Desorption Induced By Electronic Transitions Research Articles

Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices

Non-thermal desorption from icy grains containing H$_2$CO has been invoked to explain the observed H$_2$CO gas phase abundances in ProtoPlanetary Disks (PPDs) and Photon Dominated Regions (PDRs). Photodesorption is thought to play a key role, however no absolute measurement of the photodesorption from H$_2$CO ices were performed up to now, so that a default value is used in the current astrophysical models. As photodesorption yields differ from one molecule to the other, it is crucial to experimentally investigate photodesorption from H$_2$CO ices. We measured absolute wavelength-resolved photodesorption yields from pure H$_2$CO ices, H$_2$CO on top of a CO ice (H$_2$CO/CO), and H$_2$CO mixed with CO ice (H$_2$CO:CO) irradiated in the Vacuum UltraViolet (VUV) range (7-13.6~eV). Photodesorption from a pure H$_2$CO ice releases H$_2$CO in the gas phase, but also fragments, such as CO and H$_2$. Energy-resolved photodesorption spectra, coupled with InfraRed (IR) and Temperature Programmed Desorption (TPD) diagnostics, showed the important role played by photodissociation and allowed to discuss photodesorption mechanisms. For the release of H$_2$CO in the gas phase, they include Desorption Induced by Electronic Transitions (DIET), indirect DIET through CO-induced desorption of H$_2$CO and photochemical desorption. We found that H$_2$CO photodesorbs with an average efficiency of $\sim 4-10 \times 10^{-4}$ molecule/photon, in various astrophysical environments. H$_2$CO and CO photodesorption yields and photodesorption mechanisms, involving photofragmentation of H$_2$CO, can be implemented in astrochemical codes. The effects of photodesorption on gas/solid abundances of H$_2$CO and all linked species from CO to Complex Organic Molecules (COMs), and on the H$_2$CO snowline location, are now on the verge of being unravelled.

Non-thermal ion desorption from an acetonitrile (CH3CN) astrophysical ice analogue studied by electron stimulated ion desorption.

The incidence of high-energy radiation onto icy surfaces constitutes an important route for leading new neutral or ionized molecular species back to the gas phase in interstellar and circumstellar environments, especially where thermal desorption is negligible. In order to simulate such processes, an acetonitrile ice (CH3CN) frozen at 120 K is bombarded by high energy electrons, and the desorbing positive ions are analyzed by time-of-flight mass spectrometry (TOF-MS). Several fragment and cluster ions were identified, including the Hn=1-3(+), CHn=0-3(+)/NHn=0-1(+); C2Hn=0-3(+)/CHn=0-3N(+), C2Hn=0-6N(+) ion series and the ion clusters (CH3CN)n=1-2(+) and (CH3CN)n=1-2H(+). The energy dependence on the positive ion desorption yield indicates that ion desorption is initiated by Coulomb explosion following Auger electronic decay. The results presented here suggest that non-thermal desorption processes, such as desorption induced by electronic transitions (DIET) may be responsible for delivering neutral and ionic fragments from simple nitrile-bearing ices to the gas-phase, contributing to the production of more complex molecules. The derived desorption yields per electron impact may contribute to chemical evolution models in different cold astrophysical objects, especially where the abundance of CH3CN is expected to be high.

Calibrating thermal and scanning tunnelling microscope induced desorption and diffusion for the chemisorbedchlorobenzene/Si(111)7 × 7 system

The precise calibration of thermally driven processes in scanning tunnelling microscope (STM)manipulation experiments, especially at room temperature and above, is necessary touncover an accurate picture of non-thermal dynamical processes such as desorption inducedby electronic transitions, driven by the STM current. Here we probe the displacement (thesum of desorption and diffusion) of chlorobenzene molecules chemisorbed on the Si(111)-7 × 7 surface, induced both by the STM electrical current and by heat. We also establish trulypassive imaging inspection parameters. The activation energy for pure thermal displacement is580 ± 20 meV, possibly associated with excitation to a physisorbed precursor state. STM induceddisplacement shows a marked decrease with increasing temperature, once the thermaleffects are removed.

