Paper # 070DI-0386
Topic: Diagnostics
8th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute
and hosted by the University of Utah
May 19-22, 2013
A Flash Photolysis Facility for Fundamental HO2 Studies
Jamie L. Lane, Michael A. Stichter, Nicholas P. Cernansky, and David L. Miller Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, Pennsylvania 19104 The objective of this investigation is to develop a new laser spectroscopy technique
(namely, Cavity Enhanced Magneto-Optic Rotation (CEMOR)) that is sensitive and selective
enough to allow the small peroxy radicals to be measured in combustion systems. CEMOR
combines the sensitivity of Cavity Ringdown Spectroscopy (CRDS) with the selectivity of
Magneto-Optic Rotation (MOR) to allow sensitive and selective measurements of paramagnetic
species in spectral regions congested with stable non-paramagnetic species. An essential step in
the development phase of CEMOR for measuring small peroxy radicals (i.e., HO2 and RO2) is to
generate the species of interest in a controlled environment. To accomplish this task, a lowpressure flash photolysis facility has been designed and developed to carry out CRDS, continuous
wave Cavity Ringdown Spectroscopy (cw-CRDS), and CEMOR experiments to measure HO2.
This facility provides a method for studying HO2 on a fundamental level using cw-CRDS and to
test the feasibility of HO2 measurements using CEMOR. The primary focus of the study is on the
low-lying electronic band (A2A’ßX2A”) centered near 7000 cm-1 (1428.6 nm) where HO2 has
already been detected using CRDS in this photolysis system. The laser linewidth of the present
system is too broad to resolve the spectral features of HO2 using the CEMOR diagnostic and a path
toward this application is suggested based on a narrow linewidth continuous wave laser.
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Topic: Diagnostics
1. Introduction
The motivation behind the development of Cavity Enhanced Magneto-optic Rotation
(CEMOR) spectroscopy, which combines the sensitivity of cavity ringdown spectroscopy (CRDS)
and the selectivity of magneto-optic rotation (MOR), is to provide a diagnostic technique that is
capable of sensitive and selective measurements of paramagnetic species in spectral regions that
are congested with stable non-paramagnetic species. This overlap makes diagnostic measurements
using absorption-based methods nearly impossible; particularly, quantitative measurements of HO2
and RO2 radicals in the presence of H2O and CO2. An essential step in the development of a new
laser diagnostic technique for specific species that has not been extensively studied (i.e., HO2 and
RO2) is to generate the species of interest in a controlled environment. This allows measurements
to be made on a known concentration so that fundamental spectroscopic parameters can be
determined using established diagnostic techniques (such as CRDS). This provides the basis for
establishing the feasibility of radical observation in a more complex environment using CEMOR.
This paper focuses on the design and development of a low-pressure photolysis facility for
generating HO2 radicals in a controlled environment, which provides a method for studying these
molecules on a fundamental level using CRDS (both pulsed and continuous wave) and CEMOR.
This includes testing and validating the system operations. For future work, the system can be
configured for RO2 production with only minor modifications.
In terms of HO2 diagnostics, the primary focus is on the low-lying electronic band
2
2
( A’ß A”) centered near 7000 cm-1 (1428.6 nm), and the vibrational overtone band of the OH
stretch centered near 6648.9 cm-1 (1504 nm). There are other stronger absorption bands in the
mid-infrared and UV, however, the mid-IR bands suffer from greater pressure-induced line
broadening effects, and the UV band is broad and structureless and encumbered with many
overlaps from other species. There have been a number of experiments measuring HO2 using the
near-IR 2𝜈! overtone band in photolysis systems (Taatjes and Oh, 1997; DeSain et al., 2003;
Thiebaud and Fittschen, 2006). Far less attention has been focused on the 2A’ß2A” electronic
band. The relatively few investigations have consisted of monitoring the fluorescence emission of
excited HO2, which was generated in a microwave discharge and electronically excited via
electronic energy transfer from metastable oxygen molecules, O2, 𝑎! Δ! (Tuckett et al., 1979; Fink
and Ramsay, 1997). Faraday rotation spectroscopy (FRS) has recently been used to measure HO2
near 1396.9 cm-1 (7159 nm) at Princeton by Brumfield et al. (2013), although this method requires
comparison to HITRAN database in practice. Quantitative measurements of HO2 using the flash
photolysis facility described in this work can be used to calibrate diagnostic methods under
development such as FRS and CEMOR.
