While conventional XAS measurements are performed using the multi-bunch filling pattern of the synchrotron radiation as a continuous light source, time resolved laser pump-synchrotron probe XAS experiments usually require unconventional modes of operation of the storage ring, i.e. single-bunch, few-bunches, or hybrid modes. To overcome this limitation and to operate at variable repetition rate, the use of the multi-bunch filling pattern of the storage rings has been implemented at the BACH beamline with novel and effective acquisition concepts and ideas for performing time-resolved experiments in the sub-ns time domain.
In the conventional pump-probe experiment, a single x-ray pulse cyclically probes the transient state at the laser repetition rate and at fixed delay time relative to the laser pulse. In the enhanced pump multiple-probe scheme adopted at the BACH beamline a train of consecutive x-ray pulses at ∼500 MHz repetition rate and synchronized with the laser pulse, probes the transient state at multiple delays ( ns, + 2 ns, . . . ) for its entire lifetime. The photon counting approach to measure the fluorescence yield used in the present setup takes full advantage of the multi-bunch filling pattern of the storage ring allowing to fully exploit the photon flux of the x-ray probe and to implement a stroboscopic probing time-sequence thus providing continuous snapshots of the transient state spectra. . This schema allows snapshots of the excited transient state structures.Using a novel approach which exploits the full multi bunch operation of the Elettra storage ring, the synchrotron x-ray pulses, following in time the laser excitation, are used to probe the relaxation process of the excited states allowing to observe the complete dynamical evolution at once.
The present setup is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ∼500 MHz x-ray synchrotron radiation bunches and on a detection system that discriminates and singles out the signiﬁcant x-ray photon pulses by means of a custom made photon counting unit. The laser generates sub-50 fs pulses at 800 nm. Nonlinear harmonic generation processes are used to generate pump pulses of the desired wavelength (in this case second harmonic generation). The laser trigger is synchronized to the storage ring master RF clock with a pump/probe jitter of less than 2 ps.The synchronization is maintained through a timing unit that controls and tunes the relative time delay between the pump laser pulse and the probe x-ray pulses. A measuring unit discriminates and singles out the x-ray photon bunches by means of a photon counting system. The luorescence yield spectra are acquired with an ultrafast Hamamatsu MCP detector.
IMPORTANT NOTICE: Since October 2015 RegA9000 is not anymore available at BACH. Only Mira HP. Please contact us before submitting a proposal.
|Feature of the Laser Sources||RegA9000||Mira HP Ti:Sa oscillator|
|Wavelenght||800nm; SHG: 400nm||800nm; SHG: 400nm|
|Pulse width||100 fs||100 fs|
|Pulse energy||5 μJ/pulse||25 nJ/pulse|
|Repetition rate||200-250 KHz||83.3 MHz|
The samples are positioned in an ultra-high vacuum chamber (experimental station A). The laser beam enters the experimental chamber through a quartz window with an angle of 22.5° with respect to the x-ray photon beam. A window allows viewing the sample by a video camera in order to spatially overlap the laser and SR beams. The sample is mounted on 4-degree freedom manipulator with z translation and θ rotation. The fluorescence yield x-ray detector is at - 22.5° from the x-ray photon beam and at a variable distance from the sample.
Figure 2: Diagram for the pump-probe time resolved XAS set up at the BACH beamline.