Emma Anderson

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40-100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles.

Positron interactions with water–total elastic, total inelastic, and elastic differential cross section measurements

Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV. Comparison with theory is made with both R-Matrix and distorted wave calculations, and with our own application of the Independent Atom Model for positron interactions.

Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft x-ray absorption in native and hydrogenated coronene cations, C_24H_12+m^+ m=0–7, led to the conclusion that additional hydrogen atoms protect (interstellar) polycyclic aromatic hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30–200 eV) He atoms and pyrene (C_16H_10+m^+, m=0, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

Rotationally Cold OH- Ions in the Cryogenic Electrostatic Ion-Beam Storage Ring DESIREE

We apply near-threshold laser photodetachment to characterize the rotational quantum level distribution of OH^{-} ions stored in the cryogenic ion-beam storage ring DESIREE at Stockholm University. We find that the stored ions relax to a rotational temperature of 13.4±0.2  K with 94.9±0.3% of the ions in the rotational ground state. This is consistent with the storage ring temperature of 13.5±0.5  K as measured with eight silicon diodes but in contrast to all earlier studies in cryogenic traps and rings where the rotational temperatures were always much higher than those of the storage devices at their lowest temperatures. Furthermore, we actively modify the rotational distribution through selective photodetachment to produce an OH^{-} beam where 99.1±0.1% of approximately one million stored ions are in the J=0 rotational ground state. We measure the intrinsic lifetime of the J=1 rotational level to be 145±28  s.

Spontaneous decay of small copper cluster anions, Cu−N = 3 − 6

We have measured the spontaneous neutral particle emission from beams of copper cluster anions (Cu−N, N = 3 − 6) stored in an electrostatic storage ring. These experiments have show that multiple structural isomers may be present in the beam.

Dianion diagnostics in DESIREE: High-sensitivity detection of Cn2− from a sputter ion source

A sputter ion source with a solid graphite target has been used to produce dianions with a focus on carbon cluster dianions, Cn2-, with n = 7-24. Singly and doubly charged anions from the source were accelerated together to kinetic energies of 10 keV per atomic unit of charge and injected into one of the cryogenic (13 K) ion-beam storage rings of the Double ElectroStatic Ion Ring Experiment facility at Stockholm University. Spontaneous decay of internally hot Cn2- dianions injected into the ring yielded Cn- anions with kinetic energies of 20 keV, which were counted with a microchannel plate detector. Mass spectra produced by scanning the magnetic field of a 90° analyzing magnet on the ion injection line reflect the production of internally hot C72- - C242- dianions with lifetimes in the range of tens of microseconds to milliseconds. In spite of the high sensitivity of this method, no conclusive evidence of C62- was found while there was a clear C72- signal with the expected isotopic distribution. This is consistent with earlier experimental studies and with theoretical predictions. An upper limit is deduced for a C62- signal that is two orders-of-magnitude smaller than that for C72-. In addition, CnO2- and CnCu2- dianions were detected.

Non-statistical fragmentation of large molecules in collisions with atoms

Non-statistical fragmentation processes are important when Polycyclic Aromatic Hydrocarbon (PAH) molecules, fullerenes, or other large molecules collide with atoms at center- of-mass energies from a few tens to a few hundreds of eV. The typical result is the prompt, billiard-ball-like knockout of single atoms (CHx-loss). This is distinct from the well-known statistical fragmentation patterns of these molecules, which are dominated by H- and C2H2-loss for PAHs and C2-loss for fullerenes. We have explored the role of non-statistical fragmentation of PAHs and fullerenes in a series of experimental and theoretical studies. In general, the yield of non-statistical fragments depends sensitively on their stability against secondary statistical fragmentation following knockout.

Rotationally cold (> 99% J = 0) OH − molecular ions in a cryogenic storage ring

We store a 10 keV OH− ion-beam at 13.5 ± 0.5 K in one of the DESIREE storage rings. Using photodetachment thermometry we measure the effective relative photodetachment cross section at different storage times and determine the rotational temperature of the ions to be 13.4 ± 0.2 K in agreement with the macroscopic temperature. A model cross section in the threshold range taking into account the formation of excited neutral OH molecules is calculated as a function of rotational temperature in order to justify the use of the rotational thermometry method developed earlier by the group of Roland Wester at Innsbruck University in the present case. In addition, we apply a selective photodetachment technique to produce an ion beam with more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J = 1 rotational level is measured to be 145 ± 28 s.

Fragmentation studies of Hydrogenated-Pyrene Polycyclic Aromatic Hydrocarbons in collisions with He

We have studied the fragmentation of Pyrene or hydrogenated Pyrene cations (C16H+10+x with x = 0, 5, 6 and 16) in collision with He. We find that the carbon backbone fragmentation cross section increases with the number of H atoms added. This suggests that hydrogenation does not protect PAHs in, e.g., harsh interstellar environments.

Radiative cooling of hot Cn−and CnH−molecules

We have measured the rates of neutrals produced from 10 keV Cn− or CnH− (n=2, 4, 6, 8, and 10) ion beams stored in one of DESIREEs 14 K storage rings. For n=4, 6, and 8 we observe marked differences between Cn− and CnH− cooling rates as inverse internal conversion [cf. S. Martin et al (2013) Phys. Rev. Lett. 110, 063003] processes are effective for the Cn− ions only. Knowledge of the cooling rates of these ions are important for estimates of their formation and destruction rates in cold interstellar environments.