Roaming mechanisms have recently been observed in several chemical reactions alongside trajectories that pass through a traditional transition state. Here, we demonstrate that the visible light-induced reaction NO(3) → NO + O(2) proceeds exclusively by roaming. High-level ab initio calculations predict specific NO Λ doublet propensities (orientations of the unpaired electron with respect to the molecular rotation plane) for this mechanism, which we discern experimentally by ion imaging. The data provide direct evidence for roaming pathways in two different electronic states, corresponding to both previously documented photolysis channels that produce NO + O(2). More broadly, the results raise intriguing questions about the overall prevalence of this unusual reaction mechanism.

We report correlated state distributions arising from NO 3 photolysis at 588 nm using velocity map ion imaging. State-selected NO images reveal clear evidence for two dissociation pathways for the NO þ O 2 channel. Vibrationally excited O 2 ( 3 Σ g -, ν = 5-10) is formed in coincidence with low rotational states of NO ( 2 Π) in the dominant mechanism. We also observe a small channel consisting of low vibrational levels of O 2 ( 3 Σ g -, ν=0-4) coincident with high NO rotational states. We discuss the results in the context of the recent studies of formaldehyde dissociation and postulate the involvement of roaming-type dynamics. SECTION Dynamics, Clusters, Excited StatesT he nitrate radical, NO 3 , plays an important role in atmospheric chemistry, photolyzing in the visible region to yield two product channelsThe thresholds for both channels are close in energy (0.82 kcal/mol), 1 resulting in a narrow wavelength range (585-595 nm) for the molecular channel 1 since the higher-energy radical channel 2 dominates once it is energetically accessible. 2,3 The molecular channel threshold has been assumed to arise from a tight transition state involving a three-center mechanism. 4 However, no theoretical transition state of photochemically relevant energy has been identified to our knowledge, and the mechanism of molecular elimination remains unsolved. There have been several experimental studies of NO 3 photodissociation. Total translational energy distributions obtained by Davis et al. revealed that O 2 from the molecular channel was highly excited, although it was unclear whether the energy was associated with electronic excitation ( 1 Δ g ) or vibrational excitation (ν = 5-10). 1 Interestingly, the translational energy distribution included a contribution at high energy corresponding to low internal energy states which did not exhibit well-resolved vibrational features. Correlated state distributions can often provide valuable insight into the underlying dynamics. Mikhaylichenko et al. measured correlated translational energy distributions in NO 3 photodissociation using pump-probe 1 þ1 REMPI in a molecular beam, although the velocity resolution of the experiment was not sufficient to resolve individual O 2 vibrational states. 4 The state-selected NO translational energy distributions were consistent with the work of Davis et al., 1 and the authors reported a nascent NO rotational temperature of 1400 ( 300 K.We have obtained high-resolution state-selected NO translational energy distributions arising from NO 3 dissociation using velocity map ion imaging. Our results show clear statedependent bimodal O 2 internal energy distributions, implying two distinct mechanisms in the dissociation. The results show an intriguing resemblance to the molecular elimination channel in formaldehyde photodissociation, in which one pathway evolves through a tight transition state and the second pathway "roams" over a large region of the potential energy surface before leading to intramolecular abstraction. 5 We speculate that a similar c...

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.

We present a molecular level description of NO3→ NO + O2 photodissociation for both of the experimentally observed reaction pathways using the results of ion imaging experiments and recent theoretical studies. Vector correlation and Λ doublet propensity measurements have been performed on state-selected NO fragments in order to further characterize the stereodynamics of this reaction. Previous measurements (Grubb et al., Science, 2012, 1075-1078) of relative Λ doublet propensities along with ab initio calculations revealed that both pathways arise from roaming-type mechanisms, but each pathway arises from roaming on a different electronic potential. This model, however, assumes that NO3 dissociation takes place in the molecular plane. In the present paper, we have confirmed this assumption through speed-dependent vector correlation measurements. Strong perpendicular correlations between the velocity vector v and the angular momentum vector j are observed in the NO fragment originating from both pathways, in agreement with a constrained planar dissociation. These results are discussed in light of the absence of vector correlations in other roaming systems, which have previously been characterized by an unconstrained intra-molecular abstraction. We show that geometrical constraints should in fact be quite prevalent in roaming dynamics, and are analogous to the geometrical constraints of the corresponding bimolecular abstraction reaction.

We present analytical expressions relating the bipolar moment β(Q)(K)(k(1)k(2)) parameters of Dixon to the measured anisotropy parameters of different pump/probe geometry sliced ion images. In the semi-classical limit, when there is no significant coherent contribution from multiple excited states to fragment angular momentum polarization, the anisotropy of the images alone is sufficient to extract the β(Q)(K)(k(1)k(2)) parameters with no need to reference relative image intensities. The analysis of sliced images is advantageous since the anisotropy can be directly obtained from the image at any radius without the need for 3D-deconvolution, which is not applicable for most pump/probe geometries. This method is therefore ideally suited for systems which result in a broad distribution of fragment velocities. The bipolar moment parameters are obtained for NO(2) dissociation at 355 nm using these equations, and are compared to the bipolar moment parameters obtained from a proven iterative fitting technique for crushed ion images. Additionally, the utility of these equations in extracting speed-dependent bipolar moments is demonstrated on the recently investigated NO(3) system.