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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]

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]

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]

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]

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]

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 [1+1] A˜ [sup 2]Π[sub u]-state resonance enhanced two-photon dissociation process of CS[sub 2][sup +] molecular ions has been investigated by measuring the photofragment S[sup +] excitation (PHOFEX) spectrum in the wavelength range of 424-482 nm, where the CS[sub 2][sup +] molecular ions were prepared purely by [3+1] multiphoton ionization of the neutral CS[sub 2] molecules at 483.2 nm. The PHOFEX spectrum was assigned essentially to the CS[sub 2][sup +](A˜ [sup 2]Π[sub u])←CS[sub 2][sup +](X˜ [sup 2]Π[sub g]) transition, and the dissociation mechanism of CS[sub 2][sup +] was preliminarily attributed to (i) CS[sub 2][sup +](X˜ [sup 2]Π[sub g])→CS[sub 2][sup +](A˜ [sup 2]Π[sub u]) through one-photon excitation, (ii) CS[sub 2][sup +](A˜ [sup 2]Π[sub u])→CS[sub 2][sup +](X˜[sup †]) via internal conversion process due to the vibronic coupling between the A˜ and X˜ states, (iii) CS[sub 2][sup +](X˜[sup †])→CS[sub 2][sup +](B˜ [sup 2]Σ[sub u][sup +]) through the second photon excitation, and (iv) CS[sub 2][sup +](B˜ [sup 2]Σ[sub u][sup +])→S[sup +]+CS owing to the potential curve crossing with the repulsive [sup 4]Σ[sup -] state correlated with the first dissociation limit. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]