Picosecond laser-induced fluorescence from gas-phase polycyclic aromatic hydrocarbons at elevated temperatures. II. Flame-seeding measurements

Picosecond laser-induced radiative emission from flames injected with aromatic substances has been measured spectrally and temporally resolved. The measurements were performed in various seeded regions and for different stoichiometric ratios of the surrounding gas. The wavelength of the excitation radiation was 266 nm. Changes in the lifetime and the spectral composition of the emission were observed with changes in the equivalence ratio and the position in the flame. Considerable agreement with previously reported cell measurements was obtained for those regions close to the injection zone. Temperatures were determined from spectrally and temporally resolved measurements. The comparison with elastic scattering gave reasonable results at low seeding rates for naphthalene, and is hoped to be improved even further in future experiments by increasing the time resolution and the signal-to-noise ratio of the measurements. Downstream and towards the surrounding gas, the lifetimes increased and the spectral profiles shifted and broadened towards the red. This effect increased when the equivalence ratio for the surrounding gas decreased and the oxygen concentration increased. The study was also directed towards characterizing features in the emission that could be indicative of a transition from the seeded aromatic substance to the formation of soot. An indicator for molecular or particle growth was the composition of the spectral emission in terms of UV, blue and green–yellow bands and the ratio between elastic-scattering signal and total emission signal. Spatially resolved measurements across the seeding region using a gated intensified CCD camera allowed a closer study of the molecular-growth region from the parent aromatic substance seeded to the soot formed. The fluorescence properties of dimers and their cyclodehydrogenated compounds and polymers containing aryl units are also discussed. Received: 11 July 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence for PLIF applications

The temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence were studied in support of planar laser-induced fluorescence (PLIF) imaging of temperature and mixture fraction in flows of practical interest. The temperature dependencies (300–875 K) of absorption and fluorescence were measured for gaseous 3-pentanoneat atmospheric pressure in a nitrogen bath gas using 248, 266, and 308 nm excitation. The results indicate that the fluorescence signal per unit mole fraction using 248 nm excitation is highly temperature-sensitive below 600 K, while the signal from 308 nm excitation is not temperature sensitive below 500 K. For quantitative measurements over a broad range of temperatures, one must choose excitation schemes carefully to balance the trade-off between measurement sensitivity and the amount of signal at the expected conditions. As an example of such a choice and to show the capabilities of ketone PLIF techniques, we include temperature and mixture fraction images of a 300–650 K heated air jet using near-simultaneous 308 and 266 nm excitation. Received: 29 May 2002 / Revised version: 5 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-650/723-1748, E-mail: jkoch@stanford.edu RID="**" ID="**"E-mail: hanson@me.stanford.edu     

Effective A-state fluorescence lifetime of formaldehyde in atmospheric pressure CH4/air flames

Using excitation pulses of ∼30-ps duration and a fast photomultiplier detector, effective fluorescence lifetimes of the A-stateof formaldehyde after excitation at 355 and 339 nm have been measured in the preheating zone of an atmospheric pressure, premixed methane/air flame. The fluorescence lifetimes were determined as a function of height above the exit of a slot burner and were thus probed in regions of varying gas temperature and composition. The fluorescence lifetimes were independent of the intensity of the excitation pulse and decreased as a function of height in the burner from ∼18±8 ns at 1.2 mm down to 7±1 ns at 3.8 mm. This trend of the effective fluorescence lifetime with composition and temperature in the flame can qualitatively be reproduced using calculated major species mole fractions and species-specific quenching cross sections for CH from the literature. Received: 13 June 2001 / Revised version: 27 September 2001 / Published online: 29 November 2001     

Non-linear pressure dependence of A-state fluorescence lifetime of formaldehyde

Fluorescence lifetimes of formaldehyde excited at 352 nm ( A₂ – A₁ 4₀¹ band) were measured as a function of bath gas pressure. He, N₂, O₂, CO₂ and HCHO were investigated for the bath gas and the temperature dependence between 298 and 500 K for N₂ and O₂ bath gases was also examined. It was found that the non-linear pressure dependence of the lifetime is successfully reproduced by the model formula

Analysis of （1, 2） （1, 3） and （2, 3） Bands in the c^3Ⅱu- b^3Ⅱg System of P2

We investigate the high resolution absorption spectroscopy of P2 radical, generated in ac glow discharge of PC13 buffered with helium, using optical heterodyne magnetic rotation enhanced concentration modulation spectroscopy in the visible region. The （1, 2）, （1, 3） and （2, 3） bands of c^3Ⅱu- b^3Ⅱg in the range 16620-17860cm^-1 are observed and their 3II2 3II2 subbands are rotationally analysed. A set of effective molecular constants for the Ω= 2 component of the states involved are determined.     

