Charles M. Miller

Use of two-photon excitation in resonance ionization mass spectrometry

We report the detection of a series of elements via 2 + 1 (photons to resonance + photons to ionize) resonance ionization mass spectrometry. The efficiency of this process is demonstrated, as the two-photon step can often be saturated. Examples are given where experimental observations of ion yields are compared to theoretical calculations based on a simple formalism used for two-photon transition rates, coupled with a rate-equations calculation of the total ionization rate. Advantages of 2 + 1 ionization compared with more widely used 1 + 1 processes are discussed.

Resonance ionization mass spectrometry of thorium: determination of the autoionization level structure and a re-determination of the ionization potential

Abstract We report on the use of resonance ionization mass spectrometry (RIMS) for the analysis of thorium isotopes. The 238U/230Th decay pair is useful for the dating of geologic samples in the ⩽ 350 000 year range, and details are needed on the resonance ionization spectrum of thorium in order to most efficiently effect ionization. This report describes the use of a two-color, two photon ionization process for thorium near the ionization threshold. A large number of autoionizing states are observed, a new value for the ionization potential, 50 890 ± 20cm−1 (6.310 ± 0.002 eV), is reported, and electric field effects are described.

Selective photoionization of 64Cu in the presence of 63,65 Cu

Abstract It has been proposed that a copper crystal enriched in 64Cu relative to 63Cu would provide a source of a low-energy, monoenergetic beam of positrons, with an intensity as much as three orders of magnitude greater than unenriched sources. We have evaluated the use of resonance ionization to selectively photoionize 64Cu in the presence of the stable isotopes 63Cu and 65Cu. It appears that this method may be applied with high selectivity and relatively high efficiency using rather simple laser systems. Various aspects of this technique will be discussed.

Materials analysis by laser and ion beam sputtering with resonance ionization mass spectrometry

Abstract A comparison is made between sputter (ion beam)- and laser-sampling for resonance ionization mass spectrometry (RIMS). The temporal and spatial development of the generated plumes is discussed, as well as various aspects of the multistep ionization process. Lastly, several examples are given of the application of laser-ablation/RIMS to problems in optical damage.

Measurement of trace isotopes by photon burst mass spectrometry

Progress is reported on the development of a new laser- and mass spectrometer- based technique for measurement of trace levels of radioisotopes. Significant results to date include the demonstration of high efficiency and throughput in a mass spectrometer, efficient production of metastable atoms from Ar+ and Kr+ beams, a demonstration of the photon burst detector principle with Mg+ ions, and the verification of zero background in a two- detector system.

High-efficiency resonance ionization mass spectrometric analysis by external laser cavity enhancement techniques

A novel electronic circuit has been developed that detects absorption in one of two (reference and signal) laser beams with shot noise-limited sensitivity. The authors demonstrate its use as a simple shot noise limited spectroscopy by measuring several absorption lines of I2 vapor near 670 nm with a tunable diode laser. The noise-equivalent absorption in this double-beam method is 2 times the shot-noise-to-signal ratio of the signal beam. In this case, for 1.2 mW of power, the noise-equivalent absorption in a 1-Hz bandwidth was 4.2 X 10-8. This noise level was confirmed, although measurements were sometimes limited by interference fringes and laser tuning drift. These are technical, not fundamental, problems arising from unsophisticated optical equipment. This baseband method provides absorption spectra directly, eliminating laser excess noise without the somewhat elaborate signal generation and processing equipment needed in laser frequency modulation methods. The only modulation used in the system was the 0.5 to 1.0-kHz sweep of the diode laser current used to tune the laser output wavelength. The noise-cancelling circuit itself is not complex: workable versions can be constructed from readily available components at a cost of about 10 dollars.

Selective laser ionization for mass-spectral isotopic analysis

The mass spectrometric determination of accurate atomic isotope ratios is a task greatly complicated by the presence of isobaric (same mass) interferences. While there exist classical techniques for reducing such interferences, such as ultrahigh resolution mass spectrometry, these solutions often break down in the face of particular technical difficulties, such as the determination of very large isotope ratios or very small samples.

Very high resolution saturation spectroscopy of lutetium isotopes via cw single-frequency laser resonance ionization mass spectrometry

In this paper, we discuss the use of Resonance Ionization Mass Spectrometry (RIMS) to perform isotopically selective saturation spectroscopy of lutetium isotopes. Utilizing this technique, it is shown that accurate measurements of the relative frequencies of hyperfine (HF) components for different isotopes easily can be made without the need for an isotopically enriched sample. The precision with which the HF splitting constants can be determined is estimated to be ∼5 times greater than in previous work.

High resolution, mass resolved spectra of rare isotopes

Resonance ionization mass spectrometry is used to acquire high‐resolution optical spectra of rare isotopes. Hyperfine spectra of the 2D03/2, 2D3/2 lutetium transition at 22 125 cm−1 are presented for 173–176Lu. 173Lu and 174Lu are rare isotopes whose optical spectroscopy has not previously been investigated. We required only tens of picograms of these unseparated isotopes for our studies. We confirmed that the spin of 174Lu is unity and derived values for the hyperfine constants and isotope shifts of these nuclei.