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 bond 
making or photoassociation. We have recently demonstrated coherent control of bond 
making in multi-photon femtosecond photoassociation of hot magnesium atoms, using 
linearly chirped pulses Levin et al., arXiv:1411.1542. The detected yield of 
photoassociated magnesium dimers was enhanced by positively chirped pulses which is 
explained theoretically by a combination of purification and chirp-dependent Raman 
transitions. The yield could be further enhanced by pulse optimization resulting in 
pulses with an effective linear chirp and a sub-pulse structure, where the latter 
allows for exploiting vibrational coherences. Here, we systematically explore the 
efficiency of phase-shaped pulses for the coherent control of bond making, employing 
a parametrization of the spectral phases in the form of cosine functions. We find up 
to an order of magnitude enhancement of the yield compared to the unshaped transform-
limited pulse. The highly performing pulses all display an overall temporally 
increasing instantaneous frequency and are composed of several overlapping sub-pulses.
The time delay between the first two sub-pulses almost perfectly fits the vibrational 
frequency of the generated intermediate wavepacket.These findings are in agreement 
with chirp-dependent Raman transitions and exploitation of vibrational dynamics as 
underlying control mechanisms.