Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

Abstract

Abstract. The intensity dependence of the total and specific yields of positive ions desorbed from SrF 2 under 193 nm and 308 nm excimer-laser irradiation has been investigated by the time-of-flight method. The following positive ion species have been detected: F +, Sr +, Sr ++, SrF + and SrF +. The Sr + and SrF + emission yields are found to increase as E ~, where E represents the laser energy per pulse. The exponent is related to defect-initiated neutral particle emission and gas-phase ionization. The influence of surface damage on this power dependence is investigated. The F + emission yield showed a quite different behaviour compared to that of the Sr + and SrF + emission. At both wavelengths the total positive ion emission yields saturate at a certain laser energy. In the saturation regime the SrF + emission vanishes and alternative emission of F + and Sr + was observed at both wavelengths, but the total emission yield in the saturation regime (F + + Sr +) remained constant. A Scanning Electron Microscope (SEM) was used to investigate the damage spots after laser irradiation for thermal effects. 61.80.Ba, 79.20.Ds Parallel to the development of high-power laser systems, the need for laser components, highly resistant against optical damage, grew in importance. The development of highpower laser systems, especially those operating in the visible or UV, makes it possible to investigate the interaction of intense electromagnetic waves with ionic crystals and optical materials [1]. To obtain more insight into the fundamental damaging processes for ionic crystals under intense laser light, desorbing particles and their properties are analyzed as a tool to find the basic damaging mechanism. PACS:Using irradiation wavelengths in the UV, in general, the photon energy will be below the band gap energy, when considering insulating materials. Therefore, absorption of photons in large band gap materials will take place by multiphoton excitation [2] of Valence Band (VB) electrons only, if a regular crystal lattice without any disturbances (surface, defects, impurities) is assumed. In real crystals, the surface itself, defects and impurities alter the electronic properties of the regular bulk. Their electronic states are located in the band gap and can play an important role in laser absorption processes [3][4][5]. One o f the most crucial problems is, how to quantify the influence of lattice and surface defects to the occurrence of lattice damage.A single or multiphoton absorption process results in a Conduction Band (CB) electron and a hole in the VB. The so created electron-hole pair can recombine radiatively or act as a precursor for other defects, like excitons, F-centres or H-centres [6][7][8].It is known from Electron Stimulated Desorption (ESD) and Photon Stimulated Desorption (PSD) experiments on alkali halides [9] that defects created by irradiation cause desorption of mainly neutral species. ESD of positively charged halogen ions from alkali halides [10,11] has been explained by the Knotek-Feibelman Mechanis...

Modeling of UV pulsed-laser ablation of metallic targets

“…A strong increase of the ion and electron yield as a function of the laser pulse fluence has been observed at moderate laser irradiance and up to ≈ 1.5 GW cm −2 ; then at higher laser intensities (> 1.5 GW cm −2 ) a plateau was reached. A roll-off of the total ion yield was also observed by various authors in laser ablation of different materials, and it was ascribed to plasma shielding of the sample surface [5,7,8,37,38]. The roll-off observed in the theoretical investigations reported here is in quite good agreement with the experimental findings of [22](see Fig.…”

Characterization of laser-ablation plasmas

“…Also Kreitschitz et al (1994) used a reflectron MS to investigate the laser energy dependence of total and specific yields of positive ions ablated from SrF 2 single crystals under 193 and 308 nm excimer laser irradiation. At both wavelengths the total positive ion emission yield saturated at fixed laser energies of about 1 mJ at 308 nm, and one order of magnitude less at 193 nm, respectively.…”