Maurice Monnerville

Spin–orbit branching in the photodissociation of HBr: Time-independent, time-dependent, and semiclassical calculations

The dynamics of the photofragmentation of HBr is treated within time-independent, time-dependent, and semiclassical methods. The calculated relative cross sections for formation of the two accessible fine-structure channels [Br(2P1/2) and Br(2P3/2)] agree well with the experimental results, both in magnitude and in dependence on photon excitation wavelength. For relatively small photon wavelength (λ=193 nm), vertical excitation in the Franck–Condon region populates preferentially the A 1Π state, and only three states (A 1Π, the Ω=1 components of the a 3Π and 1 3Σ+), coupled by the spin–orbit interaction, are invoved in the dissociation process. For larger photon wavelength (λ=243 nm), the product branching is governed by initial excitation in both the A 1Π state and the a 3Π(Ω=0) component. Comparison of the redistribution of the time-independent photofragment fluxes as a function of the H–Br separation with the temporal evolution of the populations within a time-dependent framework shows that the two met...

First quantum investigation of the photodissociation of the Ar–HBr complex: three-dimensional time-dependent approach

Abstract The ultraviolet photodissociation of the Ar–HBr van der Waals complex is investigated using an exact three-dimensional time-dependent wave packet approach with zero-total angular momentum. Due to the large size of the required grid, calculations were carried out up to 60 fs. This time was sufficient for calculating the absorption spectrum as well as the final kinetic energy distribution of the H atom. Both were found to be completely structureless. Thus, the dynamics is entirely dominated by the fast direct dissociation of the hydrogen atom producing hot H fragments and bound Ar–Br molecules.

Multiconfiguration time-dependent Hartree and classical dynamics studies of the photodissociation of HF and HCl molecules adsorbed on ice: Extension to three dimensions

The 3D photodissociation dynamics of HCl and HF molecules adsorbed on ice is studied by quantum and classical simulations. The quantum calculations are carried out with the multiconfiguration time-dependent Hartree (MCTDH) approach. Dynamical observables like angular distributions in the momentum space of the H fragments, absorption cross sections are computed. The results are compared with our previous 2D studies. As expected, less encapsulation of the H atom between the ice surface and the halogen atom is obtained in the 3D study, resulting in less pronounced interference structures in the photoabsorption cross sections and in a decrease of the classical rainbow peaks observed in the 2D scheme. Although the amplitudes of the oscillations corresponding to quantum interferences in the asymptotic angular distribution of the H fragment are different between the 2D and 3D results, the qualitative pattern of the oscillations is similar in the 2D and 3D approaches. In addition, a good agreement is observed for the angular distribution between the classical and the quantum calculations.

Water-Induced Organization of Palmitic Acid at the Surface of a Model Sea Salt Particle: A Molecular Dynamics Study

Marine aerosols represent the most important aerosol fraction in the Earth atmosphere. Field studies have revealed that fatty acids form an organic film at the surface of sea salt particles, altering the properties of the aerosol. By means of classical molecular dynamics simulation, the surface organization of palmitic acid (PA) on a salt surface, NaCl, has been investigated at two different temperatures, 235 and 300 K, and with relative humidity varying from 0 to 40%. Calculations show that water promotes the formation of well-ordered close-packed PA islands. As a result, some area of the salt may be covered by water only or by PA molecules supported by water. Depending on the relative humidity, the hydrophilic/hydrophobic character of the sea salt surface varies. This heterogeneous coating gives rise locally to very different surface properties and hence may affect the transfer of gas phase species to the salt and their reactivity.

Quantum and classical non-adiabatic dynamics of Li

The 3D photodissociation dynamics of LiNe system is investigated by quantum calculations using the multi-configuration time-dependent Hartree (MCTDH) method and by classical simulations with the trajectory surface hopping (TSH) approach. Six electronic states of A’ symmetry and two states of A” symmetry are involved in the process. Couplings in the excitation region and two conical intersections in the vicinity of the Franck–Condon zone control the non-adiabatic nuclear dynamics. A diabatic representation including all the states and the couplings is determined. Diabatic and adiabatic populations calculated for initial excitation to pure diabatic and adiabatic states lead to a clear understanding of the mechanisms governing the non-adiabatic photodissociation process. The classical and quantum photodissociation cross-sections for absorption in two adiabatic states of the A’ symmetry are calculated. A remarkable agreement between quantum and classical results is obtained regarding the populations and the absorption cross-sections.

_{2}^{+}

The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl2 molecule and a water molecule. Our functional form is fitted to a set of high level ab initio data using the coupled-cluster single double (triple)/aug-cc-p-VTZ level of electronic structure theory for the Cl2 - H2O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl2 interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl2 on a perfectly ordered hexagonal ice slab. The calculated adsorption energy, in the range 0.27 eV, shows a good agreement with previous experimental results.