Take the size of a Rydberg atom to be the diameter of the orbit of the excited electron. If the researchers want to perform this experiment with the rubidium atoms in a gas, with atoms separated by a distance 10 times their size, the density of atoms per cubic centimeter should be about

(a) 10⁵ atoms/cm³;

(b) 10⁸ atoms/cm³;

(c) 10¹¹ atoms/cm³;

(d) 10²¹ atoms/cm³.

In photosynthesis in plants, light is absorbed in light-harvesting complexes that consist of protein and pigment molecules. The absorbed energy is then transported to a specialized complex called the reaction center. Quantum-mechanical effects may play an important role in this energy transfer. In a recent experiment, researchers cooled rubidium atoms to a very low temperature to study a similar energytransfer process in the lab. Laser light was used to excite an electron in each atom to a state with large n. This highly excited electron behaves much like the single electron in a hydrogen atom, with an effective (screened) atomic number Z_eff = 1. Because n is so large, though, the excited electron is quite far from the atomic nucleus, with an orbital radius of approximately 1 mm, and is weakly bound. Using these so-called Rydberg atoms, the researchers were able to study the way energy is transported from one atom to the next. This process may be a model for understanding energy transport in photosynthesis.