Apparent Lifetime Research Articles

Spectrally Resolved Frequency Domain Analysis of Multi-Fluorophore Systems Undergoing Energy Transfer

Complex systems of fluorophores undergoing energy transfer can exhibit a variety of anomalous lifetime behavior when probed with frequency domain methods. When presented in traditional apparent lifetime format the data from such systems can exhibit "nodal" behavior in which the computed lifetime approaches +/-infinity. The location of the nodes is system and frequency dependent. In addition, simpler systems, not undergoing energy transfer, show ill behavior in the region of zero lifetime (tau(m)) and long lifetime (tau(pi)) due to noise in typical measurements. Here, we systematically investigate systems of multiple fluorophores with and without energy transfer to provide insight into frequency domain investigations of complex systems of fluorophores. The results of simulations are compared to data collected from a multi-fluorophore system designed to exhibit fluorescence resonance energy transfer (FRET) using imaging spectroscopic fluorescence lifetime imaging microscopy (ISFLIM). The results are applicable to both cuvette and imaging arrangements.

A New Approach for Studying Fast Biological Reactions Involving Nitric Oxide: Generation of NO Using Photolabile Ruthenium and Manganese NO Donors

Nitric oxide (NO) is recognized as one of the major players in various biochemical processes, including blood pressure, neurotransmission and immune responses. However, experimental studies involving NO are often limited by difficulties associated with the use of NO gas, including its toxicity and precise control over NO concentration. Moreover, the reactions of NO with biological molecules, which frequently occur on time scales of microseconds or faster, are limited by the millisecond time scale of conventional stopped-flow techniques. Here we present a new approach for studying rapid biological reactions involving NO. The method is based on designed ruthenium and manganese nitrosyls, [Ru(PaPy3)(NO)](BF4)2 and [Mn(PaPy3)(NO)](ClO4) (PaPy3H = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide), which upon photolysis produce NO on a fast time scale. The kinetics of the binding of the photogenerated NO to reduced cytochrome c oxidase (CcO) and myoglobin (Mb) was investigated using time-resolved optical absorption spectroscopy. The NO was found to bind to reduced CcO with an apparent lifetime of 77 micros using the [Mn(PaPy3)(NO)]+ complex; the corresponding rate is 10-20 times faster than can be detected by conventional stopped-flow methods. Second-order rate constants of approximately 1 x 10(8) M(-1) s(-1) and approximately 3 x 10(7) M(-1) s(-1) were determined for NO binding to reduced CcO and Mb, respectively. The generation of NO by photolysis of these complexes circumvents the rate limitation of stopped-flow techniques and offers a novel alternative to study other fast biological reactions involving NO.

Protonation equilibria of [formula omitted] complexes possessing electron donating sites on the diimine ligand

The protonation equilibria of [Ru(LL)(CN) 4] 2− complexes, where LL = 4,4′-dicarboxy-2,2′-bipyridine (dcb) and 2,3-bis(2-pyridyl)-pyrazine (bppz), have been investigated by spectrophotometric titration and 13C NMR spectroscopy in the acidic region. Two protonation steps with p K a values of 2.0 and 0.5 and the absorption spectra of protonated species are distinguished for [Ru(bppz)(CN) 4] 2− in water. The 13C NMR spectra unambiguously prove the first protonation step occurring on the free pyridyl nitrogen of bppz. The second protonation step occurring on the cyanide ligand under pH = 2 results in a blue shift of the MLCT absorption maximum. The weakly emitting excited state of [Ru(bppz)(CN) 4] 2− is presumably more acidic than the ground state and a complete quenching of luminescence is observed with a diffusion controlled quenching rate of 4 ± 0.5 × 10 10 s −1. Four separate protonation steps with p K a values of 3.2, 2.2, 1.1, −0.2 and the absorption spectra of the different protonated complex of [Ru(dcb)(CN) 4] 4− have also been determined. The protonation of carboxyl groups gives rise to an upfield shift of 13C NMR lines (6.1 and 4.8 ppm) of the C4 and COO − carbon atoms. The chemical shift of cyanide ligands are also affected by the protonation of carboxyl group and 1.5 ppm upfield shift was measured between pH = 4 and 2. The emission intensity and apparent lifetimes of [Ru(dcb)(CN) 4] 4− decrease to ca. 10% of the original values at pH = 2, while the position of emission maxima shifts from 666 to 715 nm. The change of emission intensity versus pH suggests that the monoprotonated form of [Ru(dcbH)(CN) 4] 3− is a weaker emitter than [Ru(dcbH 2)(CN) 4] 2−.

Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor

The mechanism and kinetics of electron transfer in isolated D1/D2-cyt(b559) photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of approximately 1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8-12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the approximately 200-ps component represents the electron transfer to the Q(A) acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are approximately 3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor Chl(acc D1). Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated RC* excited state is 170-180 ns(-1), and the rate constant of secondary electron transfer is 120-130 ns(-1).

Regulation of the Monomer-Dimer Equilibrium in Inducible Nitric-oxide Synthase by Nitric Oxide

The oxygenase domain of inducible nitric-oxide synthase exists as a functional tight homodimer in the presence of the substrate L-arginine and the cofactor tetrahydrobiopterin (H4B). In the absence of H4B, the enzyme is a mixture of monomer and loose dimer. We show that exposure of H4B-free enzyme to NO induces dissociation of the loose dimer into monomers in a reaction that follows single exponential decay kinetics with a lifetime of approximately 300 min. It is followed by a faster autoreduction reaction of the heme iron with a lifetime of approximately 30 min and the concurrent breakage of the proximal iron-thiolate bond, forming a five-coordinate NO-bound ferrous species. Mass spectrometry revealed that the NO-induced monomerization is associated with intramolecular disulfide bond formation between Cys104 and Cys109, located in the zinc-binding motif. The regulatory effect of NO as a dimer inhibitor is discussed in the context of the structure/function relationships of this enzyme.

Charge Separation Kinetics in Intact Photosystem II Core Particles Is Trap-Limited. A Picosecond Fluorescence Study

The fluorescence kinetics in intact photosystem II core particles from the cyanobacterium Thermosynechococcus elongatus have been measured with picosecond resolution at room temperature in open reaction centers. At least two new lifetime components of approximately 2 and 9 ps have been resolved in the kinetics by global analysis in addition to several known longer-lived components (from 42 ps to approximately 2 ns). Kinetic compartment modeling yields a kinetic description in full agreement with the one found recently by femtosecond transient absorption spectroscopy [Holzwarth et al. (2005) submitted to Proc. Natl. Acad. Sci. U.S.A.]. We have for the first time resolved directly the fluorescence spectrum and the kinetics of the equilibrated excited reaction center in intact photosystem II and have found two early radical pairs before the electron is transferred to the quinone Q(A). The apparent lifetime for primary charge separation is 7 ps, that is, by a factor of 8-12 faster than assumed on the basis of earlier analyses. The main component of excited-state decay is 42 ps. The effective primary charge separation rate constant is 170 ns(-)(1), and the secondary electron-transfer rate constant is 112 ns(-)(1). Both electron-transfer steps are reversible. Electron transfer from pheophytin to Q(A) occurs with an apparent overall lifetime of 350 ps. The energy equilibration between the CP43/CP47 antenna and the reaction center occurs with a main apparent lifetime of approximately 1.5 ps and a minor 10 ps lifetime component. Analysis of the overall trapping kinetics based on the theory of energy migration and trapping on lattices shows that the charge separation kinetics in photosystem II is extremely trap-limited and not diffusion-to-the-trap-limited as claimed in several recent papers. These findings support the validity of the assumptions made in deriving the earlier exciton radical pair equilibrium model [Schatz, G. H., Brock, H., and Holzwarth, A. R. (1988) Biophys. J. 54, 397-405].

