next up previous contents
Next: Features of fast ion Up: Experimental Previous: Experimental   Contents

Introduction

The chapter outlines the technique and apparatus used to record absorption spectra of molecular ions through detection of photoproduct daughter ions arising from a predissociated excited electronic state. Ions are excited by tunable laser radiation from a lower state to a metastable state which lies above the dissociation limit and has a lifetime which falls within the experimental window. Photofragment spectroscopy can not only be considered as a method for recording the `absorption spectrum' of a molecule, but also as a probe of the dissociation processes occurring within that molecule, e.g. state selective lifetime measurements and the competition between different pathways to dissociation can be studied. The experiment can also be considered in terms of atom-ion scattering as a half-collision, with `resonances' occurring when a bound state is coupled to a continuum state. (The term resonance arises from scattering terminology and is defined as the change in the character of the continuum state due to the influence of a bound state.) Laser photofragment spectroscopy therefore studies the region of intermediate separation between bound molecular states and separated atomic states. This region is often poorly probed through traditional absorption and emission techniques.

For the spectroscopic experiments, a fast ion beam of molecular ions interacts collinearly with a frequency tunable laser beam and transitions are observed by monitoring the production of fragment ions as a function of excitation wavelength (see Figure 2.1).

Figure 2.1: Principles of Laser Photofragment Spectroscopy
\resizebox{5in}{!}{
\includegraphics{figures/specprinc.eps}}

For energy release experiments, a beam of molecular ions is coaxially irradiated by laser radiation of a fixed wavelength corresponding to a spectroscopic transition. An energy release spectrum for the fragments is taken by measuring the ion current as a function of momentum (see Figure 2.2).

Figure 2.2: Principles of Energy Release Experiments, where T is the kinetic energy released upon dissociation and De the dissociation energy of the ground state.
\resizebox{5in}{!}{
\includegraphics{figures/momprinc.eps}}


next up previous contents
Next: Features of fast ion Up: Experimental Previous: Experimental   Contents
Tim Gibbon
1999-09-06