Numerical modelling of the excitation of polyatomic molecules by femtosecond laser beams

De Clercq, Ludwig Erasmus (2011-03)

Thesis (MSc)--University of Stellenbosch, 2011.

Thesis

ENGLISH ABSTRACT: The selective excitation of an arbitrary vibrational level of a polyatomic molecule, without passage through an intermediary electronic excited state is demonstrated. This was achieved by simulating the interaction of a shaped, femtosecond pulse with one vibrational mode of the molecule. The carrier frequency of the pulse is chosen near resonant to the ground-to- rst-excited vibrational transition of the mode, and the pulse shape is optimized via closed-loop feedback. The simulation concentrates on the rst few vibrationally excited states since the density of states is still low, thus ensuring that the inter-vibrational decoherence time is relatively long compared to the pulse length. While various molecules were investigated this study focuses onUF6 for which detailed spectroscopic data for the v3 vibrational mode is available in literature. A multilevel model was developed and can be adapted for any number of levels. The model reported here was limited to a vibrational quantum number of four. The spectroscopic data included anharmonic splitting as well as forbidden transitions. The effect of rotational levels was not included. A density matrix approach was followed because this will allow for the introduction of dephasing of the coherent excitation via thermalizing collisions with the reservoir, as well as inter-vibrational relaxation. The time evolution of the density matrix is given by the Von Neumann equations.

AFRIKAANSE OPSOMMING: Die selektiewe opwekking van 'n arbitrêre vibrasionele vlak van 'n poliatomies molekule sonder oorgang na 'n intermediëre elektroniese opgewekte toetstand word gedemonstreer. Dit was bereik deur die interaksie te simuleer van 'n gevormde, femtosekonde pulse met een vibrasionele mode van 'n molekule. Die draer frekwensie van die pulse is so gekies dat dit naby resonansie van die grond-tot-eerste-opgewekte vibrasionele oorgang van die mode is, die puls vorm word geoptimeer deur 'n geslote-lus terugvoer. Die simulasie konsentreer op die eerste paar vibrasionele opgewekte toestande, omdat die digtheid van toestande nog steeds laag is, dus verseker dit dat inter-vibrasionele de-koherensie tyd relatief lank is in vergelyking met die puls se lengte. Verskillende molekules was ondersoek vir die studie. Die fokus is op UF6 waarvoor gedetaileerde spektroskopiese data vir die v3 vibrasionele beskikbaar is in die literatuur. 'n Multivlak model was ontwikkel en kan aangepas word vir enige aantal van vlakke. Die model wat hier aangemeld is, is beperk tot die vibrasionele kwantum getal van vier. Die spektroskopiese data het anharmonies splitting so wel as nie toegelaatbare oorgange bevat. Die effek van rotasionele vlakke was nie in berekening geneem nie. 'n Digtheids matriks benadering was gevolg, omdat dit toelaat vir die dekoherensie. Die tyd evolusie van die digtheids matriks word gegee deur die Von Neumann vergelykings.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/6522
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