Development of a narrow-band tunable picosecond dye laser and its application to excited-state lifetime measurement of a chlorinated aromatic hydrocarbon.

The development of a distributed-feedback dye laser, with a pulse width and a line width of 25 ps and 8.78 pm, respectively, is described. Using this nearly Fourier-transform-limited pulse, we measured the first singlet-excited-state lifetime of 1,2,4-trichlorobenzene. The tunable picosecond dye laser developed herein has a potential for the lifetime measurement and the efficient multiphoton ionization of aromatic hydrocarbons with a larger number of chlorine atoms and shorter excited-state lifetimes.     

Stimulated Raman laser excitation of spontaneous resonance Raman scattering

The efficient conversion of the second and third harmonics of a Nd YAG laser to near UV radiation in the 395–500 nm range by stimulated Stokes (and anti-Stokes) Raman scattering (SRS) in a 1 m Raman oscillator containing compressed H₂ or D₂ is used as an excitation source for spontaneous resonance Raman spectroscopy (RRS). SRS excited RR spectra are shown for the anion radical of tetracyanoquinodimethane (TCNQ).     

Studies of photoinitiation and phototermination of free radical polymerization using dual pulsed laser techniques

Studies of free radical polymerization are described in which the initiation and termination events are independently controlled using two laser pulses of different wavelength. Flash photolysis of azobisisobutyronitrile (AIBN) was used to initiate the polymerization of styrene. After a predefined period the polymerization was terminated by laser photolysis of 2-naphthylmethyl 1-naphthylacetate (NMNA). The molecular weights of the polymer were controlled by the time interval between laser pulses. Irradiation with high intensity laser pulses in the presence of preformed polystyrene yields a fluorescent-labelled polymer. A two-photon process is proposed to explain the mechanism of photoaddition of naphthylmethyl groups to the polymer chain.     

Molecular sensors using a resonance Raman template

The potential use of resonance Raman spectroscopy as a molecular sensing tool is illustrated using a metalloporphyrin template and pyridine as an analyte. The equilibrium binding constant for the axial binding of pyridine to zinc tetraphenylporphyrin has been measured using resonance Raman spectroscopy. Although no new peaks are observed and the porphyrin peaks do not shift position, the quantification is made possible by the selective resonance enhancement of the template vibrations. The value for log k was determined by resonance Raman to be 3.65 +/- 0.32, which compares well with previously published values estimated using absorption data. Values for log k were determined for a series of related compounds, the picolines, and these also compare favourably with those previously reported.     

Wavelength dependence of matrix-assisted laser desorption and ionization using a tunable mid-infrared laser

Matrix-assisted laser desorption and ionization by infrared laser (IR-MALDI) is expected to be an effective methods for soft-ionization of high-molecular weight proteins and intracellular proteins. IR-MALDI is not widely used because its low sensitivity, complexity, high cost, and as it does not work well on commercial MALDI time-of-flight mass spectrometers (TOFMSs). We employed a tunable mid-infrared (MIR) laser as a light source for MALDI to investigate the IR-MALDI. The laser wavelength can be tuned within a range from 5.5 to 10.0 μm, and included several biomaterial group vibration modes. We evaluated the wavelength dependence of ionization in IR-MALDI for four matrices: succinic acid, urea, 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid) and 2,5-dihydroxybenzoic acid (DHB). These matrices contained various groups of vibration modes, and absorbed an infrared (IR) energy at a specific wavelength. The mass spectra of angiotensin II was obtained at a specific wavelength corresponding to the CO stretching and benzene ring vibration mode. In IR-MALDI, we considered the strong molecular bond attracting an electron from a neighboring hydrogen atom, possibly protonating the hydrogen atom.     

