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The Xe 5s nondipole photoelectron parameter is obtained experimentally and theoretically from threshold to 200 eV photon energy. Significant nondipole effects are seen even in the threshold region of this valence shell photoionization. In addition, contrary to previous understanding, clear evidence of interchannel coupling among quadrupole photoionization channels is found. DOI: 10.1103/PhysRevLett.91.053002 PACS numbers: 31.25.Eb, 32.80.Fb Until recently, conventional wisdom had assumed nondipole effects in photoionization were negligible at relatively low photon energies, perhaps for energies up to a few keV, but certainly for photon energies below a few hundred eV [1][2][3]. Indeed, despite indications to the contrary [4 -7], the usual practice in the field of photoionization, particularly for experiment, had been to ignore effects beyond the dipole approximation for photon energies as high as several keV. While this may be a reasonable assumption for integrated cross sections, recent work has shown it is certainly wrong for differential cross sections (i.e., photoelectron angular distributions). Experiments have shown the importance of nondipole effects in the 1-3 keV photon-energy region [8,9], in the hundreds-of-eV range [10], and, in one case, at 13 eV [11]. Concurrently, theory has predicted significant nondipole contributions to electron angular distributions for atomic valence shells down to threshold at a few tens of eV photon energy [12 -14].In addition, for dipole photoionization, interchannel coupling, which is simply configuration interaction in the continuum, has been shown to be important for most subshells of most atoms at most energies [15,16]; this work was contrary to the previous conventional wisdom that the independent-particle approximation (IPA) was generally valid away from thresholds. It had been suggested, however, that such interchannel coupling was not important in quadrupole photoionization channels [17], but recent theory has suggested that interchannel coupling can indeed be significant in quadrupole photoionization channels as well [14].To test these two ideas, significant nondipole contributions to the photoionization of a valence shell in the threshold region, and the existence of interchannel coupling effects in quadrupole photoionization channels, we have performed a benchmark experiment on the differential photoionization cross section of Xe 5s from 26 eV (close to threshold) to 200 eV to obtain the nondipole contribution to the photoelectron angular distribution which arises from interference between dipole (E1) and quadrupole (E2) channels. The differential cross section is given by [6,[18][19][20] where is the angle-integrated cross section, is the dipole anisotropy parameter, P 2 cos 3cos 2 ÿ 1=2, and and are nondipole asymmetry parameters. The coordinate axes have the positive x axis along the direction of the photon propagation vector, the z axis along the photon polarization vector, and and are the polar and azimuthal angles of the photoelectron momentum vector. ...

A curved-crystal x-ray emission spectrometer has been designed and built to measure 2-5 keV x-ray fluorescence resulting from a core-level excitation of gas phase species. The spectrometer can rotate 180 degrees, allowing detection of emitted x rays with variable polarization angles, and is capable of collecting spectra over a wide energy range (20 eV wide with 0.5 eV resolution at the Cl K edge) simultaneously. In addition, the entire experimental chamber can be rotated about the incident-radiation axis by nearly 360 degrees while maintaining vacuum, permitting measurements of angular distributions of emitted x rays.

A new phenomenon sensitive only to next-door-neighbor atoms in isolated molecules is demonstrated using angle-resolved photoemission of site-selective core electrons. Evidence for this interatomic coreto-core electron interaction is observable only by measuring nondipolar angular distributions of photoelectrons. In essence, the phenomenon acts as a very fine atomic-scale sensor of nearest-neighbor elemental identity. DOI: 10.1103/PhysRevLett.92.223002 PACS numbers: 33.60.Fy, 33.80.Eh For decades, photoelectron spectroscopy (PES) has been an established method for probing the electronic and chemical structure of matter in both gaseous and condensed phases [1]. Coupled with a tunable light source, such as synchrotron radiation, PES is often done resonantly; e.g., direct photoemission intensity from an outer (valence) orbital or energy level is modified in a narrow wavelength range by interference with resonant excitation of an electron in a deeper-lying (core) orbital. Recently, a new phenomenon, multiatom resonant photoemission (MARPE), was reported in condensed phase MnO [2 -4] in which the core-level photoemission intensity, or cross section, from one element (O) is enhanced upon resonant excitation of a core electron from a different element (Mn) in the solid. The unprecedented core-to-core interaction represented by MARPE is explained as a collective resonant effect from several nearby atoms and has been proposed as a new tool for identifying near-neighbor atoms (within 2 nm) in solids. Reports of MARPE have engendered both interest and skepticism in the photoemission community [5,6]. From an isolated-molecule point of view, MARPE is an unusual form of resonant-Auger decay and can be understood as an interatomic coupling between direct core-electron photoemission from one atom and resonant core-electron excitation of a different atom in the same molecule. Up to now, attempts to find experimental evidence for this MARPE-like effect in small gas-phase molecules by looking for variations in photoemission intensities, whether integrated (cross sections) or differential (angular-distribution parameters), have been in vain.In this Letter we report the first evidence of an interatomic coupling effect in molecular photoemission solely involving core levels. To distinguish it from the relatively longer-range MARPE effect, the inherently short-range (i.e., nearest-neighbor-only) effect in isolated molecules will be referred to as nearest-neighbor-atom core-hole transfer, or NACHT. While solid-state MARPE affects photoemission cross sections, no such intensity variations are observable in the molecular analog; the NACHTeffect is measurable only in the differential cross section, which describes the angular distributions of photoelectrons and is sensitive to the phases of the photoelectrons' continuum wave functions, as well as their magnitudes. Moreover, it is necessary to consider differential-cross-section effects beyond the usual dipole approximation (DA) for interactions between radiation and matter. The electri...

Abstract:Hazardous substances (e.g., toxic elements, oxides of nitrogen, carbon and sulfur) are discharged to the environment by a number of natural and anthropogenic activities. Anthropogenic air pollution commonly contains trace elements derived from contaminants and additives released into the atmosphere during fossil fuel combustion (automobiles, power generation, etc.) as well as physical processes (e.g., metal refining, vehicle brake wear, and tire and pavement wear). Analysis of pollutant chemical concentrations in lichens collected across the Las Vegas Valley allows documentation of the distribution of air pollution in the Valley. Analyses of lichen biomass (Buellia dispersa), when compared to windrose diagrams, shows pathways of airborne pollutant transport across the Las Vegas Valley. The west and north sectors of the Las Vegas Valley contained the lowest target contaminates (e.g., Cr, Cu, Co, Pb, Ni) and the highest NO 3 − while the east and south sectors contained the highest levels of target contaminates and lowest NO 3 − . Additionally, metals and NO 3 − detected in the east and south sectors of the valley indicate that air pollution generated in the valley is moving from the south to the north-northeast and across the valley, exiting on the north and south side of Frenchman Mountain.

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