Laser-induced Fluorescence Studies Research Articles

Fluorescence characteristics of organic tracer molecules for planar laser-induced fluorescence studies in internal combustion engines, part A: non-aromatics

Fluorescence characteristics of organic tracer molecules for planar laser-induced fluorescence studies in internal combustion engines, part A: non-aromatics

Two-photon laser-induced fluorescence study of the CO B 1Σ+ (v' = 0) state in a 4850K plasma plume: Modified molecular constants, evidence of predissociation, and J'-dependent photoionization.

We report the two-photon absorption laser-induced fluorescence rotational spectrum of the CO B 1Σ+ ← X 1Σ+ Hopfield-Birge system (v' = 0, v″ = 0) Q-branch in an ∼4850K, atmospheric pressure plasma torch plume at thermal equilibrium in both the quenching-dominated (low laser intensity) and photoionization-dominated (high laser intensity) regimes. We provide a detailed analysis of the photophysics in these two regimes using a rate equation approach and propose modeling considerations for them as well. In the experimental spectra, distinct rotational transitions up to J″ = 83 are observed, allowing analysis over a very large range of rotational states. Evidence of predissociation is observed for J' ≥ 64 and is likely due to the interaction with the D'1Σ+ electronic state, which has been proposed in the literature but never observed in the v' = 0 state. The line positions of higher rotational states show disagreement with line positions calculated from molecular constants in the available literature, suggesting the need for modifications to the constants, which are reported here. A shift in the B 1Σ+ ← X 1Σ+ absorption spectrum toward higher two-photon energy as a result of the second-order Stark shift was observed in the photoionization-dominated spectrum, and the second-order Stark shift cross section was estimated to be 7 ± 3 × 10-18 cm2. The mean photoionization cross section of the excited upper state was inferred by comparing the line broadening of the two spectra and was estimated to be 11 ± 7 × 10-18 cm2. In addition, weak J'-dependent variations of the photoionization cross section were observed and are reported here.

Optical System Design for Laser-Induced Fluorescence Study of Helium Atoms on the Linear Plasma Device

According to the characteristic spectral lines of helium atoms, an optical system for the laser-induced fluorescence study of helium atoms has been designed. The design includes a helium spectral scheme and a laser injection and fluorescence collection system. The diode laser generates a 667.8-nm laser, and the laser is injected into the linear plasma device through an optical fiber. The fluorescence collection system detects 501.6-nm fluorescence signals. Experiments were carried out on the linear plasma device during helium discharge, simulating the helium ash environment at the boundary of the fusion reactor. The fluorescence collection was realized, and the fluorescence signals showed an increasing trend with laser power. Atomic density calibration and study will be performed based on the collected signals in the next step.

Laser-Induced Fluorescence Studies on Some Edible Oils and Aromatic Frankincense Oil Excited by Blue and Violet Diode Lasers at 447 nm and 405 nm

Laser-induced fluorescence (LIF) of certain edible oils (olive, mustard, sunflower, corn, sesame, peanut, rice bran, and flaxseed) and one unique aromatic oil (frankincense oil) have been studied using compact blue and violet lasers operating at 447 nm and 405 nm, respectively. LIF studies excited at these wavelengths have not been performed before, to our knowledge. The various features of the obtained spectra and their possible molecular origins have been discussed. The presence of vitamin E has been confirmed in corn, rice bran, peanut, sunflower, and frankincense oils, and the possible origin of the double peaks in the red region for olive oil has been explained.

Electronically Excited Complex Formation in Magnesium Cluster-Halogen Atom Reactions.

A near ultraviolet transition of Mg2F has been observed in emission from the reaction between magnesium clusters, most likely Mg3, and fluorine atoms. Because there is little evidence for upper-state internal excitation, the spectrum is assigned assuming that the upper state is quenched to its lowest vibrational levels. Two of possibly three ground-state vibrational frequencies, υ1 = 516 ± 10 cm-1 and υ2 = 104 ± 10 cm-1, have been established. Dispersed laser-induced fluorescence studies extrapolating on the observed chemiluminescence indicate an excited-state symmetric stretch frequency of order 370 ± 30 cm-1. Electronic structure calculations at the CCSD(T)/CBS level predict that the ground state of Mg2F has C2v symmetry and can be described as an Mg2+F- ion pair with two Mg-F bonds. Like the MgF A-X transition that is largely a transition between Mg orbitals, the observed transition in Mg2F is largely between orbitals on the magnesium dimer ion. The asymmetric C∞v Mg2+F- complex is also a minimum and is predicted to be 6.7 kcal/mol higher in energy. Calculated structures for the Mg2Cl isomers are also presented and used to further interpret the experimental results for the reaction of Mg clusters with Cl atoms. In contrast to Mg2F, the ground state of Mg2Cl is a linear C∞v MgMgCl structure with the C2v and D∞h isomers of the MgClMg structure slightly higher in energy.

