Photodissociation Experiments Research Articles

Kinetic Energy Release Distributions for Tropylium and Benzylium Ion Formation from the Toluene Cation

Hydrogen loss from the toluene molecular ion generates benzylium (Bz+) and tropylium (Tr+) ions via two competitive and independent pathways. The corresponding kinetic energy release distributions (KERDs) have been determined under various conditions in the metastable time window for toluene and perdeuterated toluene and have been analyzed by the maximum entropy method (MEM). The isomeric fraction Tr+/Bz+ is found to be equal to 0.9 ± 0.3, in good agreement with the values obtained using photodissociation and charge exchange experiments. It is, however, in disagreement with the value 5 ± 2 deduced by Moon, Choe, and Kim (J. Phys. Chem. A 2000, 104, 458) from KERD measurements. The origin of the discrepancy is suggested to be the inadequacy of the orbiting transition state theory (OTST) for the calculation of KERDs in hydrogen loss reactions. For both channels, more translational energy is released in the reaction coordinate than would be expected on statistical grounds because of the presence of a barrier...

Photodissociation of HCl and small (HCl)m complexes in and on large Ar n clusters.

Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Ar n clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes n=140-1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are m=3 and 6 and n=8-248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for.

Signature of cluster isomers in time-resolved photodissociation experiments

The unrecognized presence of structure isomers in mass-selected cluster ensembles may obstruct investigations of the systems’ intrinsic properties, since isomers differ not only in geometry, but also in other important properties. By the same token isomers are very interesting objects in the detailed study of atomic clusters. In the present work, different scenarios of isomeric coexistence are presented. They vary in the relative values of the interconversion barrier and the dissociation energies. For some idealized cases the possibility of a distinction of isomers by photodissociation experiments is discussed. In favorable situations isomeric structures may even be selected.

Photodissociation of hydrogen iodide on the surface of large argon clusters: the orientation of the librational wave function and the scattering from the cluster cage.

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Arn clusters, with an average size n = 139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2 + 1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin-orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin-orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI-Arn surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions.

Photodissociation of Bromoform at 248 nm: Single and Multiphoton Processes

We have performed photodissociation experiments on CHBr 3 at 248 nm using VUV ionization photofragment translational spectroscopy. Prompt C-Br bond fission is the dominant single-photon dissociation channel. In addition to primary Br and CHBr 2 signals, we observe Br, CHBr, CBr, HBr, and Br 2 products attributed to secondary photodissociation of CHBr 2 and CHBr. There are three competing fragmentation channels from the photodissociation of CHBr 2 : CHBr + Br, CH + Br 2 , and CBr + HBr. The conclusion that Br 2 fragments do not arise from a single-photon channel in appreciable yield is supported by transient FM absorption measurements of the CHBr radical. Because the molecular HBr and Br 2 detachment channels are multiphoton processes, they will have very little impact on the atmospheric chemistry of CHBr 3 . We conclude that the most important photodissociation channel of CHBr 3 in the UV region is C-Br bond breaking.

Ruthenium trisbipyridine as a candidate for gas‐phase spectroscopic studies in a Fourier transform mass spectrometer

Metal polypyridines are excellent candidates for gas‐phase optical experiments where their intrinsic properties can be studied without complications due to the presence of solvent. The fluorescence lifetimes of [Ru(bpy)3]1+ trapped in an optical detection cell within a Fourier transform mass spectrometer were obtained using matrix‐assisted laser desorption/ionization to generate the ions with either 2,5‐dihydroxybenzoic acid (DHB) or sinapinic acid (SA) as matrix. All transients acquired, whether using DHB or SA for ion generation, were best described as approximately exponential decays. The rate constant for transients derived using DHB as matrix was 4×107 s−1, while the rate constant using SA was 1×107 s−1. Some suggestions of multiple exponential decay were evident although limited by the quality of the signals. Photodissociation experiments revealed that [Ru(bpy)3]1+ generated using DHB can decompose to [Ru(bpy)2]1+, whereas ions generated using SA showed no decomposition. Comparison of the mass spectra with the fluorescence lifetimes illustrates the promise of incorporating optical detection with trapped ion mass spectrometry techniques.

