Laser Applications in Chemistry and Biology: Stimulation, Observation, and Manipulation

Results from three different types of laser‐based experiments are presented. Using translationally‐excited H atoms, the reaction dynamics of H( 2 S) + O 2 ( 3 Σ g − ) → OH( 2 Π) + O( 3 P) was investigated. In order to study the influence of selective reagent translational excitation on the reactivity, absolute reaction cross sections and nascent OH product quantum‐state distributions were measured at different collision energies by means of the laser photolysis/laser‐induced fluorescence (LP/LIF) “pump‐probe” technique. The measured absolute reaction cross sections: σ R (1.0eV) = (0.20 ± 0.08) Å 2 , σ R (1.6 eV) = (0.58 ± 0.17) Å 2 , and σ R (1.9 eV) = (0.35 ± 0.14) Å 2 clearly confirm the presence of a pronounced maximum in the excitation function (Keßler, K.; Kleinermanns, K. J. Chem. Phys. 1992, 97 : 374). The experimental results are compared with results from quasiclassical and recent 3D quantum scattering calculations on ab initio potential energy surfaces. In addition, results from experiments are reported in which two‐dimensional laser light sheet techniques were applied to investigate both the NO formation in a domestic natural gas burner and fuel mixing processes in combustion engines. Images of temperature and NO concentration distributions, as well as series of images of the fuel distribution, are presented. Finally, recent results of laser applications in biology are reported. A new concept in biodiagnostics using multiplex dyes, which have different characteristic fluorescence lifetimes but identical excitation and emission spectra, to distinguish different biomolecules has been developed. First experiments are described in which multiplex dyes in capillaries were identified by steady‐state fluorescence spectroscopy using a diode laser system. In combination with a pattern recognition technique, a detection limit of about 100 molecules per 25 picoliters detection volume was determined. The possible application in DNA sequencing is discussed. Results from laser‐induced cell fusion experiments, in which the laser nanoscalpel and photonic tweezers techniques were combined, are also presented.