The measurement of trace concentrations (1-400 ppm) of polyatomic molecular impurities in helium was investigated by laser-induced breakdown spectroscopy (LIBS). For the Column III and V hydrides such as B2H6, PH6, and AsH3, the detectability limits with the present apparatus were determined to be 1, 3, and ∼1 ppm, respectively. It is expected that resorting to synchronous detection techniques will improve the sensitivity of LIBS for this application by as much as an order of magnitude.

Spectra attributed to transitions from the 5sσ a 3Σu+ metastable state of Kr2 to excited Rydberg states have been observed by intracavity and multipass absorption and by laser excitation spectroscopy. Four distinct Rydberg series converging on the X 2Σu+ core of Kr2+ have been identified. Rotational structure is not resolved in any of the observed spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ(3Πg,3Σg+) and npλ(3Πg,3Σg+)←a 3Σu+ series by analogy with similar spectra in the lighter rare gas dimers. The 6pπ 3Πg←a 3Σu+ and 8pσ 3Σg+←a 3Σu+ transitions observed in the ∼19 200–18 200 cm−1 region exhibit many bands with v′≠0 in their (v′–v″) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. All series are perturbed in a few energy regions by states built on the excited ion core states A 2Π3/2g and B 2Π3/2u. Laser excitation spectra for the 8pσ 3Σg+←a transition obtained by monitoring various 5p→5s emission lines of atomic krypton suggest that the 8pσ 3Σg+ Rydberg state of Kr2 is predissociated by a repulsive potential correlated with Kr 5p[3/2]1+Kr(1S0). The lowest ionization limit of Kr2 (relative to a 3Σu+,v=0) was determined to be 28 484±5 cm−1.

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Absolute and angular transmission measurements were made on a single sheathed fiber 0.15 mm in diameter and 2.1 m long, and an optical angular transfer function was defined and calculated. The effective transmission of the source brightness was calculated as a function of the f numbers of the entrance and output optics of the fiber, and these results were used to choose a suitable optical system for illumination of a spectrometer.