Photodissociation of HNO3 at 193, 222, and 248 nm: Products and quantum yields

Aqueous phase oxidation of bisulfite influenced by nitrate and its photolysis

Nitrate aerosol is ubiquitous in the atmosphere. Nitrate in the particulate and aqueous phase can affect various atmospheric chemical processes through its hygroscopicity and photolysis. The impacts of nitrate photolysis on the heterogeneous oxidation of SO₂ have been attracting attention. However, the influence of nitrate on heterogeneous aqueous phase formation of atmospheric sulfate aerosol is still not very clear. In this study, the effects of nitrate on aqueous phase oxidation of bisulfite under different conditions were investigated. Results show that nitrate photolysis can promote the oxidation of bisulfite to sulfate, especially in the presence of O₂. It is found that pH plays a significant role in the reaction, and ammonium sulfate has significant impacts on the enhancement of aqueous phase sulfate production through regulating the pH of solution. An apparent synergism is found among halogen chemistry, nitrate and its photochemistry and S (IV) aqueous oxidation, especially the oxidation of halide ions by nitrate and its photolysis and by the intermediate products produced by the free radical chain oxidation of S (IV) in acidic solution, leading to the coupling of the redox cycle of halogen with the oxidation of bisulfite, which promotes the continuous aqueous oxidation of bisulfite and the formation of sulfate. In addition, the role of nitrate itself in the aqueous phase oxidation of bisulfite is revealed. These results provide a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO₂ in cloud and fog droplets and haze particles.

Characterization of two photon excited fragment spectroscopy (TPEFS) for HNO3 detection in gas-phase kinetic experiments

We have developed and tested two-photon excited fragment spectroscopy (TPEFS) for detecting HNO₃ in pulsed laser photolysis kinetic experiments. Dispersed (220-330 nm) and time-dependent emission at (310 ± 5) nm following the 193 nm excitation of HNO₃ in N₂, air and He was recorded and analysed to characterise the OH(A²Σ) and NO(A²Σ⁺) electronic excited states involved. The limit of detection for HNO₃ using TPEFS was ∼5 × 10⁹ molecule cm^-3 (at 60 torr N₂ and 180 μs integration time). Detection of HNO₃ using the emission at (310 ± 5 nm) was orders of magnitude more sensitive than detection of NO and NO₂, especially in the presence of O₂ which quenches NO(A²Σ⁺) more efficiently than OH(A²Σ). While H₂O₂ (and possibly HO₂) could also be detected by 193 nm TPEFS, the relative sensitivity (compared to HNO₃) was very low. The viability of real-time TPEFS detection of HNO₃ using emission at (310 ± 5) nm was demonstrated by monitoring HNO₃ formation in the reaction of OH + NO₂ and deriving the rate coefficient, k₂. The value of k₂ obtained at 293 K and pressures of 50-200 torr is entirely consistent with that obtained by simultaneously measuring the OH decay and is in very good agreement with the most recent literature values.

Theoretical insight into the wavelength-dependent photodissociation mechanism of nitric acid

The MS-CASPT2 method is used to study O(¹D) + HONO and OH + NO₂ photodissociation pathways of HNO₃ in the four lowest electronic singlet states.

Slice imaging of nitric acid photodissociation: The O(1D) + HONO channel

We report an imaging study of nitric acid (HNO(3)) photodissociation near 204 nm with detection of O((1)D), one of the major decomposition products in this region. The images show structure reflecting the vibrational distribution of the HONO coproduct and significant angular anisotropy that varies with recoil speed. The images also show substantial alignment of the O((1)D) orbital, which is analyzed using an approximate treatment that reveals that the polarization is dominated by incoherent, high order contributions. The results offer additional insight into the dynamics of the dissociation of nitric acid through the S(3) (2 (1)A(')) excited state, resolving an inconsistency in previously reported angular distributions, and pointing the way to future studies of the angular momentum polarization.

Photochemical processes induced by vibrational overtone excitations: dynamics simulations for cis-HONO, trans-HONO, HNO3, and HNO3-H2O

