Photon energy dependence of ionization-excitation in helium at medium energies

Abstract

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?

Photoionization of heliumlike ions with excitation ofnlstates

“…At low energies near the ionization threshold, the cross sections of the ns and np excitations are comparable by magnitude, while the transitions into the nd states are smaller by two orders of magnitude. The present results are in satisfactory agreement both with measurements made on neutral helium [27] and with numerical calculations performed within the framework of the convergent closecoupling approach for He, Li + , and O 6+ targets [13]. The only exception is the particular case of n = 2 excitations, where the Feynman diagrams of two-photon exchange should be taken into account.…”

“…Since, in the perturbation theory we employ, the small parameter is assumed to be 1/Z 1, our results for helium are not supposed to be accurate enough. However, due to the fast convergency of the [27]) and theoretical (middle rows, from Ref. [13]; lower rows, present work) results for the ratio R n = σ + * n /σ + in helium.…”

“…In Table II, we compare the theoretical predictions for the cross-section ratios R n at different n and photon energies ω with experimental data of Wehlitz et al [27]. For each photon energy, the top row of numbers indicates the experimental results, with the experimental errors in parentheses; the middle row of numbers is obtained within the framework of the convergent close-coupling approach [13] using the gauge in acceleration form; and the lower row of numbers is the results of our calculations.…”

Real-time observation of collective excitations in photoemission

“…Combining temporal and spectral information, a small but finite probability for intrinsic plasmon excitation in a prototypical simple metal, magnesium, at photon energies of ω ≈ 100 eV well below the sudden shake-up regime can be unambiguously determined. The window of admissible fractions α for intrinsic plasmon-satellite excitation found is consistent to that of typical atomic shake-up excitation probabilities at comparable energies (e.g., He ∼ 7% [39,40]) providing a measure for electronic ground-state correlation in atoms.…”

Double Photoionization and Transfer Ionization of He: Shakeoff Theory Revisited

“…4. The probabilities for 2p3p are much smaller than for 2s3s; thus, we can compare the calculated 2s and 3s probabilities with the measured n 2 and n 3 probabilities [21]. The shakeup theory results deviate from the experimental data at lower energies, but are in reasonable good agreement with experiments at higher photon energies.…”