The many-body dynamics in the condensed phase often leads to structureless absorption spectra while vibrational recurrences in localized coordinates are preserved. A scheme is presented to directly extract anharmonicities and energy relaxation from the varying vibrational periods in pump-probe spectra. For the B state of I-2 a constant anharmonicity and a vibrational energy relaxation rate decreasing by one order of magnitude in the wavelength range from 540 to 583 nm are obtained. An analytical expression for compensating the wave packet dispersion due to the anharmonicity is derived and it is applied to the control of wave packets by chirped pulses. Focusing of the wave packet is experimentally observed in accordance with the anharmonicity and even after significant energy relaxation. Predissociation rates are determined from the envelopes of the pump-probe spectra.