Product Energy Distributions Research Articles

Photodissociation of methanol at 193.3 nm: Translational energy release spectra

Center-of-mass translational energy distributions of the dominant primary products resulting from 193.3 nm excitation of jet-cooled CH3OH, CH3OD, and CD3OH were obtained by using the high-n Rydberg time-of-flight (HRTOF) technique. The appearance threshold in the HRTOF spectrum yields a bond dissociation energy, D0(CH3O–H), of 105±1 kcal mol−1, in agreement with recent literature values. Translational energy release spectra from the three isotopomers exhibit progressions of 950±100 cm−1, which are attributed to excitation in the ν3 O–CH3 stretch of the methoxy product. The progressions peak at v=1, with population out to at least v=5. This differs from the results of a recent wave packet dynamics study on a calculated excited state potential energy surface [Marston et al., J. Chem. Phys. 98, 4718 (1993)], which predicted no O–CH3 stretch excitation in the methoxy fragment following photolysis of ground state methanol. The spatial anisotropy of the fragments (β∼−0.7) implies a dissociation time ≤1 ps. The impulsive model for rotational excitation is compared to the unresolved rotational contour of the vibrational peaks in the translational energy release spectra and is found to underestimate the extent of rotational excitation, though the model correctly predicts the increase in contour width observed for the O-deuterated species. The unresolved rotational contours are fit empirically. The inferred vibrational energy distributions are discussed in terms of a simple Franck–Condon model for the pseudotriatomic, Me–O–H. Implications of the vibrational and rotational photofragment distributions for the full 1 1A″ surface are discussed.

Quantum mechanical calculations of collision-induced dissociation for atom—diatom reactive systems: on the scattering boundary condition

An approximate method of Sakimoto and Onda for deriving the S-matrix elements for dissociative collisions in collinear atom—diatom reactive systems, which provided accurate total dissociation probabilities, has been tested further. To do this, kinetic energy distributions of atomic products have been calculated and compared with results given by Kaye and Kuppermann. The method of Sakimoto and Onda always gives an artificial dip structure in the kinetic energy distributions. Further studies are required to resolve the double continuum problem in dissociative collisions.

Ultrafast transient infrared absorption studies of M(CO) 6 (M  Cr, Mo or W) photoproducts in n-hexane solution

Following UV excitation, M(CO) 6 (MCr, Mo or W) species in solution lose one CO ligand, and a solvent molecule occupies the vacant site. These photochemical reactions are studied using a transient broadband IR absorption technique with 390 fs time resolution. The progress of these reactions, including loss of the M(CO) 6 reactant, appearance of a solvated M(CO) 5( n-hexane) product and vibrational energy distribution and relaxation of the product, is monitored through changes in infrared absorption by the CO ligands near 2000 cm −1.

Ab Initio Potential Energy Surface for H + OCS Reactions: Extended Basis Sets and Correlation Treatment

Abstract : Ab initio calculations using extended basis sets are presented for the potential energy surface (PES) of H + OCS. There are two major reaction channels on the PES; Reaction (I) is H(2S) + OCS(1(Sigma)) yields OH(2(Pi)) + CS(1(Sigma)), and Reaction (II) is H(2S) + OCS(1(Sigma)) yields SH(2(Pi)) + CO(1(Sigma)). Results of this study substantiate findings from an earlier quantum chemical study using a lower level of theory, including (1) the existence of 12 transition states and 6 stable 4-body intermediates; (2) the qualitative description of the PES (i.e., geometries, relative barriers, and well depths are similar to those in the earlier study); and (3) the entrance channel transition states leading to (II) are tight, as suggested by experiment. The results presented here also support the explanations of observed product energy distributions for (I) and (II) based on the earlier ab initio study. An additional transition state connecting the cis-HOCS and cis-HSCO minima was located, confirming a previous suggestion that Reaction (II) could result from hydrogen migration after HOCS formation. The current results show a substantial improvement in the quantitative agreement with experiment over the previously calculated values. Potential energy surface, Ab initio, MP4, QCISD(T), Electronic structure, Potential energy, Quantum chemistry.

A Quasiclassical Trajectory Study of Product Energy and Angular Distributions in OH + H2 (D2)

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA Quasiclassical Trajectory Study of Product Energy and Angular Distributions in OH + H2 (D2)Kimberly S. Bradley and George C. SchatzCite this: J. Phys. Chem. 1994, 98, 14, 3788–3795Publication Date (Print):April 1, 1994Publication History Published online1 May 2002Published inissue 1 April 1994https://doi.org/10.1021/j100065a039RIGHTS & PERMISSIONSArticle Views81Altmetric-Citations29LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (944 KB) Get e-Alerts

Dynamical study of competing reactions in the CN −-H 2 collisional system by a multicoincident detection

A dynamical investigation of the competing reactions involved in CN −-H 2 collisions is performed in a crossed beam experiment with a multicoincidence detection of fast neutrals and ionic products for energy ranging from 2 to 18 eV. Doubly differential cross sections are presented in form of velocity contour maps for both H − and e − production. At high energy a common mechanism is proposed in which a first N-H binary collision is followed by a rearrangement of the two others partners. At lower energy, a long-lived complex should be responsible for the reactive detachment channel. This complex would correspond to the formation of the stable molecule H 2C=N. For the mechanism involving a complex formation, the translational energy distribution of the neutral products agrees with a statistical partitioning of the energy involving, as a constraint, a frozen CN bond.

