Frequency modulation spectroscopy (FMS) with mid- and near-infrared tunable diode lasers is an attractive approach for a number of applications that require ultrasensitive detection of trace gases. The technique combines high detection sensitivity with high detection speed and is capable of detecting numerous small molecular weight compounds at levels ranging from low parts-per-million (ppm) to sub-parts-per-billion (ppb). We will review the principles of FMS and discuss its application in areas relevant to materials processing.

Applications of flash photolysis with time resolved infrared (TRIR) detection techniques to kinetics investigations of reactive organometallic intermediates are described. The apparatus used a XeCl excimer laser as the pump source and lead salt diode lasers as probe sources with an overall tuning range from 1550 to 2200 cm1. For illustration, flash photolysis studies of some ruthenium and iron carbonyl complexes relevant to the catalytic activation of carbon monoxide and of other small molecules will be reviewed.

Tunnel diodes have been used to burn holes in the N-D stretching bands of NH₃D ions dilute in ammonium sulfate crystals. The ammonium ions sit in sites of low symmetry and, consequently, there are four distinct orientations possible for the NH₃D ion. Since there is low symmetry, each site gives rise to a distinct N-D stretching band. The relative intensities of these bands measure the proportion of ions in each site.

Recent exciting advances have been made using infrared (IR) diode lasers to obtain sensitive and rapid measurements of molecular-level dynamics that control polymer material properties. Infrared diodes provide a bright, nearly monochromatic, polarized source that is excellent for use in polarimetry. Optical polarimetry is the measurement of the state of polarization of light. It is a powerful technique for studying molecular and microstructural anisotropy in a sample by measuring how the sample alters the polarization of a transmitted beam. In polymers, it is precisely such molecular anisotropy that underlies their viscoelastic properties. As theories are developed to predict the relaxation dynamics of individual parts of a polymer molecule, and of different species in polymer blends, there is increasing interest in experimental methods to distinguish these dynamics. Infrared polarimetry is extremely valuable in this regard, not only because of the direct relationship between the IR dichroism and molecular orientation, but also because it is amenable to labeling. The present paper reviews the basis of this technique and its application to study the effects of polydispersity on melt rheology, and the relaxation dynamics at different positions along a polymer chain. It concludes with an overview of research in progress and future problems to be tackled.

Tunable diode laser spectroscopy offers unique advantages for high resolution infrared reflection absorption spectroscopy (IRRAS) studies of monolayers on single crystal surfaces. Its greatest potentials lie in analyses of lineshapes less than 1 cm (FWHM), for resolving narrow features on absorption peaks, and for in situ solid-liquid interfacial studies, where high intensity is often required. Other IRRAS techniques introduce instrumental broadening sufficient to create considerable uncertainty in the intrinsic linewidth of very narrow peaks, and can mask sharp features. Using isolated or single mode output from a diode laser source provides resolution of iO3 cm, and thereby eliminates the need for instrumental deconvolution. However, the signal-to-noise (SIN) ratio is generally lower for spectra obtained with diode sources than with Fr-JR techniques, so that the most important applications using diode lasers have been studies of strong scatterers, such as CO on Pt(1 1 1). For this study, linewidths were measured for the C-O stretch mode for the ontop site of CO adsorbed at saturation coverage on a Pt(1 1 1) single crystal using diode laser sources. The measured linewidth at 100 K, using isolated modes, was 2.3 0.3 cm. The linewidth at 0 K was estimated to be 2.0 0.4 cm from variation of linewidths with temperature. Using the uncertainty principle, the 0 K linewidth corresponds to a lifetime of 2.7 0.6 ps, in reasonable agreement with recent lifetime measurements using time-resolved techniques.

Despite its toxicity, arsine remains a "workhorse" in the field of semiconductor research because it provides a relatively clean (i.e. carbon-free) means of delivering atomic As to a growth surface. However, the feasibility of using alternative arsenic sources is currently being investigated by the semiconductor community. Two liquid metalorganic alternatives, triethylarsenic and monoethylarsine, contain carbon that can appear as a significant impurity in the semiconductor material itself. We present results on the laser-induced photochemistry of the above compounds. Our efforts are focused on using lasers to generate and detect atomic hydrogen, a species that is known to be a good radical scavenger in growth environments. We also present results on the laser ablation of inorganic salts that may be useful as precursors for 111-V thin-film growth. Compounds such as K₃Ga₃As₄ and K₂Ga₂Sb₄ are irradiated with various excimer laser wavelengths, and we report on the prognosis for viable film growth using this approach.

