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Raman spectroscopy of atomic fluorine
Volume 31, number 2
OPTICSCOMMUNICATIONS
November 1979
RAMAN SPECTROSCOPY OF ATOMIC FLUORINE * John C. CUMMINGS and Daniel P. AESCHLIMAN Sandia Laboratories, Albuquerque, NM 87185, USA
Received 28 August 1979
We have measured the Raman scattering cross section of atomic fluorine for transitions between the ground fine-structure electronic states. The fluorine was contained a in heated, static chamber. Gas temperature, determined from the rotational Raman spectrum of molecular fluorine, was used as an input to calculate the degree of fluorine dissociation (assurehag thermodynamic and chemicalequilibrium). The Raman frequency shift and depolarization ratio were also determined. Our results indicate that Raman scattering can be used as a probe for atomic fluorine.
In 1975 Schlossberg [1] proposed that the concentration of atomic fluorine could be measured using either infrared absorption or Raman scattering from transitions between the fluorine ground fme-structure electronic states (2P3/2 ~ 2P1/2). Since that initial proposal there have been several unsuccessful attempts to experimentally verify Schlossberg's calculations of the absorption coefficient [2,3] and the spontaneous Raman scattering cross section [4,5]. Our interest in the problem arose during an application of Raman spectroscopy to measure species concentrations and temperature in a cw HF/DF chemical laser [5]. At that time we observed a Raman signal that we thought might be due to atomic fluorine, but both the Ramanshift frequency and intensity were somewhat different than expected. We concluded that overtone emission from vibrationally-excited HF could have been responsible for the observed signal and that further experiments under controlled conditions would be required to clarify the issue. This paper reports the results of our measurement of the Raman cross section of atomic fluorine in a heated, static chamber. The basic experimental system is described in a previous publication [6]. A cw argon-ion laser, operating at 488.0 nm and typically 1.1 w TEM00, was employed as the Raman-excitation source. This laser * This work was supported by the United States Department of Energy.
beam was expanded and then focussed inside the fluorine cell. Retroreflectors were used .for both the laser beam and the monochromator collection optics. The spectrometer was a Spex model 1401 double monochromator equipped with 1200-~/mm gratings blazed at 500 nm. A cooled and RF-shielded RCA C31034 photomultiplier (PM) tube served as detector. The PM tube output could be recorded in either digital (PARC 1121 amp/discriminator plus a PARC 1110 photon counter) or analog format (D/A converter plus an x - y recorder). A schematic diagram of the fluorine cell is shown in fig. 1. The cell had six orthogonal arms (two vertical and four horizontal) leading to a central chamber. Except for the stainless steel bellows and sapphire windows, the cell was constructed entirely from monel. The central chamber was heated with a variable power electrical heat tape. The outer region of each arm was cooled with water. The cell was designed for f / 8 optics by expanding the inner diameter of each arm from 0.64 cm at the central chamber to 2.5 cm at the sapphire window, located 20 cm from the central chamber. The experimental procedure consisted of filling the cell with F2, optimizing the detection system using the F 2 vibrational Raman signal, heating the cell to the desires temperature, and then recording F 2 rotational and F electronic Raman signals. The F 2 fill pressure t was typically 550 Torr as determined with a Validyne 165
Volume 31, number 2
OPTICS COMMUNICATIONS
~
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November 1979
b)
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6 4
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~I II ~ o~ I f f=
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MONOCHROMATOR ENTRANCE
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'~4
ARGON-ION
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Fig. 1. Schematic diagram of the Raman system and atomic fluorine cell. pressure gauge. The pressure was nearly independent of temperature since only 3% of the total volume of the cell was heated (see fig. 1). F 2 rotational Raman spectra were recorded in order to provide a measurement of the gas temperature. This measurement was very critical because it provided the input to a thermodynamic/chemical equilibrium calculation [7] that determined the degree of dissociation of F 2. Since the concentration of F is a very steep function of temperature from 5 0 0 - 9 0 0 K, the major contributor to uncertainty in our final result is the error limit associated with the gas temperature measurement. A thermocouple located between the heat tape and the cell provided a measure of the cell wall temperature. The Raman spectrum of atomic fluorine was recorded at two different spectral resolutions as shown in fig. 2. Monochromator slits with entrance/intermediate/exit widths of 1/1.1/1 mm and 10 mm height were used when the argon-ion laser was not expanded before focussing (i.e., beam waist in the fluorine cell ~1 mm). This wide slit setting provided a triangular instrument function with fwhm equal to 20 cm -1. The data recorded with the wide slits (fig. 2a) indicate that for Raman shifts from 3 8 4 - 4 2 4 cm - 1 there is only one dominant, isolated spectral feature and its location is near 404 cm - 1 . There were two problems associated with the use of the wide slits: a) the spectral resolution and signal-to-noise ratio (S/N) did not permit accurate determination of the Raman shift frequency, and b) the blackbody emission from the 166
404
424
400
404
408
FREQUENCY(CM"I)
Fig. 2. Photon counting rate as a function of Raman-shift frequency for monochromator slit widths of a) 1.0 mm and b) 0.2 mm. The background level due to PM tube noise is shown as a shaded region libelled DARK COUNT. Data symbols represent temperatures of 700-740 K (o), 770-830 K (o), and 845 K (X). Error bars indicate 20 uncertainties in the counting statistics.
