bluuebunny/crossref_metadata_2025 huggingface dataset:
Range-Resolved Heterodyne Detection Dual CO2 DIAL Measurements
Authors: Grant William B. , Brothers Alan M. , Tratt David M.
Published: February 2023

There are a number of measurements of atmospheric trace species and aerosols of potential interest in the 9 to 11 micron spectral region covered by CO2 lasers. While direct detection CO2 lidar systems have been used for such range-resolved measurements, the measurement range for such systems is limited to 2 to 4 km (Flanigan, 1986). Heterodyne detection CO2 DIAL was proposed by Kobayasi and Inaba, 1975, and first realized by Fukuda et al., 1984 and Hardesty, 1984, using single laser lidar systems. Heterodyne detection offers increased range (to 10 km or more), but demands greater system and data analysis complexity. This paper will describe measurements made using the Mobile Atmospheric Pollutant Mapping System (MAPM).
Target Analysis by Differential Reflectance Lidars
Authors: Pantani L. , Pippi I. , Cvijin P. Vujkovic , Ignjatijevic D.
Published: February 2023

The spectral reflectance, namely the fraction of the incident radiation flux reflected at a given wavelength, is an intrinsic characteristic of each natural or artificial surface. It seemed therefore interesting to investigate the potential of multiwavelength lidars in target identification by means of a spectral reflectance analysis; because of its wavelength agility the CO2 laser is a very attractive one for this kind of analysis. The aim of this paper is to present the results of spectral reflectance measurements of relevant target surface materials at the CO2 laser wavelengths in order to forecast the performances of a differential reflectance lidar.
Remote Sensing of Structure Properties in the Middle Atmosphere Using Lidar
Authors: Philbrick C. R. , Sipler D. P. , Davidson G. , Moskowitz W.P.
Published: February 2023

The results from a mobile lidar have been used to derive density and
temperature profiles between 25 and 85 km over central Alaska during
February-April 1986.
Low-Pressure Gain-Cell Laser-Detector Operation with A CO2 Transversely Excited Atmospheric (Tea) Laser
Authors: Van der Laan Jan E.
Published: February 2023

The recent development of a low-pressure CO2 gain-cell1 preamplifier for optical receivers has stimulated renewed interest in the technique in the lidar community. Early work in this area2 indicated that a gain factor of only 2.5 could be achieved using a 72-cm long gain cell. Because low gain did not appear to justify the added cost and complexity to a lidar system, interest in the technique decreased.
Monte Carlo Description of Photoionization.
Authors: Kyrala George A.
Published: February 2023

The interaction of an atom with a very intense external electromagnetic field is one of considerable current interest, particularly when the magnitude of the external applied fields are of the order of, or stronger than the internal atomic field experienced by the atomic electrons. In that case perturbation treatments of the interaction should be very difficult, if not incorrect. One reaches the point where one has to decide between which picture , among many, would be used for the interaction. Instead of using a multiphoton picture for the interaction, I chose to investigate a description of the interaction utilizing classical mechanics.
Atomic Behavior In Intense, High-Frequency Laser Fields
Authors: Gavrila M. , Kaminski J.Z. , Offerhaus M.J. , Pont M. , Van De Ree J.
Published: February 2023

A theory was recently developed to describe electron-atom interaction in intense, high-frequency laser fields 1,2,3. It applies to the intensities and frequencies already available from excimer lasers in the VUV 4, but extends beyond, to the X-ray range.
Electron Spectrum in Intense-Field Photoionization
Authors: Reiss H. R.
Published: February 2023

A nonperturbative theory of multiphoton ionization was given in 19801 by the present author, which predicted the subseguently observed phenomena of above-threshold ionization (ATI), suppression of lower-order peaks in the spectrum of ionized electrons, and major gualitative differences in the spectra of electrons ionized by circularly polarized light as compared to linearly polarized light. The theory uses a short-range binding potential with a single bound state, much like the later independent work of Muller, Tip, and van der Wiel.2 The 1980 theory is here applied to the case of photoionization of xenon by circularly polarized light of 1064 nm, for which experiments have recently been carried out.3-5
Subpicosecond Ultraviolet Multiphoton Electron Spectroscopy of Rare Gases*
Authors: Luk T. S. , Jara H. , Johann U. , McIntyre I. A. , McPherson A. , Schwarzenbach A. P. , Boyer K. , Rhodes C. K.
Published: February 2023

Recent advances in high power laser development have enabled field strengths comparable to the atomic binding field to be generated. This experimental achievement has stimulated considerable interest in the study of the nature of multiphoton processes. Previously reported results of photoelectron spectra using 10 ps, 193 nm radiation1 have shown that the photoionization mechanism for the low charge states is largely a stepwise process and the threshold intensities required to produce the charge states are in reasonable agreement with the Keldysh model. In addition, the effect of the ponderomotive potential in suppressing the lowest order ionization channel was observed.
Multiphoton Double Ionization of Ba
Authors: Zhu Y. , Eichmann U. , Gallagher T.F.
Published: February 2023

