Gas-phase Fluorescence Research Articles

Gas-Phase Fluorescence Excitation and Emission Spectroscopy of Three Xanthene Dyes (Rhodamine 575, Rhodamine 590 and Rhodamine 6G) in a Quadrupole Ion Trap Mass Spectrometer

The gas-phase fluorescence excitation, emission and photodissociation characteristics of three xanthene dyes (rhodamine 575, rhodamine 590, and rhodamine 6G) have been investigated in a quadrupole ion trap mass spectrometer. Measured gas-phase excitation and dispersed emission spectra are compared with solution-phase spectra and computations. The excitation and emission maxima for all three protonated dyes lie at higher energy in the gas phase than in solution. The measured Stokes shifts are significantly smaller for the isolated gaseous ions than the solvated ions. Laser power-dependence measurements indicate that absorption of multiple photons is required for photodissociation. Redshifts and broadening of the dispersed fluorescence spectra at high excitation laser power provide evidence of gradual heating of the ion population, pointing to a mechanism of sequential multiple-photon activation through absorption/emission cycling. The relative brightness in the gas phase follows the order R575(1.00) < R590(1.15) < R6G(1.29). Fluorescence emission from several mass-selected product ions has been measured.

Gas-phase fluorescence excitation and emission spectroscopy of mass-selected trapped molecular ions

A flexible interface to perform optical spectroscopic measurements on gaseous ions stored in a modified commercial quadrupole ion trap (QIT) mass spectrometer is described. The modifications made to the mass spectrometer did not adversely affect its operating characteristics. Gas-phase ions are produced using electrospray ionization, mass isolated and stored in the trapping mass spectrometer. The ions are subsequently irradiated with visible light from a tunable laser and dispersed fluorescence spectra are recorded simultaneously. Mass spectra are recorded after the irradiation period. This set-up allows us to track a range of possible outcomes upon photoexcitation of selected ions including fluorescence, photofragmentation and photodetachment of electrons. The experimental set-up is characterized using rhodamine 590, which is a methyl ester variant of rhodamine 6G. Fluorescence excitation and emission spectra of gaseous rhodamine 590 are measured and compared with solution-phase spectra. Excitation and emission maxima for the gaseous ions are found to lie at higher energy than for the solvated rhodamine 590. In addition, the gas-phase Stokes shift is significantly smaller than the solution-phase Stokes shift. The effects of several experimental parameters on the observed fluorescence signal are investigated, including laser power, relative number of ions, q(z) trapping parameter and buffer gas pressure. In addition to its use for the photophysical characterization of the intrinsic properties of ionic chromophores, this set-up may be used to investigate the properties of mass-selected, dye-labeled biomolecules, both alone and in well-defined complexes and clusters.

Spectroscopic characterization of the fluorobenzene/DEMA tracer system for laser-induced exciplex fluorescence for the quantitative study of evaporating fuel sprays

Quantitative analysis of laser-induced exciplex fluorescence (LIEF) requires knowledge of the spectral characteristics of all the involved substances. The temperature dependence of the ratio of exciplex and monomer in the liquid phase as well as the temperature-dependent fluorescence intensity from monomers in the gas phase must be known when quantifying signals and correcting for cross talk between liquid and vapor phase. In this work we present an extensive characterization of the fluorescence of the fluorobenzene/diethyl-methyl-amine (DEMA)/n-hexane exciplex system. We use a mixture of 2% fluorobenzene, 9% DEMA, and 89% n-hexane, which was tested before for its coevaporative behavior. The temperature dependent fluorescence of the liquid exciplex was studied in a thin layer cell with UV-light (266 nm) excitation. Cross talk of a known combination of bandpass filters was quantified for the relevant temperature range. The temperature dependence of the gas-phase absorption and fluorescence signal (fluorobenzene) was studied in a heated static cell and a heated gas stream in a nitrogen coflow, respectively. These measurements provide the background for the application of LIEF for quantitative analysis of vapor and liquid distributions in fuel sprays.

