Ultraviolet Laser-induced Fluorescence Research Articles

New S1 state vibrational and T3, 2, 1 spin–rotational assignments in the vicinity of the acetylene [formula omitted] band

Pulsed supersonic jet spectra of acetylene have been recorded in the vicinity of the A ˜ 1 A u – X ˜ 1 Σ g + V 0 3 K 0 1 band in two mutually exclusive detection channels: ultraviolet laser-induced fluorescence (UV-LIF) and surface electron ejection by laser excited metastables (SEELEM). No eigenstate of the upper state(s) can appear via transitions in both UV-LIF and SEELEM channels. The UV-LIF channel is blind to transitions into eigenstates that have lifetimes longer than a few microseconds, while the SEELEM channel is exclusively sensitive to transitions into electronically excited eigenstates that have lifetimes longer than ∼30 μs. It is observed that intersystem crossing from the à ( S 1) bright state to the dark triplet states is mediated through a single “doorway state” vibrational level of the T 3 electronic state. We have characterized the bright state, the doorway state, and the dark states (singlet as well as triplet) illuminated by the bright state. Rotational transitions that belong to several previously unassigned vibrational-symmetry-allowed but nominally Franck–Condon forbidden S 1 ← S 0 electronic-vibrational bands are detected and assigned in the UV-LIF spectrum. The UV-LIF spectrum reveals that the electronic symmetry of the T 3 state is 3 B u and the K′ value of the observed T 3 ← S 0 vibrational band segment is 1. From the SEELEM spectrum, nine vibrational levels of dark triplet states are observed, spread over only ∼2.42 cm −1. Unique N, K, and e/ f quantum numbers are assigned for all of the observed triplet spin-rovibronic levels. Electronic symmetries for seven of the nine triplet states are determined. Most of the triplet state vibrational levels observed in the SEELEM spectrum belong to the T 1 surface ( 3 B u and 3 B 2) at energy well above the barrier to linearity. Despite the high vibrational state density, the upper levels of all of the assigned spin-rovibronic transitions fall onto surprisingly regular rotational term value plots [ E′ vs. N′ ( N′ + 1)] and the intensities fall onto surprisingly smooth Boltzmann plots. The most remarkable thing about the SEELEM spectrum is its apparent regularity, combined with a vibrational density of states so high that anharmonic and a-type Coriolis coupling among the T 1 and T 2 vibrational levels is essentially complete. The spectroscopically observed density of K-assigned vibrational states is comparable to the calculated symmetry-sorted state density.

Hydrogen Atom Formation in the Photolysis of Acetone at 193 nm

An experimental and theoretical study of the formation of hydrogen atom in the photolysis of acetone at 193 nm is reported. The H atom photofragments are detected by the technique of vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy at 121.6 nm. The quantum yield for H atom formation from acetone photolysis at 193 nm is determined to be 0.039 ± 0.006. The Doppler profiles of the H atom photofragments are measured by scanning the probe laser wavelength around the resonance frequency of the H atom at 121.6 nm. The nascent kinetic energy distribution of the H atom fragments is found to be characterized by a Maxwell−Boltzmann function with the temperature of 5000 K. The spatially isotropic recoil distribution in the H atom photofragmentation is observed. The potential energy surfaces for the H atom formation processes from acetone photolysis are examined using molecular orbital calculation methods. Atmospheric implications of the present result are briefly discussed.

Quantum Yield for O(1D) Production from Ozone Photolysis in the Wavelength Range of 193−225 nm

The quantum yield values for O(1D) production from the photodissociation reaction of O3 between 193 and 225 nm at 298 ± 2 K are reported. The O(1D) photofragments have been detected directly using a technique of vacuum ultraviolet laser-induced fluorescence spectroscopy at 115.22 nm which is associated with the electronic transition of O(3s1D° − 2p1D). It has been found that the O(1D) quantum yield values decrease monotonically as the photolysis wavelength becomes shorter from 0.90 ± 0.12 (225 nm) to 0.48 ± 0.03 (193 nm). Photodissociation processes of O3 around 210 nm and its atmospheric implications have been discussed.

