Self-broadening, self-shift and self-mixing in the ν2, 2ν2 and ν4 bands of NH3

Using Fourier-transform spectra (Bruker IFS 120 HR, resolution ≈0.004 cm⁻¹) of NH₃ in nine branches of the ν₂, 2ν₂ and ν₄ bands, self-broadening and self-shift as well as self-mixing coefficients have been determined at room temperature (T=295 K) for more than 350 rovibrational lines located in the spectral range 1000–1800 cm⁻¹. A non-linear least-squares multispectrum fitting procedure, including line mixing effects, has been used to retrieve successively the line parameters from 11 experimental spectra recorded at different pressures of pure NH₃. The accuracies of self-broadening coefficients are estimated to be better than 2% for most lines. The mean accuracies of line-mixing and line-shift data are estimated to be about 15% and 25%, respectively. The results are compared with previous measurements and with values calculated using a semiclassical model based upon the Robert–Bonamy formalism that reproduces rather well the systematic experimental J and K quantum number dependencies of the self-broadening coefficients.The results concerning line mixing demonstrate a large amount of coupling between the symmetric and asymmetric components of inversion doublets mainly in the ν₄ band. The line mixing parameters are both positive and negative. More than two thirds of the lines studied here have a positive shift coefficient. However, for most of them the shift coefficients are negative in the 2ν₂ band. They are positive for the R branch of the ν₂ band and for the ^PR and ^RP branches of the ν₄ band. For the other branches they are both positive and negative. Some components of inversion doublets illustrate a correlation between line mixing and shift phenomena demonstrated by a quadratic pressure dependence of line position.     

Experimental Studies of Variations in the Maximum Usable Frequency on Oblique Sounding Paths

We present the results of experimental studies of variations in the maximum usable frequency (MUF) on the latitudinal (England–Moscow and Khabarovsk–Nizhny Novgorod) and meridional (Cyprus–Moscow) paths. It is found that the quasi-period of MUF variations ranges from 20 min to several hours in spring 2002. The absolute value of the MUF varied in the range from 0.2 to 2 MHz. We show that variations in distance–frequency characteristics of HF signals propagated over the long-distance latitudinal path are strongly affected by quasi-periodic disturbances being the ionospheric response to acoustic-gravity waves excited by the terminator.     

Analysis of the relationship between neutron dose and Cerenkov photons under neutron irradiation through Monte Carlo method

To theoretically explore the feasibility of neutron dose characterized by Cerenkov photons, the relationship between Cerenkov photons and neutron dose in a water phantom was quantified using the Monte Carlo toolkit Geant4. Results showed that the ratio of the neutron dose deposited by secondary electrons above Cerenkov threshold energy to the total neutron dose is approximately a constant for monoenergetic neutrons from 0.01 eV to 100 eV. With the initial neutron beam energy from 0.01 eV to 100 eV, the number of Cerenkov photons has a good correlation with the total neutron dose along the central axis of the water phantom. The changes of neutron energy spectrum and mechanism analysis also explored at different depths. And the ratio of total neutron dose to the intensity of Cerenkov photons is independent of neutron energy for neutrons from 0.01 eV to 100 eV. These findings indicate that Cerenkov radiation also has potential in the application of neutron dose measurement in some specific fields.     

Sensitivity of stratospheric ozone lidar measurements to a change in ozone absorption cross-sections

The effect of a change in ozone absorption cross-section data is evaluated for stratospheric ozone lidar measurements, which are regularly performed for the monitoring of the ozone layer. The change is analysed for the measurements based on both the elastic and Raman scattering of the laser light by the atmosphere. The latter technique is essentially used for measurements performed in the presence of volcanic aerosol layers in the stratosphere. The discrepancy in ozone number densities is evaluated for various ozone cross-section data sets, using an atmospheric model for the evaluation of ozone cross-section temperature dependence. Results show that the difference in both elastic and Raman DIAL retrievals is below 1.5% in absolute values from 10 to 30 km. Above 30 km, the difference, estimated for the elastic DIAL retrieval only, is maximum around 45 km, with largest differences reaching 1.8% in the tropics.     

