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A six-dimensional (6D) potential energy surface (PES) describing the molecule–surface interaction in the dissociative chemisorption system H2+Cu(100) is presented. The PES is based on slab calculations performed using the generalized gradient approximation (GGA) of density functional theory (DFT). To allow the use of the PES in dynamics calculations which can test the validity of the DFT/slab approach by comparing with available experiments on dissociative chemisorption, the PES was fit to an analytical form. The fit used describes the orientational dependence of the molecule–surface interaction above the high symmetry sites upto second order in spherical harmonics. The barriers to dissociation calculated for H2 approaching with its molecular axis parallel to the surface are all located in the exit channel. Also, for different impact sites and orientations, the height and the distance to the surface associated with the barrier correlate well with the chemisorption energy of the H-atoms in the sites to which dissociation takes place; the lowest barrier (0.48 eV) is found for dissociation over the bridge site into the hollow sites, the atomic chemisorption energy being highest in the hollow sites. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A two-dimensional (2-D) potential energy surface (PES) has been calculated for H2 interacting with the (100) face of copper. The PES is for H2 approaching with its internuclear axis parallel to the surface and dissociating over a bridge site into neighboring hollow sites. The density functional calculations were performed both within the local density approximation (LDA) and within a generalized gradient approximation (GGA). The LDA surface shows no barrier to chemisorption, but the GGA surface has a barrier of height 0.4 eV. A fit of the GGA surface has been used to calculate reaction probabilities for H2 in its v=0 and v=1 vibrational states, employing a wave packet method. The 2-D wave packet results for the v=0 and v=1 thresholds are consistent with experiment, indicating that the barrier height calculated within the GGA used is accurate. The GGA results for the value of the barrier height are also consistent with the GGA value (0.5 eV) recently obtained for H2+Cu(111) by Hammer et al. [Phys. Rev. Lett. 73, 1400 (1994)], but the GGA value recently computed for H2+Cu(100) (0.9 eV) by White et al. is too high [Phys. Rev. Lett. 73, 1404 (1994)]. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state-to-state cross sections in both the dissociated and the scattered flux. The potential energy surface is two-dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is deep in the exit channel. For this case, results show that molecules can trap near the barrier for significant times without invoking substrate degrees of freedom. This is explained in terms of trapping in dynamic wells. Finally, we assess the effect on dissociation probability following vibrational excitation of the hydrogen molecule. [ABSTRACT FROM AUTHOR]

Diffusion Monte Carlo (DMC) and path-integral Monte Carlo computations of the vibrational ground state and 10 K equilibrium state properties of the H7+/D7+ cations are presented, using an ab initio full-dimensional potential energy surface. The DMC zero-point energies of dissociated fragments H5+(D5+)+H2(D2) are also calculated and from these results and the electronic dissociation energy, dissociation energies, D0, of 752 ± 15 and 980 ± 14 cm-1 are reported for H7+ and D7+, respectively. Due to the known error in the electronic dissociation energy of the potential surface, these quantities are underestimated by roughly 65 cm-1. These values are rigorously determined for first time, and compared with previous theoretical estimates from electronic structure calculations using standard harmonic analysis, and available experimental measurements. Probability density distributions are also computed for the ground vibrational and 10 K state of H7+ and D7+. These are qualitatively described as a central H3+/D3+ core surrounded by 'solvent' H2/D2 molecules that nearly freely rotate. [ABSTRACT FROM AUTHOR]

Presents a correction to the article 'Dissociation of H[sub2] on Cu(100): Dynamics on a new two-dimensional potential energy surface,' by G. Wiesenekker et al., from a 1995 issue of the 'Journal of Chemical Physics.'

We report state-to-state quasi-classical trajectory rate constants for the D + H2 reactive collision, using the accurate H3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)]. High relative collision energies (up to ≈56 000 K) and high rovibrational levels of H2 (up to ≈50 000 K), relevant to various astrophysical media, are considered. The HD product molecule is formed in highly excited rovibrational states, over a wide collision energy range. The collision-induced dissociation channel (often overlooked in fully quantum reaction dynamics calculations) is found to be significantly populated, even at collision energies as low as 1500 K. [ABSTRACT FROM AUTHOR]

In this work, we present a density functional theory study of the structure and stability of neutral and positively-charged coronene C24Hq+ . In particular, we have investigated (i) adiabatic and vertical ionization potentials up to charge q = 9, (ii) the corresponding infrared spectra, and (iii) dissociation energies and potential energy surfaces for several hydrogen loss channels: sequential H+H, H+H+, H++H, H++H+, and direct H12 . In particular, we have investigated (i) adiabatic and vertical ionization potentials up to charge q = 9, (ii) the corresponding infrared spectra, and (iii) dissociation energies and potential energy surfaces for several hydrogen loss channels: sequential H+H, H+H+, H++H, H++H+, and direct H2 . We have found that the stability of positively-charged coronene is extremely high as a consequence of the molecule's capability to redistribute the charge all over the structure. The computed dissociation energies and fragmentation barriers show that there is competition between different hydrogen loss channels and that the relative importance of these channels depends on the charge of the molecule. From a careful analysis of the potential energy surface we conclude that the channel with the lowest barrier corresponds to the loss of H+2 . We have found that the stability of positively-charged coronene is extremely high as a consequence of the molecule's capability to redistribute the charge all over the structure. The computed dissociation energies and fragmentation barriers show that there is competition between different hydrogen loss channels and that the relative importance of these channels depends on the charge of the molecule. From a careful analysis of the potential energy surface we conclude that the channel with the lowest barrier corresponds to the loss of H2 from neutral, singly-, doubly-, and triply-charged coronene, H+ 2 from quadruply-charged coronene and H++H+ from quintuply-charged coronene. [ABSTRACT FROM AUTHOR]

