NO2 Photolysis Research Articles

Fully Quantum-State Resolved Study of NO2 Photodissociation: Correlated NO(2.PI..OMEGA., .nu. = 0,J,.LAMBDA.) + O(3Pj) Distributions

Relative O(3Pj=~,~,~) spin-orbit populations correlated with specific N0[2nQ=l/2,3/2; Y = 0; J; A = II(A’), n(A”)] product states were obtained following photolysis of NO2 at excess energies Zi? = 390, 425, and 1054 cm-I. These fully quantum state-resolved measurements were carried out by recording spatial profiles of recoiling N0(213~,J,A) products using polarized radiation for photolysis and state-selective laser ionization detection. The relative O(3P,) populations correlated with each N0(213~,J,A) state show marked fluctuations at each excess energy as a function of rotational state and A-doublet component. The relative populations also fluctuate as a function of excess energy. The O(3Pj) spin-orbit population ratios, when averaged over all measurements, exhibit distributions that are colder than statistical, in agreement with previous results. In particular, we find that, on average, O(3P~):O(3P2) population ratios correlated with the ground N0(2171/2) state are colder than the corresponding ratios correlated with the excited NO(TI3/2) spin-orbit state. These results are in agreement with the state-specific calculations of Katigiri and Kat0 [J. Chem. Phys. 1993, 99, 88051 and are discussed in terms of long-range nonadiabatic transitions among electronic states correlating asymptotically with different spin-orbit states of the ground NO(TI) + O(3P) dissociation channel. I. Introduction The photodissociation of NO:! has attracted considerable experimental and theoretical attention. Due to its unique properties, NO:! has become a model system for testing statistical theories of unimolecular reactions. Usually, in a bond fission reaction proceeding without a barrier, high density of states and intramolecular vibrational redistribution in the excited molecule lead to product state distributions that can be well described by statistical theories, i.e., the product levels are populated as per their degeneracies and quantum state-specific effects are averaged out. In contrast, the unimolecular decomposition of NO:! represents a case in which both statistical and state-specific dynamical effects are manifest.’-19 Due to vibronic chaos at excitation energies > 16 000 cm-I, which results from strong couplings between the bright 12B2 state and the ground 12A1 state via a conical inter~ection,~.~~ the eigenstates in the vicinity of the dissociation threshold (DO = 25 130.6 cm-1)lc-14 are strongly mixed and have predominantly 12A1 chara~ter.~~~~~. ~~ Dissociation following 12B2

The diurnal variation of hydrogen, nitrogen, and chlorine radicals: Implications for the heterogeneous production of HNO2

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N2O5 on sulfate aerosols agree with measured concentrations of NO, NO2, and ClO throughout the day, but fail to account for high concentrations of OH and HO2 observed near sunrise and sunset. The morning burst of [OH] and [HO2] coincides with the rise of [NO] from photolysis of NO2, suggesting a new source of HOx that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO2 results in an excellent simulation of the diurnal variations of [OH] and [HO2].

In situ measurements of the NO2/NO ratio for testing atmospheric photochemical models

Simultaneous in situ measurements of NO2, NO, O3, ClO, pressure and temperature have been made for the first time, presenting a unique opportunity to test our current understanding of the photochemistry of the lower stratosphere. Data were collected from several flights of the ER‐2 aircraft at mid‐latitudes in May 1993 during NASA's Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE). The daytime ratio of NO2/NO remains fairly constant at 19 km with a typical value of 0.68 and standard deviation of ± 0.17. The ratio observations are compared with simple steady‐state calculations based on laboratory‐measured reaction rates and modeled NO2 photolysis rates. At each measurement point the daytime NO2/NO with its measurement uncertainty overlap the results of steady‐state calculations and associated uncertainty. However, over all the ER‐2 flights examined, the model systematically overestimates the ratio by 40% on average. Possible sources of error are examined in both model and measurements. It is shown that more accurate laboratory determinations of the NO + O3 reaction rate and of the NO2 cross‐sections in the 200–220 K temperature range characteristic of the lower stratosphere would allow for a more robust test of our knowledge of NOx photochemistry by reducing significant sources of uncertainties in the interpretation of stratospheric measurements. The present measurements are compared with earlier observations of the ratio at higher altitudes.