Vacuum Ultraviolet Laser-Induced Fluorescence Detection of O(1S) Atom Produced in the 193 nm Photolysis of Ozone

The O(2p1S) atoms produced in the photolysis of O3 have been detected by a technique of laser-induced fluorescence spectroscopy with the tunable vacuum ultraviolet (VUV) laser radiation around 121.76 nm. The quantum yield value for O(1S) formation is determined to be (2.5 ± 1.1) × 10-3 in the 193 nm photolysis of O3, which is determined by comparing the VUV laser-induced fluorescence intensities of the O(3s1P1 − 2p1S0) transition with those of the H(2p2Pj ← 1s2S) transition at 121.56 nm of the H atoms generated from the photolysis of HCl at 193 nm. The O(1S) detection technique used in this study is very sensitive, and the detection limit is estimated to be 1 × 109 atoms cm-3. The contribution for the OH radical production from the reaction of H2O with O(1S) produced in the UV photolysis of O3 relative to that from the O(1D) + H2O reaction has been estimated as a function of altitude in the stratosphere, using the photolytic O(1S) quantum yield value obtained in this study and the O(1S) reaction rate coefficients reported previously. The maximum contribution of the O(1S) reaction to OH production rate is a 14% fraction of that from the O(1D) reaction at 40 km altitude at mid-latitudes, assuming the spin-forbidden dissociation process, O(1S) + O2(X3Σg-), for the formation of O(1S) in the photolysis of ozone. Importance of precise measurements of the temperature-dependent reaction rates for O(1S) has been suggested.

Evidence for the dominance of collision-induced intersystem crossing in collisions of [formula omitted] with O 2 and a determination of the H atom yields from [formula omitted], using time-resolved detection of H formation by vuvLIF

The formation of H atoms, following pulsed generation of CH 2 a 1 A 1 in the presence of O 2, has been monitored by vacuum ultraviolet laser-induced fluorescence. The rise time of H is compatible with its production from the reaction of CH 2 X 3 B 1+ O 2 , with the interaction between the singlet and O 2 leading exclusively to the formation of the triplet state by collision-induced intersystem crossing. A comparison of the intensities of the H atom signal from reaction of 1 CH 2 with H 2 and of 1 CH 2 with O 2 gives a yield of H from 3 CH 2+ O 2 of 0.8±0.2.

Tunable ultraviolet laser-induced fluorescence detection of trace plastics and dissolved organic compounds in water.

We developed a tunable (220-285-nm) UV and fixed 266-nm laser-induced fluorescence (LIF) system using a spectrometer and a cooled CCD imaging detector to measure the excitation-emission matrix spectra of various compounds in water, including quinine sulfate and plastic compound bisphenol-A. The LIF instrument was used for the fast, nonspecific determination of trace amounts of dissolved organic compounds present in natural water supplies and various brand name bottled distilled water and bottled drinking water. Plastic-related compounds that leached out of plastic utensils and containers were also detected with this instrument. The sensitivity of the system was approximately 1-2 orders of magnitude better than that for a commercial system.

Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of SO2 at 222-4 nm

The dynamics of formation of oxygen atoms after UV photoexcitation of SO2 in the gas-phase was studied by pulsed laser photolysis-laser-inducedfluorescence ‘pump-and-probe’ technique in a flow reactor. SO2 at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(3Pj) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated viaresonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(3Pj=2,1,0) fine-structure state distribution:n j=2/nj=1/nj=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NO2photolysis as a reference, a value of Φ0(3P) = 0.13 ± 0.05 for the absolute O(3P) atom quantum yield was determined. The measured O(3P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO2(~C1B2 (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.

Determination of triamterene and its main metabolite hydroxytriamterene sulfate in human urine by capillary electrophoresis using ultraviolet absorbance and laser-induced fluorescence detection

Two capillary electrophoresis methods have been developed for the direct determination of triamterene and its main metabolite hydroxytriamterene sulfate in human urine. Analytes were detected using conventional UV detection as well as laser-induced fluorescence (LIF) detection with an HeCd-laser operating at a wavelength of 325 nm. The results of both detection techniques were compared. Indeed, the limit of quantification was eightfold lower using LIF detection (50 ng/ml) in comparison to UV detection (400 ng/ml). As no interference due to endogenous urine compounds was observed, direct urine analysis was feasible. Analysis was very simple and fast—one run could be performed within less than 10 min (CE–UV method) and 2.5 min (CE–LIF method), respectively. Both assays were fully validated and applied to urine samples from a human volunteer. The results of the application of the CE–LIF method to human urine samples are presented in this publication.

Dynamic labeling during capillary or microchip electrophoresis for laser-induced fluorescence detection of protein-SDS complexes without pre- or postcolumn labeling.

