No abstract

No abstract

A gas-phase time-of-flight ͑TOF͒ apparatus, capable of supporting as many as six electron-TOF analyzers viewing the same interaction region, has been developed to measure energy-and angle-resolved electrons with kinetic energies up to 5 keV. Each analyzer includes a newly designed lens system that can retard electrons to about 2% of their initial kinetic energy without significant loss of transmission; the analyzers can thus achieve a resolving power (E/⌬E) greater than 10 4 over a wide kinetic-energy range. Such high resolving power is comparable to the photon energy resolution of state-of-the-art synchrotron-radiation beamlines in the soft x-ray range, opening the TOF technique to numerous high-resolution applications. In addition, the angular placement of the analyzers, by design, permits detailed studies of nondipolar angular distribution effects in gas-phase photoemission.

Second-order [ O(k(2)), k = omega/c] nondipole effects in soft-x-ray photoemission are demonstrated via an experimental and a theoretical study of angular distributions of neon valence photoelectrons in the 100-1200 eV photon-energy range. A newly derived theoretical expression for nondipolar angular distributions characterizes the second-order effects using four new parameters with primary contributions from pure-quadrupole and octupole-dipole interference terms. Independent-particle calculations of these parameters account for a significant portion of the existing discrepancy between experiment and theory for Ne 2p first-order nondipole parameters.

Measurements of angular distributions of K-shell electrons photoejected from molecular nitrogen are reported which reveal large deviations at relatively low photon energies (Planck's omega < or = 500 eV) from emission patterns anticipated from the dipole approximation to interactions between radiation and matter. A concomitant theoretical analysis incorporating the effects of electromagnetic retardation attributes the observed large nondipole behaviors in N2 to bond-length-dependent terms in the E1 [multiply sign in circle] (E2,M1) photoelectron emission amplitudes which are indicative of a potentially universal nondipole behavior in molecular photoionization.

We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He"*""""/He"*" = 1.6%±0.3% at /zv==2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Aberg, who predict 1.7%.

Angular distributions of valence photoelectrons showing effects due to highermultipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250-1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

We have measured the photoionization-excitation-to-photoionization ratio for He + n (n = 2-6) at several photon energies from 90 to 900 eV. By extrapolating these values we could determine the asymptotic high-energy ratios for He + n (n = 2-5) which agree with theoretical predictions. We show that the satellite-to-main-line ratios are consistent with experimental double-to-single photoionization ratios and agree well with recent measurements.Somewhat neglected in the extensive recent literature concerning the measurement of the ratio of double-to-single photoionization in helium is the important role of single ionization with excitation compared to single ionization alone. The coupling between the electrons and the incoming photon is a single-particle operator. Thus, an excitation in addition to an ionization is entirely due to electron-electron interaction and probes the electron correlation in the ground and final state. Therefore, we have undertaken the study of the intensity of helium satellites He + n (n = 2-6) relative to the main photoline (n = 1) as a function of photon energy at photon energies well above threshold. The helium satellites can be identified by the principal quantum number n of the excited state He + n because the subshells for a given principal quantum number n are almost energetically degenerate, separated only by the Lamb shift [1] which is, for example, about 6 × 10 −5 eV for n = 2. Until now, the helium satellites were mainly studied near threshold to investigate their behaviour, particularly in the resonance region below 79 eV or just above the 79 eV doubleionization threshold [2][3][4][5][6][7]. Only a few measurements were performed at medium or high photon energies and mainly concentrated on the He + n (n 3) satellites or were measured at a single photon energy only [8][9][10][11].Although extensive investigations of the helium satellites were already done, there still remain some open questions. (i) What is the photon-energy dependence of the satellite-tomain-line intensity ratio? (ii) What is the high-energy asymptotic limit of this ratio? (iii) Can the satellite-to-main-line ratios be used to determine the double-to-single photoionization ratio?