cell walls increased more rapidly than the F-atom signal at high wall temperatures. Both of these problems were mitigated by the use of narrower monochromator slits. Expansion of the laser before focussing provided a smaller beam waist (beam diameter in the cell ~0.2 mm) and permitted the use of narrow monochromator slits o f 0.2/0.3/0.2 m m width and 6 mm height. These narrow slits provided an instrument function fwhm of 4 cm - I . The data shown in fig. 2b were taken with the narrow slits and they indicate both improved spectral resolution and decreased background. We determined the Raman shift due to atomic fluorine using the narrow slits and examining both Stokes (+404 cm - 1 ) and anti-Stokes ( - 4 0 4 cm - 1 ) signals. Spectral locations were calibrated using adjacent reference lines from a neon lamp. The measured Raman-shift was 403.8 cm -1 , with an uncertainty of +0.6 cm -1 due primarily to resettability errors in the spectrometer frequency. Our Raman-shift frequency compares well with the value of 404.2 + 0.3 c m - 1 previously determined using vacuum-ultraviolet emission spectroscopy [8]. During several of the experimental runs a polarization selector was positioned in front of the monochromator entrance slit in order to determine the depolarization ratio, p, for atomic fluorine. We de-
Volume 31, number 2
November 1979
OPTICS COMMUNICATIONS
termined that p = 0.1 -+ 0.1, with the large relative uncertainty due to a low S/N with the polarizer (50% transmission) in place. The most important result of this study is the determination of the F-atom spontaneous Raman scattering cross section. In order to determine the cross section, we related the average atomic-fluorine signal at 845 K to that measured for vibrational Raman scatterhag from molecular fluorine. The gas temperature of 845 K, determined from the F 2 rotational Raman spectra, was used to calculate the equilibrium concentration of atomic fluorine. Finally, the relative response of the entire optical system was determined as a function of frequency and polarization by means of a calibrated tungsten strip lamp and the emissivity data of Latyev et al. [9]. The relationship between F 2 and F cross sections can be expressed as
aoF aOF2FI F I F N ~ r l F ~ ' I F " ~ = I a~- a~--L/-~-2JLNFJLvF2JLr/FJ' where acr/a~2 is the cross section, I the measured signal intensity, N the concentration, v the frequency, r/the relative response of the optical system, and the subscripts refer to either the atomic or molecular form of fluorine. The cross section of F 2 relative to N 2 has been previously measured [10] and hence our value for the cross section of atomic fluorine can be related to that of N 2: ao F aON2 -~-(measured) = (I.0 -+ 0.5) ~ • This value is very close to that calculated by Schlossberg [1], ao F aON2 a~2 (calculated)= 0.6 a ~ with an error bound that is approximately 50% larger. As indicated previously, the uncertainty of the measured cross section is large due to the large uncertainty in the atomic fluorine concentration. A valuable check on our interpretation of the 404 cm -1 signal is provided by observing its dependence on gas temperature. Using the measured Raman signal to calculate atomic fluorine concentration, and taking account o f the population distribution between
1018
° ~
'
I
'
1
'
!
017
1016
1015 600
I
[ 700
I
[ 800
I 900
TEMPERATURE(OK)
Fig. 3. Atomic-fluorine concentration as a function of temperature. The solid curve was computed for conditions of thermodynamic/chemical equilibrium. Data symbols represent monochromator slit widths of 1.0 mm (o) and 0.2 mm (o). Symbols with arrows represent upperbound values. Error bars indicate estimated uncertainties from all sources. the two electronic states, our results can be compared to those from the thermodynamic/chemical equilibrium computation as a function o f gas temperature. This comparison is shown in fig. 3. Since the question of spectral interference due to impurities was raised in the experiment on infrared absorption of atomic fluorine [3], it is important to note that here the data exhibit the proper temperature dependence. Unfortunately it was not possible to investigate higher temperatures than those indicated in fig. 3 due to the marked increase in reaction rate of fluorine with the monel walls above ~ 8 5 0 K. Although our results do not conclusively rule out the possibility that the 404 cm -1 spectral feature is due to an impurity, the data are consistent with the assumption that it is due to atomic fluorine. The observed Raman-shift frequency agrees well with the previous result determined from vuv emission spectroscopy [8]. The measured cross section agrees with the calculated one, although both values have large margins o f error. Finally, the growth of the 404 cm -1 signal with temperature agrees with the equilibrium calculations for atomic fluorine concentration. Our results indicate that Raman scattering can be used as 167
Volume 31, number 2
OPTICS COMMUNICATIONS
a diagnostic technique to detect and measure atomicfluorine concentration. We would like to acknowledge the valuable assistance provided by discussions with our colleagues F.K. Truby, M.E. Riley, and R.A. Hill. R.W. Willey provided technical assistance in constructing the experiment.
