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FEATURE ARTICLE Laser Control of Desorption through Selective Surface Excitation Wayne P. Hess,* Alan G. Joly, Kenneth M. Beck, and Matthias Henyk Pacific Northwest National Laboratory, P. O. Box 999, Richland, Washington 99352
Peter V. Sushko, Paolo E. Trevisanutto, and Alexander L. Shluger* Department of Physics and Astronomy, UniVersity College London, London WC1E 6BT, UK ReceiVed: May 6, 2005; In Final Form: June 27, 2005 W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jpcbfk.
We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.
1. Introduction The idea of using lasers to control photochemical reactions has long intrigued chemists due to the appeal of bond selective chemistry. The promise of activating or breaking specific bonds through laser excitation and attaching new functional groups at chosen positions would allow versatile new synthetic schemes to be realized. Unfortunately, to this day the dream of lasercontrolled chemistry remains generally unfulfilled. This is due primarily to rapid energy redistribution inherent in all polyatomic molecules, even those of only modest size.1,2 While it is possible for a laser pulse to localize energy in specific bonds for short periods, it is difficult to induce a desired reaction before the localized “activation energy” leaves the selected bond and is redistributed throughout the molecule. Once energy redistribution occurs, the enhancement of the desired reaction channel is seldom observed. Nonetheless, reaction control has been successfully demonstrated in small molecules using mode-selective (vibrational) excitation,3,4 localized electronic excitation,5-7 and phase control.8 Laser control is currently pursued through active control9 schemes described as coherent control,10 quantum control,11 and two-pulse12 control. Sophisticated phase control methods rely upon interference between competing quantum mechanical pathways to modulate reactive channel branching. In this approach the transition dipole matrix element is exploited through interference in a multilaser excitation process used to * Corresponding authors. E-mail:
[email protected]; a.shluger@ ucl.ac.uk.
prepare an excited precursor state. Phase control theory13 has been verified experimentally in atoms,14 molecules,15 and clusters.16 Ultrafast laser dynamics studies have used shaped pulses designed to control multiple photon processes.17,18 These methods have recently been reviewed.19-23 In comparison, coherent quantum control in condensed-matter systems remains relatively unexplored,24,25 yet the ability to manipulate solids coherently is of critical importance for building future quantum information devices. New coherent methods are being developed to prepare, manipulate, and interrogate electron dynamics26 and quantum states using several complementary classes of excitations such as electron spins in solids, excitons in semiconductor quantum dots, vibrational states in molecular crystals, and surface plasmons in metal nanostructures.20-22,25 In application to photostimulated desorption processes, one can consider mainly incoherent control strategies where the main control parameters are photon energy and laser fluence.27 There have been several successful applications of incoherent control27 on metal and semiconductor surfaces.28,29 Recently, lasers and other excitation sources30-38 have been used to excite particular surface features of ionic crystals and to study surface decomposition, molecular dissociation, and formation of chemically active surface sites.39 Ultraviolet (UV) and electron irradiation of these materials can induce surface decomposition and emission of particles in different charge and electronic states, and with different velocity distributions.30-33,40-44 Several papers on the photodynamics of methyl iodide adsorbed on “inert”,45,46 semiconducting,47,48 or metal49,50 surfaces have shown that
10.1021/jp0523672 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/20/2005
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Wayne Hess received his Ph.D. in Physical Chemistry from the University of Colorado in 1988, working under the guidance of Professor Stephen Leone. His thesis research involved laser-based studies of gas-phase photodissociation dynamics. He then completed a postdoctoral appointment with Frank Tully at Sandia National Laboratory in Livermore, California, where he studied bimolecular reaction kinetics of CN and OH radicals with alcohols and alkanes. He began his professional career at Pacific Northwest National Laboratory (PNNL) in 1990 and is currently Associate Director of Chemical Structure and Dynamics. Along with a staff of scientists, postdoctoral associates, students, and external collaborators, he is responsible for conducting fundamental research for the Department of Energy Chemical Sciences Division. Most recent efforts involve laser control and site-selective photodesorption from wide band-gap ionic crystals and metal oxides. Current research interests also include understanding the ultrafast dynamics and catalytic properties of metal oxide thin films and nanoparticles.