Development of Low-Cost Prototype N<sub>2</sub> Laser System and Laser-Induced Fluorescence of Pyranine

This work aims to a detailed description of the development of a prototype Transversely Excited Atmospheric (TEA) nitrogen laser and its high-tension electrical pump source, along with its application study of Laser-Induced Fluorescence (LIF). The high-tension pump source is designed and simulated by using NI Multisim to study the voltage behavior at different points. The high-tension pump source is constructed using the flyback transformer. The open-air laser cavity is designed and simulated by using Zemax Optic Studio. Blumlein transmission line equivalent of nitrogen laser is designed in NI Multisim, voltage and current behavior across laser cavity and spark gap are simulated. The air is used as a lasing medium, as it contains 78% molecular nitrogen. The L-shaped electrodes are used as a cavity in the construction of this N₂ laser system. An ignition system in the form of the low inductance spark gap is built using the two bolts. Generally, the current passes through gas either by transverse or longitudinal discharge; in this work, the transverse discharge technique is used. Nitrogen laser produces a beam with a center wavelength of 337.1nm. Laser-induced fluorescence spectrum of the Pyranine is taken which shows its fluorescence in the green region with a maximum peak at the wavelength of 567.5nm. Pyranine is made up of a mixture of C₁₆H₇Na₃O₁₀S₃ so some other peaks can also be seen in the fluorescence spectrum with low intensity.

Fluorescence-based Techniques to Study the Structure and Dynamics of Mass-selected Biomolecular Ions.

Laser-induced fluorescence studies on mass-selected biomolecules are a promising route to understand their properties in the gas phase and probe their intrinsic properties in a solvent-free environment. Fluorescence has been used to investigate the conformation and dynamics of gaseous biomolecular ions. With Förster Resonance Energy Transfer (FRET), it is now possible to obtain sensitive intramolecular distance information from large biomolecules, like proteins, with high chemical specificity. With growing interest and applications, gas-phase fluorescence measurements can shed greater light on the characteristics of proteins in the gas phase. Compared to the solution phase measurements, gas-phase fluorescence can also help understand the influence of solvent interactions on the protein structure and function.

A high-repetition-rate shock tube for transient absorption and laser-induced fluorescence studies of high-temperature chemical kinetics.

A newly constructed high-repetition-rate shock tube designed for kinetic studies of high-temperature reactions using spectroscopic methods is described. The instrument operates at a 0.2-Hz cycle rate with a high reproducibility of reaction conditions that permits extensive signal averaging to improve the quality of kinetic trace data. The density and temperature of the gas behind the reflected shock wave are examined by probing the product formation from reference reactions. Two types of experimental techniques are implemented: transient absorption spectroscopy and time-resolved laser-induced fluorescence. Both methods are shown to be suitable for kinetic measurements of elementary reactions, as illustrated by their application in thermal decomposition reactions of the benzyl radicals and trifluoromethane.

Imaging and time-resolved study of laser-induced fluorescence of dysprosium atoms injected into superfluid helium

We study the time evolution of the cloud of Dy atoms dispersed in superfluid helium and investigate the possibility of using these atoms as tracers for the imaging of superfluid-helium flows. The atoms are produced by pulsed laser ablation and are visualized via their laser-induced fluorescence. It turns out that the observation time is limited by the optical pumping of the atomic population into a nonabsorbing metastable state which is also responsible for the saturation of the fluorescence yield observed earlier.

A strong interaction between torsion and vibration in S0 and S1m-fluorotoluene.