References [ 1] H. Schlossberg, Air Force Cambridge Research Laboratories Report #AFCRL-TR-75-0522 (Oct. 1, 1975); also J. Appl. Phys. 47 (1976) 2044. [2] J.P. Moran and R.B. Doak, Initiation phenomena in pulsed chemical lasers, Aerodyne Research Inc. Report #ARI-RR-139 (Oct. 1978), U.S. Navy Contract #00173-7842-1088.
168
November 1979
[3] B.D. Crane, Ph.D. thesis, Univ. of Arizona (1979). [4] R.J. Jensen and K. Boyer, LASL Report L-JUMP-75; 179 (Nov. 1975), U.S. Air Force Project # AFWL-75254. [5] J.C. Cummings, D.P. Aeschliman and A.J. Mulac, JQSRT 19 (1978) 493. [6] D.P. Aeschliman, J.C. Cummings and R.A. Hill, JQSRT 12 (1979) 293. [7] C.A. Powars and R.M. Kendall, User's Manual Aerotherm Chemical Equilibrium (ACE) Computer Program, Aerotherm Corp., Mountain View, CA (May 1969). [8] K. Liden, Ark. Phys. 1 (1949) 229. [9] L.N. Latyev, V.Ya. Chekhovskoi and E.N. Shestakov, High Temperatures-High Pressures, Vol. 2 (1970) pp. 175-181. [10] J.M. Hoell, Jr., F. Allario, O. Jarrett, Jr. and R.K. Seals Jr., J. Chem. Phys. 58 (1973) 2896.
OPTICSCOMMUNICATIONS
November 1979
RAMAN SPECTROSCOPY OF ATOMIC FLUORINE * John C. CUMMINGS and Daniel P. AESCHLIMAN Sandia Laboratories, Albuquerque, NM 87185, USA
Received 28 August 1979
We have measured the Raman scattering cross section of atomic fluorine for transitions between the ground fine-structure electronic states. The fluorine was contained a in heated, static chamber. Gas temperature, determined from the rotational Raman spectrum of molecular fluorine, was used as an input to calculate the degree of fluorine dissociation (assurehag thermodynamic and chemicalequilibrium). The Raman frequency shift and depolarization ratio were also determined. Our results indicate that Raman scattering can be used as a probe for atomic fluorine.
In 1975 Schlossberg [1] proposed that the concentration of atomic fluorine could be measured using either infrared absorption or Raman scattering from transitions between the fluorine ground fme-structure electronic states (2P3/2 ~ 2P1/2). Since that initial proposal there have been several unsuccessful attempts to experimentally verify Schlossberg's calculations of the absorption coefficient [2,3] and the spontaneous Raman scattering cross section [4,5]. Our interest in the problem arose during an application of Raman spectroscopy to measure species concentrations and temperature in a cw HF/DF chemical laser [5]. At that time we observed a Raman signal that we thought might be due to atomic fluorine, but both the Ramanshift frequency and intensity were somewhat different than expected. We concluded that overtone emission from vibrationally-excited HF could have been responsible for the observed signal and that further experiments under controlled conditions would be required to clarify the issue. This paper reports the results of our measurement of the Raman cross section of atomic fluorine in a heated, static chamber. The basic experimental system is described in a previous publication [6]. A cw argon-ion laser, operating at 488.0 nm and typically 1.1 w TEM00, was employed as the Raman-excitation source. This laser * This work was supported by the United States Department of Energy.
