Observation of Resonant Excess Photon Detachment Via a Window Resonance in the Negative Cesium Ion

Excess photon absorption (EPA) has been a subject of intense investigation over the past decade in strong field atomic physics (see overviews by Ref. [1–2]). In EPA the atom absorbs more photons than the minimum number required to reach the ionization limit. The process manifests itself in the energy spectrum of the ejected photoelectrons as a series of peaks separated in energy by the photon energy. In the case of neutral atoms the process is often denoted Above Threshold Ionization (ATI) and was observed for the first time in 1979 by Agostini and coworkers [3]. The similar process in negative ions, known as excess photon detachment (EPD), represents a qualitatively new situation in strong-field atomic physics since the additional electron in a negative ion is bound in a short-range potential in contrast to the long-range Coulomb potential. The requirement of conservation of both energy and momentum forbids the absorption of photons by a free electron. Hence, the absorption of any excess photons has to occur as an integral part of the detachment process, where the electron is still able to exchange momentum with the remaining atomic core. In the case of photodetachment of negative ions this is a much more stringent requirement than in the case of photoionization in which the long-range Coulomb potential makes photon-electron interactions possible to rather large distances from the atomic core. Also, the short-range potential implies that negative ions can exist only in a finite number of bound states, in general only in the ground state. This lack of excited bound states removes complications in the multiphoton-detachment process due to transient resonances [1].