Lowest-frequency Line Research Articles

МІЖЗОРЯНЕ СЕРЕДОВИЩЕ ТА ДЕКАМЕТРОВА РАДІОСПЕКТРОСКОПІЯ

Purpose: The analytical review of the main results of research in the new direction of the low-frequency radio astronomy, the interstellar medium radio spectroscopy at decameter waves, which had led to astrophysical discovery, recording of the radio recombination lines in absorption for highly excited states of interstellar carbon atoms (more than 600). Design/methodology/approach: The UTR-2 world-largest broadband radio telescope of decameter waves optimally connected with the digital correlation spectrum analyzers has been used. Continuous modernization of antenna system and devices allowed increasing the analysis band from 100 kHzto 24 MHz and a number of channels from 32 to 8192. The radio telescope and receiving equipment with appropriate software allowed to have a long efficient integration time enough for a large line series simultaneously with high resolution, noise immunity and relative sensitivity. Findings: A new type of interstellar spectral lines has been discovered and studied, the interstellar carbon radio recombination lines in absorption for the record high excited atoms with principal quantum numbers greater than 1000. The line parameters (intensity, shape, width, radial velocity) and their relation ship with the interstellar medium physical parameters have been determined. The temperature of line forming regions is about 100 K, the electron concentration up to 0.1 cm–3 and the size of a line forming region is about 10 pc. For the first time, radio recombination lines were observed in absorption. They have significant broadening and are amplified by the dielectronic-like recombination mechanism and are also the lowest frequency lines in atomic spectroscopy. Conclusions: The detected low-frequency carbon radio recombination lines and their observations have become a new highly effective tool for the cold partially ionized interstellar plasma diagnostics. Using them allows obtaining the information which is not available with the other astrophysical methods. For almost half a century of their research, a large amount of hardware-methodical and astrophysical results have been obtained including a record number of Galaxy objects, where there levant lines have been recorded. The domestic achievements have stimulated many theoretical and experimental studies in other countries, but the scientific achievements of Ukrainian scientists prove the best prospects for further development of this very important area of astronomical science. Key words: low-frequency radio astronomy; radio telescope; interstellar medium; radio recombination lines; carbon; hydrogen; spectral analyzer

Evidences for non-equivalent centers in mixed chain nickelates (Nd xY 1− x) 2BaNiO 5

Absorption spectra of the mixed chain nickelates (Nd x Y 1− x ) 2BaNiO 5 were measured for different x. Experimentally two evidences for the presence of non-equivalent rare-earth centers (NEREC) were found. First, the linewidths show a very strong broadening for x when the calculated distribution over NEREC is wide. Second, a complicated lineshape of the lowest-frequency line in the region of the 4I 9/2→ 4I 11/2 electronic transition is in good agreement with statistics of NEREC. A simple modeling for the line broadening due to NEREC is suggested.

Raman scattering from Ag and B1g phonons in Bi2Sr2Can-1CunO2n+4 (n=1,2).

Raman measurements were performed on intergrowth-free ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ (2:2:0:1) and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (2:2:1:2) crystals. The phonon spectra of both phases are similar; however, two lowest-frequency phonon lines corresponding to the 28- and 47-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lines in 2:2:1:2 were observed at 20 and 41 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in 2:2:0:1, and, in both cases, these two Raman features appear exclusively in the z(yy)z\ifmmode\bar\else\textasciimacron\fi{} configuration. In addition, two weak ${\mathit{A}}_{\mathit{g}}$ lines at 109 and 129 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which are absent in spectra of 2:2:0:1, were resolved for 2:2:1:2. An overall assignment can be achieved by attributing the two lowest-frequency lines to the first- and second-order scattering features of the amplitude mode of the superstructural modulation, and the other lines to the optical-phonon modes of the distorted orthorhombic subcell. A careful comparison between the spectra of the single- and double-${\mathrm{CuO}}_{2}$-layer Bi compounds enables us to clarify some controversies on assigments of certain phonon modes, especially those in the ${\mathrm{CuO}}_{2}$ layer.

Reorientational Motion of Hydrogen Bonded Octahedral Complex Anions in Hydrazinium Hexachlorostannate(IV), (N2H5)2SnCl6, as Studied by 35Cl NQR Spin-Lattice Relaxation Measurements

Abstract The temperature dependences of NQR frequencies and spin-lattice relaxation times, T1Q, of 35Cl in (N2H5 ) 2SnCl6 were measured. Four NQR signals distributed in a fairly wide frequency range were observed, the lowest-frequency line exhibiting an anomalous positive temperature coefficient. The highest-frequency line showed a steep temperature dependence. These results could be interpreted by considering the intermolecular interaction between CI and - NH2 in N2H5 + . Below ca. 250 K, T1Q of the upper three signals exhibited a gradual decrease upon heating, explainable by lattice vibrations, while a shallow T1Q minimum, ascribable to the modulation of electric quadrupole interaction, was observed at ca. 110K for the lowest-frequency signal. The reorientation of [SnCl6]2- was elucidated to be highly anisotropic, in that it reorients about one of the CI-Sn-Cl axes much more frequently than about the other two axes. The activation energies were determined to be 62 and 94 kj mol -1 for the respective reorientations.

