Berkeley Lab


Photoelectron and Auger-electron angular distributions of fixed-in-space CO2

F.P. Sturm et al.
Phys. Rev. A 80, 032506 (2009)

We report a kinematically complete experiment of carbon 1s photoionization of CO2 including Auger decay and fragmentation. By measuring in coincidence of CO2 C(1s) photoelectrons and ion fragments using synchrotron light at several energies above the C(1s) threshold, we determine photoelectron angular distributions as well as Auger-electron angular distributions with full solid angle in the molecular fixed frame. We confirm recent unexpected results showing an asymmetry of the photoelectron angular distribution along the molecular axis after ionization of the C(1s) orbital. Our high statistics and high resolution measurement unveils asymmetric features in the photoelectron angular distribution which change as a function of the kinetic energy release. This finding provides strong evidence that varying C–O bondlengths are the main cause for these asymmetries. The Auger-electron angular distributions do not show strong correlation with the photoelectrons.

Evidence of interatomic Coulombic decay in ArKr after Ar 2p Auger decay

Y. Morishita, …, Th. Weber et al.
J. Phys. B: At. Mol. Opt. Phys. 41, 99, 025101, (2008)

We have identified interatomic Coulombic decay (ICD) processes in the ArKr dimer following Ar 2p Auger decay, using momentum-resolved electron–ion–ion coincidence spectroscopy and simultaneously determining the kinetic energy of the ICD electron and the KER between Ar2+ and Kr+. We find that the spin-conserved ICD processes in which Ar2+(3p-33d) 1P and 3P decay to Ar2+(3p-2)1D and 3P, respectively, ionizing the Kr atom, are significantly stronger than the spin-flip ICD processes in which Ar2+(3p-33d) 1P and 3P decay to Ar2+(3p-2)3P and 1D, respectively.

The Simplest Double Slit: Interference and Entanglement in Double Photoionization of H2

D. Akoury, …, Th. Weber et al.
Science 41, 318, 949 – 952, (2007)

The wave nature of particles is rarely observed, in part becauseof their very short de Broglie wavelengths in most situations.However, even with wavelengths close to the size of their surroundings,the particles couple to their environment (for example, by gravity,Coulomb interaction, or thermal radiation). These couplingsshift the wave phases, often in an uncontrolled way, and theresulting decoherence, or loss of phase integrity, is thoughtto be a main cause of the transition from quantum to classicalbehavior. How much interaction is needed to induce this transition?Here we show that a photoelectron and two protons form a minimumparticle/slit system and that a single additional electron constitutesa minimum environment. Interference fringes observed in theangular distribution of a single electron are lost through itsCoulomb interaction with a second electron, though the correlatedmomenta of the entangled electron pair continue to exhibit quantuminterference.

Experimental Separation of Virtual Photon Exchange and Electron Transfer in Interatomic Coulombic Decay of Neon Dimers

Friday, 12 October 2007

T. Jahnke, …, Th. Weber et al.
Phys. Rev. Lett. 99, 99, 153401-1 to 4, (2007) Weinvestigate the interatomic Coulombic decay (ICD) of neon dimers followingphotoionization with simultaneous excitation of the ionized atom (shakeup) ina multiparticle coincidence experiment. We find that, depending on theparity of the excited state, which determines whether ICD takesplace via virtual dipole photon emission or overlap of thewave functions, the decay happens at different internuclear distances, illustratingthat nuclear dynamics heavily influence the electronic decay in theneon dimer.

Few-Photon Multiple Ionization of Ne and Ar by Strong Free-Electron-Laser Pulses

Monday, 14 May 2007

R. Moshammer, …, Th. Weber et al.
Phys. Rev. Lett. 98, 99, 203001-1 to 4, (2007) Journal of Ultrafast Science June 2007 Few-photonmultiple ionization of Ne and Ar atoms by strong vacuumultraviolet laser pulses from the free-electron laser at Hamburg wasinvestigated differentially with the Heidelberg reaction microscope. The light-intensity dependenceof Ne2+ production reveals the dominance of nonsequential two-photon doubleionization at intensities of I<6×1012 W/cm2 and significant contributions of three-photonionization as I increases. Ne2+ recoil-ion-momentum distributions suggest that twoelectrons absorbing “instantaneously” two photons are ejected most likely intoopposite hemispheres with similar energies.

Single photon induced symmetry breaking of H2 dissociation

Friday, 02 February 2007

F. Martin, …, Th. Weber et al
Science 41, 315, 629 – 633, (2007) H2, the smallest and most abundant molecule in the universe,has a perfectly symmetric ground state. What does it take tobreak this symmetry? We found that the inversion symmetry canbe broken by absorption of a linearly polarized photon, whichitself has inversion symmetry. In particular, the emission ofa photoelectron with subsequent dissociation of the remainingH +2 fragment shows no symmetry with respect to the ionic H+and neutral H atomic fragments. This lack of symmetry resultsfrom the entanglement between symmetric and antisymmetric H+2 states that is caused by autoionization. The mechanisms behind this symmetry breaking are general for all molecules.

Complete photo-fragmentation of the deuterium molecule

Tuesday, 13 July 2004

Th. Weber et al.
Nature 41, Vol. 431, 437-440, (2004) and Nature 41, Vol. 443, 1014, (2006) (Corrigendum)All properties of molecules—from binding and excitation energies to their geometry—are determined by the highly correlated initial-state wavefunction of the electrons and nuclei. Details of these correlations can be revealed by studying the break-up of these systems into their constituents. The fragmentation might be initiated by the absorption of a single photon by collision with a charged particle or by exposure to a strong laser pulse: if the interaction causing the excitation is sufficiently understood, the fragmentation process can then be used as a tool to investigate the bound initial state. The interaction and resulting fragment motions therefore pose formidable challenges to quantum theory. Here we report the coincident measurement of the momenta of both nuclei and both electrons from the single-photon-induced fragmentation of the deuterium molecule. The results reveal that the correlated motion of the electrons is strongly dependent on the inter-nuclear separation in the molecular ground state at the instant of photon absorption.

Fully Differential Rates for Femtosecond Multiphoton Double Ionization of Neon

Friday, 28 May 2004

M. Weckenbrock, …, Th. Weber et al.
Phys. Rev. Lett. 92, 213002 (2004) We have investigated the full three-dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the recollision energy of the first electron is on the order of the ionization potential. The momentum correlation in the direction perpendicular to the laser field depends on the time difference of the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize.

Correlated Electron Emission in Multiphoton Double Ionization

Thursday, 08 June 2000

Th. Weber et al.
Nature Vol. 405, 658 (2000) Electronic correlations govern the dynamics of many phenomena in nature, such as chemical reactions and solid state effects, including superconductivity. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from an atom—induced by the impact of a single photon, a charged particle or by a short laser pulse—has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser fields that are comparable in intensity to the nuclear fields have extremely high probabilities for double ionization; this has been attributed to electron–electron interaction. Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are emitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm-2). Increasing the laser intensity causes the momentum correlation between the electrons to be lost, implying that a transition in the laser–atom coupling mechanism takes place.

Recoil-Ion Momentum Distributions for Single and Double Ionization of Helium in Strong Laser Fields

Monday, 17 January 2000

Th. Weber et al.
Phys. Rev. Lett. 84, 443 (2000) We have measured the momentum distributions of singly and doubly charged helium ions created in the focus of 220 fs, 800 nm laser pulses at intensities of (2.9–6.6)×1014 W/cm2. All ions are emitted strongly aligned along the direction of polarization of the light. We find the typical momenta of the He2+ ions to be 5–10 times larger than those of the He1+ ions and a two peak structure at the highest intensity.