Lowest Rotational Transitions Research Articles

Scattering resonances in the rotational excitation of HDO by Ne and normal-H2: theory and experiment.

The rotational excitation of a singly deuterated water molecule (HDO) by a heavy atom (Ne) and a light diatomic molecule (H2) is investigated theoretically and experimentally in the near-threshold regime. Crossed-molecular-beam measurements with a variable crossing angle are compared to close-coupling calculations based on high-accuracy potential energy surfaces. The two lowest rotational transitions, 000 → 101 and 000 → 111, are probed in detail and a good agreement between theory and experiment is observed for both transitions in the case of HDO + Ne, where scattering resonances are however blurred out experimentally. In the case of HDO + H2, the predicted theoretical overlapping resonances are faithfully reproduced by experiment for the 000 → 111 transition, while the calculated strong signal for the 000 → 101 transition is not detected. Future work is needed to reconcile this discrepancy.

First sample of N2H+ nitrogen isotopic ratio measurements in low-mass protostars

Context. The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and N2H+ is particularly important because it is directly linked to molecular nitrogen N2. However, theoretical models still do not provide an exhaustive explanation for the observed 14N/15N values. Aims. Recent theoretical works suggest that the 14N/15N behaviour is dominated by two competing reactions that destroy N2H+: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if the N2H+ recombination rate is lower with respect to that for N15NH+, the rarer isotopologue is destroyed more quickly. In prestellar cores, due to a combination of low temperatures and high densities, most CO is frozen in ices onto the dust grains, leading to high levels of depletion. On the contrary, in protostellar cores, where temperature are higher, CO ices evaporate back to the gas phase. This implies that the N2H+ isotopic ratio in protostellar cores should be lower than that in prestellar cores, and consistent with the elemental value of ≈440. We aim to test this hypothesis, producing the first sample of N2H+∕N15NH+ measurements in low-mass protostars. Methods. We observe the N2H+ and N15NH+ lowest rotational transition towards six young stellar objects in the Perseus and Taurus molecular clouds. We model the spectra with a custom python code using a constant Tex approach to fit the observations. We discuss in the Appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratios. Results. Our analysis yields an average of 14N/15N|pro = 420 ± 15 in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Conclusions. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy N15NH+ faster than N2H+, leading to high isotopic ratios in prestellar cores where carbon monoxide is frozen onto dust grains.

Pure Rotational Spectrum of CN+

Abstract The pure rotational spectrum of the elusive CN+ cation has been observed for the first time using a cryogenic ion trap apparatus and applying an action spectroscopy scheme. For the 12C14N+ species, the three lowest rotational transitions have been observed, each of which exhibits hyperfine structure from the presence of the 14N nucleus. The rare C15N+ isotopologue has been studied up to the J = 4 − 3 transition. The observations conclusively confirm that CN+ occupies a 1Σ+ electronic ground state. Given the ubiquity of the CN radical in space, CN+ is an appealing candidate for future radio astronomical searches.

Resonance Enhanced Multiphoton Ionization Detected Millimeter-Wave Absorption: The 115 GHz Line of CO.

We investigate the potential for sensitively detecting millimeter-wave absorption through multiphoton ionization using the lowest rotational transition in CO (X1Σ, J = 1 ← J = 0) at 115 GHz as a diagnostic tool. This transition represents an almost perfect realization of a quantum mechanical two-level system. In the experiment, the output of a powerful continuous millimeter-wave source with sub-kilohertz resolution propagates counter to a pulsed molecular beam, causing Doppler shifted resonances. Absorption is detected by monitoring the involved rotational levels through resonance enhanced multiphoton ionization (REMPI). Observed polarization, saturation and Doppler effects demonstrate that the interaction with the millimeter-wave field is coherent and can be simulated well using the results for the two-level system. The combination of these results indicates that the millimeter-wave field is approximated well by a Gaussian beam with beam waist around 15 mm and constant effective electric field amplitude on the molecular beam axis. Time-correlated frequency modulation in the form of a short rectangular pulse is used to follow the molecular trajectory from the source to the laser interaction region. The experiments clearly demonstrate a great potential of extending high-resolution molecular beam spectroscopy with mass specific REMPI detection into the sub-terahertz or terahertz regimes.

