Ions, isotopes, and metal cyanides: Observational and laboratory studies

Abstract

Chemistry in the interstellar medium is very different from the processes which take place in terrestrial settings. Environments such as circumstellar envelopes, molecular clouds, and comets contain diverse and complex chemical networks. The low temperatures (10-50 K) and densities (1-10⁶ cm⁻³) allow normally unstable molecules to exist in significant quantities. At these temperatures, the rotational energy levels of molecules are populated, and thus these species can be detected by millimeter-wave radio astronomy. The detection and quantification of interstellar molecules, including metal cyanides and molecular ions, is the basis of this dissertation work. While conducting observations of CN and ¹³CN to determine the ¹²C/¹³C ratio throughout the Galaxy, it was found that the ratios in photon-dominated regions (PDRs) were much higher than those in nearby molecular clouds. This can be explained by isotope-selective photodissociation, in which the ¹²CN molecules are self-shielded. However, the chemistry in these regions is poorly understood, and other processes may be occurring. In order to understand one of the chemical networks present in PDRs, observations of HCO⁺, HOC⁺, and CO⁺ were made toward several of these sources. Previous studies indicated that the HCO⁺/HOC⁺ ratio was much lower in PDRs, due to the presence of CO⁺. The new observations indicate that there is a strong correlation between CO⁺ and HOC⁺ abundances, which suggests that other molecular ions which have not been detected in molecular clouds may be present in PDRs. There is a significant obstacle to the detection of new interstellar molecular ions, however. The laboratory spectra are virtually unknown for many of these species, due to their inherent instability. Thus, techniques which can selectively detect ionic spectra must be utilized. One such method is velocity modulation, which incorporates an AC electrical discharge to produce and detect ions. Previously, velocity modulation spectroscopy was employed only at infrared wavelengths. The final phase of this dissertation work was to design, build and test a velocity modulation spectrometer which functions at millimeter/sub-mm wavelengths. This system was then used to measure the previously unknown pure rotational spectrum of SH⁺ (X3Σ⁻).