Focusing effects in nonlinear resonant four-wave mixing (RFWM) MIR and FIR generation

Summary The influence of pump beam geometry in a NH₃ superradiant Raman FIR laser with RFWM generation of MIR emission has been analysed. We have observed that, in general, MIR radiation is emitted in a stimulated configuration, so pump and MIR beam overlap is basically important to exhibit an efficient RFWM. In particular, strong focusing, also in the presence of self-focusing, shows a lowering of the threshold of Raman FIR emission, as expected, plus a reduction of RFWM MIR emission. COMB-Arrich., CRE Frascati.     

Enhanced field emission characteristics of boron doped diamond films grown by microwave plasma assisted chemical vapor deposition

Boron doped diamond films were synthesized on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) technique. The effect of B₂O₃ concentration varied from 1000 to 5000 ppm on the field emission characteristics was examined. The surface morphology and quality of films were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The surface morphology obtained by SEM showed variation from facetted microcrystal covered with nanometric grains to cauliflower of nanocrystalline diamond (NCD) particles with increasing B₂O₃ concentration. The Raman spectra confirm the formation of NCD films. The field emission properties of NCD films were observed to improve upon increasing boron concentration. The values of the onset field and threshold field are observed to be as low as 0.36 and 0.08 V/μm, respectively. The field emission current stability investigated at the preset value of ∼1 μA is observed to be good, in each case. The enhanced field emission properties are attributed to the better electrical conductivity coupled with the nanometric features of the diamond films.     

Raman and IR study of high-pressure atomic phase of nitrogen

Atomic phase of nitrogen has been studied up to pressure 250 GPa and temperature 3300 K using a shear diamond anvil cell. This phase was synthesized both from azide NaN₃ and molecular N₂. The atomic phase has been interpreted as a cubic gauche (CG) structure by means of Raman and IR absorption spectroscopy procedures. The phase transition to CG begins at pressure 50 GPa and room temperature for NaN₃ and at 127 GPa for N₂. Observed pressure dependencies and degeneration of phonon modes, the selection rules for IR and Raman spectra, as well equilibrium pressure between molecular N₂ and atomic phase of nitrogen agree well with theoretical predictions for CG.     

A vibrational analysis of trichloroiodomethane

The infrared and Raman spectra of CCl₃I have been recorded. Band assignments have been made on the basis of the infrared results, and the Raman spectrum has been interpreted in terms of the fundamentals of CCl₃I, with additional bands being assigned to decomposition products of CCl₃I. A Urey-Bradley force constant calculation has been made, transferring results from the earlier work of Ngai and Mann.     

Synthesis and characterization of polycaprolactone-grafted carbon nanotubes via click reaction

Polycaprolactone (PCL) was covalently grafted on the surface of carbon nanotubes by a simple click reaction of propargyl-terminated PCL (propargyl-PCL) and carbon nanotubes (CNTs) containing azide groups (MWNT-N₃). Propargyl-PCL was synthesized by the ring-opening polymerization of ε-caprolactone using propargyl alcohol and stannous octoate. MWNT-N₃ was prepared from MWNT having 2-bromoisobutyryl groups (MWNT-Br) with sodium azide by azidation. The melting temperature of propargyl-PCL was shifted to the high temperature in PCL-grafted MWNT. The thermal stability of PCL-grafted MWNT was enhanced as compared to that of propargyl-PCL. PCL was coated on the surface of MWNT with a high density of PCL chains, which showed good solubility of PCL-grafted MWNT in organic solvents. PCL-grafted MWNT was characterized with FT-IR, ¹H NMR, Raman, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscopy.     

Nanocrystal and interface defects related photoluminescence in silicon-rich Al2O3 films

In this study, silicon nanocrystal-rich Al₂O₃ film has been prepared by co-sputtering a silicon and alumina composite target and subsequent annealing in N₂ atmosphere. The microstructure of the film has been characterized by infrared (IR) absorption, Raman spectra and UV-absorption spectra. Typical nanocrystal and interface defects related photoluminescence with the photon energy of 1.54 (IR band) and 1.69 eV (R band) has been observed by PL spectrum analysis. A post-annealing process in oxygen atmosphere has been carried out to clarify the emission mechanism. Despite the red shift of the spectra, enhanced emission of the 1.69 eV band together with the weak emission phenomenon of the 1.54 eV band has been found after the post-annealing. The R band is discussed to originate from silicon nanocrystal interface defects. The IR band is concluded to be a coupling effect between electronic and vibrational emissions.     

Surface enhanced Raman scattering from tris (bipyridine) ruthenium (II) adsorbed on silver-modified p-GaAs

Both surface enhanced Raman scattering (SERS) and surface enhanced resonant Raman scattering (SERRS) have been observed from Ru(bpy)₃²⁺ adsorbed on p-GaAs (1 0 0) after the electrodeposition of Ag particles onto the semiconductor surface. For the enhancement factor for SERS a lower limit of 10⁴ has been determined.     

Minerals from Macedonia. XXII. Laser‐induced fluorescence bands in the FT‐Raman spectrum of almandine mineral

Two strong bands centered at 446 and 607 cm⁻¹ have been observed in the FT‐Raman spectrum of almandine [Fe₃Al₂(SiO₄)₃] excited with 1064 nm, which were completely absent in the corresponding dispersive Raman spectra obtained using 488, 514.5 and 532 nm excitation. Furthermore, the mentioned strong bands have not been registered in the anti‐Stokes side of the FT‐Raman spectrum, and were therefore assigned to laser‐induced fluorescence bands. Their appearance is related to the presence of rare‐earth element traces as impurities in the almandine sample. Additionally, the FT‐Raman (and dispersive Raman) spectrum of the isomorphous spessartine [Mn₃Al₂(SiO₄)₃] mineral has been introduced, which did not show the presence of these fluorescence emission bands. The purity of the minerals was confirmed by study of their powder X‐ray diffraction (PXRD) patterns. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrical performance and Raman scattering characterization of GaAs planar photoconductors

Raman scattering has been used to characterize the effects of processing techniques on the photosensitive area of GaAs planar photoconductors. The longitudinal optic (LO) phonon observed in the first-order Raman spectra was studied to probe the consequences of both chemical and plasma etching on the crystalline quality of an n-doped GaAs photosentitive area. This method has also been used to examine the stress at the interface of a Si₃N₄ film and an n-doped GaAs surface. For this purpose, three specially designed GaAs planar photoconductors (SN 353, AN 344, SN 344) have been fabricated. Electrical measurements on the photoconductors support the results of Raman study. First, as expected, increasing the number of processes leads to slightly poorer electrical performance. Secondly, a significant strain-induced shift in the GaAs LO phonon frequency has been observed under the Si₃N₄ film for the AN 344 photoconductor. However, the electrical performances of this device clearly indicate no dramatic change in the value of the n-doped surface potential.     

Surface plasmon polariton contributions to strokes emission from molecular monolayers on periodic Ag surfaces

Surface enhanced Raman scattering from molecular monolayers adsorbed on a holographic grating and covered by Ag has been studied. The coupling of surface plasmon polaritons to light through the grating produces emission similar to the continuum observed in other surface Raman experiments. The angular dependence of the surface Raman scattering shows that enhanced Raman cross-sections can arise from scattering of surface plasmon polaritons into light by the localized molecular vibrations.     

Oxygen permeation properties and the stability of La0.6Sr0.4Fe0.8Co0.2O3 studied by Raman spectroscopy

A partial oxidation test was performed using a La_0.6Sr_0.4Fe_0.8Co_0.2O₃ (LSFCO6482) membrane in the surface limited conditions at a temperature range of 800–920 °C and in a syngas side atmosphere of CH₄ diluted with Ar (100%, 25.6% and 1%) in order to investigate chemical stability and oxygen permeation properties. On the surface, deposited carbon was observed in the test and it was revealed to be graphite by an in-situ Raman measurement. It was combusted by oxygen fed through the membrane. Segregation of the membrane on both surfaces was observed in all specimens after the test. Moreover, significant decomposition occurred on Air side surface of the membrane which tested in a syngas side atmosphere of 25.6% and 100%. However, the significant decomposition did not affect to be oxygen permeation properties, but it did affect its mechanical strength.     

New phenomena related to pulsed far-infrared superradiant and Raman emissions

We report on new effects in relation to optical pumping of far-infrared (FIR) superradiance and Raman emissions in CH₃F, CH₃CN, D₂O, NH₃ by rapidly truncated 10 μm CO₂ laser pulses and optical-free-induction decay (OFID) 30 ps-10μm-CO₂ laser pulses. Thus, we have found a drastic reduction of the FIR-pulse duration which is closely related to the fast truncation of about 10 ps of the plasma shutter used in our OFID 10 μm-CO₂ laser system. The forward emissions exhibit the on-set of swept-gain superradiance and the appearance of Raman emission with increasing pressure in the FIR cell. This implies line competition between the superradiant and Raman emissions and thus results in different emission regimes which we have investigated systematically. The backward emissions are superradiant over the whole range of our investigations and show high quality with respect to stability and reproducibility which is important for applications.Furthermore, we have found that the CO₂-pump radiation and the generated FIR Raman emission interact mutually which results in anticorrelated fluctuations of the two fields. We have been able to interpret this effect as a result of periodic back-and-forth fluctuations of the Λ-like three-level molecular systems.Finally, we have observed the development of OFID of the truncated CO₂-pump pulses in the FIR-laser gases. This effect has been thoroughly investigated and as a result we have generated for the first time ps-10 μm-CO₂-OFID pulses with FIR-laser gases as spectral filters instead of the usual hot CO₂ gas. New phenomena and advantages of our new OFID system based on FIR laser gases are discussed.     