CRDS measurements of HO2 in the photolysis facility have been conducted using an OPO
laser system. The CRDS experimental results have demonstrated a signal response that is sensitive
to HO2 concentrations. They also indicate that the near-IR linewidth of the OPO laser system used
in these experiments is insufficient for resolving individual absorption features of HO2. Attempts
to measure HO2 using CEMOR have been inconclusive at this point using the current experimental
arrangement. Details of the system and experimental method along with the diagnostic work that
has been done will be discussed. Additionally, facility upgrades to allow the continuation of the
diagnostic development will be discussed.
An essential step towards making quantitative measurements of HO2 and RO2 in a
combustion environment is to first generate these species in a controlled environment. A flash
photolysis technique has been employed to generate a “clean”, room temperature source of HO2
radicals (Taatjes and Oh, 1997; Thiebaud et al., 2007; Glover and Miller, 2005), which consists of
2 Paper # 070DI-0386
Topic: Diagnostics
dissociating chlorine molecules with 355 nm photons in the presence of methanol (CH3OH),
oxygen (O2), and helium (He). Chlorine radicals (Cl) react with CH3OH to produce HO2 via the
following first-order reaction sequences:
!! (R-0)
𝐶𝑙
2𝐶𝑙
!
𝐶𝑙 + 𝐶𝐻! 𝑂𝐻
𝐶𝐻! 𝑂𝐻 + 𝑂!
𝐶𝐻! 𝑂𝐻 + 𝐻𝐶𝑙
𝐻𝑂! + 𝐶𝐻! 𝑂
(R-1)
(R-2)
The reaction rate coefficients for reactions (1) and (2) are 5.7 x 10-11 and 9.1 x 10-12 cm3
molecule-1 s-1, respectively (Atkinson et al., 1997). Methanol is in excess of the Cl radical so that
HO2 is produced in a 1:1 ratio with Cl radicals. HO2 depletion takes place mostly through the
well-known self-reaction pathway producing hydrogen peroxide and oxygen (R-3).
𝐻𝑂! + 𝐻𝑂! ⟶ 𝐻! 𝑂! + 𝑂!
(R-3)
The second-order reaction rate coefficient for (R-3) has been measured to be 2.1 x 10-12
cm molecule-1 s-1 (Taatjes and Oh, 1997). A similar method can be used to generate C3 peroxy
radicals, where propane replaces methanol in the photolysis cell (Zalyubovsky et al., 2005; Melnik
et al., 2010).
3
2. Methods
Photolysis Cell Components
The chosen mechanism for generating peroxy radicals requires the use of Cl2 gas as an
oxidizer. Furthermore, HCl is produced in the reaction sequence yielding HO2. Cl2 and HCl are
highly corrosive and therefore special considerations had to be made in the design of the flash
photolysis cell (FPC). Accordingly, the custom-built photolysis cell (Fig. 1) was constructed of
virgin grade PTFE (polytetrafluoroethylene) to provide maximum resistance against the corrosive
gases used in these experiments. A requirement in the design of the FPC facility was to have a
sealed, low-pressure system that would provide sufficient overlap of the probe beam with the
photolysis beam to allow measurements to be made. The photolysis beam is the 355 nm
wavelength laser beam used to dissociate Cl2 molecules and the probe beam is the laser beam used
to detect peroxy radicals. Details in the design and construction of the FPC facility are outlined
below.
The cell body was machined out of a 6” diameter solid cylindrical blank of PTFE. It is
essentially a double-flanged pipe that has an internal diameter of 3” and outer diameter of 3.484”
and an overall length of 20.994”. The cell has been designed with an integral electromagnetic coil
to generate an axial magnetic field within the cell. The coil housing was machined from a 3”
diameter cylindrical blank of PTFE and has an inside diameter of 1.4” and a total length of 5.9”.
