Physical Review A

Coherent excitation of the singlet-triplet mixed 1s4f state of helium

Jingbo B. Wang, Jim Williams, A. T. Stelbovics, J. E. Furst, D. H. Madison

Published Oct 1, 1995• 15 citations

Abstract

In this paper, we present a detailed theoretical description for the coherent electron-impact excitation, the subsequent time evolution, and the cascading decay process of the singlet-triplet mixed $1s4f$ state of helium. The excitation amplitude and phase of each sublevel of this state are related to measurable coincidence intensities and polarizations of the emitted photons. It is found that the intensity and polarization of the emitted photons are time modulated due to the singlet and triplet mixing in the $1s4f$ state.

DOI: 10.1103/physreva.52.2885

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Physical Review A

Coherent excitation of the singlet-triplet mixed 1s4f state of helium

Jingbo B. Wang, Jim Williams, A. T. Stelbovics, J. E. Furst, D. H. Madison

Published Oct 1, 1995• 15 citations

Publisher

Abstract

In this paper, we present a detailed theoretical description for the coherent electron-impact excitation, the subsequent time evolution, and the cascading decay process of the singlet-triplet mixed

1s4f

state of helium. The excitation amplitude and phase of each sublevel of this state are related to measurable coincidence intensities and polarizations of the emitted photons. It is found that the intensity and polarization of the emitted photons are time modulated due to the singlet and triplet mixing in the

1s4f

state.

DOI: 10.1103/physreva.52.2885

Related papers

Coherence analysis of the He 1s4f state

Jingbo B. Wang, C M Maloney, D. H. Madison

Journal of Physics B Atomic Molecular and Optical PhysicsJan 1, 19992 citations

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A. Mihelič, M. Žitnik, Kevin Charles Prince +2 more

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 ( $N=1$ ) 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 $1s6p\phantom{\rule{0.16em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}P$ 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.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 ($N=1$) 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 $1s6p\phantom{\rule{0.16em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}P$ 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.

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Špela Krušič, K. Bučar, A. Mihelič +1 more

We present a theoretical study of self-amplification of extreme ultraviolet light in a helium gas target. Resonant excitation to the $3a\phantom{\rule{0.16em}{0ex}}^{1}P^{o}$ doubly excited state below the $N=2$ threshold by $100\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ free-electron laser pulses is followed by fluorescence at $40.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ photon energy. The process is modeled by a three-level atomic system coupled to the continuum and described by the Maxwell-Bloch equations. The evolution of state populations and radiation fields is studied over a wide range of pump intensities and target pressures, clearly showing the regions of spontaneous emission, amplification, and saturation of light emitted in the forward direction. The treatment includes both coherent and SASE pump pulses. In both cases the amplified pulses are temporally coherent and exhibit a duration similar to that of the pump pulses.We present a theoretical study of self-amplification of extreme ultraviolet light in a helium gas target. Resonant excitation to the $3a\phantom{\rule{0.16em}{0ex}}^{1}P^{o}$ doubly excited state below the $N=2$ threshold by $100\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ free-electron laser pulses is followed by fluorescence at $40.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ photon energy. The process is modeled by a three-level atomic system coupled to the continuum and described by the Maxwell-Bloch equations. The evolution of state populations and radiation fields is studied over a wide range of pump intensities and target pressures, clearly showing the regions of spontaneous emission, amplification, and saturation of light emitted in the forward direction. The treatment includes both coherent and SASE pump pulses. In both cases the amplified pulses are temporally coherent and exhibit a duration similar to that of the pump pulses.

Physical Review AJan 23, 20194 citations

In this paper, we present a detailed theoretical description for the coherent electron-impact excitation, the subsequent time evolution, and the cascading decay process of the singlet-triplet mixed

1s4f

1s4f

state.

N=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

1s6p\phantom{\rule{0.16em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}P

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 Stokes parameters

{P}_{1}

{P}_{2}

{P}_{3}

, and

{P}_{4}

have been measured in a scattered electron-polarized cascade photon correlation study of the

4^{1}F

state of helium excited by 29.6 eV electron impact. This is the first detailed experimental study of electron impact excitation leading to the transfer of three units of angular momentum. The data are expected to be influenced by the breakdown of

\mathrm{LS}

coupling for $L\ensuremath{\ge}3$ states, even in a simple system like helium, providing a further challenge for theory.

Doubly excited states below the

N=2

ionization threshold are populated by exciting helium atoms in a supersonic beam with monochromatized synchrotron radiation. The fluorescence decay of these states triggers a radiative cascade back to the ground state with large probability to populate long lived singlet and triplet helium metastable states. The yield of metastables is measured using a multichannel plate detector after the beam has passed a singlet-quenching discharge lamp. The variation of the yield observed with the lamp switched on or off is related to the triplet-singlet mixing of the doubly excited states.

(1snp)^{3}P_{J}(J=0, 1, 2)

of the helium atom. After a review of previous optical spectroscopic work and theory of this fine structure, an old unsolved mystery concerning the polarization of collision light is discussed. The theory of the Zeeman effect is worked out in a form applicable to the microwave experiments. Expected transition frequencies and rf matrix elements, intensity changes and shape of resonance curves, Stark effect and singlet-triplet mixing are also considered. An estimate is made of the signal to be expected in the experiment described in the following paper.

We present a theoretical study of self-amplification of extreme ultraviolet light in a helium gas target. Resonant excitation to the

3a\phantom{\rule{0.16em}{0ex}}^{1}P^{o}

doubly excited state below the

N=2

threshold by

100\phantom{\rule{0.28em}{0ex}}\mathrm{fs}

free-electron laser pulses is followed by fluorescence at

40.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}

photon energy. The process is modeled by a three-level atomic system coupled to the continuum and described by the Maxwell-Bloch equations. The evolution of state populations and radiation fields is studied over a wide range of pump intensities and target pressures, clearly showing the regions of spontaneous emission, amplification, and saturation of light emitted in the forward direction. The treatment includes both coherent and SASE pump pulses. In both cases the amplified pulses are temporally coherent and exhibit a duration similar to that of the pump pulses.