Infrared spectroscopy of CO on NaCl(100). II. Vibrational dephasing and band shapes

Abstract

Temperature dependence of the fundamental vibrational transition of CO on NaCl(100) single crystal surfaces has been measured between 4 and 55 K using Fourier-transform infrared spectroscopy. The absorption profiles at various temperatures are very nearly symmetric. At 4 K, the transition is centered at 2154.93 cm−1, and the full-width at half-maximum (FWHM) is 0.17 cm−1; at 55 K, the transition shifts to 2155.14 cm−1, and the FWHM broadens to 0.39 cm−1. The temperature-dependent frequency shift and broadening of the fundamental transition are interpreted by a vibrational phase relaxation process, in which the coupling between the stretching mode of CO and a low frequency CO-surface mode causes the dephasing of the excited state vibrational motion. The Boltzmann factor associated with the spectroscopic temperature dependence allows us to determine that the surface mode has a frequency of 40 cm−1 . We associate this mode with the frustrated translational motion of CO over the NaCl(100) surface. The observed temperature-dependent absorption band undoubtedly is a consequence of collective motions among the CO stretching vibrations within the monolayer. However, the current model, that assumes this collective motion is coupled by the CO transition dipoles, cannot explain the data. Further theoretical work will be required to understand the infrared spectroscopy of CO on NaCl(100). Below 10 K, the CO fundamental absorption becomes temperature independent. However, the limiting bandwidth and bandshape depend on the NaCl(100) crystal surface preparation. For one set of crystals, the 4 K bandshape was Lorentzian with a FWHM of 0.17 cm−1 . For another set, the 4 K bandshape was Gaussian with a FWHM of 0.11 cm−1. We believe these bandwidths, narrower than those reported previously for any adsorbate system, are limited by heterogeneities of the monolayer and/or substrate. The 0.11 cm−1 bandwidth provides a lower limit of 45 ps for the lifetime of vibrationally excited CO on NaCl(100).

Imaging polycrystalline and smoke MgO surfaces with atomic force microscopy: a case study of high resolution image on a polycrystalline oxide

“…On a recent contribution [40], it is reported that the FWHM of CO adsorbed on MgO(0 0 1) single crystals cut in vacuo is greater than the figure obtained for CO on halide single crystals [41,42]. This means that, although electron micrographs and atomic force images of MgO single crystals are suggesting that only regular (0 0 1) faces are exposed, the real faces must still be quite inhomogeneous at the atomic level, due to the presence of steps one or two atomic layers thick and of kinks, as revealed by high resolution AFM [19,43], LEED and STM [44].…”

Probing defect species on real surfaces from the analysis of the spectral profile of admolecules

“…The present approach could therefore be improved to account for the lateral interactions, and it should also include a better treatment of the dynamical effects connected to the presence of defects on the surface to provide direct comparison with experiments [13][14][15][16][17], using CO and CO 2 as probes of MgO. We are aware that these effects are poorly described here since the relaxation processes responsible in particular for the line broadening in the infrared spectrum are assumed to be the same for perfect and real surfaces.…”

“…Molecules such as CO, CO 2 , NH 3 , etc. physisorbed on ionic and semiconducting surfaces are well-known [13][14][15][16][17] to display strong intensity spectral signals and they can be used as efficient probes of surface morphology. Moreover, favoring the admolecule migration towards the defects on the surface by controlled temperature annealing should enhance the defect signature and provide optimal conditions for the analysis of the defect nature and concentration.…”

Infrared spectroscopy of monolayer CH4 on NaCl(100)

“…8 is consistent with a vibrational dephasing mechanism. 40 , [63][64][65][66] Line shift and broadening can occur when the phase of a vibration is interrupted by an external perturbation. In adsorbed systems dephasing arises from the anharmonic coupling of a high-frequency internal mode (e.g., the molecular vibration) to a thermally excited low-frequency, high-amplitude external mode of the same symmetry (e.g., a surface vibration) .40 If the lifetime of the low-frequency vibration is shorter than the high-frequency mode, then the vibrational motion of the high-frequency excited state will be interrupted and its phase shifted by the thermal fluctuation of the populations in the external states.…”

“…Treatment of this mechanism as applied to surface molecules results in the following equations for the linewidth and absorption frequency: [63][64][65][66] (2'1TC) (8V)21'g exp( -EJkT)…”