The magnetic coil housing was designed with two o-rings on each end to allow the magnetic coil
to be pressed inside the photolysis chamber, providing a vacuum tight seal between the corrosive
gases and the magnetic wire. The leads of the coil wire are fed through an open port on the top of
the reactor chamber to allow connection to the circuit.
3 Paper # 070DI-0386
Topic: Diagnostics
Figure 1. Flash Photolysis cell for generating HO2 and RO2 radicals. The
system has been designed to allow CRDS and CEMOR diagnostic to be
performed.
To reduce the dead volume within the cell body two 2.9” OD x 1.4” ID inserts were
machined to slide within the ends of the cell and press against the electromagnetic coil housing.
Each insert has a 0.75” groove machined lengthwise to allow the photolysis beam to pass through
the reactor. The two inserts along with the electromagnetic coil housing reduce the inside diameter
of the reaction chamber from 3” to 1.4”, allowing for a faster refresh rate of the gases flowing
through the cell, ultimately decreasing the data collection time.
The ends of the reaction chamber are sealed with a 6” diameter blind flange compressed
against an o-ring. The two blind flanges were machined out of 6” diameter cylindrical blanks of
PTFE. Each blind flange is outfitted with an extension arm for mounting the CRDS mirrors and
has an adjacent port for mounting a photolysis window. The CRDS mirrors rest on a soft o-ring
and are held in place with a PTFE washer and three micro adjusters (Thor Labs). Resting the
CRDS mirrors on an o-ring allows the cell to be sealed while still allowing fine adjustments to the
cavity alignment using the micro adjusters. Each extension arm has a gas port in front of the
mounting surfaces for the CRDS mirrors. This allows a stream of helium to flow across the
reflective surfaces of the two mirrors to provide protection from the corrosive gasses. Additionally,
each extension arm is connected to an optical mounting post (just below the CRDS mirror housing)
to ensure a stable CRDS cavity. The photolysis windows are mounted against a soft o-ring to
provide a vacuum tight seal.
Gas Scrubbing System
As the experiments in this investigation require the use of Cl2 gas as the oxidizing agent for
the reaction process, appropriate gas handling measures had to been considered. Cl2 is a highly
toxic gas and can be lethal at concentrations as low as 30 ppm. Cl2 reacts with H2O in the body to
produce hydrochloric acid (HCl) and hydrochlorous acid (HClO). Because of the toxic nature of
Cl2 gas, a gas scrubbing system has been developed to remove the excess Cl2, Fig. 2. It is made up
of two 500 mL gas-washing bottles that are filled with a 2M solution of NaOH in H2O. At room
temperature Cl2 reacts with NaOH via the following mechanism:
Cl2 + 2NaOH à NaCl + NaOCl + H2O
(R-4)
Hydrochloric acid is a by product of the reaction sequence of Cl with CH3OH and is
removed in the scrubber system via the common acid-base reaction that produces a salt and water:
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Topic: Diagnostics
HCl + NaOH à NaCl + H2O
(R-5)
Additionally, a vacuum cold trap (charged with dry ice) is positioned down stream of the
gas scrubbers to remove moisture before entering the vacuum pump.
Figure 2. Photolysis cell gas scrubber system for Cl2 and HCl removal.
Flash Photolysis Experimental Facility
The photolysis facility for CRDS and CEMOR diagnostics is illustrated in Fig. 3. The
basis of the system is to generate HO2 radicals by photolytically dissociating chlorine molecules in
the presence of methanol, oxygen, and helium. Chlorine molecules in these experiments are
dissociated with approximately 70 mJ of the third harmonic output (355 nm) from a Continuum
Precision 8000 laser. The system was designed to optimize the overlap of the photolysis beam
with probe beam. The photolysis beam is the third harmonic beam (355 nm) that dissociates Cl2.
The probe beam is the idler beam from the OPO, which is used for the diagnostics. The system is
composed of a PTFE reaction chamber in which the photolysis beam crosses the probe beam at an
angle of 5° (Fig. 4). This translates to an interaction pathlength of 17.2 cm; given a photolysis
beam diameter of 1.5 cm.
