Ultraviolet Laser-induced Fluorescence Research Articles

Behavior of F Atoms and CF2 Radicals in Fluorocarbon Plasmas for SiO2/Si Etching

Densities of F atoms and CF2 radicals were measured in CF4, C2F6 and CHF3 plasmas by vacuum ultraviolet laser absorption and laser-induced fluorescence techniques, respectively, using an RF (400 kHz) plasma source. In the measurement of source gas dependence, the F atom density became highest in CF4 and lowest in CHF3, while the tendency was reversed for the CF2 radical density. Dilution of the source gases with O2 caused increases in the F atom density and decreases in the CF2 density, but dilution with H2 brought about significant decreases in the F atom density. The density of CF2 radicals was largely affected by the chamber wall conditions, with or without polymer deposition. The decay time constants of these species were also measured during the pulsed afterglow, and the results were qualitatively consistent with the behavior of the densities. However, some quantitative disagreements remain, suggesting that the loss rates of these species in active plasmas are different from the rates in the afterglow.

State-to-state inelastic scattering from vibrationally activated OH–H2 complexes

State-selective infrared excitation of o-H2–OH via the pure OH overtone transition has been used to induce a half-collision inelastic scattering event between the OH radical and ortho-H2 under restricted initial orientation conditions. The time evolution and final state distribution of the OH products from vibrational predissociation have been evaluated by ultraviolet probe laser-induced fluorescence measurements. The half-collision scattering takes place with ∼3350 cm−1 of energy available to the OH (v=1)+o-H2 products, an energy that exceeds the classical barrier to reaction. The OH (v=1) products are preferentially populated in high rotational levels with a distribution that is consistent with an energy gap law. A significant fraction of the OH fragments are promoted to the excited spin–orbit state in the predissociation process. A strong lambda-doublet propensity is also found, indicating that the OH unpaired pπ orbital is preferentially aligned perpendicular to the rotational plane of the OH products. Finally, the OH rotational and fine structure distributions are compared with those obtained in previous full collision inelastic scattering studies at energies below the threshold for reaction.

Photofragmentation of OClO(Ã2A2ν1ν2ν3) → Cl(2PJ) + O2

The photodissociation dynamics of OClO(2A2 ν1ν2ν3) → Cl(2PJ) + O2 is investigated in the wavelength region between 360 and 450 nm. Observation of nascent chlorine atom fragments produced in their electronic ground state is performed by means of three-photon excitation with subsequent recording of the vacuum ultraviolet laser-induced fluorescence. Mode-specific branching ratios between the dominant ClO(X2ΠΩ) + O(3PJ) and the minor Cl(2PJ) + O2 channel are obtained. The observed quantum yield for Cl production is determined to be below 3.6% at photolysis wavelengths between 365 and 450 nm depending on the respective vibrational level of OClO(2A2ν1ν2ν3). At dissociation wavelengths below 365 nm, a sharp rise in the chlorine formation is observed resulting from a secondary photolysis step where vibrationally excited ClO(X2ΠΩ, v ≥ 4) radicals are dissociated.

Ultraviolet laser-induced fluorescence detection strategies in capillary electrophoresis: determination of naphthalene sulphonates in river water

Various UV-laser-induced fluorescence detection strategies for capillary electrophoresis (CE) are compared, i.e. two UV-laser systems (a pulsed laser providing up to 25 mW of tunable emission, applied at 280, 290 and 325 nm, and a continuous wave (cw) laser providing up to 100 mW of 257 nm emission) and different methods to collect the fluorescence emission signal and to reduce the background. Attention is focused on the determination of amino- and hydroxy-substituted naphthalene sulphonates (NS) in river water; these analytes exhibit native fluorescence upon UV excitation. Optimum results were obtained by applying only a minor portion of the available (average) laser powers, viz. 0.7 mW at 280 nm for the pulsed laser, and 5 mW for the cw laser. For emission collection, the most favourable results were obtained with a mirror-based microscope objective, which facilitates efficient spatial filtering and does not produce impurity fluorescence upon UV-laser irradiation. For standard solutions, the cw laser gave around 20-fold better detection limits (10 −9–10 −10 M) than the pulsed laser. For river water, excitation of interferences (presumably humic acids, which exhibit native fluorescence) could be much better suppressed if the pulsed laser was used with selective excitation at 280 nm. Therefore, for real-sample analysis the latter combination is to be preferred. The set-up was used for the identification and quantification (at the 1–35 μg l −1 level) of NS in a river Elbe sample.

