2018ApJS..236...34Y

This work investigates the transition dipole moments (TDMs) and transition probabilities of electric dipole emissions between the X²Π, B²Σ⁺, B^'2Π, D^'2Π, C²Σ^–, C^'2Δ, F²Σ⁺, and P²Π states of the PO radical. The TDMs of 23 pairs of states are calculated by the internally contracted multireference configuration method with the aug-cc-pV6Z basis set. The vibrational band origins, Franck-Condon factors, and Einstein coefficients of all the spontaneous emissions are evaluated. The rotationless radiative lifetimes of the vibrational levels are approximately 10^–7-10^–8 s for the B²Σ⁺, C²Σ^–, C^'2Δ, P²Π, and F²Σ⁺ states; 10^–4-10^–5 s for the B^'2Π state; and 10^–1-10^–2 s for the D^'2Π state. The Einstein coefficients of many emissions are large for the B²Σ⁺-X²Π, B^'2Π-X²Π, C^'2Δ-X²Π, C²Σ^–-X²Π, F²Σ⁺-X²Π, P²Π-X²Π, P²Π-B^'2Π, and P²Π-D^'2Π systems. Almost all the spontaneous emissions arising from the D^'2Π state are very weak. The vibrational band origins of these emissions extend from the UV into the far-infrared spectra. The radiative lifetimes and vibrational band origins are compared with available experimental and theoretical values. According to the radiative lifetimes and transition probabilities obtained in this paper, some guidelines for detecting these states spectroscopically are proposed. The TDMs and transition probabilities reported here are considered to be reliable and can be used as guidelines for detecting similar transitions, especially those in interstellar space.