The Chemistry Of Very Large Laser Sparks

We report on experiments simulating chemical processes in early Earth’s atmosphere initiated by events with high-energy density, e.g. lightning or impact of extraterrestrial objects. Large-scale laser spark was created in the mixture of molecular gases (N2, H2O, CO or CO2) in the high-power laser laboratory PALS. Transient chemical composition was analyzed in-situ by optical emission spectroscopy. Hot plasma core of the spark was simulated and analyzed in a supplementary experiment, using a double stream pulse jet apparatus (gas-puff target). Introduction Sufficient concentration of complex organic molecules was probably one of the important conditions for origins of life. One of existing hypotheses explains synthesis of these complex molecules due to events with high-energy density, e.g. lightning [Chyba, 1991] or impact of extraterrestrial objects [Chyba, 1992] in early Earth’s atmosphere. Chemistry of these events have been simulated in laboratory discharge [Miller, 1955; Schlesinger, 1983; Miyakawa, 1999; Scattergood, 1989], or laser-induced plasma [Davis, 1980; Jebens, 1992; McKay, 1997; Civǐs, 2004]. These experiments verified the possibility of synthesis of amino-acids in different gas mixtures. However, most of these experiments should repeat the initiation event many times in the same medium to obtain measurable effect. In experiment reported here we used much higher energy per laser pulse (hundreds of J), and prepared laser plasma with dimensions of tens centimeters. Thus we could investigate changes in chemical composition initiated even with one or a few pulses. It makes our conditions closer to situation in real atmosphere. Use of large gas cell also improves volume/surface ratio and reduces interaction of plasma with walls. Laser spark triggered in a gas (laser-induced dielectric breakdown, LIDB) is a threshold nonlinear process. Plasma is created in the volume illuminated by radiation with intensity equal or higher than certain threshold intensity. For pulses with very high energy it means that the spark spreads from the focal point towards the incoming laser beam, as is demonstrated in Figure 1. Two processes are responsible for gas ionization: a) collisional ionization by electrons accelerated by laser electric field, b) multiphoton ionization. First process causes exponential increase of electron density (avalanche). Both the processes need high intensities, but multiphoton ionization has minor importance for experiments with long wavelength radiation (λ > 1μm). Energy from plasma core is distributed to the cold surrounding gas by several channels: emission WDS'06 Proceedings of Contributed Papers, Part II, 86–90, 2006. ISBN 80-86732-85-1 © MATFYZPRESS