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Rotational structure of emission bands of the PO molecule in the region 5300-3800 A is analyzed.The spectrum is attributed to 5 electronic transitions A2Z+-B2Z+, F2Z+-B2Z+, G 2 Z + -B 2 Z + , H 2 Z + -B 2 Z + , and I 2 Z + -B 2 Z + , where F, G , H , and I are the new electronic states and A and B are the upper states of the well-known y and B bands respectively. Practically all the new ' Z states are found to be perturbed. A qualitative account of these perturbations together with a deperturbation of certain levels is given. A number of cases of predissociation are also observed. This predissociation is attributed to the presence of 411, and A"Z+ states, which dissociate to the ground state atomic products. From this an upper limit of the dissociation energy of the ground state of PO is determined to be Do = 49 536 cm-'. The A, D, E, G, H , and I states of this molecule are assigned as Rydberg states corresponding to the 04s, n4p, 63d, 04p, 03d, and 05s orbitals, respectively. From them a value of 67 570 cm-' is evaluated for the first ionization potential of PO. All the electronic states established for this molecule are described in terms of electron configurations.La structure de rotation des bandes d'emission de la molCcule PO, dans la region spectrale allant de 5300 a 3800 A a CtC analysee. Le spectre est attribue a 5 transitionselectroniques A2Z+-B2Z+, F 2 Z + -B 2 Z + , G2Z+-B2Z+, H2Z+-B2Z+ and 12Z+-B2Z+, ob F, G , H , et I sont les nouveaux Ctats electroniques, et oh A et B sont les niveaux superieurs des bandes bien connues y et B. Presque tous les nouveaux etats ' 2 apparaissent comme etant perturbes. On donne une explication qualitative de ces perturbations et du fait que certains niveaux ne sont pas perturbes. Un certain nombre de cas de predissociation sont aussi observes. Cette predissociation est attribute a la presence d'etats 411, et A J 2 Z + , lesquels se dissocient en atomes dans I'etat fondamental. Ce qui donne, comme limite superie~~re de l'energie de dissociation de I'etat fondamental de la molecule de PO, Do = 49 536 cm-'. Les etats A, D, E, G , H et I de cette molecule sont identifies cornme etats de Rydberg correspondant aux orbitales 04s, n4p, 63d, 04p, 0 3 d e t 0 5 4 respectivement. De ces orbitales une valeur de 67 570 cm-' est deduite pour le premier potentiel d'ionisation de la molecule de PO. Tous les etats electroniques etablis pour cette molecule sont dtcrits en termes des configurations electroniques.

Four new bands in the red region are observed which have been described in terms of A2Σ+–B2Σ+ and F2Σ+–B2Σ+ systems. A rotational analysis together with deperturbation calculation of one band at 6763 Å has shown that A2Σ+ (ν = 7) and F2Σ+ (ν = 0) vibrational levels are involved in a homogeneous perturbation. The rotational structure study of three bands of a new transition I2Σ+–A2Σ+ has been carried out. From the study of heterogeneous perturbations observed in the I vibrational levels, it has been suggested that the perturbing state is a 2Π state arising from the 3d complex.

A new transition, A2Σ+–B2Σ+, of the PO molecule has been observed in the region 7000–12 000 Å. From the rotational analysis of the bands numerous perturbations have been observed in the ν = 0–5 levels of the A2Σ+ state. These perturbations have been shown successfully to be partly due to the B′2Π1 state and partly due to a new 4Σ− state. A shift has been detected in the A2Σ+, ν = 3, 4, and 5 levels, which has been attributed to a vibrational perturbation produced by the F2Σ+ state. A new G2Σ+–A2Σ+ transition also is reported.