Exploring Fluorescence and Fragmentation of Ions Produced by Electrospray Ionization in Ultrahigh Vacuum

Fluorescence spectroscopy and mass spectrometry have been extensively used for characterization of biomaterials, but usually separately. An instrument combining fluorescence spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) has been developed to explore both fluorescence and mass spectrometric behavior of ions produced by electrospray ionization (ESI) in ultra high vacuum (<5 x 10(-9) mbar). Using rhodamine 6G (R6G) as a sample, the instrument was systematically characterized. Gas-phase fluorescence and mass spectral signal of the same ion population are detected immediately after each other. Effects of gas pressure, ion density, and excitation laser power on the fluorescence signal intensity and mass spectral fragmentation patterns are discussed. Characteristic times of ion photodissociation in ultra high vacuum were recorded for different irradiation powers. Photofragmentation patterns of rhodamine 6G ions in the Penning trap of an FTICR spectrometer obtained by photoinduced dissociation (PID) with visible light and sustained off-resonance irradiation collision-induced dissociation (SORI-CID) were compared. The lowest energy dissociation fragment of rhodamine 6G ions was identified by relating PID patterns of rhodamine 6G and rhodamine 575 dyes at various irradiation powers. The unique instrument provides a powerful platform for probing the intramolecular relaxation mechanisms of nonsolvated ions when interacting with light, which is of great fundamental interest for better understanding of their physical and chemical properties.

Time-dependent density functional investigation on electronic spectra of 4′- N-dimethylamino-3-hydroxyflavone

The lower singlet excited states for 4′- N-dimethylamino-3-hydroxyflavone (DMA3HF) have been investigated. Theoretical calculations have been performed by using time-dependent density functional theory. The B3LYP and MPW1PW91 functionals with 6-31G(d,p) basis set have been used to compute transition energies. Ground-state geometries are optimized by using density functional theory with both B3LYP and MPW1PW91 functionals combined with 6-31G(d,p) basis set. The first excited state geometries are optimized by using configuration interaction all single-excitations methods with 6-31G(d,p). Vertical absorption energy and excited state electronic dipole moments calculations show that the singlet excited states S 1 of the normal of DMA3HF has the character of a large charge separation and effective charge transfer. Calculated emission energies for both the normal and tautomer show a large Stokes shift between the absorption and fluorescence maxima. The present calculations suggest that DMA3HF appears dual fluorescence in gas phase. Our results obtained explain the strong ‘dual-fluorescence’ feature that involves the excited-state intramolecular proton transfer and charge transfer of DMA3HF.

Clear Evidence of Fluorescence Resonance Energy Transfer in Gas-Phase Ions

Fluorescence resonance energy transfer (FRET) is a distance-sensitive method that correlates changes in fluorescence intensity with conformational changes, for example, of biomolecules in the cellular environment. Applied to the gas phase in combination with Fourier transform ion cyclotron resonance mass spectrometry, it opens up possibilities to define structural/conformational properties of molecular ions, in the absence of solvent, and without the need for purification of the sample. For successfully observing FRET in the gas phase it is important to find suitable fluorophores. In this study several fluorescent dyes were examined, and the correlation between solution-phase and gas-phase fluorescence data were studied. For the first time, FRET in the gas phase is demonstrated unambiguously.

Experimental station for gas phase fluorescence spectroscopy

The details of an experimental setup for gas phase atomic and molecular fluorescence measurements using synchrotron radiation are described in this article. The most significant part of the apparatus is an optical arrangement, which allows for simultaneous measurements of dispersed as well as total fluorescence intensity using an effusive gas jet and an inbuilt gas cell assembled in a convenient plug and measure configuration. The first measurements concerning fluorescence of the N2 molecule around the N 1s edge obtained with this setup are presented.

WHERE ARE THE LIMITS OF THE GAS-PHASE FLUORESCENCE ON THE POLYCYCLIC AROMATIC COMPOUND ANALYSIS?

The aim of this work has been centered on the development of a detection system which could be useful to determine polycyclic aromatic hydrocarbon (PAH) emissions in power generation processes where PAHs are released to the atmosphere in the gas phase. In this way, a suitable sensor based on gas phase fluorescence spectroscopy was designed to facilitate the PAH analysis, avoiding previous steps necessary to determine liquid samples such as extraction, concentration, and cleaning. This gas cell was applied for two experimental installations at laboratory scale. One of them was used to determine adsorption capacities and breakthrough curves of PAH binary mixtures and the other installation allowed the separation of PAH mixtures by using, previous to the gas phase fluorescence spectroscopy, a packed column.

