Quantum transitions in 3He adsorbed on carbon nanotube

In gases adsorbed on smooth surfaces, the binding energy E ₁ of the lowest quantum state (first layer) is usually much lower than that of the excited state (second layer), E ₂ . The typical energy difference for helium on graphite amounts to about 50 K which gives a vanishingly small Boltzmann factor e ^{-(E ₂ -E ₁ )/k _B T} at temperatures below 1 K, and no atom should be present on the excited level (second layer), according to statistical physics. However, in our investigations on sub-monolayer of ³ He adsorbed on a carbon nanotube, we have detected helium atoms traveling freely along the tube. Free particles were shown to be responsible for quantized satellite peaks appearing in mechanical oscillation spectra next to the main resonance. We demonstrate that side peaks reflect transitions between different longitudinal quantum states of free helium atoms on the oscillating tube. Radial quantum fluctuations in AC electric field have been considered in the framework of Rabi oscillations and shown to lift some amount of particles above the dense first layer. Besides, zero-point pressure at moderate densities promotes radial fluctuations significantly.