Laser-induced desorption mechanism

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Using fs laser excitation at 620 nm, a 2PC in Y of 0.5ps [399] implicates hot electrons, probably thermalized at Te, as the mechanism for desorption induced by the fs laser (Section 2.6.2). Rotational state distributions are nearly Boltzmann characterized by Tf. The 2PC of internal state distributions was also obtained. Rather surprisingly, significant differences in these 2PC were obtained for T and the state-resolved yield for the two spin-orbit states and this was qualitatively rationalized by a DIMET picture [399]. Where overlap in experiments exist, the qualitative results are similar to those for fs laser induced desorption of NO/Pd(lll) [400,401]. For this latter system, the absolute yield Y is large at typical fluencies used in the experiments and a very hot vibrational distribution was observed (Tv = 2900 K). 

The next natural steps are the study of the mechanisms of photodetachment of dye chromophore ions from molecules adsorbed on surfaces and the search for conditions necessary to effect preferably the MPI of the labeled chromophores while causing no laser-induced desorption of intact molecules. This is necessary in order to make it possible to irradiate a large molecule on a surface repeatedly and thus accumulate information on the location of the chromophores in its various parts. 

Haglund, R.F. (1998) Mechanisms of laser-induced desorption and ablation. In Laser Ablation and Desorption, edited by Miller, J.C., Haglund, R.F. San Diego, CA Academic Press, pp. 15-138. 

The mechanisms that lead to such laser desorption are now believed to be collective, non equilibrium processes in the condensed phase (26). In this respect they are closer to processes that must be assumed to lead to ion generation in SIMS and plasma desorption rather than to the thermal laser induced ion generation discussed above, even though the spectra are often indistinguishable for all different laser techniques. The recently reported observation of metal ion (Cu, Ag, Mg etc.) attachement for desorption with high power, short pulse lasers (10, 11, 12) also points to the similarity with SIMS. 

Kiperman [31] also warns that detection of free radicals in the postcatalyst volume in itself cannot serve as concrete proof of their direct participation in the process. The relation also has to be revealed between the nature of these radicals formed in the volume and the intermediates of the true heterogeneous component of the reaction. Obviously, sophisticated analytical and characterization procedures are needed to elucidate the nature of the species reacting on and desorbing from a catalytic surface. A powerful tool to study adsorption and desorption of radicals from surface is laser-induced fluorescence, applied to hydroxyl and oxygen radicals by a number of researchers cf. Ref 37. Such techniques will continue to aid in the elucidation of heterogeneous-homogeneous mechanisms. 

Abdelrehim IM, Thornburg NA, Sloan JT, CaldweU TE, Land DP (1995) Kinetics and mechanism of benzene formation from acetylene on Pd(lll) studied by laser-induced thermal-desorption Fouiier-transform mass-spectrometry. J Am Chem Soc 117 9509... 

Apart from desorption, surface reaction with adsorbate can be stimulated by the laser irradiation. In this chapter we will demonstrate the formation of new surface species by the CO2 laser induced reaction of CDF3 with the surface of SIO2 (17,18). In order to elucidate the mechanism of the reaction especially to determine the surface species, ir spectroscopy was used. A systematic investigation was performed Including the determination of reaction yields as a function of the laser frequency, the laser intensity and the gas pressure as well as the reaction products, and the determination of the correlation between the excited species and the reaction path. 

Since its discovery, laser polymer processing has become an important field of applied and fundamental research. The research can be separated into two fields, the investigation of the ablation mechanism and its modeling and the application of laser ablation to produce novel materials. Laser ablation is used as an analytical tool in matrix-assisted laser desorption/ionization (MALDI) [12,13] and laser-induced breakdown spectroscopy (LIBS) [14] or as preparative tool for pulsed laser deposition (PLD) of synthetic polymers [15,16] and of inorganic films [17,18],... 

