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Re, Johnson; Pospieszalska, M. K.; Wl, Brown

doi:10.1103/physrevb.44.7263

Cited by 41 publications

(12 citation statements)

References 46 publications

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“…For example, high densities of electronic excitation are known to accompany desorption arising from bombardment of surfaces by very energetic particles [24]. Further, the general concept of a strong enhancement to rates from repetitive electronic excitation may be significant in a variety of physical processes beyond that of desorption from surfaces.…”

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confidence: 99%

Desorption induced by multiple electronic transitions

1992

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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...

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confidence: 99%

Desorption induced by multiple electronic transitions

1992

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“…The sputtering of solid oxygen has been a test bed for theories of thermal spikes, in particular because Y at high energies shows an interesting nonlinear dependence on S e , which has been described as a transition between linear and quadratic with increasing S e . [6][7][8] A difficulty of using solid oxygen to test models is the apparent discrepancy between the Y͑S e ͒ dependence obtained with MeV ions, 6,7 keV ions, 9 and keV electrons. 10 As in the case of the loop behavior, this raises the question of the existence of factors other than S e that determine the Y͑E͒ behavior.…”

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confidence: 99%

Role of electron capture in ion-induced electronic sputtering of insulators

et al. 2007

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Measurements of the sputtering yield of solid O 2 by 25-240-keV H + show that it is double valued in its dependence on electronic stopping power. We propose that this is because the electronic sputtering yield is dominated by repulsion of ions in the ionization track of the projectile which, at low velocities, is augmented near the surface due to the additional ionization resulting from electron captures. This process may also be responsible for enhanced radiation damage in insulators, in particular in the production of fission tracks. Ionization effects in insulators by fast ions are evident in a wide range of areas, including sputtering, dating by nuclear fission tracks, destruction of interstellar matter by cosmic rays, and medical therapy using ion beams. Some of the most basic information on these effects has come from the study of the sputtering of condensed gases 1 and fission tracks, 2 both occurring through the conversion of the electronic energy stored in electronic excitations and electron-hole pairs into repulsion between lattice atoms or molecules. Atoms set in motion when repulsive states evolve can be ejected directly or initiate a collision cascade of recoil atoms in the solid leading to sputtering or the amorphization of the ion track. The transfer of electronic to recoils depends strongly on specific details of the material, and its description can only be done in very few cases, due to the lack of knowledge of the excited electronic states in the solid state. For this reason, the understanding of ionization effects is usually limited to their correlation to the linear energy loss or electronic stopping cross section S e = dE / Ndx as the ion enters the solid. Here, dE / dx is the energy loss per unit path length and N is the atomic density. In the case of sputtering, it is often assumed that the yield Y ͑molecules ejected per incident ion͒ is proportional to the energy deposited near the surface ͑several monolayers, depending on the material͒, which can be considered to be proportional to the energy lost by the ion in that region. However, there is evidence that parameters other than the stopping power affect the sputtering yields for a given material. Since S e increases at low E, passes through a maximum, and then falls at high E, there are two ion energies for a given value of S e and one would expect the same sputtering yield in both cases if the deposited energy was the only ion property of importance. Instead, what is found is that there is a "loop" in the Y͑S e ͒ dependence where, for the same S e , Y is double valued, being larger at low than at high ion energies, as well documented for Ar ͑Ref. 1͒ and H 2 O. 3 In addition, Y is often not proportional to S e but depends on a higher power n ͑Y ϰ S e n ͒, with n up to 4, depending on the type of condensed gas and the type and energy of the projectile. 1 For instance, n = 1 for Ar, and n = 2 for water ice, a case of great astrophysical importance because of the role of sputtering in the erosion of surfaces and formation of atmospheres arou...

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“…Given this huge difference in sputtering efficiencies, we are most probably dominated in our measurements by the N 2 ice behaviour. Our measurement is reported together with measurements at lower stopping power from the literature [10,28,29,30] in Fig. 8.…”

Section: Sputtering Yield and Depth For Nmentioning

confidence: 64%

“…As a consequence it is difficult to employ the IR absorption method. In the literature, probably due to these experimental challenges, the sputtering yield of N 2 shows marked variations, and debates occurred [10] in the 90' concerning a particular behaviour of the N 2 ice sputtering yield as a function of the stopping power for low stopping power projectiles. A transition from a linear to quadratic evolution was hypothesised but uncertainties remains on its transition range.…”

Section: Introductionmentioning

confidence: 99%