Resonance Raman spectra of cytochrome C and oxyhemoglobin using pulsed laser excitation and optical multichannel detection

The resonance Raman spectra of dilute cytochrome c and oxyhemoglobin solutions obtained with high peak power (>1 MW) laser excitation at 532.0 nm and optical multichannel analysis (OMA) are presented. The frequencies and intensities of bands in resonance Raman spectra obtained under these conditions are directly comparable to spectra derived from 0.15 W peak power excitation using cw lasers. No anomalous spectral features are observed. These pulsed-laser/OMA spectra are used to comment on the applicability of such techniques for time-resolved resonance Raman spectroscopy of biopolymers.     

Infrared matrix-assisted laser desorption and ionization by using a tunable mid-infrared free-electron laser

Initial results of infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry of proteins by using the Vanderbilt free-electron laser as the source of selective vibrational excitation are reported. The ability of this laser to initiate desorption and ionization by excitation of specific vibrational modes is demonstrated. For the first time it is shown that IR-MALDI mass spectrometry at wavelengths other than those available from conventional fixed-frequency IR lasers, that is, 2.79 (Er:YSGG), 2.94 (Er:YAG), and 9.3-10.6 μm (CO₂), is feasible and exhibits similar performance. IR-MALDI mass spectra were taken in the wavelength ranges 2.8-4 and 5.5-6.5 μm, covering the absorption bands of the O-H and C=O stretch vibrations typical of many organic compounds such as succinic acid, fumaric acid, or nicotinic acid, which were used as matrices in these studies. A comparison between these results and Er:YAG/YSGG MALDI data are given. The potential of IR-MALDI at wavelengths near the C=O stretch vibration and the possibilities for studies of the IR-MALDI mechanisms by using this kind of tunable source are discussed.     

Raman resonance effect in liquid water

Raman spectroscopy is a technique preferably used for studies of water structure because the proportions of intensities of main OH stretching modes are thought to reflect well a network of "intermonomer" hydrogen bonds as well as its disturbance by the presence of some solutes. The work presented herein demonstrates how the intensity ratio of two main components (around 3200 and 3400 cm (-1)) depends on the excitation wavelength in the visible range. Polarized Raman spectra indicate that the component at ca. 3200 cm (-1) is in resonance with light from the red range, which is in agreement with the presence of vibrational overtones in UV-vis absorption spectrum of water. These results are the first report on the occurrence of the Raman resonance effect in liquid water.     

Stand-off Raman detection using dispersive and tunable filter based systems

Small, transportable Raman systems are being developed for stand-off Raman measurements at intermediate ranges (e.g. <20 m) for planetary measurements. Four variations of stand-off Raman systems are described that use a small telescope for light collection that is either fiber-optic or lens-coupled to a detection system. The performance of an acousto-optic tunable filter for wavelength selection and spectral imaging is tested by comparing signal-to-noise ratio and throughput to similar measurements using a conventional spectrograph, and by measuring a variety of organic and inorganic mineral samples at distances up to 15 m. We also determine optimal ICCD gate widths for acquiring remote Raman spectra under high ambient light conditions.     

The dushinsky effect in resonance Raman spectroscopy

The effect of normal-coordinate rotation on resonance Raman excitation profiles and depolarization dispersion curves is investigated theoretically for two nontotally symmetric modes of the same symmetry.     

Ultraviolet-induced flashover of a plastic insulator using a pulsed excimer laser

Ultraviolet-induced flashover has been observed over stressed, angled, acrylic insulators illuminated by a short (60 ns) pulse of excimer laser light at 249 nm. Flashover has been observed at ultraviolet fluences of 5–65 mJ/cm² for electric field stresses approximately 10–30% of static breakdown stress. Insulators at positive angle (conventional configuration) exhibit a reduced tolerance to ultraviolet light versus insulators at negative angle (unconventional configuration) by approximately a factor of 2, while the presence of impurities at the triple point reduces the tolerance to ultraviolet even further. Flashover is related to the fluence, rather than the power density, for short pulses, and the production of photoelectrons is a likely mechanism for the initiation of flashover.     

Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100 degrees C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures.     

Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100° C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures.     

