Vidhya Krishnamurthi

Ionization and excitation of hydrogen molecules by fast proton impact

The ratio of ionization - excitation to single ionization of hydrogen molecules caused by fast proton impact was measured over a wide velocity range (v = 6 - 24 au) using the coincidence time-of-flight technique. This ratio, %, is independent of the collision velocity at high velocities. It differs from the ratio of total to production mostly due to a large contribution from the dissociation of the electronic ground state of the molecular ion. The dissociation fraction of was measured and compares well with our calculations using the Franck - Condon approximation.

Double and single ionization of hydrogen molecules by fast-proton impact

The ratio of double to single ionization of hydrogen molecules caused by fast-proton impact was measured over a wide velocity range (v = 4-24 au) using the coincidence time-of-flight technique. The value of this ratio for hydrogen molecules at the high-velocity limit was determined to be 0.18+0.01-0.02 using the q/v dependence suggested by McGuire. This ratio is smaller by about a factor of 1.8 for hydrogen than for helium over the measured energy range and by about 1.4 at the high-velocity limit. This difference between the two targets is due mainly to the single-ionization cross section, which was measured to be larger by a factor of 1.79 ± 0.05 for hydrogen molecules than for helium. The double-ionization cross section, in contrast, is similar for both helium and hydrogen targets. It is suggested that single ionization of hydrogen molecules is more likely due to its smaller binding energy while the stronger electron-electron interaction in helium compensates for the smaller probability of proton impact ionization and leads to roughly equal double ionization of both targets. For both hydrogen and helium targets, the double- to single-ionization ratio is smaller for proton impact than for equal-velocity electron impact over the measured velocity range.

Projectile charge dependence of ionization and fragmentation of CO in fast collisions

Single and multiple ionization cross sections of CO have been measured as a function of the projectile charge. The experiment was carried out at fixed collision energies of 1 MeV using the coincidence time-of-flight technique. The cross sections increase rapidly for small q and more slowly for large q. The branching ratios of all the transient molecular ions and the kinetic energy distribution of the dissociating transient exhibit similar projectile charge dependence. These observed trends are explained quite satisfactorily using a simple model based on perturbation theory and the independent electron approximation. The model predicts that the average value of the projectile - electron interaction strength in collisions resulting in target ionization varies quickly for small q and more slowly for large q.

One and two electron processes in fast collisions between protons and hydrogen molecules

Abstract The ratio of double to single ionization of hydrogen molecules caused by fast proton impact was measured over a wide velocity range (v = 6–24 a.u.) using the coincidence time-of-flight technique. This ratio, ∼ 0.13% at high velocities, is about a factor of 2 smaller than the same ratio for the He target. This difference is due mainly to the larger single ionization cross section for hydrogen molecules, i.e. σ+(H2) ∼ 2 σ+(He). The double ionization cross section, on the other hand, is similar for both He and H2. Furthermore, the ratio of double to single ionization of the hydrogen molecule by proton impact is smaller than the one caused by electron impact at the same velocity in a similar way as for the helium target.

Charge dependence of one and two electron processes in collisions between hydrogen molecules and fast projectiles

The ratio of double- to single-ionization (DI/SI) as well as the ratio of ionization-excitation to single-ionization (IE/SI) in hydrogen molecules was studied by examining the effect of the projectile charge on these processes. The DI/SI and IE/SI ratios were measured using the coincidence time of flight technique at a fixed velocity (1 MeV/amu) over a range of projectile charge states (q = 1-9,14,20). Preliminary results indicate that for a highly charged F{sup 9+} projectile the DI/SI and IE/SI ratios are 6.8% and 24.7%, respectively, a large increase from the ratios of 0.13% and 1.95%, respectively, for H{sup +} projectiles. For low charge states, the DI/SI is negligible relative to the IE/SI ratio, while for more highly charged projectiles the DI/SI ratio becomes comparable to the IE/SI ratio. This indicates that double-ionization increases much more rapidly with projectile charge than ionization-excitation.

Projectile charge dependence of ionization and dissociation of CO in fast collisions

Experiments have been carried out to study how changes in the interaction strength (defined as q/vb) of a fast ion-molecule colision affect the ionization and dissociation of the molecular target, in this case CO. The coincidence time-of-flight technique was used for collisions at fixed velocity (energy of 1 MeV/amu). The interaction strength was changed by varying the charge of the projectile ion. The cross sections for single and multiple ionization of CO increase rapidly for small q, approximately as q2n (where n is the number of ionized electrons), and more slowly for larger values of q. A rather simple theoretical model based on the independent electron approximation and perturbation theory is in good agreement with the data [1]. The dissociation patterns of the transient COQ+ molecular ions also exhibit a dependence on the projectile charge which is qualitatively explained by the same model.

HIGHLY CHARGED ION-MOLECULE COLLISIONS

Experimental studies of collisions between highly charged ions and molecules have been carried out using the coincidence time of flight technique at high and low collision velocities. We have studied ionization of the target molecule and electron capture caused by ion impact. In general the trends are similar for molecular and atomic targets, and thus one can use the knowledge about one kind of target to help understand the other. Twoelectron processes, like ionization-excitation and double-ionization, become almost as important as single-electron processes in collisions between highly charged ions and two-electron targets. It is shown that in collisions between highly charged ions and molecules the high energy tail of the distribution of kinetic energy released in the dissociation into ion-pairs is associated with one or two electrons in highly excited states.

An experimental method for evaluating the H2 and D2 contamination levels in a HD bottle

Abstract Studies of one- and two-electron processes in collisions with hydrogen molecules using the time-of-flight technique are simplified by using the heteronuclear HD isotope because the H+ and D+ fragments can be easily distinguished from each other. The difference in the time-of-flight of H+ and D+ enable the fragments to be measured in coincidence with each other. However, it is hard to determine the purity of the HD target, in particular the H2 contamination, because the H2+ molecular ions coincide with the D+ fragments. A method is suggested to determine the purity of a HD target to a precision of a few percent. This method is based on measuring the yield of very slow ( 1 MeV). The H2 contamination is then determined by subtracting the theoretically evaluated contribution of the ground-state dissociation of the HD+ molecular ion from the measured sum of both H+ and D+ low-energy fragments.

Scaling of single ionization cross sections of molecules with the charge of fast projectiles

An experimental investigation has been carried out to probe the scaling of single ionization cross sections of molecular targets with the projectile charge q. The cross sections scale like a sigma 0qalpha power law, similar to the case of atoms. In all cases the fitted sigma 0 is in agreement with the measured proton impact cross sections, as predicted by perturbation theory within the independent electron approximation. The exponent alpha is found to be slightly smaller than the theoretically predicted value of 2. The same scaling law was found to hold for bare and dressed projectiles indicating that screening is approximately complete for these soft collisions.