Infrared determination of C for phosphine via perturbation-allowed Δ |k − l|= ± 3 transitions in the 3ν2 band

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Abstract
The 3ν2(A1) and 4ν2−ν2(A1) infrared bands of PH3 have been measured with high resolution. In the ν2=3 state an interaction between the K and K±3 levels gives rise to perturbation allowed ΔK = 3 transitions through a weak high order interaction. Since only one component of the K = 3 levels is of the proper symmetry to interact with the K = 0 levels there is a splitting of the K = 3 levels. Also detectable is the splitting of the K = 3 levels of the ground vibrational state. The measurements have been combined with microwave measurements to give accurate values for the ground state rotational constants B0, C0, D0J, D0JK, D0K, H0J, H0JJK, and H0JKK. The absence of observable inversion effects sets an upper limit of about 0.02 cm−1 for the inversion splitting of the 4ν2 level.