Study of the interaction of potassium atoms with the sapphire surface with the use of an ultrathin spectroscopic cell

The effect of the dielectric surface on the $^{39}$K $D_1$ line ($F_g =1,2 \rightarrow F_e=1,2$ transitions) at nanometer distances has been experimentally studied for the first time. A nanocell that is filled with atomic potassium and has a wedge gap has been used to study the effect of atoms at distances of 50–800 nm from the technical sapphire window surface. At distances $L < 100$ nm from the sapphire surface, the van der Waals interaction strongly broadens atomic transitions and their frequencies are redshifted. The second derivative method applied to the absorption spectra of the nanocell has allowed the first measurement of the van der Waals interaction coefficient $C_3=(1.2\pm0.3)$ kHz $\mu$m$^3$ for the $^{39}$K $D_1$ line. It has been shown that the dipole–dipole interaction between $^{39}$K atoms results in the additional redshift at an increase in the density of atoms for the nanocell thickness $L< 70$ nm. The results obtained are important for the development of submicron devices containing free atoms.