Photodissociation Of NO2 Research Articles

O atom photoproduction dynamics and reactions in cryogenic solids

The photodissociation of N2O and the resulting production of O atoms have been studied in N2 matrices and in Xe matrices as a function of N2O concentration. A previously developed kinetic model accurately predicts the maximum attainable O atom concentration in dilute and moderately doped Xe matrices but severely overestimates the maximum concentration produced in a highly doped matrix. This deviation is the result of O atom reactions with locally high concentrations of N2O, which are present in highly doped matrices. The reaction products observed subsequent to photolysis of a neat N2O matrix and N2 matrices doped with isotopically labelled N2O confirm the occurrence of O atom chemistry and are compatible with O(1D) as the reactive species. N2O photodissociation is less efficient in more crystalline matrices and there is evidence that this is due to an enhanced cage effect.

Photodissociation of NO2 at 355 and 351 nm Investigated by Photofragment Translational Spectroscopy

The photodissociation of jet-cooled NO:! at 355 and 351 nm has been investigated by polarized high-resolution photofragment translational spectroscopy. The translational energy distributions P(&) of the nascent photofragment pairs NO + 0 were derived from the measured time-of-flight (TOF) distributions. By comparison of P(ET) with the available energy, the population of vibrationally excited NO was determined to be P(v=O) = 62 f 3% and P(v=l) = 38 F 3% at 355 nm and P(v=O) = 57 f 3% and P(v=l) = 43 F 3% at 351 nm. These findings are consistent with the trend predicted by statistical models of the dissociation process. Nonstatistical decay dynamics are, however, indicated for the rotational degrees of freedom as manifested by bimodal (or multimodal) rotational distributions of NO(v=O, 1). The recoil anisotropy parameter p as a function of fragment translational energy was obtained from the polarized TOF spectra and was found to depend on the photolysis wavelength and on the vibrational state of the NO product: p(v=O) = 1.42 and p(v=l) = 1.25 at 355 nm, whereas p(v=O) = 1.77 and p(v=l) = 1.48 at 351 nm. The NO rotational alignment Af) measured by the laser-induced fluorescence method is at high J values close to the theoretical (perpendicular-type) limit of -0.4 at both photolysis wavelengths. This result, in conjunction with the values of p, implies that the photon absorption occurs via the :!B:! - electronic transition and that the photodissociation takes place essentially without anisotropy loss and hence on a subpicosecond time scale. The observed dependence of p on the product vibrational state suggests two different decay pathways with quantum mechanical effects playing an important role.

Subthermal nitric oxide spin‐orbit distributions in the thermosphere

The two NO(X²Π, υ=1, Ω=1/2,3/2) spin‐orbit populations in the Earth's thermosphere have been found to depart by more than a factor of 2 from the ratio expected from thermal equilibrium. The effective temperature describing the observed population distribution is as much as 700 K lower than the local kinetic temperature. Absolute NO(υ=1, J, Ω) column densities were derived from high‐resolution (1 cm−1) infrared earthlimb spectra for tangent altitudes up to 200 km, obtained in the CIRRIS 1A Space Shuttle experiment. Nonlinear least‐squares synthetic spectral fitting was used to analyze the NO Δυ=1 fundamental band emissions near 5.3 µm. The spin‐orbit distribution represents a third degree of freedom, along with vibration and rotation, that is not in equilibrium with the local kinetic temperature. These observations may significantly impact the interpretation of band‐integrated measurements of NO in the upper atmosphere, for which equilibrium sublevel distributions have been assumed. The subthermal distribution is most likely produced in the collisional uppumping of NO(υ=0) by O atoms, which is the major source of NO(υ=1) in the thermosphere. This inference suggests that the present effect is related to the subthermal spin‐orbit distributions observed in laboratory studies of NO2 photodissociation.

Extreme-ultraviolet photodissociation of N2O in superexcited states

We demonstrate the observation of neutral dissociation, which provides the possibility of a spectroscopy of highly lying superexcited states. The yield spectrum of the undispersed fluorescence radiation of wavelength λf in the region 113≤λf≤180 nm from excited neutral fragments in the photodissociation of N2O is presented in the region of excitation photon wavelength λex in the region 30≤λex≤111 nm (photon energy region 41.3–11.2 eV). We show the evidence of selective or preferential neutral dissociation in the decay of superexcited N2O in competition with autoionization; in particular, the evidence for an important role of neutral dissociation of superexcited N2O followed by the production of ionic fragments. We also show the neutral dissociation of superexcited states with the character of double-holed doubly excited states located for 30≤λex≤60 nm. The aspect of superexcitation (bound channel of electron promotion) in the region of inner-valence excitation is discussed in relation with free (continuum) channels.

