Ab initio potential energy surfaces, total absorption cross sections, and product quantum state distributions for the low-lying electronic states of N2O

Photodissociation Dynamics of Nitrous Oxide near 145 nm: The O(1S0) and O(3P J=2,1,0) Product Channels

We report the study of photodissociation dynamics of nitrous oxide in the vacuum ultraviolet region, using the time-sliced velocity map ion imaging technique. Ion images of the O(¹S₀) and O(³P _J=2,1,0) products were measured at nine photolysis wavelengths from 142.55 to 148.79 nm. The product channels O(¹S₀) + N₂(X¹Σ_g⁺) and O(³P _J=2,1,0) + N₂(A³Σ_u⁺) have been observed. For these dissociation channels, the total kinetic energy releases of the dissociated products were acquired. With vibrational structures of the N₂ coproducts partially resolved in the experimental images, the branching ratios of different vibrational states of the N₂ coproducts were determined, and the vibrational state specific anisotropy parameters (β values) were derived. Analysis shows that the O(¹S₀) + N₂(X¹Σ_g⁺) channel is primarily formed via nonadiabatic couplings between the C (¹Π) state and the higher-lying D (¹Σ⁺) state of the N₂O. A moderate rotational excitation and high vibrational excitation of N₂(X¹Σ_g⁺) products have been observed through this pathway. On the other hand, for the O(³P _J=2,1,0) + N₂(A³Σ_u⁺) channels, where a slightly higher rotational excitation of N₂ coproducts have been observed, the possible pathway would be via nonadiabatic couplings from the C (¹Π) state to the lower-lying A(¹Σ^-)state.

Theoretical ab Initio Study of the Series of N2X+ Cations with X = F, Cl, Br, and I. New Insights on the "Unusual" N2F+ Species

The series of cations N₂X⁺(X̃¹Σ⁺), with X = F, Cl, Br, I, has been theoretically studied by variational multireference CI and coupled-cluster techniques in conjunction with basis sets of quintuple-ζ quality. We report electronic and geometric structure data and harmonic frequencies as well as binding energies and potential energy curves. A new rationalization is provided for the bonding mode in N₂F⁺, which provides an explanation for the unusually short N-F bond.

VUV Photodissociation Dynamics of Nitrous Oxide: The N((2)DJ=3/2,5/2) and N((2)PJ=1/2,3/2) Product Channels

We report on an experimental study of the vacuum ultraviolet photodissociation dynamics of nitrous oxide as a function of photolysis wavelength. In this study, both the N((2)DJ) + NO(X(2)Π) and N((2)PJ) + NO(X(2)Π) product channels were investigated using the time-sliced velocity ion imaging technique. Images of the N((2)DJ=5/2,3/2) and N((2)PJ=3/2,1/2) products were measured at seven and ten, respectively, photolysis wavelengths between 124.44 and 133.20 nm. The vibrational states of the NO products were partially resolved in the acquired raw ion images. The total kinetic energy release and the branching ratios of different vibrational states of NO products were determined. The vibrational state distributions of NO were found to be inverted for the N((2)DJ=5/2,3/2) and N((2)PJ=3/2,1/2) product channels. This phenomenon indicates that the N-O bond is highly vibrational excited during the breaking of the N-N bond. Vibrational state resolved anisotropic parameters β in both the N((2)DJ) and the N((2)PJ) channels were acquired. The small β values (around 0.5) in the dissociation process suggest that transition states in a bent configuration play an important role in the formation of N + NO products.

Intermediate photofragment distributions as probes of non-adiabatic dynamics at conical intersections: application to the Hartley band of ozone

Quantum dynamics at a reactive two-state conical intersection lying outside the Franck–Condon zone is studied for a prototypical reaction of ultraviolet photodissociation of ozone in the Hartley band.

