Resonance enhanced fluorescence applications

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

In addition to laser fluorescence excitation, several other laser spectroscopic methods have been found to be useful for the state-selective and sensitive detection of products of reactive collisions resonance-enhanced multiphoton ionization [58], coherent anti-Stokes Raman scattering [M], bolometric detection with laser excitation [30], and direct infrared absorption [7]. Several additional laser techniques have been developed for use in spectroscopic studies or for diagnostics in reacting systems. Of these, four-wave mixing [ ] is applicable to studies of reaction dynamics although it does have a somewhat lower sensitivity than the techniques mentioned above. 

UV spectroscopy in molecular beams involves either laser-induced fluorescence (LIP) or resonance-enhanced multiphoton ionization (REMPI) methods. The latter method has the advantage that the resulting ionized molecules can be mass-analysed in a TOP mass spectrometer. The application of either REMPI or LIP spectroscopy requires the molecule of interest to incorporate a UV chromophore, such as an aromatic moiety. The DNA and RNA bases adenine, guanine, cytosine, thymine and uracil are aromatic molecules with well-known UV absorptions. Of the 20 naturally occurring proteinogenic amino acids, 3 - phenylalanine, tyrosine and tryptophan - feature an aromatic side chain, as do many neurotransmitter molecules. To study molecules that lack a UV chromophore, such as peptides without Trp, Tyr and Phe residues and carbohydrates, a UV chromophore needs to be chemically attached [47, 48, 74]. 

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