Isomerization reaction of the azobenzene TBA.
(a) Two-color 2PPE spectra of TBA adsorbed on Au(111) measured with 2.2 and 4.4 eV photons, the 1. scan spectrum (photon dose of ), and the spectrum after UV-light (4.4 eV) exposure with a photon dose of . The spectra are displayed as 2PPE intensity vs final state energy with respect to the Fermi level , (with the work function). (b) Photon energy dependence of the peaks labeled A and C in the 2PPE spectrum of TBA/Au(111). The symbols are experimental data, and the solid lines are fitting curves. The kinetic energy of peak A varies with , indicating that it originates from an unoccupied intermediate state. The kinetic energy of peak C remains constant, i.e., it arises from an unoccupied final state.
One-color 2PPE spectrum of 0.9 ML TBA adsorbed on Au(111) taken at a photon energy of 4.21 eV. Inset: Photon energy dependence of peaks D–F. The symbols are experimental data and the solid lines are fitting curves. The kinetic energies of peaks D and E vary with , indicating that the peaks originate from occupied initial states. They can be assigned to the HOMO and HOMO-1. The peak labeled F shows no energy dependency; therefore it is an unoccupied final state
UV-visible absorption spectra of TBA in cyclohexane before and after illumination with UV light at 3.96 eV. Exposure at 3.96 eV leads to a decrease in the absorbance around 3.81 eV, which is due to the trans/cis isomerization.
Effective cross section for the photoinduced trans/cis isomerization of TBA adsorbed on Au(111) as a function of photon energy (see text).
The energies of the observed photoemission spectral features in 0.9 ML TBA adsorbed on Au(111). All features are referenced to the Fermi level of Au(111).
(a) Proposed excitation mechanism for the photoinduced trans/cis isomerization of TBA adsorbed on Au(111) via the creation of a positive ion resonance. Thereby photoexcitation at photon energies above leads in the first step to electron-hole pair formation. The holes in the Au -band relax to the top of the -band (step 2) followed by a hole transfer to the HOMO of TBA (step 3). (b) Formation of holes in the Au -bands for different photon energies. For photon energies , i.e., twice the energy of the -band edge, the population of holes at the top of the -band is enhanced due to Auger-decay processes (see text).
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