Overview:
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My Research History |
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My Current Activities |
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Hot New Results |
As a graduate student I have been working in the research group of Prof. Uwe Becker at the Technical University Berlin and performed experiments using synchrotron radiation in the VUV energy region at BESSY (Berlin, Germany) and HASYLAB (Hamburg, Germany). The goal of the experiments was to study electron-electron interactions in a Coulomb potential. Since the photoelectric operator is a single particle operator, multi-electron processes are entirely due to correlation effects. Electron correlations manifest themselves as, e.g., "satellite lines" in electron spectra, e.g., the photoionization process is accompanied by an excitation process, or another ionization process (double photoionization). These experiments led to the discovery of Auger lines in the valence shell of Ne and Ar and are described in my thesis "Angle-resolved photoelectron spectrometry of multi-electron processes after photoionization".
After receiving my Ph.D. (Dr. rer. nat.) in 1991 I continued working in Prof. Becker's group at the Fritz-Haber-Institute of the Max-Planck Society. During that time I made the first measurements on the energy- and angular-distribution of electrons after double photoionization. Then, further studies followed on Auger transitions in the outer valence shell, and atomic decay processes in molecules. Another subject of my research was the investigation of fundamental processes in helium such as ionization-excitation and double ionization. In order to study double-ionization processes in greater detail, a setup for electron-electron coincidences was developed.
With a Feodor-Lynen-Fellowship of the Alexander v. Humboldt Foundation I joined the group of Prof. Ivan Sellin at the University of Tennessee, Knoxville (USA) in 1994 and stayed until 1997. I designed and built an electron time-of-flight (TOF) spectrometer and an atomic metal vapor oven which was succesfully tested for magnesium in 1998. In 1995 I applied for beam time at the National Synchrotron Light Source (NSLS, Brookhaven, NY) to perform experiments in the x-ray region using my electron TOF spectrometer as well as a commercial angle-resolving cylindrical mirror analyzer (CMA). The primary objective of my experiment at NSLS was to explore the feasibility of angle-resolved Auger-electron -- Auger-electron coincidence measurements when both electrons are emitted in a cascade-like decay process after creating an inner shell hole, e.g. by exciting an Ar 1s electron to a 4p orbital with monochromatized synchrotron radiation and detecting the Ar KLL Auger lines in coincidence with the LMM Auger lines.
As a member of a participating research team (PRT) headed by Prof. D. W. Lindle, at the Advanced Light Source (ALS, Berkeley, CA) I continued my research also in the VUV energy region. I have measured the satellite-to-1s line ratio of helium over a wide photon energy range in order to investigate the high-energy behavior. As it turned out, it was possible to calculate from these satellite-to-1s ratios the double-to-single photoionization ratio up to 300eV. In the same collaboration I participated in experiments regarding the angular distribution of photoelectrons beyond the dipole approximation. As we found out, higher order corrections to the dipole approximation may be necessary for photon energies as low as 300eV if the angular distribution is measured outside the plane perpendicular to the synchrotron beam.
In addition, I was conducting experiments using an ion beam from a Tandem Van de Graaff accelerator at the Oak Ridge National Laboratory (ORNL, Oak Ridge, TN) in order to investigate ionization and excitation of various gases after ion-atom collisions. We studied, for instance, the emission yield of electrons autoionizing from doubly-excited states of helium after Cq+-ion collisions for q=3,4,5,6 at 1.67MeV/u and obtained the angular dependence of the profiles of two of the autoionization resonances.
During my time at the University of Tennessee I participated in a collaboration with Profs. U. Becker, J. Levin, and N. Berrah conducting an experiment at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The goal of this experiment was to measure the asymptotic high-energy double-to-single ionization ratio of helium at 57keV, which is, at this energy, not due to the photoeffect but the Compton effect. I had also the opportunity to participate in experiments at the Photon Factory (Tsukuba, Japan) through a Japan-U.S. collaboration initiated by Profs. Azuma and Sellin. We studied doubly- and triply-excited states of atomic lithium ("hollow lithium") using an ion TOF spectrometer. With the help of a cw dye laser we were also able to study laser-excited Li atoms, i.e., even parity states of Li unreachable from the ground state by dipole excitation alone. By changing the polarization of the laser beam we were able to select different magnetic sublevels and could see clear differences in the corresponding ion-yield spectra.
In 1997 I was awarded a JSPS- (Japan Society for the Promotion of Science) Fellowship which enabled me to continue the research that I started during my previous stay in Japan. During the tenure of my fellowship at the Photon Factory I have investigated the triple-photoionization process of Li for the first time; a process that is of comparable fundamental importance as the double-photoionization process of helium. In a follow-up experiment we concentrated on the treshold region of the triple photoionization process. In another experiment I studied the line profile of a double-autoionizing resonance (Li 2s22p --> 1s + 2e-). The observed asymmetry of the line profile shows the direct interaction of a doubly-excited state with the double-ionization continuum, since a 2-step decay of the resonance is energetically not possible.
At the Photon Factory I have also built two electron TOF spectrometers and employed them to test the Wannier threshold law in a new approach using He photosatellite lines.
