TRANSIENT ABSORPTION-SPECTRA OF VIBRATIONALLY EXCITED OH OD GROUPS INMORDENITE ZEOLITES - EFFECT OF XE ADSORPTION/

Transient spectra following the upsilon=1<--0 excitation of the OH str etching mode of Bronsted acidic hydroxyl groups in normal and deuterat ed mordenite zeolites have been measured by a two-color picosecond pum p-probe technique. When the OH and OD stretching bands were excited by resonant IR pulses, transient bleaching of the fundamental band (upsi lon=1<--O) and the transient hot band (upsilon=2<--1) absorption signa l were observed. For the OH and OD groups free of adsorbates, the spec tral widths of the pump-induced signals were broadened by similar to 4 cm(-1) due to pure dephasing from the convoluted width of the pump an d probe IR pulses, When the OH and OD groups were adsorbed by a Xe ato m, the frequencies of the nu(OH) and nu(OD) bands shifted to lower fre quency 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. Actual ly, the widths of the transient signals were about the same as those o f the linear absorption signals. An adiabatic approximation has been e mployed to interpret the observed broadening/shift of the IR bands by making use of the results of DFT (density functional theory) calculati ons performed on a model cluster consisting of a Xe atom and an H3Si-( OH)-A1H(3) substrate. The result agreed well with the observed spectra l features. (C) 1996 American Institute of Physics.