Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied—most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

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Imaging the dynamics of gas phase reactions

In recent years, the photodissociation dynamics of aryl halides has been a subject of intensive studies, which is closely related to the atmospheric chemistry. Here we present a review on the photochemistry of aryl halides, with emphasis on the recent progress in photodissociation dynamics at 266 nm by using photofragment translational spectroscopy. The ab initio calculations have also been employed to investigate those photodissociation processes. It has been found that the photodissociation of aryl halides at 266 nm is attributed to the nonadiabatic process via intersystem crossings from bound singlet excited state to triplet excited state and/or via internal conversion from bound singlet excited state to ground state. Also, the substitution effects in the photodissociation dynamics of aryl halides are discussed.

Photochemistry of aryl halides: Photodissociation dynamics

Visualising a collision between an atom or a molecule or a photodissociation (half-collision) of a molecule on a single particle and single quantum level is like watching the collision of billiard balls on a pool table: Molecular beams or monoenergetic photodissociation products provide the colliding reactants at controlled velocity before the reaction products velocity is imaged directly with an elaborate camera system, where one should keep in mind that velocity is, in general, a three-dimensional (3D) vectorial property which combines scattering angles and speed. If the processes under study have no cylindrical symmetry, then only this 3D product velocity vector contains the full information of the elementary process under study.

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Imaging chemical reactions – 3D velocity mapping

We present measurements of the laser induced spatial alignment of two diatomic molecules, iodine (I2) and bromine (Br2). Dynamic alignment is inferred from the angular distribution of the ionic fragments from multi-electron dissociative ionization (MEDI). The angular distributions were determined for different pulse durations and energies of the short infrared laser pulses that induce the MEDI, and were measured using a velocity map ion imaging detector. The width of the angular distribution of the fragments with respect to the laser polarization axis depends only weakly on the laser pulse energy, and decreases rapidly for longer pulse lengths (a few picoseconds) at constant pulse energy. The interpretation of this result in terms of dynamic alignment is supported by an extended field ionization Coulomb explosion model that includes the rotation of the molecule induced by the laser field.

https://iopscience.iop.org/article/10.1088/0953-4075/34/23/332/pdf

Spatial alignment of diatomic molecules in intense laser fields: I. Experimental results

Excited state properties of fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene have been investigated theoretically using multireference CASSCF and CASPT2 methods. Experimentally, chlorobenzene and bromobenzene are known to exhibit one fast dissociation channel, whereas iodobenzene exhibits two fast dissociation channels. The calculations indicate that the chlorobenzene, the bromobenzene, and the slower iodobenzene dissociation channels are due to intersystem crossings from a bound (π,π*) singlet excited state to a repulsive (n,σ*) triplet excited state. The faster iodobenzene dissociation channel is instead found to be caused by a direct dissociation of an antibonding (n,σ*) singlet excited state. The CASPT2 calculations predict that the onset of fluorobenzene photodissociation should occur around 196 nm, with a single time constant longer than 1 ns. CASSCF geometries and accurate MSCASPT2 calculated vertical excitation energies are presented for the ground state as well as the first excited singlet and triplet states of all the monohalobenzenes.

Theoretical Study of the Fast Photodissociation Channels of the Monohalobenzenes

The method of velocity map imaging is used to investigate the photoinduced dynamics of iodobenzene in weak laser fields as well as in fields of sufficient intensity to produce multiphoton excitation without dressing the potential energy surfaces. The speed and angular distributions of the recoiling iodine atoms observed with weak fields at 266 and 304 nm are in good agreement with previous studies, showing evidence of dissociation via n,σ* and π,π* states. Density functional theory calculations are used to determine the energies of the excited states. Ion and electron images produced with intense visible radiation (532−609 nm, 3 × 1010 to 5 × 1012 W/cm2) show both two-photon threshold dissociation and higher order (four- to seven-photon) ionization processes. A unique characteristic of the I+ images obtained with intense visible radiation is a continuous, hourglass-shaped feature peaked at zero kinetic energy, displaying highly nonstatistical speed and angular distributions. A mechanism that is consistent...

Above-Threshold Effects in the Photodissociation and Photoionization of Iodobenzene†

The problem of dissociative ionization at intermediate intensities ( ${10}^{10}--{10}^{12}\mathrm{W}{\mathrm{cm}}^{\ensuremath{-}2}$) was studied using the example of ${\mathrm{I}}_{2}$ and the technique of velocity map imaging. Several new phenomena were observed, including a continuous distribution of recoil energies peaked at zero-kinetic energy, a set of constant dissociative ionic states, and strong anisotropy of the fragment velocity distribution that is diminished by intermediate resonances.

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Yan Du

Papers

Above-Threshold Effects in the Photodissociation and Photoionization of Iodobenzene†

Above-Threshold Dissociative Ionization in the Intermediate Intensity Regime