D. L. Donohue

Mass spectrometry studies of the ionization of organic molecules by low-energy positrons

Abstract Positrons having energies in the range of 2.0–10 eV have been used to ionize large organic molecules. For positrons having energies above 2–3 eV ionization occurs by positronium formation. In this paper new results are reported that describe the ionization of molecules at energy below the positronium formation threshold. Contrary to expectations, ionization by positrons having energies below the positronium threshold has been found to induce extensive fragmentation in sigma-bonded molecules. The necessary energy for ionizing the molecules and inducing fragmentation is supplied through the positron-electron annihilation process.

Spectral studies of actinide elements by resonance ionization mass spectrometry

Resonance ionization mass spectrometry (RIMS) was applied to a spectral study of Th, U, Np, Am, and Cm in the wavelength region of 580–607 nm. One or more analytically useful RIMS peaks were observed for all these actinides except Cm. From the data presented, spectral interference among these elements can be assessed. Several new high-lying energy levels for these elements have been cataloged.

A quadratic potential time-of-flight mass spectrometer for use with slow positron ionization sources and other large-volume ion sources

A time‐of‐flight spectrometer has been specially designed for measuring the masses of ions produced by low‐energy positrons interacting with organic molecules in a Penning trap. To make the flight times insensitive to the starting positions of the ions in the trap, acceleration was done using a potential that varied as the square of the distance of the ion from the detector. The containment of the positrons in the Penning trap for extended time periods effected very long collision paths between the positrons and the molecules. The entire length of the Penning trap source could be sampled. These features produced high sensitivity, enabling the measurement of ionization processes with small cross sections. The spectrometer is useful for mass spectrometry of ions produced by processes other than positron interaction. Both large and small volume sources can be accommodated.

Alternate ionization pathways for resonance ionization mass spectrometry

Abstract Resonance ionization mass spectrometry (RIMS) has been evaluated for use in the mass analysis of europium. Many more RIMS-active optical transitions were found than can be predicted on the basis of an equilibrium thermal distribution of initial atomic states. These transitions involve high-lying initial atomic states ( > 13000 cm−1). It is proposed that these states are populated by hybrid resonant transitions involving dimer molecules. Such a process may be a general phenomenon in RIMS.

Determination of isotopic ratios of neodymium by resonance ionization mass spectrometry

Abstract Measurements of neodymium isotope ratios have been made using laser-induced resonance ionization mass spectrometry. Results are presented for samples at the 3 μg and 40 ng levels, along with the wavelengths used for ionization. Analytical precision, bias, and elemental selectivity are reported which demonstrate the potential usefulness of this technique.

Separated isotopes as internal standards in spark source mass spectrometry

Abstract The use of separated stable isotopes as internal standards in spark source mass spectrometry is described. This method is useful for solution samples and complements the more classical isotope dilution technique. For powdered or insoluble samples, the isotopes are dried onto a conducting matrix material-usually high purity silver powder-followed by homogenization of the sample with this «spiked’ matrix. The resulting mixture can then be pressed into solid electrodes and analyzed by spark source mass spectrometry. This technique has proven useful for the determination of mononuclidic elements such as Co, As, and Mn in a variety of samples, and for multielement analysis of coal ash and fly ash, with a minimum of sample handling and resultant contamination. Use of an isotope which has a mass near to that of the element being measured gives more precise and accurate analyses than the common technique of using one internal standard.

Positron ionization mass spectrometry. II: ionization by fast positrons

Abstract The ionization of polyatomic molecules in their interaction with positrons is discussed with respect to the likely mechanisms expected at various positron kinetic energies and the rationale behind the study of these processes. These mechanisms include impact ionization, those that involve the formation of positronium, and those that involve capture of the positron without positronium formation. Data are shown for ionization by fast positrons. These results constitute the first examples of the mass analysis of ions from polyatomic molecules ionized by fast (3-keV) positrons. Mass spectra derived from the bombardment of several different targets with 3-keV positrons, 70-eV electrons, and 2.7-keV electrons are compared. These comparisons support the expectation that the underlying mechanism for ionization in this collision energy region is very similar for electrons and positrons.

Positron ionization mass spectrometry. I : Instrumentation

Abstract Initial studies have been performed to investigate the use of positrons to ionize polyatomic molecules for mass spectrometric analysis. A positron beam facility has been constructed which converts forward-scattered gamma radiation from a 150-MeV accelerator into positrons of a selected energy by means of a tungsten metal moderator. The positrons are transported by solenoidal magnetic fields into the ionization volume of a time-of-flight mass spectrometer where they interact with molecules of a target gas. The initial mass spectral results are apparently due to positron impact ionization and compare directly with electron impact ionization data acquired with the same apparatus. Further studies will seek to extend the energy range of the positrons into the region below the ionization energy of the target molecules, where other mechanisms of ionization are likely.

Resonance ionization photoelectron spectroscopy of lanthanide elements

The photoelectron spectra of Sm, Nd, and Pr have been measured following laser‐induced resonance ionization. The results obtained shed light on unexplained ionization processes observed by many workers in the field of resonance ionization mass spectrometry. Electron energy spectra can provide information about the initial energy level in a multiple‐photon sequence leading to ionization. Also present is information about the low‐lying energy levels of the atomic ions produced. A number of identified and unknown processes are presented for these three elements along with the results of power dependence measurements.

Prediction and Identification of Multiple-Photon Resonant Ionization Processes

Many single-color, multiple-photon transitions leading to ionization are observed for lanthanide and actinide elements in experiments using resonance ionization mass spectrometry (RIMS). It is desirable both to identify the energy levels involved in observed transitions and to be able to predict in advance their location. A computer code, ETRANS, has been written to perform these functions. Examples of both types of operation are given.