Phase transitions and anionic dynamics of dimethylammonium hexachlorotellurate(IV) as studied by pulsed nuclear quadrupole resonance of chlorine

The temperature dependence of 35Cl NQR frequencies has been reinvestigated for dimethylammonium hexachlorotellurate(IV) using an FT NQR spectrometer. A new line is observed just above the lowest-frequency line already reported for the intermediate-temperature phase. In the lowest-temperature phase, the temperature dependence is definitely determined by finding new lines. The NQR frequencies can be precisely measured even in the vicinity of the phase transitions. The temperature dependence of the nuclear quadrupole relaxation time, T 1Q of 35Cl and 37Cl nuclei has also been observed. In a temperature range 64–160 K, the isotope ratio, T 1Q( 37Cl)/ T 1Q( 35Cl) is 1.5 for each line, suggesting that the quadrupolar relaxation arises mainly from the libration of the complex anion. Above 160 K, T 1Q decreases very rapidly with increasing temperature for each line. This can be interpreted in terms of the onset of the reorientation of the anion as a whole, which is responsible for the fade-out phenomenon of the two lines occurring near 220 K. The nature of the phase transitions and the anionic dynamics are discussed in detail. Especially, the phase transition at 96 K which is rather unusual and is explained through softening of the librations and the rotational displacement of the anions successively operated.

Low-frequency Raman study of para-terphenyl influence of the isotopic substitution and interpretation of the spectra in both phases

We report a low-frequency Raman study of deuterated para-terphenyl versus temperature in both phases. From observed and calculated deuteration effects and potential calculations, we have interpreted the spectra of the low-temperature phase. The features of the high-temperature spectra are analysed taking into account orientational disorder. The particular behaviour of the lowest-frequency line is carefully analysed in both phases.

Nuclear quadrupole resonance of bromine in some tribromides

The NQR of bromine in some tribromides was observed at various temperatures. Ammonium, rubidium, and cesium tribromides yielded three widely separated frequencies, indicating that the Br 3- ion has a considerably asymmetric structure in crystals. These resonance frequencies were observed for tetra- n-propylammonium tribromide at liquid N 2 temperature, indicating the presence of nearly symmetric Br 3- ions in crystals. Tetra- n-butylammonium tribromide showed two frequencies attributable to the central and terminal bromine atoms of a symmetric Br 3- ion. From the observed frequencies, charge distribution in Br 3- was evaluated. The lowest-frequency line of ammonium tribromide showed an extraordinarily large positive temperature coefficient of resonance frequencies. This anomaly was interpreted in terms of the effect of neighboring cations and the formation of weak hydrogen bonds.

Optic-mode coupling in ferroelectric gadolinium molybdate

Separate measurements of the A 1(TO) and A 1(LO) Raman spectra of ferroelectric gadolinium molybdate at 80°K and above have elucidated the origin of the anomalous temperature dependence of the two lowest frequency lines in the A 1(TO) spectrum. The observed behavior is postulated to be the result of coupling among modes at 44.5, 51.5, and 83 cm −1 (at 80°K). The 44.5 and 83 cm −1 modes become the degenerate, soft zone-boundary modes of the paraelectric phase while the 51.5 cm −1 mode changes to B 2 symmetry. The two lowest frequency lines are the same as those observed previously in i.r. absorption.

Nuclear quadrupole resonance of chlorine in sodium tetrachloroiodate(III) dihydrate

The nuclear quadrupole resonance of chlorine in sodium tetrachloroiodate(III) dihydrate was observed between 77 and 365°K. Four resonance lines were observed indicating the presence of four kinds of crystallographically nonequivalent chlorine atoms in the crystals. Unlike other resonance lines, the lowest-frequency line shows a positive temperature coefficient of resonance frequency. X-Ray powder patterns indicate the isomorphism of this complex with sodium tetrachloroaurate(III) dihydrate. With the aid of X-ray crystal analysis of the latter compound, it is concluded that the resonance line showing the positive temperature coefficient is attributable to chlorine atoms involved in hydrogen bonding.

The fluorescence of the uranyl compounds and the raman spectrum of the uranyl ion

The Raman spectrum of a saturated solution of uranyl chloride has been observed to consist of three lines having frequency shifts of 211 cm.-1, 865 cm.-1 and 909 cm-1. The intense line at 865 cm.-1 is highly polarised and accompanied by a faint companion. The lowest frequency line at 211 cm.-1 is also found to be a doublet. From the appearance of the Raman lines in the compound it has been concluded that the UO2 ++ ion has a symmetrical bent structure. The frequency shift of 865 cm.-1 coincides with the wave number separation of the fluorescent bands in the solid, suggesting that the subsidiary bands in fluorescence arise from the modulation of the principal electronic transition by the symmetric vibration of the uranyl group.