A square-wave wavelength modulation system for automatic background correction in carbon furnace atomic emission spectrometry

A square-wave wavelength modulation system, based on a rotating quartz chopper with four quadrants of different thicknesses, has been developed and evaluated as a method for automatic background correction in carbon furnace atomic emission spectrometry. Accurate background correction is achieved for the residual black body radiation (Rayleigh scatter) from the tube wall and Mie scatter from particles generated by a sample matrix and formed by condensation of atoms in the optical path. Intensity modulation caused by overlap at the edges of the quartz plates and by the divergence of the optical beam at the position of the modulation chopper has been investigated and is likely to be small.     

A Compact,High Speed Ion Beam Chopper

Abstract

A simple, compact pulse circuit was designed for use as an ion beam chopper with both synchronous and asynchronous triggering. The rise and fall times of the beam-on pulse are 60 ns and 320 ns, respectively, with a beam suppression potential of 180 V. The beam chopper was interfaced to a spark-source mass spectrometer for which the precision of analyses can be improved by reducing effects of sample or spark inhomogeneities. Additionally, the synchronous mode of chopping provides a means for temporal analysis of the spark.     

Lifetime of alkyl macroradicals in irradiated ultra-high molecular weight polyethylene

The interaction of UHMWPE with an electron beam in vacuum and in the presence of oxygen has been investigated. UHMWPE irradiated to various doses was examined with Electron Paramagnetic Resonance (EPR) and Fourier Transform infra-red (FTIR) spectroscopies. EPR was used to explore the nature of radicals produced in UHMWPE upon irradiation and to follow their decay as a function of time. Hydroperoxides formation and distribution throughout the samples were studied with FTIR spectroscopy.A correlation between the rate of decay of macroradicals and that of hydroperoxide formation has been proposed. Accordingly, the lifetime of secondary alkyl macroradicals in the amorphous phase of the polymer was found to range approximately from 3 to 10 h.     

Trace analysis of pollutants in water using the photothermal interferometry as HPLC detector

A new procedure including high performance liquid chromatography in combination with photothermal interference spectroscopy as detection device (HPLC/PIS) has been proposed, optimized and its figures of merit for pesticide residue analysis are shown. The flowing sample under study is set in one arm of a Mach-Zehnder interferometer, and its refractive index is modulated by a periodically chopped continuous wave argon ion laser. As chopper, an acousto optical modulator has been introduced to switch the excitation laser beam between different lines (457 nm, 488 nm, 514 nm) simultaneously. Thus a multi component analysis can be realized either by using an HPLC-system in front of the PIS device or by a multi line Ar(+)-laser, directly. The limit of detection of the HPLC/PIS system reached 71 microg/l of the pesticide di-nitro-ortho-cresol (DNOC).     

The energy of the twisted excited singlet state of tetraphenylethylene: Picosecond optically detected calorimetry

The time-dependent diffraction of a non-resonant probe pulse following crossed beam excitation of tetraphenylethylene in cyclohexane is observed as a damped oscillation superposed on an exponential rise. The heat released by the formation and the decay of the “phantom” twisted excited singlet state is the time-dependent source term for coherent acoustic wave generation. A three-state model is employed to calculate the time-dependent diffraction from the thermally induced phase grating. With this model, the energy of the “phantom” state is found to lie 3.17 eV above the ground state in cyclohexane. It is demonstrated that this non-resonant transient grating technique can be used for sub-nanosecond optically detected photoacoustic calorimetry.     

Direct photoionization measurements of slow decay rates of vibrationally overexcited (CF3)3CI molecules in the ground electronic state in a beam

A direct photoionization technique is proposed to measure the dissociation rates of overexcited polyatomic iodides in the ground electronic state. Measurements are carried out in a molecular beam, which enables one to avoid thermal drift of the reagents and thus widen the range of the measurements toward longer lifetimes. Here the first results are reported of measurement of unimolecular decay rates of (CF₃)₃CI whose overexcitation in the electronic ground state has been effected by the IR multiple-photon excitation technique.     

Radiative lifetimes of excited W I levels

Time-resolved laser-induced fluorescence spectroscopy has been used to measure the lifetimes of 47 selected W I levels with energies between 27800 and 48200 cm^?1. A high-current hollow cathode produces an effusive beam of tungsten atoms and ions in the ground states and in metastable states. Selected states are populated with tunable dye laser pulses. The subsequent fluorescence is measured by means of a Tektronix 1 GHz transient digitizer. By taking into account the separately measured response function of the system the lifetimes can be determined from the full decay curve. All measurements are performed with linearly polarized laser beams adjusted to the magic angle in order to exclude systematic error sources. The decay curves also have been inspected for quantum beats and for saturation effects. A comparison with the scarce literature data is given.     

