Periodic Reporting for period 4 - CRESUCHIRP (Ultrasensitive Chirped-Pulse Fourier Transform mm-Wave Detection of Transient Species in Uniform Supersonic Flows for Reaction Kinetics Studies under Extreme Conditions)
Reporting period: 2021-03-01 to 2022-08-31
Complex gas-phase reactive chemical systems exist both in nature (e.g. Earth’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 transient species called radicals – highly reactive 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 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, 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 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 objective of this project is to take one of these, the revolutionary chirped pulse Fourier transform millimetre wave (CPFTMW) rotational spectroscopy technique 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 gas-phase environments.
Initial experiments combining these 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 our group in Rennes and Bob Field from MIT. The main objective of the CRESUCHIRP project is to develop the CPUF technique and increase its sensitivity to enable its routine use for detecting transient species and measuring product branching ratios at low temperatures.
At the conclusion of this action, a completely new laboratory has been constructed with a dedicated CRESU chamber, equipped with high power high repetition rate excimer laser photolysis and two state of the art chirped pulse microwave spectrometers. The relatively high pressure of the CRESU flows meant that the original project had to be modified to enable lower pressure detection. This was achieved by the end, resulting in the quantitative determination of branching ratios for reactions of astrochemical interest. The core team of young researchers can be seen in the attached photo, as well as a photo showing an overview of the new CRESUCHIRP lab.
Measuring so-called product branching ratios, 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 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 objective of this project is to take one of these, the revolutionary chirped pulse Fourier transform millimetre wave (CPFTMW) rotational spectroscopy technique 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 gas-phase environments.
Initial experiments combining these 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 our group in Rennes and Bob Field from MIT. The main objective of the CRESUCHIRP project is to develop the CPUF technique and increase its sensitivity to enable its routine use for detecting transient species and measuring product branching ratios at low temperatures.
At the conclusion of this action, a completely new laboratory has been constructed with a dedicated CRESU chamber, equipped with high power high repetition rate excimer laser photolysis and two state of the art chirped pulse microwave spectrometers. The relatively high pressure of the CRESU flows meant that the original project had to be modified to enable lower pressure detection. This was achieved by the end, resulting in the quantitative determination of branching ratios for reactions of astrochemical interest. The core team of young researchers can be seen in the attached photo, as well as a photo showing an overview of the new CRESUCHIRP lab.
The first year of the project (2016-17) was dedicated to building a new laboratory in Rennes, recruiting staff, and designing the chirped pulse microwave spectrometers. Following on from this during 2018-19, construction of the new instruments was commenced. LIF experiments were performed on reactions of astrochemical interest resulting in two publications. During 2019-20, the first chirped pulse microwave spectrometer (operating in the so-called E-band, covering frequencies from 60 to 90 GHz) was assembled and tested resulting in a publication. These tests revealed a need to operate at lower pressures (see Figure on the effect of pressure on OCS Free Induction Decays or FIDs), and significant modifications were proposed to the original project to mitigate this effect. These included skimmed molecular beam sampling of the reaction products, as well as a more long-term project for a high repetition rate pulsed CRESU. In addition, a dedicated CRESU chamber was designed and built. During 2020-2021, the second spectrometer (operating in the Ka band) was constructed and tested resulting in a publication. The first reaction products from the CN + C2H6 and CN + C2H2 reactions were observed directly in cold He and Ar flows (at 10 K and 30 K), and the resulting article is nearing submission at the time of writing. The observation of HCN (see Figure for preliminary results) and its unstable isomer HNC from laser photolysis of acrylonitrile in cold (10-70 K) He flows enabled measurement of He pressure broadening cross sections in situ by CPFTMW time-resolved spectroscopy, providing vital benchmarking of theoretical calculations essential to the interpretation of astronomical observations. The COVID-19 pandemic resulted in significant delays, but we were able to implement the skimmed molecular beam sampling chamber and fully characterise it by detection of acrylonitrile product from the CN + C2H4 reaction, and another article describing this work is in preparation. During the last year of the project (2021-22) reaction products from the CN + C3H6 reaction were detected and quantified at two different temperatures (35 K and 50 K), and experiments to investigate pressure dependence are currently underway. This reaction is of significant astrochemical interest and is thought to be occurring in cold molecular clouds. The HNC/HCN pressure broadening results along with theoretical calculations by colleagues here in Rennes were published in Nature Chemistry. Finally, we completed construction of a prototype pulsed CRESU with a hydraulically actuated spinning disk valve (see photo), and obtained preliminary results yielding a 5 K He flow at much lower pressure than before (see Figure). This will provide the basis for further projects.
The results of this project have been disseminated by team members in a large number (in excess of 50) of contributed and invited oral presentations, and poster presentations as well as by peer-reviewed scientific publications, both already appeared (7) and in preparation (3-5), as well as via the project website.
The results of this project have been disseminated by team members in a large number (in excess of 50) of contributed and invited oral presentations, and poster presentations as well as by peer-reviewed scientific publications, both already appeared (7) and in preparation (3-5), as well as via the project website.
New designs of chirped pulse microwave spectrometers beyond the state of the art using high power solid state amplifiers were realised, tested and put into service.
The simultaneous generation of the isomeric species HCN and HNC by laser photodissociation of acrylonitrile in cold He flows and their detection by chirped pulse spectroscopy is completely novel and highly innovative, and has resulted in a high-impact publication.
Skimmed molecular beam sampling and detection of reaction products from the cold CRESU flow into a lower-pressure detection chamber is a major new development. This has led to quantitative measurements of branching ratios at different, well-defined low temperatures (35 K and 50 K) for a multichannel reaction of astrochemical interest.
A prototype of a new pulsed CRESU system based on a hydraulically-actuated high-speed spinning disk valve resulting in lower pressure flows has been realised, outperforming all other existing systems.
The simultaneous generation of the isomeric species HCN and HNC by laser photodissociation of acrylonitrile in cold He flows and their detection by chirped pulse spectroscopy is completely novel and highly innovative, and has resulted in a high-impact publication.
Skimmed molecular beam sampling and detection of reaction products from the cold CRESU flow into a lower-pressure detection chamber is a major new development. This has led to quantitative measurements of branching ratios at different, well-defined low temperatures (35 K and 50 K) for a multichannel reaction of astrochemical interest.
A prototype of a new pulsed CRESU system based on a hydraulically-actuated high-speed spinning disk valve resulting in lower pressure flows has been realised, outperforming all other existing systems.