Theory of single-molecule fluorescence spectroscopy of two-state systems

We derive an explicit expression for the probability density of the amplitu des of the biexponential fluorescence decay of a two-state system observed in single-molecule experiments of finite duration T. The shape of the proba bility density of the amplitudes depends on (k(1) + k(2))T, where the k(i) are the interconversion rate constants. It approaches a delta function in t he limit T --> infinity. When the interconversion rates in the ground and e xcited states differ, the single-molecule and bulk experiments will give di fferent results even in the limit of infinite duration of the experiment. ( C) 1999 American Institute of Physics. [S0021-9606(99)00318-9].