Raman spectroscopy study of REH3 under pressure

The Raman spectroscopic studies of two rare earth trihydrides: Y H₃, HoH₃, have been performed in the pressure range from ambient up to 16 GPa and 25 GPa respectively. For the first time samples of REH₃ in the form of powder have been studied by Raman spectroscopy using the Diamond Anvil Cell (DAC) technique. A rapid decrease of Raman activity has been observed for the hydrides under pressure values in the vicinity of structural phase transition. Metallization as a possible reason for the observed dramatic change of the REH₃ Raman activity has been discussed.     

Structure and lattice dynamics of heterostructures based on bismuth ferrite and barium strontium titanate

The surface morphology, lattice parameters, and Raman spectra have been investigated at the stages of successive formation of a three-layer heterostructure consisting of epitaxial layers Ba_0.8Sr_0.2TiO₃ (BST) and (Bi_0.98Nd_0.02)FeO₃ (BNFO). The structural distortions arising from sequential deposition of BST/BNFO/BST layers of equal thickness have been determined using X-ray diffraction. It has been found that the degree of tetragonal distortion of the BST film on MgO increases after deposition of the BNFO film on the BST film surface, indicating the appearance of compressive stresses in BST. Based on the analysis of polarized Raman spectra, it has been shown that, in the BNFO layer, there is a new phase state which is not observed in the bulk samples. It has been demonstrated that the degree of tetragonality of the BST film grown on the BNFO surface is higher than that of the BST film grown directly on MgO.     

Indirect exciton luminescense and Raman scattering in CdI2

Intrinsic luminescence and Raman scattering in 4HCdI₂ have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.     

Surface‐enhanced Raman scattering of 4‐mercaptopyridine on sub‐monolayers of α‐Fe2O3 nanocrystals (sphere,spindle, cube)

We report surface‐enhanced Raman scattering (SERS) spectra from 4‐mercaptopyridine (4‐Mpy) adsorbed on sub‐monolayers of α‐Fe₂O₃ nanocrystals (sphere, spindle, cube). The maximum enhancement factor has been estimated to be about 10⁴ compared to that of 4‐Mpy in solution. A possible mechanism has been proposed that the charge transfer between the α‐Fe₂O₃ nanocrystals and the 4‐Mpy molecules is most likely responsible for the observed enhancement of Raman intensity of adsorbed 4‐Mpy molecules as surface plasmon resonances have not occurred. Copyright © 2009 John Wiley & Sons, Ltd.     

Spectroscopic identification of the reaction intermediates in the thermal decomposition of trimethylindium at GaAs(100) surfaces

High resolution electron energy loss spectroscopy (HREELS) has been used to study the adsorption and thermal decomposition of trimethylindium (TMIn) on Ga-terminated GaAs(100) surfaces. HREEL spectra recorded for adsorption at room temperature are dominated by strong CH₃ deformation and stretching modes and indicate that the surface species is based on methyl groups. The intensities of these bands decrease with increasing temperature consistent with a primary decomposition route involving the loss of CH₃ groups from the surface. A small upward shift in the frequency of the symmetric and asymmetric CH₃ deformation modes is also observed with increasing temperature and indicates that decomposition takes place via an exchange reaction in which CH₃ groups switch from In to Ga due to the stronger Ga-C bond. At temperatures greater than 350°C, the spectra are dominated by CH₂ rocking, deformation and stretching vibrations. The presence of a surface methylene species at elevated temperatures suggests a second, minority decomposition pathway which involves dehydrogenation of surface CH₃ groups to CH₂.     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

Emission Spectra Inside the Ammonia s-Multiplets Pumped by a Line-Narrowed Tunable CO2 Laser

By using a line narrowed high power tunable laser we have analysed the FIR emission spectrum inside some , compact sQ, sR k-multiplets. Jumps between adjacent k-systems are observed , although tunable emissions of dominant lines are produced. The more favourable gain overlap of the K Raman lines in the fundamental vibrational level induces an off-resonance dominance of this process respect to the standard Raman scattering in the excited vibrational level. The AC Stark splitting has been also observed when the CO₂ laser is posed in resonance with the absorption lines.     

Emission and absorption spectra of 2,3,4-, 2,3,5- and 2,4,5-trifluorobenzaldehyde vapors

Emission and absorption spectra have been measured for 2,3,4-, 2,3,5- and 2,4,5-trifluorobenzaldehyde vapors at different temperatures along with the Raman spectra of the pure samples. The observed emission is shown to consist of the T₁(n, π∗) phosphorescence accompanied by the weak S₁(n. π∗) delayed fluorescence for all the molecules. The vibrational analyses of the fluorescence and phosphorescence spectra based on the results of the DFT calculation at the B3LYP/6-311+G(p,d) level indicate that the observed emission and absorption originates from only the stable conformer (anti). The measured Raman spectra also agreed well with those calculated for the stable (anti) conformer.