Orbital polarization from DC slice imaging: S(1D2) alignment in the photodissociation of ethylene sulfide

Abstract DC slice imaging [Rev. Sci. Instrum. 74 (2003) 2530], a recently developed high-resolution `slicing' approach that directly provides the full 3D product distribution in imaging experiments, has been adapted to yield the absolute speed-dependent angular momentum alignment anisotropy parameters in a photodissociation experiment. We present the theoretical machinery for interpretation of a small basis set of sliced experimental images recorded under different laser polarization geometries and then demonstrate its application in the study of electronic orbital angular momentum alignment of excited state S( 1 D 2 ) atoms following 193.3 nm photodissociation of ethylene sulfide. We find the slicing approach to be a highly sensitive and widely applicable technique for the study of orbital polarization, even in instances where the magnitude of such effects is small. The approach outlined here represents a direct route to the determination of recoil-angle dependent orbital alignment for a continuous range of photofragment kinetic energy release.

Quantum Monte Carlo study of singlet–triplet transition in ethylene

A theoretical study is reported of the transition between the ground state (1Ag) and the lowest triplet state (1 3B1u) of ethylene based on the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo method. Using DMC trial functions constructed from Hartree–Fock calculations, complete active-space self-consistent field and multiconfiguration self-consistent field wave functions, we have computed the atomization energy and heat of formation of both states and the adiabatic and vertical energy differences between these states using both all-electron and effective core potential DMC methods. The ground-state atomization energy and heat of formation are found to agree with experiment to within the error bounds of the computation and experiment. Predictions by the DMC method of the triplet-state atomization energy and heat of formation are presented. The adiabatic singlet–triplet energy difference is found to differ by 5 kcal/mol from the value obtained in a recent photodissociation experiment.

Photodissociation of HBr in and on Arnclusters: the role of the position of the molecule

Photodissociation experiments are carried out for single HBr molecules which are embedded in the interior or absorbed on the surface of large Arn clusters. For the embedded case the size dependence is measured for the average sizes 〈n〉 = 51, 139, 230, 290 and 450. For the surface case and the average size 〈n〉 = 139 the source temperature is varied from T = 163 K to 263 K. The measured kinetic energy of the H atom fragments exhibits peaks at zero and 1.3 eV which mark completely caged and unperturbed fragments, respectively. The results are compared with Molecular Dynamics simulations which account for the quantum librational delocalization of the HBr molecule. The location of the molecule in/on the cluster is obtained from a trajectory study of the pick-up process under realistic conditions. For the embedded case corresponding to a co-expansion experiment, three argon layers are sufficient to completely hinder the H atom, in perfect agreement with the calculations. For the pick-up experiment, the large change of the source temperature leads to very small changes of cluster temperature dependent properties. Events starting from the second shell have a higher exit probability than those coming from the surface.

State-to-state photodissociation of OCS (ν2=0,1|JlM). I. The angular recoil distribution of CO (X 1Σ+;v=0|J)

State-to-state photodissociation experiments of OCS at 230 nm are reported using hexapole state selection of the parent molecule and velocity map ion imaging of the angular recoil of the CO photofragment. The role of the initial rovibrational state (ν2=0,1|JlM) of OCS on the angular recoil distribution is investigated. The CO (X 1Σ+;v=0|J) rotational distribution as well as the angular recoil anisotropy parameter β of the CO photofragment are reported for dissociation of single rovibrational (ν2=0,1|JlM) quantum states of OCS. A strong dependence of the anisotropy parameter β on the initial bending state, ν2=0 or 1, of OCS is observed. The effects of the initial bending state of OCS are rationalized in terms of the strong angular dependence of the transition dipole moment function of OCS for the 1 1Σ−(1 1A″) and 1 1Δ(2 1A′) excited state surfaces involved in the dissociation at 230 nm. The state-to-state imaging experiment provides a revised and improved determination of the binding energy of OCS (ν1,ν2,ν3=0,0,0|J=0)→CO (X 1Σ+;v=0|J=0)+S (1D2), D0=(4.284±0.009) eV.

