Study of the spectrum of (3, 1) band in the d3Δ-a3Πsystem of CO

Six hundred and sixty-one spectral lines of the (3, 1) band in the d3Δ-a3Π system of CO in the range of 13,410-13,920 cm−1 were observed employing optical heterodyne magnetic rotation enhanced concentration modulation spectroscopy. Improved molecular constants of the upper vibrational level involved were obtained by a non-linear least-squares weighted fitting procedure using effective Hamiltonians with those of the lower levels fixed at the values obtained in the sub-millimeter region, and the improvements are around one order of magnitude.