Quantitative analysis by resonant laser ablation with optical emission detection: Resonant laser-induced breakdown spectroscopy

Resonant laser ablation (RLA) is a solid sampling technique that makes use of radiation trapping, and desorption induced by electronic transitions (DIET), to produce enhanced numbers of analyte atoms in the laser-induced plasma (LIP). This is achieved by tuning the laser ablation wavelength to a gas-phase resonant transition of the analyte. In this paper, RLA was coupled with detection of optical emission in the LIP to perform resonant laser-induced breakdown spectroscopic (RLIBS) experiments with a miniature, portable spectrometer. The experiments were designed to continue an examination of the proposed mechanism of RLA, and to perform an initial assessment of the applicability of RLIBS for quantitative analysis. The study indicated that for a multi-component sample, such as steel, the signal of the wavelength-targeted atom was enhanced the most compared to other constituents, and this supported the hypothesis that the DIET phenomenon is involved. Also, RLIBS experiments indicated that ablation yields of other components were enhanced by resonant ablation of the major component, which supported the contribution of radiation trapping to the RLA phenomenon. A linear, positive slope of the RLA induced atom yield as a function of concentration suggested that the RLA mechanism allows for quantitative analysis of solid samples. Calibration graphs were created for tungsten in spectrographic steels using RLIBS. A limit of detection of 4% was calculated for tungsten in steel.

DIET processes on ruthenium surfaces related to extreme ultraviolet lithography (EUVL)

The aim of this work is to provide insights into desorption induced by electronic transitions (DIET) processes that affect the reflectivity of ruthenium-capped Mo/Si multilayer mirrors working under EUVL (extreme ultraviolet lithography) operating conditions [high vacuum, and 13.5 nm (92 eV) photons]. Critical issues are associated with possible oxidation of the 2 nm thick Ru capping layer due to the inevitable background pressure of H 2O, and carbon build up due to background hydrocarbons. In the present work, we discuss aspects of the radiation-induced surface chemistry of Ru irradiated by 100 eV electrons and 92 eV photons. The cross section for electron-stimulated desorption of oxygen from O-covered Ru is ∼6 × 10 −19 cm 2. Carbon accumulation several nm thick occurs on the Ru surface during electron irradiation in methyl methacrylate (MMA) vapor, a model background impurity hydrocarbon. Radiation damage by low-energy secondary electrons is believed to dominate over direct photoexcitation of adsorbates under EUVL conditions. The secondary electron yield from Ru varies strongly with photon energy, and is ∼0.02 electrons/photon at 92 eV.

Vibrationally enhanced associative photodesorption of molecular hydrogen from Ru(0 0 0 1)

The effect of vibrational excitation, for example by infrared (IR) laser pulses, on the photodesorption of H 2 and D 2 from a Ru(0 0 0 1) surface has been investigated theoretically. Based on information from first principle electronic structure theory, a minimal two-mode and two-state model is developed for Desorption Induced by Electronic Transitions (DIET) in the single-excitation limit. In the model, the finite excited state lifetime of a few femtoseconds (fs) is accounted for by a lifetime averaging scheme. Using the vibrational ground state as initial state for averaging, the energy partitioning into different degrees of freedom and isotope effects are investigated. We then consider vibrationally excited states and vibrational wavepackets as initial states, which are found to have a large influence on the outcome of the reaction. To show that IR excitation of the adsorbates is feasible, we use optimal control theory to design pulses in the sub-picosecond range. For these, vibrational relaxation in the ground state due to coupling of adsorbate vibrations to electron–hole pairs of the metal is accounted for. Our major result is that isotope- and bond-selective control of photoreactions should be possible to some extent, even in strongly dissipative media.

Exciton induced photodesorption in rare gas solids

This paper reviews our progress on the desorption induced by electronic transitions(DIET) in rare gas solids by selective excitation of valence excitons. Observation ofmetastable atoms desorbed by excitonic excitation gives us direct information onthe exciton-induced desorption processes in rare gas solids. The validity of threedesorption mechanisms, cavity ejection, excimer dissociation, and internal sputtering, isdemonstrated by systematic measurements of kinetic energies and angular distributions ofdesorbed particles. The absolute yield of total and partial desorption was measured,which can lead us to the quantitative understanding of exciton-induced desorptionprocesses.