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Topic: Diagnostics
(a)
(b)
Figure 3. (a) CRDS photolysis facility developed for generating HO2 radicals. Cl2 is dissociated
with 355 nm photons in the presence of CH3OH, O2 and He, which react to form HO2. The
photolysis cell has been illustrated without the magnetic coil and inserts for clarity. (b) Photolysis
cell setup for CEMOR.
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Topic: Diagnostics
Figure 4. Overlap geometry of the probe beam with photolysis beam.
The laser diagnostics conducted in the PFC is dependent on the kinetics of the radicals
being generated, therefore a timing scheme had to be implemented to allow precise control over
the delay of the probe beam relative to the photolysis beam (Fig. 3 (a)). The timing of the lasers is
controlled by externally triggering the flash lamps and Q-switch in both of the YAG lasers via
TTL signals. Each YAG laser used in this facility requires two TTL signals (5V DC, 10 µs
duration positive-going pulses that are ≤ 10 Hz) that can be delayed relative to one another to
allow external timing control. For the experiments conducted in this facility, active control of the
lasers timing relative to one another required synchronizing several function generators. The
output power of the YAG is dependent on the Q-switch delay relative to the flash lamps firing. A
290 𝜇𝑠 delay between the flash lamp trigger and the Q-switch trigger is the optimum timing delay
to produce maximum power output from the photolysis laser (Precision 8000) and pump laser
(Precision 9010) used in these experiments. The output energy of the photolysis laser is more
energy than is needed for the experiments. Therefore, the Q-switch trigger delay, relative to the
flash lamp trigger, is increased to reduce the energy output of the third harmonic beam to an
average energy of 70 mJ.
Once the flash lamp/Q-switch trigger pair for each of the lasers is adjusted to achieve the
desired output energies, each trigger pair must be adjustable relative to one another without
changing the delay of flash lamp/Q-switch trigger of the respective laser. As such, the timing
between the probe beam pulse and the photolysis pulse can be adjusted for optimum HO2
concentrations. Additionally, the reaction kinetics can be monitored if desired.
Cl2, O2, and He gas flow rates are controlled via rotometers that are configured for lowpressure applications. The gases are introduced into a mixing chamber and are then passed on and
into the top of the reactor near the far end. The gases are pulled through the system via a
diaphragm vacuum pump and exit through the bottom of the reactor at the opposite end. A
capacitance manometer (Setra, Model 720) connected to a digital readout is used to monitor the
pressure of the system and is positioned near the exit of the cell. The wetted parts of the
capacitance manometer are Inconel®, which is resistant to the corrosive gasses used in the
experiments. Temperatures are monitored near the exit with a type-K Inconel® thermocouple.
The pressure of the systems is actively controlled via a butterfly valve that is positioned between
the vacuum cold trap and vacuum pump (note: it is important to have the valve completely closed
when engaging the vacuum pump because of the gas scrubber system. Once the pump has been
engaged then the valve can be gradually opened to begin lowering the pressure of the cell to the
desired level). Each of the CRDS mirror extension arms have a port just in front of the CRDS
mirrors to allow a stream of He to circulate in front of the mirrors and down the extension arm.
This prevents any of the reacting gases from damaging the reflective coating on the CRDS mirrors.
7 Paper # 070DI-0386
Topic: Diagnostics
Methanol is injected into the reactor neat. A syringe pump delivers the methanol onto a heated
column that has a cross flow of helium, which is then directed into the mixing chamber with the
other gases.
An additional requirement in the design of the FPC was the construction of a stable optical
resonator for the CRDS and CEMOR diagnostics. The assembled cell has a CRDS cavity length
of 82 cm. For diagnostics of the HO2 electronic band, the cavity is formed with two 99.99%
reflective (centered at 1430 nm wavelength), 25.4 mm diameter x 1 m radius of curvature, CRDS
mirrors (Los Gatos Research). The probe beam is coupled to the TEM00 mode of the optical cavity
via mode coupling optics (f1 and f2). The relative positions of f1 and f2 with the optical cavity are
calculated via a ray transfer matrix approach.