In an effort to gain more insight into the process of multiphoton multiple ionization, several experiments have recently been carried out to determine the wavelength dependence of the double ionization of Sr and Ba. 1-3 These studies were not as conclusive as one might hope, in that the observed resonances in the production of Sr++ and Ba++ did not quite match the energy levels of Sr+ and Ba+. Here we report experiments in which we have compared the spectra for the production of Ba by an intense laser with and without the population of specific states of Ba by modest secondary lasers. The results of these experiments show clearly that Ba++ is produced by first producing real intermediate states of Ba+ which are multiphoton ionized through intermediate resonances in Ba+.
Multiple Excitation of Atomic Electrons
Authors: Greene Chris H.
Published: February 2023

The strong interactions between simultaneously-excited electrons require nonperturbati ve theoretical techniques for their successful description. Two such nonperturbative techniques which have been pursued vigorously in recent years are the adiabatic treatment in hyperspherical coordinates and the R-matrix method. Of these, the hyperspherical treatment, which singles out the total electronic moment of inertia as an adiabatic coordinate, has been more valuable in providing a visual picture of the global evolution of electron correlations in energy and radius. The R-matrix method has evolved instead into a simple, efficient, and accurate procedure for quantitative calculations of photoionization and scattering processes. To date, however, the applications of this finite-volume variational method have been restricted primarily to the lower-lying doubly-excited channels of any given atom or molecule.
Regularities of Negative Ion Resonances
Authors: Clark Charles W. , Buckman Stephen J.
Published: February 2023

The effects of electron correlation in atomic and molecular systems are perhaps most pronounced in negative ions. No long-range Coulomb field is present to bind an additional electron to a parent state of a neutral atom. The stability of a negative ion state is thus usually attributed to one of two alternative mechanisms, both of which invoke electron correlation phenomena: i) polarization of the parent atom provides an attractive potential in which an electron can be weakly bound; ii) two electrons orbit a singly-charged positive ion in a highly correlated fashion, so that each electron is effectively subject to the Coulomb potential of a fractional charge. These correspond respectively to motions in the valleys and upon the ridge of a hyperspherical potential surface. Both mechanisms seem capable of generating the long series of resonances converging on the parent ionization limit, which have been observed in most negative ion systems.1
Inhibited autoionization in multi continua systems
Authors: Lange V. , Renz G. , Eichmann U. , Zetsche A. , Sandner W.
Published: February 2023

The phenomenon of "inhibited autoionization”,1,2 which has recently received considerable attention, occurs whenever the direct autoionization amplitude of a given Rydberg series is cancelled by interference with indirect autoionization through a neighboring perturber state. Inhibited autoionization thus requires at least a three channel system, one open and two closed channels, where it was first observed as dramatic reduction of the linewidth of the autoionizing Rydberg series in the vicinity of the perturber.2
Many-Electron Dynamics of Heavy Atoms in Intense Laser Fields. Effects of Screening and Correlation
Authors: Wendin Göran , Jönsson Lars , L'Huillier Anne
Published: February 2023

In this talk we shall discuss the role of collective effects in multiphoton-multielectron ionization of heavy atoms. In recent work /1,2/ we have developed a theoretical description of multiphoton ionization which takes into account effects of many-electron screening and introduces an effective laser intensity. This is accomplished by introducing a RPA-type of dynamically screened electron-photon coupling. Since every electron-photon interaction operator is screened, our approach includes multiply excited intermediate levels and provides a description of dynamic non-linear response.
Dynamics in Intense Fields - Beyond the Dipole Approximation
Authors: Bandrauk André D. , Kalman O.F.
Published: February 2023

We have recently shown in a series of papers [1-7] that coupled equations methods of quantum collision theory can be conveniently used to investigate processes such as direct photodissociation [1-2], Resonance Raman Scattering in weak and strong fields [3-5], and higher order nonlinear spectroscopies [6-7] for diatomics involving several well isolated excited electronic states. This was done using the dressed molecule picture [8-12] of molecule-radiation interaction, wherein photon states are explicitly included into the theoretical description. All these methods are adequate only in the case of well isolated electronic states. There is an urgent need to derive a priori the most efficient representation for general electron-nuclear-radiation field systems such as occurs in strong field laser chemistry. In recent work we have examined this problem in an effort to incorporate as much as possible the electromagnetic field into the dynamics [13-14]. One might surmise that classical approaches should work sufficiently well at the high field intensities described here, and much work has been pursued in that direction. As we have pointed out previously, this involves treating both the molecule and the field classically. For electron-radiation interactions one would prefer a quantum formulation, since electronic states are, as a result of their large excitation energies, true quantum states. Furthermore, quantum mechanics leads to a linear theory of interactions whereas classical mechanics is a highly nonlinear theory [15]. Thus using a quantum formulation of matter- field interactions [16-17], we have been able to exploit methods of early (non-covariant) quantum electrodynamics (QED). In particular we have shown that the Bloch-Nordsieck (BN) representation [18-20] (which leads naturally to the concept of coherent states in momentum space was very convenient as a method of introducing strong field effects directly into the quantum dynamics of molecular systems. Coulomb gauge

(

A
→

.

p
→

)

and Electric Field Gauge

(

E
→

.

