Figure 1: Femtosecond X-ray diffuse scattering from lead telluride.

(a) Reference scattering from PbTe before photoexcitation. The overlaid black lines represent the Brillouin zone boundaries and two zones of interest, G=(1 3) and G=(0 4), are labelled. The white arrows point along the respective high-symmetry Δ (Γ to X) wavevector directions in each zone. The intensity of the X-ray scattering in this frame is denoted as I₀(Q) and in arbitrary units (a.u.). (b–d) Differential scattering δI(τ;Q)=I(τ;Q)−I₀(Q), in which the reference frame is subtracted from the pattern at each time delay τ following ultrafast infrared excitation. The time delays shown here are τ=0, 0.5 and 2.5 ps. Within 0.5 ps, a strong decrease in intensity is seen along the Δ line in the G=(1 3) BZ while an increase in intensity is seen along Δ in the G=(0 4) BZ.