Rotational spectra of large Rydberg Matter clusters K37, K61 and K91 give trends in K–K bond distances relative to electron orbit radius

Rotational spectra of planar clusters of Rydberg Matter (RM) have recently been reported for the first time, for clusters K19 in excitation levels nB = 4, 5 and 6 [L. Holmlid, Mol. Phys. 105 (2007) 933–939]. From such spectra, the average bond distances in the clusters can be determined, and good agreement with theoretical predictions is found. K19 is an example of a sixfold symmetric planar RM cluster, while other similar clusters have 37, 61 and 91 atoms. Rotational bands of the heavy clusters K37, K61 and K91 are easily identified since they appear as closely spaced groups of lines on top of electronic, probably spin-flip transitions. The bond lengths for the heavy clusters are in the range 3–10 nm, found with a relative precision of approximately 2 × 10−3. The cluster K37 (nB = 8) is the largest cluster observed with diameter of 58.9 nm and bond length of 9.82 ± 0.02 nm. The dimensional ratio d/rn, i.e. the ratio between the experimental interatomic distance and the theoretical electron orbiting radius, is larger for higher excitation levels, where the classical limit is approached and thus the electronic motion is more strongly confined to the cluster plane. The dimensional ratio is also larger for smaller clusters, which is probably due to larger edge effects giving larger average bond distances. The average value of d/rn is 2.865 ± 0.032, close to the theoretically predicted value 2.9. Intensity alternation for every second rotational line is often observed.