## 3 Results and Discussion  
Counterfeit Euro, U.S. dollar, and New Israeli Shekel banknotes, of different values and batches, were studied vs. authentic ones. Different bill areas were tested, e.g., low-colored frame and homogeneous well-defined colored areas (green, metallic, etc. )All available banknotes were introduced in the MEES instrument and tested. MEES spectra depended on the tested location (white, colored, hologram, etc.) When measuring MEES spectra of the same site in different authentic banknotes of the same kind, excellent reproducibility was obtained. When measuring MEES spectra of a counterfeit banknote, in zones that are supposed to be identical (along the frame or in areas of the same color and shape), also excellent reproducibility was noted. (In 10 measurements, the reproducibility of peak location was of 0.2 nm, and the reproducibility of peak intensity was of ca 7%.) Therefore, we are confident that the differences in the spectra are significant and not related to instrumental factors or to local (invisible) dirt on the banknotes. We cannot exclude the possibility that the relative intensities of the peaks are affected by instrumental factors, but the reproducibility, on one hand, and the difference in the spectra of different banknotes definitely indicate that MEES provides a significant fingerprint of the rested site. We first present the results from 20 Euro bills, which are the second common paper bills in Europe [3]. Figure 2 presents MEES spectra collected from the lightly colored area (frame) and colored (silivery) hologram at the right-hand side of the bill. A clear distinction between the spectra of the authentic and counterfeit banknotes is evident. 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 1. The differences in the peaks indicate differences in the chemical composition at the tested sites. Based on previous investigations of MEES, this method is very sensitive to the molecular composition at the irradiate spot [21; 22]. Therefore, although the chemical composition of banknotes is not in the public domain, the differences can be attributed to the local chemistry. Note that these differences are observed neither by the naked eye nor by reflectance spectroscopy in the visible range. \begin{table}
\begin{tabular}{c c c} \hline
**Peak/nm** & **Authentic** & **Counterfeit** \\ \hline
218.8 & + & - \\ \hline
222.6 & - & + \\ \hline
226 & + & - \\ \hline
229.6 & + & - \\ \hline
231.2 & + & - \\ \hline
247.4 & - & + \\ \hline
257 & + & - \\ \hline \end{tabular}
\end{table} 
Table 1: List of main peaks which are missing in the authentic 20 Euro banknote and present in the counterfeit (and vice versa). Figure 2: Comparing MEES spectra of counterfeit and authentic 20 Euro banknotes, at the same site (the lightly colored area in the frame). The variety of peaks observed in the MEES spectra of banknotes is characteristic to this method [27]. Assignment of some peaks was only possible for simple molecules, such as nitrates, where the molecular energy levels are well known. Most MEES peaks cannot yet be assigned to specific transitions, since they involve high molecular energy states, which have not been calculated so far. In the case of banknotes, even their chemical composition is not known, so peak assignment is out of question. Nevertheless, MEES still can provide a fingerprint of the banknotes and be used for distinction between authentic and counterfeit ones. For more detailed examination, a 100 Euro banknote was thoroughly investigated. Firstly, we measured MEES spectra at two zones: \"Zone A\": the low-color zone on the left-hand side (red dotted in Figure 3a) and \"Zone B\": the green zone at the right-hand side (purple dashed in Figure 3a). The measurements were carried out on authentic and on counterfeit banknotes, in the same zones. The results are shown in Figure 3b.