Distinguishing mechanisms behind the droplet-size dependence of the Na+–He attachment rate
Ascertain the dominant mechanism, or quantify the relative contributions, responsible for the observed increase of the helium attachment rate r with helium nanodroplet size in the Poisson analysis of Na+He_n yields—specifically: (i) determine whether longer lift-off times t_lift in larger droplets allow additional binding and dissipation before ejection; (ii) determine whether larger droplets dissipate solvation energy faster due to their higher density of states; and (iii) determine whether softer Coulomb repulsion in larger droplets reduces vibrational excitation and thus post-ejection evaporation of Na+He_N complexes, leading to apparent larger detected n at earlier times.
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The fit also shows that the helium attachment rate increases when the droplet size increases. This could be because the increase of $t_\text{lift}$ with droplet size leaves \ce{Na+He_$N$} more time to pick up additional He atoms and to dissipate energy for larger complexes. Another explanation for this observation is that larger droplets may be able to dissipate the solvation energy faster since the density of states in the droplet increases with size. Thirdly, the softer Coulomb explosion for the larger droplets will add less vibrational energy to the ejected \ce{Na+He_{$N$} ions. As such, the ions will have a smaller probability for evaporating He atoms, which in turn leads to detection of larger \ce{Na+He_{$n$} complexes at earlier times. Currently, we are not able to distinguish between these three mechanisms.