Research Group Prof. Dr. G. Friedrichs

Shock Tube / Frequency Modulation Spectroscopy

Spectroscopystossrohr

Frequency Modulation Spectroscopy

Compared to conventional cw laser absorption detection using a difference amplification scheme (Amin = 1.5 × 10-3), frequency modulation (FM) spectroscopy with a minimum detectable absorption of Amin < 1 × 10-4 provides a significant higher sensitivity for radical detection behind shock waves. The sensitivity of FM spectroscopy stems from the following reasons.

  •     At high modulation frequencies, the 1/f noise of the detection laser is negligible.
  •     State-of-the-art lock-in techniques enable shot-noise limited detection sensitivity.
  •     As a single beam technique, FM spectroscopy is less sensitive to shock tube induced noise components (e.g., shock
        induced birefringence of shock tube windows, beem steering effects).
     

fm_principle


FM Principle

FM spectroscopy is an optical heterodyne technique capable of sensitive detection of absorption and dispersion of not too much broadened spectral features. It also provides an intrinsically fast time response. The light of a continuous narrow-bandwidth laser is phase (or frequency) modulated by an electrooptic modulator. Whereas the corresponding frequency spectrum of the single mode laser light before the modulation is given by the sole frequency of the single mode detection laser (carrier frequency), the modulated light exhibit additional side bands (see Figure). The individual bands have a spacing equal to the applied modulation frequency. For a purely phase (or frequency) modulated laser beam, the (intensity) beating of the side bands against the carrier frequency perfectly cancels out such that the detection of the laser beam intensity (using a fast photodiode) generates a time independent dc signal (no amplitude modulation!). However, when the modulated light passes through an absorbing sample such that the different light components are absorbed to a different extent, the exiting laser light next to its phase modulation also shows a little bit of amplitude modulation. Absorption (or dispersion) transforms phase modulation into amplitude modulation. Now, the generated photodiode signal, next to a dc offset, also shows a superimposed ac current. Demodulation of this ac component using standard demodulation techniques results in the FM signal, which is (among other things) proportional to the concentration of the detected species.

FM Setup

fm_setupThe left Figure depicts a schematic diagram of the FM spectrometer used for radical detection behind shock waves. Main features of the FM system are:
 

  •     - High modulation frequency of 1.0 GHz.
  •     - High modulation index up to M = 2.0 (resonant EOM).
  •     - High total optical power up to 15 mW laser power.
  •     - Accurate phase adjustment by combination of two-
  •       polarizer setup with phase shifter.


Quantitative Detection

iop_setupQuantitative concentration measurements require the knowledge of the FM factor Δf and the electronic gain G of the FM spectrometer.

IFM = 0.5 × I0 × Δf × σ c l × G  

Here, IFM is the intensity of the demodulated FM signal, I0 is the total laser intensity, σ the absorption cross section, and l the absorption path length. Whereas the FM factor can be calculated based on the spectral lineshape of the detected absorption feature, the gain factor G has to be determined experimentally from

  •     a FM signal of a species with known absorption cross section.
  •     a simultaneously measured FM and absorption signal.
  •     a scanning etalon setup.
  •     the specifications of all electronic devices.
     

The Figure demonstrates the determination of the gain factor G by a direct comparison of absorption and FM data. The data were taken using a low pressure iodine cell.

Contributing researchers:  N. Faßheber, S. Hesse, G. Friedrichs and M. Colberg, J. Dammeier (former)

Shock Tube Investigations of High Temperature Reaction Kinetics

Combustion systems are characterized by a complicated interaction of flow and transport processes and a large number of elementary chemical reactions. The purpose of chemical kinetics is to unravel the underlying complex reaction mechanisms and to investigate the products and rates of selected reactions as function of temperature and pressure. In particular, the formation and the reactions of short-lived reactive intermediates (atoms and radicals) have to be well known. These intermediates maintain the combustion process and determine the end product distribution.

stossrohr_bunt_web

Fig. 1: Top: schematic setup of a shock tube. Bottom: pressure/temperature-time profile during a shock tube experiment and typical concentration-time profiles of a) thermal or b) photolytic generation of the detected species.

