Isotope Tuning of Fermi Resonance and Vibrational Hybridization in Surface-Adsorbed CO2 Probed by Tip-Enhanced Raman Spectroscopy

Fermi resonance in CO2 serves as a prototypical example of anharmonic vibrational coupling, where the fundamental vibrational mode of C–O symmetric stretching (v1) mixes quantum-mechanically with the nearly degenerate overtone (2v2) of C–O bending, resulting in energy level splitting, spectral peak shifting, intensity redistribution, and band bifurcation in vibrational spectra. Here, we carry out a comprehensive investigation of this cubic anharmonic interaction in surface-adsorbed CO2 clusters using low-temperature ultrahigh-vacuum Tip-Enhanced Raman Spectroscopy (TERS), in combination with isotope tuning. In addition to clear observation of well-resolved Fermi dyads for both C16O2 and C18O2 isotopologues, TERS enables direct observation of the Raman-inactive bending mode (v2) by breaking the conventional Raman selection rule. As a result, comprehensive and important Fermi resonance parameters of CO2 can be determined from the frequency domain directly, including, e.g., the frequency detuning, the Fermi coupling strength, and the mixing angle for the vibrational hybridization. Furthermore, isotope substitution from C16O2 to C18O2 is found to reduce and even reverse the frequency detuning, which modifies the quantum mixing significantly through the change in the mixing angle and leads to a redistribution of vibrational character between the Fermi dyad states. Such an isotope-induced shift in the mixing angle modifies the balance between constructive and destructive interference, resulting in the observed intensity asymmetry of the Fermi dyads. In addition, these Fermi dyads are found to be somewhat sensitive to cluster morphologies and local adsorption environment. These results demonstrate that TERS is a powerful tool for quantitatively probing anharmonic vibrational interactions such as Fermi resonance at the nanoscale, while isotope substitution can provide an effective route to tune vibrational hybridization and quantum interference for molecules adsorbed on surfaces.