Experimental Study of the Nonstationarity of an Oblique Ionospheric Sounding Signal on a Mid-Latitude HF Radiopath

We consider issues relating to the nonstationarity of an oblique ionospheric sounding signal. Experimental data obtained with the Doppler goniometric facility of the Kazan State University are used. The equipment, methods of measurement, and the algorithms used for analysis of the experimental data are described. Typical coherence ranges of an ionospheric signal measured in the daytime at different frequencies on the Moscow – Kazan path are presented. Diurnal variations in the coherence range and the diurnal mean distributions of the Doppler shift and its drift velocity are analyzed. The relative contribution of short- and long-period variations to the signal distortions is examined using wavelet transform.     

Complex Experimental Evaluation of the Characteristics of Propagation of HF Signals Along Midlatitude Paths of Different Length and Orientation

We outline a complex approach for measuring distance–frequency, angular, spectral, and statistical characteristics of HF signals and present the results of experimental studies of the turbidity coefficient, frequency-spread bandwidth, variations in the Doppler frequency shift, bearing, and elevation angle in a wide frequency range for the Khabarovsk–Rostov-on-Don, Moscow–Rostov-on-Don, and Cyprus–Rostov-on-Don sounding paths under various geophysical conditions. It is shown that the most probable values of the turbidity coefficient for one-hop paths in the afternoon range from 2 to 4. The turbidity coefficient decreases to about 0.6 in the twilight hours. The turbidity coefficient tends to decrease with the path length. The average frequency-spread bandwidth at the 95% level of the received signal power is found to be minimal for illuminated path and does not exceed a value from 0.1 to 0.3 Hz. The frequency-spread bandwidth increases in the sunset hours or under nonuniform illumination, but does not exceed 4 Hz. We estimated the range of Doppler frequency shift variations which are due mainly to the influence of medium-scale traveling ionospheric disturbances and do not exceed 2 Hz in the experiments. The method of one-position location of a radiation source is tested by the results of angular measurements on the basis of the IRI-2001 ionospheric model.     

Instrumentation with polarized neutrons

Neutron scattering with polarization analysis is an indispensable tool for the investigation of novel materials exhibiting electronic, magnetic, and orbital degrees of freedom. In addition, polarized neutrons are necessary for neutron spin precession techniques that path the way to obtain extremely high resolution in space and time. Last but not least, polarized neutrons are being used for fundamental studies as well as very recently for neutron imaging. Many years ago, neutron beam lines were simply adapted for polarized beam applications by adding polarizing elements leading usually to unacceptable losses in neutron intensity. Recently, an increasing number of beam lines are designed such that an optimum use of polarized neutrons is facilitated. In addition, marked progress has been obtained in the technology of ³He polarizers and the reflectivity of large-m supermirrors. Therefore, if properly designed, only factors of approximately 2–3 in neutron intensity are lost. It is shown that S-benders provide neutron beams with an almost wavelength independent polarization. Using twin cavities, polarized beams with a homogeneous phase space and P>0.99 can be produced without significantly sacrificing intensity. It is argued that elliptic guides, which are coated with large m polarizing supermirrors, provide the highest flux.     

Experimental investigation of the line center of Hg intercombination spectral line 253.7 nm perturbed by Kr

The collision broadening and shift of the Hg intercombination spectral line 253.7 nm (6¹S₀–6³P₁) perturbed by Kr has been investigated using a high-resolution scanning Fabry–Perot interferometer. The values of the pressure broadening and shift coefficients β and δ, respectively, for the studied line have been obtained. The obtained coefficients β and δ are compared with their corresponding published experimental values and also those calculated using Lindholm–Foley impact theory.     

