Laser-induced fluorescence of benzyl radicals in the gas phase

The excitation spectrum of the laser-induced fluorescence of benzyl has been observed in the gas phase. Fluorescence lifetimes of 880 ± 10 ns at zero pressure were obtained for the s, t and 6a¹₀ bands of the (1 ²A₂—1 ²B₂, 2 ²B₂—1 ²B₂) transitions of benzyl-h₇. The fluorescence lifetime of the t band in the corresponding transition of benzyl-d₇ was 1340 ns.     

Laser-induced fluorescence of jet-cooled complexes of carbazole and N-ethyl carbazole with alkyl cyanides

Fluorescence excitation spectra of complexes of carbazole and N-ethyl carbazole with a series of six alkyi cyanides, in a supersonic jet, are presented. The former chromophore undergoes hydrogen bonding with these ligands; N-ethylation of the chromophore prevents hydrogen bonding, but formation of complexes apparently similar in terms of both intermolecular interaction strength and geometry is observed.     

Doublet-doublet fluorescence of benzyl,p-methylbenzyl and p-chlorobenzyl radicals in solution

The doublet-doublet fluorescence spectra of benzyl, p-methylbenzyl and p-chlorobenzyl radicals were detected in the 450–650 nm region upon the double-pulse (248 nm→308 nm) excitation of corresponding benzyl chlorides in hexane at room temperature. The respective fluorescence lifetimes were determined to be ≈ 1 ns or less (benzyl), 14 ns (p-methylbenzyl) and 81 ns (p-chlorobenzyl). Extensively temperature-dependent non-radiative relaxation was confirmed for these benzyl radicals with close-lying lowest doublet excited states.     

The fluorescence lifetimes of the methyl-substituted benzyl,triphenylmethyl and diphenylmethyl radicals

Time resolved fluorescence of some methyl-substituted benzyl, triphenylmethyl, and diphenylmethyl radicals trapped in rigid solvents at low temperature has been observed. These radicals were excited by pulsed N₂ laser at 337 nm. It is found that these radicals exhibit very long fluorescence lifetime. The long lifetimes of these radicals seem to indicate that the first doublet-doublet electronic transitions of the radicals have a forbidden character.     

Fluorescence lifetimes of benzyl and deuterium-substituted benzyl radicals

Fluorescence lifetimes of C₆H₅CH₂, o-, m-, p-DC₆H₅CH₂, C₆H₅CD₂, C₆D₅CH₂, and C₆D₅CD₂ have been measured at 77 K in rigid glasses. Very long decay times are observed for all the radicals. There is a dramatic deuterium effect upon ring deuteration; alkyl deuteration lengthens the lifetime if the phenyl ring is already deuterated, but otherwise exhibits a slight inverse deuterium effect. The oscillator strength of the ²A₂ ? ²B₂ transition is calculated to be no larger than 8 × 10^?4.     

Pyrene measurements in sooting low pressure methane flames by jet-cooled laser-induced fluorescence

This paper presents in detail the study we carried out concerning the pyrene measurement by jet-cooled laser-induced fluorescence (JCLIF) in different sooting low pressure methane flames. The aim of this paper is both to demonstrate the potentialities of this technique for the measurement of such moderately sized polycyclic aromatic hydrocarbons under sooting flame conditions and to provide new experimental data for the understanding and the development of chemical models of the soot formation processes. Several concentration profiles of pyrene measured in different sooting flame (various pressure and equivalence ratio) are presented. The validation of the JCLIF method for pyrene measurements is explained in detail as well as the calibration procedure, based on the standard addition method, which has been implemented for the quantification of the concentration profiles. Sensitivity lower than 1 ppb was obtained for the measurement of this species under sooting flame conditions.     

Excitation and emission spectra of jet-cooled naphthylmethyl radicals

Gas phase excitation and emission spectra of three naphthylmethyl radical chromophores are presented. These resonance-stabilized species, 1-naphthylmethyl, 2-naphthylmethyl, and α-acenaphthenyl, each possessing an sp(2) carbon adjacent to a naphthalene moiety, are studied by resonant two-color two-photon ionization, laser induced fluorescence, and dispersed fluorescence spectroscopy. Identification of the radicals is made through a combination of dispersed fluorescence and density functional theory calculations. All three species possess spectra in the 580 nm region. The possible relevance to unidentified spectroscopic features such as the diffuse interstellar bands and emission from the Red Rectangle nebula is discussed.     

Laser-induced fluorescence excitation spectra of CCl2 and CFCI radicals in the gas phase

Laser-induced fluorescence excitation spectra in the gas phase have been obtained for CCl₂ and CFCI radicals. The observed spacings in the excitation spectra reported here agree well with previous absorption work in the matrix.     

Substituent effects on the lifetime and fluorescence of excited diphenylmethyl radicals in solution

The room-temperature lifetimes of ring-substituted excited diphenylmethyl radicals have been shown to correlate well with the energy gap between the fluorescent lowest excited doublet and the ground state. Attempts to correlate the lifetimes with Hammett constants were unsuccessful.     

Laser-induced breakdown spectroscopy for branching ratio and atomic lifetime measurements in singly-ionized neodymium and gallium

Precision laboratory astrophysics measurements can be made in laser-induced plasmas created for laser-induced breakdown spectroscopy. Branching ratios from highly-energetic levels in singly-ionized neodymium may be measured by observing spontaneous emission in laser-induced plasmas in an argon environment at decreased pressures (~ 7.7 mbar). Utilizing a broadband Èchelle spectrometer with a spectral range from 200–840 nm, the spontaneous emission intensities from hundreds of transitions originating in 138 energy levels in Nd I, Nd II, and Nd III have been observed simultaneously, allowing the determination of branching ratios for these energy levels for branches greater than 1% in the visible wavelength range. In this study, eight branching ratios from the 23,229.991 cm^− 1 level in Nd II were measured and compared to previously determined values as a method for optimizing experimental conditions such as buffer gas pressure and observation delay time. The branching ratios of the eight branches were found to be in excellent agreement with three previously determined values from both experiment and theory. A plan to utilize this laser-induced plasma apparatus to measure the lifetime of the 4s5p³P₂ level at 118,727.89 cm^− 1 in singly-ionized gallium using a cascade-photon-coincidence method is also presented. Utilizing a solid Ga target ablated in a helium environment, “start photons” at 541.6 nm from a transition into the 4s5p³P₂ level and “stop photons” at 633.4 nm from a transition out of that Ga II energy level were observed. Single-photon detection will be accomplished using avalanche photodiodes with narrowband interference filters and delay times between the detection of coincident photons from these two transitions will be measured.     

Laser-induced dispersed fluorescence detection of polycyclic aromatic compounds in soil extracts separated by capillary electrochromatography

Polycyclic aromatic hydrocarbons (PAHs) and nitrogen containing aromatic compounds (NCACs) are characterized in soil extracts and laboratory standards by capillary electrochromatography (CEC) with laser-induced dispersed fluorescence (LIDF) detection using a liquid-nitrogen cooled charge-coupled device detector. The LIDF detection technique provides information on compound identity and, when coupled with the high separation efficiencies of the CEC technique, proves useful in the analysis of complex mixtures. Differences in fluorescence spectra also provide a means of identifying co-eluting compounds by using deconvolution algorithms. Detection limits range from 0.5 to 96x10(-10) M for selected PAHs and 0.9-3.7x10(-10) M for selected NCACs. Soil extracts are also injected onto the CEC column to evaluate chromatographic method performance with respect to complex samples and the ability to withstand exposure to environmental samples.     

Single-molecule fluorescence lifetime and anisotropy measurements of the red fluorescent protein,DsRed, in solution

Fluorescence lifetime and anisotropy measurements were made on the red fluorescent protein (DsRed) from tropical coral of the Discosoma genus, both at single-molecule and bulk concentrations. As expected from previous work, the fluorescence lifetime of DsRed in solution is dependent on laser power, decreasing from an average fluorescence lifetime in the beam of about 3.3 ns at low power (3.5 ns if one extrapolates to zero power) to about 2.1 ns at 28 kW/cm2. At the single-molecule level, exciting with 532 nm, 10 ps laser pulses at 80 MHz repetition rate, DsRed particles entering the laser beam initially have a lifetime of about 3.6 ns and convert to a form having a lifetime of about 3.0 ns with a quantum yield of photoconversion on the order of 10(-3) (calculated in terms of photons per DsRed tetramer). The particles then undergo additional photoconversion with a quantum yield of roughly 10(-5), generating a form with an average lifetime of 1.6 ns. These results may be explained by rapid photoconversion of one DsRed monomer in a tetramer, which acts as an energy transfer sink, resulting in a lower quantum yield for photoconversion of subsequent monomers. Multiparameter correlation and selective averaging can be used to identify DsRed in a mixture of fluorophores, in part exploiting the fact that fluorescent lifetime of DsRed changes as a function of excitation intensity.     

Spectroscopy of the A(1B2)-X(1A1) transition of jet-cooled fluorobenzene: laser-induced fluorescence, dispersed fluorescence, and pathological Fermi resonances

A detailed study of the S1((1)B2)-S0((1)A1) electronic transition of jet-cooled fluorobenzene has been carried out using laser-induced fluorescence and dispersed fluorescence (DF) spectroscopies. Analysis of over 40 single vibronic level DF spectra resulted in the assignment of 16 fundamental frequencies in the excited electronic state. Progressions in totally symmetric modes, particularly in the ring-breathing mode nu9, feature in both types of fluorescence spectrum. There is also significant activity in non-totally-symmetric modes, with activity in Franck-Condon (FC)-allowed overtones, FC-forbidden combinations induced by Duschinsky mixing, and symmetry-forbidden transitions induced by the same Herzberg-Teller vibronic coupling mechanism that induces the benzene S1-S0 transition. Fermi resonances (FRs) are extensive throughout the spectrum, especially in the important FC-active a1 modes. A consequence of these extensive FRs is that several important previous assignments are shown to be incorrect and have been reassigned here. Ab initio and density functional theory calculations have also been performed to support the experimental assignments.     

