microARPES

Figures of Merit

Spot Size Measurement: The sample was a circular 5 µm gold dot, evaporated onto a polished copper sample holder. The detector was set to collect only Au 4f electrons excited by first order light. The exit slit was set to 10µm (HxV), which should result in a nominal beam cross section of 4 x 8 µm (due to the 0.4 and 0.76 demagnification of the horizontal and vertical KB mirrors, respectively). The mirror tolerances are specified to maintain a nominal 10 x 10 μm beam cross section. (The horizontal profile is lengthened by a factor 1/cos(54.75°), resulting from the angle between the photon beam and the sample surface).

The measured spot profile is therefore a convolution of the 5µm target and the measured beam size. The measured spot profiles in the figure below are well-fitted by a gaussian peak. Removing the 5µm target size by quadrature results in an actual beam cross section of 8.7 x 10.9 µm, consistent with the beamline optics specifications. There is no detectable tail, indicating the extremely fine quality of the optic surfaces.

The measured spot profile isthe normal cross section of the beam. The resulting spot profile on the actual sample surface is further determined by the incidence geometry of the beam onto the sample, and will thus change as the sample is rotated. For instance, the geometry of the chamber has the incident photon beam and the central axis of the electron analyzer at an angle of 54.75° in the horizontal plane. Therefore measuring normal emission from the sample requires the beam to be incident on the sample surface at 54.75° degree from normal, broadening the effective horizontal spot profile by a factor 1/cos(54.75°) ~ 1.7.