Evidence of Superelastic Electron Collisions fromH2+Studied by Dissociative Recombination Using an Ultracold Electron Beam from a Cooler Ring

Abstract

The dissociative recombination rate of H 2 1 was measured as a function of the storage time with an ultracold electron beam in a storage ring. The H 2 1 ions vibrationally relax with time, and almost reach the vibrational ground state at a time of 25 s after injection. The magnitude of the ground-state population after 25 s far exceeds that expected based on the initial ground-state population and its decay. This indicates the existence of a superelastic collision process, where electrons are scattered from excited molecular ions and gain energy by vibrational deexcitation. PACS numbers: 34.80.GsAtomic-physics research using ion-storage rings has made great progress since the early 1990s, especially concerning dissociative recombinations (DR) of molecular ions [1]. For molecular ions having permanent dipole moments, vibrationally cold ions are obtained simply by storing the excited ions for a time of 1 sec or less. Experiments using these ions, of which the vibrational quantum number is defined as y 0, have enabled precise comparative studies between experiment and theory. However, those ions without dipole moments do not radiate infrared rays during storage, and their deexcitation would not be expected during a short storage time. This has been a difficulty in studying homonuclear diatomic molecular ions. The simplest molecular ion without a permanent dipole moment is H 2 1 . The DR of H 2 1 has been studied with storage rings in Heidelberg [2], Stockholm [3,4], and Aarhus [5,6]. The population of the vibrational states varies according to the experimental conditions: The populations in Heidelberg and Stockholm are similar to each other, while the population in Aarhus is different. The vibrational deexcitation of infrared-inactive ions depends on both the intensity and quality of the electron beam as well as on the storage time. This is the reason why the populations are different among these storage rings. There are two possible mechanisms by which the vibrational distribution of the stored ions is affected during collisions using a low-energy electron beam. One is DR and the other is the vibrational deexcitation due to electron impact, which is called a superelastic collision (SEC):where y is the vibrational quantum number and E rel is the relative energy between the electron and the ion. A SEC does not affect the circulating ion current, while DR decreases it. Although DR has been studied extensively both experimentally and theoretically, SEC has only been predicted theoretically [7,8], and so far there has been no experimental evidence [4]. It is theoretically predicted that the cross sections of SEC increase with a decrease in the electron energy, and even far exceeds those of DR in N 2

Dissociative recombination and low-energy inelastic electron collisions of the helium dimer ion

“…Collisions with both residual gas molecules and electrons are known to affect the rovibrational populations in stored beams of molecular ions [47,48,49,50]. The relevant elementary reactions between the stored He intact, but changing its rovibrational excitation.…”

Cooling of molecular ion beams

“…Cross sections as a function of the storage time were observed for D + 2 and H + 2 ions already in early work at storage rings, including CRYRING [28,29] where significant increases of the relative population in v = 0 were found during storage times of up to ∼40 s. It was first suspected that the effect would be due to the preferential destruction of vibrationally excited ions either by collisions with rest gas molecules [28] or by the dissociative recombination process itself [29]. Later, similar measurements at TARN II [30] finally demonstrated that indeed H + 2 ions in higher vibrational states were converted into ground-state (v = 0) ions during electron cooling, giving evidence for electron-induced vibrational de-excitation ("super-elastic collisions").…”

Rotational state effect and fragmentation of small polyatomic molecular ions

“…Moreover, due to the super-elastic collisions in the electron cooler, further deexcitation of vibrationally excited states could be expected [20]. Measurements were made after 3 s and 17 s of storage.…”