Free Exciton States Research Articles

J-aggregate formation of long-chain merocyanine dye in various mixed monolayers and its exciton behaviours in Langmuir-Blodgett films

J-Aggregate formation of a long-chain merocyanine dye in mixed monolayers with various lipid matrices such as glycerides, phospholipids and cholesterol was investigated. From visible absorption spectra and fluorescence microscopy, different extents of J-aggregate formation together with various morphologies have been observed in the LB films, depending on the matrix agents and the compositions. By applying the Urbach rule it has been revealed that exciton-phonon coupling is weak and fluorescence occurs by radiative annihilation from the free-exciton state. From the decay curves, fluorescence lifetimes of 40 and 230 ps were obtained, which were prolonged with increasing content of arachidic acid as a matrix. The fluorescence lifetime was shortened in the presence of an electron acceptor.

Branching of exciton relaxation to the free and self-trapped exciton states

The transient free-exciton luminescence and unrelaxed self-trapped exciton luminescence from pyrene have been studied. It is found that above 120 K, the structural phase-transition point, most of the photoproduced excitons relax directly towards the self-trapped state and only a small fraction of them relax to the bottom of the free-exciton band. At temperatures below 60 K no exciton relaxes to the bottom of the free-exciton band except in the case where excitons are produced close to the bottom of the free-exciton band. When free-exciton luminescence is absent, luminescence due to unrelaxed self-trapped excitons (transient self-trapped excitons) has been confirmed. The self-trapping barrier height at low temperatures is estimated to be about 360 cm −1, which should be compared to the height, 260 cm −1, at high temperatures.

Time-Resolved Excitonic Luminescence Processes in Poly(phenylenevinylene)

Integrated luminescence spectra, time-resolved luminescence spectra, and luminescence decay times of stretch-oriented poly(phenylenevinylene) (PPV) are studied in the temperature range 300∼12 K. The reflectance spectrum at room temperature is also studied. The luminescence spectrum is composed of vibronic bands and a broad band, which are interpreted as caused by radiative annihilation of free and selftrapped excitons, respectively. Temperature dependence of the intensity of free-exciton luminescence and temperature dependence of the decay time are interpreted in terms of relaxation of excitons. The height of a potential barrier, which separates the free exciton state and the self-trapped state, is found to be 350 cm -1 . At 20 K, exciton relaxation toward the self-trapped state occurs by a quantum-mechanical tunneling process. The tunneling rate obtained is (90 ps) -1 .

Resonance Raman Spectra and Time-Resolved Emission Spectra in Exciton-Phonon Systems: Calculation in the Dynamical Coherent Potential Approximation

Light scattering spectra, both stationary response for monochromatic excitation and time-resolved response for pulse excitation, at zero temperature are calculated in the model of a Frenkel exciton coupled to Einstein phonons, by using the dynamical coherent potential approximation. For resonant excitation, calculated spectra are characterized by hot luminescences from a nearly free exciton state and also from vibronic states of a self-trapped exciton. Calculated time-resolved spectra manifest exciton-phonon dynamics involving these states, such as configurational tunneling between the free state and the self-trapped state.

Selective excitation of localised excitons in ZnSe1-xTexsolid solutions

Employing selective monochromatic laser excitation, the photoluminescence in ZnSe1-xTe/sub /x alloys with Te concentrations 1%<or=x<or=2.3% is investigated. The structured emission band is interpreted as being due to recombination of excitons localised at different types of small Te clusters. The energy spectrum of these excitons is inhomogeneously broadened. The details of exciton-lattice interaction revealed under resonant excitation of these exciton states strongly favour the model of large-radii excitons tightly bound through the hole. For excitation above the exciton gap, resonant Raman scattering by longitudinal optical (LO) phonons is observed. The measured degree of polarisation is used to discriminate between this process and the narrow-line LO resonance luminescence found under selective excitation of the localised states. The smooth transformation from Raman scattering to resonance fluorescence indicates that, owing to strong interaction between the cluster and band states, no energy gap exists between the free and localised exciton states.

Dynamics of the self-trapping process in cesium iodide

To describe the autolocalization at V k-centres in simple cubic alkali halides the adiabatic potentials are investigated. The barriers separating free and self-trapped excitonic states are analyzed for a wide range of the input parameters, and the rates of the respective multi-phonon non-radiative transitions are derived. The obtained results are consistent with the experimental data in CsI.

