Detection and reaction kinetics of reactive species in particle-forming environments using IR modulation spectroscopy by the example of dusty plasmas

• Applicant: Professor Dr. Gernot Friedrichs
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 401081403

• Final Report Year: 2023

Final Report Abstract

Within the scope of the project, different spectrometers based on frequency modulation spectroscopy (FMS) have been set up for the sensitive and time-resolved absorption measurement of gas phase species in the mid-IR spectral range (MIR). Both single-tone and two-tone modulation schemes (TTFMS) were characterized with respect to the attainable detection sensitivity. For it, the narrowbandwidth light of two tunable MIR laser systems (an optical parametric oscillator, cw-OPO, or interband cascade laser, ICL) was modulated with external electro-optical modulators at high frequencies in the range of 500 MHz and the resulting signal was recorded by means of full-digital two-phase demodulation with a novel UHF lock-in amplifier. As previously shown for single-tine FM, a detailed theoretical treatment of TTFMS shows that it is possible to perform calibration-free absorption measurements with two-tone methods as well by using so-called FM factors. The various spectrometer configurations were successfully applied for different gas-phase measurements in a room-temperature flow reactor (CH4, HCl and OH detection), at high temperatures behind shock waves (HCN detection), and in particular in dust-forming C2H2/Ar plasmas. It has been demonstrated that despite strong background absorption or scattering effects occurring in the plasma reactor during particle formation, it is possible to perform quantitative measurements by taking advantage of the derivative nature of FMS. Time-resolved measurements of C2H2 (acetylene) as well as first simultaneous measurements of C2H2 and C4H2 (diacetylene) under varied plasma conditions and over several dust formation cycles allow first interesting insights into the complex plasma chemistry. Spectroscopic and kinetic measurements of C2H radicals, which play a key role in the formation of the first long-chain hydrocarbons and hence for particle growth, were also carried out in the flow reactor. However, due to the overall low particle densities in the low pressure plasma and despite the very good sensitivity of the FM detection system, it has not yet been possible to detect C2H during plasma discharge. It turned out that pronounced baseline effects occurred when using the cw-OPO laser, making averaging of the measured signal over longer time scales difficult an ineffective. This problem can be circumvented by using an interband cascade laser (which only became available at the end of the project), such that the next project steps are already mapped out.

Publications

• Single-tone mid-infrared frequency modulation spectroscopy for sensitive detection of transient species. Optics Express, 27(19), 26499.
  Stuhr, Michael; Faßheber, Nancy & Friedrichs, Gernot
  
• Correlation of the dynamics with void transition events of the growth chain of nanodust in a reactiv argon-acetylene plasma; SMuK21, DPG Meeting: Matter and Cosmos Section, Germany, September 2021.
  S. Groth, N. Faßheber, G. Friedrichs & F. Greiner
  
• Quantitativ and Sensitiv Mid-Infrared Frequency Modulation Detection of HCN Behind Shock Waves 10th European Combustion Meeting; Napoli, Italy, April 2021.
  M. Stuhr, S. Hesse & G. Friedrichs
  
• Quantitativ detection of C2H2 in a dusty plasma environment using sensitiv mid-IR frequency modulation spectroscopy; SAMOP21: DPG Meeting: Atomic, Molecular, Plasma Physics and Quantum Optics Section, Germany, September 2021.
  M. Pal, M. Stuhr, N. Faßheber, A. Petersen, F. Greiner & G. Friedrichs
  
• Quantitative and Sensitive Mid-Infrared Frequency Modulation Detection of HCN behind Shock Waves. Fuels, 2(4), 437-447.
  Stuhr, Michael; Hesse, Sebastian & Friedrichs, Gernot
  
• Shedding light on dusty plasma chemistry using sensitiv laser diagnostics; GDCh Christmas Colloquium, Sektion Chemie, Christian-Albrechts-Universität zu Kiel, Dezember 2021.
  M. Pal
  
• Mid-infrared Frequency Modulation Detection of HCN and Its Reaction with O Atoms behind Shock Waves. The Journal of Physical Chemistry A, 126(50), 9485-9496.
  Stuhr, Michael & Friedrichs, Gernot
  
• Mid-IR frequency modulation spectroscopy for sensing C2H2 in a dusty plasma. Optical Sensors and Sensing Congress 2022 (AIS, LACSEA, Sensors, ES), LM4B.4. Optica Publishing Group.
  Pal, Mithun; Stuhr, Michael; Faßheber, Nancy; Petersen, Andreas; Greiner, Franko & Friedrichs, Gernot
  
• Optical detection of C4H2 in an Ar/C2H2 dusty plasma using mid-IR ICL-based frequency modulation spectroscopy. Optica Sensing Congress 2023 (AIS, FTS, HISE, Sensors, ES), STu2D.7. Optica Publishing Group.
  Pal, Mithun; Faßheber, Nancy; Petersen, Andreas; Greiner, Franko & Friedrichs, Gernot
  
• Quantitative and calibration-free mid-IR two-tone frequency modulation spectroscopy (TTFMS); Bunsen-Tagung Berlin, June 2023.
  M. Pal, M. Stuhr & G. Friedrichs