Understanding adsorbate dynamics on surfaces is crucial to many phenomena, ranging from heterogeneous catalysis to sensing devices. Over the past 30 years, time-resolved vibrational and electronic spectroscopies, based on femtosecond laser pulses, have made many important contributions to our understanding, with the investigation of vibrational excitation, diffusion, desorption, and reaction [1
]. Due to the high absorption cross-section of metal surfaces, compared to most adsorbates, the photon energy is first deposited in the metal electrons followed by energy transfer to phonons and the adsorbate. The interpretation of spectra, therefore, generally rests on the fact that hot electrons exist for around 1 ps before equilibration with phonons. Thus, any sub-picosecond response is thought to represent a signature of nonadiabatic dynamics, where energy is directly transferred between hot electrons and the adsorbate vibrational modes, while changes on the tens-of-picoseconds timescale relate to hot phonons and adiabatic dynamics.
Due to the large energy difference between the typically observed internal stretching (IS) mode of CO or NO and the energy scale of hot electrons, the generally accepted model involves direct energy transfer between hot electrons and low energy vibrational modes, which, in turn, couple anharmonically to the observed high energy mode. The majority of studies have focused on the vibrational dynamics of CO, which, so far, have been generally attributed to coupling between hot electrons and the frustrated rotation (FR) mode; for example, on Ru(001) [4
] and Pt(111) [5
]. The origin of the observed redshift of the C–O stretch, while the surface electrons are hot, is thought to be caused by excitation of the frustrated translational (FT) mode, which is indirectly heated through the frustrated rotation, as modeled by Ueba and Persson [6
]. The FR is then thought to be responsible for transferring the molecule into the transition state for diffusion [8
] or desorption [9
]. This model was further supported by ultrafast photoelectron spectroscopy, which suggested that the CO-Ru bond coordination increases in the first picosecond after pump excitation, due to the excitation of the FR mode [10
]. A phase-resolved measurement of the pump-perturbed free-induction decay of CO/Pt(111), in addition, proposed excitation of the CO-metal (M) mode, by noticing that the redshift of the C–O frequency was followed by a rapid reverse (blue) shift in the first picosecond, under desorption conditions [11
]. Phase-resolved sum frequency measurements of CO/Cu(100) were even used to extract different mode temperatures for IS and FR modes [12
In recent years, theory has made great strides in including non-adiabatic effects in adsorbate dynamics on metals [13
], and is now beginning to challenge this picture of energy transfer with the surprising result that phonons contribute significantly to the vibrational and desorption dynamics on sub-picosecond timescales [14
]. Experimental approaches have focused on modifying the degree of coupling between hot electrons and high energy vibrational modes, by studying CO adsorbed on nanoparticles [17
] and addressing the influences of coverage [19
], adsorption site [20
], and surface temperature [21
Here, we attempt to further our understanding of nonadiabatic coupling between electrons and vibrations and modify the vibrational dynamics by decoupling CO from the bulk metal through coordination to a metalloporphyrin on a metal substrate. The interest in these systems stems from the profound influence which small ligands can have on the adsorbed metalloporphyrin’s electronic and magnetic properties. For example, coordination of CO to bulk ruthenium tetraphenylporphyrin (RuTPP) increases the RuTPP excited state lifetime 100 times, by switching the relaxation pathway from a singlet to a triplet state [22
]. For metalloporphyrins adsorbed on metal substrates, coordination of NO decouples the metal ion from the metal substrate, partly reversing changes in the electronic structure caused by interaction with the underlying metal and, in addition, changes the spin state of the metal ions [23
]. A recent inelastic tunneling study of CO, coordinated to RuTPP adsorbed on Cu(110), revealed that CO desorption by injection of holes from a scanning tunneling microscope tip proceeds in an unusual two-carrier process, which could be related to CO decoupling the Ru ion from the copper surface [24
]. Existing time-resolved studies have only been carried out on CO ligands at heme proteins. Upon absorption of photons by the heme, CO is photodissociated and transfers to a nearby docking site [25
]. Ultrafast visible pump mid-IR probe measurements showed that CO rotated upon dissociation and moved to the new site in less than a picosecond [28
Here, we investigate the nonadiabatic vibrational dynamics of CO from CO-RuTPP/Cu(110) under photodesorption conditions. Thermal and laser desorption are compared, showing that CO-RuTPP/Cu(110) exhibits facile laser desorption, despite possessing a higher thermal desorption temperature than CO/Cu(110). Visible pump-sum frequency probe spectroscopy reveals that coupling to hot electrons is significantly altered by introducing the RuTPP monolayer to Cu(110). The frequency of the C–O stretch mode of CO-RuTPP shows a blue shift during coupling to hot electrons under photodesorption conditions. This phenomenon is more easily explained if the nonadiabatic dynamics of CO/Cu(110) are not caused by anharmonic coupling of the internal stretch to low frequency vibrations, but instead by charge transfer to the CO 2π* state.
