Electron spin resonance spectroscopy of molecules in large precessional motion: A case of H6+ and H4D2+ in solid parahydrogen

We have measured electron spin resonance (ESR) spectra of H6+ and H4D2+ ions produced in γ-ray irradiated solid parahydrogen. Anisotropic hyperfine-coupling constants for H6+ and H4D2+ determined by the analysis of ESR lines at 4.2 K were −0.06 and −0.12 mT, respectively, which were opposite in sign to and much smaller than theoretical results of 1.17-1.25 mT. Although no change was observed in H6+, the constant for H4D2+ increased to be 1.17 mT at 1.7 K, which is very close to the theoretical value. We concluded that H6+ both at 4.2 and 1.7 K and H4D2+ at 4.2 K should be in a large precessional motion with the angle of 57-59°, but the precession of H4D2+ is stopped at 1.7 K.