Physics
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.
Comparison of GLF23 and Weiland Models for Turbulent-Driven Toroidal Momentum Transport
Integrated modeling simulations using the GLF23 model indicate that the EB shear driven by toroidal rotation can have a significant impact upon the fusion performance of ITER [1]. The focus of this work is to advance the understanding of toroidal momentum transport by carrying out a systematic comparison of the toroidal momentum diffusivity computed by the GLF23 [2] and Weiland [3] models. The GLF23 model is used to compute toroidal momentum transport driven by ion temperature gradient (ITG) and trapped electron mode (TEM) in the quasilinear approximatin. The Weiland model, in addition to ITG/TEM toroidal momentum transport in the quasilinear approximation, includes non-linear contributions to transport such as a momentum pinch effect that is driven by the Reynolds stress. Benchmarking of both momentum transport models against experimental data is carried out with the PTRANSP code. Also, a direct comparison between GLF23 and the Weiland model is also carried out using a stand-alone code, where the parametric dependence of the momentum diffusivities can be determined in a straightforward manner. The variation of toroidal momentum transport with respect to plasma parameters such as temperature gradient, radial gradient of toroidal velocity, plasma beta, collisionality, and magnetic shear is examined. The EB flow shear that is needed to stabilize anomalous transport depends on the threshold and stiffness of the transport models.
[1] G.M. Staebler and H.E. St. John, Nucl. Fusion 46 (2006) L6.
[2] R.E. Waltz el al., Physics of Plasmas 4 (1997) 2482.
[3] J. Weiland and H. Nordman, Proc. 33rd EPS Conf. on Plasmas Physics, ECA Vol 30I, P2.186 (2006).
Investigation of colloidal interactions in nanoparticle
Colloidal interaction parameters such as virial coefficients or bulk modulus are traditionally measured by scattering methods. However, experimental difficulties often limit the range of applications of these methods to idealized systems. Multiple optical tweezers have also been used to study interparticle forces, but this has been limited to micron size individual particles at infinite dilution. We propose a new approach to investigate many body interactions of sub-micron colloidal particles in native suspensions with a single optical trap. Using a blinking optical trap and confocal detection of optical signals, this approach can be used to measure many body interactions in suspensions of colloidal particles in the range of tens to hundreds nanometers in size.