E. P. Benis

Exploring intense attosecond pulses

After introducing the importance of non-linear processes in the extreme-ultra-violet (XUV) spectral regime to the attosecond (asec) pulse metrology and time domain applications, we present two successfully implemented techniques with excellent prospects in generating intense asec pulse trains and isolated asec pulses, respectively. For the generation of pulse trains two-color harmonic generation is exploited. The interferometric polarization gating technique appropriate for the generation of intense isolated asec pulses is discussed and compared to other relevant approaches.

The hemispherical deflector analyser revisited. I. Motion in the ideal 1/r potential, generalized entry conditions, Kepler orbits and spectrometer basic equation

Abstract We re-examine the orbits of non-relativistic charged particles in a hemispherical deflector analyser (HDA) assuming an ideal 1/r potential. The particles start their trajectory within the HDA at the arbitrary entry radius r0, within a circular entry aperture centered at R0 at an arbitrary potential V0=V(R0). We present a vector treatment of the trajectories deriving many useful relations expressed as a function of the launching angle α. Refraction at the potential boundary at the entry of the HDA (modelled by an idealized step potential) is also considered and found to be important when V0≠Vp, where Vp is the plate voltage used for preretardation. We derive the analyser’s generalized basic equation for deflection through 180° for which the principal reference ray is an ellipse rather than a circle as in the conventional HDA treatment. Both the conventional HDA, for which R 0 = R and V0=Vp, as well as the paracentric HDA for which R 0 ≠ R and V0≠Vp, where R is the mean radius, are thus described as special cases of the same trajectory equation. Our results are expected to be of interest to all fields of electron spectroscopy, but particularly to those utilizing modern spherical sector analysers with sizeable interradial separation for accommodating large area position-sensitive detectors. This investigation is part of a concerted effort to investigate the refocusing properties of the paracentric HDA recently reported by Benis and Zouros [Nucl. Instr. & Meth. A 440 (2000) 462].

Improving the energy resolution of a hemispherical spectrograph using a paracentric entry at a non-zero potential

Abstract We demonstrate by ray tracing analysis that the focusing properties of a hemispherical spectrograph depend critically on the placement of the electron entrance position R 0 and the value of the potential V 0 at R 0 . Improvements over conventional spectrographs using R 0 = R , the mean radius and V 0 =0 are predicted for particular combinations of R 0 and V 0 . A spectrograph with R =101.6 mm and an entrance at R 0 =82.55 mm using a zoom lens and a two-dimensional position sensitive detector (2D-PSD) with 40 mm multichannel plates and resistive anode encoder has been used to confirm the predicted qualitative behaviour. These results suggest that improvement in energy resolution may be attained without the use of fringing field correctors.

Optimal energy resolution of a hemispherical analyzer with virtual entry

For an ideal hemispherical deflector analyzer (HDA) utilizing a virtual entry aperture whose size is controlled by an injection lens, the “slit” and angular contributions to the overall base resolution RB are not independent, but constrained by the Helmholtz–Lagrange law. Thus, RB becomes a function of the linear lens magnification ∣ML∣ and has a minimum, RBo¯≡RB(∣ML∣o), at the optimal magnification ∣ML∣=∣ML∣o. RBo¯ and ∣ML∣o are shown to be analytic expressions of basic experimental parameters. RBo¯ is thus the ultimate resolution that can be attained in this case. The generality and simplicity of this result should be very helpful in the efficient design and performance evaluation of any modern HDA.

Laser-induced field-free alignment of the OCS molecule

We investigate the dynamical alignment of jet-cooled OCS molecules induced by a short laser pulse. The alignment is measured through the orientational contribution of the optical Kerr effect using a second weak laser pulse as a probe. Maximum alignment is observed at conditions close to saturation of ionization. The results are analysed with a quantum mechanical model solving for the rotational dynamics.

