Photodissociation channels for N2O near 130 nm studied by product imaging

VUV absorption spectra of water and nitrous oxide by a double-duty differentially pumped gas filter

The differentially pumped rare-gas filter at the end of the VUV beamline of the Swiss Light Source has been adapted to house a windowless absorption cell for gases. Absorption spectra can be recorded from 7 eV to up to 21 eV photon energies routinely, as shown by a new water and nitrous oxide absorption spectrum. By and large, the spectra agree with previously published ones both in terms of resonance energies and absorption cross sections, but that of N2O exhibits a small shift in the {\tilde{\bf D}} band and tentative fine structures that have not yet been fully described. This setup will facilitate the measurement of absorption spectra in the VUV above the absorption edge of LiF and MgF2 windows. It will also allow us to carry out condensed-phase measurements on thin liquid sheets and solid films. Further development options are discussed, including the recording of temperature-dependent absorption spectra, a stationary gas cell for calibration measurements, and the improvement of the photon energy resolution.

Vacuum ultraviolet photodissociation dynamics of N2O via the C1Π state: The N(2Dj=5/2, 3/2) + NO(X2Π) product channels

We study the vacuum ultraviolet photodissociation dynamics of N₂O via the C¹Π state by using the time-sliced velocity map ion imaging technique. Images of N(²D_{j=5/2, 3/2}) products from the N atom elimination channels were acquired at a set of photolysis wavelengths from 142.55 to 148.19 nm. Vibrational states of the NO(X²Π) co-fragments were partially resolved in experimental images. From these images, the product total kinetic energy release distributions (TKERs), branching ratios of the vibrational states of NO(X²Π) co-fragments, and the vibrational state specific angular anisotropy parameters (β) have been determined. Notable features were found in the experimental results: the TKERs show that the NO(X²Π) co-fragments are highly vibrationally excited. For the highly vibrationally excited state of NO(X²Π), a bimodal rotational structure is found at all the studied photolysis wavelengths. Furthermore, the vibrational state specific β values of both spin-orbit channels (j = 3/2, 5/2) clearly show a monotonic decrease as the vibrational quantum number of NO(X²Π) increases. These observations suggest that multiple dissociation pathways play a role in the formation of the N(²D_{j=5/2, 3/2}) + NO(X²Π) products: one corresponds to a fast dissociation pathway through the linear state (the C¹Π state) following the initial excitation to a slightly bent geometry in the vicinity of the linear C¹Π configuration, leading to the low rotationally excited components with relatively large β values; the other corresponds to a relatively slow dissociation pathway through the bent C(3¹A') or C(3¹A″) state, leading to moderately rotationally excited NO(X²Π) products with smaller β values.

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.

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.

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Probing the barrier for CH2CHCO→CH2CH+CO by the velocity map imaging method

This work determines the dissociation barrier height for CH2CHCO --> CH2CH + CO using two-dimensional product velocity map imaging. The CH2CHCO radical is prepared under collision-free conditions from C-Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent CH2CHCO radicals that do not dissociate to CH2CH + CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl(2P(3/2)) and Cl(2P(1/2)) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P(E(T)) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the CH2CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C-Cl bond energies for the precursor to produce CH2CHC*O or C*H2CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl + CO. The experimentally determined barrier for CH2CHC*O --> CH2CH + CO is 21+/-2 kcal mol(-1), and the computed energy difference between the CH2CHC*O and the C*H2CHCO forms of the radical gives the corresponding barrier for C*H2CHCO --> CH2CH + CO to be 23+/-2 kcal mol(-1). This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the C*H2CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable CH2CHC*O radicals, most with an internal energy distribution ranging from approximately 3 to approximately 21 kcal mol(-1). We discuss the results with respect to the prior work on the O(3P) + propargyl reaction and the analogous O(3P) + allyl system.