[en] External mechanical forces and ultrashort optical pulses can be used to steer and control chemical
reactions. We demonstrate that an external mechanical force can not only enhance or inhibit the
[4+2] cycloreversion of furan/maleimide Diels-Alder adducts, but also shift its mechanism from
concerted to stepwise, mediated by diradical species. To better understand how attosecond light
pulses can control photochemical reactions, we derived an expression for the force exerted on the
nuclei by the electronic coherences created through interaction with the light and show that they
can be used to steer nuclear motion. We propose a computational scheme based on singular value
decomposition to simplify the simulation of an ensemble of molecules with random initial
orientations interacting with an attopulse. We show that a few principal orientations only suffice
to describe the ensemble dynamics and that electronic coherences are robust with respect to the
ensemble averaging. We also show that conical intersections control the relaxation of the excited
ethylene cation, shaping the yields of fragments after dissociation. Our findings contribute to a
better understanding of control mechanisms in the fields of mechanochemistry and attochemistry.
