techniques used by the group

PEPICO

Schematic of the PEPICO apparatus

Photoionisation is by wavelength-selected XUV (extreme ultraviolet, 10-50 eV) light from atomic discharges in He or Ne; wavelengths of 58.4, 46.1, 30.4 nm are often used. Electrons are energy analysed in a hemispherical electrostatic analyser with a position-sensitive detector to detect many energies at once. Resolution is controlled by the selected pass energy, typically 50 meV at 5 eV pass. Ions are mass analysed by pulsed-field time-of-flight (resolution ~150) in coincidence with the electrons. The kinetic energy release distributions in fragmentation can be found by peak shape analysis.

PEPICO apparatus

PEFCO

The PEPICO apparatus can be modified by replacing the ion detection system by a sensitive photomultiplier. Fluorescence photons from parent ions or fragment ions can be detected in coincidence with the energy-analysed photoelectrons, allowing determination of quantum yields and lifetimes.

VIPCO (Velocity Imaging Photoionisation COincidence)

By combining velocity imaging with coincidence methods we have an experiment in which positive and negatively charged particles (several of each) from individual events can be detected with energy, mass and angular analysis. All energies, masses and angles are detected all the time, giving a huge multiplex advantage; the collection efficiencies are up to 50% for all particles making the device extraordinarily sensitive. So far this apparatus has been used mainly to study photoelectron angular distributions from molecules effectively "fixed in space", by selecting coincidences between electrons and fragment ions from rapid dissociations (axial recoil). The apparatus can also take PEPICO spectra with high resolution for ion energies and is the key to studying interesting but minor photoionisation channels such as the ones producing negative ions. It can be used to do the PSD-PEPIPICO experiments too.

VIPCO apparatus

ETOF (Electron Time-Of-Flight)

To get spectra of doubly charged ions we have built a long (5.6 m) magnetic bottle TOF apparatus. Ionisation is by light from a pulsed gas discharge, which is being developed as part of the project. The detection efficiency can exceed 50%, as electrons are gathered from all directions of emission. The object is to collect complete two-electron spectra from double ionisation of atoms and molecules, showing direct and indirect double ionisation pathways as well as the spectra of the product ions.

ETOF apparatus

Coincidence apparatus

To do all these various coincidence experiments we devise special tools. Our position- sensitive detectors are built using printed-circuit boards and tapped delay lines. The complex electronics of old are largely replaced by our multichannel time-to-digital converter cards for personal computers, which are now sold commercially by RoentDek Handels GmbH. Electronic devices to greatly improve time resolution and a short (ns) pulse high repetition rate XUV lamp are under development.

The scheme of the instrument incorporating the detector. Ions are formed at the intersection of a gas jet and a light beam, and are accelerated by constant fields to the particle multiplier and delay-line anode. Time zero for ion flight times is taken from the photoelectron signal, while time zero for the position-related delay of each ion is taken from the rear face of the particle multiplier.