Photoelectron Spectroscopy

PES Experiment

PES stands for photoelectron spectroscopy, which in the case of our lab is performed on negative ions. Here's how negative ion photoelectron spectroscopy works: a laser is used to detach the extra electron from a mass-selected beam of negative ions, and the kinetic energies of the electrons are measured in a hemispherical energy analyzer. By scanning over the electron kinetic energies (up to the photon energy) we get a spectrum of photoelectron counts versus kinetic energy. Typical electron energy resolution is about 6 meV (that is, 50 cm-1 or 0.14 kcal/mole). Optimally, a photoelectron energy spectrum consists of a series of resolved peaks, corresponding to transitions from vibrational levels of the ground electronic state of the anion to various vibrational/electronic states of the neutral. Analysis of the spectrum can yield the adiabatic electron affinity, vibrational frequencies, and electronic term energies of the neutral molecule. Analysis of the peak intensities gives information concerning geometry differences between anion and neutral. Since in most cases the electron photodetachment takes place too quickly for the atoms to rearrange, the intensities of peaks within an electronic state are governed by the overlap of the anion and neutral vibrational wave functions (that is, Franck-Condon factors).

Planar Singlet States of COT are Transition States for Ring-Inversion and Bond-Shifting Processes

The difference in activation energy for the two processes is the energy difference between the two planar singlet states of COT

The PES lab has a long and successful history. Nearly the entire electron affinity tables in the CRC handbook is made up of measurements from this laboratory (or from the threshold lab), including most of the atoms and more than a hundred molecules. In molecular photoelectron spectroscopy, the vibrations and electronic state excitation energies can often be measured, and, since the selection rules for photoelectron spectroscopy differ from other, more conventional methods, these measurements are often unobtainable in other ways. The differences in structure between the negative ions and the neutral are normally determined using this method, telling us fundamental properties for adding an extra electron. In recent years we have investigated some systems that rearrange very quickly upon photoelectron detachment (specifically, 1,2-hydrogen shifts), small metal cluster properties, singlet-triplet splittings in diradicals, and transitions from stable anion geometries to transition states in the neutral.

The following is an example of the photoelectron spectrum of a large organic anion -- the C8H8 anion.

Photodetachment of COT-

The photoelectron spectrum shows clearly the transition state of the inversion.

This experiment has also been successfully used to help teach an undergraduate physical chemistry laboratory course, where the students (juniors and seniors) come into the lab, learn about the experiment, obtain spectra of previously unstudied molecules, and help publish a paper about the project.