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Holger S. P. Müller

Publications and source records attributed to Holger S. P. Müller.

At least 19 recordsLinked to original sources
arXiv2026

Tentative Detection of FeS toward Orion SrcI

Iron sulfide (FeS) is a key refractory compound in astrophysical environments. It is commonly found in meteorites and evidence of FeS has only been reported in solid form in a protoplanetary environment. Furthermore, FeS is of astrobiological interest since it acts as a precursor to the formation of pyrite (FeS$_2$), which plays a key role in anaerobic metabolism. We present millimeter-wave spectroscopic calculations for gaseous FeS and report its tentative detection toward the massive disk Orion SrcI. If confirmed, this would constitute the first ever detection of gaseous FeS in the interstellar medium. Our data are consistent with a FeS column density of $\sim1.7 \times 10^{15}$ cm$^{-2}$. Comparing to other sulfur reservoirs, the amount of sulfur locked in FeS is more than three orders of magnitude lower than that contained in SO$_2$ and SiS. However, FeS emission may arise from a different region as it is the case for the metallic salt NaCl. Our results support the presence of FeS in interstellar grains and are consistent with grain destruction playing an important role in the origin of the FeS emission. The observations, however, are still insufficient to discern whether FeS is a main sulfur reservoir in molecular clouds or a primary carrier of refractory iron-bearing material.

astro-ph.GA↗
arXiv2026

Hyperfine-Resolved Rovibrational and Rotational Spectroscopy of OH$^+$ ($X ^3Σ^-$)

The OH$^+$ ($X ^3Σ^-$) radical cation has been investigated by combining a 4 K 22-pole ion trap apparatus with high-resolution IR and THz radiation sources. Applying different types of action spectroscopic methods, the fundamental vibrational band in the 3 $μ$m range and the spin manifold of the $N=1 \leftarrow 0$ rotational transition around 1 THz have been extended and refined. Additionally, the spin manifold of the $N=2 \leftarrow 1$ rotational transition, scattered around 2 THz, has been measured for the first time with microwave accuracy. Although all hyperfine components of the pure rotational transitions are affected by considerable Zeeman splittings, a simulation of their contours allowed us to extract the field-free center frequencies with high accuracy. A global fit combining rovibrational and pure rotational transitions from the literature with those newly obtained in this work was performed, leading to improvements in the spectroscopic constants of OH$^+$, particularly those in the ground vibrational state.

physics.atm-clus↗
arXiv2026

Rotational Spectroscopy as a Tool to Study Vibration-Rotation Interaction: Investigations of $^{13}$CH$_3$CN and CH$_3$$^{13}$CN up to $v_8 = 2$ and a Search for $v_8 = 2$ Transitions toward Sagittarius B2(N)

Methyl cyanide, CH$_3$CN, is present in diverse regions in space, in particular in the warm parts of star-forming regions where it is a common molecule. Rotational transitions of $^{13}$CH$_3$CN and CH$_3$$^{13}$CN in their $v_8 = 1$ lowest excited vibrational states ($E_{\rm vib} \approx 520$ K) are quite prominent in Sagittarius B2(N). In order to be able to search for transitions of the next higher vibrational state $v_8 = 2$, we recorded spectra of samples enriched in $^{13}$CH$_3$CN and CH$_3$$^{13}$CN up to $v_8 = 2$ in the 35 to 1091~GHz region and reinvestigated existing spectra of CH$_3$CN in its natural isotopic composition between 1085 and 1200 GHz. Perturbations caused by near-degeneracies in $K = 4$ of $v_8 = 2^0$ and $K = 2$ of $v_8 = 2^{-2}$ yielded accurate information on the energy spacing of 22.93 and 21.79 cm$^{-1}$ between the $l$-components of $^{13}$CH$_3$CN and CH$_3$$^{13}$CN, respectively. Fermi-type interaction between $K = 13$ and 14 of $v_8 = 1^{-1}$ and $v_8 = 2^{+2}$ probe the energy differences between the two states of both isotopomers. In addition, a $ΔK \pm2$, $Δl \mp1$ interaction between the ground vibrational state of $^{13}$CH$_3$CN and $v_8 = 1^{+1}$ provides information on their energy spacing. Furthermore, we obtained improved or extended ground state rotational transition frequencies of $^{13}$CH$_3$$^{13}$CN and extensive data for $^{13}$CH$_3$C$^{15}$N and CH$_3$$^{13}$C$^{15}$N. Finally, we report the results of our search for transitions of $^{13}$CH$_3$CN and CH$_3$$^{13}$CN in their $v_8 = 2$ states toward Sagittarius B2(N).