We report results of a two dimensional laser induced fluorescence study of torsional states, low frequency vibrations, and combinations of torsion with low frequency vibration in m-fluorotoluene up to 560 cm-1 in S0 and 350 cm-1 in S1. Evidence is presented for interactions between torsion and low frequency vibrations in both S0 and S1, demonstrating that the coupling of torsion and vibration observed previously in toluene and p-fluorotoluene extends to a molecule with a threefold torsional barrier. This barrier is low in S0 (20 cm-1) and modest in S1 (116 cm-1). The methyl torsion-vibration interaction is much larger for the mode involving out-of-plane wagging of the methyl group with respect to the planar frame compared with the analogous out-of-plane fluorine atom motion. Methyl group out-of-plane modes were found to be most important for torsion-vibration interactions in toluene and p-fluorotoluene, and the evidence is accumulating that this motion is fundamental in torsion-vibration interactions. Fits of the experimental band positions yield torsion-vibration coupling constants, torsional potential terms (V3 and V6), and rotational constants (F) for the methyl torsion in S0 and S1. The inclusion of torsion-vibration coupling primarily affects V6 and F: |V6| is reduced and F increased, as was seen previously for the G12 molecules, toluene and p-fluorotoluene. The torsional barrier height does not appear to influence the magnitude of the torsion-vibration interaction: the coupling constants for the out-of-plane CH3 wag mode are almost the same in S0 and S1 (15.5 cm-1 and 14.0 cm-1, respectively).

Laser induced fluorescence studies on the distribution of surfactants during drop/interface coalescence

The spatiotemporal distribution of fluorescent surfactants on the merging interfaces during the coalescence of an aqueous drop with an organic/aqueous flat interface was studied experimentally with high-speed laser induced fluorescence. The aqueous phase was a 46% glycerol solution, while the organic phase was a 5 cSt silicone oil. A fluorescently tagged surfactant was used at a concentration of 0.001 mol/m3 in the aqueous phase. To vary the concentration of surfactants on the interfaces, the drop and the flat interface were left to stand for different times before the coalescence experiments (different interface ages). It was found that when a drop rested on the interface, the surfactants adsorbed on the interfaces were swept outwards by the draining liquid film between the drop and the flat interface and reached a peak value at 0.75Rh away from the centre of the film, where Rh is the horizontal drop radius. After the film rupture, the concentration of the surfactants at the tip of the meniscus increased. Once the film had retracted, the concentration of the surfactants peaked at the meniscus at the bottom of the drop. As the liquid in the drop started to merge with its homophase, the drop formed a cylinder from the upward capillary waves on the drop surface. The surfactant concentration was found to be low at the top of the liquid cylinder as the interface was stretched by the convergence of the capillary waves. Subsequently, the cylinder began to shrink and the top part of the drop acquired a high surfactant concentration.

Infrared spectroscopy of the NO3 radical from 2000 to 3000 cm−1

The present paper reports high-resolution spectroscopic study of the 14NO3 radical in the 2000–3000 cm−1 region, where eight E′-A2′ bands from the ground state are observed. Three bands at 2206, 2246, and 2377 cm−1 are analyzed for the first time, and assigned to the ν1+3ν4, 2ν2+2ν4, and ν3+3ν4 bands, respectively. Bands at 2024, 2155, 2518, and 2585 cm−1 are reassigned to the ν1+ν3, 2ν3, ν1+ν3+ν4, and 2ν3+ν4 bands, respectively, by adopting the new ν3 vibrational frequency of 1055 cm−1 lower than the previous ν3=1492 cm−1. The band at 2902 cm−1 is observed for the first time and assigned to the ν1+ν3+2ν4 band which is the ν1 combined band with the 1927 cm−1 band. Band intensities observed in the 2000–3000 cm−1 region are attributed to the intensity borrowing from the B̃2E′–X̃2A2′ electronic transition through the vibronic interaction. Although the ν3 fundamental band has not been observed due to the cancelation of vibrational intensity and borrowed intensity, the 2ν3 band becomes stronger than ν3 by a factor of more than 50. Perturbation effects are recognized for the bands observed except for the 2206 cm−1 and 2377 cm−1 bands, and are analyzed by taking into account the Coriolis interaction in the most cases. However, the 2024 cm−1 band is free from the Coriolis interaction, and the v1–v3 interaction is incorporated in the analysis, leading to the 2ν1 frequency of 2008.8 cm−1, which is close to the energy value of 2010 cm−1 observed by a laser induced fluorescence study.

Kinetics of gas phase OH radical reaction with thiophene in the 272–353 K temperature range: A laser induced fluorescence study

Absolute rate coefficient for the gas phase reaction of OH radical with thiophene has been measured using the laser photolysis (LP) – laser induced fluorescence (LIF) technique. The kinetics measurements were done over the temperature range of 272–353K and pressure range of 20–30Torr. The bimolecular rate coefficients obtained were pressure independent in the above pressure range and showed Arrhenius type behaviour which were fitted to the expression k(T)=(4.53±0.55)×10−12exp[(250±30)/T]cm3molecule−1s−1. The reaction shows weak negative temperature dependence. These results were discussed with available literature and the tropospheric lifetime of thiophene was calculated.

According theory and experiment in CaH: Laser-induced fluorescence study of new B/B′–X bands in the UV region

Despite the astrophysical importance of calcium monohydride (CaH), a long-standing discrepancy exists between the experimental and theoretical analysis of its first two excited 2Σ+ states. In a bid to resolve this discrepancy, we observed the rotationally-resolved laser-induced fluorescence spectrum of CaH in the 23,300–27,800cm−1 region. We assigned all newly observed vibrational levels, and five levels previously assigned to the D state, to the B/B′ state. The level properties alternate strongly with vibrational excitation and this new assignment brings the experimental vibronic structure into remarkably good agreement with the predictions of Carlsund-Levin et al. (2002).

Observation of a new channel, the production of CH3, in the abstraction reaction of OH radicals with acetaldehyde.

Using laser flash photolysis coupled to photo-ionization time-of-flight mass spectrometry (PIMS), methyl radicals (CH3) have been detected as primary products from the reaction of OH radicals with acetaldehyde (ethanal, CH3CHO) with a yield of ∼15% at 1-2 Torr of helium bath gas. Supporting measurements based on laser induced fluorescence studies of OH recycling in the OH/CH3CHO/O2 system are consistent with the PIMS study. Master equation calculations suggest that the origin of the methyl radicals is from prompt dissociation of chemically activated acetyl products and hence is consistent with previous studies which have shown that abstraction, rather than addition/elimination, is the sole route for the OH + acetaldehyde reaction. However, the observation of a significant methyl product yield suggests that energy partitioning in the reaction is different from the typical early barrier mechanism where reaction exothermicity is channeled preferentially into the newly formed bond. The master equation calculations predict atmospheric yields of methyl radicals of ∼9%. The implications of the observations in atmospheric and combustion chemistry are briefly discussed.

Direct observation of methyl rotor and vib-rotor states of S toluene: A revised torsional barrier due to torsion-vibration coupling

We report a two dimensional, laser induced fluorescence study of the lowest 345 cm(-1) region of S0 toluene. Methyl rotor levels of 00 up to m = 6 and of 201 up to m = 4 are observed. The rotor levels of 00 and 201 have quite different energy spacings that are well fit by a model that includes strong torsion-vibration coupling between them. The model requires that the rotor barrier height be revised from -4.84 cm(-1) (methyl hydrogens in a staggered conformation) to +1.57 cm(-1) (eclipsed conformation). However, the 3a2″ state lies below the 3a1″ state as expected for a staggered conformation due to energy shifts associated with the torsion-vibration coupling. It is shown that the rotor wave-functions exhibit little localization at the torsional energy minima. The variation in the m = 0 wavefunction probability distribution with torsional angle is shown to be very similar for the previously accepted negative V6 value and the torsion-vibration coupling model as this coupling shifts the phase of the wavefunction by 30° compared with its phase for V6 alone. The presence of a strong Δυ = ± 1 torsion-vibration coupling involving the lowest frequency vibrational mode provides a potential pathway for rapid intramolecular vibrational energy redistribution at higher energies.