A quasi-classical study of the endoergic Au+(1S) + H2(X1Σg+) → AuH+ (2Σ+) + H(2S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au++H+H, is the dominant reaction channel for energies above the H2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion. [ABSTRACT FROM AUTHOR]

We report the first six-dimensional quantum dynamical study of the dissociative adsorption of H[sub 2] on a (110) surface. We have performed quantum coupled-channel calculations for the system H[sub 2]/Pd(110) based on a potential energy surface (PES) that was derived from ab initio electronic structure calculations. In particular, we have focused on the effects of the corrugation and anisotropy of the PES on the H[sub 2] dissociation probability. Our results agree well with the available experimental data for the sticking probability as a function of the initial kinetic energy and the angle of incidence. Because of the coupling between the anisotropy and corrugation of the potential energy surface our calculations predict an unusual rotational heating and a rather small rotational alignment in desorption. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The BH[sub 5] molecule contains a weak two-electron-three-center bond and it requires extremely high level of theories to calculate the energy and structure correctly. The potential energy of the hydrogen scrambling in BH[sub 5] has been generated by the multiconfiguration molecular mechanics algorithm with 15 high-level Shepard interpolation points, which would be practically impossible to obtain otherwise. The high-level interpolation points were obtained from the multicoefficient correlated quantum mechanical methods. The more high-level points are used, the better the shape of the potential energy surface. The rate constants are calculated using the variational transition state theory including multidimensional tunneling approximation. The potential energy curve for the BH[sub 5] dissociation has also been calculated, and the variational transition state was located to obtain the dissociation rate constants. Tunneling is very important in the scrambling, and there is large variational effect on the dissociation. The rate constants for the scrambling and the dissociation are 2.1×10[sup 9] and 2.3×10[sup 12] s[sup -1] at 300 K, respectively, which suggests that the dissociation is three orders of magnitude faster than the scrambling. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Coupled-cluster [CCSD(T)] calculations of the potential energy surface of the Cl[SUP-]H[SUB2] complex were carried out, subsequently averaged over the vibrational motion of the H[SUB2] moiety and then fitted. The lower bend-stretch levels of the anion complex were then determined, for the H[SUB2] molecule in its ground and first excited vibrational levels. The resulting rotational constants agree with earlier experimental values, almost to within the experimental precision. The predicted dissociation energies are 5-8arger than the experimentally derived values. The bend-stretch wave function of the lowest state of the Cl[SUP-]oD[SUB2] complex encompasses a considerably larger range of bending motion than in the complexes with oH[SUB2] and pD[SUB2] . [ABSTRACT FROM AUTHOR]

In this letter we present preliminary results of full six dimensional quantum dynamics calculations for dissociative adsorption of a hydrogen molecule on a Cu(111) surface. We utilize the time-dependent wave-packet approach to simulate the dissociation process on a full dimensional LEPS potential energy surface which has incorporated the latest ab initio data [Hammer et al. Phys. Rev. Lett. 73, 1400 (1994)]. We use a novel partitioning of the angular momentum operator in the split-operator method so that a direct product DVR can be rigorously implemented. The most interesting observation in the present rigorous quantum dynamics study is the site-averaged effect, i.e., the averaged dissociation probability of the four dimensional calculations over the three symmetric impact sites strongly resembles the exact dissociation probability of the six dimensional calculations. In accord with the low dimensional calculations, initial vibrational excitation of H2 effectively reduces the translational threshold energy. The rotational orientation effect observed in the four dimensional studies remains in the present full dimensional dynamics with the cartwheel orientation yielding dramatically lower dissociative efficiency than the energetically equivalent helicopter orientation. We focus on normal incident scattering. The diffractive scattering and more detailed results will be presented in a later paper.. [ABSTRACT FROM AUTHOR]

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H2 and D2 on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (Ts = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found. [ABSTRACT FROM AUTHOR]

(We present results of classical trajectory calculations on dissociative adsorption and reflection of H[SUB2] impinging on a frozen Pd(110) surface. The six-dimensional potential energy surface is obtained by interpolation of density functional theory (DFT) calculations using the corrugation reducing (procedure. The comparison with earlier work for H[SUB2]/Pd(111) shows that DFT calculations predict that the Pd(110) surface, though more open, is less reactive than the Pd(111) one. This lower reactivity is associated with a more prominent role of dynamic trapping, at low energies, and with the fact that trapping may end up in reflection as well as in dissociation. [ABSTRACT FROM AUTHOR]