PAH Mixture Removal from Hot Gas by Porous Carbons. From Model Compounds to Real Conditions

This paper reports an investigation of the abatement of polycyclic aromatic hydrocarbon (PAH) emissions using different carbonaceous materials as adsorbents. PAH model compounds (naphthalene and phenanthrene) were initially used to obtain a deeper understanding of the complex process of PAH adsorption in industrial hot gas cleaning systems. First, the adsorption of binary mixtures of these compounds was carried out at the laboratory scale in an experimental rig specially designed for this purpose, where gas-phase fluorescence is used for detection. Experimental conditions, mainly in terms of temperature and contaminant concentration, close to those observed in flue gases from energy generation systems were applied. The PAH adsorption process was analyzed in terms of experimentally obtained breakthrough curves. After the detection parameters had been optimized, the influence of the structural and chemical properties of the activated carbon was studied. It was found that the higher the adsorbent porosity, t...

Study of the Adsorption of Polyaromatic Hydrocarbon Binary Mixtures on Carbon Materials by Gas-Phase Fluorescence Detection

This is the first time that the adsorption of binary mixtures of polycyclic aromatic hydrocarbons (PAHs) has been studied. This study was carried out at laboratory scale in an experimental apparatus specially designed for this aim: with gas-phase fluorescence, a new detection procedure was used. Experimental conditions, mainly in terms of temperature (150°C) and contaminant concentration (approximately 1 ppmv), close to the ones observed in energy generation systems, were applied. The PAH adsorption process interpretation was carried out by recording the experimentally obtained breakthrough curves. After the detection parameter optimization, the influence of adsorbate characteristics in the hot gas cleaning of PAH was studied. In this system, the adsorption of 10 binary mixtures of five PAH [naphthalene (Np), fluorene (Fu), phenanthrene (Phe), fluoranthene (Fl), and pyrene (Py)] on an activated carbon was investigated. It was found that the adsorbent efficiency is always determined by the breakthrough of...

Time-dependent density-functional theory investigation of the formation of the charge transfer excited state for a series of aromatic donor–acceptor systems. Part I

Singlet excitation energies for a series of acceptor para-substituted N,N-dimethyl-anilines that are dual (4DMAB-CN, 3M4MAB-CN, MHD) and nondual (4AB-CN, 3M4AB-CN, 4MAB-CN, 3M4DMAB-CN, HHD, and MMD) fluorescent have been performed using the TDDFT method. The aim of this study is to investigate the influence of changing donor groups as well as the addition of methyl groups to the benzene moiety, on the fluorescence behavior of these molecules. Calculations of excitation energies have been performed with both B3LYP and MPW1PW91 functionals using a 6-311*(2p,d) (Bg) basis set. For all systems, ground-state geometries were optimized using density-functional theory with the Becke three parameter Lee–Yang–Parr functional combined with a 6-31G(d) (Sm) basis set. In addition, 4AB-CN, 4DMAB-CN, and MMD ground-state geometry has also been optimized using the MPW1PW91 functional with the Sm basis set. For all molecules, the potential energy surface (PES) has been investigated following the twisting intramolecular charge transfer (TICT) model proposed in the literature as a possible mechanism to explain the fluorescence behavior. Both 4AB-CN and HHD molecules have been computed to be nondual fluorescent in full agreement with experimental spectra. The single band observed in the gas-phase fluorescence spectra of 3M4DMAB-CN, and MMD is clearly understood by the form of the PES of the charge transfer excited state that presents a minimum for the perpendicular structure. The qualitative picture of the PES along the twisting angle is in full agreement with experimental observations. The dual fluorescence of 4DMAB-CN and 3M4MAB-CN is explained, within the TICT model, by a double mechanism proposed by Serrano et al. [J. Am. Chem. Soc. 117, 3189 (1995)], that involves the presence of two low-lying states close enough in energy. The nondual fluorescence of 4MAB-CN is explained by the height of the energy barrier (larger than for 4DMAB-CN and 3M4MAB-CN). Finally, the dual fluorescence of the MHD molecule can be fully understood by a double mechanism within the TICT model. 3M4AB-CN is computed nondual fluorescent like 4AB-CN and HHD, but no experimental data has been reported in the literature so far. Our calculations give new evidence in favor of the TICT model as an explanation for the occurrence of dual fluorescence.