Building blocks of dust: A coordinated laboratory and astronomical study of the archtype AGB carbon star IRC+10216

This article provides an overview of recent astronomical studies and a closely coordinated laboratory program devoted to the study of the physics and chemistry of carbon-rich Asymptotic Giant Branch stars, as illustrated by the archtype star IRC+10216. The increased sensitivity and angular resolution of high altitude, ground-based millimeter-wave interferometers in the past few years has enabled molecular astronomers to determine the excitation and spatial distribution of molecules within a few stellar radii of the central star where the molecular seeds of dust are formed, and thus to critically assess the physicochemical mechanisms of dust formation and growth. However the astronomical studies are crucially dependent on precise laboratory measurements of the rotational spectra — both in the ground and vibrationally excited states of the normal and rare isotopic species as well — of the principal molecules in the inner region which appear to contain only two or three heavy atoms.Much remains to be done by laboratory spectroscopists as evidenced by the large number of unassigned millimeter-wave rotational lines that are observed in the inner envelope of IRC+10216. To assess whether vibrationally excited SiC2 and HCN are the carriers of some of the unassigned features observed in this source, millimeter-wave laboratory spectra are compared with Cycle 0 observations from the Atacama Large Millimeter-wave Array (ALMA) under experimental conditions that produce both species. Also highlighted are ongoing laboratory studies of the silicon carbides SiC2 and SiCSi in their ground and excited vibrational states, and c-SiC3 in its ground vibrational state. Following the initial detection of c-SiC3 and SiCSi in the outer molecular envelope of IRC+10216, the laboratory spectroscopy was extended to higher frequency in support of the recent interferometric measurements. Thirty-two new millimeter-wave rotational transitions of SiCSi with J⩽48,Ka⩽3 and upper level energies Eu⩽484 K in the range from 178 to 391 GHz, and 35 new transitions of c-SiC3 with J⩽38,Ka⩽20 and Eu⩽875 K between 315 and 440 GHz were measured in the laboratory. In addition, five or six rotational transitions in one quanta of each of the three fundamental vibrational modes of SiCSi and the two lowest rotational transitions in the previously unexplored C–C stretching mode (ν1) of SiC2 were measured in the normal and doubly substituted 13C isotopic species. Areas of continued emphasis and the importance of complementary experimental methods and production techniques to detect and assign vibrational transitions are also highlighted.

Laboratory rotational ground state transitions of NH3D+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 NH$_3$D$^+$ ($J_K = 1_0 - 0_0$), and the two hyperfine components of the ground state transition of CF$^+$($J = 1 - 0$) were measured with a relative precision better than $10^{-7}$. 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 NH$_3$D$^+$ 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 $J = 1 - 0$ transition towards the Horsehead PDR.

Terahertz-visible two-photon rotational spectroscopy of coldOD−

We present a method to measure rotational transitions of molecular anions in the terahertz domain by sequential two-photon absorption. Ion excitation by bound-bound terahertz absorption is probed by absorption in the visible on a bound-free transition. The visible frequency is tuned to a state-selective photodetachment transition of the excited anions. This provides a terahertz action spectrum for just few hundred molecular ions. To demonstrate this we measure the two lowest rotational transitions, J=1<-0 and J =2<-1 of OD- anions in a cryogenic 22-pole trap. We obtain rotational transition frequencies of 598596.08(19) MHz for J=1<-0 and 1196791.57(27) MHz for J=2<-1 of OD-, in good agreement with their only previous measurement. This two-photon scheme opens up terahertz rovibrational spectroscopy for a range of molecular anions, in particular for polyatomic and cluster anions.

Rotational spectra of 34S- and 33S-containing isotopologues of laser ablation generated tin monosulfide

Abstract Using a newly constructed source, tin monosulfide is produced by the reaction between carbonyl sulfide and the products resulting from the ablation of tin metal by the pulsed output of a frequency-doubled Nd:YAG laser ( λ = 532 nm). Entrained in argon carrier gas, SnS is introduced into the cavity of a Balle–Flygare Fourier transform microwave spectrometer that has a frequency range encompassing the two lowest rotational transitions ( J = 1 − 0 and 2 − 1) for this molecule. Except for 115 Sn 34 S, spectra are obtained in natural abundance for all 34 S-containing isotopologues with naturally occurring tin isotopes. Additionally, spectra were obtained for the rare isotopologues, 115 Sn 32 S and 120 Sn 33 S. Resolution of the hyperfine structure in the latter species allows a more precise determination of its 33 S nuclear quadrupole coupling constant. Spectra for all species containing 34 S are analyzed in terms of mass-independent Dunham parameters and are combined with all available isotopologues to determine Born–Oppenheimer breakdown terms for both tin and sulfur.

The rotational spectrum of CN−

The rotational spectrum of the molecular negative ion CN(-) has been detected in the laboratory at high resolution. The four lowest transitions were observed in a low pressure glow discharge through C(2)N(2) and N(2). Conclusive evidence for the identification was provided by well-resolved nitrogen quadrupole hyperfine structure in the lowest rotational transition, and a measurable Doppler shift owing to ion drift in the positive column of the discharge. Three spectroscopic constants (B, D, and eQq) reproduce the observed spectrum to within one part in 10(7) or better, allowing the entire rotational spectrum to be calculated well into the far IR to within 1 km s(-1) in equivalent radial velocity. CN(-) is an excellent candidate for astronomical detection, because the CN radical is observed in many galactic molecular sources, the electron binding energy of CN(-) is large, and calculations indicate CN(-) should be detectable in IRC+10216-the carbon star where C(6)H(-) has recently been observed. The fairly high concentration of CN(-) in the discharge implies that other molecular anions containing the nitrile group may be within reach.

Rotational spectrum and carbon-13 hyperfine structure of the C3H, C5H, C6H, and C7H radicals

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the singly substituted carbon-13 isotopic species of C(5)H, C(6)H, and C(7)H. Hyperfine structure in the rotational transitions of the lowest-energy fine structure component ((2)Pi(12) for C(5)H and C(7)H, and (2)Pi(32) for C(6)H) of each species was measured between 6 and 22 GHz, and precise rotational, centrifugal distortion, Lambda-doubling, and (13)C hyperfine coupling constants were determined. In addition, resolved hyperfine structure in the lowest rotational transition (J = 32-->12) of the three (13)C isotopic species of C(3)H was measured by the same technique. By combining the centimeter-wave measurements here with previous millimeter-wave data, a complete set of (13)C hyperfine coupling constants were derived to high precision for each isotopic species. Experimental structures (r(0)) have been determined for C(5)H and the two longer carbon-chain radicals, and these are found to be in good agreement with the predictions of high-level coupled-cluster calculations. C(3)H, C(5)H, and C(7)H exhibit a clear alternation in the magnitude and sign of the (13)C hyperfine coupling constants along the carbon-chain backbone. Because the electron spin density is nominally zero at the central carbon atom of C(3)H, C(5)H, and C(7)H, and at alternating sets of carbon atoms of C(5)H and C(7)H, owing to spin polarization, almost all of the (13)C coupling constants at these atoms are small in magnitude and negative in sign. Spin-polarization effects are known to be important for the Fermi-contact (b(F)) term, but prior to the work here they have generally been neglected for the hyperfine terms a, c, and d.

Glitters of Warm H2in the Cold Interstellar Medium

We present the scientific case for a high resolution spectro-imager in the mid-IR that would be dedicated to the mapping of H 2 emission in its four lowest rotational transitions, at 28.2, 17.0, 12.3 and 9.7 micron with a spectral resolution ~10 4 sufficient to provide kinematical distances in galaxies. The proposed instrument on a 2 m-class telescope will be most sensitive to H 2 line emission simultaneously extended and structured at small scale , in gas at temperatures higher than 80 K. Colder H 2 , which may contribute most of the baryonic dark matter in galaxies, will be traced by the emission of glitters of warm H 2 heated, throughout the medium, by the dissipation of omnipresent turbulence. The main scientific objectives are to (i) directly measure the mass and temperature distribution of the warm H 2 , far from star forming regions, in particular in low metallicity environments where the traditional tracers of H 2 (CO, dust emission) fail, (ii) trace the dissipation of turbulence in the perspective of building a global view of the star formation process in galaxies and (iii) trace baryonic dark matter in the form of cold H 2 in a large sample of galaxies along the Hubble sequence.

Interferometric Observation of the Highly Polarized SiO Maser Emission from thev= 1,J= 5-4 Transition Associated with VY Canis Majoris

We used the Submillimeter Array to image the SiO maser emission in the v = 1, J = 5-4 transition associated with the peculiar red supergiant VY Canis Majoris. We identified seven maser components and measured their relative positions and linear polarization properties. Five of the maser components are coincident to within about 150 mas (~200 AU at the distance of 1.5 kpc); most of them may originate in the circumstellar envelope at a radius of about 50 mas from the star along with the SiO masers in the lowest rotational transitions. Our measurements show that two of the maser components may be offset from the inner stellar envelope (at the 3 σ level of significance) and may be part of a larger bipolar outflow associated with VY CMa identified by Shinnaga et al. The strongest maser feature at a velocity of 35.9 km s-1 has a 60% linear polarization, and its polarization direction is aligned with the bipolar axis. Such a high degree of polarization suggests that maser inversion is due to radiative pumping. Five of the other maser features have significant linear polarization.

Substantial reservoirs of molecular hydrogen in the debris disks around young stars.

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the 'debris' disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value.

A Multitransition HCO+Study in NGC 2264G: Anomalous Emission of theJ = 1 → 0 Line

We present multitransition observations of the HCO+ molecule toward the very young star-forming region associated with the NGC 2264G molecular outflow. Anomalous emission is observed in the lowest rotational transition; the J = 4 → 3 and J = 3 → 2 transitions clearly trace the dense core encompassing the exciting source of the molecular outflow, whereas the HCO+ J = 1 → 0 is barely detected at a much lower intensity and has a much broader line shape. Analysis of the data strongly suggests that the HCO+ J = 1 → 0 emission arising from the core is being absorbed efficiently by a cold low-density envelope around the core or a foreground cloud. This result seems exceptional, yet the J = 1 → 0 HCO+ and HCN emission from other dense cores (especially those in giant molecular clouds) may be affected. In these cases, the rare isotopes of these molecules and higher rotational transitions of the main isotopes should be used to study these regions. Two quiescent clumps, JMG 99 G1 and G2, are detected in the blue lobe of the NGC 2264G molecular outflow, close to shock-excited near-IR H2 knots. These clumps belong to the class of radiatively excited clumps, i.e., the radiation from the shock evaporates the dust mantles and initiates a photochemical process, enhancing the emission of the HCO+.

A Survey of [TSUP]30[/TSUP]S[CLC]i[/CLC]O Emission from Evolved Stars

We present the first detection of 30SiO v = 1, J = 1–0 maser emission toward the Mira variable TX Cam among 18 surveyed evolved stars. The line width of this maser is much narrower than that of 30SiO v = 0, J = 1–0 masers and the intensity is also weaker, indicating that it is unsaturated. The detection of the 30SiO maser emission in the lowest rotational transition of the first vibrationally excited state, together with nearly simultaneous observations of the large number of 29SiO and 28SiO masers, provides good constraints for establishing the SiO maser pumping model of TX Cam. The global features of 30SiO and 29SiO v = 1, J = 1–0 lines are similar, but detailed characteristics, such as their half-widths, their peak intensity ratios of the v = 1, J = 1–0 line to the v = 0, J = 1–0 line, and their intensity variation with time, are different. New detection of the 30SiO v = 0, J = 1–0 and J = 2–1 lines is also reported in six evolved stars: TX Cam, R Cas, χ Cyg, W Hya, U Lyn, and WX Psc. They show a different masing characteristics of the 30SiO that depends on both sources and transitions. This is probably related to a critical condition for masing.

Carbon Monoxide and Dust Column Densities: The Dust‐to‐Gas Ratio and Structure of Three Giant Molecular Cloud Cores

We have observed emission in the three lowest rotational transitions of the optically thin species C18O and the dust continuum emission at three millimeter/submillimeter wavelengths. By employing the proper combination of the intensities of the three lowest rotational transitions of C18O, we can obtain the total molecular column density, with relatively little sensitivity to density and temperature variations along the line of sight. We use the line and continuum data to determine column densities of the dust and gas across three giant molecular cloud cores. We find that two of the three sources, M17 and Cepheus A, have the same gas column density-to-dust optical depth ratio, given by log [N(C18O)/τ(790 μm)] = 18.8. In the third source, the Orion molecular cloud, the gas-to-dust ratio is typically a factor of 3 lower than in the other two sources. The gas-to-dust ratio shows only a small (factor ≤ 3) variation across the region of M17 that we have mapped and a comparable reduction at the center of Cepheus A relative to the cloud edge. We have good evidence for the correlation of the continuum emission in different bands for the Orion molecular cloud and find the frequency dependence of the optical depth in the densest regions near the embedded sources to be given by τ ∝ ν1.9. For positions away from the embedded sources, there is a larger scatter in the data points, with a suggestion that the frequency dependence is steeper, such that τ ∝ ν2.4. This may be an indication of a change in the grain properties between less dense and very dense regions and is consistent with the results of grain growth. Using standard values for the fractional abundance of C18O relative to H2, the mean densities of the cloud cores are 3-5 × 104 cm-3 . These regions appear to be close to virial equilibrium. The dense gas [revealed by multiple transition studies of tracers such as CS and HC3N to have n(H2) 106 cm-3] has a volume filling factor of a few percent. Assuming a fractional abundance of C18O equal to 1.7 × 10-7, we find that the 790 μm dust optical depth to mass column density ratio for M17 and Cepheus A is 0.0062 cm2 g-1, while the average value for the Orion molecular cloud is a factor of 3 larger.

