## 4 Discussion  
The obtained results are in good agreement with general considerations based on UV-spectroscopic properties of these compounds given in Reference [33] and REMPI spectroscopic investigations presented in References [34; 35; 36]. For single-core aromatics, the most prominent transition into the excited state is S\({}_{0}\)\(\rightarrow\) S\({}_{1}\). For benzene the origin of this S\({}_{0}\)\(\rightarrow\) S\({}_{1}\) transition is symmetry forbidden and located at 262.5 nm. Only vibrationally-induced electronic transitions are allowed at 259.3 nm and  
Figure 7: Relative REMPI cross sections (\(\sigma_{\mathrm{rel,tol}}\); toluene was used as a reference substance) at 248 nm. For 1,2,4-trimethylbenzene, only data of the gas standard were available. 247.3 nm and can be assigned to the transition into the \(v_{6}\) and \(v_{1}\) modes. Herein, benzene showed the narrowest absorption bands of all the investigated aromatic species. These sharp absorption bands were broadened and shifted to longer wavelengths by the insertion of electron-donating substituents (e.g., -CH\({}_{3}\), -OH, -OCH\({}_{3}\)). For example, the allowed \(\mathsf{S}_{0}\rightarrow\mathsf{S}_{1}\) transitions of toluene and \(p\)-xylene were at 266.8 nm and 272.2 nm, respectively. Therefore, the ionization efficiencies for single-core aromatics (except benzene) were higher at 266 nm compared to those at 248 nm. Larger PAHs showed broadened absorption bands with a bathochromic shift (compared to benzene), whereas the density of electronic states increased with the number of condensed aromatic rings. Additionally, the energy of singlet states decreased. Therefore, transitions into higher states (S\({}_{2}\), S\({}_{3}\), etc.) are possible and much more stabilized. Consequently, PAHs showed higher ionization yields compared to single-core aromatics. The ionization rate was then determined by the second absorption step, leading to deviations from the quadratic intensity dependence [35]. Additionally, naphthalene and acenapthene were ionized more efficiently at 266 nm (\(\mathsf{S}_{0}\rightarrow\mathsf{S}_{2}\)), while PAHs such as phenanthrene and pyrene showed increased ionization efficiencies at 248 nm (\(\mathsf{S}_{0}\rightarrow\mathsf{S}_{3}\)). Due to the lower ionization yield of toluene and the increased REMPI response, phenanthrene and pyrene showed very high \(\sigma_{\text{rel,tol}}\) at 248 nm. For measurements at 266 nm, the obtained \(\sigma_{\text{rel,tol}}\) were in good agreement with previous results obtained using PITOF 2 (Table 1), which were verified by gas chromatographic (GC) measurements by [PERSON] et al. [23]. In their study, the direct comparison of emission factors (obtained by REMPI-TOFMS or GC-MS) from different vehicle engines shows variations mostly below 10%. Though these results did not focus on intensity-dependent behavior, these \(\sigma_{\text{rel,tol}}\) were successfully used in other studies [25; 37]. In our study, laser intensities larger than \(10^{7}\) W/cm\({}^{2}\) led to an increased fragmentation of all single-core aromatics and some PAHs (e.g., 1-methylnaphthalene and acenaphthene). The fundamental model (ladder-switching-model) of this fragmentation behavior was published by [PERSON] et al. [17]. Herein, due to the high photon density inside of the ionization volume, the initially produced ions absorbed more photons and were excited into higher ionic states. This additional input of energy eventually leds to fragmentation. For toluene, the tropylium ion (\(m/z\) 91) is a prominent fragment, which is produced by rearrangement of the deprotonated toluene ion [38]. The ratio between the tropylium ion and the molecular ion of toluene can be drawn upon to assess the hardness of the ionization process. In our measurements, the ratio of the signal intensities obtained at \(m/z\) = 91 and 92 was approximately 70% at \(10^{8}\) W/cm\({}^{2}\).