Asymmetric Photoelectron Momentum Distribution from Photoionization of Carbon Monoxide

Asymmetry in the photoelectron momentum distribution (PMD) is a remarkable feature of strong-field and attosecond science especially when produced from heteronuclear diatomic molecules due to their electric dipole moments. It has been studied from the nonlinear process of multiphoton single-ionization of the CO molecule$^{1}$ by exploring its laser-polarization dependence, to the linear process of single-photon single-ionization (dubbed photoionization) of the CO molecule$^{2,3}$ by examining its Stereo-Wigner time delay, and the nitric oxide (NO) molecule$^{4}$ by looking at the shape resonance and photoionization time delay. While NO is an open-shell molecule, CO is a closed-shell one, and the rather small energy gap between its highest occupied molecular orbital (HOMO) and the orbital below it (HOMO-1) compared to the broader UV pulse bandwidth renders possible single ionization by one-photon transition from these inner and outer orbitals. Using the \emph{ab initio} R-matrix method with time dependence (RMT)$^{5}$, we have examined photoionization of CO by a single UV laser pulse for several orientations of the linear polarization vector with respect to the molecular axis. The PMD is found to be entangled (mixed) because electrons are emitted from these inner and outer initial orbitals. We show that the asymmetry of the HOMO is strongly revealed in the entangled PMD for any molecular orientation. Such basic feature characteristic of HOMO can be used as a marker to determine the degree-of-mixture between the ionization signals due to HOMO and HOMO-1$^6$.

[1] Zhu et al, Optica \textbf{19}, 24198 (2011)

[2] Chacon and Ruiz, Optics Express \textbf{26}, 4548 (2018)

[3] Vos et al., Science \textbf{360}, 1326 (2018)

[4] Gong et al., Phys. Rev. X \textbf{12}, 011002 (2022)

[5] A. C. Brown \textit {et al.}, Comp. Phys. Comm., {\bf 250}, 107062 (2022)

[6] H. B. Ambalampitiya and J. M. N. Djiokap, (manuscript submitted)