## 2 Experimental  
### MEES Apparatus and Spectra  
At present, there is no MEES instrument commercially available. Our home-made setup is schematically described in Figure 1. The apparatus has been previously described ([PERSON] et al. 2010), and only slight modifications were needed for application to banknotes. Briefly, the radiation source was a solid state OPO laser (NT342/3/UVE EKSPLA, Vilnius, Lithuania), generating wavelengths in the range of 210-350 nm at a 10 Hz repetition rate and 5 ns pulse duration. The laser pulse energy at each wavelength was monitored with a pyroelectric sensor (Ophir LaserStar PE-10-sh-v2, Israel). The pulse energy was in the range of 300-350 \(\mu\)J in all measurements. The tested banknotes were placed at a distance of 150 mm from a focusing lens of f = 180 mm, resulting in a spot diameter of ca. 250 \(\mu\)m. At such soft focusing, no breakdown took place. Figure 1: The experimental setup for measuring MEES spectra. The banknote was placed on a metal electrode, at an electrical field of 1.8 kV cm\({}^{-1}\) (high voltage power supply: PS325, Stanford Research Systems, USA). This electrical field allowed for very high electron collection efficiency. Multiphoton ionization at the laser spot resulted in emitting electrons from the surface. The current was amplified ([PERSON] amplifier, 428, 106 V/A) and digitized (Tektronix, TDS 220 oscilloscope). The time dependent photocurrent was integrated, normalized by the actual pulse energy at each wavelength, and presented as the total photocharge as a function of wavelength. The measurements and data acquisition were fully automated and controlled by a LabVIEW(tm) program. All measurements were carried out under ambient conditions, i.e., room temperature (22 \(\pm\) 2 \({}^{\circ}\)C), 60-80% humidity and air (or nitrogen) pressure of 750 \(\pm\) 5 Torr. In MEES setup, the tested surface is irradiated by a slightly focused beam of a pulsed UV laser. The energy of a single photon is not sufficient for ionization; however, multiphoton absorption takes place, resulting in electron extraction. The electrons are collected by a positive electrode placed above the surface. The current is amplified and recorded as a function of time. The integrated current provides the total photo-charges generated by each laser pulse. The photo-charges are recorded as a function of laser wavelength. Thus, MEES is the presentation of the photo-charges (normalized by the laser pulse energy) as a function of laser wavelength. While scanning the laser wavelength, we reveal numerous resonant transitions between electronic and vibronic states of the surface molecules. Since the transitions are induced by multiphoton absorption, the number of possible transitions is much higher than in single photon processes. The peaks in MEES represent the sequence of transitions, whose result is ionization. While this rather simplified description, is valid for multiphoton ionization in the gas phase, the actual processes taking place in the solid phase have not been investigated in detail and probably include additional effects. Assignment of MEES peaks to molecular transitions requires knowing the molecular energy levels. MEES peaks of simple molecules were in agreement with some of the calculated levels. However, MEES reveals additional peaks, corresponding to higher energy levels, not yet calculated. MEES peaks are also sensitive to molecular vibrational states and even to isotopic substituents. Application of MEES to complex materials, such as natural products or banknotes, results in numerous peaks that cannot be yet interpreted. However, they are repeatable and can be used as a unique spectral fingerprint of the sample composition. Therefore, MEES can be used for identification and authentication purposes. In principle, MEES is not totally a nondestructive method, since the tested surface is irradiated by UV light. However, the sample is placed on a conductive electrode, which supplies electrons for fast recombination.