Ultrafast molecular dynamics of strongly adsorbed species on oxide surface

In this contribution, the ultrafast dynamics of molecules strongly attached to oxide surfaces are studied using the femtosecond pump-probe mass spectrometry technique. As a model system, the molecular dynamics of CD₃I with random orientations on amorphous CeO₂ thin films is studied. The reaction dynamics is monitored through the detection of reaction intermediates and final products with time-, mass-, and kinetic-energy resolution. In contrast to the previous studies on surface-aligned CD₃I on crystalline oxide films, in the current investigation, in which CD₃I has random adsorption geometries, the CD₃⁺ and I⁺ transient signal intensities decay after the radicals are formed. The decay time constant of I⁺ (8.0 ps) coincided with the formation dynamics of a new peak at mass 254 amu corresponding to I₂⁺. The decay time constant of CD₃⁺ (2.5 ps) coincides with the formation dynamics of a new peak at 144.5 amu, corresponding to a highly energetic CD₃I⁺, which is reformed at the surface due to the increased collision between the I and CD₃ fragments, facilitated by the random adsorption geometry of the CD₃I molecules. In addition, power dependence measurements and kinetic energy-resolved mass spectra are employed to gain insights into the surface reaction dynamics.