49 research outputs found
Laboratory rotational ground state transitions of NHD and CF
Aims. This paper reports accurate laboratory frequencies of the rotational
ground state transitions of two astronomically relevant molecular ions, NH3D+
and CF+. Methods. Spectra in the millimeter-wave band were recorded by the
method of rotational state-selective attachment of He-atoms to the molecular
ions stored and cooled in a cryogenic ion trap held at 4 K. The lowest
rotational transition in the A state (ortho state) of NHD (), and the two hyperfine components of the ground state transition of
CF() were measured with a relative precision better than
. Results. For both target ions the experimental transition
frequencies agree with recent observations of the same lines in different
astronomical environments. In the case of NHD the high-accuracy
laboratory measurements lend support to its tentative identification in the
interstellar medium. For CF the experimentally determined hyperfine
splitting confirms previous quantum-chemical calculations and the intrinsic
spectroscopic nature of a double-peaked line profile observed in the transition towards the Horsehead PDR.Comment: 7 pages, 2 figure
Infrared predissociation spectroscopy of the hydrocarbon cations C3H+, C2H+, and C3H2+
Reactive hydrocarbon cations play an important role in the astrochemistry of the interstellar medium, but spectroscopic data, needed for their identification in astronomical observations, is sparse. Here we report the first gas-phase vibrational spectra of the linear CH (), the radical cation CH (), and the linear-/cyclic-CH ( /A, resp.). Broadband spectra were recorded by Ne- and He-messenger infrared-predissociation (IR-PD) action spectroscopy in a cryogenic (~K) ion trap instrument (FELion) in the ~{\wn} range using a free electron laser and a MIR-OPO at the FELIX (Free-Electron Laser for Infrared eXperiments) laboratory. The band positions (determined with a precision of ~\wn) covering the C-H and C-C stretching as well as several bending modes are compared to high-level (CCSD(T) with large basis sets) quantum-chemical calculations with an emphasis on anharmonic effects and on the influence of the rare-gas messenger atom. The experimental and theoretical data provide a solid basis for subsequent IR high-resolution studies, with the ultimate goal to predict and measure accurate rotational spectra for a radio-astronomical search of these molecular ions in space
VIBRATIONAL AND ROTATIONAL SPECTROSCOPY IN CRYOGENIC ION TRAPS
Reactive molecular ions play a central role in the chemistry of the interstellar medium and in planetary atmospheres. Spectroscopic studies of these often elusive ions yield fundamental insights on their geometrical and electronic structure, and provide vibrational and rotational signatures needed for their identification in space. Cryogenic ion traps have proven to be ideal tools for the development of sensitive spectroscopic schemes of mass-selected, cold, and isolated molecular ions. Recent progress on these so-called action spectroscopic methods allows not only to probe electronic and vibrational excitation processes, but also to record high-resolution purely rotational molecular spectra\footnote{ S. Br\"{u}nken, L. Kluge, A. Stoffels, O. Asvany, and S. Schlemmer, Astrophys. J. Lett., 783, L4 (2014); A. Stoffels, L. Kluge, S. Schlemmer, and S. Br\"{u}nken, A\&A 593, A56 (2016); S. Br\"{u}nken, L. Kluge, A. Stoffels, J. Pèrez-Rios, and S. Schlemmer, J. Mol. Spectrosc., 332, 67 (2017)}, which are a direct prerequisite for radio-astronomical detections of new species in space as will be demonstrated with selected examples. In addition, details of broadband infrared experiments on several astrophysically important hydrocarbon cations ranging in size from comparatively small systems (e.g., CH, CH, and CH) to PAH cations will be given, using the unique combination of a cryogenic ion trap instrument\footnote{ O. Asvany, S. B\"{u}nken, L. Kluge, and S. Schlemmer, Appl. Phys. B, 114, 203 (2014)} interfaced to the free electron lasers at the FELIX Laboratory
Infrared action spectroscopy as tool for probing gas-phase dynamics: Protonated Dimethyl Ether, (CH)OH, formed by the reaction of CHOH with CHOH
Methanol is one of the most abundant interstellar Complex Organic Molecules
(iCOMs) and it represents a major building block for the synthesis of
increasingly complex oxygen-containing molecules. The reaction between
protonated methanol and its neutral counterpart, giving protonated dimethyl
ether, (CH)OH, along with the ejection of a water molecule, has
been proposed as a key reaction in the synthesis of dimethyl ether in space.