r
→

)

representations were shown to be poor zeroth order approximations for the dressed molecular eigenstates in the presence of strong fields, i.e., the adiabatic coupled equations for the latter two gauges define the appropriate adiabatic state as unperturbed (zero field) molecular states [13-14].
Electron Angular Distributions and Branching Ratios of the SrI 5p3/2ns1/2 and 5p1/2ns1/2 J = 1 Autoionizing States
Authors: Zhu Y. , Xu E.Y. , Gallagher T.F.
Published: February 2023

Measurements of the energy distributions, Branching ratios, and angular distributions of electrons ejected from SrI 5p3/2ns1/2 and 5p1/2ns1/2 J=1 doubly excited states with n ranging from 10 to 20 in the autoionization to the SrII 5s, 4d, and 5p ion state have been made by the techniques of Isolated-Core-Excitation and Time-of-Flight Spectroscopy. The experiment consists of three Nd:YAG laser pumped tunable dye lasers which excite the Sr atoms in an atomic beam from the ground state to the 5p3/2ns1/2 or 5p1/2ns1/2 J=1 autoionizing states. The autoionizing electrons are analyzed in energy and angle by a time of flight analyzer, and the electrons and residual ions are collected by two microchannel plate detectors respectively and sent to the computer through the Boxcar Integrators. The measurements show that the doubly excited SrI 5p3/2ns1/2 and 5p1/2ns1/2 J=1 states autoionize predominantly to the excited SrII 4d ion states with a p or f electron. The asymmetry parameter varies continuously within the line profile of the 5p3/2ns1/2 J=1 states as the manifestation of the localized interaction between the SrI 5p3/2ns1/2 and 5p1/2ns1/2 J=1 autoionizing series. This variation is distinguished from that caused by the interference between the excitation to the bound part of the autoionizing state and underlying continua.
Multiphoton Isolated Core Excitation
Authors: Story J.G. , Van Woerkom L.D. , Cooke W.E.
Published: February 2023

In a number of recent experiments, the Isolated Core Excitation (ICE)1 method for producing doubly excited Rydberg states has been extended to two photon core excitations.2,3 One particularly novel experiment showed that it is possible to use two photon excitation even in the case where the single photon excitation moment is zero.3 Furthermore, an analytical, closed form expression for this two photon core excitation moment has been reported.4 This work extends that analysis to the general case, where the core has absorbed n independent photons.
Electric Field Effects on Doubly Excited Autoionizing States
Authors: Kelleher D. E. , Saloman E. B. , Cooper J. W. , Clark C. W.
Published: February 2023

We have observed electric fields to have pronounced effects on autoionizing resonances in the continuum region above the first ionization threshold. These effects are observed predominantly between field-coupled levels of opposite parity that have significantly different zero-field widths.
Multiple Excitation and Ionization of Atoms by Strong Lasers: Is There Any New Physics?
Authors: Lambropoulos P.
Published: February 2023

Although, under certain conditions a transition probability per unit time averaged over the laser pulse may be useful in interpreting general features of experimental results, in general events during the evolution of the pulse must be followed in detail because total ionization of various ionic species is apt to occur.1 As a result, at different stages of the pulse, processes of different order may appear. The end result is ionization. But a number of other processes such as wave mixing and lasing can coexist or even compete with ionization under the appropriate conditions. Employing calculated generalized cross sections, sequential ionization is calculated and shown to be the dominant mechanism under the conditions of recent experiments.2,3 But even in sequential processes, multielectron excitations may play a role in determining the cross section and the state of the ion. Examples of calculations in multielectron atoms illustrate the role of more then one electron excitation. Doubly excited states, for example, can in principle be employed in a sequence of transitions leading to a high lying doubly excited state. The probability of such transitions is estimated and the importance of various competing processes leading to sequential electron ejection is also discussed. In all cases, the structure of a particular atom and the frequency of the laser are found to play an essential role in the details of the process. Specific results pertaining to He, C, Sr and Xe are discussed as examples of the behavior of multielectron atoms in strong laser fields.
Halos and Singularities
Authors: Tape Walter
Published: February 2023

This talk is a somewhat more sophisticated version of the expository article of Tape (1982), with added technical flesh and more halos, both real and theoretical. The message is the same: Most refraction halos are physical manifestations of singularities of mappings, and the crude nature of these halos is anticipated by singularity theory. Infralateral arcs, supralateral arcs, and upper and lower tangent arcs tend to be "folds," while many Lowitz arcs have "pleats" as well as folds. The circular halos are instances of a higher dimensional singularity.
Brightness of Rainbows and Halos
Authors: Gedzelman Stanley David
Published: February 2023

The principal geometric features of most atmospheric optical phenomena such as rainbows and halos can be explained merely by finding the angular distribution of light scattered by single particles. If, however, we wish to ex plain the brightness features of the various phenomena, the single particle approach is not sufficient since they are produced by clouds or swaths of particles. Thus their bright ness is highly dependent on the optical depth of the clouds (Gedzelman, 1980, 1982).
The Faceted Snow Crystal
Authors: Hallett John
Published: February 2023

