Vibrational excitation, desorption process

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

The vibrationally excited precursor AB/s/(fs) can decay not only via energy transfer to the bulk but also via a chemical transformation (desorption of B and reaction with the formation of D and C/s/). These chemical processes can be characterized by the chemical lifetime Tch, which can be estimated in the framework of the statistical RRKM theory (see, e.g., Refs. [50, 51]) using the reaction parameters of reagents B and A/s/, precursor AB/s/, and transition complexes determined based on the results of quantum-chemical calculations. Such estimates were performed for many reactions of interest for the growth of metal oxide films [20]. It appeared that in the wide temperature range... 

The impulse model is applied to the interpretation of experimental results of the rotational and translational energy distributions and is effective for obtaining the properties of the intermediate excited state [28, 68, 69], where the impulse model has widely been used in the desorption process [63-65]. The one-dimensional MGR model shown in Fig. 1 is assumed for discussion, but this assumption does not lose the essence of the phenomena. The adsorbate-substrate system is excited electronically by laser irradiation via the Franck-Condon process. The energy Ek shown in Fig. 1 is the excess energy surpassing the dissociation barrier after breaking the metal-adsorbate bond and delivered to the translational, rotational and vibrational energies of the desorbed free molecule. 

The flow of energy from laser-heated snbstrate electrons into the adsorbate results in the excitation of adsorbate vibrational modes. For desorption of diatomics from various surfaces, it has been snggested that excitation of the frustrated rotation is responsible for the desorption process [9,14, 39]. Our results are consistent with those observations. We cannot, strictly speaking, dismiss (a contribution from) the Pt-CO stretch vibration, but the frustrated translational mode can be exclnded based on the independently determined electron-coupling times found to be 2.5 and 4 ps for terrace-and step-adsorbed molecnles, respectively (see the Sect. 10.3.2). These coupling times are much longer than those describing the very rapid desorption process. 

Similar experiments with more tightly held adsorbates require higher bias voltages [54—59]. In this way, chlorobenzene adsorbed on Si(l 11) was subjected to the selective dissociation of C—Cl bonds, and it was concluded that a two-electron mechanism is operating that couples vibrational excitation and dissociative electron attachment processes. As can be seen from Fig. 4.9, the yield of desorption increases linearly with the electron current indicating a single-electron process, while for dissociation the yield increases with the second power. 

Many MALDI ions are vibrationally excited or hot (i.e., they are metastable on the time scale of detection in the mass spectrometer) as they emerge from the desorption plume (for further information, see also Chapter 1, Section 1.6). This is due to three separate processes ... 

On the basis of the dressed photon theory and the important role of electric dipole-forbidden molecular vibrational excitations, recent theoretical studies have proposed a simple model to describe atom or atom-cluster desorption due to the dressed photons from a nanometric particle deposited on a substrate [49]. Assuming an anharmonic potential for each atomic binding, an effective atom-nanodot potential was evaluated to determine the desorption energy and the stabilized dot size. The model shows that electric dipole-forbidden molecular vibrational excitations play an important role in the phonon-assisted process, which could potentially lead to a novel fabrication method, in addition to controlling the size and position of nanostructures [51]. 

A H2 molecule impinging on the surface can be physisorbed or dis-sociatively chemisorbed or rejected. In order to stick on the surface and to dissociate the molecule has to dissipate its kinetic, rotational and vibrational energy by the excitation of phonons and possibly of electron-hole pairs in the substrate. Thus, the metal lattice and possibly the conduction electrons serve as a heat bath in the adsorption (and desorption) process. The ratio of the number of molecules which stick on the surface to those which impinge on the surface is called sticking coefficient s. It depends on Jthe coverage 0. The initial sticking coefficient for 0 = 0 is of the order of 0.2 to 0.5 for H2 on clean transi-... 

Although the dynamical problem of the reacting molecular system is of the same complexity as that encountered in gas-phase reaction dynamics, the presence of a surface adds additional processes and phenomena. Such phenomena are, for instance, not only the importance of the structure, including corrugation, steps, and surface anomalities, but also the interaction with the possible excitation processes in the solid, such as phonon (surface vibrations) and electronic excitations. Also for charge transfer and other nonadiabatic electronic processes in the gas phase, the importance of the surface temperature adds additional features to the problem. Aside from this, the various processes of interest occur on different time scales, from fast reactive chemisorption processes on the sub-pico second time scale to the relatively slow diffusion and desorption processes. Thus different theoretical tools are needed in order to describe the variety of processes and the large time span one needs to cover. Also the many-body problem of the solid combined with the few-body gas-phase problem makes it necessary to introduce different methods for treating the dynamics, from classical trajectories and... 

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. 

Mechanism (27) distinguishes fast photoinitiated processes, viz. (a) and (b) leading to vibrationally or electronically excited O from the slower thermally assisted desorption in step (c) and from the recapture of electrons by 02, which together would account for t 100 ms and low 0. 

Localized chemical processes, such as desorption and ablation, stimulated by resonant laser pulse-surface layer interaction have been discovered recently. In this lecture the essential theoretical features of the desorption induced by resonant excitation of adsorbate vibrations with laser infrared and their influence on yield, rate, and quantum efficiency are presented. Results on selective damage to pigmented biological structures by short resonant optical and ultraviolett laser pulses are briefly reported. 

In principle, also a laser infrared pulse should be able to deposit sufficient energy into the vibration of the adsorptive bond so that the molecule desorbs. However, the vibration of the adsorptive bond will be very enharmonic, particularly at high excitation, so that desorption induced by monochromatic infrared will be very unlikely, except at enormous laser intensities, calculated to be in the order of >10 Wcm". (3) In addition there must be considerable broadening of the vibrational energy levels of the adsorptive bond. Then process (2) is of little resonant character. In general process (3), the direct interaction between the adsorbent and laser infrared, will heat up the system leading to thermal desorption. This is particularly true for metals, where extremely fast relaxation of electronic excitation into the phonon bath within lO s takes place. Laser induced thermal desorption is possible at all laser frequencies at which appreciable absorption of light occurs in the adsorbent.(4)... 

---