## 3 Results
### Determination of relPICS for 266 nm  
The intensity-dependent ionization behavior of selected aromatic and polycyclic aromatic species at 266 nm is shown in Figure 4. The slope of the regression line reflects the intensity dependence of the REMPI signal for each compound. Exact quadratic behavior was not observed for all compounds, which may have originated from the geometrical effects of the laser beam. Additionally, saturation occurred for phenanthrene and pyrene at higher laser intensities. However, for single-core and small polycyclic aromatics, similar trends appeared. These similarities in the ionization behavior between analyte and reference substance led to constant values of \(\sigma_{\text{rel,tol}}\) over a wide intensity range. In contrast, an intensity-dependent course of \(\sigma_{\text{rel,tol}}\) was determined for the case where the ionization behavior differs between both substances. Figure 4: Intensity-dependent REMPI response of selected aromatic and polycyclic aromatic compounds; values on the left side correspond to a lens position of 3.5 cm relative to the focal point, whereas data points on the right side were determined at the focus position. The slope of the regression line is given for each compound; similar values for the analytes compared to toluene (reference) result in constant relPICS over a wide intensity range. Figure 3: Evaluation of the sample inlet system. (**a**) Comparison of theoretical (calculation based on Equation (6); dotted lines) and experimental (symbols) toluene concentrations for different sample flow rates as a function of nitrogen flux. (**b**) Signal response and stability for selected single-core aromatics and polycyclic aromatic hydrocarbons (PAHs) at different sample flow rates. A graphical overview of the obtained \(\sigma_{\text{rel,tol}}\) is given in Figure 5.  
The investigated single-core aromatic compounds show mostly intensity-independent \(\sigma_{\text{rel,tol}}\)-Benzene (\(\sigma_{\text{rel,tol}}\approx 0.13\)) had the lowest response of all the investigated compounds. The methylation of the benzene ring leads to a higher \(\sigma_{\text{rel,tol}}\) in order of the number of methyl groups (\(p-\text{xylene}\) :  
Figure 5: Relative REMPI cross sections (\(\sigma_{\text{rel,tol}}\); toluene was used as a reference substance) at 266 nm (**a**) obtained by PTTOF 1; (**b**) obtained by PTTOF 2. Clear intensity-dependent trends of \(\sigma_{\text{rel,tol}}\) of PAHs were obtained, whereas for small aromatic compound constant values of \(\sigma_{\text{rel,tol}}\) were determined. For 1,2,4-trimethylbenzene, only data of the gas standard were available. \(\sigma_{\rm rel,tol}=2.4\); \(1,2,4-\) trimethylbenzene : \(\sigma_{\rm rel,tol}\approx 4.2\)). Due to the increase of the electron density of the chromophoric center and the resonance stabilization of the ionic state, the insertion of other substituents with a positive mesomeric effect (+\(M\)-effect) led to increasing \(\sigma_{\rm rel,tol}\) compared to benzene (phenol: \(\sigma_{\rm rel,tol}=3.5\); guaiacol: \(\sigma_{\rm rel,tol}=7.1\)). For indole (\(\sigma_{\rm rel,tol}=35\)) and benzofuran (\(\sigma_{\rm rel,tol}=17\)), relPICs increased as a result of oxygen being replaced by nitrogen. With the exception of naphthalene, intensity-dependent \(\sigma_{\rm rel,tol}\) were observed for larger aromatic ring systems even at lower laser intensities. Higher numbers of condensed rings led to higher values of \(\sigma_{\rm rel,tol}\), whereas the effect of methylation was reduced. Thus, \(\sigma_{\rm rel,tol}\) of naphthalene with an approximate value of 21 showed a slight intensity-dependent behavior at laser intensities above \(10^{7}\) W/cm\({}^{2}\), whereas pyrene had a strong intensity-dependent \(\sigma_{\rm rel,tol}\) in the range of approximately 25-225. The used solvents slightly influenced \(\sigma_{\rm rel,tol}\) (see for example fluorene in Figure 5a), which might be due to the different solubilities of the investigated compounds.