Influence of the initial angular distribution on strong-field molecular dissociation

Yu, Youliang; Zeng, Shuo; Hernández, José Falcón; Wang, Yujun; Esry, B. D.

doi:10.1103/physreva.94.023423

Cited by 2 publications

(3 citation statements)

References 21 publications

(39 reference statements)

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“…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 shown, rotational dynamics are crucial for the understanding of the angular distribution of the final ion fragments. Even at lower intensities where ionization is negligible and pure dissociation is the dominating fragmentation process, it was shown that molecular rotations play a role [20][21][22][23][24] for pulses as short as ∼ 5 fs.For our studies, we choose to focus on the simplest stable polar molecule, HeH + , sketched in Fig. 1(d).…”

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“…However, as several works employing semiclassical methods [7,18,19] have shown, rotational dynamics are crucial for the understanding of the angular distribution of the final ion fragments. Even at lower intensities where ionization is negligible and pure dissociation is the dominating fragmentation process, it was shown that molecular rotations play a role [20][21][22][23][24] for pulses as short as ∼ 5 fs.…”

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Strong-field polarizability-enhanced dissociative ionization

et al. 2018

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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...

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Strong-field polarizability-enhanced dissociative ionization

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“…We here follow up on and extend our previous modeling of DI of oxygen molecules [1,2,8,14,16,36], based on the propagation of nuclear vibrational wave packets in one nuclear degree of freedom, the internuclear distance, by allowing for molecular rotation. Investigating the effects of coherent ro-vibrational excitation [5,9,11,12,39,40] on the dissociation dynamics of O + 2 , we assume rapid ionization of unaligned O 2 molecules, i.e., random alignment angles θ between the molecular axis and IR-laserpolarization direction, launching a ro-vibrational nuclear wave packet Ψ(R, θ, 0) in the O + 2 (a 4 Π u ) state [4]. We numerically propagate Ψ(R, θ, t > 0) through a 50 fs delayed linearly polarized IR probe pulse to follow its evolution subject to IR-field-induced dipole couplings between the O + 2 (a 4 Π u ) and O + 2 (f 4 Π g ) states.…”

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Molecular bond stabilization in the strong-field dissociation of O2+

2020

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We theoretically examine the rotational and vibrational dynamics of O + 2 molecular ions exposed to intense, short laser pulses for conditions realized in contemporary pump-probe experiments. We solve the time-dependent Schrödinger equation within the Born-Oppenheimer approximation for an initial distribution of randomly aligned molecular ions. For fixed peak intensities, our numerical results show that total, angle-integrated O + 2 → O( 3 P) + O + ( 4 S 0 ) dissociation yields do not monotonically increase with increasing infrared-probe pulse duration. We find this pulse-durationdependent stabilization to be consistent with the transient trapping of nuclear probability density in a light-induced (bond-hardening) potential-energy surface and robust against rotational excitation. We analyze this stabilization effect and its underlying bond-hardening mechanism (i) in the time domain, by following the evolution of partial nuclear probability densities associated with the dipolecoupled O + 2 (a 4 Πu) and O + 2 (f 4 Πg) cationic states, and (ii) in the frequency domain, by examining ro-vibrational quantum-beat spectra for the evolution of the partial nuclear probability densities associated with these states. Our analysis reveals the characteristic timescale for the bond-hardening mechanism in O + 2 and explains the onset of bond stabilization for sufficiently long pulse durations.

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Influence of the initial angular distribution on strong-field molecular dissociation

Cited by 2 publications

References 21 publications

Strong-field polarizability-enhanced dissociative ionization

Strong-field polarizability-enhanced dissociative ionization

Molecular bond stabilization in the strong-field dissociation of O2+

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