Studies of 1fs scale dynamics in atoms and molecules

Abstract

Coherent light pulses of few to hundreds of femtoseconds (fs) duration haveprolically served the eld of ultrafast phenomena. While fs pulses addressmainly dynamics of nuclear motion in molecules or lattice in the gas, liquidor condensed matter phase, the advent of attosecond (asec) pulses has inrecent years provided direct experimental access to ultrafast electron dynamics.However, there are processes involving nuclear motion in molecules andin particular coupled electronic and nuclear motion that occur in the few fsor even sub-fs time scale. Electronic excitations in molecules are commonlyin the VUV/XUV spectral region. Until recently most of the XUV sourceswere lacking either sucient pulse energy ( High Order Harmonic Generation(HOHG) sources) or ultrashort pulse duration (free electron lasers (FEL) [1]),thus preventing access to XUV-pump-XUV-probe measurements in the 1fsor asec temporal scale. In this work, by loose focusing a multi-cycle, highpower, fs Infra-Red (IR) laser (35fs, 160mJ=pulse, 805nm) into a Xenongas jet, the non-linear interaction results in frequency up-conversion and,under proper experimental conditions for Phase Matching (PM), in the formationof Attosecond Pulse Trains (APT). The application of InterferometricPolarization Gating (IPG) [88, 89] to this scheme allows the generation ofenergetic, broadband, coherent extreme-ultraviolet (XUV) continuum radiation.Although the temporal properties of the XUV pulse are aected by thelack of Carrier-Envelope Phase (CEP) stabilization [86], the XUV intensityis high enough to induce XUV multi-photon absorption in atomic [75] ormolecular system.So far, experimental eorts on this time scale have been restricted to XUVIRpump-probe schemes [34, 37, 41, 44, 73, 94], or in-situ electron-ion collisionmethods [59]. As the IR eld induced potential may distort the molecular potential[33], the use of IR-free technique to investigate the intrinsic moleculardynamics is of central importance.The present work focuses on the developmentof techniques for the investigation of such dynamics utilizing solelyXUV radiation and demonstrates proof of principle time resolved and XUVpump - XUV probe experiments, tracking ultrafast molecular dynamics.In order to induce the aforementioned XUV multi-photon processes, highXUV photon ux is required. In order to increase the XUV intensity, thesetup was upgraded to allow the use of 3m or 6m lenses for loose focus con-guration [36], and of a Barium Borate (BBO) crystal, for 2-color drivenHOHG [9, 14, 19, 25, 40, 95]. Although in our case the XUV intensity doesnot benet from the 2-color generation scheme, there is the possibility in theHOHG process to control the electron re-collision trajectories by acting on the parameters of the two laser elds [10]. With this option, the use of longtrajectories is convenient due to their higher energy content. The temporalcharacterization of the HOH obtained by the long trajectories in a 2-colordriving eld is an unexplored eld of investigation. We succeeded in thistask, recording 2nd order Interferometric Volume Auto Correlation [84, 85]traces by using, for the rst time, a multiply ionized atom [8, 87] as a nonlineardetector. The results show clearly the existence of a structure on the1fs scale.Exploiting the short duration of the above mentioned high energy pulses, allthe optically allowed excited states of H2 are coherently populated. Monitoringthe pump-probe delay dependent yield of protons, nuclear and electronic1fs scale dynamics are subsequently investigated and compared to the resultsof ab initio calculations. The revealed dynamics reects the intrinsicmolecular behavior as the XUV probe pulse, despite its still high intensity,hardly distorts the molecular potential. It was further feasible to follow theopening of the dissociative ionization channel through the 2+g (2pu) state,due to the stretching of the molecule. This is visible as a build-up of non-zerokinetic energies proton signal during the rst fs of the delay time.Utilizing individual harmonics, single (5th+5th; 7th+7th harmonic) and twocolor(5th + 9th harmonic) time resolved spectroscopy has been performedin molecular Oxygen, aiming at settling existing discrepancies of previousexperiments [23, 64]. Finally an attempt in observing energy resolved directtwo photon atomic double ionization [43, 48] through energy resolved electronspectroscopy has identied necessary actions and improvements for asuccessful implementation of such an experiment.