Thermionic power-law decay of excited aluminum-cluster anions and its dependence on storage-device temperature

The decay of excited aluminum-cluster anions (Al${}_n^{-}$, $n=4$ and 5) has been investigated in a cryogenic linear ion-beam trap. A power-law is found to accurately reproduce the time dependence of the observed decay rates at early storage times, although the exponents are significantly larger than the typically observed $1/t$ decay. It is shown that the power-law exponent is, at most, weakly dependent on the cluster electron affinity and heat capacity. A previous power-law exponent model for small clusters is also shown to be in disagreement with both investigated species. The attribution of a drop in the decay rate at later times to radiative cooling as observed in larger molecules also does not appear justified in our case. A strong dependence of the power-law exponent on the ambient temperature was observed.

Figure 1

Absolute time dependence of the neutral product rate from Al${}_5^{-}$ ions stored at two different trap temperatures (dots, $<$15 K; crosses, 300 K). The vertical line marks the time at which the ions enter the temperature variable region. Background arising from residual gas collisions in the trap was fitted and extrapolated to earlier times. Several exponential decay curves (gray dotted lines) illustrate the superposition of individual decaying levels in the observed power-law. A sample statistical error bar is shown around 6 ms in the $<$15 K decay rate.

Figure 2

Intensity of decay neutrals after background subtraction is plotted as a function of time for both (a) Al${}_5^{-}$ and (b) Al${}_4^{-}$ beams stored at different trap temperatures (dots, $<$15 K; crosses, 300 K). A power-law with an exponential reduction at a given time was used to fit the data. The vertical line represents the time at which the ions enter the temperature variable region.