High-Resolution Ion-Imaging Studies of the Photodissociation of the BrCl+ Cation

The Potential Energy Profile of the HBr+ + HCl Bimolecular Collision

Recently, the HBr⁺ + HCl bimolecular reaction has been exploited by guided ion beam studies to probe the effect of rotational excitations and collision energies on the dynamics of the ion-molecule reactions. The current manuscript employs high-level ab initio calculations and reports the potential energy of pathways leading to various products, including HBr + HCl⁺, H₂Cl⁺ + Br, H₂Br⁺ + Cl, and H₂ + BrCl⁺. The study shows that the intermediates involved in this reaction are connected by low-lying transition states, thus frequent isomerizations and diverse products are expected. Further, this manuscript screens the performance of 192 different combinations of computationally efficient methods and basis sets in order to identify the optimal quantum chemical method for further dynamics simulations.

Invited Review Article: Photofragment imaging

Photodissociation studies in molecular beams that employ position-sensitive particle detection to map product recoil velocities emerged thirty years ago and continue to evolve with new laser and detector technologies. These powerful methods allow application of tunable laser detection of single product quantum states, simultaneous measurement of velocity and angular momentum polarization, measurement of joint product state distributions for the detected and undetected products, coincident detection of multiple product channels, and application to radicals and ions as well as closed-shell molecules. These studies have permitted deep investigation of photochemical dynamics for a broad range of systems, revealed new reaction mechanisms, and addressed problems of practical importance in atmospheric, combustion, and interstellar chemistry. This review presents an historical overview, a detailed technical account of the range of methods employed, and selected experimental highlights illustrating the capabilities of the method.

Photofragment Imaging, Spectroscopy, and Theory of MnO. J Phys Chem A 2018;122:8047-8053. [PMID: 30226771 DOI: 10.1021/acs.jpca.8b07849]

Density functional and ab initio calculations, along with photodissociation spectroscopy and ion imaging of MnO⁺ from 21,300 to 33,900 cm^-1, are used to probe the photodissociation dynamics and bond strength of the manganese oxide cation (MnO⁺). These studies confirm the theoretical ground state (⁵Π) and determine the spin-orbit constant ( A' = 14 cm^-1) of the dominant optically accessible excited state (⁵Π) in the region. Photodissociation via this excited ⁵Π state results in ground state Mn⁺ (⁷S) + O (³P) products. At energies above 30,000 cm^-1, the Mn⁺ (⁵S) + O (³P) channel is energetically accessible and becomes the preferred dissociation pathway. The bond dissociation energy ( D₀ = 242 ± 5 kJ/mol) of MnO⁺ is measured from several images of each photofragmentation channel and compared to theory, resolving a disagreement in previous measurements. MRCI+Q calculations are much more successful in predicting the observed spectrum than TD-DFT or EOM-CCSD calculations.

A velocity map imaging mass spectrometer for photofragments of fast ion beams

We present the details of a fast ion velocity map imaging mass spectrometer that is capable of imaging the photofragments of trap-cooled (≥7 K) ions produced in a versatile ion source. The new instrument has been used to study the predissociation of N₂O⁺ produced by electric discharge and the direct dissociation of Al₂⁺ formed by laser ablation. The instrument's resolution is currently limited by the diameter of the collimating iris to a value of Δv/v = 7.6%. Photofragment images of N₂O⁺ show that when the predissociative state is changed from ²Σ⁺(200) to ²Σ⁺(300) the dominant product channel shifts from a spin-forbidden ground state, N (⁴S) + NO⁺(v = 5), to a spin-allowed pathway, N^*(²D) + NO⁺. The first photofragment images of Al₂⁺ confirm the existence of a directly dissociative parallel transition (²Σ⁺_u ← ²Σ⁺_g) that yields products with a large amount of kinetic energy. D₀ of ground state Al₂⁺ (²Σ⁺_g) measured from these images is 138 ± 5 kJ/mol, which is consistent with the published literature.

Imaging studies of the photodissociation of NH3+ and ND3+ cations

Velocity map ion imaging methods have been used to study the photofragmentation dynamics of state-selected NH3+ and ND3+ cations. The cations were prepared in selected nu2+ bending vibrational levels of the ground (x2A'') electronic state by two-photon resonant, three-photon ionization of NH3(ND3), via several different nu2' levels of the and ' Rydberg states. Subsequent excitation to the A2E state by absorption of a 207.6 nm photon resulted in N-H(D) bond fission and NH2+(ND2+) fragment ion formation. These fragments exhibit isotropic recoil velocity distributions, which peak at low kinetic energy but extend to the maximum allowed by energy conservation. Such findings accord with conclusions from earlier electron induced photoionization and photoelectron-photoion coincidence studies of NH3 at similar total energies (defined relative to the ground-state neutral) and, as previously, can be rationalized in terms of excitation to the Jahn-Teller distorted state, rapid radiationless transfer via one or more conical intersections linking the and state potential energy surfaces (PESs) and subsequent unimolecular decay on the latter PES. Weak NH2+ and NH+ fragment ion signals are also observed when exciting with the ionization laser only; imaging these fragment ions provides some insights into their likely formation mechanisms.