A tertiary two-state allosteric model for hemoglobin

The two-state allosteric model of Monod, Wyman, and Changeux (MWC) provides an excellent description of homotropic effects in a vast array of equilibrium and kinetic measurements on cooperative ligand binding by hemoglobin. However, in contrast to experimental observations, the model does not allow for alteration of the ligand affinity of the T quaternary structure by allosteric effectors. This failure to explain heterotropic effects has been appreciated for over 30 years, and it has been generally assumed to result from tertiary conformational changes in the absence of a quaternary change. Here we explore a model that preserves the essential MWC idea that binding without a quaternary conformational change is non-cooperative, but where tertiary conformations of individual subunits play the primary role instead of the quaternary conformations. In this model, which we call the ‘tertiary two-state (TTS) model’, the two affinity states correspond to two tertiary conformations of individual subunits rather than the two quaternary conformations of the MWC two-state allosteric model. Ligation and the R quaternary structure bias the subunit population toward the high affinity tertiary conformation, while deligation and the T quaternary structure favor the low affinity tertiary conformation. We show that the model is successful in quantitatively explaining a demanding set of kinetic data from nanosecond carbon monoxide photodissociation experiments at times longer than ∼1 μs. Better agreement between the model and the submicrosecond kinetic data may result from detailed considerations of the distribution and dynamics of conformational substates of the two tertiary conformations. The model is consistent with the results of solution, gel, and single crystal oxygen binding studies, but underestimates the population of doubly-liganded molecules determined in low-temperature electrophoresis experiments.

How many molecules make a solution?

Water clusters are studied in order to investigate the evolution of solution phase chemistry from the molecular level to bulk. When water clusters are studied in a Fourier transform ion cyclotron resonance (FI-ICR) mass spectrometer, their fragmentation under the influence of infrared black-body radiation must be considered and can be used as a tool to monitor the destabilization of particular arrangements as a function of cluster size. As examples for solution phase chemistry, dissolution of acids, metal ion oxidation and metal halide precipitation, acid-base catalysis and hydrolysis are discussed. All these solution phase reactions proceed on the single-ion level in small water clusters. In accordance with spectroscopic and thermochemical data, gas phase ion chemistry of small water clusters rapidly approaches bulk behaviour, if the ion high concentration and pH value of the cluster are taken into account. Examination of competing reaction pathways as a function of cluster size in hydrated electron clusters reveals the crucial influence of entropy. Recent photodissociation experiments by Metz and Cocrorteers provide experimental evidence for the salt bridge mechanism for charge separation in hydrated dication clusters.

Measurement of bipolar moments for photofragment angular correlations in ion imaging experiments

A general numerical method is given to extract angular correlations from photodissociation experiments with ion imaging detection. The angular correlations among the transition dipole moment of the parent molecule, μ, the photoproduct recoil velocity, v, and its angular momentum, j, are parametrized analytically using the semiclassical bipolar moment scheme due to Dixon. The method is a forward-convolution scheme which allows quantitative extraction of all measurable bipolar moments and can be applied in experiments with both linearly and circularly polarized probe light. It avoids the cylindrical symmetry limitations of the inverse Abel transform method, traditionally used for extracting photoproduct recoil anisotropy and speed distribution from imaging data. The method presented here also takes into account the possibility of multiple photodissociation channels. The features of the method are illustrated in a two-color 1+1′ REMPI-ion imaging study of the NO photoproduct trajectories resulting from the 650 nm photodissociation of 2-chloro-2-nitrosopropane (CNP). A comparison between experimental and synthetic images is presented for selected experimental geometries. The experimental images for CNP and the results from their fit confirm earlier TOF studies showing that the recoil speed distribution is bimodal with the low and high speed components having average values of approximately 500 and 910 m/s. These components have been previously assigned to dissociation from the S0 and T1 electronic states of the parent molecule, respectively. The experimental results from the current study also confirm that for the high-speed component the product NO velocity vector, v, is preferentially perpendicular to its angular momentum, j [β00(22)=−0.21], and that there is no significant correlation between v and the transition dipole moment μ of the CNP molecule [β02(20)=−0.02].

