An investigation of polarized atomic photofragments using the ion imaging technique

Abstract

The Goverment reserves for itself and others acting on its behalf a rayalty free, nonexclusive, irrevocable, world-wide license for goverrxnental purposes t o publish, distribute, translate, &plicate, exhibit, and perform ary such data aopyri*ted ky the antractor. Chapter 2 describes the photofragment ion imaging technique, which we have used to study the atomic v-J correlation in chlorine and ozone dissociation. With this technique, a three-dimensional velocity vector distribution of stat e-selectively ionized photofragments is mapped into a two-dimensional spatial distribution. Photofragments are ionized with a resonant multiphoton technique, which also provides sensitivity to angular momentum polarization. This polarization is observed as small modulations in the ion images, which depend on the probe laser polarization and on the symmetry of the chosen spectroscopic probe transition. 2Chapter 3 outlines a method for isolating and describing the contribution to the image signal which is due exclusively to angular momentum alignment. This method is based on recently-developed theoretical tools in combination with wellknown two-photon linestrength expressions. We show that four molecular excitation processes-parallel and perpendicular incoherent excitation and two types of coherent excitation-each contribute a unique spatial structure to the ion images. These mechanisms are represented by a set of alignment anisotropy parameters, which are analogous to the familiar velocity anisotropy parameter ,f3.Ion imaging results are presented and discussed in Chapter 4. We show that the alignment contribution to images of ground state chlorine atom photofragments results primarily from a perpendicular optical excitation in the parent Cl2 molecule.In order to adequately simulate the data, it was necessary to include contributions from both incoherent and coherent excitation. We also argue that nonadiabatic transitions must have occurred in the asymptotic region of the molecular potential energy curves. Preliminary measurements of polarized O( 'D2) from ozone dissociation show a strong polarization effect, which depends on the vibrational state of the 0 2 partner fragment.Chapter 5 discusses a different set of experiments on the three-fragment dissociation of azomethane. We measured appearance times and kinetic energy release for two vibrational quantum states of the methyl radical photofragments and have characterized multiphoton processes which would otherwise complicate the analysis of these data. The results are consistent with a previously proposed mechanism of a concerted dissociation following randomization of internal energy.

Coherence in polyatomic photodissociation: Aligned O(3P) from photodissociation of NO2 at 212.8 nm

“…The persistence of these coherences on the long (nanosecond) timescale of the experiment means that the oxygen atom 'remembers' the original plane of the molecule even in the asymptotic region. We note that this is likely to be a common feature of polyatomic pho~ todissociation, and is not specific to N0 2 • It is interesting to contrast this observation with coherence effects observed in photodissociation of diatomic molecules [21,22,18,29]. In the diatomic case, coherent excitation of two dissociative states of different symmetry leads to quantum mechanical interference, yielding an electron cloud in the recoiling atom possessing azimuthal asymmetry, and observed as oscillations in the angular distributions when probed by polarized lasers.…”

“…A detailed quantitative analysis of this data in terms of these alignment parameters will require linestrength factors for the probe transition which are not yet available. However, qualitative insight may be obtained from the results directly, as we have shown in the Cl 2 case [21].…”

“…We have recently developed a rigorous quantum mechanical approach to treating angular momentum polarization in photodissocation as manifested in Doppler profiles (20] and in ion image data (21,17,22] based on the work of Siebbeles et al (23], directly connecting the experimental results to alignment parameters having explicit physical significance. These parameters characterize distinctly both the coherent and incoherent contributions as well as the symmetry of the transition involved, either 'parallel' or 'perpendicular'.…”

“…These parameters characterize distinctly both the coherent and incoherent contributions as well as the symmetry of the transition involved, either 'parallel' or 'perpendicular'. The initial application of these methods (21,17,22] was to the case of Cl 2 dissociation, in which was shown direct evidence of a coherent perpendicular contribution to the photodissociation event. In this Report, we present measurements of the angular distribution of orbital alignment for the ground state oxygen atom from photodissociation of N0 2 at 212.8 nm obtained using the velocity map imaging technique [24].…”

“…This kinetic energy distribution is shown in Fig. 2 [21,17,22]. A detailed quantitative analysis of this data in terms of these alignment parameters will require linestrength factors for the probe transition which are not yet available.…”