Comparison with recent theoretical work

Stimulated by the orientational isomerization reported here, several theoretical stud-ies [142,162–164] have been published dealing with different aspects of the isomerization dynamics and the properties of the two isomers in the CO/NaCl(100) system. In principle, three possible scenarios exist that could explain how the isomerization occurs although the activation barrier remains large even for high vibrational states according to the electrostatic model (see Section 5.1.4): (1) The DOFs that are frozen in the electrostatic model, in particular the distance from the surface, could play an important role in lowering the barrier. (2) The isomerization dynamics cannot be fully captured by the vibrationally adiabatic model, which accurately describes the vibrational frequency shifts. In this case, weak anharmonic coupling between the C-O stretching vibration and the frustrated rotation of CO could transfer energy to the frustrated rotational mode in a relaxation or VEP event, which is used to overcome the activation barrier. This scenario was also proposed in Ref. [162]. (3) Tunneling effects at energies below the isomerization threshold could play a role, although this explanation seems less likely given the relatively high mass of CO.

Ref. [142] reports density functional theory (DFT) calculations based on the PBE functional with different dispersion corrections that can reproduce the C-down adsorption structure of CO on NaCl(100) with good accuracy. The DFT calculations predict stable O-down structures (𝜃 =120-145°for different functionals) with harmonic vibrational frequencies that are red-shifted with respect to the C-down structures and the unperturbed gas phase molecule by a few wavenumbers, in agreement with the experiment. In addition, the authors show that the relative stability of the O-down and C-down structures inverts in favor of O-down for high vibrational states if CO is positioned above Na⁺, similar to what is predicted by the electrostatic model (see Section 5.1.4). It should, however, be noted that the DFT calculations are performed for a single CO molecule inside(1×1)and(2×2)unit cells with periodic boundary conditions. Therefore, all CO molecules flip simultaneously, which will not quantitatively represent the experiment.

Ref. [163] reports a PES for a cluster of 12 CO molecules adsorbed on a single, frozen surface layer of NaCl, which is composed of contributions from a CO-NaCl PES (diatom-diatom potential with the Na-Cl distance fixed at the NaCl bulk value) [163]

and a recently published full-dimensional CO-CO PES [162]. Also this work, which is based on highly accurate wavefunction methods, predicts a stable O-down isomer with

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5.2 Discussion a similar tilt angle of the C-O bond axis (51°) as derived from the electrostatic model (46°). The tilt angle is, however, lower than the experimentally determined value for the buried monolayer (∼69°, see Appendix C). According to the calculations in Ref. [163], the O-down isomer becomes stabilized for large C-O bond distances, resembling the outer turning points of high vibrational states. Furthermore, the transition state for high vibrational states lies at large distances from the surface centered around 𝑧 ≈ 5.5 Å, which is significantly further away than the equilibrium bond distances for C-down (𝑧 ≈3.0 Å) and O-down (𝑧 ≈ 2.8 Å). The authors compare the isomerization further away from the surface with a roaming reaction, a unimolecular reaction channel in the gas phase based on near-dissociation events. [165,166]

The authors of Ref. [163] also performed quasiclassical trajectory calculations for high vibrational states (𝑣 =22) in which isomerization and desorption events are observed.

However, it should be noted that these trajectories start at the equilibrium adsorption geometry of the C-down isomer in the vibrational ground state, which is significantly shifted from the equilibrium geometry in high vibrational states. Isomerization (and desorption) is observed because the energy of this starting geometry lies above the isomerization threshold. This “direct excitation” to𝑣 =22 associated with additional potential energy is inconsistent with the experimental conditions, in which high vibrational states will be populated by sequential one-quanta energy transfer processes—potentially allowing for dissipation of the additional energy deposited in the low frequency rotational mode. The work does, however, show that considering other DOFs, in particular the 𝑧-distance, is an important factor to decrease the barrier.

Finally, DFT calculations based on the PBE-D2 functional in Ref. [164] confirm many of the findings in Refs. [142,162,163] but also consider the possibility of a different isomerization mechanism. The authors found that the vibrational states obtained for a 2D potential, depending only on the𝑧-distance and the tilt angle, and a 3D potential, which is additionally a function of the C-O bond distance, exhibit several quasi-degenerate pairs of states. For these pairs, one of the states is located in the C-down well and the other state is located in the O-down well. If these quasi-degenerate states also exist for the full-dimensional PES and have appreciable overlap, it can potentially lead to isomerization below the classical barrier. So far, this mechanism is only hypothetical as no overlap was observed for two dimensions. It could, however, be relevant to the lifetimes for back-conversion of the O-down isomer to the C-down isomer in the vibrational ground state, where vibrationally non-adiabatic CO(𝑣) →CO(𝑣−1) transitions cannot occur.

The discussed theoretical studies clearly lead to an improved understanding of the orientational isomerization. However, all these studies predict similarly high barriers for isomerization in high vibrational states as the electrostatic model and are thus not able to

Chapter 5 Laser-induced orientational isomerization in the CO/NaCl(100) monolayer explain how isomerization from C-down to O-down can occur. So far, breakdown of the vibrationally adiabatic approximation seems to be the most likely explanation, leading to energy transfer from the CO stretching vibration to the frustrated rotational modes.

However, also tunneling contributions cannot be excluded at the present stage.

Another alternative mechanism involving vibration-to-electronic (V-E) energy transfer from high vibrational states of the electronic ground state to the 𝑎 ³Π state is also conceivable, as it could potentially reduce the isomerization barrier due to the large magnitude of the dipole moment in the electronically excited state (1.4 D). [161]

This hypothesis has not been considered in the theoretical calculations and also the experimental emission spectra presented in this chapter do not contain information on the excited electronic state. It could, however, be tested experimentally by either monitoring infrared fluorescence after direct excitation of the𝑎³Πstate or by observing UV phosphorescence from the𝑎³Πstate populated through VEP and subsequent V-E transfer.

In conclusion, further experiments and calculations are needed for a complete understanding of the orientational isomerization dynamics. Pump-probe experiments that directly track the isomerization dynamics from quantum states of the C-down isomer to states of the O-down isomer would be incredibly helpful to investigate the role of vibrationally non-adiabatic transitions or the𝑎³Πstate. Such experiments are, however, challenging due to the coupled vibrational dynamics when the high vibrational states are prepared via VEP. Furthermore, the repetition rate of the experiment would have to be adapted to allow for back-conversion of the O-down orientation before each laser excitation cycle. Therefore, molecular dynamics calculations based on the above-mentioned full-dimensional PESs are invaluable to interpret the experimental observations presented in this chapter.

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C H A P T E R 6

Mid-infrared light harvesting to promote orientational CO isomerization

In Chapters 4 and 5, a thorough understanding of the VEP dynamics within the monolayer was obtained. In this chapter, the vibrational energy transport across isotopically layered CO monolayer-multilayer samples will be investigated. I will show that vibrational energy transport can be controlled by choosing the right isotopic substitution patterns. For a

13C¹⁸O monolayer sample with about 100¹²C¹⁶O overlayers on top, energy transport from the overlayer to the monolayer results in higher vibrational excitation than possible with direct excitation of the monolayer and can efficiently drive the orientational CO isomerization discussed in Chapter 5. This effect will be discussed in analogy to solar light harvesting systems. Furthermore, the mechanism for vibrational energy transport across the interface will be discussed. The results presented in this chapter have been first published in Ref. [130] by Springer Nature.

6.1 Results

6.1.1 Preparation and characterization of isotopically layered samples