Archive for May, 2008
Theoretical Studies of Dissociative Recombination
We are currently investigating the dissociative recombination (DR) of electrons with molecular
ions such as N2H+ and C3H+3 . Both these ions exist in the interstellar medium, and accurate DR rate constants are needed for astrophysical models. Elaborate electronic structure calculations of potential surfaces for e− + N2H+ → N2 + H have been carried out in the linear geometry [D. Talbi, Chem. Phys.
332, 289 (2007)]. Additional work is necessary to determine the autoionization width Γ, which is essential for a dynamics calculation. We are using the block diagonalization method to determine both diabatic potential curves and Γ; the status of the calculations will be presented at the conference. In addition, preliminary work on the C3H+3 molecular ion has investigated the normal modes of the motion. We expect that energy flow into and out of the vibration of a single CH bond may influence
the overall DR dynamics, and we account for this effect using an appropriate quantum mechanical wave function for the initial state.
Bound-free Emission in the NaK Molecule
We are extending the analysis of the bound-free emission from the 4 3Σ+ electronic state to the a(1) 3Σ+ repulsive state of the NaK molecule. In previous work, Burns et al. [J. Chem. Phys. 119, 4743 (2003)] measured spectra from initial vibrational levels up to v = 8, determined a refined potential for the 4 3Σ+ state, and obtained relative values of the transition dipole moment function M(R) in the range R ∼ 26 3.8 Å to 4.6 Å. Recent measurements include data for many additional vibrational levels up to v = 34 of the 4 3Σ+ state. The new data provide information about M(R) for larger values of R, including a region where theoretical calculations have predicted sharp structure due to an avoided crossing. Using a version of R. J. Le Roy’s code BCONT that we modified, we will obtain values M(R) for a larger range of R, and we will refine the inner repulsive wall of the potential for the a(1) 4 3Σ+ state.
Maximizing Third Order Optical Polarizability of Small Organic Molecules Through Donor-Acceptor Substitution
We show that a donor-acceptor or push-pull system is an important design strategy for maximizing the off-resonant third-order optical nonlinearity (gamma) in small organic molecules, cyanoethynylethenes. Our work details the two competing contributions to gamma that depend on the conjugation length: the energy corresponding to the first optical transition, and the strength of the transition dipole matrix elements. This competition leads to the weak power-law dependence of gamma that we measured, and depends on the number of conjugated electrons that separate the donor and acceptor (N^1.5). Our molecules have record high nonlinearity relative to their molecular mass and fall within a factor of 50 of their theoretical limits, making them highly attractive candidates for all-optical device integration.