Ab initio study of the reactions of Ga(2P, 2S, and 2P) with methane

The interactions of Ga(2P:4s(2)4p1, 2S:4s(2)5s1, and 2P:4s(2)5p1) with CH4 is studied by means of Hartree-Fock self-consistent-field (SCF) calculations using relativistic effective core potentials and multiconfigurational-SCF plus multireference variational and perturbational on second-order Möller-Plesset configuration interaction calculations. The Ga atom 2P(4s(2)5p1) state can spontaneously insert into the CH4. In this interaction the 4 2A potential energy surface is initially attractive and becomes repulsive only after meeting with the 3 2A surface, adiabatically linked with the Ga(2S:4s(2)5s1) + CH4 fragments. The Ga atom 2S(4s(2)5s1) excited state inserts in the C-H bond. In this interaction the 3 2A potential energy surface initially attractive, becomes repulsive after meet the 2 2A' surface linked with the Ga(2P:4s(2)4p1) + CH4 fragments. The two 2A curves (2 2A and X 2A) derived from the interaction of Ga(2P:4s(2)4p1) atoms with methane molecules are initially repulsive. The 2 2A curve after an avoided crossing with the 3 2A curve goes smoothly down and reaches a minimum: after this point, it shows an energy barrier. The top of this barrier is located below the energy value of the Ga(2S:4s(2)5s1) + CH4 fragments. After this energy top the 2 2A curve goes down to meet the X 2A curve. The 2 2A curve becomes repulsive after the avoided crossing with the X 2A curve. The X 2A curve becomes attractive only after its avoided crossing with the 2 2A curve. The lowest-lying X 2A potential leads to the HGaCH3 X 2A intermediate molecule. This intermediate molecule, diabatically correlated with the Ga(2S:4s(2)5s1) + CH4 fragments, which lie 6 kcal/mol, above the ground-state reactants, the dissociation channels of this intermediate molecule leading to the GaH + CH3 and H + GaCH3 products. These products are reached from the HGaCH3 intermediate without activation barriers. The work results suggest that Ga atom in the first excited state in gas-phase methane molecules could produce better quality a-C:H thin films through CH3 radicals, as well as gallium carbide materials.