Resonance and Nonresonant Laser Ionization of Sputtered Uranium Atoms from Thin Films and Single Microparticles:  Evaluation of a Combined System for Particle Trace Analysis

• Published in: Analytical chemistry (Washington) Vol. 75; no. 13; pp. 3175 - 3181
• Main Authors: Erdmann, Nicole, Betti, Maria, Kollmer, Felix, Benninghoven, Alfred, Grüning, Carsten, Philipsen, Vicky, Lievens, Peter, Silverans, Roger E, Vandeweert, Erno
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.07.2003
• Subjects: Analytical chemistry; Atoms & subatomic particles; Chemistry; Exact sciences and technology; Ions; Lasers; Spectrometric and optical methods; Trace elements; Uranium; Trace analysis; Particle size; Autoionizing state; Selectivity; Sputtering; Thin film; Trace element; Sensitivity; Uranium; Microparticle; Uranium oxide; Multiphoton ionization; Resonance ionization mass spectroscopy
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac0264426

The resonance and nonresonant laser ionization of uranium atoms sputtered from thin metal films and individual micrometer-size uranium oxide particles, respectively, was studied to evaluate a new setup for the analysis of actinide-containing micrometer-size particles. Experiments using nonresonant (193-nm) ionization of atoms and molecules sputtered from micrometer-size uranium oxide particles have shown that the uranium detection efficiencies for sputtered neutral atoms are ∼2 orders of magnitude higher than for secondary ions. In uranium particles of 0.5-μm diameter, 6 × 106 atoms of 235U were easily detected and the isotopic ratio of 235U/238U = 0.0048 ± 4.6% is in excellent agreement with the certified value. The use of two-color, two-step resonance ionization of the sputtered neutral uranium atoms from thin films was investigated. Several excitation schemes were tested, and a significant population of several low-lying metastable states after ion sputtering was observed. Autoionizing states for double-resonant ionization were determined, and the high selectivity of ionization schemes involving these autoionizing states was illustrated by comparing the flight-time distributions of different sputtered species obtained both by resonance and nonresonant multiphoton (355-nm) laser postionization. Ideally, the options for resonance as well as nonresonant ionization would be combined in a single setup, to obtain a large gain in sensitivity and selectivity. Thus, information about the main components as well as specific isotopic information of a trace element could be obtained from the same single particle.