We have systematically studied a simple technique that accurately determines number of atoms in a magneto-optical trap, Absorption energy of a laser field that interacts with cold atoms is a direct measurement of atom number. The measured energy neither depends on the detuning, intensity, and polarization of the laser field nor is affected by other system parameters. Our work also demonstrates that such technique can be applied to study the phenomenon of coherent population trapping.

We report an experimental observation of narrow and high-contrast spectra. which are induced by interacting dark resonances and have been predicted in Phys. Rev. A 60, 3225 (1999). Spectra are measured with cold (87)Rb atoms produced by a magneto-optical trap. In this experimental system, a coupling laser and a weak probe laser form a three-level Lambda -type configuration of electromagnetically induced transparency (EIT); a microwave drives a magnetic-dipole transition between the fourth level and the ground state that is coupled with the excited state by the coupling laser. The observed spectral profile of probe absorption exhibits a very sharp peak emerging inside a narrow EIT dip. Such spectral feature provides more opportunities in manipulating atomic-optical response.

We have studied the structural stability of thin silver films with thicknesses of N = 1 to 15 monolayers, deposited on an Fe(100) substrate. Photoemission spectroscopy results show that films of N = 1, 2, and 5 monolayer thicknesses are structurally stable for temperatures above 800 kelvin, whereas films of other thicknesses are unstable and bifurcate into a film with N +/- 1 monolayer thicknesses at temperatures around 400 kelvin, The results are in agreement with theoretical predictions that consider the electronic energy of the quantum well associated with a particular film thickness as a significant contribution-to the film stability.