Temperature Dependence of the Quantum Yields for the Photolysis of NO2 Near the Dissociation Limit

Temperature Dependence of the Quantum Yields for the Photolysis of NO2 Near the Dissociation Limit

CO(v,J) product state distributions from the reaction O(3P)+OCS→CO+SO

The title reaction was studied by probing CO[v,J] product state distributions with a tunable infrared diode laser. Only the fastest O(3P) atoms formed by 355 nm photolysis of NO2, i.e., those corresponding to NO[2Π1/2, v=0], have sufficient energy to surmount the activation barrier. Thus these experiments were performed under monoenergetic, near-threshold conditions. The CO[v,J] distributions were extracted from the relative slopes of the transient infrared absorption signals. Little vibrational excitation is observed. The vibrational branching ratio is [v=1]/[v=0]≊0.05; CO[v≥2] was not detected. In contrast, rotational degrees of freedom are much hotter. The nascent CO[v=0] rotational distribution is well characterized by a temperature of TR=4400±390 K. Total CO internal excitation, on the average 3150 cm−1, accounts for 16% of the energy available to products. Surprisal analyses based on several models of energy disposal show that the nascent distributions are nonstatistically cold. The results are consistent with direct attack at the S atom to form an OSCO intermediate, followed by rapid C–S bond scission which exerts a torque on the CO product.

Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 10. Reductions of stratospheric NO2 due to Pinatubo aerosols

Visible absorption spectroscopy was employed for stratospheric measurements at McMurdo Station, Antarctica, during the summer and fall seasons in 1990, 1991, 1992, and 1993. Observed column amounts of NO2 were as much as 50% smaller in 1992 and 1993 than in 1990 and 1991. The measured decreases in NO2 are believed to be due to the hydrolysis of N2O5 on the enhanced volcanic aerosols injected by the massive eruption of Mount Pinatubo in mid‐1991. Such decreases in NO2 are broadly consistent with observations at other latitudes and with model calculations. However, substantially reduced NO2 was also observed during the polar day period in summer under conditions of continuous local solar illumination. The formation of the key N2O5 intermediate and reduction of NO2 at this time of year probably occurs via two mechanisms: (1) excursions of air parcels to lower latitudes where nighttime chemistry can produce N2O5 and (2) conversion of a small fraction of the NO3 formed during the day to N2O5. Model calculations show that the latter mechanism can be important in spite of the fast daytime photolysis of NO3.

Absorption of solar radiation by O2: Implications for O3 and lifetimes of N2O, CFCl3, and CF2Cl2

An accurate line‐by‐line model is used to evaluate effects of absorption in the Schumann‐Runge bands of O2 on transmission of ultraviolet radiation. Allowing also for absorption in the Herzberg continuum, the model is shown to provide a reliable simulation of observed transmission in the spectral interval 192 to 200 nm. The model is used to evaluate rates for photolysis of N2O, CFCl3, and CF2Cl2, and to infer global loss rates (1.22×l010 kg N yr−1, 7.21×107 and 3.04×l07 kg Cl yr−1, respectively) and instantaneous lifetimes (123, 44, and 116 years, respectively) appropriate for 1980. A parameterized version of the line‐by‐line model enabling rapid evaluation of transmission in the Schumann‐Runge region is described. Photochemical calculations employing the parameterization and constrained by data from the Atmospheric Trace Molecule Spectroscopy experiment are used to examine the budget of odd oxygen. Consistent with previous studies, it is shown that photochemical loss of odd oxygen exceeds production by photolysis of O2 for altitudes above 40 km. The imbalance between production and loss is shown to be consistent with a source of odd oxygen proportional to the product of the mixing ratio and photolysis rate of ozone, which suggests that processes involving vibrationally excited O2 may play an important role in production of odd oxygen.

Laser-induced gratings in free jets. II. Photodissociation dynamics via photofragment transient gratings

We describe the use of photofragment transient gratings to measure populations, velocities, and translational anisotropies of nascent, state-selected photofragments. This technique is a variant of two-color laser-induced grating spectroscopy (LIGS); scanning the probe laser gives spectra that reveal fragment-state populations and scanning the grating laser, with the probe laser monitoring a specific fragment state, produces photofragment excitation (PHOFEX) spectra. The temporal behavior of the photofragment grating is used to determine speeds and translational anisotropies of the photofragments. We have demonstrated the photofragment grating method on the NO(2Π1/2, v″=0, J″) fragments produced from the near-threshold photolysis of jet-cooled NO2. The grating PHOFEX spectrum yields the threshold for production of NO(2Π1/2, v″=0, J″=0.5, e)+O(3P2) from NO2 (N″=0) as 25 128.5±0.2 cm−1. For photolysis at 126 cm−1 above threshold, the NO rotational distribution obtained from the probe spectrum is in good agreement with that determined from a LIF spectrum recorded simultaneously. In addition, analysis of the grating decays for state-selected NO fragments gives their speeds and anisotropies; the speeds are in excellent agreement with the known values, even for fragments with as little as 7 cm−1 of translational energy.

