Broad central line observed in electron spin resonance experiments on hydrogen atom isotopes in solid molecular films of hydrogen isotopes

We report on an observation and investigation of a broad central line (BCL) in electron-spin resonance (ESR) studies of hydrogen and tritium atoms embedded in solid molecular films of hydrogen isotopes (${\mathrm{H}}_{2}$ and ${\mathrm{T}}_{2}$) at a temperature of 0.1 K and in a high magnetic field $B=4.6\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. We observed that this broad central line remains in the ESR spectrum after evaporating the hydrogen films and even after heating to $T=150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ but disappears after heating to room temperature. We suggest possible nature of the ESR active centers forming this line.