Velocity Map Ion Imaging Research Articles

Resonance-state selective photodissociation dynamics of OCS + hv → CS(X1Σ+) + O(3Pj=2,1,0) via the 21Σ+ state.

Understanding vacuum ultraviolet photodissociation dynamics of Carbonyl sulfide (OCS) is of considerable importance in the study of atmospheric chemistry. Yet, photodissociation dynamics of the CS(X1Σ+) + O(3Pj=2,1,0) channels following excitation to the 21Σ+(ν1',1,0) state has not been clearly understood so far. Here, we investigate the O(3Pj=2,1,0) elimination dissociation processes in the resonance-state selective photodissociation of OCS between 147.24 and 156.48nm by using the time-sliced velocity-mapped ion imaging technique. The total kinetic energy release spectra are found to exhibit highly structured profiles, indicative of the formation of a broad range of vibrational states of CS(1Σ+). The fitted CS(1Σ+) vibrational state distributions differ for the three 3Pj spin-orbit states, but a general trend of the inverted characteristics is observed. Additionally, the wavelength-dependent behaviors are also observed in the vibrational populations for CS(1Σ+, v). The CS(X1Σ+, v = 0) has a significantly strong population at several shorter wavelengths, and the most populated CS(X1Σ+, v) is gradually transferred to a higher vibrational state with the decrease in the photolysis wavelength. The measured overall β-values for the three 3Pj spin-orbit channels slightly increase and then abruptly decrease as the photolysis wavelength increases, while the vibrational dependences of β-values show an irregularly decreasing trend with increasing CS(1Σ+) vibrational excitation at all studied photolysis wavelengths. The comparison of the experimental observations for this titled channel and the S(3Pj) channel reveals that two different intersystem crossing mechanisms may be involved in the formation of the CS(X1Σ+) + O(3Pj=2,1,0) photoproducts via the 21Σ+ state.

Vacuum ultraviolet photodissociation dynamics of OCS via the F Rydberg state: The O (3PJ=2,1,0) product channels.

We study the vacuum ultraviolet (VUV) photodissociation dynamics of carbonyl sulfide (OCS) by using the time sliced velocity map ion imaging technique. Experimental images of the dissociative O (3PJ=0,1,2) products were acquired at five VUV photolysis wavelengths from 133.26 to 139.96nm that correspond to the F Rydberg state of OCS. High vibrational states of the carbon monosulfide (CS) co-products are partially resolved in the images. The product total kinetic energy releases, angular distributions, and the product state branching ratios were derived from the experimental images. Notably, it is found that the anisotropic parameters change systematically with the photolysis wavelength. The anisotropic parameters and the product state branching ratios are significantly sensitive to the J quantum number of the O (3PJ) products. The phenomenon indicates that multiple nonadiabatic pathways are strongly involved in the photodissociation processes.

Multiparticle Cumulant Mapping for Coulomb Explosion Imaging.

We extend covariance velocity map ion imaging to four particles, establishing cumulant mapping and allowing for measurements that provide insights usually associated with coincidence detection, but at much higher count rates. Without correction, a fourfold covariance analysis is contaminated by the pairwise correlations of uncorrelated events, but we have addressed this with the calculation of a full cumulant, which subtracts pairwise correlations. We demonstrate the approach on the four-body breakup of formaldehyde following strong field multiple ionization in few-cycle laser pulses. We compare Coulomb explosion imaging for two different pulse durations (30 and 6fs), highlighting the dynamics that can take place on ultrafast timescales. These results have important implications for Coulomb explosion imaging as a tool for studying ultrafast structural changes in molecules, a capability that is especially desirable for high-count-rate x-ray free-electron laser experiments.

A direct liquid sampling interface for photoelectron photoion coincidence spectroscopy.

