Speaker
Peter Lambropoulos
(University of Crete)
Description
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 multiphoton (MP) processes evolved through a series of
stages, involving harmonic generation, resonantly enhanced
multiphoton ionization (REMPI), effects of field
correlations in non-linear processes, to mention a few. With
the advent of sub-picosecond, Fourier limited and eventually
few-cycle sources, over the last 20 years, the paradigm
shifted to what is now known as strong field phenomena and
attosecond physics, in terms of the single-active electron
approximation, dominated by recollision dynamics. Thus
multiphoton processes per se went into a dormant stage,
leaving behind certain tools such as REMPI. The appearance
of strong, sub-picosecond, radiation in a broad frequency
range, from the XUV to hard X-rays, has now revived MP
concepts and techniques, but in an entirely new context.
Whereas traditional MP processes under infrared to UV
radiation involved exclusively valence electrons, in the new
paradigm it is inner electrons that dominate the chain of
events triggered by the exposure to strong, short wavelength
radiation.
After a brief historical review of the main stages and concepts of MP processes under long wavelength radiation, I discuss their connection to the short wavelength context, using specific examples of observations in the XUV to soft X-ray range, in order to illustrate the similarities and differences between the two paradigms. In particular the operational meaning of concepts such as strong field, short pulse, sequential versus direct multiple ionization, double resonance, etc. are discussed and illustrated through the application to specific systems.
After a brief historical review of the main stages and concepts of MP processes under long wavelength radiation, I discuss their connection to the short wavelength context, using specific examples of observations in the XUV to soft X-ray range, in order to illustrate the similarities and differences between the two paradigms. In particular the operational meaning of concepts such as strong field, short pulse, sequential versus direct multiple ionization, double resonance, etc. are discussed and illustrated through the application to specific systems.