The analysis of proteins under denaturing conditions is routinely performed with SDS-polyacrylamide gel electrophoresis. The automated capabilities of CE, use of nongel sieving matrixes, and on-line optical detection by either ultraviolet (UV) absorption or laser-induced fluorescence (LF) promise to revolutionize this method. While direct on-line detection of proteins is possible as a result of their intrinsic ability to absorb light in the UV part of the spectrum (detection sensitivity comparable to Coomassie Blue staining of gels), LIF provides more powerful detection but requires pre- or postcolumn fluorescence labeling of the proteins. The development of a protocol analogous to that used for double-stranded DNA analysis, where fluorescent intercalating dyes are simply included in the separation medium, would simplify size-based protein analysis immensely. This would avoid the complications associated with covalent modification of the proteins but still exploit the sensitivity of LIF detection. We demonstrate that this is possible with CE and microchip detection by incorporating, into the run buffer, a fluorescent dye that interacts hydrophobically with protein-SDS complexes. Key to this is a dye that fluoresces significantly when bound to protein-SDS complexes but not when bound to SDS micelles. Comparison of electropherograms from CE-based denaturing protein analysis with UV and LIF detection indicates that the presence of the fluor does not alter separation of the proteins. Moreover, comparison with electropherograms generated from microchip electrophoresis with LIF detection shows that equivalent patterns can be obtained. Despite the unoptimized nature of this separation system, a dynamic labeling protocol that allows for LIF detection for proteins is attractive and has the potential to circumvent the tedious labeling steps typically required.

Kinetics of the reactions of [formula omitted] and [formula omitted] atoms with C3H8,C3D8, n-C4H10, and i-C4H10 at 298 K

The title reactions were studied using laser flash photolysis in conjunction with vacuum ultraviolet laser-induced fluorescence techniques. Separate monitoring of the two spin orbit states, Cl∗(2P1/2) and Cl(2P3/2), was used to measure the kinetics of the chemical reactions of Cl(2P3/2) and Cl∗(2P1/2), and physical quenching of Cl∗(2P1/2). The rate constants for chemical reactions of Cl∗(2P1/2) atoms with C3H8, n-C4H10, and i-C4H10 are approximately 30% of those for Cl(2P3/2) atoms. There was no observable chemical reaction of Cl∗(2P1/2) atoms with C3D8 and C2H6 (<17% of Cl(2P3/2) reaction rate).

Capillary Electrophoresis-based Heteroduplex Analysis with a Universal Heteroduplex Generator for Detection of Point Mutations Associated with Rifampin Resistance in Tuberculosis

Slab gel heteroduplex analysis (HDA), a popular scanning method for genetic mutations, uses DNA fragments typically generated by PCR to create homo- and heteroduplex molecules with conformational differences and sequence-dependent electrophoretic profiles. Use of a universal heteroduplex generator (UHG) enhances the subtle variations caused by single-base substitutions. The HDA-UHG slab gel format was modified for an efficient capillary-based method. The effect of staining dyes TOPRO5 and YOPRO1 on the analysis of heteroduplexes was studied, as well as ultraviolet absorbance and laser-induced fluorescence (LIF) detection methods. In addition, the entangled polymers hydroxyethyl cellulose, methyl cellulose, and linear polyacrylamide were evaluated as separation matrices. This assay was able to detect the presence of Mycobacterium tuberculosis and its rifampin susceptibility directly from clinical specimens in dramatically reduced analysis time (30 min vs 2.5 h). Optimized conditions included 0.3% methyl cellulose as the separation matrix, on-line staining using 1 micromol/L YOPRO1, and LIF detection for quantitative and reproducible analysis of single-base substitutions in the rifampin resistance-determining region of rpoB that give rise to the rifampin-resistant phenotype of M. tuberculosis. We generated 95% confidence limits using the wild-type sequence and used these limits to determine rifampin susceptibility in samples. Capillary electrophoresis, combined with the HDA-UHG technique, may be of value for rapid and efficient clinical diagnosis of rifampin-resistant tuberculosis strains.

Activation of the CH stretching vibrations in CH4–OH entrance channel complexes: Spectroscopy and dynamics

