Markus Guehr Abstract

Markus Guehr Stanford University

"What Can We Learn About Electronic Structure From High Harmonic Generation"

Markus Guhr, Stanford PULSE Institute, SLAC National Accelerator Laboratory

High harmonic generation (HHG) of laser radiation in atoms and molecules is generally described in terms of a simple one-electron model. The electric field component of the laser deforms the atomic or molecular potential and a part of the electron wave function tunnels out of the highest occupied orbital. The liberated electron wave packet accelerates in the laser field and finally coherently recombines with the initially ionized orbital, leading to the emission of photons in the extreme ultraviolet region. The periodicity of the process in the laser field leads to reshaping of the emitted spectrum in terms of odd harmonics of the driving laser frequency.

The electronic structure of the highest occupied orbital shapes the harmonic amplitude and also its phase. For atomic argon, the well known Cooper minimum observed in photionization appears in the HHG spectrum. For molecular nitrogen, the spectrum is more complicated and we find that a one-electron model cannot describe our results. In that case, we show evidence that also a second, lower bond orbital contributes to HHG.

Due to the large sensitivity on electronic structure, it would be attractive to use HHG as a spectroscopic tool for transient excited electronic states. We present our latest results on Bragg scattering schemes in HHG that promise an efficient separation of harmonic signals from ground and excited electronic states. At the end of the talk, I will point to important applications of high harmonic spectroscopy.