Correlation between P chemical shift tensor and local structure in lithium cyclohexaphosphates Li6P6O18·3H2O and Li6P6O18

To understand the surprising behavior between the variations of the P′–P–P″ angles and the correlated variations of the O′–P–O″ ones, two lithium cyclohexaphosphate compounds Li₆P₆O₁₈·3H₂O and Li₆P₆O₁₈ are studied by solid state nuclear magnetic resonance (NMR) spectroscopy. The two compounds exhibit the same [P₆O₁₈]⁶⁻ ring anions but with 3m or internal symmetry, respectively. Such symmetries induce local distortions that are exhibited by NMR spectroscopy. One-dimensional (1D) NMR gives information on structural sites of ⁷Li and ³¹P ions and the crystallographic non-equivalencies are observed. Nevertheless, in the anhydrous compound, X-ray diffraction and NMR results do not completely agree and some discrepancy exists between the number of sites observed with the first technique and the number of lines exhibited in the NMR spectra either for ⁷Li or ³¹P nuclei. This problem is elucidated by using 2D double quantum NMR spectroscopy coupled with theoretical considerations. We find that the ³¹P chemical shift tensor is dependent on the deviations of the O–P–O angles from those in the regular tetrahedron. Within the same empirical model, we suggest that the surprising behavior between the variations of the P′–P–P″ and the ones of the O′–P–O″ is related to the overall charge on the PO₄ group. We also find the positions of the isotropic lines for ⁷Li essentially depend on the site co-ordination of this nuclei.     

Trace Detection of Low Concentrations of Ozone Using a CO<Subscript>2</Subscript> Laser with Automatic Tuning over the Spectrum

A technique of detection of ozone, using a CO₂ laser with rapid automated tuning over 8–10 generation lines in the 9.4–9.6-µm spectral range, has been developed. Its experimental verification has shown a higher accuracy of detecting low concentrations of O₃ in comparison with the traditional technique of differential absorption on two lines (on-line and off-line). Using the technique suggested, the content of ozone in the vicinity of city roads with heavy traffic has been measured.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 251–255, March–April, 2005.     

Pressure Lineshift and Broadening Coefficients in the 2ν3 Band of OCS

In this study we report the first measurements of the pressure-induced lineshift coefficients due to Ar, He, O₂, and N₂ for 22 rovibrational lines from P(53) to R(53), belonging to the 2ν₃ band of ¹⁶O¹²C³²S at 4100 cm⁻¹. The lineshift results were obtained from the simultaneous record of the pressure-broadened and pure low-pressure OCS lines, using a tunable difference-frequency laser spectrometer. For four lines of the 2ν₃ band we also report Ar-, He-, O₂-, and N₂-broadening coefficients by fitting Voigt and Rautian profiles to the measured shapes of these lines. The broadening and shift coefficients are compared to the results of theoretical calculations based on the semiclassical Robert–Bonamy formalism and two different isotropic and anisotropic intermolecular potentials. For OCS–Ar we also consider the Smith–Giraud–Cooper model including all orders of the interaction within the peaking approximation. In all cases, the calculated broadening coefficients are in reasonable agreement with the experimental data. By considering adjustable parameters for the vibrational dependence of the isotropic potential, the general trends of the lineshifts with J can be roughly predicted, except at low J values where an asymmetry behavior for P and R branches is generally observed.     

Ultra cold neutron life times in glow discharge cleaned bottles

Life times of ultra cold neutrons in tubes made of various materials have been increased with improvement of surface cleanliness. With glow discharge cleaning in vacua of only 10^–4–10^–3 torr, life times for long storage periods were obtained that differ by less than a factor of 2 from calculated values in the case of stainless steel and aluminium and by a factor of 3 for quartz. Surface recontamination is slow at the achieved level and approaches a stable value well below that before cleaning. A temperature dependence of ultra cold neutron reflection loss rates was observed. A direct correlation between reflection losses and the surface hydrogen concentration was established. We conclude that for smooth surfaces of materials with low reaction cross section, losses are mainly caused by upscattering on tightly bound hydrogen. The probability is discussed that the remaining losses are due to still incomplete cleanliness of the surface caused by hydrogen recontamination from the bulk and the atmosphere.     