Collision induced dissociation and four center exchange reaction in H[sub 2](v[sub 1]=high)+H[sub 2](v[sub 2]=low) are studied by means of time-dependent wave packet calculations and within a three-degree-of-freedom reduced dimensionality model. The role of both—vibrationally excited and vibrationally cold—collision partners is examined varying v[sub 1] between 10 and 14 and v[sub 2] between 0 and 2, respectively. From the analysis of the results, a clear picture of the main mechanisms of dissociation and reaction has been obtained, and the regions of the potential energy surface most sensitive to the dynamics have been identified. In this way, reaction bottlenecks are found to significantly depend on the initial v[sub 1] state, owing to the anharmonicity of these v[sub 1] states near dissociation and the different regions of the potential explored by the associated wave packets. The topography of such bottlenecks provide a basis for the existence of tunneling in (v[sub 1]=10,12, v[sub 2]=0–2) reactions. Regarding the dissociation process, we find that there are two main mechanisms for dissociation; one where the unbroken diatom recoils with respect to dissociated fragments, and the second, where the diatom passes through the dissociated fragments. These mechanisms are responsible of a double peak observed in some dissociation probabilities. For (v[sub 1]=14, v[sub 2]=0–2) reagents, new processes appear with non-negligible probabilities: (i) inelastic collision by insertion of the cold diatom into the vibrationally excited one and (ii) dissociation of the initially cold diatom. These features, together with the observation of structures in all channel probabilities, suggest that four-atom complexes are formed during collision. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations of the energy and angular momentum dependence of the rate constant for the dissociation of C6H6+ into C6H5+ and an H atom are reported. In these variational calculations both the definition of the reaction coordinate and its value are independently optimized. A model potential-energy surface which interpolates between a Morse potential at short range and an ion-induced dipole potential at long range is employed in these variational calculations. The fully optimized variational results indicate that the transition state for this dissociation occurs at separation distances of about 3–4 Å and that the available phase space in the transition state is typically a factor of 5 lower than that predicted by phase space theory. Experimental measurements were made of the time-resolved product ion intensity resulting from the laser-induced dissociation of a thermal (≊375 K) distribution of benzene cations. An ion cyclotron resonance trap was used over a range of photolysis wavelengths from 266 to 285 nm. The observed time dependences in the product ion signals are a result of both dissociative and radiative relaxation processes with a deconvolution procedure yielding estimated dissociation rate constants. Satisfactory agreement between the theoretical and experimental results, including the previous experimental results of Neusser and co-workers [J. Phys. Chem. 93, 3897 (1989), and references cited therein] is obtained for an assumed dissociation energy of 3.88 eV to the lowest triplet state of C6H5+. [ABSTRACT FROM AUTHOR]

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time-of-flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm. [ABSTRACT FROM AUTHOR]

Calculations of the total and partial decay widths are presented for the vibrational predissociation of H2HF. The calculation is done for the total angular momentum J=0 of the molecule and for the dissociation process H2HF(v=1)→H2(v=0)+HF(v=0). A time-dependent golden rule wave packet method is employed in the numerical calculation for the decay widths. The lifetime of the complex is determined from our calculation to be about 600 ns for para- and 1600 ns for ortho-H2HF. These values are much larger than the experimentally measured value of 27 ns for ortho-H2HF. The large discrepancy in lifetime for H2HF is in sharp contrast to the excellent agreement in lifetime for D2HF between theory and experiment, though the same potential energy surface is used in both calculations. We also present the rotational state distribution of the fragments H2 and HF following the vibrational predissociation of H2HF. It is found that about 58% of the final rotational population is in j=4 states of H2 for para-H2HF and about 48% is in j=5 states of H2 for ortho-H2HF. [ABSTRACT FROM AUTHOR]

