Photodissociation
What is photodissociation and what is it useful for?
Photodissociation is the use of light to break chemical bonds. It is a highly versatile means of initiating chemical reactions, since reactive species (radicals, ions) can be produced in large concentrations. There is lot of photodissociation (and reactions) going on throughout our atmosphere, so it is often studied and exploited by atmospheric chemists.
Photodissociation is the use of light to break chemical bonds. It is a highly versatile means of initiating chemical reactions, since reactive species (radicals, ions) can be produced in large concentrations. There is lot of photodissociation (and reactions) going on throughout our atmosphere, so it is often studied and exploited by atmospheric chemists.
What can photodissociation tell us about chemical bonds?
Our group is interested in the physics of bond scission, and we have made fundamental contributions to both experiments and theory. By using polarized light to break bonds and then again polarized light to probe and detect the fragments, we can learn a significant amount about the dynamics of bond breaking. In particular we can learn about the potential energy surfaces (a bit like a mountain range) that the products travel across from beginning to end. For a general overview, see ref. [1].
Our group is interested in the physics of bond scission, and we have made fundamental contributions to both experiments and theory. By using polarized light to break bonds and then again polarized light to probe and detect the fragments, we can learn a significant amount about the dynamics of bond breaking. In particular we can learn about the potential energy surfaces (a bit like a mountain range) that the products travel across from beginning to end. For a general overview, see ref. [1].
Optical production and detection of spin-polarized H atoms
In collaboration with Peter Rakitzis (FORTH) we have developed optical methods for measuring the spin polarization of H atoms in the gas phase [2,3]. The images below illustrate the method. The pump laser pulse uses either left (LCP) or right (RCP) circularly polarized light to photolyse HBr (or HCl) molecules in the gas phase. A polarized probe laser pulse (LCP) in combination with a linear polarizer and photomultiplier tube (PMT) is then capable of detecting the degree of spin-polarization of the H atom products.
In collaboration with Peter Rakitzis (FORTH) we have developed optical methods for measuring the spin polarization of H atoms in the gas phase [2,3]. The images below illustrate the method. The pump laser pulse uses either left (LCP) or right (RCP) circularly polarized light to photolyse HBr (or HCl) molecules in the gas phase. A polarized probe laser pulse (LCP) in combination with a linear polarizer and photomultiplier tube (PMT) is then capable of detecting the degree of spin-polarization of the H atom products.
How polarized are the H atoms?
The degree of polarization can be measured using polarization parameters (a). The graph below shows that the polarization of spin along the direction of travel of the atoms (the z axis) is very high, and agrees well with theoretical calculations. There is also a small component of spin perpendicular to the direction of travel (which we call the x axis). For further details see refs [2] and [3].
The degree of polarization can be measured using polarization parameters (a). The graph below shows that the polarization of spin along the direction of travel of the atoms (the z axis) is very high, and agrees well with theoretical calculations. There is also a small component of spin perpendicular to the direction of travel (which we call the x axis). For further details see refs [2] and [3].
Why is this important?
Our method outstrips current methods of production by several orders of magnitude in several areas. For example the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory produces spin polarized H atoms using a Stern Gerlach method (magnetic fields).
Our method outstrips current methods of production by several orders of magnitude in several areas. For example the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory produces spin polarized H atoms using a Stern Gerlach method (magnetic fields).
Theory behind measurements of photofragment polarization
Our group has also made several contributions to the theory underlying photofragment polarization. In our work on determination of the helicity of oriented photofragments, we deal with confusion between definitions of left and right circularly polarized light [4]. More recently, Rakitzis and Alexander have given a polarization-parameter model in the molecular frame that can be used to describe product angular momentum polarization from the one-photon photodissociation of polyatomic molecules [5]. The equations allow researchers to extract the degree of orientation of alignment from experiments, and to present them in a convenient form. This work was chosen by Editors of The Journal of Chemical Physics as one of 80 to represent 'outstanding work' published in the journal from 1933-2013 (their 80th anniversary). Our citation appears among legendary authors including Eyring, Mulliken, Marcus, Pople, and many more.
Our group has also made several contributions to the theory underlying photofragment polarization. In our work on determination of the helicity of oriented photofragments, we deal with confusion between definitions of left and right circularly polarized light [4]. More recently, Rakitzis and Alexander have given a polarization-parameter model in the molecular frame that can be used to describe product angular momentum polarization from the one-photon photodissociation of polyatomic molecules [5]. The equations allow researchers to extract the degree of orientation of alignment from experiments, and to present them in a convenient form. This work was chosen by Editors of The Journal of Chemical Physics as one of 80 to represent 'outstanding work' published in the journal from 1933-2013 (their 80th anniversary). Our citation appears among legendary authors including Eyring, Mulliken, Marcus, Pople, and many more.
JCP_80th_anniversary.pdf |