The spectrum and structure of singlet CH 2

Abstract

The red absorption bands of CH$_2$, previously shown to correspond to a transition between singlet states, have been analysed in detail. In the lower electronic state $\tilde a^1$A$_1$ the molecule is a strongly asymmetric top with rotational constants A$_{000}$ = 20$\cdot$14, B$_{000}$ = 11$\cdot$16 and C$_{000}$ = 7$\cdot$06 cm$^{-1}$. From these constants a CH distance of 1$\cdot$11 $\overset{\circ}{\mathrm A}$ and an HCH angle of 102$\cdot$4$^\circ$ is obtained. In the upper state b$^1$B$_1$ the molecule is nearly linear, having an HCH angle of about 140$^\circ$ and a CH distance of 1$\cdot$05 $\overset{\circ}{mathrm A}$. Most of the observed absorption bands correspond to excitation of the bending vibration $\nu'_2$ in the upper state (v'$_2$ = 6, 8, $\ldots$, 18). In some of them in addition the symmetric stretching vibration $\nu_1$ is singly excited. The levels 1, v$_2$-4, 0 cause strong perturbations in the levels 0, v$_2$, 0 for v$_2$ = 8, 10, 12, 14. In the lower state only one excited vibrational level 0, 1, 0 has been established which yields $\nu''_2$ = 1352$\cdot$6 cm$^{-1}$. Fragments of some other singlet absorptions in the near ultraviolet and in the vacuum ultraviolet have been observed and are briefly described. The upper state of the near ultraviolet system is probably the predicted $^1$A$_1$ state corresponding to $^1\Sigma^+$ in the linear conformation.