DIET process

The rotational and the translational freedom appear after desorption of adsorbed molecules and each energy is kept without any disturbance before detection in the present experimental condition, since there is no collision and the lifetime of the excited states for a desorbed molecule is long. The experimental data can be analyzed by a simple model using the impulse scheme, con fi ned to the momentum transferred from the substrate to an adsorbate atom, in which the form of the excited-state PES and the transition process need not be assumed [68, 69]. The energy released from the excited state is converted to the momentum and this energy is transferred impulsively. The desorption also occurs impulsively. This simple model sheds hght on the property of the intermediate excited state, and the intermediate excited state plays an important role in the DIET process. 

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. 

In the impulse model, the excess energy Ek is transferred to an NO molecule as the momentum p0 given only to an N atom. Here, p0 is normal to the surface and Ek = p /2m, where m is mass of the N atom. Recoil of substrate Pt atoms can be ignored, because the mass of a Pt atom is much larger than that of an N atom. After desorption the momentump0 is converted to the linear momentum of the center of mass, P, and the linear momentum of the internal coordinate, p. A relationship p0 = m dri/df is satisfied in the impulse model and it can be approximated to dr2/df = 0 at the moment of the Pt-N bond breaking, where and r2 are the position vectors of N and O atoms, respectively, in an adsorbed NO molecule. 

The internal momentum p is divided to pt + pa, where pt is the radial part corresponding to the vibrational motion andpa is the angular part corresponding to the rotational motion, i.e. pa is converted to the rotational angular momentum of the nuclear motion L = r x pa. Hence, the energy Ek is delivered to translational (Et), rotational (Et), and vibrational ( v) energies of the desorbed NO molecule corresponding to P,pa and pt, respectively. That is, 

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A DIET process involves tliree steps (1) an initial electronic excitation, (2) an electronic rearrangement to fonn a repulsive state and (3) emission of a particle from the surface. The first step can be a direct excitation to an antibondmg state, but more frequently it is simply the removal of a bound electron. In the second step, the surface electronic structure rearranges itself to fonn a repulsive state. This rearrangement could be, for example, the decay of a valence band electron to fill a hole created in step (1). The repulsive state must have a sufficiently long lifetime that the products can desorb from the surface before the state decays. Finally, during the emission step, the particle can interact with the surface in ways that perturb its trajectory. 

Initial femtochemistry theoretical models were simply generalizations of those for the single-photon DIET processes, i.e., as dynamics induced by multiple electronic transitions (DIMET) [100]. The idea is simply that even if the excited state residence is too short to cause excitation to a ground state continuum after resonant scattering (tR < tc), it can still cause some vibrational excitation in the ground state. If resonant... 

Thus, in this review we present the desorption phenomena focused on the rotational and translational motions of desorbed molecules. That is, we describe the DIET process stimulated by ultraviolet (UV) and visible nanosecond pulsed lasers for adsorbed diatomic molecules of NO and CO from surfaces. Non-thermal laser-induced desorption of NO and CO from metal surfaces occurs via two schemes of DIET and DIMET (desorption induced by multiple electronic transitions). DIET is induced by nanosecond-pulsed lasers and has been observed in the following systems NO from Pt(0 0 1) [4, 5],... 

Figure 1 Schematic energy diagram for the DIET process due to the MGR model illustrating the relaxation and desorption processes. Electronic excitation due to laser irradiation occurs via the Franck-Condon transition. After a residue time t at the intermediate excited state, relaxation occurs with an excess energy ZA surpassing the surface barrier for desorption. The value of depends strongly on t, and no desorption occurs when t is shorter than the critical residence time tc. The Absicissa is the adsorbate-substrate distance.

Figure 2 Schematic energy diagram representing the DIET process due to Antoniewicz model, in which the intermediate excited state is a negative ion. The parameters are similar to those given in Fig. 1. The Absicissa is the adsorbate-substrate distance.

Laser-induced desorption via the DIET process is a structure-sensitive phenomenon. Firstly, we describe the recent results for adsorbed NO on Pt(l 1 1), since the adsorption structure of this system has been misunderstood for a long time. Adsorbed species giving rise to the 1490 cm-1 NO stretching vibrational mode had been believed to be adsorbed at bridge sites [34, 35]. Recently it has been shown that this species is adsorbed at the threefold fee hollow site. This problem was pointed at first using LEED analysis by Materer et al. [36, 37]. A similar problem is the occupation of the fee and hep threefold hollow sites in a ratio of 50/50 described by Lindsay et al. [38] on the basis of a photoelectron diffraction investigation of NO on Ni(l 1 1) at a coverage of 0.25 monolayer. 

The threshold energy of the photostimulated desorption process gives us an important information regarding the excitation mechanism. Desorption from Pt(l 11) via the DIET process has been observed... 

Table 6 Laser-induced desorption and dissociation of NO and CO on Pt, Pd, and Ni due to the DIET process [81].

The peak temperature in TDS of NO desorption from the alloy is 220 K in contrast with 310 K for the on-top species from Pt(l 11), i.e. lower by 90 K due to alloying. This lowering of the TDS peak is also observed for CO on the alloy at saturation, which reveals the peak at about 330 K, lower by 80 K than that on Pt(l 1 1) [87]. This weakening of the chemisorption s bond for the on-top species is consistent with the attenuation of the interaction with the dyy and dy / orbitals caused by the d band filling of the eg state. Flowever, laser-induced desorption of CO from the alloy is not observed. Therefore, the desorption activity in the DIET process is indifferent to the strength of the chemisorption bond. 

Finally, we would like to make a scenario of the desorption activity for NO and CO desorption from Pt(l 1 1) and Pt(l 1 1)-Ge surface alloy. This scenario will be extended to a general concept of desorption in the DIET process of simple molecules from metal surfaces. The lifetime and the critical residence time in the intermediate excited state followed by desorption are important keys for solving what is the origin of the desorption activity in the DIET process from metal surfaces. The excited molecules are not desorbed, if the residence time in the excited state is shorter than the critical residence... 

Eq. (20.36) is a relation of (cumulative) reaction probability, that is, reaction dynamics, and hot electron dynamics. Because the rate of energy transfer is much higher in DIET than DIMET per single hot electron attachment, the eTST is suitable for a DIET process as a first approximation. 

Adsorption of NO on a metal surface is important in heterogeneous catalysis [99], and the photodesorption of N O on an Ag(l 11) surface is an intensely studied DIET process. The action spectra (reaction probability) have been measured by several groups, and the data analyzed [100-102]. Recently, Carlisle and King have observed four different ordered dimer phases by STM, named as the a, p, y and 8 phases [103], Furthermore theoretical calculations also support a stable dimer as an adsorbate, the photoactive species can be considered as a dimer. Recently Kidd et al. carried out a comparative study of the action spectra of NO and OCS photodesorption on Ag(l 11), and analyzed it using a phenomenological model [ 100]. Their analysis can be summarized as follows ... 

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