Energy transfer in ion-Rydberg charge exchange

Abstract

The charge transfer process between slow ions and Rydberg atoms was studied using resonant laser excitation to detect specific energy states of the collision products. The collisions were studied for ions of charge q = 2 - 11 at a velocity of v = 0.100 a.u. and for velocities v = 0.031 - 0.130 a.u. at a charge of q = 3. The Rydberg target was excited by three lasers into a final state where the primary quantum number could be varied from nt = 7 to nt = 26. The apparatus used to implement the detection scheme is discussed in detail. The apparatus includes a device to remove the primary ion beam prior to the detector while transmitting the q-1 charge transfer beam. The development and characterization of this device are described in detail. Also described is the flexible low noise detector developed to measure the laser excited state in the collision product. The data collected show a clear resonance in the capture cross section to a particular energy state as the binding energy of the Rydberg target is varied. The resonance varies significantly over the range of charges and velocities studied and agrees very well with the classical trajectory Monte Carlo (CTMC) theory over nearly the entire range of measurements. Only at the lowest and highest velocities studied are any significant disagreements seen. The measurement shows a slightly larger width than CTMC theory at the lowest two velocities studied, v = 0.031 and 0.046 a.u. At the highest velocity studied v=0.130 a.u. significant disagreement with CTMC theory is seen. However, the measured resonance position clearly disagrees with the predictions of the classical over barrier (COB) model. The variation of width with both charge and velocity is similar to what would be expected from a simple uncertainty principle argument, however the overall agreement with CTMC suggests a quantum mechanical argument may not be necessary to explain this behavior.