Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

Abstract

Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by the photo-excited surface plasma. Data have been interpreted in terms of the dynamics of an expanding surface, i.e., a density gradient rapidly forming along the normal surface. This allowed us to determine the characteristic time (τ 1 ps) for hydrodynamic expansion in photo-excited a-C. This finding suggests an extremely narrow time window during which the system can be assumed to be in the isochoric regime, a situation that may complicate the study of photo-induced metastable phases of carbon. Data also showed a weak dependence on the probing EUV wavelength, which was used to estimate the electronic temperature (T e ≈ 0.8 eV) of the excited sample. This experimental finding compares fairly well with the results of calculations, while a comparison of our data and calculations with previous transient optical reflectivity measurements highlights the complementarities between optical and EUV probing.

Free Electron Laser Measurement of Liquid Carbon Reflectivity in the Extreme Ultraviolet

“…The optical (251 nm) pump power is sufficient to induce sample damage, while the EUV probe power is not. This has been verified previously, based on power densities, visual inspection, and reflectivity measurements [22].…”

“…As shown in Figure 2, s-polarization has a time constant of ~1 ps, while p-polarization has a time constant of ~5 ps. Previous work estimated the plasma frequency to correspond to an energy t 5 eV, which is substantially below that of the probe energy [22]. However, at the longer probe wavelengths used here, the reflectivity should be higher which may lead to our observed change in time constant.…”

“…In this case, Z is the effective charge, T E is the electron temperature, M is mass of the carbon atom, and K b is the Boltzmann constant. Given the large scatter in the experimental data, when including all of the points and using the s-polarization f value of 0.6, an expansion velocity of 9.5 ± 1 nm/ps is obtained, substantially lower than both the previously determined 17 ± 4 nm/ps and the speed of sound in graphite 18 nm/ps [14,22,26]. However, due to the limited number of points and the large error bars, this value is sensitive to the removal of outlier points, and a value within error of the speed of sound can also be obtained.…”

“…The calculated surface expansion velocity can then be used to calculate the electronic temperature, as detailed in Ref. [22]. While the surface temperature calculated from a fit of all data is closer to 0.4 eV than to the 0.8 eV value reported…”

“…where f is a parameter depending on the probe polarization and the exact form of the density gradient, and υ is the velocity of the expanding surface [14,22,25]. This result implies that the time constant should increase linearly with the wavelength of the probe.…”