Dissociation Quantum Yield Research Articles

Methane photochemistry and haze production on Neptune

A numerical model was used to study methane photochemistry in the stratosphere of Neptune. The observed mixing ratio of methane, 2%, forces photolysis to occur near the CH 4 homopause. For an assumed nominal value of the eddy mixing coefficient of 10 6 cm 2 sec −1 at the CH 4 homopause, the predicted average mixing ratios of C 2H 6 and C 2H 2, 1.5 × 10 −6 and 6 × 10 −7, respectively, agree well with observations in the infrared. The acetylene and ethane abundances are weakly dependent upon the strength of the eddy mixing and directly proportional to it. Haze production from methane photochemistry results from the formation of hydrocarbon ices and polyacetylenes. The calculated mixing ratios of C 2H 6, C 2H 2 are large enough to cause condensation to their respective ices near the tropopause. These hazes are capable of providing the necessary aerosol optical depth at the appropriate pressure levels required by observations of Neptune in the visible and near IR. Polyacetylene formation from C 2H 2 photolysis is limited by the low quantum yield of dissociation for acetylene, efficient recycling of its photolysis products by the other hydrocarbons, and the greatly reduced solar flux at Neptune. Comparisons of model predictions to Uranus show both a lower ratio of polyacetylene production to hydrocarbon ice and a lower likelihood of UV postprocessing of the acetylene ice to polymers on Neptune compared to Uranus. This is in agreement with the observed difference in the single scattering albedo of the stratospheric aerosols in the visible between Uranus and Neptune, with the aerosols on Neptune being brighter.

Dissociation of 2,2′‐substituted thioindigo whites, and recombination of their radicals in solution and in polymer matrix

AbstractThe dissociation of 2,2′‐disubstituted thioindigo whites into captodative radicals and the recombination of these radicals have been examined in solution and in polymeric matrix. For the methyl‐ and ethyl‐substituted TIW the quantum yield of dissociation in solution is equal to 4 × 10−4 and 5 × 10−4, respectively. Using the iodine scavenging method the quantum yield of formation of free radicals was almost 3 to 4 × 10−4, i.e. 20–25% radicals recombine in cage. For the isopropyl and aryl TIWs partial dissociation occurs already at room temperature. Their dissociation enthalpies have been evaluated in solution on the basis of the ESR signal surfaces at different temperatures. For the aromatic substituted Δ Hdiss amounts to 88–95 kJ mol−1, while for i‐PrTIW ΔHdiss is 150 kJ mol−1. The behavior of these compounds was studied in three different matrices: polymethyl methacrylate (PMMA), polypropyl methacrylate (PPMA), and polystyrene (PST). In the cases of the aryl substituted derivatives, cage dimensions of the TIW‐radical pairs were measured at low temperature, and their apparent dissociation enthalpies above their glass transition temperatures were evaluated using ESR spectrometry; they are equal to 118 ± 3 kJ mol−1 in PPMA and around 155 kJ in PMMA and PST. The decay kinetics of the radicals after photolysis of the TIWs below Tg were interpreted on the basis of Waite's equations for diffused controlled reactions. The great influence of steric effects in the cases of the 2‐orthochlorophenyl and the 2,4‐dichlorophenyl TIWs is stressed as well as the importance of the merostabilization of the captodative radicals on these kinetic characteristics. The nature of the polymeric matrix is also underlined; below Tg the diffusion rate constant is only one‐third in PST compared to PMMA. The radicals generated from Ph TIW and its para‐nitro and p‐methoxy derivatives show a strong inhibiting effect toward the polymerization of methyl methacrylate, while the alkyl TIWs behave as free radical initiators.

The effect of vibrational relaxation on infrared multiphoton excitation

Abstract A model description of vibrational relaxation (redistribution) (VR) in the quasicontinuum (QC) of polyatomic molecules, subject to infrared multiphoton excitation (IR MPE) has been introduced, based on the kinetic equations technique. This model has been applied to the derivation of expressions for the population distributions as well as dissociation quantum yields (QYs) of polyatomic molecules under IR MPE, with special emphasis on the dependence of the obtained QYs on exciting laser pulse intensity and duration as compared to the VR rate constant. Good agreement has been obtained between the theoretically derived dependences of the dissociation QYs in SF 6 on buffer gas pressure and experimental data.

The apparent quantum yield of T-state human hemoglobin. Contribution of protein and heme to rates of oxygen reactions.

