The consequences of diffractively spreading beams in ultrafast THz spectroscopy

There are established methods for calculating optical constants from measurements using a broadband terahertz (THz) source. Applications to ultrafast THz spectroscopy have adopted the key assumption that the THz beam is treated as a normal incidence plane-wave. We show that this assumption results in a frequency-dependent systematic error, which is compounded by distortion of the beam on introduction of the sample.

Measurement of propagation constant in waveguides with wideband coherent terahertz spectroscopy

A quasi-optical technique for characterizing micromachined waveguides is demonstrated with wideband time-resolved terahertz spectroscopy. A transfer-function representation is adopted for the description of the relation between the signals in the input and output port of the waveguides. The time-domain responses were discretized, and the waveguide transfer function was obtained through a parametric approach in the z domain after describing the system with an autoregressive with exogenous input model. The a priori assumption of the number of modes propagating in the structure was inferred from comparisons of the theoretical with the measured characteristic impedance as well as with parsimony arguments. Measurements for a precision WR-8 waveguide-adjustable short as well as for G-band reduced-height micromachined waveguides are presented. (C) 2003 Optical Society of America.

High signal to noise ratio THz spectroscopy with ASOPS and signal processing schemes for mapping and controlling molecular and bulk relaxation processes

Asynchronous Optical Sampling has the potential to improve signal to noise ratio in THz transient sperctrometry. The design of an inexpensive control scheme for synchronising two femtosecond pulse frequency comb generators at an offset frequency of 20 kHz is discussed. The suitability of a range of signal processing schemes adopted from the Systems Identification and Control Theory community for further processing recorded THz transients in the time and frequency domain are outlined. Finally, possibilities for femtosecond pulse shaping using genetic algorithms are mentioned.

Spectroscopy of ultrathin epitaxial rutile TiO[sub 2](110) films grown on W(100)

Epitaxial ultrathin titanium dioxide films of 0.3 to similar to 7 nm thickness on a metal single crystal substrate have been investigated by high resolution vibrational and electron spectroscopies. The data complement previous morphological data provided by scanned probe microscopy and low energy electron diffraction to provide very complete characterization of this system. The thicker films display electronic structure consistent with a stoichiometric TiO2 phase. The thinner films appear nonstoichiometric due to band bending and charge transfer from the metal substrate, while work function measurements also show a marked thickness dependence. The vibrational spectroscopy shows three clear phonon bands at 368, 438, and 829 cm(-1) (at 273 K), which confirms a rutile structure. The phonon band intensity scales linearly with film thickness and shift slightly to lower frequencies with increasing temperature, in accord with results for single crystals. (c) 2007 American Institute of Physics.

Ultrafast charge separation in a photoreactive rhenium-appended porphyrin assembly monitored by picosecond transient infrared spectroscopy

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated as [Re(CO)(3)(Pic)Bpy-MgTPP][OTf] and [Re(CO)(3)(Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)(3)(bpy) units and the metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption (TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the TRIR spectra by a low-frequency shift in the nu(CO) bands of the Re(CO)(3) moiety similar to that observed by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO)(3)(Pic)Bpy-MTPP][OTf] results in characteristic TRVIS spectra of the S-1 state of the porphyrin that decay with a time constant of 17 ps (M = Mg) or 24 ps (M = Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence of triethylamine.

Low-lying excited states and primary photoproducts of [Os-3(CO)(10)-(s-cis-L)] (L = cyclohexa-1,3-diene, buta-1,3-diene)] clusters studied by picosecond time-resolved UV/Vis and IR spectroscopy and by density functional theory

Combined picosecond transient absorption and time-resolved infrared studies were performed, aimed at characterising low-lying excited states of the cluster [Os-3(CO)(10)(s-cis-L)] (L= cyclohexa-1,3-diene, 1) and monitoring the formation of its photoproducts. Theoretical (DFT and TD-DFT) calculations on the closely related cluster with L=buta-1,3-diene (2') have revealed that the low-lying electronic transitions of these [Os-3(CO)(10)(s-cis-1,3-diene)] clusters have a predominant sigma(core)pi*(CO) character. From the lowest sigmapi* excited state, cluster 1 undergoes fast Os-Os(1,3-diene) bond cleavage (tau=3.3 ps) resulting in the formation of a coordinatively unsaturated primary photoproduct (1a) with a single CO bridge. A new insight into the structure of the transient has been obtained by DFT calculations. The cleaved Os-Os(1,3-diene) bond is bridged by the donor 1,3-diene ligand, compensating for the electron deficiency at the neighbouring Os centre. Because of the unequal distribution of the electron density in transient la, a second CO bridge is formed in 20 ps in the photoproduct [Os-3(CO)(8)(mu-CO)(2)- (cyclohexa-1,3-diene)] (1b). The latter compound, absorbing strongly around 630 nm, mainly regenerates the parent cluster with a lifetime of about 100 ns in hexane. Its structure, as suggested by the DFT calculations, again contains the 1,3-diene ligand coordinated in a bridging fashion. Photoproduct 1b can therefore be assigned as a high-energy coordination isomer of the parent cluster with all Os-Os bonds bridged.

