Persistent infrared hole burning spectroscopy of NH3D+ doped in [(NH4)x,Rb1−x]2SO4 mixed crystals

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Abstract
[(NH4)x,Rb1−x]2SO4 mixed crystals (0.16≤x≤1) were doped with NH3D+. Four of the eight N–D stretching bands of the NH3D+ ion gradually disappear with increasing Rb+ ion concentration while the widths of the N–D stretching bands increase, indicating that Rb+ ions first substitute NH4+ ions only in one type of crystal site, and that addition of Rb+ ions introduces glasslike disorder into the (NH4)2SO4-type crystalline structure. Infrared hole burning has been demonstrated in the broadened N–D stretching band of NH3D+ ion using a combination of a diode laser and a Fourier-transform infrared spectrometer. The initial hole width decreases proportionally with the center frequency of the hole at all Rb+ ion concentrations and agrees with the measurements of the [(NH4)x,K1−x]2SO4 mixed crystals. The similar proportionality, long known for many hydrogen-bonded systems in solution, suggests that the widths observed in solution are homogeneous. A longer irradiation time (≳10 min), however, leads to a wider spectral hole. Measured hole decay rates decrease with decrease of the center frequency of the hole, showing that the rotational tunneling barrier increases with the strength of the hydrogen bond. The change of the rotational tunneling barrier with Rb+ ion concentration is also observed as a change of the hole decay rate (more than tenfold in the experimental range). On the other hand, the hole burning quantum efficiency shows little change with the Rb+ ion concentration, or temperature. The observed steady holeburning quantum efficiency supports the infrared hole burning mechanism proposed in our previous study: The configurational change of the hole burning must occur in the excited vibrational state.