Velocity Map Ion Imaging Technique Research Articles

Imaging the Pair-Correlated HNCO Photodissociation: The NH(a1Δ) + CO(X1Σ+) Channel

The NH(a(1)Δ) + CO(X(1)Σ(+)) product channel for the photodissociation of HNCO at 201 nm was investigated using the sliced velocity map ion imaging technique with the detection of NH(a(1)Δ) products via (2 + 1) resonance enhanced multiphoton ionization (REMPI). Images were measured for the NH(a(1)Δ) rotational states in the ground and vibrational excited states (v = 0 and 1). Correlation between the NH(a(1)Δ) and CO rovibrational state distributions were determined from these images. Experimental results show that the vibrational distribution of the CO fragment in the NH(a(1)Δ) + CO(X(1)Σ(+)) channel peaks at v = 1. The negative anisotropy parameter measured for the NH(a(1)Δ) (v = 0 and 1|j) products indicates a direct dissociation process for the N-C bond cleavage in the S1 state. A bimodal CO rotational distribution was observed, suggesting that HNCO dissociates in the S1 state in two distinctive pathways.

Conical intersection seam and bound resonances embedded in continuum observed in the photodissociation of thioanisole-d3

Herein, the multi-dimensional nature of the conical intersection seam has been experimentally revealed in the photodissociation reaction of thioanisole-d3 (C6H5SCD3) excited on S1, giving C6H5S·(Ã or X̃]) +·CD3 products. The translational energy distribution of the nascent·CD3 fragment, reflecting the relative yields of the C6H5S·(Ã) and C6H5S·(X̃) products, was measured at each S1 vibronic band using the velocity map ion imaging technique. Direct access of the reactant flux to the conical intersection seam leads to the increase of the nonadiabatic transition probability resulting in sharp resonances in the X̃/ÃC6H5S·product branching ratio at several distinct S1 vibronic bands. The nature of the S1 vibronic bands associated with such dynamic resonances was clarified by the mass-analyzed threshold ionization spectroscopy. The bound state embedded in continuum generated by the conical intersection is observed as a distinct dynamic resonance, revealing the nature of the nuclear motion responsible for the nonadiabatic coupling of two potential energy surfaces at the conical intersection. The multi-dimensional facets of the conical intersection seam in terms of its detailed structure and dynamic role are discussed with the aid of theoretical calculations.

Using Ion-velocity Map Imaging Technique to Study Photodissociation of 2-Bromopentane

The photodissociation dynamics of 2-bromopentane at ∼234 nm has been investigated by utilizing ion-velocity map imaging technique. The mapped images of Br(2P3/2) (denoted as Br) and Br(2P1/2) (denoted as Br*) fragments were analyzed by means of the speed and angular distributions, respectively. The speed distributions can be fitted with two Gaussian components which are correlated to the two independent reaction paths on the excited potential energy surfaces (PES). The high-energy component is from the prompt dissociation along the C—Br stretching mode, while the low-energy one is related to the dissociation from the coupling of the C—Br stretching and bending modes. Relative quantum yield is measured to be 0.892 for Br in the photodissociation of 2-bromopentane at 234 nm. Combining the anisotropy parameter with the relative quantum yield of Br and Br* fragments, the contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products Br and Br* were derived. The effect of alkyl branching on the mechanism of photodissociation was discussed by comparing the photodissociation processes of four isomers of bromopentane.

Exploring quantum phenomena and vibrational control in σ* mediated photochemistry

Non-adiabatic dynamics involving 1πσ* or 1nσ* excited electronic states play a key role in the photochemistry of numerous heteroatom containing aromatic (bio-)molecules. In this contribution, we investigate more exotic phenomena involved in σ* mediated dynamics, namely: (i) the role of purely quantum mechanical behavior; and (ii) manipulating non-adiabatic photochemistry through conical intersections (CIs) with ‘vibration-specific control’. This is achieved by investigating S–CH3 bond fission via a 1nσ* potential energy surface (PES) in thioanisole (C6H5SCH3). Using a combination of time- and frequency-resolved velocity map ion imaging techniques, together with ab initio calculations, we demonstrate that excitation to the 1ππ* ← S0 origin [1ππ*(ν = 0)] results in S–CH3 bond fission on the 1nσ* PES, despite an (apparent) energetic barrier to dissociation formed by a CI between the 1ππ* and 1nσ* PESs. This process occurs by accessing ‘classically forbidden’ regions of the excited state potential energy landscape where the barrier to dissociation becomes negligible, aided by torsional motion of the S–CH3 group out of the plane of the phenyl ring. Control over these dynamics is attained by populating a single quantum of the S–CH3 stretch mode in the 1ππ* state [1ππ*(ν7a = 1)], which mirrors the nuclear motion required to promote coupling through the 1ππ*/1nσ* CI, resulting in a marked change in the electronic branching in the C6H5S radical products. This observation offers an elegant contribution towards a vision of ‘quantum control’ in photo-initiated chemical reaction dynamics.

