Fluorescence and self-absorption free excitation spectroscopy of carbon monoxide isotopomers in the vacuum ultraviolet

De Bruyn, Andre (2017-03)

Thesis (MSc)--Stellenbosch University, 2017

Thesis

ENGLISH ABSTRACT : Carbon Monoxide (CO) is the most abundant heteronuclear diatomic molecule in the universe and has been of great interest in the fields of astrophysics and astronomy. CO is additionally, a prototype molecule for theoretical models. By using a supersonic free-jet expansion it is possible to conduct spectroscopy of this molecule under collision-free and low temperature conditions similar to that found in space. The radiation used in this study to conduct spectroscopy is tuneable coherent vacuum ultraviolet (VUV) light which is generated by four-wave sum frequency mixing in a magnesium vapour medium. For the first time in our laboratory, the fluorescence of CO upon rovibronic excitation has been investigated using a scanning monochromator. Vibrationally resolved fluorescence spectra resulting from population of single rovibronic levels in the A1Π(v 0 = 3, 4) bands for 12C 16O and the A1Π(v 0 = 3) bands for 13C 16O, 12C 18O were recorded. The theoretical models of Morse and Dunham were applied to calculate vibrational constants of the X1Σ +(v 00 = 0) state, Franck-Condon factors, r-centroid values and the r-centroid dependence of the electronic transition moments for the A1Π(v 0 = 3, 4) − X1Σ +(v 00 = 0) bands for the different isotopomers. Using the tuneable coherent VUV light it was possible to record the rotationally resolved excitation spectra for X1Σ +(v 00 = 0) − A1Π(v 0 = 3) and X1Σ +(v 00 = 0) − A1Π(v 0 = 4) bands of 12C 16O. In the high-resolution excitation spectra, several forbidden singlet-triplet rovibronic transitions of the X1Σ +(v 00 = 0) − a 03Σ +(v 0 = 14) band were recorded. Two new measuring techniques were developed in this study. The first measuring technique allows for an accurate measurement of the amount of VUV light generated during excitation measurements. The second measuring technique allows for recording of self-absorption free rotationally resolved excitation measurements. These two techniques additionally resulted in a significant improvement in spectral resolution. Using the new measuring techniques it was possible to accurately determine the rotational temperature of the supersonic gas jet. Using the temperature, limiting values for the Mach number, gas flow speed and effective density in the supersonic gas were calculated.

AFRIKAANSE OPSOMMING : Koolstofmonoksied (CO) is die volopste heterokern diatomiese molekuul in die heelal en is van groot belang in die velde van astrofisika en sterrekunde. CO is daarbenewens 'n prototipe molekuul vir teoretiese modelle. Deur die gebruik van vrye supersoniese gasstraal is dit moontlik om spektroskopie van hierdie molekule uit te voer onder botsings vrye en lae temperatuur toestande soortgelyk aan die toestande in die ruimte. Die lig wat in hierdie studie gebruik word om spektroskopie uit te voer is afstembare koherente vakuum ultraviolet (VUV) lig wat gegenereer word deur vier-golf som-frekwensie vermenging in 'n magnesiumdamp medium. Vir die eerste keer in ons laboratorium, is die uoressensie van CO na rovibroniese opwekking ondersoek met behulp van 'n skanderende mono-kromator. Vibrasioneel-opgeloste uoressensie spektra as gevolg van die bevolking van enkel rovibroniese vlakke in die A1Π(v 0 = 3, 4) bande van 12C 16O en die A1Π(v 0 = 3) band van 13C 16O, 12C 18O is opgeneem. Die teoretiese modelle van Morse en Dunham is toegepas om vibrasie konstantes van die X1Σ +(v 00 = 0) toestand, Franck-Condon faktore, r-swaartepunt waardes en die r-swaartepunt afhanklikheid van die elektroniese oorgangsmoment vir die A1Π(v 0 = 3, 4)−X1Σ +(v 00 = 0) band van die verskillende isotopomere. Die gebruik van afstembare koherente VUV lig het dit moontlik gemaak om die rotasioneel-opgeloste opwekkingspektra van die X1Σ +(v 00 = 0) − A1Π(v 0 = 3) en X1Σ +(v 00 = 0) − A1Π(v 0 = 4) bande van 12C 16O te meet. In die hoë- resolusie opwekkingspektra is verskeie verbode singlet-triplet rovibroniese oorgange van die X1Σ +(v 00 = 0) − a 03Σ +(v 0 = 14) band aangeteken. Twee nuwe meettegnieke is in hierdie studie ontwikkel. Die eerste meettegniek laat toe dat die hoeveelheid VUV lig wat gegenereer word tydens die opneem van opwekkingspektra akkuraat gemeet word. Die tweede meettegniek maak laat die opname van self-absorpsie-vrye rotasioneel-opgeloste opwekkingspektra toe. Hierdie twee tegnieke het bykomend gelei tot 'n beduidende verbetering in die spektrale resolusie. Die gebruik van die nuwe meettegnieke het dit moontlik gemaak om die rotasionele temperatuur van die supersoniese gasstraal akkuraat te bepaal. Die temperatuur is gebruik om limiet waardes vir die Mach-getal, gas vloei spoed en effektiewe digtheid in die supersoniese gasstraal te bereken

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101289
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