Review
Time- and angle-resolved two-photon photoemission studies of electron localization and solvation at interfaces

https://doi.org/10.1016/j.progsurf.2004.08.001Get rights and content

Abstract

We review recent work in the study of interfacial electronic states and electron–adsorbate interactions with time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. Results for interfaces between noble-metal surfaces and organic as well as inorganic dielectric overlayers are presented. Layer structure and thickness have pronounced effects on the spatial extents, binding energies, and dynamics of image-potential states (IPS) and molecular orbitals. The transition from delocalized to localized states in the plane of the interface can also occur dynamically through electron-induced nuclear motion in the overlayer. An important example of this class of phenomenon is the formation of polaronic states from initially delocalized IPS, which occurs on sub-picosecond timescales. Dynamic energy relaxation through electron solvation by molecular dipoles, the analogue of the dynamic Stokes shift in bulk solvents, may also accompany localization on ultrafast timescales, and has become the focus of much recent experimental and theoretical interest. The interplay of a static layer structure and an evolving potential from the adsorbate creates a rich environment for interfacial electron dynamics, as evidenced by the alcohol/Ag(1 1 1), nitrile/Ag(1 1 1), and D2O/Cu(1 1 1) systems. We conclude with a discussion of recent attempts to determine the spatial extent of localization parallel to the interface for electrons following localization and solvation using experimental measurements of the photoelectron distribution in momentum space.

Introduction

The study of the electronic properties of interfaces between dissimilar materials constitutes a major research area of fundamental interest and technological significance. Chemical and physical processes occurring at interfaces may be understood on a microscopic level in terms of the associated energy levels and dynamics of the interfacial electrons. Hot electrons, for example, can induce surface chemical reactions or desorption of adsorbates [1]. Molecular anion formation via electron transfer from the metal substrate has been implicated as the initial step in these processes [2], [3]. Electron trapping and scattering at the interface can drastically affect carrier transport properties and the performance of devices [4], [5].

The sensitivity of interfacial electrons to the composition of the interface may also be exploited to probe the dynamics of adsorbed molecules. A strong electron–adsorbate interaction can cause an electron-induced adsorbate reorganization [6], [7], [8] similar to electron solvation in liquids [9], [10]. Studies of solvation at an intrinsically asymmetric environment such as a two-dimensional interface is particularly interesting because the reduced dimensionality and hindered solvent motion can result in electron dynamics distinct from those in the isotropic material [11], [12], [13], [14]. Characterizing the energy levels of interfacial electrons as a function of the adsorbate reorganization time can elucidate the time scales and mechanisms of both the electrons’ response as well as the molecular motions of the adsorbate overlayer. To address these issues, an investigation of the properties associated with films of molecular thickness in a well-controlled fashion is necessary.

Much of the research on electrons at interfaces has involved the study of image-potential states (IPS). The IPS are bound by the interaction between an electron outside of a surface and the polarization it induces at that surface (Fig. 1). The polarization can be treated formally by replacing it with the oppositely-charged image of the electron reflected across the surface plane, a configuration that reproduces the boundary conditions of the original problem [15], [16]. The resultant one-dimensional Coulombic potential, the image potential (IP), supports an infinite series of hydrogenic states. For metal surfaces, the energies of IPS in electron volts are given byEn=-0.85eV(n+a)2+V0,where n represents the principal quantum number and a is the quantum defect parameter [17], [18], [19]. The series converges to V0, which is equal to the vacuum energy for a uniform surface. Image-potential state electrons reside only a few angstroms outside of the interface. The hydrogenic model [20] gives an expectation value of 〈z = 6a0n2, where a0 is the Bohr radius [21]. This distance is roughly 3 Å and 12 Å for n = 1 and n = 2 IPS, respectively, making them sensitive to changes in the interfacial electrostatic potential. It has been shown that the surface electronic structure modified by the overlayer also changes the energy and lifetime of the IPS [22]. For a more detailed description of the properties of IPS and calculations of their energies, the reader is referred to review articles by Memmel [19], Fauster and Steinmann [17], Harris et al. [22], and Osgood and Wang [23].

