Peter Ljungberg

Velocity measurements by flow tagging employing laser enhanced ionisation and laser induced fluorescence

A new flow tagging technique for measurements of gas velocities over a wide range of flow conditions is proposed. This method employs the laser-based spectroscopic techniques of laser-enhanced ionisation (LEI) and laser induced fluorescence (LIF). Initially the flow to be studied is seeded with a small amount of sodium. The flow is then tagged by utilising LEI to deplete a substantial fraction (80%) of the neutral sodium species in a well defined region upstream of the flow. Finally, single laser pulse planar LIF is used to create a two-dimensional (2D) inverse image of the depleted tagged region downstream of the flow at a specific time delay. The feasibility of this method was demonstrated in two different types of air-acetylene flame by measuring instantaneous 2D flow structures. Flame velocities were determined with an accuracy of better than 10%.

Direct Determination of Thalliilm in Natural Waters by Laser Induced Fluorescence in a Graphite Furnace

Abstract The thallium content in some natural waters (lakes, rivers and drinking water) in South-West Sweden has been determined by Laser-Induced Fluorescence in a Graphite Furnace (LIF-GF). The high sensitivity of the LIF-GF technique eliminated the need for any sample pre-treatment or sample pre-concentration (except for acidification with small amounts of HNO3), thus reducing the number of sources of contamination. It was found (by the standard addition technique) that the Tl content of the samples investigated ranges from 8 ppt for drinking water up to 56 ppt for water from the Viskan river. No matrix interferences from concomitant elements in the samples could be detected with the present experimental set-up. This allowed for a calibration against matrix-free reference solutions.

Lifetime measurements of metastable states of Au, Bi, Cd, Mg, Pb, and Sr in an acetylene/air flame by laser-enhanced ionization spectrometry

The collisional dominated lifetimes of 9 metastable states of Au, Bi, Cd, Mg. Pb, and Sr in an acetylene/air flame have been measured by the step-wise delayed laser-enhanced ionization spectrometry technique. First, the dependency of the lifetime of the metastable 6p23P1 state in Pb upon both flame composition and height in the flame were studied. It was found that the lifetime was significantly shorter, both for lean as well as for rich flames, as compared to a stoichiometric composition. The lifetime was furthermore found to be dependent of the height in the flame with a maximum around 10 mm. The lifetimes were not significantly affected by the electron concentration in the flame. The lifetimes for the other metastable states were determined for one given flame composition (φ ≈ 0.9 φstoich) and one given height (20 mm) only. The measured lifetimes were found to be between 5 ns and 3.1 μs. The results show clearly that the lifetimes of metastable states are significantly shortened in the flame (as compared to natural lifetimes). The results also indicate that the metastable states which are of the same parity as the ground configuration (Au, Bi and Pb) in general have longer (collisional) lifetimes (85 ns to 3.1 μs) than those which are forbidden to decay to the ground configuration only by violation of spin conservation (Cd, Mg and Sr) (5–33 ns).

Detection of titanium in electrothermal atomizers by laser-induced fluorescence. Part 1. Determination of optimum excitation and detection wavelengths

Abstract A detailed investigation has been performed of suitable excitation and detection wavelengths of Ti for the technique of laser-induced fluorescence in electrothermal atomizers. Fluorescence spectra (most often in the 250–350 nm region) from the 39 excitation wavelengths conjectured to be the most important (in the 220–270 nm region) have been investigated in more detail by the use of an intensified CCD detector. The fluorescence spectra were found to have a rich occurrence of peaks (about 50 each), of which many cannot be found in existing atomic wavelength data in the literature. All of the peaks found (with one exception) could be identified thanks to a home-made program that calculates atomic wavelengths from existing energy level data. Most of the spectra are dominated by “indirect” transitions (i.e. fluorescence from an upper level that is different from the one accessed by the laser) despite the prediction of existing “direct” transitions. This indicates that the collisional redistribution processes the excited levels, in general, are faster than typical fluorescence rates for Ti in graphite furnaces. Suitable excitation and detection wavelength combinations are given. One such suitable choice is excitation at 264.662 nm since the fluorescence following this excitation consisted of peaks of almost equal magnitude in three different wavelength regions (around 295, 319 and 338 nm).

DensMat: fully time-resolved simulation of two-step pulsed laser excitation of atoms in highly collisional media☆

A program that simulates and displays the level populations of atomic systems exposed to dual-wavelength (i.e. two-colour) pulsed laser excitation in highly collisional media (such as flames and plasmas) has been developed. The program is based upon a previously published fully time-dependent density-matrix model that describes step-wise excitations of atoms with degenerate states under collision-dominated conditions, and which thus goes beyond the rate-equations formalism. This model can predict such phenomena as Rabi flopping and a.c.-Stark splitting, shifting and broadening. The program can be used as a prediction tool for laser-enhanced ionization, laser-induced fluorescence, fluorescence dip spectroscopy and other two-colour laser-based spectroscopic experiments. The program provides the user with a flexible four-level atomic system, configurable as a one- or two-step excitation ladder, along with an ionization continuum and non-laser-connected level(s) that may act as trap(s) or metastable level(s). Parameters such as level degeneracy, collisional rates and laser pulse widths, shapes, wavelengths, intensities and bandwidths are accessible to the user. The program can display both the time development of the level populations and also level populations versus laser wavelength. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by a disk containing the program DensMat, an associated on-line help file and manual, an installation program, and data files pertaining to the examples illustrated in this article. The program runs under Windows 3.1 on an IBM-compatible computer.

