Photoluminescence Induced by 6.4 eV Photons in High Purity Silica
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PHOTOLUMINESCENCE INDUCED BY 6.4 eV PHOTONS IN HIGH PURITY SILICA T. E. Tsai*, and H. B. Lin** *GEO Centers, Inc. Fort Washington, MD 20744 **Naval Research Laboratory, Washington, DC 20375 ABSTRACT photoluminescence in the visible range in high purity silica rods at room temperature was studied using 6.4 eV photons as the excitation source. Electron spin resonance (ESR) was used to monitor the induced paramagnetic centers. Broad luminescence bands at 2.7 eV, 2.2 eV and 1.9 eV were observed. The relative intensities of these bands were found to be intensity dependent. The fluence dependence of these bands were also studied. The results are discussed in relationship to the excitonic mechanism for defect generation in silica. INTRODUCTION The origins of luminescence bands in irradiated high purity silica are not yet clearly understood', 2 ,3 . The origin of 6.7 eV luminescence band, induced by high energy electron irradiation', was proposed by Griscom3 but no experimental confirmation has yet been reported. The 4.3 eV emission band was recently demonstrated to be associated with a 5.0 eV absorption band due to one of the two types of oxygen vacancy defects On the contrary, Tohmon, et. al.6 attributed the emission at 4.3 eV to a defect other than oxygen vacancy. The 2.7 eV blue photoluminescence band in the visible region was proposed' to be due to the transition from a triplet excited state to the singlet ground state of a two coordinated Si in silica. The absorption band at 5.0 eV and the luminescence band at 4.3 eV were argued to be singlet-to-singlet transitions of the 2 same defect. Nagasawa, et. al. 8 and Griscom reported that the 5.0 eV absorption is mostly originated from an oxygen vacancy defect. On the other hand, Siegl 9 and Griscom3 proposed that the 2.7 eV emission stimulated by pulsed electron irradiation is due to the emission from transient Si E' centers of self-trapped excitons. A detailed model for the self-trapped exciton responsible for the 2.7 eV emission was suggested by Griscom3 . Experimental data consistent with this model were reported by Hayes, et. al.10 . and by Itoh, et. al.". However, based on the observation that a 2.7 eV photoemission band can be observed in unirradiated oxygendeficient silica, Tohmon, et. al.' 2 proposed that it is the transition from the excited triplet state to ground singlet state of a neutral oxygen-vacancy defect. The 1.9 eV red luminescence band was suggested by Skuja and Silin13 to be associated with 2.0 eV and 4.75 eV absorption bands arising from a non-bridging oxygen hole center (NBOHC). However, the growth curves for the 4.75 eV absorption and 1.9 eV luminescence bands were found to be markedly different when high purity fused silica was irradiated with vacuum-ultraviolet photons", suggesting that they arise from different centers. Experimental data supporting the association of the 1.9 eV emission band with NBOHC were reported by Siegl and Marrone ' and by Nagasawa, et. al.", 17. However, data suggesting that it arises from different defect centers in low O
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