Interaction of 157-nm excimer laser light with fused silica, polytetrafluoroethylene and calcium fluoride

Emission of positive ions, neutral atoms and molecules and negative ions was observed during 157-nm irradiation of fused silica. Positive ion emission from fused Silica is best explained by a hybrid mechanism involving (a) excitation of an antibonding chemical state (Menzel-Gomer-Redhead mechanism) and (b) acceleration of the positive ion by repulsive electrostatic forces due to the photoionization of nearby electron traps. Neutral emission results from the absorption of single 157 nm photons that cleave strained bonds to produce defects that subsequently diffuse to the surface. There then react with dangling bonds to release neutral atoms and molecules. Negative ions are created after the laser pulse, by electron attachment to the neutral particles.;Time- and mass-resolved measurements on neutral particles emitted PTFE during exposure to 157-nm laser radiation. We observe neutral (CF2) N radicals for all N up to the limit of our mass filter (N = 6). These radicals are emitted directly from the surface with high kinetic energies of about 0.6 eV of the fastest (CF2)N radicals consistent with photochemical scission of the polymer backbone. A great majority of emitted material is in the form of heavier fragments ranging from 300 to 2000 amu. The presence of heavy and light particles in the cloud of emitted particles gives rise to some unique behavior in the way it expands. The trajectories of the v heaviest fragments are preferentially directed at large angles away from the surface normal---often nearly parallel to the substrate surface. This behavior is due to density and pressure gradients associated with the Gaussian intensity profile of the laser beam.;Transient absorption was observed in Calcium Fluoride during 157-nm irradiation with peaks at 153 nm, 220 nm and 375 nm under 157-nm irradiation due to the Hs- center, Ca1+ interstitial and the F-center respectively. Capture of a hole by Hs- center, Ca1+ interstitial destroys the absorption and trapping of an electron at Hi0 center and the Ca2+ interstitial re-induces absorption. Aggregation of F-centers to form M-centers leads to loss of absorption at 375 nm and the dissociation of the M-center when irradiation is restored re-induces absorption.