Vibrational Action Spectroscopy of Solids: New Surface-Sensitive Technique

Vibrational action spectroscopy employing infrared radiation from a free-electron laser has been successfully used for many years to study the vibrational and structural properties of gas phase aggregates. Despite the high sensitivity of this method no relevant studies have yet been conducted for solid sample surfaces. We have set up an experiment for the application of this method to such targets, using infrared light from the free-electron laser of the Fritz Haber Institute. In this Letter, we present first results of this technique with adsorbed argon and neon atoms as messengers. We were able to detect surface-located vibrations of a thin ${\mathrm{V}}_2{\mathrm{O}}_3(0001)$ film on Au(111) as well as adsorbate vibrations, demonstrating that this method is highly surface sensitive. We consider that the dominant channel for desorption of the messenger atoms is direct inharmonic vibrational coupling, which is essentially insensitive to subsurface or bulk vibrations. Another channel is thermal desorption due to sample heating by absorption of infrared light. The high surface sensitivity of the nonthermal channel and its insensitivity to subsurface modes makes this technique an ideal tool for the study of surface-located vibrations.

Figure 1

Scheme of the experimental setup for vibrational action spectroscopy of surfaces.

Figure 2

Comparison of different spectra of ${\mathrm{V}}_2{\mathrm{O}}_3(0001)/\text{Au}(111)$. Spectra (b) and (c) were measured from high to low energy with the slowly decreasing background intensity being mostly due to desorption of rare gas from the cryostat and the sample holder. Spectrum (e) is the sample temperature rise for spectrum (c) relative to the sample temperature before infrared irradiation. The scale on the right ordinate axis applies to curve (e) only.

Figure 3

Action spectra with neon as messenger gas of (a) vanadyl-terminated ${\mathrm{V}}_2{\mathrm{O}}_3(0001)/\text{Au}(111)$, (b) 1 Å iron on ${\mathrm{V}}_2{\mathrm{O}}_3(0001)/\text{Au}(111)$ (deposited at 300 K), and (c) methanol on ${\mathrm{V}}_2{\mathrm{O}}_3(0001)/\text{Au}(111)$ (dosed at 8 K followed by a flash to 200 K to remove methanol above the first layer).

Figure 4

(a) Action spectrum of rutile ${\mathrm{TiO}}_2(110)$ with neon as messenger gas, (b) sample temperature rise.