J. C. Levin

Beyond the dipole approximation: angular-distribution effects in valence photoemission

Angular distributions of valence photoelectrons showing effects due to higher-multipole 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.

Photofragmentation of third-row hydrides following photoexcitation at deep-core levels

The relaxation dynamics of HCl, DCl, H2S, and D2S following photoexcitation in the vicinities of the Cl and S K-shell thresholds ( ;2.8 keV for Cl, ;2.5 keV for S! were studied by means of ion time-of-flight mass spectroscopy. In all cases, the onset of pre-edge core-shell photoionization precedes the formation on resonance of a significant amount of neutral hydrogen as well as postcollision-interaction effects above threshold. Examination of the width of the H 1 peak in spectra taken with the analyzer parallel and perpendicular to the polarization vector of the incident light indicates that on resonance, the photofragmentation asymmetry parameter, b, is approximately two for HCl, and is clearly positive for H2S. @S1050-2947~98!08211-0# PACS number~s!: 33.80.Gj, 33.80.Eh Time-of-flight mass spectroscopy and coincidence measurements of atoms and molecules are relatively wellunderstood techniques @1‐3#. Coupled with the high bright

New Window Resonances in the Potassium 3s Photoabsorption Spectrum

The photoion spectrum of atomic potassium was measured over the 3 s → n p excitation region with the photoion time-of-flight method and monochromatized synchrotron radiation. An unusual spectrum wi...

K-shell photoionization of Be-like carbon ions : experiment and theory for C2+

Absolute cross-section measurements for K-shell photoionization of Be-like C2+ ions have been performed in the photon energy range 292 325 eV. These measurements have been made using the photon ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory. Absolute measurements compared with theoretical results from the R-matrix method indicate that the primary C2+ ion beam consisted of 62 percent ground-state (1s22s2 1S) and 38 percent metastable state (1s22s2p 3Po) ions. Reasonable agreement is seen between theory and experiment for absolute photoionization cross sections, resonance energies and autoionization line widths of K-shell-vacancy Auger states.

Common window resonance features in K and heavier alkaline atoms Rb and Cs

A previous study of subvalence s -shell photoionization of potassium [Koide et al. : J. Phys. Soc. Jpn. 71 (2002) 1676] has been extended to the cases of heavier alkaline atoms Rb and Cs. We have measured the photoion time-of-flight spectra using monochromatized synchrotron radiation. Dual windows resonance structure previously observed in K was also found in Rb and Cs, suggesting that thouse structure are general features in alkaline atoms. We have observed also the Rydberg series of resonances that appear in dual windows. Our data analysis shows that the resonance widths are broad when compared with its rare gas neighbors. Based on multiconfiguration Dirac–Fock calculations, the Rydberg series of resonances were assigned to the 4 s 1 4 p 6 5 s 5 p excitations embedded in the 4 p 5 5 s continua for Rb and to the 5 s 1 5 p 6 6 s 6 p excitations embedded in the 5 p 5 6 s continua for Cs.

Sequential photoionization of ions using synchrotron radiation and a Penning ion trap

Abstract Sequential photoionization has the potential of making available for study highly charged low energy ions. Synchrotron radiation was used to create a multicharged xenon ion target for further ionization by synchrotron radiation inside a Penning ion trap. Evidence of sequential photoionization was seen, though the yields were small. Improvements in the apparatus and radiation from third generation synchrotrons are expected to increase sequential photoionization yields significantly.

Studies of Ar photoion charge-state distributions near the K-shell threshold

An ion time-of-flight (TOF) spectrometer was used to measure Ar photoion charge-state distributions near the K-shell threshold (hν=3206.3 eV). When photons interact with argon atoms producing 1s holes, the atoms typically decay by KLL Auger-electron emission so that two holes in the L-shell are created; the atoms can further decay by LMM Auger electron emission. The result is a cascade-like decay process. When cascade decays take place, the final photoion charge-state depends on the decay path. Previous measurements performed by our group and others showed large discrepancies in the lowest and highest charge fractions measured. The measured charge-state fractions, however, are sensitive to several experimental parameters that can adversely affect the values of the charge-state fractions. We therefore performed a quantitative study of the variation of the measured charge-state fractions with the experimental parameters. Branching ratios for several decay processes were obtained from measurements of the photoion charge-state fractions as a function of the photon energy as well as the fluorescence yield, which is now in much better agreement with other determinations than was the case heretofore.

Synchrotron-radiation experiments with recoil ions

Studies of atoms, ions and molecules with synchrotron radiation have generally focused on measurements of properties of the electrons ejected during, or after, the photoionization process. Much can also be learned, however, about the atomic or molecular relaxation process by studies of the residual ions or molecular fragments following inner‐shell photoionization. Measurements are reported of mean kinetic energies of highly charged argon, krypton, and xenon recoil ions produced by vacancy cascades following inner‐shell photoionization using white and monochromatic synchrotron x radiation. Energies are much lower than for the same charge‐state ions produced by charged‐particle impact. The results may be applicable to design of future angle‐resolved ion‐atom collision experiments. Photoion charge distributions are presented and compared with other measurements and calculations. Related experiments with synchrotron‐radiation produced recoil ions, including photoionization of stored ions and measurement of sh...

Photoion Auger-electron coincidence measurements near threshold

Abstract The vacancy cascade which fills an atomic inner-shell hole is a complex process which can proceed by a variety of paths, often resulting in a broad distribution of photoion charge states. We have measured simplified argon photoion charge distributions by requiring a coincidence with a K-LL or K-LM Auger electron, following K excitation with synchrotron radiation, as a function of photon energy, and report here in detail the argon charge distributions coincident with K-L1L23 Auger electrons. The distributions exhibit a much more pronounced photon-energy dependence than do the more complicated non-coincident spectra. Resonant excitation of the K electron to np levels, shakeoff of these np electrons by subsequent decay processes, double-Auger decay, and recapture of the K photoelectron through postcollision interaction occur with significant probability.

Angle-resolving electron-electron coincidence setup

We introduce a novel angle-resolving electron-spectrometer system for coincidence studies of gas-phase targets using the timing structure of synchrotron radiation. The experimental setup, mainly consisting of a cylindrical mirror analyzer with a position sensitive detector and an electron time-of-flight spectrometer is particularly useful in cases where low-kinetic-energy electrons are measured in coincidence with high-kinetic-energy electrons. The experimental method and corresponding electronics are discussed. As an example, we present argon LMM Auger electron spectra taken in coincidence with KL2,3L2,3 and KL1L2,3 Auger electrons after resonant 1s→4p photoexcitation.