## 1 Introduction  
Money is still a most essential commodity, despite recent growth in the use of electronic financial transactions. Banknotes, due to their wide use and susceptible nature, are the most favorite target of defrauders and scammers. Recently, due to some technological advancements, the problem of counterfeit banknotes has increased on a large scale and seriously threatens the economy, financial institutions, and consumers. Earlier, only printing houses had the facility to print counterfeit paper notes, but today, everybody can do it at their home, by using appropriate commercially available scanners and printers. Therefore, the amount of production of fake banknotes is increasing every year [1; 2; 3; 4].  
Two basic ways are used to detect counterfeit banknotes: the manual method and analytical/forensic methods. In the manual method the currencies are inspected with the naked eye or by touch. Detection is based on the physical appearance of the banknotes. The forensic methods are based on science or technology for authentication of the banknotes. Modern banknotes are prepared with many features, which allow for machine authentication. While forensic detection of counterfeit banknotes is in most cases an easy task, several cases of sophisticated forgeries, in which authentication is very difficult, have been reported.  
Among the currently used forensic technologies are multispectral imaging such as Video Spectra Comparator (VSC) and AFM coupled Raman spectroscopy. The VSC can detect low-level forgeries but is blind for more sophisticated cases. Other multispectral imaging methods, which are based on reflection spectroscopy may be useful in some cases; however, since these devices are only sensitive to the dyes used, sophisticated criminals can prepare their dye mixtures in a way that mimics the original spectra. Instruments working in several wavelength ranges are rare but are much more difficult to overcome.  
AFM coupled Raman spectroscopy is a very powerful analytical tool. Its main drawbacks in banknotes applications are its time consuming and expensive operation. Moreover, the method can only be used for scanning small areas. It also suffers of limited sensitivity to trace compounds.  
When countries face such criminal activities, authentication becomes very difficult, and it can only be based on observation of minor details, revealed using uncommon detection technologies. Therefore, application of new authentication technologies is of interest. Actually, most powerful analytical techniques (e.g., SIMS) can be applied in difficult cases; however, new methods that can scan banknotes in seconds and under ambient conditions, providing detailed and sensitive information, are still needed. Moreover, even when authentication is simple, fingerprinting of the fake banknotes, including the ink characteristics and the printing process, is of interest. This information is needed for identifying the source and for forensic purposes.  
A rather new and uncommon analytical technology is Multiphoton Electron Extraction Spectroscopy (MEES), which has been based of previous analytical applications of multi-photon ionization under ambient conditions: starting with gas phase applications [5; 6; 7; 8], liquids [9], aerosols [10; 11; 12], and solid surfaces [13; 14; 15; 16; 17; 18; 19; 20; 21]. MEES is a quantitative analytical spectroscopy, based on detection of photoelectrons extracted in multiphoton process, as a function of laser irradiation wavelength. It was proved to be more sensitive and more informative than most common optical spectroscopies [22; 23; 24; 25; 26; 27]. This method can directly be applied to surfaces, without any preparations and preprocessing. Thus far, it was applied to detecting a variety of compounds, including polycyclic aromatic hydrocarbons (PAH), pesticides, pharmaceutical compounds, explosives, and narcotic drugs. MEES method has never been applied to banknotes, and this is the first attempt to utilize it for authentication purposes and fingerprinting of counterfeit banknotes.