The vibrational spectroscopy and decay dynamics of CH4–OH reactant complexes have been studied in the CH4 symmetric and antisymmetric stretching regions (ν1 and ν3). The vibrational spectra have been obtained using both infrared and stimulated Raman excitation with ultraviolet probe laser-induced fluorescence detection. Stimulated Raman excitation of CH4–OH in the symmetric stretching region reveals two blended Q branch features at 2912.5 and 2911.8 cm−1. An extremely weak infrared spectrum is also seen in the CH4 symmetric stretching region, which is induced by the presence of the nearby OH partner. Infrared excitation in the asymmetric stretching region results in an intense, yet enormously broad spectrum centered at 3020 cm−1 that extends over 40 cm−1. The appearance of the spectra in the ν1 and ν3 regions has been explained in terms of a model in which the CH4 unit undergoes internal rotation within the CH4–OH complex. The ν1 features are attributed to transitions involving two different nuclear spin states of CH4. In the ν3 region, the CH4–OH complex can undergo a multitude of allowed transitions, each associated with a rovibrational transition of free methane, which give rise to the enormous span of the spectrum. The vibrational spectra also exhibit extensive homogeneous broadening (⩾1 cm−1) arising from the rapid decay of vibrationally activated CH4–OH complexes due to vibrational predissociation and possibly reaction. The OH fragments are produced with minimal rotational excitation, indicating that the dominant inelastic decay channel involves near-resonant vibrational energy transfer within the CH4 unit from the initially prepared CH stretch to an overtone bend (2ν4) state.

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.

Photodissociation of O3 around 309 nm

Photodissociation reaction of O3 following ultraviolet photon excitation around the thermodynamic threshold of the dissociation channel of O(1D) + O2(a1Δg) at 309.44 nm has been studied under room-temperature and jet-cooled conditions. Both O(1D) and O(3P) photoproducts are detected by a technique of vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy. Photofragment excitation (PHOFEX) spectra for the O(1D) and O(3P) atoms are obtained by scanning the photolysis laser wavelength between 297 and 316 nm while monitoring VUV-LIF signal intensities at 115 and 130 nm for O(1D) and O(3P), respectively. From the behavior of the PHOFEX spectra for the O(3P) and O(1D) atoms around the threshold into O(1D) + O2(a1Δg), the existence of an exit barrier along the O−O2 dissociation coordinate in the photoexcited electronic state of O3 is suggested. Analysis of the line widths in the jet-cooled PHOFEX spectra of O(1D) and O(3P) suggests that the quasi-bound states below the barrier and above the thresho...

Direct kinetic measurements on reactions of atomic carbon, C(3P), with O2 and NO at temperatures down to 15 K

A CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus has been used to measure rate coefficients for the reactions of C (3P) with O2 and NO at temperatures from 295 K down to 15 K. C (3P) atoms were, for the first time in a kinetic study, monitored directly by vacuum ultraviolet laser-induced fluorescence. The rate coefficients for both reactions increase as the temperature is lowered, matching the expressions k(C+O2)=(4.9±0.8)×10−11 (T/298 K)−(0.32±0.08) cm3 molecule−1 s−1 and k(C+NO)=(1.5±0.4)×10−10 (T/298 K)−(0.16±0.14) cm3 molecule−1 s−1.

OH vibrational activation and decay dynamics of CH4–OH entrance channel complexes

Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH4–OH complexes that have been stabilized in the entrance channel to the CH4+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH4–OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) double-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm−1 (origins), along with combination bands involving the simultaneous excitation of OH stretching and intermolecular bending motions. The infrared spectra exhibit extensive homogeneous broadening arising from the rapid decay of vibrationally activated CH4–OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH4–OH prepared with one and two quanta of OH excitation, respectively, have been extracted from the infrared spectra. The nascent distribution of the OH products from vibrational predissociation has been evaluated by ultraviolet probe laser-induced fluorescence measurements. The dominant inelastic decay channel involves the transfer of one quantum of OH stretch to the pentad of CH4 vibrational states with energies near 3000 cm−1. The experimental findings are compared with full collision studies of vibrationally excited OH with CH4. In addition, ab initio electronic structure calculations have been carried out to elucidate the minimum energy configuration of the CH4–OH complex. The calculations predict a C3v geometry with the hydrogen of OH pointing toward one of four equivalent faces of the CH4 tetrahedron, consistent with the analysis of the experimental infrared spectra.

Relaxation Processes of Translationally Hot O(1D) by Collisions with O2

The collisions of translationally hot O(1D) with O2 result in two processes, translational energy relaxation and electronic quenching to O(3P). These two processes were studied in a gas cell at room temperature using the vacuum ultraviolet laser-induced fluorescence technique. The initial hot O(1D) atoms were produced by the photodissociation of N2O at 193 nm, which have average translational energies of 18.1 kcal mol-1 in the laboratory frame. Time-resolved measurements of the Doppler profiles for the hot O(1D) atoms revealed the translational energy relaxation process, whereas the quenching process was investigated by measuring both the decrease of the O(1D) concentration and the increase of the product O(3P) concentration at various delay times after the photochemical formation of the hot O(1D) atoms. From the simulation employing an elastic hard-sphere collision model with a Monte Carlo method, the hard-sphere diameter for the translational energy relaxation process of hot O(1D) by collisions with O2 ...