2. Materials and Methods
Sum frequency experiments were performed with an amplified 10 Hz femtosecond laser system (TSA-10, Spectra Physics, Santa Clara, CA, USA) inside a UHV chamber, as described previously [19
]. One optical parametric amplifier (TOPAS, Light Conversion, Vilnius, Lithuania) generates 4 μJ, 200 fs mid-IR pulses, while a second TOPAS creates a 150 fs pump beam with wavelengths of 532 nm, 800 nm, or 400 nm. The remainder is passed through an etalon (SLS Optics Ltd, Tromode, Isle of Man) to produce an upconversion pulse of about 7 cm−1
spectral width, time-shifted by 1.3 ps to reduce the non-resonant sum frequency signal [29
]. To fit resonant sum frequency data, we used a sum of Lorentzian peak shapes for χ(2)
, convoluted with a Gaussian peak with the up-conversion pulse width.
For pump-probe measurements, a single Cu(110) crystal was cleaned by 1 keV Ar+ bombardment, followed by annealing to 600 K. Ruthenium tetraphenylporphyrin (RuTPP, Sigma Aldrich Company Ltd, Gillingham, UK) was used as purchased and sublimed at 500 K onto the Cu(110) surface, which was held at 300 K during deposition. The RuTPP coverage was estimated from TPD. The CO was dosed from the background at a substrate temperature of 100 K. All pump-probe sum frequency spectra shown were recorded at a 100 K substrate temperature. Unpumped sum frequency spectra were recorded every four time-delay points, to confirm long term stability of the CO and RuTPP layer during pulsed laser irradiation. Sum frequency spectra of CO adsorbed on bare Cu(110) were acquired after dosing, while pump-probe spectra on RuTPP-covered Cu(110) were acquired under 10−8 mbar CO partial pressure to replenish CO desorbing from RuTPP.
The STM image, shown in Figure 1
, was acquired in a separate chamber with a low-temperature STM (Scienta Omicron Inc, Taunusstein, Germany) at 4.7 K.
The introduction of a RuTPP layer onto Cu(110) changed the ultrafast dynamics of CO drastically. Femtosecond laser pulses were seen to desorb CO easily from RuTPP, even though the desorption energy from RuTPP was higher than from Cu(110). The typical redshift of the internal stretch disappeared at short delay times and was replaced by a blue shift at a higher pump fluence. The femtosecond-induced desorption had characteristics of both nascent as well as hot electrons, while the vibrational dynamics seemed to be driven by hot electrons alone. This is not necessarily a contradiction—if only a small fraction of the molecules is in a precursor state to desorption, then the sum frequency spectrum will be dominated by molecules adsorbed on a hot surface, as discussed by others [4
The frequency blue shift of the internal stretch in the 1–2 ps delay time range, the relevant delay range for desorption, is unusual in pump-probe studies of CO. Inoue et al. reported, for CO/Pt(111) under desorption conditions, a very fast redshift of the internal stretch, followed by a reverse (i.e., blue) shift [11
]. A possible reason is that the pump pulse excites the adsorbed CO enough to initially diffuse across the surface, where it collides with other CO molecules, thus transferring lateral to normal momentum, resulting in an excitation of the surface-CO vibration and thus a frequency blueshift. A similar picture was derived from ultrafast electronic spectroscopy on CO/Ru(0001) [9
], which showed evidence for an intermediate state prior to desorption on a picosecond time scale. This precursor state would have a weaker Ru-CO bond and could, thus, show a C–O blueshift, but is hidden in sum frequency spectra, as explained above. The frequency redshift of the C–O stretch mode of CO-RuTPP, observed with long delay times, is purely thermal and indicates a very small temperature increase (~40 K), which is too low to cause desorption.