Control of Photoinduced Energy- and Electron-Transfer Steps in Zinc Porphyrin−Oligothiophene−Fullerene Linked Triads with Solvent Polarity

The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C(60)) linked triads (ZnP-nT-C(60)) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C(60), an energy transfer took place from the (1)ZnP moiety to the C(60) moiety, generating ZnP-nT-(1)C(60). In polar solvents, the CS process also took place directly via the (1)ZnP moiety, generating ZnP(*+)-nT-C(60)(*-), as well as the energy transfer to the C(60) moiety. After this energy transfer, an indirect CS process took place from the (1)C(60) moiety. In the less polar solvent anisole, the radical cation (hole) of ZnP(*+)-nT-C(60)(*-) shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 10(8) s(-1); then, the final CS states ZnP-nT(*+)-C(60)(*-) were lasting for 6-7 mus. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT(*+)-C(60)(*-) and ZnP(*+)-nT-C(60)(*-) were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 mus) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nT(*+)-C(60)(*-) was confirmed with apparent short lifetimes (0.6-0.8 mus), which can be explained by the fast hole shift to more stable ZnP(*+)-nT-C(60)(*-) followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C(60); that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H(2)P-nT-C(60), in which H(2)P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C(60) are approximately 30 times longer than those in o-DCB.

Charge Recombination Fluorescence in Photosystem I Reaction Centers fromChlamydomonas reinhardtii

The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Müller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Müller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii.

A transient molecular probe for characterizing the surface properties of TiO2 nanoparticle in colloidal solution

A transient molecular probe for characterization of the surface properties of TiO2 nanoparticles in colloidal solution developed recently in our laboratory is briefly reviewed. The probe molecule is all-trans-retinoic acid (ATRA) adsorbed at the TiO2 nanoparticle surface. After photoexcitation, the photoinduced interfacial charge recombination would generate ATRA triplet state (ATRAt) with a substantial quantum yield. It is found that the triplet–triplet absorption spectrum of ATRA adsorbed molecule is sensitive to its binding form with the surface Ti atom through the carboxylic group, as well as to the polarity of the medium. Especially the apparent lifetime of ATRAt at the TiO2 surface changes substantially when the local polarity around the TiO2 nanoparticle changes. Therefore the ATRAt monolayer adsorbed at the surface can be used as a transient molecular probe for the surface binding forms, coordination state of the surface Ti atoms and the light-induced wettability change of the TiO2 nanoparticle.

Determining oxygen diffusion coefficients in polymer films by lifetimes of luminescent complexes measured in the frequency domain.

Polymer films doped with luminescent ruthenium complexes are proving to be important oxygen sensors. We describe a technique using lifetime measurements in the frequency domain for determining the diffusion coefficient of oxygen through various polymer supports. These fundamental measurements will allow for more rational design of improved sensors. Three types of polymers were doped with [Ru(4,7-diphenyl-1,10-phenanthroline)3]Cl2. We monitored the luminescence versus time after applying a step increase in the oxygen pressure at the surface of the film. We modeled the decrease in apparent lifetime as a function of time using the diffusion coefficient of oxygen in the polymer as the only adjustable parameter. The model accurately predicted the lifetime versus time curves, and diffusion coefficients agreed well with those obtained from intensity measurements. The advantages and disadvantages of the lifetime technique to those used earlier are discussed.

Why a long-lived fireball can be compatible with Hanbury-Brown–Twiss measurements

The common interpretation of HBT data measured at top SPS energies leads to apparent source lifetimes of 6-8 fm/c and emission duration of approximately 2-3 fm/c. We investigate a scenario with continuous pion emission from a long-lived (~17 fm/c) thermalized source in order to show that it is not excluded by the data. Starting from a description of the source's spacetime expansion based on gross thermodynamical properties of hot matter (which is able to describe a number of experimental observables), we introduce the pion emission function with a contribution from continuous emission during the source's lifetime and another contribution from the final breakup and proceed by calculating the HBT parameters R_out and R_side. The results are compared with experimental data measured at SPS for 158 AGeV central Pb-Pb collisions. We achieve agreement with data, provided that some minor modifications of the fireball evolution scenario are made and find that the parameter R_out is not sensitive to the fireball lifetime, but only to the duration of the final breakup, in spite of the fact that emission takes place throughout the whole lifetime. We explicitly demonstrate that those findings do not alter previous results obtained within this model.

