Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Solving the CH$_4^-$ riddle: the fundamental role of spin to explain
  metastable anionic methane   When atoms or molecules exist in the form of stable negative ions, they play
a crucial role in the gas phase chemistry. Determining the existence of such an
ion, its internal energy and its stability are necessary prerequisites to
analyze the role of this ion in a particular medium. Experimental evidence of
the existence of a negative methane ion CH$_4^-$ has been provided over a
period of 50 years. However, quantum chemistry had not been able to explain its
existence, and a detailed recent study has shown that the experimentally
observed species cannot be described by the attachement of an electron in the
ground state of CH$_4^-$. Here we describe CH$_4^-$ as being a metastable
species in its lowest quartet spin state and we find that this species is a
CH$_2^-$-:H$_2$ exciplex with three open shells, lying 5.8 eV above the methane
singlet ground state but slightly below the dissociation fragments. The
formation of charged exciplexes is a novel mechanism to explain small molecular
anions with implications in a plethora of basic and applied research fields.

37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
32
The categories of this paper are: atom-ph
31
The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
24
The categories of this paper are: dg-ga
20
The categories of this paper are: acc-phys
28
The categories of this paper are: alg-geom
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg