Lifetime-separated spectroscopy: Observation and rotational analysis of the BaO A′ 1Π state

The BaO A′ 1Π−X 1Σ+ band system has been investigated using laser−induced fluorescence. The BaO is formed in the gas−phase reaction Ba + CO2 under single−collision conditions, and the A′ 1Π and A 1Σ+ states are simultaneously excited by a pulsed tunable dye laser with a 0.2 Å bandwidth. The weak, long−lifetime A′−X fluorescence is separated from the strong, short−lifetime A−X fluorescence by delaying the observation of the A′−X emission until the A−X emission dies away. The lifetime of the BaO A′ 1Π state is found to be 9±1 μsec, more than an order of magnitude longer than that of the the BaO A 1Σ+ state. We have observed the (v′,0) band progression for 9 ? v′ ⩽ 18 as well as the (10,1) and (13,1) bands of the A′−X system, and they are found to be represented by the A′ state vibrational constants ν00 = 17 573±10 cm−1, ω0 = 442.38±1.1 cm−1 and ω0x0 = 1.693±0.025 cm−1. The (12,0), (17,0), and (18,0) bands have been rotationally analyzed, yielding the rotational constants B12 = 0.20849±0.00022 cm−1, B17 = 0.20290±0.00012 cm−1, and B18 = 0.20109±0.00014 cm−1. These results confirm that the shapes of the potential curves for the A′ 1Π and a 3Π states of BaO are remarkably similar for low vibrational levels. We also conclude that the A′ 1Π state is not the emitter responsible for the many−line emission spectrum seen in the Ba + N2O and Ba + O3 reactions at low pressures, nor is it the ’’dark’’ precursor responsible for the high photon yields observed in these chemiluminescent reactions at high pressures.