Vibrational distribution of CH3 produced by the reaction of O(1D2) atom with CH4

The nascent vibrational distribution of CH3 produced by the reaction O(1D2)+CH4→OH+CH3 (ΔH0=−182 kJ mol−1) has been investigated by infrared diode laser kinetic spectroscopy. The reaction was initiated by the generation of O(1D2) atoms by excimer laser photolysis of N2O or O3 at a total pressure of 200 mTorr, and the ν2 (out-of-plane bending) bands of CH3, v2=1←0 up to 4←3, were measured as functions of time. The vibrational distribution of ν2 (v≤3) was found to be noninverted and much less excited than a prior distribution. The fraction of the available energy released to the ν2 vibration, <fv(CH3ν2)≳, is estimated to be about 0.019. A similar noninverted vibrational distribution of CH3 was observed in the reaction of CH4 with O(3P) (ΔH0=8 kJ mol−1), which is known to be a direct hydrogen abstraction reaction. The results may suggest that the CH3 moiety is gradually relaxed to a planar structure during these reactions.

Photodissociation of nitrogen dioxide adsorbed on lithium fluoride (001)

The photochemistry of NO2 physisorbed on single-crystal LiF(001) at 100 K has been studied at lambda1 = 248 nm. The adsorbate was examined by polarized FTIR in both the presence and absence of lambda1 radiation. In the absence of UV irradiation the adlayer is composed of dimeric (NO2)2. In the presence of UV, FTIR shows that some N2O3 is formed. Photodissociations (PDIS) giving both NO(g) and molecular NO2(g) were the predominant mechanisms as determined by time-of-flight mass spectrometry (TOF-MS) and resonantly enhanced multiphoton ionization (REMPI). The main objective of this work was the characterization of the photoproduct, NO, internal state distribution by 1 + 1 REMPI. Vibrational levels from v'' = 0 to 9 were probed with rotational resolution using a tunable laser, lambda2. The rotational distributions for each vibrational level could be described by one Boltzmann temperature. The spin-orbit states of NO(g) were equally populated in all vibrational levels. The lambda doublet states, PI(A') and PI(A''), were equally populated. The principal observation was that the vibrational distribution in NO(g) was inverted and bimodal with a peak in v'' = 0 and a second substantial peak in v'' = 3-4, qualitatively resembling but quantitatively different from that for photolysis of NO2(g). Time delays between the two lasers were used to probe the translational energy of the NO(g) photofragment in specified states of internal excitation. The translational energy distributions were invariant over all vibrational levels, except v'' = 0 for which much slower fragments were observed. This complete determination of the energy distribution in the degrees of freedom of the NO(g) from photodissociation of adsorbate has implications for the identity of the photolyzing species and the dynamics of photodissociation. Two mechanisms for photoformation of NO2(g) were found: one at low coverages and one at higher coverages, the former giving peak translational energies approximately 1.2 kcal/mol and the latter 0.4 kcal/mol.

Latitudinal variation of measured NO2 photolysis frequencies over the Atlantic Ocean between 50� N and 30� S

Using a filter radiometer, the meridional profile of the NO2 photolysis frequency, J(NO2), was measured between 50° N and 30° S during the cruise ANTVII/1 September/October 1988 of the research vessel Polarstern on the Atlantic Ocean. Simultaneously, global broadband irradiance and acrosol were monitored. Clean marine background air with low aerosol loads (bsp=(1–2)×10-5 m-1) was encountered at the latitudes 25° N–30° N and 18° S–27° S, respectively. Under these conditions and an almost cloudless sky J(NO2) reached 7.3×10-3 s-1 (2π sr) for a zenith angle of 30°. Between 30° N and 30° S, the latitudinal variation of the J(NO2) noontime maxima was less than ± 10%, while the mean value at noon was 7.8×10-3 s-1. For the set of all data between 50° N and 30° S, a nearly linear correlation of J(NO2) vs. global broadland irradiance was found. The slope of (8.24±0.03)×10-5 s-1/mW cm-2 agrees within 10% with observations in Julich (51° N, 6.2° E).

Generation of SnO by photoproduction and -mobilization of oxygen atoms in krypton matrices doped with N2O and Sn

SnO molecules are photochemically formed by irradiation of a Sn: N2O mixture in solid krypton. A KrxO complex which is produced by VUV photolysis of N2O at 8.8 eV has been identified from a new absorption band at 6.9 eV. The mobilization of oxygen in Sn:N2O:Kr matrices, photoinduced by excitation of the KrxO species, was studied by trapping the oxygen at Sn atoms and by using the SnO generation as a monitor. Photolysis studies in Sn:N2O:Kr matrices as well as in Sn:Kr/N2O:Kr sandwich structures revealed an extraordinary high mobility of oxygen. A qualitative model of the mobilization scheme is discussed.