Formation and photodissociation of M(+)-C(6)H(6) (M(+) = V(+) and Ta(+)) and Ta(+)-C(6)H(4) complexes in a time-of-flight mass spectrometer.

A series of cyclic hydrocarbons were introduced to react with V(+) and Ta(+) using a pulsed beam expansion source in a time-of-flight mass spectrometer. The third-row metal Ta(+) displayed high reactivity in dehydrogenation to form benzyne complexes, whereas benzene complexes were the terminal products for V(+). M(+)-C(6)H(6) (M(+) = V(+) and Ta(+)) and Ta(+)-C(6)H(4) were selected to perform the photodissociation experiments. In contrast to the V(+) fragment formation via simple cleavage of the V(+)-C(6)H(6) bond, a photoinduced loss of C(2)H(2) occurred in both the Ta(+)-C(6)H(6) and Ta(+)-C(6)H(4) complexes. Plausible explanations involved in the formation of Ta(+)-C(6)H(6) and Ta(+)-C(6)H(4) complexes are given for observing such photo-induced dissociation. The observed photodissociation in Ta(+)-C(6)H(6) is analogous to the dissociative process previously investigated in metal ion-molecule reactions. The photodissociation spectrum of Ta(+)-C(6)H(4) was obtained by recording the appearance of Ta(+)-C(4)H(2) as a function of wavelength and yielded a dissociation energy of 91 +/- 1 kcal mol(-1).

(2+1 ′) mass analyzed threshold ionization (MATI) spectroscopy of the CD 3 radical

The mass analyzed threshold ionization (MATI) spectrum of CD 3, recorded using two-colour (2+1 ′) multiphoton excitation via the Q-branch transition to the 3p z Rydberg state, is reported. The fast CD 3 radicals are produced by photodissociation of CD 3I at 280 nm. The spectroscopy provides a possible route to state-selection of rovibronic levels of the CD 3 + ion, or for Rydberg tagging of the CD 3 fragments in a photodissociation experiment. The MATI spectrum is simulated using a simple Hund's case (b) to Hund's case (d) intensity model. The variable parameters optimized in the model are the ratio of radial transition moments to the s and d channels (1:1), the MATI bandwidth (∼15 cm −1) and asymmetric lineshape (determined by the discrimination field and the high- n Rydberg decay), and the ionization potential of CD 3 (79287±5 cm −1) .

Metal-ligand interactions: Gas-phase transition metal cluster carbonyls

Experimental studies of the interactions of small transition-metal cluster anions with carbonyl ligands are reviewed and compared with neutral and cationic clusters. Under thermal conditions, the reaction rates of transition-metal clusters with carbon monoxide are measured as a function of cluster size. Saturation limits for carbon monoxide addition can be related to the geometric structures of the clusters. Both energy-resolved threshold collision-induced dissociation experiments and time-resolved photodissociation experiments are used to measure metal-carbonyl binding energies. For platinum and palladium trimer anions, the carbonyl binding energies are assigned to different geometric binding sites. Platinum and palladium cluster anions catalyse the oxidation of carbon monoxide to carbon dioxide in a full catalytic cycle at thermal energies.

