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Journal of Physics B Atomic Molecular and Optical Physics

Electron impact coherence parameters for the (...6s5d1D2) to (...6s6p1P1) excitation in138Ba determined by superelastic scattering measurements

Y Li, P W Zetner

Published Dec 14, 1995• 14 citations
Publisher

Abstract

Superelastic electron scattering involving the de-excitation of laser-excited 138 Ba (...6s6p 1 P 1 ) to the lower-lying (...6s5d 1 D 2 ) metastable level has been investigated. These measurements have been used to extract the electron impact coherence parameters which describe excitation out of an isotropic (...6s5d 1 D 2 ) level to the higher-lying (...6s6p 1 P 1 ) state. Studies have been carried out for an electron impact energy of 20 eV and a range of scattering angles up to 45 degrees . This investigation represents a significant extension of coherence parameter measurements to inelastic transitions between excited atomic levels.

DOI
10.1088/0953-4075/28/23/020

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Superelastic electron scattering from the laser excited 52P3/2 state of rubidium

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Contained within this thesis are the collective theoretical and experimental methodologies required for the investigation of superelastic electron scattering from the laser excited 52P312 level of atomic rubidium. Superelastic electron scattering is the process by which an incident electron collides with an excited atom, in this case optically prepared rubidium, subsequently de -exciting the atom and gaining the relevant kinetic energy. By observing the different superelastic electron scattering rates as a function of laser polarisation, in conjunction with performing pseudo Stokes parameter analysis, the alignment and orientation of the rubidium 52P state caused by electron impact excitation is deduced. As a consequence of this experimental study, rubidium is at present the heaviest alkali to which the superelastic electron scattering technique has been applied. The motivation for performing an electron scattering experiment with a rubidium target, pertains to the possible relativistic effects that arise due to the heavy nucleus of the target atom. Recent fully relativistic first order distorted wave Born approximation (RDW) scattering calculations, for electron impact excitation have shown that rubidium has exhibited strong relativistic effects. As this calculation predicts radically different behaviour when compared to a conventional non relativistic first order distorted wave Born approximation (DWBA) calculation, an experimental investigation to determine the validity of each theory is justified A continuous wave Titanium Sapphire ring laser was used to provide the coherent single mode laser radiation that was resonantly tuned to excited the 52S1/2 - 52P3/2 rubidium transition. The laser excitation process, for both the 85Rb and 87Rb isotopes, was theoretically modelled using a full quantum electrodynamic (QED) model. This modelling of the rubidium atom within the influence of a resonant laser field was used to calculate values for the optical pumping parameters K, K' and K". These parameters are necessary as they provide information about the depolarisation of the excited 2P state caused by the laser. An unpolarised electron beam with an incident electron energy of 18.4eV was used to collisionally de -excite the optically prepared rubidium. Superelastic electron scattering measurements were made for an angular range of 5�-120�, allowing experimental determination of the Stokes parameters, the natural frame parameters and the collision frame parameters associated with electron impact excitation of the 52P state of rubidium. At small scattering angles the DWBA and RDW theories predict similar values for the aforementioned parameters. One of the key aims of this study was the collection of electron scattering data beyond 25�, an experimental limitation previously encountered on this superelastic apparatus. This aim was achieved and experimental data has been collected to maximum angle of 120�. A comparison between the experimental data and the DWBA and RDW scattering theories, reveals that while the RDW theory consistently outperforms the DWBA, there are areas where both theories do not agree with experiment.

