Rick Clinite

Synchronization of femtosecond laser and electron pulses with subpicosecond precision

The temporal evolution of electron shadow images, formed by the projection of primary femtosecond electron pulses (probe) over a metal target and perturbed by the transient space-charge field near the target surface induced by the excitation of femtosecond optical pulses (pump), is recorded in real time. By quantitatively analyzing the evolution of these shadow images as a function of pump-probe delay times, we are able to synchronize the femtosecond laser and electron pulses with sub-ps precision. This approach is independent of the structural dynamics under investigation and can be applied to a variety of diffraction setups and target materials using laser pulses of different wavelengths.

Measurement of femtosecond electron pulse length and the temporal broadening due to space charge

The temporal width of ultrashort electron pulses as a function of beam intensity was measured on the femtosecond time scale with a customized streak camera. The results show that the temporal profile of an electron pulse is Gaussian at low beam intensity and progressively evolves to a top-hat shape due to space charge broadening as the beam intensity increases. The strong correlation between the pulse width and beam intensity observed in our streaking measurements agrees very well with the mean-field calculation and supports the main conclusion of previous theoretical studies that the space charge broadening plays a determinant role.

Electronic Thermal Expansion and the Coherent Acoustic Phonons Generation

We have investigated the thermal expansion dynamics of metal films using femtosecond electron diffraction. We show that the electronic thermal expansion from the transient heating of conduction electrons contributes significantly in driving coherent acoustic phonons.