Electron-impact excitation and ionization cross sections for ground state and excited helium atoms

Ralchenko, Yu.; Janev, R. K.; Katō, Takako; Fursa, Dmitry V.; Bray, Igor; Heer, F. de

doi:10.1016/j.adt.2007.11.003

Cited by 133 publications

(80 citation statements)

References 21 publications

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“…In recent years, accurate cross section data for neutral helium have been assessed thoroughly. 16 The model for helium has been widely used for the measurement of n e and T e by the line intensity ratio method, [17][18][19][20][21][22][23][24][25] which gives consistent results compared with those from other diagnostics. [19][20][21][22] Based on a CR model for neutral helium together with a linear approximation technique, we investigated the relationship between fluctuations in three helium lines, He I 667.8 nm (3 1 D !…”

Section: Introductionmentioning

confidence: 97%

Relations between light emission and electron density and temperature fluctuations in a helium plasma

Howard

Thapar

2011

Physics of Plasmas

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The relations between three atomic lines, He I 667.8 nm (3 1 D ! 2 1 P), 706.5 nm (3 3 S ! 2 3 P), and 728.1 nm (3 1 S ! 2 1 P), and the underlying fluctuations in a helium plasma are investigated for the quantitative interpretation of optical observations in plasma fluctuation measurements. Frequency dependent fluctuation amplitude ratios and phase delays between the line emission fluctuation and the electron density and temperature fluctuations are calculated based on a quasi-static collisionalradiative model and a linear approximation technique. For frequencies up to the upper limit of practical interest (<1 MHz), the fluctuation amplitude ratios and phase delays are similar to those directly evaluated by the quasi-static model. It is found that the difference between the results from the linear approximation technique and from the quasi-static model is due to the absence of metastable fluctuations. Contributions of the 2 1 S and 2 3 S metastable fluctuations to the three helium line emission fluctuations are analyzed. The linearity between fluctuations in the line emission and in the electron density and temperature is valid for fluctuation levels higher than 10%.

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Section: Introductionmentioning

confidence: 97%

Relations between light emission and electron density and temperature fluctuations in a helium plasma

Howard

Thapar

2011

Physics of Plasmas

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“…1(d)]. The cross sections for such inelastic processes are very large (2000-3000 Mb [26]); thus, we assume that they take place with close to unit probability. As a result, an almost continuous electron distribution is expected since the primary electron can lose energy due to inelastic collisions as depicted in Figs.…”

Section: Prl 112 073401 (2014) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 99%

Novel Collective Autoionization Process Observed in Electron Spectra of He Clusters

et al. 2014

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The ionization dynamics of He nanodroplets irradiated with intense femtosecond extreme ultraviolet pulses of up to 10 13 W=cm 2 power density have been investigated by photoelectron spectroscopy. Helium droplets were resonantly excited to atomiclike 2p states with a photon energy of 21.4 eV, below the ionization potential (I p ), and directly into the ionization continuum with 42.8 eV photons. While electron emission following direct ionization above I p is well explained within a model based on a sequence of direct electron emission events, the resonant excitation provides evidence of a new, collective ionization mechanism involving many excited atomiclike 2p states. With increasing power density the direct photoline due to an interatomic Coulombic decay disappears. It indicates that ionization occurs due to energy exchange between at least three excited atoms proceeding on a femtosecond time scale. In agreement with recent theoretical work the novel ionization process is very efficient and it is expected to be important for many other systems. With the advent of short-wavelength free-electron lasers (FELs) the interaction between intense, high-energy light pulses and matter has become a very active field of research [1][2][3] and one of the most exciting topics in atomic and molecular science. Key questions are related to ionization dynamics on an atomic level, answers to which will help to develop an understanding of processes in more complex systems. In pioneering experiments and theoretical studies, various new phenomena such as absorption enhancement [1,4], bleaching [3,5,6], as well as modification [7] and suppression [8] of electron emission were discovered.At high power densities a nanoscale sample, such as a large molecule or cluster, can absorb a large number of photons and the system undergoes a transition to a highly excited, nonequilibrium state. Ionization in this case is strongly interlinked with correlated electron dynamics, either due to multielectron collisions with energy exchange [7] or by novel types of autoionization processes related to interatomic Coulombic decay (ICD), as predicted recently [9]. According to this work, clusters resonantly irradiated by intense light pulses with photon energies insufficient to ionize the atoms by single photon absorption are efficiently autoionized due to the energy exchange between two excited electrons [ Fig. 1(a)]. As a result, an unusual form of a collectively excited, plasmalike state may be formed which is expected to autoionize on a fs-ps time scale [9]. Initial evidence for such an ionization process in Ne clusters has been reported recently [10,11].In this Letter we report a study of electron emission from He clusters irradiated by intense pulses from the new seeded-FEL FERMI [12] at power densities where such collective autoionization (CAI) processes are expected to occur [13]. He droplets were either resonantly excited to the 2p atomiclike state [14], which is well below the ionization potential (I p ), or excited into the continuum. The elect...

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“…28,50 The appearance of additional features in the PES of the Ne surface and of Ne clusters as compared to the PES of free Ne atoms was ascribed to satellite lines resulting from energy loss of the outgoing photoelectrons due to the creation of the lowest cluster excitations ("excitons"). 28 The probability P inel for such collisions in our experiments can be estimated using the well-known inelastic scattering cross sections σ inel , 51 P inel = σ inel ρ He N R He N . Here, ρ He N = 0.0218 Å −3 denotes the density of He droplets 52 and R He N = (3N/(4πρ He N )) 1/3 = 32 Å is the droplet radius for a mean droplet size N = 2900.…”

Section: Inelastic Photoelectron-helium Collisionsmentioning

confidence: 99%

Extreme ultraviolet ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization, and electron energy-loss spectra

Buchta

Krishnan

Bräuer

et al. 2013

The Journal of Chemical Physics

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Electron-impact excitation and ionization cross sections for ground state and excited helium atoms

Cited by 133 publications

References 21 publications

Relations between light emission and electron density and temperature fluctuations in a helium plasma

Relations between light emission and electron density and temperature fluctuations in a helium plasma

Novel Collective Autoionization Process Observed in Electron Spectra of He Clusters

Extreme ultraviolet ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization, and electron energy-loss spectra

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