Visible Light Induced Electron Transfer and Long-Lived Charge Separated State in Cyanine Dye/Layered Titanate Intercalation Compounds

Visible light induced guest-to-host electron transfer and formation of long-lived charge separated state were demonstrated in cyanine dye/layered titanate intercalation compounds. 1,1‘-Diethyl-2,2‘-cyanine (dye-1) and 5,5‘-dichloro-3,3‘,9-triethyl-thiacarbocyanine (dye-2) were intercalated into layered titanates, CsxTi2-x/4□x/4O4 (□ = vacancy, x = 0.7) and Na2Ti3O7, by cation exchange reactions with alkylammonium-exchanged forms as the intermediates. The formation of the intercalation compounds was confirmed by XRD, elemental analysis, and visible absorption and fluorescence spectroscopies. The intercalated dyes formed J-aggregates in the interlayer spaces for all the dye/titanate systems. The visible light induced electron transfer was shown by the appearance of ESR signal, which was ascribed to the radical dications of the dyes, and by the quenching of the fluorescence of the dye aggregates. Apparent lifetimes of the charge separated states were estimated from the decay curve of the ESR signal. The char...

Reaction of Cr Atoms with O2at Low Pressures: Observation of New Chemiluminescence Bands from CrO2*

Ground and low-lying electronic states of Cr atoms in the gas phase were generated from photolysis of <TEX>$Cr(CO)_6$</TEX> vapor in He or Ar using an unfocussed weak UV laser pulse and their reactions with <TEX>$O_2$</TEX> and <TEX>$N_2O$</TEX> were studied. When 0.5-1.0 Torr of <TEX>$Cr(CO)_6$</TEX> /<TEX>$O_2$</TEX> /He or Ar mixtures were photolyzed using 295-300 nm laser pulses, broadband chemiluminescence peaked at ~420 and ~500 nm, respectively, was observed in addition to the atomic emissions from <TEX>$z^7P^{\circ}$</TEX>, <TEX>$z^5P^{\circ}$</TEX>, and <TEX>$y^7P^{\circ}$</TEX> states of Cr atoms. When <TEX>$N_2O$</TEX> was used instead of <TEX>$O_2$</TEX>, no chemiluminescence was observed. The chemiluminescence intensities as well as the LIF intensities for those three low-lying electronic states (<TEX>$a^7S_3,\;a^5S_2\;and\;a^5D_J$</TEX>) showed second-order dependence on the photolysis laser power. Also, the chemiluminescence intensities were first-order in <TEX>$O_2$</TEX> pressure, but the presence of excess Ar showed a strong inhibition effect on them. Based on the experimental results, the chemiluminecent species in this work is attributed to <TEX>$CrO_2^*$</TEX> generated from hot ground state Cr atoms with <TEX>$O_2$</TEX>. The apparent radiative lifetimes of the chemiluminescent species and collisional quenching rate constants by <TEX>$O_2$</TEX> and Ar also were investigated.

25 nm resolution single molecular fluorescence imaging by scanning near-field optical/atomic force microscopy

High-resolution single molecular near-field fluorescence images were observed by scanning near-field optical/atomic force microscopy (SNOM/AFM). We modified the SNOM/AFM for both high-resolution fluorescence imaging and high-resolution topographic imaging. The imaged fluorophore, Alexa 532, is prepared with a poly-methyl-methacrylate (PMMA) film coating. A fluorescence resolution of 25 nm was obtained with a simultaneous topographic image of a flat surface. A sample prepared with a lower PMMA concentration exhibited a rough surface in the micro area. The results for the flat surface indicated that the fluorescence resolution is worst in the rough surface sample, that the maximum fluorescence intensities for the individual fluorophore are similar, and that the decay rate is faster. Thus, we concluded that the morphological effect is an important factor in fluorescence image resolution and the apparent lifetimes of the fluorescence molecules.

