Note: Vibrationally mediated photodissociation of carbon dioxide cation

Dissociation dynamics of carbon dioxide cation (CO2 +) in the C2Σg + state via [1+1] two-photon excitation

The dissociation dynamics of CO₂ ⁺ in the C²Σ_g ⁺ state has been studied in the 8.14-8.68 eV region by [1+1] two-photon excitation via vibronically selected intermediate A²Π_u and B²Σ_u ⁺ states using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces an internally cold mass selected ion sample of CO₂ ⁺. Total translational energy release (TER) and two-dimensional recoiling velocity distributions of fragmented CO⁺ ions are measured by time-sliced velocity map imaging. High resolution TER spectra allow us to identify and assign three dissociation channels of CO₂ ⁺ (C²Σ_g ⁺) in the studied energy region: (1) production of CO⁺(X²Σ⁺) + O(³P) by predissociation via spin-orbit coupling with the repulsive 1⁴Π_u state; (2) production of CO⁺(X²Σ⁺) + O(¹D) by predissociation via bending and/or anti-symmetric stretching mediated conical intersection crossing with A²Π_u or B²Σ_u ⁺, where the C²Σ_g ⁺/A²Π_u crossing is considered to be more likely; (3) direct dissociation to CO⁺(A²Π) + O(³P) on the C²Σ_g ⁺ state surface, which exhibits a competitive intensity above its dissociation limit (8.20 eV). For the first dissociation channel, the fragmented CO⁺(X²Σ⁺) ions are found to have widely spread populations of both rotational and vibrational levels, indicating that bending of the parent CO₂ ⁺ over a broad range is involved upon dissociation, while for the latter two channels, the produced CO⁺(X²Σ⁺) and CO⁺(A²Π) ions have relatively narrow rotational populations. The anisotropy parameters β are also measured for all three channels and are found to be nearly independent of the vibronically selected intermediate states, likely due to complicated intramolecular interactions in the studied energy region.

A cryogenic cylindrical ion trap velocity map imaging spectrometer

A cryogenic cylindrical ion trap velocity map imaging spectrometer has been developed to study photodissociation spectroscopy and dynamics of gaseous molecular ions and ionic complexes. A cylindrical ion trap made of oxygen-free copper is cryogenically cooled down to ∼7 K by using a closed cycle helium refrigerator and is coupled to a velocity map imaging (VMI) spectrometer. The cold trap is used to cool down the internal temperature of mass selected ions and to reduce the velocity spread of ions after extraction from the trap. For CO₂ ⁺ ions, a rotational temperature of ∼12 K is estimated from the recorded [1 + 1] two-photon dissociation spectrum, and populations in spin-orbit excited X²Π_g,1/2 and vibrationally excited states of CO₂ ⁺ are found to be non-detectable, indicating an efficient internal cooling of the trapped ions. Based on the time-of-flight peak profile and the image of N₃ ⁺, the velocity spread of the ions extracted from the trap, both radially and axially, is interpreted as approximately ±25 m/s. An experimental image of fragmented Ar⁺ from 307 nm photodissociation of Ar₂ ⁺ shows that, benefitting from the well-confined velocity spread of the cold Ar₂ ⁺ ions, a VMI resolution of Δv/v ∼ 2.2% has been obtained. The current instrument resolution is mainly limited by the residual radial speed spread of the parent ions after extraction from the trap.