Dispersed fluorescence spectroscopy and fluorescence lifetime measurements of excited vibrational levels of CdCH3

Laser-induced excitation and dispersed fluorescence spectra (originating from the 0⁰, 2¹ and 3¹ vibrational levels of the electronic state) of jet-cooled CdCH₃ are presented and assigned. The observation of emission from these levels is a result having important implications for the dynamics of photodissociation of Cd(CH₃)₂. Fluorescence lifetimes for emission from the 0⁰, 2¹ and 3¹ vibrational levels are also presented.     

Photolysis of bromo- and chloro-substituted benzyl derivatives. Competition between ionic and radical pathways

Photolysis of 1-fluoro-2-halo-1,2-diphenylethanes was studied in solutions of tetrahydrofuran, acetonitrile, and cyclohexane. The effect of free radical inhibitor and metal hydrides on products formation as well as their ratio was analyzed to elucidate the reaction pathway. In the first step homolytic C-X bond cleavage occurs from a single excited state, resulting in a biradical pair. Further reaction path depends on the type of the halogen bonded and on the solvent polarity. Electron transfer within the radical pair cage is apparently more rapid for bromides than for chlorides and is opposite as expected on the basis of the relative electronegativities of chlorine and bromine. As radicals approach each other, they fall into ionic or radical product channels. This is influenced by solvent polarity, resulting in the larger yield of ionic products in the case of acetonitrile as in the case of less polar cyclohexane.     

Laser fluorescence excitation band profiles of jet-cooled tropolone

The fluorescence excitation spectrum of the A¹B₂−X¹A₁ transition of tropolone was investigated as a function of pulsed laser intensity, I. For 0.5 < I < 4.5 MW/cm² the weak fluorescence intensity was found to increase approximately as aI + bI². The band profiles, which showed no evidence of resolvable rotational fine structure, were modeled by computations assuming a Lorentzian lineshape for the individual rotational transitions.     

High-temperature thermal decomposition of benzyl radicals

The thermal decomposition of the benzyl radical was studied in shock tube experiments using ultraviolet laser absorption at 266 nm for detection of benzyl. Test gas mixtures of 50 and 100 ppm of benzyl iodide dilute in argon were heated in reflected shock waves to temperatures ranging from 1430 to 1730 K at total pressures around 1.5 bar. The temporal behavior of the 266 nm absorption allowed for determination of the benzyl absorption cross-section at 266 nm and the rate coefficient for benzyl decomposition, C6H5CH2 --> C7H6 + H. The rate coefficient for benzyl decomposition at 1.5 bar can be described using a two-parameter Arrhenius expression by k1(T) = 8.20 x 10(14) exp(-40 600 K/T) [s(-1)], and the benzyl absorption cross-section at 266 nm was determined to be sigma(benzyl) = 1.9 x 10(-17) cm2 molecule(-1) with no discernible temperature dependence over the temperature range of the experiments.     

The flourescence lifetime of the benzyl radical

Time resolved fluorescence of the benzyl, monomethylbenzyl and dimethylbenzyl radicals strapped in rigid solvents at low temperatures has been observed using the second harmonic of the ruby laser at the exciting source. The fluorescence lifetimes of these radicals are very long (10^?7–10^?6 sec), which are influenced considerably by the methyl substituents. The long fluorescence lifetimes of the benzyl radical and its methyl derivatives are interpreted in terms of the forbidden character of the first doublet-doublet electronic transition.     

Laser-induced temperature dependent triplet state lifetime of rubreneperoxide

A number of photophysical properties of three different types of rubreneperoxides have been measured experimentally by flash spectroscopy technique, including the two-photon absorption, fluorescence, delayed fluorescence and temperature dependent triplet-triplet absorption spectra. Excited singlet and triplet state lifetimes are temperature dependent. Lowest triplet state lifetimes were measured from 77 K to 50 degrees C. Experimental observations showed that as we decreased the temperature of rubreneperoxides, most of the molecules migrate to the lowest vibrational and rotational energy levels of the ground electronic state. Similar migration is also observed for the lowest triplet state. Therefore at 77 K, we can get the clean absorption an emission spectra and decay curves for the lowest triplet state. At 50 degrees C, due to the P- and/or E-type of delayed fluorescences, decay of T(1) state, in other words disappearance of the T(1) state is becoming faster than at low temperature (below room temperature).