1s-2p Excitation of Atomic Hydrogen by Electron Impact Studied Using the Angular Correlation Technique

Abstract

Excitation of the 2p state of atomic hydrogen by electron impact was studied using the electronphoton angular correlation technique with the aim of resolving a long-standing and serious discrepancy between theories and previous experiments at large scattering angles. At a scattering angle of 100 ± , where the discrepancy was greatest, the present result shows excellent agreement with the theoretically predicted correlations. [S0031-9007(97)04312-3] PACS numbers: 34.80.Dp "There has been a very long-standing discrepancy at large scattering angles between theory and measurements of the 2 2 P angular correlation parameters for 54.4 eV electron-impact excitation of atomic hydrogen. In our view, this is the most outstanding problem in fundamental electron-atom scattering. The development of most electron-atom scattering theories use the e-H system as a testing ground for dealing with the more sophisticated problems. The discrepancy with experiment here undermines the basic building blocks of such theories. For this reason it is imperative that this problem be resolved as soon as possible" [1]. This quote is typical of a number made by various authors in recent years and it would be difficult for the present authors to put the present experimental study more vividly in context.Study of electron impact excitation processes using either the electron-photon angular correlation method, in which the angular distribution of the decay photon is measured in coincidence with the electron scattered in a particular direction, or the electron-photon polarization correlation method, where the polarization state of the emitted radiation is determined for a specific scattered electron momentum, is now well established [2]. In principle correlation methods can provide a complete quantum mechanical description of excitation processes. In practice this is true for only a limited number of cases, for example, S -P excitation in helium. Spin averaged correlation studies, such as those reported here, for S -P excitation in hydrogen are incomplete because the different spin channels can be resolved using spin polarized beams. Nevertheless these measurements provide the most stringent test of theoretical models for excitation of hydrogen presently available.Experimentally, correlation measurements for hydrogen have been hampered by the difficulties of producing suitable beams of hydrogen atoms, stable over a long period of time and by the depolarization of the radiation due to fine structure. Only three independent experimental research groups have made correlation measurements for the ͑1S -1P͒ excitation process [3][4][5][6][7][8][9][10]. Of these only the data of Weigold et al. [5] and Williams [6,7] have been made over a wide range of electron scattering angles at an incident electron energy of 54.4 eV. There is accord between these two sets of data at all scattering angles but because of the large statistical uncertainties in the data of Weigold et al. [5] further discussion of that work is not given here. Conversely this excitation proc...

H(2p) excitation by 54.4 eV electrons

“…Here we also have an earlier set of measurements due to Chormaic et al (1993) which suggest a different trend as a function of angle and which are even further away from Weigold et al (1980), Williams (1981Williams ( , 1986, Chormaic et al (1993), O'Neill et al (1998), Yalim et al (1997) and Gradziel and O'Neill (2004). The CCC-L calculations are due to Bray and Stelbovics (1992).…”

Convergent calculations of double ionization of helium: From(γ,2e)to(e,3eKheifets1, Bray2 2004Phys. Rev. A391230View full textAdd to dashboardCite

“…Hence, when there is disagreement with experiment as discussed in Ref. [1], which may be even explained by others [2] or improved upon [3], we find that new experiments [4,5] are supportive of the original CCC results. The predictive power of the CCC approach to double photoionization (DPI or ␥ ,2e) was demonstrated more recently when initial disagreement with experiment [6] was resolved in favor of the CCC calculations [7].…”

Electron Scattering by Atoms, Ions, and Molecules

“…Note that the angular correlation data of Yalim, Cvejanovic and Crowe (1997) and the polarization correlation data of Williams and Mikosza (2006) lie consistently within one standard deviation of theory.…”

“…The reduced Stokes parameters, P 1 , P 2 and P 3 for the H(2p) state at an incident electron energy of 54 eV. Closed squares, Yalim, Cvejanovic and Crowe (1997); closed and open circles, Williams and Mikosza (2006); closed triangles, Gradziel and O'Neill (2004); open triangles, O'Neill et al (1998);open squares, Nic Chormaic, Chwirot and Slevin (1993); open diamonds, Williams (1986); full line, the PECS theory of Bartlett et al (2005). Adapted from Williams and Mikosza (2006 More detailed reviews of angular and polarization correlation and superelastic scattering studies were given by Andersen, Gallagher and Hertel (1988) Andersen et al (1997), and Andersen and Bartschat (2001).…”