Real-time observation of nonadiabatic surface dynamics: the first picosecond in the dissociation of NO on iridium

Ultrafast laser pulses on Ir{111} cause a highly temperature-dependent redshift of the intramolecular stretch frequency of adsorbed NO. The time-resolved spectral changes are driven by charge transfer of hot electrons to the NO 2pi*d antibonding orbital, which leads to bending of NO and internal bond weakening. The nonadiabatic change in the NO adsorption geometry follows the charge transfer within a time scale of 700 femtoseconds. This geometrical change is the same as the mechanism predicted for thermally induced dissociation.     

Hot hole-induced dissociation of NO dimers on a copper surface

We use reflection-absorption infrared spectroscopy (RAIRS) to study the photochemistry of NO on Cu(110) in the UV-visible range. We observe that the only photoactive species of NO on Cu(110) is the NO dimer, which is asymmetrically bound to the surface. RAIRS shows that photoinduced dissociation proceeds via breaking of the weak N-N bond of the dimer, photodesorbing one NO(g) to the gas phase and leaving one NO(ads) adsorbed on the surface in a metastable atop position. We model the measured wavelength-dependent cross sections assuming both electron- and hole-induced processes and find that the photochemistry can be described by either electron attachment to a level 0.3 eV above the Fermi energy E(F) or hole attachment to a level 2.2 eV below E(F). While there is no experimental or theoretical evidence for an electron attachment level so close to E(F), an occupied NO-related molecular orbital is known to exist at E(F) - 2.52 eV on the Cu(111) surface [I. Kinoshita, A. Misu, and T. Munakata, J. Chem. Phys. 102, 2970 (1995)]. We, therefore, propose that photoinduced dissociation of NO dimers on Cu(110) in the visible wavelength region proceeds by the creation of hot holes at the top of the copper d-band.     

Broadband femtosecond sum-frequency spectroscopy of CO on Ru1010 in the frequency and time domains

CO on Ru[1010] was investigated by broadband femtosecond sum-frequency spectroscopy at 200 K. Approximately half of the frequency shift of 71 cm(-1) over the coverage range from 0.15 to 1.22 monolayers is shown to originate from dipole-dipole coupling, with the remainder due to a chemical shift. Despite low adlayer-surface registration at the highest coverages, the linewidth of the C-O stretch is comparatively low, and is described by homogeneous broadening according to sum-frequency free-induction decay measurements in the time domain. This can be explained by the dominance of the CO dipole coupling strength over the static disorder present in a coincidence structure. As the coverage decreases below 0.3 monolayer, the linewidth increases considerably, indicative of inhomogeneous broadening. Supported by a concomitant frequency change we suggest that at low coverages CO molecules form chains of irregular length in the [0001] direction, as has been shown for other surfaces with similar symmetry.     

The determination of an inhomogeneous linewidth for a strongly coupled adsorbate system

We present a set of experiments that provide a complete mapping of coherent and incoherent vibrational relaxation times for a molecule on a metal surface, CO/Ir{111}. Included is the first detection of a midinfrared photon echo from a metallic surface, some 15 years after the analogous measurement on a semiconductor surface, which sets a precedent for the ability to manipulate and rephase polarization on a subpicosecond time scale on surfaces. For the C-O stretch in a strongly dipole-coupled CO layer we obtain a total linewidth of 5.6 cm-1, composed of a homogeneous width of 2.7 cm-1 and an inhomogeneous contribution of 3.0 cm-1. Pure dephasing is negligible at liquid nitrogen temperatures, making CO/Ir{111} an attractive model system for quantum computing.     

Inducing non-adiabatic effects through coadsorption: CO + NO on iridium

We compare the response of the intramolecular C–O stretch of carbon monoxide alone and coadsorbed with nitric oxide on Ir{1 1 1} following femtosecond laser heating with the help of time-resolved vibrational sum frequency (SF) spectroscopy. The C–O stretch of a pure CO layer couples anharmonically to the CO frustrated translation and its frequency adiabatically follows the temperature of the iridium surface. In a mixed CO/NO layer, the C–O frequency exhibits non-adiabatic coupling to the hot iridium electrons with a friction coefficient that depends on the electron temperature and the CO:NO ratio. Two possible scenarios emerge: NO causes a static tilt of the CO with a tilt angle depending on the relative coverage. This increases the degree of bonding of the CO 2π^* orbital to the iridium surface, which in turn increases the degree of non-adiabatic coupling. Alternatively, the C–O frequency reflects transient changes in the bonding configuration of the neighboring NO. The latter interaction could be the primary step in the direct reduction of NO by CO to form CO₂.