Rotational analysis of the 0–0 band of the²A₁–²B₁electronic transition of PH₂

The long wavelength bands of the visible absorption spectrum of PH$_2$ have been rephotographed with much greater intensity than in the earlier work of Ramsay. A detailed rotational analysis has been carried out for the 0-0 band. The transition is of the type $^2$A$_1$-$^2$B$_1$ and the rotational constants lead to the following molecular parameters: ground state, $r''_0$ = 1.429 $\overset{\circ}{\mathrm A}$, valence angle = 91$^\circ$ $40'$; excited state, $r'_0$ = 1.401 $\overset{\circ}{\mathrm A}$, valence angle = 123$^\circ$ $4'$. The spectrum shows doublet splittings up to 9 cm$^{-1}$. The upper state is subject to very large centrifugal distortion which modifies the term values by ca. 70 cm$^{-1}$ at $K'_a$ = 7. 464 lines have been assigned of which 373 belong to branches with $\Delta$K$_a$ = $\pm$ 1, 89 to branches with $\Delta$K$_a$ = $\pm$ 3 and two to branches with $\Delta$K$_a$ = \pm 5. The relative intensities of the various branches agree well with the values calculated from rigid rotor line strengths, when due allowance is made for the large change in the asymmetry parameter on excitation ($\Delta\kappa$ = -1.4). Observation of the 0-1 and 1-1 bands leads to a $\Delta$G($\frac{1}{2}$) value of $v''_2$ = 1101.4 cm$^{-1}$.