Synchrotron-Based Ultrafast Optical Pump/X-Ray Probe Measurements of Lattice Dynamics in Photo-Excited Pt Thin Films

A synchrotron-based optical pump/x-ray probe measurement technique has been used to study the structural evolution of elemental Pt thin films following photo-excitation by ps duration optical laser pulses. High quality x-ray diffraction (XRD) patterns were obtained with a time-resolution of less than 100 ps (corresponding to the duration of a single x-ray pulse) and with time delays relative to the pump pulse ranging from 10’s of ps to ms. These measurements revealed a very rapid lattice expansion normal to the sample for several 100 ps following photo-excitation followed by a much less rapid decrease. The film response to excitation energies less than about 50 mJ/cm² was nearly reversible while greater excitation energies produced irreversible stress relaxation, and eventually film oblation. Assuming that the lattice expansion can be modelled in terms of a bulk thermal expansion coefficient, the maximum observed lattice temperature was about 1500 K.