Alan Joly was born in Pittsburgh, Pennsylvania. He obtained a B.S. in Chemistry from the University of Rochester and a Ph.D. in physical chemistry from the Massachusetts Institute of Technology under the direction of professor Keith Nelson. After graduation from MIT, he joined the staff at the Pacific Northwest National Laboratory (PNNL), first as a postdoctoral fellow and later as a research scientist. His research interests include time-resolved studies of semiconductor metal oxides, nanostructures, and thin-films, laser-induced desorption from insulating crystals, and time-resolved luminescence studies of semiconductor nanoparticles.
adsorption alters the relative electronic state yield compared with the analogous gas-phase branching. Other work has focused on the photochemical reactions of adsorbed species on surfaces where the surface serves as a template to orient reactants favorably.51 One of the potential applications of photostimulated surface desorption is controlled surface modification. There is considerable interest in applying laser control principles to manipulate atomic surface structure. Early examples are found in laser desorption studies of GaP and GaAs semiconductors. The emission of Ga atoms was linked to particular surface defect
Hess et al.
Kenneth M. Beck earned his B.A. in Astronomy/Physics from Northwestern University. He received his Ph.D. with Professor Robert J. Gordon in Physical Chemistry at the University of Illinois, Chicago. Among his publications, Ken has authored chapters for two books. In one he explores the chemical kinetics of polyatomic molecular vibrational relaxation, for which his Ph.D. research defined the modern propensity rules. In another, he reviews the utility of multiphoton excitation in the study of molecular energy transfer phenomena. Ken postdoc’d for Professor Marsha I. Lester at the University of Pennsylvania, studying weakly-bound complexes of the -OH radical. Later, Ken became a faculty member at the University of Central Florida, holding a joint appointment in the Department of Chemistry and the Department of Physics. He was affiliated faculty in the Center for Research and Education in Laser and Optics (CREOL). Ken joined PNNL in 1996, where he has earned two U.S. patents for Battelle. Ken’s interests include working on photoinduced processes in solids, nanocomposite thin films, geologic interfaces, and biological samples. Primary projects involve photostimulated desorption of atomic and molecular species from metal oxide crystals and thin films and observing the underlying dynamics of electron-hole pairing and exciton formation and evolution.
Matthias Henyk was born in Weimar, Germany, in 1969. He obtained a diploma of physics at the Technical University of Berlin in 1996, after diploma work at the Max-Born-Institute for Nonlinear Optics and Ultra Fast dynamics (Germany). He received the Dr. rer. nat. degree from the Technical University of Cottbus (Germany) with a thesis about laser-induced charged particle emission from dielectric targets. Now he is a postdoctoral researcher at the Pacific Northwest National Laboratory. His current interests include characterization and deposition of thin films, nonlinear optics, ultrafast lasers, and control and dynamics of laser desorption.
sites such as adatoms, corner kinks, and vacancies.52 Furthermore, certain of these defect types could be removed from the surface under selective irradiation without causing further defect formation or other material damage. Tanimura et al. showed that adatoms could be removed from the reconstructed Si (111)(7 × 7) surface following resonant 2.0 eV laser irradiation.53 These studies confirmed that laser excitation could be used to remove defects and to form more perfect surfaces. On the other
Feature Article
Peter V. Sushko was born in Sillama¨e, Estonia in 1973. He has received his BSc (1994) and MSc (1996) degrees from St. Petersburg State University, Russia. From 1997 to 1999 he was a graduate student jointly at the Royal Institution of Great Britain and University College London (UCL). Since 2000 he has been working as a Research Fellow, and from 2005 as a Senior Research Fellow at UCL. His research interests are concerned with theoretical modelling of ground and excited states of defects and mechanisms of photoinduced processes in the bulk and at surfaces of insulators and wide band-gap semiconductors, as well as with development of embedded cluster approach for these systems. He has co-authored about 50 papers and book chapters and one patent.
Paolo E. Trevisanutto was born in 1972 in Rome. He graduated from the University of Rome “La Sapienza” where he completed M.Sc. degree in Physics in 2000. From 2000 to 2003 he worked as a software developer for Marconi Communications. In 2003 he started his Ph.D. studies at the University College London under the supervision of Prof. A. L. Shluger and Dr. P. V. Sushko. His research interests are focused on the theoretical aspects of mechanisms of photoinduced processes in insulators and photoinduced desorption of insulating surfaces.
