The rate of decay from state two depends on the number of atoms that were collected in that state from all previous intervals, so the number of atoms in state 2 is ∫ ∫ − − ∞ ∞ 0 n e − − t / τ τ P 12 d t ; The rate of decay of atoms from state two is proportional to the number of atoms present in that state, while the constant of proportionality is decay constant λ λ . Performing the integration rate of decay of atoms from state two is obtained as: