Transient IR reflection-absorption spectroscopy with diode lasers has been used to characterize the adsorption kinetics and the nature of the bonding of CO on Cu(100). Although the strong dynamic dipole coupling between the CO molecules adversely affect the absorption intensity and frequency, it can be used to reveal the interadsorbate interactions. It was found that the adsorption of CO on Cu is correlated, resulting from a repulsive interaction between the first and second nearest neighboring sites. A model based on static screening of the charge transferred from CO to Cu is proposed to account for all the observations. Furthermore, a nonresonant reflectivity change, which results only from chemisorption, but not physisorption, was identified in both the IR and visible regions.

Diffusion of ammonia on Re(001) was studied utilizing optical second harmonic diffraction from surface coverage grating. The diffusion process at initial coverages NH₃/Re>0.15 could be simulated only if coverage dependent diffusion constant has been considered. The resulting diffusivity D((theta) )equalsD₀ exp(-E_m((theta) )/RT), is defined by a barrier for diffusion E_m((theta) )equalsE₀-(omega) Z(theta) , with activation energy at zero coverage E₀equals3.4+/- 0.8 kcal/mol, D₀equals2.8(DOT)10^-3cm²sec^{- 1}. The repulsion energy between pair of nearest neighbor molecules of (omega) equals0.4+/- 0.06 kcal/mol is obtained if an average of three nearest neighbors is assumed at the edge of the grating. The coverage effect on the diffusivity is found to correlate quantitatively with dipole-dipole repulsion between nearest neighbors. This results in a constant corrugation ratio E_diff/E_des, over the entire coverage range.

Optical interference techniques were used to measure the real index of refraction of nitric acid/ice films representative of type I and type II polar stratospheric clouds (PSCs). Possible candidates for type I PSCs include amorphous HNO₃/H₂O mixtures, as well as crystalline nitric acid trihydrate, dihydrate (NAD), and monohydrate (NAM). Amorphous and crystalline model PSC films were grown in vacuum by vapor deposition on single-crystal Al₂O₃ substrates at low temperatures. The real indices of refraction at (lambda) equals632 nm were measured for these films using the time-dependent optical interference during film deposition. The stoichiometries of the HNO₃/H₂O films were determined using laser induced thermal desorption techniques. For the amorphous films at 130 K, the refractive indices increased with increasing nitric acid content. The values ranged from nequals1.31+/- 0.01 for pure ice to nequals1.47+/- 0.01 for nearly pure nitric acid. A Lorentz-Lorenz analysis was in good agreement with the measured refractive indices of the amorphous HNO₃/H₂O films as a function of HNO₃ mole fraction. Growth of HNO₃/H₂O films at 175 K resulted in the formation of either crystalline NAM or NAD. The crystalline indices were substantially higher than their amorphous analogs. The crystalline refractive indices at 175 K were in nequals1.52+/- 0.01 for NAD and nequals1.54+/- 0.01 for NAM.

Single photon laser ionization time-of-flight mass spectroscopy is used to monitor the gaseous fluxes of Ga, As₂, and As₄, which are relevant in molecular beam epitaxy (MBE) of GaAs. This noninvasive and real-time technique measures densities, and hence fluxes, of multiple chemical species impinging on or scattered from a substrate during conventional MBE. With single photon ionization at 118 nm (10.5 eV), the energy is sufficient to ionize the species, but insufficient to ionize and fragment. The lack of molecular fragmentation greatly simplifies the interpretation of mass spectra. Additionally, the probe geometry permits simultaneous film growth monitoring using RHEED. Results will be presented on the probing of scattering and desorption of III-V MBE species during GaAs growth. This technique promises to be a valuable in-situ diagnostic for III-V and II-VI MBE.

