Detection of thePe1Series of Doubly Excited Helium States belowN=2via the Stark Effect

Abstract

The Stark effect on the doubly excited states of helium below the N=2 threshold has been studied by vacuum ultraviolet fluorescence yield spectroscopy. Two new series of states are observed at moderate fields (<10 kV/cm), and assigned to the previously unobserved even 1pe series, and a group of 1De series. The 1Se states are observed indirectly via their mixing with nearby 1 po states. The observations at moderate field contradict theoretical predictions that field strengths about an order of magnitude greater are necessary to observe the Stark effect on He doubly excited states at low quantum numbers.

Ab initiostudy of high-lying doubly excited states of helium in static electric fields: Complex-scaling generalized pseudospectral method in hyperspherical coordinates

“…In addition to fundamental interest, the energies, lifetimes, and oscillator strengths of these doubly excited resonance states are also of significance in astrophysics and plasma physics [3]. More recently there is considerably experimental [4][5][6][7] and theoretical [8][9][10][11] interest in the study of the effect of static electric fields on low-lying (n 8) doubly excited states of helium atoms below the N = 2 threshold. The presence of the dc electric field allows the exploration of new sets of doubly excited states which cannot be accessed by photoexcitation directly.…”

“…In the presence of external dc electric fields, these dark states become accessible by means of the Stark mixing with the 1 P o states. For example, the even 1 P e series of doubly excited states have been recently observed and measured [6,11] in weak dc electric fields (F < 10 kV/cm). In addition, a dramatic electric field effect has been also reported in the fluorescence yield spectrum of the doubly excited states in He in the weak dc electric field regime (∼few kV/cm) [5,6,13].…”

“…For example, the even 1 P e series of doubly excited states have been recently observed and measured [6,11] in weak dc electric fields (F < 10 kV/cm). In addition, a dramatic electric field effect has been also reported in the fluorescence yield spectrum of the doubly excited states in He in the weak dc electric field regime (∼few kV/cm) [5,6,13]. To our knowledge, however, the study of the resonance energies and autoionization widths of higher-lying doubly excited states (n > 10) in the presence of dc electric fields has not yet been achieved by either experimental or theoretical methods.…”

High Resolution Multiphoton Spectroscopy by a Tunable Free-Electron-Laser Light

“…The dipole-allowed subset of resonances can be excited by synchrotron light and has been extensively studied over the years [15][16][17][18], improving our understanding of many-body interactions in these systems. However, the dipole-forbidden states have been less explored [19,20].…”

“…The DES correlation types can be abbreviated by letters [8]: for the 1 S e and 1 D e states converging to the N ¼ 2 threshold, that are in the focus of this work, na is equivalent to nð1; 0Þ þ , whereas nb stands for nð−1; 0Þ þ and nð0; 1Þ 0 quantum numbers of states associated with the 1 S e and 1 D e symmetry, respectively. In the last decade, the fluorescence decay channel [29], with branching to the 1s2s metastable states [30], was proven a sensitive tool for studying DES with high resolution synchrotron sources in the field-free [31][32][33][34], and static electric [20,35,36] and magnetic [37] field environment.…”