astro-ph.GA↗
arXiv2026

Millimeter and submillimeter spectroscopy of methylallene, CH$_3$CHCCH$_2$

Small polycyclic aromatic hydrocarbons and somewhat larger cyano derivatives were detected in the cold dark cloud TMC-1 recently. Their formation from smaller hydrocarbons is not well understood, in part because abundances of many species are not known. Methylallene, CH$_3$CHCCH$_2$, may be one of the building blocks, but its rotational spectrum was characterized only to a very limited extent. We recorded rotational transitions in the 36$-$501 GHz region to extend the existing line list of methylallene and thus enable searches for the molecule in space. Quantum-chemical calculations were carried out to evaluate initial spectroscopic parameters. We obtained transition frequencies with $J \le 61$ and $K_a \le 21$ and resolved the internal rotation splitting of the CH$_3$ group at least partially. As a result, a full set of distortion parameters up to sixth order along with two octic ones were determined, as well as parameters describing the internal rotation of the methyl group. The spectroscopic parameters are accurate enough to identify methylallene up to 720 GHz, sufficient for searches even in the warm interstellar medium.

astro-ph.GA↗
arXiv2025

Leveraging MMW-MMW Double Resonance Spectroscopy to Understand the Pure Rotational Spectrum of Glycidaldehyde and 17 of Its Vibrationally Excited States

Broadband measurements of glycidaldehyde in the frequency ranges 75-170 and 500-750 GHz were recorded to extend previous analyses of its pure rotational spectrum in the microwave region. The rotational parameters of the ground vibrational states for the main isotopologue and the three singly 13C-substituted isotopologues were considerably improved, and additional higher-order parameters were determined. To identify new vibrationally excited states in the dense and convoluted spectrum, an updated version of the double-modulation double-resonance spectroscopy technique was used. Connecting transitions with a shared energy level into series and expanding these via Loomis-Wood plots proved to be a powerful method, which allowed the identification of 11 new vibrationally excited states in addition to the already known aldehyde torsions, v21 = 1 to v21 = 6. Interactions between several vibrational states were observed, and three interacting systems were treated successfully. Rotational transitions of glycidaldehyde were searched for in the imaging spectral line survey ReMoCA obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming region Sgr B2(N). The observed spectra were modeled under the assumption of local thermodynamic equilibrium (LTE). Glycidaldehyde, an oxirane derivative, was not detected toward Sgr B2(N2b). The upper limit on its column density implies that it is at least six times less abundant than oxirane in this source.

physics.chem-ph↗
arXiv2024

Submillimeter-wave spectroscopy of the CH$_3$O radical

The methoxy radical, CH$_3$O, has long been studied experimentally and theoretically by spectroscopists because it displays a weak Jahn-Teller effect in its electronic ground state, combined with a strong spin-orbit interaction. In this work, we report an extension of the measurement of the pure rotational spectrum of the radical in its vibrational ground state in the submillimeter-wave region (350$-$860 GHz). CH$_3$O was produced by H-abstraction from methanol using F-atoms, and its spectrum was probed in absorption using an association of source-frequency modulation and Zeeman modulation spectroscopy. All the observed transitions together with available literature data in $v = 0$ were combined and fit using an effective Hamiltonian allowing to reproduce the data at their experimental accuracy. The newly measured transitions involve significantly higher frequencies and rotational quantum numbers than those reported in the literature ($f < 860$ GHz and $N \leq 15$ instead of 272 GHz and 7, respectively) which results in significant improvements in the spectroscopic parameters determination. The present model is well constrained and allows a reliable calculation of the rotational spectrum of the radical over the entire microwave to submillimeter-wave domain. It can be used with confidence for future searches of CH$_3$O in the laboratory and the interstellar medium.