Colored haloes are produced by snow crystals with well defined facets, whose size, crystallographic orientation, and uniformity depend on changing environment, fall velocity and crystalline defect structure.
Terrestrial Images transmitted by the Novaya Zemlya Effect
Authors: Lehn Waldemar H.
Published: February 2023

Superior mirages are seen when the temperature of a stably stratified atmosphere increases with elevation. A classification into three types has been suggested1: (i) strong nonuniform refraction over short ranges (haf gerdingar), (ii) mild uniform refraction over medium ranges, giving the earth a flat or saucer-shaped appearance (hillingar), and (iii) long range optical ducting beneath a thermocline (the Novaya Zemlya effect).
Variations in atmospheric refractive index associated with ground-based turbulence at the Mauna Kea Observatory
Authors: Erasmus D. Andre
Published: February 2023

Horizontal and vertical variations in the refractive index structure parameter (Cn2) within the surface boundary layer in the Mauna Kea summit area are quantified. The contribution of ground-based turbulence to the value of Cn2 integrated through the entire atmosphere is determined.
A Study into the Mechanism of ION-Assisted Etching
Authors: Sanders F.H.M.
Published: February 2023

During the last few years, the rapidly growing need for ion-assisted, anisotropic etching of a.o. silicon and its compounds for VLSI circuits, has stimulated basic research on the processes underlying the largely empirical etch recipes. Part of the plasma etch process can be simulated in an experiment, where the combined effect of the exposure of a film (Si) to chemically active gases (XeF2, C12, SF6), and a low-energy ion beam (Ar+) has been investigated. Special attention will be paid to the reaction of Si with C12 alone, Si(C12), and under simultaneous exposure to an Ar+ ion beam, Si(C12,Ar+), as a function of the target temperature T, the Ar+ ion energy Ep, and the C12 flux ϕC12, using mass spectrometry and time-of-flight analysis.
Second Harmonic Generation Spectroscopy of Molecules Adsorbed on Oxide Surfaces
Authors: Van Wyck N. E. , Koenig E. W. , Byers J. D. , Ho Z. Z. , Hetherington W. M.
Published: February 2023

The observation and analysis of surface second harmonic generation (SHG) phenomena by Shen and coworkers (1) suggest that SHG spectroscopy is an effective interface - specific approach to detecting electronic spectra of surface and interfacial species. Since SHG is only possible in regions lacking inversion symmetry, the interface between two such materials will be the only source of an SHG signal. If a dye laser beam at frequency ω is focused upon this interface and the harmonic at 2ω is monitored as the laser is scanned in frequency, then an electronic spectrum of interfacial species can be generated because of the great enhancement of the second order susceptibility, χ(2), when 2ω concides with an electronic transition. In this way, spectra of species at solid-solid, solid-liquid, solid-vapor, liquid-liquid and liquid-vapor interfaces can be considered.
Photochemical Deposition of Sn for In-Situ Selected Area Doping of MBE GaAs (001)
Authors: Kowalczyk Steven P. , Miller D.L.
Published: February 2023

Ultraviolet (UV) initiated microchemistry has recently emerged as a possible new technique in microfabrication for solid-state electronic applications.1-3 Much of the recent research has centered on delineated deposition of metals by UV photolysis.4 One intriguing possibility that this capability opens up is that of selected area doping in conjunction with molecular beam epitaxy (MBE) technology.
Doppler Shift Laser Fluorescence Spectroscopy of Sputtered and Evaporated Atoms under Argon Ion Bombardment.
Authors: Husinsky W. , Betz G. , Girgis I.
Published: February 2023

Doppler Shift Laser Fluorescence Spectroscopy DSLFS with cw., single mode, tunable dye lasers is an excellent tool to study particles released from surfaces during the interaction of atoms and ions with solids /1,2/. From energy distributions of sputtered particles obtained by DSLFS, the mechanism for the particle release can be derived in many cases /2,3/. In particular, nonlinear and inelaitic effects in sputtering have been investigated extensively in recent publications /2,4,5/. In this paper we present measurements of energy and state distributions of Ca, Cr and Zr atoms released from the respective metal targets under 15 keV Argon ion bombardment. The influence of the target temperature and the oxygen concentration on the surface has been studied.
Infrared Laser-induced Photodesorption and Ultraviolet Laser-induced Thermal Desorption of Methyl Fluoride from Polycrystalline Copper
Authors: Viswanathan R. , Hussla I. , Burgess D. R. , Stair P. C. , Weitz E.
Published: February 2023

Laser-induced desorption of methyl fluoride from a polycrystalline copper surface has been achieved by either direct excitation of the γ3 vibrational mode of adsorbed methyl fluoride using a pulsed CO2 laser at 10μ or via heating of the copper substrate using a KrF excimer laser operating at 248 nm. We have previously demonstrated that at ultraviolet wavelengths, the reflectivity of copper is low enough that the KrF laser pulse of ~ 200 mJ deposits enough energy into the surface to desorb CO from either single crystal (100) or polycrystalline copper [1]. Time-of-flight (TOF) spectra have been characterized with respect to the CO translational velocity as a function of laser power density and CO coverage [2]. In the present work, preliminary results of such measurements are presented for the CH3F-Cu system.
Direct Laser Pumping of Adatom-Surface Vibrational Modes
Authors: Riley Merle E. , Diestler Dennis J.
Published: February 2023

