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From the first observation of electron spin resonance of irradiated proteins' in 1955 it was clear that the electron vacancy, or electron spin density, caused by the irradiation must be able to migrate through the protein from the multiple sites where the ionizing particles, or quanta, strike to the few sites such as the cystine sulfur where the spin density is finally detected. These original observations were carried out at room temperature. In the present work it is shown that such migrations do not, in fact, occur significantly at the temperature of liquid nitrogen, 770K. Thus the migration of electron holes in the valence shell of proteins requires an activation energy, assistance from the molecular motions of excited vibrational or torsional oscillational states.Studies similar to the ones reported here on the proteins have been made on most of the amino acids, many di-and tripeptides, and on the nucleic acids and their constituents. The results and conclusions drawn from them, which in many respects are similar to those for the proteins, will be published elsewhere. Here we show only the results on the few peptides and amino acids which give resonances like those of the proteins and which are essential for comparison with them. A preliminary report2 on the present results was given at a meeting of the American Physical Society in 1959.Experimental Procedure.-All measurements reported here were made at a microwave frequency of 9,000 Mc/sec. The samples were irradiated in a liquid nitrogen flask with a kilocurie cobalt 60 gamma ray source. Dosages of the order of 5 million r were employed. For the observations at 770K the sample was inserted under liquid nitrogen into the tip of a liquid air flask made of low-loss glass which fitted into a hole in the microwave cavity.Magnetic modulation was employed with a phase-sensitive amplifier tuned to the second harmonic of the modulation frequency. With the small amplitude modulation employed, this gave a response which represents the second derivative of the actual aborpption curve. The second derivative curve has its peak at the peak absorption of the resonance curve and minima on either side at approximately the half-power points of the actual absorption curve.Some of the earlier curves obtained and shown here are on curved coordinate paper, whereas later curves are on rectangular coordinate paper. These differences cause no confusion and will be evident from the figures.Results.-As found in the original observations1 and elaborated in the accompanying paper, the electron spin resonance pattern indicates that unpaired spins usually occur at only one site in a given protein irradiated and observed at room temperature and at only two different sites for a variety of proteins. In contrast, we have found that if the proteins are irradiated at the temperature of liquid nitrogen (770K) and their spin resonance observed at that temperature, without allowing the sample to warm up between irradiation and observation, distinguish-1137

Radiation-induced NH 3 C1 and NH 3 Br defect centers have been studied in ammonium halide single crystals by EPR and optical spectroscopy. The defects are most likely formed by removal of H° from an ammonium ion with subsequent bonding of NH 3 + to X~ along a (111) direction. In the NH3CI center, NH3 4 " is a nearly planar group loosely bonded to a Cl~~; in the NH 3 Br center there is more tendency toward sp z hybridization of the N orbitals, and stronger covalency within the nitrogen-halogen bond. The molecular orbital description of these centers shows a strong similarity to that of mixed VK color centers in the alkali halides. Optical absorptions due to the NH3CI and NH 3 Br centers were observed at 3.31 and 3.14 eV, respectively. The NH 3 C1 defects were observed to undergo thermally induced reorientations which are strongly influenced by the ordering of the surrounding ammonium ions at low temperature. INTRODUCTIONT HE ammonium halides are a suitable system for the extension of studies of radiation-induced color centers which have in the past been studied primarily in the alkali halides. 1 Recent electron-paramagnetic-resonance and optical-absorption experiments in the ammonium halides have led to the identification of two color centers which are also found in the alkali halides: (1) the F center, which is an electron trapped at a negative ion vacancy 2 ; (2) the VK center, a hole shared by two nearest-neighbor halide ions. 3 -4 A third center has been identified as NH 3 X, where X represents a halogen atom. 5 ' 6 In this paper we give a fuller description of the structure of this center in ammonium chloride, deuterated ammonium chloride, and ammonium bromide single crystals. The kinetics of the reorientation behavior, dealt with briefly here, will be described more completely in a subsequent publication. 7 CRYSTAL STRUCTUREBoth NH4CI and NH 4 Br have the CsCl structure at room temperature, 8 as shown in Fig. 1. Below 242.8°K in the chloride, 234.5°K in the bromide, both materials undergo a X transition of the order-disorder type involving the relative orientations of adjacent ammonium ions within the lattice. 9 The tetrahedral NH 4 + ions can be oriented in two equivalent positions of minimum electrostatic potential within the cube of nearestneighbor halide ions. In the completely ordered state 1 of NH 4 C1, all NH 4 + ions have the same relative orientation, for a given domain, with respect to the crystallographic axes; the space-group symmetry is T d l {PiZm). ND4CI has similar structural changes. The NH 4 Br crystal structure becomes tetragonal below the X point, though this is the result of a slight change of 0.4% of the lattice parameter along one cube direction. Additionally, the Br~ ions are displaced about 3.0% from planes perpendicular to the distortion axis, lying alternately in positive and negative directions along the tetragonal axis. The space-group symmetry is Dih 7 (P/nmm). In the ordered NH 4 Br crystal, adjacent NH 4 + ions in a plane perpendicular to the distortion axis have opposite orie...

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