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First measurements of the differential positronium-formation cross-sections

Positrons are the antimatter version of electrons and so their fate in a matter world is ultimately to annihilate. However, prior to this, a positron may combine with an electron to form a matter-antimatter hybrid called positronium. This is akin to a hydrogen atom with the proton replaced by a positron. Fundamental to our understanding of the physical universe, positron and positronium are these days also acknowledged as being fantastically useful in practical applications such as probing material properties and medical diagnostics. However, there is still much that we do not know for sure about the details of the interactions of these particles with ordinary matter. For example if, in a collision with an atom or molecule, a positron captures an electron, in which directions is the positronium likely to travel and with what probability? More...

Published: Jun 17, 2015 12:35:19 PM

CO2 Satellite

New calculations to improve carbon dioxide monitoring from space

How light of different colours is absorbed by carbon dioxide (CO2) can now be accurately predicted using new calculations developed by a UCL-led team of scientists. This will help climate scientists studying Earth’s greenhouse gas emissions to better interpret data collected from satellites and ground stations measuring CO2. More...

Published: Jun 15, 2015 10:29:10 AM

Watt Steam Engine

On quantum scales, there are many second laws of thermodynamics

New research from UCL has uncovered additional second laws of thermodynamics which complement the ordinary second law of thermodynamics, one of the most fundamental laws of nature. These new second laws are generally not noticeable except on very small scales, at which point, they become increasingly important. More...

Published: Feb 10, 2015 11:55:53 AM

Professor W Roy Newell

Email w.r.newell@ucl.ac.uk
Telephone +44 (0) 20 7679 7140

Research Profile

The general theme of the current research is the interaction of laser radiation with atomic matter, collisional processes in laser fields and atomic scattering phenomena. While the work is of a fundamental nature there is ample application in environmental physics and the physics of fusion plasmas. The use of computers for data transfer, modelling and covariance mapping is a path-way to techniques used in commerce.

Intense Laser Fields

In the interaction of short pulse (10-9 s - 10-14 s) high intensity radiation with molecules we study the fragmentation processes of molecules in laser fields of 1017  W/cm2 . This intensity is greater than that existing between the electron and proton in the ground state of atomic hydrogen. The dynamics of the molecule, re-orientation and symmetry changes are determined using a newly developed momentum imaging technique in which the molecular structure is imaged on a detector using the Coulomb explosion of the molecule. In a 30 fs pulse all natural molecular rotation and vibration is frozen during the interaction. New processes of molecular interactions with intense light fields are revealed.

Collision Processes in Laser Fields

In this area of research we study the interaction of free electrons with atoms and molecules while dressed by a laser field. The electron and atom can virtually exchange photons with the laser field before and after a collision. Only when a real collision occurs does the virtually dressed electron retain the extra photon energy and the structureless electron speeds up. This is Free-Free scattering. Additionally the combined energy, E, of an electron and n virtually absorbed photons, E + nhv, can cause excitation of real states of energy Ex when E < Ex . This process is Simultaneous Electron Photon Excitation (SEPE) and is the electron-photon analogue of two photon excitation.

Scattering Phenomena

Using High Resolution Electron Energy Loss Spectroscopy (HREELS) we measure the cross sections for excitation processes in molecules of atmospheric interest. In particular we study scattering from molecular excited states.


Good laser facilities providing 6 ns pulses with 0.8J/pulse at 1064 nm and 532 nm and 30 to 200 ps pulses with 60 microJ/pulse are available; also a continuous 400 W CO2 laser system is operational. In addition to the laser systems, several atomic scattering apparatus, HREELS and TOF, are available. Modern computer controlled data acquisition systems are in use.

Femtosecond laser pulses are available at the ASTRA facility at RAL.

The group currently has one PDRA and three research students. There are established collaborations with RAL, QUB and TMU.

Page last modified on 06 mar 08 18:49