ESR investigation of a stable trapped hydrogen atom in X-ray-irradiated beta-tricalcium phosphate at room temperature

A stable trapped hydrogen atom in X-ray-irradiated beta-tricalcium phosphate (beta-Ca3(PO4)2, beta-TCP) was successfully detected at room temperature. This hydrogen atom is stable at ambient temperature for several months. Hyperfine structure of the hydrogen atom and superhyperfine structures of the two phosphorus atoms were observed by means of electron spin resonance (ESR) spectroscopy. Electron spin-echo (ESE) of the hydrogen atom was observed in X-ray-irradiated beta-TCP. At room temperature, relaxation times of the hydrogen atom in X-ray-irradiated beta-TCP were very long (phase memory time TM = 19.4 mus, spin-lattice relaxation time T1 = 75.8 mus) compared with those of usual paramagnetic species. The most important facts are the detections of ESE and electron spin-echo envelope modulation (ESEEM) at room temperature. At room temperature, the observations of ESE and ESEEM and the estimations for the relaxation times (TM, T1) of the hydrogen atom were carried out for the first time until now. TM was able to be measured from room temperature to 9 K. The short relaxation time TM below 20 K might be explained by the quantum tunneling effect of the hydrogen atom. Fourier transformation of the electron spin-echo envelope modulation (FT-ESEEM) at room temperature suggests the overlapping of the wave functions between the hydrogen atom and the two phosphorus atoms. The site of the hydrogen atom in the X-ray-irradiated beta-TCP was discussed on the basis of the continuous wave ESR (CW-ESR) and pulse-ESR analyses.