Photolysis Cell Electromagnet
The magnetic field within the photolysis cell is generated with an electromagnetic coil
(Fig. 5). The magnetic coil was built using a 22 gauge magnetic wire that was consecutively
wrapped around a PTFE housing over a length of 11.7 cm It consists of 11 layers of magnetic wire
with 160 turns per layer corresponding to a total of 1760 turns over the 11.7 cm and a total
resistance of 23.08 ohms. The housing has a total length of 15 cm. The coil is connected to a
triggering circuit to allow the magnetic field to be turned on and off in correspondence with the
laser pulse (~0.2 Hz). This reduces the amount of heat generated by the coil. The components of
the magnetic coil circuit are a 30 V power supply, solid state relay, and function generator. The
triggering scheme for the circuit is depicted in Fig. 6. Once the circuit has been triggered the
response time is approximately 0.1 µs (Fig. 7) (i.e., a delay of 0.1 µs to maximum voltage). The
response time must be taken into consideration in the timing scheme for the experimental facility,
as the applied magnetic field needs to correlate with the probe beam.
Figure 5. Electromagnet for the photolysis cell.
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Topic: Diagnostics
(a)
(b)
Figure 6. Triggering scheme for the magnetic coil. The relay trigger has been intentionally offset by approximately -10 V
for clarity. Image (b) is an expanded view of (a).
9 Paper # 070DI-0386
Topic: Diagnostics
Figure 7. Voltage applied to the magnetic coil has a rise
time of approximately 0.1 µs, which is limited by the
response time of the relay.
To gain insight into the magnetic flux density generated by the magnetic coil a modeling
investigation was conducted using a finite element analysis approach. The results are depicted in
Fig. 8. The magnetic flux density ranges from approximately 0.016 to 0.009 T for the applied
currents of 1.3 to 0.8 amps, respectively (note: 1.3 amps is the maximum current that should be
applied to the magnetic coil, and corresponds to an applied voltage of 30 V for the 23.08 ohm
magnetic coil).
10 Paper # 070DI-0386
Topic: Diagnostics
(a)
(b)
Figure 8. (a) Magnetic field produced in the photolysis cell with an applied current
of 1.3 amps. (b) Magnetic field along the centerline of the photolysis cell solenoid
for several applied currents.
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Topic: Diagnostics
3. Results and Discussion
Near-IR CRDS diagnostic of HO2 and H2O have been performed in the FPC at a
temperature of 298 K. For the HO2 experiments, He, Cl2, O2, and CH3OH, were introduced into the
reactor at partial pressures of 126.5 Torr, 0.5 Torr, 2 Torr, and 1 Torr, respectively. The total flow
rate of the gases through the system was set at 1 SLPM. This required the photolysis laser to be
trigged at a rate of 0.2 Hz to ensure a fresh gas mixture for each measurement. Because of the
slow data acquisition rate required for the HO2 experiments (~5 s per data point) this investigation
has been limited to a spectral window of 10 cm-1 over the 2A’ß2A”electronic band of HO2. The
step size for the laser scan was set to 0.01 cm-1. Each data point was averaged over 3 ringdown
events resulting in a data acquisition time of 15 s per data point. This translates to approximately
4.2 hr for each scan displayed in Fig. 9 (with the exception of the baseline scan, which was taken
at 10 Hz).
Figure 9. Near IR CRDS scan of HO2 in the photolysis cell at a total
pressure of 130 Torr (i.e., 1 Torr CH3OH, 0.5 Torr Cl2, 2 Torr O2,
126.5 Torr He).
The partial pressures of the gas components for the HO2 experiment translate to: [Cl2] = 3.8
x 1014 cm-3, [CH3OH] = 3.24 x 1016 cm-3, and [O2] = 6.48 x 1016 cm-3. The concentration scheme
was chosen to ensure that CH3OH was in excess of Cl radicals and that O2 was in excess of
CH3OH. As such, HO2 is generated in a 1:1 ratio with Cl radicals, assuming 100% reaction
efficiency. To estimate the expected concentration of HO2 being produced under these
experimental conditions the absorption cross section of Cl2 at 298 K must be considered. Cl2 has a
355 nm absorption cross section of 1.598 x 10-19 cm2 at 298 K.
