Real-Time Observation of Molecular Motion on a Surface

Abstract

The laser-induced movement of CO molecules over a platinum surface was followed in real time by means of ultrafast vibrational spectroscopy. Because the CO molecules bound on different surface sites exhibit different C-O stretch vibrational frequencies, the site-to-site hopping, triggered by excitation with a laser pulse, can be determined from subpicosecond changes in the vibrational spectra. The unexpectedly fast motion--characterized by a 500-femtosecond time constant--reveals that a rotational motion of the CO molecules, rather than pure translation, is required for this diffusion process. This conclusion is corroborated by density functional theory calculations.

Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)

“…The observed quite large population change of hot chemisorbed CO molecules transferring towards higher coordination sites suggests a broad lateral distribution of vibrationally excited chemisorbed molecules consistent with a rather shallow corrugation in the lateral potential energy surface; this is similar to what has been determined by temperature-dependent line shape variations of, e.g., O 1s spectra for CO on Ni(100) [28,29]. This can also be interpreted as large amplitudes of coupled frustrated translations and rotations like those observed in the vibrational spectroscopy SFG experiments [1][2][3][4]. We conclude that the majority of molecules are vibrationally excited at this fluence but do not leave the chemisorbed state.…”

“…Figure 3 shows selected AIMD snapshots representing the relevant processes leading to CO desorption; because of the high cost of these simulations, too few trajectories for reliable statistics could be run. Similar to the case of CO on Pt(533) [1] our simulations show that translational motion is activated by the frustrated rotational vibrations, visualized by the severely tilted CO molecules in snapshot (a) in Fig. 3.…”

“…2(c). The temporal evolution of the spectroscopic features resembles the transient responses observed with vibrational spectroscopy for CO that were interpreted as a heating of the molecule leading to diffusion [1] or desorption [2][3][4] largely driven by frustrated rotations, that typically couple to the electron system on a subpicosecond time scale [1,27]. The apparent stronger interaction evidenced by the increasingd intensity and the redshift with intensity lowering of the 2~Ã indicates that the majority of CO molecules moves away from the on-top site towards more highly coordinated bridge and hollow sites [8,22].…”

“…This technique has been successfully utilized, e.g., to monitor transient excitation of external adsorbate vibrational modes that couple to diffusion [1] and to desorption reactions [2][3][4].…”

Vibrational overtone excitation in electron mediated energy transfer at metal surfaces

“…Developing better understanding of interfacial energy conversion at the atomic level is particularly promising in studies at the gas-solid interface. Here, sophisticated dynamics methods including molecular beams 8 and laser spectroscopy 9 can be combined with surface analytical probes to create well defined experiments, the observations of which can be directly compared to first principles theory. [10][11][12] This approach has led to significant insights which have been reviewed on several occasions.…”