Dyson norms in XUV and strong-field ionization of polyatomics: Cytosine and uracil

The extreme-ultraviolet (XUV) and strong-field valence ionization of cytosine and uracil is considered. We examine some simple estimates of the relative yields of the cation states populated following ionization and compare these to the results of a recently developed ab initio–type numerical model designed to compute strong-field ionization of molecules, the so-called time-dependent resolution in ionic states (TD-RIS) method. In analogy with one-photon XUV ionization, where the photoionization matrix elements can be related to the Dyson orbitals, we construct estimates for the yield of strong-field ionization (SFI) to different cation states based on the Dyson orbital norms and the Keldysh tunneling ionization rate. In the case of XUV ionization, the Dyson norms are shown to be good predictors of the relative cation yields when compared with the TD-RIS yields. The Dyson- and Keldysh-based models underestimate the yield to excited cation states in the case of SFI. The increased yield to the excited cation states in the TD-RIS results is attributed to the inclusion of multielectron effects and continuum structure not present in the simple models. The molecular Ammosov-Delone-Krainov (MO-ADK) method of calculating SFI is also considered. This later method is seen to agree more closely with the Dyson- and Keldysh-based estimates as it also fails to capture the multielectron effects and continuum structure included in the TD-RIS approach.

Figure 1

Comparison of the Dyson norms (left) with the TD-RIS XUV ionization yields (right) for ionization from the lowest two singlet states of cytosine (top, S${}_0$; bottom, S${}_1$).

Figure 2

Comparison of the Dyson norms (left) with the TD-RIS XUV ionization yields (right) for ionization from the lowest three singlet states of uracil (top, S${}_0$; middle, S${}_1$; bottom, S${}_2$).

Figure 3

Comparison of the Dyson-Keldysh estimate (left) with the half-cycle strong-field ionization yields (right) for ionization from the lowest two singlet states of cytosine (top, S${}_0$; bottom, S${}_1$).

Figure 4

Comparison of the Dyson-Keldysh estimate (left) with the half-cycle strong-field ionization yields (right) for ionization from the lowest three singlet states of uracil (top, S${}_0$; middle, S${}_1$; bottom, S${}_2$).

Figure 5

Comparison of the ionization calculations with (open circles) and without (solid circles) the Coulomb-mediated interchannel couplings for ionization from the S${}_1$ of cytosine.

Figure 6

Ionic-state-resolved ionization rates for cytosine and uracil computed using the MO-ADK [12, 13] method.

Figure 7

Dominant electronic configurations for relevant states of the neutral and the ion for cytosine (top) and uracil (bottom). The participating orbitals are shown as well. Whenever there is an approximately equal contribution from more than one configuration, both of them are shown in the figure. The orbitals shown are taken from the MCSCF calculation on the neutral molecule and are only qualitative for the ions.

Figure 8

Nonadiabatic bound-state dynamics.

Figure 9

Ionization of the S${}_2$ state of uracil for a full pulse (4.8-fs FWHM envelope) and a half-cycle pulse.