Off-center self-trapped excitons and creation of lattice defects in alkali halide crystals

Theoretical analysis of the kinetics of low-temperature defect recombination in alkali halide crystals

“…About 95% of such anion FDs are shortlived ones (10 -11 −10 -1 s) [3][4][5][6][7][9][10][11], while the accumulation of so-called long-lived structural defects which are stable for hours, days and months plays a crucial role in radiation-induced material degradation, therefore, being a limitation for many applications [12][13][14][15][16]. It is experimentally proved that low-temperature irradiation leads to the creation of two types of FD pairs: a classical Frenkel pair is defined as a positively charged anion vacancy (v a , α center) and an interstitial halide ion ( , a i − I center), while a pair of neutral FDs consists of an F center (an electron trapped by an anion vacancy, v a e) and an H center (a dihalide molecule 2 X − located in one anion site, 0 ) a i [1][2][3][4][5][6][7][8][9][17][18][19].…”

“…It is generally accepted that radiation instability of the majority of alkali halide crystals (AHCs) is determined by the creation of interstitial-vacancy (i-v) pairs of Frenkel defects (FDs) in an anion sublattice via the decay of self-trapping excitons or the recombination of conduction band electrons with self-trapped holes (V k centers), i.e., the so-called excitonic and electron-hole (e-h) mechanisms of FDs creation (see [1][2][3][4][5][6][7][8] and references therein). About 95% of such anion FDs are shortlived ones (10 -11 −10 -1 s) [3][4][5][6][7][9][10][11], while the accumulation of so-called long-lived structural defects which are stable for hours, days and months plays a crucial role in radiation-induced material degradation, therefore, being a limitation for many applications [12][13][14][15][16].…”

Material characterization and modification using helium lon microscopy : various examples

“…The radiationless transition of an off-center STE from an excited state can provide sufficient energy for the displacement of a Cl − 2 ion and the formation of a nearest-neighbor F -H pair. This process of STE conversion into an F -H pair is effective at room temperature [133,135]. While H-centers are mobile in NaCl already above 80 F-centers are mobile only above 500 K [137].…”

“…However, color-center production is more effective via electronic excitations [131,132]. A model for the creation of F -H pairs due to electronic excitations was suggested by Hersh [104] and Pooley [107], and was extensively studied later [112,122,133,134]. During irradiation a Cl − ion can lose an electron forming a free exciton.…”

“…A free exciton rapidly transforms into a self-trapped exciton as a result of the bonding of the Cl atom to a neighboring Cl − ion. Subsequently, an STE in an off-center configuration acts as precursor for the F -H pair formation [112,133]. The radiationless transition of an off-center STE from an excited state can provide sufficient energy for the displacement of a Cl − 2 ion and the formation of a nearest-neighbor F -H pair.…”