Desorption induced by multiple electronic transitions

Abstract

A new mechanism is introduced to describe desorption from surfaces under conditions of strong electronic excitation. When repetitive excitations occur within the relaxation time for the adsorbate-surface vibration, the process of desorption induced by multiple electronic transitions may provide an enhancement of orders of magnitude over a single-excitation mechanism. This generalization of the classic Menzel-Gomer-Redhead picture encompasses within one formalism both single-excitation processes and a thermal limit. The mechanism may be operative in desorption by femtosecond laser pulses. PACS numbers: 68.45.Da, 42.65.Re, 78.90.+t, 82.65. -i Desorption induced by electronic transitions (DIET) [1] is a subject of great fundamental [2] and practical significance [3] in surface science. A general framework for DIET processes was introduced in 1964 by Menzel, Gomer, and Redhead [4]. The initial step in this process is a Franck-Condon transition from the ground potential energy surface (PES) Vg of the adsorbate complex to an excited state V, . While moving on the excited PES, the adsorbate converts potential energy into kinetic energy. In a semiclassical picture, desorption results whenever the adsorbate remains in the excited state for the critical time r, needed to attain positive total energy upon return to the ground state. Trajectories for adsorbates experiencing motion on the excited PES for times less than and greater than r, are illustrated in Figs. 1(a) and 1(b), respectively. Both trajectories start with a vertical transition from Vg to V, . In 1(a), the adsorbate is accelerated toward the surface after excitation [5], but is deactivated before r, . The energy gained results in vibrational excitation in the ground state, but is insufficient for desorption. In 1(b), the adsorbate remains on V, for a time greater than r,. It then escapes from the ground-state well after deactivation. The efficiency of this process depends critically on the excited-state lifetime r. The probability of desorption P can be estimated by the likelihood of the adsorbate remaining on the excited PES for a time t & r" i.e. , as P -exp( -r,/r). Since adsorbate lifetimes on excited PES are typically quite short, low DIET yields are often observed.In this Letter, we discuss a generalization of the usual DIET picture to include desorption induced by multiple electronic transitions (DIMET). As in DIET, the initial step is a Franck-Condon transition to the excited PES [ Fig. 1(c)]. If the adsorbate remains on the excited PES for a time t & r" then it cannot escape from the well upon returning to the ground state V~. While in the conventional DIET scheme this particle will not contribute to the desorption yield, in DIMET it may be reexcited and desorb. Figure 1(c) shows such a case. The adsorbate makes another Franck-Condon transition, but this time from a vibrationally excited level of the ground PES.The additional energy associated with the vibrational excitation allows the adsorbate to desorb more readily than after the first excitation. O...

Ultrafast Electron Diffractive Voltammetry: General Formalism and Applications

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“…1(b)]. Unlike in phonon-mediated processes [15], distinct isotope effects in the reaction yield are characteristic for processes involving electronic excitation, e.g., in electron stimulated desorption [16] as well as in surface femtochemistry [2], and have to be attributed to the mass-dependent distance the system travels on the electronically excited state during its short lifetime [17]. The principle underlying this process is illustrated by Fig.…”

“…As a consequence, an adsorbate derived affinity level above E F may become partially occupied, and the system is transferred to another electronically excited potential. The gradient on this potential initiates nuclear motion during the short lifetime (typically 10 fs [17]). Relaxation occurs by electron transfer back to the substrate, and thereby the system returns to the ground state potential.…”

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