Molecules Of Astrophysical Interest Research Articles

Defining the temperature of an isolated molecule.

The microcanonical temperature of an isolated molecule is derived in terms of Boltzmann and Gibbs volume entropies within the quantum harmonic vibrational and equivalent degenerated model approximations. The effects of the entropy functional choice and various approximations are examined. The difference between Boltzmann and Gibbs volume temperatures is negligible for molecules bigger than ten atoms. However, it is significant for smaller systems, opening a way to probe them experimentally. A simple, analytical expression of the temperature as a function of the vibrational energy is provided, allowing predictions with a ±3% margin of error compared to the exact harmonic estimate. The microcanonical temperature is discussed and exemplified with polycyclic aromatic hydrocarbon molecules and other molecules of astrophysical interest.

A Large-scale Approach to Modeling Molecular Biosignatures: The Diatomics

Abstract This work presents the first steps to modeling synthetic rovibrational spectra for all molecules of astrophysical interest using a new approach implemented in the Prometheus code. The goal is to create a new comprehensive source of first-principles molecular spectra, thus bridging the gap for missing data to help drive future high-resolution studies. Our primary application domain is for molecules identified as signatures of life in planetary atmospheres (biosignatures), but our approach is general and can be applied to other systems. In this work we evaluate the accuracy of our method by studying four diatomic molecules, H2, O2, N2, and CO, all of which have well-known spectra. Prometheus uses the transition-optimised shifted Hermite (TOSH) theory to account for anharmonicity for the fundamental ν = 0 → ν = 1 band, along with thermal-profile modeling for the rotational transitions. To this end, we expand TOSH theory to enable the modeling of rotational constants. We show that our simple model achieves results that are a better approximation of the real spectra than those produced through an harmonic approach. We compare our results with high-resolution HITRAN and ExoMol spectral data. We find that modeling accuracy tends to diminish for rovibrational transition away from the band origin, thus highlighting the need for the theory to be further adapted.

Study of the frequency factor in the thermal desorption of astrophysical ice analogs: CH4, C2H4, C2H6, CH3OH, CO, CO2, H2O and N2

In this work the frequency factor and the influence of the temperature on this parameter, for zeroth order desorption processes, has been experimentally determined for eight molecules of astrophysical interest. In the literature, this parameter has been estimated indirectly, obtaining values that differ by as much as three orders of magnitude from different authors. As a consequence, there are very different desorption rates reported for the same molecule and additionally its temperature dependence has been systematically neglected.The frequency factor is widely used to model the dynamics of these species under low temperature conditions present in some astrophysical environments. The method reported in this work is based on the analysis of the signal of a quartz crystal microbalance acting as a sample-holder, which is able to directly detect molecules desorbing from it.Two different types of desorption experiments were necessary for this study. In a first set of experiments, carried out at a constant rate of warming up, the desorption energy is obtained. The second set of experiments were performed at several constant temperatures to calculate the frequency factor and its relationship with temperature. The reasons for some anomalous behaviour have been analyzed. The dependence of the frequency factor on temperature should be taken into account when the Polanyi-Wigner equation is used for desorption processes. Every molecule has to be independently studied as no global tendency is found for the variation of the frequency factor with temperature.

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of shielding types depends on the species in question and the dust optical properties. The new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of rates to variation of temperature and isotope, and cross section uncertainties, are tested. Tests were conducted with an interstellar-cloud chemical model, and find general agreement (within a factor of two) with the previous iteration of the Leiden database for the ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

Sensitivity to a possible variation of the proton-to-electron mass ratio of torsion–rotation transitions in acetone (CH3)2CO

The notion that certain transitions of molecules with intramolecular large amplitude motions exhibit an enhanced sensitivity to the proton-to-electron mass ratio variation is changing the paradigm for searching for drifting constants from the optical to the microwave domain. It also stimulates a search for the new molecules that possess enhanced sensitivities to a variation of the proton-to-electron mass ratio. In this paper, we calculate the sensitivity coefficients for acetone, a molecule of astrophysical interest having a C2v equilibrium configuration with two equivalent methyl tops undergoing internal rotation. The calculations were done for rotational transitions belonging to the ground and the first excited (ν12≈75cm−1) torsional states of acetone, since both these states were detected during observations of interstellar molecular clouds. For rotational transitions with linestrengths above 1.0 and lower-level rotation excitation energies below 10cm−1 the sensitivity coefficients in this molecule range from Kμ=−0.69 to −1.23 for the ground state and from Kμ=−1.98 to +6.07 for the first excited torsional state.

Refractive index and density of ammonia ice at different temperatures of deposition

The optical properties and density of ammonia have not been exhaustively studied despite the relevance of these characteristics in astrophysical scenarios. This work presents new data pertaining to the real part of the refractive index of ammonia at 632.8nm and the density at different temperatures of deposition ranging from 13K to ammonia’s desorption temperature of approximately 110K. The results indicate a significant variability for both parameters versus temperature: an increase of 50% for the density and of 10% for the refractive index as the temperature increases in the range of 13–60K; at temperatures greater than 60K, a constant value is reached for both parameters. This initial variation and the following plateau reflect structural differences in the form of ammonia at low and high temperatures of deposition that can play an important role in the interaction of ammonia with other molecules of astrophysical interest.

