Production of cold atoms via photdissociation of small molecules

Thesis

This thesis describes the development of a relatively novel technique for the gen- eration and subsequent trapping of cold species. Molecules in a pulsed supersonic expansion are photolysed, such that the centre-of-mass velocity vector of one of the fragments is equal in magnitude but opposed in orientation to the lab-frame velocity of the precursor molecule. This technique, known as ‘Photostop’, leaves a fraction of the fragments with a narrow velocity distribution, centered around zero velocity in the lab-frame. They can be shown to have zero velocity by changing the time between photodissociation and ionisation; fragments with a high kinetic energy will leave the ionisation volume prior to interrogation. The underlying velocity distribu- tion is uncovered by using the velocity-map imaging technique, and the temperature of the fragments can be determined.

The method was originally optimised for the molecular case. Cold NO has been produced from the dissociation of NO₂ molecules, and a single rotational state has been shown to remain in the ionisation volume 10 μs after dissociation, implying a sample temperature of 1.17 K. Using the optimised experimental conditions de- rived from the velocity cancellation of NO, the atomic case is demonstrated for the dissociation of Br₂ to give zero-velocity Br fragments. The Br atoms are seen for delay times in excess of 100 μs, showing the greater applicability of the method to the atomic case. The temperature of the residual atoms is shown to be in the milliKelvin regime, as determined through detailed Monte Carlo simulation of the motion of the stopped atoms. The possibility of trapping the ultracold Br atoms in a magnetic field is explored, and a quadrupolar trap created between two per- manent bar magnets is demonstrated to confine the atoms spatially, within the ion extraction optics, for delays in excess of 1 ms.

The Photostop technique is intended to be a stepping stone on the way to widening the number of chemical species available for study in the ultracold regime. The possibility of improvements to the experiment is considered, in order to increase the efficiency of the experiment such that the number density becomes high enough to be viable as a source of atoms for use in cold chemical reactive studies. The possibility of extending the method so as to be used as a tunable velocity source of atoms is also discussed.

Authors: Doherty, W; William G Doherty

• Publication date: 2012
• DOI: 10.5287/ora-5jj6jmyoe
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: photodissociation; ultracold; cold; bromine; atoms; lasers
• Subjects: Chemistry & allied sciences; Photochemistry and reaction dynamics; Physical & theoretical chemistry; Atomic and laser physics