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Physical Review A

Effect of electric fields on the decay branching ratio of Pe1 doubly excited states in helium measured by time-resolved fluorescence

M. Žitnik, Francis Jean Penent, P. Lablanquie, A. Mihelič, K. Bučar, Robert Richter, Michele Alagia, Stefano Stranges

Published Nov 22, 2006• 8 citations
Publisher

Abstract

We have measured the lifetimes of $^{1}P^{e}$ $(n=9--12)$ doubly excited states in static electric fields $(1--6\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm})$ by observing the decay of the fluorescence signal as a function of time. The effects of the field on these helium states below the second ionization threshold are twofold: their excitation becomes possible due to the Stark mixing with the optically allowed $^{1}P^{o}$ series, and their lifetime is strongly modified by the opening of the autoionization channel, not accessible in zero field. Although the electric field represents only a tiny perturbation of the atomic potential, a substantial shortening of the lifetimes below $100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ is observed. This is the simplest quantum system where the ratio of autoionization to fluorescence decay probability can be effectively controlled by an electric field of moderate strength.

DOI
10.1103/physreva.74.051404

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We have measured the lifetimes of 1Pe^{1}P^{e}1Pe (n=9−−12)(n=9--12)(n=9−−12) doubly excited states in static electric fields (1−−6kV∕cm)(1--6\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm})(1−−6kV∕cm) by observing the decay of the fluorescence signal as a function of time. The effects of the field on these helium states below the second ionization threshold are twofold: their excitation becomes possible due to the Stark mixing with the optically allowed 1Po^{1}P^{o}1Po series, and their lifetime is strongly modified by the opening of the autoionization channel, not accessible in zero field. Although the electric field represents only a tiny perturbation of the atomic potential, a substantial shortening of the lifetimes below 100ps100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}100ps is observed. This is the simplest quantum system where the ratio of autoionization to fluorescence decay probability can be effectively controlled by an electric field of moderate strength.
The effect of a weak electric field on helium doubly excited states has been analyzed by time resolved fluorescence spectroscopy as a function of the E-field amplitude and relative orientation with respect to synchrotron light polarization. In addition to the possible excitation of parity forbidden states (i.e. 2pnp 1P e ), the effect of the weak Stark mixing induces a strong variation of fluorescence signal and decay lifetime for the states with an unfavored autoionization decay. For 2pnd 1 P o states a strong quenching of the fluorescence decay is observed when the E-field is parallel to the incident light polarization due to their mixing with 2sns 1 S e dark states.
The fluorescence yield quenching of low-lying doubly excited 2pnd2pnd2pnd 1Po^{1}{P}^{o}1Po states is observed to depend strongly on a dc electric field strength and its orientation with respect to the polarization of the incoming photon beam. The reduction of the yield accompanied by the lifetime shortening is attributed to the Stark mixing with the neighboring 2sns2sns2sns 1Se^{1}{S}^{e}1Se states, which redirects the 2pnd2pnd2pnd 1Po^{1}{P}^{o}1Po decay to the prompt autoionization channel. For $n\ensuremath{\ge}4$, the lifetimes decrease from several hundred picoseconds down to several tens of picoseconds when an electric field in the kV/cm range is applied parallel to the photon probe polarization. Practically no lifetime change is observed for polarization perpendicular to the electric field direction. The results of the complex-scaling calculations are in a good agreement with the experimental data.
The doubly excited singlet states of He below the N=2N=2N=2 threshold may decay by autoionization or fluorescence. In the fluorescence decay channel, most decay cascades consist of emission of three photons, of which the first is a VUV photon, the second is in or near the visible range, and the last is another VUV photon. We have studied the fluorescence channel decay dynamics of the (2,0n)(2,{0}_{\mathrm{n}})(2,0n​), (2,1n)(2,{1}_{\mathrm{n}})(2,1n​) and $(2,\ensuremath{-}{1}_{\mathrm{n}})$ 1P^{1}\mathrm{P}1P, n=3−−7n=3--7n=3−−7, states by wavelength dispersed photon-induced fluorescence spectroscopy. We have detected the photons in the second step of the cascade and determined the branching ratios for the strongest lines in this step. From these data we are able to calculate the branching ratios of the first step in the cascade. The results are in good agreement with calculations of the main decay channels of the higher resonances, but about 20--30 % lower, and so we are able to describe quantitatively the whole fluorescence cascade of the above-mentioned doubly excited states.
We present a theoretical framework for the calculation of photon-yield spectra from radiative-cascade decays of photoexcited helium atoms in a static electric field for incident photon energies below the first (N=1N=1N=1) ionization threshold. The present theory takes full account of the singlet-triplet mixing among the singly excited states with principal quantum numbers $n\ensuremath{\lesssim}10$. The model predicts the enhancement of transition probabilities to triplet final states due to the field-induced mixing. In particular, for a field strength of 9.17 kV/cm, polarization of incident photons perpendicular to the field, and a 4 meV broad excitation function tuned to the 1s6p1P1s6p\phantom{\rule{0.16em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}P1s6p1P states, the $1s6\ensuremath{\ell}\ensuremath{\rightarrow}1s2p\phantom{\rule{0.16em}{0ex}}{}^{3}\phantom{\rule{-0.16em}{0ex}}P$ and $1s5\ensuremath{\ell}\ensuremath{\rightarrow}1s2p\phantom{\rule{0.16em}{0ex}}{}^{3}\phantom{\rule{-0.16em}{0ex}}P$ emitted photon yields are predicted to be about 70 times and 20 times lower, respectively, than the photon yields for transitions to the corresponding singlet final states.
The helium atom, with one nucleus and two electrons, is a prototypical system to study quantum many-body dynamics. Doubly excited states, or quantum states in which both electrons are excited by one photon, showcase electronic-correlation mediated effects. In this paper, the natural lifetimes of the doubly excited 1 P o 2 s n p Rydberg series and the 1 S e 2 p 2 dark state in helium in the 60–65 eV region are measured directly in the time domain with extreme-ultraviolet/near-infrared noncollinear attosecond four-wave-mixing (FWM) spectroscopy. The measured lifetimes agree with lifetimes deduced from spectral linewidths and theoretical predictions, and the roles of specific decay mechanisms are considered. While complex spectral line shapes in the form of Fano resonances are common in absorption spectroscopy of autoionizing states, the background-free and thus homodyned character of noncollinear FWM results exclusively in Lorentzian spectral features in the absence of strong-field effects. The onset of strong-field effects that would affect the extraction of accurate natural lifetimes in helium by FWM is determined to be approximately 0.3 Rabi cycles. This study provides a systematic understanding of the FWM parameters necessary to enable accurate lifetime extractions, which can be utilized in more complex quantum systems in the future. Published by the American Physical Society 2024
Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the (N = 1,2,3 and 4) ionization thresholds. The measurements have been made using an unconventional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are prevented from reaching the detector. For , it appears that the formation, via photoexcitation, of relatively long-lived, doubly excited Rydberg states is a necessary step in the production of the observed signal. Four processes that might account for the production of the excited neutral particles are considered. The simplest, that atoms in the initial doubly excited Rydberg states are themselves directly observable, is considered unlikely. However, the lifetimes of the photoexcited double Rydberg states may be increased through the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of collisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpretation of other photon impact measurements.