Imaging the Photodissociation Dynamics of CO2 in a Low-Energy Region of the 1Δu ←X1Σg+ Absorption Band

Abstract

Carbon dioxide (CO₂) photodissociation dynamics in a low-energy region of the ¹Δ_u ←X¹Σ_g⁺ absorption band (149.55-160.55 nm) is investigated using velocity map imaging with state-selective detection of O(¹D) photoproducts from the dominant CO(X¹Σ⁺) + O(¹D) channel. Total kinetic energy release (TKER) spectra and photofragment angular distributions are obtained at a series of discrete photolysis wavelengths corresponding to structured features in the vacuum ultraviolet absorption band. The TKER spectra exhibit rich structures that can be assigned to rovibrational state distributions of the CO(X¹Σ⁺) co-products, including contributions from both rotationally hot vibrationally cold and vibrationally excited channels extending to the energetic limits. A small but significant contribution from vibrationally excited CO₂ present in the molecular beam is identified through the appearance of above-threshold TKER features and additional spectral intensity in specific kinetic energy regions. The presence of vibrationally excited CO₂ in the molecular beam enables access to additional features in the TKER spectra that are less apparent under colder beam conditions. The partitioning of excess vibrational energy into translational, vibrational and rotational degrees of freedom is found to vary with photolysis wavelength, accompanied by systematic changes in the anisotropy parameters of selected CO(v, low j) channels. These observations are consistent with wavelength-dependent changes in the underlying dissociation dynamics and suggest that initial vibrational excitation of the parent molecule could influence the energy disposal and angular distributions of the photofragments. The results provide a comprehensive picture of CO₂ photodissociation dynamics in the low-energy VUV region and offer new insight into the vibronic structure and state-dependent behavior of the ¹Δ_u absorption band.