Energy conversion between CO and porphyrins on surfaces studied by ultrafast vibrational and scanning tunneling spectroscopies

Omiya, Takuma
Energy conversion between CO and porphyrins on surfaces studied by ultrafast vibrational and scanning tunneling spectroscopies. PhD thesis, University of Liverpool.

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Energy conversion between carbon monoxide (CO) and ruthenium tetraphenyl porphyrin (RuTPP) on Cu(110) surface has been investigated by means of vibrational sum frequency generation spectroscopy (SFG) and scanning tunneling microscopy (STM) in order to understand vibrational and chemical dynamics at surfaces. The study revealed that introducing porphyrins has considerable effect on the energy conversion between CO and copper, and also on the adsorbate dynamics, i.e. desorption of CO. CO/Cu(110), was first studied, showing that energy conversion between CO and copper becomes faster with increasing coverage of CO. This coverage dependence can be explained by the modification of the potential energy surface (PES) and gradual filling of density of states (DOS) around the Fermi level (EF). The results also indicate that the frustrated translation mode cannot be the dominant vibrational mode for electron coupling. For the study of the energy conversion between CO and porphyrin on Cu(110), the adsorption structure of RuTPP is first investigated using an STM, revealing that the ruthenium atom occupies the short bridge site of Cu(110). With increasing coverage of RuTPP molecules, surface supramolecular organization was formed and it was compared to theoretically calculated structures. The calculated structures are used for the modeling of the PES and DOS. The first discovery from CO-RuTPP/Cu(110) is the modification of the PES for the C-O stretch mode, showing a larger Morse anharmonicity cai_e and lower dissociation energy De than on a bare copper. The anharmonic constants are compared for various surfaces, which suggest the importance of considering local electric fields and the vibrational Stark effect to explain the large anharmonicity of oxidized and porphyrin covered surfaces. Inserting RuTPP also changes the desorption mechanism of CO by inelastic tunneling process from a one-carrier to a two-carrier process with lower threshold bias voltage. The resonant electron tunneling from occupied states of CO-RuTPP to an STM tip triggers CO desorption. The two-carrier process can be explained by tunneling of a second hole into an excited state, which was created by a hole tunneling into an adsorbate HOMO. On the other hand, facile laser-induced-desorption of CO was observed from CO-RuTPP/Cu(110), although, it shows a larger desorption energy of CO than on bare Cu(110). This can be explained by the enhancement of hot electron coupling via the DOS around EF. The coupling between the C-O stretch mode and hot electrons is also changed from a frequency redshift to a blueshift, indicating that the CO-Ru bond weakens, which can be caused by excitation of the CO-Ru stretch or bending of CO.

Item Type: Thesis (PhD)
Additional Information: Date: 2015-09 (completed)
Subjects: ?? QD ??
Depositing User: Symplectic Admin
Date Deposited: 22 Dec 2015 10:15
Last Modified: 17 Dec 2022 01:39
DOI: 10.17638/02036279