The shock tube technique has been proven to be a very powerful method for investigating gas phase reactions at high temperatures. With shock tubes, the experimental pressure (0.1-1000 bar) and temperature (500-15000 K) can be easily varied over a wide range. Essentially, a shock tube apparatus (Fig. 1) is just a several meters long tube, which is divided by a diaphragm into a driver (high pressure) and a driven (low pressure) section. The test gas mixture, typically a highly diluted mixture of reactants in argon, is found in the driven section. The driver section is filled with helium or hydrogen until the diaphragm bursts. A shock wave is formed, which propagates downwards the tube at supersonic speed and heats and compresses the test gas within less than 1 μs (incident shock wave). The shock wave is reflected at the end wall and the preheated testgas is heated and compressed again (reflected shock wave). The resulting pressure- and temperature-time-profile is shown in the lower left part of Fig. 1. Typically, the constant conditions behind the reflected wave last for roughly 1 ms - long enough for studying chemical reactions, which are mostly fast at high temperatures. Experimental pressures and temperatures behind the shock waves are calculated from the measured shock wave velocity and the initial conditions using a 1-dimensional shock tube code. Small, independent corrections account for boundary layer perturbations.

In many cases, the species of interest are generated thermally by the decomposition of suitable precursor molecules (Fig. 1a), but often a photolytic production is also feasible (Fig. 1b). Finally, real-time detection of the concentration-time-profiles is accomplished through optical windows by means of a variety of sensitive spectroscopic absorption or emission techniques. We often use the highly sensitive laser absorption based frequency modulation (FM) spectroscopy for quantitative detection of small radicals behind shock waves. With FM spectroscopy the radicals methylene (1CH2) and formyl (HCO), which are both of considerable importance in combustion chemistry, could be detected behind shock waves for the first time.

Kiel Shock Tube

Our hydrogen driven shock tube, which is designed for the investigation of elementary gas phase reactions of small radicals (e.g., NH2, HCO, SiH2, 1CH2, NCN, CN), can be operated at temperatures of 700 K < T < 3500 K and pressures of 0.75 bar < p < 3.5 bar. Radicals are detected by means of FM spectroscopy, UV absorption, and (very soon) IR absorption.

   

shock_tube

    Shock tube: 

    8 m long, 8 cm inner diameter
    electro-polished stainless steel
    shock velocity measured by four piezo-electric pressure sensors
    pmin < 10-7 mbar, oil free turbomolecular drag / diaphragm backing pump
    combination

    Sample preparation:

    18 l stainless steel tank
    oil free vacuum (turbomolecular drag / diaphragm backing pump)
    flow system with mass flow controllers

    Detection systems: 

    Nd:YVO4 laser pumped cw ring-dye laser system
    FM spectroscopy setup
    UV absorption setup
    IR absorption setup
    excimer laser for "side-on" photolysis

NOx Formation: The NCN Pathway

Nitrogen oxides, NOx, are major atmospheric pollutants generated in combustion processes by different mechanisms. For example, under rich combustion conditions, prompt-NO formation is initiated by the reaction of small hydrocarbon radicals with nitrogen stemming from the combustion air. In contrast to textbook knowledge, it has been theoretically and experimentally shown in recent years that the cyanonitrene radicals (NCN) plays a key role in the reaction sequence

CH + N2 → NCN + H → … → NOx .

ncn_sourceIn order to model NOx formation via the NCN pathway, NCN chemistry needs to be implemented into combustion mechanisms. However, as rate constant data for NCN high temperature chemistry were only scarcely available, first implementations mostly relied on theoretical or estimated data. In our group, we use the thermal decomposition of the very toxic and highly explosive NCN3 as a quantitative source for NCN radicals allowing us to directly measure the rate constants of NCN reactions at combustion relevant temperatures for the first time. Concentration-time profiles of NCN radicals were detected at λ = 329.1302 nm by UV difference laser absorption spectroscopy in shock tube experiments [1,3]

ncn_rates_web
By simulating the experimental NCN concentration-time for experiments with different reaction mixture composition the rate constants of several bimolecular NCN reactions could be directly measured [1-7]

Especially the fast reactions NCN + O and NCN + H (and its product branching ratios) are very important to reliably model overall prompt NO concentrations in combustion processes.