Photochemistry of Atmospheric Trace Gases

The complex scheme of atmospheric radical chain reactions is driven by a relatively small number of photodissociation processes. The most important ones in the troposphere are the formation of O(1D) by the photolysis of ozone (frequency J1), and the photolysis of NO2 (frequency J2). It is shown that uncertainties in J1 and J2 lead to large uncertainties in model-predicted species concentrations, particularly in OH levels. Methods of calculating/measuring J1 and J2 will be discussed.

O(3P) atom lifetimes and mobilities in xenon matrices

The concentration of O atoms in xenon matrices at 32 and 40 K is followed by monitoring 736 nm emission from XeO* excimers. Emission is induced by excitation of O atom Xe pairs with 193 or 248 nm radiation. O atoms are initially produced via UV photolysis of N2O. At 32 K, approximately 50% of the atoms present after the initial probe pulse persist after eight days. The most likely mechanism for O atom decay is O+O recombination. At both temperatures, the time dependence of the concentration of O atom decay cannot be fit to a single bimolecular decay. It can, however, be fit to two (or more) bimolecular decays. Based on this mechanism, diffusion coefficients for the longer time scale diffusion process are 5.4×10−18 and 2.0×10−17 cm2/s at 32 and 40 K, respectively. Other possible explanations for the observed O atom time dependence are discussed.

A numerical study of the effects of air chemistry on fluxes of NO, NO2, and O3 near the surface

A numerical model was developed to investigate modifications of the vertical fluxes of NO, NO2, and O3 by rapid atmospheric chemical reactions in a horizontally homogeneous, neutrally stratified atmospheric surface layer. The one‐dimensional model coupled second‐order budget equations for fluxes with conservation equations for concentrations and incorporated simplified homogeneous gas kinetics describing the photolysis of NO2 and the recombination of NO and O3. The effects of hydrocarbons were ignored. Simulations were carried out for representative chemical concentrations at the top boundary, moderate atmospheric turbulent mixing, surface removal rates typical for vegetation in the daytime, and a range of NO emission rates from the surface. Results indicate that the effective eddy diffusivity for NO and NO2 can be significantly altered by chemical reactions and that the fluxes of NO and NO2 can vary strongly with height but that the impact of rapid atmospheric chemical reactions on O3 is small relative to its total flux. When the NO2‐to‐NO concentration ratio aloft is greater than the ratio for the photostationary state, photodecomposition of NO2 dominates the reaction cycle. In this case, NO flux increases with height even when the flux at the surface is zero, while the downward NO2 flux increases in magnitude with height and the magnitude of the downward O3 flux decreases. When the value of [NO2]/[NO] aloft is smaller than the photostationary value, these trends with height are reversed. Emission of NO at the surface strongly influences NO flux near the surface, while chemical reactions tend to determine the flux at heights above a few meters. A significant effect on NO2 occurs at the intermediate heights, but the relative change in O3 flux is minor.

Photochemistry and environment Part XIV. Phototransformation of nitrophenols induced by excitation of nitrite and nitrate ions

The excitation of NO2−, HNO2 or NO3− in the presence of nitrophenol induces hydroxylations in the ortho or para positions with respect to the phenol function. No nitration is observed in contrast with phenol and hydroxybiphenyls. The reaction can be attributed to hydroxyl radicals formed in the photolysis of NO2−, HNO2 and NO3−. With nitrite, the reaction is more efficient at 253.7 nm than at longer wavelengths. This effect is not observed with HNO2. When NO2− or HNO2 are excited in the presence of nitrophenol, the formation of dihydroxynitrobenzenes competes with the scavenging of hydroxyl radicals by nitrite ions (or HNO2) and the efficiency of the reaction is significantly influenced by the relative concentrations.

