Abstract-The degree of optical excitation of porphycene in solution can be controlled by manipulating a linear chirp of high intensity ultrashort laser pulses while keeping their energies and spectra constant. Transient absorption, which reflects molecular populations in the ground and the first singlet excited state, varies by over 100% between optimal positively and negatively chirped excitation pulses. The results are discussed invoking coherent vibrational wavepacket dynamics.Porphycene, a synthetic organic molecule ( Fig. 1), attracts considerable interest of researchers primarily due to intramolecular double hydrogen transfer occurring near its internally-positioned nitrogens [1]. This process has been successfully studied with pump-probe ultrafast techniques, yielding information on both ground-and excited-state hydrogen transfer rates in a variety of porphycene derivatives [2,3]. In this work, we focus on population control in porphycene and show that the transient absorption signal related to populations in both ground and excited state can be modified by more than 100% by varying only the linear chirp of a high intensity pump pulse while keeping both its energy and spectrum constant. This result comprises an example of coherent control, in which coherent properties of electromagnetic fields are mapped onto matter wave functions. Exploiting either interference effects of continuous wave radiation or time-ordered interactions with short pulses, the evolution of a system can be steered towards the desired product [4]. Our approach employs interactions of pulses with coherent vibrational wavepackets, mirroring the strategy used previously to alter the fluorescence yield [5] or vibrational coherence [6] in a laser dye LD690. In effect, * E-mail: pciacka@fuw.edu.pl interactions taking place during the first few hundreds of femtoseconds after excitation can be used to affect the behaviour of a molecule nanoseconds later. A prerequisite for the aforementioned phenomena to occur is a strong pumping field to fulfil the saturated absorption condition, when more than 50% of illuminated molecules are excited. It enables multiple interactions of photons with each molecule. To facilitate that, our femtosecond transient absorption setup used in hydrogen transfer studies [3] was accordingly modified. Briefly, two non-collinear optical parametric amplifiers at the output of a femtosecond Ti:Sapphire regenerative amplifier served as independently-tunable sources of beams for pumping and probing the sample. A commercial acousto-optic programmable dispersive filter (AOPDF) [7] acted as a spectral phase shaper that allowed pump pulses to be both positively and negatively chirped. Out of necessity to compensate the positive dispersion of AOPDF and other optical elements, the obtainable negative chirp before the onset of undesirable changes to the pulse spectrum was around −4000fs 2 , while the positive dispersion range was in excess of 7000fs 2 . AOPDF synchronized with the regenerative amplifier also served as a chopper blo...