Optically excited near-surface phonons of TiO(2) (110) observed by fourth-order coherent Raman spectroscopy.

We observed the fourth-order and third-order optical responses in the time domain on a TiO(2) (110) surface covered with trimethyl acetates. Coherent vibrations assignable to near-surface phonon modes were present at 179, 191, 359, 440, 507, 609, and 823 cm(-1) in the fourth-order responses. The amplitude and phase of each mode were determined with different azimuths and polarizations of pump and probe light pulses. Vibrational assignments and possible mechanisms to excite the vibrations were discussed.