Photodissociation of Ozone at 248 nm and Vacuum Ultraviolet Laser-Induced Fluorescence Detection of O(1D)

Ozone molecules, O3, were photodissociated in the presence of N2 at 248 nm (KrF laser) to O(1D)+ O2(1∆). The O(1D) atoms were detected by generating vacuum ultraviolet laser-induced fluorescence (VUV LIF) for the 3s1D0 -2p 1D transition at 115.2 nm. The 115.2 nm probe laser was generated by frequency tripling (ω vuv=3ω) of the 345.6 nm PTP dye laser in a Xe gas cell.

Dynamics of H Atom Formation in the Photodissociation of Chloromethanes at 193.3 nm

Using the laser photolysis/vacuum−ultraviolet laser-induced fluorescence (LP/VUV−LIF) “pump-and-probe” technique the dynamics of H atom formation after photoexcitation of chloromethanes at 193.3 nm were studied in the gas phase at room temperature under collision-free conditions. For all chloromethanes, H atoms were detected by (2p2P ← 1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα ≈ 121.6 nm) generated by resonant third-order sum−difference frequency conversion of pulsed-dye-laser radiation. However, only in the cases of CH3Cl and CH2Cl2 were the H atoms found to originate from a primary photodissociation step. Absolute quantum yields for the formation of primary H atoms were measured by means of a calibration method to be φH(CH3Cl) = (1.2 ± 0.6) × 10-2 and φH(CH2Cl2) = (0.2 ± 0.1) × 10-2. From H atom Doppler profiles measured under single-collision conditions, the average translational energy released to the H + CH2Cl and H + CHCl2 products in the center-of-mass system was determined to be: ET(H−CH2Cl) = (86.6 ± 14.2) kJ/mol and ET(H−CHCl2) = (84.3 ± 8.9) kJ/mol. On the basis of available thermochemical data, the corresponding fraction of the available energy released as product translational energy was determined to be fT(H−CH2Cl) = (0.44 ± 0.07) and fT(H−CHCl2) = (0.41 ± 0.04). In the CHCl3 photodissociation, primary H atom formation was not observed. The H atoms detectable after laser irradiation of CHCl3 at 193.3 nm were found to originate from secondary photodissociation of the CHCl2 radical.

Micellar electrokinetic capillary chromatography with laser-induced fluorimetric detection of amines in beer.

Capillary electrophoresis with simultaneous ultraviolet absorbance and laser-induced fluorescence detection is applied to identify and quantify selected amines in beer following derivatization with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole. Quantitation was performed using the method of standard addition in order to avoid pH-dependent variations in the reactivity of the derivatizing agent with the added benefit of verifying peak identity. An inexpensive and "easy-to-use" on-column fiber optic fluorescence detection cell is described and implemented in the analysis.

Ultraviolet laser-induced fluorescence of human stomach tissues: detection of cancer tissues by imaging techniques.

The background for this work was several literature reports on applications of the fluorescence methods to detection and localization of human cancers. The objective of our study has been to investigate if such an approach could be applied for the detection of gastric cancers. Our study was designed in such a way that spectrally resolved images of laser-induced fluorescence of human gastric mucosa were collected and assessed from a point of view of elaborating an algorithm allowing for a differentiation between malignant and premalignant lesions and areas of normal mucosa. The method involved exciting the autofluorescence with ultraviolet light (325 nm, He-Cd laser). The images were recorded in vitro in six regions of a visible spectrum using a cooled CCD camera. The material for study was 21 resected specimens for which altogether 72 surface areas were examined. The main result is the observation that a difference of the fluorescence intensities measured at 440 nm and 395 nm, both normalized to intensity measured at 590 nm, differs significantly for the tissues of interest. Using that difference as a diagnostic parameter, it was possible to classify malignant tumor tissues with a sensitivity of 96% and a predictive value of 42%, whereas the same approach applied to abnormal but not tumor stomach tissues gave values of 80%, and 98%, respectively.

Detection of O(3P2) by Vacuum Ultraviolet Laser Spectroscopy

O atoms in the 3P2 state were detected by vacuum ultraviolet laser-induced fluorescence (VUV LIF) technique. The O(3P2) atoms were produced by the collision between the O atoms in the 1D excited state and collision partner N2. O atoms in the 1D excited state were obtained by photodissociation of N2O at 193 nm laser light. As O(3P2) species has their first transition in the VUV region, the probe laser was used to excite O(2p3Pj) atoms to the O(3s3So) state at 130.212 nm. A four wave difference frequency mixing (2ω1−ω2) process using Kr as nonlinear media was employed to generate 130.212 nm probe laser light.

Herbicide analysis by micellar electrokinetic capillary chromatography

Capillary electrophoresis with ultraviolet detection (CE-UV) and laser-induced fluorescence detection (CE-LIF) was used for analysis of a group of herbicides that have widespread use in the USA. CE-UV was employed for simultaneous determination of atrazine, simazine, alachlor and metolachlor in water. In addition, CE-UV was also suitable for analysis of dicamba, 2,4-D and chlorimuron ethyl. Dicamba, 2,4-D and chlorimuron ethyl were also analyzed using CE-LIF following derivatization with fluorescent reagents. Dicamba and 2,4-D were derivatized with 4-bromomethyl-7-methoxycoumarin and chlorimuron ethyl was derivatized with dansyl chloride following hydrolysis. The detection limit with CE-UV for atrazine, simazine and metolachlor was 0.1 μg/l and for alachlor was 1.0 μg/l. The estimated detection limit with CE-LIF for dicamba, 2,4-D and chlorimuron ethyl was 10 ng/l. Our results demonstrate that CE provides a powerful new analytical tool for herbicide analysis.

18] Two-dimensional liquid chromatography—capillary electrophoresis techniques for analysis of proteins and peptides

Publisher Summary The probability of resolving all the components in a sample decreases, as the number of components in that sample increases. Mathematical models, set forth by Giddings, predict that, as sample complexity increases, complete separation of component peaks can be achieved only with high-efficiency separations. Even the most efficient one-dimensional (1D) techniques have insufficient peak capacity for the separation of highly complex biological samples. Two-dimensional (2D) separations have peak capacities approximately equal to the product of the peak capacities of the component 1D separations and are, therefore, preferable for these samples. This chapter discusses three approaches to 2D coupled-column designs. Each approach is comprehensive as defined and each uses a form of liquid chromatography, as the first dimension, with capillary zone electrophoresis, as the second. Liquid chromatography and capillary zone electrophoresis (CZE) are complementary techniques, because their solvent systems are compatible, but their separation mechanisms are different. The interface between the two systems is also simple: either an automated valve or a fluid tee. Detection is by ultraviolet (UV) absorption or laser-induced fluorescence (LIF). The first approach couples microcolumn size-exclusion chromatography (SEC), with CZE, for the separation of proteins. The second approach combines reversed-phase liquid chromatography (RPLC) and CZE for the separation of tryptic peptides. The last approach couples RPLC with fast-CZE, a variation of CZE, enabling fast 2D separations.

Photodissociation Processes of Ozone in the Huggins Band at 308−326 nm: Direct Observation of O(1D2) and O(3Pj) Products

Both O(1D) and O(3Pj) atoms produced in the photodissociation of O3 were directly detected, using a vacuum ultraviolet laser-induced fluorescence technique. The photofragment yield spectra of the O(1D) and O(3Pj) atoms were recorded at 298 K, scanning the photodissociation laser wavelength across the range 308−326 nm in the Huggins band system of O3. The photofragment yield spectrum for O(3Pj) has the vibrational structure as appearing in the absorption spectrum in the Huggins band, while the spectrum for O(1D) is smooth and has no structure. The wavelength dependence of the quantum yield for O(1D) from the O3 photolysis exhibits a dip at every peak of the vibrational structure in the absorption spectrum in the Huggins band. The partial cross sections for the O(1D) production process in the photoabsorption of O3 were determined. The existence of the tail around 315 nm in the wavelength dependence of the quantum yield of O(1D) produced from O3 photolysis was verified. Our results indicate that the tail ari...

Photofragmentation of ClNO in the A-Band: Velocity Distribution and Fine-Structure Branching Ratio of Cl(2Pj) Atoms

The photodissociation of ClNO in the 211−248 nm region has been studied by using photofragment imaging spectroscopy and vacuum ultraviolet laser-induced fluorescence. From the structure of the images of the nascent Cl(2P3/2) and Cl*(2P1/2) atoms, a fragment total translational energy distribution and an anisotropy parameter were derived. Near 235 nm the fragment translational energy distributions exhibit a distinctly bimodal shape. Relative intensities of the slow and fast components vary for two different products, Cl and Cl*. Both components of the Cl* distribution and the major slow component of the Cl state have the anisotropy parameter β = 1.5 ± 0.2 and 1.8 ± 0.2, respectively. The Cl*/Cl branching ratios are 0.93 ± 0.10, 0.45 ± 0.05, and 1.08 ± 0.12 at λ = 213, 235, and 248 nm. The present results provide evidence of strong effects during molecule fragmentation both at internuclear distances close to the equilibrium position due to avoided crossing and at large interfragment separations.

