Hyperfine Rather Than Spin Splittings Dominate the Fine Structure of the \u3cem\u3eB\u3c/em\u3e \u3csup\u3e4\u3c/sup\u3eΣ\u3csup\u3e-\u3c/sup\u3e–\u3cem\u3eX\u3c/em\u3e \u3csup\u3e4\u3c/sup\u3eΣ\u3csup\u3e-\u3c/sup\u3e Bands of AIC

Laser-induced fluorescence and wavelength resolved emission spectra of the B 4Σ−–X 4Σ− band system of the gas phase cold aluminum carbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high pressure argon. High resolution spectra show that each rotational line of the 0-0 and 1-1 bands of AlC is split into at least three components, with very similar splittings and intensities in both the P- and R-branches. The observed structure was reproduced by assuming bβS magnetic hyperfine coupling in the excited state, due to a substantial Fermi contact interaction of the unpaired electron in the aluminum 3s orbital. Rotational analysis has yielded ground and excited state equilibrium bond lengths in good agreement with the literature and our own ab initio values. Small discrepancies in the calculated intensities of the hyperfine lines suggest that the upper state spin-spin constant λ′ is of the order of ≈0.025–0.030 cm−1

The optical spectroscopy of extraterrestrial molecules

The ongoing quest to identify molecules in the interstellar medium by their electronic spectra in the visible region is reviewed. Identification of molecular absorption is described in the context of the elucidation of the carriers of the unidentified diffuse interstellar bands while molecular emission is discussed with reference to the unidentified Red Rectangle bands. The experimental techniques employed in undertaking studies on the optical spectroscopy of extraterrestrial molecules are described and critiqued in the context of their application.Comment: 21 pages, 9 figures, Invited review Australian Journal of Chemistry, accepted for publicatio

Absorption spectroscopy of massselected hydrocarbon and boron species in 6 K neon matrices

Matrix isolation spectroscopy is an important experimental technique used to characterize spectroscopically unstable species like ions and radicals. Species of interest can be frozen in a matrix for a long time and investigated using different spectroscopic methods in a wide range of wavelengths. Electronic and vibrational absorption spectroscopy provides information about the location and strength of transitions for the interrogated species, which is essential for subsequent gas phase studies. It is an important link in the following research sequence: ab initio calculations → matrix isolation studies → gas phase studies → comparison with astrophysical data. One should emphasize the “symbiosis” between matrix and gas phase research. Due to matrix effects like shifts and broadening, direct comparison of matrix spectra with those of ISM is not possible (gas phase measurements are needed); on the other hand, gas phase techniques are usually very restricted in their spectral range, and hence the need for matrix data as the basis for searching for transitions and their assignments. Several species which contain atoms that are abundant in cosmos were investigated in cold matrices as a part of this PhD. Using an electron impact cation source and diacetylene as a precursor it was possible to produce positively charged carbon chains terminated by one or several H atoms (C6H+, C8H+, C6H4+, C4H3+, C6H3+, C8H3+). However, one can not obtain bare carbon cations in this way (the probability that a carbon chain will not capture at least one H atom is very small at the given experimental conditions). Thus, different chlorinated hydrocarbons were used as precursors to produce such species as Cn+ (n = 6 – 9) and chlorine terminated carbon chains CnCl+ (n = 3 – 6). The disadvantage here is the need to always find a proper precursor for each ion. (Every single precursor has its own physical properties, e.g. melting temperature, and requires some modification in the experimental set-up.) It was also possible to spectroscopically characterize the B3 molecule in neon matrices. Either Cs sputter anion source or a laser ablation source without mass-selection were used for production. However, larger boron compounds were elusive in the case of the anion source, and laser vaporization alone proved difficult since one can not make any proper assignments without mass-selection. Therefore, one part of this work was devoted to the development of a laser ablation source, suitable to be coupled with the existing mass-selection experimental set-up. Such a source can provide a breakthrough in matrix isolation spectroscopy of species like larger boron molecules (> B3) and bare carbon cations. This source promises to be quite a universal tool, which can produce many species from one precursor; in contrast to the chlorinated hydrocarbons example. With this source, one will also be able to obtain efficient matrix concentrations of small species like C3+; this cation did not reveal any electronic absorptions in matrices, most likely due to its insufficient production in the cation source. Some progress in construction of this source has already been achieved. Bare carbon cations Cn+ (n = 1 – 8) were produced, however their yield must be significantly increased (e.g. the application of a pulsed valve is a promising solution)