Laser-induced acoustic desorption-electrospray ionization mass spectrometry (LlAD-ESl/MS) is a technique combining an electrospray and a pulsed laser beam to characterize solid and liquid samples with minimal sample preparation [48]. Although the instrumental setup of LIAD-ESl is similar to that of ELDl, the laser intensity required for LIAD (i.e., 10 W/cm ) is higher than that for ELDl, and the desorption mechanism of LIAD is also different from that of LD. In LIAD, the sample is not desorbed by direct laser irradiation, but by acoustic and shock waves induced by the laser irradiation. A pulsed laser beam with a flux energy of several mJ is used to irradiate the rear of a thin metal foil (e.g., AL Ti, Cu, and Ta 5-25 pm thickness) to generate acoustic and shock waves [48-50]. These laser-induced... 

An ultrafast laser can excite the surface electrons and these can induce chemistry as well as desorption prior to their being rapidly ( 1 ps) thermalized with the phonons (Bonn et ai, 1999). The electronic mechanism for laser-induced desorp-tion(Gomer, 1983 Gadzuk, 1988 Avouris and Walkup, 1989) is through the temporary formation of the negative ion of the adsorbate. The ion is pnlled sttongly toward the surface while its equiUbrium distance tends to increase. Shortly thereafter the charge is returned to the surface and a vibrationally excited neutral is ejected. 

It should be noted that whilst most of the particle-induced desorption techniques are simple to perform, they are only partially understood in theory, i.e. there exists no clear understanding of the mechanisms involved in ion formation from organic molecules in the condensed phase. It is, however, striking that different desorption ionization (DI) techniques [including FAB and LSIMS but also plasma desorption (PD) and laser desorption (LD)] produce reasonably similar mass spectra from nonvolatile organic molecules. Similarities in the spectra produced with incident beams of keV atoms or ions, or MeV particles and photons, can only be explained by similar ion formation and ion dissociation processes occurring after the initially very different physical excitation process. A general mechanism for the DI process can be presented, at least schematically, by... 

Pt(l 11) [6-8], Cu(l 1 1) [9] and Ag(l 1 1) [9], and CO fromPt(00 1) [10] andPt(l 1 1) [11,12]. On the other hand, these molecules are not desorbed from Ni and Pd metal surfaces in spite of the isoelectronic character of the metals Ni, Pd and Pt [13,14]. Desorption induced by subpicosecond-pulsed laser takes place via multiple correlated (and partially coherent) electronic transitions DIMET. DIMET is a very different mechanism from DIET [15-17] and in DIMET the vibrational excitation during the multiple electronic transitions leads to the desorption. Desorption via multiple vibrational transitions has also been observed using an infrared laser [18]. However, these topics are not described in this review. 

Photo-induced reaction on a metal surface usually consists of several elementary reactions and it is difficult to model the whole reaction process. However, any reactions need to be triggered by electronic excitation. As stated in Section 20.1.4, the major mechanism is indirect excitation thus we focus on modeling the indirect excitation reaction. Since desorption from the surface is one of the simplest processes and can be a prototype for other complex surface reactions, DIET or DIME are clearly the best to study [10, 48, 53, 57, 96]. In photochemistry, continuous wave or nanosecond lasers lead to DIET, where desorption increase linearly with fluence. In contrast, the DIMET process is caused by intense and short laser pulses on the picosecond or femtosecond time scale, with nonlinear dependence on fluence. Since the fluence is proportional to the number of created hot electrons in the bulk, linear... 

As discussed above, every laser exposure of a sample leads to the removal of a bulk volume - that is, many monolayers of matrix molecules of the sample. The term desorption is, therefore, somewhat ill-chosen for this process, and was so even for the field desorption for which it was originally coined. Ablation (removal of bulk material from surfaces) is the more specific term, and is used interchangeably with desorption throughout this chapter. The processes of material ablahon and the ionizahon of a minor fraction of the matrix and analyte molecules are, no doubt, intimately intertwined, and both take place on a micrometer geometric and a nanosecond time scale. It is experimentally very difficult - if not impossible - to sort out the complex contributions of the physical processes induced by the laser irradiation in all detail. Despite this complexity, it is of considerable merit to treat the ablation and ionization mechanisms separately. From such a discussion, some basic understanding can be derived, particularly, because the vast majority of the ablated material comes off neutral. 

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