We report the first observation of the dynamic optical Stark shift on a femtosecond time scale. Pump pulses of 50-fs duration with peak intensities of ${10}^{11}$-${10}^{12}$ W/${\mathrm{cm}}^2$ were used to cause a Stark shift of the $S_0$ to $S_1$ transition in the organic dye molecule rhodamine-B. The perturbed spectra were observed with the use of probe pulses of 6-fs duration. The inferred effective transition dipole moment of 7 D is in good agreement with the value derived from the absorption line strength. At the higher pump powers, line-shape distortions and a reduction in the apparent line strength are also noted and discussed.

• Received 29 December 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.2462