Detection of the 3.4 micron emission feature in Comets P/Brorsen-Metcalf and Okazaki-Levy-Rudenko (1989r) and an observational summary

The 3.4 micron emission feature due to cometary organics was detected in Comets P/Brorsen-Metcalf and Okazaki-Levy-Rudenko (1989r). Features-to-continuum ratios in these two comets were higher than those expected from the trend seen in other comets to date. Three micron spectra of eight comets are reviewed. The 3.4 micron band flux is better correlated with the water production rate than with the dust production rate in this sample of comets. High feature-to-continuum ratios in P/Brorsen-Metcalf and Okazaki-Levy-Rudenko can be explained by the low dust-to-gas ratios of these two comets. The observations to date are consistent with cometary organics being present in all comets (even those for which no 3.4 micron feature was evident) at comparable abundances with respect to water. The emission mechanism and absolute abundance of the organics are not well determined; either gas-phase fluorescence or thermal emission from hot grains is consistent with the heliocentric distance dependence of the 3.4 micron band flux. There is an overall similarity in the spectral profiles of the 3.4 micron feature in comets; however, there are some potentially significant differences in the details of the spectra.

Indirect fluorescence detection in capillary gas chromatography

Post-column addition of naphthalene vapour to the effluent from a wide-bore capillary gas chromatographic column, prior to gas-phase fluorescence detection, was used to detect fluorescent and non-fluorescent species as positive and negative peaks, respectively. Quenching constants were determined for nitrobenzene and cyclohexanone using indirect detection.

Thermovision monitoring of gas-phase fluorescence induced by cw CO 2 laser radiation

Applying thermovision monitoring, spatial distribution of gas-phase fluorescence in the wavelength region 2–5.4/um induced by CO 2 laser radiation is displayed. Experimental results demonstrate that the spatial distribution of the fluorescence is effected not only by the convection in a reaction cell but it also depends on V-T energy transfer relaxation time. Relatively slow relaxation compared to fast convection can cause that dominant portion of excited molecules is quenched on the cell walls. The used thermovision technique allows to determine both the threshold and the steady-state of convection generated by cw CO 2 laser beam absorption.

A proposal for the partial pressure measurement of oil molecules by fluorescence spectroscopy

Vacuum pumps which rely on oil to effect pumping, lubrication or sealing can cause severe contamination of pumped systems when, in molecular flow or at low gas throughouts, back-migration of oil occurs. The technique of gas-phase fluorescence spectroscopy conceptually offers a new method to give a measure of the identity and molecular density for a number of commonly-used vacuum fluids. To assess feasibility, a systematic study has been undertaken of the uv-visible absorption and fluorescence of a range of industrial pump oils, using both pure liquids and dilute solutions. From the quantum yield and molar absorptivities, the detection limit of vapour-phase fluorescence using photon counting has been estimated. The practicality of using this technique is discussed.

The heliocentric evolution of cometary infrared spectra: Results from an organic grain model

Observations of Comets Halley and Wilson reveal an emission feature peaking near 3.4 μm, characteristic of C-H stretching in hydrocarbons. We have previously (Chyba and Sagan 1987a, Nature (London) 330, 350–353) fit this feature with a simple two-component thermal emission model for dust in the cometary coma (one component corresponding to large, cool, optically thick particles, the other due to smaller, hotter, organic grains) by employing laboratory spectra of the organic residue produced by the irradiation of carbon-bearing ices. This procedure yields optical depths in agreement with limits from spacecraft data. One remarkable result of such modeling is that at ∼1 AU emission features at wavelengths longer than 3.4 μm are largely overwhelmed (or “diluted”) by continuum emission. The large particle optical depth is ∼10 2 times that of the emitting organics, so that, relative to the continuum, only near the continuum minimum can the emitting organics make a significant contribution. At ∼1 AU, the 3.4-μm feature is the sole feature near that minimum, lying at the intersection of the curves for particle thermal emission and scattered sunlight. Thus, since as a comet moves away from perhelion the intersection of the scattered solar spectrum and the comet's thermal emission spectrum will move to longer wavelengths, we predicted (Chyba and Sagan 1987a) that the 3.4-μm feature is diluted while those at longer wavelengths are progressively revealed—so long as the comet retains its coma. We now quantitatively develop this model and find agreement with observational data for Comet Halley for certain plausible values of optical constants. Thus the observed heliocentric evolution of the 3.4-μm feature provides information on the composition, and perhaps structure, of the organic grains in Comet Halley. In addition, we argue that the heliocentric evolution of organic features will differ in the cases of thermal emission from small grains and gas-phase fluorescence. Therefore observations of cometary spectral evolution can in principle distinguish between solid or gas-phase origins for these features.

