Velocity Map Ion Imaging Research Articles

Vibrational state-selected photodissociation of ClO+

We report on the UV photodissociation of specific vibrational states (v=2–45) of ClO+ using velocity map ion imaging. The high vibrational states of ClO+ are prepared via a double resonant scheme through the ClO (A 2Π) state and ion-pair states followed by photoionization with a third photon. The absorption of a fourth photon results in photodissociation of the ClO+ into two dominant asymptotic channels. The Cl+ and O+ fragment ion images reveal information on both the energetics of high-lying cation vibrational states and the low-lying dissociative electronic states that correlate to Cl+(3P)+O(3P) and Cl(2P)+O+(4S) asymptotic channels. We also report ab initio potentials for the bound ClO+ and ion-pair states as well as calculations of the ClO+ excited states relevant to the photodissociation process.

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

The paper reports experimental measurements and theoretical calculations of rotational-state-resolved differential scattering cross-sections (DCS) for collisions between electronically excited NO(A2Σ+) molecules and rare gas atoms. The experimental NO(A2Σ+) + Ar and NO(A2Σ+) + He state-resolved product scattering distributions are determined using velocity-mapped ion imaging. The ion images are analysed to determine the state-resolved DCS, which are compared with new theoretical DCS calculated using quantum scattering methods on ab initio electronic potential energy surfaces. Both collision systems are imaged simultaneously; this constraint on the collision energies of the two systems aids the comparison to theory. The experimental and calculated DCS agree well for the NO(A2Σ+)/He system. For the NO(A2Σ+)/Ar scattering system, the experiments do not recover the degree of forward-scattering theoretically predicted and significant differences in the positions of the observed and predicted rotational rainbow features are apparent at large scattering angles, particularly for the most rotationally inelastic collision channels investigated: ΔN = 12,14.

Photodissociation of 2-Bromobutane at ∼265 nm by Ion-velocity Map Imaging Technique

The photodissociation dynamics of 2-bromobutane has been investigated at 264.77 and 264.86 nm by ion-velocity map imaging technique coupled with resonance-enhanced multi-photon ionization. The speed and angular distributions have been derived from the velocity map images of Br and Br*. The speed distributions of Br and Br* atoms in the photodissociation of 2-bromobutane at ∼265 nm can be fitted using only one Gaussian function indicating that bromine fragments were produced via direct dissociation of C—Br bond. The contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products (Br and Br*) were discussed. It is found that the nonadiabatic 1Q1 ← 3Q0 transition plays an important role for Br photofragment in the dissociation of 2-C4H9Br at ∼265 nm. Relative quantum yield of 0.621 for Br(2P3/2) at ∼265 nm in the photodissociation of 2-bromobutane is derived. By comparing the photodissociation of 2-C4H9Br at ∼265 nm and that that at ∼234 nm, the anisotropy parameter β(Br) and β(Br*), and relative quantum yield Φ(Br) decrease with increasing wavelength, the probability of curve crossing between 3Q0 and 1Q1 decreases with increasing laser wavelength.

Branching ratio measurements of the predissociation of 12C16O by time-slice velocity-map ion imaging in the energy region from 108 000 to 110 500 cm−1

Direct branching ratio measurements of the three lowest dissociation channels of (12)C(16)O that produce C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) are reported in the vacuum ultraviolet region from 108,000 cm(-1) (92.59 nm) to 110,500 cm(-1) (90.50 nm) using the time-slice velocity-map ion imaging and nonlinear resonant four-wave mixing techniques. Rotationally, resolved carbon ion yield spectra for both (1)Σ(+) and (1)Π bands of CO in this region have been obtained. Our measurements using this technique show that the branching ratio in this energy region, especially the relative percentages of the two spin-forbidden channels, is strongly dependent on the particular electronic and vibrational energy levels of CO that are excited.

Electronic polarization effects in the photodissociation of Cl2

Velocity mapped ion imaging and resonantly enhanced multiphoton ionization time-of-flight methods have been used to investigate the photodissociation dynamics of the diatomic molecule Cl(2) following excitation to the first UV absorption band. The experimental results presented here are compared with high level time dependent wavepacket calculations performed on a set of ab initio potential energy curves [D. B. Kokh, A. B. Alekseyev, and R. J. Buenker, J. Chem. Phys. 120, 11549 (2004)]. The theoretical calculations provide the first determination of all dynamical information regarding the dissociation of a system of this complexity, including angular momentum polarization. Both low rank K = 1, 2 and high rank K = 3 electronic polarization are predicted to be important for dissociation into both asymptotic product channels and, in general, good agreement is found between the recent theory and the measurements made here, which include the first experimental determination of high rank K = 3 orientation.

