In situ observation of the ultrafast lattice dynamics of graphite under ion irradiation

We develop a pump-probe experiment system, in which vibrational dynamics of a solid sample under ion irradiation can be measured in real time. In situ observation enables us to monitor small changes induced by ion irradiation, without being influenced by the irreproducibility of the sample quality or the experimental configuration. We apply the experimental system to investigate the femtosecond dynamics of the coherent E_2g1 phonon of graphite under 5 keV He⁺ irradiation. A slight decrease in the dephasing rate of the phonon at the initial stage, as well as a downshift followed by an upshift of the phonon frequency, are clearly demonstrated, all of which were ambiguous in the ex situ experiment due to the poor reproducibility of the surface quality. This technique could also be applied to study femtosecond vibrational dynamics in real time during thermal annealing, film deposition with e.g. ablation and sputter, and molecular adsorption on substrates.