Fast energy delocalization upon vibrational relaxation of a deuterated zeolite surface hydroxyl

In this time-resolved study of vibrational dynamics of deuterated surface hydroxyls at acid sites in the zeolite Mordenite, we investigate the O-D T_l vibrational lifetime and transient band shifts. It is shown that after infrared excitation of the stretching mode of a surface hydroxyl, the excess energy is rapidly distributed over delocalized low-energy lattice modes upon de-excitation. This is asserted from the observation that nonexcited hydroxyls are perturbed by the relaxation of their excited counterparts immediately after this relaxation. This observation can be made owing to better resolution in transient transmission spectroscopy obtained by deuteration of the surface hydroxyls. This assignment allows for accurate estimates of lattice temperatures after relaxation of the vibration. Further, from the observation that the vibrational lifetime is dependent on frequency (increasing from 25 to 70 ps with increasing frequency), it is concluded that the deuterated acidic protons are hydrogen bonded to lattice oxygen atoms in the zeolite.