Excimer Laser Flash Photolysis Research Articles

Effects of Fe(III)-concentration, speciation, excitation-wavelength and light intensity on the quantum yield of iron(III)-oxalato complex photolysis

Abstract Iron(III)oxalato complexes do frequently occur in the environment, specifically in surface waters, in atmospheric waters (clouds, rain, fog) or in waste waters. Due to their high photo-reactivity and their absorption overlap with the actinic spectrum, Fe(III)oxalato complex photochemistry is widespread and of broad interest. Fe(III)oxalato complex photolysis in deaerated solutions using single excimer laser flash photolysis at 308 and 351 nm and continuous Hg(Xe)-lamp irradiation at 313, 366 and 436 nm was quantified via Fe(II) quantum yield measurements with phenanthroline complexometry and UV–vis detection. Measured Fe(II) quantum yields showed a dependence on initial Fe(III)ferrioxalate concentration and irradiation energy at below millimolar concentrations. Individual molar extinctions (in l mol−1 cm−1) and individual quantum yields (Φ) were determined for initial Fe(III) concentrations of 4.85 × 10−4 M for the 1:2 (FeOx2−) and 1:3 (FeOx33−) complexes applying a regression analysis for solutions containing variable ratios of 1:2 and 1:3 complexes: ɛ1:2, 308 nm = 2300 ± 90, ɛ1:3, 308 nm = 2890 ± 40, Φ1:3, 308 nm = 0.93 ± 0.09; ɛ1:2, 351 nm = 1040 ± 30, ɛ1:3, 351 nm = 1120 ± 20, Φ1:3, 351 nm =0.88 ± 0.08; ɛ1:2, 313 nm = 2055 ± 111, ɛ1:3, 313 nm = 2663 ± 37, Φ1:3, 313 nm = 0.12 ± 0.05; ɛ1:2, 366 nm = 753 ± 357, ɛ1:3, 366 nm = 709 ± 10, Φ1:2, 366 nm = 1.17 ± 1.46, Φ1:3, 366 nm = 0.91 ± 0.09; ɛ1:2, 436 nm = 55 ± 9, ɛ1:3, 436 nm = 22 ± 2, Φ1:2, 436 nm = 1.40 ± 0.40, Φ1:3, 436 nm = 1.00 ± 0.20. Individual quantum yields for the 1:2 complex could only be determined for the excitation wavelengths 366 and 436 nm due to non-linearity of the data for 308, 351 and 313 nm. The non-linearity is ascribed to complicated interactions of secondary reactions involving Fe(III)oxalato educt-complexes, carboxyl radicals and Fe(II)-radical complexes. The 1:2 complex has generally a higher quantum yield compared to the 1:3 complex at all considered wavelengths.

Degradation processes in the cellulose/N-methylmorpholine-N-oxide system studied by HPLC and ESR. Radical formation/recombination kinetics under UV photolysis at 77 K

Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC) and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature, radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide type radicals ∼CH2–NO•–CH2∼ and/or ∼CH2–NO•–CH3∼ at the first and methyl •CH3 and formyl •CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g., Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO.

Electron spin resonance study of radicals generated in cellulose/N‐methylmorpholine solutions after flash photolysis at 77 K

The processes of radical formation in N-methylmorpholine-N-oxide monohydrate (NMMO) and cellulose/NMMO solutions were studied by ESR at 77 K under high-power UV (lambda = 248 nm) excimer laser flash photolysis. Radicals mainly generated were attributed to the nitroxide type radicals -CH2-NO*-CH2- and -CH2-NO*-CH3 at the first step and methyl *CH3 and formyl *CHO radicals at the second step of the photoreaction. Kinetic studies of these radicals revealed that formation and recombination rates of the radicals depend on the cellulose concentration in cellulose/NMMO solutions and the concentration of additional ingredients, e.g. Fe(II) and propyl gallate. Even at frozen state temperature, acceleration or quenching of radical reaction processes was found. The proposed scheme of UV light-induced NMMO degradation during irradiation based on ESR data correlates well with independently obtained results based on high-performance liquid chromatography (HPLC). The analysis of degradation products by HPLC, e.g. aminoethanol and acetaldehyde, supports the assumption concerning a radical-initiated ring opening of NMMO.

