The first citation is a scientist’s coming of age. For the first time, you were referenced as a credible source of the edge of human knowledge.
Last week, my undergraduate research advisor, Vitaly Kresin, emailed me a “nice paper”, saying “I think you might enjoy it.” He was spot on. I could barely read the abstract before I started jumping around and squealing like a little girl.
During my senior year at USC, Vitaly and I published theoretical results of modeling alkali metal clusters inside helium nanodroplets. We called it “Critical sizes for the submersion of alkali clusters into liquid helium” and published in Physical Review B.
The awesome thing is that our intuition for the problem was right. We expected that small alkali clusters end up sitting in dimples on the surface of helium droplets (this was experimentally confirmed for sodium dimers and probably trimers at the time), and large clusters must sink into the center of the droplet.
The new paper is “The submersion of sodium clusters in helium nanodroplets: Identification of the surface → interior transition” by An der Lan et al., in The Jounal of Chemical Physics. Here is the abstract:
The submersion of sodium clusters beyond a critical size in helium nanodroplets, which has recently been predicted on theoretical grounds, is demonstrated for the first time. Confirmation of a clear transition from a surface location, which occurs for alkali atoms and small clusters, to full immersion for larger clusters, is provided by identifying the threshold electron energy required to initiate Nan cluster ionization. On the basis of these measurements, a lower limit for the cluster size required for submersion, n ≥ 21, has been determined. This finding is consistent with the recent theoretical prediction.
Referenced in the abstract? I couldn’t believe it! They go on to reference me and Prof. Kresin and our results for the critical submersion size of sodium clusters. The experiment demonstrates “excellent agreement” with our prediction for sodium.
In the grand scheme of science, this is not exactly earth-shattering – this is like a tiny screw in the giant tower of human knowledge. I doubt anyone besides Vitaly and I will sleep more soundly knowing this. But it’s my tiny screw and I’m proud of it. And I’m so glad that I had the opportunity to do some good theory work with a brilliant advisor.
If you care to read on, here are some other goodies from An der Lan et al.:
“Thus, at some critical size the cluster may become submerged in the helium rather than adopt a surface location. Recently, Stark and Kresin have described and employed a theoretical model which anticipates such a critical cluster size for alkali submersion^9. For example, Na_n clusters are predicted to preferentially enter ^4 He nanodroplets once n ≥ 21, whereas K_n clusters require approximately 78 atoms to become fully solvated.”
“The ion yield curves for Na^19+ and smaller clusters are consistent with a surface location for the corresponding neutral clusters, whereas those for Na^21+ and larger clusters are consistent with an interior location for the corresponding neutral clusters. These findings provide strong evidence for a switch from a surface location to an interior location at a certain sodium cluster size. For completeness, we show aggregated data for the much weaker even cluster ions in Figure 5, where the signal from all Na^n+ ions with n > 20 have been summed together and similarly all of those with n ≤ 20. Again this is consistent with a change in cluster location at around the n = 20 or 21 size.”
"Thus, because of the potential for ion fragmentation, we can only firmly establish a lower limit of n ≥ 21 for sodium cluster submersion in ^4 He droplets. The lower limit is in excellent agreement with a recent theoretical prediction made by Stark and Kresin, whose model suggested that the minimum Nan cluster size for submersion in ^4 He droplets occurred at n = 21^9.