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Discusses major developments in radioastronomy since 1945. Topics include proposed facilities, very-long-baseline interferometric array, millimeter-wave telescope, submillimeter-wave telescope, and funding for radioastronomy facilities and projects. (JN)

The rotational spectra of two small silicon sulfides, silanethione H2SiS and the disilicon sulfide ring Si2S, have been detected in the centimeter band by Fourier transform microwave spectroscopy of a molecular beam; lines of H2SiS were also observed in the millimeter band up to 377 GHz in a glow discharge. Precise rotational and centrifugal distortionconstants have been determined for the normal and a number of the more abundant rare isotopic species of both closed-shell molecules. Theoretical equilibrium (re) structures of H2SiS and Si2S were derived from coupled-cluster calculations that included triple and quadruple excitations, core correlation, and extrapolation to the basis-set limit. The re structures agree to within 5×10-4 Å and 0.1°with empirical equilibrium (reemp) structures derived from the experimental rotational constants, combined with theoretical vibrational and electronic corrections. Both H2SiS and Si2S are good candidates for radioastronomical detection in the circumstellar shells of evolved carbon-rich stars such as IRC+10216, because they are fairly polar and are similar in composition to the abundant astronomical molecule SiS. [ABSTRACT FROM AUTHOR]

By means of Fabry-Pérot Fourier transform microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a cis- and a trans-geometry, is reported. The search for both isomers was based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects. At a similarly high level of theory, the cis-isomer is calculated to be the global minimum on the potential energy surface, but the trans-isomer is predicted to lie only a few kcal/mol higher in energy. A total of seven lines, including a- and b-type transitions, has been observed for both isomers, and precise rotational constants have been derived. Because sulfur dioxide, SO2, is a widespread and very abundant astronomical species, and because it possesses a large proton affinity, HOSO+ is an excellent candidate for radioastronomical detection. [ABSTRACT FROM AUTHOR]

Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space. [ABSTRACT FROM AUTHOR]

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2–OCS, ortho-H2–OCS, HD–OCS, para-D2–OCS, and ortho-D2–OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2–OCS relative to ortho-H2–OCS increases exponentially with decreasing normal H2 component in H2/He gas mixtures, making the observation of para-H2–OCS in the presence of the more strongly bound ortho-H2–OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2–OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2–OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD–OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2–OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2–OCS. [ABSTRACT FROM AUTHOR]

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH2, orthoH2, HD, orthoD2, and paraD2) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH2, where I=0. For the two j=1 species, orthoH2-OCS and paraD2-OCS, nuclear hyperfine coupling constants are found to be da=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD2-OCS yield the quadrupole coupling constants eqQa=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD2 in the complex are slightly hindered producing a small nonspherical distribution. For orthoD2-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex. [ABSTRACT FROM AUTHOR]

Vibrational excitation and relaxation of five linear polyatomic molecules, OCS, OC3S, HC3N, HC5N, and SiC2S, have been studied by Fourier transform microwave spectroscopy in a supersonic expansion after the application of a low-current dc electric discharge. For each chain, the populations in bending and stretching modes have been characterized as a function of the applied discharge current; for stable OCS and HC3N, vibrational populations were studied as well in the absence of a discharge. With no discharge present the derived vibrational temperatures are slightly below T, the temperature of the gas before the supersonic expansion (i.e., 300 K). In the presence of the discharge, vibrational excitation occurs via inelastic collisions with the electrons and the vibrational temperatures rise as the applied current increases. Global vibrational relaxation is governed by rapid vibration–vibration (VV) energy transfer and slow vibration–translation (VT) energy transfer. The latter process is rate-determining and depends primarily on the wave number of the vibration. Vibrational modes with wave numbers near and below kT/hc (where T=300 K and kT/hc∼210 cm-1) are efficiently cooled by VT transfer because a sufficient number of collisions occur in the initial stages of the supersonic expansion. Vibrational modes with wave numbers around 450 cm-1 appear to be inefficiently cooled in the molecular beam; at these energies VV and VT rates are probably comparable. For high-frequency vibrations, VV energy transfer dominates. For the longer chains OC3S and HC5N, higher-lying modes are generally not detectable and vibrational temperatures of most lower-lying modes were found to be lower than those of OCS and HC3N, suggesting that as the size of the molecules increases, intermode VV transfer becomes more efficient, plausibly due to the higher density of vibrational levels. New high resolution spectroscopic data have been obtained for several vibrationally excited states of OC3S, HC3N, and HC5N. Rotational lines of the 13C and 15N isotopic species of HC5N have been measured, yielding improved rotational and centrifugal distortion constants; 14N nitrogen quadrupole coupling constants for the isotopic species of HC5N with 13C have been determined for the first time. [ABSTRACT FROM AUTHOR]