Here, gas phase vibrational spectra of the (CH)OH reaction product
and of the [CHO] intermediate complex(es), formed under
different pressure and temperature conditions, are presented. The widely
tunable free electron laser for infrared experiments, FELIX, was employed to
record their vibrational fingerprint spectra using different types of infrared
action spectroscopy in the cm frequency range, complemented
with measurements using an OPO/OPA system to cover the O-H stretching region
cm. The formation of protonated dimethyl ether as a product
of the reaction is spectroscopically confirmed, providing the first gas-phase
vibrational spectrum of this potentially relevant astrochemical ion.Comment: 15 pages, 6 figures, Molecular Physics, Published online: 22 Jun
2023, for associated data files see Zenodo repository at
https://doi.org/10.5281/zenodo.786855
Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands
The so-called aromatic infrared bands are attributed to emission of
polycyclic aromatic hydrocarbons. The observed variations toward different
regions in space are believed to be caused by contributions of different
classes of PAH molecules, i.e. with respect to their size, structure, and
charge state. Laboratory spectra of members of these classes are needed to
compare them to observations and to benchmark quantum-chemically computed
spectra of these species. In this paper we present the experimental infrared
spectra of three different PAH dications, naphthalene,
anthracene, and phenanthrene, in the vibrational fingerprint
region 500-1700~cm. The dications were produced by electron impact
ionization of the vapors with 70 eV electrons, and they remained stable against
dissociation and Coulomb explosion. The vibrational spectra were obtained by IR
predissociation of the PAH complexed with neon in a 22-pole cryogenic
ion trap setup coupled to a free-electron infrared laser at the Free-Electron
Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic
density-functional theory calculations for both singly and doubly charged
states of the three molecules. The experimental band positions showed excellent
agreement with the calculated band positions of the singlet electronic ground
state for all three doubly charged species, indicating its higher stability
over the triplet state. The presence of several strong combination bands and
additional weaker features in the recorded spectra, especially in the
10-15~m region of the mid-IR spectrum, required anharmonic calculations to
understand their effects on the total integrated intensity for the different
charge states. These measurements, in tandem with theoretical calculations,
will help in the identification of this specific class of doubly-charged PAHs
as carriers of AIBs.Comment: Accepted for publication in A&
Detection of Vibrationally Excited CO in IRC+10216
Using the Submillimeter Array we have detected the J=3-2 and 2-1 rotational
transitions from within the first vibrationally excited state of CO toward the
extreme carbon star IRC+10216 (CW Leo). The emission remains spatially
unresolved with an angular resolution of ~2" and, given that the lines
originate from energy levels that are ~3100 K above the ground state, almost
certainly originates from a much smaller (~10^{14} cm) sized region close to
the stellar photosphere. Thermal excitation of the lines requires a gas density
of ~10^{9} cm^{-3}, about an order of magnitude higher than the expected gas
density based previous infrared observations and models of the inner dust shell
of IRC+10216.Comment: Accepted for publication in ApJ Letter
Submillimeter narrow emission lines from the inner envelope of IRC+10216
A spectral-line survey of IRC+10216 in the 345 GHz band has been undertaken
with the Submillimeter Array. Although not yet completed, it has already
yielded a fairly large sample of narrow molecular emission lines with
line-widths indicating expansion velocities of ~4 km/s, less than 3 times the
well-known value of the terminal expansion velocity (14.5 km/s) of the outer
envelope. Five of these narrow lines have now been identified as rotational
transitions in vibrationally excited states of previously detected molecules:
the v=1, J=17--16 and J=19--18 lines of Si34S and 29SiS and the v=2, J=7--6
line of CS. Maps of these lines show that the emission is confined to a region
within ~60 AU of the star, indicating that the narrow-line emission is probing
the region of dust-formation where the stellar wind is still being accelerated.Comment: 5 pages, 5 figures, Accepted for publication in Ap
PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula
The JWST has captured the most detailed and sharpest infrared images ever
taken of the inner region of the Orion Nebula, the nearest massive star
formation region, and a prototypical highly irradiated dense photo-dissociation
region (PDR). We investigate the fundamental interaction of far-ultraviolet
photons with molecular clouds. The transitions across the ionization front
(IF), dissociation front (DF), and the molecular cloud are studied at
high-angular resolution. These transitions are relevant to understanding the
effects of radiative feedback from massive stars and the dominant physical and
chemical processes that lead to the IR emission that JWST will detect in many
Galactic and extragalactic environments. Due to the proximity of the Orion
Nebula and the unprecedented angular resolution of JWST, these data reveal that
the molecular cloud borders are hyper structured at small angular scales of
0.1-1" (0.0002-0.002 pc or 40-400 au at 414 pc). A diverse set of features are
observed such as ridges, waves, globules and photoevaporated protoplanetary
disks. At the PDR atomic to molecular transition, several bright features are
detected that are associated with the highly irradiated surroundings of the
dense molecular condensations and embedded young star. Toward the Orion Bar
PDR, a highly sculpted interface is detected with sharp edges and density
increases near the IF and DF. This was predicted by previous modeling studies,
but the fronts were unresolved in most tracers. A complex, structured, and
folded DF surface was traced by the H2 lines. This dataset was used to revisit
the commonly adopted 2D PDR structure of the Orion Bar. JWST provides us with a
complete view of the PDR, all the way from the PDR edge to the substructured
dense region, and this allowed us to determine, in detail, where the emission
of the atomic and molecular lines, aromatic bands, and dust originate
PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar
(Abridged) Mid-infrared observations of photodissociation regions (PDRs) are
dominated by strong emission features called aromatic infrared bands (AIBs).
The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 m. The
most sensitive, highest-resolution infrared spectral imaging data ever taken of
the prototypical PDR, the Orion Bar, have been captured by JWST. We provide an
inventory of the AIBs found in the Orion Bar, along with mid-IR template
spectra from five distinct regions in the Bar: the molecular PDR, the atomic
PDR, and the HII region. We use JWST NIRSpec IFU and MIRI MRS observations of
the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288).
We extract five template spectra to represent the morphology and environment of
the Orion Bar PDR. The superb sensitivity and the spectral and spatial
resolution of these JWST observations reveal many details of the AIB emission
and enable an improved characterization of their detailed profile shapes and
sub-components. While the spectra are dominated by the well-known AIBs at 3.3,
6.2, 7.7, 8.6, 11.2, and 12.7 m, a wealth of weaker features and
sub-components are present. We report trends in the widths and relative
strengths of AIBs across the five template spectra. These trends yield valuable
insight into the photochemical evolution of PAHs, such as the evolution
responsible for the shift of 11.2 m AIB emission from class B in
the molecular PDR to class A in the PDR surface layers. This
photochemical evolution is driven by the increased importance of FUV processing
in the PDR surface layers, resulting in a "weeding out" of the weakest links of
the PAH family in these layers. For now, these JWST observations are consistent
with a model in which the underlying PAH family is composed of a few species:
the so-called 'grandPAHs'.Comment: 25 pages, 10 figures, to appear in A&