Velocity Map Imaging of Dissociative Ionization and Coulomb Explosion of CH3I Induced by a Femtosecond Laser

Characterizing the multi-dimensional reaction dynamics of dihalomethanes using XUV-induced Coulomb explosion imaging

Site-selective probing of iodine 4d orbitals at 13.1 nm was used to characterize the photolysis of CH2I2 and CH2BrI initiated at 202.5 nm. Time-dependent fragment ion momenta were recorded using Coulomb explosion imaging mass spectrometry and used to determine the structural dynamics of the dissociating molecules. Correlations between these fragment momenta, as well as the onset times of electron transfer reactions between them, indicate that each molecule can undergo neutral three-body photolysis. For CH2I2, the structural evolution of the neutral molecule was simultaneously characterized along the C-I and I-C-I coordinates, demonstrating the sensitivity of these measurements to nuclear motion along multiple degrees of freedom.

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.

Slicing Newton spheres with a two-camera 3D imaging system

We demonstrate a simple approach to achieve three-dimensional ion momentum imaging. The method employs two complementary metal-oxide-semiconductor cameras in addition to a standard microchannel plates/phosphor screen imaging detector. The two cameras are timed to measure the decay of luminescence excited by ion hits to extract the time of flight. The achieved time resolution is better than 10 ns, which is mainly limited by camera jitters. A better than 5 ns resolution can be achieved when the jitter is suppressed.

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.

Ultrafast Imaging of Molecular Dynamics Using Ultrafast Low-Frequency Lasers, X-ray Free Electron Lasers, and Electron Pulses

The requirement of high space-time resolution and brightness is a great challenge for imaging atomic motion and making molecular movies. Important breakthroughs in ultrabright tabletop laser, X-ray, and electron sources have enabled the direct imaging of evolving molecular structures in chemical processes, and recent experimental advances in preparing ultrafast laser and electron pulses resulted in molecular imaging with femtosecond time resolution. This Perspective presents an overview of the versatile imaging methods of molecular dynamics. High-order harmonic generation imaging and photoelectron diffraction imaging are based on laser-induced ionization and rescattering processes. Coulomb explosion imaging retrieves molecular structural information by detecting the momentum vectors of fragmented ions. Diffraction imaging encodes molecular structural and electronic information in reciprocal space. We also present various applications of these ultrafast imaging methods in resolving laser-induced nuclear and electronic dynamics.

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.

Multi-channel photodissociation and XUV-induced charge transfer dynamics in strong-field-ionized methyl iodide studied with time-resolved recoil-frame covariance imaging

The photodissociation dynamics of strong-field ionized methyl iodide (CH3I) were probed using intense extreme ultraviolet (XUV) radiation produced by the SPring-8 Angstrom Compact free electron LAser (SACLA). Strong-field ionization and subsequent fragmentation of CH3I was initiated by an intense femtosecond infrared (IR) pulse. The ensuing fragmentation and charge transfer processes following multiple ionization by the XUV pulse at a range of pump-probe delays were followed in a multi-mass ion velocity-map imaging (VMI) experiment. Simultaneous imaging of a wide range of resultant ions allowed for additional insight into the complex dynamics by elucidating correlations between the momenta of different fragment ions using time-resolved recoil-frame covariance imaging analysis. The comprehensive picture of the photodynamics that can be extracted provides promising evidence that the techniques described here could be applied to study ultrafast photochemistry in a range of molecular systems at high count rates using state-of-the-art advanced light sources.

Absorption spectra at the iodine 3d ionisation threshold following the CHxI+ (x = 0-3) cation sequence

Yields of atomic iodine I^q+ (q≥ 2) fragments resulting from photoexcitation and photoionisation of the target cations CH_xI⁺ (x = 0-3) have been measured in the photon-energy range 610 eV to 670 eV, which comprises the threshold for iodine 3d ionisation. The measured ion-yield spectra show two strong and broad resonance features due to the excitation of the 3d_3/2,5/2 electrons into εf states similar to atomic iodine. In the 3d pre-edge range, electrons are excited into molecular orbitals consisting of iodine, carbon, and hydrogen atomic orbitals. These transitions have been identified by comparison with literature data and by simulations using time-dependent density functional theory (TDDFT) with the KMLYP functional. The ion-yield spectrum for CH₃I⁺ resembles the spectrum of IH⁺ [Klumpp et al., Phys. Rev. A, 2018, 97, 033401] because the highest occupied molecular orbitals (HOMO) of the H and CH₃ fragments both contain a single vacancy, only. For the molecular cations with higher number of vacancies in the valence molecular orbitals CH_xI⁺ (x = 0-2), a stronger hybridisation of the molecular orbitals occurs between the organic fragment and the iodine resulting in a change of bonding from a single σ bond in CH₃I⁺ to a triple bond including two π orbitals in CI⁺. This is reflected in the resonance energies of the observed absorption lines below the iodine 3d excitation threshold.