Probing ultrafast dynamics in photoexcited pyrrole: timescales for 1πσ* mediated H-atom elimination

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250-200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1(1)pisigma* (1A2) state; and (ii) initial photoexcitation to the higher energy 1 pipi* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1(1)pisigma* (1A2) at 250 nm, we observe a single time-constant of 126 +/- 28 fs for N-H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1(1)pisigma* (1A2) surface in the vertical Franck-Condon region, followed by non-adiabatic coupling through a 1(1)pisigma*/S(0) conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N(X)) together with H-atoms. At 238 nm, direct excitation to, and N-H dissociation along, the 1(1)pisigma* (1A2) surface is observed to occur with a time-constant of 46 +/- 22 fs. Upon initial population of the 1pipi* (1B2) state at 200 nm, a rapid 1pipi* (1B2) --> 1(1)pisigma* (1A2) --> N-H fission process takes place within 52 +/- 12 fs. In addition to ultrafast N-H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S(0) ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 +/- 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C-H bond cleavage.

The vibrationally mediated photodissociation of Cl2

The photodissociation of vibrationally excited Cl(2)(v = 1) has been investigated experimentally using the velocity mapped ion imaging technique. The experimental measurements presented here are compared with the results of time-dependent wavepacket calculations performed on a set of ab initio potential energy curves. The high level calculations allow prediction of all the dynamical information regarding the dissociation, including electronic polarization effects. Using a combination of theory and experiment it was found that there was negligible cooling of the vibrational degree of freedom of the parent molecule in the molecular beam. The results presented are compared with those following the photodissociation of Cl(2)(v = 0). Although the same electronic states are found to be important for Cl(2)(v = 1) as for Cl(2)(v = 0), significant differences were found regarding many of the observables. The overall level of agreement between theory and experiment was found to be reasonable and confirms previous assignments of the photodissociation mechanism.

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.

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.

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.

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).

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.

Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

The SH + CH(3) product channel for the photodissociation of CH(3)SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH(3) products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH(3) formed in the ground and excited vibrational states (v(2) = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH(3) channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH(3) (v(2) = 0, 1, and 2) products is indicative of a fast dissociation process for C-S bond cleavage. Two kinds of slower CH(3) products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH(3)S (X(2)E) radical via cleavage of the S-H bond in CH(3)SH, followed by probe laser photodissociation of nascent CH(3)S radicals yielding CH(3)(X(2)A(1), v(2) = 0-2) + S((3)P(j)/(1)D) products.

Energy transfer of highly vibrationally excited phenanthrene and diphenylacetylene

The energy transfer between Kr atoms and highly vibrationally excited, rotationally cold phenanthrene and diphenylacetylene in the triplet state was investigated using crossed-beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques. Compared to the energy transfer between naphthalene and Kr, energy transfer between phenanthrene and Kr shows a larger cross-section for vibrational to translational (V → T) energy transfer, a smaller cross-section for translational to vibrational and rotational (T → VR) energy transfer, and more energy transferred from vibration to translation. These differences are further enlarged in the comparison between naphthalene and diphenylacetylene. In addition, less complex formation and significant increases in the large V → T energy transfer probabilities, termed supercollisions in diphenylacetylene and Kr collisions were observed. The differences in the energy transfer between these highly vibrationally excited molecules are attributed to the low-frequency vibrational modes, especially those vibrations with rotation-like wide-angle motions.

Energy transfer of highly vibrationally excited biphenyl

The energy transfer between Kr atoms and highly vibrationally excited, rotationally cold biphenyl in the triplet state was investigated using crossed-beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques. Compared to the energy transfer of naphthalene, energy transfer of biphenyl shows more forward scattering, less complex formation, larger cross section for vibrational to translational (V→T) energy transfer, smaller cross section for translational to vibrational and rotational (T→VR) energy transfer, larger total collisional cross section, and more energy transferred from vibration to translation. Significant increase in the large V→T energy transfer probabilities, termed supercollisions, was observed. The difference in the energy transfer of highly vibrationally excited molecules between rotationally cold naphthalene and rotationally cold biphenyl is very similar to the difference in the energy transfer of highly vibrationally excited molecules between rotationally cold naphthalene and rotationally hot naphthalene. The low-frequency vibrational modes with out-of-plane motion and rotationlike wide-angle motion are attributed to make the energy transfer of biphenyl different from that of naphthalene.

Exploring the Time Scales of H-Atom Elimination from Photoexcited Indole

Recent spectroscopic measurements have shown that following excitation of indole molecules above the (1)pipi*-(1)pisigma* conical intersection, photoinduced N-H bond cleavage results in a range of H-atom kinetic energy release. H-atoms with large amounts of kinetic energy were attributed to direct dissociation whereas those with low kinetic energy were attributed to indirect pathways such as statistical unimolecular decay. With use of a combination of femtosecond pump-probe spectroscopy and velocity map ion-imaging techniques, both energy and time-resolved photoinduced H-atom elimination at 200 nm has been measured. The results show that H-atoms with both high and low kinetic energies are generated on an ultrafast time scale, <200 fs, suggesting that on the time frame of our measurements (<200 ps) there appears to be a direct route to H-atom formation yielding H-atoms with low kinetic energies.

