Fast-scanning Spectrometer Research Articles

Rotational spectrum and interstellar detection of the first torsionally excited state of methylamine

Context. Methylamine (CH3NH2) was first detected in the interstellar medium (ISM) toward Sgr B2 almost 50 years ago by observation of rotational transitions in its torsional ground state. Methylamine exhibits two large-amplitude motions (LAMs), the methyl torsion and amine wagging, which complicate the spectral analysis, especially in excited vibration states. The lack of an accurate model of the two coupled LAMs has also hampered the identification in the ISM of rotational transitions in excited vibrational states. Aims. The aim of this work is to study the terahertz and microwave rotational spectra of methylamine experimentally and theoretically in order to provide a reliable basis for the detection of its rotational transitions in the first torsionally excited state, υt = 1, in the ISM. Methods. The terahertz spectrum of methylamine was measured from 150 to 1520 GHz with the Lille fast scan spectrometer. Using a new “hybrid” Hamiltonian model, we were able to analyze the nuclear quadrupole hyperfine structure and to accurately fit the rotational spectrum of the υt = 1 state of methylamine. We used the imaging spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to search for rotational transitions of methylamine in its first torsionally excited state toward the high-mass star forming region Sgr B2(N). The observed spectra are modeled under the assumption of local thermodynamic equilibrium (LTE). Results. Accurate spectral predictions were obtained for the ground and first excited states of CH3NH2. We report the first interstellar detection of methylamine in the υt = 1 state toward the offset position Sgr B2(N1S) in the hot molecular core Sgr B2(N1). The LTE parameters derived previously from the rotational emission of methylamine in its torsional ground state toward Sgr B2(N1S) yield synthetic spectra of methylamine in the υt = 1 state that are fully consistent with the ALMA spectra and allow us to identify five rotational lines of this state.

Rotational spectroscopy of malononitrile and its corresponding monoisocyanide isomer, isocyanoacetonitrile

Context. Nitriles constitute almost 20% of the molecules observed in the interstellar medium, whereas only one dinitrile and one isocyanonitrile compound have been detected up to now. The lack of detections of such compounds may be partially explained by the lack of accurate spectroscopic data on their rotational spectra.Aims. Two small seven-atom dinitriles, malononitrile NCCH2CN and isocyanoacetonitrile NCCH2NC, were chosen as target species for this study. For malononitrile the goal of the study is to systematize all the previous measurements, and to extend the measurements to the sub-millimeter wavelength range. The spectrum of isocyanoacetonitrile has not been studied before.Methods. The rotational spectra of the two molecules were measured in the frequency range 150–660 GHz using the Lille fast-scan spectrometer. The spectroscopic study was supported by high-level theoretical calculations on the structure of these molecules and their harmonic force field.Results. Accurate frequency predictions for malononitrile and isocyanoacetonitrile were calculated on the basis of the analysis of their rotational spectra. The influence of the spin statistics on the intensities of the lines of malononitrile was taken into account. The provided line lists and sets of molecular parameters meet the needs of astrophysical searches for the two molecules.

The Millimeter‐ and Submillimeter‐wave Spectrum of Methyl Formate (HCOOCH3)

Methyl formate (HCOOCH3), a threefold internal rotor, is a well-known interstellar molecule located in hot cores of giant molecular clouds. Over a decade ago, we published separate analyses of the rotational-torsional spectra of methyl formate in its A and E ground torsional substates. The analysis of the A substate was undertaken with a standard asymmetric top Hamiltonian, while that of the E substate was undertaken with a principal axis method (PAM), which was less than entirely satisfactory. In the interim, we have measured many new lines of the rotational-torsional spectrum of both the ground A and E torsional substates of this species through the rotational quantum number J = 50 using two spectrometers: our standard klystron-based system and a new fast scan spectrometer (FASSST). The newly measured lines have been combined with previous data to form a data set consisting of over 2000 lines that has been fitted to the accuracy of our experiments using a modification of the internal axis method (IAM) we have previously applied to methanol. The spectral constants obtained from the fit have allowed us to predict the frequencies of many additional lines up to 700 GHz in frequency and up to J = 50 in rotational quantum number.

Simultaneous multi-element determination with coherent forward scattering spectrometry employing chromatically corrected polarizers and a fast scanning spectrometer

