Strong-field polarizability-enhanced dissociative ionization

Abstract

We investigate dissociative single and double ionization of HeH + induced by intense femtosecond laser pulses. By employing a semi-classical model with nuclear trajectories moving on field-dressed surfaces and ionization events treated as stochastical jumps, we identify a strong-field mechanism wherein the molecules dynamically align along the laser polarization axis and stretch towards a critical internuclear distance before getting dissociative ionized. As the tunnel-ionization rate is greater for larger internuclear distance and for aligned samples, ionization is enhanced. The strong dynamical rotation is traced back to a maximum in the parallel component of the internucleardistance-dependent polarizability tensor. Qualitative agreement with our experimental observations is found. Finally the criteria for observing the isotope effect for the ion angular distribution is discussed.PACS numbers: 33.80. Wz, 33.80.Eh, 42.50.Hz The ionization and dissociation of small molecules in intense laser fields is of fundamental interest and has captured the attention of physicists for many years [1][2][3]. When the ratio of the laser frequency to the peak electric field is sufficiently small, the ionization process can be considered as an electron tunneling through the instantaneous barrier formed by the field and the Coulomb potential of the system [4]. In molecules, such tunnelionization rates depend on the spatial separation between the nuclei [5-9], as well as the molecular orientation with respect to the laser polarization axis, where the ionization rate follows the shape of the highest-occupied molecular orbital [10][11][12]. In addition to these fixed-nuclei properties, the molecules will dynamically rotate and stretch in the field, potentially leading to fragmentation [8]. Here, we theoretically identify a new fragmentation pathway that involves the combination of the aforementioned strong-field dynamics. Namely, due to the force resulting from the internuclear-distance-dependent polarizability tensor, the molecule is simultaneously aligned and stretched towards a specific internuclear distance in the field-dressed ground state before being ionized. We denote it as polarizability-enhanced dissociative ionization (PEDI) and support our findings with experimental data.Polarizability effects have been explored extensively in strong-field physics, e.g. it has regularly appeared in the interpretation of strong-field ionization experiments [13,14], and the anisotropic polarizability is often exploited in molecular alignment experiments [15][16][17]. In dissociative ionization studies involving short laser pulses (tens of femtoseconds (fs) duration), often only the vibrational degrees of freedom are considered, while the rotational dynamics are disregarded. This is based on the intuition that the field-free rotational timescale of picoseconds is much greater than the vibrational timescale of fs and thus rotational motion can be safely neglected.However, as several works employing semiclassical methods [7,18,19] have show...

Post-Ionization Dynamics of the Polar Molecule OCS in Asymmetric Laser Fields

“…Although the asymmetry amplitude slightly decreases compared to the main peak in the OC + + S + channel, which we attribute to the effects of molecular rotation due to a higher potential barrier along the O-C bond breaking. This result indicates that the orientation of the parent OCS 2+ ions is mainly determined by the ionization step, even for different dissociation channels as discussed in the previous studies of polar molecules (Ohmura and Tachiya, 2008;Holmegaard et al, 2010;Dimitrovski et al, 2011;Li et al, 2011;Wu et al, 2012;Ohmura et al, 2014;Li et al, 2016;Wustelt et al, 2018;Yue et al, 2018;Endo et al, 2019;Ohmura et al, 2019).…”

“…In the case of polar (asymmetric) molecules on the contrary, the fragment ejection direction in asymmetric laser fields has been exclusively explained by the anisotropy of the tunneling ionization rate, which is determined by the shape of the ionizing molecular orbital (usually the highest occupied molecular orbital, HOMO) and the molecule's permanent dipole moment (Ohmura and Tachiya, 2008;Holmegaard et al, 2010;Dimitrovski et al, 2011;Li et al, 2011;Wu et al, 2012;Ohmura et al, 2014;Li et al, 2016;Wustelt et al, 2018;Yue et al, 2018;Endo et al, 2019;Ohmura et al, 2019), rather than by post-ionization interactions. Although post-ionization interactions should be important even for the dissociation of polar molecules, their contributions are generally buried under the strong anisotropy of the tunneling ionization rate.…”

Characterizing Sub-Cycle Electron Dynamics of Polar Molecules by Asymmetry in Photoelectron Momentum Distributions

“…1(b), results only in Q2 and Q4 show large amplitudes. It has been shown that vibrating HeH + stretches rapidly towards larger internuclear distances R due to vibration excitation [6,29,30]. For the present cases, with calculating R-dependent ionization probability as in [29], our analyses show that the main contributions to ionization occur around R = 2 a.u.…”

Local most-probable routes and classic-quantum correspondence in strong-field two-dimensional tunneling ionization

“…It should be stressed that in this paper, we focus our discussions on atoms. For molecules with multi-center structure, some complex effects emerge in strong-field ionization, such as quantum interference between the molecular centers during tunneling [53][54][55], permanent dipole induced large Stark effect [56][57][58], laser induced nuclear quick stretching [59][60][61][62][63], and permanent dipole induced direct vibration excitation [64][65][66], etc.. For the purpose of achieving more precise control and measurement of the electron motion within the molecule with 2D laser fields, studies on influences of molecular structure and structure-related effects on properties of 2D tunneling are also expected.…”