Vibrational Relaxation Lifetime of a Physisorbed Molecule at a Metal Surface

Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1–3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For $\mathrm{CO}(v=1)$ adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is $49\pm 3\mathrm{ps}$. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances.

Figure 1

Experimental setup to measure the vibrational lifetime of CO on Au(111) by surface pump-probe sum frequency generation. (Left) schematic diagram. (Right) Vibrational isolation for cryostatic cooling of the sample.

Figure 2

(a) IRRA and (b) sum frequency generation spectra of monolayer CO on Au(111) at 25 K; the experimental data (full circles) of (a) and (b) were fitted with a Lorentzian line shape function and Eq. (2), respectively (red lines), using a common resonance frequency of $v_{01}=2131{\mathrm{cm}}^{-1}$ and a line width of $\mathrm{\Delta }$ $v_{01}=6.5{\mathrm{cm}}^{-1}$. (c) The ${\mathrm{IR}}_{\text{pump}}$-induced change in the SFG spectrum obtained 35 ps after exciting the sample with $30\mu \mathrm{J}$ (full circles) and $60\mu \mathrm{J}$ (open circles) pump pulses at $2131{\mathrm{cm}}^{-1}$ is shown ($T=35\mathrm{K}$).

Figure 3

Time-dependent SFG response of monolayer CO on Au(111) at 35 K with resonant excitation at ${\nu }_{\mathrm{pump}}=2131{\mathrm{cm}}^{-1}$ and probing at (a) ${\nu }_{\mathrm{probe}}=2131{\mathrm{cm}}^{-1}$ and (b) $2104{\mathrm{cm}}^{-1}$, respectively (black points). Red lines are obtained from least-square fits, yielding a vibrational lifetime of $\tau =49\pm 3\mathrm{ps}$. (a) Green data points show the SFG response when the surface is excited off resonance from the CO $0\to 1$ transition (${\nu }_{\mathrm{pump}}=2112{\mathrm{cm}}^{-1}$). Blue squares represent the cross-correlation between the visible and IR pump pulse with a FWHM of 27 ps.