4188
J. Phys. Chem. 1996, 100, 4188-4192
Fragmentation and Recombination of Molecules during Laser Vaporization of Cryogenic Films Gabriela C. Weaver† and Stephen R. Leone* JILA, National Institute of Standards and Technology and UniVersity of Colorado, and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0440 ReceiVed: September 15, 1995; In Final Form: December 1, 1995X
Laser vaporization of cryogenically condensed films of ICl and Cl2 is studied by investigating the beams of neutral atoms and molecules formed with high translational kinetic energies. The fragments resulting from the vaporization process of these molecules are examined and compared to the fragmentation patterns of molecules studied previously, XeF2 and NO. It is found that ICl shows a large amount of fragmentation and recombination during vaporization, which forms Cl2 and I2. This is also true for XeF2 and may be true for Cl2, but does not occur for NO. The degree of recombination is affected by the thickness of the cryogenic film being ablated and depends on the bond energy of the molecule with respect to the photon energy.
Introduction Traditionally, the generation of fast neutral species with energies in the range 1-20 eV has been performed with supersonic nozzle sources.1 This is a practical method for achieving fast neutral particles, but the velocities are limited by the terminal velocity of molecules in an expansion.2 For supersonic beams of pure Cl2, which can be used for kinetic energy-enhanced etching, the translational kinetic energies cannot be much higher than 1 eV. Working with seeded beams of Cl2 in He, Teraoka and Nishiyama3 have achieved Cl2 kinetic energies as high as 3.0 eV. However, seeding has the disadvantage that the carrier gas may not be a desirable component in the reaction being studied. This is definitely the case for H2 carrier gas in regard to semiconductor surfaces. In addition, if the concentration of seed gas is very low, the reaction may be difficult to observe. For much higher energies, ion beam neutralization techniques can be used, but the densities are limited by space-charge repulsions at the lower energy range (
