Velocity map ion imaging study of Ar2+ photodissociation

Photodissociation Spectroscopy and Photofragment Imaging of the Mg+(Benzene) Complex

Tunable laser photodissociation spectroscopy and photofragment imaging experiments are employed to investigate the spectroscopy and dissociation dynamics of the Mg+(benzene) ion-molecule complex. When excited with ultraviolet radiation, Mg+(benzene) photodissociates efficiently, producing both Mg+ and benzene+ fragments, with branching ratios depending on the wavelength. The wavelength dependence of these processes are similar, with intense resonances at 330 and 241 nm and weaker features at 290 and 258 nm. Comparisons of the experimental spectra to those predicted by computational chemistry at the TD-DFT level allow assignment of these to metal ion-based (330 and 241 nm), charge-transfer (290 nm), and benzene-based (258 nm) transitions. However, the observation of the benzene cation fragment at all wavelengths, which can only result from charge-transfer, indicates unanticipated excited state dynamics. Spectroscopy experiments are complemented by photofragment imaging to investigate these dynamics. The high kinetic energy release indicates that multiphoton absorption based on the intense atomic resonances is responsible at least in part for the dissociation processes.

Photodissociation Spectroscopy and Photofragment Imaging to Probe Fe+(Benzene)1,2 Dissociation Energies

Tunable laser photodissociation spectroscopy measurements and photofragment imaging experiments are employed to investigate the dissociation energy of the Fe+(benzene) ion-molecule complex. Additional spectroscopy measurements determine the dissociation energy of Fe+(benzene)2. The dissociation energies for Fe+(benzene) determined from the threshold for the appearance of the Fe+ fragment (48.4 ± 0.2 kcal/mol) and photofragment imaging (≤49.3 ± 3.2 kcal/mol) agree nicely with each other and with the value determined previously by collision-induced dissociation (49.5 ± 2.9 kcal/mol), but they are lower than the values produced by computational chemistry at the density functional theory level using different functionals recommended for transition-metal chemistry. The threshold measurement for Fe+(benzene)2 (43.0 ± 0.2 kcal/mol) likewise agrees with the value (44.7 ± 3.8 kcal/mol) from previous collision-induced dissociation measurements.

Photodissociation Spectroscopy and Photofragment Imaging of the Fe+(Acetylene) Complex

Tunable laser photodissociation spectroscopy in the 700-400 nm region and photofragment imaging experiments are employed to investigate the Fe⁺(acetylene) ion-molecule complex. At energies above a threshold at 679 nm, continuous dissociation is detected throughout the visible wavelength region, with regions of broad structure. Comparison to the spectrum predicted by time-dependent density functional theory (TD-DFT) indicates that the complex has a quartet ground state. The dissociation threshold for Fe⁺(acetylene) at 679 nm provides the dissociation energy on the quartet potential energy surface. Correction for the atomic quartet-sextet spin state energy difference provides an adiabatic dissociation energy of 36.8 ± 0.2 kcal/mol. Photofragment imaging of the Fe⁺ photoproduct produced at 603.5 nm produces significant kinetic energy release (KER). The photon energy and the maximum value of the KER provide an upper limit on the dissociation energy of D₀ ≤ 34.6 ± 3.2 kcal/mol. The dissociation energies determined from the spectroscopy and photofragment imaging experiments agree nicely with the value determined previously by collision-induced dissociation (38.0 ± 2.6 kcal/mol). However, both values are significantly lower than those produced by computational chemistry at the DFT level using different functionals recommended for transition-metal chemistry.

Photoinduced Charge Transfer in the Zn-Methanol Cation Studied with Selected-Ion Photofragment Imaging

The Zn+(methanol) ion molecule complex produced by laser vaporization is studied with photofragment imaging at 280 and 266 nm. Photodissociation produces the methanol cation CH3OH+ via excitation of a charge-transfer excited state. Surprisingly, excitation of bound excited states produces the same fragment via a curve crossing prior to separation of products. Significant kinetic energy release is detected at both wavelengths with isotropic angular distributions. Similar experiments are conducted on the perdeuterated methanol complex. The Zn+ cation is a minor product channel that also exhibits significant kinetic energy release. An energetic cycle using the ionization potentials of zinc and methanol together with the kinetic energy release produces an upper limit on the Zn+-methanol bond energy of 33.7 {plus minus} 4.2 kcal/mol (1.46 {plus minus} 0.18 eV).

Photofragment Imaging of Carbon Cluster Cations: Explosive Ring Rupture

Carbon cluster cations (C_n⁺) produced by laser vaporization are mass selected and photodissociated at 355 nm. Multiphoton dissociation of smaller ions leads to the elimination of neutral C₃, as in previous work, whereas larger clusters exhibit more varied fragmentation channels. Photofragment velocity-map imaging detects significant kinetic energy release (KER) in the various n - 3 cation fragments. Small cations (n = 6 or 7) with linear structures produce moderate KER, whereas larger cations (n = 10, 11, 12, 15, or 20) having monocyclic ring structures produce much higher KER values. Such high KER values are unanticipated, as optical excitation should produce a wide distribution of internal energies. These carbon clusters have a surprising ability to absorb multiple photons of ultraviolet radiation, achieving a state of extreme excitation prior to dissociation. The remarkable nonstatistical distribution of energy is apparently influenced by the significant ring strain that can be released upon photodissociation.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion

Vibrational predissociation processes of the H₂O⁺Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H₂O⁺ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H₂O⁺ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H₂O⁺ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H₂O⁺ fragment ions was discussed based on the steric configuration of the H₂O⁺ and Ar units at the moment of dissociation.

