Ultrafast XAFS measurements on laser excited Ge films

Laser-pump/x-ray-probe measurements were made on Ge films with nominal time-delay steps of 18 ps. The response of the Ge lattice to the excitation of a high efficiency 200 fs pulse laser operating at 800 nm wavelength and at the Advanced Photon Source (APS) ring frequency of 272 kHz is probed by x-ray absorption fine structure (XAFS) measurements which determine the time dependences of the local structure. Initially the lattice response is dominated by a large increase of the RMS nearest neighbor bond disorder that decays within a single delay step. The next nearest neighbor RMS disorder shows an increase delayed by ∼ 30 ps, consistent with only optical modes dominating the initial lattice response and other phonons being excited more slowly. The different rate of excitation of optical than the rest of phonons require a different mechanism for exciting the optical phonons. Because the laser excites electrons from binding to anti-binding states, a Franck-Condon like effect is suggested as the cause of this initial response, instead of the standard hot electron-hole coupling to phonons.