Asymmetric photoelectron momentum distribution of carbon monoxide

The photoelectron momentum distribution (PMD) for the linear (in intensity) process of one-photon single ionization of carbon monoxide is examined in the polarization plane for several orientations of the linear polarization vector with respect to the molecular axis. For a pulse bandwidth broader than the energy gap between the highest occupied molecular orbital (HOMO) and HOMO-1, the PMD is found to be mixed with HOMO and HOMO-1 signatures because electrons are emitted from these initial orbitals. We show that the asymmetry of the HOMO is strongly revealed in the PMD for any molecular orientation. The backward-forward asymmetry in the PMD and the normalized asymmetry are used as probes of the degree of mixing.

Figure 1

(a) The asymmetric HOMO ($5\sigma$) and (b) quasi-symmetric HOMO-1 ($1\pi$) of the CO molecule. The threshold energies of the computed $X{}^2\mathrm{\Sigma }^{+}$ and $A{}^2\mathrm{\Pi }$ doublet states are $14.98\mathrm{eV}$ and $17.68\mathrm{eV}$, in good agreement with the experimental data [46]. (c) The molecular orientation scheme used for our study, where $\mathrm{\Theta }$ is the angle between the molecular axis $\mathbf{R}\left|\right|\widehat{\mathbf{z}}$ and the polarization vector $\mathbf{e}$. (d) The variation of the normalized backward-forward asymmetry as a function of $\mathrm{\Theta }$.

Figure 2

The PMDs when the laser polarization vector $\mathbf{e}$ is parallel [(a),(c),(e)] and perpendicular [(b),(d),(f)] to the molecular axis $\mathbf{R}\left|\right|\widehat{\mathbf{z}}$, which is directed from the carbon atom to the oxygen atom. The laser is propagating along the $x$ axis and the polarization vector lies on the $yz$ plane. In both cases, the contributions from all occupied orbitals in the active space are shown in (a) and (b). The individual contributions to the PMD from HOMO [(c),(d)] and HOMO-1 [(e),(f)] are also shown.

Figure 3

Buildup of the forward-backward asymmetry in the PMD in Fig. 2 from the HOMO when the molecular axis $\mathbf{R}\left|\right|\widehat{\mathbf{z}}$ is parallel to the laser polarization vector $\mathbf{e}$. The computed partial contributions to the PMD are shown for (a) $l=1$, (b) $l=2$, and (c) $l=3$. Here, $l$ is the angular momentum of the continuum electron. The isotropic ring-like partial contribution from $l=0$ (not shown) is at least an order of magnitude smaller. Shown in (d), (e), and (f) are the mixture between the partial contributions from $l=1,2$, $l=1,3$, and $l=1,2,3$, respectively. Note that $m=0$ for all $l$ values above because the laser pulse is linearly polarized.