Results are reported of a four-dimensional dynamics study on the dissociation of H2 on Cu(100). The potential-energy surface was taken from density functional calculations, which employed the generalized gradient approximation and a slab representation for the surface. Reaction occurs preferentially in impacts near the bridge and hollow sites. Collisions near top sites promote vibrational excitation. The conclusion that vibrationally inelastic scattering and reaction occur preferentially on different sites can be generalized to other low index Cu surfaces. Resonances affect the reaction in the 4D model through a mechanism in which the molecule, trapped by excitation of the molecular bond which is weakened at the surface near top sites, is allowed more time to tunnel through the barrier to reaction. The calculated dependence of the diffraction probabilities on incidence energy suggests that a measurement of low-order diffraction would be able to determine whether the minimum barrier to reaction occurs for impacts on the bridge sites (as theory predicts) or on the hollow or top sites. The calculated reaction probabilities are in good agreement with experiment, further improvement being expected from including the rotational degrees of freedom in the model. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We develop a quantal model for studying four-center reactions, A2+B2→2AB, and collision induced dissociation A2+B2→A+B2+A. The method involves using hyperspherical coordinates to describe vibrations of the A2 and B2 bonds and a global vibration and rotation of the exchange products. Application to the H4 system is presented, using a realistic potential energy surface. The reaction goes through a four-center linear transition state located just above the dissociation threshold. In the energy range studied (5–5.5 eV), collision induced dissociation competes with the four-center reaction and is the dominant process. It is found that vibrational energy, originally deposited in one of the diatomic partners, is much more efficient than translational energy in promoting reaction. Vibrational and rotational final distributions show that the products are internally hot. This simple quantal model, yet very demanding computationally, illustrates in detail many features of the H4 dynamics above the dissociation threshold, and could serve to study other four center reactions with trapezoidal or linear transition states. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This is the first of a series of papers in which we investigate the unimolecular dissociation of hydroperoxyl. Using the DMBE IV potential energy surface [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)], in the present study 726 bound states of HO2(X) up to the H+O2 dissociation threshold are calculated in an attempt to access the extent of the coupling between the modes of the system. The first approach involves an analysis of the nodal structure of the wave functions. While the wave functions for the lowest states are regular and assignable, the degree of mixing and complexity rapidly increases with energy. The wave functions close to the dissociation threshold are mostly irregular without any clear cut nodal structure and fill the entire coordinate space available. Nevertheless, a small number of regular states, that are associated with large excitation in the O2 stretching coordinate and no or only little excitation in the other modes, are found even at high energies. The second approach used to study the degree of intramolecular coupling is an analysis of the energy spectrum. The nearest neighbor level spacing distribution, which probes the short-range correlation, as well as the Σ2 and Δ3 statistics, which are sensitive to the long-range correlations in the spectrum, are investigated and compared to the distributions predicted for regular and irregular spectra. Both of these approaches indicate that the system is almost totally irregular with a Brody parameter of about 92%. In addition, the sum of states at a particular energy, which is extremely important in all statistical models for unimolecular dissociation, is approximately calculated from the volume of classical phase space and found to be in excellent agreement with the exact quantum mechanical result. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Lee–DePristo model for the dissociative chemisorption of H2 on Ni(100) has an interesting behavior. The potential energy surface has a minimum corresponding to a molecular, physisorbed state. This minimum is shallow and at 300 K the lifetime of physisorbed H2 is extremely short. One is then justified in treating the kinetics as a direct dissociation and ignoring physisorption. At 100 K physisorbed H2 has a long lifetime and one is forced to consider a two-step kinetics: H2 is first physisorbed and then dissociated. While chemical kinetics describes easily these two limiting cases, it offers little guidance for the intermediate temperature. We show here how the correlation function theory deals with this situation by providing equations which cover all temperature regimes. The theory is general and can be used in all cases when intermediates with shallow wells participate in the reaction mechanism. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio predictions of the molecular geometry, harmonic vibrational frequencies, and dissociation energies are reported for the germanium hydride cluster ion GeH+7. Seven stationary points were located on the potential energy surface (PES) of GeH+7 using the self-consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included triple excitations [CCSD(T)] methods in conjunction with a double-ζ plus polarization (DZP) and a triple-ζ plus polarization TZ(3d1f,1p) quality basis set. The most stable structure has a C2 symmetry with the two H2 subunits rotating freely about the symmetry axis of the GeH+3 fragment. Our best estimate of the dissociation energy for GeH+7, taking into account the zero point vibrational energy (ZPVE) is 3.10 kcal/mol compared to 4.6 and 1.2 kcal/mol obtained, respectively, for the SiH+7 and CH+7 cluster ions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A simple picture of the hydrogen dissociation/associative desorption dynamics on Cu(111) emerges from a two-parameter, full dimensionality microcanonical unimolecular rate theory (MURT) model of the gas-surface reactivity. Vibrational frequencies for the reactive transition state were taken from density functional theory calculations of a six-dimensional potential energy surface [Hammer et al., Phys. Rev. Lett. 73, 1400 (1994)]. The two remaining parameters required by the MURT were fixed by simulation of experiments. These parameters are the dissociation threshold energy, E0=79 kJ/mol, and the number of surface oscillators involved in the localized H2/Cu(111) collision complex, s=1. The two-parameter MURT quantitatively predicts much of the varied behavior observed for the H2 and D2/Cu(111) reactive systems, including the temperature-dependent associative desorption angular distributions, mean translational energies of the associatively desorbing hydrogen as a function of rovibrational eigenstate, etc. The divergence of the statistical theory’s predictions from experimental results at low rotational quantum numbers, J≤=5, suggests that either (i) rotational steering is important to the dissociation dynamics at low J, an effect that washes out at high J, or (ii) molecular rotation is approximately a spectator degree of freedom to the dissociation dynamics for these low J states, the states that dominate the thermal reactivity. Surface vibrations are predicted to provide ∼30% of the energy required to surmount the activation barrier to H2 dissociation under thermal equilibrium conditions. The MURT with s=1 is used to analytically confirm the experimental finding that ∂ “Ea(Ts)”/∂Et=-1 for eigenstate-resolved dissociative sticking at translational energies Et≤E0-Ev-Er. Explicit treatment of the surface motion (i.e., surface not frozen at Ts=0 K) is a relatively novel aspect of the MURT theoretical approach. [ABSTRACT FROM AUTHOR]

This paper presents the results of a detailed study of dissociation and recombination of H2 over the temperature range 1000 to 5000 K. The chemical processes are modeled by solving the master equation for the concentrations of the full set of rovibration states. All of the state-to-state energy transfer rate coefficients required by the master equation are evaluated by the quasiclassical trajectory method using a potential energy surface which is a fit to ab initio electronic structure calculations. The analysis of the results of the master equation to obtain the phenomenological rate coefficients was carried out using several techniques. This is required because there is a mixture of third bodies and their concentrations change as the reaction proceeds. The methods based on one-way fluxes are not reliable, while the methods based upon an eigenvalue solution of the linearized master equation are in reasonable agreement with our preferred method, which is based on fitting the concentrations from a two component master equation. The results from various methods of determining the phenomenological rate coefficient for H2 as a third body are in good agreement with experimental estimates. The recombination rate coefficients are most sensitive to the collision induced dissociation rates of initial states with moderate values of vibrational and rotational quantum numbers. A comparison of the orbiting resonance theory to the accurate results shows that this simple model is not valid under the conditions of the present study. [ABSTRACT FROM AUTHOR]

We present calculations for the dissociative adsorption of hydrogen molecules on a Cu surface as a function of initial translational energy and vibrational quantum state. Classical, semiclassical, and fully quantum calculations are performed and the results compared. The potential energy surface was based upon a total energy calculation for H2 on a small Cu cluster and has been previously employed in dynamical simulations. Our results show that for low primary beam energies, dissociation occurs primarily via tunneling through the activation barrier in the vibrational coordinate. Populating the initial vibrational states is shown to enhance reactivity, but not simply by a total energy shift. By changing the hydrogen isotope it is shown that tunneling effects can persist up to quite high molecular masses. This occurs because the activation barrier lies in the vibrational coordinate, where the reduced mass of the molecule determines the dynamics. [ABSTRACT FROM AUTHOR]