Capturing the photo-induced dynamics of nano-molecules by X-ray free electron laser induced Coulomb explosion

We performed reaction dynamics simulations to demonstrate that the vibrational dynamics of C₆₀ induced by infrared (IR) pulses can be traced by triggering Coulomb explosion with intense femtosecond X-ray free electron laser (XFEL) probe pulses. The time series of the angular anisotropy β(t) of fast C⁺ and C²⁺ fragments of C₆₀ ⁶⁰⁺ produced by such an XFEL pulse reflects the instantaneous structure of C₆₀ vibrationally excited by IR pulses. The phases and amplitudes of excited vibrational modes and the coupling between excited modes can be successfully extracted from the expansion of β(t) in terms of vibrational modes. This proof-of-principle simulation clearly demonstrates that various information of the structures and reaction dynamics of large clusters or biomolecules can be retrieved by decomposing the experimentally determined β(t) into vibrational modes.

Theoretical and experimental studies on hydrogen migration in dissociative ionization of the methanol monocation to molecular ions H3 + and H2O. RSC Adv 2019;9:16683-16689. [PMID: 35516392 PMCID: PMC9064428 DOI: 10.1039/c9ra02003a]

The dissociative ionization processes of the methanol monocation CH3OH+ to H3 + + CHO and H2O+ + CH2 are studied by ab initio method, and hydrogen migration processes are confirmed in these two dissociation processes. Due to the positive charge assignment in dissociation processes, the fragmentation pathways of CH3OH+ to H3 + CHO+ and CH3OH+ to H2O + CH2 + are also calculated. The calculation results show that a neutral H2 moiety in the methanol monocation CH3OH+ is the origin of the formation of H3 +, and the ejection of fragment ions H3 + and H2O+ is more difficult than CHO+ and CH2 + respectively. Experimentally, by using a dc-slice imaging technique under an 800 nm femtosecond laser field, the velocity distributions of fragment ions H3 +, CHO+, CH2 +, and H2O+ are calculated from their corresponding sliced images. The presence of low-velocity components of these four fragment ions confirms that the formation of these ions is not from the Coulomb explosion of the methanol dication. Hence, the four hydrogen migration pathways from the methanol monocation CH3OH+ to H3 + + CHO, CHO+ + H3, H2O+ + CH2, and CH2 + + H2O are securely confirmed. It can be observed in the time-of-flight mass spectrum of ionization and dissociation of methanol that the ion yields of fragment ions H3 + and H2O+ are lower than CHO+ and CH2 + respectively, which is consistent with the theoretical results according to which dissociation from the methanol monocation to H3 + and H2O+ is more difficult than CHO+ and CH2 + respectively.

Coulomb Explosion Imaging for the Visualization of a Conical Intersection

Coulomb explosion imaging is proposed as a method to directly map the presence of conical intersections encountered by a propagating wave packet in a molecular system. The case of choice is the nonadiabatic coupling between two dissociative surfaces in the methyl iodide molecule, probed by Coulomb explosion with short, intense near-infrared pulses causing multiple ionization. On-the-fly multidimensional trajectory calculations with surface hopping using perturbation theory and including spin-orbit coupling are performed to visualize the dynamics through the conical intersection and compare with experimental results. The possibilities and limitations of the technique are examined and discussed.

KR, Ziaee F, Xue S, Le AT, Ben-Itzhak I, Rolles D, Rudenko A. Time-resolved imaging of bound and dissociating nuclear wave packets in strong-field ionized iodomethane

We report the results of a time-resolved coincident ion momentum imaging experiment probing nuclear wave packet dynamics in the strong-field ionization and dissociation of iodomethane (CH₃I).

BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields

By using a dc-slice imaging technique, photodissociation of 1,2-C₂H₄BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl⁺ in an 80 fs laser field. The occurrence of fragment ion BrCl⁺ in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl⁺ in this experiment. The relative quantum yield of the BrCl⁺ channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl⁺ and its partner ion C₂H₄⁺, we concluded that BrCl⁺ came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C₂H₄BrCl²⁺. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl⁺ detachment from 1,2-C₂H₄BrCl²⁺ has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C₂H₄BrCl²⁺ and the products BrCl⁺ + C₂H₄⁺. In this entire dissociation process, the C–Br and C–Cl bond lengths were observed to elongate asymmetrically, that is, the C–Br chemical bond broke firstly, and subsequently a new Br–Cl chemical bond started to emerge while the C–Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl⁺ detachment.

Concerted elimination of the molecular ion BrCl⁺ from Coulomb explosion of 1,2-bromochloroethane was studied theoretically and experimentally.

Disentangling Strong-Field Multielectron Dynamics with Angular Streaking

The study into the interaction between a strong laser field and atoms/molecules has led to significant advances in developing spectroscopic tools in the attosecond time-domain and methods for controlling chemical reactions. There has been great interest in understanding the complex electronic and nuclear dynamics of molecules in strong laser fields. However, it is still a formidable challenge to fully model such dynamics. Conventional experimental tools such as photoelectron spectroscopy encounter difficulties in revealing the involved states because the electron spectra are largely dictated by the property of the laser field. Here, with strong field angular streaking technique, we measure the angle-dependent ionization yields that directly reflect the symmetry of the ionizing orbitals of methyl iodide and thus reveal the ionization/dissociation dynamics. Moreover, kinematically complete measurements of momentum vectors of all fragments in dissociative double ionization processes allow access to electron-momentum correlations that reveal correlated multielectron dynamics.

Spin-Orbit State-Selective C-I Dissociation Dynamics of the CH3I+ X̃ Electronic State Induced by Intense Few-Cycle Laser Fields

Studies of ultrafast molecular dynamics induced by intense laser fields can reveal new approaches to manipulating chemical reactions in the strong-field regime. Here, we show that intense few-cycle laser pulses can induce the spin-orbit state-selective C-I dissociation of the iodomethane cation (CH₃I⁺) in the X̃ electronic state. Irradiation of CH₃I by 6 fs laser pulses with peak intensities of 1.9 × 10¹⁴ W/cm² followed by femtosecond extreme ultraviolet probing of the iodine 4d core-level transitions reveals dissociation of the CH₃I⁺ X̃ ²E_1/2 state with a time constant of 0.76 ± 0.16 ps. By contrast, the X̃ ²E_3/2 spin-orbit ground state does not exhibit any appreciable dissociation on the picosecond time scale. The observed spin-orbit state-selective dissociation of the X̃ state is rationalized in terms of the laser-induced coupling to the à state. Our results suggest that the intense-laser control of photodissociation channels can be potentially extended to spin-orbit split states.

A velocity map imaging study of the photodissociation of the methyl iodide cation

The photodissociation dynamics of the methyl iodide cation has been studied using velocity map imaging and ab initio theory to disentangle the dissociation mechanisms.

Coincidence ion imaging with a fast frame camera

A new time- and position-sensitive particle detection system based on a fast frame CMOS (complementary metal-oxide semiconductors) camera is developed for coincidence ion imaging. The system is composed of four major components: a conventional microchannel plate/phosphor screen ion imager, a fast frame CMOS camera, a single anode photomultiplier tube (PMT), and a high-speed digitizer. The system collects the positional information of ions from a fast frame camera through real-time centroiding while the arrival times are obtained from the timing signal of a PMT processed by a high-speed digitizer. Multi-hit capability is achieved by correlating the intensity of ion spots on each camera frame with the peak heights on the corresponding time-of-flight spectrum of a PMT. Efficient computer algorithms are developed to process camera frames and digitizer traces in real-time at 1 kHz laser repetition rate. We demonstrate the capability of this system by detecting a momentum-matched co-fragments pair (methyl and iodine cations) produced from strong field dissociative double ionization of methyl iodide.

Coulomb explosion and dissociative ionization of 1,2-dibromoethane under an intense femtosecond laser field

Coulomb explosion and dissociative ionization of 1,2-dibromoethane are experimentally investigated in a near-infrared (800 nm) femtosecond laser field by dc-slice imaging technology.