T. Gießel

Pump-probe electron spectroscopy using an integrating electron detector is only possible, if the repetition rates of pump and probe beam match. For laser-pump synchrotron-radiation-probe experiments this is in general not the case. In the so-called "multibunch mode" at BESSY (when the storage ring is filled with several hundred bunches) the repetition rate of the synchrotron radiation pulses is 500 MHz. In the less frequently available "single bunch mode" this repetition rate is reduced to 1.25 MHz. Since 2004, BESSY provides a "hybrid mode", which is a combination of "multibunch mode" and "single bunch mode". The regenerative amplified laser system we are using is operated at ~208.3 kHz.

To overcome the problem of the different repetition rates of pump and probe pulses, we developed two different detection schemes:

1. Pump - Multiple-probe Setup for multibunch mode (time domain: ns to µs)
2. Pump -(Single)-probe Setup for single bunch and hybrid mode (time domain: ps to ns)

For studying the dynamics of processes in the ns to µs time region we probe the laser-excited state of the sample in a parallel fashion using all the synchrotron pulses in multi bunch mode, which are separated from each other by 2 ns. The pump - multiple-probe setup is sketched below. The photoelectrons generated by the synchrotron light are energy dispersed by a hemispherical analyzer, detected by channeltrons and read out preserving the information of their arrival time relative to a trigger signal derived from the storage ring master clock.

Experimental setup for pump-multiple-probe spectroscopy

The time-of-arrival histogram shows the characteristic filling structure of the BESSY storage ring. The time resolution of the electron detector is demonstrated to be about 1 ns, which allows to assign the electron signal to specific bunches in multi bunch mode, where consecutive pulses are separated by only 2 ns.

for further details see:
T. Gießel D. Bröcker, P. Schmidt, and W. Widdra, Rev. Sci. Instr. 74(2003)11

In the pump - multiple-probe scheme the probing takes place on a 2 ns-time-delay-grid. For smaller time delay steps we apply the conventional pump - (single)-probe scheme. Photoelectrons from only one synchrotron probe pulse per laser pump pulse are detected by setting an electronic gate to the read-out-signal. This detection scheme is limited to single bunch and hybrid mode. The "right" bunch is identified by multiple-probing of e.g. the laser-induced changes of the surface potential (relaxation time in the range of several hundred nanoseconds).