Fourier transform microwave spectroscopy has been used to investigate vibrational excitation and relaxation of diatomic molecules produced by an electric discharge in the throat of a supersonic nozzle. Rotational transitions of SO, SiO, and SiS, in vibrational states up to v=33 for [sup 32]S[sup 16]O, v=45 for [sup 28]Si[sup 16]O, and v=51 for [sup 28]Si[sup 32]S in their ground electronic states have been detected. The isotopic species [sup 33]S[sup 16]O, [sup 34]S[sup 16]O, [sup 29]Si[sup 16]O, [sup 28]Si[sup 18]O, [sup 29]Si[sup 32]S, and [sup 28]Si[sup 34]S have also been observed in highly excited vibrational states. Microwave transitions include up to v=22 for the second lowest excited electronic state b [sup 1]Σ[sup +] of SO (∼10 510 cm-1 above ground) have also been detected. Effective vibrational temperatures have been derived for each species, and a general model is proposed to qualitatively explain the observations. Vibrational excitation is caused by inelastic collisions with the hot electrons produced in the discharge. The subsequent vibrational populations are largely determined by vibration–vibration energy transfer via molecule–molecule binary collisions. Two regions can be inferred from the data: one characterized by a temperature of around 1000 K and a second region with a temperature of several thousand degrees Kelvin. Improved Dunham coefficients and correction terms for the breakdown of the Born–Oppenheimer approximation have been determined for b [sup 1]Σ[sup +] SO, X [sup 1]Σ[sup +] SiO, and X [sup 1]Σ[sup +] SiS. Nuclear spin-rotation hyperfine structure for the [sup 29]Si isotopic species of SiO and SiS has been observed in all highly excited vibrational states. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

On the basis of extensive coupled cluster calculations, the rotational spectra of the linear silicon- and sulfur-containing carbon chains SiC[sub 2n] S (n = 1 -3) in their singlet electronic ground state and SiC[sub 3]S in its triplet electronic ground state have been detected and characterized by means of molecular beam Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined to high accuracy as well as the spin-spin coupling constant for triplet SiC[sub 3] S. In addition, the [sup 29]Si, [sup 34]S, and both [sup 13]C isotopic species of SiC[sub 2]S have been detected, allowing the determination of both an effective r[sub 0] structure, as well as a mixed experimental-theoretical structure resulting from the combination of the measured rotational constants with the vibrationrotation coupling constants calculated ab initio. Several rotational satellite lines have also been observed for SiC[sub 2]S and, on the basis of their predicted vibration-rotation and l-type doubling constants, were assigned to the two highest-frequency stretching modes υ[sub 1] and υ[sub 2], and to a progression in the second lowest-frequency bending mode υ 4 up to 3 υ 4. Equilibrium structures and various spectroscopic properties are predicted for all SiC[sub n]S species with n = 1 - 8. [ABSTRACT FROM AUTHOR]

Rotational transitions for omicron-benzyne were measured with a pulsed-beam, Fourier transform microwave spectrometer for all unique, singly substituted (super 13)C and single-D isotopomers. The new data combined with previous data for the normal isotopomer, provide a complete set of structural parameters for this molecule and the r(sub s) substitution coordinates and the coordinates from a least-squares fit are reported.

The rotational spectrum of thiozene that is S3 by Fourier transform microwave (FTM) spectroscopy of a molecular beam is detected. It is analyzed that the symmetry of S3 can detect its structure with the two geometrical parameters.

The design and performance of a cryogenic (T=77 K) microwave spectrometer are reported. This instrument, being a time domain experiment in microwave molecular spectroscopy, consists of three basic components: a near-confocal Fabry-Perot-type resonator, a pulsed microwave excitation Fourier transform system, and a pulsed molecular-beam source in a coaxially oriented beam-resonator arrangement. With the recently developed spectrometer, which employs confocal reflectors cooled to liquid-nitrogen temperature, we were able to reduce the system noise of the instrument significantly. The high sensitivity of this next generation spectrometer allows the detection of transient species produced in a dc-discharge nozzle, other short-lived molecules and complexes at low abundance, or nearly nonpolar molecules. We will discuss the spectrometer design with respect to critical considerations of the resonator theory in the context of cryogenic operation, as well as important aspects of classical circuit theory applied to excitation and detection of molecular systems in a resonator. Following the description of the dc-discharge source, the performance of the instrument is demonstrated. [ABSTRACT FROM AUTHOR]

Discusses the major developments in the field of radioastronomy since 1945. Historical background on the discovery of radioastronomy; Facilities identified as necessary for future progress in U.S. radioastronomy; Details on very long baseline interferometery.

By means of Fabry-Pérot Fourier transform microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a cis- and a trans-geometry, is reported. The search for both isomers was based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects. At a similarly high level of theory, the cis-isomer is calculated to be the global minimum on the potential energy surface, but the trans-isomer is predicted to lie only a few kcal/mol higher in energy. A total of seven lines, including a- and b-type transitions, has been observed for both isomers, and precise rotational constants have been derived. Because sulfur dioxide, SO(2), is a widespread and very abundant astronomical species, and because it possesses a large proton affinity, HOSO(+) is an excellent candidate for radioastronomical detection.

Unsubstituted silanethione, H(2)Si=S, has been characterized experimentally for the first time by means of rotational spectroscopy; the equilibrium structure of this fundamental molecule has been evaluated through a combination of experimental data from a total of ten isotopologues and results of high-level coupled-cluster calculations.

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH(2), orthoH(2), HD, orthoD(2), and paraD(2)) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH(2), where I=0. For the two j=1 species, orthoH(2)-OCS and paraD(2)-OCS, nuclear hyperfine coupling constants are found to be d(a)=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD(2)-OCS yield the quadrupole coupling constants eqQ(a)=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD(2) in the complex are slightly hindered producing a small nonspherical distribution. For orthoD(2)-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex.