Differential Relaxation Effects and Multielectron Excitation for Molecularly Chemisorbed CO on Platinum

Synchrotron radiation ($h\nu =80-170\mathrm{}$ eV) was used for photoemission studies of CO chemisorption on a Pt crystal. We report the first unambiguous observation of the $3\sigma$ molecular level for CO adsorbed on a metal surface and note a relaxation shift 2.4 eV larger for the $3\sigma$ than for the $4\sigma$ as compared to the $3\sigma -4\mathrm{}\sigma$ splitting in the gas phase. Furthermore, we observe energy-loss structure between the $3\sigma$ and $4\sigma$ levels, reminiscent of the shakeup satellites in transition-metal carbonyls.