抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Wave-particle duality is an inherent peculiarity of the quantum world. The double-slit experiment has been frequently used for understanding different aspects of this fundamental concept. The occurrence of interference rests on the lack of which-way information and on the absence of decoherence mechanisms, which could scramble the wave fronts. Here, we report on the observation of two-center interference in the molecular-frame photoelectron momentum distribution upon ionization of the neon dimer by a strong laser field. Postselection of ions, which are measured in coincidence with electrons, allows choosing the symmetry of the residual ion, leading to observation of both, gerade and ungerade, types of interference.

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Double-slit photoelectron interference in strong-field ionization of the neon dimer.

National Institute of Standards and Technology における超伝導研究及び生活

Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

Attosecond Physics

Recoil-ion and electron momentum spectroscopy is a rapidly developing technique that allows one to measure the vector momenta of several ions and electrons resulting from atomic or molecular fragmentation. In a unique combination, large solid angles close to 4π and superior momentum resolutions around a few per cent of an atomic unit (a.u.) are typically reached in state-of-the art machines, so-called reaction-microscopes. Evolving from recoil-ion and cold target recoil-ion momentum spectroscopy (COLTRIMS), reaction-microscopes—the `bubble chambers of atomic physics'—mark the decisive step forward to investigate many-particle quantum-dynamics occurring when atomic and molecular systems or even surfaces and solids are exposed to time-dependent external electromagnetic fields. This paper concentrates on just these latest technical developments and on at least four new classes of fragmentation experiments that have emerged within about the last five years. First, multi-dimensional images in momentum space brought unprecedented information on the dynamics of single-photon induced fragmentation of fixed-in-space molecules and on their structure. Second, a break-through in the investigation of high-intensity short-pulse laser induced fragmentation of atoms and molecules has been achieved by using reaction-microscopes. Third, for electron and ion-impact, the investigation of two-electron reactions has matured to a state such that the first fully differential cross sections (FDCSs) are reported. Fourth, comprehensive sets of FDCSs for single ionization of atoms by ion-impact, the most basic atomic fragmentation reaction, brought new insight, a couple of surprises and unexpected challenges to theory at keV to GeV collision energies. In addition, a brief summary on the kinematics is provided at the beginning. Finally, the rich future potential of the method is briefly envisaged.

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Recoil-ion and electron momentum spectroscopy: reaction-microscopes

Theory of Photoionization: VUV and Soft Xray Region M.Ya. Amusia. Manyelectron Effects in Photoionization M. Kutzner. Fundamental Aspects of Atomic Photoionization with High Brightness Light Sources A.F. Starace, S.T. Manson. Core Relaxation Effects in Molecular Photoionization J. Schirmer, et al. Partial Cross Sections and Angular Distributions U. Becker, D.A. Shirley. High Resolution Electron Spectrometry of Atoms M.O. Krause, C.D. Caldwell. Valence Ionization Processes in the VUV Region J. Berkowitz, et al. Photoionpair Formation J. Berkowitz. Electronic and Nuclear Relaxation of Core Excited Molecules I. Nenner, P. Morin. Resonances and Nearthreshold Processes G.C. King, K.H. Schartner. Resonant and Nonresonant Auger Recombination H. Aksela, et al. Laserbased UV and VUV Spectroscopy of Doubly-excited Atoms J.P. Connerade. Ion Yield Spectroscopy with Soft Xrays T. Hayaishi, P. Zimmermann. Coincidence Measurements on Ions and Electrons J.H.D. Eland, V. Schmidt. Spin Polarization in Photoionization U. Heinzmann, N.A. Cherepkov. Photoionization of Excited and Ionized Systems F.J. Wuilleumier, J.B. West. Photoionization of Oriented Systems and Circular Dichroism G. Schonhense, J. Hormes. Index.

VUV and Soft X-Ray Photoionization

Most of the low-energy electrons emitted from a material when it is subjected to ionization radiation are believed to be directly ionized secondary electrons. Coincidence measurements of the electrons ejected from water clusters suggests many are produced by a quantitatively new mechanism, known as intermolecular Coulombic decay. Low-energy electrons are the most abundant product of ionizing radiation in condensed matter. The origin of these electrons is most commonly understood to be secondary electrons1 ionized from core or valence levels by incident radiation and slowed by multiple inelastic scattering events. Here, we investigate the production of low-energy electrons in amorphous medium-sized water clusters, which simulate water molecules in an aqueous environment. We identify a hitherto unrecognized extra source of low-energy electrons produced by a non-local autoionization process called intermolecular coulombic decay2 (ICD). The unequivocal signature of this process is observed in coincidence measurements of low-energy electrons and photoelectrons generated from inner-valence states with vacuum-ultraviolet light. As ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table2,3, these results could have implications for our understanding of ionization damage in living tissues.

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A hitherto unrecognized source of low-energy electrons in water

Recent photoelectron spectroscopy results from gas phase ${\mathrm{C}}_{60}$ exhibit the same partial cross section variation with photon energy as has been observed in its solid phase. We assume that the variations originate from a fullerene specific ability to form a spherical standing wave of the final state electron by intramolecular interference or virtual reflection at the center of the photoionized molecule. The calculated photon energies of the cross section minima based on the boundary conditions of the standing wave agree fairly well with experimental data.

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Oscillations in the Photoionization Cross Section of C 60

Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction ('gerade' and 'ungerade') are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites. Here we observe the coherence of core electrons in N_2 through a direct measurement of the interference exhibited in their emission. We also explore the gradual transition to a symmetry-broken system of localized electrons by comparing different isotope-substituted species—a phenomenon analogous to the acquisition of partial 'which-way' information in macroscopic double-slit experiments.

Isotope-induced partial localization of core electrons in the homonuclear molecule N2

Angular distributions of C(1s) photoelectrons emitted from oriented CO molecules were measured using a new method: mass and angle-resolved photoelectron-photoion coincidence spectroscopy. Along with the angular momentum composition of the photoelectron wave, the experiment reveals pronounced forward-backward asymmetries in the photoemission intensity with respect to the molecular orientation. This asymmetry, being observed for the first time in molecular photoionization, is interpreted in terms of multiple scattering effects. Calculations using the MS-X{alpha} method show good quantitative agreement with the presented experimental results. {copyright} {ital 1997} {ital The American Physical Society}

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Uwe Becker

Papers

VUV and Soft X-Ray Photoionization

A hitherto unrecognized source of low-energy electrons in water

Oscillations in the Photoionization Cross Section of C 60

Isotope-induced partial localization of core electrons in the homonuclear molecule N2

Demonstration of Strong Forward-Backward Asymmetry in the C1s Photoelectron Angular Distribution from Oriented CO Molecules