A schematic of the MeV-UED beamline is shown above. The femtosecond MeV electron bunch is generated in a S-band photocathode rf gun. The rf gun is powered by a pulse-forming-network-based modulator and a 50 MW S-band klystron. The rf amplitude and phase stability are typically 2x10-4 rms and 30 fs rms, respectively. Approximately 0.5 mJ of IR laser light is used to produce UV light to generate electrons from the copper photocathode. The UV beam illuminates the photocathode at 70 degrees angle of incidence. Immediately after photocathode a magnetic solenoid lens provides the primary electron beam focusing.
The rf photocathode gun is separated from the sample chamber environment by two differential pumping stages. The first stage holds pop-in diagnostics and an electron beam collimator. The available collimator sizes are: 100, 200 and 500 um diameter. A second magnetic solenoid lens is installed after the differential pumping sections (not shown). This allows the electron beam focus to be optimized at either the sample position or the detector position. A holey laser in-coupling mirror is installed in a vacuum cross (not shown) directly before the sample chamber. The sample chamber for gas phase experiment is a rectangular chamber with internal dimensions: 22.5” x 14.5” x 14.56” with interaction point located ~1.6 m from the photocathode.Two 1000 L/s turbo molecular pumps are mounted to the top of the gas sample chamber.
The primary diagnostic detector is mounted 4.6 m from the photocathode. The detector consists of a P43 phosphor screen, which is mounted in vacuum at normal incidence to the electron beam. The phosphor screen is imaged via a flat in-vacuum mirror, downbeam of the screen, using a commercial EMCCD (Andor iXon Ultra) mounted outside vacuum.