We report on the first (7-value measurements in charge-transfer collisions using recoil longitudinal momentum spectroscopy. This method is not limited to relatively low beam energies and is easily adaptable to captures involving any number of transferred electrons. A very monoenergetic beam is not necessary. For a 50-keV Ar ,5+ on Ar collision system, Q values corresponding to single through quintuple electron capture were measured and found to be in good agreement with the predictions of the molecular classical overbarrier model. PACS numbers: 34.70.+e, 34.50.Fa When a multiply charged ion moving at a velocity below that of typical outer shell atomic electrons encounters a neutral target, the dominant electron removal process is electron capture. The change in electronic energy, or Q value, for such a process is a direct measure of the distribution of final states populated on the projectile, which is in turn one of the most crucial tests of any theoretical description of the process. Direct measurement of the energy gain of the projectile has often been used to determine this final-state distribution [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Since the fractional energy resolution practically achievable rarely exceeds 10 ~3, this technique has usually been limited to projectiles of 10 keV and less, and to single and double capture. For higher energies, measurement of Q through direct energy change of the projectile becomes increasingly more difficult [17]. It has been pointed out previously that recoil ion longitudinal momentum can yield direct information on the Q value and electron mass transfer in fast-ion-atom collisions [18]. In this work we report the first experimental use of recoil longitudinal momentum spectroscopy to obtain Q values for capture. The technique is not limited to collision energies below a few keV, and can be used in situations for which the beam energy is not very well determined. This technique is readily applicable to any charge-transfer number, and is not affected by the kinematic broadening due to autoionization of the projectile following the collision. We apply the method to measure Q values for up to fiveelectron transfer for Ar 15 " 1 " ions on Ar.For collisions involving /-electron capture, conservation of both energy and momentum results in a simple relation between the Q value of the collision and the momentum transfer to the recoil given by (in a.u.)(1) where v is the projectile velocity, P\\ and P± are the longitudinal and transverse (relative to the beam direction) components of the momentum transfer to the recoil, and M\ and Mi are the projectile and recoil masses, respectively. The term iv 2 /2 appears due to the fact that the captured electrons are moving with the projectile at velocity r just after the collision. We will show later that Q' is much smaller than QQ and to a very good approximationsuch that it is sufficient to measure P\\ to obtain Q values. We wish to emphasize that this technique applies only to pure electron capture collisions, and bre...

We have used time-of-flight coincidence techniques to study multielectron reactions in 10-keV/u (U=0.632 a.u.) Ar + (5 ( q ( 17) on Ar collisions. Absolute cross sections for total charge-transfer (o. ), Projectile charge-change (o. q q l, ), recoil Production (crq ), and Phenomenological cross sections (O. q qk) have been obtained by normalizing to cross sections reported in the literature [H. Klinger, A. Miiller, and E. Salzborn, J. Phys. B 8, 230 (1975)]. The data have been used to critically test the predictions of the molecular classical overbarrier model (MCBM) [A. Niehaus, J. Phys. B 19, 2925 (1986)] and rather impressive agreements have been obtained. In particular, the predictions of target outer-shell excitation seem to have supporting evidence in this set of data. A stabilization scheme for the multiply excited projectile following charge transfer is proposed to complement the MCBM predictions and the gross features of the Anal reaction products are fairly accounted for. In addition, enhanced electron loss from the projectile-target system is observed in hard collisions for low-charged projectiles (q 8) and is attributed to inner-shell excitation via molecular-orbital promotion.PACS number(s): 34.70.+e, 34.50.Fa

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