Interaction of wide-band-gap single crystals with 248-nm excimer laser irradiation. IX. Photoinduced atomic desorption from cleaved NaCl(100) surfaces

Abstract

Neutral atomic sodium and chlorine emissions from cleaved, single-crystal NaCl͑100͒ surfaces due to pulsed, 248-nm excimer laser irradiation have been characterized by time-resolved, quadrupole mass spectroscopy. At laser fluences below the threshold for optical breakdown, the resulting time-of-flight signals are consistent with particles emitted in thermal equilibrium with a laser-heated surface. Activation energy measurements made by varying the substrate temperature are consistent with F-H pair formation under UV excitation. By varying the laser fluence and estimating the effective surface temperature from the time-of-flight signals, additional activation energy measurements were made. The corresponding rate-limiting step is attributed to a thermally assisted, photoelectronic process involving atomic steps. Atomic force microscope images of surfaces irradiated at low fluences show monolayer islands that are created by the aggregation of material desorbed from steps. At somewhat higher fluences, monolayer pits due to F-center aggregation are also observed.

Interaction of wide band gap single crystals with 248nm excimer laser radiation. XII. The emission of negative atomic ions from alkali halides

“…F centers formed in the bulk diffuse to the surface, where they contribute to the formation of pits ͑observed by atomic force microscopy͒ 21-23 and step roughening. 2,24 This process would provide high densities of surface electron traps, suitable for alkali ion adsorption and subsequent positive emission. Adsorbed positive ions are presumably provided by diffusion from kink sites along surface steps.…”

“…1.͒ For nanosecond pulse durations, these emissions typically require surface and near surface defects, such as anion vacancies and steps. [2][3][4] In many cases, anion vacancies and steps are created by the laser itself. For instance, pulsed 248 nm radiation can produce pits on NaCl͑100͒ surfaces via vacancy aggregation.…”

Emission of Negative Potassium Ions from Single Crystal Potassium Bromide during Exposure to 248-nm Excimer Laser Radiation

“…In recent work, we have shown that neutral emission intensity from NaCl during 248-nm irradiation increases with bulk sample temperature in an Arrhenius fashion. The activation energy corresponds to the thermal activation energy for thermally assisted F center formation due to bandgap excitations (requiring the absorption of two-photons, which we verified).…”

“…The resulting effective surface temperature is T = 2700 K, which corresponds to a translational kinetic energy of ∼0.2 eV. (By way of comparison, effective surface temperatures of 2800 K were observed for Na 0 from NaCl during 248-nm irradiation at a somewhat lower fluence …”

“…(By way of comparison, effective surface temperatures of 2800 K were observed for Na 0 from NaCl during 248-nm irradiation at a somewhat lower fluence. 2 ) Electron attachment to these neutrals, if it occurred, would not appreciably alter the kinetic energy of the resulting negative ion. We have previously shown that the ( charge cloud (here delineated by the K + distribution) contains large numbers of electrons.…”