Trace analysis of pollutants in water using the photothermal interferometry as HPLC detector

A new procedure including high performance liquid chromatography in combination with photothermal interference spectroscopy as detection device (HPLC/PIS) has been proposed, optimized and its figures of merit for pesticide residue analysis are shown. The flowing sample under study is set in one arm of a Mach-Zehnder interferometer, and its refractive index is modulated by a periodically chopped continuous wave argon ion laser. As chopper, an acousto optical modulator has been introduced to switch the excitation laser beam between different lines (457 nm, 488 nm, 514 nm) simultaneously. Thus a multi component analysis can be realized either by using an HPLC-system in front of the PIS device or by a multi line Ar⁺-laser, directly. The limit of detection of the HPLC/PIS system reached 71 g/l of the pesticide di-nitro-ortho-cresol (DNOC).     

Substrate and Positional Selectivity of Free Phenylium Ions toward Arenes in the Gas and Liquid Phases

A nuclear technique, based on the spontaneous decay of tritiated precursors that allows the generation of free carbenium ion of exactly the same nature in different environments, has been exploited in a comparative study of aromatic phenylation by free phenylium ions, both in the gas phase at various pressures and in the liquid phase. The differences between the reactivity pattern of the phenylium ion in the two environments can essentially be reduced to significant ion-neutral electrostatic interaction in the gas phase and, to the much greater efficiency of collisional stabilization, in the condensed phase, allowing a larger fraction of the excited ionic intermediates, from the highly exothermic attack of phenylium ion on the competing arenes, to survive dissociation and isomerization. The mechanism of the major competitive processes promoted by phenylium-ion attack on arenes, i.e. phenyldehydrogenation and phenyldephenylation, are discussed, together with the substrate and positional selectivity displayed by the nuclear-decay-formed phenylium ion toward the selected arenes. The kinetic and mechanistic features of aromatic phenylation, deduced from the present decay experiments, are compared with those of related phenylation reactions carried out in the dilute gas state and in solution.     

Quantitative strain mapping applied to aberration--corrected HAADF images.

A systematic distortion in high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images, which may be caused by residual electrical interference, has been evaluated. Strain mapping, using the geometric phase methodology, has been applied to images acquired in an aberration-corrected STEM. This allows this distortion to be removed and so quantitative analysis of HAADF-STEM images was enabled. The distortion is quantified by applying this technique to structurally perfect and strain-free material. As an example, the correction is used to analyse an InAs/GaAs dot-in-quantum well heterostructure grown by molecular beam epitaxy. The result is a quantitative measure of internal strain on an atomic scale. The measured internal strain field of the heterostructure can be interpreted as being due to variations of indium concentration in the quantum dot.     

Possibilities and limitations of the time-correlated single photon counting technique: a comparative study of correction methods for the wavelength dependence of the instrument response function

One of the principal reasons for poor fits in time-correlated single photon counting fluorescence decay measurements is the wavelength dependence of the instrument response function. Over the years several correction methods have been developed to account for or correct for this effect. The most recent and widely used procedures are critically compared in this paper. A comparison of the channel shift technique, the pseudoscatterer technique, the excitation pulse-shape mimic technique and the delta function convolution method demonstrates that the latter method is the superior one. This method requires the measurement of the fluorescence decay of a reference compound with single exponential decay kinetics under identical conditions as used for the sample. A modified functional form is used to describe the sample decay law. Since no experimental data are altered in any way, a correct residual analysis is possible. Simulations show that fluorescence lifetimes can be recovered accurately, as long as the reference decay time is sufficiently different from that of the sample. Biexponential decays can be resolved successfully when the two decay times are well separated and different from that of the reference. The difference between the standard normal variates of chi-square for single and double exponential fits can be used to distinguish between mono- and bi-exponential decays. The usefulness of the delta function convolution method is demonstrated by fluorescence decay measurements of a series of different samples with very short decay times. Fluorescence lifetimes as short as 10 ps could be resolved accurately and reliably.     

Coherent optical transients observed by polarization switching

A transient signal analogous to the optical free induction decay is observed when a laser beam tuned to an absorption line of I₂ is switched from left to right circular polarization. The polarization transient amplitude decays at a rate equal to twice the usual transverse decay rate, and can be detected with sensitivity similar to that achieved in polarization spectroscopy.     

Single crystal HPGe (80%) versus BGO shielded CLOVER detector for high precision decay rate measurements: a comparative study

In this study, various detector configurations have been investigated in order to explore the optimal condition for decay rate measurements of radioactive samples using gamma spectroscopy technique. A limitation of detecting low energy gamma rays from decaying radioactive nuclei, is the Compton background which can be significantly reduced by rejecting Compton scattered events through active Bismuth germanate (BGO) shielding. On the other hand, for a CLOVER detector without BGO shielding, one can place the radioactive samples very close to the detector for enhancing geometrical efficiency. A single crystal High Purity Germanium (HPGe) detector can also be used for decay rate measurements. In order to measure the decay rate of nuclei decaying via gamma emission with reasonable intensity, optimal close geometry options have been investigated for various HPGe detector configurations.

