Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VIII. Laser desorption of molecular ions from MgO

We present unambiguous evidence for direct, nonthermal laser desorption of molecular ions from the surface of an ionic wide band gap material, single crystal MgO. Using time resolved mass spectroscopy, we measure positive ion s emitted during pulsed excimer laser irradiation at 248 nm. Species observ ed (besides singly and doubly charged Mg) include: Mg-2(+), MgO+, MgO2+ , M g2O+ and (MgO)(2)(+). The kinetic energies determined from time of flight m easurements are species dependent, nearly independent of fluence, and range from 5 to 9 eV. The ion intensities display strongly nonlinear fluence dep endencies, proportional to the fluence to the nth power, where n ranges fro m 3 to 4. These kinetic energies and fluence dependences support a defect m ediated photoelectronic desorption mechanism. Emission is attributed to mol ecular ion precursors adsorbed at special sites associated with surface ele ctron traps. Emission occurs when the electron trap is photoionized via the sequential absorption of 3-4 photons. (C) 2001 American Institute of Physi cs.