Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

Vibrational sum-frequency generation (VSFG) spectroscopy was used to investigate the orientation and azimuthal anisotropy of the C–H stretching modes for propynyl-terminated Si(111) surfaces, Si—C≡C—CH_3. VSFG spectra revealed symmetric and asymmetric C–H stretching modes in addition to a Fermi resonance mode resulting from the interaction of the asymmetric C–H bending overtone with the symmetric C–H stretching vibration. The polarization dependence of the C–H stretching modes was consistent with the propynyl groups oriented such that the Si—C≡C– bond is normal to the Si(111) surface. The azimuthal angle dependence of the resonant C–H stretching amplitude revealed no rotational anisotropy for the symmetric C–H stretching mode and a 3-fold rotational anisotropy for the asymmetric C–H stretching mode in registry with the 3-fold symmetric Si(111) substrate. The results are consistent with the expectation that the C–H stretching modes of a –CH_3 group are decoupled from the Si substrate due to a −C≡C– spacer. In contrast, the methyl-terminated Si(111) surface, Si–CH_3, was previously reported to have pronounced vibronic coupling of the methyl stretch modes to the electronic bath of bulk Si. Vacuum-annealing of propynyl-terminated Si(111) resulted in increased 3-fold azimuthal anisotropy for the symmetric stretch, suggesting that removal of propynyl groups from the surface upon annealing allowed the remaining propynyl groups to tilt away from the surface normal into one of three preferred directions toward the vacated neighbor sites

Vibrational Sum Frequency Spectroscopic Investigation of the Azimuthal Anisotropy and Rotational Dynamics of Methyl-Terminated Silicon(111) Surfaces

Polarization-selected vibrational sum frequency generation spectroscopy (SFG) has been used to investigate the molecular orientation of methyl groups on CH_(3)-terminated Si(111) surfaces. The symmetric and asymmetric C–H stretch modes of the surface-bound methyl group were observed by SFG. Both methyl stretches showed a pronounced azimuthal anisotropy of the 3-fold symmetry in registry with the signal from the Si(111) substrate, indicating that the propeller-like rotation of the methyl groups was hindered at room temperature. The difference in the SFG line widths for the CH_3 asymmetric stretch that was observed for different polarization combinations (SPS and PPP for SFG, visible, and IR) indicated that the rotation proceeded on a 1–2 ps time scale, as compared to the 100 fs rotational dephasing of a free methyl rotor at room temperature

Vibrational Sum Frequency Generation (VSFG) Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces

The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120° jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 °C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 ± 360 cm^(–1) and an attempt frequency of (2.9 ± 4.2) × 10^(13) s^(–1) for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation

Orientational Dynamics in Sum Frequency Spectroscopic Line Shapes

We present a general response function formalism describing the contribution of orientational dynamics of molecules at interfaces to spectroscopic line shapes in vibrational sum frequency generation (SFG). When reorientation occurs on the time scale comparable to vibrational dephasing, its dynamics can be extracted from polarization-selected SFG spectral line shapes. Unique features of orientational motion at interfaces are (1) the anisotropic case-specific equilibrium orientational distribution and (2) possible dynamic anisotropy (e.g., different in-plane versus out-of-plane relaxation rates), both of which must be taken into account. Within the small-step rotational diffusion model, we present solutions for two cases, the weak-confinement model, applicable when the deviations from the isotropic case are not severe, and the wobbling-in-a-cone model, which considers a hard-wall orienting potential. SFG line shapes are calculated for a rod-like chromophore as a function of the rotational diffusion rate. For certain equilibrium orientational distributions, orientational dynamics may result in anomalous bi-Lorentzian line shapes (two Lorentzians of different widths centered and the same frequency)

Vibrational Sum-Frequency Spectrum of the Water Bend at the Air/Water Interface

We present the spectrum of the water bend vibrational mode (ν₂) at the air/water interface measured using vibrational sum-frequency generation (SFG). The blue-shift of the ν₂ frequency from the gas phase value reports on the hydrogen bonding in the interfacial region. The ν₂ line shape of surface water is inhomogeneously broadened and structured. The dominant feature is the least blue-shifted and relatively narrow Lorenztian, tentatively assigned to water molecules straddling the interface, those with free OH bonds. This feature appears at different frequencies in the SFG spectra recorded using different polarization combinations (SSP and PPP for SFG-visible-IR), pointing to possible orientational inhomogeneity. Weaker features are observed at higher frequencies, tentatively assigned to fully H-bonded water molecules. Small but measurable changes of the line shape with temperature are observed in the 0 to +20 °C range