Laser-reduced fluorescence study of the carbon monoxide nd triplet Rydberg series: Experimental results and multichannel quantum defect analysis

The ndσ and ndδ (n = 4 ...9, ν = 0) triplet Rydberg series of carbon monoxide have been observed for the first time and rotationally resolved with an accuracy of 0.03 cm^-1 using a three-step excitation scheme. A multichannel quantum defect theory (MQDT) analysis shows strong perturbations for n = 5, 8, and 9 and predicts the positions of the sσ and dπ levels. There is significant mixing between the sσ and dσ states, with a mixing angle between 42 and 45 degrees, as well as some smaller interaction with the n p series. The ionization limit for CO is found to be 113 027.5(3) cm^-1 .