•Jan 1, 1998•0 citations
Superelastic electron scattering involving the de-excitation of laser-excited 138 Ba (...6s6p 1 P 1 ) to the lower-lying (...6s5d 1 D 2 ) metastable level has been investigated. These measurements have been used to extract the electron impact coherence parameters which describe excitation out of an isotropic (...6s5d 1 D 2 ) level to the higher-lying (...6s6p 1 P 1 ) state. Studies have been carried out for an electron impact energy of 20 eV and a range of scattering angles up to 45 degrees . This investigation represents a significant extension of coherence parameter measurements to inelastic transitions between excited atomic levels.
The results of a joint experimental and theoretical study concerning elastic electron scattering by laser-excited 138 Ba( ... 6s6p 1 P 1 ) atoms are described. These studies demonstrate several important aspects of elastic electron collisions with coherently excited atoms, and are the first such studies. From the measurements, collision and coherence parameters, as well as cross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the collision frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10°, 15° and 20° scattering angles. The convergent close-coupling (CCC) method was used within the non-relativistic LS -coupling framework to calculate the magnetic sublevel scattering amplitudes. From these amplitudes all the parameters and cross sections at 20 eV impact energy were extracted in the full angular range in 1° steps. The experimental and theoretical results were found to be in good agreement, indicating that the CCC method can be reliably applied to elastic scattering by 138 Ba( ... 6s6p 1 P 1 ) atoms, and possibly to other heavy elements when spin-orbit coupling effects are negligible. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic electron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable 138 Ba atoms were also obtained.
Measurements of superelastic scattering of electrons by laser-excited 138Ba^{138}\mathrm{Ba}138Ba(. . .6s6p 1P1^{1}P_{1}1P1​) atoms were carried out. An asymmetry observed by us and previously by Register et al. [Phys. Rev. A 28, 151 (1983)] has been explained using a model of scattering from a target with finite dimensions. This model employed coherence parameters which we calculated in the distorted-wave approximation. The results indicated that the interpretation of coherence experiments in terms of scattering from a pointlike target can lead to serious errors in the deduction of coherence parameters at low scattering angles.
Experimental studies concerning elastic and inelastic electron scattering by coherently ensembles of Ba (...6s6p (sub 1)P(sub 1)) atoms with various degrees of alignment will be described. An in-plane, linearly-polarized laser beam was utilized to prepare these target ensembles and the electron scattering signal as a function of polarization angle was measured for several laser geometries at fixed impact energies and scattering angles. From these measurements, we derived cross sections and electron-impact coherence parameters associated with the electron scattering process which is time reverse of the actual experimentally studied process. This interpretation of the experiment is based on the theory of Macek and Herte. The experimental results were also interpreted in terms of cross sections and collision parameters associated with the actual experimental processes. Results obtained so far will be presented and plans for further studies will be discussed.
Superelastic electron scattering from laser-excited 138Ba^{138}\mathrm{Ba}138Ba(. . .6s6p 1^{1}1P) is investigated both theoretically and experimentally. The theoretical framework is developed and incorporated into a model of a realistic scattering geometry of finite spatial extent. Model calculations predict that the dependence of the superelastic scattering intensity on the linear polarization direction of the laser beam can be severely distorted due to the existence of a finite scattering volume. Our measurements confirm the presence of this distortion and are well described by the model calculations. Conclusions are drawn concerning the influence of this effect on the extraction of electron-impact coherence parameters.
Contained within this thesis are the collective theoretical and experimental methodologies required for the investigation of superelastic electron scattering from the laser excited 52P312 level of atomic rubidium. Superelastic electron scattering is the process by which an incident electron collides with an excited atom, in this case optically prepared rubidium, subsequently de -exciting the atom and gaining the relevant kinetic energy. By observing the different superelastic electron scattering rates as a function of laser polarisation, in conjunction with performing pseudo Stokes parameter analysis, the alignment and orientation of the rubidium 52P state caused by electron impact excitation is deduced. As a consequence of this experimental study, rubidium is at present the heaviest alkali to which the superelastic electron scattering technique has been applied. The motivation for performing an electron scattering experiment with a rubidium target, pertains to the possible relativistic effects that arise due to the heavy nucleus of the target atom. Recent fully relativistic first order distorted wave Born approximation (RDW) scattering calculations, for electron impact excitation have shown that rubidium has exhibited strong relativistic effects. As this calculation predicts radically different behaviour when compared to a conventional non relativistic first order distorted wave Born approximation (DWBA) calculation, an experimental investigation to determine the validity of each theory is justified A continuous wave Titanium Sapphire ring laser was used to provide the coherent single mode laser radiation that was resonantly tuned to excited the 52S1/2 - 52P3/2 rubidium transition. The laser excitation process, for both the 85Rb and 87Rb isotopes, was theoretically modelled using a full quantum electrodynamic (QED) model. This modelling of the rubidium atom within the influence of a resonant laser field was used to calculate values for the optical pumping parameters K, K' and K". These parameters are necessary as they provide information about the depolarisation of the excited 2P state caused by the laser. An unpolarised electron beam with an incident electron energy of 18.4eV was used to collisionally de -excite the optically prepared rubidium. Superelastic electron scattering measurements were made for an angular range of 5�-120�, allowing experimental determination of the Stokes parameters, the natural frame parameters and the collision frame parameters associated with electron impact excitation of the 52P state of rubidium. At small scattering angles the DWBA and RDW theories predict similar values for the aforementioned parameters. One of the key aims of this study was the collection of electron scattering data beyond 25�, an experimental limitation previously encountered on this superelastic apparatus. This aim was achieved and experimental data has been collected to maximum angle of 120�. A comparison between the experimental data and the DWBA and RDW scattering theories, reveals that while the RDW theory consistently outperforms the DWBA, there are areas where both theories do not agree with experiment.