Space charge effects in photoemission electron microscopy using amplified femtosecond laser pulses.

Linear and nonlinear photoemission microscopy is used to study the origin of space charge effects that are frequently observed if amplified femtosecond lasers are used for generation of photoelectrons. Space charge effects are apparent in the width of the photoemission spectra, but also create image blur. The onset threshold for space charge effects is determined by recording the width of photoemission spectra and by finding the conditions under which spectral broadening is just less than the energy resolution of the microscope. The principal findings are independent if harmonics of the fundamental of the fs laser pulses are used, but the space charge effects are found to be more dominant at lower repetition rates. By inserting apertures into the electron path, the place at which space charge effects occur can be localized.