The interactions of Ga(P-2:4s(2)4p(1), S-2:4s(2)5s(1), and P-2:4s(2)5p(1)) 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 Moller-Plesset configuration interaction calculations. The Ga atom P-2(4s(2)5p(1)) state can spontaneously insert into the CH4. In this interaction the 4 (2)A' potential energy surface is initially attractive and becomes repulsive only after meeting with the 3 (2)A' surface, adiabatically linked with the Ga(S-2:4s(2)5s(1))+CH4 fragments. The Ga atom S-2(4s(2)5s(1)) excited state inserts in the C-H bond. In this interaction the 3 (2)A' potential energy surface initially attractive, becomes repulsive after meet the 2 (2)A' surface linked with the Ga(P-2:4s(2)4p(1))+CH4 fragments. The two (2)A' curves (2 (2)A' and X (2)A') derived from the interaction of Ga(P-2:4s(2)4p(1)) atoms with methane molecules are initially repulsive. The 2 (2)A' curve after an avoided crossing with the 3 (2)A' 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(S-2:4s(2)5s(1))+CH4 fragments. After this energy top the 2 (2)A' curve goes down to meet the X (2)A' curve. The 2 (2)A' curve becomes repulsive after the avoided crossing with the X (2)A' curve. The X (2)A' curve becomes attractive only after its avoided crossing with the 2 (2)A' curve. The lowest-lying X (2)A' potential leads to the HGaCH3 X (2)A' intermediate molecule. This intermediate molecule, diabatically correlated with the Ga(S-2:4s(2)5s(1))+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. (C) 2004 American Institute of Physics.