Persistent infrared holeburning spectroscopy of (NH4)2SO4–K2SO4 mixed crystals

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Abstract
Infrared holeburning has been observed from mixed crystals. NH3D+ ions were imbedded in (NH4)2SO4–K2SO4 mixed crystals, the sample irradiated at the N–D stretching frequency, and resulting spectral hole then continuously monitored by a Fourier‐transform infrared (FTIR) spectrometer. The holewidth decreases proportionally to the center frequency of the hole; within the range of 2240–2300 cm−1, the width changes from 10 to 5 cm−1 at 7.5 K. A longer irradiation time (≳10 min), however, leads to a wider spectral hole and also comparatively larger antiholes. The holewidth also shows a T2 type increase with temperature over the range of 7.5–45 K. Decay of the hole (hole filling) is not exponential, indicating that there is a distribution of decay rates. The holeburning quantum efficiency remains almost unchanged with temperature. Similarly, it remains almost constant with the variation of the mole‐fraction of (NH4)2SO4 in the solid solution [in the range of 5%–100% mole‐fraction of (NH4)2SO4], while the hole decay rate varies more than 20‐fold. The constant holeburning quantum efficiency is surprising, considering that the phonon density increases by several orders of magnitude in the experimental temperature range and that the variation of the (NH4)2SO4 concentration changes the height of the rotational barrier of the ammonium ion. Thus, the holeburning must occur by conformational change in the excited vibrational state.