Photodissociation of N2O: Energy partitioning

Abstract

The energy partitioning in the UV photodissociation of N 2 O is investigated by means of quantum mechanical wave packet and classical trajectory calculations using recently calculated potential energy surfaces. Vibrational excitation of N 2 is weak at the onset of the absorption spectrum, but becomes stronger with increasing photon energy. Since the NNO equilibrium angles in the ground and the excited state differ by about 70• , the molecule experiences an extraordinarily large torque during fragmentation producing N 2 in very high rotational states. The vibrational and rotational distributions obtained from the quantum mechanical and the classical calculations agree remarkably well. The shape of the rotational distributions is semi-quantitatively explained by a two-dimensional version of the reflection principle. The calculated rotational distribution for excitation with λ = 204 nm and the translational energy distribution for 193 nm agree well with experimental results, except for the tails of the experimental distributions corresponding to excitation of the highest rotational states. Inclusion of nonadiabatic transitions from the excited to the ground electronic state at relatively large N 2 -O separations, studied by trajectory surface hopping, improves the agreement at high j.

Ultraviolet photodissociation of OCS: Product energy and angular distributions

“…Suzuki et al 8 discussed the possibility of nonadiabatic A→X transitions and argued that the bimodality of the rotational state distributions is the result of such transitions. A similar, but much less pronounced effect was investigated by Schmidt et al 14 for N 2 O.…”

“…It is strongest at the longest wavelength, but becomes smaller with decreasing λ. At 222 nm, for example, the trajectories cross the well at a larger bond length R, hit the repulsive wall around 90 • also at larger R, and as a consequence they are 14 The agreement of the calculations using the original A and B PESs with the measured P j is poor: the calculated distributions peak at rotational states which are significantly too high. For example, the main peak of the measured distribution 8 for 248 nm occurs at j = 33 while the quantum mechanical distributions for A and B peak at 38 and 39, respectively.…”

“…The N 2 O vibrational distributions are also cold at long wavelengths. With decreasing wavelength, however, P v becomes inverted and the peak gradually shifts to higher v. 14 The shape of the A-state PES, which includes a valley leading toward the N+NO product channel that is absent in OCS, readily explains the strong wavelength dependence of P v for N 2 O.…”

“…Following our theoretical work on the UV photodissociation of N 2 O, [13][14][15][16][17] which has the same number of 16 valence electrons, we recently began to study the complex photodissociation dynamics of OCS. 18,19 We calculated global three-dimensional PESs for the four lowest singlet states (letters in parentheses denote abbreviations used below)-1 1 A (X), 2 1 A (A), 1 1 A (B), and 2 1 A (C),-and the four lowest triplet states-1 3 A (a), 2 3 A (d), 1 3 A (b), and 2 3 A (c)-employing the multi-configuration reference internally contracted configuration interaction (MRCI) theory.…”

The ultraviolet spectrum of OCS from first principles: Electronic transitions, vibrational structure and temperature dependence

“…We have studied the photodissociation of N 2 O in a series of papers and essentially all experimental results have been well reproduced. [41][42][43][44][45][46][47][48][49] A consistent and nearly complete picture has thus been obtained. This paper is part of a similar investigation of the UV photodissociation of OCS.…”