Effect of the oxygen pressure on the microstructure and optical properties of ZnO films prepared by laser molecular beam epitaxy

A series of ZnO films were prepared on the Si (1 0 0) or glass substrate at 773 K under various oxygen pressures by using a laser molecular beam epitaxy system. The microstructure and optical properties were investigated through the X-ray diffraction, Raman spectrometer, scanning electron microscope, ultraviolet–visible spectrophotometer and spectrofluorophotometer. The results showed that ZnO thin film prepared at 1 Pa oxygen pressure displayed the best crystalinity and all ZnO films formed a columnar structure. Meanwhile, all ZnO films exhibited an abrupt absorption edge near the wavelength of 380 nm in transmission spectra. With increasing the oxygen pressure, the transmission intensity changed non-monotonically and reached a maximum of above 80% at 1 Pa oxygen pressure, based on which the band gaps of all ZnO films were calculated to be about 3.259–3.315 eV. Photoluminescence spectra indicated that there occurred no emission peak at a low oxygen pressure of 10⁻⁵ Pa. With the increment of the oxygen pressure, there occurred a UV emission peak of 378 nm, a weak violet emission peak of 405 nm and a wide green emission band centered at 520 nm. As the oxygen pressure increased further, the position of UV emission peak remained and its intensity changed non-monotonically and reached a maximum at 1 Pa. Meanwhile the intensity of green emission band increased monotonically with increasing the oxygen pressure. In addition, it was also found that the intensity of UV emission peak decreased as the measuring temperature shifted from 80 to 300 K. The analyses indicated that the UV emission peak originated from the combination of free excitons and the green emission band originated from the energy level jump from conduction band to O_Zn defect.     

Study of ozone concentration variations in the upper stratosphere using millimeter wave spectroscopy methods

New results of the study of ozone concentration variations in the upper stratosphere over Moscow in the layer at a height of 40 km, which is most sensitive to anthropogenic impacts, are presented. Changes in the ozone concentration and its relation to other atmospheric parameters in cold periods of 2008–2009 and 2009–2010 are analyzed. It was shown that there exist regions with decreased ozone content in the polar vortex and outside of it in air with higher temperature in the upper stratosphere. These phenomena cause deformations of vertical ozone distribution profiles and an appreciable shift of the maximum of the relative content of ozone to lower stratosphere layers.     

Determination of Self-, Air-, and Oxygen-Broadening Coefficients of Pure Rotational Absorption Lines of Ozone and of Their Temperature Dependencies

Room temperature measurements of self-, air-, and oxygen-broadening coefficients are reported for 101 pure rotational absorption lines for ¹⁶O₃ covering a range of 7≤J″≤34 and 3≤K₋₁″≤11 in the spectral region 50 to 90 cm⁻. In addition, the temperature dependence of the coefficients has been determined for most of these lines. A total of 14 high-resolution Fourier transform far-IR spectra (0.004 cm⁻ resolution) of ozone/air/dioxygen mixtures were recorded at various temperatures (212, 252, and 296 K). The broadening coefficients and corresponding temperature exponents were deduced by analyzing all of the 14 spectra simultaneously for each absorption line using a nonlinear least-squares fitting technique. Several sources of systematic errors were taken into account: the values of the partial pressures of the species involved, the value of the total optical path length, the sample temperature, thermal emission from sources other than the spectrometer source (e.g., sample emission), the effect of convolution with the instrument function, and intensity contributions from weak and neighbouring absorption lines. The variation of the determined broadening coefficients and temperature exponents with the rotational quantum numbers J″ and K₋₁″ has been examined. Furthermore the determined pressure-broadening parameters have been compared to the data reported in the 2000 edition of the HITRAN compilation.     