     

Radiative lifetimes of Fe I levels of astrophysical interest

Time-resolved laser-induced fluorescence has been used to measure the lifetimes of 43 selected Fe I levels with energies between 25900 and 52000 cm^?1. A beam of iron atoms from a hollow cathode discharge was excited with a tunable dye laser pulse and the fluorescence was measured by means of a fast transient digitizer (1 GHz). The full decay curve could be used for a determination of the lifetimes by including the separately measured response function of the system in the evaluation procedure. A comparison with literature data is given.     

Influence of spontaneous decay on resonantly enhanced two-photon ionisation of D (2s)

The wavelength dependence of the two-photon ionisation of metastable D(2s) by a laser beam has been measured near the 2s ? 3p resonant frequency. A sharp structure is observed in the broad resonance profile at quite moderate intensity. Its origin is shown to be the spontaneous decay of the 3p state that competes with photoionisation.     

Investigation of the optimum optical design for pulsed-laser crossed-beam thermal lens spectrometry in infinite and finite samples

Optimization of the optical design for pulsed-laser crossed-beam thermal lens (PLCBTL) spectrometry has been investigated. Experiments have been carried out with large samples as well as for very small samples in a microchannel and using different lens combinations to focus the probe and excitation beams. The results have been interpreted in terms of the influence of the excitation beam size as well as the degree of mode-mismatching of the excitation and probe beams on the optimum sample position and on the amplitude and decay of the photothermal signal. A semi-empirical formula that describes the influence of the sample position with respect to the probe beam waist has been established. We have shown that the optimum signal is inversely proportional to the waist of the excitation beam and is independent of the sample size as long as the size of the excitation beam is smaller than the microchannel. Time-resolved experiments have also shown that when the excitation beam is smaller than the sample, the signal decay depends not only on the size of the excitation beam but also on the mode-mismatching factor. Otherwise, the temporal characteristics are closely related to the size of the microchannel.     

Lifetimes of triplet state alkylbenzenes in the vapor phase

Rate constants for triplet state decay of C₆H₆, C₆D₆ and 15 alkylbenzenes in the vapor phase have been found, using a flash-sensitized biacetyl phosphorescence technique, to range between 800 s^?1 and ? 18 000 s^?1. Only benzene has a significant positive activation energy for decay. Kinetic and spectroscopic evidence supports a photoisomerization decay channel in t-butylbenzene. Comparison of lifetimes with molecular size shows that increased density of rotational levels does not account for rapid decay of the triplet state. This contradicts a recent suggestion to explain short lifetimes of triplet state aromatic hydrocarbons in the vapor phase, relative to the long lifetimes of the same molecules in low temperat matrices. Evidence suggests that the dominant decay paths for triplet state alkylbenzenes are different in room temperature vapors and low temperature matrices.     

Transient absorption measurement system using pulsed energetic ion

This article reports a highly sensitive transient absorbance measurement system using pulsed energetic ions. The ions were pulsed by a beam chopper, which was synchronized with the cyclotron, and accelerated to the desired energy around 18 MeV/u. H, He, C and Ne ions can be used for the transient absorption measurement. The optical system can measure an absorbance smaller than 1.0×10⁻⁴ in the wavelength range of 400–740 nm.     

Time-resolved spectroscopy of long-path fluorescence and scattering for measuring total absorption of fluids

Optical methods are appropriate tools to detect organic micro-pollutants in fluids. A new technique is introduced which uses the decay of interaction processes like fluorescence and elastically scattered radiation by a fluid. Principally two different parameters are determined:

1. (i) the decay-time of the conventional interaction τ_C, which occurs at relatively short path-lengths of the incidence beam in the fluid, and

2. (ii) the decay-time τ_MP of the multi-path-saturation interaction originating at long path-lengths, e.g. in multi-path-reflection cuvettes, where the incidence beam is fully absorbed by the fluid.

A relation between the decay-time and the absorption coefficient of a fluid is theoretically derived. A simple preliminary experiment is performed considering distilled water polluted with non-fluorescent azobenzene and fluorescent quinine-sulphate. A nitrogen laser has been used to generate the fluorescence and scattering signals. The reciprocal value of the difference between the decay-time of the multi-path and conventional signals, 1/(τ_MP − τ_C), yields the total absorption coefficient directly. In comparison to the conventional absorption technique the decay-time method is characterized by a higher sensitivity.