Active feedback scheme for minimization of helicity-dependent instrumental asymmetries

A method for the active feedback reduction of optical instrumental intensity asymmetries is presented. It is based on the fast chopping of two spatially separated beams of light with orthogonal linear polarizations that are recombined and passed through a quarter-wave plate to yield a single beam with rapidly flipping helicity. Active electro-optic feedback has been successfully employed to maintain this asymmetry below 10−5

Active feedback scheme for minimization of helicity-dependent instrumental asymmetries

A method for the active feedback reduction of optical instrumental intensity asymmetries is presented. It is based on the fast chopping of two spatially separated beams of light with orthogonal linear polarizations that are recombined and passed through a quarter-wave plate to yield a single beam with rapidly flipping helicity. Active electro-optic feedback has been successfully employed to maintain this asymmetry below 10−5

Assignment of resonances in dissociative recombination of HD+ ions: high-resolution measurements compared with accurate computations

The collision-energy resolved rate coefficient for dissociative recombination of HD+ ions in the vibrational ground state is measured using the photocathode electron target at the heavy-ion storage ring TSR. Rydberg resonances associated with ro-vibrational excitation of the HD+ core are scanned as a function of the electron collision energy with an instrumental broadening below 1 meV in the low-energy limit. The measurement is compared to calculations using multichannel quantum defect theory, accounting for rotational structure and interactions and considering the six lowest rotational energy levels as initial ionic states. Using thermal equilibrium level populations at 300 K to approximate the experimental conditions, close correspondence between calculated and measured structures is found up to the first vibrational excitation threshold of the cations near 0.24 eV. Detailed assignments, including naturally broadened and overlapping Rydberg resonances, are performed for all structures up to 0.024 eV. Resonances from purely rotational excitation of the ion core are found to have similar strengths as those involving vibrational excitation. A dominant low-energy resonance is assigned to contributions from excited rotational states only. The results indicate strong modifications in the energy dependence of the dissociative recombination rate coefficient through the rotational excitation of the parent ions, and underline the need for studies with rotationally cold species to obtain results reflecting low-temperature ionized media.Comment: 15 pages, 10 figures. Paper to appear in Phys. Rev. A (version as accepted

Isotope effect for the mutual neutralization reaction at low collision energies: He+ + H−

We measured the branching ratios of the He+ + H− and He+ + D− mutual neutralization at a collision energy below 25 meV. Those are correctly reproduced using the Landau-Zener model applied to potential energy curves computed by an anion-centered asymptotic model. The analyticity of both models allows getting a deeper insight into the reaction. It allows defining a low collision energy regime for the mutual neutralization at which the collision energy no longer affects the branching ratios. Using those models, we explain how heavier isotopes favor the production of higher excited states

Production of excited atomic hydrogen and deuterium from H2, HD and D2 photodissociation

We have measured the production of Lyman α and Balmer α fluorescence from atomic H and D for the photodissociation of H2, HD and D2 by linearly-polarized photons with energies between 22 and 64 eV. We discuss systematic uncertainties associated with our data, and compare our results with previous experimental results and ab initio calculations of the dissociation process. We comment on the discrepancies

State-to-state measurements of low-energy ion-molecule and ion-ion collisions via 3D momentum imaging

info:eu-repo/semantics/nonPublishe

Intense-laser-field ionization of molecular hydrogen in the tunneling regime and its effect on the vibrational excitation of H2+.

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 H+2 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

Production of excited atomic hydrogen and deuterium from H2, HD and D2 photodissociation

We have measured the production of Lyman α and Balmer α fluorescence from atomic H and D for the photodissociation of H2, HD and D2 by linearly-polarized photons with energies between 22 and 64 eV. We discuss systematic uncertainties associated with ou

DR rate coefficient measurements using stored beams of H₃⁺ and its isotopomers

In studies of the rate coefficient of the dissociative recombination of H3+ and its isotopomers, the rovibrational excitation of the molecular ions was found to play an important role, in particular when employing the technique of heavy-ion storage rings. The dependence of the DR rate on rotational excitation was investigated in recent experiments at the Test Storage Ring TSR in Heidelberg through time-resolved measurements on D2H+ and H3+ over long storage times. For both molecules, an influence of rotational excitation on the DR rate was observed. The level of excitation in turn was found to be dominated by radiative coupling to the surrounding 300 K background for D2H+. In the case of H3+, a strong influence of electron collisions on the excitation level was found, whereas an additional influence of collisions with residual gas in the storage ring cannot be excluded