Time-sliced Velocity Map Imaging Research Articles

Note: Vibrationally mediated photodissociation of carbon dioxide cation

The photodissociation dynamics of carbon dioxide cation, CO2(+), mediated by its different Ã(2)Πu,1/2(υ1,υ2,0) vibronic states has been investigated by means of time-sliced velocity map imaging. Through analysis of the recorded translational energy release spectra of photofragment CO(+), we found that the photodissociation of CO2(+) exhibits drastic change in a rather narrow energy region. A conformational barrier in the CO2(+)(Ã(2)A1) state is suggested to be ∼5600 cm(-1) relative to the CO2(+)(Ã(2)Πu,1/2(0,0,0)) state, in reasonable agreement with previous prediction.

Accessing Multiple Conical Intersections in the 3s and 3px Photodissociation of the Hydroxymethyl Radical

The photodissociation dynamics of the hydroxymethyl radical (CH2OH, CH2OD, and CD2OD) following excitation to the 3s and 3p(x) Rydberg states is studied using time-sliced velocity map imaging of hydrogen photofragments. Dissociation takes place on the ground potential energy surface reached via conical intersections from the excited states, and formaldehyde and hydrxymethylene are identified as reaction products. The major product, formaldehyde, has a bimodal internal energy distribution. The largest fraction has high kinetic energy (KE), modest rotational excitation, and vibrational excitation mainly in the CO stretch and the CH(D)2 deformations modes (scissors, wag, and rock). The minor fraction has lower KEs and a higher rovibrational excitation that is unresolved. A bimodal internal energy distribution in the formaldehyde fragment has been predicted by Yarkony [J. Chem. Phys. 2005, 122, 084316] for a conical intersection along the O-H bond coordinate. The hydroxymethylene product state distributions depend strongly on the nature of the excited state. In dissociation via the 3s state, the hydroxymethylene products have broad rovibrational state distributions and are produced at low yield. As suggested by Yarkony, they may be produced in the same dissociation events that give rise to low KE formaldehyde. In these events, the bound region of the PES is sampled following the conical intersection along O-H(D). The hydroxymethylene yield is low near its threshold and increases slowly with excitation energy to the 3s state, but its internal energy distribution remains broad and the contributions of the cis- and trans-isomers cannot be resolved. The mechanism changes markedly when exciting to the 3p(x) state. The hydroxymethylene products have less rotational excitation and show separate contributions of cis- and trans-isomers. The trans-isomer is found to be a minor product relative to the higher-energy cis-isomer, as predicted by Yarkony for conical intersections along the C-H coordinate. It appears that the efficiency of dissociation via conical intersections along the O-H and C-H coordinates depends on the initial excited state. While the O-H conical intersection seam (vertical cone) provides an efficient route to the ground state following excitation via the 3s or the 3p(x) Rydberg states, conical intersections along the C-H bond coordinate (tilted cone) are sampled more efficiently via 3p(x) excitation and proceed through different dynamics. The energy separations between formaldehyde and hydroxymethylene and between the cis- and trans-isomers of hydroxymethylene are determined experimentally for all the investigated isotopologs and are in good agreement with theory.

Improved sliced velocity map imaging apparatus optimized for H photofragments.

Time-sliced velocity map imaging (SVMI), a high-resolution method for measuring kinetic energy distributions of products in scattering and photodissociation reactions, is challenging to implement for atomic hydrogen products. We describe an ion optics design aimed at achieving SVMI of H fragments in a broad range of kinetic energies (KE), from a fraction of an electronvolt to a few electronvolts. In order to enable consistently thin slicing for any imaged KE range, an additional electrostatic lens is introduced in the drift region for radial magnification control without affecting temporal stretching of the ion cloud. Time slices of ∼5 ns out of a cloud stretched to ⩾50 ns are used. An accelerator region with variable dimensions (using multiple electrodes) is employed for better optimization of radial and temporal space focusing characteristics at each magnification level. The implemented system was successfully tested by recording images of H fragments from the photodissociation of HBr, H2S, and the CH2OH radical, with kinetic energies ranging from <0.4 eV to >3 eV. It demonstrated KE resolution ≲1%-2%, similar to that obtained in traditional velocity map imaging followed by reconstruction, and to KE resolution achieved previously in SVMI of heavier products. We expect it to perform just as well up to at least 6 eV of kinetic energy. The tests showed that numerical simulations of the electric fields and ion trajectories in the system, used for optimization of the design and operating parameters, provide an accurate and reliable description of all aspects of system performance. This offers the advantage of selecting the best operating conditions in each measurement without the need for additional calibration experiments.

Mode specific photodissociation of CS2+via the A2Πu state: a time-sliced velocity map imaging study

The vibrationally mediated photodissociation of CS(2)(+) cations via the A(2)Π(u)(ν(1),ν(2),0) state has been studied by means of the velocity map ion imaging technique. The measurements were made with a double resonance strategy. The CS(2)(+) cations were prepared by a (3 + 1) resonance enhanced multiphoton ionization method. The photo-fragment excitation spectrum of S(+) was recorded by scanning the photolysis laser via the A(2)Π(u)(ν(1),ν(2),0) state. By fixing the photolysis laser wavelength at the specific vibrational state, the (1 + 1) photodissociation images of S(+) photofragments from numerous vibrationally mediated states have been accumulated. The translational energy release spectra derived from the resulting images imply that the co-fragments, CS radicals, are both vibrationally and rotationally excited. The one-photon photodissociation without the vibrational state selection has also been performed. Comparing the vibrationally mediated photodissociation with one-photon photodissociation observations, clear evidence of vibrational state control of the photodissociation process is observed.

Multiphoton dissociative ionization of tert-pentyl bromide near 265 nm

We report on the photodissociation dynamics of tert-pentyl bromide near 265 nm investigated by time-sliced velocity map imaging. The speed and angular distributions have been analyzed for both the ground-state Br((2)P(3∕2)) atom (denoted Br) and the spin-orbit excited-state Br((2)P(1∕2)) atom (denoted Br*). The speed distributions of Br and Br* atoms are all found to consist of three Gaussian components, which correlate to three independent dissociation pathways on the excited potential energy surfaces: (1) the high translational energy (E(T)) component from the prompt dissociation along the C-Br stretching mode, (2) the middle E(T) component from the repulsive mode along the C-Br stretching coupled with some bending motions, and (3) the low E(T) component from the repulsive mode along the C-Br stretching coupled with more bending motions. More interestingly, we have also observed the tert-C(5)H(11)(+) ions in 263-267 nm. The near-zero kinetic energy distributions extracted from the three tert-C(5)H(11)(+) images near 265 nm show the typical characteristics that are attributable to multiphoton dissociative ionization, suggesting the existence of a neutral superexcited state of the parent tert-pentyl bromide molecule. The contribution of bromine atoms formed in this dissociative ionization channel adds in the total relative distribution of low E(T) component in the Br*(Br) formation channel, which reasonably explains the abnormal distributions observed in between the middle and low E(T) components in the Br*(Br) formation channel.

Note: Single-ultraviolet-photon dissociation dynamics of ${\rm CS}_{\rm 2}^ + (\tilde X{}^2\Pi _g)$ CS 2+(X̃2Πg) in 227–243 nm revealed by time-sliced velocity map imaging

The single-UV-photon (227–243 nm) dissociation of ${\rm CS}_2^ + (\tilde X{}^2\Pi _{g,3/2,1/2})$ CS 2+(X̃2Πg,3/2,1/2) into the S+(4S)CS(X1)Σ+ channel was investigated by means of time-sliced velocity map imaging technique. In the high-energy region of the $\tilde B{}^2\Sigma _u^ +$B̃2Σu+ state, a rather subtle photodissociation dynamics was observed, i.e., a slight variation in the excitation photon energy may lead to a drastic change in the vibronic distributions of the CS(X1Σ+) fragments originating from the two spin-orbit $\tilde X{}^{\rm 2}\Pi _g$X̃2Πg states. The photodissociation mechanisms involved here may be interpreted as being due to the complex interplay among various vibronic and spin-orbit interactions in this energy region of ${\rm CS}_{\rm 2}^ +$ CS 2+. It is quite impressive that, for such a simple triatomic cation species, ${\rm CS}_{\rm 2}^ +$ CS 2+, there exhibits such complexity and subtlety in its photodissociation dynamics.

1 + 1] photodissociation of ${\rm CS}_{\rm 2}^ + (\tilde X{}^{\rm 2}\Pi _g)$CS2+(X̃2Πg) via the vibrationally mediated $\tilde B^{\rm 2} \Sigma _u^ +$B̃2Σu+ state: Multichannels exhibiting and mode specific dynamics

Dissociation dynamics of CS(2)(+) vibrationally mediated via its B̃(2)Σ(u)(+) state, was studied using the time-sliced velocity map imaging technique. The parent CS(2)(+) cation was prepared in its X̃(2)Π(g) ground state through a [3 + 1] resonance enhanced multiphoton ionization process, via the 4pσ(3)Π(u) intermediate Rydberg state of neutral CS(2) molecule at 483.14 nm. CS(2)(+)(X̃(2)Π(g)) was dissociated by a [1 + 1] photoexcitation mediated via the vibrationally selected B̃ state over a wavelength range of 267-283 nm. At these wavelengths the C̃(2)Σ(g)(+) and D̃(2)Σ(u)(+) states are excited, followed by numerous S(+) and CS(+) dissociation channels. The S(+) channels specified as three distinct regions were shown with vibrationally resolved structures, in contrast to the less-resolved structures being presented in the CS(+) channels. The average translational energy releases were obtained, and the S(+)∕CS(+) branching ratios with mode specificity were measured. Two types of dissociation mechanisms are proposed. One mechanism is the direct coupling of the C̃ and D̃ states with the repulsive satellite states leading to the fast photofragmentation. The other mechanism is the internal conversion of the C̃ and D̃ states to the B̃ state, followed by the slow fragmentation occurred via the coupling with the repulsive satellite states.

A new crossed molecular beam apparatus using time-sliced ion velocity imaging technique

A new crossed molecular beam apparatus has been constructed for investigating polyatomic chemical reactions using the time-sliced ion velocity map imaging technique. A unique design is adopted for one of the two beam sources and allows us to set up the molecular beam source either horizontally or vertically. This can be conveniently used to produce versatile atomic or radical beams from photodissociation and as well as electric discharge. Intensive H-atom beam source with high speed ratio was produced by photodissociation of the HI molecule and was reacted with the CD(4) molecule. Vibrational-state resolved HD product distribution was measured by detecting the CD(3) product. Preliminary results were also reported on the F+SiH(4) reaction using the discharged F atom beam. These results demonstrate that this new instrument is a powerful tool for investigating chemical dynamics of polyatomic reactions.