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High Harmonic Generation

We are operating a dedicated laser laboratory to complement our steady-state investigations at BESSYII and other synchrotron radiation facilities and our time-resolved investigations at free-electron laser sources. One of the main applications in this lab is the use of femtosecond VUV photon pulses from high-order harmonic generation (HHG) of a Ti:Sapphire laser for photoelectron spectroscopy of gases, liquids and solids.

Figure 1:
Three step picture of harmonic generation: (1) The coulombic potential of a gas atom is lowered by the laser field allowing for an electron to tunnel out of the barrier to the vacuum level. (2) Once free, the electron is accelerated away from the atom and back again by the E-field of the laser. (3) The electron recombines with the ionized atom emitting radiation (taken from Diplomarbeit, Mateusz Ibek, January 2012, “reflective Off-Axis Zone Plates As Focussing Monochromators For a Femtosecond High-Order Harmonic VUV Photon Source”).

Figure 2:
Schematic representation of the experimental setup with generation of the high-order harmonics in a gas cell, with VUV focusing and with time of flight electron and ion spectrometers. BS denotes the beamsplitter and λ/2 the lambda half plates in the pump and probe parts of the laser beam [taken from Wernet et al., Rev. Sci. Instrum. 82, 063114 (2011)].

Ph. Wernet, J. Gaudin, K. Godehusen, O. Schwarzkopf, W. Eberhardt, Femtosecond time-resolved photoelectron spectroscopy with a Vacuum-Ultraviolet photon source based on laser high-order harmonic generation, Rev. Sci. Instrum. 82, 063114 (2011).