Size-selected Spectroscopy Research Articles

Structure and IR Spectra of Microhydrated Cl2 with an Excess Electron: Experiment versus Theory

A working methodology to generate theoretical IR spectra following ab initio electronic structure methods is reported. Theoretical IR spectra of Cl(2)(•-)·nH(2)O clusters (n = 1-5) are generated as a case study. Excellent agreement between the calculated and the reported experimental IR spectra based on size-selected spectroscopy is observed. It is shown that uniform scaling of calculated harmonic frequencies of these hydrated clusters fail to produce accurate IR spectra. Two different scaling factors in two different regions of O-H stretching of solvent water molecules are needed to account for the anharmonic contribution. This observation is also true for Br(2)(•-)·nH(2)O and I(2)(•-)·nH(2)O systems.

Excited States of InAs/GaAs Quantum Dots

Exciton properties of self-organized InAs/GaAs QDs are investigated. Size-selective spectroscopy reveals the excited single-exciton transition spectrum and enhanced polar exciton-LO-phonon coupling. A good qualitative understanding of the experimental results is achieved by eight-band k.p calculations. Renormalization of the single-exciton spectrum in highly populated QDs shows the importance of many-particle interactions in the strong confinement limit. The ground state transition energy decreases by ∼16 meV for QDs occupied with ∼18 excitons.

Spectroscopy of extremely cold silver clusters in helium droplets

The first highly resolved electronic spectra of small non-alkali metal clusters embedded in nanoscopic helium droplets (N≈20000) are presented. The helium droplets serve as an extremely cold liquid matrix with a temperature of 0.37 K. Resonant two-photon-ionization is used for size-selective spectroscopy on a silver cluster distribution (N≤10). Results are reported for Ag2, Ag3 and for Ag8. For the latter, a very narrow absorption spectrum is resolved corresponding to the Ag8 plasmon resonance. The linewidth of the observed resonance is a factor of two smaller than theoretical zero-temperature predictions for the plasmon line width of closed-shell metal clusters. In contrast, the lifetime of the Ag8 resonance is estimated to be of the order of nanoseconds, which is inconsistent with the plasmon picture and typical for molecular transitions.

Size-dependent electronic level structure of InAs nanocrystal quantum dots: Test of multiband effective mass theory

The size dependence of the electronic spectrum of InAs nanocrystals ranging in radius from 10–35 Å has been studied by size-selective spectroscopy. An eight-band effective mass theory of the quantum size levels has been developed which describes the observed absorption level structure and transition intensities very well down to smallest crystal size using bulk band parameters. This model generalizes the six-band model which works well in CdSe nanocrystals and should adequately describe most direct semiconductor nanocrystals with band edge at the Γ-point of the Brillouin zone.

Growth and optical properties of semiconductor nanocrystals in a glass matrix

Over the last 15 years nanocrystals embedded in a glass matrix have been a subject for studies of fundamental phenomena of quasiparticles (electrons, holes, excitons, phonons) quantum confinement in the nanosize semiconductor materials. Growth of the nanocrystals in a glass matrix is based on the thermodynamic process of the diffusion-controlled phase decomposition of oversaturated solid solutions. Three stages of the process in solutions prepared by co-melting, co-sputtering and ion-implantation techniques are discussed. It is shown that the growth technique makes it possible to vary the mean size of the particles, their size distribution and crystalline structure. The optical properties of nanocrystals of various semiconductor compounds grown in different glass matrices are discussed. Attention is given to studies of a fine structure of optical spectra at resonant size-selective spectroscopy for both “strong” and “weak” confinement regimes. Energy spectra of confined acoustic and optical phonons in a “strong” confinement regime, studied by resonant Raman scattering, are discussed.