Attosecond Reaction Microscope Laboratory
Laser sources
The laboratory is based on two high repetition rate ytterbium sources:
1) Pharos laser system delivering 20 W at 50 kHz with a pulse duration of 260 fs.
2) Amphos laser system delivering 200 W at 100 kHz with a pulse duration of 900 fs.
The pulses are compressed either by a hollow core fibre compressor or by a multi-pass cell. In both cases, a set of chirped mirrors is used to compress the temporal duration down to 20 fs. The pulse duration is characterised using a second harmonic autocorrelator and second harmonic frequency resolved optical gating.
Attosecond beamline
Attosecond pulse trains consisting of odd harmonics of the fundamental radiation are generated by focusing the femtosecond laser pulses into a gas jet. Using argon as the generating medium, we can routinely produce a spectrum of odd harmonics ranging from 20 to 50 eV. Using neon, harmonic radiation up to 100 eV can be observed.
The radiation is characterised using an extreme ultraviolet spectrometer consisting of a concave grating and an MCP/phosphor assembly. During the experiments, the inline spectrometer is used as a diagnostic to control the stability of the harmonic radiation.
Reaction Microscope
A photoion-photoelectron coincidence spectrometer is used to measure the photoelectrons and photoions generated in the photoionisation process induced by the absorption of an extreme ultraviolet photon. The spectrometer consists of two position sensitive detectors with an MCP and a delay anode. The charged particles (electrons and ions) are guided to their respective detectors by the combination of a uniform electric and magnetic field. By measuring the arrival time (i.e. the time elapsed between the ionisation instant, the instant at which the charged particle reaches the detector) and the impact position on the detector, we can reconstruct the momentum p of the particles at the ionisation instant. This instrument, usually referred to as a reaction microscope, provides the most complete characterisation of the photoionisation process in atoms and small molecules.
Ultrastable delay-line
The acquisition of pump-probe measurements over long periods of time with a reproducible delay between the two pulses requires an exceptionally stable experimental setup. To this end, we have developed an ultra-stable delay line that allows the measurement of photoelectron spectra over several days with a reproducibility better than 40 as. The system is based on two drilled plates with different hole diameters, allowing a monolithic collinear interferometer for pump-probe measurements.