Electronic spectroscopy of metal terminated carbon chains

The new experimental setup for laser induced fluorescence spectroscopy (LIF) in a gas phase was design. Electronic transitions of the linear MgC4H MgC6H and MgC4D radicals and triangular AlC2 radical have been observed. The species were prepared in a supersonic expansion by ablation of magnesium or aluminum rod in the presence of acetylene, diacetylene, or methane gas. The transitions were recorded in the 415 - 475 nm region and assigned based on previously reported mass-selective resonance enhanced ionization spectra and the rotational structure. Astrophysical implications are briefly addressed

Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the rate coefficients and their uncertainties for the major gas-phase processes -- ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination -- is reviewed. Emphasis is placed on those reactions that have been identified, by sensitivity analyses, as 'crucial' in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalysed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review

Electronic spectroscopy of unsaturated hydrocarbons and sulfur-terminated carbon chains by cavity ringdown

Unsaturated hydrocarbons are highly reactive species that are found in flames and plasma discharges being important intermediates in the formation of polyaromatic hydrocarbons (PAHs) and in the processes of chemical vapor deposition (CVD). Some particular unsaturated hydrocarbons have been identified in the interstellar medium where their role is not yet fully understood. Linear carbon chains are one of the simplest rigid nanostructures that can potentially conduct electron current. Most of the unsaturated hydrocarbons are highly reactive species and for this reason they can only be studied in situ during the short time after their formation. The species are generated in the gas phase through an electrical discharge of a precursor gas and cooled in the supersonic expansion. High-resolution optical absorption spectra are recorded using a tunable dye laser. Because the concentrations of the reactive species studied are generally low, highly sensitive detection technique has to be used. This work presents a study of highly unsaturated hydrocarbons by cavity ringdown spectroscopy (CRDS). High-resolution electronic spectra of ions and radicals allow identifying them in remote environments such as interstellar medium and providing information about the physical conditions of the environment. It is shown how the information about the structure of the unknown absorbing species can be deduced from its rotationally resolved spectrum

Electronic Spectra of Aromatic Hydrocarbons: A Deductive Approach to the Diffuse Interstellar Bands

The diffuse interstellar bands (DIBs) are a series of more than 500 interstellar absorption features, the carriers of which have remained unidentified since 1919. In order to determine which aromatic chemical species are likely to be carriers of the DIBs, trends observed in the spectroscopic features of polycyclic aromatic hydrocarbon (PAH) species with chromophores ranging from 6 to 17 carbon atoms are considered. These trends are explored for PAHs with differing charge states and multiplicities, for multiple electronic transitions. Previously unreported electronic transitions of the neutral radicals 1-naphthylmethyl, 2-napthylmethyl, 9-methylanthracene and 1-pyrenylmethyl as well as the 9-methylanthracenium+ and phenalenium+ radical cations and the closed-shell neutral molecule 1H-phenalene were recorded. The D1 ← D0 transitions of small PAH resonance stabilized radicals (RSRs) are shown to be unlikely to be responsible for the DIBs. The spectroscopic properties of larger PAH RSRs were empirically extrapolated from experimental and computational trends. The vibronic structures of these molecules were assigned. As the D1 ← D0 transitions of PAH RSRs are weak, techniques were developed to obtain the gas-phase spectra of more intense electronic transitions to higher excited-states by double-resonance spectroscopy. Several strong transitions were observed, which were then assigned using ab-initio and TD-DFT computational methods. The spectra of PAH radical cations were recorded. The recorded spectra covered a range from the mid infrared to the ultraviolet. As a result of the work presented, several classes of PAH can now be dismissed as possible carriers of the DIBs. Further avenues of research have been suggested. The role of this work as part of the ongoing search for the carriers of the DIBs will be discussed

