Laser-induced melting of a single crystal gold sample by time-resolved ultrafast relativistic electron diffraction

Abstract

We report the experimental demonstration of time-resolved relativistic electron diffraction. Single shot diffraction patterns from a single crystal gold sample were recorded using ultrashort 3.5 MeV electron bunches from a radio frequency photoinjector. By scanning the pump pulse time-delay, we studied the Bragg peaks amplitude change due to the laser-induced melting of the sample. The observed time scale matches the one predicted using a simple two temperature model of the heating of the thin foil. Time-resolved relativistic electron diffraction using megaelectronvolt electron beams with 107 particles in 100 fs bunch length opens exciting possibilities in ultrafast structural dynamics.

Structural dynamics of laser-irradiated gold nanofilms

“…For 3.0-MeV electrons, the extinction distance for (200)-order in Au is 186 nm, much larger than the sample thickness ( 35 nm); hence multiple diffraction effects are negligible. 14,31 In fact, for 3.0-MeV energy of probe electrons, the (000)-order peak intensity remains constant in our measurements, hence the kinematic theory assuming single scattering events can be applied. This is in contrast with the previous results obtained by conventional UED, where transient (000)-order attenuation, characteristic of multiple scattering processes, is unavoidable.…”

“…The rapid thermalization of electrons after absorption of the laser energy in thin gold films 14 (∼100 fs) allows us to assume a well-defined electronic temperature T e (z,t) throughout the sample, where z is the distance from the surface and t is the time. The laser spot diameter is typically much larger than the probed area, therefore lateral energy redistribution can be safely neglected.…”

“…5,7 In particular, the femtosecond structural evolution of photoexcited gold and other metals has been studied recently both experimentally and theoretically. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Different regimes of excitation have been identified, depending on the energy deposited by the laser pulse in a gold nanofilm: a low-fluence regime in which sample does not melt, where coherent acoustic phonon generation is observed; 13 a medium-fluence regime in which the sample undergoes melting, where a competition between homogeneous and heterogeneous melting is identified; 9 a high-fluence regime in which electronic effects are expected to affect the melting process. 10,11,22 The initial stages of the dynamics are extremely difficult to unravel and recent results still stir a lot of controversy 10,11,14,15 even for relatively simple gold nanofilms.…”

Melting dynamics of ice in the mesoscopic regime

“…This thermalization process takes ∼20 ps in water ice (13) and can be slower in non-H-bonded molecular crystals. The structural changes following the heating pulse have been monitored in aluminum (14) and gold (15) by electron diffraction with a sample thickness of 20 nm, where the main signatures of melting complete in a few picoseconds. Water ice melting after an ultrafast T jump has been studied by time-resolved infrared spectra (13,16), with a sample thickness of 1.6 µm and over a time window of 250 ps.…”

Ultrafast time resolved x-ray diffraction, extended x-ray absorption fine structure and x-ray absorption near edge structure

“…4. 53 It is to be noted though that the blast wave is generated during the very early stages of heating, while the sonic wave achieves its maximum tens of picoseconds later. It is estimated that when a 100 nm gold film is heated by a 100 fs optical pulse the blast force will last $1.6 ps.…”