Photographic and Spectroscopic Measurements of the Hg Mass and its Oxide Coatings in Low Pressure, RF-Excited Discharge Lamps

Charles Klimcak, Arielle Little, Kaitlin Fundell, and James Camparo

Abstract: Some atomic clocks (e.g., rubidium and mercury-ion frequency standards) rely on a metal-vapor rf-discharge lamp to generate the device’s atomic signal. Briefly, rubidium (Rb) or mercury (Hg) atoms in the lamp are excited by the discharge, creating Rb or Hg resonance light that is employed for optical pumping. Surprisingly, although rf-discharge lamps have a nearly 100-year history, understanding the subtleties of their operation and their life-limiting mechanisms is only now beginning. In part, this renewed interest in atomic clock rf-discharge lamps derives from the fact that their performance may directly impact the capabilities of global navigation satellite systems, and the satellites for those systems are expected to have decade or longer lifetimes. Crucial to studying rf-discharge lamps and their long-term reliability is the non-destructive assessment of the initial mass of the metallic element placed into the lamp. While calorimetric measurement methods have been utilized in the past for this purpose, modifying commercial calorimetric instrumentation to permit measurements in lamps can be difficult and expensive. Here, we will show that the metal fill in a Hg rf-discharge lamp can be approximately determined using a simple photographic method. Since Hg is liquid at room temperature, and has appreciable surface tension, it can be condensed in the lamp to form a single, high contact angle, contiguous droplet exhibiting a spherical cap geometry for sufficiently low masses. This droplet can then be photographed to determine its volume and mass by standard macrophotographic techniques. Although imaging through a small diameter cylindrical lamp creates significant optical distortion, we have demonstrated the attainment of a mass fill uncertainty of + 25%. While not as accurate as the calorimetric mass measurement procedure that is often used to determine rf-discharge lamp fills for Rb and Hg consumption investigations, the simplicity of this photographic method will allow researchers and Hg frequency standard suppliers to easily determine Hg mass fills in rf-discharge lamps without the need for modified calorimetric instrumentation nor personnel with calorimetric measurement expertise. An insufficient Hg fill level could result in premature frequency standard failure particularly if substantial Hg consumption by diffusion and trapping and/or chemical reaction mechanisms occurs within the standard’s required mission lifetime. Recently, we have observed evidence suggesting that chemical reactions do occur in Hg rf-discharge lamps resulting in the deposition of a thin brown coating on the interior surface of the lamp that could attenuate the deep UV resonance radiation necessary for optical pumping thereby reducing both the strength of the atomic clock signal as well as the availability of free metallic Hg in the lamp. Consequently, we have performed spectroscopic analyses of this deposited coating in the UV spectral regime to investigate absorbance at 194 nm, the wavelength of light used for both optical pumping and atomic signal monitoring in Hg+ atomic clocks. Our results indicate that the coating is composed of a HgO-type material, which we believe to be embedded in the matrix of the lamp envelope. This coating can have significant absorption at 194 nm, suggesting that further study of the coating is warranted to aid in the development of long-lived Hg+ atomic clocks.
Published in: Proceedings of the 53rd Annual Precise Time and Time Interval Systems and Applications Meeting
January 25 - 27, 2022
Hyatt Regency Long Beach
Long Beach, California
Pages: 96 - 107
Cite this article: Klimcak, Charles, Little, Arielle, Fundell, Kaitlin, Camparo, James, "Photographic and Spectroscopic Measurements of the Hg Mass and its Oxide Coatings in Low Pressure, RF-Excited Discharge Lamps," Proceedings of the 53rd Annual Precise Time and Time Interval Systems and Applications Meeting, Long Beach, California, January 2022, pp. 96-107.
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