M.Sc Thesis

M.Sc StudentLangbeheim Yonathan
SubjectObservation and Quantum Control of Resonant Two-Photon
Photoassociation of a K-Ar Collision Pair
DepartmentDepartment of Chemistry
Supervisor ASSOCIATE PROF. Zohar Amitay
Full Thesis textFull thesis text - English Version


Femtosecond quantum coherent control is a quantum-based concept and method aimed at controlling photo-induced dynamics of quantum systems using shaped femtosecond laser pulses. In the proper scheme, the pulse shape and properties are matched to the physical dynamics involved in the excitation process, yielding maximal effect. In the past three decades since the invention of broad-band laser pulses, coherent control has been demonstrated over many physical processes, however one major goal of quantum control has not yet been realized, and that is the coherent control over the outcome of chemical reactions. Our research group has recently presented for the first time coherent control on the process of bond formation, demonstrating coherent control over strong-field multiphoton femtosecond photoassociation of hot Magnesium atoms in the gas phase. As a further step towards the realization of coherent control over chemical reactions, this study investigates quantum coherent control of femtosecond photoassociation processes in a Potassium-Argon (K-Ar) collision pair. In contrast to the previous study of bond making in Magnesium, this work focuses on weak-field resonant two-photon photoassociation in a heteronuclear molecule with weakly-bound intermediate states.

Our model system is a thermal K-Ar collision pair at 600K. The corresponding two-photon excitation scheme is composed of a free-to-bound or free-to-free photoassociation step at short internuclear distances, followed by a second excitation step forming a free dissociating molecular wavepacket above the corresponding atomic asymptote. The measure for the excited population in this entire process is the fluorescence emitted from the atomic fragments in their spontaneous radiative decay into their ground state.

Experimental results for the measurement of resonant two-photon femtosecond photoassociation of K-Ar are presented here for the first time. The experimental signal was detected even for excitation in the weak field regime. This opens the door for the application of a 2nd-order perturbative description to the excitation process, and, as a result, for the corresponding understanding and powerful rational coherent control strategy that are based on a frequency-domain picture.

Next, this study presents detailed theoretical numerical results and analysis of the proposed excitation scheme by linearly chirped and colored femtosecond pulses of various spectra. Corresponding coherent control is successfully exerted over a few aspects of the excitation, including the total yield of the excited population, the rotational distribution of the excited population and the branching ratio to different final states. The corresponding excitation mechanisms are identified, including also the so-called “Franck-Condon filtering”. The best performance identified here is attributed to shaped pulses with pulse durations and temporal structures that allow synchronization with the propagation of bound molecular wavepackets on an intermediate electronic state.

In summary, the results presented in this thesis predict that a high degree of experimental quantum coherent control is feasible even in thermal conditions for weak-field resonant two-photon femtosecond photoassociation of a K-Ar collision pair. This is another step toward the realization of coherent control of chemical reactions.