Ph.D Thesis

Ph.D StudentLevin Liat
SubjectCoherent Control of Bond Making
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Zohar Amitay
Full Thesis textFull thesis text - English Version


Femtosecond coherent control utilizes shaped femtosecond pulses to induce and control quantum dynamics. A long-standing yet unrealized dream since the early days of coherent control, about 30 years ago, is the coherent control of photo-induced bimolecular chemical reactions. Realizing this dream will create a new type of photochemistry. Shaped femtosecond laser pulses act there as special "photo-catalysts" with a fist pulse inducing and controlling the formation of a chemical bond, and a second time-delayed pulse breaking the desired bonds in the generated molecule. The control knobs are the shapes of these pulses, i.e., their temporal amplitudes, phases and polarizations. Conceptually, the basic idea is to employ constructive and destructive interferences of matter waves to enhance the transitions leading from the reactants to desired products, while suppressing the transitions to all undesired products.

Control over the second step, photodissociation into target channels, has been previously demonstrated in many studies. On the other hand, and in striking contrast, no experimental study has previously demonstrated coherent control of bond making (photoassociation) between two freely colliding reactants via free-to-bound photoinduced transitions. This work presents for the first time such demonstration.

First, experimental results are presented for the formation of diatomic molecules (Mg2) with rotational and vibrational coherence in strong-field multiphoton femtosecond photoassociation of hot magnesium atoms. Then, the generated molecular coherence is utilized for demonstrating coherent control of bond making. The yield of detected magnesium dimer molecules is enhanced by positively chirped pulses (having linearly increasing instantaneous frequency) and suppressed by negatively chirped pulses. Our ab initio model shows that control is achieved by Franck-Condon filtering (purification) combined with chirp-dependent Raman transitions. Further yield enhancement is achieved by a closed-loop pulse optimization resulting in an improved pulse, which better utilizes the vibrational coherent dynamics. Last, based on the obtained insights regarding the underlying control mechanism, an extended set of shaped pulses is rationally designed to further enhance the photoassociation yield, up to an order-of-magnitude as compared to the unshaped pulse. The highly performing pulses all display an overall temporally increasing instantaneous frequency and are composed of several overlapping sub-pulses that their different time delays fit very well the photoinduced vibrational dynamics.

In summary, we demonstrate coherent control of bond making, a milestone on the way to coherent control of photoinduced bimolecular chemical reactions, and show that it is feasible even under thermal conditions.