In collaboration with N. Lamoureux and P. Desgroux (Lille University), a detailed NCN submechanism including the reactions shown in the Arrhenius plot has been developed and implemented in a global combustion mechanism [6]. Based on this new mechanism, it is possible to simulate prompt NOx formation more reliably, which is important for the design and testing new combustion engine concepts.

ncn_reaction_sheme
 

[1] "The Thermal Decomposition of NCN3 as a High Temperature NCN Radical Source: Singlet-Triplet Relaxation and Absorption Cross Section of NCN(3Σ)", J. Dammeier, G. Friedrichs, J. Phys. Chem. A 114 (2010) 12963–12971.

[2] "Direct Measurements of the Rate Constants of the Reactions NCN + NO and NCN + NO2 Behind Shock Waves", J. Dammeier, G. Friedrichs J. Phys. Chem. A 115 (2011) 14382–14390.

[3] "Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M", J. Dammeier, N. Faßheber, G. Friedrichs Phys. Chem. Chem. Phys. 14 (2012) 1030 – 1037.

[4] "A consistent model for the thermal decomposition of NCN3 and the singlet-triplet relaxation of NCN", J. Dammeier, B. Oden, G. Friedrichs, Int. J. Chem. Kinet. 45 (2013) 30-40.

[5] "Direct measurements of the total rate constant of the reaction NCN + H and implications for the product branching ratio and the enthalpy of formation of NCN", N. Faßheber, J. Dammeier, G. Friedrichs, Phys. Chem. Chem. Phys. 16 (2014) 11647-11657.

[6] "Rate constant of the reaction NCN + H2 and its role for NCN and NO modeling in low pressure CH4/O2/N2-flames", N. Faßheber, N. Lamoureux, G. Friedrichs, Phys. Chem. Chem. Phys. 17 (2015) 15876 – 15886.

[7] "Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2" N. Faßheber, G. Friedrichs, Int. J. Chem. Kinet. 47 (2015) 586-595.

Contributing Researchers: N. Faßheber, S. Hesse, G. Friedrichs and (formerly) J. Dammeier, B. Oden

Glyoxal - An Efficient HCO Source

The formyl radical (HCO) is a key intermediate along the direct oxidation pathway of hydrocarbons. The rate constants of the bimolecular reactions of HCO with the most important oxygen species O, OH, and O2 as well as the HCO thermal decomposition and the reaction HCO + H need to be precisely known in order to model the overall combustion process.

Most of the high-temperature rate constants have been determined in shock tube experiments. But often, due to the high reactivity and low thermal stability of the HCO radical, these experiments turned out to be difficult and inaccurate. Therefore, our group developed the 193 nm glyoxal photolysis as an effective and quantitative source of HCO:

(CHO)2 + hν → (H, HCO, CO, H2, CH2O)

H + (CHO)2 → HCO + CO + H2
 

In combination with a sensitive frequency modulation (FM) scheme for time-resolved HCO radical detection, we were able to directly measure and validate the rate constants of many HCO reactions. In the meantime, the recommended rate constant expressions have been implemented in many broadly used combustion mechanisms.

Glyoxal Thermal Decomposition

gloxal_web






















Interestingly and despite the fact that the HCO forming channel of the unimolecular decomposition is energetically less favorable, also the thermal decomposition of glyoxal yields high amounts of HCO radicals:

(CHO)2 + M → HCO + HCO + M        E0 = 291 kJ/mol

(CHO)2 + M → CO + CO + H2 + M    E0 = 256 kJ/mol

(CHO)2 + M → HCOH + CO + M        E0 = 254 kJ/mol

(CHO)2 + M → CH2O + CO + M         E0 = 232 kJ/mol
 

Experimentally measured HCO radical, (CHO)2, and H atom concentration-time profiles could be very well modeled with the help of unimolecular rate theory (RRKM/SACM/ME calculations). Actually, it turned out that the gyloxal thermal decomposition system is a suitable textbook example for a multi-channel unimolecular decomposition reaction with pronounced weak-collision and rotational effects. At high temperatures and pressures, HCO formations becomes the main product channel.