The influence of barrier topography on the dynamics of the H + F 2 reaction

Classical trajectory calculations on a series of three DIM potential energy surfaces and a standard LEPS surface for the reaction H + F 2 → HF + F are reported. All surfaces have the same features in collinear configurations but differ in the position, height and number of accessible saddle points in bent configurations. The dependence of reaction cross sections, product energy and angular distributions, thermal rate data and reaction mechanism on these features is investigated and discussed. The non-collinear part of the surfaces is of particular interest when it allows for microscopic branching, the various ensuing reaction mechanisms making different contributions to the product property distributions. In particular, a broadening of the HF vibrational distribution together with the appearance of a component extending to very low vibrational levels can arise from reactive encounters passing through a T-shaped transition state.

The effect of reagent translational excitation on the dynamics of the reaction H+Cl2→HCl(v′,J′)+Cl

We use fast time-resolved Fourier transform spectroscopy and low pressure infrared chemiluminescence techniques to determine the product energy distribution in the title reaction. We create the reagent hydrogen atoms with 2.3 eV of translational excitation by photofragmentation of H2S at 193 nm, and observe the time evolution of the infrared chemiluminescence from the product HCl(v′,J′) under single gas kinetic collision conditions. The initial vibrational distribution, determined from the first observation after creation of the H atoms is P(v′=1:2:3:4:5:6:7:8)=1.0:0.84:0.74:0.59:0.34:0.24:0.17:0.13. The initial HCl rotational distribution in each vibrational level is broad, showing no identifiable maximum. The fraction of the total available energy entering HCl vibration and rotation, respectively, are 0.19 and 0.10. The time evolution of the observed vibrational and rotational distributions gives information about the changes in the reaction dynamics consequent on reduction of the reagent translational energy.

Reactant rotational energy effect on energy distribution in products. The role of the electronic angular momentum

In reactions of O(1D) with CH4 clusters, CD4 and propane, monomers and clusters, preference for the 2Π32 spin-orbit state of the OH product is observed. New results on the effect of the initial rotational temperature of propane on the spin-orbit state distribution are presented. The preference for the 2Π32 state and the rotational energy effect are attributed to conservation of electronic angular momentum during curve crossing in the entrance channel and to conservation of the projection of the electronic angular momentum on the internuclear axis through the reaction. A quantitative model rationalizes all the experimental observations.

Ultrafast photodissociation of I3− in ethanol: A molecular dynamics study

A detailed molecular model of I3− photodissociation in liquid ethanol is developed. Extensive molecular dynamics trajectory calculations are used to determine product energy distribution, time-dependent spectra, and reorientation dynamics in semiquantitative agreement with experimental data.

Photodecomposition dynamics of Mo(CO)6/Si(111) 7×7: CO internal state and translational energy distributions

The rotational state and translational energy distributions of CO photodesorption products resulting from the 266 nm photolysis of Mo(CO)6 adsorbed on Si(111) 7×7 with coverages in the multilayer regime are reported. State-resolved measurements show two desorption components with highly disparate energy dispositions. Results for different surface temperatures indicate that the energy content in one component reaches quasi-equilibration with the surface temperature, which is attributed to collisional relaxation of nascent photodecomposition products within the adlayer. The other component exhibits disparate rotational and translational ‘‘temperatures’’ that are significantly greater than, and independent of, the surface temperature. These nascent photodecomposition products are influenced by both energy quenching effects and dynamical constraints imposed by the existence of the adlayer.

Dependence on scattering angle of the internal energy distribution of products of charge‐changing collisions