Vibrationally Mediated Photodissociation of C2H4+

We report the vibrationally mediated photodissociation dynamics of C2H4+ excited through the B2Ag state. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (pi, 3s) and (pi, 3p) Rydberg intermediate states in the wavelength range 298-349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C2H3+ and C2H2+ were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state.

A velocity map imaging study of the one and two photon dissociations of state-selected DCl+ cations

DCl(+)(X (2)Pi(32),v(+")=0) cations have been prepared by 2+1 resonance enhanced multiphoton ionization, and their subsequent fragmentation following excitation at numerous wavelengths in the range of 240-350 nm studied by velocity map imaging of the resulting Cl(+) products. This range of excitation wavelengths allows selective population of A (2)Sigma(+) state levels with all vibrational (v(+')) quantum numbers in the range 0< or =v(+')< or =15. Image analysis yields wavelength dependent branching ratios and recoil anisotropies of the various D+Cl(+) ((3)P(J), (1)D, and (1)S) product channels. Levels with 10< or =v(+')< or =15 have sufficient energy to predissociate, forming D+Cl(+)((3)P(J)) products with perpendicular recoil anisotropies-consistent with the A (2)Sigma(+)<--X (2)Pi parent excitation and subsequent fragmentation on a time scale that is fast compared with the parent rotational period. Branching into the various spin-orbit states of the Cl(+)((3)P(J)) product is found to depend sensitively upon v(+') and, in the case of the v(+')=13 level, to vary with the precise choice of excitation wavelength within the A (2)Sigma(+)<--X (2)Pi(13,0) band. Such variations have been rationalized qualitatively in terms of the differing contributions made to the overall predissociation rate of DCl(+)(A,v(+')) molecules by coupling to repulsive states of (4)Pi, (4)Sigma(-), and (2)Sigma(-) symmetries, all of which are calculated to cross the outer limb of the A (2)Sigma(+) state potential at energies close to that of the v(+')=10 level. Cl(+)((3)P(J)) fragments are detected weakly following excitation to A (2)Sigma(+) state levels with v(+')=0 or 1, Cl(+)((1)D) fragments dominate the ion yield when exciting via 2< or =v(+')< or =6 and via v(+')=9, while Cl(+)((1)S) fragments dominate the Cl(+) images obtained when exciting via levels with v(+')=7 and 8. Analysis of wavelength resolved action spectra for forming these Cl(+) ions and of the resulting Cl(+) ion images shows that (i) these ions all arise via two photon absorption processes, resonance enhanced at the one photon energy by the various A(v(+')<10) levels, (ii) the first A (2)Sigma(+)<--X (2)Pi absorption step is saturated under the conditions required to observe significant two photon dissociation, and (iii) the final absorption step from the resonance enhancing A(v(+')) level involves a parallel transition.

Imaging the dynamics of gas phase reactions

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.

Vibrationally Mediated Photodissociation of Ethylene Cation by Reflectron Multimass Velocity Map Imaging

A new imaging technique, reflectron multimass velocity map ion imaging, is used to study the vibrationally mediated photodissociation dynamics in the ethylene cation. The cation ground electronic state is prepared in specific vibrational levels by two-photon resonant, three-photon ionization via vibronic bands of (pi, nf) Rydberg states in the vicinity of the ionization potential of ethylene, then photodissociated through the (B 2A(g)) excited state. We simultaneously record spatially resolved images of parent C2H4+ ions as well as photofragment C2H3+ and C2H2+ ions originating in dissociation from the vibronic excitations in two distinct bands, 7f 4(0)2 and 8f 0(0)0, at roughly the same total energy. By analyzing the images, we directly obtain the total translation energy distributions for the two dissociation channels and the branching between them. The results show that there exist differences for competitive dissociation pathways between H and H2 elimination from C2H4+ depending on the vibronic preparation used, i.e., on the vibrational excitation in the ground state of the cation prior to photodissociation. Our findings are discussed in terms of the possible influence of the torsional excitation on competition between direct dissociation, isomerization, and radiationless transitions through conical intersections among the numerous electronic states that participate in the dissociation.