CRESU experiment
Encyclopedia
The CRESU experiment is an experiment
investigating chemical reactions taking place at very low temperature
s.
The technique involves the expansion of a gas or mixture of gases through a de Laval nozzle
from a high pressure reservoir into a vacuum chamber. As it expands, the nozzle collimates the gas into a uniform supersonic
beam that is essentially collision free and has a temperature that, in the centre of mass frame, can be significantly below that of the reservoir gas. Each nozzle produces a characteristic temperature. This way, any temperature between room temperature and about 10K can be achieved. There are relatively few CRESU apparatuses in existence for the simple reason that the gas throughput and pumping requirements are huge, which makes them expensive to run. Two of the leading centres have been the University of Rennes
(France) and the University of Birmingham
(UK). A more recent development has been a pulsed version of the CRESU, which requires far less gas and therefore smaller pumps. One might well ask why we should use such a complex method for producing low temperature gases when they could be produced much more easily using liquid helium. The answer is simple: most species have a negligible vapour pressure at such low temperatures and this means that they quickly condense on the sides of the apparatus. Essentially, the CRESU technique provides a "wall-less flow tube," which allows the kinetics of gas phase reactions to be investigated at much lower temperatures than otherwise possible.
Chemical kinetics
experiments can then be carried out in a "pump-probe" fashion using a laser to initiate the reaction (for example by preparing one of the reagents by photolysis of a precursor), followed by observation of that same species (for example by laser-induced fluorescence
) after a known time delay. The fluorescence signal is captured by a photomultiplier
a known distance downstream of the de Laval nozzle. The time delay can be varied up to the maximum corresponding to the flow time over that known distance. By studying how quickly the reagent species disappears in the presence of differing concentrations of a (usually stable) co-reagent species the reaction rate constant at the low temperature of the CRESU flow can be determined.
Reactions studied by the CRESU technique typically have no significant activation energy barrier. In the case of neutral-neutral reactions (i.e., not involving any charged species, ions), these type of barrier-free reactions usually involve free radical species such as molecular oxygen (O2), the cyanide radical (CN) or the hydroxyl radical (OH). The energetic driving force for these reactions is typically an attractive long range intermolecular potential.
CRESU experiments have been used to show deviations from Arrhenius kinetics
at low temperatures: as the temperature is reduced, the rate constant actually increases. They can explain why chemistry is so prevalent in the interstellar medium
, where many different polyatomic species have been detected (by radio astronomy
), but where temperatures are so low that conventional wisdom might suggest that chemical reactions do not occur.
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
investigating chemical reactions taking place at very low temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
s.
The technique involves the expansion of a gas or mixture of gases through a de Laval nozzle
De Laval nozzle
A de Laval nozzle is a tube that is pinched in the middle, making a carefully balanced, asymmetric hourglass-shape...
from a high pressure reservoir into a vacuum chamber. As it expands, the nozzle collimates the gas into a uniform supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
beam that is essentially collision free and has a temperature that, in the centre of mass frame, can be significantly below that of the reservoir gas. Each nozzle produces a characteristic temperature. This way, any temperature between room temperature and about 10K can be achieved. There are relatively few CRESU apparatuses in existence for the simple reason that the gas throughput and pumping requirements are huge, which makes them expensive to run. Two of the leading centres have been the University of Rennes
University of Rennes
The University of Rennes was a French university located in the city of Rennes. It was established by the union of the 3 faculties of the city in 1885. In 1969, it was divided in two new universities:* the University of Rennes 1...
(France) and the University of Birmingham
University of Birmingham
The University of Birmingham is a British Redbrick university located in the city of Birmingham, England. It received its royal charter in 1900 as a successor to Birmingham Medical School and Mason Science College . Birmingham was the first Redbrick university to gain a charter and thus...
(UK). A more recent development has been a pulsed version of the CRESU, which requires far less gas and therefore smaller pumps. One might well ask why we should use such a complex method for producing low temperature gases when they could be produced much more easily using liquid helium. The answer is simple: most species have a negligible vapour pressure at such low temperatures and this means that they quickly condense on the sides of the apparatus. Essentially, the CRESU technique provides a "wall-less flow tube," which allows the kinetics of gas phase reactions to be investigated at much lower temperatures than otherwise possible.
Chemical kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
experiments can then be carried out in a "pump-probe" fashion using a laser to initiate the reaction (for example by preparing one of the reagents by photolysis of a precursor), followed by observation of that same species (for example by laser-induced fluorescence
Laser-induced fluorescence
Laser-induced fluorescence is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements....
) after a known time delay. The fluorescence signal is captured by a photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
a known distance downstream of the de Laval nozzle. The time delay can be varied up to the maximum corresponding to the flow time over that known distance. By studying how quickly the reagent species disappears in the presence of differing concentrations of a (usually stable) co-reagent species the reaction rate constant at the low temperature of the CRESU flow can be determined.
Reactions studied by the CRESU technique typically have no significant activation energy barrier. In the case of neutral-neutral reactions (i.e., not involving any charged species, ions), these type of barrier-free reactions usually involve free radical species such as molecular oxygen (O2), the cyanide radical (CN) or the hydroxyl radical (OH). The energetic driving force for these reactions is typically an attractive long range intermolecular potential.
CRESU experiments have been used to show deviations from Arrhenius kinetics
Arrhenius equation
The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of the reaction rate constant, and therefore, rate of a chemical reaction. The equation was first proposed by the Dutch chemist J. H. van 't Hoff in 1884; five years later in 1889, the Swedish...
at low temperatures: as the temperature is reduced, the rate constant actually increases. They can explain why chemistry is so prevalent in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
, where many different polyatomic species have been detected (by radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
), but where temperatures are so low that conventional wisdom might suggest that chemical reactions do not occur.