Detecting handedness of spatially oriented molecules by Coulomb explosion imaging

Full control of the orientation of non-symmetric molecules using weak and moderate electric fields

We investigate the full control over the orientation of a planar non-symmetric molecule by using moderate and weak electric fields. Quantum optimal control techniques allow us to orient any axis of 6-chloropyridazine-3-carbonitrile, which is taken as prototype example here, along the electric field direction. We perform a detailed analysis by exploring the impact on the molecular orientation of the time scale and strength of the control field. The underlying physical phenomena allowing for the control of the orientation are interpreted in terms of the frequencies contributing to the field-dressed dynamics and to the driving field by a spectral analysis.

Unraveling the ultrafast dynamics of thermal-energy chemical reactions

In this perspective, we discuss how one can initiate, image, and disentangle the ultrafast elementary steps of thermal-energy chemical dynamics, building upon advances in technology and scientific insight. We propose that combinations of ultrashort mid-infrared laser pulses, controlled molecular species in the gas phase, and forefront imaging techniques allow to unravel the elementary steps of general-chemistry reaction processes in real time. We detail, for prototypical first reaction systems, experimental methods enabling these investigations, how to sufficiently prepare and promote gas-phase samples to thermal-energy reactive states with contemporary ultrashort mid-infrared laser systems, and how to image the initiated ultrafast chemical dynamics. The results of such experiments will clearly further our understanding of general-chemistry reaction dynamics.

Molecular photodissociation dynamics revealed by Coulomb explosion imaging

Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, etc). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies. 'One-colour' studies (employing strong field ionisation driven by intense near infrared or single X-ray or extreme ultraviolet laser pulses) afford routes to preparing multiply charged molecular cations and exploring how their fragmentation progresses from valence-dominated to Coulomb-dominated dynamics with increasing charge and how this evolution varies with molecular size and composition. 'Two-colour' studies use one ultrashort laser pulse to create electronically excited neutral molecules (or monocations), whose structural evolution is then probed as a function of pump-probe delay using an ultrafast ionisation pulse along with time and position-sensitive detection methods. This latter type of experiment has the potential to return new insights into not just molecular fragmentation processes but also charge transfer processes between moieties separating with much better defined stereochemical control than in contemporary ion-atom and ion-molecule charge transfer studies.

Multi-mass velocity map imaging study of the 805 nm strong field ionization of CF3I

Multi-mass velocity map imaging studies of charged fragments formed by near infrared strong field ionization together with covariance map image analysis offer a new window through which to explore the dissociation dynamics of several different highly charged parent cations, simultaneously - as demonstrated here for the case of CF₃I^Z+ cations with charges Z ranging from 1 to at least 5. Previous reports that dissociative ionization of CF₃I⁺ cations yields CF₃⁺, I⁺ and CF₂I⁺ fragment ions are confirmed, and some of the CF₃⁺ fragments are deduced to undergo secondary loss of one or more neutral F atoms. Covariance map imaging confirms the dominance of CF₃⁺ + I⁺ products in the photodissociation of CF₃I²⁺ cations and, again, that some of the primary CF₃⁺ photofragments can shed one or more F atoms. Rival charge symmetric dissociation pathways to CF₂I⁺ + F⁺ and to IF⁺ + CF₂⁺ products and charge asymmetric dissociations to CF₃ + I²⁺ and CF₂I²⁺ + F products are all also identified. The findings for parent cations with Z ≥ 3 are wholly new. In all cases, the fragment recoil velocity distributions imply dissociation dynamics in which coulombic repulsive forces play a dominant role. The major photoproducts following dissociation of CF₃I³⁺ ions are CF₃⁺ and I²⁺, with lesser contributions from the rival CF₂I²⁺ + F⁺ and CF₃²⁺ + I⁺ channels. The CF₃²⁺ fragment ion images measured at higher incident intensities show a faster velocity sub-group consistent with their formation in tandem with I²⁺ fragments, from photodissociation of CF₃I⁴⁺ parent ions. The measured velocity distributions of the I³⁺ fragment ions contain features attributable to CF₃I⁵⁺ photodissociation to CF₃²⁺ + I³⁺ and the images of fragments with mass to charge (m/z) ratio ∼31 show formation of I⁴⁺ products that must originate from parent ions with yet higher Z.

Laser-Induced Coulomb Explosion Imaging of Aligned Molecules and Molecular Dimers

We discuss how Coulomb explosion imaging (CEI), triggered by intense femtosecond laser pulses and combined with laser-induced alignment and covariance analysis of the angular distributions of the recoiling fragment ions, provides new opportunities for imaging the structures of molecules and molecular complexes. First, focusing on gas phase molecules, we show how the periodic torsional motion of halogenated biphenyl molecules can be measured in real time by timed CEI, and how CEI of one-dimensionally aligned difluoroiodobenzene molecules can uniquely identify four structural isomers. Next, focusing on molecular complexes formed inside He nanodroplets, we show that the conformations of noncovalently bound dimers or trimers, aligned in one or three dimensions, can be determined by CEI. Results presented for homodimers of CS₂, OCS, and bromobenzene pave the way for femtosecond time-resolved structure imaging of molecules undergoing bimolecular interactions and ultimately chemical reactions. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A new route for enantio-sensitive structure determination by photoelectron scattering on molecules in the gas phase

Combination of Coulomb explosion imaging, molecular frame diffraction imaging, and ab initio computations provide a route for enantio-sensitive structure determination.

Nonadiabatic Coupling Effects in the 800 nm Strong-Field Ionization-Induced Coulomb Explosion of Methyl Iodide Revealed by Multimass Velocity Map Imaging and Ab Initio Simulation Studies

The Coulomb explosion (CE) of jet-cooled CH₃I molecules using ultrashort (40 fs), nonresonant 805 nm strong-field ionization at three peak intensities (260, 650, and 1300 TW cm^-2) has been investigated by multimass velocity map imaging, revealing an array of discernible fragment ions, that is, I^q+ (q ≤ 6), CH_n⁺ (n = 0-3), CH_n²⁺ (n = 0, 2), C³⁺, H⁺, H₂⁺, and H₃⁺. Complementary ab initio trajectory calculations of the CE of CH₃I^Z+ cations with Z ≤ 14 identify a range of behaviors. The CE of parent cations with Z = 2 and 3 can be well-described using a diatomic-like representation (as found previously) but the CE dynamics of all higher CH₃I^Z+ cations require a multidimensional description. The ab initio predicted I^q+ (q ≥ 3) fragment ion velocities are all at the high end of the velocity distributions measured for the corresponding I^q+ products. These mismatches are proposed as providing some of the clearest insights yet into the roles of nonadiabatic effects (and intramolecular charge transfer) in the CE of highly charged molecular cations.