Emission of H+ and CHx+ is observed for dodecanethiolate SAMs on Au(1 11) surfaces under irradiation of the slow helium metastable atom beam at a thermal energy. Metastable atom deexcitation spectroscopy measurements show that impinging metastable helium atoms predominantly interact with up orbitals constituted mainly of carbon 2p and hydrogen Is atomic states of the methyl group at the top end of the alkane chain of the dodecanethiolate molecule. These results indicate that the hole created in the sigma(p) orbitals leads not only to C-H bond scission followed by H+ desorption but also to C-C bond scission between the methyl group and the neighboring carbon atom followed by CHx+ desorption.