ABSTRACT
In the case of gas-phase clusters, instead of measuring the attenuation of radiation, i.e., studying what the particles do to the light, one o en detects what the light does to the particles: changes in the medium induced by the absorption of photons are used to obtain spectroscopic information. As this relies on an action of the medium, such a technique is usually called “action spectroscopy.” e ratio of the population n(ν)/n0 of the initial state of the absorbing species with and without interaction with the light can be expressed as
( )n e n
and is determined by the absorption cross section σ(ν) and the laser uence φ(ν) (in photons/cm2) only. Obviously, one can gain in the action spectroscopic signal by using light sources with the highest possible uence. is is the origin of the sensitivity of action spectroscopic techniques that also allows to measure spectra if only very low particle densities can be obtained or even only a countable number of particles are under investigation. ere are di erent kinds of action that can be used to follow the absorption process. Absorption of a photon leads to excitation into higher states, which may be followed by a detectable uorescence; the depopulation of the (vibrational) ground state can be traced by state-speci c photoionization schemes (ion-dip spectroscopy). Mass spectrometric detection is amenable to ionization or fragmentation processes. In a wider sense, even photoemission spectroscopy is an “action spectroscopy” technique. Here, the energy distribution of the electrons emitted upon photoexcitation of a species contains the information on the (ro-)vibronic states. Charged species have the advantage that they can be mass selected before photoexcitation. is forms the basis for the application of anion photoelectron spectroscopy in the investigation of cluster structures (Section 9.2.2.2.1).
