REMI end station
A COLTRIMS Reaction Microscope (REMI) is available for ion and electron momentum imaging experiments in the gas phase. The spectrometer consists of one short side (˜20 cm) intended for ion detection, and one long side (˜50 cm) intended for electron detection. Both sides are equipped with time- and position-sensitive hexanode delay-line MCP-detectors with a diameter of 120 mm.
We typically carry out two different kinds of experiments with the REMI:
Coulomb explosion imaging: Here, we make use of the very intense (few mJ) and short (10 fs or less) X-ray pulses to strongly ionize the molecules. All fragment ions are detected (full angle acceptance up to kinetic energies of a few hundred eV, the typical ion detection efficiency is ˜60%). Based on experience, photon energies of 1.5 to 2.5 keV provide high pulse energies while resulting in a good signal-to-background ratio.
Ion-electron coincidence experiments: For significantly reduced X-ray pulse energies (˜few μJ), it is feasible to detect electrons in coincidence with the fragment ions. This provides the opportunity to record molecular-frame electron angular distributions. The X-ray pulse energy is limited by the number of generated electrons in this case. Electrons of typcially up to ~150 eV kinetic energy can be detected with the full solid angle acceptance.
Both types of experiments can be carried out time-resolved, when using either two-color X-ray operation or an optical laser pump pulse. However, in the case of electron detection, the use of the optical laser may require significant effort for stray light reduction. In the case of two-color and/or special short-pulse operation of the X-rays, the per-pulse energies are significantly reduced (˜0.2 to 1.0 mJ, depending on the photon energy) compared to standard operation.
A four-stage supersonic gas jet is used for sample delivery. It features piezo slits, suitable to cut down the target in the interaction point to a point where only single molecules interact with each X-ray pulse. Gas-phase or liquid targets can be delivered at room temperature. Rudimentary heating can be provided for low-vapor-pressure samples, but the setup is not suitable for solid samples that require high temperatures at this point. Three nozzles of different sizes and with separate gaslines provide the opportunity for a fast change between different samples. Cooling of the nozzles down to a few Kelvin is foreseen for producing small clusters, but has so far not yet been used.
The maximum repetition rate of the experiments is limited by the flight time of the heaviest ionic fragment (to avoid overlapping spectra) and/or by the desired pulse energy of the optical laser, needed e.g. for frequency conversion. Typically used intra-train repetition rates are of the order of 200 kHz, thus resulting in around 400 pulses per second (i.e. 40 pulses in each 10 Hz pulse train).
Users are strongly encouraged to discuss their specific use case before submitting a proposal. For further information, contact Rebecca.
The REMI was designed and built in the framework of a BMBF project in collaboration with the group of Reinhard Dörner, Frankfurt University.