We introduce an effective and flexible high vacuum interface to probe the liquid phase with photoelectron photoion coincidence (liq-PEPICO) spectroscopy at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source. The interface comprises a high-temperature sheath gas-driven vaporizer, which initially produces aerosols. The particles evaporate and form a molecular beam, which is skimmed and ionized by VUV radiation. The molecular beam is characterized using ion velocity map imaging, and the vaporization parameters of the liq-PEPICO source have been optimized to improve the detection sensitivity. Time-of-flight mass spectra and photoion mass-selected threshold photoelectron spectra (ms-TPES) were recorded for an ethanolic solution of 4-propylguaiacol, vanillin, and 4-hydroxybenzaldehyde (1 g/l of each). The ground state ms-TPES band of vanillin reproduces the reference, room-temperature spectrum well. The ms-TPES for 4-propylguaiacol and 4-hydroxybenzaldehyde are reported for the first time. Vertical ionization energies obtained by equation-of-motion calculations reproduce the photoelectron spectral features. We also investigated the aldol condensation dynamics of benzaldehyde with acetone using liq-PEPICO. Our direct sampling approach, thus, enables probing reactions at ambient pressure during classical synthesis procedures and microfluidic chip devices.

Photodissociation study of CO2 on the formation of state-correlated CO(X1Σ+, v) with O(3P2) photoproducts in the low energy band centered at 148 nm.

The spin-forbidden O(3P2) + CO(X1Σ+, v) channel formed from the photodissociation of CO2 in the low energy band centered at 148nm is investigated by using the time-sliced velocity-mapped ion imaging technique. The vibrational-resolved images of the O(3P2) photoproducts measured in the photolysis wavelength range of 144.62-150.45nm are analyzed to give the total kinetic energy releases (TKER) spectra, CO(X1Σ+) vibrational state distributions, and anisotropy parameters (β). The TKER spectra reveal the formation of correlated CO(X1Σ+) with well resolved v = 0-10 (or 11) vibrational bands. Several high vibrational bands that were observed in the low TKER region for each studied photolysis wavelength exhibit a bimodal structure. The CO(X1Σ+, v) vibrational distributions all present inverted characteristics, and the most populated vibrational state changes from a low vibrational state to a relatively higher vibrational state with a change in the photolysis wavelength from 150.45 to 144.62nm. However, the vibrational-state specific β-values for different photolysis wavelengths present a similar variation trend. The measured β-values show a significant bulge at the higher vibrational levels, in addition to the overall slow decreasing trend. The observed bimodal structures with mutational β-values for the high vibrational excited state CO(1Σ+) photoproducts suggest the existence of more than one nonadiabatic pathway with different anisotropies in the formation of O(3P2) + CO(X1Σ+, v) photoproducts across the low energy band.

Imaging rotational energy transfer: comparative stereodynamics in CO + N2 and CO + CO inelastic scattering.

State-to-state rotational energy transfer in collisions of ground ro-vibrational state 13CO molecules with N2 molecules has been studied using the crossed molecular beam method under kinematically equivalent conditions used for 13CO + CO rotationally inelastic scattering described in a previously published report (Sun et al., Science, 2020, 369, 307-309). The collisionally excited 13CO molecule products are detected by the same (1 + 1' + 1'') VUV (Vacuum Ultra-Violet) resonance enhanced multiphoton ionization scheme coupled with velocity map ion imaging. We present differential cross sections and scattering angle resolved rotational angular momentum alignment moments extracted from experimentally measured 13CO + N2 scattering images and compare them with theoretical predictions from quasi-classical trajectories (QCT) on a newly calculated 13CO-N2 potential energy surface (PES). Good agreement between experiment and theory is found, which confirms the accuracy of the 13CO-N2 potential energy surface for the 1460 cm-1 collision energy studied by experiment. Experimental results for 13CO + N2 are compared with those for 13CO + CO collisions. The angle-resolved product rotational angular momentum alignment moments for the two scattering systems are very similar, which indicates that the collision induced alignment dynamics observed for both systems are dominated by a hard-shell nature. However, compared to the 13CO + CO measurements, the primary rainbow maximum in the DCSs for 13CO + N2 is peaked consistently at more backward scattering angles and the secondary maximum becomes much less obvious, implying that the 13CO-N2 PES is less anisotropic. In addition, a forward scattering component with high rotational excitation seen for 13CO + CO does not appear for 13CO-N2 in the experiment and is not predicted by QCT theory. Some of these differences in collision dynamics behaviour can be predicted by a comparison between the properties of the PESs for the two systems. More specific behaviour is also predicted from analysis of the dependence on the relative collision geometry of 13CO + N2 trajectories compared to 13CO + CO trajectories, which shows the special 'do-si-do' pathway invoked for 13CO + CO is not effective for 13CO + N2 collisions.