Excimer laser induced deposition of InP and indium-oxide films

InP and In-oxide films have been deposited on quartz, GaAs, and InP substrates by excimer laser induced photodecomposition of (CH3)3InP(CH3)3 and P(CH3)3 vapors at 193 nm. The oxide film refractive index and stoichiometry are close to In2O3. Phosphorus incorporation in the films was greatly enhanced by focusing the laser beam to promote multiple-photon dissociation processes. These conditions also lead to enhanced carbon inclusion in the films, due to formation of species such as CH and CH2 in the gas phase. However, this carbon inclusion could be suppressed by focusing the beam onto the surface at normal incidence. In the irradiated zone InP could be deposited with P(CH3)3-to-(CH3)3InP(CH3)3 ratios of only ∼1:1. The technique offers several potential advantages over conventional metalorganic chemical vapor deposition, including lower temperature, enhanced rates, safer gases, and three-dimensional film composition control. Strong atomic In emission is observed in the gas phase above the depositing film, due to a multiple photon dissociation process. Gas phase fluorescence from P, CH, and C was also observed. These emissions give insight into the photodecomposition mechanism and also serve as a monitor of metalorganic precursor concentrations.

Excimer Laser-Induced Deposition of InP and Indium-Oxide Films

ABSTRACTInP and In-oxide films have been deposited on quartz, GaAs, and InP substrates by excimer laser-induced photodecomposition of (CH3)3InP(CH3)3 and P(CH3)3 vapors at 193 nm. The oxide film refractive index and stoichiometry are close to In2O3. Phosphorus incorporation in the films was greatly enhanced by focusing the laser beam to promote multiple-photon dissociation processes. These conditions also lead to enhanced carbon inclusion in the films, due to formation of species such as CH and CH2 in the gas phase. However, this carbon inclusion could be suppressed by focusing the beam onto the surface at normal incidence. In the irradiated zone InP could be deposited with P(CH3)3-to-(CH3)3InP(CH3)3 ratios of only ∼1:1. The technique offers several potential advantages over conventional metal-organic chemical vapor deposition (MOCVD), including lower temperature, enhanced rates, safer gases, and threedimensional film composition control.Strong atomic In emission is observed in the gas-phase above the depositing film, due to a multiple photon dissociation process. Gasphase fluorescence from P, CH, and C was also observed. These emissions give insight into the photodecomposition mechanism and also serve as a monitor of metal-organic precursor concentrations.

The Jahn–Teller distortion in the X2E″ state of sym-C6Cl3F+3 as determined from laser-induced fluorescence studies

Both gas-phase and matrix laser-induced fluorescence studies of sym-C6CL3F3+ provide detailed information on the energy-level structure of the Jahn–Teller perturbed e′ vibrations in the ground electronic state. These experimental results are analysed in terms of a multi-mode Jahn–Teller theory to yield distortion constants, unperturbed oscillator frequencies, stabilization energies and distorted geometries. These results are combined with similar results for the symmetrical ions C6F3H3+, C6Cl3H3+ and C6F6+ to reach some general conclusions about the quantitative nature of the Jahn–Teller distortion of benzenoid cations.

Argon matrix photolysis and photoionization studies of benzene. Absorption spectrum of benzene cation and benzene dimer cation

Argon/benzene samples subjected to argon resonance photoionization during condensation at 21±1 K revealed a sharp new 552.5 nm product absorption, sharp C−2 bands and a broad 900 nm absorption. The 552.5 and 900 nm absorptions were destroyed and the C−2 bands decreased by mercury arc photolysis, which suggests that the 552.5 and 900 nm absorptions are due to molecular cations. The broad 900 nm absorption is in agreement with an earlier assignment to the intervalence transition in (C6H6)2+. Near agreement between the 552.5 nm absorption energy and the difference between the sharp origins for the first two benzene photoelectron bands invites consideration of the 552.5 nm absorption for C6H6+. A tentative assignment of the sharp, weak 552.5 nm band to the vibronically allowed 2E2g+e2u←2E1g transition of C6H6+ is in accord with deuterium isotopic shifts and predictions from the gas phase fluorescence spectra of fluorine substituted benzene cations. A strong 310 nm band system is attributed to the C6H7 radical which decreases on photolysis to give 288 and 534 nm absorptions that are assigned to the C6H5 radical.