Appropriate Gate Time for Single Molecular Photon Detection Based on Optimal Signal-to-Noise Ratio Analyses

Signal-to-noise ratio of fluorescence detection from a single molecule ＆ analysed by using time-gated techniques. It is found that the optimal signal-to-noise ratio can be obtained by choosing an appropriate gate time with a certain optical background. The dependences of molecular fluorescence lifetime and the optimal signal-to-noise ratio on the appropriate gate time are respectively discussed with two kinds of background sources~ chaotic state with uniform distribution and coherent state with exponential distribution in time domain. For chaotic state background we find that a certain range for appropriate gate time can be obtained with a definite fluorescence lifetime, larger fluorescence lifetime would lower the value of optimal signal-to-noise ratios. For coherent state background we find that there is also a narrow range of appropriate gate time when lifetime of single molecule is less than that of background photons.     

Determination of resonance Raman scattering cross sections of laser dyes using a traveling-wave amplifier

Received: 1 April 1998/Revised version: 20 April 1998     

Quantitative measurements of the CH radical in sooting diffusion flames at atmospheric pressure

The potential of Laser Induced Fluorescence detection of the CH radical using C–X (0–0) excitation is investigated in a sooting methane/air diffusion flame at atmospheric pressure. Fluorescence is detected using the very narrow (<0.4 nm) Q-branch of the C–X (0–0) band, which enables the measurement of CH in sooting flames without interference from PAH fluorescence and soot emissions. Absolute concentrations are obtained using Cavity Ring Down Spectroscopy. 1D CH profiles in the sooting zone are recorded using a CCD camera with an excellent signal-to-noise ratio. The C–X (0–0) excitation associated with Q-branch detection is shown to be three times more efficient than the B–X scheme. Received: 4 March 2002 / Revised version: 5 November 2002 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +33-3/2033-6463, E-mail: eric.therssen@univ-lille1.fr     

Spectroscopic diagnostics of chemical processes: applications of tunable optical parametric oscillators

Optical parametric oscillator (OPO) and amplifier (OPA) devices are useful for spectroscopic sensing of chemical processes in laboratory, industrial, and environmental settings. This is particularly true of nanosecond-pulsed, continuously tunable OPO/OPA systems, for which we survey a variety of instrumental strategies, together with actual spectroscopic measurements. The relative merits of OPO wavelength control by intracavity gratings and by injection seeding are considered. A major innovation comprises an OPO with a ring cavity based on periodically poled lithium niobate (PPLN) and injection-seeded by a single-mode tunable diode laser (TDL). Active cavity control by an ‘intensity dip’ method yields an optical bandwidth ≤0.005 cm^-1 (150 MHz), which compares favourably with the performance of advanced grating-tuned OPO/OPA systems. A novel adaptation of this TDL-seeded PPLN OPO employs a compact, inexpensive multimode pump laser, with which it is still possible to obtain continuously tunable single-mode signal output. Cavity ringdown (CRD) spectroscopy also figures prominently, with infrared (IR) CRD spectra from both grating-scanned and TDL-seeded OPOs reported. Finally, a tunable ultraviolet (UV) source, combining a TDL-seeded passive-cavity OPO and a sum-frequency generation stage, is developed for measurements of time-resolved IR-UV double resonance spectra of acetylene and UV laser-induced fluorescence spectra of nitric oxide. Received: 28 March 2000 / Published online: 13 September 2000     

Laser spectroscopy with a pulsed, narrowband infrared optical parametric oscillator system: a practical, modular approach

-1 in a single scan. The potential of the OPO system for linear and nonlinear-optical spectroscopy is demonstrated by recording high-resolution photoacoustic absorption and coherent anti-Stokes Raman spectra of methane, as a gas and in a pulsed supersonic free jet. This narrowband tunable infrared source is shown to scan reliably with an optical bandwidth as small as 0.007 cm^-1 (210 MHz) full width half maximum (fwhm), close to the fourier-transform limit. Received: 5 February 1998/Revised version: 6 March 1998     

Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases

Spectroscopic sensing of gases can be performed with high sensitivity and photometric precision by cavity ringdown (CRD) absorption spectroscopy. Our cavity ringdown spectrometer incorporates continuous-wave (cw) tunable diode lasers, fibre-optic coupling and standard photonics and optical telecommunications components. It comprises a rapidly swept optical cavity in a single-ended optical heterodyne transmitter–receiver configuration, enabling optical absorption of gases to be recorded either as single-frequency scanned spectra or as simultaneous, multi-wavelength tailored spectra. By measuring weak near-infrared rovibrational spectra of carbon dioxide gas (CO₂), with high resolution in the vicinity of 1.53 μm, we have realised a noise-limited absorption sensitivity of 2.5×10^-9 cm^-1 Hz^-1/2. Analytical sensitivity limits (both actual and projected) and prospective gas-diagnostic applications are discussed. Our approach to cw-CRD spectroscopy offers high performance in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medical, agricultural, industrial and environmental situations. Received: 16 May 2002 / Revised version: 3 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/9850-8313, E-mail: brian.orr@mq.edu.au     

Mechanism of and method to avoid discoloration of stainless steel surfaces in laser cleaning

