Direct Optical Absorption Research Articles

Density-operator description of excitons in molecular aggregates. II. Optical line-shape selection rules of "one-dimensional" excitons and excitation localization in finite molecular chains

In a previous paper, a method based on superoperators techniques suggested by Fano and on the stochastic exciton Hamiltonian proposed by Haken et al., has been proposed for the calculation of optical line shapes and macroscopic dynamics of exciton states of an assembly of oriented dimers in a nonresonant matrix. This method is now extended to the calculation in a very simple manner for larger systems: analytical expressions are obtained for optical line shapes of exciton states in finite chains where the Born-von K\'arm\'an boundary conditions are relaxed in the nearest-neighbor constant coupling scheme. Edge effects (asymmetrical nondefinite positive contributions to the absorption) appear to be negligible only for large chains ($N\ensuremath{\gtrsim}20$) and also depend on the prepared state. Numerous line shapes are presented; molecular chains possessing simple geometrical configurations (parallel, alternate, and helix) exhibit specific optical selection rules, shown to be very sensitive to the conformation (for instance, to the angular step in helix chains). The mathematics of the selection rules are worked out extensively. The selection rules are related to the concept of collective excitations in a homogeneous photon field and with analogy to an interference experiment. For large chains the selection rules relax to those predicted for one-dimensional crystals. Using an entropylike function from information theory, we have investigated the expectation value of the degree of localization of an electronic excitation in an aggregate by direct optical absorption. The localization is discussed quantitatively: for instance, it is shown that, in the absence of traps, drastic conditions are required for the material system and the exciting light (polarization, energy, and coherence) to obtain localization on a site.

Excited-state absorption spectra of ZnS : Mn

Absorption bands have been found at 655 and 830 nm in a Mn 2+-doped ZnS crystal under intense optical excitation. From the close correlation to the orange fluorescence, these bands are ascribed to the transitions from the emitting state 4T 1( 4G) of Mn 2+. This observation of the excited-state absorption makes it possible for the first time to locate higher excited levels of impurity ions hidden behind the strong fundamental absorption of the host material by a direct optical absorption method. Assignments for the terminal states of the observed absorption transitions are discussed.

Excitation Spectra of Luminescence in Cadmium Sulfide

AbstractExcitation spectra of the excitonic luminescence in CdS are presented for three types of samples: doped, pure, and surface contaminated samples. The positions and relative magnitudes of peaks appearing in some of these spectra are analyzed and quantitatively explained in terms of indirect creation of free excitons. The different behaviour of the three types of samples investigated allows a more general discussion which leads to important discriminations between direct and indirect optical absorption and between volume and surface radiative recombination.

The fermi surface and optical properties of copper

A Fourier expanded model Hamiltonian is applied to copper. The parameters in this phenomenological Hamiltonian are adjusted to fit the de Haas-van Alphen periods, the cyclotron effective masses, and the position of the lowest direct optical interband absorption. The agreement with experiment suggests that the electron-phonon enhancement of the effective mass is small, and that the optical properties are consistent with direct wave-vector conserving transitions between one-electron states.

Effects of Current Flow on the Optical Absorption of a Semiconductor

The effect of electric-field-induced current flow on the direct optical absorption in a semiconductor is calculated. The calculations are performed specifically for InSb, under the assumption that the only effect of the electric field is to shift the distribution of carriers in k space; however, the method may be applied to any semiconductor for which the electron wave functions have a sufficiently simple analytic form, and the substitution of other angularly dependent distribution functions would not alter the interesting characteristics of the results. A change in the absorption is found which is dependent on the photon energy and varies approximately as the square of the electric field. It is also found that the absorption depends on the relative orientation of the dc electric field and the polarization vector of the light. This polarization effect is, in turn, found to vary as the square of the carrier drift velocity, and its magnitude and sign are found to depend on the relative amounts of $X$ and $Z$ components in the atomic $p$ orbitals in the carrier wave functions. Curves showing the results of calculations of the absorption change and the polarization effect for both fundamental and inter-valence-band absorption are presented. The possibility of measuring current-flow effects and the extraction from such measurements of information concerning the band structure and distribution functions of semiconductors are discussed. A comparison is made between the effect considered here and a similar effect studied experimentally by Bray and Pinson in $p$-type germanium.

Direct optical absorption selection rules for the hexagonal close-packed lattice

The direct electric dipole optical selection rules between Bloch electronic states at the symmetry points of the Brillouin zone of the hexagonal close-packed lattice are calculated. Bloch wave functions transforming according to the irreducible representations of both the single and double groups are considered.