The van der Waals force between a single atom and a rod-like molecule is investigated by introducing a one-dimensional dipole–dipole interaction atomic model. The force between a single atom and its nearest neighbor atom in a rod-like molecule is related to the correlation between them, which is obtained by solving the problem of coupled quantum oscillators. The dependence of the correlation on the coupling constant between the single atom and atoms in the molecule are presented for different molecular weight. Furthermore, the transition of the force from attractive to repulsive in the excited state is considered with regard to the correlation.
The van der Waals force between a single atom and a rod-like molecule is investigated by introducing a one-dimensional dipole–dipole interaction atomic model. The force between a single atom and its nearest neighbor atom in a rod-like molecule is related to the correlation between them, which is obtained by solving the problem of coupled quantum oscillators. The dependence of the correlation on the coupling constant between the single atom and atoms in the molecule are presented for different molecular weight. Furthermore, the transition of the force from attractive to repulsive in the excited state is considered with regard to the correlation.
We consider the van der Waals force involving excited atoms in general environments, constituted by magnetodielectric bodies. We develop a dynamical approach studying the dynamics of the atoms and the field, mutually coupled. When only one atom is excited, our dynamical theory suggests that for large distances the van der Waals force acting on the ground-state atom is monotonic, while the force acting in the excited atom is spatially oscillating. We show how this latter force can be related to the known oscillating Casimir-Polder force on an excited atom near a (ground-state) body. Our force also reveals a population-induced dynamics: for times much larger that the atomic lifetime the atoms will decay to their ground states leading to the van der Waals interaction between ground-state atoms.
We compute the conservative van der Waals forces between two atoms, one of which is initially excited, in the limit of identical atoms. Starting with the perturbative calculation of the interaction between two dissimilar atoms, we show that a time-dependent approach in the weak-interaction approximation is essential in considering the identical-atom limit in the perturbative regime. In this limit we find that, at leading order, the van der Waals forces are fully resonant and grow linearly in time, being different upon each atom. The resultant net force upon the two-atom system is related to the directionality of spontaneous emission, which results from the violation of parity symmetry. The strength of this force is much greater than that found in dissimilar atoms, raising the possibility of its experimental detection. In contrast to the usual stationary van der Waals forces, the time-dependent conservative forces cannot be written as the gradients of the expectation values of the interaction potentials, but as the expectation values of the gradients of the interaction potentials only.
We present a time-dependent calculation of the velocity-dependent forces which act on an excited atomic dipole in relative motion with respect to ground state atoms of a different kind. Both its interaction with a single atom and with a dilute atomic plate are evaluated. In either case, the total force consists of a conservative van der Waals component and a nonconservative R\"ontgen component. On physical grounds, the former corresponds to the velocity-dependent recoil experienced by the excited atom in the processes of absorption and emission of the photons that it exchanges with the ground-state atoms on a periodic basis. The latter corresponds to the time-variation of the R\"ontgen momentum, which is also mediated by the periodic exchange of quasiresonant photons. We find that, at leading order, all these interactions are linear in velocity. In the nonretarded regime the van der Waals force dominates, being antiparallel to the velocity. On the contrary, in the retarded regime the velocity-dependent forces oscillate in space, van der Waals and R\"ontgen forces are of the same order in the atom-atom interaction, and the R\"ontgen component dominates in the atom-surface interaction.
The structure and stability of atomic and molecular systems with van der Waals (vdW) bonding are often determined by the interplay between attractive dispersion interactions and repulsive interactions caused by electron confinement. Arising due to different mechanisms---electron correlation for dispersion and the Pauli exclusion principle for exchange repulsion---these interactions do not appear to have a straightforward connection. In this paper, we use a coarse-grained approach for evaluating the exchange energy for two coupled quantum Drude oscillators and investigate the mutual compensation of the attractive and repulsive forces at the equilibrium distance within the multipole expansion of the Coulomb potential. This compensation yields a compact formula relating the vdW radius of an atom to its multipole polarizabilities, ${R}_{\mathrm{vdW}}={A}_{l}^{\phantom{\rule{0.16em}{0ex}}}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\alpha}}_{l}^{2/7(l+1)}$, where l is the multipole rank and Al is a conversion factor. Such a relation is compelling because it connects an electronic property of an isolated atom (atomic polarizability) with an equilibrium distance in a dimer composed of two closed-shell atoms. We assess the accuracy of the revealed formula for noble-gas, alkaline-earth, and alkali atoms and show that Al can be assumed to be universal constants. Besides a seamless definition of vdW radii, the proposed relation can also be used for the efficient determination of atomic multipole polarizabilities solely based on the corresponding dipole polarizability and the vdW radius. Finally, our work provides a basis for the construction of efficient and minimally empirical interatomic potentials by combining multipolar interatomic exchange and dispersion forces on an equal footing.
The highly excited atoms have a strong Van der Waals interaction as compared with the ground-state atoms, which can block the excitation of neighboring atoms and forming the blockade effect. In this work, the interaction between highly excited atoms is calculated using the perturbation theory, the interaction of nS and nD pair states and the dispersion coefficients are obtained. This shows that the interactions of the SS for both Rb and Cs atoms are repulsive, whereas the interaction of DD states is repulsive for Cs and attractive for Rb.
Few-body interactions offer the opportunity to study the isolated atom to few-body coupled molecules, and to condensed matter transitions. Atoms in molecules and in condensed matters are coupled by different orders of multipole–multipole interactions, which all stem from different orders of approximations from coulomb interactions between multiple charges. The lowest order multipole–multipole interaction is the dipole–dipole interaction, which is proportional to the size of the dipole. In this article, we use Rydberg atoms, which have more than 1000 times greater electric dipoles than the ground state atoms, to study the van der Waals interaction between few bodies. In addition to the large dipoles, the kinetic energy of the atoms is significantly reduced by reducing the temperature, which makes these interactions stable and observable. Here, we report on the 2D and 3D few-body interaction potentials and possible ways of creating semistable molecules in such an ultracold Rydberg gas with a temperature of ≈100 nK. Although we use Rydberg atoms in this article, this calculation can be applied to other states too. The results reported here are useful for studying repulsive van der Waals interactions and creating ultracold molecules.