LAB PICTURES

Introduction

Our research program focuses on the experimental studies of the electronic structure and chemical dynamics of negative ions at the molecular-orbital level. Our experiments offer a new look at chemical structure and reactivity, made possible by the emergence of the new photoelectron imaging technology.

We use cluster anions as model micro-solutions -a platform for the studies of elementary chemical processes in condensed environments. We study the effects of intermolecular interactions on the electronic wave functions and, therefore, chemical bonding.

One of the cornerstones of our research program is the development of new experimental methodologies, such as the application of femtosecond photoelectron imaging to negative ions. In this pioneering approach, the integration of several techniques from different areas experimental physics and chemistry allows us to reach new levels of capabilities.

Our goal is to develop photoelectron imaging into a comprehensive method for studies of the electronic structure of anions and its femtosecond evolution in chemical reactions.

Negative-ion photoelectron imaging spectrometer. Our home-build apparatus is shown schematically in the figure:

It consists of a pulsed ion source, a time-of-flight ion mass spectrometer, and a photoelectron imaging assembly. Photoelectron images are recorded using a velocity-mapping electrostatic lens, by projecting the photoelectrons on a position-sensitive detector. From the resulting image a complete 3D velocity distribution, including the speed and angular distributions, can be uniquely reconstructed. The radial or speed distributions represent the photoelectron energy spectra. The angular distributions reflect the electronic wave function symmetry and convey information about the parent atomic or molecular orbitals.

Photoelectron Imaging

For example, the figure below shows one of the very first photoelectron images recorded in our laboratory. It was obtained in the photodetachment of I- at 267 nm with the laser polarized vertically in the image plane:

Next to the image is its Abel inversion, which is a digitally reconstructed cut through the 3D photoelectron distribution in the vector velocity space. The rings in the image correspond to the formation of two spin-orbit states of the I atom. The greater intensity of the rings near the equator (i.e., perpendicular to the laser polarization direction) is due to interference between the s and d free-electron partial waves produced in the transitions from the p atomic orbital of I-. The case of molecular cluster anion photodetachment is illustrated by the experiment on OCS-•H2O. Below is a 400 nm photoelectron image of this cluster anion:

The most revealing feature of the photoelectron image is its angular anisotropy, which is a fingerprint of the parent molecular orbital. Using group theory, it can be shown that the observed anisotropy is in agreement with the calculated shape of the parent orbital. The width of the radial distribution in the image corresponds to the bending vibrational envelope of the OCS- -> OCS transition.

Femtosecond Dynamics

The most exciting aspect of our research is the use of time-resolved photoelectron imaging to obtain a unique perspective of reaction dynamics. The application of photoelectron imaging in conjunction with ultrafast pump-probe techniques enables us to monitor the evolution of electronic structure along reactive pathways, emphasizing the evolving electronic wave functions, not just energy eigenvalues. We are interested in a wide range of chemical processes transpiring on timescales from 100 fs to 100 ps. We are particularly interested in cases involving internal conversion or dissociation via a conical intersection or avoided crossing, as straightforward opportunities to study nonadiabatic dynamics. We use 100 fs laser pulses to initiate reactive events in molecular and cluster anions and probe the evolving electronic structure by photodetaching electrons at variable pump-probe delays, recording time-dependent photoelectron images of the reaction. The following figure gives a schematic diagram of our femtosecond amplified tunable laser system (Spectra Physics).

Photofragmentation Dynamics

Tandem mass-spectrometry is used to elucidate the dynamics involved in the photofragmentation of anions. After interaction with the laser pulse, resulting ionic fragments are analyzed with a reflectron mass spectrometer (RETOF-MS)..

In addition to the experiments, important aspects of our research are electronic structure calculations and the advancement of theory of molecular anion photodetachment with respect to the photoelectron angular distributions.

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