  Inelastic neutron scattering was performed for an iron-based superconductor
LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny
amount goes into interstitial sites. By first-principle calculation, we
characterize the interstitial sites for D (and for H slightly mixed) with four
equivalent potential minima. Below the superconducting transition temperature
Tc = 26 K,new excitations emerge in the range 5-15 meV, while they are absent
in the reference system LaFeAsO0.9F0.1. The strong excitations at 14.5 meV and
11.1 meV broaden rapidly around 15 K and 20 K, respectively, where each energy
becomes comparable to twice of the superconducting gap. The strong excitations
are ascribed to a quantum rattling, or a band motion of hydrogen, which arises
only if the number of potential minima is larger than two.
