Seminars Archive

Few-photon processes in the XUV and soft x-ray range: Opportunities and Difficulties

Abstract
About fifty years ago, the first laser-induced two-photon absorption in an atomic vapor was observed in Cesium. With the subsequent developments in laser technology, the field of multi-photon processes and strong field phenomena evolved through a series of stages, the most recent one being dominated by ultrashort pulses whose absolute phase may also be controlled. In view of the recent appearance of sources of significant intensity and subpicosecond pulse duration in the XUV and shorter wavelength range, which aspects of novel atomic and molecular physics would become available for exploration? What can we learn and expect from the experience of the previous fifty years? What are the differences from and similarities with strong field interactions in the infrared and optical range? I discuss the operational meaning of the various intensity regimes in this new wavelength range, including issues of perturbative versus non-perturbative behavior, as well as time-dependent versus transition probability per unit time. Would few- or multi-photon processes be of interest and what would we learn? Since photons in the energy range of, say 40 eV and above would interact significantly with electrons below the valence shells, we need to ask what information such processes would reveal. One aspect that immediately suggests itself is the few-photon excitation of multiply excited electronic states, including those of the Auger type. If the intensities available will allow, say, two- to four-photon processes, then indeed hitherto largely unexplored manifolds of atomic excitations will become accessible. The way such manifolds couple multiple excitations to multiple continua would become accessible to experimental investigation for the first time. The demands on intensity, photonenergy range and pulse duration need to be carefully estimated. If the relative phase of beams with different but commeasured frequencies could be controlled, then additional aspects pertaining to coherent control of transitions and products of photobreakup could be profitably explored, as has been done in the optical and near UV range. The combination and synchronization of short-wavelength with optical laser pulses has already been shown theoretically to promise information on the coupling of highly excited states. It is also possible to exploit such a combination towards exploring the pulse duration and shape of the XUV pulse. Quantitative ideas along these lines will also be discussed.