The apparent quantum yield for dissociation of oxygen from T-state human hemoglobin has been determined using pulses of light 350 ns long at 540 nm. Two quantum yields were found. One was the same as for the R-state, and, like it, strongly temperature- and viscosity-dependent. The other, only slightly influenced by temperature and viscosity, was 10 times larger at 20 degrees C. Previous work (Sawicki, C. A., and Gibson, Q. H. (1977) J. Biol. Chem. 252, 7538-7547) has shown two distinct phases in binding of oxygen by T-state human hemoglobin at pH 7, 20 degrees C. When the apparent quantum yield was followed with time, the species with high quantum yield correlated with the rapidly reacting T-state species. The hemoglobin chains have different quantum yields in the T-state. Quantum yield data may serve as a measure of population of the liganded T-state in human hemoglobin, supplementing absorbance and circular dichroism data, and permit calculation of the rates of reaction at the heme in both R- and T-states.

Geminate recombination of O2 and hemoglobin.

The photolysis of HbO2 and HbCO has been studied by measuring transient absorption spectra in the Soret region after excitation with picosecond pulses at 530 nm. Dissociation occurred promptly in both cases, followed (for HbO2) by geminate recombination of ca. 40% of the photodissociated O2 with a lifetime of 200 +/- 70 psec (25 degrees C). No recombination of Hb + CO was observed up to 1200 psec after photolysis. The HbO2 and HbCO photoproduct spectra were broader, weaker, and red-shifted in comparison to the spectrum of stable Hb and Gibson's fast-reacting form, Hb. For HbO2 the spectrum was initially much broader to longer wavelengths but relaxed to a constant shape within 90 psec, whereas for HbCO there was no spectral evolution. The photophysics is analyzed by considering the effect of spin constraints as well as spin--orbit coupling and orbital correlation among the various electronic states of liganded and deoxy hemoglobins. The small quantum yield of HbO2 dissociation is not primarily due to rebinding but rather to electronic relaxation to nonreactive states.

Near-UV photodissociation of gaseous UF 6 in the presence of H 2

Mixtures of UF 6 and H 2 in different ratios have been irradiated at 360 and 400 nm by means of a filtered mercury lamp. A significant pressure drop has been observed at both excitation wavelengths due to the dissociation of UF 6 into UF 5+ F. A very high dissociation quantum yield has been found.

The photodissociation of ammonia in the absorption system. Part I. Deuterium isotope effects in the photodissociation

The photolyses of mixtures of NH3, NH2D, NHD2, and ND3 have been studied at wave lengths of 2144, 2139, 2062, and 1850 Å in the presence of C3H8 as a hydrogen atom scavenger. Quantum yields of dissociation have the same values for all four species, presumably unity. Analysis of the H2 and HD produced permitted evaluation of intramolecular deuterium isotope effects in the photodissociation of NH2D and ND2H. At the two shortest wavelengths dissociation of H was favored by a factor of 2 or 3, while at 2144 and 2139 Å the isotope effect was much larger. Implications for the mechanism of the predissociation of the Ã-state of ammonia are discussed briefly. The system does not appear to be useful for the photochemical separation of deuterium.

The photodissociation of ammonia in the absorption system. Part II. Translational excitation of the hydrogen atoms produced, and the mechanism of the predissociation

The photolyses of NH3 and ND3 have been studied at 2139, 2062, and 1850 Å in the presence of propane and ethylene. Upper limits (none was actually observed) were established for the quantum yields of molecular dissociation of D2 from ND3 of 0.003 and 0.004 at 2139 and 2062 Å, while at 1850 Å a definite yield of 0.009 was obtained. Similar results were observed with NH3. From the dependence of hydrogen yields on the ratio of ethylene to propane, it was concluded that H and D atoms were produced in the photolysis with excess translational energy. Values of the integrated reaction probability (IRP) of hot H atoms with propane were estimated to be 0.078, 0.070, and 0.045 at 2139, 2062, and 1850 Å respectively, while corresponding values for hot D atoms from ND3 were 0.083, 0.062, and 0.029. Implications of the decrease in IRP with increasing photon energy are discussed, and it is concluded that at the shorter wavelengths a second dissociation channel leading to NH2(2A1) becomes important. A mechanism for the predissociation of the Ã-state of ammonia is presented which accounts for this behaviour and for the deuterium isotope effects observed previously. It is suggested that the dissociation does not follow the state correlation rules for dissociation in the plane of the molecule, at least when the ν2 out-of-plane bending vibration in the Ã-state is excited to levels of υ2 = 2 or higher.

Transition moment, radiative lifetime, and quantum yield for dissociation of the [formula omitted] state of 81Br 2

The electric dipole moment of the B → X transition of 81Br 2 has been calculated from the measured absorption of single vibration-rotation lines with a YAG laser at 558 nm to be | R e | 2 = 0.12 D 2. The corresponding radiative lifetime is τ rad = 20 μsec. Fluorescence decay times have been measured, as a function of pressure, with a narrowband, nitrogen pumped, dye laser, for vibrational levels v′ = 16, 19, 23. The quantum yield for predissociation at zero pressure is near unity.