Geometries and electronic structures of the ground and low-lying excited states of FeCO: An ab initio study

FeCO is a molecule of astrophysical interest. We report here theoretical calculations of its geometrical parameters, electronic structures, and molecular constants (such as dipole moment and spin-orbit coupling constant) in the electronic ground state X(3)Σ(-) and the low-lying triplet and quintet excited states. The calculations were made at the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] and MR-AQCC_DK3/[5ZP ANO-RCC (Fe, C, O)] levels of theory. A multi-reference calculation was required to describe correctly the wavefunctions of all states studied. For all triplet states, the σ-donation through the 10σ molecular orbital (MO) as well as the π-back-donation through the 4π MO are observed, and the dipole moment vector points from O toward Fe as expected. However, in the excited quintet states (5)Π, (5)Φ, and (5)Δ, the almost negligible contribution of Fe 4s to the 10σ MO makes the dipole moment vector point from Fe toward O, i.e., in the same direction as in CO. In the X(3)Σ(-) state, the electron provided by the σ-donation through the 10σ MO is shared between the Fe atom and the C end of the CO residue to form a coordinate-covalent Fe-C bond. In the ã(5)Σ(-) state (the high-spin counterpart of X(3)Σ(-)), the σ-donation through the 10σ MO is not significant and so the Fe-C bond is rather ionic. The π-back-donation through the 4π MO is found to be of comparable importance in the two electronic states; it has a slightly larger magnitude in the X(3)Σ(-) state. The difference in the molecular properties of the low-spin X(3)Σ(-) and the high-spin ã(5)Σ(-) states can be understood in terms of the dynamical electron correlation effects.

One-electron capture mechanisms in collisions of He-like O and C ions with molecules of astrophysical interest

One-electron capture mechanisms in collisions of He-like O and C ions with molecules of astrophysical interest

Franck-Condon factors and r-centroids for certain band systems of astrophysical molecules SrF and ScF

The Franck-Condon factors and r-centroids, which are very closely related to vibrational transition probabilities, have been evaluated by the more reliable numerical integration procedure for the bands of B2∑+ − X2∑+, F2∑+ − X2∑+ systems of SrF and C1∑+ − X1∑+, G1Π − X1∑+ systems of ScF molecules of astrophysical interest, using a suitable potential.

Molecules of astrophysical interest: Recent submillimeter and infrared spectra

New developments in submillimeter technology are discussed: the use of new sweepers up to 180 GHz driven by backward wave oscillators. Spectroscopy in the terahertz region is carried out with BWOs. Detailed studies of line shifts and broadening are summarized.

The Berkeley Program on Molecules of Astrophysical Interest

A systematic program of laboratory analyses of selected molecular spectra of astrophysical interest started in 1958 and continues to the present time. The program includes production of spectral atlases, tabulations of spectral lines, analyses, calculations of excitation energies and molecular parameters, measurements of radiative lifetimes, and determinations of transition strengths. Work has been completed or is in progress on the spectra of ArH+, C2, carbon clusters, CN, CS, CaCl, CaH, CaS, FeD, FeH, HgH, HgD, InI, LaO, LaS, OD, OH, SH, Si2, SiC2, TiCl, TiO, TiO+, VO, YS, ZrCl, ZrO, and ZrS. The basic needs for astronomically useful data have not changed, but laboratory and analysis methods have become more sophisticated in order to cope with ever greater demands for consistency, accuracy, and breadth of information. The Fourier transform spectrometer and computer codes for analyses have enhanced our ability to satisfy some of these demands.

Laboratory measurements on molecules of astrophysical interest

The infra-red absorption bands of a representative polycyclic aromatic hydrocarbon (PAH) molecule, coronene, have been examined in the laboratory at various temperatures up to 240 °C. This paper is devoted to a systematic analysis of the temperature variations of the integrated band intensity. A tentative interpretation of the observed behaviour is given in terms of anharmonicity effects. These new laboratory results may be helpful in the context of the attribution of the «unidentified» infra-red band to PAH molecules. In addition, for the first time thermal-emission spectra of three different PAHs recorded at 240 °C are presented.

Dissociation equilibrium constants of molecules of astrophysical interest

Dissociation equilibrium constants are computed for 53 molecules containing H, C, N, O, Mg, Si, S, and Fe in a range of temperatures of astrophysical interest. The results are obtained using recent molecular constants and are compared with previously published data. The equilibrium constants are approximated by an analytical expression and the fitting coefficients are supplied.

Systematics of vibrational intensities for “diatomic” molecules: Approximate formulas

Several approximations are discussed which enable one to estimate electric dipole moment transition matrix elements for diatomic molecules based on a very limited amount of experimental data. For strongly polar molecules, a simplistic point-charge dipole model in which there is no vibrational charge flux predicts a simple relation between the permanent moment, M 0, and its derivative with respect to internuclear separation, M 1. This is found to be approximately valid for a surprisingly large number of polar molecules. Next, approximate matrix elements for the fundamental and overtone bands are presented for a linear dipole moment function. Finally, systematic trends for molecules having similar electronic structures are investigated and combined with the approximations discussed above in order to estimate the transition moments for several molecules of astrophysical interest. An extension to the ν 3 fundamental and overtone bands of CH 4 is suggestive that some of these ideas may be applicable to certain polyatomic molecules as well.