High resolution RTE measurements at 0° using a hemispherical analyser with lens and 2-D PSD

Abstract First high resolution measurements using a new electron spectrograph at 0° to the beam direction are reported. The new apparatus consists of a hemispherical analyser with a mean radius of 101.6 mm, a 4-element focusing lens and 40 mm diameter 2-D position sensitive detector (PSD). Electrons are decelerated and focused in the lens prior to analysis to improve the energy resolution. As a test of the apparatus in deceleration mode, we measured the differential cross section of the F 7+ (2p 2 ) 1 D state produced by Resonance Transfer Excitation (RTE) in collisions of 21.78 MeV F 8+ + H 2 . An effective energy resolution of 0.25% was obtained using a deceleration factor of 4. Good agreement with known cross section values was found. Presently, our spectrograph with a maximum count rate of about 4 kHz (limited by dead-time considerations) is more than 15 times faster than the conventional tandem zero-degree spectrometer at Kansas State University, while using a factor of 20 lower intensity beams.

Frequency-resolved photoelectron spectra of two-photon ionization of He by an attosecond pulse train

We present measured and calculated energy-resolved photoelectron spectra obtained through two-photon ionization of He induced by a superposition from the 9th to the 15th harmonic of a Ti:Sapph laser forming an attosecond (asec) pulse train. The reported measured spectra are a decisive step towards frequency-resolved two-XUV-photon ionization-based second-order autocorrelation (AC) of asec pulse trains, and thus towards a complete reconstruction of asec pulses.

Ionization suppression of Cl 2 molecules in intense laser fields

The strong field ionization of Cl{sub 2} molecules is investigated by using an ultrashort pulse Ti:sapphire laser. A spatial imaging technique is used in such measurements to reduce the effect of spatial integration. Cl{sub 2} shows strong ionization suppression as do other diatomic molecules having valence orbitals with antibonding symmetry (O{sub 2},S{sub 2}) when compared with the field ionization of atoms with nearly identical ionization potential. A more general molecular tunneling ionization model is proposed, and the calculations are in reasonable agreement with the measurements. Our results support that antibonding leads to ionization suppression, a trend that only F{sub 2} goes against and that needs to be further investigated.

Determination of the solid angle and response function of a hemispherical spectrograph with injection lens for Auger electrons emitted from long lived projectile states

We present SIMION 8.1 Monte Carlo type simulations of the response function and detection solid angle for long lived Auger states (lifetime τ ∼ 10(-9) - 10(-5) s) recorded by a hemispherical spectrograph with injection lens and position sensitive detector used for high resolution Auger spectroscopy of ion beams. Also included in these simulations for the first time are kinematic effects particular to Auger emission from fast moving projectile ions such as line broadening and solid angle limitations allowing for a more accurate and realistic line shape modeling. Our results are found to be in excellent agreement with measured electron line shapes of both long lived 1s2s2p(4)P and prompt Auger projectile states formed by electron capture in collisions of 25.3 MeV F(7+) with H2 and 12.0 MeV C(4+) with Ne recorded at 0° to the beam direction. These results are important for the accurate evaluation of the 1s2s2p (4)P/(2)P ratio of K-Auger cross sections whose observed non-statistical production by electron capture into He-like ions, recently a field of interesting interpretations, awaits further resolution.

Determination of the

New results are presented on the ratio concerning the production cross sections of Li-like quartet and doublet P states formed in energetic ion–atom collisions by single electron transfer to the metastable component of the He-like ion beam. Spin statistics predict a value of R m = 2 independent of the collision system in disagreement with most reported measurements of . A new experimental approach is presented for the evaluation of R m having some practical advantages over earlier approaches. It also allows for the determination of the separate contributions of ground- and metastable-state beam components to the measured spectra. Applying our technique to zero-degree Auger projectile spectra from 4.5 MeV (Benis et al 2002 Phys. Rev. A 65 064701) and 25.3 MeV (Zamkov et al 2002 Phys. Rev. A 65 062706) mixed state He-like ion collisions with H2 targets, we report new values of for boron and for fluorine. In addition, the ratios of and populations from either the metastable and/or ground state beam component, also relevant to this analysis, are evaluated and compared to previously reported results for carbon collisions on helium (Strohschein et al 2008 Phys. Rev. A 77 022706) including a critical comparison to theory.