Metal-ligand interactions: gas-phase transition metal cluster carbonyls

Experimental studies of the interactions of small transition-metal cluster anions with carbonyl ligands are reviewed and compared with neutral and cationic clusters. Under thermal conditions, the reaction rates of transition-metal clusters with carbon monoxide are measured as a function of cluster size. Saturation limits for carbon monoxide addition can be related to the geometric structures of the clusters. Both energy-resolved threshold collision-induced dissociation experiments and time-resolved photodissociation experiments are used to measure metal-carbonyl binding energies. For platinum and palladium trimer anions, the carbonyl binding energies are assigned to different geometric binding sites. Platinum and palladium cluster anions catalyse the oxidation of carbon monoxide to carbon dioxide in a full catalytic cycle at thermal energies.

Novel Cationic Selenium-Cluster Nitride Species [SenN]+ (n=1–11) Formed by Laser Ablation of a Se Target in the Presence of N2

Nitride cations of selenium clusters [SenN]+ (n=1–11) were readily produced by laser ablation of a selenium disk that was surrounded by a trace amount of nitrogen seeded in helium and followed by supersonic expansion into a high vacuum. Even at high nitrogen partial pressures, the cluster mononitride cations were found to be essentially the only nitride products in the whole size range we studied. The exception was [Se3N2]+, which is known to be a stable five-membered ring with seven π electrons. We propose that, in the laser-ablation plasma, the selenium clusters with n>2 take on a chain conformation, and that the N species links the two ends of the selenium chains, thus forming stable mononitride cations of the cyclic selenium clusters. Their stability is supported by the results of ab initio calculations (at both B3LYP/6-31+G* and MP2/6-31+G* levels) and of mass-selected cluster-ion photodissociation experiments.

Novel cationic selenium-cluster nitride species [SenN]+(n = 1-11) formed by laser ablation of a Se target in the presence of N2.

Nitride cations of selenium clusters [SenN]+ (n = 1-11) were readily produced by laser ablation of a selenium disk that was surrounded by a trace amount of nitrogen seeded in helium and followed by supersonic expansion into a high vacuum. Even at high nitrogen partial pressures, the cluster mononitride cations were found to be essentially the only nitride products in the whole size range we studied. The exception was [Se3N2]+, which is known to be a stable five-membered ring with seven pi electrons. We propose that, in the laser-ablation plasma, the selenium clusters with n > 2 take on a chain conformation, and that the N species links the two ends of the selenium chains, thus forming stable mononitride cations of the cyclic selenium clusters. Their stability is supported by the results of ab initio calculations (at both B3LYP/ 6-31 + G* and MP2/6-31 + G* levels) and of mass-selected cluster-ion photodissociation experiments.

Photodissociation and caging of HBr and HI molecules on the surface of large rare gas clusters.

Photodissociation experiments were carried out at a wavelength of 243 nm for single HBr and HI molecules adsorbed on the surface of large Nen, Arn, Krn and Xen clusters. The average size is about <n> = 130; the size ranges <n> = 62-139 for the system HBr-Arn and <n> = 110-830 for HI-Xen were covered. In this way the dependence of the photodissociation dynamics on both the size and the rare gas host cluster was investigated. The main observable is the kinetic energy distribution of the outgoing H atoms. The key results are that we do not find any size dependence for either system but that we observe a strong dependence on the rare gas clusters. All systems exhibit H atoms with no energy loss that indicate direct cage exit and those with nearly zero energy that are an indication of complete caging. The intensity ratio of caged to uncaged H atoms is largest for Nen, decreases with increasing mass of the cage atoms, and is weakest for Xen. On the basis of accompanying calculations this behaviour is attributed to the large amplitude motion of the light H atom. This leads to direct cage exit and penetration of the atom through the cluster with different energy transfer per collision depending on the rare gas atoms. The differences between HBr and HI molecules are attributed to different surface states, a flat and an encapsulated site.