It is the purpose of the present review to focus on recent work using two-photon photoemission (2PPE) to study electron localization, both static and dynamic, and the related process of electron solvation at dielectric/metal interfaces. Both IPS and the molecular orbitals (MO) of adsorbed molecules will be discussed. The angle- and time-resolved 2PPE technique will be discussed in Section 2, followed by a review of structurally-induced localization at interfaces (Section 3). Dynamic localization of electrons by nonpolar (Section 4) and polar (Section 5) adsorbates are covered separately, where the distinction is due to the significant dynamic energy relaxation caused by the latter. Section 5 also compares the localization and solvation that occur at interfaces to the analogous phenomena experienced by excess electrons in bulk liquids. Finally, a discussion of attempts to determine the spatial extent of localization parallel to the interface is presented in Section 6.

Section snippets

Experimental methods

Two-photon photoemission (2PPE) is a pump–probe spectroscopic technique for exploring interfacial electronic states. Unlike conventional photoemission, which uses one-photon processes, 2PPE is capable of examining initially unoccupied as well as initially occupied states. Initially unoccupied states, such as IPS, as studied by 2PPE are illustrated in Fig. 2a. Electrons from below the Fermi level (EF) of a metal substrate (or from an initially occupied surface [17] or adsorbate [24] state) are

Qualitative scaling arguments

Benzene, methylthiolate, and C60 each represent a case where the static structure of the layer dictates the spatial extent of the interfacial electrons. Nevertheless, electrons may localize in a dynamic process, where the mutual interaction of the electron and the layer changes the spatial extent of the electron over time.

Fig. 7 shows the dispersion of the n = 1 state of 2 ML of n-heptane/Ag(1 1 1). At 0 fs, the n = 1 IPS is delocalized in the plane of the surface, with an effective mass of m* = 1.2. At

Introductory remarks and background

The observation of dynamic localization at polar interfaces shows that the coupling of the electron to the molecular modes of an adsorbate can influence critically the dynamics of the electron. The coupling to these modes can, however, do more than simply alter the electron’s spatial extent. Polar adsorbates interact more strongly over longer ranges, and their motion causes dynamic changes in the energy of the electron through the process of solvation.

The topic of solvation dynamics is easy to

Spatial extent of localized interfacial states

In the study of localization at interfaces, the question of the spatial extent of localization arises. While estimates for higher-energy, delocalized electrons in bulk liquids have been made through modeling the reactions of the electrons with other solutes [139] and measurements of electron–hole recombination rates [140], localized states have been limited to quantum–mechanical calculations of the radius of gyration [130], [131], [132]. At interfaces, the nature of the electronic states are

Conclusions

Since the original studies of IPS at vacuum/metal interfaces, 2PPE has established itself as a valuable technique for determining the electronic properties of a variety of interfaces as a function of composition and structure. With recent demonstrations of the ability to measure the dynamics, energetics, and spatial extent accompanying electron localization, the utility of 2PPE as a probe of electron dynamics is greatly extended. Further experimental and theoretical work will likely expand the

Acknowledgement

The authors would like to thank T. Heinz for helpful discussions. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US Department of Energy, under Contract No. DE–AC03–76SF00098. The authors acknowledge the National Science Foundation support for specialized equipment used in the experiments described herein.

References (142)

  • D. Zimdars et al.

    Chem. Phys. Lett.

    (1999)
  • N. Memmel

    Surf. Sci. Rep.

    (1998)
  • R.M. Osgood et al.

    Image states on single-crystal metal surface

  • H. Petek et al.

    Prog. Surf. Sci.

    (1997)
  • S. Pawlik et al.

    Surf. Sci.

    (1998)
  • Y. Sonoda et al.

    Surf. Sci.

    (2002)
  • K.J. Gaffney et al.

    Chem. Phys.

    (2000)
  • K. Ishioka et al.

    Surf. Sci.

    (2000)
  • C. Gahl et al.

    Surf. Sci.

    (2003)
  • T. Fauster et al.

