Photoemission Spectroscopy at Liquid Microbeams with a High Harmonics Table top Radiation Source

Abstract

The use of ultrafast light pulses to visualize chemical dynamics has advanced ourfundamental understanding of chemical and biochemical processes, by providing a meansof monitoring the motion of atoms within a molecule in real time (Assmann et al. (2003)).However, closer to the chemist’s heart is the evolution of the electron density in a molecule -as molecules transform - as a function of time. In fact, the chemist’s dream is to trace orbitalsin time during a chemical transformation (Kling et al. (2006)). The most attractive mediumin this context is the liquid phase where chemistry happens (Siefermann et al. (2010)). Recentprogress in electronic spectroscopy in the condensed phase (e.g., Link, Lugovoy, Siefermann,Liu, Faubel & Abel (2009) and Link, Voehringer-Martinez, Lugovoj, Liu, Siefermann, Faubel,Grubmueller, Gerber, Miller & Abel (2009)) has been driven by the development of newultrashort pulsed light sources, as well as the development of new experimental techniques.Recent successful experiments have made use of sources of ultrafast extreme-UV as probesof ultrafast molecular and materials dynamics (Paul et al. (2001)). These new probes can inprinciple directly provide site-specific molecular dynamics or local order via photoelectronspectroscopy, providing the potential to directly observe chemical reactions in atomic-leveldetail as they occur without the need for specific chromophors. The new tools appear toprovide a powerful new window into the microscopic world, particularly as the temporalresolution and energy range of these sources continues to improve. One recent area ofresearch motivated by these goals and the quest for fundamental light matter interactionsis the generation of ultrashort EUV (extreme ultraviolet) light pulses through the processof high-order harmonic generation (HHG). HHG pushes traditional nonlinear optics to anextreme, by coherently combining many laser photons together to generate coherent beamsthat span from the UV to the keV-region of the spectrum (Krausz & Ivanov (2009)). Thepulses from HHG may be femtosecond to attosecond in duration. Thus, in the wavelengthregion, where these beams are bright and where they contain enough photons (<100 eV), theyhave been used to monitor a variety of processes in chemical and materials science and forapplications such as holographic imaging and photoelectron emission spectroscopy (Krausz& Ivanov (2009), Kapteyn et al. (2007)).As a liquid phase medium water has many unusual properties when compared with simpleorganic liquids (Head-Gordon & Johnson (2005)), and bulk water as well as single watermolecules play a decisive role in many chemical and biological systems. The dynamicalnetwork of hydrogen bondend water in chemical and biological systems has been studied