More on femtosecond bond formation

The Search and Discovery news story by Mark Wilson (Physics Today, August 2015, page 19 ) highlighted the exciting results on coherent control of bond formation by Zohar Amitay and coworkers. ¹ I have followed that work with great interest. The report emphasizes femtosecond bond formation but does not mention that this process was reported 20 years earlier. ²

The initial demonstration of femtosecond bond making and the follow-up quantum mechanical calculations ^{2 , 3} explained that the ultrashort laser pulse is capable of capturing collision pairs, which, for a short time, can absorb light with frequencies that are not resonant with the colliding atoms or with the bound precursors.

In the context of coherent control of bimolecular reactions, the photoassociation process is useful in establishing the time, orientation, and alignment of the collision that leads to the nascent molecule. Bimolecular laser control without photoassociation had been observed before, most notably in the work of F. Fleming Crim and coworkers, who controlled the bimolecular reaction between semi-deuterated water and chlorine. ⁴ Those experiments took advantage of the fact that overtone excitation of either the OH or OD bond remained localized long enough to permit selective chemistry to occur following a collision with a chlorine atom. The femtosecond photoassociation and its chirp enhancement—the work highlighted in Wilson’s report—bypass the need for long-lived intermediates and promise a fertile new field of chemical investigation.