Dynamics of CO and CN Vibrational Relaxation

For CO on Pt(lll) in acetonitrile containing 0.1 M NaCl04, the lifetime of the CO vibration increased from 1.5 to 2.1 ps with decreasing potential over a range of 2.6 V. These observations are consistent with the Blyholder model since, as the potential is adjusted to more negative values, electrons are expected to fill the 2 r orbital on CO, leading to a less efficient decay process [106]. 

However, in the work of Tadjeddine et al. in aqueous and nonaqueous electrolyte (acetonitrile), a lifetime of 1.7 ps was observed with no effect of potential, even over a range of 2 V [107, 108]. The results indicate the CO vibrational Hfe-time is not dependent on electrode potential or solvent molecules or surface orientation [Pt(lll), Pt(llO)] [2, 108]. This discrepancy is small and could be due to an improved signal-to-noise ratio in the later results. Thus, CO vibrational relaxation is presumed to be dominated by the charge transfer-like mechanism because of the interaction between the CO 2n and the Fermi level of the metal. These measurements help in developing molecular models for bonding of molecules at electrode surfaces. 

Dynamics with respect to applied potential have also been studied. Chou et al. [110] followed the oxidation of CO on Pt(lll) using in-situ SFG collecting 1 spectrum every 22 s. A -100 mV shift in the CO stripping peak was observed in the CV for a scan rate of 0.5 mV s and followed a simultaneous decrease in the SFG signal for top site CO. Also, a dramatic frequency shift and intensity increase is noted just before CO oxidation. The intensity and frequency shift are related to the Raman cross-section and coupling between CO dipoles while CO surface coverage changes, as shown in Fig. 5.18 [110]. 

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