DETERMINATION OF THE ION VELOCITY IN A RADIO-FREQUENCY ION-BEAM BY LASER-INDUCED FLUORESCENCE

Laser-induced fluorescence (LIF) has been used to measure in situ the velocity of argon ions being extracted from a filamentless capacitively coupled radio frequency ion beam source. Velocity distributions are measured by scanning a narrow-bandwidth dye laser over the Doppler-shifted absorption profile of an Ar II line while detecting the fluorescence. In the ion beam, charge-exchange processes produce a group of low energy ions manifested in an unshifted LIF peak useful for wavelength calibration. Partial reflection of the laser beam gives rise to additional LIF signals with a negative wavelength shift, thereby generating a mirror image of the originally measured ion velocity distribution. The ion velocity distribution has also been independently obtained with an electrostatic retarding field analyzer (RFA). The broad ion beam is characterized by an energy spread of about 20 eV and a mean energy between 100 and 1000 eV, depending on the extraction optics and its parameters. LIF results are compared with the RFA measurements and show excellent agreement. © 1995, American Vacuum Society. All rights reserved.