The Observation and Interpretation of Plasma Line Broadening in Molecular Rotational Spectra.

The microwave spectrometer used to study rotational transitions in molecular ions and other transient species was modified. A solenoid magnet was added to a DC glow discharge, which was our standard molecular ion source. Molecular ion signals are increased by up to a factor of 40 in the magnetically enhanced abnormal (high voltage, low current) discharge. Three new plasma sources were built: radio frequency, hollow cathode glow, and hollow cathode arc discharges. The spectrometer's frequency range was extended to 500 GHz by adding an harmonic generator and an InSb detector. This enabled us to make the first microwave observations of KrD^{+} and rm H_2D^{+}. For KrD^{+} we found: U _{01} = 250 214.364(300) MHz/amu, and the mass dependent corrections Delta _sp{01}{rm Kr} = 0.682(70) and Delta_sp{01} {rm H} = 0.1215(40). We made a detailed study of plasma induced line broadening in the J = 3 to 4 and J = 4 to 5 rotational transitions of HCO^ {+} and the J = 2 to 3 and J = 3 to 4 transitions in HCN, all in the 02^20 vibrational state. A 10 GHz microwave interferometer was used to measure the plasma density, which was varied from 10^9 to 7 times 10^ {10} cm^{-3} . The nearly degenerate l-doublets in the 02 ^20 state are substantially broadened and weakened at these densities. (The nearly non-degenerate 02^20 state is unperturbed.) This effect is due to the microscopic plasma electric fields, macroscopic fields in our discharges being too small to account for the observed broadening. The effect was modeled as a function of plasma density using models based either on the quasi-static approximation or on the impact approximation. The impact approximation was shown to be more valid. In this approximation inelastic collisions with ions are the main cause of plasma line broadening. Elastic collisions with ions and all collisions with electrons are not important. An approximate experimental value for the electric dipole moment of HCO^{+} was found. The two transitions studied gave inconsistent results: 3.0 +/- 0.5 D (J = 3 to 4) and 5.3 +/- 0.5 D (J = 4 to 5). The discrepancy is large, but the average value agrees with Botschwina's ab initio value of 3.91 D. Our discrepancy may be caused by the approximations used in the model and/or by errors in the experimentally determined linewidths.