We will, first, discuss the frequency blueshift in the light of established models, such as change of adsorption site or anharmonic coupling with low frequency vibrational modes.
The possibility of CO moving to another adsorption site has been discussed in the ultrafast photo-decarboxylation of CO-protein complexes [25
]. A potential intermediate site of CO on the RuTPP/Cu(110) surface could be a Ru-N (imine) bridge site, as observed for CoTPP/Ag(111) [45
]. However, such a bridge site occupation by CO was not observed, even at 4.7 K during STM imaging [24
]. Moreover, occupying this bridge site is expected to show a frequency redshift.
In the original anharmonic coupling model [41
], frequency changes observed following ultrafast excitation were generally expected to occur with the same sign as those observed by step-wise heating. For singly-coordinated CO, for which the most detailed data are available (e.g., [4
]), the internal stretch frequency is generally seen to decrease as it couples anharmonically to the FT, which moves CO from an atop to a multiply coordinated site, which increases backdonation and reduces the frequency. Bridge site CO, however, is thought to anharmonically couple to different low frequency modes, which might reduce the overlap between 2π* and metal orbitals, thus causing a blue shift. Persson and Ryberg analyzed the static temperature dependence of the C–O stretch mode of CO/Ni(111) [49
], showing that the redshift of atop CO is due to coupling to the FT and the blueshift of bridge site CO is due to coupling to the FR. Cook et al., similarly, detected a blueshift of the bridge site CO on Pd(100) with increasing temperature, but excluded anharmonic coupling to the FR and, instead, attributed it to multiphonon coupling [51
]. The vibrational dynamics of CO on palladium has only been studied on Pd nanoclusters [18
], and appears to show a frequency redshift—not the blueshift seen on Pd(100). Such a reversal of sign between slow and ultrafast heating could indicate a change in the vibrational mode that the internal stretch couples to. In our case, switching to coupling with the FR (i.e., bending of the C–O bond from the surface normal) could cause a frequency blue shift, due to the reduction of the backdonation from the metal d-orbital to the CO 2π* orbital, as observed for heme proteins [52
]. Bending of CO by as much as 25° can take place with an energy input of 0.09 eV or less [53
]. Thus, it is feasible that hot electrons/holes could induce bending of CO on RuTPP/Cu(110), which would cause a transient blueshift.
Ueba and Persson extended the anharmonic coupling model by adding intermode coupling between FR and FT modes [6
], as coupling to the FT alone was not sufficient to model vibrational transients when CO is diffusing [8
] or desorbing [4
]. In this model, both low frequency modes couple to hot electrons with different rates, but also couple to each other.
