Speed-Dependent Alignment and Angular Distributions of O(¹D₂)from the Ultraviolet Photodissociation of N₂O

Abstract

Photofragment ion imaging has been used to study the O(1D2) atoms produced in the ultraviolet (∼200 nm) photodissociation of nitrous oxide (N2O) in a molecular beam. The images of O(1D2) reveal a speed-dependent angular distribution resulting from both variation in the spatial anisotropy of the recoil and alignment of the electronic angular momentum of the O(1D2) fragment. The orbital alignment effects are revealed by a change in the images when different transitions of the O atom are employed in the resonance enhanced multiphoton ionization (REMPI) process. By correlating the O atom anisotropies with previously measured values for the complementary N2 fragments and comparing images collected on two different REMPI transitions, we calculate the relative branching ratios and anisotropies for the three different |m| values (defined along the product recoil axis) of the electronic angular momentum of the O(1D2) fragment. While m = 0 fragments have the highest probability for most O atom speeds, there is a significant change in the alignment and angular distributions for the slower O atoms. We use a simplified dynamical model supported by previous theoretical structure calculations to explain the measured trends and to estimate the relative branching in the excitation to two N2O electronic states.

Single-field slice-imaging with a movable repeller: Photodissociation of N2O from a hot nozzle

“…18,19,22 We observe both atomic orbital alignment for the O( 1 D) photofragment and a non-limiting β value for N 2 (v = 0, J), on average ∼0.7 for J's between 60 and 80, in agreement with previous results. Recent calculations predict that over 97% of the absorption at 204 nm involves excitation to the 2 1 A' state, i.e., a parallel transition, which means the dipole moment lies in the plane of the molecule.…”

“…As observed previously, [14][15][16][17][18][19][20][21][22] Since the corresponding sibling N 2 (J) photofragments do not show this effect, e.g., in Fig. 6 the angular distribution is maximum at θ = 0 o , this has been attributed to a preferential alignment of the O( 1 D 2 ) atomic orbital with respect to the recoil velocity.…”

“…This raises the possibility that a second type of photodissociation dynamics is responsible for the low energy tail. The angular distributions for the slow region, i.e., N 2 (J > 83), changes dramatically from positive anisotropy parameters (β) for N 2 (J < 83) to nearly zero, 18,19,22 providing further evidence that the photodissociation dynamics mechanism in this part of the KER spectrum could be different.…”

“…Also due to the large change in NNO angle upon photoexcitation, N 2 photoproducts are formed with a large amount of rotational excitation (60 < J < 80). In recent years N 2 O has been subject to a number of high resolution experiments [13][14][15][16] using VMI, [17][18][19][20][21][22] as well as high level theoretical calculations. [23][24][25][26][27][28][29][30][31][32] In the most recent VMI study of Suzuki and coworkers, 22 the authors ionized individual rotational states of the N 2 photoproduct.…”

Rotationally resolved photoelectron spectroscopy of hot N2 formed in the photofragmentation of N2O

“…5 As the fragments separate, a strong torque will be exerted on the N 2 photofragments, resulting in rotationally ''hot'' N 2 . Several experimental techniques, including resonance-enhanced multiphoton ionization ͑REMPI͒, either in conjunction with mass-resolved ion detection, 7 or ion imaging, 5,[8][9][10][11][12] have been used to probe the product channels. All these studies of the photodissociation of N 2 O have focused on elucidating the details of the various photodissociation pathways, and on measuring the energy and angular distributions of N 2 .…”