Fine-structure branching ratios N(2D5/2)/N(2D3/2) above threshold N(2D5/2.3/2)+N(2D5/2.3/2) for 14N2 and 15N2

Accurate measurements of the product spin-orbit finne-structure branching ratios are important for understanding the detailed photodissociation dynamics of small molecules. In this study, the atomic spin-orbit fine-structure branching rar tio N(2D5/2)/N(2D3/2) to the dissociation channel N(2D5/2,3/2)+N(2D5/2,3/2) is measured for the c4′1Σu+(υ′=6) and b′1Σu+(υ′=21) states of 14N2, and the b′1Σu+(υ′=20) and b′1Σu+(υ′=21) states of 15N2 by using the vacuum ultraviolet (VUV)pump VUV-probe time-sliced velocity-mapped ion imaging setup. The measurements show that the fine-structure branching ratio N(2D5/2)/N(2D3/2) is independent of the rotational level of the parent molecule (14N2 or 15N2) in each vibronic state, while it does show dependence on the vibronic characteristics. It is ~1.35 for the c4′1Σu+(υ′=6) state of 14N2 and b′1Σu+(υ′=20) state of 15N2, which are both close to the dissociation threshold N(2D5/2,3/2)+N(2D5/2,3/2); while it becomes ~1.00 for the b′1Σu+(υ′=21) state of both 14N2 and 15N2, which are relatively far above the dissociation threshold. A possible change from a statistical process near the threshold to a diabatic process far above the threshold might have occurred to be responsible for the observed vibronic dependence of the branching ratio. Detailed informations on the potential energy curves and their mutual couplings near the dissociation threshold are highly desired for understanding the present experimental measurement.

Slice imaging study of NO2 photodissociation via the 12B2 and 22B2 states: the NO(X2Π) + O(3PJ) product channel.

The state-resolved photodissociation of NO2via the 12B2 and 22B2 excited states has been investigated by using time-sliced velocity-mapped ion imaging technique. The images of the O(3PJ=2,1,0) products at a series of excitation wavelengths are measured by employing a 1 + 1' photoionization scheme. The total kinetic energy release (TKER) spectra, NO vibrational state distributions and anisotropy parameters (β) are derived from the O(3PJ=2,1,0) images. For the 12B2 state photodissociation of NO2, the TKER spectra mainly present a non-statistical vibrational state distribution of the NO co-products, and the profiles of most vibrational peaks display a bimodal structure. The β values show a gradual decrease with the photolysis wavelength increasing except for a sudden increase at 357.38 nm. The results suggest that the NO2 photodissociation via the 12B2 state proceeds via the non-adiabatic transition between the 12B2 and X̃2A1 states, leading to the NO(X2Π) + O(3PJ) products with wavelength-dependent rovibrational distributions. As for photodissociation of NO2via the 22B2 state, the NO vibrational state distribution is relatively narrow with the main peak shifting from v = 1, 2 at 235.43-249.22 nm to v = 6 at 212.56 nm. The β values exhibit two distinctly different angular distributions, i.e., near isotropic at 249.22 and 246.09 nm and anisotropic at the rest of the excitation wavelengths. These results are consistent with the fact that the 22B2 state potential energy surface has a barrier, and the dissociation process is fast when the initial populated level is above this barrier. A bimodal vibrational state distribution is clearly observed at 212.56 nm, in which the main distribution (peaking at v = 6) is ascribed to dissociation via an avoided crossing with the higher electronically excited state while the subsidiary distribution (peaking at v = 11) likely arises due to dissociation via the internal conversion to the 12B2 state or to the X̃ ground state.