Picosecond resonance fluorescence and energy transfer of localized electronic states in silver halides

In AgCl and AgBr at low temperature an elastic light scattering process is observed which is attributed to resonance fluorescence from electronic states of small non-metallic silver clusters. The time dependence and polarization are analyzed in terms of a model taking energy and cross-relaxation into account. The strong reduction of the decay time above the exciton absorption edge is quantitatively explained by resonant energy transfer between the electronic states of the clusters and the free exciton states.

Competition and coexistence of free and self-trapped excitons in aromatic molecular crystals

Picosecond spectroscopy combined with ordinary luminescence measurements has been applied to elucidate excited states in pyrene and α-perylene crystals. The low-temperature luminescence spectra of a pyrene crystal which is cooled through its structural phase transition point (120 K) differ significantly from those supercooled without phase change. The latter shows vibronic, violet luminescence (V-luminescence) bands whose characteristics are quite similar to Y luminescence bands in α-perylene. The V-state in pyrene and the Y-state in α-perylene as the origins for V- and Y-luminescence respectively are stable states below about 60 K but they are unstable at high temperatures. To explain observed facts in those crystals a three-valley potential for the lowest excited states is proposed, where V- and Y-states are considered to be a kind of self-trapped exciton states (STE-I in this paper). STE-I and STE-II (so-called excimer) states are separated by potential barriers, Barrier-I and -II, from the free-exciton state.

Free, quasifree, momentarily trapped, and self-trapped exciton states in molecular crystals.

It is shown that the self-trapped, quasifree, and momentarily trapped exciton states observed in crystals are forms of composite exciton-phonon states which arise due to nonzero exciton-phonon interaction. For anthracene crystals, the present results agree very well with experiments.

Magnetization and magnetoreflectivity measurements in Zn1-xMnxTe with 0.25

The exchange splitting of the free-exciton state in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te (0.25\ensuremath{\le}x\ensuremath{\le}0.71) was studied at helium temperature and in magnetic fields up to 5.5 T. Magnetoreflectivity measurements were used in order to determine the exciton splitting. Magnetization measurements were carried out on the same samples, at the same temperature, and in magnetic fields up to 6 T. The comparison of magnetooptical with magnetic data yields the difference between the exchange integrals \ensuremath{\alpha} and \ensuremath{\beta} of ${\mathrm{Mn}}^{2+}$ ions with conduction and valence electrons. Contrary to what was expected, \ensuremath{\alpha}-\ensuremath{\beta} decreases strongly when x increases between 0.25 to 0.71.

Coupling of ground and excited state excitations in GaAs under uniaxial stress

We report new experimental evidence of the exchange interaction between heavy and light excitons near the Brillouin zone center in GaAs. We have combined polarized photoluminescence excitation spectroscopy and uniaxial stress at liquid He temperatures in order to elucidate the structure of the free exciton states in low donor concentration GaAs grown by molecular beam epitaxy. The data indicate that the exchange interaction is small (deduced from the peak positions of the polarized spectra) but finite (due to the observation of level repulsion between the ground state of the light exciton and excited states of the heavy exciton at a stress of about 0.5 kbar).

Pressure-Induced Transformation of Excitons in AgBr1-xClx

Pressure dependence of photoluminescence and absorption spectra in AgBr 1- x Cl x ( x =1.0, 0.6 and 0.15) is investigated, using a diamond anvil cell with N 2 as pressure medium in order to generate good quality of hydrostatic pressure at 1.5 K. Pressure coefficients of self-trapped exciton states in AgCl, AgBr 0.4 Cl 0.6 and that of free exciton state in AgBr 0.85 Cl 0.15 are determined. After initial self-trapped exciton disappears at high pressures, new emission bands are found to appear in AgCl and AgBr 0.4 Cl 0.6 . These phenomena are ascribed to pressure-induced phase transition of excitons without accompanying a change of crystal structures.

Dynamical Aspects of Self-Trapping of 1s Excitons in RbI and KI

Previously direct measurements of the self-trapping time of 1 s excitons in KI was reported. In this paper, measurements for RbI are described. Picosecond pulses of the fifth harmonic (212.8 nm) of a Nd 3+ : YAG laser, which can directly generate the 1 s excitons are used as the excitation source. The rise time of the E x luminescence is measured at 4.2 K. From the observed rise time, the self-trapping time is estimated to be 2.0±0.5 ns. The height of the adiabatic potential barrier separating the free and self-trapped exciton states is measured to be 18 and 30 meV for RbI and KI, respectively. The self-trapping time and the barrier height in RbI and KI are compared, and their changes with changing material are discussed on the basis of the theoretical calculation.