The nature and disposition of surface states can have a dramatic effect on the near-surface electronic properties in semiconductor heterostructures. In particular the lack of a well-defined surface oxide in III-V materials means that surface band bending can cause surface recombination velocities to be up to 10³ larger than in Si-based materials. Raman scattering by coupled longitudinal optic phonons and 2D electron gas electrons in In_0.52Al_0.48AsIn_0.53Ga_0.47As (delta) -doped heterostructures is used to demonstrate the extreme sensitivity to surface states. The two highest frequency modes, of the three coupled electron-phonon modes expected in this system, were observed, with the L₊ mode being identified for the first time in InGaAs-based systems. The large dispersion of this mode makes it a particularly sensitive probe for changes in such properties as carrier concentration and subband energy. For structures with higher carrier concentrations coupling of the longitudinal optic phonon to multiple electron intersubband transitions is resolved. In order to passivate native surface states organic thiols are being investigated. Measurements on bulk GaAs indicate a change in the surface depletion region thickness, within the abrupt junction model, of up to 50 angstrom (ca. 30%). Changes in carrier scattering times up to 50% have also been observed.

The nonlinear optical properties of the aromatic amino acid residues naturally found in proteins are studied using resonantly enhanced second harmonic generation (SHG) in reflection from the air/water interface. In order to evaluate their potential utility as nonlinear optical probes of peptide and protein organization, tryptophan (Trp), tyrosine, and phenylalanine have been adsorbed from solution to the air/water interface in a model study. The dependence of the theoretically determined tilt angle of the resonantly enhanced ¹A->¹L_a transition dipole moment of Trp with respect to the surface normal on the assumed refractive indices of the monolayer is inferred from a SHG nulling measurement. Orientational information cannot be meaningfully deduced for tyrosine or phenylalanine as several transitions are resonantly enhanced for these residues in the near UV. The phases of the second harmonic signals from the surface adsorbed residues in comparison to the reference signal from water are also determined.

Image states on the surface of bare and adsorbate covered metals form simple, near-surface, bound electron systems. These systems provide useful model systems for understanding the electron- transfer dynamics between an adsorbate molecule and its underlying substrate. These states may be readily detected by two-photon photoemission, a technique ideally suited for high- resolution studies of excited surface states. Because of its relatively narrow linewidths, spectroscopy of the image states provides a useful method of detecting changes in surface and composition.

A new instrument for angle-resolved two-photon photoemission with exceptional sensitivity and energy resolution has allowed a detailed examination of the interaction of image-state electrons with adsorbates. In addition to measuring the electrostatic properties of molecular-thickness films, the technique serves as a probe of adsorbate growth modes, and provides new opportunities to explore the dynamics of electrons in well-controlled 2D systems.

Femtosecond time resolved two photon photoemission and above- threshold photoemission (ATP) have been used to characterize the dynamics of photoexcited electrons at single crystal Cu surfaces. The two photon photoemission studies measure nonradiative relaxation pathways of electrons near the surface, and the ATP studies demonstrate that photoemission occurs even when using light that is far below the work function. These studies provide important information regarding the extent and duration of the interaction of photoexcited electrons with surface adsorbates.

This work presents a new method to produce 3D structures, using a CO₂ laser and thermosensitive resins. This method is called thermal stereolithography and is based on the spatially selective thermal curing of a resin, a filling material and a catalyst. The results obtained with a physical model for the resin system indicates that filling material must be added to control a pressure wave that is thermally induced. A resolution of 0.1 mm was achieved for epoxy and polyester resins. The successive layering of a laser scanned spot can produce 3D solid structures out of a high viscosity liquid.

The structure of W(110) surface covered by submonolayer of C or Ba atoms is studied by thermal He diffractive scattering. For C different preparation methods give (2 X 2) and R(3 X 5) tungsten carbide structures. For Ba a periodic structure with 25 angstrom lattice spacing is observed.

We report the collision-induced desorption and dissociation of two molecular forms of ammonia bound to a Pt{111} single crystal by a beam of translationaly energetic, neutral noble gas atoms. In this experiment the probability for desorption at low coverage is measured as a function of the collision energy between an Ar atom and chemisorbed ammonia molecule and is found to be independent of the Ar beam angle of incidence. At higher initial ammonia coverages the probability of collision-induced desorption is greater since the NH₃/Pt{111} binding energy is much weaker. In addition we observe formation of surface NH_x products generated in a process known as collision-induced dissociation.