Photodissociation Dynamics of Thiophenol-d1: The Nature of Excited Electronic States along the S−D Bond Dissociation Coordinate

The S-D bond dissociation dynamics of thiophenol-d1 (C6H5SD) pumped at 266, 243, and 224 nm are examined using the velocity map ion imaging technique. At both 266 and 243 nm, distinct peaks associated with X and A states of the phenylthiyl radical (C6H5S*) are observed in the D+ image at high and low kinetic energy regions, respectively. The partitioning of the available energy into the vibrational energy of the phenylthiyl radical is found to be enhanced much more strongly at 266 nm compared to that at 243 nm. This indicates that the pipi* electronic excitation at 266 nm is accompanied by significant vibrational excitation. Given the relatively large anisotropy parameter of -0.6, the S-D dissociation at 266 nm is prompt and should involve the efficient coupling to the upper-lying n(pi)sigma* repulsive potential energy surface. The optical excitation of thiophenol at 224 nm is tentatively assigned to the pisigma* transition, which leads to the fast dissociation on the repulsive potential energy surface along the S-D coordinate. The nature of the electronic transitions associated with UV absorption bands is investigated with high-level ab initio calculations. Excitations to different electronic states of thiophenol result in unique branching ratios and vibrational excitations for the fragment of the phenylthiyl radical in the two lowest electronic states.

Exploring the Time-Scales of H-Atom Detachment from Photoexcited Phenol-h6 and Phenol-d5: Statistical vs Nonstatistical Decay

The prevalence of (1)pi sigma* states in the photochemistry of heteroaromatics is becoming increasingly clear from the recent literature. Photodissociation measurements have shown that following excitation of phenol molecules above the S(1)/S(2) conical intersection, H-atoms are eliminated with two distinct ranges of kinetic energy release. Those with high kinetic energy are attributed to direct dissociation while those with low kinetic energy are traditionally attributed to indirect dissociation or statistical unimolecular decay, both pathways giving electronic ground-state phenoxyl fragments. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination in phenol-h(6) and phenol-d(5), following excitation at 200 nm has been measured. At the lowest kinetic energies, the H-atom elimination from phenol-d(5) occurs in <150 fs, in sharp contrast to what one expects from a statistical decay process. This implies that these H-atoms are formed through a direct dissociation process yielding electronically excited phenoxyl fragments.

Probing free xenon clusters from within

Inner-shell and valence-shell photoionization of van-der-Waals Xe clusters have been measured using both angle-resolved photoelectron spectroscopy and velocity map ion imaging technique. Both techniques have been used to probe the electronic properties and the fragmentation dynamics of clusters as a function of photon energy and cluster size. In particular, the evolution of the photoelectron angular distributions and partial cross sections as a function of photon energy and cluster size has revealed cluster size effects similar to the ones found in solids. Our cluster angular distribution parameters have been compared to the free atoms. In addition, the measurement of the fragmentation dynamic of the clusters seems to be photon energy dependent.

Time-resolved velocity map ion imaging study of NH3 photodissociation

Following excitation of the A state nu(2) (')=4 mode in ammonia, we show how the time scale to dissociation of the N-H bond depends on the internal energy imparted to the NH(2) photofragment. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination can be directly related to the nonadiabatic nature of the photodissociation for high kinetic energy H atoms with evidence for adiabatic dynamics to dissociation giving the lowest energy H atoms. Extrapolation of the time scales for dissociation versus internal energy of the NH(2) photofragment implies that dissociation to the vibrationless ground state of NH(2) occurs in <50 fs, in very good agreement with frequency resolved measurements. The anisotropy of the H fragments with the highest kinetic energies seems to also suggest that the NH(2) partner fragment comes off with very low rotational excitation.

Multiphoton ionization/dissociation dynamics of formyl fluoride by velocity mapping ion imaging

The dissociation dynamics of HFCO(+) ion has been studied using the velocity map ion imaging technique. The HFCO(+) ion is prepared by one-photon resonant three-photon ionization in the region of 43100-43860 cm(-1) excitation energy. The HFCO(+) ions, produced by multiphoton ionization, have sufficient internal energy to dissociate into the F and HCO(+) fragments without further absorption of another photon. Images of HCO(+) have been recorded at various excitation energies. It is noticed that the angular distributions of HCO(+) change dramatically from parallel distribution to perpendicular distribution and then back to parallel distribution in a very narrow excitation energy region of 43 473-43 500 cm(-1). Analysis of anisotropy parameters of beta(n) (n = 2, 4 and 6) reveals that the electronic states in the three-photon excitation of HFCO are mainly: HFCO(X(1)A') --> HFCO(A(1)A'') --> HFCO(A') --> HFCO(+)(A(2)A'';B(2)A'). The purely perpendicular resonant transitions are likely responsible for the perpendicular angular distribution of the HCO(+) ion fragment.