電子遷移誘起脱離研究の先駆者，石川義興博士および太田芳雄博士の業績紹介

Dr. Yoshioki Ishikawa is a pioneer who carried out systematic electron stimulated desorption (ESD) investigations for hydrogen adsorbed on a clean platinum plate (H2/Pt) under ultrahigh vacuum (UHV) conditions in 1942. (His name is spelled as Yosioki Isikawa in the original papers.) Although his papers are referred in several papers on desorption induced by electronic transitions (DIET), these seem to be not as well-known as famous ones by Menzel, Gomer, and Redhead in 1964. We describe his pioneering work in this article. He developed an UHV apparatus made of glass with a Pt plate and an oxide cathode pumped by a small mercury pump via a liquid air trap. Clear ESD signals were observed for H2/Pt at excitation energies of 8, 12, 14, 33-46, and 46-51 eV. He assigned the desorbate to atomic hydrogen (H), because it was pumped by a glass wall baked above 350°C. Based on these results and the potential curves of H2 in the gas phase, he proposed a model for the ESD of H from H2/Pt; that is, the first step is an electronic transition of adsorbed H2, and the second is the desorption of H along the repulsive potential surface of the excited state. This was the first introduction of the notion of DIET. He also carried out an ESD study of H2O/Pt in 1943. We also introduce achievements by Dr. Yoshio Ohta, who was another pioneer in the field of DIET. He measured electron-stimulated ion desorption from an oxidized Ni plate. He found that the threshold excitation energy for ion desorption is 22 eV, and that the kinetic energy of the desorbed ion is 1∼2 eV. To explain these results, he also proposed the concept of DIET.

Photodesorption of hydrogen molecules physisorbed on Ag: Isotope dependence of translational-energy distribution

We report findings of our investigation on the non-thermal photodesorption dynamics of H 2(D 2) initially physisorbed on Ag. We photodesorbed the H 2(D 2) using a 6.4 eV pump photon, and then state-selectively probed the desorbing H 2(D 2) using the resonance-enhanced multi-photon ionization technique. The corresponding time-of-flight (TOF) spectra measured for the initial rotational states J = 1 and J = 0 indicate no particular J dependence. The mean translational temperature was ∼8 K for both H 2 and D 2. Using a coherent DIET (desorption induced by electronic transitions) model, we were able to reproduce the TOF spectra, including the observed isotope dependence.

Vibrationally assisted DIET through transient temperature rise: the case of benzene on Pt{1 1 1}

A new model for vibrationally assisted desorption induced by electronic transitions (DIET) is presented and applied to the laser-induced desorption of benzene from benzene multilayers on Pt{1 1 1}. A transient surface temperature rise initiated by laser pulse durations spanning the 150 fs to 200 ps time scale initially leads to vibrational excitation of chemisorbed benzene on Pt{1 1 1}.The remainder of the laser pulse then induces an electronic transition to an antibonding benzene-metal state from the vibrationally excited ground state. This opens up the possibility of photochemistry induced by low energy photons.

Performance of higher order Monte Carlo wave packet methods for surface science problems: A test for photoinduced desorption

Aiming to treat multidimensional quantum dissipative dynamics of adsorbates at surfaces, we consider application of several variants of the Monte Carlo wave packet method to an exemplary problem, the desorption induced by electronic transitions (DIET) of NO from a Pt(1 1 1) surface with a two-state two-dimensional model. We investigate the convergence of stochastic unravelling schemes of different order for ‘rare’ observables characteristic for this test system.

Effect of vibrational relaxation on DIET: a density matrix treatment

The influence of substrate-induced vibrational relaxation on desorption induced by electronic transitions (DIET) is studied using a density matrix formulation. The one-dimensional model describing the DIET dynamics of NO consists of two electronic states. Relaxation in both electronic and vibrational modes is simulated with dissipative Liouvillians of the Lindblad form. As expected, vibrational relaxation results in a smaller desorption yield and lower product translational temperature.