We have applied the technique of second harmonic generation to the study of the solid/vapor interface of porous toluene/water ice films at low temperature. The ice samples are condensed from the vapor onto a cold support at 90 K and are typically approximately 500 micrometers thick with an internal surface area of approximately 200 m². The ices are transparent to both the 532 nm probe laser and to the 266 nm second harmonic signal. Second harmonic light is generated from surfaces throughout the bulk of the porous sample. Significant differences in the efficiency for second harmonic generation from water and toluene enable the species specific study of annealing and adsorption in this binary system. Measurements of the coverage dependent adsorption of toluene on water ice, and the observation of a coverage dependent liquid-solid phase transition in a few monolayers of toluene adsorbed on water ice at temperatures below the bulk toluene melting temperature are presented.

The possibility of ZnSe/GaAs epitaxial layer enhancing by means of soft X-ray irradiation is reported. High intensity soft X-rays in waverange 80-120 A are produced by laser plasma source with waveguide X-ray optics. The X-ray diffraction data and photoluminescence spectra excited by He-Cd laser (T=4.2 K) are represented.

The objective of this report is aimed on both theoretical and experimental study of second harmonic generation at light reflection by a metal surface. The theory includes the effect of spatial dispersion, and the experiment exploits Nd:YAG laser to probe silver, gold, and copper sample. The comparative analysis shows nice agreement between the experimental data and results given by the theory developed here.

This work is devoted to the investigation of laser-induced decomposition mechanism of solid films and powders of nickel(II) chelate complexes with dimethylglyoxime Ni(DMG)2 and with dehydrogenated free—radical methyl derivative of 1,2—hydroxylaminooxime Ni(HAO)2. A comparison of stable gaseous products of laser-induced decay of these complexes with those of thermal decomposition have shown considerable difference between them. Sufficiently threshold character of the conversion degree dependence on the power density of XeCl excimer laser irradiation was revealed in the experiments. These two facts being correlated with the data on positive and negative ion chemical ionization mass spectrometry allow to propose a primary process to be predominantly photochemical with two-photon initiation mechanism. The vertical photoionization of one of chelate molecules is suggested to be accompanied by electron transfer to lattice and then by localization of excess electron on antibonding level of neighbouring chelate molecule resulting in its fragmentation. The secondary processes of solid chelate decomposition are determined by the pecularities of electronic and stereochemical structure of the complexes. Thin layers of these chelates applied onto Si and Si02 substrates decompose explosion-like in air under the influence of radiation pulse of XeC1 excimer laser. A plume of decomposition products is thrown from the substrate into atmosphere, and a thin layer of dielectric film with refractive index close to the value for nickel hydroxide is deposited on the substrate.

We present the results of a theoretical investigation of the fluorescence spectrum from the gas of two-level atoms strongly driven by an evanescent wave (EW). The mean time-of-flight of atoms across the EW is supposed to be less than the relaxation times in the gas volume. Assuming the atomic absorption line to be Doppler-broadened we analyze two specific cases: (1) the atoms over the whole Doppler contour are excited by the EW in the adiabatic-following regime and (2) the EW 'burns out the hole' inside the absorption line. In case (1) in addition to the well- known Mollow triplet the line at the atomic transition frequency appears due to the transient behavior of the atoms departing the surface. In case (2) the spectrum has a sub-Doppler structure sensitive to the type of the atom-surface scattering (diffuse, specular or inelastic). In both cases the spectrum depends on the sticking probability and phases of the scattering in the ground and excited states.

The process of charge formation of quasi—one-1eve1ed atomic particle outgoing from a solid body surface in an electromagnetic field is investigated on the basis of the developed "quasienergy" approach.

The nonlinear optical properties of a metal irradiated by a high power laser radiation are theoretically analyzed. It is shown that in the radiation field the deflection of the distribution function from the equilibrium one leads to sharp relative increase of the image part of the nonlinear current and, hence, to the nonlinear optical response of the surface.

