**5. Conclusion**

Traditionally, the name LIAD combine all desorption phenomena taking place when an opaque target is irradiated from the back side, ignoring differences in experimental conditions. In our opinion, it is not correct. As it was demonstrated in this work, under some experimental conditions (most commonly used in many present studies), the physical origin of the observed desorption phenomenon is not (and could not be) connected with the acoustic waves generated in the foil and most likely is defined by the film stress and

2001). The essence of this effect is the emission of charged particles and photons initiated by surface distortion (in particular by mechanical deformation, scratching, bending, etc.) and do not connected with thermal excitation. The luminescence of thin metal discs, irradiated from back side by the laser pulses (Abramova, Shcherbakov et al. 1999; Abramova, Rusakov et al. 2000) can also serve as evidence of formation of excited electronic states by the laser-

The mechanism of molecular desorption due to such laser-generated stress has been proposed earlier by Vertes (Vertes and Levine 1990; Vertes 1991) for MALDI, and was based on the thermal stress generation in the layer of organic film deposited on solid substrates. One should notice, however, a clear differences in the physical conditions between these sample volatilization methods. For MALDI, the absorption of the laser pulse energy occurs in an optically and thermally dense film, which experiences thermal stress due to its nonuniform and fast heating. In the case of LIAD, the laser radiation is absorbed by the back side of the metal foil substrate, opposite from where the sample was deposited. The amount of energy transmitted through the metal foil to the analyte layer on the front site should be so strongly attenuated, compared to the direct (front side) irradiation, that it cannot directly be sufficient for desorbing molecules with velocities observed in our experiments. On the other hand, a typical average thickness of the analyte film in LIAD can be estimated to be on the order of several molecular layers. Because of this, the specific density of energy stored in each analyte island due to intrinsic stress can be high, and during laser irradiation of the back side of the foil, the laser irradiation can simply trigger the release of this energy and to

The other experimental fact that could help in the interpretation of LIAD phenomenon is the similarity of the energy and velocity spectra of desorbed neutral molecules in case of LIAD and Electron Stimulated Desorption (ESD) (Young, Whitten et al. 1989). The primary mechanism of ESD is supposed to be the formation of the repulsive states in the surface due

Thus we could hypothesize that in case of LIAD, due to complete opaqueness of metal foil for laser radiation, the only channel for energy transfer is the formation of acoustic and thermal field by laser pulse impact. As shown above, the result can be the mechanical distortion of the analyte film followed by film cracking and delamination and the consequent formation of excited and repulsive states electronic for analyte molecules. This will increase the number of desorption sites and finally the total number of desorbed molecules as observed in the nonlinear laser intensity dependence. But because the formation of the any individual cracks is defined only by the intermolecular bonding forces in the vicinity of the crack, the translational kinetic energy of desorbed molecules should still remain independent of driving laser intensity, also matching our observations and other

Traditionally, the name LIAD combine all desorption phenomena taking place when an opaque target is irradiated from the back side, ignoring differences in experimental conditions. In our opinion, it is not correct. As it was demonstrated in this work, under some experimental conditions (most commonly used in many present studies), the physical origin of the observed desorption phenomenon is not (and could not be) connected with the acoustic waves generated in the foil and most likely is defined by the film stress and

driven stress in thin foils.

induce molecular desorption event.

to electron excitation.

recent LIAD publications.

**5. Conclusion** 

cracking due to thermal and mechanical mismatch of the analyte and substrate. Therefore, ironically, the acronym LIAD in this case does not correctly reflect the physical nature of the process.

From the other hand, we cannot exclude that some strong change of the experimental conditions can also change the relationship between various physically possible mechanisms of molecular desorption, similarly to the case of direct laser desorption, when the thermal mechanism dominates under wide range of conditions and makes other possible mechanisms undetectable. One can expect, for example, that strong increase of laser power density (10 GW/cm2 and above) will cause the corresponding increase of the foil temperature and formation of hot and dense plasma plume near its surface facing the laser. In some such cases and strongly depending on the foil material properties, the material motion could evolve from elastic into plastic regime of deformation. The acoustic wave relation could not then apply, and the velocity of the surface linear motion may strongly increase. "Large and heavy" objects weakly bound to the substrate surface may be then kinematically removed from it ("shaken-off"). This could indeed serve as an explanation of observations from recent experiments where the desorption of intact viruses and biological cells were reported (Peng, Yang et al. 2006). However, this regime cannot be connected with generation of the acoustic waves but most likely corresponds to a physically different mode of shock-wave generation (Menikoff 2007). One cannot exclude that under some experimental LIAD conditions only these shock-wave induced phenomena can be responsible for molecular desorption, particularly in the experiments where emission of ions was detected. In one of such pioneering experiments (Golovlev, Allman et al. 1997), where the laser generated pressure pulse was apparently much stronger than that in our work, because of confined ablation conditions (Fabbro, Fournier et al. 1990), the emission of both electrons and ions was observed. This may have been connected with significant surface disruption at the microscale generated by shock waves.

From another standpoint, the backside irradiation of very thin films (about two or three hundred nanometers), which also could be called LIAD, has demonstrated domination of the thermal mechanism in the desorption process (Ehring, Costa et al. 1996). It is clear that the thermal equilibrium between front and back sides in such thin films can establish within the time interval of tens of nanoseconds, and the absolute temperature difference between front and back sides is negligibly small. Thus, at some laser power densities, the front side temperature could reach the melting temperature of the metal that would be enough to cause an efficient thermal desorption of the most organic molecules.

To conclude, the variety of desorption and emission phenomena observed on the front side of thin metal foils whose back sides are subjected to pulsed laser irradiation, combined under the general name of LIAD, could, in fact, have a number of different physical origins depending on specific experimental conditions. According to our observations conducted at moderate laser power densities (0.1 - 1 GW/cm2) and foils thicknesses of about 5 – 20 µm, that appear to be the most commonly used conditions in LIAD experiments, the predominant desorption mechanism is connected with the reorganization of the deposited analyte film and the consequent breaking of molecular bonds on the edges of these cracks.
