Lifetime measurement ofHe−using an electrostatic ion trap

Abstract

The lifetimes of the metastable 1s2s2 p 4 P 5/2 level of He Ϫ , as well as the lifetime of the average of the 4 P 3/2 and 4 P 1/2 levels, have been measured using a new type of ion trap that stores keV ion beams using electrostatic fields only. The use of a pure electrostatic field avoids the complication of magnetic-field-induced mixing effects, which can interfere with the measurement of the spontaneous decay. The measured lifetime for the 4 P 5/2 state, after correction for decay induced by blackbody radiation, is 343Ϯ10 s. This value is consistent with previous experiments, and in excellent agreement with the most recent theoretical calculations. The average lifetime of the 4 P 3/2 and 4 P 1/2 is 8.9Ϯ0.2 s, which is about 20% lower than the weighted theoretical value. ͓S1050-2947͑99͒09501-3͔

The interaction of He− with fullerenes

“…Most significantly, there is a large suppression of the total He − signal, which is attributed to electron transfer to SF 6 . However, it is also found that the He − produced at the two lower energy resonances (22 and 23 eV) is more strongly affected by the addition of SF 6 than that at the high energy resonance (25 eV).…”

“…The primary anionic product detected in the gas phase by mass spectrometry is the SF 5 − fragment. Figure 5 shows how the He − ion yield is affected by addition of SF 6 . Some preliminary data of this type were briefly presented in Ref.…”

“…Before considering the impact on fullerene dianion formation, we first highlight findings for electron donation to SF 6 in helium nanodroplets. In the case of SF 6 , only monoanion formation is induced by electron transfer from He − , presumably because formation of dianions is energetically forbidden (there may also be other factors that would prevent observation of dianions, such as Coulomb explosion). The primary anionic product detected in the gas phase by mass spectrometry is the SF 5 − fragment.…”

“…8 The other spin-orbit components, corresponding to J = 3/2 and 1/2, have shorter lifetimes on the order of 10 µs. 6,7 Given that He − is short-lived, it is challenging to investigate the physics and particularly the chemistry of this ion in the gas phase. However, the recent discovery of He − in superfluid helium nanodroplets has opened up a new route a) Electronic addresses: Paul.Scheier@uibk.ac.at and andrew.ellis@le.ac.uk for exploring the properties of this unusual anion.…”

“…The lifetimes of the spin-orbit components of the 1s2s2p 4 P state have been subjected to various measurements. [4][5][6][7] The most recent and precise measurement for the lowest and longest lived spin-orbit component, the J = 5/2 state, yields a value of 359.0 ± 0.7 µs. 8 The other spin-orbit components, corresponding to J = 3/2 and 1/2, have shorter lifetimes on the order of 10 µs.…”