IR-visible sum-frequency generation as a versatile surface vibrational spectroscopic tool is described. Applications to neat water interfaces and hydrogen adsorption on diamond are used as illustrative examples.

In analogy to the linear optical method optical rotatory dispersion (ORD) which is sensitive to chirality, a surface second harmonic generation (SHG) experiment with plane polarized light is conducted to study a monolayer of chiral molecules at various interfaces. The R- and S- enantiomers of 2,2'-dihydroxy- 1,1'-binaphthyl (BN) adsorbed at the air/water, air/quartz, and liquid/liquid interfaces are studied. Using p-polarized fundamental radiation, the polarization of the SHG signal is rotated by a value (Phi) according to the surface chirality. The rotations are large: tens of degrees from a monolayer of material. R-BN and S-BN give (Phi) values of the same magnitude but opposite signs. The orientation, spectral and concentration dependences of the SHG rotation are explained in terms of the electric dipole-allowed second order nonlinear tensor. The experiments show that while SHG-ORD has origins quite distinct from ordinary ORD, it can be utilized to measure chiral structures of surfaces.

We report the results of low energy helium atom scattering studies of the surface structure of n-alkane thiol overlayers on gold surfaces. We show that the primitive unit mesh of conventionally prepared self-assembled monolayers is not a simple ((root)3X(root)3)R30deg hexagonal mesh but is in fact a 8.65X9.99 $angstrom rectangle [corresponding to a c(4(root)3X2(root)3)R30deg overlayer]. We also discuss the results of recent molecular-beam-deposition experiments which produce ordered (5(root)3X(root)3)R30deg and rectangular (root)3Xn (n$AP10) overlayers, structures quite distinct from the solution-grown c(4(root)3X2(root)3)R30deg structure. Finally, we report thermally-induced reorganization of conventionally prepared self-assembled c(4(root)3X2(root)3)R30deg monolayers into (5(root)3X(root)3)R30deg and (root)3Xn (n$AP10)lattices. These experiments suggest that the variety of structures observed results from dependence of the molecular packing upon coverage. Our observations demonstrate that the ordering of thiols on gold is not as simple as early studies have proposed; rather, adsorption of these molecules on gold results in a rich variety of structures, depending upon the coverage and, possibly, the means of preparation.

Quantum state resolved velocity distributions of NO desorbing from single crystal metal and oxide surfaces after a non-thermal excitation process with UV-photons have been studied by several groups in the past. In order to achieve a 'complete' experiment it is necessary to determine in addition the spatial distribution of the desorbing particles. We report on results of the determination of angular distributions of desorbing NO via a new experimental setup. Two systems have been studied in detail, i.e. NO on NiO(100 and NO on NiO(111). A model proposed before is employed to explain the experimental results, i.e. the observation of bimodal velocity flux distributions. The bimodal signal is consistent with the existence of two desorption channels which are predicted to exhibit different angular distributions. This prediction is verified with the new experimental setup. The comparison of the two crystallographic planes of NiO allows us to address the problem of the influence of the magnetic properties of the substrate onto the population of different spin states of desorbing NO molecules. Finally we shall report on results gained with a CO detection system based on a (1+1') REMPI process employing VUV photons. Here the system CO/Cr₂O₃(111) is studied. With this setup the resolution of rotational states in the desorbing particles is easy to achieve in contrast to the widely used (2+1) REMPI process of the same transition.

Three topics illustrate some central physics of processes produced by UV-laser and low-energy electron stimulation. First, a multi-dimensional ground-state potential energy surface (PES) for NH₃:Pd(111), computed using ab initio local-density functional theory, allows dramatically different dynamics depending on poorly-known excited-state forces. We use quantum- resolved experimental data to argue that stimulated desorption is dominated by a direct path off the surface, following placement of the wavepacket on a molecule-surface hard wall accessed by internal molecular motion. This illustrates the questionable relevance of 1D models for understanding molecule-surface dynamics. Second, we study the image-charge model of excited state forces experienced by ions produced, for example, by hot carrier attachment. Ab initio results show that this model totally fails at molecule-surface distances typical of chemisorption. Finally, we present a purely-electronic adiabatic model of excited state PESs and use it to argue that, if significant covalent interactions occur between an adsorbate and a surface, hot carrier attachment does not simply produce singly- charged ions. Instead, attachment creates excitation of the molecule-surface bond occur and, in some cases, may result in multiply-charged ions.