The interaction of laser radiation with chemical species adsorbed on solid surfaces is a process of relevance to vibrationally activated desorption and other reactive processes. For desorption, photons can either excite an internal mode of the adsorbate, A, forming A* which can convert to translational motion and removal, or the photons can directly excite the adspecies-surface bond A-S above the dissociation limit. In both cases the important competing process is vibrational deactivation due to the coupling with the modes of the solid lattice. The central question is the amount of energy that can be localized in the surface bond at a given laser intensity.
Atomic-Resolution Dynamics by Electron Microscopy
Authors: Smith David J.
Published: February 2023

Latest developments in high-resolution electron microscopes (HREMs) have made it possible to obtain localised real-space information on the atomic scale which has resulted in major insights into many important materials problems [1]. The HREM technique has also recently been shown to be applicable to the characterisation of surfaces in profile and initial surface studies of gold, catalysts and semiconductors have been very interesting.
Excimer Laser Etching and Oxidation of Silicon
Authors: Horiike Y. , Sekine M. , Horioka K. , Arikado T. , Nakase M. , Okano H.
Published: February 2023

In previous report, an anisotropic etching of undoped poly-Si by a Hg-Xe lamp or a XeCl excimer laser in a Cl2 gas was demonstrated. However, n+ Poly-Si, which is currently used as a gate material in MOS devices, is etched isotropically by photo-dissociated Cl radicals(1). For the LSI application, the anisotropic etching for n+ Poly-Si should be also achieved.
Chemical Synthesis of Ideal Electronic Surfaces on Silicon and Other Semiconductors
Authors: Yablonovitch Eli , Gmitter T.
Published: February 2023

In the past decade semiconductor surface science has successfully studied the electronic structure and reconstructions of totally bare semiconductor surfaces. Recently experimentalists have begun to assume a more active role1,2 in modifying surface chemical structures to passivate the surface, i.e., to render the surface states electronically inactive. We can expect that this work will evolve more and more toward the artificial synthesis of surface chemical-bonding structres with desired physical properties. From the point of view of electronics, it is desirable to covalently satisfy all surface bonds, thereby shifting the surface states out of the band gap and into the valence and conduction bands.
The Low Temperature Reactivity of Si(100) with NH3 and NO: Rate Determining Steps and Rate Enhancement Via Electronic Excitations
Authors: Bozso F. , Avouris Ph.
Published: February 2023

There is currently strong interest in understanding the surface chemistry and thin film growth mechanisms of technologically important semiconductors. One intriguing finding of many studies in this area is that unusually high temperatures are required for sustained surface reactivity and thin film growth. Thus, for example, silicon nitride film growth using the reaction of Si with NH3 is carried out at temperatures of 1000-1300K. The high process temperatures are often undesirable because they can also enhance the rates of unwanted processes such as dopant diffusion. For these reasons non-thermal means of reaction are actively being sought. Essential for the success of such efforts is to understand the mechanism of the reaction, particularly the nature of the rate-limiting step.
Synchrotron Radiation Excited CVD and Etching
Authors: Urisu Tsuneo , Kyuragi Hakaru
Published: February 2023

Synchrotron radiation (SR) is a unique light source having several outstanding features. These features include 1) strong intensity in the X-ray to VUV range, 2) good directionality and 3) light source is clean due to the ultra high vacuum. Laser excited CVD and etching processes are also attractive as new semiconductor fabrication techniques, but, many gas molecules used in semiconductor processes show strong absorption in the VUV range, where no laser can emit sufficient intensity of photons. This problem can be solved using SR as the light source in the photo-excited processes. It therefore holds considerable promise in the development of new materials and processing techniques in the future. In this paper, we introduce our recent experiments of SR-excited CVD and etching conducted at Photon Factory in National Laboratory for High Energy Physics, and discuss the feasibility of using SR as a light source in the photo-excited semiconductor processes.
Wavelength Dependent Activation Selectivity In Aluminum Chemical Vapor Deposition
Authors: Higashi G. S. , Blonder G. E. , Fleming C. G.
Published: February 2023

Projection patterned laser deposition offers an alternative technique to focussed beam writing in the growth of patterned films for microelectronic applications. Focussed beam writing has the advantage of extreme flexibility because it allows discretionary patterning, while the projection patterned deposition technique offers high throughput for the repetitive processes encountered in mass production.
Mass Spectral Identification of UV-Laser Photoablation Products from Polymers
Authors: Estler R. C. , Nogar N. S.
Published: February 2023

