Emission of H+ and CH
x
+ is observed for dodecanethiolate SAMs on Au(111) 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 σp orbitals constituted
mainly of carbon 2p and hydrogen 1s 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 σp 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 CH
x
+ desorption.