The stretching vibration of hydrogen adsorbed on epitaxial graphene studied by sum-frequency generation spectroscopy

IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a graphene sheet prepared on an Ir (1 1 1) crystal surface. We observe two distinct resonances at 2563 and 2716 cm−1. Similarly, lines at 1881 and 2027 cm−1 are found after D atom adsorption. The assignment to C–H stretching vibrations is discussed in view of the propensity for dimer formation reported for hydrogen adsorbed on graphene. The lines are assigned to hydrogen bound in either ortho or para configuration to the hexagonal mesh of carbon atoms.