astro-ph.GA↗
arXiv2024

Millimeter-millimeter-wave double-modulation double-resonance spectroscopy

A new millimeter- to millimeter-wave double-modulation double-resonance (MMW-MMW DM-DR) scheme has been applied to record spectra of two astronomically relevant complex organic molecules (COMs), propanal (C2H5CHO) and ethyl cyanide (C2H5CN), to demonstrate advantages of the DM-DR experimental technique. The DR technique helps to identify target transitions in a forest of lines and the implementation of a DM procedure (modulation of the pump and probe source) allows for confusion- and baseline-free spectra containing only the line(s) of interest. In particular the unambiguous assignment of weak and blended transitions in very dense MMW spectra is highlighted. Details of the observed Autler-Townes line splitting and possible future applications, such as automated analyses and adaptions of DM-DR methods to other experimental setups, are discussed.

physics.chem-ph↗
arXiv2024

Investigation of the rotational spectrum of CH$_3$$^{17}$OH and its tentative detection toward Sagittarius B2(N)

Methanol is an abundant molecule in space. The column density of CH$_3^{18}$OH is in some star-forming regions so high that the search for CH$_3^{17}$OH is promising. But only very few transition frequencies of CH$_3^{17}$OH with a microwave accuracy have been published thus far. We recorded the rotational spectrum of CH$_3^{17}$OH between 38 and 1095 GHz employing a methanol sample enriched in $^{17}$O to 20\%. A torsion-rotation Hamiltonian model based on the rho-axis method was employed to fit the data, as in our previous studies. We searched for rotational transitions of CH$_3^{17}$OH in the imaging spectral line survey ReMoCA obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming region Sgr B2(N). The observed spectra were modeled under the assumption of local thermodynamic equilibrium (LTE). The assignments cover $0 \le J \le 45$, $K_a \le 16$, and mainly the $v_ t = 0$ and 1 torsional states. The Hamiltonian model describes our data well. The model was applied to derive a line list for radio-astronomical observations. We report a tentative detection of CH$_3^{17}$OH along with secure detections of the more abundant isotopologs of methanol toward Sgr B2(N2b). The derived column densities yield isotopic ratios $^{12}$C/$^{13}$C = 25, $^{16}$O/$^{18}$O = 240, and $^{18}$O/$^{17}$O = 3.3, which are consistent with values found earlier for other molecules in Sgr B2. The agreement between the $^{18}$O/$^{17}$O isotopic ratio that we obtained for methanol and the $^{18}$O/$^{17}$O ratios reported in the past for other molecules in Sgr B2(N) strongly supports our tentative interstellar identification of CH$_3^{17}$OH. The accuracy of the derived line list is sufficient for further radio astronomical searches for this methanol isotopolog toward other star-forming regions.

astro-ph.GA↗
arXiv2024

Discovery of thionylimide, HNSO, in space: the first N-, S- and O-bearing interstellar molecule

We present the first detection in space of thionylimide (HNSO) toward the Galactic Center molecular cloud G+0.693-0.027, thanks to the superb sensitivity of an ultradeep molecular line survey carried out with the Yebes 40$\,$m and IRAM 30$\,$m telescopes. This molecule is the first species detected in the interstellar medium containing, simultaneously, N, S and O. We have identified numerous $K$$_a$ = 0, 1 and 2 transitions belonging to HNSO covering from $J$$_{\rm up}$ = 2 to $J$$_{\rm up}$ = 10, including several completely unblended features. We derive a molecular column density of $N$ = (8 $\pm$ 1)$\times$10$^{13}$ cm$^{-2}$, yielding a fractional abundance relative to H$_2$ of $\sim$6$\times$10$^{-10}$, which is about $\sim$37 and $\sim$4.8 times less abundant than SO and SO2, respectively. Although there are still many unknowns in the interstellar chemistry of NSO-bearing molecules, we propose that HNSO is likely formed through the reaction of the NSO radical and atomic H on the surface of icy grains, with alternative routes also deserving exploration. Finally, HNSO appears as a promising link between N- , S- and O- interstellar chemistry and its discovery paves the route to the detection of a new family of molecules in space.

