Ultrafast x-ray thermometry: Contrasting strain and Debye-Waller effects in platinum thin films

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Thermal energy and temperature govern a wide range of physical properties and dynamics in solids. X-ray diffraction can be used to monitor material-specific temperatures on ultrafast timescales and within nanostructures by exploiting thermal expansion or the Debye-Waller effect. Here, we simultaneously track the intensity and position changes of different out-of-plane Bragg peaks of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>16</a:mn> <a:mspace width="0.16em"/> <a:mtext>nm</a:mtext> </a:mrow> </a:math> thick Pt film on an MgO substrate upon equilibrium heating and upon femtosecond laser excitation to quantify the mean-square atomic displacement and the lattice expansion. Our comparison of these two x-ray thermometers experimentally verifies that the in-plane expansion of homogeneously excited continuous single-crystalline thin films is forbidden on picosecond timescales. This drastically changes the out-of-plane thermal expansion coefficient, i.e., the relationship between the observed lattice expansion and the corresponding temperature increase. Thus, considering the boundary conditions of in-plane lattice expansion is generally indispensable for extracting temperatures from the lattice expansion in diffraction experiments.