OH formation dynamics in 193 nm photolysis of 2-methoxyethanol: A laser induced fluorescence study

Dynamics of OH radical formation in the 193nm photolysis of 2-methoxyethanol is studied using Laser Photolysis–Laser Induced Fluorescence technique. The nascent state distribution of the OH radical is measured. The OH fragments are formed vibrationally cold, characterized by a Boltzmann-like single rotational temperature of 450±100K. The spin–orbit and Λ-doublet ratios of OH fragments are measured. The relative average translational energy of the OH channel is determined to be 17.0±3.0kcal/mol. The experimental studies along with theoretical calculations suggest a complex mechanism for OH formation consisting of at least three pathways. The prominent pathway at shorter timescale (<50ns) involves crossing over to the nearby repulsive state, whereas, at longer timescale (>1ms) involves a series of reaction with initial H3C–OCH2CH2OH bond cleavage, followed by rearrangement of OCH2CH2OH to CH2OCH2OH, and a final concerted step to generate OH and ethylene epoxide.

Laser Induced Fluorescence Studies of Blood Plasma and Tumor Tissue of Men with Prostate Tumors

Objectives: Fluorescence spectroscopy which can be used for optical tissue diagnosis of tumor pathology deserves special interest. The purpose of the work was to study blood plasma and tumor tissue of men with different forms of prostate tumors by using laser induced fluorescence. Blood plasma and tumor tissue of the patients with benign hyperplasia of the prostate (BHP), BHP with inflammation, BHP with high grade PIN (BHP with HGPIN) and adenocarcinoma of prostate (CaP) have been studied. Results: In case of blood plasma fluorescence, intensity of the plasma proteins corresponding peak (340 - 360 nm) was increasing in the following manner: control group → BHP → BHP with HGPIN → CaP. The intensity of the nicotinamide coenzymes correspond peak (440 - 460 nm) was increased in case of BHP with HGPIN and CaP patients, but decreased in case of BHP, compared to control. In case of tumor tissue, the changes of the collagen peak (390 - 400 nm) intensity have been revealed in all cases of prostate tumor tissues. These alterations point to altered collagen biosynthesis levels in different tumor tissues, that reflects the structural changes and characteristics of malignant transformation. Also the changes of the nicotinamide coenzymes peak (440 - 460 nm) intensity in all spectra of tumor tissues were observed. The highest intensity of the peak was observed in the spectra of BHP with HGPIN and in prostate cancer tissue. Conclusions: Alterations of the coenzymes peak intensities perfectly reflect and are in accordance with the specific energy metabolism of prostate epithelial cells. Normalization of fluorescent spectra from different forms of prostate tumor tissues has shown that, each form has typical spectral shape and ratio of fluorescence peaks intensities.

Laser-Induced Fluorescence Study of the S1 State of Doubly-Substituted 13C Acetylene and Harmonic Force Field Determination

In the first half of this study, rotational and vibrational constants of six Franck-Condon bright vibrational levels of S1 doubly-substituted (13)C acetylene are determined from laser-induced fluorescence spectra and an updated geometry of the trans conformer of S1 acetylene is obtained. In the second half, we determine the quadratic force field of S1 acetylene on the basis of the harmonic frequencies of four isotopologues of acetylene. The effects of both diagonal and off-diagonal x(ij) anharmonicities are removed from the input harmonic frequencies. Results from both experimental and theoretical studies of various isotopologues of acetylene (including those from the first half of this paper) are used to obtain a set of force constants that agrees well with ab initio calculations. Our set of force constants for S1 acetylene is an improvement over previous work by Tobiason et al., which did not include off-diagonal anharmonicities.

Time-resolved temperature study in a high-power impulse magnetron sputtering discharge

The gas heating dynamics is studied in a high-power impulse magnetron sputtering discharge operating in Ar-N2 gas mixtures. The time-resolved rotational temperature analysis based on the spectral transition between the B2Σu+-X2Σg+ energy levels in molecular nitrogen ion (N2+ First Negative Band) is undertaken for this purpose. The rotational temperature in the discharge is found to increase linearly during the plasma pulse being roughly independent on the nitrogen content in the examined range. Such a temperature increase is attributed to the bulk gas heating which is the result of collisions with the sputtered species. Two sputtered materials, Ti and W, are examined during the study. In the case of W sputtering, the gas heating is found to be more pronounced than in the Ti case, which is explained by more efficient energy exchange between the sputtered W atoms and the bulk gas atoms during the plasma on-time. The obtained temperature data are compared to the laser-induced fluorescence study of Ar metastable atoms performed recently in the same discharge in our group. The particularities related to gas thermalization as well as to validity of the utilized approach for characterization of the pulsed sputtering discharges are discussed.