engleski

Metastable helium production in laser produced plasma

Metastable state of a helium atom at 19.8 eV with a lifetime of around 7000 s is the longest living excited state known to present. Because of such properties these atoms act as storage of energy for various chemical processes. In collisions, metastable helium atoms can excite or ionize other particles. In our experiments we have used Nd:YAG pulsed laser at 1064 nm to ablate various rotating targets in helium atmosphere in order to measure metastable production rates. Laser energy was varied from 53 to 300 mJ and helium pressure ranged from 0.03 to 0.2 mbar. Metastable helium density was measured using cavity ring down spectroscopy at the resonant absorption from state 1s2s3S1 to 1s3p3PJ at 388.86 nm. We compared metastable relative production rates for different targets (Fe, Al, C), pressures, laser energies, spatial and temporal positions. The results show that spatial distribution at a given height above the target surface is uniform and that life time is reduced at higher pressures due to higher collision rates. Density dependences on laser energy obtained for iron and aluminium targets are different but scalable to each other. On the other hand, dependence for carbon target is different. Results are discussed within the model which assumes initial electron production by laser, subsequent collisional excitation and de- excitation of helium.

Metastables; laser produced plasma

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