Cation-π Complexes of Silver Studied with Photodissociation and Velocity-Map Imaging

Ag⁺(aromatic) ion-molecule complexes of benzene, toluene, or furan are generated in the gas phase by laser vaporization in a supersonic expansion. These ions are mass selected in a time-of-flight spectrometer and studied with ultraviolet laser photodissociation and photofragment imaging. UV laser excitation results in dissociative charge transfer (DCT) for these ions, producing neutral silver atom and the respective aromatic cation as the photofragments. Velocity-map imaging and slice imaging techniques are employed to investigate the kinetic energy release in these photodissociation processes. In each case, DCT produces significant kinetic energy, and evidence is also found for excitation of the internal rovibrational degrees of freedom for the molecular cations. Analysis of the kinetic energy release together with the known ionization energies of silver and the molecular ligands provides new information on the cation-π bond energies.

Visible photodissociation of the CO2 dimer cation: fast and slow dissociation dynamics in the excited state

Velocity and angular distributions of photofragment CO2+ ions produced from mass-selected (CO2)2+ at 532 nm excitation were observed in an ion imaging experiment. The velocity distribution was assigned to two components, fast and slow velocity components, which was consistent with the previous study by Bowers et al. The anisotropy parameters of the angular distributions for the fast and slow velocity components were experimentally determined to be βfast = 1.52 ± 0.14 and βslow = 0.46 ± 0.10, respectively. In the theoretical approach, potential energy surfaces (PESs) of (CO2)2+ were calculated along two coordinates, the intermolecular distance and mutual orientations of the CO2 monomers. In addition, molecular dynamics simulations were performed. The visible transition of the most stable staggered structure of (CO2)2+ was attributed to C[combining tilde]2Ag ← X[combining tilde]2Bu by an excited state calculation. On the PES of the C[combining tilde] state, a potential well was found in which the two CO2 monomers lay side by side to each other, in addition to a repulsive slope along the intermolecular distance. The results of the simulations confirmed that the fragment CO2+ ions with fast velocity and large anisotropy originated from the rapid dissociation of (CO2)2+ on the repulsive slope. Meanwhile, the fragment CO2+ ions with slow velocity and small anisotropy were expected to emerge from statistical dissociation after large amplitude libration of CO2 molecules which was caused by the potential well in the excited state PES.

A cryogenic cylindrical ion trap velocity map imaging spectrometer

A cryogenic cylindrical ion trap velocity map imaging spectrometer has been developed to study photodissociation spectroscopy and dynamics of gaseous molecular ions and ionic complexes. A cylindrical ion trap made of oxygen-free copper is cryogenically cooled down to ∼7 K by using a closed cycle helium refrigerator and is coupled to a velocity map imaging (VMI) spectrometer. The cold trap is used to cool down the internal temperature of mass selected ions and to reduce the velocity spread of ions after extraction from the trap. For CO₂ ⁺ ions, a rotational temperature of ∼12 K is estimated from the recorded [1 + 1] two-photon dissociation spectrum, and populations in spin-orbit excited X²Π_g,1/2 and vibrationally excited states of CO₂ ⁺ are found to be non-detectable, indicating an efficient internal cooling of the trapped ions. Based on the time-of-flight peak profile and the image of N₃ ⁺, the velocity spread of the ions extracted from the trap, both radially and axially, is interpreted as approximately ±25 m/s. An experimental image of fragmented Ar⁺ from 307 nm photodissociation of Ar₂ ⁺ shows that, benefitting from the well-confined velocity spread of the cold Ar₂ ⁺ ions, a VMI resolution of Δv/v ∼ 2.2% has been obtained. The current instrument resolution is mainly limited by the residual radial speed spread of the parent ions after extraction from the trap.

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.

Development of a linear-type double reflectron for focused imaging of photofragment ions from mass-selected complex ions

An ion imaging apparatus with a double linear reflectron mass spectrometer has been developed, in order to measure velocity and angular distributions of mass-analyzed fragment ions produced by photodissociation of mass-selected gas phase complex ions. The 1st and the 2nd linear reflectrons were placed facing each other and controlled by high-voltage pulses in order to perform the mass-separation of precursor ions in the 1st reflectron and to observe the focused image of the photofragment ions in the 2nd reflectron. For this purpose, metal meshes were attached on all electrodes in the 1st reflectron, whereas the mesh was attached only on the last electrode in the 2nd reflectron. The performance of this apparatus was evaluated using imaging measurement of Ca⁺ photofragment ions from photodissociation reaction of Ca⁺Ar complex ions at 355 nm photoexcitation. The focused ion images were obtained experimentally with the double linear reflectron at the voltages of the reflection electrodes close to the predictions by ion trajectory simulations. The velocity and angular distributions of the produced Ca⁺ ([Ar] 4p¹, ²P_3/2) ion were analyzed from the observed images. The binding energy D₀ of Ca⁺Ar in the ground state deduced in the present measurement was consistent with those determined theoretically and by spectroscopic measurements. The anisotropy parameter β of the transition was evaluated for the first time by this instrument.