H. B. Pedersen

A bent electrostatic ion beam trap for simultaneous measurements of fragmentation and ionization of cluster ions.

We describe a bent electrostatic ion beam trap in which cluster ions of several keV kinetic energy can be stored on a V-shaped trajectory by means of an electrostatic deflector placed between two electrostatic mirrors. While maintaining all the advantages of its linear counterpart [Zajfman et al., Phys. Rev. A 55, R1577 (1997); Dahan et al., Rev. Sci. Instrum. 69, 76 (1998)], such as long storage times, straight segments, and a field-free region for merged or crossed beam experiments, the bent trap allows for simultaneous measurement of charged and neutral fragments and determination of the average kinetic energy released in the fragmentation. These unique properties of the bent trap are illustrated by first results concerning the competition between delayed fragmentation and ionization of Al(n) (-) clusters after irradiation by a short laser pulse.

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

The dissociative recombination (DR) of He-3 He-4(+) 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 x 10(-9) cm(3)s(-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 similar to 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) x 10(-10) cm(3)s(-1). The corresponding branching ratios from v=0 to the atomic final states are found to be (3.7 +/- 1.2) % for 1s2s S-3, (37.4 +/- 4.0) % for 1s2s S-1, (58.6 +/- 5.2) % for 1s2p P-3, and (2.9 +/- 3.0) % for 1s2p P-1. A DR rate coefficient in the range of 2 x 10(-7) cm(3)s(-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.

Resonance structure in the electron-impact detachment cross section of caused by the formation of

Electrons have been scattered on in the energy range from 0 - 35 eV. A resonance structure in the detachment channel was observed at an energy of about 10 eV. To explain the structure, a potential curve was constructed from the isoelectronic ground-state potential curve and the repulsive Coulomb potential, and it is argued that the structure may be related to the population of a state. The state is predicted to decay primarily by electron emission since a large potential barrier prevents dissociation into .

Electron collisions and rovibrational action spectroscopy of cold H3+ molecules

Electron recombination of H3+ has found a lot of attention due to its outstanding relevance for the chemistry of the interstellar medium (ISM) and its role as a benchmark for the treatment of dissociative recombination (DR) of polyatomic ions. We report DR measurements performed at the TSR storage ring utilizing a cryogenic ion trap injector. Furthermore, a chemical probing spectroscopy technique is described that allows for a very sensitive monitoring of the populated states inside the ion injector. Since H3+ exists in two different nuclear spin modifications, a controlled manipulation of the ortho/para fraction is needed in order to perform state-selective measurements.

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 rovibrational 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.

Soft-x-ray fragmentation studies of molecular ions

Imaging of photofragments from molecular ions after irradiation by soft x-ray photons has been realized at the ion beam infrastructure TIFF set up at the FLASH facility. Photodissociation of the two-electron system HeH+ at 38.7 eV revealed the electronic excitations and the charge-state ratios for the products of this process, reflecting the non-adiabatic dissociation dynamics through multiple avoided crossings among the HeH+ Rydberg potential curves. Dissociative ionization of the protonated water molecules H3O+ and H5O+2 at 90 eV revealed the main fragmentation pathways after the production of valence vacancies in these ionic species, which include a strong three-body channel with a neutral fragment (OH + H+ + H+) in H3O+ photolysis and a significant two-body fragmentation channel (H3O++ H2O+) in H5O+2 photolysis. The measurements yield absolute cross sections and fragment angular distributions. Increased precision and sensitivity of the technique were realized in recent developments, creating a tool for exploring x-ray excited molecular states under highly controlled target conditions challenging detailed theoretical understanding.

Storage ring study of the dissociative recombination of He-2(+)

Absolute cross sections for dissociative recombination have been measured for the He-4(2)+ and (HeHe+)-He-3-He-4 isotopomers as a function of storage time at the ASTRID 2 storage ring. The recombination rate of (HeHe+)-He-3-He-4 with low-energy electrons is found to dramatically decrease with time, giving an upper limit for the thermal rate coefficient for cold ions of the order of 6 x 10(-10) cm(3) s(-1), in agreement with flowing afterglow measurements. Two-dimensional imaging of the atomic fragments points to He(1s(2) S-1) + He(1s3s S-3) as the lowest dissociation channel to be accessed in the dissociative recombination of rovibrationally hot ions.

A Model for Calculating Branching Ratios in H3+ Dissociative Recombination

The dissociative recombination (DR) of molecular ions is a key process for the formation of complex molecules in dense interstellar clouds.1,2 Among all species, H 3 + attracts particular interest owing to its key role in astrochemistry.3 Moreover, the low energy DR rate for H 3 + has been a controversial issue for many years, and there are still unexplained differences reaching orders of magnitude among the experimental and theoretical results.3

XUV photofragmentation of small water cluster cations at FLASH

Photofragmentation of small water cluster ions H+(H2O)n (n = 1..4) has been investigated at 35eV using fast beam momentum imaging in a crossed beams setup at the free electron laser FLASH at DESY in Hamburg, revealing the dominant fragmentation pathways of these systems under ionizing radiation.

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.