Atmospheric pressure laser ionization is an atmospheric pressure ionization method for mass spectrometry (MS). Laser light in the UV range is used to ionize molecules in a resonance-enhanced multiphoton ionization (REMPI) process. It is a selective and sensitive ionization method for aromatic and polyaromatic compounds. Atmospheric photoionization is the latest in development of atmospheric ionization methods.

The excitation of electrons in atoms and molecules by the absorption of one or more photons can be sufficient for the spatial separation of the electron and the atom or molecule. In the gas phase, this process is called photoionization. The combined energy of the absorbed photons in this process must be above the ionization potential of the atom or molecule.

In the simplest case, a single photon has sufficient energy to overcome the ionization potential. This process is therefore called single photon ionization, it is the basic principle of atmospheric pressure photoionization (APPI). For sufficiently high power densities of the incident light also nonlinear absorption processes like the absorption of at least two photons in a rapid sequence via virtual or real states can occur. If the combined energy of the absorbed photons is higher than the ionization potential, this multiphoton absorption process can also lead to ionization of the atom or molecule. This process is called multi-photon ionization (MPI).

The laser light sources used in APLI have power densities which allow multiphoton ionization via stable electronic states of the molecule or atom. The required power density has to be sufficiently high, so that in the lifetime of the first reached electronic state, which is in the range of a few nanoseconds, a second photon can be absorbed with a reasonable probability. Then a radical cation is formed:

This process is called resonance enhanced multi photon ionization (REMPI). In the case of APLI both absorbed photons have the same wavelength, which is called "1+1 REMPI".

Most of the organic molecules which are favorable for a photoionization method have ionization potentials smaller than approximately 10 eV. Thus APLI utilizes light with a photon energy of around 5 eV which corresponds to a wavelength of about 250 nm, which is in the ultraviolet (UV) part of the electromagnetic spectrum.

Typical laser systems used in APLI are krypton fluoride laser (λ = 248 nm) and frequency quadrupled Nd:YAG laser (λ = 266 nm).^{[citation needed]}

APLI has some special characteristics because of the ionization with UV-laser-light:^{[citation needed]}