Newly observed low-lying Ω = 1 state of PbO

High-resolution spectroscopy of lead monoxide was performed in a range of 22 400–25 300 cm−1. A new Ω = 1 state located between the a1 and A0+ states was observed, and it is labeled c1. Spectroscopic constants, including the hyperfine interaction coefficient, were determined for the a1 and c1 states. The vibrational levels of these two electronic states are located closely to each other, and the interaction between them causes gradual exchange of electronic state properties in our observation wave number range. Our observation poses a question for the band assignment for the b0− state, which has some resemblance with this c1 state.