Imaging the Dynamics of the Electron Ionization of C2F6

The dissociation of C2F6 following electron ionization at 100 eV has been studied using multimass velocity-map ion imaging and covariance-map imaging analysis. Single ionization events form parent C2F6+ cations in an ensemble of electronic states, which follow a multiplex of relaxation pathways to eventually dissociate into ionic and neutral fragment products. We observe CF3+, CF2+, CF+, C+, F+, C2F5+, C2F4+, C2F2+, and C2F+ ions, all of which can reasonably be formed from singly charged parent ions. Dissociation along the C–C bond typically forms slow-moving, internally excited products, whereas C–F bond cleavage is rapid and impulsive. Dissociation from the à state of the cation preferentially forms C2F5+ and neutral F along a purely repulsive surface. No other electronic state of the ion will form this product pair at the electron energies studied in this work, nor do we observe any crossing onto this surface from higher-lying states of the parent ion. Multiply charged dissociative pathways are also explored, and we note characteristic high kinetic energy release channels due to Coulombic repulsion between charged fragments. The most abundant ion pair we observe is (CF2+, CF+), and we also observe ion pair signals in the covariance maps associated with almost all possible C–C bond cleavage products as well as between F+ and each of CF3+, CF2+, CF+, and C+. No covariance between F+ and C2F5+ is observed, implying that any C2F5+ formed with F+ is unstable and undergoes secondary fragmentation. Dissociation of multiply charged parent ions occurs via a number of mechanisms, details of which are revealed by recoil-frame covariance-map imaging.

Observation of Competitive Nonadiabatic Photodissociation Dynamics of H2S+ Cations.

A comprehensive understanding of dissociation mechanisms is of fundamental importance in the photochemistry of small molecules. Here, we investigated the detailed photodissociation dynamics of H2S+ near 337 nm by using the velocity map ion imaging technique together with the theoretical characterizations by developing global full-dimensional potential energy surfaces (PESs). Rotational state resolved images were acquired for the S+(4S) + H2 product channel. Significant changes in product total kinetic energy release distributions and angular distributions have been observed within a small excitation photon energy range of 5 wavenumbers. Analysis based on the full-dimensional PESs reveals that two nonadiabatic pathways determined by the transition state connecting two minima on the 12A' state are responsible for the dramatic variation of observed product distributions. The current study has directly witnessed the competitive photodissociation mechanisms controlled by a critical energy point on the PES, thereby providing in-depth insight into the nonadiabatic dynamics in photochemistry.

Crossed Molecular Beam Study of the H + HD → H2 + D Reaction at 0.60 and 1.26 eV Using the Near-Threshold Ionization Velocity Map Ion Imaging

By using the 1 + 1' near-threshold ionization velocity map ion imaging technique, state-to-state reactive differential cross sections have been measured for the H + HD → H2 + D reaction. High-resolution images of the D products, with the rotational states of the H2 co-products clearly resolved, were acquired at the collision energies of 0.60 and 1.26 eV, respectively. It is found that the angular distribution is predominantly backward-scattering at the collision energy of 0.60 eV. However, at 1.26 eV, where the collision energy is higher, the angular distribution becomes forward-backward-scattering. Notably, at both collision energies, the main peaks of backward-scattered products gradually shift from backward toward sideways direction as the rotational quantum number of H2 increases. Moreover, in the forward direction, fast angular oscillations, which are induced by specific partial waves have also been observed at 1.26 eV. These features show a strong correlation between the product states and angular distributions and also indicate the unique role of partial waves in quantum reactive scattering.

