Lifetime-vibrational interference effects in the resonantly excited x-ray-emission spectra of CO

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Abstract
A systematic study of vibrational band profiles in resonantly excited x-ray-emission spectra of free molecules is made. Carbon and oxygen K-emission spectra of CO following core excitations, using narrow-bandpass synchrotron radiation, to the 2π orbital are presented. Because of the short lifetime of the core-excited states, interference effects between close-lying coherently excited vibrational levels may occur. The importance of lifetime-vibrational interference in the carbon and oxygen x-ray-emission spectra is investigated by comparing the experimental band profiles with simulated profiles. It is shown that to properly describe the band profiles in the oxygen emission spectra it is necessary to include the interference contributions, while in the carbon spectra the interference contributions are important only when the excitation energy is tuned towards higher vibrational levels. For example, when the excitation energy is tuned to the v=2 vibrational level significant interference effects are observed. It is also demonstrated how emission spectra recorded with a detuning of the excitation energy from the resonance can be used to check the shape of the photon distribution of the synchrotron radiation.