Quantum Control and reactions with SO2: Photodissociation of molecules at rest and generation of cold SO and O
The general aim is to generate a cold molecular sample by deceleration of SO2 to a standstill and to control the molecule in its external and internal degrees of freedom.
This molecular sample will be used for the investigation of cold reactions like photofragmentation.
The decelerator makes use of the force on the permanent electric dipole moment of SO2 generated by spatially inhomogeneous and time varying electrical fields to slow down the translational motion of the molecules in a series of steps.
Since SO2 offers the extraordinary possibility of dissociation at threshold, the energy of the photofragments can be varied by the choice of the predissociating level in the SO2 molecule and by external field control.
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Until present only in few experiments trapping of molecules was achieved and the applications of the cold molecules produced in subsequent experiments are very limited. We have built up a first prototype Stark-decelerator for SO2 following the example of Meijer’s group.
The SO2 molecule offers the exciting possibility to study simple reactions by photodissociation and producing new cold species like SO and O. We have shown that SO2 is, despite its high mass, a suitable candidate for Stark deceleration . A picture of a part of our first Stark decelerator is shown above.
Currently we are building an about 2 m long decelerator to stop SO2 in a well defined quantum state, which is low field seeking. The decelerator is constructed and produces the first results!
Finally, it is planned to apply a voltage difference of ±12.5 kV between opposing electrodes, the spacing between the electrodes is 2 mm only.
It is very important to have the initial SO2 beam as slow as possible in order to decrease the amount of kinetic energy; which has to be removed by the decelerator. For this purpose we use a nitrogen-cooled pulsed valve (below left).
The slow molecular beam is then focused into the Stark decelerator by an 8 cm long hexapole lens (above right). During operation there is a potential difference of 24 kV between the electrodes!
The influence of all elements of the Stark decelerator on the molecules can be modeled precisely. This is done by classical trajectory calculations of the molecule in the inhomogeneous electric field and randomized starting conditions.
What is next?
Trapping of SO2 and of fragments SO and O
When SO2 is available at zero velocity it shall be trapped electrostatically. The trap itself will be mounted in a chamber separated from the decelerator to insure low background pressure and long storage times. The trap should have sufficient optical accesses to manipulate and detect the molecules. In first experiments we will study the predissociation of SO2 and the velocity distribution of the photofragments to prove the existence of cold SO molecules.
The dissociation process can for example be manipulated by additional fields that couple the predissociating level to others. An analysis of the kinetic energy can be done by time-of-flight analysis or simultaneous trapping of SO and variation of the trap depth. An improved determination of the dissociation energy and collisional properties of the system SO2 – SO should be ascertainable in this way. In future, the Stark effect of SO can be measured with high precision or scattering experiments with SO2 or its photofragments with other particles can be done under well defined conditions.
Control of the photodissociation
- S. Jung, E. Tiemann, and Ch. Lisdat: Cold atoms and molecules from fragmentation of decelerated SO2, Phys. Rev. A 74, 040701 (R) (2006)
- S. Jung, E. Tiemann, and Ch. Lisdat: The Stark effect of the excited C 1B2 state and manipulation of dissociation channels, J. Phys. B 39, S1085 (2006)