Exponential frequency dependence of multiphonon-summation infrared absorption

The recently observed exponential frequency dependence of the infrared absorption coefficient β in NaCl- and CaF₂-structure crystals is explained in terms of multiphonon absorption processes. A calculation gives the correct frequency dependence and order of magnitude of β, and predicts the temperature dependence, which should be easy to measure.     

Detection of electron paramagnetic resonance absorption using frequency modulation

A frequency modulation (FM) method was developed to measure electron paramagnetic resonance (EPR) absorption. The first-derivative spectrum of 1,1-diphenyl-2-picrylhydrazyl (DPPH) powder was measured with this FM method. Frequency modulation of up to 1.6 MHz (peak-to-peak) was achieved at a microwave carrier frequency of 1.1 GHz. This corresponds to a magnetic field modulation of 57microT (peak-to-peak) at 40.3 mT. By using a tunable microwave resonator and automatic control systems, we achieved a practical continuous-wave (CW) EPR spectrometer that incorporates the FM method. In the present experiments, the EPR signal intensity was proportional to the magnitude of frequency modulation. The background signal at the modulation frequency (1 kHz) for EPR detection was also proportional to the magnitude of frequency modulation. An automatic matching control (AMC) system reduced the amplitude of noise in microwave detection and improved the baseline stability. Distortion of the spectral lineshape was seen when the spectrometer settings were not appropriate, e.g., with a lack of the open-loop gain in automatic tuning control (ATC). FM is an alternative to field modulation when the side-effect of field modulation is detrimental for EPR detection. The present spectroscopic technique based on the FM scheme is useful for measuring the first derivative with respect to the microwave frequency in investigations of electron-spin-related phenomena.     

Dispersion dependence of two-photon absorption transition on frequency in Si PIN photodetector

The variation of two-photon absorption (TPA) coefficient \(\beta _{\mathrm{TPA}} (\omega )\) of Si excited at difference photon energy was investigated. The TPA coefficient was measured by using a picosecond pulsed laser with the wavelength could be tuned in a wide photon-energy range. An equivalent RC circuit model was adapted to derive the TPA coefficient \(\beta _{\mathrm{TPA}} (\omega )\) . The results showed that \(\beta _{\mathrm{TPA}} (\omega )\) varied from \(4.2 \times 10^{-4}\) to \(1.17 \times 10^{-3 }\)  cm/GW in the transparent wavelength region \(1.80<\lambda <1.36\,\upmu \) m of Si. The increasing tendency of \(\beta _{\mathrm{TPA}} (\omega )\) with the incident photon energy can be qualitatively interpreted as the photon energy increases from \(E_{\mathrm{ig}}/2\) to nearly \(E_{\mathrm{ig}}\) , the electrons excited from the valance band find an increasing availability of conduction band states. Comparing with the high-energy side transitions, the TPA coefficient in low-energy side is about 10 times too small. This can be attributed that the TPA transition in low-energy side is the process of photon-assisted electron transitions from valence to conduction band occurring between different points in k-space, while is direct transition in high-energy side.     

Account of frequency dependence of the acoustic absorption coefficient in solving problems of acoustic-brightness thermometry

We perform a comparative analysis of the estimate bias in measuring the internal thermodynamic temperature and time of thermal destruction of biological tissue using the acoustothermometry method based on the model of recording of thermoacoustic radiation from a temperature-inhomogeneous medium with allowance for frequency dependence of the acoustic absorption coefficient. An expression for acoustic-brightness temperature is obtained in the case of reception of radiation by a broadband acoustothermograph. The possibilities of using the described method for determining the thermal destruction threshold of biological tissue under hyperthermia are studied. The parameter range in which frequency dependence of the acoustic absorption coefficient can be neglected is found. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 6, pp. 478–488, June 2006.     

The effect of electron state hybridization on the frequency dependence of hopping conductivity in disordered systems

The effect of electron state hybridization on phononless hopping conductivity in a disordered system of point localization centers in the intermediate frequency range corresponding to electron transitions to distant centers is discussed. When the basis of the atom-type localized functions is applied, with the hybridization of the wave functions of the distant centers not being taken into account, the frequency dependence of phononless conductivity is shown to be superlinear and monotonic in a wide frequency range. In this case, the kink in the vicinity of a crossover from a linear to a quadratic frequency dependence of the conductivity turns out to be sharper as distinct from the standard theory.     

Mid-latitude anomalies in the diurnal variation of electron density in the ionosphere

The main morphological features of the F region of the mid-latitude ionosphere as obtained from Intercosmos-19 satellite measurements are presented. The causes of the anomalies in the diurnal variation of the electron density in certain longitudinal areas at the June and December solstice in the northern (Yakutsk anomaly) and southern (Weddell Sea anomaly (WSA)) hemispheres are determined. For both anomalies, the nighttime values of the critical frequency of the F2 layer, foF2, are higher than the daytime ones. Based on Intercosmos-19 satellite data, global maps of foF2 distribution for midday and midnight local time under high solar activity are drawn. Both anomalies occupy a large area in latitude and longitude, about 100° and 30°, respectively. The maximum difference between nighttime and daytime values of foF2 in the Yakutsk anomaly area reaches 1.0–1.5 MHz, smaller than that for the WSA (3.5–4.0 MHz). In the present work, these anomalies are reproduced with the help of a global self-consistent model of the thermosphere, ionosphere, and protonosphere (GSM TIP), and the mechanisms of their formation are preliminary investigated.     

Inverse Bremsstrahlung absorption coefficient in inhomogeneous plasma with nonextensive electron velocity distribution and Tsallis exponential electron density profile

Inverse bremsstrahlung (collisional) absorption of the laser beam is studied in plasma with a generalized (q-nonextensive) electron velocity distribution and some kind of generalized electron density profile. It is shown that for some values of parameters designating the q-nonextensive electron velocity distribution function and its generalized density profile, the calculated absorption coefficient reduces to the already known cases with Maxwellian velocity distribution with linear and exponential density profiles.