Spectroscopy of the B(1)(υ^′=3-6)-X(0⁺)(υ^″=0) transitions of PbO with 10-MHz precision

We have performed high precision spectroscopy of lead oxide (PbO) molecules. The B(1)(υ^′=3-6)-X(0⁺)(υ^″=0) transitions located in 398–423 nm were investigated by using a cold and slow PbO beam. About 140 lines were measured by comparing with resonances of an ultralow expansion etalon, which allows us to determine absolute frequencies of these lines with the uncertainty of about 10 MHz. The υ^′=4 level has considerably different spectroscopic constants from other vibrational levels. This can be explained by a homogeneous perturbation due to a near-degenerate vibrational level in the a(1) state.