FIZIKA A 7 (1998) 2, 91-96

TIME-INTEGRATED PHOTOGRAPHIC STUDY OF LASER ABLATION AT LARGE IMPACT ANGLES

ŽELJKO ANDREIĆ

Ruđer Bošković Institute, Dept. of Materials Science, Thin Films Laboratory, Bijenička 54, HR-10000 Zagreb, Croatia
e-mail: andreic@rudjer.irb.hr

Received 3 April 1998; revised manuscript received 5 July 1998; Accepted 13 July 1998

The shape of the plasma cloud generated by pulsed laser ablation of solid targets when the laser beam impacts onto the target surface at large angles is investigated. Time-integrated photographs of the expanding plasma clouds clearly show that plasma is produced at large impact angles too, and that it always expands perpendicularly to the target surface. The cone angles of the expanding plasma clouds increase with the impact angle, indicating that the kinetic energy of the plasma particles is reduced at large angles. The intensity of the plasma emission drops at angles larger than about 60^° to the surface normal, indicating that the amount of plasma produced at such large angles is smaller than during ablation at small angles. The apparent discrepancy in the observed plasma formation threshold and the on-surface energy density is explained by the mechanism of plasma initiation on localized surface defects whose surfaces are placed at favourable angles to the incoming laser radiation.

PACS numbers: 52.50.Jm, 61.80.Ba
UDC 533.9

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