X-Ray Photoelectron Spectroscopy of Carbon Monoxide

Abstract

The ionization potentials for all of the electrons on carbon monoxide have been measured by x‐ray photoelectron spectroscopy. The values found (in electron volts) are $\text{1Σ}$ (oxygen $\text{1s}$ ) 542.3, $\text{2Σ}$ (carbon $\text{1s}$ ) $\text{296.2, 3Σ\hspace{0.17em}−\hspace{0.17em}38.9, 4Σ\hspace{0.17em}−\hspace{0.17em}19.8, 1π\hspace{0.17em}−\hspace{0.17em}17.2,}\mathrm{and}\text{5Σ\hspace{0.17em}−\hspace{0.17em}14.0}$ . The $\text{1Σ}$ ionization potential is 1.2 eV less than the $\text{1s}$ ionization potential for molecular oxygen; the $\text{2Σ}$ ionization potential is 5.4 eV greater than the $\text{1s}$ ionization potential in methane. These results indicate a positive charge on the carbon of carbon monoxide and a negative charge on the oxygen, in agreement with the relative electronegativities, but opposite to what might be expected from the dipole moment of carbon monoxide. The ionization potentials for the three least bound electrons are in agreement with those found by other techniques, provided that allowance is made for the fact that these values represent vertical (rather than adiabatic) ionization potentials. Although the relative intensities of the photoelectrons reflect the atomic photoelectric cross sections and the molecular orbital composition in a qualitative way, there is not quantitative agreement between the measured intensities and theoretical cross sections.