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

Abstract

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.

Correlated Product Distributions in the Photodissociation of à State NO–CH4 and NO–N2 van der Waals Complexes

“…Here the distribution falls to a minimum at N = 5, with weak minima also indicated at N = 0 and 8. This is similar to the behavior seen for NO–Ar ,, and quite different to that recently reported for NO–N 2 [ref ] (see also section ).…”

“…The distributions are quite simple, with maxima at low N (0 and 1 for 44,375 and 44,475 cm –1 photolysis, respectively) with a monotonic decay to higher N . Very recently, Parsons et al have reported similarly shaped distributions for E a from 28 to 758 cm –1 . These authors reported average NO rotational energies at a range of available energies with which our values can be compared.…”

“…Similarly, for 44,375 cm –1 photolysis, the highest N energetically accessible, N = 6, has very little population. Parsons et al noted analogous behavior at higher available energies, and thus, it is a consistent theme of NO–N 2 dissociation. This suggests that the rotational excitation of NO is limited by the maximum torque that can be applied by the N 2 partner, as discussed by McCaffery in his model of angular momentum constraints associated with energy transfer and van der Waals molecule dissociation ( vide infra ). , …”

“…This group had earlier used VMI to determine the dissociation threshold in the A ̃ state and reported the ground state dissociation energy to be 85.8 ± 2.8 cm –1 . Most recently, they have presented NO A state product distributions for dissociation from 28 to 758 cm –1 above threshold …”

“…17 Most recently, they have presented NO A state product distributions for dissociation from 28 to 758 cm −1 above threshold. 18 Correlated product distributions are presented for the dissociation of NO−CH 4 at photolysis energies of 44,355 and 44,505 cm −1 , which correspond to ca. 50 and 200 cm −1 above the reported NO−CH 4 dissociation threshold, 15 and NO−N 2 at photolysis energies of 44,375 and 44,475 cm −1 , which correspond to accessible energies of 90 and 190 cm −1 , respectively.…”