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A methodology is described to extract the absorption coefficient spectrum and exciton oscillator strength of GaN layers and GaN/AlGaN quantum wells by analyzing microtransmittance experiments in high-quality, free-standing membranes with thicknesses in the 160–230 nm range. The absorbance of a subwavelength GaN membrane is found to be an oscillating function of its thickness, in keeping with the standing wave effect. We analyze our results using two alternative models including interference effects and extract identical absorption coefficient values. The room-temperature absorption coefficient of bulk GaN membranes at the main exciton peak is found to be 9 × 104 cm−1. In the case of GaN/AlGaN quantum wells, the enhancement and blue shift of the excitonic absorption are observed, as a result of quantum confinement. [ABSTRACT FROM AUTHOR]

Excitonic effects in 1D semiconductors can be qualitatively different from those in higher dimensions. In particular, the Sommerfeld factor, the ratio of the above-band-edge excitonic continuum absorption to free electron-hole pair generation, has been shown to be less than 1 (suppressed) in 1D systems while it is larger than 1 (enhanced) in 2D and 3D systems. Strong continuum suppression indeed exists in semiconducting single-wall carbon nanotubes, a prototypical 1D semiconductor. However, absorption spectra for carbon nanotubes are typically fit with a combination of Lorentzians and a polynomial background baseline with little physical meaning. Here, we performed absorption measurements in aligned single-chirality (6,5) carbon nanotube films. The obtained spectra were fit with our theoretical model obtained by solving the Boltzmann scattering equation involving fifty-nine different types of transitions among three different types of quasiparticles. Specifically, we took into account microscopic interactions between photons, phonons, and excitons, including their dispersions, which unambiguously demonstrated that the background absorption is due to phonon-assisted transitions from the semiconductor vacuum to finite-momentum continuum states of excitons. The excellent agreement we obtained between experiment and theory suggests that our numerical technique can be seamlessly extended to compute strongly out-of-equilibrium many-body dynamics and time-resolved spectra in low-dimensional materials. [Display omitted] [ABSTRACT FROM AUTHOR]

The electronic absorption spectra of dithizone were recorded in various solvents and the absorption maxima of the dye were found to be dependent on solvent polarity. The molecule showed two absorption peaks in the visible region, the higher wavelength band was assigned to the keto form and the lower wavelength band is an overlap of the enol and the keto forms. The absorption data fitted well to Kamlet–Abboud–Taft equation. Studies done in a binary mixture of solvents (acetone + i -PrOH and dichloromethane + i -PrOH) showed that the probe molecule tends to form a hydrogen-bonding solvated complex with the more polar solvent. The values of preferential solvation constant (K PS ) were also calculated and it was found that the solute molecule preferentially solvated in i -PrOH. DFT calculations showed that the keto form absorbance occurred at 608 nm and was assigned to n → π* transition. The first-excited state electron densities suggested that the excited state was favorably stabilized in less polar solvents. [ABSTRACT FROM AUTHOR]