Lifetime Of Excited Electrons Research Articles

Femtosecond Time‐Resolved Measurement of Electron Relaxation at Metal Surfaces

AbstractTime resolved two‐photon photoemission, based on the equal pulse correlation technique, is used to measure the energy relaxation of the photoexcited electrons in various polycrystalline metal samples. While for Ag the inelastic lifetime of excited electrons is, within our time resolution, in agreement with the Fermi‐liquid theory, the result obtained for Au is very different. Scanning tunneling microscopy studies reveal that the enhanced inelastic lifetime of gold is not caused by surface effects. The measured inelastic relaxation time for transition metals with unoccupied d‐orbitals is shorter as compared to the noble metals, indicating that the inelastic decay depends on the number of unoccupied d‐orbitals below the probed energy state.

Observation of the self-quenching of thermalized-electron-mediated photochemistry on semiconductor surfaces

We show that the “thermalized” electron-mediated photodissociation of CH 3Br on GaAs(110) is self-quenched as the reaction occurs. The photoreaction rate, as measured by the ejected CH 3 flux, decreases rapidly as the surface is irradiated. Photoluminescence measurements on the substrate show that the reduction in the rate is due to a decrease in the lifetime of excited electrons which have relaxed to the conduction band edge. This reduced carrier lifetime results from enhanced surface recombination via Br-induced surface states. It is suggested that such self-quenching may be a common consideration for thermalized, photoexcitedcarrier-induced reactions on semiconductor surfaces.

Novel collective excitation spectrum and lifetime of electrons in a tunneling semiconductor superlattice

We present theoretical results for the effect of electron tunneling on the collective excitation spectrum and lifetime of electrons in a semiconductor superlattice. Using a tight-binding envelope function and taking account of the nearest-neighbor-layer overlap enables us to derive the dynamical dielectric tensor, the zero of which yields the peculiar dispersion relations. The quasiparticle lifetime of excited electrons is evaluated by using the plasmon-pole approximation and is shown to display novel behavior.

Negative hopping photoconductivity in lattice models

In the present paper a consistent theory of absolute negative photoconductivity discovered recently in concentrated ruby crystals is proposed for the model where two-level centres form a periodic lattice in three-dimensional space. An explicit expression for the current is obtained by the density matrix method and its frequency dependence is analysed. The conditions for realisation of the effect are considered depending on the degree of compensation of the dielectric. Some peculiarities of the effect are analysed on the basis of an exactly solved one-dimensional model at an arbitrarily long excited electron lifetime.

Feldeffekt und photoleitung an ZnO-einkristallen

It has been shown that atomic hydrogen produces an n-type accumulation layer on the surface of ZnO crystals. The high surface-conductivity associated with this layer can be lowered by adsorption of oxygen or by heating in a vacuum. In this way the surface conductance, which is large compared to the bulk conductance at 90°K, is varied over a range of five orders of magnitude. Three different methods are used to change carrier density at the surface without changing the gas coverage: field-effect, photoexcitation with chopped light and photoexcitation with steady illumination. From field-effect (change of carrier density due to transverse electric fields) and from surface photoconduction, using chopped light in the intrinsic absorption region, a drift mobility is derived. The results of these two methods are in good agreement. The drift mobility increases with increasing surface conductivity for several orders of magnitude and reaches 30 cm 2/Vsec. Measurements of surface photoconduction, using continuous irradiation in the intrinsic absorption region, combined with the drift mobility, yield information on the density and lifetime of excited electrons at the surface. As the irradiation intensity is increased through four orders of magnitude, the lifetime decreases from 0.5 sec to 7 × 10 −4 sec. However, if the surface conductivity is changed by four ciders of magnitude by adsorption or desorption, the lifetime remains constant and is therefore independent of adsorbed gases. A consistent explanation of the results is possible with a surface model containing two sets of continuously distributed surface states, one just below the conduction band and one just above the valence band. An estimation for the density of these fast surface states is given.

Number of Traps and Behavior of Excited Electrons in Luminescent Materials

The number of traps per unit volume in different types of luminescent materials, and the prolonged lifetimes of excited electrons due to trapping, were studied by means of curves of the rise of fluorescence with time under constant high-energy electron and alpha-particle excitation. These rise curves were taken after various dark decay periods and after different infrared irradiation conditions. They were found to be different in shape for different types of phosphors. Lifetimes longer than one week were found to occur in most of the zinc cadmium sulfides. The number of traps per unit volume responsible for lifetimes longer than about a minute was found to be of the order of ${10}^{15}$ (${10}^{\ensuremath{-}7}$ mole fraction) in the zinc cadmium sulfides, and the number responsible for lifetimes longer than a week was of the same order of magnitude. There is no direct correlation between infrared stimulability and the number of traps in which electrons remain for some time. Substances with extremely high stimulability (Fonda type) and those with extremely low stimulability both contained about the same number of these traps per unit volume. Materials with large concentrations of Ni exhibited no more such traps than those with smaller concentrations. Lack of phosphorescence is caused either by the preponderence of excited electrons of very long lifetime or by the radiationless recombination of electrons of short lifetime. Examples of each are given. It was found further that most of the electrons which remain in traps for long periods could be ejected by infrared radiation of wavelength greater than 1.25 \ensuremath{\mu}, indicating that retrapping of both electrons and positive holes is of major importance. The different effects of alpha particle excitation and electron excitation on the filling of the traps is discussed.