In May 1999, I started working as Staff Scientist at the Synchrotron Radiation Center (SRC) in Stoughton, Wisconsin. As a Principal Investigator of the NSF funded project "Double and triple photoionization studies of few-electron systems" I investigate atomic systems such as He, Li, and Be. A new experimental setup was assembled and tested for the investigations of multiple photoionization in Apr. 2000.
The probability for multiple ionization of atoms and molecules is a sensitive probe for the interaction among the ejected electrons. While He is a textbook case for 2-electron photoionization, Li could become a similar example for 3-electron ejection, which is expected to be much different from the 2-electron ejection. This study is related to former experiments regarding the double-to-single photoionization ratio of He. The triple-to-single ionization ratio is expected, but not proven, to show a different, much stronger decrease towards higher photon energies than the double-to-single photoionization ratio of He does.
Recently I have extended my investigations to beryllium that is more difficult to handle but has a unique electron configuration with 2 inner and 2 outer s-electrons. While the quadruple-photoionization process will most likely be not observable due to its extremely low cross section, the measurement of the double and triple photoionization cross section in Be is potentially very interesting. The double-to-single photoionization ratio can be determined over a wide energy range before Auger processes contribute to the double-ionization process and it can be compared to the double-to-single photoionization ratio of He. The influence of the remaining 1s electrons in Be can be discovered by comparison with He. Also, due to the much different binding energy of the two 2s electrons compared to the two 1s electrons of He, a different strength in the electron correlation is anticipated.
The triple photoionization of Be can be viewed as a complementary experiment to the triple photoionization of Li and Ne. In the case of Ne all three electrons are ejected from the same subshell (2p) and in the case of Li we have two strongly bound and one weakly bound electron. However, in the case of Be we have two strongly bound and two weakly bound electrons. Above the first triple-ionization threshold one strongly bound and both weakly bound electrons are removed which can result in a different threshold behavior of the triple photoionization cross-section and these results may lead to a generalized description of the triple-photoionization process.
A more challenging project is the investigation of the energy sharing among the ejected electrons after triple photoionization of Li. The energy sharing between two ejected electrons is symmetric for energy-conservation reasons. If one electron is fast, the other one has to be slow, i.e., the sum of both kinetic energies equals the available excess energy. This situation is different for the triple-photoionization process, where the energy sharing of three electrons is unknown. Lithium is ideally suited for such an experiment since 2-step (i.e. Auger decay) processes cannot contribute to the triple-ionization cross section, and the results can be obtained directly from the data. After the initial experiment in 1998 - detection of triply-charged Li ions for the first time ever - the next step in differentiation is to energy-analyze one of the electrons created in the triple-photoionization process. In order to distinguish between electrons associated with double-photoionization from electrons associated with triple photoionization, the detection of electrons needs to be done in coincidence with the associated triply-charged ion.
1) A bouncing electron in a bucky ball
We have found a new phenomenon that may allow us to measure the size
of some molecules via ionization by X-rays. When C60,
consisting of 60 carbon atoms arranged in a soccer-ball shape, is
irradiated by a photon of the right energy, one of the electrons can bounce
between two of the carbon atoms before leaving the molecule. This effect
became apparent by measuring the probability of kicking out two electrons and
comparing that to the probability of kicking out just one electron for
different energies of the impacting photon.
The resulting curve,
shown in the lower panel on the right, displays that ratio as a function
of the inverse of the square root of the excess energy (= photon energy
minus double-ionization threshold). The such transformed energy axis allows us
to divide the ratio by a smooth curve (we have used two 3rd-order polynomials).
As can be seen in the upper panel, some "humps" at certain energies emerge
and their positions can easily be related to geometrical distances of the
molecule using the formula for the de Broglie wavelength.
The "C" in the figure denotes the distance between two carbon atoms,
which is 1.44 Angstrom, and "D" denotes the diameter of the cluster,
which is 7.07 Angstrom. Other molecules with a similar structure are
expected to exhibit the same behavior.
2) A new scaling law for double photoionization
It all started in April 2001 when we measured the Be+ and Be2+ ion yields
and determined the double-to-single photoionization ratios between 28 and
40 eV photon energy (black data points in figure on the left).
Also shown in that figure is the double-to-single photoionization ratio of
He [J.A.R. Samson et al., 1998] multiplied by 0.636 on an energy
scale that is in units of the energy difference between the corresponding
double and single ionization thresholds Delta E (purple curve).
For Be the energy unit Delta E is
27.5 - 9.3 = 18.2 eV while for He Delta E is 79.0 - 24.6 = 54.4 eV.
Since we did not know if the similarity in the energy dependence is just
accidental, we tested our scaling law on lithium.
The red points are the ratios for Li when one 1s and one 2s
electron is double photoionized [Wehlitz et al., 2002]. In this case,
Delta E = 81.03 - 64.41 = 16.62 eV. Also included are the
(less accurate) data for Na from Rouvellou et al. (green points).
All data sets line up very well and clearly exhibit a scaling law.
It is worthwhile to mention that this scaling also seems to work for molecular
hydrogen although, in that case, the error bars are quite large.
Interestingly, we have also found a scaling procedure for the absolute ratio.
The ratio appears to be proportional the square root of the double-ionization
threshold minus the square root of the single-ionization threshold.
Two major questions remain to be asnwered: 1) Does this scaling law work
at higher photon energies? and 2) Is it applicable to inner-shell
double photoionization?