Photochemical processes in HNO3, HNO3-H2O, and cis- and trans-HONO following overtone excitation of the OH stretching mode are studied by classical trajectory simulations. Initial conditions for the trajectories are sampled according to the initially prepared vibrational wave function. Semiempirical potential energy surfaces are used in "on-the-fly" simulations. Several tests indicate at least semiquantitative validity of the potential surfaces employed. A number of interesting new processes and intermediate species are found. The main results include the following: (1) In excitation of HNO3 to the fifth and sixth OH-stretch overtone, hopping of the H atom between the oxygen atoms is found to take place in nearly all trajectories, and can persist for many picoseconds. H-atom hopping events have a higher yield and a faster time scale than the photodissociation of HNO3 into OH and NO2. (2) A fraction of the trajectories for HNO3 show isomerization into HOONO, which in a few cases dissociates into HOO and NO. (3) For high overtone excitation of HONO, isomerization into the weakly bound species HOON is seen in all trajectories, in part of the events as an intermediate step on the way to dissociation into OH + NO. This process has not been reported previously. Well-established processes for HONO, including cis-trans isomerization and H hopping are also observed. (4) Only low overtone levels of HNO3-H2O have sufficiently long liftimes to be spectrocopically relevant. Excitation of these OH stretching overtones is found to result in the dissociation of the cluster H hopping, or dissociation of HNO3 does not take place. The results demonstrate the richness of processes induced by overtone excitation of HNO(x) species, with evidence for new phenomena. Possible relevance of the results to atmospheric processes is discussed.

Resonance Raman and density functional theory investigation of the photodissociation dynamics of the A-band absorption of (E)-β-nitrostyrene in cyclohexane solution

Resonance Raman spectra were obtained for (E)-beta-nitrostyrene in cyclohexane solution with excitation wavelengths in resonance with the charge transfer (CT)-band absorption spectrum. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion predominantly along the nominal NO(2) symmetric stretch mode (nu(14)), the nominal C=C stretch mode (nu(8)), the nominal benzene ring stretch mode (nu(9)), accompanied by a smaller amount of motion along the nominal ONO symmetric bend/benzene ring stretch mode (nu(34)), the nominal CCH in-plane bending mode (nu(20)), the nominal HC=CH in-plane bending mode (nu(18)), the nominal NO(2) asymmetric stretch mode (nu(11)), the nominal C-N stretch/benzene ring breathing mode (nu(27)), and the nominal CCC trigonal bending mode (nu(25)). A preliminary resonance Raman intensity analysis was done and these results for (E)-beta-nitrostyrene were compared to results previously reported for several nitrobenzene and trans-stilbene compounds. The differences and similarities between the CT-band resonance Raman spectra and vibrational reorganizational energies for (E)-beta-nitrostyrene relative to those for nitrobenzene and trans-stilbene were briefly discussed.

Reaction of HO with Glycolaldehyde, HOCH2CHO: Rate Coefficients (240−362 K) and Mechanism

Absolute rate coefficients for the title reaction, HO+HOCH2CHO-->products (R1), were measured over the temperature range 240-362 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. Within experimental error, the rate coefficient, k1, is independent of temperature over the range covered and is given by k1(240-362 K)=(8.0+/-0.8)x10(-12) cm3 molecule-1 s-1. The effects of the hydroxy substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for HOCH2CHO throughout the troposphere. As part of this work, the absorption cross-section of HOCH2CHO at 184.9 nm was determined to be (3.85+/-0.2)x10(-18) cm2 molecule-1, and the quantum yield of HO formation from the photolysis of HOCH2CHO at 248 nm was found to be (7.0+/-1.5)x10(-2).

Resonance Raman intensity analysis of the excited state proton transfer dynamics of 2-nitrophenol in the charge-transfer band absorption

Resonance Raman spectra were obtained for 2-nitrophenol in cyclohexane solution with excitation wavelengths in resonance with the charge-transfer (CT) proton transfer band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion along more than 15 normal modes: the nominal CCH bend+CC stretch nu(12) (1326 cm(-1)), the nominal CCC bend nu(23) (564 cm(-1)), the nominal CO stretch+NO stretch+CC stretch nu(14) (1250 cm(-1)), the nominal CCH bend+CC stretch+COH bend nu(15) (1190 cm(-1)); the nominal CCH bend+CC stretch nu(17) (1134 cm(-1)), the nominal CCC bend+CC stretch nu(22) (669 cm(-1)), the nominal CCN bend nu(27) (290 cm(-1)), the nominal NO(2) bend+CC stretch nu(21) (820 cm(-1)), the nominal CCO bend+CNO bend nu(25) (428 cm(-1)), the nominal CC stretch nu(7) (1590 cm(-1)), the nominal NO stretch nu(8) (1538 cm(-1)), the nominal CCC bend+NO(2) bend nu(20) (870 cm(-1)), the nominal CC stretch nu(6) (1617 cm(-1)), the nominal COH bend+CC stretch nu(11) (1382 cm(-1)), nominal CCH bend+CC stretch nu(9) (1472 cm(-1)). A preliminary resonance Raman intensity analysis was done and the results for 2-nitrophenol were compared to previously reported results for nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone. The authors briefly discuss the differences and similarities in the CT-band absorption excitation of 2-nitrophenol relative to those of nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone.