Synthesis, spectroscopy and spectroelectrochemistry of chlorocarbonyl {1,2-bis[(2,6-diisopropylphenyl)imino]-acenaphthene-kappa(2)-N,N '}rhodium (I)

Reaction of the dinuclear complex [{Rh(CO)(2)}(2) (mu-Cl)(2)]with an alpha-diimine ligand, 1,2- bis[(2,6-diisopropylphenyl) imino] acenaphthene (iPr(2)Ph-bian), produces square-planar [RhCl(CO)(iPr(2)Ph-bian)]. For the first time, 2: 1 and 1: 1 alpha-diimine/dimer reactions yielded the same product. The rigidity of iPr(2)Ph-bian together with its flexible electronic properties and steric requirements of the 2,6-diisopropyl substituents on the benzene rings allow rapid closure of a chelate bond and replacement of a CO ligand instead of chloride. A resonance Raman study of [RhCl(CO)(iPr(2)Ph-bian)] has revealed a predominant Rh-to-bian charge transfer (MLCT) character of electronic transitions in the visible spectral region. The stabilisation of [RhCl(CO)(iPr(2)Ph-bian)] in lower oxidation states by the pi-acceptor iPr(2)Ph-bian ligand was investigated in situ by UV-VIS, IR and EPR spectroelectrochemistry at variable temperatures. The construction of the novel UV-VIS-NIR-IR low-temperature OTTLE cell used in these studies is described in the last part of the paper.

Comparative study of Barium Fluoride thin films produced by ion-beam sputtering and thermal evaporation

Coatings and filters for spaceflight far-infrared components require a robust, non-absorptive low-index thin film material to contrast with the typically higher refractive index infrared materials. Barium fluoride is one such material for the 10 to 20µm wavelength infrared region, however its optical and mechanical properties vary depending on the process used to deposit it in thin film form. Thin films of dielectric produced by thermal evaporation are well documented as having a lower packing density and refractive index than bulk material. The porous and columnar micro structure of these films causes possible deterioration of their performance in varied environmental conditions, primarily because of the moisture absorption. Dielectric thin films produced by the more novel technique of ion-beam sputtering are denser with no columnar micro structure and have a packing density and refractive index similar to the bulk material. A comparative study of Barium Fluoride (BaF2) thin films made by conventional thermal evaporation and ion-beam sputtering is reported. Films of similar thicknesses are deposited on Cadmium Telluride and Germanium substrates. The optical and mechanical properties of these films are then examined. The refractive index n of the films is obtained from applying the modified Manifacier's evvelope method to the spectral measurements made on a Perkin Elmer 580 spectrophotometer. An estimate is also made of the value of the extinction coefficient k in the infrared wavelength transparent region of the thin film. In order to study the mechanical properties of the BaF2 films, and evaluate their usefulness in spaceflight infrared filters and coatings, the thin film samples are subjected to MIL-F-48616 environmental tests. Comparisons are made of their performance under these tests.