astro-ph.GA↗
arXiv2023

Shocking Sgr B2(N1) with its own outflow: A new perspective on segregation between O- and N-bearing molecules

We want to investigate the influence of the powerful outflow driven by the hot core Sgr B2(N1) on the gas molecular inventory of the surrounding medium. We used the data taken as part of the 3 mm imaging spectral-line survey ReMoCA (Re-exploring Molecular Complexity with ALMA). Integrated intensity maps of SO and SiO emission reveal a bipolar structure with blue-shifted emission dominantly extending to the SE from the centre of the hot core and red-shifted emission to the NW. This is also prominently observed in emission of other S-bearing molecules and species that only contain N as a heavy element, including COMs, but also CH3OH, CH3CHO, HNCO, and NH2CHO. For a selection of COMs and simpler species, spectra were modelled under the assumption of LTE and population diagrams were derived at two positions, one in each outflow lobe. From this analysis, we obtained rotational temperatures, which are in a range of ~100-200K, and column densities. Abundances were subsequently compared to predictions of astrochemical models and to observations of L1157-B1, a position located in the well-studied outflow of the low-mass protostar L1157, and the source G+0.693-0.027, located in the Sgr B2 molecular cloud complex. Given the short distance of the analysed outflow positions to the centre of Sgr B2(N1), we propose a scenario in which a phase of hot-core chemistry (i.e. thermal desorption of ice species and high-temperature gas-phase chemistry) preceded a shock wave. The subsequent compression and further heating of the material resulted in the accelerated destruction of (mainly O-bearing) molecules. Gas-phase formation of cyanides seems to be able to compete with their destruction in the post-shock gas. Abundances of HCnN (n=3,5) are enhanced in the outflow component pointing to (additional) gas-phase formation. To confirm such a scenario, appropriate chemical shock models need to be run.

astro-ph.GA↗
arXiv2023

Millimetre and submillimetre spectroscopy of isobutene and its detection in the molecular cloud G+0.693

Isobutene ((CH$_3$)$_2$C=CH$_2$) is one of the four isomers of butene (C$_4$H$_8$). Given the detection of propene (CH$_3$CH=CH$_2$) toward TMC-1, and also in the warmer environment of the solar-type protostellar system IRAS 16293$-$2422, one of the next alkenes, isobutene, is a promising candidate to be searched for in space. We aim to extend the limited line lists of the main isotopologue of isobutene from the microwave to the millimetre region in order to obtain a highly precise set of rest frequencies and to facilitate its detection in the interstellar medium. We investigated the rotational spectrum of isobutene in the 35$-$370 GHz range using absorption spectroscopy at room temperature. Quantum-chemical calculations were carried out to evaluate vibrational frequencies. We determined new or improved spectroscopic parameters for isobutene up to a sixth-order distortion constant. These new results enabled its detection in the G+0.693 molecular cloud for the first time, where propene was also recently found. The propene to isobutene column density ratio was determined to be about 3:1. The observed spectroscopic parameters for isobutene are sufficiently accurate that calculated transition frequencies should be reliable up to 700 GHz. This will further help in observing this alkene in other, warmer regions of the ISM.

astro-ph.GA↗
arXiv2023

Rotational spectroscopy of the thioformaldehyde isotopologues H$_2$CS and H$_2$C$^{34}$S in four interacting excited vibrational states and an account on the rotational spectrum of thioketene, H$_2$CCS

