Absorption spectroscopy of carbon and sulfur chains in 6 K neon matrices

Matrix isolation is challenged by a variety of laser-based techniques, which, in the gas phase, have demonstrated their ability to obtain high-resolution vibrational and electronic spectra of transient molecules, free of possible perturbation by the matrix environment. Despite this challenge, there are important reasons why matrix studies of transient molecules are, in fact, complementary to lasers studies and should continue to yield valuable contributions to the study of chemically reactive intermediates. While lasers are tuneable over limited spectral ranges, observations in rare-gas matrices provide a broad spectral survey, extending from the FIR to the VUV. Thus, all of the products can be detected. Matrix-shifts from the gas-phase band center are typically small enough to provide a guide for the choice and tuning of lasers for more detailed gas-phase studies. Also, while isotopic substitution is crucial to positively identify a particular system, these studies are more readily conducted in matrices. Matrix absorption observations are also helpful in spectral assignments, due to the fact that spectral contributions from hot bands are eliminated; at cryogenic temperatures all absorptions originate from the molecular ground state.
Unfortunately, a direct comparison of the spectra obtained from matrix isolation measurements can not be conclusive; however, the localized electronic transitions and vibrational patterns provide crucial information for gas phase investigations. Therefore, matrix absorption observations, if complemented by appropriate gas phase studies, will serve as a guide for investigation of astrophysically relevant molecules in the interstellar medium.
The results of the spectroscopic studies on Cn+ n=6−9, obtained during the present PhD work, locate the wavelength range and the relative intensities of their electronic absorptions. Additionally the photobleaching experiments permit a comparison with the transition intensities of their neutral counterparts. Neutral l-C6 and l-C8 have an electronic transition with origin at 511 and 640 nm in a neon matrix but the oscillator strength is an order of magnitude smaller than for the 646 nm band system of l-C6+ or 890 nm of l-C8+. In view of this and the UV radiation field in the interstellar medium, these linear cations may be more readily detected than their neutral chains in the optical region where the diffuse absorption bands are observed. On the other hand, in the UV both l-C8 (around 277 nm) and l-C8+ (308 nm) have strong electronic transitions of comparable intensity. The neutral chains l-C7 and l-C9 have much stronger electronic transitions than l-C7+ and l-C9+ both in the UV and visible, and should therefore be the easier ones to detect. Based on this work, one should search for the strong absorption near 250 and 295 nm for l-C7 and l-C9, respectively; their calculated oscillator strengths are very large. Thus gas phase spectra of these neutral and cation carbon chains are now required.
Matrix isolation is not powerful in structure determination of the produced species. For example, it is difficult to conclude exclusively from the matrix spectroscopic data which isomer, linear or cyclic, is produced while using a cyclic precursor, but, as experience shows, linear species are more readily generated when linear precursors are used. A helpful technique includes introducing a Ne/N2O mixture for deposition. N2O plays a dual role in the matrix: as a reactant and as an electron scavenger. Gas-phase ion mobility and bimolecular reactivity studies indivate that the linear isomers are more reactive. Comparing spectra obtained using pure Ne for deposition with those using the Ne/N2O mixture, as was in the case of Cn+ cations, allows reasonable confidence as to which of the species (cyclic or linear) were collected.
One of the beauties of the cesium sputter source is its use of pure rods, specifically, rods which do not contain any hydrogen atoms. The absence of hydrogen in a precursor simplifies the mass selection process. Thus, it was possible to obtain B3 molecule after mass-selected deposition of B3− anion with subsequent irradiation. The main problem which occurs with anions’ deposition is the ability to keep a stable current. After a couple of minutes of deposition the latter drops. Irradiating the matrix while depositing is help, but in this way only neutral species are obtained. The solution comes with introducing a new source, which could eliminate the current dropping. This new source (for example Xe+ source) has to be put in the matrix chamber and directed onto the matrix substrate. In this way Xe cations are also collected in the matrix and further arriving of anions should not be suppressed.
The electron impact ion sources are straightforward to operate. A stable discharge, obtained with gaseos precursors, makes the deposition process easier. The disadvantage here is in choosing a different precursor for each new species of interest (preferentially without a hydrogen atom). That is why one should think about introducing some new additional sources for ions production. One of the possible candidates is a magnetron sputtering source, which gives not high but sufficient current. A symbiosis of a stable current and mass selection is the heart of the matrix isolation. The magnetron sputtering source has at least two electrically mutually isolated stationar bar-shaped target arrangements mounted one alongside the other and separated by respective slits. Each of the target arrangements includes a respective electric pad so that each target arrangement may be operated electrically independently from the other target arrangement. Each target arrangement also has a controlled magnet arrangement for generating a time-varying magnetron field upon the respective target arrangement. The magnet arrangements may be controlled independently from each others. The source further has an anode arrangement with anodes alongside and between the target arrangements and/or along smaller sides of the target arrangements. This will allow studying of new species in the matrix, such as: Cn+-, CnO+-, CnN+-, MeCn+-, Sn+-, Bn+- as well as their neutral analogues.