Lifetime anomaly of charge-density excitations in a simple molten salt

Charge-density excitations of a simple ionic liquid exhibit an anomalous decrease of their spectral linewidth with increasing wave number. We explain this phenomenon on the basis of charge-current decay processes. We show that it is not sufficient to identify the frequency-integrated current-decay rate as a decreasing function of wave number. Notably, the frequency dependence of this rate, which is governed by mode-decay processes, plays a crucial role.