The photodesorption of Na atoms from Na clusters deposited on dielectric surfaces is investigated via pulsed laser excitation and cw two-photon laser-induced fluorescence detection. The combination of pulsed excitation and cw detection within the focii of two counterpropagating lasers provides high spatial resolution (allowing one to obtain accurate angular and kinetic energy distributions) while at the same time preserving the high temporal resolution given by the pulsed laser. Moreover, spatial overlap of desorption and detection lasers on the surface facilitates to follow directly the conversion of initial surface plasmon excitation into bond-breaking in the clusters or phonon excitation in the substrate. It is found that the final step of photodesorption of Na from large Na clusters bound to mica surfaces can be described via multiphonon scattering of the desorbing particles from the surface of the clusters (with nearly zero initial kinetic energy). By use of lithium fluoride as the supporting substrate it is observed that the activation energy for photodesorption from the clusters depends on the Debye temperature of the supporting substrate. This emphasizes the need to include coupling between clusters and substrate in order to understand the overall desorption process.

Time resolved IR spectroscopy is used to monitor the stretch mode of CO on Cu(111) following femtosecond visible excitation. A time dependent shift in the stretch mode complex frequency is observed and is attributed to low frequency adsorbate-like modes becoming excited via coupling to substrate electrons and phonons. At low levels of excitation it is found that the frustrated translation is coupling to the bulk electron reservoir with a rate (gamma) _eequals167(24) GHz, and to the bulk phonon reservoir with a rate (gamma) _Lequals145(56) GHz. At higher excitation levels deviations from the model are observed.

Recent measurements of the photodesorption of isotope-labeled ammonia from GaAs(100) provided the first evidence for UV photodesorption from electronically-quenched, but vibrationally- excited adsorbates. The present study characterizes the quantum state distribution of photodesorbed NH₃ using resonance- enhanced multiphoton ionization spectroscopy and time-of-flight spectroscopy. The translational and rotational distributions can be approximated by Boltzmann temperatures of 300+/- 20 and 350+/- 50 K, respectively, with a relative population of 6+/- 2 % in the (nu) ₂equals1 vibrational level, corresponding to a vibrational temperature of 490+/- 60 K. The observed nonthermal distribution, in conjunction with measurement on coadsorbed isotope mixtures, rules out the possibility of a resonant heating mechanism, which was proposed in earlier IR laser induced desorption studies. It suggests that UV photo-induced vibrational predesorption of NH₃ from GaAs occurs on a time scale shorter than that for vibrational relaxation to substrate phonons. Possible energy transfer mechanisms responsible for desorption are discussed.

The paradigm of surface femtochemistry, laser induced desorption of molecules adsorbed on metal surfaces is considered, first from a `traditional' point of view and then from a contemporary one, suggested by experiments over the past five years. The key is laser excitation of substrate electrons followed by inelastic resonant scattering from the adsorbate. Theory is developed within the framework of Gaussian wavepacket dynamics.

The processes of desorption and dissociation for O₂ on Pd(111) under femtosecond laser irradiation have been investigated. Desorption is characterize by a high yield, a nonlinear fluence dependence, and a dominant subpicosecond feature in two-pulse correlation measurements. These observations are consistent with a process driven by the high substrate electronic temperature produced by the femtosecond laser pulse. The correlation measurements also reveal the existence of a weaker feature persisting >10 ps which is attributed to an enhancement of the desorption rate by adsorbate vibrational excitation. Under the same conditions where efficient desorption is occurring, an upper limit of 5% is found for the dissociation of molecular oxygen. This is in contrast to the high branching ratio for dissociation found in thermal activation and conventional photoactivation for the same system. Explanations for the anomalous branching ratio in the femtosecond surface chemistry for O₂/Pd(111) within a model involving multiple cycles of electronic excitation are examined.

