5. Action Spectroscopy: Toward a Structural Probe for Ions in the Gas Phase

5.1 Principle of action spectroscopy

The density of ions in the trap makes it impossible to perform optical absorption spectroscopy (measuring the extinction of the laser beam due to interaction with the ions in the trap). It is necessary to perform action spectroscopy: the effect of the laser on the ions is measured by mass spectrometry as a function of the laser’s wavelength. An action spectrum can be obtained by:

• photodissociation spectroscopy;

• electron photodetachment spectroscopy.

Photodissociation spectroscopy involves measuring the dissociation rate of an ion as a function of wavelength: