a conical intersection on the amplitude of the dissociative wave function at bent geometries. The results provide a critical comparison between the long-standing empirical surfaces for ICN photodissociation and more recent ab initio potential energy surfaces for ICN’s first absorption band. In the experiment, we observe an increase in branching from 49% to 58% I(2P3/2) products when the temperature of the ICN parent photodissociated is increased in going from a 100 °C vs a 400 °C nozzle expansion. We analyze the angular distributions of the photofragments to eliminate the possibility that the change in branching is due to an increased contribution from direct absorption to the electronic state correlating with I(2P3/2) products. We discuss the relationship between these studies, which mediate the effective off-diagonal potential coupling by forcing the nuclear dynamics to sample more strongly coupled regions of the intersecting potential surfaces, with intramolecular electron transfer studies which examine how the coupling depends on the particular conformer. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of difluorodiiodomethane CF2I2 following 248 nm excitation were studied using the time-of-flight crossed laser-molecular beam technique. There is clear evidence that CF2I2 undergoes exclusively a simultaneous three-body dissociation. Two different reaction channels are observed: the dominant one (86%) yields CF2+I(2P1/2)+I(2P3/2) while the less efficient one (14%) produces the same fragments but in the ground state CF2+I(2P3/2)+I(2P3/2). The angle I–C–I between the recoil velocity vectors of the two departing I atoms was determined to be 120 °. The measured anisotropy parameters of βCF2=-0.8 for the CF2 fragments and βI=+1.1 for the I atoms (in both reaction channels) imply that the excited state symmetry of CF2I[ATOTHER]@B|[/ATOTHER]2 is B1 (molecular symmetry C2v) and also indicate that the excited state lifetime is significantly shorter than a rotational period. Furthermore, the dissociation energy for the rupture of both C–I bonds was determined to be D0≤53 kcal/mol. Based on this dissociation energy a heat of formation for CF2I2 of ΔH0f, 0 K=-46 kcal/mol was calculated. [ABSTRACT FROM AUTHOR]

Molecule–surface collision-induced dissociation (CID) has been studied for n-C3F7NO and i-C3F7NO molecular beams scattered from MgO(100) at incident kinetic energies (Eincident) up to 7 eV. The NO fragment was detected state selectively using two-photon, two-frequency ionization, and rotational and spin–orbit distributions are reported for several Eincident values. State and angle-resolved signals were integrated to give CID yields, which increased sharply with Eincident . In most cases, rotational excitation could be described by separate temperatures for each spin–orbit state. The upper 2∏3/2 state was underpopulated relative to statistical predictions (e.g., for n-C3F7NO at Eincident =5.0 eV, the spin–orbit temperature was ∼170 K, while Trot was ∼500 K). The CID results are compared to NO state distributions derived from the photodissociation of expansion-cooled molecules under collision-free conditions, at different energies (E°) above D0. These distributions were measured for both n-C3F7NO and i-C3F7NO up to E°∼4500 cm-1, and rotational excitation within each spin–orbit state was statistical, except at E°≥3000 cm-1. As with CID, a low [2∏3/2]/[2∏1/2] ratio was observed, and the reaction mechanism is probably unimolecular decomposition via the lowest triplet surface T1 with little or no exit channel barrier. The pronounced similarities between the CID and photodissociation results suggest that common reaction mechanisms may be operative. All of the CID results are compatible with direct inelastic scattering followed by unimolecular reaction on the S0 and/or T1 potential surfaces. [ABSTRACT FROM AUTHOR]

The H+N3→NH(X 3∑-,a 1Δ, b 1∑+)+N2 reaction has been studied in a molecular beam-gas scattering arrangement in order to determine the nascent product state distribution. The NH product in specific rovibronic/fine-structure states has been detected by laser fluorescence excitation. The relative cross sections for formation of various vibrational levels in the a 1Δ electronic state were determined to equal 1:1.0±0.3:1.4±0.3:≤1.5 for v=0 through 3, inclusive, while the v=0 to v=1 population ratio in the X 3∑- state was found to be 1:0.015±0.003. The rotational distributions in all vibronic levels were found to be characterized by temperatures near 300 K, suggestive of relaxation of the nascent rotational distributions. By comparison of the populations of a specific pair of X 3∑- and a 1Δ state levels and with summation over the derived rovibrational distributions, an electronic state branching ratio of 3.2±1.3 was obtained for the X 3∑- to a 1Δ electronic state branching ratio. An upper limit of ≤0.02 was also derived for the ratio of the b 1∑+ v=0 to a 1Δ v=0 populations. These results are compared with NH fragment distributions observed in the photodissociation of HN3(X 1A’) and with our expectations based on our fragmentary knowledge of HN3 potential energy surfaces. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The photodissociation processes of benzene following excitation at 193 and 248 nm have been studied by molecular beam photofragmentation translational spectroscopy. When benzene was excited to the 1 B1u state by absorption at 193 nm, dissociation occurred through three primary channels, C6H5+H (80%), C6H4+ H2 (16%), and C5H3+CH3 (4%), following internal conversion to the vibrationally excited ground state. When benzene was excited to the 1 B2u state at 248 nm, two primary dissociation channels, C6H4+H2 (96%), and C5H3+CH3 (4%), were observed. Photodissociation to produce two C3H3 was induced by two photon absorption of benzene at both 193 and 248 nm. Numerous secondary photodissociation processes of the primary photoproducts were also observed at both 193 and 248 nm. [ABSTRACT FROM AUTHOR]