A time-dependent quantum wave packet method has been applied to studying the process of activated dissociative chemisorption of H2 on Ni(100) surface. The Ni surface is treated as static and the effect of weak surface correlation is neglected in our dynamics calculation. The three-dimensional calculation is fully quantum mechanical without any reduced dimension approximation. An empirical London–Erying–Polanyi–Sato (LEPS) potential surface has been used and modified in our dynamics calculation to produce a reasonable barrier height compatible with experiment. Sticking probabilities have been computed as a function of initial normal incident kinetic energy and are compared to experimental results as well as another 3D quantum dynamics calculation. Good agreement has been found between our theoretical calculation and molecular beam experiment in the energy dependence of the dissociation probability. It is also found that vibrational excitation of H2 enhances dissociation probability, in agreement with the prediction of an earlier theoretical calculation. Snapshots of the wave function are plotted that provide intimate details of the dissociation dynamics in time and space. [ABSTRACT FROM AUTHOR]

Metastable levels of OH–Ar which lie significantly above the OH (A 2Σ+)+Ar dissociation limit are identified. The rates of internal rotational predissociation and the resultant OH (A 2Σ+) product rotational distributions are determined. Comparison with theoretical calculations of the predissociative states provides a sensitive test of the short-range interaction potential. [ABSTRACT FROM AUTHOR]

Numerous processes in atmospheric and combustion chemistry produce the vinoxy radical (•CH2CHO). To understand the fate of this radical and to provide reliable energies needed for kinetic modeling of such processes, we have examined its reaction with O2 using highly reliable theoretical methods. Utilizing the focal point approach, the energetics of this reaction and subsequent reactions were obtained using coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] extrapolated to the complete basis set limit. These extrapolated energies were appended with several corrections including a treatment of full triples and connected quadruple excitations, i.e., CCSDT(Q). In addition, this study models the initial vinoxy radical + O2 reaction for the first time with multireference methods. We predict a barrier for this reaction of approximately 0.4 kcal mol−1. This result agrees with experimental findings but is in disagreement with previous theoretical studies. The vinoxy radical + O2 reaction produces a 2-oxoethylperoxy radical which can undergo a number of unimolecular reactions. Abstraction of a β-hydrogen (a 1,4-hydrogen shift) and dissociation back to reactants are predicted to be competitive to each other due to their similar barriers of 21.2 and 22.3 kcal mol−1, respectively. The minimum-energy β-hydrogen abstraction pathway produces a hydroperoxy radical (QOOH) that eventually decomposes to formaldehyde, CO, and •OH. Two other unimolecular reactions of the peroxy radical are α-hydrogen abstraction (38.7 kcal mol−1 barrier) and HO2• elimination (43.5 kcal mol−1 barrier). These pathways lead to glyoxal + •OH and ketene + HO2• formation, respectively, but they are expected to be uncompetitive due to their high barriers. [ABSTRACT FROM AUTHOR]

We have performed calculations on the dissociative chemisorption of H2 on un-reconstructed and reconstructed Au(111) with density functional theory, and dynamics calculations on this process on un-reconstructed Au(111). Due to a very late barrier for dissociation, H2 + Au(111) is a candidate H2-metal system for which the dissociative chemisorption could be considerably affected by the energy transfer to electron-hole pairs. Minimum barrier geometries and potential energy surfaces were computed for six density functionals. The functionals tested yield minimum barrier heights in the range of 1.15-1.6 eV, and barriers that are even later than found for the similar H2 + Cu(111) system. The potential energy surfaces have been used in quasi-classical trajectory calculations of the initial (v,J) state resolved reaction probability for several vibrational states v and rotational states J of H2 and D2. Our calculations may serve as predictions for state-resolved associative desorption experiments, from which initial state-resolved dissociative chemisorption probabilities can be extracted by invoking detailed balance. The vibrational efficacy ηv=0→1 reported for D2 dissociating on un-reconstructed Au(111) (about 0.9) is similar to that found in earlier quantum dynamics calculations on H2 + Ag(111), but larger than found for D2 + Cu(111). With the two functionals tested most extensively, the reactivity of H2 and D2 exhibits an almost monotonic increase with increasing rotational quantum number J. Test calculations suggest that, for chemical accuracy (1 kcal/mol), the herringbone reconstruction of Au(111) should be modeled. [ABSTRACT FROM AUTHOR]

Direct-dynamics variational transition-state theory calculations are studied at the MPWB1K/6-311++G(d,p) level for the four parts of reactions. The first part is hydrogen or deuterium abstraction in the reactions of CH3 + CH4, CH3 + CD4, and CH3D + CH3. The second part involves C-C bond formation in these reactions. The third one is the reactions of CH3CH3 + H and CH3CD3 + D to form of H2, HD, and D2. The last one is the dissociation of C-C bonds in the last group of reactions. The ground-state vibrational adiabatic potential is plotted for all channels. We have carried out direct-dynamics calculations of the rate constants, including multidimensional tunneling in the temperature range T = 200-2200 K. The results of CVT/μOMT rate constants were in good agreement with the experimental data which were available for some reactions. Small-curvature tunneling and Large-curvature tunneling with the LCG4 version were used to include the quantum effects in calculation of the rate constants. To try to find the region of formation and dissociation of bounds we have also reported the variations of harmonic vibrational frequencies along the reaction path. The thermally averaged transmission probability (P(E)exp (-ΔE/RT)) and representative tunneling energy at 298 K are reported for the reactions in which tunneling is important. We have calculated kinetic isotope effect which shows tunneling and vibrational contributions are noticeable to determine the rate constant. Nonlinear least-squares fitting is used to calculate rate constant expressions in the temperature range 200-2200 K. These expressions revealed that pre-exponential factor includes two parts; the first part is a constant number which is important at low temperatures while the second part is temperature dependent which is significant at high temperatures. [ABSTRACT FROM AUTHOR]