Time-slice velocity-map ion imaging studies of the photodissociation of NO in the vacuum ultraviolet region

The time-slice velocity-map ion imaging and the resonant four-wave mixing techniques are combined to study the photodissociation of NO in the vacuum ultraviolet (VUV) region around 13.5 eV above the ionization potential. The neutral atoms, i.e., N((2)D(o)), O((3)P(2)), O((3)P(1)), O((3)P(0)), and O((1)D(2)), are probed by exciting an autoionization line of O((1)D(2)) or N((2)D(o)), or an intermediate Rydberg state of O((3)P(0,1,2)). Old and new autoionization lines of O((1)D(2)) and N((2)D(o)) in this region have been measured and newer frequencies are given for them. The photodissociation channels producing N((2)D(o)) + O((3)P), N((2)D(o)) + O((1)D(2)), N((2)D(o)) + O((1)S(0)), and N((2)P(o)) + O((3)P) have all been identified. This is the first time that a single VUV photon has been used to study the photodissociation of NO in this energy region. Our measurements of the angular distributions show that the recoil anisotropy parameters (β) for all the dissociation channels except for the N((2)D(o)) + O((1)S(0)) channel are minus at each of the wavelengths used in the present study. Thus direct excitation of NO by a single VUV photon in this energy region leads to excitation of states with Σ or Δ symmetry (ΔΩ = ±1), explaining the observed perpendicular transition.

The ultraviolet photodissociation of CS2: The S(1D2) channel

The photodissociation of CS(2) has been investigated using velocity-map ion imaging of the S((1)D(2)) atomic photofragments following excitation at 193 nm and at longer wavelengths close to the S((1)D(2)) channel threshold. The experiments probe regions both above and below the energetic barrier to linearity on the (1)Σ(u) (+)((1)B(2)) potential energy surface. The imaging data in both regions indicate that the electronic angular momentum of the S((1)D(2)) atom products is unpolarized, but also reveal different dissociation dynamics in the two regions. Excitation above the barrier to linearity yields an inverted CS((1)Σ(+)) vibrational population distribution, whereas the long-wavelength state-to-state results following excitation below the barrier reveal CS((1)Σ(+))(v, J) coproduct state distributions which are consistent with a statistical partitioning of the energy. Below the barrier, photofragment excitation spectra point to an enhancement of the singlet channel for K = 1, relative to K = 0, where K is the projection of the angular momentum along the principal axis, in agreement with previous work. However, the CS cofragment product state distributions are found to be insensitive to K. It is proposed that dissociation below the barrier to linearity occurs primarily on a surface with a significant potential energy well and without an exit channel barrier, such as that for the ground electronic state. However, oscillatory structure is also observed in the kinetic energy release distributions, which is shown to be consistent with a mapping of parent molecule bending motion. This could indicate the operation of competing direct and indirect dissociation mechanisms below the barrier to linearity.

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.

Combining two VUV lasers with the time-sliced velocity mapped ion imaging method for studying the photodissociation of small molecules

A new experimental setup is described that allows us to study the state-selected photodissociation dynamics of small molecules of astrophysical interest using two independently tunable VUV radiation sources.

Competing 1πσ* mediated dynamics in mequinol: O–H versus O–CH3 photodissociation pathways

