Search Results (6 total)

Rate coefficients for dissociative recombination (DR), dissociative excitation (DE), and vibrational excitation between the helium dimer ion ⁴He₂⁺ and electrons from a few meV up to 40 eV were measured using fast (3.8 and 8.3 MeV) ion beams stored for up to 85 s. Vibrational relaxation to greater than 95% in the v=0 level was achieved by collisions with cold electrons over 50 s. Low-energy, strongly v-dependent DR rate coefficients are given for v=0, 1, and ≥2. The rate coefficients at higher energies for v=0, with DR and DE given on an absolute scale, are compared to results from recent wave-packet calculations on the fast dissociation dynamics of the doubly excited helium dimer, where the three processes occur as competing reaction channels. Also given are rate coefficients for vibrationally superelastic electron collisions at near 10 meV average energy and the approximate vibrational excitation cross section for fast collisions with the residual gas.

A three-dimensional imaging technique installed at the Heidelberg Test Storage Ring (TSR) has been used to investigate the three-body breakup channels occurring in the dissociative recombination process of the methylene ion CH₂⁺. By selecting dissociation planes perpendicular to the molecular beam direction the dissociation kinematics could be measured with unprecedented momentum resolution. Release energies, the relative branching ratio, and the kinematical correlations between the three fragments were determined for the two energetically allowed channels: C(³P)+H(²S)+H(²S) and C(¹D)+H(²S)+H(²S).

The energy-resolved rate coefficient for the dissociative recombination (DR) of H₃⁺ with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to 500  μeV by using a cryogenic GaAs photocathode. This and a previous cold-H₃⁺ measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate H₃⁺ in the two lowest, (J,G)=(1,1) and (1,0) rotational states prevailing also in cold interstellar matter. The use of para-H₂ in the ion source, expected to enhance para-H₃⁺ in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

The dissociative recombination (DR) of ³He  ⁴He⁺ has been investigated at the heavy-ion Test Storage Ring (TSR) in Heidelberg by observing neutral products from electron-ion collisions in a merged beams configuration at relative energies from near-zero (thermal electron energy about 10 meV) up to 40 eV. After storage and electron cooling for 35 s, an effective DR rate coefficient at near-zero energy of 3×10⁻⁹  cm³s⁻¹ is found. The temporal evolution of the neutral product rates and fragment imaging spectra reveals that the populations of vibrational levels in the stored ion beam are nonthermal with fractions of ∼0.1–1  % in excited levels up to at least v=4, having a significant effect on the observed DR signals. With a pump-probe-type technique using DR fragment imaging while switching the properties of the electron beam, the vibrational excitation of the ions is found to originate mostly from ion collisions with the residual gas. Also, the temporal evolution of the DR signals suggests that a strong electron induced rotational cooling occurs in the vibrational ground state, reaching a rotational temperature near or below 300 K. From the absolute rate coefficient and the shape of the fragment imaging spectrum observed under stationary conditions, the DR rate coefficient from the vibrational ground state is determined; converted to a thermal electron gas at 300 K it amounts to (3.3±0.9)×10⁻¹⁰  cm³s⁻¹. The corresponding branching ratios from v=0 to the atomic final states are found to be (3.7±1.2)  % for 1s2s  ³S,(37.4±4.0)  % for 1s2s  ¹S,(58.6±5.2)  % for 1s2p  ³P, and (2.9±3.0)  % for 1s2p  ¹P. A DR rate coefficient in the range of 2×10⁻⁷  cm³s⁻¹ or above is inferred for vibrational levels v=3 and higher. As a function of the collision energy, the measured DR rate coefficient displays a structure around 0.2 eV. At higher energies, it has one smooth peak around 7.3 eV and a highly structured appearance at 15–40 eV. The small size of the observed effective DR rate coefficient at near-zero energy indicates that the electron induced rotational cooling is due to inelastic electron-ion collisions and not due to selective depletion of rotational levels by DR.

H₂ molecules were ionized by Ti:sapphire (45 fs, 800 nm) and Nd-doped yttrium aluminum garnet lasers (6 ns, 1064 nm). The relative populations of the vibrational levels of the H₂⁺ ions were determined and found to be concentrated in the lowest vibrational levels. Tunneling ionization calculations with exact field-modified potential curves reproduce the experimental results. The reason for the departure from conventional Franck-Condon-like distributions is the rapid variation of the ionization rate with internuclear distance.

We demonstrate that the dissociative recombination of D₂H⁺ with low-energy electrons depends on the rotational energy of the molecular ion such that highly excited ions have a larger rate coefficient than colder ones. Observations on an ion beam continuously interacting with electrons at low relative velocity indicate that excited rotational levels are preferentially depleted which, in competition with radiative heating due to blackbody radiation, provides an opportunity for controlling the rotational temperature of stored molecules.