HCO Detection
 
HCO concentration-time profiles have been detected by FM spectroscopy at a wavelength of λ = 614.752 nm. Absorption cross sections and line shape data for quantitative FM measurements of HCO have been determined for the Q(6)P(1) absorption line of the A2A''-X2A'(0900 ←  0010) transition.

hco_detection
[1] Room Temperature and Shock Tube Study of the Reaction HCO + O2 using the Photolysis of Glyoxal as an Efficient HCO Source, M. Colberg, G. Friedrichs, J. Phys. Chem. A 110 (2006) 160-170.

[2] Wide Temperature Range (T = 295 K and 770-1305 K) Study of the Kinetics of the Reactions HCO + NO and HCO + NO2 using Frequency Modulation Spectroscopy, J. Dammeier, M. Colberg, G. Friedrichs, Phys. Chem. Chem. Phys. 9 (2007) 4177-4188.

[3] HCO Formation in the Thermal Unimolecular Decomposition of Glyoxal: Rotational and Weak Collision Effects, G. Friedrichs, M. Colberg, J. Dammeier, T. Bentz, M. Olzmann, Phys. Chem. Chem. Phys. 10 (2008) 6520-6533.

[4] Glyoxal Oxidation Mechanism: Implications for the reactions HCO + O2 and (CHO)2 + HO2, N. Faßheber, G. Friedrichs, P. Marshall, P. Glarborg, J. Phys. Chem. A 119 (2015) 7305 – 7315.

Contributing Researchers: N. Faßheber, G. Friedrichs and (formerly) M. Colberg, J. Dammeier
 

Shock Tube Investigations of High Temperature Reaction Kinetics

Combustion systems are characterized by a complicated interaction of flow and transport processes and a large number of elementary chemical reactions. The purpose of chemical kinetics is to unravel the underlying complex reaction mechanisms and to investigate the products and rates of selected reactions as function of temperature and pressure. In particular, the formation and the reactions of short-lived reactive intermediates (atoms and radicals) have to be well known. These intermediates maintain the combustion process and determine the end product distribution.

stossrohr_bunt_web
 
Fig. 1: Top: schematic setup of a shock tube. Bottom: pressure/temperature-time profile during a shock tube experiment and typical concentration-time profiles of a) thermal or b) photolytic generation of the detected species.

The shock tube technique has been proven to be a very powerful method for investigating gas phase reactions at high temperatures. With shock tubes, the experimental pressure (0.1-1000 bar) and temperature (500-15000 K) can be easily varied over a wide range. Essentially, a shock tube apparatus (Fig. 1) is just a several meters long tube, which is divided by a diaphragm into a driver (high pressure) and a driven (low pressure) section. The test gas mixture, typically a highly diluted mixture of reactants in argon, is found in the driven section. The driver section is filled with helium or hydrogen until the diaphragm bursts. A shock wave is formed, which propagates downwards the tube at supersonic speed and heats and compresses the test gas within less than 1 μs (incident shock wave). The shock wave is reflected at the end wall and the preheated testgas is heated and compressed again (reflected shock wave). The resulting pressure- and temperature-time-profile is shown in the lower left part of Fig. 1. Typically, the constant conditions behind the reflected wave last for roughly 1  - long enough for studying chemical reactions, which are mostly fast at high temperatures. In many cases, the species of interest are generated thermally by the decomposition of suitable precursor molecules (Fig. 1a), but often a photolytic production is also feasible (Fig. 1b). Finally, real-time detection of the concentration-time-profiles is accomplished through optical windows by means of a variety of sensitive spectroscopic absorption or emission techniques. A recent example is the application of the highly sensitive laser absorption based frequency modulation (FM) spectroscopy for quantitative detection of small radicals behind shock waves. With FM spectroscopy the radicals methylene (1CH2) and formyl (HCO), which are both of considerable importance in combustion chemistry, could be detected behind shock waves for the first time [1,2].
Both formaldehyde (CH2O) and formyl radicals (HCO) lie on the main oxidation pathway of hydrocarbons with CH3 as the chain center. Methane oxidation, for example, proceeds in the following steps:

