Transient absorption spectra of vibrationally excited OH/OD groups in mordenite zeolites: Effect of Xe adsorption

Abstract

Transient spectra following the v=1←0 excitation of the OH stretching mode of Bro/nsted acidic hydroxyl groups in normal and deuterated mordenite zeolites have been measured by a two-color picosecond pump–probe technique. When the OH and OD stretching bands were excited by resonant IR pulses, transient bleaching of the fundamental band (v=1←0) and the transient hot band (v=2←1) absorption signal were observed. For the OH and OD groups free of adsorbates, the spectral widths of the pump-induced signals were broadened by ∼4 cm−1 due to pure dephasing from the convoluted width of the pump and probe IR pulses. When the OH and OD groups were adsorbed by a Xe atom, the frequencies of the ν(OH) and ν(OD) bands shifted to lower frequency and at the same time the widths of both the transient bleaching and the transient hot band absorption were much broader than those of the isolated hydroxyl groups by the factor of more than three. Actually, the widths of the transient signals were about the same as those of the linear absorption signals. An adiabatic approximation has been employed to interpret the observed broadening/shift of the IR bands by making use of the results of DFT (density functional theory) calculations performed on a model cluster consisting of a Xe atom and an H3Si–(OH)–AlH3 substrate. The result agreed well with the observed spectral features.