Multi-mode ultrafast electron diffraction
The home-built MMUED apparatus works with two experimental layouts of transmission and reflection, for observing specimen forms of thin film and bulk (surface), respectively. Pulsed electrons are tuned to parallel or convergent beams, producing Bragg or Kikuchi diffractions, respectively. The combinations of layouts and incidence beam profiles provide the feasibility of studying diverse sample morphologies. Moreover, MMUED is integrated with functions of cleaned surface preparation and characterization, and in-situ sample temperature control. The apparatus consists of six parts, which are the optical system, the ultrahigh vacuum (UHV) chambers, the fs e-gun (electron source), the sample transfer and manipulation system, the surface preparation system, and the imaging system.
The Optical system consists of an fs laser (Legend Elite Duo HE+, Coherent Inc.), an optical parametric amplifier (TOPAS-Prime, Light Conversion), a frequency tripler (TPH-fs, Minioptics), a liner motor stage (PRO190LM-1000, Aerotech Inc.) and many optical elements. The fs laser delivers pulses of 35 fs width, 800 nm wavelength, 5 kHz repetition rate, and 12.5 W output power. The TOPAS tunes the fs laser pulses in a wavelength span of 240-2400 nm, which covers the ultraviolet, visible and infrared bands, for exciting specimens with different band gaps. The tripler converts input pulses of 800 nm to 266 nm, to illuminate the photocathode of fs e-gun for photon-generated electron pulses. The liner motor stage constructs an optical delay line to produce the time sequence required for the pump-probe measurement.
The UHV chambers provide the vacuum condition required for operations of the apparatus, and connect all parts. The chamber system consists of a main chamber (diffraction chamber), a sample preparation chamber, a load-lock chamber, a sample storage chamber, and other vacuum equipment, capable to generate a UHV condition with air pressure lower than 5×10-8 Pa.
The fs e-gun uses a lanthanum hexaboride (LaB6) single crystal as the photocathode. The extracted electrons are accelerated under a voltage of 30 kV, carrying a wavelength of 0.07 Å. The e-gun is designed with several stages of electric lens and electron-optics after the anode, collimating the electron beam. The convergence angle of electron beam is adjustable by selecting the voltage ratio between the condenser lens and the objective lens, to emit parallel beam or convergent beam.
The sample transfer and manipulation system runs with adjustments in 5 degrees of freedom, which are three-dimensional translation motions and two rotations of azimuthal and polar angles, to transfer, and accurately position and orientate the sample. The sample state is equipped with temperature control by electron beam heating, and liquid nitrogen and liquid helium cooling.
The surface preparation system includes a wobble stick, a quadrupole mass spectrometer, a sputter gun, and a low-energy electron diffraction and Auger electron spectrometer combo.
The imaging system consists of a microchannel plate (MCP) based vacuum image detector, installed on the main chamber, and an imaging CCD camera outside the chamber, capable to capture very weak signals of diffracted electron.