Recently, much attention has been given to the laser photoablation of polymeric films because of applications to the etching processes involved in the microelectronics industry. For a number of compounds, including PMMA ( polymethylmethacrylate), PET (poly (ethylene terephthalate)), polycarbonate, and polyimide, it has been observed1 that short wavelength (193 nm) laser ablation results in precise and clean etching, while long wavelength (≥248 nm) ablation can result in melting, charring and burning. It has been proposed that the differences cited above in the ablative photodecomposition (APD) are a manifestation of the variation of photochemical quantum yields with wavelength, or possibly with intensity. If the quantum yield(s) for various photolytic processes are in fact wavelength dependent, one would expect that dependence to be manifest as a change in the distribution of volatilized products. We report here the direct observation of such quantum yield variations via mass spectrometric studies of the photoablation products.
Laser Probes of Surface Diffusion
Authors: Hall R. B.
Published: February 2023

Pulsed-laser-induced thermal desorption (LITD) can remove adsorbates from a localized region of the surface. The kinetics of migration of adsorbates against this concentration gradient can be followed using a second LITD pulse, delayed in time, to measure the concentration of adsorbates that have diffused into that area from surrounding regions of the surface. Such measurements, when made as a function of surface temperature and of adsorbate concentration, reveal fundamental information on the nature of adsorbatesurface and adsorbate-adsorbate interactions.
Surface Photoprocesses in Laser Assisted Etching and Film Growth
Authors: Houle F. A.
Published: February 2023

When simple molecules adsorbed on clean crystals surfaces interact with light dynamical studies can reveal a great deal about the photophysics of absorption, energy relaxation, and stimulated desorption, as well as the dynamics of photochemical reactions.1 In typical photochemical etching and deposition systems, however, surfaces are usually rough and disordered, and reactions involve a wide range of adsorbates which can undergo both photochemical and spontaneous processes. Although this level of complexity would seem to preclude obtaining as detailed a picture as is possible for the methyl halides, for example, rather simple experiments can provide direct and indirect information on important surface photoprocesses. In this talk early efforts to understand microscopic aspects of two very different photochemical systems will be described, with a particular focus on data pertaining to the influence of electronic excitation on adsorption of reactants and desorption of products.
Optical Second Harmonic Generation from Clean and Adsorbate-Covered Ni(110)*
Authors: Hamilton J. C. , Anderson R. J. M. , Williams L.R.
Published: February 2023

Optical second harmonic generation (SHG) is a surface and interface probe useful in environments as diverse as ultrahigh vacuum (UHV), electrochemical cells, atmospheric pressure reactors, and solid-solid interfaces. This wide applicability is due to the ability of visible or near-visible light to reach these various interfaces. The interface sensitivity arises since for centrosymmetric media, the second order nonlinear susceptibility is non-zero only where the centrosymmety is broken, ie. at interfaces. A recent review article documents a wide range of applications of this technique.1
IR-Visible Sum-Frequency Generation from Adsorbates on Metal and Dielectric Surfaces
Authors: Miraqliotta Joseph , Polizzotti R. S. , Rabinowitz P. , Hall R. B.
Published: February 2023

IR-Visible sum frequency generation (SFG) is a surface specific, nonlinear optical process that can provide information on structure of molecular adsorbates at both liquid-solid and gas-solid interfaces [1,2]. The nonlinear susceptibility of the surface region is resonantly enhanced when the input IR field is tuned across a vibrational transition in a molecular adsorbate, provided this transition is both Raman and IR-active [3]. We have used SFG to study the C-H stretching region (2800-3000 cm-1) of a variety of molecular adsorbates on both metal and dielectric surfaces. Internal reflection of the input IR and visible fields at a fused silica/air interface provides a two-orders of magnitude enhancement to the SFG intensity in comparison to an external reflection configuration. Analysis of the SFG dependence on the polarization of the input fields allows a determination of the average polar orientation of e.g. the terminal methyl groups in a stearic acid monolayer (Fig. 1) or of CH3 and CO groups on metal surfaces. We will discuss results that show the ability of SFG to detect modifications in the monolayer induced by temperature, and by chemical and mechanical interactions.
Ion- And Thermal-Induced Reactions on a Refractory Metal Surface
Authors: O’Brien W.L. , Rhodin T.N.
Published: February 2023

Ion-induced reactions on a variety of surfaces have been studied by a number of research groups during the past decade. Typically, gas phase product distributions, both of abundance and kinetic energy, are measured during exposure to inert ions and a reactive gas. From these results the emission mechanisms of the various gas phase products have been determined. Knowledge of the emission mechanisms, together with additional information on the “ion mixing” process, has allowed some conclusions to be obtained on the ion-induced surface reactions.
Laser Bilayer Etching of GaAs Surfaces*
Authors: Maki P. A. , Ehrlich D. J.
Published: February 2023

In the present work we describe an in-situ excimerlaser-patterned projection etching process used on a molecular beam epitaxy machine which demonstrates the basic processing steps required for 3-D structures in GaAs. The system has two ultrahigh-vacuum chambers which are isolated by a gate valve. In one chamber, GaAs is grown by elemental-source molecular beam epitaxy. In the second chamber, chlorine gas is introduced through a leak valve, and the sample is exposed to an ArF excimer laser (193 nm) through a single-element optical projection system. Samples are transferred between the two chambers under ultrahigh-vacuum conditions.
In Situ Plasma-Surface Diagnostics
Authors: Mitchell Annette , Gottscho Richard A. , Downey S.W. , Scheller Geoffrey R.
Published: February 2023