Comment on ‘‘State-specific unimolecular reaction of NO2 just above the dissociation threshold’’ [J. Chem. Phys. 99, 254 (1993)

Spectroscopic studies of NO2 photoinitiated unimolecular decomposition by Miyawaki et al. indicate a very loose transition state just above threshold. If extrapolated to higher energies, this contradicts our time resolved studies of NO2 photodissociation that demonstrate a tighter transition state. We point out that both sets of data are consistent with variational RRKM theory, which predicts tightening of the transition state with increasing energy.

Response to ‘‘Comment on ‘State-specific unimolecular reaction of NO2 just above the dissociation threshold’ ’’ [J. Chem. Phys. 100, 4714 (1994)

In this comment, the authors have given a response to the foregoing comment by Wittig and Ionov on our previous paper [J. Chem. Phys. 99, 254 (1993)] entitled ‘‘State-specific unimolecular reaction of NO2 just above the dissociation threshold.’’ Discussion was made by referring to recent theoretical papers on the NO2 photodissociation to harmonize the results obtained from the time-domain experiment with our observations by the frequency-domain experiment and to clarify the problem to be solved for further understanding dynamics of the unimolecular reaction dynamics.

Vector correlations in the reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P)

The reaction O(3P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3P) has been studied using translationally aligned oxygen atoms formed from the 355 nm polarized photodissociation of NO2. The nascent CO product was detected by laser-induced fluorescence (LIF) with sub-Doppler resolution in order to extract the pair correlations between the reagent and product relative velocities k and k′ and the product rotational angular momentum J′. Previous theories interpreting the Doppler profiles of photodissociation products in terms of vector correlations have been extended to the case of bimolecular reactions. The system studied was seen to yield a close to isotropic distribution of product velocities k′ about the k direction, and a rotational alignment of J′ with k close to zero. The CO molecule departs with its rotational angular momentum vector J′ aligned preferentially perpendicular to the product relative velocity k′, hence exhibiting a negative k′, J′ correlation. Further insight has been gained on these results by quasiclassical trajectory (QCT) calculations on a London–Eyring–Polanyi–Sato (LEPS) potential energy surface (PES).

Photoinduced chemical reaction in NO2–C2H4Van der Waals complex: detection of vinyloxyl and formyl radicals and hydrogen atoms

The photodissociation of NO2 within the NO2–C2H4 complex excited at 355 nm leads to the formation of the vinyloxyl radical CH2CHO. A nonstatistical rotational and vibrational state distribution has been measured. The kinetic energy of the H atom produced in the same reaction has been obtained through the Doppler profile of the Lyman α transition. The HCO product issued from the reaction in the gas phase is observed but not in the complex. These results can be explained by the selection of the reaction path in the Van der Waals complex excitation.

Doppler‐Resolved Laser Probing of Photon‐Initiated Bimolecular Collisions O(1D) + CH4 → OH(v,N) + CH3

AbstractThe stereodynamics of the reaction O(1D) + CH4 → OH(v ≤ 4,N) + CH3 at collision energies of ˜ 40 kJ mol−1, has been probed via Doppler‐resolved, polarized laser‐induced fluorescence spectroscopy. Velocity‐aligned, reagent O(1D) atoms were generated under bulb conditions via polarized laser photodissociation of N2O. Analysis of the Doppler profiles of nascent OH(v,N) fragments has allowed both scalar and vectorial product correlations to be determined, including differential scattering cross sections and rotational alignments. These imply the operation of a ‘delayed’ collision mechanism, dominated by the deeply attractive potential energy surface associated with the insertion to form CH3OH; the intermediacy of a long‐lived complex is excluded, however. Rotational angular momentum in the recoiling OH fragments is preferentially aligned perpendicular to, and azimuthally distributed about, their recoil velocity. The rotational excitation is thereby attributed to bending motion in the C···O···H plane during the reagent collisions. The scalar product pair correlations establish a near‐zero translational exoergicity and anticorrelated internal energy distributions in the two reaction products.

