Thesis (MSc)--University of Stellenbosch, 2011. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6522 |
| Date | 03 1900 |
| Creators | De Clercq, Ludwig Erasmus |
| Contributors | Botha, Lourens R., Rohwer, Erich G., University of Stellenbosch. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 96 p. : ill. |
| Rights | University of Stellenbosch |
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