FRESH Lecture: Observation of long-lived molecular vibrations during adsorption and desorption on a metal surface
- 13 september 2018
- Gorlaeus Building
2333 CC Leiden
The most common mechanism of catalytic surface chemistry is that of Langmuir and Hinshelwood (LH). In the LH mechanism, reactants adsorb, become thermalized with the surface and subsequently react . At the same time, molecular vibration is well known to enhance the rates of gas-phase chemical reactions as the motion associated with vibration – for example, bond stretching may guide the atoms of the reactant molecule efficiently to the transition state. However, the measured vibrational lifetimes for molecules adsorbed at metal surfaces are in the range of a few picoseconds which is attributed to electronically nonadiabatic coupling of molecular vibration to electron-hole pairs in the metal [2-4]. As a consequence, vibrational promotion of LH chemistry is rarely observed – an exception is LH reactions occurring via a molecular physisorbed intermediate.
In this work, we have directly detected adsorption and subsequent desorption of vibrationally excited CO molecules from a Au(111) surface . Our results show that CO (v = 1) survives on a Au(111) surface for ~10-10 sec. Such long vibrational lifetimes for adsorbates on metal surfaces are unexpected and pose an interesting challenge to the current understanding of vibrational energy dissipation on metal surfaces. They also suggest that vibrational promotion of surface chemistry might be more common than is generally believed.
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