The photolysis beam has a total absorption pathlength of 53.5 cm and a diameter of
1.5 cm. This transforms to a total photolysis beam volume of 94.5 cm3. Based on the absorption
cross section of Cl2, the absorption pathlength of the photolysis beam, and Cl2 concentration, the
Beer-Lambert law yields a total absorbance of 0.138. This translates to approximately 10 mJ of an
initial 70 mJ photolysis pulse being absorbed as it passes through the photolysis cell. 10 mJ of
355 nm laser energy contains an average of 1.8 x 1016 photons. This corresponds to the total
number of photons absorbed by the Cl2 gas over the photolysis beam pathlength. In considering
the total volume in which the 10 ns FWHM photolysis pulse traces out as it propagates through the
reactor, it can be estimated that the concentration of Cl radicals being generated at these
12 Paper # 070DI-0386
Topic: Diagnostics
experimental conditions corresponds to: [Cl] = 3.6 x 1014 cm-3 (remembering that for every photon
absorbed by Cl2, two Cl radicals are produced). The estimated concentration of HO2 being
produced in this experimental arrangement is: [HO2] = 3.6 x 1014 cm-3.
Based on the rate coefficients for reactions (R-1) and (R-2), HO2 is expected to reach a
maximum concentration at approximately 2 µs after the photolysis pulse for the gas concentrations
used in these experiments. HO2 concentrations deplete by more than 50 % after 4 ms in
accordance with (R-3). CRDS scans have been performed over a portion the 2A’ß2A”electronic
band of HO2 for delay times of 30 and 100 µs relative to the photolysis pulse (Fig. 9). The CRDS
absorption scans at the respective delays has demonstrated a response to HO2. HO2 exhibited a
strong absorption signal near 6998 cm-1 (1428 nm). While there was a CRDS signal response to
HO2, the linewidth of the OPO laser was insufficient to resolve the rovibronic spectra.
Multiexponential behavior in the CRDS signal was noticed during the diagnostic measurements.
This is indicative of the laser linewidth being broader than the resonance feature (KohseHöinghaus and Jeffries, 2002). For CRDS diagnostics, it is imperative for the linewidth of the
laser to be less than the resonance feature (Zalicki and Zare, 1995). As such, HO2’s complex
electronic structure could not be resolved with the quality of radiation used for the diagnostics. The
decrease in the measured absorbance in moving from a delay of 30 to 100 us is due the depletion in
HO2 concentrations from the HO2 self-reaction pathway (R-3).
CEMOR diagnostics were attempted on HO2 for the absorption band illustrated in Fig. 12.
The experiments did not produce a signal response. The non-responsive signal from the CEMOR
diagnostics has been attributed to the relatively weak magnetic field produced by the magnetic coil
(~ 0.016 Tesla), as the sensitivity of CEMOR is dependent on the applied external magnetic field
strength (i.e., the amount of Zeeman splitting of HO2 energy levels is dependent on the external
field strength) (Lane, 2012).
The H2O experiments consisted of flowing humid air (~25 % relative humidity) through the
photolysis cell at a flow rate of 1 SLPM and a total pressure of 60 Torr. The objective of the H2O
experiments was to get an estimate on the OPO laser linewidth. Because of the continuous flow of
H2O, experiments could be conducted at a comparatively higher data acquisition rate (10 Hz)
compared to the HO2 experiments (0.2 Hz). As such, CRDS scans between 6990 cm-1 and
7050 cm-1 were logged during the diagnostics. Each data point was obtained at a wavelength step
size of 0.01 cm-1 and averaged over 10 ringdown events. H2O absorption within this spectral
region results from the vibrational overtone and combination bands (2ν3,ν1+ν3, 2ν1). In general,
vibrational bands are less complex than electronic bands. The comparatively less complex
structure of H2O vibrational bands allowed the rotational lines to be resolved at a pressure of
60 Torr (Fig. 10 (a)), albeit with multiexponential behavior of the CRDS ringdown signal. The
multiexponential behavior can be specified by the mean square error (MSE) resulting from the fit
of the CRDS signal to a single exponential function (Fig. 10 (b)). Ideally, the MSE remains
constant when scanning over an absorption feature, retaining a good single exponential profile.