I– V and deep level transient spectroscopy studies on 60 MeV oxygen ion irradiated NPN transistors

NPN transistors have been irradiated by 60 MeV oxygen ions in a fluence ranging from 5 × 10 10 to 1 × 10 13 ions/cm 2. The DC current gain ( h FE), excess base current ( ΔI B =I B post −I B pre ), excess collector current ( ΔI C =I C post −I C pre ) and collector saturation current (I C Sat ) of the ion irradiated transistors were studied systematically. The h FE of the transistors were found to be decreased drastically after ion irradiation. A significant increase has been observed in the collector current ( I C) along with the increase in the base current ( I B) after ion irradiation. The I C Sat of the ion irradiated transistors were also decreased significantly after irradiation. The radiation induced trap levels in the collector base depletion region of NPN transistors were studied by deep level transient spectroscopy technique and different types of trap levels were observed. The results obtained on the activation energy, density of trap levels, apparent capture cross section, introduction rate and space charge layer lifetime of different defects for different total fluence are presented and discussed.

Low-Temperature Study of Photoinduced Energy Transfer from Tryptophan Residues of Escherichia coli Alkaline Phosphatase to Bound Terbium

Low-temperature phosphorescence (LTP) decay measurements have been carried out in wild-type Escherichia coli alkaline phosphatase (AP), metal-depleted AP (apoAP), and terbium-substituted AP (TbAP) at 77, 4.2, and 1.2 K. Over this temperature range, Tb emission monitored at 542.4 nm decays with two apparent lifetime components of 16−20 ms and 130−170 ms when TbAP is excited at 280 nm where Tb absorbs negligibly. These lifetimes are orders of magnitude greater than exhibited by TbCl3 at low temperature and by TbAP at room temperature; we attribute this Tb luminescence in TbAP largely to energy transfer from the triplet state of initially excited tryptophan. LTP and ODMR at 1.2 K suggest that Trp109 and Trp220 have similar environments in AP and in apoAP. Trp109 is found to occupy two distinct sites in both AP and apo AP that exhibit resolved ODMR transitions. Comparison of the phosphorescence spectra of AP and TbAP shows clearly that the sensitized Tb emission observed in TbAP is due selectively to energy transfer (ET) from Trp109 to Tb and not significantly from Trp220. Evaluation of the results also suggests that ET takes place through space by Dexter's exchange mechanism, where the lowest triplet state of Trp acts as donor and the 7F6 ground state of Tb acts as acceptor. The ratio of the two ET rate constants suggests that two sites of Trp109 (possibly those resolved by ODMR) act as donors to either or both metal-binding sites, M2 or M3, rather than one Trp109 site transferring energy to Tb at either site M2 or M3.

Spin–orbit induced predissociation of the B 2Σ g− and 2 2Σ g+ states of the O 2+ ion

Highly correlated MRCI electronic wavefunctions are used to calculate the potential energy functions, the non-adiabatic and the spin–orbit coupling elements for the B 2Σ g − and neighboring states of the O 2 + ion. The results are employed in calculations of the spectroscopic constants and the spin–orbit induced predissociation lifetimes using the complex scaling method. The dominant predissociation path correlating with the O + O + atoms in their ground states is found to proceed via the f 4Π g state. The path leading to the O( 1D) + O +( 4S) asymptote involves couplings with the 2 2Π g and 2 4Π g states. The weak vibrational progression embedded in the progression of the B-state is assigned to the 2 2Σ g + state. The predissociative lifetimes are calculated to be distinctly longer than the apparent experimental lifetimes reported previously.

PH dependence of the reduction of dioxygen to water by cytochrome c oxidase. 1. The P(R) state is a pH-dependent mixture of three intermediates, A, P, and F.

Recent studies on cytochrome oxidase have indicated that the putative "peroxy" intermediate in the catalytic cycle (P(R)) is a mixture of intermediates, including P and F [Sucheta, A., et al. (1998) Biochemistry 37, 17905-17914], and the bench-made P and F forms appear to have the same redox state (Fe(a3)(4+)=O(2-)), but a different protonation state [Fabian, M., and Palmer, G. (2001) Biochemistry 40, 1867-1874]. To explore the possibility that the putative P(R) state is a pH-dependent mixture of intermediates, we investigated the reduction of dioxygen to water by the fully reduced cytochrome oxidase at pH 6.2, 7.5, and 8.5 in the visible and Soret regions (350-800 nm) using the CO flow-flash technique. Singular value decomposition and global exponential fitting of the time-resolved absorption difference spectra resolved five apparent lifetimes. The fastest three (1.5, 13, and 34 micros) were independent of pH, while the two slowest rates (80-240 micros and 1.1-2.4 ms) decreased by a factor of 2-3 as the pH increased. When the time-resolved spectra were analyzed using a unidirectional sequential model, the spectra of the reduced enzyme and the dioxygen-bound intermediate, compound A, were found to be pH-independent. However, the putative P(R) intermediate was best represented by a pH-dependent mixture of compound A, P, and F. The ferryl form was favored at low pH. The subsequent intermediate is a ferryl with a pH-dependent electron transfer equilibrium between heme a and Cu(A), the reduced heme a being favored at low pH. These results suggest a pH-dependent reaction mechanism of the reduction of dioxygen to water by the fully reduced enzyme that is more complex than previously proposed.

PH dependence of the reduction of dioxygen to water by cytochrome c oxidase. 2. Branched electron transfer pathways linked by proton transfer.

Recent time-resolved optical absorption studies in our laboratory have indicated that the putative peroxy intermediate formed during the reduction of dioxygen to water by cytochrome oxidase (P(R)) is a pH-dependent mixture of compound A, P, and F [Van Eps, N., et al. (2003) Biochemistry 42, 5065-5073]. This conclusion is based on a kinetic analysis of flow-flash time-resolved data using a unidirectional sequential scheme with five apparent lifetimes. To account for this observation, we propose a more complex kinetic model that consists of branched pathways, one branch producing the 607 nm P form and the other the 580 nm F form. The two pathways are interconnected, and the rate of exchange between the two is pH-dependent. The kinetic analysis and testing of the new model involves a novel algebraic approach which transforms the intermediates of the complex branched scheme into intermediates comparable to those derived on the basis of a sequential model. The branched model reproduces the experimental data very well and is consistent with a variety of experimental observations. The two branches may arise from two structurally different CO or O(2) conformers or protein conformers, which could lead to different accessibilities of proton donors to the binuclear center.

Collision-Free Lifetimes of SO(B3Σ-, v‘ = 0, 1, and 2) and Vibrational Level Dependence of Deactivation by He

Collision-free lifetimes and deactivation rate constants of SO(B3Σ-, v‘ = 0, 1, and 2) by collisions with He have been determined. SO(X3Σ-) was generated in the photolysis of SO2 at 193 nm (ArF laser) and excited to a single rovibrational level of B3Σ- with a pulsed ultraviolet laser. We recorded dispersed fluorescence spectra, and found that 0−12 (350.8 nm), 1−14 (368.5 nm), and 2−17 (401.2 nm) bands are suitable for detecting fluorescence from single vibrational level of interest. Time profiles of wavelength-resolved fluorescence were recorded at various buffer gas pressures (5−165 Torr of He). Deconvolution by the integrated phase plane method derives apparent fluorescence lifetime at a given pressure. Total pressure dependence of fluorescence decay rates leads to collision-free lifetimes and rate constants for deactivation of SO(B3Σ-, v‘ = 0, 1, and 2) by collisions with He. Lifetimes of SO(B3Σ-) have been determined to be (28 ± 2) ns (v‘ = 0), (30 ± 3) ns (v‘ = 1), and (27 ± 4) ns (v‘ = 2). Rate coef...