Spectroscopic Detection and Characterization of Jet-Cooled Transient Molecules

Transient molecules are of great importance due to their critical role as intermediates in the semiconductor industry, in upper atmosphere reactions, and in astrochemistry. In the present work, reactive intermediates were produced in the laboratory by applying an electric discharge through a suitable precursor gas mixture and studied by means of their laser-induced fluorescence and emission spectra. The band systems of and have been studied in detail. The energy levels of both isotopologues were fitted with a Renner-Teller model, and the isotope relations have been used to test the validity of the derived parameters. The A2Πu - X 2Πg electronic transition of jet-cooled has been detected and shown to originate from the Ω=3/2 spin-orbit component of v=0 of the ground state. For the first time, the 0-0 band has been identified and vibrational assignments have been made. Our ab initio studies show that the extensive observed perturbations are due to spin-orbit interaction between A2Πu(3/2) and B2Δu(3/2) states. The experimental data were fitted to an effective Hamiltonian and yielded the spin-orbit coupling term =240 cm-1. LIF and emission spectra of the transition of N2O+ have been recorded. Both spin-orbit components of the band were studied at high resolution and rotationally analyzed, providing precise molecular constants. Emission spectra provided extensive data on the ground state vibrational levels which were fitted to a Renner-Teller model including spin-orbit and Fermi resonance terms. The previously unknown electronic spectrum of the H2PO radical has been identified. Ab initio predictions were used to aid in the analysis of the data. The band system is assigned as the electronic transition. The excited state molecular structure was determined by rotational analysis of high resolution LIF spectra. The band systems of the HBCl and DBCl free radicals have been studied in detail. This electron promotion involves a linear-bent transition between the two Renner-Teller components of what would be a 2Π electronic state at linearity. Ab initio potential energy surface calculations were used to help in assigning the LIF spectra which involve transitions from the ground state zero-point level to high vibrational levels of the excited state

Computational investigation of the photochemistry and spectroscopy of cyclic aromatic hydrocarbons in interstellar ice analogs

This thesis describes the photochemistry and ultraviolet (UV) spectroscopy of cyclic aromatic hydrocarbons such as benzene and naphthalene, along with small water clusters and crystalline water ice clusters. Firstly, benzene and naphthalene interactions with small water hexamer (W6) clusters, and then benzene interactions with crystalline water ice clusters are investigated. This thesis primarily focuses on the applications of a range of computational chemistry techniques to investigate and characterize excited states of these complex systems, which are generated following one-photon absorption. Benzene and naphthalene, as prototypical polycyclic aromatic hydrocarbons (PAHs), and water and crystalline ice clusters, taken as representative of interstellar ices, could also be considered as useful model systems to replicate polycyclic aromatic hydrocarbons (PAHs) in interstellar ices, and to study their behaviour under UV processing. From coupled cluster (CC) benchmark studies on small water clusters up to water the pentamer, it is shown that that highly correlated linear-response coupled cluster methods such as CCSD and CC3 are important to consider while studying electronic excitations, as electron correlation effects play an important role in such systems, with double excitations playing a dominant role. However, triple excitations contributions calculated are negligible with CCSD and CC3 methods converging monotonically to similar results. The aggregation effect on water at CCSD level has shown a blue shift of ~ 0.7 eV in the central water molecule of water pentamer (C2v) relative to water monomer (C2v), and is in good agreement with the experimental blue shift of ~ 1 eV in condensed phase. For both benzene- and naphthalene-bound water W6 clusters, we have calculated interesting features of benzene- and naphthalene-mediated electronic excitations of the water W6 cluster at wavelengths where photon absorption cross section of water is negligible i.e., above 170 nm. These excitations were originally absent in the isolated water W6 cluster. Similar features are calculated for benzene-bound crystalline ice clusters, which also illustrate the effect of cyclic aromatic hydrocarbons on electronic excitations of ice clusters, and are also observed experimentally. The brightest → ∗ electronic transition of benzene and naphthalene is calculated to be red-shifted in wavelength and occurs with lower intensities after interacting with the water W6 and ice clusters. The degeneracy of this transition is also slightly broken in benzene. We have observed new electronic transition features such as charge transfer (CT), and locally diffuse Rydberg type excitation in these complexes. We have found a good performance of hybrid DFT functionals i.e. M06-2X and CAM-B3LYP in calculating vertical excitation energies of these complexes using time dependent density functional theory (TD-DFT). Further, diffusion studies of the deuterium (D) atom have shown the importance of surface morphology in generating different potential sites and hopping characteristics of the D atom on crystalline and amorphous ice surfaces. D2 formation is found to be efficient on the amorphous ice surface, with longer residence times of the D atom indicating a possibility of the deuterium atom getting trapped in such sites. There is then a further possibility of the diffusing D atom to recombine with the trapped D atom to form a D2 molecule. However, such D atom trapping is a rare possibility on crystalline surface, as hopping is fast and thus the recombination process is not efficient on crystalline ice surface