The 308 nm laser induced photodesorption of physisorbed and chemisorbed O₂ on Pt(111) has been examined. Photodesorption of physisorbed O₂ occurred with 9 X 10^-20 cm² cross section, and exhibited a sharp approximately cos³⁴$ angular distribution and a structured O₂ translational energy distribution, P_(delta )(E_(Tau )), which extended to E_(Tau )equals1.2 eV. In contrast, photodesorption of chemisorbed O₂ occurred with 1.8 X 10^{-20 cm(superscript 2} cross section, and exhibited a broad approximately cos$ angular distribution and a O₂P_(delta )(E_(Tau )) which extended to only E_(Tau )equals0.8 eV. Photodesorption mechanisms involving photoexcitation of substrate electrons are discussed.

This paper demonstrates that electronically resonant inelastic electron scattering can be used to probe the vibrational dynamics of chemisorbed species, and that the formation of a transient negative ion during scattering can lead to significant enhancement in the probability for vibrationally inelastic scattering. We specifically examine CO/Ni(111) chemisorbed in a c(4X2) structure. Our measurements consist of the incident energy dependencies for vibrationally inelastic scattering from different adsorbate modes, as well as the final angular distributions for electrons emerging from the transiently formed resonance. We also observe the presence of weak first overtone scattering for the CO intramolecular stretch under resonant scattering conditions. All of the observations are consistent with the formation of a (Sigma) shape resonance in the vicinity of 18 eV which is slightly lower in energy, and has a shortened lifetime, than in the gas phase. Discussions are presented which emphasize that the presence (or absence) of vibrational excitation in a given vibrational coordinate following negative ion formation can be used to infer important details about femtosecond nuclear coordinate evolution for the system in the excited state.

Photodissociation dynamics of N₂O adsorbed on Si(100) has been compared with that on Pt(111). N₂O is adsorbed molecularly on both of the surfaces at lower than 95 K. Upon the irradiation of excimer laser pulses at 193 and 248 nm, adsorbed N₂O is dissociated to oxygen and N₂. While oxygen remains on the surfaces, N₂ desorbs from the surfaces. Translational energy distributions of N₂ are measured by time-of-flight (TOF) spectroscopy. The TOF distributions of N₂ desorbing from Si(100) as well as Pt(111) show nonthermal multiple velocity components. There are some differences in the N₂ desorption characteristics between the two cases. In particular, the translational energy distribution of N₂ fragments from Si(100) depends on the desorption angle. Furthermore, the N₂ desorption from Si(100) is peaked at approximately 30 degree(s) from the surface normal. These peculiar features observed in N₂O photodissociation on Si(100) are discussed in relation to the adsorption structure of N₂O.

The mass spectral fragmentation of l-tryptophan has been investigated as a function of desorption technique and photoionization method using 7 keV Ar⁺ static sputtering, 355 nm pulsed laser-induced desorption, and thermal desorption at 150 to 200 degree(s)C in conjunction with 118 nm (10.49 eV), 266 nm (4.66 eV), and 355 nm (3.50 eV) laser positionization. In addition, ion-stimulated desorption of neutrals has been compared with positive secondary ion data. Molecular fragmentation is dominated by the internal energy contribution of the desorption process rather than photoionization method; fragmentation is maximal with ion-stimulated desorption and minimal with thermal desorption. Furthermore, single-photon ionization with 118 nm generally results in less fragmentation than multiphoton ionization (MPI). Finally, the effect of laser pulse width on the 266 nm MPI of thermally-desorbed neutral species has been explored. The use of 35 ps rather than 5 ns laser pulses at 266 nm has been explored. The use of 35 ps rather than 5 ns laser pulses at 266 nm has been found to cause a decrease in the molecular fragmentation of l-tryptophan. These results have implications for the surface analysis of labile organic compounds.