Fourier transform measurements of SO2 absorption cross sections: II.: Temperature dependence in the 29 000–44 000 cm (227–345 nm) region

This paper is the second of a series that reports results on the measurements of the absorption cross section of SO₂ in the UV/visible region at high resolution and that investigates high temperatures in support to planetary applications. Absorption cross sections of SO₂ have been obtained in the 29 000–44 000 cm⁻¹ spectral range (227–345 nm) with a Fourier transform spectrometer at a resolution of 2 cm⁻¹ (0.4500 cm MOPD and boxcar apodisation). Pure SO₂ samples were used and measurements were performed at room temperature (298 K) as well as at 318, 338 and 358 K. Temperature effects in this spectral region are investigated and are favorably compared to existing studies in the literature. Comparison of the absorption cross section at room temperature shows good agreement in intensity with most of the literature data, but shows that most of the latter suffer from inaccurate wavelength scale definition. Moreover, literature data are often given only on restricted spectral intervals. Combined with the data described in the first part of this series of papers on SO₂, this new data set offers the considerable advantage of covering the large spectral interval extending from 24 000 to 44 000 cm⁻¹ (227–420 nm), at the four temperatures investigated.     

Influence of Nonlinear Gain and Nonradiative Recombination on the Quantum Noise in InGaAsP Semiconductor Lasers

Effects of nonlinear gain and nonradiative recombination on characteristics of intensity and phase noises of InGaAsP lasers emitting in a wavelength of 1.55 Μm are investigated. The investigations are performed based on a self consistent numerical approach that takes into account the cross correlation of fluctuations among the injected electron number, photon number and the optical phase. Time variations of the fluctuations of the intensity and the shift of the lasing frequency are statistically analyzed. The corresponding frequency dependencies of intensity and frequency noises are characterized. The results show that both intensity and frequency noises around the relaxation frequency are suppressed when counting the nonlinear gain in the rate equations. The intensity noise is enhanced in the low frequency regime by increasing the nonradiative recombination. The results fit well with those predicted by the small-signal analysis.     

Influence of the interaction of intramolecular and crystalline oscillations on the form of the phononless line of impurity molecules

The temperature broadening and shift of phononless lines of the impurity centers due to interaction of intraimpurity and crystal oscillations are investigated. It is shown that the width of phononless lines as a function of the mode-interaction parameter has a maximum, which is reached at parameters corresponding to the appearance of unstable oscillations. Numerical calculations show that the rate of temperature broadening is limited at the level of 0.5 cm^–1/deg.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 45–50, December, 1987.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Anodization of aluminium thin films on pSi and annihilation of strong luminescence from Al2O3

Photoluminescence (PL) of Al₂O₃ films obtained by anodization of thermally evaporated and annealed thin Al films on p⁺⁺Si in 0.3 M oxalic acid has been investigated. Thermal annealing at 200–950 °C under the dry nitrogen atmosphere was used for deactivation of luminescence centres. Luminescence from as grown films was broad and located at 425 nm. This luminescence reached to highest level after annealing at 600 °C. Maximum 10 min was required for full optical activation and prolonged annealing up to 4 h did not change the luminescence intensity. Because of deep levels, absorption band edge of as grown films was shifted to the lower energy which is 3.25 eV. Annealing above 800 °C reduced the PL intensity and this observation was correlated with the blue shift of band edge as the defects annealed out. Disappearing PL intensity and blue shift of band edge absorption after annealing at 950 °C was mainly attributed to the oxygen-related defects and partly to impurities that may be originated from oxalic acid. AFM results did not show any hexagonally ordered holes but uniformly distributed nanosized Al₂O₃ clusters that were clearly seen. XRD measurements on as grown Al₂O₃ showed only [1 1 0] direction of α phase. Debye–Scherer calculation for this line indicates that cluster size is 35.7 nm. XRD and AFM pictures suggest that nanocrystalline Al₂O₃ are embedded in amorphous Al₂O₃.