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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - CRESUCHIRP (Ultrasensitive Chirped-Pulse Fourier Transform mm-Wave Detection of Transient Species in Uniform Supersonic Flows for Reaction Kinetics Studies under Extreme Conditions)

Teaser

Summary

Complex gas-phase reactive chemical systems exist both in nature (e.g. Earthâ€™s atmosphere, interstellar molecular clouds, extraterrestrial planetary atmospheres) and in technological applications (e.g. flames, internal combustion engines). The chemical evolution of these environments turns principally around the reactivity of highly reactive transient species called radicals â€“ atoms and molecules usually possessing one or more unpaired electrons. In order to understand and model this evolution, and eventually be able to predict future behaviour requires knowledge of the efficiency (or rate constants) of typically many thousands of reactions involving these species, over very wide ranges of temperature from a few degrees above absolute zero in the case of interstellar clouds to many thousands of degrees in combustion. For the vast majority of these reactions between a radical species and another molecule, there exist multiple product channels, yet to date most experiments only measure the overall disappearance rate constants of the reactants and not the individual product branching ratios to the different product channels. This means that it is impossible to use these data to model these natural and man-made complex chemical environments.
Measuring so-called product branching ratios, and the related problem of the quantitative detection of short-lived transient intermediates, in particular at very low or very high temperatures, is one of the greatest challenges in modern chemical kinetics. Why is this so hard? Because it requires a detection technique that is able to detect multiple species at the same time (a multiplex technique) and quantitatively relate their concentrations, while being rapid enough (microsecond time scale) to follow individual chemical reactions in isolation, thus avoiding the interference that would otherwise result (the so-called isolation technique).
The most sensitive modern technique currently used in gas phase chemical kinetics experiments, laser-induced fluorescence (LIF), is essentially unable to provide this quantitative cross-comparison between products, rendering it unsuitable for this purpose. A number of other techniques are currently being developed to accomplish this feat. All of these new techniques suffer from a lower typical sensitivity compared to LIF and require technical breakthroughs and lengthy development to render them suitable for this application. The objective of this project is to take one of these, the revolutionary chirped pulse Fourier transform millimetre wave (CPFTMW) rotational spectroscopy technique invented recently by Brooks Pate and co-workers at the University of Virginia and combine it with a pioneering tool invented and developed in Rennes, the CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique which generates large volumes of cold gas which helps to enhance the sensitivity of the CPFTMW technique as well as simulating cold environments.
Initial experiments combining the CPFTMW and CRESU techniques (the Chirped Pulse in Uniform supersonic Flow or CPUF technique) were performed in the group of Arthur Suits (then Wayne State University, Detroit) in collaboration with the Rennes group and Bob Field from MIT. The main objective of the current CRESUCHIRP project is to develop the CPUF technique and increase its sensitivity to enable its use for detecting transient species and measuring product branching ratios at low temperatures, and if possible at higher temperatures relevant to combustion.

Work performed

During the first 12 months of the project (September 2016 â€“ August 2017) the following work was performed and results achieved:
- additional funding was sought and obtained from Rennes Metropole and the University of Rennes 1 to enable the creation of a dedicated laboratory and office/control room for the CRESUCHIRP project
- the new CRESUCHIRP laboratory and associated laser laboratory and was designed, tendered and constructed
- a new CRESU chamber was designed
- a tender was launched for the purchase of a high power high repetition rate ultraviolet excimer laser
- calculations were performed in collaboration with the group of Stephen Klippenstein on the formation of HC5N (a likely target for CPFTMW detection) in space environments by the first postdoc recruited to the project to compare with experimental results already obtained
- complex and lengthy negotiations with a commercial company to build a chirped pulse spectrometer revealed finally that this option would not be suitable for our application
- the successful recruitment of a postdoc with experience in building a chirped pulse spectrometer was therefore undertaken, along with a younger postdoc also with microwave experience

During the second period of 12 months (September 2017 â€“ August 2018) the following work was performed:
- the two postdocs joined the group in September, additional funding obtained from Brittany Region enabled us to envisage building two CPFTMW spectrometers rather than one as initially planned
- the new excimer laser was received and installed
- we designed and specified a completely new spectrometer in the E-band (60-90 GHz), involving complex and detailed negotiations with dozens of suppliers (September â€“ November 2017)
- tenders were prepared for the main equipment items for this spectrometer (December -January 2017), and orders placed (January-March 2018)
- a similar process was then undertaken for a Ka band (26.5-40 GHz) spectrometer (March-June 2018) including a revolutionary solid state power amplifier (SSPA)
- construction of the E-band spectrometer was commenced as the components arrived, but the delivery of the final stage amplifier was delayed until July 2018
- two M2 students joined the project (February â€“ June 2018) and were recruited to start as PhD students in the Autumn
- a new method of production of Laval nozzles by 3-D photolithographic printing was implemented within one of these M2 projects, enabling new Laval nozzles to be produced with optimised design for the CRESUCHIRP application
- within the other M2 project a room temperature flow cell was designed and constructed in preparation for testing of the spectrometers
- a postdoc with experience in the CRESU technique provided training for the rest of the CRESUCHIRP team in the CRESU technique (January-June 2018) and characterisation of the newly designed and printed Laval nozzles

During the period from September 2018 to date the following work was performed and results obtained:
- initial tests of the E-band spectrometer (September-December 2018) revealed that some components were not performing to specification
- replacements were ordered and a second version of the spectrometer implemented (December 2018 â€“ January 2019)
- using these new components, the first test results were obtained in a room temperature flow cell (February 2019)
- these test results revealed what had been suspected but not demonstrated, that as the pressure was increased from values used in previous chirped pulse spectroscopic experiments to those typical of CRESU experiments the free induction decay (recorded signal) was not only reduced in intensity but also changed in form (its decay becoming exponential). This can be seen on the attached graphic. Reflections of the excitation chirped pulse at short times interfere with this short duration rapidly decaying FID signal, limiting severely the sensitivity of the technique.
- the E-band spectrometer was installed in combination with the CRE

Final results

The detailed study of the production of HC5N by low temperature neutral-neutral reactions is very novel and is currently being written up for publication. The theoretical calculations underpinning this work were carried out within the CRESUCHIRP project and we hope to measure directly the production of HC5N using the CPUF technique.

The new results on pressure broadening at room temperature in a flow cell obtained with the new E-band chirped pulse spectrometer are also completely novel and will be published with a full description of this novel and unique instrument.

The first, pioneering detection of photodissociation products in the cold CRESU flow necessitated the use of a high repetition rate detection which is completely unique â€“ this is a major advance that will be followed by a careful campaign to increase the experimental signal to the level where bimolecular reaction products can be observed.

In order to achieve this increase we envisage the application of a number of technical advances that are currently underway in the group. Once this is achieved, a wide range of fast bimolecular reactions will be studied to determine their product branching ratios at low temperatures.