An investigation of the rotational spectrum of the interstellar molecule thioformaldehyde between 110 and 377 GHz through a pyrolysis reaction revealed a multitude of absorption lines assignable to H$_2$CS and H$_2$C$^{34}$S in their lowest four excited vibrational states besides lines of numerous thioformaldehyde isotopologues in their ground vibrational states reported earlier as well as lines pertaining to several by-products. Additional transitions of H$_2$CS in its lowest four excited vibrational states were recorded in selected regions between 571 and 1386 GHz. Slight to strong Coriolis interactions occur between all four vibrational states with the exception of the two highest lying states because both are totally symmetric vibrations. We present combined analyses of the ground and the four interacting states for our rotational data of H$_2$CS and H$_2$C$^{34}$S. The H$_2$CS data were supplemented with two sets of high-resultion IR data in two separate analyses. The $v_2 = 1$ state has been included in analyses of Coriolis interactions of low-lying fundamental states of H$_2$CS for the first time and this improved the quality of the fits substantially. We extended furthermore assignments in $J$ of transition frequencies of thioketene in its ground vibrational state.

physics.chem-ph↗
arXiv2023

Rotational spectroscopy of oxirane-\textit{2,2}-$d_2$, $c$-CD$_2$CH$_2$O, and its tentative detection toward IRAS 16293$-$2422~B

We prepared a sample of oxirane doubly deuterated at one C atom and studied its rotational spectrum in the laboratory for the first time between 120~GHz and 1094~GHz. Accurate spectroscopic parameters up to eighth order were determined, and the calculated rest frequencies were used to identify $c$-CD$_2$CH$_2$O tentatively in the interstellar medium in the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293$-$2422. The $c$-CD$_2$CH$_2$O to $c$-C$_2$H$_4$O ratio was estimated to be $\sim$0.054 with $T_{\rm rot} = 125$ K. This value translates to a D-to-H ratio of $\sim$0.16 per H atom which is higher by a factor of 4.5 than the $\sim$0.036 per H atom obtained for $c$-C$_2$H$_3$DO. Such increase in the degree of deuteration referenced to one H atom in multiply deuterated isotopologs compared to their singly deuterated variants have been observed commonly in recent years.

astro-ph.GA↗
arXiv2022

Rotational spectroscopy of mono-deuterated oxirane ($c$-C$_2$H$_3$DO) and its detection towards IRAS 16293$-$2422 B

We prepared a sample of mono-deuterated oxirane and studied its rotational spectrum in the laboratory between 490 GHz and 1060 GHz in order to improve its spectroscopic parameters and consequently the calculated rest frequencies of its rotational transitions. The updated rest frequencies were employed to detect $c$-C$_2$H$_3$DO for the first time in the interstellar medium in the Atacama Large Millimetre/submillimetre Array (ALMA) Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293$-$2422. Fits of the detected lines using the rotation diagrams yield a temperature of $T_{\rm rot} = 103 \pm 19$ K, which in turn agrees well with 125 K derived for the $c$-C$_2$H$_4$O main isotopologue previously. The $c$-C$_2$H$_3$DO to $c$-C$_2$H$_4$O ratio is found to be $\sim$0.15 corresponding to a D-to-H ratio of $\sim$0.036 per H atom which is slightly higher than the D-to-H ratio of species such as methanol, formaldehyde, ketene and but lower than those of the larger complex organic species such as ethanol, methylformate and glycolaldehyde. This may reflect that oxirane is formed fairly early in the evolution of the prestellar cores. The identification of doubly deuterated oxirane isotopomers in the PILS data may be possible judged by the amount of mono-deuterated oxirane and the observed trend that multiply deuterated isotopologues have higher deuteration rates than their mono-deuterated variants.

astro-ph.GA↗
arXiv2022

LLWP -- A new Loomis-Wood Software at the Example of Acetone-13C1

Acetone-13C1 is a complex organic molecule with two internal methyl (-CH3) rotors having relatively low effective barriers to internal rotation of about 249 cm-1. This leads to two low-lying torsional modes and five internal rotation components resulting in a dense and complicated spectrum. In this study, measurements of acetone-13C1 were performed with an isotopically enriched sample in the frequency range 37-1102 GHz. Predicted spectra of acetone-13C1 created with ERHAM allow for future radio astronomical searches. Loomis-Wood plots are one approach to improve and fasten the analysis of such crowded spectra. In this study, the new Loomis-Wood software LLWP was used for fast and confident assignments. LLWP focuses on being user-friendly, intuitive, and applicable to a broad range of assignment tasks. The software is presented here and can be downloaded from llwp.astro.uni-koeln.de.

physics.chem-ph↗
arXiv2022

Resolving desorption of complex organic molecules in a hot core: Transition from non-thermal to thermal desorption or two-step thermal desorption?

