Electron-ion correlations in strong-field molecular ionization with shaped ultrafast laser pulses

Geißler, Dominik; Weinacht, Thomas

doi:10.1103/physreva.89.013408

Cited by 5 publications

(2 citation statements)

References 27 publications

(31 reference statements)

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“…In the Discussion we address this study and offer as a possible explanation the fact that fragment formation for the polycyclic molecules studied requires at least two chemical bonds to be broken. More recently, the role of intermediate Freeman resonances in strong-field ionization of polyatomic (halogenated methane) molecules has been studied, finding clear signals in the photoelectron spectrum for the presence of resonance-enhanced ionization vs nonresonant ionization. − Electron–ion correlation studies aided by velocity map imaging have helped to determine direct as well as indirect reaction paths and pulse shaping has helped to find Freeman resonances. − …”

Section: Introductionmentioning

confidence: 99%

Polyatomic Molecules under Intense Femtosecond Laser Irradiation

et al. 2014

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Interaction of intense laser pulses with atoms and molecules is at the forefront of atomic, molecular, and optical physics. It is the gateway to powerful new tools that include above threshold ionization, high harmonic generation, electron diffraction, molecular tomography, and attosecond pulse generation. Intense laser pulses are ideal for probing and manipulating chemical bonding. Though the behavior of atoms in strong fields has been well studied, molecules under intense fields are not as well understood and current models have failed in certain important aspects. Molecules, as opposed to atoms, present confounding possibilities of nuclear and electronic motion upon excitation. The dynamics and fragmentation patterns in response to the laser field are structure sensitive; therefore, a molecule cannot simply be treated as a "bag of atoms" during field induced ionization. In this article we present a set of experiments and theoretical calculations exploring the behavior of a large collection of aryl alkyl ketones when irradiated with intense femtosecond pulses. Specifically, we consider to what extent molecules retain their molecular identity and properties under strong laser fields. Using time-of-flight mass spectrometry in conjunction with pump-probe techniques we study the dynamical behavior of these molecules, monitoring ion yield modulation caused by intramolecular motions post ionization. The set of molecules studied is further divided into smaller sets, sorted by type and position of functional groups. The pump-probe time-delay scans show that among positional isomers the variations in relative energies, which amount to only a few hundred millielectronvolts, influence the dynamical behavior of the molecules despite their having experienced such high fields (V/Å). High level ab initio quantum chemical calculations were performed to predict molecular dynamics along with single and multiphoton resonances in the neutral and ionic states. We propose the following model of strong-field ionization and subsequent fragmentation for polyatomic molecules: Single electron ionization occurs on a suboptical cycle time scale, and the electron carries away essentially all of the energy, leaving behind little internal energy in the cation. Subsequent fragmentation of the cation takes place as a result of further photon absorption modulated by one- and two-photon resonances, which provide sufficient energy to overcome the dissociation energy. The proposed hypothesis implies the loss of a photoelectron at a rate that is faster than intramolecular vibrational relaxation and is consistent with the observation of nonergodic photofragmentation of polyatomic molecules as well as experimental results from many other research groups on different molecules and with different pulse durations and wavelengths.

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Section: Introductionmentioning

confidence: 99%

Polyatomic Molecules under Intense Femtosecond Laser Irradiation

et al. 2014

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“…However, some efforts have been focused at gaining control mechanistic insights involving electron and nuclear wavepacket dynamics in the strong laser field ionization. In particular, some studies have used various pump–probe techniques, photoelectron spectroscopic methods, , and/or velocity map imaging (VMI) measurements of photoelectrons in coincidence with photoions, − in combination with electronic structure calculations and quantum dynamical simulations, , which have provided rich information on control mechanisms. These studies often call for sophisticated apparatus and detailed additional knowledge about the system in order to understand the physics.…”

Section: Introductionmentioning

confidence: 99%

Gaining Mechanistic Insight with Control Pulse Slicing: Application to the Dissociative Ionization of CH₂BrI

Rey-de-Castro

Rabitz

2017

J. Phys. Chem. A

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In quantum control experiments with shaped femtosecond laser pulses, adaptive feedback control is often used to identify pulse shapes that can optimally steer the quantum system toward the desired outcome. However, gaining mechanistic information can pose a challenge due to the varied structural features of the control pulses and/or the often complex nature of the associated simulations of the experiments. In this article, we introduce control pulse slicing (CPS) as an easy-to-implement experimental analysis tool that can be employed directly in the laboratory without the need for modeling, to gain mechanistic insights about control experiments, regardless of whether the pulse is optimal or chosen by other means. As an illustration, we apply CPS to dissociative ionization of CHBrI with mass spectral detection, where two pulses with similar intensities are investigated, with each capable of distinctively controlling the ratio of Br/CHBr. These two control pulses were, respectively, first identified with closed loop and open loop procedures, and then the multispecies experimental data was analyzed with CPS. By comparing the dynamical evolution of the observed multiple fragment ion yields upon slicing scans of the two distinct pulses, we were able to reveal insights about the control mechanism for manipulating the objective ratio. In addition, we also identified the relationship between the temporal structures of the control pulses and the associated key reaction pathways involved in ionic as well as neutral electronic states, in spite of the signals only directly being from the ionic species. The CPS technique is not limited to controlled fragmentation mass spectrometry, and it may be applied to gain mechanistic insights in any control experiment, reflected in the nature of the recorded signal.

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Photoelectron Circular Dichroism of Bicyclic Ketones from Multiphoton Ionization with Femtosecond Laser Pulses

et al. 2014

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Photoelectron circular dichroism (PECD) is a CD effect up to the ten-percent regime and shows contributions from higher-order Legendre polynomials when multiphoton ionization is compared to single-photon ionization. We give a full account of our experimental methodology for measuring the multiphoton PECD and derive quantitative measures that we apply on camphor, fenchone and norcamphor. Different modulations and amplitudes of the contributing Legendre polynomials are observed despite the similarity in chemical structure. In addition, we study PECD for elliptically polarized light employing tomographic reconstruction methods. Intensity studies reveal dissociative ionization as the origin of the observed PECD effect, whereas ionization of the intermediate resonance is dominating the signal. As a perspective, we suggest to make use of our tomographic data as an experimental basis for a complete photoionization experiment and give a prospect of PECD as an analytic tool.

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Electron-ion correlations in strong-field molecular ionization with shaped ultrafast laser pulses

Cited by 5 publications

References 27 publications

Polyatomic Molecules under Intense Femtosecond Laser Irradiation

Polyatomic Molecules under Intense Femtosecond Laser Irradiation

Gaining Mechanistic Insight with Control Pulse Slicing: Application to the Dissociative Ionization of CH₂BrI

Photoelectron Circular Dichroism of Bicyclic Ketones from Multiphoton Ionization with Femtosecond Laser Pulses

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Electron-ion correlations in strong-field molecular ionization with shaped ultrafast laser pulses

Cited by 5 publications

References 27 publications

Polyatomic Molecules under Intense Femtosecond Laser Irradiation

Polyatomic Molecules under Intense Femtosecond Laser Irradiation

Gaining Mechanistic Insight with Control Pulse Slicing: Application to the Dissociative Ionization of CH2BrI

Photoelectron Circular Dichroism of Bicyclic Ketones from Multiphoton Ionization with Femtosecond Laser Pulses

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Gaining Mechanistic Insight with Control Pulse Slicing: Application to the Dissociative Ionization of CH₂BrI