The spin–orbit effect on potential surfaces of NO2 photodissociation

Potential energy surfaces for photodissociation reaction NO2→NO(2Π)+O(3P) have been studied by ab initio calculations. The effect of spin–orbit interaction on the potential surface features was studied near the product region. All the 12 potential surfaces asymptotically correlated to the NO(2Π)+O(3P) limit were obtained by the state-averaged complete-active-space-self-consistent-field (CASSCF) method. The adiabatic potential surfaces including the spin–orbit interaction were constructed using the full Breit–Pauli Hamiltonian. It was found that the lowest two states are attractive, while all the other states are repulsive. Assuming that NO2 undergoes the photodissociation on the ground state surface, we obtained the bending-rotation energy levels along the dissociation coordinate, and the transition state for each bending level was determined. The potential barriers for the vibrationally adiabatic energy curves were consistent with the recent experiments. Using a simplified model based on the infinite order sudden approximation (IOSA) and the Franck–Condon approximation, the product fine structure distribution was estimated, which is in good agreement with the experimental results.

Reaction of O(1D) with ethylene: Vibrational and rotational state distribution of product OH

Reaction of O(1D) with ethylene was studied under low pressure flow conditions. The O(1D) was formed by photodissociation of N2O by an ArF laser and rotational and vibrational state distributions of product OH were determined by laser induced fluorescence. The rotational distributions of the v=0 and 1 levels showed a bimodal feature. A major part of the OH had a rotational energy higher than the statistical expectation, and about 20% of total available energy appeared as the rotation of OH. About 20% of the total OH was characterized by a rotational distribution that corresponded to a specific temperature, i.e., 546 K and 526 K for v=0 and 1, respectively. The relative populations of the first two vibrational levels were measured to be 1.00 and 0.30, and no inversion was observed. No propensity was observed between formation of the two spin–orbit states, and the Π(A′) state was slightly more favored than the Π(A″) state. These results were explained by a mechanism in which O(1D) inserts into a CH bond of ethylene and OH is eliminated from a bend geometry. The same reaction was studied under cluster conditions where a van der Waals complex of N2O–C2H4 was converted into the reactant pair, O(1D)–C2H4. The rotational distribution of OH formed under these conditions showed little difference from that from the bimolecular reaction.

An investigation of the 355 nm photodissociation of NO2 by state-resolved photofragment imaging

The 355 nm photodissociation of NO2 cooled in a supersonic beam has been investigated by state-resolved photofragment imaging. The NO and O(3PJ) photofragments were state-selectively ionized and projected onto a two-dimensional, position-sensitive detector to obtain speed and angular distributions. The speed distribution of the O(3P2) fragment displays two peaks corresponding to oxygen produced in coincidence with NO(υ=0) and NO(υ=1). The angular distributions for the O(3P2) and for the NO in several vibrational and rotational levels can be characterized by an anisotropy parameter of β=1.2±0.3. This value, while higher than that measured previously, is consistent with a dissociation lifetime on the order of 200–400 fs and with the colder rotational temperature of the current beam experiment. The rotational distributions of the NO product are found to be in good agreement with other recent measurements.

Dynamics of the oxygen combination reaction on Pt(111) initiated by photodissociation of N2O at 193 nm: O*+O(ad)→O2(g)

The desorption of O2 is observed when a Pt(111) surface with the coadsorbates of oxygen atoms and N2O is irradiated with 193 nm photons. This indicates that an oxygen atom produced by photodissociation of N2O reacts with a chemisorbed oxygen adatom to form an oxygen molecule. The dynamics of the photoinitiated combination reaction of oxygen is studied by time-of-flight spectroscopy.

Photoproduction and dynamics of oxygen atoms in xenon matrices

The photodissociation of N2O doped in Xe matrices and the subsequent dynamics of atomic oxygen production have been studied. The O atom concentration is monitored via the laser-induced fluorescence of XeO exciplexes produced by the 193 nm excitation of Xe/O pairs. The O atom photoproduction cross section for 193 nm irradiation of N2O is 6.4±1.0×10−20 cm2 at 27 K, comparable to the gas phase value of 1.1×10−19 cm2. Dissociation of XeO exciplexes generates kinetically hot O atoms which are mobile. This photoinduced mobility can lead to O atom loss by recombination. The extent of O atom production as a function of laser irradiation is governed by a competition between the rates of photoproduction and photoinduced loss. The effects of temperature, concentration, and laser fluence on the production of O atoms are considered. The efficiency of photoinduced O atom loss increases significantly with increasing temperature. An ultraviolet absorption spectrum of XeO has been obtained with an absorption cross section of 1.9±0.4×10−16 cm2 at 248 nm.

