Where A + stands for analyte cations such as BTPP + . A -is analyte anions such as Cl -. A + ⁞A -, A + ⁞[M-H] -, A -⁞[M+H] -are ion pairs. In solid phase, isolated ions predominate due to stabilization by the neutral polar matrix molecules. They are close to 100%. Upon laser irradiation, the ions will form ion pairs, which are regarded as neutral species. The equilibrium constants for the ion pair formation are similar to the cases discussed earlier.