Doubly excited states of helium observed inN- andl-specific partial photoionization cross-sections using lifetime-resolved fluorescence spectroscopy

Abstract

We have used a lifetime-resolved fluorescence technique to obtain the first measurements of partial photoionization cross-sections of ground state neutral helium with both N-and l-specific final ion state separation. Structure is observed in the He 4lnl doubly excited state regions and the resulting resonance profiles and energy positions qualitatively compared to previously reported theoretical calculations. Due to the sensitive nature of the N-and l-specific partial cross-sections, states from the (4, −2) n series are observed for the first time.The doubly excited states of helium have long been of interest since the first observations [1, 2] both experimentally and theoretically, particularly since becoming accessible to study in tunable photoexcitation with the advent of synchrotron radiation sources in the 1960s [3]. Most recently there have been very high resolution experimental studies of the doubly excited states observed in total ion yield [4-6] and fluorescence photon and metastable singly excited atom yield [7-10] and their photoexcitation and subsequent decay has also been investigated in an electric field [11]. A review of theoretical results is given by Tanner et al [12]. Most of the recent studies have concentrated on the states in Rydberg series which converge on the 'N = 2' threshold, i.e. the threshold above which photoionization can produce He + in an excited state. There are 2N −1 'optically allowed' 1 P o series converging on each excited ion threshold of ion state principal quantum number N, and for the lowest N = 2 threshold all of these have been observed in photoion yield, VUV photon yield and metastable yield. Ultrahigh resolution measurements [9,10] have also recently revealed the importance of triplet states. States converging on higher thresholds are more difficult to study since there are a larger number of 3 Author to whom any correspondence should be addressed.

Theoretical photoionization partial cross sections and angular distribution parameters of doubly excited helium

“…This is in particular valid for the resonance 4, 0 5 in σ 3p . The line shapes of the present calculations agree better with these measurements than those of Sánchez and Martín shown in [2], in particular in the case of σ 3d . The relative magnitudes from the measurements demonstrate that the PCS σ 3d always dominates the σ 3s , an observation that agrees well with our results, as discussed above.…”

“…Here we present the n-and l-specific PCSs, σ nl , and ADPs, β nl , for the photoionization process between the ionization thresholds I 2 and I 4 of the He + ion leading to He +(nl) final states, with n = 1, 2, 3 and l = s, p, d. These theoretical results are compared with the results of very recent experimental l-specific studies [2,3], a comparison confirming the quality of the present theoretical approach and of the wavefunctions employed.…”

“…Very recently, Harries et al [2] reported on the first measurements of l-specific PCSs σ 2s , σ 2p , σ 3s , σ 3p and σ 3d below the N = 4 threshold using a lifetime-resolved fluorescence technique; statistically improved data were later presented in [3]. The measurements of [2] are displayed in figure 10 along with the present calculations convoluted with a Gaussian of 10 meV (FWHM) in order to simulate the experimental resolution of the experiments. The relative intensities of the measurement are given on the right ordinate axis.…”

“…These facts have caused substantial interest in l-specific PCSs and ADPs from both the theoretical and the experimental point of view. Very recently, Harries et al [2,3] reported on the first analysis of l-specific PCSs below the N = 4 ionization threshold using a lifetime-resolved fluorescence technique. These measurements of l-specific PCSs allow a critical test of the present approach and the wavefunctions employed, which is the first motivation of this paper.…”

Asymmetries in production of He+(n=2)with an intense few-cycle attosecond pulse

“…3 are essentially constant for electrons ejected within a few degrees of the z axis. Finally, although distinguishing excitations to the 2s or 2p states requires detection of fluorescence from these states [39][40][41], Fig. 3 shows that P n=2 (ϕ) is significant for both pulse durations and that R n=2 (ϕ) is far larger than R n=1 (ϕ).…”