Ultrafast X-ray scattering is one of the primary tools to track intrinsic crystallographic evolution with atomic accuracy in real time. However, its application to study nonequilibrium structural properties at the two-dimensional limit remains a long-standing challenge due to a significant reduction of diffraction volume and complexity of data analysis. Here, we report femtosecond surface X-ray diffraction in combination with crystallographic model-refinement calculations to quantify the ultrafast structural dynamics of monolayer WSe2 crystals supported on a substrate. We found the absorbed optical photon energy is preferably coupled to the in-plane lattice vibrations within one picosecond whereas the out-of-plane lattice vibration amplitude remains unchanged during the first ten picoseconds. The model-assisted fitting suggests an asymmetric intralayer spacing change upon excitation. The observed nonequilibrium anisotropic structural dynamics agrees with first-principles modelling in both real and momentum space, marking the distinct structural dynamics of monolayer crystals from their bulk counterparts.
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I.-C.T., Q.Z., K.S., G.C., X.X. and H.W. acknowledge support from the Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract no. DE-SC0012509. A.K., H.K., A.N., F.S., R.K.K. and P.V. acknowledge support of the Computational Materials Sciences Program funded by the US Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0014607. E.M.M., C.N., A.M.L. and T.F.H. acknowledge support by the Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515. F.E. gratefully acknowledges grant LPDS 2013-13 from the German National Academy of Sciences Leopoldina. NAQMD simulations were performed at the Argonne Leadership Computing Facility under the Department of Energy, the Innovative and Novel Computational Impact on Theory and Experiment Program and at the Center for High Performance Computing of the University of Southern California. Use of the Linac Coherent Light Source, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515. Work at the Advanced Photon Source and the Center for Nanoscale Materials was supported by the US Department of Energy, Office of Science, under contract no. DE-AC02-06CH11357. S.Sa. was supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory.