Rotational Line Spectra Research Articles

Self-consistent dust and non-LTE line radiative transfer with SKIRT

We introduce Monte-Carlo-based non-local thermodynamic equilibrium (non-LTE) line radiative transfer calculations in the three-dimensional (3D) dust radiative transfer code SKIRT, which was originally set up as a dust radiative transfer code. By doing so, we developed a generic and powerful 3D radiative transfer code that can self-consistently generate spectra with molecular and atomic lines against the underlying continuum. We tested the accuracy of the non-LTE line radiative transfer module in the extended SKIRT code using standard benchmarks. We find excellent agreement between the S KIRT results, the published benchmark results, and the results obtained using the ray-tracing non-LTE line radiative transfer code MAGRITTE, which validates our implementation. We applied the extended SKIRT code on a 3D hydrodynamic simulation of a dusty active galactic nucleus (AGN) torus model and generated multiwavelength images with CO rotational-line spectra against the underlying dust continuum. We find that the low-J CO emission traces the geometrically thick molecular torus, whereas the higher-J CO lines originate from the gas with high kinetic temperature located in the innermost regions of the torus. Comparing the calculations with and without dust radiative transfer, we find that higher-J CO lines are slightly attenuated by the surrounding cold dust when seen edge-on. This shows that atomic and molecular lines can experience attenuation, an effect that is particularly important for transitions at mid- and near-infrared wavelengths. Therefore, our self-consistent dust and non-LTE line radiative transfer calculations can help the observational data from Herschel, ALMA, and JWST be interpreted.

The Case for Methyl Group Precession Accompanying Torsional Motion

For molecules containing a methyl group, high precision fits of rotational line data (microwave spectra) that encompass several torsional states require considerably more constants than are required in comparable rigid molecules. Many of these additional terms are ‘torsion-rotation interaction’ terms, but their precise physical meaning is unclear. In this paper, we explore the physical origins of many of these additional terms in the case where the methyl group is attached to a planar frame. We show that torsion-vibration coupling, which has been observed in toluene and several substituted toluenes, provides the dominant contribution to a number of the torsion-rotation constants in toluene. It is further demonstrated that this coupling is intimately related to precession of the methyl group. A number of the constants required in the high resolution fits of rotational line data are shown to arise as a natural consequence of methyl precession. By considering several molecules whose rotational line spectra have been fit to high precision, we demonstrate that the experimental evidence is consistent with the occurrence of methyl group precession. Quantum chemistry calculations of the optimised molecular structures at key torsional angles provide further evidence that methyl precession occurs. There is both a torsional angle dependent tilt of the Cmethyl-frame bond and of the methyl group principal rotation axis relative to the Cmethyl-frame bond.

Radiative transfer of HCN: interpreting observations of hyperfine anomalies

Molecules with hyperfine splitting of their rotational line spectra are useful probes of optical depth, via the relative line strengths of their hyperfine components.The hyperfine splitting is particularly advantageous in interpreting the physical conditions of the emitting gas because with a second rotational transition, both gas density and temperature can be derived. For HCN however, the relative strengths of the hyperfine lines are anomalous. They appear in ratios which can vary significantly from source to source, and are inconsistent with local thermodynamic equilibrium. This is the HCN hyperfine anomaly, and it prevents the use of simple LTE models of HCN emission to derive reliable optical depths. In this paper we demonstrate how to model HCN hyperfine line emission, and derive accurate line ratios, spectral line shapes and optical depths. We show that by carrying out radiative transfer calculations over each hyperfine level individually, as opposed to summing them over each rotational level, the anomalous hyperfine emission emerges naturally. To do this requires not only accurate radiative rates between hyperfine states, but also accurate collisional rates. We investigate the effects of different sets of hyperfine collisional rates, derived via the 'proportional method' and through direct recoupling calculations. Through an extensive parameter sweep over typical low mass star forming conditions, we show the HCN line ratios to be highly variable to optical depth. We also reproduce an observed effect whereby the red-blue asymmetry of the hyperfine lines (an infall signature) switches sense within a single rotational transition.

Probing the structure and dynamics of molecular clusters using rotational wave packets.

Rotational wave packets of the weakly bound C(2)H(2)-He complex have been created using impulsive alignment. The coherent rotational dynamics were monitored for 600 ps enabling extraction of a frequency spectrum showing multiple rotational energy levels up to J = 4. spectrum has been combined with ab initio calculations to show that the complex has a highly delocalized structure and is bound only by ca. 7 cm(-1). The experiments demonstrate how highly featured rotational spectra can be obtained from an extremely cold environment where only the lowest rotational energy states are initially populated.

Formation of coherent rotational wavepackets in small molecule-helium clusters using impulsive alignment.

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. Aligned rotational wavepackets were generated by non-resonant interaction with intense femtosecond laser pump pulses and then probed using Coulomb explosion by a second, time-delayed femtosecond laser pulse. By means of a Fourier transform a rich spectrum of rotational eigenstates was derived. For the smallest cluster, C(2)H(2)-He, we were able to establish essentially all rotational eigenstates up to the dissociation threshold on the basis of theoretical level predictions. The C(2)H(2)-He complex is found to exhibit distinct features of large amplitude motion and very early onset of free internal rotor energy level structure.

Sub-Doppler laser absorption spectroscopy of the A2Π i − X2Σ + (1, 0) band of CN: Measurement of the 14N hyperfine parameters in A2Π CN

Measurements of multiple rotational lines in the (1, 0) band of the A 2Π i − X 2Σ + “red” system of the cyanogen radical (CN) at sub-Doppler resolution are reported. The CN radical was produced by 193 nm photodissociation of NCCN (ethane dinitrile) and detected with a Ti:sapphire ring laser operating near 10 900 cm −1. The sample was exposed to a weak, frequency-modulated probe beam and a strong, counterpropagating bleach laser beam. Demodulated probe laser signals display absorption and dispersion features derived from Doppler-free saturation of the hyperfine components as the laser scans across the central region of a Doppler-broadened rotational line spectrum. Hyperfine-resolved transition frequencies were combined with known ground-state X 2Σ hyperfine term values to determine A 2Π state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A 2Π state. All the expected hyperfine and 14N quadrupolar parameters were determined and their values analyzed in terms of a simple molecular orbital picture of the bonding in the radical. Higher sensitivity obtained with 400 kHz amplitude modulation of the bleach laser and additional phase-sensitive detection allowed hyperfine splittings in some rotational lines of 13C 14N to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the 13C hyperfine parameters.

Coherent laser spectroscopy of RF resonances in liquid

Four-photon polarization spectra of double distilled water subjected to a special treatment in a cavitation chamber and 20% aqueous solution of hydrogen peroxide were recorded in the range ±8 cm−1. All recorded spectra contain narrow (< 0.3 cm−1) resonances corresponding to the frequencies of the rotational spectrum of ortho and para spin isomers of the H2O molecule. Numerical simulation of the spectra obtained made it possible to quantitatively estimate the contribution of the rotational spectrum to the coherent scattering signal. It was found that the contribution of the para spin isomer of the H2O molecule to the rotational line spectrum decreases in an aqueous solution of the α-chymotrypsin protein. Apparently, this decrease indicates the selectivity of interaction of biopolymer molecules with different spin isomers.

SiO line emission from C-type shock waves: interstellar jets and outflows

We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si--bearing material in refractory grain cores, which are taken to be olivine; we calculate also the rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady--state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si being released into the gas phase. More realistic assumptions concerning the initial fractional abundance of O2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG `slab' models. The fractional abundance of SiO is deduced. Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.

Late Stages of Stellar Evolution

A large fraction of ISO observing time was used to study the late stages of stellar evolution. Many molecular and solid state features, including crystalline silicates and the rotational lines of water vapour, were detected for the first time in the spectra of (post-)Asymptotic Giant Branch (AGB) stars. Their analysis has greatly improved our knowledge of stellar atmospheres and circumstellar environments. A surprising number of objects, particularly young planetary nebulae with Wolf-Rayet (WR) central stars, were found to exhibit emission features in their ISO spectra that are characteristic of both oxygen-rich and carbon-rich dust species, while far-IR observations of the PDR around NGC 7027 led to the first detections of the rotational line spectra of CH and CH+.

The Use of Monte Carlo Simulation Methods to Calculate the Radiation of Jets of Combustion Products in View of Rotational Spectral Structure

Monte Carlo simulation algorithms are given which are intended for the calculation of the spectral emissivity of jets of combustion products of solid-propellant rocket motors (SRM). A distinguishing feature of these algorithms is the inclusion of the rotational structure of the spectrum of combustion products. Analysis is made of the prospects for the use of line-by-line (LBL) methods of calculations of the spectral emissivity and hybrid methods which combine simulation methods with statistical models of rotational-line spectrum.

Magnetic moments of hindered rotators by off‐center impurity ions

AbstractOff‐center impurity ions in solids often perform rotations around their regular lattice sites. Unlike quasifree rotors with rotational line spectra subject to textbook attention in quantum mechanics, the off‐center species are hindered rotors with spectra quantized in rotational bands. These bands occur because of tunneling through barriers arising along the orbital path. For an off‐center ion rotating along a planar orbit, such as the Li+ impurity nearest‐neighboring an F center in alkali halide, the hindered rotation will give rise to specific magnetic moments that couple to and quantize external magnetic fields normal to the orbital plane. We present a simple theory and estimates of Li+ magnetic dipoles and rotational bands to find conditions for an experimental verification. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

High‐Resolution Electron‐Impact Emission Spectrum of H 2 . I. Cross Sections and Emission Yields 900–1200 A

High-resolution (Δλ = 115 mA) emission spectra of molecular hydrogen produced by electron-impact excitation at 100 eV have been obtained in the wavelength range 900-1200 A. The emission spectra can be assigned to transitions between the X 1Σ ground state and the 1sσgnpσu 1Σu and 1sσgnpπu 1Πu Rydberg states. Synthetic rotational line spectra based on the excitation function of Liu et al. and calculated 2pσ B 1Σ-X 1Σ, 2pπ C 1Πu-X 1Σ, 3pσ B' 1Σ-X 1Σ, and 3pπD 1Πu-X 1Σ transition probabilities of Abgrall et al. are generally found to be in good agreement with the experimental spectra in the regions where emissions from 1sσgnpσu (n ≥ 4) and 1sσgnpπu (n ≥ 4) states are negligible. Emission cross sections for D 1Π, D' 1Πu, and D'' 1Π, B' 1Σ, and B'' 1Σ states are obtained by measuring the emission intensities from these states relative to those from the B 1Σ,C 1Πu, and D 1Π states. A high- resolution EUV calibration technique is established. At 100 eV the emission cross sections of the D 1Πu, D' 1Πu, D'' 1Πu, B' 1Σ, and B'' 1Σ states are measured to be (2.8 ± 0.4) × 10-18, (6.3 ± 1.3) × 10-19, (5.9 ± 1.7) × 10-20, (2.1 ± 0.3) × 10-18, and (1.6 ± 0.4) × 10-19 cm2, respectively. In addition, the vibrational emission cross sections have been compared to the estimated excitation cross sections to obtain the predissociation yields for selected vibrational levels of the D 1Π, D' 1Π and D'' 1Π states. The B' 1Σ state is inferred to have very significant excitation into the H(1s)+H(2p,2s) dissociative continuum.

Variability of the atmospheric N2O line (J=3→4)

We analyze the results of observations of the radio emission of atmospheric N2O in the line J=3→4 at frequency near 100.5 GHz. Observations aimed at monitoring the nitrous oxide content at altitudes h>20 km were performed in Nizhny Novgorod from March to May, 1997. We discover variations of the optical-depth decrement in the line, which can be explained by the redistribution of nitrous oxide over the altitude range h=20–55 km. This assumption does not contradict published data on in-situ measurements of N2O density at h=5–35 km. Using the obtained data we calculate the rotational-line spectra of atmospheric N2O. We point out that the measured decrements of the optical depths are greater than the calculated ones. We discuss observations of the N2O lines in the mm-wave band as a promising method for monitoring the vertical mixing in the atmosphere.

ChemInform Abstract: Intensity Distribution in Rotational Line Spectra. Part 1. Experimental Results for Doppler‐Free S1 ← S0 Transitions in Benzene.

ChemInformVolume 20, Issue 7 Physical Organic Chemistry ChemInform Abstract: Intensity Distribution in Rotational Line Spectra. Part 1. Experimental Results for Doppler-Free S1 ← S0 Transitions in Benzene. H. SIEBER, Inst. Phys. Theor. Chem., TU Muenchen, D-8046 GarchingSearch for more papers by this authorE. RIEDLE, Inst. Phys. Theor. Chem., TU Muenchen, D-8046 GarchingSearch for more papers by this authorH. J. NEUSSER, Inst. Phys. Theor. Chem., TU Muenchen, D-8046 GarchingSearch for more papers by this author H. SIEBER, Inst. Phys. Theor. Chem., TU Muenchen, D-8046 GarchingSearch for more papers by this authorE. RIEDLE, Inst. Phys. Theor. Chem., TU Muenchen, D-8046 GarchingSearch for more papers by this authorH. J. NEUSSER, Inst. Phys. Theor. Chem., TU Muenchen, D-8046 GarchingSearch for more papers by this author First published: February 14, 1989 https://doi.org/10.1002/chin.198907048AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume20, Issue7February 14, 1989 RelatedInformation

ChemInform Abstract: Intensity Distribution in Rotational Line Spectra. Part 2. Analysis of the Fluctuations in the Doppler‐Free S1 ← S0 Spectrum of Benzene.

ChemInformVolume 20, Issue 7 Physical Organic Chemistry ChemInform Abstract: Intensity Distribution in Rotational Line Spectra. Part 2. Analysis of the Fluctuations in the Doppler-Free S1 ← S0 Spectrum of Benzene. Y. M. ENGEL, Y. M. ENGEL Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this authorR. D. LEVINE, R. D. LEVINE Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this author Y. M. ENGEL, Y. M. ENGEL Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this authorR. D. LEVINE, R. D. LEVINE Fritz Haber Res. Cent. Mol. Dyn., Hebrew Univ., Jerusalem 91904, Isr.Search for more papers by this author First published: February 14, 1989 https://doi.org/10.1002/chin.198907049AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue7February 14, 1989 RelatedInformation

Intensity distribution in rotational line spectra. II. Analysis of the fluctuations in the Doppler-free S1←S spectrum of benzene

The fluctuations in the intensities of rotational bands in the S1 manifold of benzene are analyzed in detail. The extent of mixing is found to increase with energy and those bands which could not be assigned are found to have an intensity distribution characteristic of the chaotic limit. At a given energy, those bands which borrow intensity due to resonance coupling are more mixed and the C6D6 bands are more strongly coupled than those of C6H6.

Intensity distribution in rotational line spectra. I. Experimental results for Doppler-free S1←S transitions in benzene

Completely resolved Doppler-free rotational line spectra of six vibronic two-photon bands in benzene C6 H6 and C6 D6 are presented. The excited final states possess different excess energies in S1 (1567 to 2727 cm−1 ) and are embedded in dense manifolds of background states with differing densities of states (1&amp;lt;ρ&amp;lt;60 1/cm−1 ). The bands are analyzed by a statistical procedure. The intensity distribution of several hundreds of lines of each band is investigated. It is found that all weakly perturbed bands display a similar, peaked intensity distribution while in strongly perturbed bands the number of lines decreases monotonically with increasing intensity. The origin of this difference is discussed in terms of coupling to the many background states.

Stratospheric observation of far IR pure rotational lines of hydroxyl

The hydroxyl radical, OH, has a key role in many atmospheric and ionospheric processes and has been detected in molecular clouds in space. The well-known near IR vibration–rotation bands of this molecule, named after their discoverer, Meinel, were first identified in atmospheric observations1 while much of the impetus for precise microwave data has come from the identification of radiation in this wavelength range from OH in space2. However, although increasingly precise measurements have now been reported from the UV3 through the IR4 to the microwave spectral regions5, the far IR pure rotational line spectrum has been virtually ignored, and no observations of this relatively simple spectrum have been reported between 5 and 500 cm−1. Here we present measurements of spectra between 30 and 110 cm−1 and deduce stratospheric OH column densities from the observed spectral lines.

The properties of liquid nitrogen

The light-scattering ‘anisotropy’ spectrum of liquid nitrogen has been measured for the liquid along the liquid-vapour coexistence line from 69·4 K (almost the triple point) to near the critical point at 125 K and for the gas under pressure at 128 K. The spectrum is very broad (HWHH∼50 cm-1) due to rapid molecular reorientation. The molecular anisotropy spectrum is approximately gaussian, corresponding to a correlation time for molecular reorientation of order 2 × 10-13 s at 80 K which is comparable with that obtained from nuclear magnetic resonance. A gaussian rather than a lorentzian form arises because molecular reorientation is not a ‘slow’ variable. Above the critical temperature the molecular anisotropy spectrum can be roughly described as a collision-broadened rotational line spectrum. The spectrum and its time Fourier Transform are analysed in terms of the dynamical correlation of orientation of the molecules. A component of the spectrum due to induced polarization is separated from the anisotropy contribution by a study of the far wing of the spectrum and is observed on the Stokes side out to about 250 cm-1. This depends exponentially on frequency shift and is interpreted in terms of molecular collision dynamics. The correlation time for this motion is about a factor three shorter than that for molecular reorientation. The spectra are markedly asymmetric after all experimental corrections for asymmetry have been made. The asymmetry is shown to correspond to the detailed balance factor. It is pointed out that this factor should be allowed for in the case of induced scattering, in particular. A theoretical analysis is given of the effect of correlation of molecular orientation on the light-scattering spectrum for centrosymmetric linear molecules. In particular it is shown that the normalized second moment of the spectrum is unaffected by correlation of orientation.

A Low Temperature Bolometer Heterodyne Receiver for Millimeter Wave Astronomy

Liquid helium cooled InSb hot electron bolometers are used in a balanced mixer configuration as detectors for an imageless microwave receiver. The system is designed for mounting at the prime focus of the National Radio Astronomy Observatory (NRAO) 11 m antenna at Kitt Peak, Arizona, and is suitable for the study of rotational line spectra of interstellar gas molecules. Currently the operating frequency is in the 90–140 GHz band where the double sideband system noise temperature is 250 K.