hand, electron and photon irradiation creates rectangular monatomically deep pits on terraces of alkali halides through desorption of halogen and alkali atoms.44 These pits have been used for studying the modes of assembling organic molecules.54 Photodesorption or photoexcitation may also significantly affect surface reactivity and the nature and concentration of chemically active surface sites, particularly at oxide surfaces. For many catalytic materials, the chemical activity is related to the composition and detailed atomic-level structure. The demonstration of site-selective excitation leads naturally to concepts of site-selective photocatalysis. In principle, particular active surface sites may be “turned on” by selective photoexcitation leading to controlled enhancement of specific reactive channels. For instance, the Prof. E. Kno¨zinger group has demonstrated that corner sites of surface anions on MgO powders can be selectively excited, by 4.6 eV photons, to induce reactions of adsorbed H2 molecules.39
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Alexander L. Shluger graduated from the Latvia State University, Riga, in 1976. He received his Ph.D and Doctor of Science degrees in Chemical Physics from the L. Karpov Physics and Chemistry Research Institute, Moscow, in 1981 and 1988, respectively. He became a professor at Latvia University in 1994 and joined the Royal Institution of Great Britain, London in 1991. He joined the faculty of the University College London in 1996, where he is a Professor of Physics from 2004. He is a Fellow of Canon Foundation from 1993 and a Fellow of the Institute of Physics from 1999. His main research interests concern the mechanisms of defect related processes in the bulk and at surfaces of insulators. Current research is focused on theoretical studies of defects in oxides and the mechanisms of photoinduced processes at oxide surfaces, and on modelling of images and manipulation of molecular species at insulating surfaces using atomic force microscopy.
Our understanding of photoinduced desorption of insulating surfaces has developed over many years.55-57 Research has been carried out mainly on alkali halides due to the relative simplicity of their crystalline and electronic structures and a wealth of studies of radiation-induced bulk processes. Several mechanisms of photo- and electron-stimulated desorption of alkali halides have been discussed in the literature.33,38,58-63 It is clear that to achieve control over photostimulated surface processes one must have significant understanding of the reactive mechanism. However, most laser modifications of insulating surfaces have been accomplished largely without a detailed understanding of surface electronic structure. This understanding has been recently achieved for alkali halides through a combination of theory and experiment. In particular, controlled laser desorption of halogen atoms,30,64-66 using carefully chosen photon energies, induces specific dynamics allowing for selectivity in both the kinetic energy and electronic state distributions of desorbed atoms. Electronic structure calculations provided significant insight into the nature of the pertinent electronic excited states and the dynamics of the fundamental processes. In this paper we discuss the mechanisms of laser control of photodesorption from alkali halides using frequency selected laser pulses. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be controlled. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We then describe initial efforts to extend laser control to MgO, a cubic metal oxide. The technological importance of metal oxides is well established in fields such as catalysis, electronics, geochemistry, and high Tc superconductors. The detailed surface structure often dictates the important chemical or physical properties. Therefore, methods that manipulate surface structure on an atomic scale are likely to be increasingly valuable. If these methods can be applied successfully to MgO then extension to other important
19566 J. Phys. Chem. B, Vol. 109, No. 42, 2005 metal oxides such as TiO2, ZrO2, CuO, and NiO, is foreseeable. Application of this approach to controlling the yield and state distributions of desorbed species requires detailed knowledge of the atomic structure, spectroscopic properties, and electronic structure of transient surface exciton species. These studies integrate results of detailed electronic structure calculations and ultrasensitive laser pump-probe experiments within a developing surface exciton desorption model. If this model is extendable to other insulators, selective excitation of surface excitons could become a general method for controlling surface processes and modifying atomic surface structures of a wide variety of materials. 