A new coherent forward scattering spectrometer for simultaneous multi-element determination on up to 20 atomic lines has been constructed and evaluated. The apparatus consists of a continuum primary source, calcite Glan-Taylor polarizers equipped with a laboratory-designed chromatic correction for the wavelength range 214–766 nm, an electrothermal atomizer with magnet and autosampler and a laboratory-constructed wavelength modulated polychromator with medium resolving power. Light intensities of up to 20 resonance lines in the wavelength range of 214–500 nm are transferred from the focal plane to an array of 20 miniature photomultipliers by optical fiber-bundles. The instrumentation is controlled by a computer. Owing to modular construction the graphite furnace can be exchanged by a flame. Simultaneous multi-element determinations of Ag, Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mn, Na, Ni, Pb, Sr, Tl and Zn are carried out. The received analytical curves cover 1.5–2.5 orders of magnitude per atomic line, which is in the same order as with multi-element measurements with electrothermal atomic absorption spectrometry (ETAAS). Further working range expansions are demonstrated with determining on resonance lines with different strengths. The detection limits for the strongest resonance line of most elements are in the μg l −1-range and are one order of magnitude higher than those measured with commercially available ETAAS instrumentation when determining four elements simultaneously. The crossed-to-open extinction ratio of the chromatically corrected Glan-Taylor polarizers is determined to approximately 2.5×10 −5 under installed conditions with the graphite furnace and its two windows in between. The spectral transmissions of these polarizers and the optical fiber-bundles are measured with a photometer. It shows a steep decay for wavelengths below 220 nm.

Dimethyl Ether: Laboratory Assignments and Predictions through 600 GHz

Dimethyl ether (CH3OCH3) is a well-known interstellar molecule with a large abundance in hot core regions. We have measured many rotational-torsional transition lines of this molecule in the 100-550 GHz frequency range using two experimental techniques: a klystron-based traditional millimeter-wave spectrometer and a new fast scan spectrometer (designated "FASSST") with a voltage-tunable backward wave oscillator. The lines arising from the ground vibrational state have been combined with previous data, mainly at lower frequencies, to form a global data set consisting of over 1600 lines that have been assigned and fitted by an effective Hamiltonian method. The spectral constants obtained from the fit allow us to predict the frequencies of almost 6000 additional lines of dimethyl ether through 600 GHz.

The Millimeter‐ and Submillimeter‐Wave Spectrum of Ethylene Oxide (c‐C2H4O)

The cyclic molecule ethylene oxide (c-C2H4O) has recently been detected in the interstellar source Sgr B2N. Previous laboratory work on the rotational spectrum of this molecule extends only to a frequency of 123 GHz. We report here the extension of the laboratory rotational spectrum of this species through the frequency range 262-358 GHz using a new fast scan spectrometer (FASSST). The newly measured lines have been combined with previous data at lower frequencies to form a data set consisting of 662 lines that has been assigned and fitted via a standard semirigid asymmetric top analysis. The spectral constants obtained from the fit have allowed us to predict the frequencies of many additional lines.

A Millisecond Time-Scale UV-VIS Spectrophotometer Based on a Double-Diode Array Detector Coupled to a Stopped-Flow Instrument

Abstract Details are reported of the construction, calibration and testing of a fast-scanning spectrometer consisting of a double-diode array optical simultaneous multichannel analyzer (DDA-OSMA) coupled to stopped-flow instrument. This set-up enables the recording of high quality absorbance spectra as well as absorbance-time traces as a function of wavelength, on a millisecond time scale. The electron-transfer reaction between Fe(CN)6 3- and L-ascorbic acid is used to demonstrate the performance of the system.

Solvatochromic and photochromic characteristics of new 1,3-dihydrospiro[2H-indole-2,2′-[2H]-bipyrido[3,2-f][2,3-h][1,4]benzoxazines

The mono-oxime of 1,10-phenanthroline-5,6-quinone reacts with 1,3,3-trimethyl-2-methyleneindoline, derivatives to give the corresponding spirooxazines, i.e. 1,3-dihydrospiro[2H-indole-2,2′-[2H]-bipyrido[3,2-f][2,3-h][1,4]benzoxazines]. These compounds exhibit particular solvatochromic behaviour, strongly dependent on the substitution pattern. At room temperature and without any light excitation, an equilibrium is established between closed and opened forms. The photochromic characteristics have been determined using flash photolysis coupled to a fast scanning spectrometer allowing the determination of rate constants for thermal bleaching, visible absorption and photocoloration. The coloured opened forms, either photoinduced or arising from the solvation effect, appear to be spectroscopically identical. Structural parameters of the short-lived photomerocyanines have also been obtained.

A Wide Range, Rapid Scanning Spectrometer for Emission and Absorption Measurements

A new, very wide spectral range, fast scanning spectrometer that combines high speed with good resolving power has been developed. The instrument operates with an f number in the vertical and horizontal plane of f/3 and f/17, respectively, and with a useful slit height of 3 cm. The theoretical resolving power in first order is 14,000 using an 1180-lines/mm grating. The optical layout is basically an asymmetrical Czerny-Turner arrangement with the grating at the center of curvature. The scanning element is a six sided, air turbine driven mirror that allows the whole spectrum from zero to 1.2 micro in first order to be scanned in about 70 microsec at mirror speeds of 1500 rps. The wide variation in the spectral response of the instrument presents recording problems, but ways of circumventing or diminishing those problems are suggested. A high intensity, end-on, pulsed flash tube has been used as a source for absorption spectroscopy. Some experiments are described, where the system has been appli d to emission or absorption measurements.