The dissociation of NO-Ar(A) from around threshold to 200 cm(-1) above threshold

NO (A) Rotational State Distributions from Photodissociation of the N2-NO Complex

We have recorded the resonance-enhanced multiphoton ionization spectrum for NO (A) products from photodissociation of the N₂-NO complex. We made measurements at excitation energies ranging from 28 to 758 cm^-1 above the threshold to produce NO (A) + N₂ (X) products, and the resulting spectra reveal the NO (A) rotational states formed during dissociation, allowing us to determine the rotational state distribution. At the lowest available energies, 28 and 50 cm^-1 above threshold, we observed contributions from NO (A) rotational states that exceed the available energy and must originate from excitation due to hotbands of the complex. At all higher energies, we did not observe any energetically disallowed NO (A) rotational states, and for all available energies above 259 cm^-1 the observed rotational transitions do not extend to the maximum allowed by energy conservation. Furthermore, the observed distributions were typically biased toward low rotational states, in contrast with expectations from vibrational predissociation. From the rotational state distributions, we determined the average fraction of energy partitioned into NO (A) rotation, f_{NO rot, ave}, to be 0.088 at the highest available energy, and this fraction increased as the available energy decreased. By combining the average NO (A) rotational energy along with the average center-of-mass translational energy from our previous work, we determined the average rotational energy for the undetected N₂ (X) photoproduct. The results showed that the N₂ fragment has a higher average rotational energy relative to the NO fragment. Finally, we found that the NO (A) rotational state distribution was colder than expected for a statistical dissociation.

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 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 ¹Σ_g⁺)-NO (A ²Σ⁺) ← N₂ (X ¹Σ_g⁺)-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(E_T). The maximum internal energy of these complexes was consistent with the ground-state dissociation energy.

Three-dimensional potential energy surfaces of ArNO (X̃ 2Π)

Until now, the potential energy surfaces (PESs) of the ArNO complex found in the literature were two-dimensional, with the NO interatomic distance being fixed. In this work, we present the first accurate three-dimensional ground state X̃ ²Π PESs (both A' and A″) of ArNO computed at the CCSD(T)/CBS level of theory. The equilibrium geometries and the well depths (D_e) are compared to several other electronic structure methods. We found that using the multireference method, MRCI-F12 makes the surfaces much shallower (by 25%) and the depth of the surfaces does not agree with experimental data. The explicitly correlated coupled-cluster method underestimates the well depth as well. Analytic representations for both A' and A″ surfaces were fit to 4380 ab initio points to within 2.71 cm^-1. A three-dimensional Numerov propagator method in Delves coordinates is used to compute the bound state spectrum up to J_tot = 6.5. The recommended dissociation energies are D₀ = 97.2 cm^-1 for the adiabatic ground state and D_e = 133.7 (128.1) cm^-1 for A' (A″).

Rotational and angular distributions of NO products from NO-Rg (Rg = He, Ne, Ar) complex photodissociation

We present the results of an investigation into the rotational and angular distributions of the NO à state fragment following photodissociation of the NO-He, NO-Ne, and NO-Ar van der Waals complexes excited via the à ← X̃ transition. For each complex, the dissociation is probed for several values of Ea, the available energy above the dissociation threshold. For NO-He, the Ea values probed were 59, 172, and 273 cm(-1); for NO-Ne they were 50 and 166 cm(-1); and for NO-Ar they were 44, 94, 194, and 423 cm(-1). The NO à state rotational distributions arising from NO-He are cold, with most products in low angular momentum states. NO-Ne leads to broader NO rotational distributions but they do not extend to the maximum possible given the energy available. In the case of NO-Ar, the distributions extend to the maximum allowed at that energy and show the unusual shapes associated with the rotational rainbow effect reported in previous studies. This is the only complex for which a rotational rainbow effect is observed at the chosen Ea values. Product angular distributions have also been measured for the NO à photodissociation product for the three complexes. NO-He produces nearly isotropic fragments, but the anisotropy parameter, β, for NO-Ne and NO-Ar photofragments shows a surprising change in sign from negative to positive as Ea increases within the unstructured excitation profile. Franck-Condon selection of a broader distribution of geometries including more linear geometries at lower excitation energies and more T-shaped geometries at higher energies can account for the changing recoil anisotropy. Two-dimensional wavepacket calculations are reported to model the rotational state distributions and the bound-continuum absorption spectra.