28 research outputs found
Carbon K-shell Photo Ionization of CO: Molecular frame angular Distributions of normal and conjugate shakeup Satellites
We have measured the molecular frame angular distributions of photoelectrons
emitted from the Carbon K shell of fixed-in-space CO molecules for the case of
simultaneous excitation of the remaining molecular ion. Normal and conjugate
shake up states are observed. Photo electrons belonging to normal \Sigma
-satellite lines show an angular distribution resembling that observed for the
main photoline at the same electron energy. Surprisingly a similar shape is
found for conjugate shake up states with \Pi -symmetry. In our data we identify
shake rather than electron scattering (PEVE) as the mechanism producing the
conjugate lines. The angular distributions clearly show the presence of a
\Sigma -shape resonance for all of the satellite lines.Comment: 8 pages, 2 figure
Interatomic Coulombic Decay following Photoionization of the Helium Dimer: Observation of Vibrational Structure
Using synchrotron radiation we simultaneously ionize and excite one helium
atom of a helium dimer (He_2) in a shakeup process. The populated states of the
dimer ion (i.e. He^[*+](n = 2; 3)-He) are found to deexcite via interatomic
coulombic decay. This leads to the emission of a second electron from the
neutral site and a subsequent coulomb explosion. In this letter we present a
measurement of the momenta of fragments that are created during this reaction.
The electron energy distribution and the kinetic energy release of the two He^+
ions show pronounced oscillations which we attribute to the structure of the
vibrational wave function of the dimer ion.Comment: 8 pages, 5 figure
Single photon double ionization of the helium dimer
We show that a single photon can ionize the two helium atoms of the helium
dimer in a distance up to 10 {\deg}A. The energy sharing among the electrons,
the angular distributions of the ions and electrons as well as comparison with
electron impact data for helium atoms suggest a knock-off type double
ionization process. The Coulomb explosion imaging of He_2 provides a direct
view of the nuclear wave function of this by far most extended and most diffuse
of all naturally existing molecules.Comment: 10 pages, 5 figure
Time-Resolved Coherent Photoelectron Spectroscopy of Quantized Electronic States on Metal Surfaces
Time-resolved two-photon photoemission in combination with the coherent excitation of several quantum states was used to study the ultrafast electron dynamics of imagepotential states on metal surfaces. For a (100) surface of copper, the spectroscopy of quantum beats made previously unresolved high-order states (quantum number n Ն 4) experimentally accessible. By exciting electrons close to the vacuum level, electron wave packets could be created and detected that described the quasi-classical periodic motion of weakly bound electrons. They traveled more than 200 Å away from the surface and oscillated back and forth with a period of 800 femtoseconds. Photoelectron spectroscopy has developed into one of the most versatile and successful tools for surface studies. Particularly attractive features of this technique are the high surface sensitivity associated with the low escape depth of the photoelectrons and the capability of angle-resolved photoemission to completely characterize electronic states in energy and momentum space (1). Recently, these features have been combined with ultrafast laser excitation for direct time-domain investigations of electron dynamics at surfaces (2). Here, we demonstrate another facet of this powerful technique, the investigation of coherence phenomena in real time. In contrast to experimental methods that rely merely on intensities, coherent spectroscopies offer the unique capability of accessing not only the amplitudes but also the phases of the wave functions of interest (3). This technique dramatically increases the amount of information that one is able to obtain about the temporal evolution of fast processes. In this report, we discuss the dynamics of image-potential states, that is, the quantized excited states of electrons that exist in front of many metal surfaces (4, 5). Using femtosecond time-resolved two-photon photoemission (2PPE), we observed the interference between the wave functions of neighboring eigenstates and the quasi-classical motion of electron wave packets created by the coherent superposition of several quantum states. Recently, the imaging of the static charge density of related surface electronic (ground) states in real space with the scanning tunneling microscope has attracted considerable interest (6); the present results reveal the dynamical evolution of excited electrons in real time. Image-potential states are conceptually rather simple. An electron at a distance z in front of a conducting metal surface experiences an attractive force F(z) ϭ Ϫe 2 /(2z) 2 identical to that produced by a positive (mirror image) charge at a distance z inside the metal converging toward the vacuum energy, where the influence of the surface potential on the binding energy E B ϭ ϪE n is approximated by a quantum defect 0 Յ a Յ 0.5. Experimentally, image-potential states have been studied with 2PPE on many metal surfaces including surfaces covered with adsorbates and metallic overlayers (5, 7-11). One photon with energy ប a (ប is Planck's constant h divided by 2 and is the photon frequency times 2) excites an electron out of an occupied state below the Fermi energy E F into the image-potential state n. A second photon with energy ប b excites the electron to an energy above E vac The experimental setup consisted of a 80-MHz Ti:sapphire laser system that generated infrared (IR) pulses of 70-fs duration. Frequency-tripled 95-fs ultraviolet (UV) pulses from this laser were used for the excitation step (ប a ϭ 4.7 eV). The photoelectrons were emitted by the fundamental IR pulses (ប b ϭ 1.57 eV) and were detected in a hemispherical analyzer with an energy resolution of 30 meV and an angular acceptance of Ϯ0.6°about the surface normal. The preparation of the Cu(111) and Cu(100) samples and details of the ultrahigh-vacuum chamber have been described elsewhere (5). The samples were kept at room temperature. Typical energy-resolved 2PPE spectra of C
Spin-orbit splitting of image states
We quantify the effect of the spin-orbit interaction on the Rydberg-like
series of image state electrons at the (111) and (001) surface of Ir, Pt and
Au. Using relativistic multiple-scattering methods we find Rashba-like
dispersions with Delta E(K)=gamma K with values of gamma for n=1 states in the
range 38-88 meV Angstrom. Extending the phase-accumulation model to include
spin-orbit scattering we find that the splittings vary like 1/(n+a)^3 where a
is the quantum defect and that they are related to the probability of spin-flip
scattering at the surface. The splittings should be observable experimentally
being larger in magnitude than some exchange-splittings that have been resolved
by inverse photoemission, and are comparable to linewidths from inelastic
lifetimes.Comment: 10 pages, 4 figure
Imaging Polyatomic Molecules in Three Dimensions Using Molecular Frame Photoelectron Angular Distributions
We demonstrate a method for determining the full three-dimensional molecular-frame photoelectron\ud
angular distribution in polyatomic molecules using methane as a prototype. Simultaneous double Auger\ud
decay and subsequent dissociation allow measurement of the initial momentum vectors of the ionic\ud
fragments and the photoelectron in coincidence, allowing full orientation by observing a three-ion decay\ud
pathway, (H??, H??, CH??\ud
2 ). We find the striking result that at low photoelectron energies the molecule is\ud
effectively imaged by the focusing of photoelectrons along bond direction