  Hydrogen bond is often assumed to be a purely electrostatic interaction
between a electron-deficient hydrogen atom and a region of high electron
density. Here, for the first time, we directly image hydrogen bond in real
space by room-temperature scanning tunneling microscopy (STM) with the
assistance of resonant tunneling effect in double barrier mode. STM
observations demonstrate that the C=O:HO hydrogen bonds lifted several angstrom
meters above metal surfaces appear shuttle-like features with a significant
contrast along the direction connected the oxygen and hydrogen atoms of a
single hydrogen bond. The off-center location of the summit and the variance of
the appearance height for the hydrogen bond with scanning bias reveal that
there are certain hybridizations between the electron orbitals of the involved
oxygen and hydrogen atoms in the C=O:HO hydrogen bond.