    Chem. Phys.

    (2000)
  • T. Fauster

    Surf. Sci.

    (2002)
  • N.V. Smith et al.

    Introduction

  • T. Munakata et al.

    Chem. Phys. Lett.

    (1997)
  • T. Munakata et al.

    J. Electron. Spectrosc. Rel. Phenom.

    (1998)
  • R. Jacquemin et al.

    Solid State Commun.

    (1998)
  • P. Avouris et al.

    Annu. Rev. Phys. Chem.

    (1989)
  • W.J. Ho

    J. Phys. Chem.

    (1996)
  • J.W. Gadzuk

    Phys. Rev. Lett.

    (1996)
  • J.R. Sheats et al.

    Science.

    (1996)
  • W.R. Salaneck

    Conjugated Polymer Surfaces and Interfaces: Electronic and Chemical Structure of Interfaces for Polymer Light Emiting Diodes

    (1996)
  • N.-H. Ge et al.

    Science.

    (1998)
  • A.D. Miller et al.

    Science.

    (2002)
  • C. Gahl et al.

    Phys. Rev. Lett.

    (2002)
  • C. Silva et al.

    Phys. Rev. Lett.

    (1998)
  • J.A. Kloepfer et al.

    J. Chem. Phys.

    (2000)
  • D.M. Willard et al.

    J. Am. Chem. Soc.

    (1998)
  • B.J. Loughnane et al.

    J. Phys. Chem. B

    (2000)
  • I. Benjamin

    Chem. Rev.

    (1996)
  • W.R. Smythe

    Static and Dynamic Electricity

    (1950)
  • L.D. Landau et al.

    Electrodynamics of Continuous Media

    (1984)
  • T. Fauster et al.

    Two-photon photoemission spectroscopy of image states

  • N.V. Smith

    Phys. Rev. B

    (1985)
  • P.M. Echenique et al.

    J. Phys. Pt. C Solid

    (1978)
  • C.M. Wong et al.

    J. Phys. Chem. B

    (1999)
  • C.B. Harris et al.

    Annu. Rev. Phys. Chem.

    (1997)
  • A.D. Miller et al.

    J. Phys. Chem. A

    (2002)
  • S. Ogawa et al.

    Phys. Rev. Lett.

    (1997)
  • G. Dutton et al.

    J. Phys. Chem. B

    (2002)
  • T. Miller et al.

    Phys. Rev. Lett.

    (1996)
  • G. Dutton et al.

    J. Phys. Chem. B

    (2001)
  • W. Wallauer et al.

    Phys. Rev. B

    (1996)
  • U. Thomann et al.

    Appl. Phys. B

    (1999)
  • U. Thomann et al.

    Phys. Rev. B

    (2000)
  • K. Giesen et al.

    Ann. Israel Phys. Soc.

    (1984)
  • K.I. Shudo et al.

    Phys. Rev. B

    (2001)
  • K.I. Shudo et al.

    Phys. Rev. B

    (2002)
  • C. Kentsch et al.

    Phys. Rev. B

    (2002)
  • E. Knoessel et al.

    Phys. Rev. B

    (1998)
  • H. Petek et al.

    Phys. Rev. Lett.

    (1999)
  • S. Ogawa et al.

    Phys. Rev. Lett.

    (2002)
  • Cited by (78)

    • Influence of molecular distortion on the exciton quenching for quaterthiophene-terminated self-assembled monolayers on Au(111)

      2018, Surface Science
      Citation Excerpt :

      The structure-specific investigations on dynamics of the photoexcited states for functional molecular adlayers are important in understanding the electronic properties at organic-inorganic interfaces [1–5].

    • Time-resolved photoelectron spectroscopy

      2018, Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry
    • Theoretical Investigation on Optical Signatures and Photochemical Properties of Photocatalytic TiO<inf>2</inf> Surfaces

      2023, Handbook of Self-Cleaning Surfaces and Materials: From Fundamentals to Applications: Volumes 1 and 2
    View all citing articles on Scopus
    1

    Present address: Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, United States.

    View full text