Single-field slice-imaging with a movable repeller: photodissociation of N₂O from a hot nozzle

We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to easily resolve photo-electron recoil in (2 + 1) resonant enhanced multiphoton ionization of N2 photoproducts from N2O or from molecular beam cooled N2. Second, rapid changes between spatial imaging, velocity mapping, and slice imaging are straightforward. We demonstrate this technique's utility in a re-investigation of the photodissociation of N2O. Using a hot nozzle, we observe slice images that strongly depend on nozzle temperature. Our data indicate that in our hot nozzle expansion, only pure bending vibrations--(0, v2, 0)--are populated, as vibrational excitation in pure stretching or bend-stretch combination modes are quenched via collisional near-resonant V-V energy transfer to the nearly degenerate bending states. We derive vibrationally state resolved absolute absorption cross-sections for (0, v2 ≤ 7, 0). These results agree well with previous work at lower values of v2, both experimental and theoretical. The dissociation energy of N2O with respect to the O((1)D) + N2¹Σ(g)⁺ asymptote was determined to be 3.65 ± 0.02 eV.

Isotopomer fractionation in the UV photolysis of N(2)O: 3. 3D Ab initio surfaces and anharmonic effects

The wavelength-dependent isotopic fractionation of N(2)O is calculated, extending our previous work, Parts 1 and 2, in several aspects: (1) the fully three-dimensional ab initio electronic potential and transition dipole moment surfaces of S. Nanbu and M. S. Johnson (J. Chem. Phys. A 2004, 108, 8905) are used to calculate the absorption cross sections, instead of a 2D surface and (2) the vibrational frequencies and wave functions with anharmonicity correction are used for the ground electronic state. The results for the absorption spectrum and for the isotopic fractionation of the different isotopomers are discussed. One difference between experiments measuring the absorption coefficient (von Hessberg et al. Atmos. Chem. Phys. 2004, 4, 1237) and the others that measure instead the photodissociation is also discussed. Experiments on the quantum yield for wavelengths longer than 200 nm (>50 000 cm(-1)) would be helpful in treating the observed difference.

Cross sections for the valence shell excitations of nitrous oxide studied by fast electron impact

The generalized oscillator strengths, differential cross sections, as well as the integral cross sections for electron impact excitation of the B (1)Delta, C (1)Pi, and D (1)Sigma(+) states of N(2)O have been determined at an incident electron energy of 2500 eV. A generalized oscillator strength analysis reveals that the B (1)Delta<--X (1)Sigma(+) transition is dominated by the quadrupolar component. From the comparison to the previous experimental results, it is found that the first Born approximation is not satisfied for the C (1)Pi excitation while it is valid for the D (1)Sigma(+) excitation at an incident electron energy of 200 eV. The BE-scaled integral cross section for the B (1)Delta excitation from its threshold to 5000 eV was calculated based on its generalized oscillator strength, and the present integral cross sections for the excitation of the C (1)Pi and D (1)Sigma(+) states are in good agreement with the calculations using the BEf-scaling approach.

UV photo-dissociation and photodesorption of N2O on Ag(111)

Nanosecond laser induced photoreactions of N2O adsorbed on Ag(111) have been studied by temperature programmed desorption (TPD) and mass-selected, angle-dependent time-of-flight (MS-TOF) measurements of neutral desorbing particles. N2O molecules in the first monolayer are thermally inert but photo-dissociate into N2 + O, or photodesorb molecularly or dissociatively, at photon energies above 3.5 eV. We have found that TOF spectra of photodesorbed N2 as well as of N2O measured at hν = 4.7 eV consist of two velocity components. The desorption flux of the fastest component of N2O peaks ∼ 25° off the surface normal, whereas the others are directed in the surface normal. Origins and photo-excitation as well as photodesorption mechanisms of the N2O and N2 signals are discussed.