Supramolecular assemblies of metal ions in complex bimetallic and trimetallic salts based on hexacyanoferrate(III) ion

Two complex heterometallic salts with formulae Tl-6[Fe(CN)(6)](1) (33)(NO3)(OH) (1) and [Co(bpy)(2)(CN)(2)](2){[Ag(CN)(2)](0) (5)[Fe(CN)(6)](0) (5)} 8H(2)O (2) have been synthesized and fully characterized Single crystal X-ray analyses reveal that compound 1 is comprised of discrete Tl+ cations and [Fe(CN)(6)](3-) anions together with OH- and NO3- anions Compound 2 contains [Co(bpy)(2)(CN)(2)](+) cations and {[Ag(CN)(2)][Fe(CN)(6)]}(-) anions together with eight molecules of water of crystallization Both structures form unprecedented three-dimensional supramolecular networks via non covalent interactions Another important observation is that the stereochemically active inert (lone) pair present on Tl+ plays little role in controlling the structure of 1 The water molecules in 2 play important roles in providing stability organizing a supramolecular network through hydrogen bonding In the syntheses of 1 and 2 Fe(II) is oxidized to Fe(III) and Co(II) to Co(III) respectively facilitating the formation of the salts that are obtained Both compounds exhibit photoluminescence emission in solution near the visible region.

A simple fluid cell for the study of aqueous solutions using THz time-domain spectroscopy

We describe a fluid cell for the measurement of aqueous solutions of biomolecules adapted particularly for the requirements of THz time-domain spectroscopy. The design is simple, requires small-volume samples, avoids cross-contamination and is inexpensive.

Tuning the self-assembly of the bioactive dipeptide L-carnosine by incorporation of a bulky aromatic substituent

The dipeptide L-carnosine has a number of important biological properties. Here, we explore the effect of attachment of a bulky hydrophobic aromatic unit, Fmoc [N-(fluorenyl-9-methoxycarbonyl)] on the self-assembly of Fmoc-L-carnosine, i.e., Fmoc-Beta-alanine-histidine (Fmoc-BetaAH). It is shown that Fmoc-BetaAH forms well-defined amyloid fibril containing Beta sheets above a critical aggregation concentration, which is determined from pyrene and ThT fluorescence experiments. Twisted fibrils were imaged by cryogenic transmission electron microscopy. The zinc-binding properties of Fmoc-BetaAH were investigated by FTIR and Raman spectroscopy since the formation of metal ion complexes with the histidine residue in carnosine is well-known, and important to its biological roles. Observed changes in the spectra may reflect differences in the packing of the Fmoc-dipeptides due to electrostatic interactions. Cryo-TEM shows that this leads to changes in the fibril morphology. Hydrogelation is also induced by addition of an appropriate concentration of zinc ions. Our work shows that the Fmoc motif can be employed to drive the self-assembly of carnosine into amyloid fibrils.

Infrared linear dichroism spectroscopy on amyloid fibrils aligned by molecular combing

We report the use of molecular combing as an alignment method to obtain macroscopically oriented amyloid fibrils on planar surfaces. The aligned fibrils are studied by polarized infrared spectroscopy. This gives structural information that cannot be definitively obtained from standard infrared experiments on isotropic samples, for example, confirmation of the characteristic cross-beta amyloid core structure, the side-chain orientation from specific amino acids, and the arrangement of the strands within the fibrils, as we demonstrate here. We employed amyloid fibrils from hen egg white lysozyme (HEWL) and from a model octapeptide. Our results demonstrate molecular combing as a straightforward method to align amyloid fibrils, producing highly anisotropic infrared linear dichroism (IRLD) spectra.

Assessment of the consistency of H2O line intensities over the near-infrared using sun-pointing ground-based Fourier transform spectroscopy

We report on the consistency of water vapour line intensities in selected spectral regions between 800–12,000 cm−1 under atmospheric conditions using sun-pointing Fourier transform infrared spectroscopy. Measurements were made across a number of days at both a low and high altitude field site, sampling a relatively moist and relatively dry atmosphere. Our data suggests that across most of the 800–12,000 cm−1 spectral region water vapour line intensities in recent spectral line databases are generally consistent with what was observed. However, we find that HITRAN-2008 water vapour line intensities are systematically lower by up to 20% in the 8000–9200 cm−1 spectral interval relative to other spectral regions. This discrepancy is essentially removed when two new linelists (UCL08, a compilation of linelists and ab-initio calculations, and one based on recent laboratory measurements by Oudot et al. (2010) [10] in the 8000–9200 cm−1 spectral region) are used. This strongly suggests that the H2O line strengths in the HITRAN-2008 database are indeed underestimated in this spectral region and in need of revision. The calculated global-mean clear-sky absorption of solar radiation is increased by about 0.3 W m−2 when using either the UCL08 or Oudot line parameters in the 8000–9200 cm−1 region, instead of HITRAN-2008. We also found that the effect of isotopic fractionation of HDO is evident in the 2500–2900 cm−1 region in the observations.