## 3 Results and Discussion
Clear and significant differences between the spectra measured from the authentic and counterfeit banknotes, in both sampled zones, are observed. The list of the main peaks which are missing in the authentic banknotes and present in the counterfeit (and vice versa) is presented in Table 2 for Zone A and in Table 3 for Zone B. In Zone A, a large dissimilarity is quite clear. Zone A is characterized by pretty-low MEES intensities (note the scale), and the spectrum of the counterfeit banknote is higher at most wavelengths. This may indicate the use of a different ink type [28]. In Zone B, the overall MEES amplitudes are  
Figure 3: (**a**) The two tested zones in 100 Euro banknotes; (**b**) MEES spectra of 100 Euro banknotes in Zone A (left) and in Zone B (right). much higher than in Zone A, but also in this case, the spectrum of the counterfeit one is higher than that of the authentic one in wavelengths above 225 nm. The distinction between the authentic and counterfeit bills, based on the spectral peaks is unquestionable. Note that MEES signals of the counterfeit banknotes look noisy, even in Zone B, where the overall intensities are higher. We repeated the measurements several times and scanned in different directions and conditions and reached the conclusion that almost all peaks, except for the very smallest, are repeatable and are not noise. This is related to the fact that MEES spectra represent much more molecular transitions, as compared to single photon spectroscopies. The method reveals many resonances, which result in the many real peaks. Next, we examined U.S. dollars, in two zones: \"Zone A\", on the frame of the bill, red dotted in Figure 4a, and \"Zone B\", a green colored zone, purple dashed in Figure 4a. These zones were analyzed in both authentic and counterfeit banknotes. The corresponding MEES spectra are shown in Figure 4b. Once again, unambiguous results have been observed. For both tested zones, MEES spectra can easily differentiate between authentic and counterfeit banknotes. The list of the main peaks which are missing in the authentic banknotes and present in the counterfeit (and vice versa) is presented in Table 4 for Zone A and in Table 5 for Zone B. \begin{table}
\begin{tabular}{c c c} \hline
**Peak/nm** & **Authentic** & **Counterfeit** \\ \hline
225.4 & - & + \\ \hline
225.9 & - & + \\ \hline
228 & - & + \\ \hline
238.8 & - & + \\ \hline
239.4 & + & - \\ \hline
242.9 & - & + \\ \hline \end{tabular}
\end{table} 
Table 2: List of main peaks which are missing in Zone A of the authentic 100 Euro banknote and present in the counterfeit (and vice versa). \begin{table}
\begin{tabular}{c c c} \hline
**Peak/nm** & **Authentic** & **Counterfeit** \\ \hline
210.1 & - & + \\ \hline
210.5 & + & - \\ \hline
214.2 & - & + \\ \hline
225.9 & - & + \\ \hline
233.4–235.9 & - & + \\ \hline \end{tabular}
\end{table} 
Table 3: List of main peaks which are missing in Zone B of the authentic 100 Euro banknote and present in the counterfeit (and vice versa). Another interesting observation is that in all tested banknotes, the MEES signals are significantly higher for the counterfeit, as compared to the authentic ones (the larger intensity differences are in the shorter wavelength range). Since the exact ink type or composition of authentic banknotes is not in the public domain, we can only assume that those who forge banknotes do not have access to the authentic ink and most probably use commercial/home-made inkjet inks or toners (for laser printing). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Peak/nm** & **Authentic** & **Counterfeit** \\ \hline
215 & - & + \\ \hline
218 & - & + \\ \hline
229 & - & + \\ \hline
231 & - & + \\ \hline
250 & + & - \\ \hline
292 & + & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: List of main peaks which are missing in Zone A of the authentic 1 USD banknote and present in the counterfeit (and vice versa).