Observation of rotationally dependent fine-structure branching ratios near the predissociation threshold N(2D5/2,3/2) + N(2D5/2,3/2) of 14N2.

Photofragment spin-orbit fine-structure branching ratios have long been predicted to depend on the rotational quantum number J' by theory near the dissociation thresholds of several diatomic molecules, while this has rarely been observed in any photodissociation experiments yet. Here, we measured the fine-structure branching ratios N(2D5/2)/N(2D3/2) produced in the N(2D5/2,3/2) + N(2D5/2,3/2) channel at the b'1Σu +(v = 20) state of 14N2 by using our vacuum ultraviolet (VUV)-pump-VUV-probe time-sliced velocity-mapped ion imaging setup. It is found that 14N2 almost exclusively dissociates into the spin-orbit channel N(2D5/2) + N(2D3/2) at low rotational levels and gradually approaches the statistical or diabatic limit by distributing all possible spin-orbit channels at higher rotational levels. The strongly rotationally dependent fine-structure branching ratios should be due to the increasing strength of nonadiabatic Coriolis interaction among various dissociative states in the so-called "recoupling zone" as J' increases. They are supposed to provide unprecedented information on the near threshold photodissociation dynamics of 14N2.

Anisotropy Measurements from the Near-Threshold Photodissociation of the N2-NO Complex.

We have used velocity map ion imaging to measure the angular anisotropy of the NO (A) products from the photodissociation of the N2-NO complex. Our experiment ranged from 108 to 758 cm-1 above the threshold energy to form NO (A) + N2 (X) products, and these measurements reveal, for the first time, a strong angular anisotropy from photodissociation. At 108 cm-1 above the photodissociation threshold, we observed NO (A) photoproducts recoil preferentially perpendicular to the laser polarization axis with an average anisotropy parameter, β = -0.25; however, as the available energy was increased, the anisotropy increased, and at 758 cm-1 above the threshold energy, we found an average β = +0.28. The observed changes in the angular anisotropy of the NO (A) photoproduct are qualitatively similar to those observed for the photodissociation of the Ar-NO complex and likely result from changes in the region of the excited state potential energy surface accessed during the electronic excitation. At the lowest available energy, we also noted a large contribution from hotband excitation; however, this contribution decreased as the available energy increased. The outsized contribution at the lowest available energy may result from hotbands having better Franck-Condon overlap with the excited electronic state near threshold. Finally, we contrast the experimental center of mass translational energy distribution with a statistical energy distribution determined from phase space theory. The experimental and statistical distributions show pronounced disagreement, particularly at low kinetic energies, with the experimental one showing less dissociation resulting in high rotational levels of the fragments.

Photodissociation dynamics of CO2 + hv → CO(X1Σ+) + O(1D2) via the 3P1Πu state.

The vacuum ultraviolet (VUV) photodissociation of CO2 is important to understand the primary photochemical processes of CO2 induced by solar VUV excitation in the Earth's atmosphere. Here, we report a detailed study of vibrational-state-specific photodissociation dynamics of the CO(X1Σ+) + O(1D2) channel via the 3P1Πu state by using the time-sliced velocity-mapped ion imaging apparatus combined with the single VUV photoionization detection scheme. By recording the sliced images of the O(1D2) photoproducts formed by VUV photoexcitation of CO2 to the individual vibrational structure of the 3P1Πu state, both the vibrational state distributions of the counterpart CO(X1Σ+) photoproducts and the vibrational-state-specific product anisotropy parameters (β) are determined. The experimental results show that photodissociation of CO2 at 108.22, 107.50, 106.10, and 104.76nm yields less anisotropic (β > 0) and inverted distributed CO(X1Σ+, v) photoproducts. The possible dissociation mechanism for the CO(X1Σ+) + O(1D2) channel may involve the non-adiabatic transition of excited CO2* from the initially prepared state to the 31A' state with potential energy barriers. While at 108.82 and 107.35nm, the vibrational distributions are found to have the population peaked at a low vibrational state, and the anisotropy parameters turn out to be negative. Such variation indicates the possibility of another non-adiabatic dissociation pathway that may involve Coriolis-type coupling to the low-lying dissociative state. These observations show sclear evidence of the influence of the initially vibrational excitations on the photodissociation dynamics of CO2 via the 3P1Πu state.