Cage effects and steric hindrance in van der Waals solids, with application to alkyl iodide photolysis in rare gas hosts

Molecular luminescence in the near ir is produced by 259–290 nm pulsed ’’photodissociation’’ of CH3I, CD3I, and perfluoroalkyl iodide guests in rare gas hosts at 4.2 °K. For CD3I and CH3I in Ne, the 15 member progression observed is assigned as fluorescence into high lying (near v″=37) vibrational states of the methyl–iodide stretch in the ground electronic state. Photoselection studies confirm the transition dipole lies along the methyl–iodine bond. The photodissociation cage effect is near complete, with only a minor quantum yield of permanent dissociation. Detailed analysis of the spectra suggests that the 0+ ’’repulsive’’ excited state is actually slightly (<2000 cm−1) chemically bound, and that the ground state potential is negligibly perturbed from its free molecule shape. A simple semiquantitative theory of the cage effect is proposed. In the impulsive limit, the excited state dynamics reflect a strong cage effect. However, in the adiabatic limit, the absorption and emission spectra reflect only a weak, van der Waals cage effect. The theory is applied to known examples of photodissociation in solids, and to rearrangements of larger molecules. A cage potential experiment of Schnepp and Dressler is reinterpreted.

Flash spectroscopy with mercury resonance radiation. Part 4.—Measurement of mercury hydride yields with a nitric oxide titration

By comparing the yields of HNO in the Hg-flash-photosensitized decomposition of H2+ NO mixtures with those formed in H2+ C2H4+ NO mixtures, k4/(k3+k4) for the reactions, Hg(63P1)+[graphic omitted], was recorded as 0.67(± 0.04). Without added C2H4, both HgH and H are converted to HNO. Added C2H4 scavenges atomic hydrogen and the only HNO forming reaction is HgH + NO → HNO + Hg. In similar experiments with D2 the equivalent k4/(k3+k4) was recorded as 0.76(± 0.05). The HNO and DNO were monitored photoelectrically via their systems in the far ultra-violet. The absolute quantum yield for dissociation of H2 by Hg(63P1) was recorded as 0.93(± 0.10) by comparing the HNO yields in H2+ NO mixtures with those in C3H8+ NO mixtures. Less than 5% of the reaction of C2H5 with NO produces HNO at high pressure, and there is no fast removal of HNO by C2H4. Fragments of several new systems of HgH and HgD were detected in the far ultra-violet.

Effects of ultraviolet irradiation (2537 A) on the structure of human thyroglobulin.

The effect of in vitro ultraviolet irradiation at 2537 Å on the structure of highly purified human thyroglobulin has been investigated by following the ultracentrifugal pattern and the appearance of free sulfhydryl groups in the irradiated protein. Thyroglobulin (19 S) dissociates into two 12 S subunits when it is irradiated with doses of a few hundred Einsteins per mole. The quantum yields of dissociation vary between 0.5 × 10−3 and 2 × 10−3, depending on the experimental conditions. At higher ultraviolet doses, the protein is partially aggregated and denatured. The extent of dissociation is largely dependent on the ionic strength and the pH of the medium. Doses up to 1,500 Einsteins per mole cause only a partial (30%) splitting of the 19 S molecules, at neutral pH in 0.1 M KCl, whereas at pH 10 and low salt (Γ/2=0.0042), the same dose produces almost complete dissociation into 12 S subunits. Ultraviolet light also causes the rupture of some of the disulfide bonds of native thyroglobulin. On irradiation with several hundred Einsteins per mole protein, as many as 21 –SH groups per molecule can be titrated. The quantum yields for –SH production and destruction of cystine residues are both 0.05. No direct correlation has been found between the extent of dissociation of thyroglobulin and the number of –S–S– bonds cleaved, nor between the reassociation of the 12 S subunits and the reoxidation of –SH groups. It is concluded that two 12 S units present in human thyroglobulin are not linked by interchain disulfide bridges; the disulfides cleaved by ultraviolet irradiation, however, may play an important role in maintaining the subunit organization of the protein molecule.

The Primary Quantum Yield of Dissociation of Iodine in Hexane Solution

The primary quantum yield for the photo-dissociation of iodine in degassed hexane at 25° has been calculated from measurements of the mean lifetime of the chains involved in the exchange of iodine atoms with trans-diiodoethylene. The primary quantum yield to an accuracy of ±15 percent is 0.59 at 436 mμ and 0.37 at 578 mμ. The deviation of this quantity from unity is affected both by the ``primary recombination'' of atoms which fail to escape from the original ``cage'' of solvent molecules, and by the ``secondary recombination'' of the original atoms by diffusion during a time which is very short compared to that in which either is apt to encounter an atom from another molecule. The specific rate constant for the recombination of iodine atoms in hexane solutions has been determined and has been shown to be about one fifth of the rate constant for collision in the gas phase. The presence of air decreases the rate constant for the photochemical exchange of iodine with diodoethylene by a factor of four and increases the apparent mean lifetime of iodine atoms by a factor of one hundred.