G. Lapicki. Department of Physics, East Carolina University, Greenville, North Carolina 27858. Received 8 August 1995; revised manuscript received 10 ...
PHYSICAL REVIEW A
VOLUME 53, NUMBER 4
APRIL 1996
L 3 -subshell alignment in gold and bismuth induced by low-velocity carbon ions D. Mitra, M. Sarkar, D. Bhattacharya, M. B. Chatterjee, and P. Sen Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Calcutta 700 064, India
G. Kuri and D. P. Mahapatra Institute of Physics, Bhubaneswar, Orissa 751 005, India
G. Lapicki Department of Physics, East Carolina University, Greenville, North Carolina 27858 ~Received 8 August 1995; revised manuscript received 10 November 1995! Angular distribution of the L l x-ray line (3s 1/2→2 p 3/2) in Au and Bi induced by 3–9 MeV carbon ions has been measured. Values of the alignment parameter (A 2 ) of the L 3 (2 p 3/2) subshell have been obtained from the data as a function of the carbon ion energy. The data have been compared with the calculations of the standard perturbed-stationary-state ~PSS! theory with energy-loss ~E!, Coulomb deflection ~C!, and relativistic ~R! corrections ~ECPSSR! and ECPSSR with the intrashell ~IS! effect included as a multiplicative factor. From this comparison it is evident that an account for the IS coupling substantially improves agreement between theory and measurement. The effect of simultaneous multiple ionization of the M and higher shells on the measured A 2 values is discussed. PACS number~s!: 34.50.Fa
I. INTRODUCTION
Alignment of inner shell vacancies induced by ion-atom collisions has been studied rather extensively during the past decade @1–3#. Mehlhorn discovered @4# that when a beam of charged particles ionizes an atom, the ions with vacancies in subshells whose total angular momentum j. 12 could be aligned with respect to the beam direction. The beam direction acts as the quantization axis and the nonstatistical population of the various magnetic substates gives rise to alignment. The alignment reflects the spatial anisotropy of the ionized state and manifests itself in the anisotropic angular distribution ~and polarization! of the characteristic x rays ~and Auger electrons! emitted from the aligned ions. The alignment parameter could easily be obtained from the angular distribution of the x-ray lines. The main reason for studying vacancy alignment is that this parameter yields information about the ionization process which is impossible to obtain from total cross section measurements. Being sensitive to the dynamics of the collision, alignment studies present a good testing ground for the various collision models, e.g., plane wave Born approximation ~PWBA!, semiclassical approximation ~SCA!, perturbed stationary state ~PSS! approach including correction factors for projectile energy loss ~E!, coulomb deflection of the projectile ~C!; and the relativistic motion ~R! of the orbital electrons ~ECPSSR!. In the majority of the alignment studies undertaken so far, it is the total alignment averaged over all the scattering angles that has been investigated. In this domain great progress has been made in understanding the behavior of the alignment as a function of the collision velocity and the projectile atomic number. It has now become clear that to describe the alignment as well as subshell cross section ratios, it will be necessary to go beyond the first-order perturbation treatment @5#. In addition, a method was proposed to account for intrashell ~IS! coupling @6#. The efficacy of this coupling has been 1050-2947/96/53~4!/2309~5!/$10.00
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addressed in a few recent publications @7,8#. We now understand the qualitative nature of the collision induced vacancy alignment quite well. We can also quantitatively predict the total alignment in a broad energy range for various collision partners to a reasonable accuracy, e.g., within 10–15 %. However, our understanding and knowledge of vacancy alignment induced by low velocity heavy ions (Z 1 .2) is far from satisfactory. For scaled velocities V[( v 1 / v L3 ),0.1, where v 1 is the projectile velocity and v L3 @ 5(Z 2 24.15)/2 # is the orbital velocity of the L 3 electron, the ECPSSR predictions are lower than the experimental values by factors of 3–5. We intend to do a detailed study of this region. The present work is the first of this series, wherein the vacancy alignment of the 2 p 3/2 subshell of Au and Bi induced by 3–9 MeV C 21,31,41 ions has been investigated. This energy region, in terms of the reduced velocity of the projectile, corresponds to 0.081