Effect of Vibrational Excitation on the Collisional Removal of Free Radicals by Atoms: OH(v=1) + N

The collisional removal of vibrationally excited OH(upsilon=1) by N(4S) atoms is investigated. The OH radical was prepared by 193 nm photolysis of H2O2, and N(4S) atoms were generated by a microwave discharge in N2 diluted in argon. The concentrations of OH(upsilon=0 and 1) were monitored by laser-induced fluorescence as a function of the time after the photolysis laser pulse. The N(4S) concentration was determined from the OH(upsilon=0) decay rate, using the known rate constant for the OH(upsilon=0) + N(4S) --> H + NO reaction. From comparison of the OH(upsilon=0 and 1) decay rates, the ratio of the rate constant k(upsilon=1)(OH-N) for removal of OH(upsilon=1) in collisions with N(4S) and the corresponding OH(upsilon=0) rate constant, k(upsilon=0)(OH-N) was determined to be 1.61 +/- 0.42, yielding k(upsilon=1)(OH-N) = (7.6 +/- 2.1) x 10(-11) cm3 molecule(-1) s(-1), where the quoted uncertainty (95% confidence limits) includes the uncertainty in k(upsilon=0)(OH-N). Thus, the collisional removal of OH(upsilon=1) by N(4S) atoms is found to be faster than for OH(upsilon=0).

Quantum yields for OH production in the photodissociation of HNO3 at 248 and 308 nm and H2O2 at 308 and 320 nm

The quantum yields for OH formation from the photolysis of HNO(3) were measured to be (0.88 +/- 0.09) at 248 and (1.05 +/- 0.29) at 308 nm and of H(2)O(2) to be (1.93 +/- 0.39) at 308 and (1.96 +/- 0.50) at 320 nm. The quoted uncertainties are at the 95% confidence level and include estimated systematic uncertainties. OH radicals were produced using pulsed laser photolysis and monitored using pulsed laser-induced fluorescence. Quantum yields were measured relative to the OH quantum yields from a reference system. The measured quantum yields at 248 nm are in agreement with previous direct determinations. The quantum yield values at 308 and 320 nm are the first direct quantum yield measurements at these wavelengths and confirm the values currently recommended for atmospheric model calculations. Rate coefficients (at 298 K) for the OH + H(2)O(2) and OH + HNO(3) + M (in 100 Torr of N(2)) reactions were measured during this study to be (1.99 +/- 0.16) x 10(-12) cm(3) molecule(-1) s(-1) and (1.44 +/- 0.12) x 10(-13) cm(3) molecule(-1) s(-1), respectively.

Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH AΣ+2 (v=4) radicals

Fluorescence-dip infrared spectroscopy, an UV-IR double-resonance technique, is employed to characterize the line positions, linewidths, and corresponding lifetimes of highly predissociative rovibrational levels of the excited A (2)Sigma(+) electronic state of the OH radical. Various lines of the 4 <--2 overtone transition in the excited A (2)Sigma(+) state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (2)Sigma(+) (v = 4) state are determined, along with the vibrational frequency for the overtone transition. Homogeneous linewidths of 0.23-0.31 cm(-1) full width at half maximum are extracted from the line profiles, demonstrating that the N = 0 to 7 rotational levels of the OH A (2)Sigma(+) (v = 4) state undergo rapid predissociation with lifetimes of < or =23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions.

Photochemistry of Molecules Relevant to the Atmosphere: Photodissociation of Nitric Acid in the Gas Phase

This Minireview gives an account of the photochemical decay of nitric acid HNO3 in the gas phase, which has been well investigated under bulk and molecular-beam conditions. Due to the importance of this molecule in atmospheric chemistry, attention was paid to the irradiation regions around 300 and 200 nm, where solar photolysis of HNO3 is expected to be particularly efficient. While the low-energy region is characterized by the products OH and NO2, the high-energy region gives rise to a variety of photochemical decay pathways, dominated by channels which lead to the products HONO + O in different electronic states.

Infrared spectrum and stability of a π-type hydrogen-bonded complex between the OH and C2H2 reactants

A hydrogen-bonded complex between the hydroxyl radical and acetylene has been stabilized in the reactant channel well leading to the addition reaction and characterized by infrared action spectroscopy in the OH overtone region. Analysis of the rotational band structure associated with the a-type transition observed at 6885.53(1) cm(-1) (origin) reveals a T-shaped structure with a 3.327(5) A separation between the centers of mass of the monomer constituents. The OH (v = 1) product states populated following vibrational predissociation show that dissociation proceeds by two mechanisms: intramolecular vibrational to rotational energy transfer and intermolecular vibrational energy transfer. The highest observed OH product state establishes an upper limit of 956 cm(-1) for the stability of the pi-type hydrogen-bonded complex. The experimental results are in good accord with the intermolecular distance and well depth at the T-shaped minimum energy configuration obtained from complementary ab initio calculations, which were carried out at the restricted coupled cluster singles, doubles, noniterative triples level of theory with extrapolation to the complete basis set limit.