We have demonstrated the use of a standing-wave laser beam to focus chromium atoms as they deposit onto a silicon surface. A permanent array of Cr lines has been fabricated, with line width 65 nm, spacing 213 nm, and height 34 nm. The array covers an area of 0.4 mm X 1 mm, and was deposited in approximately 10 minutes. The lines made in this way constitute a proof-of- principle of an entirely new approach to nanostructure fabrication, with potential for extremely small feature size coupled with massive parallelism.

Laser-assisted chemical etching reactions of Cl₂ with semiconductor surfaces are investigated using a cw supersonic Cl₂ molecular beam coupled with an angle-resolved time-of- flight (TOF) technique. TOF spectra of reaction products are measured as a function of the detection angle, laser fluence, and normal component of incident molecules' translational energy. Our results show that the differences in the above etching reactions at 355 and 1064 nm radiations can be explained by the different mechanisms of laser-stimulated desorption process. The flux of the reaction products linearly increases with increasing the normal component of the incident translational energy of Cl₂ molecules. It implies that the mechanism of Cl₂ chemisorption on the surfaces might be a direct translationally activated dissociative chemisorption.

The chemistry, driven by 6.4 eV photons, of ammonia, phosphine, and arsine, adsorbed on GaAs(100), is compared. When molecularly adsorbed XH₃ (XequalsN, P, and As) at low temperatures, ca. 100 K, is irradiated, molecular desorption and hydride bond cleavage occurs. The branching between these two channels varies strongly with X; desorption is favored for NH₃ and As-H Bond cleavage favored for AsH₃. The H atoms within the photochemically formed XH₂ groups can also be removed with photons, but only by extensive irradiation.

The surface modification of semiconductors by laser-induced surface electromagnetic wave etching was investigated. With the novel etching method using a holographic exposure system, submicron periodic dot structures were fabricated directly on semiconductor substrates (n-InP, n-GaAs). Making the best of laser polarization dependence in this etching system, a variety of surface modification could be obtained on the surface of InP and GaAs. Especially, in the case of using s-polarization laser lights, periodic submicron dot structure could be fabricated directly with a single step process without any mask process. The size of dots by the etching depended on the incident laser wavelength, and the smallest size of dots was 80 nm. The exponential growth of the SEW grating with the positive gain was demonstrated experimentally. As the etched depth of the SEW grating approached to the laser wavelength, the saturation of the growth due to self-limiting effect was also demonstrated experimentally for the first time.

Semiconductor nanocrystals containing 10² to 10³ atoms show significant electronic quantum size effects at energies near the band gap. In solid state terms, this happens because of the large delocalization lengths (ie, small effective masses) that naturally occur for electrons and holes at band extrema in bulk crystals. This size dependence also has a clear, physically equivalent explanation in the language of molecular orbitals. Close analogies exist with the spectroscopy of large aromatic hydrocarbons. In this short manuscript, I compare two nanocrystal systems that have become prototypes for direct and indirect gap behavior: CdSe and Si. Despite the fact that both materials are tetrahedrally hybridized semiconductors of similar band gaps, the effects of quantum confinement are quite different.

The dynamics of photocarrier diffusion was studied by a two color transient grating technique in reflection geometry. A single exponential decay feature was observed immediately following the electron excitation pulse and was attributed to band edge carrier diffusion.

The surface sensitive laser technique second harmonic generation has been utilized to probe the adsorption of p-tolualdehyde (TA) at the ice/vacuum interface. The non-Langmurian adsorption behavior can be explained by a model in which the TA initially adsorbs as a weakly bound precursor species that is highly mobile and eventually settles to form a more strongly bound species. The relative sticking coefficient for p-tolualdehyde on the ice surface was found to be 40% smaller than that on a NaCl crystal.

Pulsed excitation, time correlated single photon counting, and time gated detection are used in near-field optical microscopy to enhance fluorescence images and measure the fluorescence lifetimes of single molecules of Rhodamine 6G on silica surfaces. Time gated detection is used to reject prompt scattered background and to improve the image signal to noise ratio. The excited state lifetime of a single Rhodamine 6G molecule is found to depend on the position of the near-field probe. We attribute the lifetime variations to spontaneous emission rate alterations by the fluorescence reflected from and quenching by the aluminum coated probe.