Noble-Gas Loss in Alkali Rf-Discharge Lamps and Its Possible Dependence on Electron Temperature

Abstract

The optically pumped, vapor-cell atomic clock is a work horse of precise timekeeping, finding applications onboard global navigation satellites as well as at cellular communications base stations. At the heart of the device is the relatively simple alkali rf-discharge lamp, which enables the production of the atomic-clock signal and the sensing of the atoms' response to resonant microwaves. In the lamp, electrons extract energy from an rf-field via elastic collisions with noble-gas buffer atoms; the energetic electrons then ionize Rb atoms; finally, resonant light for optical pumping is produced through Rb + /electron recombination at the lamp's glass walls. Unfortunately, recent research has shown that alkali rf-discharge lamps slowly lose their buffer gas, giving rise to a life-limiting mechanism for the rf-discharge lamp and hence the atomic clock. Here, we review the literature on buffer-gas loss in alkali rf-discharge lamps and discuss a likely mechanism for the process. We then discuss the dependence of the discharge's electron temperature on rf-frequency, and how this might be used as a critical test of the proposed mechanism.Index Terms-Alkali discharge lamp, atomic clocks, inductively-coupled plasma, noble-gas ionization, Rb atomic clock.

Spectral emission from the alkali inductively-coupled plasma: Theory and experiment

“…(12) for the electron density is that n e (r) is independent of [Rb] (i.e., lamp temperature). Further, since previous studies have found that T e ∼ 1/ν o 2 , 29 Eq. (17) suggests that all other things being equal η ∼ ν o .…”

“…15,23 Additionally, the very high energy of xenon's first two excited states (i.e., 6s[3/2] 1 and 6s[3/2] 2 implies that most electron/Xe collisions will be elastic in nature given the average electron kinetic energy of ∼ 0.65 eV. 29 To model the buffer-gas excitation, for clarity of notation we will denote the two lowest-energy excited states of Xe as a and b (listed in terms of increasing angular momentum): 6s[3/2] 1 → a and 6s[3/2] 2 → b . For these two states it is worth noting that b is metastable with a lifetime (against spontaneous emission) of 42 sec, 32 while a has a decay rate, Γ a , of 1.5×10 5 s -1 even in FIG.…”

Measuring buffer-gas pressure in sealed glass cells

“…This buffer-gas loss not only alters the lamp's operating characteristics, it can eventually lead to the lamp's failure, since without elastic collision partners the electrons cannot extract energy from the rf-field. At present, the most viable theory for buffer-gas loss in discharge lamps involves the Noble-gas Ion Capture (NIC) mechanism of Jaduszliwer and co-workers [17,18]. Briefly, the NIC mechanism posits that the few very high energy…”