Double Photoexcitation of Helium in a Strong dc Electric Field

Abstract

We report the first experimental measurements of the effect of an applied field on the photoexcitation and autoionization of doubly excited states of helium. Ground-state photoionization spectra have been measured in the region below the He+(N=2) threshold with static electric fields of up to 84.4 kV/cm across the interaction region. The results are compared to the theoretical calculations of Chung et al. [J. Phys. B 34, 165 (2001)]], which are the only calculations available in this regime. Transitions to several states in the N=2, n=6 manifold are assigned, and a wealth of new structure is observed. Our data show that many more series are mixed in by the field than those predicted by theory.

Femtosecond induced transparency and absorption in the extreme ultraviolet by coherent coupling of the He 2s2p (1Po) and 2p2 (1Se) double excitation states with 800nm light

“…While previous demonstrations of EIT reveal a dramatic increase in transparency on a sub-linewidth scale, and are further accompanied by phenomena such as slowed light and enhanced nonlinear optical susceptibilities [2], the EIT-like phenomenon observed in this work is characterized solely by an increase in transmission over the entire unperturbed lineshape. Existing experimental studies on the interaction of electric fields with the He double excitation states are confined to measuring the effect of weak (10 V/cm) to moderate (10 5 V/cm) static (dc) electric fields on the photoabsorption [10] and fluorescence spectra [11,12]. These studies show the effect of dc Stark mixing on the photoabsorption cross-section and fluorescence/autoionization branching ratios of these states, as well as to reveal states that cannot be observed in the field-free photoabsorption spectra.…”

Photoionization of Ca in a static electric field

“…Detailed calculations were carried out to show the variation of structure profiles for the overlapping doubly excited autoionization series of He [10]. Shortly thereafter, measurement was carried out in the spectral region below the He + (N = 2) threshold with fields up to 84.4 kV/cm, which led to the observation of more resonance structures than predicted by the theory [11]. To interpret the experimental spectra, Tong and Lin [12] proposed a propensity rule to determine the doubly excited resonances to be preferentially populated.…”

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

“…The first observation of the effect of dc electric field on the photoexcitation spectrum of He doubly excited states was performed by Harries et al [4], who measured the Stark shifts and splittings in strong dc fields (up to 84.4 kV/cm) in the region of the 6a − 8a 1 P o resonances below the N = 2 threshold. Most theoretical works in the recent past have dealt with dc fields in this strong field regime [8][9][10]12].…”

“…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.…”