Product Branching Ratios for O(3P) Atom and ClO Radical Formation in the Reactions of O(1D) with Chlorinated Compounds

Product branching ratios for the formation of O(3P) atoms and ClO radicals in the reactions of the electronically excited oxygen O(1D) atoms with chlorofluorocarbons, chloromethanes, and hydrofluoromethanes were measured at room temperature. O(3P2) atoms and ClO radicals were detected directly using the technique of vacuum ultraviolet laser-induced fluorescence. The results are compared with previous investigations, and reaction mechanisms for the product branchings are discussed. Sums of the branching ratios for O(3P) atom and ClO radical formation are almost unity for O(1D) + chlorofluorocarbons, while the sums are less than unity for O(1D) + chloromethanes. Upper limit values of the O(3P) formation yield for O(1D) + H2O, D2O, and CH4 were also determined.

State-specific unimolecular dissociation dynamics of HFCO. II. CO rotational distribution and Doppler widths

Rovibrational state distributions and Doppler widths of CO fragments formed from unimolecular dissociation of HFCO in its ground electronic state are measured by vacuum ultraviolet laser-induced fluorescence, following state-selective preparation of the molecule in a single quantum state in the energy region of 2000 to 3000 cm−1 above the dissociation threshold by stimulated emission pumping. CO fragments are rotationally hot and distributed over J≤15 to J=63 with distributions peaking at J=45 to 50 depending upon the initial HFCO dissociative state. Although CO rotational distributions are significantly different for different initial states, about 20% of the total available energy is released on average as rotational energy of CO for all three initial states studied. The yield of CO(v=1) fragments is determined to be about 10% and CO(v≥2) fragments are not observed. The average Doppler width of CO fragments is 0.85 cm−1, which indicates that ∼50% of the total available energy is released as translation. The CO product state distributions and Doppler widths may be rationalized using a modified impulsive model with the ab initio transition state geometry. The dependence of product state distributions on the initial HFCO quantum state may reflect incomplete intramolecular vibrational energy redistribution.

Reaction dynamics studies of a simple tetraatomic system: AA + BA ↔ A + ABA

We have studied the reaction of translationally excited hydroxyl radicals with molecular hydrogen at different center-of-mass energies. H/D atoms produced in the reaction have been detected under single collision conditions by means of vacuum ultraviolet laser-induced fluorescence at the Lyman-α transition. By calibrating the H/D signals from the reaction against H atom signals from the H2S and HCl photolysis, respectively, the following absolute reactive cross sections were determined: OH + H2 → H + H2O: σR(0.17 eV) = (0.08 ± 0.03) Å2, σR(0.22 eV) = (0.60 ± 0.30) Å2; OH + D2 → D + HOD: σR(0.28 eV) = (0.22 ± 0.05) Å, σR(0.37 eV) = (0.43 ± 0.09) Å2. With translationally excited H atoms, we have also studied the reverse reaction system, H + D2O. In this case the OD product radicals were detected under single collision conditions with quantum state resolution by means of laser-induced fluorescence. By calibrating the OD signals from the reaction against OH signals from the H2O2 photolysis, absolute reaction cross sections were measured for H + D2O → OD + HD: σR(1.5 eV) = (0.07 ± 0.04) Å2, σR(1.8 eV) = (0.10 ± 0.03) Å2, and σR(2.2 eV) = (0.11 ± 0.03) Å2. At different center-of-mass collision energies nascent population distributions of the OD product fine-structure components were determined. It has been found that at all collision energies OD radicals are produced exclusively in their vibrational ground state, with only a small amount of the total available energy appearing in the rotational degree of freedom. A comparison of the dependence of the reaction cross section on the translational energy shows that relative reagent translation is more effective to promote reactivity in the reaction OH + H2 and OH + D2 than in the reaction H + D2O. This, together with a preferential population of the symmetric 2Π(A′) Λ-doublet state (with the unpaired π orbital lying in the plane of rotation) at high OD rotational quantum numbers, suggests that the reaction H + D2O → OD + HD proceeds through a planar transition state via a direct mechanism, where the OD moiety acts as a spectator, and where the reaction barrier is located at a "later" position of the reaction coordinate.

