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The driving force for CO desorption can be explained by either the electric field effect or inelastic scattering of the tunneling electrons. We can exclude an electric field effect, because it should result in a linear dependence of the desorption yield versus electric field (sample bias per tip-sample distance)322, unlike the dependence shown in figure 6.4.

Next, we discuss the direct excitation of vibrational modes by IET as a possible origin of CO desorption. The desorption yield is too low at the sample voltage of below 1 V to measure the yield within a reasonable time scale. The desorption energy of CO from RuTPP/Cu(110) is 0.75 eV which indicates that the C-O stretch mode with a vibrational energy of 250 meV is the most likely of all the vibrational modes to realize desorption. However, direct excitation of the C-O stretch mode is inefficient because of the lack of LDOS28 at the vibrational energy range around 250 meV. We can explain the increase in the desorption yield observed below -1 V and the subsequent plateau by the peak observed by STS around Vs=-1.1 V: while STS detects the LDOS at a specific Vs, the reaction yield in

action spectroscopy reflects the integral over the same LDOS. An overlaid plot of the LDOS integral for both bare RuTPP and CO-RuTPP shows the close correspondence between these two measurements in figure 6.4. The integral traces in figure 6.2 also show

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that we are not able to separate whether the process originates in a CO or a RuTPP-related peak. In either case, the IET process is enhanced by the CO-RuTPP HOMO or the pure RuTPP HOMO. A second threshold is observed at -1.5 V and corresponds again to a two electron process. We conclude that the IET process from the STM tip plays a dominant role in the desorption of CO between the Vs range of -1.075 and -1.5V. Since we are operating at

negative bias voltages, the underlying process must be a hole injection into the occupied states, which then leads to desorption of CO.

Desorption induced by hole injection and other chemical reactions have been previously reported in studies of adsorbates on metal substrates317,323-327. Applying a positive bias voltage does not show desorption of CO within a reasonable time scale, which can be assigned to a low LDOS of unoccupied states as seen in figure 6.2. It should be noted that applying a high voltage (Vs>1.5 V) can cause CO desorption non-locally which is assigned

to the electric field effect as previously reported244,245,316,317. CO desorption from RuTPP on Cu(110) has been already observed to begin at Vs=-1.075 V of figure 6.4. In contrast, CO

desorption from a copper surface requires a bias voltage higher than 2.4 V58. This difference cannot be explained by different desorption barriers, because the CO desorption temperature of CO-RuTPP is 80 K higher than that of CO/Cu(110) (as shown in figure 7.1 in next chapter). The desorption mechanism of CO from a copper surface is electron injection to the unoccupied 2* state, which occurs in a single-electron process58. A two-carrier process was not observed from CO on a copper surface, which can be explained by the short lifetime of the electronically excited state of around 0.8 to 5 fs58. We propose that the lower desorption threshold voltage of CO-RuTPP/Cu(110) is due to the two-carrier process as observed in figure 6.5.

The two-carrier induced IET process for a single molecule reaction in such a wide Vs range

is unique and so far has not been reported. At this moment, it is unable to conclude the details of desorption mechanism. Here, a probable scenario is described. Since our reaction thresholds are in the energy range of electronic excitations, we propose that the first hole excites CO-RuTPP to an electronically excited (positive ion) state. The second electron then leads to desorption by injecting a further hole. A single hole in the CO-RuTPP HOMO at -0.8 V would unlikely result in desorption: the HOMO of CO-metalloporphyrins is governed by the dxy orbital which has little overlap with the orbitals of CO

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as shown in figure 6.6, while an efficient IET process would require the localization of the tunneling electrons at the target chemical bond27. However, once a hole is injected in the CO-RuTPP or RuTPP HOMO, lower occupied states might become energetically accessible to the second hole. For example, HOMO-1 and HOMO-2 originate from the hybridization between metal dxz/dyz and CO * orbitals

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centre metal-CO. Since these states contain the target chemical bond, their excitation could lead to relatively efficient desorption via a vibrational excitation in analogy to desorption induced by electronic transition (DIET)66 or by simply withdrawing electrons from bonding states to induce repulsive CO-Ru potential. Such a process would be aided by an increase in the lifetimes of electronically excited states, which could originate in an electronic decoupling of ruthenium from the copper substrate by CO adsorption, analogous to what has been observed for CO on FePc on Au(111)243.

As an alternative mechanism, two holes could be created in two different orbitals. If one hole is created in the RuTPP derived occupied orbital at -1.1 V, and another is injected into the CO-RuTPP derived occupied orbital at -0.8 eV, and these orbitals show little spatial overlap, the ensuing repulsion between two positive ions could drive CO desorption.

The difference in desorption mechanism between CO from RuTPP and NO from CoTPP317 could lie in the degree of hybridization between CO/NO and TPP orbitals. NO is adsorbed in a tilted geometry, which means stronger hybridization between orbitals, which could make desorption possible over a wider energy range through carrier injection into any molecular orbitals.

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6.5 Conclusion

CO desorption by inelastic tunneling from CO-RuTPP on Cu(110) has been studied with a combination of STS and reaction yield measurements. Scanning tunneling spectroscopy shows different HOMO levels of both RuTPP and CO-RuTPP. CO desorption by inelastic tunneling has been observed by creating holes in the occupied states of CO-RuTPP/Cu(110). An efficient desorption compared to CO/Cu(110) is attributed to enhanced inelastic scattering through resonant tunneling. The two-carrier process is probably caused by tunneling of a second hole into an excited stated created by a hole tunneling into a RuTPP or CO-RuTPP HOMO. The desorption of CO from RuTPP/Cu(110) is further studied by femtosecond laser spectroscopy which is described in the next chapter.

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Chapter 7

Photodesorption of CO from CO-RuTPP on