Dynamics and vector correlations of vacuum ultraviolet (VUV) photodissociation of CO2 at 155 nm

Wavelength-Dependent Dynamics of the O(1D2) Channel in the 1Δu State Photodissociation of CO2

The wavelength-dependent dynamics of the O(1D2) channel, formed by photoexcitation of CO2 to the 1Δu state at 143.53-153.03 nm, is investigated by using the time-sliced velocity-mapped ion imaging method. The measured ionic peaks of the O(1D2) images are analyzed to determine the total kinetic energy release (TKER) spectra and image anisotropy parameters. The structures observed in the TKER spectra can be directly assigned to the ro-vibrational state distributions of the counter CO photofragments. Compared to those observed at 157 and 155 nm, the highly rotationally excited CO photofragments still obviously appear in v = 0 and 1, but the fraction of rotational excitations is significantly reduced. Conversely, the CO photofragments exhibit substantially higher vibrational excitations, implying that the nearly linear 21A' state also contributes to dissociation in addition to the bend configuration. The image anisotropy parameters display an extremely slow decreasing trend with an increase of the CO ro-vibrational state besides those for the highest ro-vibrationally excited CO photofragments. Nevertheless, the nonaxial recoil effect, suggested in previous photodissociation studies of CO2 and other triatomic molecular systems, is still appropriate to explain the observations of internal energy dependences of image anisotropy parameters.

Photodissociation of CO2 via the 1Πg state: Wavelength-dependent imaging studies of O(1D2) photoproducts

Photodissociation of CO2 via the 1Πg state is investigated using a time-sliced velocity-mapped ion imaging apparatus combined with a tunable vacuum ultraviolet photolysis source. The main O(1D2) + CO(X1Σ+) channel is directly observed from the measured images of O(1D2) photoproducts at 129.08-134.76 nm. The total kinetic energy release spectra determined based on these images show that the energetic thresholds for the O(1D2) + CO(X1Σ+) photoproducts correspond to the thermochemical thresholds for the photodissociation of CO2(v2 = 0) and CO2(v2 = 1). One significant difference among the CO(X1Σ+, v) vibrational distributions for the predominant CO2(v2 = 0) dissociation is that the population of CO(v = 0) becomes favorable at 130.23-133.45 nm compared to the Boltzmann-like component (v > 0) that always exists at 129.08-134.76 nm. The wavelength dependences of the overall β are found to follow the variation trend of the CO(v = 0) abnormal intensity. The vibrational state-specific β values present a roughly decreasing trend with an increase in v, whereas β(v = 0) appears to be significantly larger than β(v = 1) at 130.23-133.45 nm compared to 134.76 and 129.08 nm. The non-statistical CO(v = 0) with larger β values at 130.23-133.45 nm implies that an additional pathway may open through the conical intersection coupling to the dissociative 21A'  state, except for the ever-existing pathway that yields the Boltzmann-like component. In contrast, at 129.08 nm, the restoration of the statistical equilibrium in the CO(X1Σ+, v) vibrational distribution may be caused by the emergence of novel dissociation pathways arising from the participation of the 31A″ state.

Ultrafast CO2 photodissociation in the energy region of the lowest Rydberg series

We present a detailed theoretical survey of the electronic structure of excited states of the CO₂ molecule, with the aim of providing a well-defined theoretical framework for the quantum dynamical studies at energies beyond 12 eV from the ground state. One of the major goals of our work is to emphasize the need for dealing with the presence of both molecular valence and Rydberg states. Although a CASSCF/MRCI approach can be used to appropriately describe the lowest-lying valence states, it becomes incapable of describing the upper electronic states due to the exceedingly large number of electronic excitations required. To circumvent this we employ instead the EOM-CCSD monoconfigurational method to describe the manifold of both valence and Rydberg states in the Franck-Condon region and then a matching procedure to connect these EOM-CCSD eigensolutions with those obtained from CASSCF/MRCI in the outer region, thus ensuring the correct asymptotic behavior. Within this hybrid level of theory, we then analyze the role of valence and Rydberg states in the dynamical mechanism of the photodissociation of quasi-linear CO₂ into CO + O fragments, by considering a simple but effective 1D multistate non-adiabatic model for the ultrafast C-O bond break up. We show evidence that the metastability of the Rydberg states must be accounted for in the ultrafast dynamics since they produce changes in the photodissociation yields within the first tens of fs.