Measurements of Photon Polarization and Angular Correlations forHe+-He Collisions Coincidence Technique Using an Ion-Photon

Abstract

communicated during the construction of the apparatus 8 and the experimental data is satisfactory. The differences might be due to uncertainties of the experimental resonance cross sections or to the possible existence of a 6 2 S 1/2 autoionization transition 9 near 1300 A. According to the measurement the value P = -50% [where Q s ' = 0, Eq. (4)] is at the wave number 64 400 ± 600 cm" 1 . Since from Eq. (2) e 0 = -# 0 for Q s ' = 0, Eq.(3) together with the measurement yields T 0 q 0 = 5500 ±1200 cm" 1 for the 6 2 P 1/2 autoionization transition.We have shown that polarized electrons can be obtained from autoionizing transitions and that the polarization can be utilized for the analysis of the autoionization resonances. It is worth noting that the intensity distribution 1(A) of the ionizing light (depending on the light source and the absorption coefficients of the optical devices) does not have to be known for the measurement of the wavelength-dependent P(X), since the electron polarization is obtained from the ratio of the electron intensities in the two counters of the Mott detector.The interpretation of excitation processes in ion-atom collisions at moderate energies (a few hundred eV) is based on a molecular description of the colliding-particle system. In most cases, the primary excitation mechanism consists in an interaction between ground and excited molecular states, which occurs at a rather small internuclear distance (a few bohrs). In general, many coherent scattering amplitudes, corresponding to several output channels, are produced at the same time by the collision and these amplitudes can interfere at large or infinite internuclear distances. Thus, in spite of the fact that the primary excitation mechanisms are now generally We gratefully acknowledge support by the Deutsche Forschungsgemeinschaft. 1 U. Fano, Phys. Rev. _178, 131 (1969).transitions from the 6 2 P 3 / 2 state can be neglected because this state is only populated with a probability of less than 3xio' 4 . 3 N.well understood, transition probabilities-or inelastic differential cross sections--for excitation to one specific atomic level are often difficult to determine because of these "secondary" interactions occurring at large distances. For the particular case of the excitation of a 1 P state of helium, in a He + -He collision, for a specific scattering angle (the ion is assumed to remain in the \S state), different and coherent amplitudes a m (6, <*>) are obtained for the different magnetic sublevels m = -1, 0, + 1, with the symmetry relation a i = -a~i ( see Macek and Jaecks 1 ), As the collision time at moderate energies (10~1 5 sec) is much shorter than the lifetime of the excited The polarization of photons from the transition 3 3 P -* 2 3 S and the angular distribution of radiation from the decay 2 l P~+ 1*5 emitted in He + +He collisions at an incident energy of 150 eV and an ion scattering angle of 13.5° were measured by detecting the scattered ions and photons in delayed coincidence. From these measurements we conclude that t...

Observation of the HeI 33 P polarization after medium energy He+ ? Ne collisions with impact parameter selection

“…When the rate of random coincidences R=f21f22S2At, where At is the conversion time (A t ~ 10-8 s), is set equal to C one limits S to S= 1~At, which then yields for our example C~, 10-2 s-1. This is a value typically observed in most particle photon coincidence experiments [9]. In contrast no such limitation exists in our quasi-coincidence technique of Figure 2a.…”

“…'.".,-:'. ': ... surface atom In comparison to a standard coincidence experiment [9] in Figure 2c this quasi coincident technique in Figure 2a is limited to the observation of excited states of the projectile but offers great advantages for such studies with respect to the signal strength by several orders of magnitude. Consider only the true coincidences of an experiment in Figure 2c.…”

On mechanisms of He I collisional excitation in He-He system

“…(Note that the sublevels with M l = ±1 are mainly occupied at the interaction of helium ions with helium atoms [20,21]). Probably, the decrease of the polarization degree at the increase of the collision energy evidences of the increase of the magnetic sublevels population with M l = ±1.…”

Determination of alignment tensor components for the H(2p) charge exchange excitation in proton-argon collisions

“…Williams [5]). Experimental data for excitation in ion-atom [6][7][8] and atom-atom collisions [9] have als0 been published. Few data are available for charge exchange processes which, so far, are limited to the charge exchange excitation of He(33p) in He +-atom or He § collisions [I0, 11].…”