Upon irradiation with laser light of specific wavelengths and intensities optical fibers can undergo a slow (minutes to hours) permanent change, which results in new optical phenomena that were not present before irradiation. These phenomena include photoinduced refractive index gratings^1,2 and photoinduced second harmonic generation^1-4 in doped silica fibers. Since these phenomena are the result of a photoinduced periodic modification of the optical properties of the material with a periodicity that is determined by the wavelength(s) of irradiation, they are examples of photoinduced organization in glasses. Although the physics of these phenomena is not understood in detail, there is ample experimental evidence that the presence of optically active centers in the glass is of crucial importance for the observation of these phenomena. The optically active centers are due to defects or dopants in the glass. Here we discuss photoinduced SHG in Ge-doped silica fibers and the use of luminescence spectroscopy to characterize different types of Ge-related defects in the fiber.

Polarized Raman spectra of spin-oriented and drawn fibers of high-tenacity polyethylene and polyethylene terephthalate have been measured in order to correlate bulk properties with microscopic structural order. Band-fitting is pivotal to separating components of closely spaced lines. In the case of polyethylene terephthalate, glycol bands just below ^~1100 cm-i correlate in intensity with orientation as derived front optical birefringence. Separate bands can be correlated with the trans crystalline and trans amorphous phases. Polarized spectra of a fiber of polyethylene with high orientation and crystallinity can be used as a basis with which to compare partially crystalline materials. Since the amorphous phase of oriented partially crystalline polymers determines the practical strength of these fibers, the Raman data that provides information otherwise unobtainable can be useful in their characterization.

Multiresonant nonlinear spectroscopy has been used to achieve narrowing of inhomogeneously broadened spectral transitions. Multiply enhanced nonparametric spectroscopy (MENS) has been used to observe the narrowing in pentacene doped benzoic acid. The signal dependence of pentacene and benzoic acid lines on sample absorbance, detuning and laser intensity are studied. Recent papers from our group have established that nonlinear spectroscopy offers unique capabilities for dissecting individual spectral features of components in mixtures, narrowing inhomogeneously broadened spectral bands, and measuring mode mixing between vibrational states [1-22]. This paper will present additional information that complements previously published work. The reader is encouraged to consult the original literature for the main body of the work.

This paper presents an experimental study of the influence of some organic solvents (toluene, carbon tetrachloride, benzene, absolute methanol) and concentrations of the suspensions containing oxizincquinolate and POPOP in the above mentioned organic liquids in the third harmonic generation (THG) and the induced fluorescence by three-photon processes.

An investigation of the time-resolved photoluminescence decay in glow-dischargingly deposited hydrogenated amorphous carbon is presented, the dependence upon substrat temperature, emission energy, and excitation energy has been investigated. Our measurements suggest the exciton recombination rather than the pair process. The photoluminescence decay time decreases with increasing substrat temperature, as result of enhancing nonradiative recombination. Comparisons with hydrogenated amorphous silicon are also made.

The dynamics of the carrier recombination in a-Si:H/a-SiN₃:H multilayers has been investigated with picosecond photoluminescence spectroscopy. The thermalization of photogenerated carriers is a direct hopping process. The decay time cutoff, the mobility edge and the bandtail width vary non-monotonously with a turning point near x=O.85, which is attributed to the changes of the build-in field and the multilayer structure with nitrogen content.

The results show that some nonlinear interaction procses (x2 and x3) occur in LiB₃0₅, the upper limits of temporal parameters of the refractive index grating are 35ps, and no optical coupling effect is observed.

Basing on the results of the ESR, RS and PL, we propose the following structural model for polyacenic conducting polymer prepared by pyrolysis of model phenolformaldhyde (PF). Sample is consist of condense aromatic rings in short range order in which sp² and sp³ two phase carbon exist. Small sp² cluster will directly couple forming extented larger sp² cluster by carbonating and nucleating with the rise of Tp. The stability of sp³ bond is bad so that it would easily change to the stable sp² bond in condense aromatic rings. The weight of the sp³ bond carbon get small with Tp rising. On the other hand, larger size sp² cluster form resulting in electrical conductivity increasing and thermal active energies decreasing. We suggest that the size of the condense aromatic rings is proporational to Tp. Small (large) cluster has large (small) optical gap. PL spectra shift to lower energies with the rise of Tp. The PL intensity of the samples is correlated with the content of hydrogan, the number of the dangling bonds and the weight of the sp³ bond carbon.

Recent advances in detector technology have enabled sensitivity limitations inherent in surface Raman scattering from thin organic films on metal surfaces to be overcome. The application of both surface enhanced Raman scattering (SERS) and normal Raman scattering of organic monolayer films on metal surfaces in electrochemical and ambient environments is an area of active research in this laboratory. The use of surface selection rules for the development of detailed molecular pictures of these interfaces is discussed in this report. The application of these rules for determination of the orientation of self-assembled monolayers on Ag and Au surfaces in both electrochemical and ambient environments is used to demonstrate the approach. In addition, extension of surface Raman methods to the study of monolayer organic films on Pt surfaces is reported.