Recently, occurrence of coalescence point was reported in H2+ undergoing multiphoton dissociation in strong laser field. We have applied parametric equations of motion and smooth exterior scaling method to study the coalescence phenomenon of H2+. The advantage of this method is that one can easily trace the different states that are changing as the field parameters change. It was reported earlier that in the parameter space, only two bound states coalesce [R. Lefebvre, O. Atabek, M. Sindelka, and N. Moiseyev, Phys. Rev. Lett. 103, 123003 (2009)]. However, it is found that increasing the accuracy of the calculation leads to the coalescence between resonance states originating from the bound and the continuum states. We have also reported many other coalescence points. [ABSTRACT FROM AUTHOR]

Through a combined experimental and theoretical approach, we study the nonadiabatic dynamics of the prototypical ethylene (C2H4) molecule upon π → π* excitation with 161 nm light. Using a novel experimental apparatus, we combine femtosecond pulses of vacuum ultraviolet and extreme ultraviolet (XUV) radiation with variable delay to perform time resolved photo-ion fragment spectroscopy. In this second part of a two part series, the XUV (17 eV < hν < 23 eV) probe pulses are sufficiently energetic to break the C-C bond in photoionization, or to photoionize the dissociation products of the vibrationally hot ground state. The experimental data is directly compared to excited state ab initio molecular dynamics simulations explicitly accounting for the probe step. Enhancements of the CH2+ and CH3+ photo-ion fragment yields, corresponding to molecules photoionized in ethylene (CH2CH2) and ethylidene (CH3CH) like geometries are observed within 100 fs after π → π* excitation. Quantitative agreement between theory and experiment on the relative CH2+ and CH3+ yields provides experimental confirmation of the theoretical prediction of two distinct conical intersections and their branching ratio [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A. 113, 13656 (2009)]. Evidence for fast, non-statistical, elimination of H2 molecules and H atoms is observed in the time resolved H2+ and H+ signals. [ABSTRACT FROM AUTHOR]

Collision induced dissociation (CID), four center reaction (4C), and single exchange reaction (SE) in H2 (v1 = high) + D2 (v2 = low) were studied by means of time-dependent wave packet approach within a full-dimensional model. Initial state-selected total reaction probabilities for the three competitive processes have been computed on two realistic global potential energy surfaces of Aguado-Suárez-Paniagua and Boothroyd-Martin-Keogh-Peterson (BMKP) with the total angular momentum J = 0. The role of both vibrationally excited and rotationally excited reagents was examined by varying the initial vibrational and rotational states. The vibrational excitation of the hot diatom gives an enhancement effect on the CID process, while the vibrational excitation of the cold diatom gives an inhibition effect. The rotational excitation of both reagents has a significant effect on the reaction process. The 4C and SE probabilities are at least one order of magnitude smaller than the CID probabilities over the energy range considered. Isotope substitution effects were also studied by substituting the collider D2 by H2 and HD on the BMKP potential energy surfaces. The CID process is most efficient for the H2 + D2 combination and least efficient for the H2 + H2 combination and is different for the 4C and SE processes. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations with a polarizable multistate empirical valence-bond model were carried out to investigate NaOH dissociation and pairing in water bulk and at the air-water interface. It was found that NaOH readily dissociates in the bulk and the effect of the air-water interface on NaOH dissociation is fairly minor. Also, NaOH complexes were found to be strongly repelled from the air-water interface, which is consistent with surface tension measurements. At the same time, a very strong preference for the hydroxide anion to be oriented toward the air was found that persisted a few angstroms toward the liquid from the Gibbs dividing surface of the air-water interface. This was due to a preference for the hydroxide anion to have its hydrogen pointing toward the air and the fact that the sodium ion was more likely to be found near the hydroxide oxygen than hydrogen. As a consequence, the simulation results show that surfaces of NaOH solutions should be negatively charged, in agreement with experimental observations, but also that the hydroxide has little surface affinity. This provides the possibility that the surface of water can be devoid of hydroxide anions, but still have a strong negative charge. [ABSTRACT FROM AUTHOR]

We investigate and analyze the vibrational properties, including hydrogen/deuterium isotope effects, in a fundamental organic hydrogen bonded system using multiple experimental (infrared multiple photon dissociation and argon-tagged action spectroscopy) and computational techniques. We note a qualitative difference between the two experimental results discussed here and employ ab initio molecular dynamics simulations to explain these results. A deeper understanding of the differences between the isotopically labeled systems arises from an analysis of the simulated cluster spectroscopy and leads to a system-bath coupling interpretation. Specifically, when a few active modes, involving the shared hydrogen/deuterium stretch, are identified and labeled as “system,” with all other molecular vibrational modes being identified as “bath” modes, we find critical differences in the coupling between the system modes for the shared proton and shared deuteron cases. These differences affect the energy repartitioning between these modes resulting in a complex spectral evolution as a function of temperature. Furthermore, intensity borrowing across modes that are widely distributed in the frequency domain plays an important role on the simulated spectra. [ABSTRACT FROM AUTHOR]

