Demos.FemtoChem.HOD
Selective Bond Cleavage in HOD
The water isotope HOD is considered a prototypical system for laser control of chemical reactions. When electronically exciting the molecule, one of the bonds with the oxygen can be broken, leaving an OH or OD radical behind. The goal is to exert isotope control, i. e. selectively cleave either the O-H or the O-D bond.
Unshaped Laser Pulses
By just applying an ordinary (e.g. Gaussian, cw) laser pulse, the cleavage of the O-H bond is preferred. The underlying mechanism is that the smaller reduced mass of the O-H bond leads to a faster dissociation of this bond as compared to the O-D bond. As a simple model for this case, we applied a sin² pulse with a FWHM of 25 fs and an intensity of 18 TW/cm². Both the contour plot and the branching ratio show that much more probability density ends up in the H + OD channel.
Shaped Laser Pulses
In an article by A.K.Tiwari et al., a laser pulse was numerically optimized for preferred cleavage of the O-D bond. At an intensity similar to the case of the unshaped pulse, the fraction of OH molecules after the pulse is significantly increased. From the contour plot and the pulse shape it becomes apparent, that a first burst electronically excites the molecule and de-excites it again, thus creating a vibrational wave packet in the electronic ground state. Subsequent bursts are timed such that they excite the molecule again when breakage of the O-D bond is favored.