Desorption induced by electronic transitions of Na from SiO 2: relevance to tenuous planetary atmospheres

Motivated by controversy concerning the origins of Na vapor in the atmospheres of Mercury and the Moon, we have studied the desorption induced by electronic transitions (DIET) of Na adsorbed on model mineral surfaces, i.e. amorphous, stoichiometric SiO 2 films. We find that electron stimulated desorption (ESD) of atomic Na occurs for electron energy thresholds as low as ∼4 eV, that desorption cross-sections are high (∼1×10 −19 cm 2 at 11 eV), and that desorbing atoms are ‘hot’, with suprathermal velocities. Photon stimulated desorption (PSD) of atomic Na is observed to have a threshold energy of ∼4 eV, and the desorption cross-section for hν≈5 eV is ∼3×10 −20 cm 2. The data are interpreted in terms of charge transfer to adsorbed Na + to form neutral Na 0, which desorbs from the surface. Desorption of Na + is observed with a threshold energy of ∼25 eV in ESD, associated with excitation of the O 2s energy level. The estimated Na desorption rate from the lunar surface via ESD by solar wind electrons is a small fraction (a few per cent) of the rate needed to sustain the Na atmosphere. However, the solar photon flux at energies ≥5 eV exceeds the solar wind electron flux by orders of magnitude; there are sufficient ultraviolet photons incident on the lunar surface to contribute substantially to the lunar Na atmosphere via PSD of Na from the surface.

Pulsed UV laser induced desorption of ions from aluminum

A study of pulsed UV laser induced desorption (LID) has been performed on an Al(111) sample. The positive ion desorption was investigated at low laser fluence, in a regime in which the ion yield exhibits a highly non-linear dependence on the laser fluence. The peak of the kinetic energy distribution of the desorbed ions has been measured to be about 15 eV. This result is consistent with the conjecture that the ion departing the metal surface can acquire a kinetic energy kick from a process associated with plasmon annihilation. The Al + ion kinetic energy peak is asymmetric and about 3 eV full-width at half-maximum (FWHM). This experiment indicates that plasmon excitation can play a significant role in laser stimulated desorption induced by electronic transitions (DIET).

Desorption induced by electronic transitions (DIET) of neutral fragments from chemisorbed biological molecular systems

Low-energy electron stimulated desorption of neutral fragments from oligomers chemisorbed onto a gold surface is investigated within the 1–30 eV range. The oligonucleotides are anchored to the surface via a sulfur-bound technique similar to the procedure used in molecular self-assemblies. We show that under electron impact the dissociation of DNA bases occurs, leading to the production of CN, OCN and/or H 2NCN neutral species, which are the most intense observable yields. The incident electron energy dependence of these desorbed neutral species exhibits typical signatures of dissociative electron attachment initiated by the formation of shape and core-excited resonances below 20 eV, whereas usually non-resonant processes (i.e. dipolar dissociation or dissociative ionization) become predominant above 20 eV.

Direct and indirect DIET and DIMET from semiconductor and metal surfaces: what can we learn from 'toy models'?

Desorption induced by electronic transitions (DIET) and its variant DIMET (M = 'Multiple'), are among the simplest possible "reactions" of ad-species involving ultra-short lived electronically excited states at surfaces. The non-adiabatic bond-cleavage can be enforced, for example, with laser irradiation or with electrons or holes emitted from the tip of a scanning tunnelling microscope (STM). The transient creation of excited intermediates can proceed directly (localised to the adsorbate-substrate complex), or indirectly (i.e., through the substrate). To understand the basic processes, simple one-mode two-state "toy models" such as the Menzel-Gomer-Redhead (MGR) or the Antoniewicz scenarios have proven very useful in the past. We adopt and extend MGR- and Antoniewicz-type models together with numerically exact open-system density matrix theory to address a few actual problems/experiments in DI(M)ET: (1) Direct, laser-induced desorption of H(D) from Si(100) surfaces which has been realised in the continuous-wave DIET regime only recently [T. Vondrak and X.-Y. Zhu, Phys. Rev. Lett., 1999, 82, 1967], is studied and compared to so-far hypothetical femtosecond laser desorption. The possibility of controlling the reaction by shaping the laser pulses is addressed. (2) For the same system, temperature effects are studied for electron- or hole-stimulated desorption with an STM [T. C. Shen, C. Wang, G. C. Abeln, T. R. Tucker, J. W. Lyding, Ph. Avouris and R. E. Walkup, Science, 1995, 268, 1590; C. Thirstrup, M. Sakurai, T. Nakayama and K. Stokbro, Surf. Sci., 1999, 424, L329]. A modified version of Gadzuk's "sudden transition and averaging" approach is adopted which accounts for temperature dependent excited state lifetimes. (3) For photodesorption of NO from Pt(111), based on quantum dynamical simulations possible experimental tests involving static electric fields are suggested to address the relevance of the recently challenged [F. M. Zimmermann, Surf. Sci., 1997. 390, 174], "negative ion resonance" model of the Antoniewicz type.

