Hamburg 2009 – scientific programme

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A: Fachverband Atomphysik

A 11: Attosecond Physics I

A 11.3: Talk

Tuesday, March 3, 2009, 15:00–15:15, VMP 6 HS-B

Strong field Cooper minimum in High Harmonic Generation — •Markus Gühr, Joseph P. Farrell, Brian K. McFarland, and Philip H. Bucksbaum — Stanford PULSE Institute, SLAC National Accelerator Lab, Menlo Park, CA 94025 and Physics Department, Stanford University, Stanford CA 94305, USA

The so called Cooper minimum is a prominent feature in the VUV photoionization cross section. It results from a cancellation of the transition matrix element from the atomic ground state to the continuum states at certain photon energies and is accompanied by a π phase jump in the matrix element [1]. We observe the Cooper minimum in high harmonic generation (HHG) spectrum of argon. In addition to the spectral amplitude modulation we also observe a spectral phase jump of π around the Cooper minimum. The recombination step of HHG can be described by the inverse photoionization matrix element and we apply the photoionization theory to simulate our data. In the experiment, we observe a shift of the HHG Cooper minimum with respect to the photoionization Cooper minimum of about 5 eV. We explain this shift by a strong-field mixing of the continuum states during the recombination step in HHG. The results show the opportunity for controlled attosecond pulse shaping.
J. W. Cooper, Physical Review 128, 681 (1962)