An experimental study of the depopulation of Na 30s Rydberg atoms under electron bombardment has been carried out. Electron energies E ranged from 105 to 660 eV corresponding to collision velocities 80 to 200 times the mean orbital speed of the excited 30s electron. The collision-induced final states were identified by means of selective field ionization (SFI). 30p, 29p and a small trace of 28p were clearly observed in the adiabatic SFI distributions. Collision-induced diabatic SFI signals were absent, in contrast with a previous study that used slow ionic projectiles. 30s-30p and 30s-29p absolute cross sections were obtained from measurements of the collisional depopulation of the 30s state and measurements of the electron beam current-density-profiles. The cross sections ranged from 0.33 to 1.69 * l0-10 cm2. The results agreed with calculations based on the dipole approximation with first-order perturbation theory and the Born approximation. Cross sections were also measured in external electric fields up to 15.75 V cm-1 for the higher collision energies (422 to 660 eV), but unlike the previous study no field dependence of cross sections for final-state distributions was apparent.

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An electrostatic analyzer positioned at various angles has been used to measure absolute values of cross sections for electron production in collisions of neutral hydrogen atoms with helium as a function of the angle and energy of the ejected electrons. These cross sections are compared with corresponding ones for proton and electron impact to infer the mechanisms of electron production. A prominent peak in the energy distribution due to electron loss from the projectile occurs at an electron velocity equal to that of the projectile. At very low electron energies the similarity of the energy distribution to that due to electron impact suggests that the large cross section there is due to ionization of the target by the electron carried by the projectile. As with proton impact, the cross sections at low projectile velocities fall off approximately exponentially with the energy of the ejected electron, in agreement with the predictions of the electron promotion model. An electrostatic analyzer positioned at various angles has been used to measure absolute values of cross sections for electron production in collisions of neutral hydrogen atoms with helium as a function of the angle and energy of the ejected electrons. These cross sections are compared with corresponding ones for proton and electron impact to infer the mechanisms of electron production. A prominent peak in the energy distribution due to electron loss from the projectile occurs at an electron velocity equal to that of the projectile. At very low electron energies the similarity of the energy distribution to that due to electron impact suggests that the large cross section there is due to ionization of the target by the electron carried by the projectile. As with proton impact, the cross sections at low projectile velocities fall off approximately exponentially with the energy of the ejected electron, in agreement with the predictions of the electron promotion model. ©1980 The American Physical Society P H Y S I C A L R E V I E W A V O L U M E 2 1 , N U M B E R 2 F E B R U

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Ar+ and Na+ ion beams bombarding Na ns and np Rydberg-state targets at impact velocities near the Bohr-orbital velocity of the target atoms (i.e. , 0 =1) induce transitions to (n -1)l states (l & 2) with larger-than-geometric cross sections. Depopulation of ns states proceeds directly into the full n -1, I & 2 manifold rather than populating the np or (n -1)p states as the first of a sequence of dipole-allowed steps. Depopulation of np states leads to a distribution of final states that is dominated by the nearer or high-l part of the n -1 manifold. n dependences of the cross section for ns depopulation are given at several energies for n =32-41 and at a single energy for np, n =26-32. The absolute cross section for Na(36s) depopulation falls gradually but steadily for reduced velocities 0' increasing from 0.3 to 1.35. Bombardment of Na 39s and 39p states in applied fields 0-18.5 V/cm reveals that the Anal-state distributions become narrower as manifold states are Stark shifted close to the initial states. No theoretical results are available for direct comparison with these experiments, but the method of coupled channels seems to be the most suitable candidate.