Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy. Delocalized excitons are detected in the atomic systems at particle densities where the mean interatomic distance (>10 μm) is much greater than the laser wavelength and multi-particle coherences should destructively interfere over the ensemble average. With a combined experimental and theoretical analysis, we identify an effective interaction mechanism, presumably of dipolar nature, as the origin of the excitonic signals. Our study implies that even in highly-dilute thermal atom ensembles, significant transition dipole–dipole interaction networks may form that require advanced modeling beyond the nearest neighbor approximation to quantitatively capture the details of their many-body properties.
Part I: Spectroscopy. 1: Spectroscopy of molecular beams: an overview. 2: Magnetic and electric resonance spectroscopy. 3: Beam-maser spectroscopy. 4: Quantum amplifiers and oscillators. 5: Metrology with molecular beams. 6: Infrared laser spectroscopy. 7: Visible and UV spectroscopy: physical aspects. 8: Photofragment spectroscopy. 9: Fourier-transform microwave spectroscopy. 10: Fourier-transform methods: infrared. Part II: Surface Scattering. 11: General principles and methods. 12: Elastic scattering of atoms. 13: Rotational inelastic scattering. 14: Single phonon inelastic helium scattering. 15: Multiple phonon inelastic scattering. 16: Scattering from disordered surfaces. 17: Reactive scattering
We prove that groups acting geometrically on delta-quasiconvex spaces contain no essential Baumslag-Solitar quotients as subgroups. This implies that they are translation discrete, meaning that the translation numbers of their nontorsion elements are bounded away from zero.