The N2 pressure broadening coefficient of the J = 1 ← 0 transition of ¹H35Cl measured by tunable far infrared (TuFIR) spectroscopy

A tunable far infrared (TuFIR) laser sideband spectrometer has been used to determine the N2 pressure broadening coefficient of the lowest rotational transition of ¹H35Cl between 0 and 1.8 Torr at room temperature (298 K). The parameters defining a Voigt lineshape were fitted to the J = 1 ← 0 transition using a Levenberg‐Marquardt non‐linear least squares fitting routine. The average N2 broadening coefficient of the three components was determined to be 4.10(10)/(MHz/Torr), 0.104(2)/(cm−1/atm).

An ab initio study of the HNCN radical

The equilibrium structure for the ground state of the HNCN radical is calculated at the levels of the self-consistent field theory (SCF), the second-order Mo/ller–Plesset perturbation approximation (MP2), and the full single and double excitation coupled cluster theory including all connected triples in a noniterative manner [CCSD(T)], using various extended basis sets starting from 6–311 G(d,p). At the CCSD(T) level, the outer C–N bond is more than 0.1 Å shorter than the inner one and the N–C–N group departs from linearity by 6°. The total N–C–N length is in good agreement with the experimental value [Herzberg and Warsop, Can. J. Phys. 41, 286 (1963)], however, the H–N–C angle is about 6° smaller. The N–H bond is very close to a normal N–H bond but is about 0.2 Å smaller than the experimental estimate. Except for the smaller H–N–C angle, the geometrical parameters for HNCN closely parallel those for the triplet HCCN molecule. The dipole moment, harmonic frequencies, electric quadrupole, and Fermi contact coupling constants of HNCN are also calculated. The calculated harmonic frequencies confirm the preliminary assignments of Wu, Hall, and Sears [J. Chem. Soc. Faraday Trans. 89, 615 (1993)]. The quadrupole coupling constants for the inner and outer N atoms are comparable, implying a complex pattern of hyperfine split components in the lowest rotational transitions. The present calculation may thus serve as a useful guide for the interpretation of the rotational spectrum.

A search for the rotational transitions of H2D+ at 1370 GHz and H3O+ at 985 GHz

A search was made for the 1370 GHz lowest rotational transition of the molecular ion H2D+ in NGC 2264, W3, and the IRc2 region of M42. No emission lines were seen, but an absorption feature was detected toward IRc2. The column density and fractional abundance were calculated using a tentative identification of the line as the transition of para H2D+. The LSR velocity and the measured line width are consistent with the dynamical parameters of the hot core source. Physical parameters deduced from the data differ from those derived from millimeter-wave observations of the hot core condensation. It is suggested that significant amounts of low-density gas are associated with this region and that the material is cold enough for enhanced deuterium fractionation to occur. A search was also made for the 985 GHz transition of ortho H3O+ in W3 and IRc2 with negative results.

Molecular parameters of chromium hydride in its X6Σ + state determined by far-infrared laser magnetic resonance spectroscopy

The far-infrared laser magnetic resonance spectrum of the CrH radical in the v = 0 level of its X 6Σ + state has been studied in detail. Signals associated with the five lowest rotational transitions have been detected. Nearly 500 resonances for 52CrH have been assigned and used to determine molecular parameters which describe the rotational energy, the fine and proton hyperfine splittings, and the Zeeman effect. These parameters are well determined and their implications for the structure of CrH are discussed. A fourth-order correction to the spin-spin coupling has been identified for the first time. Precise zero-field rotational transition frequencies have been calculated which may allow the detection of the CrH radical in the interstellar medium.