DIMET electronic transition

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. 

DIMET Dynamics Induced by Multiple Electronic Transitions... 

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],... 

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. 

Associative desorption of hydrogen or the reaction between CO and O on a Ru(0 001) initiated by intense IR fs laser pulses, as outlined at the end of Chapter 3, are reactions caused by heating up the electron gas of the metal. The repetitive transition between ground-state and excited potentials in the DIMET process is responsible for the occurrence of the reaction that can still be considered as thermal since the electron gas equilibrates rapidly to an electron temperature and the process is still dominated by the ground-state potential. It is characteristic of this mechanism that the yield increases stronger than linear with the photon flux, reflecting its multiple excitation mechanism. 

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