| Summary: | Thesis (PhD)--Stellenbosch University, 2003. === ENGLISH ABSTRACT: A tunable narrow-bandwidth laser source combined with a supersonic gas jet as sample is wellsuited
for obtaining high-resolution spectra of cold isolated molecules and complexes. In the
present study such a laser source in the vacuum ultraviolet was applied to the spectroscopic
investigation of rare carbon monoxide (CO) isotopomers and CO-noble gas van der Waals
complexes in supersonic gas pulses seeded with natural CO gas.
Tunable coherent vacuum ultraviolet radiation was generated by two-photon resonant fourwave
sum-frequency mixing of two pulsed dye laser beams in a magnesium vapour medium.
Laser induced fluorescence excitation spectra of the A(v'=3)-X(v"=0) vibronic band of CO
molecules in a noble gas (neon or argon) jet were obtained by measuring the total undispersed
fluorescence from the irradiated sample volume in dependence of the excitation wavelength. The
dynamics of the flow-cooling process in the supersonic jet were investigated and the experimental
parameters optimised using the rotational temperature of 12C160as determined from rotational
line intensities. Rotational temperatures as low as 2 ± I K were observed.
Spectroscopic detection of the rare 12C170and 12C180isotopomers was facilitated by the low
rotational temperature and high spectral resolution. Six rotational lines of 12C170and four of
12C180were detected in the A(v'=3)-X(v"=0) vibronic band. This demonstrates the low detection
limit (circa 3 parts per million) obtained in the experiment. The line wavelengths were
determined to an accuracy of 0.2 pm using the well-documented 12C160and 13C160lines for
calibration. The spectral results on 12C170are, to our knowledge, the first rotationally resolved
laboratory measurements published on the A-X band of this isotopomer. Accurate wavelength
data of the stable isotopomers of CO is of importance in the interpretation of astrophysical
observations of CO in the interstellar medium. The newly determined 12C170wavelengths were
successfully applied to a recent problem in astrophysics (Astrophys. J. Lett. 2003).
The conditions in a supersonic jet facilitate the study of weakly bound van der Waals
complexes, of which CO-noble gas complexes are prototypes. However, there is no experimental
data available on the electronic excitation spectra of the CO-noble gas complexes, lying in the
vacuum ultraviolet region. In the present experiment evidence of extensive complexation of the CO in the noble gas jet has been found, but in the spectral region around the A(v'=3)-X(v"=O)
band of CO no distinct spectral features that could be associated with these complexes were
observed. Having considered the existing knowledge on CO and CO-noble gas complexes and
experimental studies on the excitation and dissociation dynamics of Iz-noble gas complexes, we
regard complex induced inter-system crossing or electronic predissociation as the most likely
causes for these observations.
The results on the rare CO isotopomers demonstrate the potential of our experimental setup
for high-resolution, isotope and state selective spectroscopy in the vacuum ultraviolet with a high
sensitivity for fluorescent species. The availability of the now well-characterised experimental
setup in our laboratory opens the way for further investigation of molecular or complex species
with spectroscopic features in the vacuum ultraviolet region. === AFRIKAANSE OPSOMMING: Vakuum ultraviolet laser spektroskopie van CO
molekules in 'n supersoniese gasstraal:
'n Afstembare smal bandwydte laserbron en 'n supersoniese gasstraal as monster is 'n geskikte
kombinasie vir hoë-resolusie spektroskopie van geïsoleerde afgekoelde molekules en komplekse.
In hierdie studie is so 'n laserbron in die vakuum ultraviolet gebruik in die spektroskopiese
ondersoek van skaars koolstofmonoksied (CO) isotopomere en CO-edelgas van der Waals
komplekse in supersoniese gaspulse wat 'n klein persentasie natuurlike CO gas bevat.
Afstembare koherente vakuum ultraviolet lig is verkry deur twee-foton resonante vier-golf
som-frekwensie vermenging van twee gepulseerde kleurstoflaserbundels in 'n magnesiumdamp
medium. Laser-geïnduseerde fluoressensie opwekkingspektra van die A(v'=3)-X(v"=0)
vibroniese band van die CO molekules in die edelgasstraal (neon of argon) is uitgemeet deur die
totale fluoressensie van die beligte gasmonster, sonder golflengteskeiding, te meet as funksie van
die opwekkingsgolflengte. Die dinamika van die vloeiverkoelingsproses in die supersoniese
gasstraal is ondersoek en die eksperimentele parameters geoptimeer deur gebruik te maak van die
rotasionele temperatuur van 12Cl60 soos bepaal uit die intensiteitsverhoudings van die
rotasielyne. Rotasionele temperature tot so laag as 2 ± 1 K is waargeneem.
Spektroskopiese waarneming van die skaars 12C170 and 12Cl80 isotopomere is moontlik
gemaak deur die lae rotasionele temperatuur en die hoë spektrale resolusie. Ses rotasielyne van
12C170 en vier van 12C180 is waargeneem in die A(v'=3)-X(v"=0) vibroniese band. Dit
demonstreer die lae deteksielimiet (ongeveer 3 dele per miljoen) wat bereik kon word. Die
golflengtes van die lyne is bepaal met 'n akkuraatheid van 0.2 pm deur die bekende lyne van
12C160en 13C160vir kalibrasie te gebruik. Die resultate ten opsigte van 12C170 is sover vasgestel
kon word die eerste rotasioneel-opgeloste laboratorium metings van die A-X band van hierdie
isotopomeer. Akkurate golflengte data vir die stabiele CO isotopomere is van belang vir die
interpretasie van die astrofisiese waarnemings van CO in die interstellêre medium. Die nuwe
12C170 golflengtes is suksesvol aangewend in die oplossing van 'n onlangse
interpretasieprobleem in astrofisika (Astrophys. J. Lett. 2003).
Die toestande in 'n supersoniese gasstraal maak die bestudering van swak-gebonde van der
Waals komplekse moontlik. Hoewel CO-edelgas van der Waals komplekse as prototipes beskou word, is daar geen eksperimentele data beskikbaar oor die elektroniese opwekkingspektra van
hierdie spesies, wat in die vakuum ultraviolet gebied lê, nie. In hierdie studie is daar
eksperimentele getuienis gevind vir uitgebreide kompleksering van CO in die edelgasstraal, maar
in die spektraalgebied rondom die A(v'=3)-X(v"=O) band van CO is geen duidelike spektrale
kenmerke wat met hierdie komplekse geassosieer kan word, waargeneem nie. Na oorweging van
die bestaande kennins oor CO en CO-edelgas komplekse en eksperimentele studies oor die
opwekking en dissosiasie-dinamika van Iz-edelgas komplekse, beskou ons kompleksgeïnduseerde
intersisteemoorgange of elektroniese predissosiasie as die waarskynlikste redes vir hierdie
waarnemings.
Die resultate oor die skaars CO isotopomere toon die potensiaal van ons eksperimentele
opstelling vir hoë-resolusie, isotoop- en toestandselektiewe spektroskopie in die vakuum
ultraviolet met uitstekende sensitiwiteit vir fluoresserende spesies. Die beskikbaarheid van hierdie
nou deeglik gekarakteriseerde eksperimentele opstelling in ons laboratorium maak verdere
ondersoek na molekulêre of kompleks-spesies met spektroskopiese kenmerke in die vakuum
ultraviolet moontlik.
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