Supersonic jet spectroscopy of laser desorbed molecules

• Main Author: Plows, Fiona L.
• Published: University of Edinburgh 1998
• Subjects: 543.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.660649

Conventional supersonic jet spectroscopy is limited by the thermal stability of the molecules under investigation. The molecules of interest are heated and the resultant vapour mixed with the carrier gas before cooling. This method has the drawback that only volatile and/or thermally stable molecules may be studied. Laser desorption, in contrast, utilises an infra-red laser pulse to desorb molecules directly from the solid state into the gas phase, and enables thermally labile molecules to be vaporised without decomposition. Thus, the use of laser desorption allows a greater range of molecules to be investigated using the technique of jet cooling. A specialised desorption 'faceplate' was used in a supersonic jet apparatus to interface the laser desorption and collisional cooling processes. Molecules were thus desorbed directly into the path of the pulsed supersonic jet, such that they were entrained in the jet carrier gas and cooled. These cold and isolated molecules were then investigated as in a conventional free jet, using laser-induced fluorescence (LIF) and resonant two-photon ionisation (R<SUP>2</SUP>PI) spectroscopy. The S<SUB>1</SUB>←S<SUB>0</SUB> excitation spectra of various molecules have been investigated and well-resolved vibronic spectra obtained. The efficacy of this technique has been validated using carbazole, benzoic acid and para-amino benzoic acid. The potential of laser desorption supersonic jet spectroscopy as an analytical tool was demonstrated by the detection of carbazole desorbed directly from real-world samples such as pollution filters and contaminated soils. A nonresonant model of pulsed IR laser desorption using the 10.6μm line of a CO<SUB>2 </SUB>laser is proposed for a sample of several mm thickness. Molecules are desorbed from the surface of the sample via absorption of the laser radiation into the near-surface bulk phonons. This energy is transferred to the bonds holding the surface molecules to the bulk leading to desorption of the molecule.