Mario Cacciatore

Eley-Rideal and Langmuir-Hinshelwood recombination coefficients for oxygen on silica surfaces

The energetics and dynamics of the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) recombination reaction of oxygen atoms on β cristobalite have been studied within a semiclassical collisional model. The calculated recombination coefficient γ for the E-R reaction at T s = 1000 K is in satisfactory agreement with the experimental value, whereas at T s = 600 K a satisfactory agreement is found between the L-H γ value and the experimental one. Results on the energy exchanges between the formed O 2 molecules and the silica surface are reported. Site-specific effects as well as the influence of the top layer surface structure are also pointed out and discussed.

Semiclassical calculation of VV and VT rate coeffecients in CO

Abstract We have calculated 66 rate constants for vibration-vibration (V-V) and vibration-translation/rotation (V-T/R) energy transfer in CO by using a semiclassical collision model. Extensive comparison with experimental data and more approximate theoretical predictions is carried out.

Hydrogen atom recombination on graphite at 10 K via the Eley–Rideal mechanism

Abstract This work reports results on the recombination of hydrogen atoms on a graphite surface at 10 K obtained using a detailed semi-classical molecular dynamics method in connection with the use of recently proposed ab initio potential energy surfaces. The calculated recombination probabilities together with the vibrational distributions of the formed H 2 molecules obtained assuming that the surface reaction proceeds via the Eley–Rideal mechanism can have an impact on the chemistry of H 2 formation in interstellar space.

Dynamical relaxation of H2(v,j) on a copper surface

Abstract Dynamical studies of hydrogen-copper collisions are carried out using an analytical potential energy surface fitted to the data of recent ab initio calculations. We have investigated the role of tunnelling as well as the importance of electron-hole pair excitation and particularly the effect of screening in collisions between the hydrogen molecule and the copper surface.

Theoretical semiclassical investigation of the vibrational relaxation of CO colliding with 14N2

Abstract Vibrational state-to-state rate constants for CO deactivation in collisions with 14 N 2 are calculated in the temperature interval T =100–1000 K. The vibrational quantum number of CO and N 2 runs in the interval 0–40 so that sufficiently wide samples of the VV and VT/R relaxation matrix for this important collisional system are obtained. The relative importance of one-quantum and multi-quantum transitions is also investigated. The theoretical results reproduce quite well the υ dependence of the transition probabilities experimentally determined by Hancock and Smith for the process: CO(υ - 1)+N 2 (υ′=1)→CO(υ)+ N 2 (υ′=0) - Δ E thus confirming that rate constants can be predicted by the semiclassical dynamical model used in this study within an accuracy of ≈ 30%.

Kinetic processes in non-equilibrium carbon monoxide discharges. I. Vibrational kinetics and dissociation rates

Abstract Vibrational distributions and dissociation rates of carbon monoxide under non-equilibrium conditions have been calculated by solving the vibrational master equation coupled to the plasma chemistry and to the Boltzmann equation for the electron energy distribution function. The results show the predominance of a vibrational mechanism in dissociating CO and the strong coupling between the vibrational distribution of CO and the concentration of the species (C, O and CO 2 ), generated by the dissociation process, due to the large V-T (vibration-translation) energy transfer of these species. Finally the dependence of the vibrational distribution and of the dissociation rates on the recombination is analyzed by means of a simple model which allows the insertion of loss terms for the species C, O and CO 2 in the relevant plasma chemistry equations.

Semiclassical calculation of vibrational relaxation of CO colliding with 4He and 3He

Abstract The rate constant for the deactivation of CO (υ = 1) colliding with 4 He and 3 He is calculated in the range 100–1000 K. Agreement with experiment can be obtained using a hybrid isotropic potential. The isotopic ratio k 10 ( 3 He) k 10 ( 4 He) is predicted to be strongly temperature dependent at low temperatures.

Semiclassical calculation of the reaction probability for the process C + O → CO on a Pt(111) surface

Abstract The formation of CO on a Pt(111) surface is studied using a semiclassical approach. The reaction probability is calculated as a function of kinetic energy of the carbon atom and initial oxygen position on the surface.

Vibration-to-translation energy exchanges in H2 colliding with highly vibrationally excited H2 molecules

Rate constants Kν,ν−1ν,ν for vibrational deactivation of H2 in highly excited vibrational states have been calculated using a semiclassical dynamical model. The results show a strong increase of K1,00,0 as well as Kν,ν−1ν,ν as a function of the vibrational quantum number ν. The dependence of K1,0ν,ν on ν could open new perspectives in non-equilibrium vibrational kinetics of H2.

Oxygen Adsorption on β-Cristobalite Polymorph: Ab Initio Modeling and Semiclassical Time-Dependent Dynamics

The adsorption dynamics of atomic oxygen on a model beta-cristobalite silica surface has been studied by combining ab initio electronic structure calculations with a molecular dynamics semiclassical approach. We have evaluated the interaction potential of atomic and molecular oxygen interacting with an active Si site of a model beta-cristobalite surface by performing DFT electronic structure calculations. As expected, O is strongly chemisorbed, E(b) = 5.57 eV, whereas molecular oxygen can be weakly adsorbed with a high-energy barrier to the adsorption state of approximately 2 eV. The binding energies calculated for silica clusters of different sizes have revealed the local nature of the O,O(2)-silica interaction. Semiclassical collision dynamic calculations show that O is mainly adsorbed in single-bounce collisions, with a smaller probability for adsorption via a multicollision mechanism. The probability for adsorption/desorption (reflected) collisions at the three impact energies is small but not negligible at the higher energy considered in the trajectory calculations, about P(r) = 0.2 at E(kin) = 0.8 eV. The calculations give evidence of a complex multiphonon excitation-deexcitation mechanism underlying the dynamics of stable adsorption and inelastic reflection collisions.