Effect of clouds on the photodissociation of NO2: Observations and modelling

The role of clouds in photodissociation is examined by both modelling and observations. It is emphasized that the photodissociation rate is proportional to the actinic flux rather than to the irradiance. The actinic flux concerns the energy that is incident on a molecule, irrespective of the direction of incidence. The irradiance concerns the energy that is incident on a plane.

Multiconfigurational time-dependent Hartree study of complex dynamics: Photodissociation of NO2

The multiconfigurational time-dependent Hartree (MCTDH) approach is applied to an example showing very complex dynamics: the wave-packet dynamics on the three-dimensional B2 potential-energy surface of NO2. The ability of the MCTDH scheme to describe accurately the severe splitting of the wave packet on a saddle-shaped surface is demonstrated. Internal checks of the MCTDH calculation enable us to assess the degree of convergence without the need to resort to a numerically exact wave-packet calculation. As a representative observable the photodissociation spectrum is calculated and discussed. The A1/B2 vibronic coupling is neglected in our study, but the dynamics on the diabatic B2 surface is treated in its full three dimensionality.

The stereochemistry of the O(1D)+N2O→NO+NO reaction via velocity-aligned photofragment dynamics

Velocity-aligned, superthermal O(1D) atoms generated via the photodissociation of N2O have been employed to investigate the stereodynamics of the title reaction. The power of this experimental technique, when coupled with Doppler-resolved, polarized laser-induced fluorescence probing of the reaction products, is demonstrated by reference to the specific reaction channel leading to NO(υ′=0)+NO(υ′=16,17), which is shown to proceed via direct stripping dynamics. Furthermore, the observed product-state selective linear and angular momenta disposals imply that the reaction is stereodynamically constrained to occur via collinear collision geometries.

Comparison between the Photo Induced Chemical Reaction in the NO2 – C2H4 van der Waals Complex and the O + C2H4 Gas Phase Reaction

AbstractThe photodissociation of NO2 within NO2–C2H4 complexes or small clusters excited at 355 and 266 nm leads to the formation of vibrationally and rotationally cold vinoxy radicals (CH2CHO). On the other hand, the vinoxy radicals produced in gas phase thermal collisions are vibrationally and rotationally hot. Comparison between these experiments seems to show that two different competitive reaction paths are responsible for the different internal energy content in the vinoxy radical.

Actinometer and Eppley radiometer measurements of the NO2 photolysis rate coefficient during the Mauna Loa Observatory photochemistry experiment

Measurements of the apparent first‐order rate coefficient for NO2 photodissociation (jNO2) were made using a chemical actinometer during the month of May 1988 at the Mauna Loa Observatory, Hawaii. Simultaneous measurements of the ultraviolet irradiance (E), obtained with an Eppley radiometer, allowed extensive testing of the semi‐empirical relationships between E and jNO2 proposed by Madronich (1987a). More than 3700 simultaneous measurements of jNO2 and E were obtained for solar zenith angles ranging from 4–90 degrees, and for different sky conditions (including clear skies, partial cloud cover, arid valley clouds below the horizon). For overhead clear skies, the NO2 photodissociation rate coefficient derived from Eppley radiometer data, here denoted j′, was in good agreement with actinometric measurements, j′/jNO2=1.01±0.05(1σ). The actinometer‐radiometer relationship holds reasonably well even when low‐lying valley clouds are present. For the periods of overhead intermittent clouds, the j′values track jNO2 values well, but the observed ratio shows significantly more scatter and the average is somewhat less than unity: 0.93 ± 0.09 (1σ). Measurements taken with and without upward scattered and reflected radiation show that valley clouds can contribute to the jNO2 values for the conditions encountered during the Mauna Loa Observatory Photochemistry Experiment.

The influence of photodissociated N2O in pulsed-laser deposition of Y-Ba-Cu-O films

Pulsed-laser deposition of YBa2Cu3O7 in N2O permits lowering of the substrate temperature with respect to deposition in O2 atmosphere. Additional photodissociation of N2O near the substrate surface deteriorates the superconducting properties of deposited films.