Markus Drescher

Time-resolved atomic inner-shell spectroscopy

The characteristic time constants of the relaxation dynamics of core-excited atoms have hitherto been inferred from the linewidths of electronic transitions measured by continuous-wave extreme ultraviolet or X-ray spectroscopy. Here we demonstrate that a laser-based sampling system, consisting of a few-femtosecond visible light pulse and a synchronized sub-femtosecond soft X-ray pulse, allows us to trace these dynamics directly in the time domain with attosecond resolution. We have measured a lifetime of 7.9-0.9+1.0 fs of M-shell vacancies of krypton in such a pump–probe experiment.

Atomic transient recorder

In Bohrs model of the hydrogen atom, the electron takes about 150 attoseconds (1 as = 10-18 s) to orbit around the proton, defining the characteristic timescale for dynamics in the electronic shell of atoms. Recording atomic transients in real time requires excitation and probing on this scale. The recent observation of single sub-femtosecond (1 fs = 10-15 s) extreme ultraviolet (XUV) light pulses has stimulated the extension of techniques of femtochemistry into the attosecond regime. Here we demonstrate the generation and measurement of single 250-attosecond XUV pulses. We use these pulses to excite atoms, which in turn emit electrons. An intense, waveform-controlled, few cycle laser pulse obtains ‘tomographic images’ of the time-momentum distribution of the ejected electrons. Tomographic images of primary (photo)electrons yield accurate information of the duration and frequency sweep of the excitation pulse, whereas the same measurements on secondary (Auger) electrons will provide insight into the relaxation dynamics of the electronic shell following excitation. With the current ∼750-nm laser probe and ∼100-eV excitation, our transient recorder is capable of resolving atomic electron dynamics within the Bohr orbit time.

Laser-based apparatus for extended ultraviolet femtosecond time-resolved photoemission spectroscopy

A novel laser-based apparatus is presented utilizing high harmonic radiation for visible pump–EUV probe experiments on ultrafast processes. True femtosecond temporal resolution is achieved by a monochromator making use of dedicated narrowband multilayer mirrors rather than gratings for selection of single harmonic orders in the photon energy range between 66 and 73 eV. First applications of this new light source for electron spectroscopy on gas phase helium and xenon demonstrate the selection of a single high harmonic order with the intensity ratio between the selected and its adjacent harmonic not exceeding 10:1. A pump–probe study of hot electron production on a solid Pt(110) surface yields a cross-correlation corresponding to a temporal system resolution of 100 fs.

Spin resolved auger electron spectroscopy after photoexcitation with circularly polarized radiation from the BESSY crossed undulator

Abstract The new crossed undulator beamline U2-FSGM at BESSY enables spin resolved spectroscopy of electrons emitted after excitation of atoms, molecules and solids by circularly polarized radiation, even beyond the energy limit of normal-incidence monochromators. It is thus possible to perform spin resolved Auger spectroscopy of an enlarged number of elements. We present first results obtained at the U2-FSGM in the gas phase and from solids. For the gas phase we show spin resolved data for 5p- and 4d-photoionization of Xe and for the N4,5O2,3O2,3 Auger decay in Xe which agree within the uncertainties with theory. They are measured by means of a newly developed time-of-flight apparatus including spin polarization analysis. For solids we present spin resolved M3VV Auger spectra obtained at Cu(100) in a normal incidence, normal emission setup. On the low energy side of the Auger peak they show a change of the preferential spin direction not expected within an atomic model of the Cu M3VV Auger decay. In the center of the peak the preferential spin direction is antiparallel to the spin of the photons exciting the primary M3 hole states.

The Kr M4,5N1N2,3 1P1 Auger decay: measurement of the transferred spin polarization and analysis of Auger amplitudes

The angular distribution of the transferred spin polarization of Kr M4,5N1N2,3 1P1 Auger electrons after primary ionization with circularly polarized light has been measured and the intrinsic parameters δ1 and ξ1 have been determined from experimental results. The explicit equation interconnecting the intrinsic parameters for Auger decay from a (d5/2)-1 core hole has been derived and is used for a detailed analysis of the transitions in terms of Auger amplitude ratios and phase shift differences. The results indicate that the final ionic state is well described by a 1P symmetry. Additional constraints imposed on the intrinsic parameters for singlet states prevent unambiguous determination of Auger amplitudes from experiment even in the limiting case of pure LS coupling.

Spin-resolved electron spectroscopy of the xenon N4,5 O2,3 O2,3 Auger lines

The spin polarization of the Xe N4,5 O2,3 O2,3 Auger lines was measured after ionization by circularly polarized synchrotron radiation of 93.8 eV photon energy. The circularly polarized radiation was produced by converting the linearly polarized light from a conventional undulator by a Mo/Si transmission multilayer acting as a quarter-wave plate. Nearly perfect circular polarization was obtained. In the framework of the two-step model of Auger decay the orientation of the primary hole states 4d-1 2D3/2 and 4d-1 2D5/2 can be derived from the spin polarization of the 1S0 Auger lines. Furthermore, the intrinsic parameters for all lines in this Auger group corresponding to the other 5p-2 final states are determined and compared with theory.

Applicability of monochromatized high harmonic extended ultraviolet radiation for inner-shell photoelectron spectroscopy

Abstract A laboratory light source for extended ultraviolet (EUV) radiation was realized utilizing high harmonic radiation from intense femtosecond laser light. Utilizing a multilayer monochromator three harmonic orders with photon energies ranging from 66.7 to 72.9 eV can individually be selected to perform inner-shell photoelectron spectroscopy on gas phase as well as surface targets. Examples of spectroscopic studies on free xenon atoms, clean W and GaAs surfaces and I 2 or W(CO) 6 adsorbates emphasize different properties of this new light source like photon flux, spectral resolution and purity, polarization and sensitivity.

A laser light source for circularly polarized VUV radiation

A laser-based light source for circularly polarized VUV radiation generated by nonlinear sum-frequency mixing is described. By choosing an appropriate nonlinear resonant process a light flux of up to photons at a bandwidth smaller than 1.2 is achieved. The determination of the polarization state is performed by means of using a four-mirror analyser. A degree of circular polarization greater than 0.99 was found for several mixing processes. These properties make this instrument an ideal source for high-resolution spectroscopic studies of polarization-dependent photo-ionization or photoemission at photon energies up to 14 eV.

PFI-ZEKE spectroscopy of the 2Π spin-orbit components of HBr+

Abstract The PFI-ZEKE method was applied for examination of the rotational structure of the 2 Π spin-orbit components of HBr + (v + = 0). Precise absolute values for the position of rotational thresholds were obtained. Additional lines not compatible with ionization thresholds are attributed to slow electrons from autoionizing states attached to neutral molecules and field-ionized by the extraction pulse.

Photoelectron dynamics of HI ionized by coherent VUV radiation

Abstract The asymmetry parameter β of the photoelectron angular distribution in photoionization of the molecule HI was measured using a tunable coherent VUV source. The target was cooled to a rotational temperature of less than 20 K by a supersonic beam expansion. The data show pronounced resonance structure which is compared to results on a target at nearly room temperature and to an ab-initio calculation by H. Lefebvre-Brion and coworkers.