Thomas A. Dixon

The laboratory microwave spectrum of the cyanide radical in its X 2Σ+ ground state

The microwave absorption spectrum of the CN molecule in the v=0 and v=1 vibrational states of the electronic ground state has been obtained in glow discharges in nitrogen–cyanogen mixtures at room temperature. Zeeman modulation is used for detection of the signal, and the microwave source is phase locked and digitally programmed by a computer. For each vibrational state the frequencies of the seven strongest hyperfine components of the N=0→1 rotational transition have been extracted from a careful regression analysis of the complex line shapes observed in the digitized spectral data. For the v=0 state the resulting rest frequencies and molecular parameters are in good agreement with, but more precise and accurate than, the values obtained from earlier radioastronomical studies. For the excited vibrational state the present work provides the first determination of the hyperfine parameters. The precision of the results for both states is sufficient to give a reliable measure of the variation of the spin–rot...

Laboratory microwave spectrum and rest frequencies of the N/sub 2/H/sup +/ ion

The J = 1 reverse arrow 0 multiplet of N/sub 2/H/sup +/ has been detected in laboratory glow discharges in mixtures of hydrogen and nitrogen. This constitutes the first observation of this species in the laboratory by any spectroscopic method. A pattern of three lines, due to the quadrupole hyperfine splitting of the outer nitrogen, was observed, and the frequencies of these three lines were measured with the following results: F = 2 reverse arrow 1, ..nu.. = 93,173.70 +- 0.04 MHz, F = 1 reverse arrow 1, ..nu.. = 93,171.88 +- 0.04 MHz, and F = 0 reverse arrow 1, ..nu.. = 93,176.13 +- 0.22 MHz. These are in very good agreement with previous astronomical measurements.

The molecular structure of HCO+ by the microwave substitution method

Precise measurements of the frequencies of the J = 0–1 transition of HCO+, H13CO+, HC18O+, DCO+, D13CO+, and DC18O+ have been used to determine a substitution (rs) molecular structure for HCO+. The bond distances obtained are rs(CO) = 1.1071(2) A and rs(CH) = 1.0930(1) A. This is the first such microwave rs structure to be determined for any molecular ion. The determined bond distances and the transition frequencies can be compared to the results of previous high quality ab initio calculations and excellent agreement is found. An initial attempt to characterize the Doppler shifts of the HCO+ frequency due to ion drift in a dc discharge and a measurement of the J = 1→2 transition of DCO+, and thus its centrifugal distortion constant, are also reported here.

The microwave spectrum of isotopically substituted CO+ ion

The frequencies of the N = 0–1 transitions of C18O+ and 13CO+ have been measured, and those of the main isotopic form have been remeasured. The hyperfine parameters of 13CO+ have been obtained and compared to those previously determined in the laser‐ion beam experiments of Carrington et al. The isotopic variations of the spin‐rotation parameter γ have been studied, and evidence for breakdown of the Born–Oppenheimer approximation has been observed in the isotopic dependence of the rotational constant B0.

A microwave substitution structure for protonated nitrogen N2H

Microwave observations of the J = 0–1 transitions of N2H+, 15N2H+,and 15N14ND+, N2D+, 15N2D+, and 15N14NH+ have been used to determine a substitution structure for N2H+. The resultant structural parameters are rs(NN) = 1.0947(4) A and rs(NH) = 1.0320(1) A, which agree beautifully with those from available ab initio calculations. The frequencies obtained for the 15N species provide a basis for their future radioastronomical detection and identification.

Pressure broadening of the HCO+J=0–1 transition by hydrogen

Measurements have been made of the broadening of the HCO+ J=0–1 microwave absorption line by hydrogen. The linewidths (Δν, half‐width at half‐intensity) were obtained from a nonlinear least squares analysis of the individual spectra, taken over the pressure range of 4–40 mTorr. The pressure dependence of the linewidth, dΔν/dP was determined to be 29.6±3 MHz/Torr for the HCO+ transition broadened by H2 near 100 °K. This corresponds to a collisional cross section of 182±20 A2 for HCO+–H2. Also, a new and more reliable measurement is reported for broadening of the J=7–8 transition of 18OCS, which gives dΔν/dP=7.11±0.15 MHz/Torr for self‐broadening at 296 °K.

The microwave spectrum of CO in the a 3Π state. I. The J=0–1 transitions in CO, 13CO, and C18O

The J= 0–1 transitions of a 3Π carbon monoxide have been measured in several isotopic forms and vibrational states: CO (v=0–4), 13CO (v=0–3), and C18O (v=0–4). These new data were combined with the many lambda doublet transition frequencies previously available from the molecular beam electric resonance experiments of Klemperer and co‐workers and fit to a model Hamiltonian essentially the same as that used for previously published fits of combined optical–lambda doubling data. Many different fits were made including single state–single isotope, multistate–single isotope, single state–multi‐isotope, and multistate–multi‐isotope ones. Among the important conclusions are (1) that the explicit perturbation due to the v′=0 level of the a′ 3∑+ state must be included for adequate fits of the v=0–3 data (and of course the more strongly perturbed v=4 data), (2) that the spin–orbit coupling constant A can be determined well from just the microwave–rf data if several isotopic forms are fitted together, and (3) that ...

A computer controlled microwave spectrometer system

A digitally programmable microwave source, assembled from various commercially available components and covering the range 8.0–40.0 GHz in four bands, has been interfaced to a small computer, and software has been developed for the acquisition, storage, retrieval, and statistical analysis of data. This system has been used in conjunction with Stark and Zeeman modulated microwave spectrometers, yielding high resolution, high sensitivity, high accuracy, and operation with a considerable degree of automation and flexibility. The programming time is typically 5–15 msec, and the frequency may be set in increments of 100 Hz or less. The long term frequency stability and accuracy is 2×10−9/month, and the short term stability or spectral purity appears to be excellent, in agreement with theoretical expectations. The software provides for signal averaging, baseline subtraction, convenient display and frequency measurement, and for transfer of data, via magnetic tape, to a large scale computer, where it can be anal...