We observe the electric fields caused by charge distributions during femtosecond laser ablation from a silicon
(100) surface. Femtosecond electron pulses passing near the ablation site serve as a probe of the electric field
generated by the emitted charges and countercharges on the sample surface. The density map of the electron pulse
downstream from the sample contains information about the charge distributions. We invert this information
by fitting the beam maps using a simple charge distribution model. Under the present excitation conditions
(390 nm, 150 fs, 5.6 J/cm2), we observe the emission of 5.3×1011 electrons/cm2 within 3 ps of the excitation
pulse, leading to self-acceleration of the emitted electrons to 2% of the speed of light. Preliminary experiments
on a metal sample display even faster dynamics.
The ability to watch atoms move in real time - to directly observe transition states - has been referred to as "making the molecular movie". Femtosecond electron diffraction is ideally suited for this purpose since it records the atomic structure of the sample with sub-Angstrom spatial resolution and femtosecond temporal resolution. Many-body simulations of ultrashort electron pulse propagation dynamics allowed the development of sources for femtosecond electron pulses with sufficient number density to perform near single shot structure determinations, a requirement for studies of irreversible processes. We have obtained atomic level views of melting of thin films of aluminum and gold under strongly driven conditions. The results are consistent with a thermally driven phase transition and the observed time scales reflect the different electron-phonon coupling constants for these metals.
Recent technical advances in electron gun design have further improved the temporal resolution of femtosecond electron diffraction. New electron pulse characterization techniques use direct laser-electron interaction and electron-electron interaction to determine the temporal overlap of the pump and probe pulses as well as the time resolution of the system. These advances have made femtosecond electron diffraction capable of observing transition states in molecular systems. The camera for "making the molecular movie" is now in hand.
The picosecond barrier to high brightness electron pulses has been broken. Electron diffraction harbors great potential for providing atomic resolution to structural changes at critical points — a real-time view of atomic motions during structural transitions. Femtosecond electron pulses of sufficient number density to execute nearly single-shot structure determinations are needed. This requirement places severe constraints on the electron pulse propagation. A new photoactivated electron gun design has been developed based on an N-body numerical simulation and mean-field calculation of the electron wavepacket propagation that is capable of less than 600 femtosecond electron pulses with high enough brightness to provide structural details in the small shot number limit. Time-resolved diffraction studies with this new instrument have focused on strongly driven solid-liquid phase transitions of aluminum as a model problem of a structural transition. The signal to noise and available diffraction orders were sufficiently high to give direct access to fluctuations leading to the disordering or melting process and the associated radial distribution function.
This work gives atomic level details of a solid-liquid phase transition, i.e., we can literally watch the atoms move during melting. The promise of atomically resolving transition state processes is at hand and applications along this line will be discussed.