SERS of CO2 on Cold-Deposited Cu: An Electronic Effect at a Minority of Surface Sites⊥

We compare the results of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) of carbon dioxide (CO2) on cold-deposited copper films. The SERS spectra of CO2 on copper films deposited at 40 K display neutral species at SERS active sites with bands not observed by Raman spectroscopy of CO2 gas, but identical to the loss bands of gaseous CO2 in electron energy loss spectroscopy. The absence of one component of the Fermi doublet of CO2 in SERS proves that the local electromagnetic field enhancement at SERS active sites cannot deliver signals above the noise level. The activated anionic CO2− is observed by transient electron transfer from the anionic molecule to the copper metal at a subgroup of SERS active sites, which are annealed below 200 K. The IRRAS spectra show only the expected infrared (IR) active modes of neutral CO2 representing the “majority species” of adsorbed CO2.