One of the major goals of chemical physics is to understand the nature of chemical bonds in detail. How they are formed and how they can be broken are key elements of chemistry. We have been using light to break chemical bonds (photodissociation) and to probe the products formed. We have been developing very powerful experimental methods that use polarized light to study photodissociation.

Photodissociation of H₂O₂

Our group has demonstrated quantum interference between multiple dissocation pathways using a room temperature sample of hydrogen peroxide. The molecules were dissociated using linearly polarized UV light, and the resulting OH radicals were found to be oriented with respect to the velocity of the recoiling OH molecules. The orientation is a measure of the helicity of the fragments (analagous with spin polarization in nuclear physics). We showed that the degree of orientation changes with photolysis wavelength, a signature of quantum coherence (quantum beat) effect. See this poster, and the published papers [1, 2] for more details.

Theory of photodissociation

To make measurements of photofragment polarization requires a robust set of tools for analysing the experimental data. We have developed a formalism for calculating the effect of long-range quadrupole-quadrupole interactions on atomic polarization [3]. Our formalism breaks the problem down into simple components, making the calculation of the expected adiabatic polarization of the products very simple. We have also shown that the polarization coherence (such as that measured for ICl or for hydrogen peroxide, above) can be as sensitive to the shape of the dissociating potential energy path as much as 10 cm^-1 per Angstrom [4], many times more sensitive than current ab initio calculations for heavy atomic systems (see Fig.1 below). We have devised a theoretical framework for determining the absolute helicity of photofragments, based on knowing the polarization of the light (left or right circularly polarized, LCP or RCP) used [5]. Among other things, this work has corrected some errors in the literature concerning polarization (in particular, the spherical tensor notation).

Figure 1: the effects of very small changes in the PES (left figure) on the degree of orientation of Cl atoms (right figure) are shown (Circles are experimental points, dashed lines are theory).

Spin-polarized H atoms

In collaboration with colleagues at FORTH, Crete (Prof. T. Peter Rakitzis), we have been working on optical methods for production and detection of spin-polarized H atoms (SPH). The production of SPH is of particular use for particle physics (SPH target samples are used to measure polarization of particle beams). The measurement of polarization normally requires a bulky, expensive, Stern-Gerlach experiment, which relies on separating the atoms by a magnetic field. By contrast, our optical scheme offers nanosecond time resolution, sub-micron spatial resolution, and all in a table-top setup (Fig. 2). The degree of polarization that we achieve is very high, and is a stringent test of state of the art ab initio calculations (Fig. 3). More details can be found in the published papers [6, 7].

References:

1. A. J. Alexander, Phys. Rev. A. 66, 060702 (2002): "Interference between dissociating states in H₂O₂ and HOCl causes orientation of OH diatomic products". doi:10.1103/PhysRevA.66.060702.
2. A. J. Alexander, J. Chem. Phys. 118, 6234 (2003): "Photofragment angular momentum polarization from dissociation of hydrogen peroxide near 355 nm". doi:10.1063/1.1557920.
3. A. J. Alexander, Phys. Chem. Chem. Phys. 7, 3693 (2005): "Calculation of adiabatic polarization of atomic photofragments under the influence of long range quadrupole-quadrupole interactions". doi:10.1039/b509864e.
4. A. J. Alexander, T. P. Rakitzis, Mol. Phys. 103, 1665 (2005): "Effects of long-range potentials on polarization of chlorine atoms from photodissociation of ICl". doi:10.1080/00268970500074985.
5. A. J. Alexander, J. Chem. Phys. 123, 194312 (2005): "Determination of the helicity of oriented photofragents".doi:10.1063/1.2122667.
6. D. Sofikitis, L. Rubio-Lago, A. J. Alexander, T. P. Rakitzis, Europhys. Lett. 81, 68002 (2008) "Nanosecond control and high-density production of spin-polarized hydrogen atoms". doi:10.1209/0295-5075/81/68002
7. D. Sofikitis, L. Rubio-Lago, L. Bougas, A. J. Alexander, T. P. Rakitzis, J. Chem. Phys. 129, 144302 (2008): "Laser-detection of spin-polarized hydrogen from HCl and HBr photodissociation: Comparison of H- and halogen-atom polarizations ". doi:10.1063/1.2989803