Solving the ${\mathrm{CH}}_4^{-}$ Riddle: The Fundamental Role of Spin to Explain Metastable Anionic Methane

Several types of experiments showed the existence of negative methane ions ${\mathrm{CH}}_4^{-}$ over a period of 50 years but the nature of this elusive species remains unknown. A benchmark study has shown that the experimentally observed species cannot be described by the attachment of an electron in the doublet ground state of ${\mathrm{CH}}_4^{-}$. Here we find ${\mathrm{CH}}_4^{-}$ as being a metastable species in its lowest quartet spin state, a ${\mathrm{CH}}_2^{-}:{\mathrm{H}}_2$ exciplex with three open shells lying ca. 10 eV above the methane singlet ground state but slightly below the dissociation fragments. The formation of charged high-spin exciplexes is a novel mechanism to explain small molecular anions with implications in a plethora of basic and applied research fields.

Figure 1

Energy evolution of the excited quartet state of ${\mathrm{CH}}_4^{-}$ at 300 K. The deep about 8 $ps$ indicates the appearance of the ${\mathrm{CH}}_2^{-}:{\mathrm{H}}_2$ exciplex. Metastability with respect to the separate ${\mathrm{CH}}_2^{-}(S=3/2)+{\mathrm{H}}_2$ ($S=0$) fragments was found, at least, over the next 1 $ns$.

Figure 2

Vibrational spectrum of the lowest quartet state of ${\mathrm{CH}}_4^{-}$ at 300 K; the lowest frequency peak at $180{\mathrm{cm}}^{-1}$ corresponds to the intermoeity stretching mode of the ${\mathrm{CH}}_2^{-}:{\mathrm{H}}_2$ exciplex. Intensity (arb. units) vs energy (${\mathrm{cm}}^{-1}$).