Using the high angular resolution provided by the ALMA interferometre we want to resolve the COM emission in the hot molecular core Sagittarius B2(N1) and thereby shed light on the desorption process of Complex Organic Molecules (COMs) in hot cores. We use data taken as part of the 3 mm spectral line survey Re-exploring Molecular Complexity with ALMA (ReMoCA) to investigate the morphology of COM emission in Sagittarius B2(N1). Spectra of ten COMs are modelled under the assumption of LTE and population diagrams are derived for positions at various distances to the south and west from the continuum peak. Based on this analysis, resolved COM rotation temperature and COM abundance profiles are derived. Based on the morphology, a rough separation into O- and N-bearing COMs can be done. Temperature profiles are in agreement with expectations of protostellar heating of an envelope with optically thick dust. Abundance profiles reflect a similar trend as seen in the morphology and, to a great extent, agree with results of astrochemical models that, besides the co-desorption with water, predict that O-bearing COMs are mainly formed on dust grain surfaces at low temperatures while at least some N-bearing COMs and CH$_3$CHO are substantially formed in the gas phase at higher temperatures. Our observational results, in comparison with model predictions, suggest that COMs that are exclusively or to a great extent formed on dust grains desorb thermally at ~100 K from the grain surface likely alongside water. Non-zero abundance values below ~100 K suggest that another desorption process is at work at these low temperatures: either non-thermal desorption or partial thermal desorption related to lower binding energies experienced by COMs in the outer, water-poor ice layers. In either case, this is the first time that the transition between two regimes of COM desorption has been resolved in a hot core.

astro-ph.GA↗
arXiv2022

Rotational spectroscopy of isotopic oxirane, $c$-C$_2$H$_4$O

We studied the rotational spectrum of oxirane in a sample of natural isotopic composition in selected regions between 158 GHz and 1093 GHz. Investigations of the isotopologs with one $^{13}$C or one $^{18}$O were the primary focus in order to facilitate searches for them in space. We also examined the main isotopic species mainly to look into the performance of Watson's A and S reductions both in an oblate and in a prolate representation. Even though oxirane is a rather asymmetric oblate rotor, the A reduction in the III$^l$ representation did not yield a satisfactory fit, as was observed frequently earlier for other molecules. The other three combinations yielded satisfactory fits of similar quality among each other; the A reduction in the I$^r$ representation required two parameters less than both S reduction fits.

physics.chem-ph↗
arXiv2021

(Sub-)millimeter-wave spectroscopy of gauche-propanal

A detailed analysis of (sub-)millimeter-wave spectra of the vibrational ground state ($\upsilon=0$) combined with the energetically lowest excited vibrational state ($\upsilon_{24}=1$; aldehyde torsion) of gauche-propanal (g-C$_2$H$_5$CHO) up to 500 GHz is presented. Both vibrational states, $\upsilon=0$ and $\upsilon_{24}=1$, are treated with tunneling rotation interactions between their two respective tunneling states, which originate from two stable degenerate gauche-conformers; left- and right-handed configurations separated by a small potential barrier. Thanks to double-modulation double-resonance (DM-DR) measurements, important but weak $c$-type transitions connecting the tunneling states could be unambiguously assigned. In addition, Coriolis interaction as well as Fermi resonance between the two vibrational states needed to be taken into account to derive fits with experimental accuracy using Pickett's SPFIT program in a reduced axis system (RAS). Based on the rotational analysis, the fundamental vibrational frequency $ν_{24}$ of gauche-propanal is redetermined to 68.75037(30) cm$^{-1}$.

physics.chem-ph↗