The vibrational assignment for the A 2Π–X 2Σ+ band system of the SiN radical: The 0–0 bands of 29SiN and 3SiN

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Abstract
The vibrational assignment for the A–X system of the SiN radical was reexamined by observing the spectra in the 5 μm region of 29SiN and 30SiN in natural abundance. The original assignment of Mulliken for the A state was found to be correct, rather than the one currently accepted, i.e., the 5 μm spectrum was assigned to the 0–0 band of the A–X system. The term value of the A state was thus modified to Te(A)=2032.4(1) cm−1 and the equilibrium internuclear distance in the A state was recalculated to be 1.641 879(20) Å, with the uncertainty in parentheses which was primarily due to that in Planck’s constant. The vibrational and isotopic variations observed for the Λ‐type doubling and the spin–orbit interaction constants of the A state and for the spin–rotation interaction constants in the X state were explained by treating the electronic matrix elements of the spin–orbit interaction and of the orbital angular momentum between the A/X, B/A, and D/X states as parameters.