PI occurs when an atom or molecule absorbs a photon, usually from a laser, and ejects an electron to form a cation. In order to induce ionization, the energy of the photon has to exceed the ionization energy (IE) of the chemical species.(56)PI efficiency (PIE), the ratio of number of ions formed to the number of photons absorbed by the molecule, typically increases as photon energy approaches the IE of the molecule. However, PIE is reduced considerably if the dissociation energy of the molecule is lower than its IE and the absorption of a photon results in dissociation and not ionization of the molecule. Hence, PI can only be used to ionize those samples that have lower IE than dissociation energy.(57) PI has been applied to ionize vaporized molecules from ambient particles as well as particles made in a controlled laboratory environment. Laser desorption ionization (LDI), a type of PI, has been used to ionize atmospheric particles that have been collected on a filter and re-vaporized for prior to ionization.