Eckart Hasselbrink

Wavelength dependence of the photochemistry of O2 on Pd(111) and the role of hot electron cascades

The photochemistry of O2 adsorbed on Pd(111) has been studied as a function of the energy of the exciting photons in the range of hν=3.9–6.4 eV. The obtained data are reproduced by a proposed kinetic model considering photostimulated desorption and dissociation, whereby the latter gives rise to additional displacement processes. This modeling yields rate constants and, hence, cross sections for the respective processes. It is found that the cross sections for all processes rise exponentially with photon energy. This result motivates a study of the distribution of hot electrons generated by laser irradiation, and its decay via electron‐hole pair scattering events resulting in a broad secondary electron distribution. The energetic and spatial distribution of these electrons is calculated. The probability for these electrons to tunnel into an adsorbate affinity level is modeled and compared to the observed photo cross sections.

Cross sections and NO product state distributions resulting from substrate mediated photodissociation of NO2 adsorbed on Pd(111)

Ultraviolet irradiation of NO2 adsorbed on top of a NO saturated Pd(111) surface causes the photodissociation of NO2/N2O4 and results in the desorption of NO molecules. This process has been studied using excitation energies between 3.5 and 6.4 eV. At a photon energy of 6.4 eV, a cross section of 3×10−18 cm2 is found. Using laser‐induced fluorescence to detect the desorbed NO molecules, fully state‐resolved data detailing the energy channeling into different degrees of freedom has been obtained. Two desorption channels are found, one characterized by nonthermal state populations, and one showing accommodation to the surface. The yield of the fast channel shows a marked increase above 4 eV photon energy. The slow channel is interpreted as being due to NO molecules which, after formation, undergo a trapping–desorption process. A polarization experiment indicates that the photodissociation is initiated by excitation of metal electrons rather than direct absorption by the adsorbate.

Dynamics of the ultraviolet photochemistry of water adsorbed on Pd(111)

UV‐laser irradiation (hν=6.4 eV and 5.0 eV) of the water bilayer adsorbed on a Pd(111) surface leads to molecular desorption and to conversion of the adsorbed state as manifested in thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time‐of‐flight measurements show that water molecules desorb with a translational energy of about 600 K for both photon energies indicating a nonthermal process. While desorption is largely suppressed with adsorbed multilayers, conversion within the first layer still proceeds. The dependence of the desorption yield on angle of incidence and polarization of the light reveals substrate excitations as the dominant primary step. A strong variation of cross sections with isotopic substitution is observed. This is interpreted as evidence for the operation of a mechanism involving excitation onto an isotope‐independent excited potential energy surface followed by rapid deexcitation to the ground state so that, of the total number...

Coupling of the rotational and translational degrees of freedom in molecular DIET: A classical trajectory study

Abstract Classical trajectories have been calculated to address recent observations in laser-induced desorption of molecules: in particular that the mean translational energy increases with rotational energy of the desorbed molecule. A model is discussed which explains rotational excitation on the basis of an anisotropic repulsive interaction in the excited state. The observed correlation is a consequence of the lifetime spread in the excited state resulting in the fact that for those molecules quenched later more potential energy is transferred into translational and rotational energy. Calculated rotational state and velocity distributions are in semiquantitative agreement with experimental findings.

The adsorbate state specific photochemistry of dioxygen on Pd(111)

The ultraviolet‐photochemistry of molecularly adsorbed oxygen on Pd(111) has been studied using pulsed laser light with 6.4 eV photon energy. Three processes occur upon irradiation: desorption of molecular oxygen, conversion between adsorption states, and dissociation to form adsorbed atomic oxygen. By using time‐of‐flight spectroscopy to detect the desorbing molecular oxygen and post‐irradiation thermal desorption spectroscopy (TDS) to characterize the adsorbate state, a detailed picture of the photochemical processes is obtained. The data indicate that the O2 molecules desorbing with low translational energies from the saturated surface as well as the conversion of adsorbed molecules between binding states are induced by the photoinduced build‐up of atomic oxygen on the surface. Analysis of a proposed reaction model reproduces the observed data and yields detailed rates. Polarization analysis indicates that the photochemical processes are initiated by electronic excitations of the substrate.

O2/Pd(111). Clarification of the correspondence between thermal desorption features and chemisorption states

The system O₂/Pd (111) was probed with EELS and thermal desorption spectrometry. The coincidence of 3 peaks in thermal desorption and the O-O region of the EELS spectrum was noted previously and a one-to-one correspondence between the states resolved in EELS and the desorption features was assumed. The authors show that this assumption is incorrect. The lowest binding energy state correlates with the highest frequency vibrational state. The lowest vibrational frequency state acts as a precursor to dissocn. The 2 higher temp. thermal desorption features both arise from the remaining state.

Polarization probe of excitation mechanisms in surface photochemistry

Abstract Based on classical electrodynamic theory, cross sections for surface photochemical processes are derived as a function of the angle of incidence. Using s- and p-polarized light, the expected primary excitation mechanisms. i.e. direct excitation and substrate excitation, can be distinguished. Two examples are shown: for O2 on Pd(111), photo-induced desorption and dissociation at 6.4 eV are unambiguously due to substrate excitation, while for Cl2CO on Pt(111), direct excitation plays an important role in photolysis at 4.4 eV.

Beam investigations of D2 adsorption on Si(100): On the importance of lattice excitations in the reaction dynamics

The adsorption of D2 on Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D2 sticking coefficient S on surface temperature Ts and nozzle temperature Tn. The sticking coefficient increases gradually in the range 300≤Tn≤1040 K. The influence of increased v=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on Ts is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5±0.1)×10−5 at Ts=630 K. The decrease in the effective sticking coefficient beyond this Ts is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excita...

Ultraviolet‐laser induced dissociation and desorption of water adsorbed on Pd(111)

Ultraviolet‐laser irradiation (6.4 eV and 5.0 eV) of the first layer of water adsorbed on a Pd(111) surface at 90 K leads to desorption of H2O and to conversion of the adsorbed state as manifested in the thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time‐of‐flight measurements show that water molecules desorb with the same translational energy of about 600 K for both photon energies. While desorption is suppressed with adsorbed multilayers, conversion within the first layer still proceeds.

On the interaction of excited alkali atoms with rare gas targets in scattering processes

A model potential calculation has been applied to evaluate scattering experiments for Na and K in the groundstate and the resonance state interacting with Ar. The model potential has only two free parameters which are determined by a best fit of the interatomic potentials to experimental results. Satisfactory agreement between calculated and experimental results is found for the differential cross sections in the groundstate and the excited state, for satellites in theK(4P),K(5P) and Na(3P) line profile, for the van der Waals constantsC(6) andC(8), the alkali ion-rare gas interaction and the vibrational energy levels of the Na-Ar molecule. As maior advantages we point out that for a given pair of atoms all these calculated data are given with one single pair of values for the free parameters and that with this set also the interatomic potentials for the higher alkali states (specifically up to 5f for Na and K) are obtained.