Coherent control of bond making: the performance of rationally phase-shaped femtosecond laser pulses

Abstract

The first step in the coherent control of a photoinduced binary reaction is\nbond making or photoassociation. We have recently demonstrated coherent control\nof bond making in multi-photon femtosecond photoassociation of hot magnesium\natoms, using linearly chirped pulses [Levin et al., arXiv:1411.1542]. The\ndetected yield of photoassociated magnesium dimers was enhanced by positively\nchirped pulses which is explained theoretically by a combination of\npurification and chirp-dependent Raman transitions. The yield could be further\nenhanced by pulse optimization resulting in pulses with an effective linear\nchirp and a sub-pulse structure, where the latter allows for exploiting\nvibrational coherences. Here, we systematically explore the efficiency of\nphase-shaped pulses for the coherent control of bond making, employing a\nparametrization of the spectral phases in the form of cosine functions. We find\nup to an order of magnitude enhancement of the yield compared to the unshaped\ntransform-limited pulse. The highly performing pulses all display an overall\ntemporally increasing instantaneous frequency and are composed of several\noverlapping sub-pulses. The time delay between the first two sub-pulses almost\nperfectly fits the vibrational frequency of the generated intermediate\nwavepacket.These findings are in agreement with chirp-dependent Raman\ntransitions and exploitation of vibrational dynamics as underlying control\nmechanisms.\n