I am doing CO adsorption experiment. When I collect the CO gas phase spectrum there are two peaks for CO. One at around 2114 cm-1 and the other one is around 2170 cm-1. I don't know the reason why there are two peaks for CO there.
Dear Fereshteh Rezvani
Thank you very much for your interesting technical question. I think you recorded the IR spectrum at high temperature at which some CO molecules was ionized to CO+ ion. From the molecular orbital diagram of CO, it is easily understood that, one electron is removed from an antibonding type molecular orbital of CO to form CO+ ion. This obviously results C-O bond stronger for which the band at higher frequency ( ~2170 cm-1) is observed. The lower frequency band (2114 cm-1) may correspond to the neutral CO molecule. Please check the relative intensity of the bands. For clarification, I would like to suggest you to record IR spectra of CO varying the temperature. Following article may help you a lot. Good luck with your research!
Article Infrared spectra of carbon monoxide, carbon dioxide, nitric ...
Infrared spectrum of adsorbed CO: CO partial pressure = 0.05 atm; (a) T = 33°C; (b) T = 96°C. also be observed as the temperature is raised. The intensity of the band at 2140 cm-l decreases slowly with increasing temperature and above 167°C a new band appears at 2170 cm-l.
I would think that the explanation is much simpler than the ones stated above. Factor in that CO is in the gaseous state which means that with high spectral resolution you see typically a lot of peaks due to the rotational fine structure with P- and R-branch (cf. https://en.wikipedia.org/wiki/Rotational%E2%80%93vibrational_spectroscopy#/media/File:Vib_rot_CO.png). Once you decrease resolution, you get two bands, one for the P- and one for the R-branch with a gap in between. Finally, if you decrease the resolution further, you obtain a single band.