O(3Pj) atom formation from photodissociation of ozone in the visible and ultraviolet region

The photodissociation of ozone at 266, 308, and 532 nm has been studied for [Formula: see text] probing O(3Pj) atomic photofragments by a vacuum ultraviolet laser-induced fluorescence method. Angular distributions and average kinetic energies are determined by measuring Doppler profiles of the O(3Pj) photofragments. Anisotropy parameters β for the angular distributions are 0.81 ± 0.10 at 266 nm, 0.60 ± 0.10 at 308 nm, and −0.68 ± 0.09 at 532 nm. These values are consistent with the assignment of the photoexcited states, that is, 1B2 in the ultraviolet and 1B1 in the visible region. Average center-of-mass translational energies are 44, 38, and 21 kcal/mol for photodissociation at 266, 308, and 532 nm, respectively. The j-branching ratios of O(3Pj) produced from the photodissociation at 266 nm are (j = 2)/(j = 1)/(j = 0) = (0.55 ± 0.03)/(0.32 ± 0.03)/(0.12 ± 0.03), which are close to the state degeneracy (2j + 1) ratios. At 308 and 532 nm the branching ratios are (j = 2)/(j = 1)/(j = 0) = (0.66 ± 0.03)/(0.27 ± 0.03)/(0.09 ± 0.01) and (0.74 ± 0.03)/(0.20 ± 0.02)/(0.05 ± 0.01), respectively. Population of the j = 2 level increases with decreasing photon energies. The ratios obtained are discussed in terms of the adiabaticity of the potential surfaces during bond breakup as a function of the relative speed of separation.

Absolute rate constants and reactive cross sections for the reactions of O( 1D) with molecular hydrogen and deuterium

The dynamics of the reaction of O( 1D) with molecular hydrogen and deuterium has been investigated using “superthermal” O( 1D) atoms generated by laser photolysis of N 2O at 193 nm. H/D atom products have been detected under single-collision conditions by vacuum ultraviolet laser-induced fluorescence at the Lyman-α transition. With a calibration method using HCl photolysis as a source of well-defined H atom concentrations, the following absolute rate constants k and absolute reactive cross sections σ R have been determined: k= (2.7±0.6) × 10 −10 cm 3 s −1 molecule −1 and σ R(0.12 eV) =7.6±1.5 Å 2 for the reaction O( 1D)+H 2→OH+H, and k=(2.3±0.5)×10 −10 cm 3 s −1 molecule −1 and σ R(0.18 eV)=7.0±1.4 Å 2 for the reaction O( 1D)+D 2→OD+D.

Ultraviolet laser-induced fluorescence of colonic polyps

Ultraviolet laser-induced fluorescence was examined in vivo to determine whether the technique can reliably distinguish between hyperplastic and adenomatous polyps of the colon. Spectra from 86 normal colonic sites, 35 hyperplastic polyps, and 49 adenomatous polyps were recorded in vivo. Polyp type was independently determined by two senior pathologists who were unaware of the fluorescence measurement. A multivariate linear regression analysis was used to differentiate spectra from hyperplastic and adenomatous polyps and resulted in a sensitivity, specificity, predictive value positive, and predictive value negative for identifying adenomatous polyps of 86%, 80%, 86%, and 80%, respectively. These values were not significantly different from the accuracy of routine clinical pathology. Thus, ultraviolet laser-induced fluorescence appears to show promise as a means for distinguishing tissue types. However, further experience is needed before its routine clinical use can be recommended. Significant changes in the fluorescence spectra occurred postmortem, suggesting that future studies of laser-induced fluorescence of colonic tissue must use data acquired in vivo.

Ultraviolet laser-induced fluorescence of colonic polyps

Ultraviolet laser-induced fluorescence was examined in vivo to determine whether the technique can reliably distinguish between hyperplastic and adenomatous polyps of the colon. Spectra from 86 normal colonic sites, 35 hyperplastic polyps, and 49 adenomatous polyps were recorded in vivo. Polyp type was independently determined by two senior pathologists who were unaware of the fluorescence measurement. A multivariate linear regression analysis was used to differentiate spectra from hyperplastic and adenomatous polyps and resulted in a sensitivity, specificity, predictive value positive, and predictive value negative for identifying adenomatous polyps of 86%, 80%, 86%, and 80%, respectively. These values were not significantly different from the accuracy of routine clinical pathology. Thus, ultraviolet laser-induced fluorescence appears to show promise as a means for distinguishing tissue types. However, further experience is needed before its routine clinical use can be recommended. Significant changes in the fluorescence spectra occurred postmortem, suggesting that future studies of laser-induced fluorescence of colonic tissue must use data acquired in vivo. (Gastroenterology 1992 Apr;102(4 Pt 1):1155-60)