Lifetimes of image-potential states on Cu(100) and Ag(100) measured by femtosecond time-resolved two-photon photoemission

Shumay, I. L.; Höfer, U.; Reuß, Ch.; Thomann, U.; Wallauer, W.; Fauster, Thomas

doi:10.1103/physrevb.58.13974

Cited by 139 publications

(125 citation statements)

References 25 publications

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“…Effective masses for n ¼ 1 and 2 have been taken as m Ã ¼ 1 following the measured dispersion of these states [4]. The use of realistic effective masses leads to excellent agreement with the measured lifetime for n ¼ 1 while for n ¼ 2; 3 the theory gives lifetimes which are longer than published experimental values [218,231]. However, recent measurements on a better sample (see Fig.…”

Section: Image-potential Statesmentioning

confidence: 59%

“…2 in earlier work . [36,218] are systematically shorter than predicted theoretically (see discussion of Table 8 in Section 6.2). High-quality samples are needed to obtain longer lifetimes as shown in Fig.…”

Section: Momentum Dependence Of Lifetimesmentioning

confidence: 82%

“…One striking example of the limitations of the penetration approximation appears when comparing Cu(1 0 0) and Ag(1 0 0) surfaces which have a very similar electronic structure. Due to a slightly smaller s-p-gap, the penetration of the wave functions in Ag(1 0 0) is slightly larger than in Cu(1 0 0), whereas the experimental decay rates of the image-potential states are nearly 40% smaller [218]. Only after properly accounting for the decay into the surface plasmon, as described below, many-body calculations can explain the experimental trend.…”

Section: Image-potential Statesmentioning

confidence: 93%

“…The use of different photon energies ho a 6 ¼ ho b is preferable, because the low-energy background can be reduced [4,216]. The asymmetric, shifted time-resolved 2PPE signal of a two-color experiment allows for the extraction of intermediate-state lifetimes by a factor of 5 shorter than the cross-correlation [217,218]. A similar factor applies to the case of interferometric single-color experiments [175].…”

Section: Two-photon Photoemissionmentioning

confidence: 94%

“…Before we are going to discuss the unique physics of 2PPE measurements, we will shortly present the experimental requirements [218][219][220]. The experiment basically consists of a laser system and an electron energy analyzer.…”

Section: Two-photon Photoemissionmentioning

confidence: 99%

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Section: Image-potential Statesmentioning

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Section: Momentum Dependence Of Lifetimesmentioning

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Decay of electronic excitations at metal surfaces

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Time‐resolved two‐photon photoemission

Fauster¹

2003

Solid‐State Photoemission and Related Methods

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In this chapter time resolution will be introduced to photoemission experiments. This means not just a series of regular photoemission measurements to monitor the changes of a sample with time. In two-photon photoemission the time delay between the two photons is an additional experimental parameter which can be controlled with femtosecond resolution, i.e., on a time scale relevant for electronic processes. At the same time, the introduction of the second photon leads to the participation of an additional electronic state in the energy diagram for photoemission. Energy diagramIn Figure 8.1 the transition from initial state |1 to an intermediate state |2 after the absorption of the photon with energy hν a is shown. The second photon of energy hν b excites the electron out of the intermediate state into the final state |3 . If the final state energy E 3 is above the vacuum energy E vac , the electron might leave the surface and its kinetic energy E kin is measured with an electron energy analyzer. Obviously, the initial state has to be occupied, i.e., its energy E 1 should be below the Fermi energy E F . The intermediate state on the other hand has to be empty at first. In most cases it is desirable to keep the photon energy hν a below the work function Φ = E vac − E F , because this constitutes the threshold for one-photon photoemission. This applies also to the other photon, because processes with the role of the two photons interchanged are possible as well. Energy-resolved spectroscopyThe spectrum sketched at the right of Fig. 8.1 shows a peak at the final state energy E 3 . The cutoff at kinetic energy zero corresponds to the vacuum level E vac of the sample. Electrons with the highest energy after the absorption of the photons with energies hν a and hν b appear at kinetic energy E max − E vac = hν a + hν b − Φ. These limits are familiar from regular photoemission spectroscopy and can be used to determine the work function from the width of the spectrum. Similarly, the momentum k of the electron parallel to the surface is conserved for single-crystal surfaces. For electrons emitted at an angle ϑ with respect to the surface normal one obtainshk = √ 2mE kin sin ϑ. Umklapp processes would add reciprocal lattice vectors of the surface to the momentum conservation, but play usually no role in twophoton photoemission. One has to keep in mind that the kinetic energies are generally quite

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Exciton Formation and Decay at Surfaces and Interfaces

Muntwiler

Zhu

2010

Dynamics at Solid State Surfaces and Interfaces

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IntroductionExcitons are the primary optical excitations of a wide range of low dielectric materials, including semiconductors, insulators, and molecular solids. In contrast to free, independent charge carriers in the conduction or valence band of a semiconductor, excitons consist of a correlated pair of an electron in the conduction band and a hole in the valence band. Correlation arises due to their mutual Coulomb interaction, which lowers the total energy of the quasiparticle with respect to two independent particles. In molecules, correlation is particularly strong because electron wave functions are intrinsically confined to a small spatial region. These two limiting cases are described by models known as the Mott-Wannier (MW) exciton [1] and the Frenkel exciton (FE) [2], respectively. Although the physics of excitons in inorganic semiconductors and isolated molecules has been developed since the 1930s, recent progress in the development of applications of organic optoelectronic devices has renewed scientific interest in excitons and their formation, decay, and energy transfer dynamics in organic semiconductors, and, particularly, at interfaces of organic semiconductors.In this chapter, we will discuss recent experimental results of time-resolved twophoton photoelectron spectroscopy (TR-2PPE) of excitons at the interface of two model organic semiconductors. We start with an introduction about classification of excitons and photophysics of organic semiconductors. In Section 15.2, we will review a few, mostly phenomenological, theoretical concepts that have been used to model excitons in organic semiconductors. In Section 15.3, the technique of timeresolved two-photon photoelectron spectroscopy, particularly its application to excitons, will be discussed. The results from two model systems, thin films of C 60 and of pentacene, will then be presented and discussed in Sections 15.4 and 15.5, respectively.

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Lifetimes of image-potential states on Cu(100) and Ag(100) measured by femtosecond time-resolved two-photon photoemission

Cited by 139 publications

References 25 publications

Decay of electronic excitations at metal surfaces

Decay of electronic excitations at metal surfaces

Time‐resolved two‐photon photoemission

Exciton Formation and Decay at Surfaces and Interfaces

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