Experimental configuration for ultrafast pump-double-probe spectroscopy. The two probe pulses arrive at two different angles θ₁ = 19.5° and θ₂ = 34.5°, respectively. The pump pulse arrives in-between the two probe pulses, at an angle θ_P  = (θ₂ + θ₁)/2. In the figure, the vertical black line labelled SP is the surface’s perpendicular, from which the angles are measured. The two probe pulses are measured by the two photodiodes D1 and D2, respectively, while the reflected pump pulse is blocked by the beam blocker B.