Knowledge Bank

High resolution Fourier transform emission spectrum of the $B2\Sigma +-X2\Sigma +$ band of the $PN+$ ion was observed in the ultra violet region around $31000cm-1$ with Bruker IFS120HR. The rotationally resolved spectrum of $PN+$, produced by an ac discharge of the $PCl3,N2$, and He gas mixture, was recorded for about 11 h accumulating the interferogram 1600 times. The monochromator placed just before a photomultiplier worked effectively as an optical filter with a band width of $200cm-1$. The (0,0) band lines split into doublet due to the spin-rotation splitting were assigned to the R- and P- branch lines of N'' up to 35. The determined band origin $\nu 0=31058.9963(81)cm-1$ is consistent with the previous low resolution result [1]. The rotational, centrifugal distortion constants for both states were determined as well as the spin-rotation constant for the ground state. The $B2\Sigma +$ state has weaker bond than that of the ground state, since the bond length of the $B2\Sigma +(v=0)$ state (1.570 {\AA}) is by 0.076 {\AA} longer, and the force constant ($k=2.90$ mdyne/{\AA}) is by about 70% smaller than those of the ground state. Present results support the ab initio calculation which predicts a shallow minimum for the $B2\Sigma -$ state due to avoided crossing [2]. 1. I.K. Ahmad and P.A. Hamilton, J. Mol. Spectrosc., 163, 214 (1994). 2. F. Grein, Chem. Phys. Lett., 120, 383 (1998).