The Adsorbate Electron Affinity Dependence of Femtosecond Electron Dynamics at Dielectric/Metal Interfaces

Abstract

The two photon photoemission technique has been utilized to investigate the adsorbate electron affinity dependence of interfacial electron transfer dynamics. The comparison of calculated and experimental results highlight a brief discussion of a dielectric continuum model. For the n-heptane/Ag(lll) and the benzene/ Ag(lll) interfaces, the model effectively reproduces experimental image potential state properties. The model fails to adequately describe the influence of anthracene adsorbates on image potential state lifetimes, as demonstrated by the experimental and theoretical results. The experimental n = 1 lifetime of 1200 femtoseconds and binding energy of -0.53 eV for the anthracene bilayer differ greatly from the calculated values of 40 femtoseconds and -1.4 eV. The application offemtosecond (fs) laser technology to two photon photoemission (TPPE) has transformed the experimental investigation of electron dynamics in metals and at metal surfaces. Indirect information has long been accessible via linewidth analysis of static spectra obtained from inverse photo emission, 1 and high resolution electron energy loss spectroscopy studies;' but the difficulty in distinguishing between phase and energy relaxation, as well as the complicating influence of inhomogeneous broadening, stresses the importance of directly measuring electron dynamics.The two photon photoemission technique populates unoccupied electronic states with a pump pulse and photoemits the excited state electrons with a probe pulse, as depicted in Fig. 1(a). The binding energy of the unoccupied electronic states can be determined by measuring the kinetic energy of the photoemitted electrons and then subtracting the energy of the probe pulse photon. The excited state lifetimes can be determined by measuring the number of electrons detected as a function of time delay between the arrival of the pump and the probe pulses at the sample. The direct observation of excited state lifetimes constitutes the most significant advantage of TPPE (see Fig. 1).Time resolved TPPE has been utilized to study many aspects of electron dynamics in metals and at metal surfaces. Numerous experimental investigations of hot electron dynamics have been conducted to date in an attempt to determine the influence of the metal band structure on electron dynamics.I" The phase resolved interferometric two photon photoemission studies of Petek and coworkers have directly determined the energy dependence of both population and phase decay for noble metal surfaces 3 As expected from Fermi liquid theory, electrons with energies near the Fermi level have the longest

Electronic Band Formation at Organic−Metal Interfaces: Role of Adsorbate−Surface Interaction

“…Harris and co-workers recently carried out 2PPE studies of Ag(111) adsorbed with a series of aromatic molecules with increasing electron affinity: benzene, naphthalene, and anthracene. 16 However, these experiments did not reveal any molecular resonance. Instead, the authors showed the sensitivity of image states to the presence of molecular layers, depending on the energetics of image states relative to molecular bands.…”

“…Will such a molecular resonance show free electron like dispersion parallel to the surface as is observed for modified image states? Harris and co-workers recently carried out 2PPE studies of Ag(111) adsorbed with a series of aromatic molecules with increasing electron affinity: benzene, naphthalene, and anthracene . However, these experiments did not reveal any molecular resonance.…”

Femtosecond Dynamics of Electrons Photoinjected into Organic Semiconductors at Aromatic-Metal Interfaces

“…10,19,20 The dielectric continuum model, however, does not reproduce the experimental findings for naphthalene and anthracene on Ag(111). 21 An alternative description of the adsorbate electronic structure, based on the two-band nearly free electron model, has successfully reproduced the primary experimental results. Analysis of the calculation provides an explanation for experimental findings, as well as the failure of the dielectric continuum model.…”

“…The primary experimental trends seen in the present study have been successfully accounted for with theoretical model calculations. Dielectric continuum descriptions of adsorbate electronic structure have been successful in explaining the influence of numerous adsorbates on image potential states. ,, The dielectric continuum model, however, does not reproduce the experimental findings for naphthalene and anthracene on Ag(111) . An alternative description of the adsorbate electronic structure, based on the two-band nearly free electron model, has successfully reproduced the primary experimental results.…”

Watching Electrons Transfer from Metals to Insulators using Two Photon Photoemission

“…Adsorbates with negative electron affinities should from a barrier that pushes the electron away from the surface, increasing its lifetime. This model has been successfully used to explain the IPS energetic positioning and lifetime for several adsorbate systems including noble gasses 22,23 , linear acenes 24 , and alkanes 21,25 . This model, however, also has several inadequacies.…”