## 1 Introduction  
Over the past decades, the investigation of the environmental issues and health-related effects of polycyclic aromatic hydrocarbons (PAHs) has become an important research field. Due to their high biological and ecological importance, analytical techniques for a fast and sensitive determination of these compounds are in the state of continuous improvements. Resonance enhanced multiphoton ionization (REMPI) coupled to time-of-flight mass spectrometry (TOFMS) is a powerful technique in this respect, particularly for online measurements of PAHs in gaseous samples. Since its very first application in the late 1970s [1; 2; 3], REMPI-TOFMS has been evolved as a versatile tool for the selective and sensitive analysis of PAHs in combustion processes and environmental monitoring [4; 5; 6; 7; 8; 9; 10; 11]. To date, some profound papers have been published, reviewing the fundamentals of REMPI as well as REMPI-MS in detail [12; 13; 14]. In respect to the detailed information given in those publications, only a short overview is presented in this work. REMPI occurs by the simultaneous or sequential absorption of two or more photons including resonant molecular excited states. The energy of each photon is below the ionization threshold of the molecules. REMPI can be classified with respect to the number and energy of the photons involved in the ionization process. Figure 1a illustrates the ionization schemes of one of the most common REMPI techniques, which is known as \((1+1)\) one-color REMPI. Herein, molecules are excited into their intermediate states by absorbing one photon with a distinct wavelength. The following absorption of a second photon of the same energy, within the lifetime of this intermediate state, leads to ionization. Based on the molecular electronic structure, REMPI is particularly selective for PAHs. For these compounds, the intermediate states are accessible via UV-photons, whereas other components (e.g., N\({}_{2}\), O\({}_{2}\), H\({}_{2}\)O, alkanes, etc.) are not ionized. A quantitative description of REMPI rates is given by Equation (1) and is known as the formal intensity law. This expression originates from kinetic considerations discussed in References [15; 16; 17]. As expressed in the equation, the ionization rate (\(R\)) depends on the squared intensity (\(I\)) of the irradiating laser beam. \[R=\frac{N}{M_{0}}=\frac{\sigma_{1}(\omega)\cdot\sigma_{2}(\omega)}{2}\cdot I^ {2}\cdot t_{\rm P}^{2}\, \tag{1}\]  
here, \(N\)--number of ions; \(M_{0}\)--number of initial molecules; \(\sigma_{1}\)--photoabsorption cross section; \(\sigma_{2}\)--photoionization cross section; \(\omega\)--angular frequency; \(t_{\rm P}\)--laser pulse duration. However, this expression only holds well for moderate laser intensities (\(\approx\)10\({}^{4}\)--10\({}^{6}\) W/cm\({}^{2}\)), where no saturation effects occur, and for resonant intermediate states that are long-lived compared to the pulse length of the laser. If one of the ionization steps is saturated, the quadratic intensity dependence of the ionization rate will change to exhibit a more linear behavior. Besides this intrinsic intensity dependency, geometrical effects of the laser beam may result in a non-integer deviation from the formal intensity law, as was shown by [PERSON] et al. [18; 19]. Fragmentation of molecular ions when using REMPI is controllable and depends on the applied laser intensity. With moderate laser intensities, REMPI is a very soft ionization method, producing  
Figure 1: (**a**) Ionization scheme of \((1+1)\) one-color resonance enhanced multiphoton ionization (REMPI) (\(\lambda_{1}\)—wavelength; \(k_{1}\)—rate constant of the transition; \(\sigma_{1,2}\)—photoabsorption/photoionization cross section; \(I\)—laser intensity; nrad—non-radiational decay; rad—radiational decay).