Deactivation of excited electronic states through coupling to dissociative (1)πσ* states in heteroaromatic systems has received considerable attention in recent years, particularly as a mechanism that contributes to the ultraviolet (UV) photostability of numerous aromatic biomolecules and their chromophores. Recent studies have expanded upon this work to look at more complex species, which involves understanding competing dynamics on two different (1)πσ* potential energy surfaces (PESs) localized on different heteroatom hydride coordinates (O-H and N-H bonds) within the same molecule. In a similar spirit, the work presented here utilizes ultrafast time-resolved velocity map ion imaging to study competing dissociation pathways along (1)πσ* PESs in mequinol (p-methoxyphenol), localized at O-H and O-CH(3) bonds yielding H atoms or CH(3) radicals, respectively, over an excitation wavelength range of 298-238 nm and at 200 nm. H atom elimination is found to be operative via either tunneling under a conical intersection (CI) (298 ≥ λ ≥ 280 nm) or ultrafast internal conversion through appropriate CIs (λ ≤ 245 nm), both of which provide mechanisms for coupling onto the dissociative state associated with the O-H bond. In the intermediate wavelength range of 280 ≥ λ ≥ 245 nm, mediated H atom elimination is not observed. In contrast, we find that state driven CH(3) radical elimination is only observed in the excitation range 264 ≥ λ ≥ 238 nm. Interpretation of these experimental results is guided by: (i) high level complete active space with second order perturbation theory (CASPT2) calculations, which provide 1-D potential energy cuts of the ground and low lying singlet excited electronic states along the O-H and O-CH(3) bond coordinates; and (ii) calculated excitation energies using CASPT2 and the equation-of-motion coupled cluster with singles and doubles excitations (EOM-CCSD) formalism. From these comprehensive studies, we find that the dynamics along the O-H coordinate generally mimic H atom elimination previously observed in phenol, whereas O-CH(3) bond fission in mequinol appears to present notably different behavior to the CH(3) elimination dynamics previously observed in anisole (methoxybenzene).

Communication: Branching ratio measurements in the predissociation of 12C16O by time-slice velocity-map ion imaging in the vacuum ultraviolet region

The first direct branching ratio measurement of the three lowest energy dissociation channels of CO that produce C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) is reported. Rotational resolved carbon ion yield spectra for two Π bands (W(3sσ)(1)Π (v(') = 3) at 108,012.6 cm(-1) and (1)Π(v(') = 2) at 109,017 cm(-1)) and two Σ bands ((4sσ)(1)Σ(+)(v(') = 4) at 109,452 cm(-1) and (4pσ)(1)Σ(+)(v(') = 3) at 109,485 cm(-1)) of CO were obtained. Our measurements show that the branching ratio in this energy region is strongly dependent on the electronic and vibrational energy but it is independent or just weakly dependent on the parity and rotational energy levels. To our knowledge, this is the first time that the triplet channel producing O((1)D) has been experimentally observed and this is also the first time that a direct measurement of the branching ratio for the different channels in the predissociation of CO in this energy region has been made.

Photodissociation of 2-Bromobutane by Ion-velocity Map Imaging Technique

The photodissociation dynamics of 2-bromobutane has been investigated at 233.62 and 233.95 nm by ion-velocity map imaging technique coupled with resonance-enhanced multiphoton ionization. The speed and angular distribution of Br and Br* fragments were determined from the map images. The two Gaussian components, shown in the speed distributions of Br and Br* atoms, are suggested to attribute to the two independent reaction paths of photodissociation for 2-bromobutane at 233.62 and 233.95 nm. The high-energy component is related to the prompt dissociation along the C-Br stretching mode, and the low-energy component to the dissociation from the repulsive mode with bending and C-Br stretching combination. The contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products (Br and Br*) were discussed. Relative quantum yield of 0.924 for Br(2P3/2) at about 234 nm in the photodissociation of 2-bromobutane is derived.

Dissociation energy and vibrational predissociation dynamics of the ammonia dimer

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (ν(NH,S)) or the intramolecular antisymmetric N-H stretch (ν(NH,A)) of the ammonia dimer ((NH(3))(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D(0) = 660 ± 20 cm(-1) for the dimer. Fitting the distributions shows that rotations are excited up to their energetic limit and determines the correlation of the fragment vibrations. The fragments NH(3)(ν(2) = 3(+)) and NH(3)(ν(2) = 2(+)) have a vibrational ground-state partner NH(3)(ν = 0), but NH(3)(ν(2) = 1(+)) appears in partnership with another fragment in ν(2) = 1. This propensity is consistent with the idea of minimizing the momentum gap between the initial and final states by depositing a substantial fraction of the available energy into internal excitation.