Vacuum ultraviolet photodissociation dynamics of OCS via the F Rydberg state: The S(3PJ = 2, 1, 0) product channels

Vacuum ultraviolet (VUV) photodissociation dynamics of carbonyl sulfide was investigated experimentally by using a tunable photolysis light source and the time-sliced velocity map ion imaging technique. Ion images of S(3PJ =2, 1, 0) dissociation products were measured at five photolysis wavelengths from 133.26 nm to 139.96 nm, corresponding to the F Rydberg state of OCS. Two dissociation channels: S(3PJ)+CO(X1Σ+) and S(3PJ)+CO(A3Π) were observed with the former being dominant. The vibrational states of CO co-products were partially resolved in the ion images. The product total kinetic energy releases, anisotropy parameters (β), and the branching ratios of high-lying CO vibrational states were determined for the S(3PJ )+CO(X1Σ+) channel. We found that the anisotropy parameters suddenly changed from negative to positive when OCS was excited to the higher vibrational levels of the F state. Furthermore, the anisotropy parameters for S(3PJ) products of J = 2, 1, 0 were even different. These anomalous phenomena may result from the simultaneous existence of both parallel and perpendicular dissociation mechanisms, suggesting the involvement of other electronic states with different symmetry in the initially-excited energy region. This work provides a further understanding of the nonadiabatic couplings in the VUV photodissociation process of OCS.

Vacuum ultraviolet photoexcitation and photofragment spectroscopic studies of 14N15N between 109000 and 117500cm-1.

In this study, we employed a newly built time-slice velocity-map ion imaging setup, equipped with two tunable vacuum ultraviolet (VUV) laser sources, to obtain the first comprehensive high-resolution photoexcitation and photofragment excitation spectra of 14N15N in the VUV photon energy range 109 000-117 500cm-1. The spectroscopic simulation program PGOPHER was used to analyze the rotationally resolved spectra. Band origins, rotational constants, and isotope shifts compared with those of 14N2 have been obtained for 31 electric-dipole-allowed vibrational states of 14N15N in the aforementioned energy range. These spectroscopic parameters are found to depend on the vibrational quantum number irregularly. Systematic perturbations of the rotational transition energies and predissociation rates within individual absorption bands have also been observed. These are proved to be caused by the strong homogeneous interactions between the valence b'1Σu + state and the Rydberg cn ' 1Σu + states, and between the valence b1Πu states and the Rydberg o3 1Πu states. Heterogeneous interactions between the Rydberg cn 1Πu states and cn ' 1Σu + states also play an important role.

Photodissociation dynamics of CF3CHO: C-C bond cleavage.

The photodissociation dynamics of jet-cooled trifluoroacetaldehyde (CF3CHO) into radical products, CF3 + HCO, was explored using velocity mapped ion imaging over the wavelength range 297.5nm ≤λ≤ 342.8nm (33613-29172 cm-1) covering the entire section of the absorption spectrum accessible with solar actinic wavelengths at the ground level. After initial excitation to the first excited singlet state, S1, the radical dissociation proceeds largely via the first excited triplet state, T1, at excitation energies above the T1 barrier. By combining velocity-mapped ion imaging with high-level theory, we place this barrier at 368.3 ± 2.4 kJ mol-1 (30780 ± 200 cm-1). After exciting to S1 at energies below this barrier, the dissociation proceeds exclusively via the ground electronic state, S0. The dissociation threshold is determined to be 335.7 ± 1.8 kJ mol-1 (28060 ± 150 cm-1). Using laser-induced fluorescence spectroscopy, the origin of the S1 ← S0 transition is assigned at 28903 cm-1. The S0 dissociation channel is active at the S1 origin, but the yield significantly increases above 29100 cm-1 due to enhanced intersystem crossing or internal conversion.

