Radiative lifetimes of metastable states of negative ions

Abstract

We present a technique for measuring the radiative lifetimes of metastable states of negative ions that involves the use of a heavy-ion storage ring. The method has been applied to investigate the radiative decay of the np 3 2 P 1/2 levels of Te − ͑n =5͒ and Se − ͑n =4͒ and the 3p 3 2 D state of Si − for which the J =3/2 and 5/2 levels were unresolved. All of these states are metastable and decay primarily by emission of E2 and M1 radiation. Multi Configuration Dirac-Hartree-Fock calculations of rates for the transitions in Te − and Se − yielded lifetimes of 0.45 s and 4.7 s, respectively. The measured values agree well with these predicted values. In the case of the 2 D state of Si − , however, our measurement was only able to set a lower limit on the lifetime. The upper limit of the lifetime that can be measured with our apparatus is set by how long the ions can be stored in the ring, a limit determined by the rate of collisional detachment. Our lower limit of 1 min for the lifetime of the 2 D state is consistent with both the calculated lifetimes of 162 s for the 2 D 3/2 level and 27.3 h for the 2 D 5/2 level reported by O'Malley and Beck and 14.5 h and 12.5 h, respectively, from our Breit-Pauli calculations.

Negative Ions in Cold Storage

“…These belong to the same PRL 114, 143003 (2015) P H Y S I C A L R E V I E W L E T T E R S week ending 10 APRIL 2015 atomic group in the periodic table as S − . The present error bars for Te − and Se − are about a factor of 5 smaller than those of the only available previous experiments [8]. Both sets of experimental results are close to the theoretical results [8] and in agreement with each other, as shown in Table I.…”

“…We will explain how lifetime measurements are performed by means of laser probing of the metastable population in circulating 10 keV 32 S − ion beams and how we take beam loss due to electron detachment in the small amounts of residual gas into account. In addition, we present measurements on Te and Se − and demonstrate that our new method is a factor of 5 more precise than the previous ones [8,25] due to our much longer storage times.…”

“…Further, cryogenic cooling was shown to eliminate all measurable effects of the black-body radiation field for the 4 P o 5=2 levels in 4 He − with its inherent lifetime of a few tenths of a millisecond [24]. The only previous lifetime measurements of a truly bound metastable state are for the 2 P o 1=2 level in Te − [8,25] and in Se − [8]. These were measured in a room-temperature magnetic storage ring (pressure 10 −11 -10 −12 mbar) to have lifetimes of up to a few seconds.…”

Resolving a discrepancy between experimental and theoretical lifetimes in atomic negative ions

“…Halogen-like negative ions have recently been investigated [1,7,8]. These have ground states of the form np 5 2 P 3/2 , and the np 5 2 P 1/2 as the only excited states.…”

“…A lingering problem [7,8] in these studies seemed to be a disagreement between experimental [1] and theoretical lifetimes from multiconfiguration Hartree-Fock calculations [9], which casts some doubt on the identification of this transition in S − (where n = 3 …”