Ph.D Thesis

Ph.D StudentRybak Leonid
SubjectThe Atom-Atom Binary Reaction of Multi-Photon Femtosecond
Photoassociation: Coherence and Control
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Zohar Amitay
Full Thesis textFull thesis text - English Version


Coherent control utilizes the broadband nature of femtosecond pulses and the dynamics of wave- packets in order to direct the quantum system from its initial state to a desired final state(s). The control is achieved by shaping the pulses to manipulate the interferences between the initial-to-final quantum pathways. The pulses are shaped by changing the amplitude, phase and/or polarization of their different spectral components.

Coherent control was initiated as a scheme to steer a chemical reaction to its desired outcome. To effectively control a reaction, one needs to form a coherent superposition of ro-vibrational states on the excited potential surface. A vibrational coherence is of utmost importance since it controls the making or breaking of chemical bonds. In our study we have studied multi-photon photoassociation. The main advantage of employing multi-photon transitions is that the variety of molecules that can be photoassociated by NIR pulses is significantly larger for multi-photon transitions compared to one-photon excitation.   

This research studies femtosecond coherent control of free-to-bound photoassociation. Achieving such control will open the way to femtosecond coherent control of photoinduced bimolecular chemical reactions. Our present interest is in temperatures of hundreds of Kelvin. Controlling hot photoassociation also addresses fundamentals of coherent control of initial state that is a thermal mixture of different eigenstates.

In the current thesis we demonstrate the first-time-ever successful experimental formation of diatomic molecules with ro-vibrational coherence in the process of hot photoassociation. This corresponds to the generation of the photo-associated molecules in a coherent superposition of ro-vibrational molecular states. Our results are also the first-time-ever successful experimental demonstration of inducing the coherent femtosecond photoassociation process via multi-photon excitations.

In the second part of this study we demonstrate the first-time-ever successful experimental demonstration of coherent control of the photoassociation process. A very strong enhancement of the photoassociation yield has been achieved using the family of linearly-chirped femtosecond pulses. By using these pulses we have been able to control and significantly enhance the photoassociation yield . Then, using another family of shaped femtosecond pulses (with V-shape spectral phases), we have also demonstrated combined control of both the photoassociation yield and the generated vibrational-rotational coherence. Several mechanisms were offered as an explanation for the enhancement of the PA yield. The most plausible one is based on the matching of the instantaneous frequency of the excitation pulse to the energy differences between the intermediate and the final state.