2 O₃. Based on Auger Electron Spectroscopy analyses and optical microscopic observations, γ-Fe₂O₃ and Fe₃O₄ are formed on the stainless steel surface in laser cleaning in the air. Since laser can induce high temperature rise in stainless steel surfaces, the above phenomenon can be explained by a thermochemical reaction between oxygen in the air and the stainless steel. With increasing laser fluences, the temperature rise in the irradiated area of stainless steel surface increases, which enhances oxygen diffusion into the surface and oxidation reaction within the irradiated area. In order to avoid discoloration of stainless steel surfaces, a vacuum system was used to reduce the oxidation reaction between oxygen and stainless steel. Received: 7 June 1996/Accepted: 30 September 1996     

Bound states and dipole polarizability of hydrogen molecular ion H2 in weakly coupled hot plasmas

The effects of hot-dense plasmas on the bound states and the dipole polarizability of the ground state of Coulomb three-body molecular ion H⁺₂ have been investigated using highly correlated basis functions and by considering the Debye shielding approach of plasma modeling. The ground S state and the first excited P state energies along with the dipole polarizability for different shielding parameters are reported.     

A theoretical model for laser removal of particles from solid surfaces

Received: 13 November 1996/Accepted: 30 January 1997     

Single-shot laser-induced fluorescence imaging of formaldehyde with XeF excimer excitation

Single-shot formaldehyde laser-induced fluorescence (LIF) imaging measurements in a technical scale turbulent flame have been obtained using XeF excimer laser excitation in the ?¹A₂-˜X¹A₁ transition at 353.2 nm. Measurements have been carried out in a 150 kW natural gas swirl burner where formaldehyde distribution fields have the potential, in combination with OH concentration fields, to visualize the heat release distribution and therefore give an optimal visualization of flame-front positions. The extended areas where formaldehyde was detected in the swirl flame indicates the presence of low temperature chemistry in preheated gas pockets before ignition. Received: 31 January 2000 / Revised version: 2 March 2000 / Published online: 5 April 2000     

Observation of iodine transitions using the second and third harmonics of a 1.5-μm laser

This paper presents spectroscopic measurements of iodine at 778 nm and 518 nm performed by second and third harmonics of a 1.5-μm diode laser, generated by quasi-phase matching in a periodically poled lithium niobate waveguide. Sub-Doppler spectroscopy by a 780-nm source was also demonstrated, and shows the potential of the system to reach high levels of frequency stability by locking the laser to the iodine transitions. The suggested method significantly improves the number of frequency references available for stabilizing 1550-nm lasers. Received: 4 February 2002 / Revised version: 14 May 2002 / Published online: 8 August 2002     

Absolute frequency measurement of a CO2/OsO4 stabilized laser at 28.8 THz

A cw carbon dioxide laser operating on the 10 μm R(0)_I transition (28.832 THz) was frequency stabilized by a servo lock to the saturated absorption dip of the Q(15) transition of ¹⁸⁸OsO₄. The laser frequency was measured with a cesium-clock-based frequency chain. In addition, the absorption line frequencies Q(14) of ¹⁸⁸OsO₄ and sQ(4,3) of ¹⁵NH₃, were measured relative to the frequency of Q(15). Received: 21 August 2000 / Revised version: 15 January 2001 / Published online: 30 March 2001     

Exhaust-gas imaging via planar laser- induced fluorescence of sulfur dioxide

The potential use of planar laser-induced fluorescence (PLIF) of sulfur dioxide (SO₂) for visualization of exhaust-gas distributions is outlined and demonstrated. Strong absorption features in the UV spectral range allow excitation of SO₂ with the fourth harmonic of a Nd:YAG laser at 266 nm. Fluorescence emissions are mostly red-shifted and can be easily detected in single-shot imaging arrangements with a good signal-to-noise ratio. This study uses a premixed methane/air flame that is doped with SO₂ to demonstrate the technique. The signal strength has a pronounced temperature dependence for excitation at 266 nm. Received: 14 January 2002 / Revised version: 30 January 2002 / Published online: 14 March 2002     

Electronic absorption and emission spectra of Alq3 in solution with special attention to a delayed fluorescence

Tris(8-quinolinolato)aluminum(III) (Alq₃) shows electronic absorption bands at 378, 360 (in a 1:1 mixed solvent of methanol and ethanol (ME) at 77 K), 334, 316, 300, 263, 255.8, and 233 nm in ethanol at room temperature. According to the polarized fluorescence excitation spectrum together with MO calculations, for instance, the 360 nm band is assigned to an LL CT transition (an intramolecular charge transfer transition between two ligands), and the 378 nm band to an LM/ML CT one (an intramolecular charge transfer transition between ligand and metal). Alq₃ shows a broad fluorescence band peaking at around 478 nm in the ME matrix at 77 K. The emission spectrum measured with a phosphoroscope has two emission bands at 567 and 478 nm. The 567 nm band accompanies vibronic bands at 578 and 605 nm, being safely assigned to a phosphorescence of Alq₃. The lifetimes of the 478 and 567 nm bands are both 5.4 ms. The lifetime of the 478 nm band together with the band position and its band shape indicate that this band can be assigned to a delayed fluorescence.