Action Spectroscopy

Action spectroscopy allows to measure optical properties of small isolated systems indirectly - through the action that is caused by the interaction. For example a regular absorption measurement involves shining light through a sample and measuring the reduction in the intensity of light as a result of passing through the sample:

In action spectroscopy, an isolated molecule is irradiated by light, and one can detect that a single photon is observed by the fact that the photon causes the molecule to dissociate, for example by emitting an electron or by fragmenting:

These fragments can then be detected extermely efficiently. Moreover, by working with ions one can mass-select, trap, accumulate and thermalize the ions prior to the experiment. Therefore action spectroscopy measurements are Extremely Clean: we know exactly which ion we are working with and the ions are completely isolated from the surroundings. In addition to measuring the fact that photo-absorption has occured, action spectroscopy yields a great deal of additional information such as the kinetic energy and angular distribution of the photo-products, the mass of the charged fragements, and the rates for photodissociation.