The nonlinear optical process of second harmonic generation (SHG) is an inherently surface sensitive technique for studying the interface of two centrosymmetric media. This surface selectivity has led to its application as an in situ probe of chemisorption, molecular orientation, and adsorbate organization at interfaces. This paper describes the use of resonant SHG to measure molecular adsorption and orientation at solid-air, solid-liquid, and liquid-liquid surfaces. The polarization and phase dependence of the resonant SHG from molecules at these surfaces can be related to the average molecular orientation at the interface. Perturbation theory calculations using pi-electron wavefunctions are used to identify the molecular nonlinear polarizability tensor elements required in the orientation calculation.

Monolayer films of alkanethiolates CH₃(CH₂)_nSH at Au(111) films on mica were examined by scanning tunneling microscopy (STM) (n equals 1,9,17) and AFM (n equals 1 - 17). The resulting atomically resolved images reveal the packing arrangement of the overlayer. Observed images correspond to a hexagonally packed array of adsorbates with respective nearest- and next-nearest-neighbor spacings of 0.50 +/- 0.02 nm and 0.87 +/- 0.04 nm with STM and 0.52 +/- 0.03 nm and 0.90 +/- 0.04 nm with the AFM. This packing agrees with the expected ((root)3 X (root)3)R30 degree(s) adlayer structure of the adsorbate. We believe the STM images reflect the arrangement near the gold-bound sulfur interface, whereas the AFM images reveal the arrangement of the alkyl chains.

A silver-coated capillary substrate has been prepared by means of chemical reduction of silver ions. Its performance as a surface-enhanced Raman scattering (SERS) active substrate has been compared with those of chemically prepared silver/glass and of silver islands using p- chlorobenzoic acid (PCBA) as an analyte. The adsorption behavior of PCBA on or near the surface of silver films has partially been characterized. The dependence of SERS intensity on the pH and the concentration of the analyte solution may be interpreted on the basis of chemical enhancement mechanism.

Coherent anti-Stokes Raman spectroscopy (CARS) in conjunction with a two-stage light-gas gun has been used to obtain vibrational spectra of shock-compressed liquid N2, 02, CO, and their mixtures, as well as liquid N20. The experimental spectra are compared to spectra calculated using a semiclassical model for CARS intensities to obtain vibrational frequencies, peak Raman susceptibilities, and linewidths. The derived spectroscopic parameters suggest thermal equilibrium of the vibrational populations is established in less than a few nanoseconds after shock passage. Vibrational temperatures obtained are compared to those derived from equation-of-state calculations. The variation of the vibrational frequency shift at pressure with species concentration in mixtures is investigated.

Multiresonant, nondegnerate, four wave mixing spectroscopy involves three simultaneous resonances that can be used to achieve spectral selectivity in chemical measurements. There are nonlinear analogs to the familiar Raman, infrared, absorption, and fluorescence. Unlike the familiar methods, the multiple resonances of the nonlinear methods can be used to dissect the spectra into the modes and sites that are responsible for the spectral structure.

We have used the novel technique of absorption detected magnetic resonance (ADMR) to investigate the photoexcitation properties in conducting polymers and fullerene C₆₀ films. In conducting polymers we have identified both charged and neutral long-lived photoexcitations with distinct different properties in polymers with degenerate (trans polyacetylene; t-(CH)_x) and nondegenerate (polyparaphenylene vinylene; PPV) ground state backbone structure. In t-(CH)_x the photocarriers are charged solitons and the neutral photoexcitations are spin ½ soliton pairs, whereas the photoexcitations in PPV form charge bipolarons and triplet excitons. The neutral photoexcitations in C₆₀/polystyrene "glass" are triplet excitons identified in ADMR through the triplet "powder-pattern" around g = 2.003.

The detection of CH* radicals was made by the laser induced fluorescence method. CH* radicals were formed through chemical sputtering processes of isotropic graphite targets irradiated by hydrogen ions in the low keV range and at temperatures around 700K. Fluorescent spectra of CH* radicals induced by a flashlamp pumped dye pulse laser were obtained by a photo-multiplier tube as a function of wavelength. The detailed structure of the chemical and electronic states of CH* radicals can be extracted from highly resolved fluorescent spectra.