Two-step degenerate four-wave mixing as a means to decrease pre- and post-filtering effects in optically thick media

The use of crossed beam 2-Step Degenerate Four-Wave Mixing (2S-DFWM) for decreasing pre- and post-filtering effects under optically thick conditions has been investigated. 2S-DFWM is a technique in which the DFWM process is performed between two excited states of which the lower one is populated by an ordinary laser excitation from a low lying, highly populated state. Experiments were performed on Au in an acetylene/air flame. We have shown that under conditions where ordinary (one-step) DFWM experiments are significantly affected by pre- and post-filtering effects (i.e., partly absorption of the pump, probe or signal beams prior to or after the interaction region) the 2S-DFWM technique can give virtually interference free signals. A variety of different laser beam and flame configurations have been investigated. It was found that the use of a crossed beam geometry, where the first step exciting laser is incident upon the interaction region perpendicular to the DFWM beams, completely eliminated severe pre- and post-filtering effects occurring for an ordinary single-step DFWM scheme in an optically thick medium.

A density-matrix treatment of step-wise laser excitations of atoms with degenerate states in high collisional media

Abstract A theory, based on the density-matrix formalism, for two-step excitation of atoms between three levels with arbitrary degeneracies in collision-dominated media by pulsed laser light of arbitrary bandwidth is presented. First, a general set of density-matrix equation is formulated within the rotating-wave approximation for a three level system, in which each level consists of an arbitrary number of sub-states. The system, composed of N 2 equations (where N is the total number of sub-states of the atomic levels involved), is then reduced to six full time-dependent equations by a summation of the diagonal elements for each level and by an averaging of the non-diagonal elements for each transition. The high collisional rates are assumed to wash out all coherence between the sub-states within each level. It is assumed that the laser light is linearly polarized and has a Lorentzian shape (the finite bandwidths of the lasers are included as phase-fluctuations given by the phase-diffusion model). The equations are solved in the steady-state limit of the non-diagonal elements, yielding time dependent rate-equation-like population-transfer equations. An analytical solution in closed form of the full steady-state situation is also given.

Degenerate four-wave mixing from laser-populated excited states.

Degenerate four-wave mixing (DFWM) from laser-populated excited states, i.e., two-step DFWM (2S-DFWM), has been performed to investigate the possibility of increasing signal quality (i.e., strength or signal-to-noise ratio) when species with low transition probabilities or far-UV transitions are to be detected or when large beam areas are used. Gold atoms, aspirated into an air-acetylene flame, were chosen as a suitable species for this investigation. The 2S-DFWM signal strength was found to be comparable to the ordinary (one-step) DFWM signals for moderately high UV-light intensities but substantially better for low UV-light intensities. This finding implies that DFWM detection of species with lower transition probabilities in the first step as compared with gold (<10(-8) s(-1)) can benefit from the 2S-DFWM technique when moderate or low UV-light intensities are available. Additional possible advantages of using 2S-DFWM are also discussed.

Laser-induced fluorescence in a graphite furnace as a sensitive technique for assessment of traces in North Arctic atmospheric aerosol samples

An improved version of the highly sensitive laser-based spectroscopic trace-element detection technique, laser-induced fluorescence in a graphite furnace, LIF–GF (often also referred to as laser-excited atomic fluorescence spectrometry in electrothermal atomizer, ETA–LEAFS) has been used to assess the trace-element content of Al and Pb in size-fractionated aerosol samples from the Norwegian Arctic. The ordinary LIF–GF technique has been modified for improved selectivity by the incorporation of a multi-channel intensified CCD detector (ICCD) which makes constant monitoring of various background signals possible (scattered laser light, concomitant fluorescence light, and black body radiation). It is shown that the sensitivity and selectivity of the LIF–GF–ICCD technique is sufficient for efficient detection of the trace contents of Al and Pb in dissolved aerosol samples from the Norwegian Arctic (0–75 pg for each furnace heating). The Al and Pb concentrations in air from Ny Alesund, Svalbard, at the time of sampling (March–April 1992) were found to be 1–50 ng m–3.

Analysis of the 3d64s(6D)6d subconfiguration of Fe I by laser-enhanced ionisation and emission spectroscopy

Laser-enhanced ionisation spectrometry has been used to confirm 36 levels within the highly-excited 3d64s(6D)6d subconfiguration in Fe I that have been established by grating spectrometry. The new levels are used to identify ~ 200 lines in the UV grating and Fourier transform spectra, most of which are also observed in the solar spectrum.