Towards understanding the hydrogen molecule in ZnO

The hydrogen molecule H-2 (or hidden hydrogen) in ZnO is studied by Raman scattering spectroscopy. It is shown that H-2 is practically a free rotator stable up to 700 degrees C, which is formed by two mobile interstitial hydrogen atoms. The concentration profile of the hydrogen molecule anticorrelates with hydrogen substituting for oxygen (H-O) created at the sample surface during the high temperature hydrogenation. The H-2 dissociation at elevated temperatures in the bulk regenerates interstitial (H-BC) hydrogen, whereas the formation of H-O and hydrogen out-diffusion are dominant decay channels of the molecule near the surface. The latter mechanisms are responsible for the lower stability of H-2 in the subsurface region. An ortho-para conversion between the nuclear spin states 1 and 0 of the molecule at 79 K was found to occur within 7.5 h, whereas the back conversion at room temperature occurs faster than 0.5 h. A shift in frequency of the H-2 local vibrational mode with annealing time and temperature is associated with the thermal anneal of lattice imperfections. The coupling of transitions between rotational states of the molecule and lattice phonons influences the ro-vibrational properties. Interstitial lattice sites and/or larger vacancy clusters are preferred trapping centers for H-2 in ZnO.