Abstract
In a seminal paper, Keldysh published a theory of ionization of atoms and solids in intense sinusoidally varying laser fields. In this paper the Keldysh parameter was introduced, which separates optical field ionization into tunnel ionization and multi-photon ionization for and , respectively. Keldysh showed that the Keldysh parameter can be written as a product of laser frequency ω and of the Keldysh tunnel time , i.e. . Over the past 50 years there has been much debate about the meaning and the significance of the Keldysh tunnel time and of the tunnel time in general. This debate has been rekindled by recent attosecond experiment measurements of the tunnel time that are in contradiction with the Keldysh tunnel time and with other definitions of the tunnel time. This paper focuses on an analytical and numerical investigation of the Keldysh tunnel time. It is proven that the tunnel time in atoms represents a response time needed for the groundstate wavefunction to react to the laser field and to develop into the quasi-static resonance state, where full tunnel ionization occurs. Based on this definition, methods to measure the tunnel time of atoms in intense fields are discussed.
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