Collision-Energy Dependent Stereodynamics of Dissociative Charge Exchange Reaction between Ar+ and CO.

Long-distance charge-dipole attraction between atomic ion and randomly oriented polar molecule potentially makes the molecular orientation, which profoundly influences the products' kinetics of collisional reaction. Using the three-dimensional ion velocity map imaging technique, here we report a collision-energy dependent stereodynamics of dissociative charge exchange reaction Ar+ + CO → Ar + O + C+ in a range of 7.46-9.97 eV. At the lowest collision energy, the most C+ products are forward-scattered and are along the collision axis and are attributed to three different dissociation channels including the predominant one experiencing the rotating intermediate ArC+. At the high collision energies, the remarkably diffusive distribution of C+ arises from the prompt dissociation of the rebounded CO+. The different dynamic processes arising from the nearly collinear collision are elaborated explicitly on the basis of the data analyses using the Doppler kinetics models.

Superposition-state N2 + produced in the intermolecular charge transfer from low-energy Ar+ to N2.

Molecular electronic or vibrational states can be superimposed temporarily in an extremely short laser pulse, and the superposition-state transients formed therein receive much attention, owing to the extensive interest in molecular fundamentals and the potential applications in quantum information processing. Using the crossed-beam ion velocity map imaging technique, we disentangle two distinctly different pathways leading to the forward-scattered N2 + yields in the large impact-parameter charge transfer from low-energy Ar+ to N2. Besides the ground-state (X2Σg +) N2 + produced in the energy-resonant charge transfer, a few slower N2 + ions are proposed to be in the superpositions of the X2Σg +-A2Πu and A2Πu-B2Σu + states on the basis of the accidental degeneracy or energetic closeness of the vibrational states around the X2Σg +-A2Πu and A2Πu-B2Σu + crossings in the non-Franck-Condon region. This finding potentially shows a brand-new way to prepare the superposition-state molecular ion.

Photodissociation of the N2-NO Complex between 225.8 and 224.0 nm.

Our primary goal was to measure the NO (A) photoproduct appearance energy and ground-state dissociation energy of the N2-NO complex. We recorded velocity map ion images of NO photofragments resulting from the dissociation of the N2-NO complex excited between ∼225.8 and 224.0 nm, which ranged from the photodissociation threshold to about 342 cm-1 above the threshold. In the experiment, one photon dissociated the complex through the N2 (X 1Σg+)-NO (A 2Σ+) ← N2 (X 1Σg+)-NO (X 2Π) transition, and a second photon nonresonantly ionized the NO (A) photoproduct. The lowest-energy photons near 225.8 nm did not have sufficient energy to photodissociate the lowest excited state of the complex; however, dissociation was observed with increasing photon energy. On the basis of the experiments, we determined the appearance energy for the NO (A) photoproduct to be 44 284.7 ± 2.8 cm-1. From the appearance energy and the NO A ← X origin band transition, we determined a ground-state dissociation energy of 85.8 ± 2.8 cm-1. As we increased the photon energy, the excess energy was partitioned into rotational modes of the diatomic products as well as product translational energy. We found good agreement between the average fraction of rotational energy and the predictions of a simple pseudo three atom impulsive model. Finally, at all photon energies, we observed some contribution from internally excited complexes in the resulting P(ET). The maximum internal energy of these complexes was consistent with the ground-state dissociation energy.