Ultraviolet absorption and fluorescence measurements in turbulent spray flames

Studies of elemental and organic carbon formation/oxidation in diffusion or rich premixed combustion systems focus mostly on soot particles and polycyclic aromatic hydrocarbons. In recent years, a new class of carbon compounds, in the form of molecular clusters (nanoparticles) of organic composition has been inferred, detected, and modeled within and at the exhausts of a variety of combustion systems. This new category of pollutants seems to be related to the formation processes of ultrafine particles found at the exhaust of practical combustion systems, which are considered dangerous to human health. Heavy molecular weights and extremely small sizes of nanoparticles prevent their detection by ordinary techniques (e.g., GC-MS), particularly in combustion conditions. The paper reports on optical investigations based on ultraviolet (UV) extinction spectroscopy and laser-induced fluorescence (LIF) of turbulent flames and exhausts of a spray burner, involving processes by far more complex than gaseous laminar flames. The light source for extinction was the plasma emission, resulting from the optical breakdown of a Q-switched Nd:YAG laser pulse, which featured intense, fast, and bright broadband UV radiation. Extinction spectra in the UV band range of 200–500 nm and LIF UV-visible spectra, excited at λ=266 nm, were measured and analyzed at different heights above the nozzle, which allowed us to infer the formation/oxidation of species strongly absorbing in the far UV region and fluorescing at longer wavelengths. Resulting data were correlated with the spectral extinction of air-diluted exhausts.

H2 Production in the 440-nm Photodissociation of Glyoxal

H2 has been detected following the photolysis of glyoxal at 440 nm using the techniques of vacuum ultraviolet laser-induced fluorescence and (2 + 1) resonance enhanced multiphoton ionization. It is thus confirmed that a fraction of the glyoxal excited at this wavelength dissociates into three photofragments: HCOCOH → H2 + 2 CO. The most populated vibrational level of those observed was H2 (v = 1), and in this level rotational states from J = 0−9 were detected. Doppler profiles of these lines provide estimates of the translational energy and show a v||J correlation. Of the available energy to the H2 + 2CO products, 3.1% appears as rotational energy in H2 (v = 1), 17.8% appears as the H2 (v = 1) vibration, and 46.8% appears as H2 (v = 1) translation. Excitation of the 702 band produces somewhat more of the H2 + 2CO channel than does excitation of the 000, 501, or 801 bands. These observations are consistent with a model in which trans→cis isomerization precedes dissociation. Rotational excitation with v||J is caused by the v7 torsional motion. The small degree of rotational excitation, the production of H2 in v = 1 and v = 2, and the translational energy distribution are all consistent with ab initio calculations of the transition state structure.

Determination of the heat of formation of O3 using vacuum ultraviolet laser-induced fluorescence spectroscopy and two-dimensional product imaging techniques

Two different techniques, vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy and two-dimensional (2D) ion counting product imaging, have been used to determine the bond energy for the dissociation of jet-cooled O3 into O(1D)+O2(a 1Δg). The photofragment excitation (PHOFEX) spectrum for O(1D) products is recorded by detecting the VUV-LIF signal associated with the 3s 1D0–2p 1D transition at 115.22 nm while scanning the photolysis laser wavelength between 305 and 313 nm. A clear cut-off corresponding to the appearance threshold into O(1D)+O2(a 1Δg) is observed in this PHOFEX spectrum. The 2D image of the O(1D) products from the O3 photolysis near 305 nm is measured using an ion-counting method, with the detection of O(1D) atoms by [2+1] resonance enhanced multiphoton ionization (REMPI) at 205.47 nm. The kinetic-energy distribution obtained from the 2D image shows rotational structure due to the O2(a 1Δg,v″=0) fragment. The bond energy into O(1D)+O2(a 1Δg) has been obtained from the rotational assignments in the kinetic-energy distribution. The two different experimental approaches give consistent results and an accurate value of the bond dissociation energy into O(1D)+O2(a 1Δg) is found to be 386.59±0.04 kJ/mol. The standard heat of formation of O3, ΔfH0(O3)=−144.31±0.14 kJ/mol, has also been calculated from the bond energy obtained, in conjunction with thermochemical data for O2 molecule and O atom. The uncertainty for the ΔfH0(O3) value obtained in the present study is smaller than the previous value which has been used widely.