Effect of reagent rotation on product energy disposal in the light atom transfer reaction O(3P)+HCl(v=2,J=1,6,9)→OH(v′,N′)+Cl(2P)

A rovibronic-state-to-rovibronic-state experiment has been performed on the reaction O(3P)+HCl(v=2,J=1,6,9)→OH(v′,N′)+Cl(2P). The O(3P) atoms are produced with a known energy by photolysis of NO2. The HCl(v=2,J) molecules are prepared by IR excitation of thermal HCl using an optical parametric oscillator. All energetically accessible OH rovibrational product levels are probed by laser-induced fluorescence for each prepared HCl rotational level. The OH(v′=0,N′) rotational distribution shows a dip at N′=11, the depth of which decreases with increasing HCl rotational excitation. The available energy of reaction is partitioned so that 40% appears as OH vibration (V′), 32% as OH rotation (R′), and 28% as product translation (T′). This energy partitioning does not change with HCl rotation, in contrast to the general expectation for light atom transfer reactions of approximate conservation of internal angular momentum (R→R′). A substantial vibrational inversion is observed, in agreement with the vibrational adiabaticity (V→V′) expected for such reactions.

Product rotational alignment for the reaction O(3P)+ CS(X1Σ+)→ CO(X1Σ+)+ S(3P)

Rotational alignment of the CO(X 1Σ+, v′= 14) product of the O(3P)+ CS(X 1Σ+) reaction has been measured relative to the velocity vector k of the reagents. O(3P) atoms were produced with k aligned in the laboratory frame by pulsed laser photolysis of NO2, and the CO product was detected by polarised laser-induced fluorescence. Transformation of the measured laboratory-frame rotational alignments to the required values of the alignment parameter 〈P2(J′·k)〉 were carried out using previously determined values of the translational anisotropy for the photodissociation of NO2, making allowances for both the thermal distribution of CS radicals and the spread of recoil energies of the O-atom fragment. Values of 〈P2(J′·k)〉 were measured for J′ between 12 and 35, and found to be close to zero to within the range 0 ± 0.25, with the mean value being slightly positive. Measurements of the Doppler profiles of the transitions are in qualitative agreement with those predicted for an isotropic distribution of product velocities about the k direction. These preliminary results illustrate the scope of laser based methods of extracting quantum-state-resolved data on scattering dynamics under experimental conditions which do not involve the use of molecular beam methods.

An intercomparison of airborne nitrogen dioxide instruments

Results from an airborne intercomparison of techniques to measure tropospheric levels of nitrogen dioxide (NO2) are discussed. The intercomparison was part of the National Aeronautics and Space Administration's Global Tropospheric Experiment and was conducted during the summer of 1986. Instruments intercompared included a two‐photon nitric oxide (NO) laser‐induced fluorescence system with laser photolysis of NO2 to NO, an NO/O3 chemiluminescence detector using FeSO4 for conversion of NO2 to NO, an NO/O3 chemiluminescence detector with arc lamp photolysis of NO2 to NO, and a tunable diode laser multipath absorption system. All intercomparisons were for NO2 mixing ratios of <200 pptv with most at mixing ratios of <100 pptv. The FeSO4 converter was found to convert peroxyacetyl nitrate (PAN) to NO, resulting in NO2 values a factor of 2–3 higher than reported by the other techniques. Thus the FeSO4 converter data are excluded from the analyses. Intercomparison data were analyzed in various mixing ratio ranges. Good correlation was observed between the remaining three instruments for those data sets which included mixing ratios to 100 or 200 pptv, showing on the average a 30–40% level of agreement among the techniques. However, when the data were restricted to mixing ratios of <50 pptv, little correlation among the measurements was observed. Even though correlations were poor at mixing ratios of <50 pptv, the tunable diode laser system tended to be high compared to data reported by the two‐photon laser and arc lamp chemiluminescence systems, and agreement between the latter two instruments was generally better than 20 pptv with an equal tendency for one to be high relative to the other.

An airborne compatible photofragmentation two‐photon laser‐induced fluorescence instrument for measuring background tropospheric levels of NO, NOx, and NO2

Reported on is a photofragmentation two‐photon laser‐induced fluorescence (PF/TP‐LIF) sensor with a demonstrated capability for making simultaneous measurements of NO/NOx/NO2. This instrumental technique is based on the direct spectroscopic detection of NO via the TP‐LIF methodology. The measurement of NOx (NO+NO2), and subsequently NO2, is accomplished by the photofragmentation of NO2 at 353 nm via a XeF excimer laser. The resultant NO photofragment is then quantitatively detected using the TP‐LIF technique. The laser photolysis efficiency for NO2 is typically 60% for NO2 photolysis cell residence times of less than 0.5 s. The limit of detection (S/N=2) for the described PF/TP‐LIF instrument is 3.5 pptv for NO (2 min integration time) and 10 pptv for NO2 (6 min integration time). An assessment of chemical interferences in this system revealed no significant problems. The extension of the PF/TP‐LIF methodology to the simultaneous detection of multiple NxOy, species is discussed.