Ion-pair dissociation of N2O in the 16.25-16.41 eV: dynamics and electronic structure

The ion-pair dissociation dynamics of N(2)O -->(XUV) N(2)(+)(X (2)Sigma(g)(+), v) + O(-)((2)P(j)) at 16.248, 16.271, 16.389, and 16.411 eV have been studied using the velocity map imaging method and tunable XUV laser. The electronic structures of the ion-pair states have been studied by employing the ab initio quantum chemical calculation. The translational energy distributions and the angular distributions of the photofragments have been measured. The results show that about 40% of available energies are transformed into the translational energies, and the first excited vibrational states are populated most strongly for all four excitation energies. The anisotropy parameters beta are approximately 1. The ab initio calculations at the level of CASSCF6-311++g(3df) show that the equilibrium geometries of the ion-pair states are nonlinear with bond lengths R(N-N) = 1.10 A, R(N-O) = 2.15 A, and bond angle N-N-O = 103 degrees, respectively. The ion-pair states are formed by electron migration from the bonding sigma orbital of N[triple bond]N to the antibonding sigma orbital localized primarily on the O atom. Combining the experimental and theoretical results, it is concluded that the ion-pair dissociation occurs via predissociation of Rydberg states with (1)Sigma(+) symmetry, which converges to the ion-core N(2)O(+)(A (2)Sigma(+)).

Photodissociation dynamics of OCS at 248nm: The S(D21) atomic angular momentum polarization

The dissociation of OCS has been investigated subsequent to excitation at 248 nm. Speed distributions, speed dependent translational anisotropy parameters, angular momentum alignment, and orientation are reported for the channel leading to S((1)D(2)). In agreement with previous experiments, two product speed regimes have been identified, correlating with differing degrees of rotational excitation in the CO coproducts. The velocity dependence of the translational anisotropy is also shown to be in agreement with previous work. However, contrary to previous interpretations, the speed dependence is shown to primarily reflect the effects of nonaxial recoil and to be consistent with predominant excitation to the 2 (1)A(') electronic state. It is proposed that the associated electronic transition moment is polarized in the molecular plane, at an angle greater than approximately 60 degrees to the initial linear OCS axis. The atomic angular momentum polarization data are interpreted in terms of a simple long-range interaction model to help identify likely surfaces populated during dissociation. Although the model neglects coherence between surfaces, the polarization data are shown to be consistent with the proposed dissociation mechanisms for the two product speed regimes. Large values for the low and high rank in-plane orientation parameters are reported. These are believed to be the first example of a polyatomic system where these effects are found to be of the same order of magnitude as the angular momentum alignment.

Slice imaging of the quantum state-to-state cross section for photodissociation of state-selected rovibrational bending states of OCS (v2=0,1,2∣JlM)+hν→CO(J)+S(D21)

Using hexapole quantum state-selection of OCS (v(2)=0,1,2/JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v(2)=0,1,2/JlM)+hnu-->CO(J)+S((1)D(2)) was measured for bending states up to v(2)=2. The population density of the state-selected OCS (v(2)=0,1,2 /JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60-67. Integrating over all J states the authors find sigma(v(2)=0):sigma(v(2)=1):sigma(v(2)=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle.

Photodissociation of nitrous oxide starting from excited bending levels

The photodissociation dynamics of N2O in the wavelength region of 203-205 nm was studied by velocity map ion imaging. A speed resolution of 0.8% was obtained using standard projection imaging and subpixel centroiding calculations. To investigate N2O dissociation starting from the excited bending levels in the ground electronic state, a supersonic molecular beam and an effusive beam were used. The photoabsorption transition probability from the first excited bending level in the wavelength region of 203-205 nm was estimated to be seven times greater than that from the ground vibrational level.

Three-isotope plot of fractionation in photolysis: A perturbation theoretical expression

The slope of the three-isotope plot for the isotopomer fractionation by direct or nearly direct photodissociation is obtained using a perturbation theoretical analysis. This result, correct to first order in the mass difference, is the same as that for equilibrium chemical exchange reactions, a similarity unexpected a priori. A comparison is made with computational results for N2O photodissociation. This theoretical slope for mass-dependent photolytic fractionation can be used to analyze the data for isotopic anomalies in spin-allowed photodissociation reactions. Earlier work on chemical equilibria is extended by avoiding a high-temperature approximation.