This extended model has been used by us, previously, to explain why vibrational transients of CO/Cu(110) show stronger coupling to hot electrons with increasing CO coverage [19
]. The degree of coupling is thought to change with coverage because CO coverage increases charge density around the carbon atom. This produces a stronger coupling between FR and hot electrons, and the FT-FR intermode coupling then generates a stronger redshift. This interpretation is supported by Helium atom scattering for CO on Cu(100), which also found a gradual increase in coupling between FT and FR modes with increasing CO coverage, as neighboring CO molecules led to a low-frequency motion that was more wagging- than translation-like [54
]. CO-RuTPP at 0.04 ML has a coverage that is roughly half the value of the lowest coverage transients, measured previously on Cu(110) for 0.1 ML. These showed a maximum redshift of 3 cm−1
at a short delay time, for a comparable fluence as the one used in Figure 5
a. It is, therefore, conceivable that the transient measured at 0.04 ML CO-RuTPP could show even less of a redshift at short delay times, although the change in coupling times with coverage observed on Cu(110) suggests that the difference would not be large. The redshift could be further reduced, if the intermode coupling between FT and FR is very different for CO on Cu(110) compared to CO on RuTPP. This is feasible: Since the macrocycle is relatively flat and the phenyl rings are relatively far away from CO adsorbed in the center, the low frequency motions of CO might acquire a much more harmonic character than on a metal surface. This could drastically reduce FT-FR intermode coupling, explaining the lack of a redshift at lower fluences. As the fluence increases, direct coupling of the internal stretch to a different low frequency mode might then become possible, as discussed above. Since both a bending of CO, as induced by FR excitation, or a lengthening of the Ru-CO bond, as induced by external stretch excitation, could lead to a blueshift, and since both external modes have relatively similar frequencies [55
], it would be difficult though to deduce which mode is involved from the transients alone.
Overall, it is possible to explain the cause of the transients by weaker anharmonic coupling between frustrated rotation and translation and stronger anharmonic coupling between frustrated rotation and internal stretch. Nevertheless, this just shifts the unexplained cause from the observed transient to the degree of anharmonic coupling.
A more promising approach has recently emerged from theory [14
]. Novko et al. saw the influence of two different mechanisms in calculated ultrafast transients of CO/Cu(100). First, nonadiabatic coupling (NC) between the internal stretch and hot electrons is caused by charge transfer to the CO 2π*, which softens the C–O bond and occurs when the electron bath is much hotter than the phonon bath. Such a bond softening by charge transfer was proposed, previously, as a possible origin of the fast transients of NO/Ir(111) [56
]. Charge transfer from a hot electron bath would explain why we see no significant dependence on pump wavelength, as shown in Figure 6
. The influence of a charge transfer mechanism could be significantly reduced if the CO 2π* shifts up in energy. This is expected, as the 2π* state has been found at 4 eV above EF
for low CO coverages on Ru(001) [57
], but at only 1.9 eV for low CO coverages on Cu(100) [58
]. The second mechanism, electron-mediated phonon-phonon coupling, is caused by electron-hole pairs, which can bridge the energy gap between the IS, coherently excited by the infrared probe pulse and all the vibrational modes available in the system. In this mechanism, coupling to the incoherent IS initially dominates, while, at later times, the influence of FR, FT, and surface phonon modes is felt. This mechanism is predicted to lead to a blue shift when coupling to hot electrons, although the blueshift is much weaker than the redshift caused by charge transfer from hot electrons. For CO-RuTPP, the low frequency RuTPP modes could provide another reservoir of low-frequency vibrational modes to couple to, which, alongside the suppression of the NC mechanism, could lead to an observable distinct blueshift. The model can also easily explain the coverage-dependent nonadiabacity of CO/Cu(110), without resorting to intermode coupling—the 2π* states broaden with coverage which would increase the NC contribution and, thus, increase the redshift at higher coverage.
This model is in line with other theoretical approaches, which have shown the importance of low-frequency surface phonons acting on the same timescale as electron-hole pairs for CO/Ru(001) [16
], and increased energy transfer from hot electrons into adsorption sites with higher local density of states [59
Finally, we address the question why we observe facile laser desorption of CO from RuTPP when thermal desorption implies a higher binding energy, compared to Cu(110). This becomes feasible if desorption is caused by transfer of electrons or holes into states which do not exist on Cu(110) alone. Prime candidates would be the RuTPP HOMO at EF
–0.8 eV and the CO-induced band around the Fermi level of CO-RuTPP/Cu(110) (which likely arises from dz2
character around the C atom) observed by scanning tunneling spectroscopy [24
]. If both bands were involved, then the RuTPP HOMO might be responsible for an adsorbate-mediated character of the photodesorption, while the CO-induced state near EF
would be sensitive to the hot electron distribution [60
]. Clearly, more detailed desorption experiments are required to answer this question fully.