AbstractThe internal energy distributions arising from charge‐changing collisions were measured as a function of scattering angle, θ, for the ‘thermometer’ molecule W(CO)6. The experiments were performed by modifying a reverse‐geometry mass‐analyzed ion kinetic energy (MIKE) spectrometer by adding angle‐resolving slits which allow measurement of the scattering angle in the non‐focusing plane of the instrument. Charge exchange of W(CO) with benzene to give W(CO), and charge stripping of W(CO) on collision with O2 to give W(CO), were studied at 6 keV with the product ions being collected over laboratory scattering angles selected in the range 0–0.60°. The results show that charge‐changing collisions accompanied by scattering have the potential for depositing extremely large internal energies. The observation of the W(CO)2+ ion formed in dissociative charge stripping of W(CO) shows that it is possible to deposit at least 27 eV into the colliding W(CO) ion; of this energy, 15 eV is used for the charge‐stripping process, leaving 12 eV of internal energy in the nascent W(CO). Even greater internal energies (more than 15 eV) can be deposited into scattered W(CO) produced by charge exchange of the doubly charged ion. The availability of such high internal energies has potential use in causing dissociation of refractory ions such as those of biomolecules. The average internal energy, εAVE, deposited increases with the scattering angle at a rate of 10 eV degree−1 for charge exchange, and at approximately 5 eV degree−1 for charge stripping and for simple collision‐induced dissociation (CID). This observation suggests that non‐zero angle charge stripping and CID may occur via similar mechanisms in which direct vibrational activation occurs in small impact parameter collisions which also lead to angular scattering. The higher internal energies and larger εAVE vs. θ dependence observed for charge exchange are consistent with the formation of the products upon a highly repulsive surface associated with coulombic repulsion between the separating products.

Emission from OH(A) produced in the dissociative recombination of H2O+ with electrons

The production of OH(A) from the dissociative recombination of H2O+ with electrons has been studied in a crossed jet experiment. The branching ratio into the OH(A)+H product channel has been estimated to be 7.6%+8%/−4% for an electron temperature of ∼1000 K. The internal energy of the OH(A) has been determined from modeling the emission spectrum. The vibrational distribution is 1.00:0.87 for v=0:v=1. The estimated error is ±15% for both states. The rotational temperatures were found to be 4000 K for both v=0 and 1. The estimated error is +500, −1000 K. The present work represents one of the first measurements of the internal energy distribution of the molecular products of the dissociative recombination of a molecular cation and an electron.

ChemInform Abstract: Dynamics of Ethylene Photodissociation from Rovibrational and Translational Energy Distributions of H2 Products

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Dynamics of ethylene photodissociation from rovibrational and translational energy distributions of H2 products

The dynamics of H2 elimination from ethylene was studied via a pump and probe technique utilizing an ultrahigh resolution vacuum ultraviolet laser system. H2 product internal and translational energy distributions were obtained for the photodissociation at 193 nm. The distribution of energy in H2 product from the dissociation of (1,1)-dideuteroethylene is also presented. Two separate H2 elimination channels are inferred: a 1,1 elimination producing the vinylidene radical and a 1,2 elimination producing the acetylene molecule. Differences between the vibrational, rotational, and translational energy distributions for these two channels are discussed and correlations between product internal and translational energy are presented. A comparison with ab initio calculations of the transition state configurations for these processes is made. We suggest that the H2 elimination process may be nonstatistical in nature. The D atom elimination from C2D4 was examined and kinetic energy distribution for this product measured.

Beam-gas study of bismuth–fluorine reactions using laser-induced fluorescence of BiF

Relative rotational and vibrational populations have been extracted by computer simulation of laser excited fluorescence spectra of BiF (X0+) formed in a thermal molecular beam of bismuth reacting with molecular fluorine under single-collision conditions. The observed rotational distribution is colder than the prior distribution calculated assuming three product fragments. The observed vibrational distribution is also cold, indicating that comparatively little energy is available to the ground state molecules probed. Since the dependence of the detected BiF (X0+) product on the beam time of flight points toward Bi2 as the major beam reactant, the product energy distributions presented here suggest a mechanism in which BiF (X0+), Bi (4S), and F (2P) are formed in an end-to-end attachment of Bi2 and F2. There were no indications that BiF (X0+) can be readily formed by reaction of atomic bismuth.

Vibrational energy of the monoalkyl zinc product formed in the photodissociation of dimethyl zinc, diethyl zinc, and dipropyl zinc

The gas-phase photodissociation of (CH3)2Zn, (C2H5)2Zn, and (n-C3H7)2Zn has been examined at 248 nm using laser-induced fluorescence to detect the monoalkyl zinc radical and zinc atom photoproducts. For each compound, the monoalkyl zinc radical is the primary photoproduct and is formed sufficiently hot that it spontaneously dissociates to an alkyl radical and a Zn atom without absorption of a second photon. Photodissociation was examined in the presence of He buffer gas to measure the probability of quenching the secondary spontaneous dissociation of the monoalkyl zinc species. For all three dialkyl zinc compounds, the probability of quenching the secondary dissociation step increases substantially over the He pressure range of 0–400 Torr. The quenching probability vs He pressure was fit using RRKM theory in conjunction with a time-dependent master equation, treating the nascent vibrational energy distribution of the monoalkyl zinc product as an adjustable function. The quenching data for C2H5Zn and n-C3H7Zn can be fit only if it is assumed that these species are formed with a hot, narrow vibrational energy distribution, much narrower than that predicted by phase-space theory. A dissociation mechanism involving crossover from an optically prepared singlet state to a repulsive triplet state is proposed to explain this observation. Spontaneous dissociation of CH3Zn is quenched much more strongly by He than is calculated using any reasonable vibrational energy distribution function for CH3Zn. This is attributed to the inapplicability of RRKM theory to reactions involving very low-state-density molecules like CH3Zn.