High-resolution infrared spectra of the C-H asymmetric stretch vibration of jet-cooled methoxy radical (CH3O)

The high-resolution infrared spectrum of jet-cooled methoxy radical (CH3O) in the region 2850–2940 cm−1 has been recorded at about 26 K in a pulsed slit-jet expansion producing the radical by excimer laser flash photolysis. A perpendicular band centered near 2885 cm−1 was assigned to a portion of the C-H asymmetric stretch vibration (ν4) of CH3O by means of ground-state combination differences. Five ΔP=+1 subbands with P″=−1.5, −0.5, 0.5, 1.5, 2.5, and J″ up to 9.5 have been assigned. Because of the low jet temperature, the assigned transitions all arise from the ground E3/2 fine-structure component of the lowest vibronic state. Preliminary analysis yields values of A, B, and D of 5.232(12), 0.913(8), and 2.5(1.5)×10−5 cm−1, respectively. A small hyperfine splitting was observed in the P″=0.5 series transitions.

High-resolution infrared spectra of jet-cooled allyl radical (CH2–CH–CH2): ν2, ν3, and ν14 C–H stretch vibrations

The high-resolution infrared spectrum of jet-cooled allyl radical (CH2–CH–CH2) in the region 3010–3045 cm−1 has been recorded in pulsed slit jet expansions using both modulated discharge and excimer laser flash photolysis methods to produce the radicals. Over 400 transitions are observed and assigned to the ν2, ν3, and ν14 C–H stretch vibrations. The band origins have been determined to be 3033.8745(6), 3023.4605(6), and 3020.32(1) cm−1, respectively. Spectral analysis indicates that all three upper states are perturbed, with the perturbations in the ν14 upper state so pervasive as to make least-squares analysis difficult. The gas phase frequencies reveal a somewhat unusual, in comparison with the other bands, matrix shift to the blue for ν2 of 18 cm−1.

Dynamics of Silylenes in Polysilylenes with Structural Defects by Excimer Laser Flash Photolysis

Abstract The dynamics of silylene extrusion reaction was observed in polysilylene with Si branchings by using KrF excimer laser flash photolysis technique. The initial yield of silyl radicals was seen to be dependent on the backbone structure of polysilylenes, whereas the yield of silylenes was almost proportional to the number of the linear Si units in their backbones.

Absorption spectroscopy of singlet CH2 near 11 200 cm–1

New spectra of methylene, CH2, in the near infrared region of the singlet band system have been obtained. They were detected by laser transient absorption spectroscopy following excimer laser flash photolysis of ketene, CH2CO, at 308 nm. The new data provide information on the nature and energies of rotational levels in three vibronic states that lie between approximately 11 500 and 13 000 cm–1 above the zero point level of the lower, ã1A1, state. Taken together with previous measurements, a fairly complete picture of the vibronic energy levels of CH2 in this region can be built up. There is generally good agreement with the recent computational studies on the singlet states of CH2, but some evidence for discrepancies for levels with K = 3 and higher near the minimum of the [Formula: see text]1B1 potential surface. PACS No.: 33.20Ea

Reduction Potential of the Sulfhydryl Radical: Pulse Radiolysis and Laser Flash Photolysis Studies of the Formation and Reactions of ·SH and HSSH·- in Aqueous Solutions

Formation and reactions of the ·SH/·S- and HSSH·-/HSS·2- radicals in aqueous solutions have been studied by excimer laser flash photolysis and by pulse radiolysis. Acidic H2S solutions can be photolyzed with 193 nm laser pulses and produce a transient species with λmax at 240 nm, ascribed to the ·SH/·S- radical. Solutions of SH- can be photolyzed also with 248 nm laser pulses to produce the ·SH/·S- radical. The same radical is formed by oxidation of SH- ions with SO4·- and CO3·- radicals. At pH > 5, ·SH/·S- reacts with SH- (kf = 4 × 109 L mol-1 s-1, kr = 5 × 105 s-1) to form HSSH·-/HSS·2-, with λmax at 380 nm. Both ·SH/·S- and HSSH·-/HSS·2- react rapidly with O2; the former produces SO2·- (k = 5 × 109 L mol-1 s-1), and the latter produces O2·- (k = 4 × 108 L mol-1 s-1). Both radicals react with olefinic compounds. The monomeric radical oxidizes Fe(CN)64-, SO32-, ClO2-, and chlorpromazine. The dimeric radical is a weaker oxidant toward ferrocyanide but reduces N-methylpyridinium compounds. The reduction potential for the dimeric radical at pH 7 was determined from one-electron transfer equilibria with Mo(CN)83- and with the 4-methoxyaniline radical cation and found to be 0.69 V vs NHE. From the equilibrium constant K = [HSS·2-]/[SH-][·S-] = 8 × 103 L mol-1, the reduction potential for (·S-,H+/SH-) is calculated to be 0.92 V.

Rate coefficient for the reaction FCO+FC(O)O 2→2 FC(O)O at 296 K

The kinetics of the reaction FCO+FC(O)O 2→2 FC(O)O has been studied. The reactants were generated by KrF excimer laser flash photolysis of mixtures of (FCO) 2 and O 2 in excess of M=SF 6 at 296 K. The three radicals were monitored by absorption spectroscopy. Assuming that the reaction proceeds through a complex-forming process, the measured rate coefficient, (2.1±0.5)×10 −11 cm 3 molecule −1 s −1, agrees with that predicted by a simplified version of the statistical adiabatic channel model using a ratio α/ β=0.50 between the looseness and the Morse parameters. A rate coefficient of (3.7±0.2)×10 −12 cm 3 molecule −1 s −1 was determined for the FC(O)O+FC(O)O 2+M→(FCO) 2O 3+M reaction. Employing isodesmic reaction schemes at the B3LYP/6-311++G(d,p) level of theory, the enthalpy of formation of FC(O)O 2 and (FCO) 2O 2 are calculated to be −76.1±4 and −202.9±4 kcal mol −1, respectively.

Photo-Induced Reaction Dynamics in Poly(di-n-hexylsilylene) by Excimer Laser Flash Photolysis

Abstract KrF excimer laser induced photo-dissociation reaction was observed for poly(di-n-hexylsilylene) (PDHS) in several solvents at room temperature by using transient absorption spectroscopy technique within the wavelength range at 340-1600 nm. Transient absorption strongly depended on solvents suggesting that observed near UV absorption was ascribed to polysilylated silyl radicals in cyclohexane and tetrahydrofran (THF), and electron photoejection was also observed in CH2Cl2 giving radical cations of PDHS.

Kinetics of the reaction of Cl atoms with FC(O)O and FC(O) OCl

The kinetics of the Cl + FC(O) O + M → FC(O)OCl + M recombination reaction has been studied by using an ArF excimer laser flash photolysis technique over the 3–650 mbar pressure range of the bath gases He and SF 6 at 295 K. The reaction was found to be close to the second-order regime at total pressures greater than about 100 mbar. The derived limiting high-pressure rate coefficient is (3.1 ± 0.3) × 10 −11 cm 3 molecule −1 s −1. For the Cl + FC(O)OCl → FC(O)O + Cl 2 reaction a rate coefficient of (1.5 ± 0.2) × 10 −11 cm 3 molecule −1 s −1 was determined. An analysis of the results in terms of the simplified versions of the statistical adiabatic channel model has carried out.

Experimental and theoretical study of the recombination reaction of FC(O)O radicals

Abstract The kinetics of the recombination reaction of FC(O)O radicals has been experimentally and theoretically investigated. The FC(O)O radicals were generated by excimer laser flash photolysis of mixtures of (FCO) 2 and O 2 in excess of CO at 297 K and monitored by absorption spectroscopy at 545 nm. In agreement with the predictions from unimolecular rate theory, no appreciable falloff effects were observed for total pressures ranging from 117 to 727 Torr. The measured limiting high pressure rate coefficient, k rec, ∞ = (7.0 ± 1.1) × 10 −13 cm 3 molecule −1 s −1 , agrees very well with that predicted by a simplified version of the statistical adiabatic channel model. The influence of the dominant potential energy surface features on k rec, ∞ was examined between 200 and 600 K.

Quantum yield for carbon monoxide production in the 248 nm photodissociation of carbonyl sulfide (OCS)

Tunable diode laser absorption spectroscopy has been coupled with excimer laser flash photolysis to measure the quantum yield for CO production from 248 nm photo‐dissociation of carbonyl sulfide (OCS) relative to the well‐known quantum yield for CO production from 248 nm photolysis of phosgene (Cl2CO). The temporal resolution of the experiments was sufficient to distinguish CO formed directly by photodissociation from that formed by subsequent S(³PJ) reaction with OCS. Under the experimental conditions employed, CO formation via the fast S(¹D2) + OCS reaction was minimal. Measurements at 297K and total pressures from 4 to 100 Torr N2 + N2O show the CO yield to be greater than 0.95 and most likely unity. This result suggests that the contribution of OCS as a precursor to the lower stratospheric sulfate aerosol layer is somewhat larger than previously thought.

Rate Constant Measurement of the Recombination Reaction C3H3 + C3H3

Using the technique of infrared kinetic absorption spectroscopy, the second-order rate constant for the recombination reaction of the propargyl radical (C[sub 3]H[sub 3] + C[sub 3]H[sub 3]) has been measured and found to have the value (1.2 [+-] 0.2) x 10[sup [minus]10] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] at 295 K. The radical was produced in a flow cell by excimer laser flash photolysis ([lambda] = 193 nm) of the precursors C[sub 3]H[sub 3]Cl or C[sub 3]H[sub 3]Br and detected using time-resolved IR absorption. Absolute concentrations of C[sub 3]H[sub 3] were determined by comparing the C[sub 3]H[sub 3] absorption intensity with that of the Br atom. This calibration scheme was checked by producing methyl radicals by photolysis of methyl bromide and comparing the rate constant for methyl recombination thus obtained with literature values. The quantum yield for HCl production from the photodissociation of C[sub 3]H[sub 3]Cl at 193 nm was determined to be 0.07 [+-] 0.01. 47 refs., 10 figs., 1 tab.

Absolute rate constants of R+NO(+M)→RNO(+M) reactions. II. Measurements for R=C 2F 5, i-C 3F 7, n-C 4F 9, and t-C 4F 9 at T = 295 K

Absolute rate constants of the reactions C 2F 5+NO(+M)→C 2F 5NO(+M) (4), i-C 3F 7+NO(+M)→ i-C 3F 7NO(+M) (5), n-C 4F 9+NO(+M)→ n=C 4F 9NO(+M) (6) and t-C 4F 9+NO)(+M)→ t-C 4F 9NO(+M) (7) for M=He over the pressure range 10–500 Torr have been measured at T=295 K by means of time-resolved intractivity absorption of He-Ne laser radiation combined with excimer laser flash photolysis. The fall-off curves k([M]) have been constructed using a procedure based on Troe's formalism. The limiting high-pressure rate constants obtained are k 4,∞ = (1.8 ± 0.2) × 10 −11 cm 3/s, k 5,∞ = (1.7±0.2) × 10 −11 cm 3/s, k 6,∞ = (1.6±0.2) × 10 −11 cm 3/s and k 7,∞ = (1.25±0.20) × 10 −11 cm 3/s. A statistical adiabatic channel model (SACM) calculation of k ∞ for the R+NO→RNO reaction has been performed for several radicals R. Experimental k ∞ values are described by SACM with 0.50<α/β<0.62.

Kinetics and thermochemistry of R + hydrogen bromide .dblarw. RH + bromine atom reactions: determinations of the heat of formation of ethyl, isopropyl, sec-butyl and tert-butyl radicals

The reactions of alkyl radicals (R = CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, and t-C{sub 4}H{sub 9}) with HBr have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry. R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetics of reverse reactions, Br + RH (R = C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, n-C{sub 4}H{sub 10}, i-C{sub 4}H{sub 10}), have also been investigated using laser flash photolysis/resonance fluorescence methods. These results, combined with previously obtained kinetic information, were used in second- and third-law thermochemical calculations to obtain accurate determinations of the heats of formation of the C{sub 2}-C{sub 4} alkyl radicals involved. Second- and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol{sup {minus}1}). The heats of formation of the radicals thus obtained are in excellent agreement with values obtained from studies of dissociation/association equilibria, within 2.6 kJ mol{sup {minus}1}. Recommended alkyl-radical heats of formation (with uncertainties) at 298 K are provided that are based on an assessment of all the results of the current study and a review of other recent determination (kJ mol{sup {minus}1}): C{sub 2}H{sub 5},more » 121.0 {+-} 1.5; i-C{sub 3}H{sub 7}, 90.0 {+-} 1.7; sec-C{sub 4}H{sub 9}, 67.5 {+-}2.2; t-C{sub 4}H{sub 9}, 51.3 {+-} 1.8. Accurate determination of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol{sup {minus}1}): primary C-H in C{sub 2}H{sub 6} (422.8 {+-} 1.5); secondary C-H in C{sub 3}H{sub 8} (412.7 {+-} 1.7) and n-C{sub 4}H{sub 10} (411.1 {+-} 2.2); tertiary C-H in i-C{sub 4}H{sub 10} (403.5 {+-} 1.8). 44 refs., 4 figs., 8 tabs.« less

Study of kinetic processes in the excimer laser flash photolysis of TiCl 4

The production of titanium atoms and TiCl n species in the laser flash photolysis of TiCl 4 was investigated by the time-resolved fluorescence spectroscopy. In addition to the dissociation TiCl n → TiCl n− + Cl, there is significant production of Cl 2, detected by UV absorption spectroscopy and formed by the reaction TiCl n → TiCl n− +Cl 2. The formation of atomic titanium is preferred at high laser fluences and in the presence of hydrogen, which removes reactive atomic chlorine from the reaction mixture.

Photoamination of phenanthrene in the presence of p-dicyanobenzene—a flash photolysis study

The photoamination of phenanthrene (PhH) with ethylenediamine (EDA) in the presence of p-dicyanobenzene (DCB) was studied in a 9:1 acetonitrile—water mixture using a nanosecond excimer laser flash photolysis system. Time-resolved transient absorption spectra and decay curves at different absorption wavelengths were analysed. Five different transients, 3PhH*, PhH ∓+, DCB ∓−, [PhH +·DCB −] and PhH +−EDA], were observed. The efficiency of the photoamination depends on two important steps. The first is an electron transfer via an exciplex-type transient between PhH and DCB and the subsequent formation of an ion pair. The second step is a nucleophilic attack by the amine on the ion pair. This reaction competes with the dissociation of the ion pair.

Absolute rate constants of R + NO (+ M) → RNO(+ M) reactions (R = C 3F 7, CF 3, CH 3) at T = 295 K measured by time-resolved intracavity absorption of He-Ne laser radiation

Absolute rate constants of the reactions C 3F 7 + NO(+ M) → C 3F 7NO(+ M) (1), CF 3+NO(+M)→CF 3NO(+M) (2), and, CH 3 + NO(+ M) → CH 3NO(+ M) (3) for M=He over the pressure range 10–720 Torr have been measured at T = 295 K by means of time-resolved intracavity absorption of He-Ne laser radiation combined with excimer laser flash photolysis. The fall-off curves κ([M]) have been constructed using a procedure based on Tree's formalism. The limiting low- and high-pressure rate constants obtained are: κ 1∞=(1.76±0.20)×10 −11 cm 3/s; κ 20=(4.4±1.0)×10 −29 cm 6/s; κ 2∞=(1.83±0.20)×10 −11 cm 3/s; κ 30=(2.3±2.2)×10 −30 cm 6/s; κ 3∞=(1.7±1.0)×10 −11 cm 3/s.

Spectrokinetic determination of fluorosulphate radical absorption cross-sections

The absorption cross-sections of fluorosulphate radicals (FSO 3) were measured in the wavelength range 340 – 450 nm at 295 K using a technique of combined ArF excimer laser flash photolysis and absorption spectroscopy. The absorption cross-sections of fluorine fluorosulphonate (FSO 2OF) between 193 and 360 nm are also reported.