CH4    =>    CH3    =>    CH2O   =>   HCO   =>   CO    =>   CO2

Under radical-rich conditions, formaldehyde is mainly formed by the reaction of methyl radicals with oxygen atoms. Subsequent abstration reactions of H, OH, O and CH3 yield formyl radicals. By reactions of HCO with H, O2 and OH, and also through its unimolecular decomposition, carbon monoxide and eventually carbon dioxide are formed. The high temperature decomposition of CH2O provides a simple system to investigate some of these reactions. The chain mechanism of the CH2O decay can be described over a wide range of temperatures and pressures by only five reactions:

CH2O        +      M           HCO       +         H        +              M        (1a)
CH2O        +      M           H2           +        CO      +              M        (1b)
CH2O        +      H           H2           +        HCO                               (2)
HCO          +      M           H              +       CO       +              M        (3)
HCO          +      H           H2            +       CO                                  (4)
HCO          +      HCO     CH2O      +       CO                                  (5)

By means of sensitive vacuum-UV-absorption detection of CH2O at 174 nm and frequency modulation detection of HCO at wavelength around 614 nm, the rate of reaction (2) was directly measured at high temperatures for the first time (CH2O detection, C2 H5I as H atom source, T=1510 - 1960 K) and the rate of reaction (3) could be measured at temperatures of 835 - 1230 K (HCO detection, photolysis of CH2O mixtures) [2, 3]. Furthermore, measurements of reactions (4) and (5) at lower temperatures (HCO detection, photolytic production of HCO) and the detection of CH2O and HCO profiles during the thermal decomposition of pure formaldehyde mixtures behind shock waves provided additional information about the rates of reactions (1a) and (3) [2,4]. Altogether, sensitive detection methods and extensive experimental data made it possible to separate the strongly coupled reactions and to obtain a consistent set of rate constants.

ch2o_total_web

Fig. 2: Left: schematic potential curve of the thermal decomposition of formaldehyde (CH2O) with H2 + CO and H + HCO as reaction products, respectively. Right: calculated branching fraction as function of pressure and temperature; based on two-channel RRKM calculation.

As a final step, the rate of reaction (1b), which becomes more important at low formaldehyde concentrations, and also the branching fraction β of the initiation step (reactions (1a) and (1b)) were calculated using statistical theories of unimolecular reactions. Similar threshold energies and different energy dependencies of the decay rates of the two reaction channels with loose (1a) and tight (1b) transition states, respectively, induce a distinct pressure and temperature dependence of the branching fraction β (Fig. 2). A two-channel RRKM calculation, which takes into account rotational effects and "weak collisions" (master equation analysis) reveals that at temperatures from 1400 to 3000 K and at low pressures (1 mbar) reaction (1b) with H2 and CO as products is the main channel. However, with increasing pressure, channel (1a) eventually dominates and at very high pressures (1 kbar)
 
[1] G. Friedrichs, H. Gg. Wagner, Quantitative FM Spectroscopy at High Temperatures: The Detection of 1CH2 behind Shock Waves, Z. Phys. Chem. 214, 1723-1746 (2000).

[2] G. Friedrichs, J. T. Herbon, D. F. Davidson, R. K. Hanson, Quantitative Detection of HCO behind Shock Waves: The Thermal Decomposition of HCO, Phys. Chem. Chem. Phys. 4, 5778-5788 (2002).

[3] G. Friedrichs, D. F. Davidson and R. K. Hanson, Direct Measurements of the Reaction H + CH2O H2 + HCO by means of V-UV Detection of Formaldehyde behind Shock Waves, Int. J. Chem. Kinet. 34, 374-386 (2002).

[4] G. Friedrichs, D. F. Davidson, R. K. Hanson, Validation of a Thermal Decomposition Mechanism of Formaldehyde by Detection of CH2O and HCO behind Shock Waves, Int. J. Chem. Kinet. 36, 157-169 (2004).