In this Letter we report the first direct determination of the wave function of the intrinsic shallow electron center in silver chloride. A model is suggested in which an electron is shallowly trapped by two adjacent silver ions on a single cationic site. The information has been obtained by pulsed electron nuclear double resonance at 95 GHz and illustrates the potential of high-frequency EPR techniques for the study of defects in semiconductors.Intrinsic defect centers in inorganic crystals have attracted considerable attention, because of their importance for material science. In particular, the F center in alkali halides has occupied a central place. The identification of its structure was eventually solved by electron nuclear double resonance (ENDOR) spectroscopy.The results proved that this center consists of an electron trapped in the Coulombic field of a (substitutional) anion vacancy and that the alternative model, which assumed the presence of an electron bound to an interstitial cation, had to be rejected [1].The identification of the structure of the intrinsic shallow electron center in silver halides resembles the problem concerning the F center. Shallow electron centers are created upon uv excitation and are believed to play an important role in the latent image formation process [2,3]. They can be studied at low temperatures and were first observed in 1969 by Brandt and Brown using uv-induced infrared absorption spectroscopy [4]. It was suggested that the intrinsic binding core consists of an interstitial silver ion [5], but other models include a substitutional silver ion at a surface kink or an internal jog [2]. The intrinsic shallow electron centers have also been observed by EPR spectroscopy and they give an isotrupic, structureless line at g -1.88 in AgC1 [6] and g -1.49 in AgBr [7]. Until now, both optical and EPR spectroscopy have been unable to establish the structure of the binding core. In this Letter we report the first ENDOR spectra of intrinsic shallow electron centers in AgCl. They allow us to determine the spatial delocalization of the loosely bound electron and to propose a model for the binding core. The conclusions are supported by similar ENDOR results on the self-trapped exciton (STE) and on electrons shallowly bound to divalent cationic impurities. The ENDOR spectra were obtained via a method which is based on the stimulated echo (SE) pulse sequence [8]. Here a vr/2 r vr/2 T vr/2 micro-w-ave p-ul-se sequence is applied resonant with the EPR signal of the shallow electron center at g = 1.878. The SE signal is produced at time r after the third 7r/2 pulse. A radio frequency (RF) pulse between the second and third vr/2 pulse induces the nuclear transitions and its effect is monitored as a decrease of the SE intensity. The ENDOR spectra were recorded at 1.2 K with a spectrometer operating at a microwave frequency of 95 GHz. The spectrometer and its specific advantages for this work will be described in detail elsewhere [9]. The main experiment has been performed on an undoped AgC...