Self-trapping of hot and thermalized excitons in solid xenon

The first direct measurement of the self-trapping rate, Γ ST, of thermalized excitons in solid Xe is presented. The temperature dependence of Γ ST is explained by thermal assisted tunneling through the self-trapping barrier separating free and self-trapped exciton states. The results agree with recent theories on self-trapping. The ratio of luminescence intensity of free and self-trapped states disagrees with the measured self-trapping rates and displays the role of self-trapping of hot excitons.

Bound and free exciton states in InP

Photoluminescence (PL), PL excitation (PLE) and photoconductivity (PC) studies in the near band edge spectral region of InP are reported. Very well resolved donor bound exciton (Do,X) lines are observed at low excitation intensity. Selective excitation of the (Do,X) “two electron” PL satellites is studied. A comparison is made between the relative strength of the different exciton features in PLE and PC.

Near-gap energy levels of InP-luminescence and photoconductivity study

Near-band-gap bound and free exciton states in high purity InP are investigated using photoluminescence (PL) and photoconductivity (PC) techniques. Direct comparison is made between features observed in PL excitation and PL spectra. The effects of excitation intensity and applied electric field on the PL spectra are studied and comparison is made with corresponding phenomena in GaAs. Values for the shallow donor binding energy and for the binding energy of excitons at ionised donors are derived.

Optical Absorption and Exciton Lattice Interaction in α- and β-Perylene

The shape of the lowest exciton absorption band in α- and β-perylene due to the B 2u ←A g transition, the halfwidth of the absorption band, and the steepness parameter of the Urbach tail are investigated. From linear temperature dependence of the halfwidth and the Lorentzian line-shape, the transition A u ←A g is well explained by the exciton band picture. The lattice relaxation behavior of excitons after they are formed is described by the strong exciton-phonon coupling scheme. Based on the steepness parameter obtained, 0.93 in α-perylene and 1.38 in β-perylene, and the amount of the Davydov splitting obtained, 2040 cm -1 in α-perylene and 960 cm -1 in β-perylene, the lattice relaxation energy is estimated to be 1530 cm -1 and 523 cm -1 respectively. Also the energy difference between the free and self-trapped exciton states is expected to be 620 cm -1 in α-perylene and 43 cm -1 in β-perylene.

Hot-exciton exciton on excitation spectra in NaI

Two series of oscillation structure with the period of LO phonon energy are observed in the 2 s exciton absorption region of the excitation spectra of free and self-trapped exciton luminescence. This structure is explained by the usual cascade relaxation process of 2 s hot-excitons with LO phonon emission. Two oscillation series are due to different relaxation processes of 2 s hot-excitons; (1) self-trapping to π state, (2) scattering to the 1 s free exciton state.

Resonant Electronic Raman Scattering and Electronic Structures of Bound Exciton Complex in ZnSe

Electronic Raman scattering of shallow donors in cubic ZnSe has been studied for the incident photon energy in the vicinity of the n =2 free exciton level. Raman scattering intensities for 1s→2s and 1s→2p excitations of donor electrons are resonantly enhanced as the photon energy coincides with the excited states of exciton-neutral donor complex. Since these excited states are located within the continuous dispersion of free exciton state, the large enhancement in the Raman scattering intensity has been attributed to the mixing of these excited states with the free exciton state. The resonant behavior of the electronic Raman scattering shows that these excited states correspond to the lowest lying electronic excited states of the exciton neutral donor complex.

Excitation mechanisms and excited states of the I1 bound exciton in CdS

AbstractFor the neutral‐acceptor‐bound exciton emission (I1) of CdS three new excitation lines with light E ⟂ c are found at 1.2, 2.5, and 3.7 meV on the high energy side of the I1 line. The nearest one is interpreted as due to the simultaneous excitation of the bound exciton and a transverse‐acoustic phonon. The other two are understood as the direct creation of the I1 bound exciton in excited states with one of the equal holes belonging to a higher configuration. With light E ‖ c the excitation mechanisms via the free exciton states AL and AF and the bound exciton states I1B and I1B are discussed. An excitation intensity dependent energy shift of the long wavelength tail of the band edge is reported for the first time.