Wave-packet analysis of laser-induced half-collision processes

The usual approach to calculating multiphoton collision and half-collision processes uses the interaction picture and introduces a classical time-dependent field into the Hamiltonian for the scattered particles, which is further simplified using the Floquet ansatz. In particular, the laser-induced decay of an initial bound state is derived from a half-collision loquet ansatz that utilizes a complex quasienergy, whose imaginary part is identified with the laser-induced decay constant of the bound state. This interpretation presupposes a pure exponential decay of the initial-state population and yields a Lorentzian distribution of product-state energies in the infinite-depletion limit. Here we demonstrate how a full-collision Floquet ansatz can be derived from a fully quantal wave packet constructed to represent scattering in the presence of a coherent state of the laser field. The wave packet utilizes the set of the time-independent close-coupled wave functions for scattering in the field generated by quantized radiation number states. The resultant Floquet scattering states are energy normalized and stationary, and the quasienergy is real. We show how to use these states to construct a coherent-state wave packet that describes the decay of an arbitrarily prepared bound state, and yields a half-collision Floquet ansatz without any commitment to a complex quasienergy. The product energy and quantum-state distribution in the infinite-depletion limit are obtained without approximation to exponential decay. Further we show how the Fourier transform of the infinite-depletion line shape gives the temporal behavior of the initial bound-state population, often with distinctly nonexponential behavior. Such results are demonstrated for the above-threshold ionization of H, and the above-threshold dissociation of ${\mathrm{H}}_{2}^{+}$. In the following paper [R. W. Heather and F. H. Mies, Phys. Rev. 44, XXXX (1991)], excellent agreement is found between the present fully quantal wave-packet approach, utilizing the time-independent scattering wave functions, and the comparable solutions to the time-dependent Schr\"odinger equation for a corresponding classical time-dependent laser field.

Laser photodecomposition of sintered YBa2Cu3O6+x: Ejected species population distributions and initial kinetic energies for the laser ablation wavelengths 351, 248, and 193 nm

We present experimental results of photoablated product and kinetic energy (KE) distributions from the ultraviolet laser ablation of a sintered YBa2Cu3O6+x wafer at the laser wavelengths 351, 248, and 193 nm. Data is presented which spans the laser fluence range beginning at the threshold for species ejection (50 mJ/cm2) to nearly that required for the formation of an above surface plasma (800 mJ/cm2). The goal of this experiment was to measure changes in the photophysical process as the incident laser fluence was increased above the threshold value. Our results show, that near the laser threshold fluence, the photoejected products consist of small atomic and oxide species. An unforeseen result for the UV laser wavelengths used, is the lack of CuO+/CuO and free oxygen (O+/O, O+2/O2) in the mass spectrum. In addition, the product distributions are dependent on the laser wavelength. Measured also, at laser threshold fluence, are the nascent photoejected cation kinetic energy distribution. Here our results show that the KE distribution is independent of both the laser fluence and the wavelength. The mean kinetic energy, 〈KE〉, exceeds 3 eV and cannot be explained by a thermal excitation process. With increasing laser fluence (50% above threshold), we detected the photodissociation of the ejected oxide species, and the appearance of the O− anion. Unlike the KE of the cation species, the O− kinetic energy is nearly thermal (<1 eV). With additional increases in the laser fluence, we measure the photoejection of slow KE neutral species and the simultaneous KE enhancement of the laser ablated ions (KE≳30 eV). At the highest laser fluences used in this experiment, it was noted that atomic cluster formations were enhanced. These compounds are presumably formed in the expanding ablated plume. In summary, our results show that, at threshold laser fluences, the photoejection process is via non thermal excitation. With increasing laser fluence the ejected species mass spectrum includes products from the plume photolysis and the plume chemistry.

Oscillations in the collision-energy dependence of partial cross sections and of product energy distributions for the backward scattering in the reaction D+H 2(ν=1, j=0)→HD(ν′, all j′)+H

Results of three-dimensional quasiclassical trajectory calculations for the backward scattering in the reaction D+H 2(ν=1, j=0)→HD(ν′, all j′)+H are reported. Significant oscillations in the collision-energy dependence of partial cross sections for the range of scattering angles 170°–180° are observed. These oscillations are stronger than those found earlier for the ground vibrational-state reaction D+H 2(ν=0, j=0)→HD(ν′, all j′)+H, and are accompanied by oscillations in product energy distributions.