Monday, November 19, 2012 at 1:00pm to 2:00pm
Bernhard Adams of APS will give the CLASSE Seminar on Monday, November 19 at 1:00 pm entitled:
"The Hard X-ray Streak Camera at the Advanced Photon Source"
Abstract: The combination of picosecond time resolution and sub-nanometer spatial resolution is a powerful tool to elucidate the elementary processes of chemistry and condensed-matter physics. The very fastest coherent electron dynamics take place at time scales of a few femtoseconds, and can thus only be addressed with free-electron lasers. However, incoherent electron dynamics and vibrational processes take place on the 100-fs to few-ps scale. Often a one or two ps time resolution is necessary and sufficient to distinguish from the next level of larger-scale conformational dynamics, taking place on the 100-ps time scales and longer. The native time resolution of synchrotron-radiation facilities is insufficient for study of few-ps processes, and currently an x-ray streak camera is required.
The talk will present the hard-x-ray streak-camera program at the Advanced Photon Source and will showcase a few experimental examples. In particular we discuss cooperative molecular dynamics in the photoexcitation of iron pentacarbonyl ( Fe(CO)_5 ) that occur within a few ps.
Important to the success of the program was not only the streak camera itself (with 1-2 ps time resolution it is hardly the world's fastest), but also the supporting infrastructure of optimized beamline optics, pre-adjusted and rapidly interchangeable setups, laser, x-ray chopper, etc.
The program is still undergoing rapid evolution: we recently replaced the photoconductive switch that is commonly used to generate the deflection voltage with power-RF electronics to reduce the timing jitter and run the streak camera at repetition rates up to 88 MHz, i.e., far beyond the capabilities of a drive laser for the photoconductive switch. Future plans include operation with grazing incidence on the photocathode for higher photon efficiency, improved electron optics, and upgrades to the beamline x-ray optics.