Photodesorption of NO from a metal surface: quantum dynamical implications of a two-mode model

Open-system density matrix theory is used to study the dissipative quantum dynamics of photoinduced, hot-electron mediated desorption of NO from Pt(111). Two electronic states are considered and both the DIET (Desorption Induced by Electronic Transitions) and DIMET (Desorption Induced by Multiple Electronic Transitions) limits addressed. Besides the center-of-mass motion of the molecule perpendicular to the surface plane, the vibrational motion of the NO molecule is accounted for. In a first, methodological part the interrelations between various “direct” and “jumping wave packet” methods to solve the underlying open-system Liouville-von Neumann equations of motion, are emphasized. In a second, applied part we investigate (i) the relevance of the popular “negative ion resonance” model, (ii) the role of coordinate-dependent electronic relaxation, and (iii) the implications of multiple-electronic excitations.

Desorption Induced by Electronic Transitions at Metal Surfaces.

An attempt is made to provide a physical basis for DIET (desorption induced by electronic transitions) at metal surfaces. For this purpose, a theoretical description for DIET is given from a microscopic point of view. From the calculation for the dependence of the DIET probability on the penetration depth of the laser light and on the band-width of the electron system of the metal substrate, it is suggested that the spatial locality of electronic excitations affects the probability significantly. It is shown, furthermore, that the DIET probability increases, as the shape of PES (potential energy surface) is changed such that the number of adsorbate bound-states in the PES is decreased. As an order of magnitude, the result of calculation for the DIET probability of NO from Cu(111) agrees with the probability estimated experimentally.

Controlled surface photochemistry: Bond- and isotope-selective photodesorption of neutrals by adsorbate vibrational preparation with infrared laser pulses

The possibility of controlling surface photochemistry by the selective vibrational preparation of adsorbates with infrared (ir) laser pulses is investigated theoretically. In particular, the selective ir plus ultraviolet (uv) light-induced desorption of different isotopomeric neutral adsorbates from metal surfaces is studied with the help of nuclear density matrix theory. As a concrete example the system NH3/ND3/Cu(111) is chosen. In a first step of the “vibrationally mediated chemistry” advocated here, based on computed two-mode dipole functions and model potentials, optimal infrared laser pulses are designed to selectively excite the umbrella mode ν2 of either adsorbed NH3 or ND3. In a second step, an uv/visible photon enforces an electronic transition, leading, after ultrafast quenching, to desorption induced by electronic transitions (DIET). It is argued that despite strong dissipation, the proper vibrational preparation not only increases desorption yields substantially, but also allows for an almost complete separation of both isotopomers.

Quantum dynamical aspects of rotationally and vibrationally mediated photochemistry in matrices and at surfaces HCl/DCl in Ar and NH3/ND3 at Cu(111)

We investigate two extensions of the concept of vibrationally mediated chemistry, from small molecules in the gas phase to small molecules in matrices and at surfaces using, as examples, the systems HCl/DCl in Ar and NH 3 /ND 3 at Cu(111). The transition from isolated systems to the condensed phase calls for new quantum dynamical techniques and allows us to predict new phenomena. For matrix isolation, we propagate three-dimensional wavepackets representing photo-dissociated H or D atoms which penetrate from the initial cage into the lattice provided by the matrix. The cage-exit probabilities are found to depend not only on the initial vibrational, but also on the rotational states, owing to the environmental (O h ) symmetry provided by the (fcc) lattice of the matrix. As a consequence, we suggest the extension from vibrationally to rotationally, or rovibrationally mediated chemistry for matrix-isolated molecules. For molecules at surfaces, we adopt Gadzuk's jumping wavepacket plus incoherent averaging scheme, applied to an extended two-dimensional Antoniewicz-type model for the surface–molecule bond plus the vibrational coordinate which lends itself to preferential vibrational excitation (here the umbrella mode of ammonia). The desorption depends selectively on the initial vibrational state. As a consequence, we suggest the extension of traditional desorption induced by electronic transitions (DIET) to a vibrationally mediated IR–UV DIET scheme which may be used e.g. for enrichment of specific isotopomers at surfaces.