Small aromatic molecules studied by spectroscopy : what fundamental research tells us

Abstract

Aromatic molecules form an extensively studied group of molecules. They can be found in a large number of everyday materials, such as fuels, pharmaceuticals, pesticides, and fertilizers. Understanding the fundamentals about aromatic molecules can therefore help to understand the way these molecules behave, or react in these materials. This thesis discusses fundamental research on a few small aromatic molecules. Spectroscopic techniques are used to get information on the structure and behavior of these molecules. By studying electronic transitions, properties of both the ground and the electronically excited state can be uncovered from the measured spectra. The first technique used is rotationally resolved laser induced fluorescence (LIF) spectroscopy. A continuously tunable, narrow band UV source excites the molecules to an electronically excited state. As the frequency of the UV source is scanned, the fluorescence from the excited state is detected by a photomultiplier. In this way an excitation spectrum is recorded, which contains information on both the ground and the electronically excited state. The resolution, that is necessary to resolve the rotational transitions of the smallest aromatic molecules, is achieved by combining the narrow band UV source with a molecular beam machine. The spectra are governed by the rotational constants, A, B, and C, which are determined by the structure of the molecule that is studied. This technique is used to study the effect of the internal rotation of the methyl group on the overall rotation in 3- and 5-methylindole, to get structural information about o-fluorophenol, and to study the proton tunneling in the benzoic acid dimer. Another technique that is used is resonance enhanced multiphoton ionization (REMPI). One (frequency doubled) dye laser excites an electronically excited state, while the second UV source subsequently ionizes the molecules. The ions that are formed are detected by a microchannel plate. The use of an infrared laser as a third laser source, makes it possible to study vibrational transitions. This technique is used to measure the rotational contour and vibrational IR spectra of the lowest triplet state of pyrazine