We have studied the influence of preadsorbed CO on the dissociative adsorption of H2 on Ru(0001) with density functional theory calculations. For a coverage of 1/3 ML CO, we investigated different possible reaction paths for hydrogen dissociation using nudged elastic band and adaptive nudged elastic band calculations. One reaction path was studied in detail through an energy decomposition and molecular orbital type of analysis. The minimum barrier for H2 dissociation is found to be 0.29 eV. At the barrier the H–H bond is hardly stretched. Behind this barrier a molecular chemisorption minimum is present. Next, the molecule overcomes a second barrier, with a second local chemisorption minimum behind it. To finally dissociate to chemisorbed atoms, the molecule has to overcome a third barrier. To move along the reaction path from reactants to products, the hydrogen molecule needs to rotate, and to significantly change its center-of-mass position. The procedure of mapping out reaction paths for H2 reacting on low-index surfaces of bare metals (computing two-dimensional elbow plots for fixed impact high-symmetry sites and H2 orientations parallel to the surface) does not work for H2+CO/Ru. The first barrier in the path is recovered, but the features of the subsequent stretch to the dissociative chemisorption minimum are not captured, because the molecule is not allowed to change its center-of-mass position or to rotate. The dissociative chemisorption of H2 on CO/Ru(0001) is endoergic, in contrast to the case of H2 on bare Ru(0001). The zero-point energy corrected energies of molecularly and dissociatively chemisorbed H2 are very close, suggesting that it may be possible to detect molecularly chemisorbed H2 on (3×3)R30°CO/Ru(0001). The presence of CO on the surface increases the barrier height to dissociation compared with bare Ru(0001). Based on an energy decomposition and molecular orbital analysis we attribute the increase in the barrier height mainly to an occupied-occupied interaction between the bonding H2 σg orbital and the (surface-hybridized) CO 1π orbitals, i.e., to site blocking. There is a small repulsive contribution to the barrier from the interaction between the H2 molecule and the Ru part of the CO covered Ru surface, but it is smaller than one might expect based on the calculations of H2 interacting with a clean Ru surface, and on calculations of H2 interacting with the CO overlayer only. Actually, the analysis suggests that the Ru surface as a subsystem is (slightly) more reactive for the reaction path studied with CO preadsorbed on it than without it. Thus, the results indicate that the influence of CO on H2 dissociation on Ru is not only a simple site-blocking effect, the electronic structure of the underlying Ru is changed. [ABSTRACT FROM AUTHOR]

Ab initio quantum chemical methods have been employed to investigate the structure, stability, charge redistribution, and harmonic vibrational frequencies of rare gas (Rg=He, Ne, Ar, Kr, and Xe) containing HRgCO+ ion. The Rg atoms are inserted in between the H and C atoms of HCO+ ion and the geometries are optimized for minima as well as transition state using second order Mo\ller–Plesset perturbation theory, density functional theory, and coupled-cluster theory [CCSD(T)] methods. The HRgCO+ ions are found to be metastable and exhibit a linear structure at the minima position and show a nonlinear structure at the transition state. The predicted ion is unstable with respect to the two-body dissociation channel leading to the global minima (HCO++Rg) on the singlet potential surface. The binding energies corresponding to this channel are -406.4, -669.3, -192.3, -115.4, and -52.2 kJ mol-1 for HHeCO+, HNeCO+, HArCO+, HKrCO+, and HXeCO+ ions, respectively, at CCSD(T) method. However, with respect to other two-body dissociation channel, HRg++CO, the ions are found to be stable and have positive energies except for HNeCO+ at the same level of theory. The computed binding energies for this channel are 15.0, 28.8, 29.5, and 29.1 kJ mol-1 for HHeCO+, HArCO+, HKrCO+, and HXeCO+ ions, respectively. Very high positive three-body dissociation energies are found for H+Rg+CO+ and H++Rg+CO dissociation channels. It indicates the existence of a very strong bonding between Rg and H atoms in HRgCO+ ions. The predicted ions dissociate into global minima, HCO++Rg, via a transition state involving H–Rg–C bending mode. The barrier heights for the transition states are 22.7, 10.1, 13.1, and 15.0 kJ mol-1 for He, Ar, Kr, and Xe containing ions, respectively. The computed two-body dissociation energies are comparable to that of the experimentally observed mixed cations such as ArHKr+, ArHXe+, and KrHXe+ in an electron bombardment matrix isolation technique. Thus HRgCO+ cations may also be possible to prepare and characterize similar to the mixed cations (RgHRg′)+ in low temperature matrix isolation technique. [ABSTRACT FROM AUTHOR]

It is well-known, both theoretically and experimentally, that alloying MgH2 with transition elements can significantly improve the thermodynamic and kinetic properties for H2 desorption, as well as the H2 intake by Mg bulk. Here, we present a density functional theory investigation of hydrogen dissociation and surface diffusion over a Ni-doped surface and compare the findings to previously investigated Ti-doped Mg(0001) and pure Mg(0001) surfaces. Our results show that the energy barrier for hydrogen dissociation on the pure Mg(0001) surface is high, while it is small/null when Ni/Ti are added to the surface as dopants. We find that the binding energy of the two H atoms near the dissociation site is high on Ti, effectively impeding diffusion away from the Ti site. By contrast, we find that on Ni, the energy barrier for diffusion is much reduced. Therefore, although both Ti and Ni promote H2 dissociation, only Ni appears to be a good catalyst for Mg hydrogenation, allowing diffusion away from the catalytic sites. Experimental results corroborate these theoretical findings, i.e., faster hydrogenation of the Ni-doped Mg sample as opposed to the reference Mg- or Ti-doped Mg. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of a HX (X=Cl,F) molecule adsorbed on a hexagonal ice surface at T=0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures. [ABSTRACT FROM AUTHOR]

Within the framework of Bohm’s reformulation of quantum physics we revisit the activated dissociation of hydrogen molecules at metal surfaces. The quantum-mechanical force, which accounts for most of quantum effects in the method, and is caused by nonlocal, topographical properties of the wave function, is computed using time-dependent wave packets obtained using conventional, spectral methods. Driven by a combination of the classical force together with the quantum force, trajectories carrying probability density either succeed in overcoming the barrier for dissociation or are scattered back into the gas phase. The Bohmian picture for the dissociation process has enabled us to develop a novel mechanism to account for vibrationally enhanced molecular dissociation. This is relevant to the recently observed promotion of dissociation of very highly vibrationally excited NO molecules at Cu surfaces. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies dissociation processes in three-dimensional helium+hydrogen collisions using a semiclassical approach which treats the relative radial motion in classical mechanics and other motions in quantum mechanics. Centrifugal sudden approximation assumed to solve the semiclassical equation; Comparison of results with previous full quantum mechanical calculations.

