Resonant Photoionization of CO2 up to the Fourth Ionization Threshold

Abstract

We present a comprehensive theoretical study of valence-shell photoionization of the CO₂ molecule by using the XCHEM methodology. This method makes use of a fully correlated molecular electronic continuum at a level comparable to that provided by state-of-the-art quantum chemistry packages in bound-state calculations. The calculated total and angularly resolved photoionization cross sections are presented and discussed, with particular emphasis on the series of autoionizing resonances that appear between the first and the fourth ionization thresholds. Ten series of Rydberg autoionizing states are identified, including some not previously reported in the literature, and their energy positions and widths are provided. This is relevant in the context of ongoing experimental and theoretical efforts aimed at observing in real-time (attosecond time scale) the autoionization dynamics in molecules.

Figure 1

Schematic representation of the molecular orbitals of CO₂ and the cationic electronic states resulting from the removal of an electron from different molecular orbitals.

Figure 2

Computed total photoionization cross sections of CO₂ as a function of the photon energy. The comparison between length (red solid line) and velocity (blue solid line) gauges is also depicted. Green dashed line: energy position of each ionization threshold.

Figure 3

Partial photoionization cross sections for photon energies between the first (X²Π_g) and second (A²Π_u) ionization thresholds. (a) Molecular axis is placed parallel to the light polarization vector, i.e., ¹Σ_u⁺ final symmetry. (b) Model calculation limiting the number of channels included in the close coupling to the first two ionization thresholds for the ¹Σ_u⁺ final symmetry. (c) Molecular axis is placed perpendicular to the light polarization vector, i.e., ¹Π_u final symmetry. The corresponding resonances are indicated with nl labels.

Figure 4

Same as Figure 3, but for photon energies between the second (A²Π_u) and third (B²Σ_u⁺) ionization thresholds. (a) Molecular axis is placed parallel to the light polarization vector. (b) Molecular axis is placed perpendicular to the light polarization vector.

Figure 5

Same as Figure 3, but for photon energies between the third (B²Σ_u⁺) and fourth (C²Σ_u⁺) ionization thresholds. (a) Molecular axis is placed parallel to the light polarization vector. (b) Molecular axis is placed perpendicular to the light polarization vector.

Figure 6

Energies of the different Rydberg series identified in the computed photoionization cross section as a function of (n – δ)⁻². (a) Light polarization vector parallel to the molecular axis, i.e., ¹Σ_u⁺ final symmetry. (b) Light polarization vector perpendicular to the molecular axis, i.e., ¹Π_u final symmetry.

Figure 7

Molecular frame photoelectron angular distributions, at a fixed azimuthal angle ϕ, as a function of the photon energy. Top row: Molecular axis is placed parallel to the light polarization vector, with ϕ = 0, for the channels (a) X²Π_g, (b) A²Π_u, and (c) B²Σ_u⁺. Bottom row: Molecular axis is placed perpendicular to the light polarization vector, with ϕ = π/2, for the channels (d) X²Π_g, (e) A²Π_u, and (f) B²Σ_u⁺. Results have been normalized by a constant factor for a better visualization.

Figure 8

(a) Total cross section for single-photon ionization as a function of the photon energy. Panels (a)–(d) depict the cross section correlated to each ionization channel (see insets). Black points: Experimental results from refs (34) and (37). Solid blue line: Theoretical from ref (40). (e) Branching ratios for various ionization channels compared with the experimental results from refs (34), (35), and (37).

Figure 9

Photoelectron angular distribution asymmetry parameter β for single-photon ionization as a function of the photon energy. Each panel depicts the β parameter correlated to each ionization channel (see insets). Yellow asterisks, green dots, and black squares are experimental results from refs (36) and (38) and, ref (37), respectively. Blue dots: theoretical results from ref (40). Red line: Presents results.