RESONANT LASER IONIZATION IN THE STUDY OF MOLECULAR EXCITED STATES
DUNCAN, MICHAEL ANSEL
Doctor of Philosophy
Resonance enhanced two-photon ionization (R2PI) has been developed as an analytical detection in supersonic molecular beams and subsequently applied to current problems in molecular spectroscopy and excited state dynamics. The method consists of tunable dye laser ionization followed by time-of-flight mass spectroscopy detection of ions. Ionization requires resonant absorption of two photons and so the method is selective for molecular resonances. Parent ions only are formed which makes detection and identification straightforward. Such resonant ionization with commonly available laser powers can approach unit detection efficiency, which makes the technique suitable for environments with low sample densities. It is especially useful for systems which for various reasons cannot be studied by fluorescence. This technique has been applied to the S(,1) spectroscopy of bromobenzene and naphthalene and to the spectroscopy of naphthalene and benzene near the first ionization potential. It has also been used to measure excited state lifetimes for the statistical limit molecules benzene and toluene and for the intermediate case molecules biacetyl and benzoquinone. The triplet lifetimes of toluene and benzene are much shorter in the collision-free environment than previously estimated, due to intersystem crossing to the ground state which is promoted by vibrational energy in the triplet manifold. The dependence of the rate of intersystem crossing for toluene on triplet vibrational energy is not as steep as expected from current theories, due to intramolecular vibrational relaxation in the triplet manifold. Biacetyl and benzoquinone decays can both be described in the intermediate strong coupling case, although weakly coupled triplet states are also necessary in the decay scheme. The non-radiative decay rate of benzoquinone, unlike that of toluene, increases exponentially with vibrational energy in the triplet manifold. The successful application of the R2PI technique in these experiments, as well as others which have been attempted, demonstrate that this is a versatile method which will continue to be important in basic research and in practical analytical chemistry.