At a pressure of 60 Torr and 298 K, the linewidth of H2O is defined by its Doppler
linewidth. The Doppler FWHM of H2O lines at the experimental conditions are 0.0194 cm-1. An
H2O absorption line was fit to a Gaussian lineshape function, which had demonstrated a FWHM of
approximately 0.614 cm-1 (Fig. 10). This has indicated that the H2O linewidths should be limited
by the laser linewidth used in the diagnostics. Subtracting the calculated Doppler linewidth from
the experimentally measured linewidth demonstrates an approximate laser linewidth of 0.595 cm-1.
As an additional note, HO2 lines at a pressure of 130 Torr and temperature of 298 K are
best represented by a Voigt lineshape function. HO2 has a calculated Voigt linewidth of
13 Paper # 070DI-0386
Topic: Diagnostics
approximately 0.05 cm-1 FWHM at the respective conditions. Based on the H2O data obtained in
the photolysis cell, the laser has an insufficiently narrow linewidth to perform the needed
spectroscopic analysis on HO2. To allow spectroscopic analysis of the individual rovibronic lines
of HO2 a narrower linewidth laser will need to be implemented into the facility.
(a)
(b)
Figure 10. (a) Near-IR CRDS scans of H2O in air (25% relative humidity at 20oC and 60 Torr) at a flow rate of 1 SLPM
through the photolysis cell. (b) The Mean Square Error (MSE) (lower) resulting from the CRDS fit function for each H2O
data point. An increase in the MSE while scanning over a resonance features indicates a deviation from a single
exponential and is a good indication of the multiexponential behavior resulting from the linewidth of the laser being broader
than the resonance feature. For a single exponential fit of the CRDS signal the MSE would remain constant.
Figure 11. H2O line fit to a Gaussian lineshape function (adj. R2 = 0.980).
The fit function indicates a FWHM linewidth of 0.614 cm-1. The calculated
Doppler linewidth at the experimental conditions is 0.0194. Note: this
image is an expanded view of a section of Fig. 10 (a).
4. Conclusions
A flash photolysis facility has been designed and built for generating HO2 radicals and can
be modified relatively easy to allow proxy radical production. The system has been designed to
14 Paper # 070DI-0386
Topic: Diagnostics
allow both CRDS and CEMOR diagnostics to be performed. Each of system components has been
successfully tested for operation. CRDS scans have been performed over a portion the
2
A’ß2A”electronic band of HO2 for delay times of 30 and 100 µs relative to the photolysis pulse.
The CRDS absorption scans at the respective delays has demonstrated a response to HO2 kinetics.
HO2 exhibited a strong absorption signal near 6998 cm-1 (1428 nm). While there was a CRDS
signal response to HO2, the linewidth of the OPO laser was insufficient to resolve the rovibronic
spectra. Also, CEMOR was attempted on HO2 and did not produce a signal response. The nonresponsive signal has been attributed to the relatively weak external magnetic field applied for the
diagnostics. Future experiments will require the application of a stronger magnetic field.
Furthermore, the inability to resolve the complex electronic structure of the 2A’ß2A” band of HO2
using CRDS has indicated a requirement to move to a narrower linewidth laser system to allow the
required spectroscopic studies of this molecule. It is recommended at this point to transition the
diagnostics to continuous wave diode lasers, which offer extremely narrow linewidths
(~0.0001 cm-1).
5. Acknowledgements
The authors would like to acknowledge the partial support of this effort under NSF Grant
CBET-1142312.
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absorption cross sections of the 𝐴ß𝑋 transition in organic peroxy radicals by dual-wavelength
cavity ring-down spectroscopy,” J. Phys. Chem. A, 114: 11583-11594.
15 Paper # 070DI-0386
Topic: Diagnostics
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