Impact-parameter dependence ofK-vacancy production in chlorine-argon collisions

We have measured the probability per collision for the production of Cl and Ar $K$ vacancies in the bombardment of argon gas targets by 15- and 30-MeV chlorine projectiles. By detecting the Cl and Ar $K$ x rays in coincidence with Cl ions scattered to a known angle we have deduced the dependence of the probability on impact parameter $b$. The sum of Ar and Cl $K$-vacancy production probabilities, presumably a measure of $2p\sigma$ vacancy production, is found to depend in magnitude on the charge state of the Cl projectile, but the shape of the summed probability curve, when plotted versus $b$, is not charge-state dependent. The measured shape is close to that expected for $K$-vacancy production via $2p\sigma$-$2p\pi$ rotational coupling. The data suggest that, for these collisions, the production of $2p\pi$ vacancies at large internuclear distances is important and that a two-step mechanism for the production of the $K$ vacancies is operating.