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

Abstract

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).