Studies the energetics of hydrogen[sub 2] dissociative absorption on and associative desorption from Si(001). Clusters used in density functional theory calculations; Exchange-correlation functionals; Estimates of the activation barriers and reaction energy; Differences in calculated activation energies.

Ab initio configuration interaction calculations are performed to study the dissociative adsorption of H2 on a Ni(111) surface. The lattice is modeled as an embedded three-layer 41-atom cluster. Ni 3d orbitals are explicitly included on seven Ni atoms on the surface. H is preferentially chemisorbed at a threefold site on Ni(111) and the calculated binding energy of 62 kcal/mol, H–Ni distance of 1.86 Å, and H vibrational frequency of 1176 cm-1 are in excellent agreement with experimental data. H adsorbed at bridge and on-top Ni sites is 2.5 and 8.1 kcal/mol less stable, respectively. The heat of reaction H2 (gas)→2 H (ads) is calculated to be 22.0 kcal/mol exothermic. When two H atoms are adsorbed as nearest neighbors to the same Ni atom, threefold sites are preferred with H atoms adsorbed at fcc–fcc, hcp–hcp, or across atom fcc–hcp sites. These structures are consistent with the observed (2×2)-2H low energy electron diffraction pattern. The average adsorption energy per H is calculated to be 62 kcal/mol for the across atom case. Adsorption of H at a threefold site on Ni(111) has a tendency to block adsorption at adjacent threefold sites; coadsorbed H atoms at adjacent sites are unbound by 13.5 kcal/mol relative to H2 at infinite separation. For three H atoms chemisorbed at separated threefold sites sharing a Ni atom with the equal H–H lateral distances of 2.48 Å, the calculated average adsorption energy is 60 kcal/mol per H. Dissociation of H2 at an on-top Ni site has the lowest activation barrier of 1.6 kcal/mol. For the transition state, the H–H bond length, H-surface height, and H–H vibrational frequency are 1.22 Å, 1.38 Å, and 2506 cm-1, respectively. Molecular H2 adsorbed over an on-top site is bound by 3.4 kcal/mol with a slightly stretched H–H bond length of 0.79 Å, H-surface height of 1.85 Å, and H–H stretching frequency of 3396 cm-1. [ABSTRACT FROM AUTHOR]

The dissociative adsorption of H2 on copper is studied using classical trajectory and quantum wave-packet calculations. A multidimensional effective-medium potential is used which takes the molecular orientation as well as the surface structure into account. The emphasis in this work is on the effects of including all six molecular degrees of freedom in calculations of the sticking coefficient and the implications on comparisons with experimental data. We find that six-dimensional effects, among other things, lead to substantial shifts of the sticking onset energies. The change of onset energy due to vibrational excitation or isotope exchange is also sensitive to six-dimensional effects. This means that a two-dimensional analysis of the relation between observed onset energies and potential barriers sometimes may be misleading. The agreement between calculated and experimental sticking curves is improved by including six dimensions. The surface atom motion, however, is found to have a negligible effect on the sticking coefficient at surface temperatures used in recent molecular beam measurements. By comparisons in reduced dimensionality we find that a quasiclassical calculation gives a reasonable account of the true quantum sticking coefficient in the present case. There are significant quantum effects but these appear small compared to the multidimensional effects. A multidimensional combined quantum and classical approach seems to be appropriate for the dissociation dynamics problem. [ABSTRACT FROM AUTHOR]

Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm-1–23 000 cm-1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser-induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well-resolved transition lines measured by high-resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specificity in the dissociation rates is observed. For states with approximately the same total energy those with higher excitation in the out-of-plane bending mode (ν6) dissociate more slowly than others. For rotationless levels in the tunneling region, the A’ states decay much more rapidly than the A‘ states indicating symmetry-induced mode specificity.The dissociation rates in HFCO almost surely exhibit mode specificity because the ν6 mode is weakly coupled to the reaction coordinate. The enhancement of this coupling by Coriolis forces appears to produce the unprecedentedly large rotational level dependence of the rates. However, the spectroscopically observed coupling of ν6 to the background of vibrationally mixed levels is not significantly increased by rotation. Thus states with nearly all of the excitation energy in the reaction coordinate appear not to be strongly mixed into the background states. The large effect of... [ABSTRACT FROM AUTHOR]

The state-to-state vibrational predissociation dynamics of the hydrogen fluoride dimer has been investigated in detail using a newly developed instrument which gives both initial state selection and photofragment state determination. Results are reported for a wide variety of initial states associated with the ν1 and ν2 vibrations. The final state distributions universally indicate that the preferred dissociation channels correspond to the production of one HF fragment that is highly rotationally excited and another that is not. This is explained in terms of an impulsive dissociation mechanism which proceeds from a geometry close to that of the equilibrium structure of the dimer. We find that nearly degenerate initial states can have rather different final state distributions. In particular, there is a distinct difference between the upper and lower members of the tunneling doublet, which is most likely attributable to their related symmetries. The dissociation energy (D0) is determined to be 1062±1 cm-1. [ABSTRACT FROM AUTHOR]