Dr Jan R.R. Verlet
(email at email@example.com)
1. Research Interests
Our research aims to develop a mechanistic understanding of how anions cope when they possess high levels of excess energy. For example, how does DNA dispose of the energy it gains following the absorption of light or how does an electron acceptor keep an electron after it has acquired it? These are important fundamental questions central to understanding how energy can be harvested and how fundamental chemical processes work. To undestand these processes on the molecular level, we perform time-resolved methods on isolated and solvated anions.
Following excitation of a molecular anion, the energy can be converted into a directed mechanical action via a molecular transformation such as isomerisation or to heat by populating highly excited vibrational modes in lower-lying electronically excited states. Both processes require the motion of atoms within the molecule. To study how the energy is transferred to the nuclear dynamics, we monitor the excited state dynamics on the time-scale of the motion of the atoms – that is on a femtosecond timescale (10-15 seconds). Using laser pulses of femtosecond (10-15 seconds) duration, the atoms in a molecule are effectively frozen – similar in concept to the speed of a shutter in a camera. This allows us to monitor in real time molecular processes such as vibrations and non-radiative decay.
Our specific interest is in anions and we have developed the first femtosecond photoelectron imaging experiment that combines electrospray ionisation as a source of anions. This has allowed us to study virtually any anion, including anions with more than one charge (polyanions). Some recent examples are given below.
2. Time-resolved photoelectron imaging of important chromophores
In biology, chemistry and technology, there are many chromophores (molecules or parts of molecules that absorb light) that perform a specific action upon the absorption of photons. In nature, many of these are anions and by isolating the chromophore and studying its dynamics, we can understand the intrinsic properties which in turn allow us to understand how the environment alters these dynamics. As recent examples, we have studied the spectroscopy and dynamics of: the chromophore of the green fluorescent protein (GFP) which is a key fluorescent marker in biology; the indigo chromophore which is a well known dye used in Celtic paint (see Brave Heart) and denim jeans; nucleotides which are the building blocks of DNA.
3. Photoelectron imaging of polyanions
How does an electron leave a multiply charged anion? Unlike anions, neutrals or cations, photodetachment of an electron from a polyanion will leave an anionic species behind. This means that when the photoelectron leaves, it experiences the Coulomb potential of the remaining anion which “guides” the photoelectron in a specific direct. We can measure this directionality using photoelectron imaging methods. We have shown that one can use this to learn about electron tunnelling through the barrier as well as a tool to monitor large-amplitude motion in molecular system. This is qualitatively shown in the figure on the right where then changes in photoelectron anisotropy signify the rotation of a molecular dianion. Initially, the molecule is aligned and photoelectrons are guided in the direction perpendicular to the molecular frame. As time evolves and the molecule rotates, the connetion between lab and molecular frame is lost and the image is isotropic.
4. Ultrafast spectroscopy of radical anions
Many processes in nature and chemistry involve the transfer of an electron. A common electron accepting molecule is a quinone-derivative. Indeed, many of these have been shown to undergo electron transfer processes at rates that exceed theoretical predictions. One explanation for this is the electron transfer into electronically excited states of the anion radical of the quinone. We have studied the dynamics of excited states in a small number of such systems and have shown that excited states at threshold are very short-lived with lifetime of ~ 100 fs. This is equivalent to the time it takes to make a single vibration and indicates that electronic energy is lost very efficiently by internal conversion Hence, as soon as an electron is attached into one of these short lived states, the excess energy can be rapidly converted into internal vibrational energy of the system, which can then be dissipated to the surroundings.
5. Hydrated electrons at surfaces
Arguably the most fundamental anion is the hydrated electron – and electron bound within a cavity formed in water. But what happens if we bring this electron to the surface of water? In the gas-phase, an excess electron can bind to the surface of a water cluster. We have extended this to the ambient water/air interface. Using time-resolved second-harmonic generation spectroscopy, we have shown that hydrated electron can reside at the water/air interface, but remain below the dividing surface, within the first nanometer. Currently, we are extending these studies to gain full quantitative understanding of how an electron solvates at aqueous interfaces and to extend this to other interfaces. We are also developing new gas-phase methods to study electrons at the interface of aqueous nano-particles.
“Communication: Photoelectron spectroscopy of the model GFP chromophore anion” D. A. Horke and J. R. R. Verlet Phys. Chem. Chem. Phys. 14, 8511 (2012)
“Femtosecond photoelectron imaging of aligned polyanions: Probing molecular dynamics through the electron-anion” D. A. Horke, A. S. Chatterley and J. R. R. Verlet J. Phys. Chem. Lett. 3,834 (2012)
“Effect of internal energy on the repulsive Coulomb barrier of polyanions” D. A. Horke, A. S. Chatterley and J. R. R. Verlet Phys. Rev. Lett. 108, 083003 (2012)
“Excited states in electron-transfer reaction products: Ultrafast relaxation dynamics of an isolated acceptor radical anion” D. A. Horke, G. M. Roberts and J. R. R. Verlet J. Phys. Chem. A 115, 8369 (2011)
“Hydrated electrons at the water/air interface” D. M. Sagar, C. D. Bain and J. R. R. Verlet J. Am. Chem. Soc. 132, 6917 (2010)
"Observation of large water-cluster anions with surface-bound excess electrons", J.R.R. Verlet, A.E. Bragg, A. Kammrath, O. Cheshnovsky and D.M. Neumark,Science 307, 93 (2005)