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Register a new userDetermination of accurate rest frequencies and hyperfine structure parameters of cyanobutadiyne, HC$_5$N
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Very accurate transition frequencies of HC$_5$N were determined between 5.3 and 21.4 GHz with a Fourier transform microwave spectrometer. The molecules were generated by passing a mixture of HC$_3$N and C$_2$H$_2$ highly diluted in neon through a discharge valve followed by supersonic expansion into the Fabry-Perot cavity of the spectrometer. The accuracies of the data permitted us to improve the experimental $^{14}$N nuclear quadrupole coupling parameter considerably and the first experimental determination of the $^{14}$N nuclear spin-rotation parameter. The transition frequencies are also well suited to determine in astronomical observations the local speed of rest velocities in molecular clouds with high fidelity. The same setup was used to study HC$_7$N, albeit with modest improvement of the experimental $^{14}$N nuclear quadrupole coupling parameter. Quantum chemical calculations were carried out to determine $^{14}$N nuclear quadrupole and spin-rotation coupling parameters of HC$_5$N, HC$_7$N, and related molecules. These calculations included evaluation of vibrational and relativistic corrections to the non-relativistic equilibrium quadrupole coupling parameters; their considerations improved the agreement between calculated and experimental values substantially.
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We report the first interstellar detection of DC$_7$N and six $^{13}$C-bearing isotopologues of HC$_7$N toward the dark cloud TMC-1 through observations with the Green Bank Telescope, and confirm the recent detection of HC$_5$$^{15}$N. For the average of the $^{13}$C isotopomers, DC$_7$N, and HC$_5$$^{15}$N, we derive column densities of 1.9(2)$times$10$^{11}$, 2.5(9)$times$10$^{11}$, and 1.5(4)$times$10$^{11}$ cm$^{-2}$, respectively. The resulting isotopic ratios are consistent with previous values derived from similar species in the source, and we discuss the implications for the formation chemistry of the observed cyanopolyynes. Within our uncertainties, no significant $^{13}$C isotopomer variation is found for HC$_7$N, limiting the significance CN could have in its production. The results further show that, for all observed isotopes, HC$_5$N may be isotopically depleted relative to HC$_3$N and HC$_7$N, suggesting that reactions starting from smaller cyanopolyynes may not be efficient to form HC$_{n}$N. This leads to the conclusion that the dominant production route may be the reaction between hydrocarbon ions and nitrogen atoms.
The $1 ^3Sigma_u^- leftarrow X^3Sigma_g^-$ transition of linear HC$_5$H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer HC$_{2n+1}$H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the $1 ^1 A_1 leftarrow X ^1 A_1$ transition of the cumulene carbene pentatetraenylidene H$_2$C$_5$ (B).
A better understanding of sulphur chemistry is needed to solve the interstellar sulphur depletion problem. A way to achieve this goal is to study new S-bearing molecules in the laboratory, obtaining accurate rest frequencies for an astronomical search. We focus on dithioformic acid, HCSSH, which is the sulphur analogue of formic acid. The aim of this study is to provide an accurate line list of the two HCSSH $trans$ and $cis$ isomers in their electronic ground state and a comprehensive centrifugal distortion analysis with an extension of measurements in the millimetre and submillimetre range. We studied the two isomers in the laboratory using an absorption spectrometer employing the frequency-modulation technique. The molecules were produced directly within a free-space cell by glow discharge of a gas mixture. We measured lines belonging to the electronic ground state up to 478 GHz, with a total number of 204 and 139 new rotational transitions, respectively, for $trans$ and $cis$ isomers. The final dataset also includes lines in the centimetre range available from literature. The extension of the measurements in the mm and submm range lead to an accurate set of rotational and centrifugal distortion parameters. This allows us to predict frequencies with estimated uncertainties as low as 5 kHz at 1 mm wavelength. Hence, the new dataset provided by this study can be used for astronomical search.
The Vienna Atomic Line Database (VALD) has been supplemented with new data and new functionality -- the possibility of taking into account the effect of hyperfine splitting (HFS) of atomic levels in the analysis of line profiles. This has been done through the creation of an ancillary SQL database with the HFS constants for atomic levels of 58 isotopes of 30 neutral and singly-ionized atoms. The completeness of the collected data and new opportunities for studies of stars of various spectral types is analyzed. The database enables analysis of splitting of up to 60% of lines with measurable effects in the ultraviolet ($lambdagtrsim1000$~AA), and up to 100% of such lines in the optical and infrared ranges ($lambdalesssim25000$~AA) for A--M stars. In the spectra of hot O--B stars, it is necessary to use laboratory measurements for atoms in the second and higher stages of ionization.
We have investigated energies, magnetic dipole hyperfine structure constants ($A_{hyf}$) and electric dipole (E1) matrix elements of a number of low-lying states of the triply ionized tin (Sn$^{3+}$) by employing relativistic coupled-cluster theory. Contributions from the Breit interaction and lower-order quantum electrodynamics (QED) effects in determination of above quantities are also given explicitly. These higher-order relativistic effects are found to be important for accurate evaluation of energies, while QED contributions are seen to be contributing significantly to the determination of $A_{hyf}$ values. Our theoretical results for energies are in agreement with one of the measurements but show significant differences for some states with another measurement. Reported $A_{hyf}$ will be useful in guiding measurements of hyperfine levels in the stable isotopes of Sn$^{3+}$. The calculated E1 matrix elements are further used to estimate oscillator strengths, transition probabilities and dipole polarizabilities ($alpha$) of many states. Large discrepancies between present results and previous calculations of oscillator strengths and transition probabilities are observed for a number of states. The estimated $alpha$ values will be useful for carrying out high precision measurements using Sn$^{3+}$ ion in future experiments.
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