A new method to combine aqueous sample introduction with matrix assisted laser desorption mass spectrometry (MS) for interfacing liquid-chromatographic techniques, such as capillary electrophoresis, to MS is described. Aqueous sample solution is introduced directly into the ion source of a time-of-. flight (TOF) mass spectrometer through a fused silica capillary; evaporative cooling results in ice formation at the end of the capillary. The ice can be made to extrude continuously by using localized resistive heating. With direct laser desorption, molecular ions from proteins as large as bovine insulin (5734 Da) can be produced. Two-step desorption/photoionization with a variety of wavelengths is demonstrated, and has the advantages of improved resolution and shot-to-shot reproducibility. Ion structural information is obtained using surface-induced dissociation with an in-line collision device in the reflectron mirror of the TOF instrument. Product ion resolution of ~70 is obtained at m/z77. Extensive fragmentation can be produced with dissociation efficiencies between 7-15% obtained for molecular ions of small organic molecules. Efficiencies approaching 30% are obtained for larger peptide ions.

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29 ≤ n ≤ 45 indicating 3 or 4 different structures. Fullerenes first appear at C₃₀⁺ and begin to dominate above C₄₅⁺. From small to large the isomeric progression is from linear to rings to fullerenes. Preliminary results on some theoretically calculated carbon cluster structures and their simulated mobilities are presented.

State-selected molecular beams of ND₃ in antisymmetric inversion states (formula available in paper) are scattered on a graphite (0001) surface at T₈ = 130K. A preponderance of symmetric inversion states, (formula available in paper), is observed in the directly scattered molecules. The observed effect is an interference on a surface potential between the left and right inversion configurations of a single molecule which are oppositely oriented with respect to the surface. A procedure leading from the experimental data on this molecular self-diffraction to an approximate analytical molecule-surface potential is suggested. This procedure, which is based on the infinite order sudden approximation (Gerber et. al., J. Chem. Phys. 73, 4397 (1980)), requires experimental study of the inversion symmetry change for several initial rotational states as a function of translational energy.

New techniques such as matrix assisted laser desorption, electrospray ionization, and fast atom or ion bombardment make it possible to obtain quasimolecular (e.g. protonated) ions of thermally fragile biomolecules without significant decomposition or fragmentation. These ions can be studied using a variety of mass spectrometric techniques, most powerful among which is Fourier transform ion cyclotron resonance spectroscopy (FT-ICR). One of the important advantages of FT-ICR is the ability to store ions for long periods of time, facilitaüng studies of processes such as bimolecular reactions and laser photodissociation. One significant application of FT-ICR has been to study the acid-base properties of molecules in the gas phase. In this article we demonstrate a new method to probe the acid-base properties of local sites in biomolecules using a novel application of infrared multiphoton dissociation. The usual method of studying proton transfer reactions with appropriate reference bases does not work well with large molecules due to their propensity for radiative bimolecular cluster formation. The difficulty which this introduces can be circumvented by dissociating the proton bound dimer by infrared multiphoton excitation with a cw CO2 laser. Infrared heating assists dissociation alongthe lowest energy pathway and fragments the dimer to leave the proton preferentially on the more basic site. We illustrate this technique by demonstrating that the proton affinity of N-acetyl glycine is intermediate between glycine and alanine.

Sources of laser-induced even-even and odd-even isotopic selectivity in the resonance ionization mass spectroscopy of Os and Ti have been investigated experimentally for various types of transitions. A set of conditions with regard to laser bandwidth and frequency tuning, polarization state and intensity was obtained for which isotopic selectivity is either absent or reduced below the 2&percent; level.

Vibrational super-equilibrium excitation and simultaneously effect of rotational cooling have been measured for CO₂ molecules desorption from titanium foil surface by methods of diode-laser spectroscopy. CO₂ molecules were desorbed when foil was heated up by current impulse. CO₂ molecules binding energy with titanium surface approximately is equal to 0.22eV. The vibrational and rotational temperatures are equal to 100±10 K and 220±20 K accordingly, for the surface temperature range 120 K-180 K.

The intensity of surface enhanced Raman scattering (SERS) from benzoic acid and benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a strong dependence on the rate at which the Ag film was deposited. Slowly deposited Ag films give a superior SERS response, by up to a factor of 10, to quickly deposited films, with films deposited at an intermediate rate yielding intermediate results. Careful examination using STM indicates that little distinction can be made between the differently prepared films in terms of surface roughness. By contrast TEM measurements reveal that the average metal grain dimension in slowly deposited Ag films is about 3-4 times greater than that in their quickly deposited counterparts. On the premise that the fundamental excitation of importance to the enhancement mechanism is the surface plasmon polariton (SPP) it is argued that the contrast in Raman scattering efficiency is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities. Corollary data on elastic SPP-photon scattering obtained in a related experiment are also presented.

The vibration spectra of NAB and self-frequency-doubling NYAB laser crystals were measured and analysed. It is pointed out that the V₃(E') mode of BO₃^3- groups may play a role as the "accepting mode" in the nonradiative transition process and the V₁(A_1g) mode of Nd0₆ structure as the "promoting mode", and the effect of the latter on the nonradiative transition would be principal.