where N a and N b are number of occupied energy levels of E a and E b respectively, where E b > E a . Then, using: d N b d t = − − A b a N b − − N b u ( ω ω b a ) B b a + N a u ( ω ω b a ) B a b = − − N b w b → → a s . e m i − − N b w b → → a e m i + N a w a → → b a b s