Communication: Imaging wavefunctions in dissociative photoionization

The dissociative ionization dynamics of excited electronic states of the xenon dimer, Xe(2), have been studied using velocity map ion imaging (VMI). A one-colour, (2+1) resonant excitation scheme was employed to first excite and then ionize selected vibrational levels of the Xe(2) 6p (2)[(1)∕(2)](0) 0(g)(+) Rydberg state. Cationic fragments were then detected by the VMI. The data provide an outstanding example of the reflection principle in photodissociation with the full nodal structure of the Rydberg state wavefunctions clearly observed in the final Xe(+) kinetic energy distributions without the need for scanning the excitation energy. Fitting of the observed distributions provides detailed and precise information on the form of the Xe(2)(+) I((1)/(2)g) potential energy curve involved which is in excellent agreement with the results of photoelectron imaging studies [Shubert and Pratt, J. Chem. Phys. 134, 044315 (2011)]. Furthermore, the anisotropy of the product angular distributions yields information on the evolution of the electronic character of the ionic state with internuclear separation, R. The combination of the nature of dissociative ionization and the extent of the bound state wavefunctions provide information over an unusually wide range of internuclear separation R (ΔR > 0.75 Å). This would normally require scanning over a considerable energy region but is obtained in these studies at a fixed excitation energy.

Communication: Vacuum ultraviolet laser photodissociation studies of small molecules by the vacuum ultraviolet laser photoionization time-sliced velocity-mapped ion imaging method

We demonstrate that the vacuum ultraviolet (VUV) photodissociation dynamics of N(2) and CO(2) can be studied using VUV photoionization with time-sliced velocity-mapped ion imaging (VUV-PI-VMI) detection. The VUV laser light is produced by resonant sum frequency mixing in Kr. N(2) is used to show that when the photon energy of the VUV laser is above the ionization energy of an allowed transition of one of the product atoms it can be detected and characterized as the wavelength is varied. In this case a β parameter = 0.57 for the N((2)D°) was measured after exciting N(2)(o(1)Π(u), v(') = 2, J(') = 2) ← N(2)(X(1)Σ(g) (+), v(") = 0, J(") = 1). Studies with CO(2) show that when there is no allowed transition, an autoionization resonance can be used for the detection of a product atom. In this case it is shown for the first time that the O((1)D) atom is produced with CO((1)Σ(+)) at 92.21 nm. These results indicate that the VUV laser photodissociation combined with the VUV-PI-VMI detection is a viable method for studying the one-photon photodissociation from the ground state of simple molecules in the extreme ultraviolet and VUV spectral regions.

Energy transfer of highly vibrationally excited naphthalene: Collisions with CHF3, CF4, and Kr

Energy transfer of highly vibrationally excited naphthalene in the triplet state in collisions with CHF(3), CF(4), and Kr was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. Highly vibrationally excited naphthalene (2.0 eV vibrational energy) was formed via the rapid intersystem crossing of naphthalene initially excited to the S(2) state by 266 nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited naphthalene. In comparison to Kr atoms, the energy transfer in collisions between CHF(3) and naphthalene shows more forward scatterings, larger cross section for vibrational to translational (V → T) energy transfer, smaller cross section for translational to vibrational and rotational (T → VR) energy transfer, and more energy transferred from vibration to translation, especially in the range -ΔE(d) = -100 to -800 cm(-1). On the other hand, the difference of energy transfer properties between collisional partners Kr and CF(4) is small. The enhancement of the V → T energy transfer in collisions with CHF(3) is attributed to the large attractive interaction between naphthalene and CHF(3) (1-3 kcal/mol).

Exploring Ultrafast H-Atom Elimination versus Photofragmentation Pathways in Pyrazole Following 200 nm Excitation

The role of ultraviolet photoresistance in many biomolecules (e.g., DNA bases and amino acids) has been extensively researched in recent years. This behavior has largely been attributed to the participation of dissociative (1)πσ* states localized along X-H (X ═ N, O) bonds, which facilitate an efficient means for rapid nonradiative relaxation back to the electronic ground state via conical intersections or ultrafast H-atom elimination. One such species known to exhibit this characteristic photochemistry is the UV chromophore imidazole, a subunit in the biomolecules adenine and histidine. However, the (1)πσ* driven photochemistry of its structural isomer pyrazole has received much less attention, both experimentally and theoretically. Here, we probe the ultrafast excited state dynamics occurring in pyrazole following photoexcitation at 200 nm (6.2 eV) using two experimental methodologies. The first uses time-resolved velocity map ion imaging to investigate the ultrafast H-atom elimination dynamics following direct excitation to the lowest energy (1)πσ* state (1(1)A" ← X(1)A'). These results yield a bimodal distribution of eliminated H-atoms, situated at low and high kinetic energies, the latter of which we attribute to (1)πσ* mediated N-H fission. The time constants extracted for the low and high energy features are ~120 and <50 fs, respectively. We also investigate the role of ring deformation relaxation pathways from the first optically bright (1)ππ* state (2(1)A' ← X(1)A'), by performing time-resolved ion yield measurements. These results are modeled using a (1)ππ* → ring deformation → photofragmentation mechanism (a model based on comparison with theoretical calculations on the structural isomer imidazole) and all photofragments possess appearance time constants of <160 fs. A comparison between time-resolved velocity map ion imaging and time-resolved ion yield measurements suggest that (1)πσ* driven N-H fission gives rise to the dominant kinetic photoproducts, re-enforcing the important role (1)πσ* states have in the excited state dynamics of biological chromophores and related aromatic heterocycles.

Alkylation Effects on the Energy Transfer of Highly Vibrationally Excited Naphthalene

The energy transfer of highly vibrationally excited isomers of dimethylnaphthalene and 2-ethylnaphthalene in collisions with krypton were investigated using crossed molecular beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques at a collision energy of approximately 300 cm(-1). Angular-resolved energy-transfer distribution functions were obtained directly from the images of inelastic scattering. The results show that alkyl-substituted naphthalenes transfer more vibrational energy to translational energy than unsubstituted naphthalene. Alkylation enhances the V→T energy transfer in the range -ΔE(d)=-100~-1500 cm(-1) by approximately a factor of 2. However, the maximum values of V→T energy transfer for alkyl-substituted naphthalenes are about 1500~2000 cm(-1), which is similar to that of naphthalene. The lack of rotation-like wide-angle motion of the aromatic ring and no enhancement in very large V→T energy transfer, like supercollisions, indicates that very large V→T energy transfer requires special vibrational motions. This transfer cannot be achieved by the low-frequency vibrational motions of alkyl groups.

Photodissociation dynamics of Cl 2O at 235 nm using velocity map ion imaging

We have investigated the photodissociation dynamics of expansion-cooled Cl 2O at 235 nm using photofragment velocity map ion imaging coupled with resonance-enhanced multiphoton ionization (REMPI) to measure the speed and angular distributions of the Cl( 2P 3/2) and Cl( 2P 1/2) products. We observe speed-dependent spatial anisotropy for both Cl fragments from the imaging data and attribute this to a fraction of nascent ClO, which undergoes secondary photodissociation. We have employed a Monte–Carlo forward-convolution scheme to analyze the data and obtain the speed distribution and anisotropy parameters for the ClO + Cl( 2P 3/2,1/2) channels. We find that the speed distribution of the Cl( 2P 3/2) + ClO channel is bimodal with an anisotropy parameter of 1.2 ± 0.2, whereas the speed distribution of the Cl( 2P 1/2) + ClO channel consists of a single component with an anisotropy parameter of 1.0 ± 0.2. We have determined branching ratios for Cl( 2P 3/2) + ClO and Cl( 2P 1/2) + ClO channels to be 0.97 ± 0.02:0.03 ± 0.02. Based on the translational energy distribution of Cl( 2P 3/2) + ClO channel, 28% of the parent molecule undergoes three-body dissociation.

Ion Imaging Study of NO3Radical Photodissociation Dynamics: Characterization of Multiple Reaction Pathways

The photodissociation of NO(3) has been studied using velocity map ion imaging. Measurements of the NO(2) + O channel reveal statistical branching ratios of the O((3)P(J)) fine-structure states, isotropic angular distributions, and low product translational energy consistent with barrierless dissociation along the ground state potential surface. There is clear evidence for two distinct pathways to the formation of NO + O(2) products. The dominant pathway (>70% yield) is characterized by vibrationally excited O(2)((3)Σ(g)(-), v = 5-10) and rotationally cold NO((2)Π), while the second pathway is characterized by O(2)((3)Σ(g)(-), v = 0-4) and rotationally hotter NO((2)Π) fragments. We speculate the first pathway has many similarities to the "roaming" dynamics recently implicated in several systems. The rotational angular momentum of the molecular fragments is positively correlated for this channel, suggesting geometric constraints in the dissociation. The second pathway results in almost exclusive formation of NO((2)Π, v = 0). Although product state correlations support dissociation via an as yet unidentified three-center transition state, theoretical confirmation is needed.