2. Experimental Method Experiments are performed on single crystals of alkali halides that are cleaved in air and subsequently mounted in a ultra high vacuum (UHV) chamber with a base pressure of 10-9 Torr. The cleaved crystals are heated to 650 K to clean and anneal the surface. The crystals are then irradiated at room temperature, with unfocused nanosecond UV laser pulses, to induce desorption of both halogen and alkali metal atoms. The desorbed atoms are ionized by a second focused nanosecond laser positioned approximately 3.8 mm above and parallel to the crystal surface. The second laser pulse is derived from a Nd:YAG pumped frequency-doubled dye laser which can be tuned to ionize either the ground or spin-orbit excited halogen atoms through a resonantly enhanced multiphoton ionization (REMPI) scheme. Alkali metal atoms are probed through nonresonant ionization by the probe pulse. The resulting ions are then extracted into a time-of-flight (TOF) mass spectrometer and detected using a chevron microchannel plate detector. The microchannel plate output is amplified in a video amplifier (×10) and sent to a digital oscilloscope. Velocity profiles of a particular mass are obtained by integrating the ion yield in the TOF spectrum as a function of delay between the two laser pulses. The obtained velocity profiles can be converted to kinetic energy distributions using the appropriate Jacobian transformation for the detection system. Point-tunable UV excitation pulses between 5.5 and 7.5 eV are produced using stimulated Raman scattering of Nd:YAG laser harmonics in hydrogen gas.The selected Raman-shifted excitation pulse is spatially separated from the other wavelengths using a CaF2 prism, and then directed into the UHV chamber and onto the sample. The power of the excitation pulses is measured using a pyroelectric detector and the excitation laser pump fluences maintained between 5 and 100 µJ/cm2, depending on the wavelength. At these fluences, we estimate that less than 10-5 surface atoms desorb per pulse; nonetheless, the halogen atom yield decreases slowly with irradiation dose. X-ray photoelectron spectroscopy shows no evidence of stoichiometric changes such as surface metallization or colloid formation following irradiation. Two-pulse excitation experiments utilize the fourth harmonic of a Nd:YAG laser to create defect centers in a KBr single crystal. A second Nd:YAG laser operating at the third harmonic is delayed by 20 ns and overlapped spatially with the first pump pulse. The second laser excites the defect centers produced by the first laser leading to bromine atom emission. The bromine atoms are then detected using the previously described REMPI scheme. Experiments performed on MgO utilize films grown by reactive ballistic deposition.67 The MgO films are deposited with an oxygen background pressure of 1.3 × 10-6 Torr, while magnesium is evaporated from an effusion cell held at 300-
Hess et al. 360 °C. The deposition angle ΘD between substrate and effusion cell is typically fixed at 76° and the deposition rate is 3-10 Å/s. The film morphology is characterized by means of X-ray diffraction (XRD) and electron microscopy. After deposition, the MgO thin film samples are transferred in vacuo (∼10-9 Torr) into a UHV chamber for irradiation. Laser irradiation of MgO films is performed at 266 nm (4.66 eV) obtained using either the frequency quadrupled output of a nanosecond Nd:Yag laser or the frequency tripled output of a Ti:sapphire femtosecond laser. The desorbed oxygen atoms are detected again using a (2+1) REMPI scheme combined with time-of-flight (TOF) mass spectrometry. XRD measurements of the films indicate preferred growth along the 〈111〉 crystal axis, and smaller contributions showing 〈100〉 or random orientation. 3. Hyperthermal Desorption of Halogen Atoms Under most irradiation conditions, the velocity distribution of desorbed halogen atoms is bimodal and consists of thermal and hyperthermal components.33 The thermal component has been explained by formation of bulk H centers, their subsequent diffusion to the surface, and eventual thermally activated desorption of neutral halogen atoms (as described in section 4A).33,56-58 The hyperthermal component has been shown theoretically to result from direct decomposition of excitons near the surface into a surface F center and desorbed neutral halogen atom.58 The surface exciton is thought to be energetically and spectroscopically distinct from the bulk exciton68 such that selective surface excitation is possible. However, despite much evidence, the existence of alkali halide surface excitons has not been generally and unambiguously proven using established surface-sensitive spectroscopic techniques. Laser desorption combined with laser ionization is a highly sensitive technique capable of probing these elusive surface exciton states. Figure 1 shows the normalized halogen-atom velocity profiles from both KBr and NaCl single crystals following excitation at different wavelengths. In both alkali halides, the lower energy excitation corresponds to excitation in the Urbach tail region where no significant bulk absorption occurs but surface absorption is strong. Similarly, for both alkali halides, the higher energy excitation is strongly absorbed in the material bulk. The velocity profiles depend markedly on photon energy in each case. Excitation using the lower energy photons (in the Urbach tail region) produces only a narrow hyperthermal component. That is, lower energy photoexcitation produces only the higher energy velocity component. Excitation using higher energy photons produces the hyperthermal component and a second thermal component observed at longer delay time. The higher-energy photons excite both the crystal surface and the bulk material inducing both hyperthermal and thermal halogen-atom emission. Hyperthermal halogen-atom desorption is a signature of surface electronic excitation, while thermal desorption indicates excitation within the bulk. Only weak 6.41 eV laser pulses (fluence