Monitoring surfaces during plasma treatment is important both for device processing and understanding basic plasma chemistry. In situ techniques offer the advantage of real-time analysis allowing instant feedback during process development and control. Additionally, removal of the substrate from the plasma is not required as in ex situ diagnostics thereby ensuring that the surface composition has not changed. We have developed two in situ surface diagnostics, photoemission optogalvanic spectroscopy (POGS) and second harmonic generation (SHG), to monitor real time changes on surfaces in contact with plasmas.
Structure and Dynamics of Strong Chemisorption on Si(111) by Helium Scattering
Authors: Doak R. B.
Published: February 2023

Atomic He scattering has been used to investigate strong chemisorption onto Si(111). The diversity of surface structure and dynamics is illustrated with results for As, Ga, and Cu chemisorption.
The Use of Diffuse He Atom Scattering in the Characterisation of Single Step Edge Defects
Authors: Hinch B.J. , Lock A. , Toennies J.P.
Published: February 2023

The technique of helium atom scattering is intrinsically very sensitive to all types of surface defects. With dynamic sensitivites of order 105, or more, one is able to observe and characterise very low densities of either point[1] or linear type defects[2]. The diffuse elastic scattering from isolated step edge defects, for example, is able to give detailed information about the characteristic shape of a step edge, as observed by the thermal energy He probe[3]. The linear nature of a step edge ensures that the intensity is scattered predominately along certain crystallographic azimuths. The large angle diffraction, from the non-periodic defect, is dominated by the strongly repulsive part of the interaction potential. Hard wall calculations may be used to deduce the shape of single step edges. Experimentally, only a very small incident-energy dependence is found. However, it has been shown that a soft potential calculation may improve the optimum fits, or comparison between calculation and experiment[4]
Laser-assisted Cleaning of GaAs Surfaces Prior to Metalorganic Vapour Phase Epitaxy
Authors: Rodway D C , Mackey K J , Smith P C , Vere A W
Published: February 2023

Low temperature Metalorganic Vapour Phase Epitaxy (MOVPE) is critically dependent on the correct chemical and physical preparation of the substrate before growth. The major contaminants on GaAs substrate surfaces are oxygen, in the form of native oxides, and carbon. Chemical etching in 5H2SO4: 1H2O2: 1H2O (1) followed by thermal cleaning (annealing at 560-580°C) removes the oxygen but leaves carbon. Other techniques, such as plasma irradiation or ion bombardment (2), remove C and O but may introduce surface damage and change the surface stoichiometry. One technique which is employed for carbon removal in both Si (3) and III-Vs (4) is reactive etching in an ozone atmosphere. We have taken advantage of the fact that ArF excimer radiation can be used to decompose O2 to yield O3 to devise an in situ cleaning technique for GaAs substrates prior to MOVPE growth. We report here the results of an Auger study of the contaminant species and the effectiveness of their removal as a function of laser power and oxygen partial pressure.
Heteroepitaxial Growth of GaAs Films by Alternating Ion Beam Deposition
Authors: Zuhr R. A. , Haynes T. E. , Pennycook S. J. , Appleton B. R.
Published: February 2023

The formation of thin films by the direct deposition of ions from a low-energy ion beam (ion beam deposition or IBD) has been shown to be an effective technique for promoting the epitaxial growth of elemental semiconductors at low temperatures.(1-3) It has also been used to produce layers of stoichiometric oxides and silicides by deposition of a single element at temperatures lower than those required for thermal formation of the same compounds.(4,5) IBD, with a mass- and energy-analyzed ion beam, has many advantages for thin film fabrication. It provides high purity, isotopic selectivity, control of the dose rate, sub-monolayer control of the dose, and control of the incident ion energy and the resultant effect on the chemistry and crystal structure of the film. In this work the IBD concept is extended significantly by the deposition of multiple elements onto a single substrate to produce heteroepitaxial growth of a III-V semiconductor, GaAs, on elemental single crystal substrates, Si and Ge.
Reaction of Silicon with Chlorine and UV Laser Induced Chemical Etching Mechanisms
Authors: Sesselmann W. , Hudeczek E. , Bachmann F.
Published: February 2023

Reaction of silicon with chlorine is studied by angle-resolved X-ray photoelectron spectroscopy. Chlorine surface concentration and penetration depth are determined. Excimer laser induced etching exhibits specific differences at 308nm and 248nm radiation. At 308nm the etch rates are strongly affected by gas phase parameters like chlorine gas pressure and gas flow. At 248nm higher etch rates are observed and the reactivity of silicon with molecular chlorine turns out to be rate limiting.
Electron and Photon-Beam Induced Thin-Film Growth
Authors: Bozso Ferenc
Published: February 2023

