We investigate the laser-assisted photoelectric effect from a solid surface. By illuminating a $\mathrm{Pt}(111)$ sample simultaneously with ultrashort 1.6 and $42\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum, and accurately extract their amplitudes over a wide range of laser intensities. Our results agree with a simple model, in which soft x-ray photoemission is accompanied by the interaction of the photoemitted electron with the laser field. This strong effect can definitively be distinguished from other laser surface interaction phenomena, such as hot electron excitation, above-threshold photoemission, and space-charge acceleration. Thus, laser-assisted photoemission from surfaces promises to extend pulse duration measurements to higher photon energies, as well as opening up measurements of femtosecond-to-attosecond electron dynamics in solid and surface-adsorbate systems.

Laser-assisted photoemission from surfaces

Since the first demonstration of the generation of attosecond pulses (1 as = 10−18 s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron–nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

https://re.public.polimi.it/bitstream/11311/1219390/1/ROPP-101457_Proof_hi.pdf

Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics

Time-resolved valence band photoelectron spectroscopy with a temporal resolution of 135 fs is used to map the entire occupied valence electronic structure of photoexcited gas-phase Br2 molecules during dissociation. The observed shifting and mixing of valence energy levels defines a transition period where the system appears to be intermediate between atoms and molecules. The surprisingly short bond breaking or dissociation time is determined by monitoring in real time how the photoelectron multiplet structure of the free atom arises from the valence states of the photoexcited molecule.

Real-time evolution of the valence electronic structure in a dissociating molecule

The light-emission mechanism and lifetime of an organic light-emitting device (OLED) are determined by the properties of the organic material in the devices. [1–3] In OLEDs, the emission mechanism and the phenomena that degrade emission are affected by various factors, including the energy levels of the organic material, the charge-carrier trapping and transport characteristics, recombination in the light-emitting layer (EML), and their interplay. [4–6] Moreover, in OLEDs based on phosphorescent organic materials, the long-life excited states and higher luminous quantum yields of the phos-phorescent material mean that different mechanisms, such as preferentially direct charge-carrier trapping and exciton migration, dominate the process of light emission and recombination. However, the doping and charge-trapping behavior of phosphorescent organic materials and their multilayers have yet to be thoroughly studied. In multilayered devices, transport and blocking layers are used to confine excitons that are originally formed in the dye-doped emission layer, enabling efficient emission to be maintained. Therefore, both for small-molecule and polymeric-type phosphorescent organic materials, the OLED structures used for charge balancing and exciton confinement are important for stable device operation and higher efficiency, because the properties of these devices strongly depend on the interactions between the layer of phosphorescent material and adjoining layers. [7–8] Greater knowledge of the physics underlying the operation of multi-layered OLEDs is essential for the commercial development of highly efficient large-area OLEDs with long lifetimes. Previously, using 4,4 ′ - N , N ′ -dicarbazole-biphenyl (CBP) as host and fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy) 3 ) and iridium(

Confinement of Charge Carriers and Excitons in Electrophosphorescent Devices: Mechanism of Light Emission and Degradation

A complete model for the quantitative simulation of electrical and optical characteristics for organic light-emitting devices with fluorescent dopant in the host is presented. This simulation model consists of three parts: charged carrier transport model, exciton model, and emission and optical model. In the first part, we include not only charge carrier trapping but also direct carrier recombination phenomena on the fluorescent dopant. In the second part, Forster [Discuss. Faraday Soc.27, 7 (1959)]energy transfer from the host molecule to the dopant molecule is included in exciton model. In addition, the quenching phenomena related to dopant concentration and electrode are also considered in this study. In the optical model, the thin-film optics is applied to calculate the interference effect of the device. Results for several multilayer devices with different fluorescent dopant concentrations are presented. On the basis of the experimental data of a typical doped device, we have found good agreement between the simulation results and the experimental data.

Electrical and optical simulation of organic light-emitting devices with fluorescent dopant in the emitting layer

In this work, we use excite-probe photoelectron spectroscopy to study the decay of electronic excitation in tris(8-hydroxy quinoline) aluminum (Alq) doped with the organic dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). Ultrashort laser pulses are used to photoexcite electrons into unoccupied molecular orbitals, and the ensuing decay rate is directly observed using photoelectron spectroscopy. Decay of the electronic excitation is studied as a function of DCM doping percentage and excitation intensity. The decay rate is seen to increase with both doping percentage and excitation intensity. These data are explained using a model including Forster transfer, stimulated emission, concentration quenching, and bimolecular singlet–singlet exciton annihilation. In this model, we find that it is necessary to include a very fast (faster than predicted in standard Forster transfer theory) excitation transfer of a fraction of the excitation from the Alq to the DCM, where that fraction corresp...

Excitation dynamics of dye doped tris(8-hydroxy quinoline) aluminum films studied using time-resolved photoelectron spectroscopy

The lowest unoccupied molecular orbital (LUMO) in the two blue-light-emitting organic luminescent materials bis(2-methyl-8-quinolinolato)(para-phenyl-phenolato)aluminum and 1,4-bis(2,2-diphenylvinyl)biphenyl was studied by femtosecond laser pump-and-probe photoemission and compared with tris(8-hydroxyquinoline)aluminum. We have determined the energy gap between the LUMO and the highest occupied molecular orbital and studied the LUMO decay dynamics in these materials. The differences in decay rates are shown to be related to the morphology of the evaporated films.

Electron dynamics in unoccupied molecular orbitals of two blue-light-emitting organic electroluminescent materials

Summary form only given. The electroluminescent organic material, tris(8-hydroxyquinoline)aluminum (Alq), is foremost in organic light emitting device (OLED) fabrication. Doping Alq with highly fluorescent dyes enhances the emission, and allows color tuning. Furthermore, DCM doped Alq films have been shown to lase at low photoexcitation thresholds, and have the potential for electrical pumping. These applications benefit from detailed understanding of the Alq-DCM excitation transfer, the recombination dynamics, and the energy gap values. Each of these properties were studied in the work.

Ultrafast photoelectron spectroscopy of dye-doped organic films

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Excitation dynamics of dye doped tris(8-hydroxy quinoline) aluminum films studied using time-resolved photoelectron spectroscopy

Electron dynamics in unoccupied molecular orbitals of two blue-light-emitting organic electroluminescent materials

Ultrafast photoelectron spectroscopy of dye-doped organic films