Does an overlayer/underlayer transition occur for H Ru(001)?

The reflectivity for low energy electrons (8-35 eV) has been studied from clean, hydrogen and deuterium covered Ru(001) faces. This technique is sensitive to hydrogen adsorbed at room temperature, which is nearly invisible in UPS. Explanations for the difference between low temperature and room temperature hydrogen layers are examined in the light of these results; a previously suggested overlayer/underlayer transition model can be ruled out.