Suppressed Molecular Ionization for a Class of Diatomics in Intense Femtosecond Laser Fields

Abstract

It is shown that the puzzling experimental observation of suppressed molecular ionization, in intense laser fields, of O2, and its absence in N2, is a symmetry induced dynamical effect. More generally, it is predicted that the ionization signal of the class of homonuclear diatomic molecules having valence orbitals with an antibonding symmetry (e.g., sigma(u), pi(g)) would be suppressed, but not those with a bonding symmetry (e.g., sigma(g), pi(u)). The suppression effect can be visualized as due to an effective destructive interference between the two subwaves of the ionizing electron emerging from the two atomic centers.

High-Harmonic Spectroscopy: From Femtosecond to Attosecond Molecular Dynamics

“…[17] Recent experiments show that strongfield ionization does not exclusively prepare the ground state of the cation [4][5][6]28] but that excited states can also be populated. The probability of accessing a certain ionic state is exponentially sensitive to I p [29] but also to the shape of orbitals, [30] which can enhance the contribution of a deeply bound orbital. When ionization accesses two states of the ion and the laser-induced dynamics in the ion is neglected, highharmonic generation has two contributions…”

Role of proton dynamics in efficient photoionization of hydrocarbon molecules

“…Because field ionization takes place in bursts every laser half-cycle, for a given pulse intensity and molecular orientation and in the absence of saturation effects the yield of these ions should monotonically increase with the pulse duration. To illustrate this we have calculated the yield of C 2 H 4 2+ , assuming that the two electrons are removed sequentially, as a function of laser pulse duration at the experimental laser peak intensity using tunnel-ionization theory [55] and the tabulated ionization potentials of ethylene [54], and taking ionization saturation into account, but neglecting effects caused by the structure of molecular orbitals [2,3]. The results of these calculations are shown by the black dashed line in Fig.…”

“…One striking difference is that the ionization rate in molecules sensitively depends on the orientation of the molecule with respect to the laser polarization axis, as well as on the character of the molecular orbital from which an electron is removed [1][2][3][4][5][6][7][8]. In addition, the electronic energy levels in molecules can be quite closely spaced such that, within a single-particle picture, ionization may not only take place from the highest occupied molecular orbital (HOMO), but also from lower-lying valence orbitals with a pronounced probability [9][10][11][12][13][14], especially when ionization from the highest-lying states is suppressed by unfavorable orientation of the orbital(s) with respect to the laser polarization direction.…”

Theory of strong-field ionization of alignedCO2

“…Full ab initio calculations of the alignment-dependent ionization are available only for H + 2 [11,12,13,14] and H 2 [15,16]. For larger molecules, despite a tremendous amount of experiments, no ab initio calculations are available, and the most widely used approaches to explain strong-field processes are the molecular tunneling theory [17] and strongfield approximation [18,19]. Calculations of alignmentdependent ionization yields based on these theories fail to explain recent experiments [9]: Tunneling theory and strong-field approximation predict the ionization yield to follow the electron density of the initial electronic state, in contrast with observations for the CO 2 molecule [9].…”