Desorption laser-induced

Vibrational interactions between molecules and a solid surface play a substantial role in many processes occurring on the surface. A large number of investigations show that the vibrational energy exchange can determine the direction and the rate of processes such as adsorption, desorption, laser-induced surface transformations, surface diffusion, chemical transformations of adsorbates, etc. [1-5]. The development of new experimental techniques gives an opportunity for detailed study of different surface processes, and in some cases for direct measurement of the molecular dynamics at surfaces for extremely short times. This is a permanent challenge for the development of the theoretical concepts for vibrational interactions on solid surfaces. 

LISC Laser-induced surface Similar to LIF chemical Pbotochemistry, desorption... 

Mobility of this second kind is illustrated in Fig. XVIII-14, which shows NO molecules diffusing around on terraces with intervals of being trapped at steps. Surface diffusion can be seen in field emission microscopy (FEM) and can be measured by observing the growth rate of patches or fluctuations in emission from a small area [136,138] (see Section V111-2C), field ion microscopy [138], Auger and work function measurements, and laser-induced desorption... 

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. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

J.L. Brand, A.A. Deckert, and S.M. George, Surface diffusion of hydrogen on sulfur-covered Ru(001) surfaces studied using laser-induced thermal desorption, Surf. Sci. 194, 457-474 (1988). 

Laser-Induced Thermal Desorption with Fourier Transform Mass Spectrometric Detection... 

Laser-Induced Temperature Jumps and Molecular Desorption... 

During the last year we have built an FTMS instrument specifically designed for laser-induced thermal desorption from single-crystal surfaces. Figure 5 is a perspective drawing of the Instrument. The chamber is pumped by a 150 1/s ion pump and has a base pressure of 2.0 X 10- torr. Gases are Introduced through sapphire-sealed leak valves from a diffusion pumped gas manifold. 

Larmor frequency, 39 353, 42 127 Laser desorption, Pt(lOO), wave types, 37 257 Laser-induced fluorescence, in detection of surface-generated gas-phase radicals, 35 150-160... 

Unfortunately, some of the analyzed molecules, as most biologically related molecules (e.g., amino acids), are solids with extremely low vapor pressures at room temperature and rapidly decompose when they are heated. For these molecules, which cannot be thermally vaporized, laser ablation or desorption have been alternatively used to produce neutral species in the gas phase.Both methodologies refer to laser-induced particle removal (laser sputtering) from a surface under the two extremes of massive and negligible rates of surface erosion, respectively. 

Laser-induced desorption can be utilized quantitatively to remove adsorbates in a localized area of a surface, without substantially altering the temperature of the surrounding sample. This provides a viable technique for measuring surface diffusion kinetics . Alternatively, sampling different... 

Fig, 7. Laser-induced heating model. The solid line represents the temperature transient calculated from Eq. (3) for a S ns FWHM laser pulse (dott trace). The instantaneous desorption rate calculated from Eq. (4) is represented by the... 

A simple model for the dynamics of nonresonant laser-induced desorption of adsorbates from surfaces has been formulated by Lucchese and Tully (LT). LT present the result of stochastic, classical trajectory calculations for thermal and laser-induced desorption of NO from LiF(100). For the LID simulations the initial temperature was set at 0 K and temperature Jumps of several thousand degrees were driven in a few picoseconds through nonspecific heating of the substrate. The interaction potential for these calculations... 

Extrapolation of the picosecond simulations would imply that isothermal and laser-induced desorption results should be in qualitative agreement for nonspecific heating pulses of 1 to 10 ns duration. Of course this may not hold if all desorption occurs during the leading edge of the laser pulse or if the desorption process is driven by a nonthermal maihanism. Nevertheless, incomplete equilibration is not expected to play a major role for translational or rotational accommodations if the residence times are longer than 10 ns. 

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