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

►Two lines are observed for the H stretch on graphene on Ir (1 1 1) using SFG. ► The two lines are due to the symmetric stretch modes in ortho and para dimers. ► The C-H bond is somewhat softer than for H-graphite. ► The line positions are close with recent theoretical predictions.