The small dimensions of novel electronic and optoelectronic device structures and their abruptly and unconventionally changing material compositions require high levels of spatial and kinetic control of the surface chemical reactions which are involved in the materials' growth or modification. These requirements exclude high temperature processes based on thermally activated chemical reactions. Directed energetic beams (electrons, photons, ions), however, can induce reactions at low temperatures, as well as provide for spatial and kinetic control in microfabrication processes. Electrons and photons of sufficient energy can cause fragmentation and desorption of adsorbed molecules or fragments of them, by inducing electronic excitations to dissociative states1. The surface species after such excitations are mostly of highly reactive radical character, which readily react with the substrate and with other radical or molecular species in the adsorbed layer. The formation of such reactive surface species in a controlled way can provides a basis for non-thermal selective area film growth.
The Elucidation of the Mechanism of the Initial Stages of Si(111)-7x7 Oxidation Using Scanning Tunneling Microscopy
Authors: Avouris Ph. , Lyo I.-W. , Bozso F. , Schubert B. , Hoffmann R.
Published: February 2023

The capability of the scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the topography and electronic structure of surfaces and adsorbate layers with atomic resolution makes it a powerful tool in the study of surface chemistry (1). Here we use STM, STS, ultraviolet photocmission spectroscopy (UPS) and electronic structure calculations to study the long-standing problem involving the nature of the initial stages of the oxidation of silicon. There have been a large number of studies on this issue utilizing a great variety of techniques. However, not only is the mechanism of oxidation still unclear, but there is not even agreement on what kind of product(s) is formed. Several different configurations have been proposed for the oxygen-containing sites in the early stages of the reaction (2). They involve oxygen atoms saturating the dangling-bonds of top-layer Si atoms, oxygen atoms inserted in back-bonds but leaving the dangling-bonds intact, or molecular forms of oxygen attached to surface atoms or bridging two surface Si atoms.
Interaction of Low-Energy Electrons With Adsorbed Molecules: Mechanisms of Energy Transfer and Dissociation
Authors: Sanche. Léon
Published: February 2023

The chemical reactivity of molecules physisorbed or chemisorbed on solid surfaces is a topic of interest in many areas of applied physics and chemistry. A large portion of the activity in this field of research has been focused on the catalytic action of surfaces; however, the recent discoveries on the possibility to enhance, trigger and even control chemical reactions with photon and particle beams incident on surfaces are now starting to shift scientific interests toward studies of the dynamics involved in these non-thermal reactions. Particularly, photon and electron beam methods are expected to surpass present macroscopic methods (e.g., plasma etching) in the manufacture of microelectronic and micromechanical devices requiring fine-line lithography [1]. One class of such techniques involves the utilization of laser photons with energies ranging from 0.1 eV in the infrared to 6 eV in the ultraviolet [2]. Many of the laser-driven surface reactions are expected to be caused by the photogeneration of low-energy electrons which transfer energy to an adsorbate causing dissociation and desorption [3-7].
Laser-Induced Reactions of Semiconductor Surfaces with Chlorine
Authors: Qin Qi-Zong , Li Yu-Lin , Lu Ping-He , Zhang Zhuang-Jian , Jin Zhong-Kao , Zheng Qi-Ke
Published: February 2023

Laser-induced chemical reaction of semiconductor with halogen and halogen compounds has attracted much attention in recent years due to its potential application in fabrication of microelectronic devices. We have reported UV and visible laser-induced reactions of Si and GaAs surfaces with chlorine using a CW molecular beam technique coupled with time-resolved mass spectrometry(1,2,). This paper will present recent studies in our laboratory on laser-induced reactions of Ge(111), Si(111), GaAs(100) and lnP(100) surfaces with chlorine molecules under 355-, 560-, and 1064-nm laser irradiations. We are particularly interested in the use of near infrared (1064-nm) laser photons as well as the promotion of reaction by raising the incident chlorine molecules’ translational energy. The objective is to achieve a better understanding of the mechanism of laser-induced reaction and its potential application in the chemical etching of semiconductor.
State-Specific Studies of the Laser-Induced Desorption of NO from Si (111)
Authors: Richter L.J. , Buntin S.A. , King D.S. , Cavanagh R.R.
Published: February 2023

A wide variety of chemical processes at semiconductor surfaces have been observed to be promoted by radiation. The possible mechanisms for the transfer of the initial photon energy to the reaction coordinate are many, including simple substrate heating, substrate carrier driver reactions, and localized adsorbate photoexcitation. State-resolved studies of laser-inducec reaction products have proven extremely illuminating as they often allow the distinction and quantification of various competing excitation mechanisms.1 We present the results of a state-resolved study of the laser-induced desorption (LID) of NO from Si (111) in which the energy partitioning in the desorbed NO is found to vary dramatically with the initial NO coverage due to the presence of competing excitation channels.
CW laser induced chemical reactions with integrated circuits
Authors: Auvert Geoffroy
Published: February 2023

By focussing a CW laser on the surface of an integrated circuit, the temperature can be locally increased if the laser wavelength is adsorbed by the silicon of the circuit [1]. This is the case when using visible or UV wavelengths. When turning on the laser beam power, the laser induced temperature increases and reaches an equilibrium temperature in less than a tenth of a microsecond for a one micron laser spot diameter [2]. The equilibrium temperature strongly depends on the optical and thermal properties of the irradiated area and is lower when irradiating a metal, which has high lateral heat losses and high reflectivity, than a semiconductor, which has low thermal conductivity and low reflectivity.