Figure 1.

Femtosecond pulse pair experiment on a single molecule. (a) Schematic illustration of the electric field of the pulse in the frequency and time domains, with the spectral phase given in red. (b) Delay-dependent single molecule fluorescence for phase differences of Δϕ = 0 and Δϕ = π between the pulses. (c) The difference and sum of the traces in b separate the phase-dependent part from the phase-independent background. (d) Fourier transformation of the difference trace yields the single molecule fluorescence excitation spectrum (top, black). Overlaid is the spectrum of the excitation pulse (dashed gray). Bottom: Ensemble absorption spectrum of terrylene in toluene.