Fabrice Dayou

Long-range multipolar potentials of the 18 spin-orbit states arising from the C(P3)+OH(X Π2) interaction

We present multipolar potentials at large intermolecular distances for the 18 doubly degenerate spin-orbit states arising from the interaction between the two open-shell systems, C((3)P) and OH(X (2)Pi). With OH fixed at its ground vibrational state-averaged distance r(0), the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance R and the angle gamma = CGH, where G represents the mass center of OH. The 18x18 diabatic potential matrix elements are built up from the perturbation theory up to second order and from a two-center expansion of the Coulombic interaction potential, resulting in a multipolar expansion of the potential expressed as a series of terms varying in R(-n). The expressions for the long-range coefficients of the expansion are explicitly given in terms of monomer properties such as permanent multipole moments, and static and dynamic polarizabilities. Accurate values for the monomer properties are used to properly determine the long-range interaction coefficients. The diagonalization of the full 18x18 potential matrix generates adiabatic long-range PESs in good agreement with their ab initio counterparts.

Spin-orbit coupling in O2(υ)+O2 collisions: I. Electronic structure calculations on dimer states involving the XΣg−3, aΔg1, and bΣg+1 states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.

Si(3P) + OH(X2Π) Interaction: Long-Range Multipolar Potentials of the Eighteen Spin−Orbit States†

Eighteen spin-orbit states are generated from the open-shell open-shell Si((3)P) + OH(X(2)Pi) interacting system. We present here the behavior of the associated long-range intermolecular potentials, following a multipolar expansion of the Coulombic interaction treated up to second order of the perturbation theory, giving rise to a series of terms varying in R(-n). In the present work, we have considered the electrostatic dipole-quadrupole (n = 4) and quadrupole-quadrupole (n = 5) interactions, as well as the dipole-induced dipole-induced dispersion (n = 6) and dipole-dipole-induced induction (n = 6) contributions. The diatomic OH is kept fixed at its ground state-averaged distance, (r)(v=0) = 1.865 bohr, so that the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance R and on the bending angle gamma = angleSiGH, where G represents the mass center of OH. From the calculated properties of the monomers, such as the dipole and quadrupole moments and static and dynamic polarizabilities, we have determined and tabulated the long-range coefficients of the multipolar expansion of the potentials for each matrix elements. The isolated monomer spin-orbit splittings have been included in the final matrix, whose diagonalization gives rise to 18 adiabatic potentials. Then, the adiabatic states have been compared to potential energies given by supermolecular ab initio calculations resulting in a general good overall agreement.

Spin-orbit coupling in O2(v)+O2 collisions: a new energy transfer mechanism.

A reduced dimensionality model is used to study the relaxation of highly vibrationally excited O(2)(X (3)Sigma(g) (-),v>/=20) in collisions with O(2)(X (3)Sigma(g) (-),v=0). Spin-orbit coupled potential energy surfaces are employed to incorporate the vibrational-to-electronic energy transfer mechanism involving the O(2)(a (1)Delta(g)) and O(2)(b (1)Sigma(g) (+)) excited states. The transition probabilities obtained show a sharp increase for v>/=26 providing the first direct evidence of the important role played by the electronic energy transfer processes in the depletion of O(2)(X (3)Sigma(g) (-),v>/=26).

On the role of the vibrational dependence of the intermolecular potential in O2(v)+ O2 Collisions

The van der Waals intermolecular potential for the dimer O2–O2 is determined as a function of both intermolecular and bond coordinates from ab initio calculations for two selected orientations of the diatomic molecules. A recent empirical method based on correlation formulae between the polarizability of the diatoms and some potential features is also employed and extended to determine a model potential which incorporates the dependence on the bond coordinate. The main features of the ab initio and empirical potential energy surfaces are compared and discussed. Quantum scattering calculations of the vibrational relaxation of O2(v) are performed by using both types of potentials for each selected geometry, providing evidence for the key role played by the representation of the vibrational dependence of the potential.

Interactions and Collision Dynamics in O 2 + O 2

We review the interaction between molecular oxygen molecules involving the X3Σ g − , a1Δ g and b1Σ g + states. The long radiative lifetimes of the excited electronic states imply that collision-induced energy transfer becomes a key mechanism to understand their role in a variety of physical and chemical processes ranging from photochemistry to nanocrystals. However, due to the open shell nature of these molecules and their weak intermolecular interactions, it is no easy to deal with problems involving these molecules both from the electronic structure calculation and from the dynamical or structural point of view. We will focus on several models recently developed in order to understand the outcome of recent experiments and observations for which the energy transfer between several electronic states play an important role as well as in the determination of accurate (rigid) full dimensional potentials of the dimer for the three lowest singlet, triplet and quintet states, where recent experiments and derived potential energy surface put ab initio theory on its edge.