  We show that a field emission tip electron source that is triggered with a
femtosecond laser pulse can generate electron pulses shorter than the laser
pulse duration (~100 fs). The emission process is sensitive to a power law of
the laser intensity, which supports an emission mechanism based on multiphoton
absorption followed by over-the-barrier emission. Observed continuous
transitions between power laws of different orders are indicative of field
emission processes. We show that the source can also be operated so that
thermionic emission processes become significant. Understanding these different
emission processes is relevant for the production of sub-cycle electron pulses.
