## 4 Discussion
Other fragments (\(m/z\) 65, 51, and 39) of toluene were also observed at these high laser intensities and showed similarities to conventional electron ionization (EI) spectra [39]. Based on EI spectra, xylene and 1,2,4-trimethylbenzene produced fragments \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Substance**} & \multicolumn{4}{c}{\(\boldsymbol{\alpha_{\text{rel,tol}}}\)} \\  & \(m/z\) & **PITOF 1** & **PITOF 2** & **PITOF 2 Used in** & **PITOF 1** \\  & **(266 nm)** & **(266 nm)** & **Reference [17] (266 nm)** & **(248 nm)** \\ \hline
**benzene** & 78 & 0.11 * (0,15 *) & 0.05 * & 0.06 & 0.91 * (0.86 *) \\
**phenol** & 94 & 3.6 * & - & 1.66 & 1.70 \\
**\(o\)-xylene** & 106 & 2.0 * & - & 2.1 & - \\
**\(p\)-xylene** & 106 & 2.5 * (2.4 *) & 3.0* & 2.3 * (3.1 *) & 2.2 * (1.96 *) \\
**indole** & 117 & 32.2 -40.7 & - & 31.1 & 51.8–74.2 \\
**benzofuran** & 118 & 15.0–18.1 & - & 13.6 & 49.3 \\
**1,2,4-trimethylbenzene** & 120 & (2.6 *) & (5.8 *) & (5.13 *) & (1.03 *) \\
**guaiocal** & 124 & 4.9–8.5 & 6.5–7.9 & 5.45 & 1.2 * \\
**naphthalene** & 128 & 13.8–21.4 & 22.9 & 25.5 & 59.4–99.3 \\
**1-methylnaphthalene** & 142 & 9.0–20 & - & 24.1 & - \\
**acenaphthene** & 154 & 6.8–21.6 & - & 28.5 & 40.4–76.8 \\
**fluorene** & 166 & 21.3–75.0 & 54.2–61.0 & 86.8 & 270–654 \\
**phenanthrene** & 178 & 11.5–163 & 99.4–144 & 179 & 976–3451 \\
**pyrene** & 202 & 14.0–223 & 130–207 & 196 & 555–1730 \\ \hline \end{tabular}  
* Averaged values; values in brackets are obtained from measurements of the gas standard.  
\end{table} 
Table 1: Overview of relative REMPI cross sections (\(\sigma_{\text{rel,tol}}\); toluene was used as a reference substance) at 266 nm and 248 nm obtained using different mass spectrometric systems. similar to those of toluene, especially in the lower \(m/z\) range. Since toluene was part of each standard solution, no satisfying discrimination between fragments originating from toluene, xylene, and 1,2,4-trimethylbenzene can be accomplished. Therefore, we only used the molecular ions to calculate the response factors of each compound. Additionally, this strategy is also more suitable for investigations of complex gas mixtures, since fragment ions of toluene (the reference substance) and analytes cannot be distinguished. Fluorene, phenanthrene, and pyrene showed no fragmentation at elevated laser intensities, which was attributed to the stabilization of molecular ions by a high degree of freedom of higher conjugated \(\pi\)-systems [40; 41]. Hence, saturation phenomena for phenanthrene and pyrene can be explained by the saturation of both absorption steps. Even though this is an interesting finding, it should be noted that such high laser intensities are not used in conventional REMPI-TOFMS experiments for the analysis of combustion processes or environmental monitoring. At moderate laser intensities (<10\({}^{7}\) W/cm\({}^{2}\)), REMPI is a very soft ionization technique, producing mostly molecular ions (for toluene: \((m/z\ 91)/(m/z\ 92)\) < 15% at 5 \(\times\) 10\({}^{6}\) W/cm\({}^{2}\)). Saturation phenomena can also explain the deviations from quadratic intensity dependence given by the formal intensity law. The intensity in some parts of the ionization volume may be high enough to induce saturation effects in one or both of the involved transitions. Therefore, the measured ionization yields are mixture of certain saturated and unsaturated proportions, resulting from geometrical effects of the laser beam [18; 42]. In order to minimize the effect of the geometrical power density distribution of the laser beam, a top hat rather than a Gaussian laser profile may be used. Absolute cross sections of toluene (\(\sigma_{1}\) = 6 \(\times\) 10\({}^{-18}\) cm\({}^{2}\), \(\sigma_{2}\) = 2 \(\times\) 10\({}^{-17}\) cm\({}^{2}\)) at 266.8 nm were published by [PERSON] et al. [22]. Therefore, based on the REMPI spectra of toluene given by [PERSON] et al. [44] (recorded using a warm effusive molecular beam inlet similar to the setup in this study) and [PERSON] et al.