The size distributions of neutral ${}^4\mathrm{He}$ clusters in cryogenic jet beams, analyzed by diffraction from a 100 nm period transmission grating, reveal magic numbers at $N=10–11$, 14, 22, 26–27, and 44 atoms. Whereas magic numbers in nuclei and clusters are attributed to enhanced stabilities, this is not expected for quantum fluid He clusters on the basis of numerous calculations. These magic numbers occur at threshold sizes for which the quantized excitations calculated with the diffusion Monte Carlo method are stabilized, thereby providing the first experimental confirmation for the energy levels of ${}^4\mathrm{He}$ clusters.

• Received 19 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.185301