Research Description

We study various processes which occur when electrons or photons interact with a molecule. These processes include elastic electron-molecule collision, inelastic electron-molecule collisions, electron impact ionization and photoionization. Our current research interests are focused on predictions of molecular frame photoelectron angular distributions (MFPAD)and the interpretation of experiments which measure these processes. Below are the angular distribution of photoelectrons for ionization of the NO molecule leading to the c 3Π state of NO+ . These are the angular distributions that were computed using the multichannel Schwinger configuration interaction(MCSCI) method. In these figures the molecule is aligned vertically and the polarization of the field is either parallel, perpendicular, or at the magic angle (54.7°) with respect to the molecule. These figures show that the MFPAD in this case has the shape of a d orbital in the case of the perpendicular excitation and is a linear combination of d and s orbitals in the parallel excitation.

Theory

 
     
 

Parallel

 

Magic Angle

 

Perpendicular

 

.

These cannot be directly compared to the experiment since there is some loss of angular resolution due to the design of the experimental apparatus. If we take our predicted angular distributions and convolute them with the instrument function we get the following distributions:

Theory Convoluted with Experiment

 
     
 

Parallel

 

Magic Angle

 

Perpendicular

 

Finally this distribution can be compared to the measured angular distribution:

Experiment

 
     
 

Parallel

 

Magic Angle

 

Perpendicular

 

The relatively good agreement between the convoluted theoretical and the experimental distributions indicates that the theory has been able to accurately predict the dynamics of the photoionization process.

We also have several projects which examine similar process for larger non-linear molecular systems. The calculations are performed using the EPolyScat computer codes. A recent project was the study of electron scattering from SF6. In the figure below, there is clearly evidence of several shape resonances at scattering energies from 7 eV to 30 eV. At the present time we are investigating the differences between the theory and experiment at very low scattering energies.

A second area of interest concerns the structure and dynamics of hydrogen bonded clusters. This work is done in collaboration with Prof. Bevan's research group where the corresponding systems are studied experimentally. We develop potential energy surfaces using both experimental data and by performing quantum mechanical electronic structure calculations. These potentials are then used in quantum mechanical calculations of the vibrational motion of the complexes with particular attention being focused on the large amplitude motion found in hydrogen bonded systems. The computed interaction potentials can be used as the basis for spectroscopically accurate interaction potentials using the process of potential morphing.