Spectroscopic analysis of works of art using a single LIBS and pulsed Raman setup

A nanosecond pulsed laser setup has been optimized to perform laser-induced breakdown spectroscopy (LIBS) and pulsed Raman spectroscopy measurements in the field of cultural heritage. Three different samples of artistic/architectural interest with different typologies have been analyzed. The results from the two techniques allowed the identification of the materials used in their manufacture or contaminating them, probably coming from atmospheric pollution and biological activity. No sampling and sample preparation was required before the measurements, and no visual or structural damage was observed. Depth profiling using LIBS was performed in one of the samples, providing elemental information along the different layers composing the object and covering its surface. The quality of the results and the rather short time needed for the measurements and for switching between techniques confirmed the instrument’s capabilities and specificity for dealing with objects of artistic or historical interest.     

Absorption measurements of OH using A cw tunable laser

We have performed absorption measurements of OH using a cw tunable laser. Results indicate that this technique, when combined with frequency modulation, promises a sensitivity of 10⁵ molecules/cm³ for OH monitoring in the atmosphere.     

Fast Raman spectroscopy of cytochrome c using intracavity resonance Raman amplification

The resonance Raman spectrum of cytochrome c (5 × 10^?4 M) was obtained using the method of resonance Raman amplification (RRA) in a dye laser resonator in 30 ns. The second harmonic of a Q-switched Nd glass laser was used as an excitation source and the spectra were recorded using a photographic plate.     

Direct measurement of the vibrational predissociation lifetime of NeBr2 in the ground electronic state

The vibrational predissociation lifetime of NeBr₂ in the ground electronic state with one vibrational quantum in the halogen stretch was measured directly in a free jet expansion. NeBr₂(X, ν = 1) was detected by optical-optical double resonance to monitor the population as a function of distance from the nozzle to the laser interaction region. The vibrational predissociation lifetime for NeBr₂(X, ν = 1) was determined to be 8 ± 3 μs.     

A resonance Raman study of the higher-lying electronic states of styrene vapor

Resonance Raman spectra of styrene vapor excited in the S₂ /S₃ and S₄ absorption systems are reported. The lack of double bond torsional scattering activity is taken as evidence of high barriers to rotation in all the excited states examined. Consequences for styrene photoisomerization are discussed. Excitation to S₂/S₃ is delocalized in nature, while S₄ excitation is benzene-like. The observed scattering intensities are largely in agreement with the jet absorption analysis and the results of Franck-Condon calculations.     

A molecular electronic Raman effect of the ferricenium ion

A molecular electronic Raman effect is reported for the ferricenium ion. The transitions occur at 213 cm^?1 and 1580 cm^?1 to spin-orbit levels of the ground electronic configuration (a_1g)²(e_2g)³. The positions of the spin-orbit states as obtained from the Raman spectrum agree with EPR data.     

Raman enhancement factor of a single tunable nanoplasmonic resonator

We have developed a novel technique to precisely determine the Raman enhancement factor in single nanoplasmonic resonators (TNPRs). TNPRs are lithographically defined metallodielectric nanoparticles composed of two silver disks stacked vertically, separated by a silica layer. At resonance, the local electromagnetic fields are enhanced at the TNPR surface, making it an ideal surface-enhanced Raman scattering (SERS) active substrate. The ability to control the dimensions of the metallic and dielectric layers offers the unique advantage of fine-tuning the plasmon resonance frequency to maximize the enhancement of the Raman signal. Furthermore, by selective shielding of the outer surface of the metallic structure, the efficiency can be further enhanced by guiding the molecular assembly to the locations that exhibit strong electromagnetic fields. We experimentally demonstrate SERS enhancement factors of (6.1+/-0.3)x10(10), with the highest enhancement factor being achieved by using an individual nanoparticle. By using nanofabrication techniques, we eliminate the issues such as large size variations, cluster aggregation, and interparticle effects common in preparing SERS substrates using conventional chemical synthesis or batch fabrication methods. TNPRs produce very controllable and repeatable SERS signals at the desired locations and, thus, make an ideal candidate for device integration.