Diffraction and microscopy with ultrashort electron pulses can reveal atomic-scale motion during matter transformations. However, the spatiotemporal resolution is significantly limited by the achievable quality of the electron source. Here we report on the emission of femtosecond single/fewelectron pulses from a flat metal surface via two-photon photoemission at 50-100 kHz. As pump we use wavelength-tunable visible 40 fs pulses from a noncollinear optical parametric amplifier pumped by a picosecond thin-disk laser. We demonstrate the beneficial influence of photon energies close to the photocathode's work function for the coherence and duration of the electron pulses. The source's stability approaches the shot noise limit after removing second-order correlation with the driving laser power. Two-photon photoemission offers genuine advantages in minimizing emission duration and effective source size directly at the location of photoemission. It produces an unprecedented combination of coherent, ultrashort and ultrastable single/few-electron wave packets for timeresolving structural dynamics.

Tilted femtosecond laser pulses, having an intensity front with an angle to the propagation direction, can be generated by a dispersive element and a lens or mirror for imaging. Here we show that conventional geometries, for example with a grating at Littrow's condition, produce significant temporal distortions over the beam profile. The aberrations are the result of a mismatch between the grating's surface and the object plane of the imaging system. This changes the chirp of the pulses over the beam profile and lengthens the pulses to picoseconds for millimeter-sized beams. The distortions can be avoided by choosing a geometry in which the propagation direction of the tilted pulses is perpendicular to the grating's surface.

An ubiquitous focusing element for charged particles is the magnetic solenoid lens. For the case of ultrashort electron pulses, we show here that misalignment of the lens, i.e. displacement or tilt, causes significant temporal aberrations on a femtosecond time scale. Pulse-lengthening is only minimized if the beam travels on the symmetry axis. We present an experimental procedure with periodic reversal of the magnetic field for aligning position and tilt with sufficient precision for reducing the aberrations to less than one femtosecond. This method will be instrumental for advancing ultrafast electron microscopy and diffraction to ultimate temporal resolutions.