Gas-Phase Ion Chemistry in Interstellar, Circumstellar, and Planetary Environments

In the last century, astronomers, physicists, and chemists have shown that the environments of space are complex. Although we have learned a great amount about the interstellar medium, circumstellar medium, and atmospheres of other planets and moons, many mysteries still remain unsolved. The cooperation of astronomers, modelers, and chemists has lead to the detection of over 180 molecules in the interstellar and circumstellar medium, and the evolution of the new scientific field of astrochemistry. Gas-phase ion chemistry can determine the stability of ions in these complex environments, provide chemical networks, and guide searches for new interstellar molecules. Using the flowing afterglow-selected ion flow tube (FA-SIFT), we have characterized the reactions of positive and negative ions that are important in a variety of astrochemical environments. The detection of CF+ in photodissociation regions highlights the importance of fluorinated species in the interstellar medium. The viability of CF+ as a possible diffuse interstellar band (DIB) carrier is discussed as related to reactions with neutral molecules in various interstellar conditions; the reactions of CF+ with twenty-two molecules of interstellar relevance were investigated. The chemical reactions of HCNH+ with H2, CH4, C2H2, and C2H4 were reexamined to provide insight into the overprediction of HCNH+ in Titan’s ionosphere by current astrochemical models. In addition, this work suggests other chemical reactions that should be included in the current models to fully describe the destruction rates of HCNH+ in Titan’s ionosphere. The reactions of polycyclic aromatic hydrocarbon (PAH) ions with H atoms and other small molecules were carried out to determine the stability of these species. In diffuse regions, where the photon flux is high, PAH cations are the dominant ionization state. This work continues our previous research to include PAHs of differing geometries as well as nitrogencontaining PAHs. Extension to larger PAH cations was made possible by the integration of the laser induced acoustic desorption (LIAD) source with the FA-SIFT. In addition, in dense environments, where the photon flux is low, anionic PAHs may exist. The detection of negative ions in the past 10 years has highlighted the importance of their inclusion in astrochemical models. We have investigated the chemistry of deprotonated PAHs with molecules of interstellar relevance to determine their chemical stability in dense regions of the interstellar and circumstellar medium. In addition to PAH anions, H− is an important species in dense interstellar environments. While the reaction of hydride anion has been recognized as a critical mechanism in the initial cooling immediately after the Big Bang, H− + H −→ H2 + e−, chemistry with neutral molecules was largely unknown. The chemistry of H− with various classes of organic molecules was investigated and conclusions are drawn based on reaction mechanisms