beam was expanded and then focussed inside the fluorine cell. Retroreflectors were used .for both the laser beam and the monochromator collection optics. The spectrometer was a Spex model 1401 double monochromator equipped with 1200-~/mm gratings blazed at 500 nm. A cooled and RF-shielded RCA C31034 photomultiplier (PM) tube served as detector. The PM tube output could be recorded in either digital (PARC 1121 amp/discriminator plus a PARC 1110 photon counter) or analog format (D/A converter plus an x - y recorder). A schematic diagram of the fluorine cell is shown in fig. 1. The cell had six orthogonal arms (two vertical and four horizontal) leading to a central chamber. Except for the stainless steel bellows and sapphire windows, the cell was constructed entirely from monel. The central chamber was heated with a variable power electrical heat tape. The outer region of each arm was cooled with water. The cell was designed for f / 8 optics by expanding the inner diameter of each arm from 0.64 cm at the central chamber to 2.5 cm at the sapphire window, located 20 cm from the central chamber. The experimental procedure consisted of filling the cell with F2, optimizing the detection system using the F 2 vibrational Raman signal, heating the cell to the desires temperature, and then recording F 2 rotational and F electronic Raman signals. The F 2 fill pressure t was typically 550 Torr as determined with a Validyne 165
Volume 31, number 2
OPTICS COMMUNICATIONS
~
10
lASER RETROREFLECTOR
WA,ER
El
COOLANT--. ". LINES ~
~, , ~ Lt > .,I If b
.
i~-
Z"LL
November 1979
b)
0)
SApPH IRE WINDOW
S HEATTAPE AND z
6 4
RAMAN SIGi'~L RETROREFLECTOR
~I II ~ o~ I f f=
~
MONOCHROMATOR ENTRANCE
SLIT
'~4
ARGON-ION
lASER BEAM
Fig. 1. Schematic diagram of the Raman system and atomic fluorine cell. pressure gauge. The pressure was nearly independent of temperature since only 3% of the total volume of the cell was heated (see fig. 1). F 2 rotational Raman spectra were recorded in order to provide a measurement of the gas temperature. This measurement was very critical because it provided the input to a thermodynamic/chemical equilibrium calculation [7] that determined the degree of dissociation of F 2. Since the concentration of F is a very steep function of temperature from 5 0 0 - 9 0 0 K, the major contributor to uncertainty in our final result is the error limit associated with the gas temperature measurement. A thermocouple located between the heat tape and the cell provided a measure of the cell wall temperature. The Raman spectrum of atomic fluorine was recorded at two different spectral resolutions as shown in fig. 2. Monochromator slits with entrance/intermediate/exit widths of 1/1.1/1 mm and 10 mm height were used when the argon-ion laser was not expanded before focussing (i.e., beam waist in the fluorine cell ~1 mm). This wide slit setting provided a triangular instrument function with fwhm equal to 20 cm -1. The data recorded with the wide slits (fig. 2a) indicate that for Raman shifts from 3 8 4 - 4 2 4 cm - 1 there is only one dominant, isolated spectral feature and its location is near 404 cm - 1 . There were two problems associated with the use of the wide slits: a) the spectral resolution and signal-to-noise ratio (S/N) did not permit accurate determination of the Raman shift frequency, and b) the blackbody emission from the 166
404
424
400
404
408
FREQUENCY(CM"I)
Fig. 2. Photon counting rate as a function of Raman-shift frequency for monochromator slit widths of a) 1.0 mm and b) 0.2 mm. The background level due to PM tube noise is shown as a shaded region libelled DARK COUNT. Data symbols represent temperatures of 700-740 K (o), 770-830 K (o), and 845 K (X). Error bars indicate 20 uncertainties in the counting statistics.
cell walls increased more rapidly than the F-atom signal at high wall temperatures. Both of these problems were mitigated by the use of narrower monochromator slits. Expansion of the laser before focussing provided a smaller beam waist (beam diameter in the cell ~0.2 mm) and permitted the use of narrow monochromator slits o f 0.2/0.3/0.2 m m width and 6 mm height. These narrow slits provided an instrument function fwhm of 4 cm - I . The data shown in fig. 2b were taken with the narrow slits and they indicate both improved spectral resolution and decreased background. We determined the Raman shift due to atomic fluorine using the narrow slits and examining both Stokes (+404 cm - 1 ) and anti-Stokes ( - 4 0 4 cm - 1 ) signals. Spectral locations were calibrated using adjacent reference lines from a neon lamp. The measured Raman-shift was 403.8 cm -1 , with an uncertainty of +0.6 cm -1 due primarily to resettability errors in the spectrometer frequency. Our Raman-shift frequency compares well with the value of 404.2 + 0.3 c m - 1 previously determined using vacuum-ultraviolet emission spectroscopy [8]. During several of the experimental runs a polarization selector was positioned in front of the monochromator entrance slit in order to determine the depolarization ratio, p, for atomic fluorine. We de-
Volume 31, number 2
November 1979
OPTICS COMMUNICATIONS
termined that p = 0.1 -+ 0.1, with the large relative uncertainty due to a low S/N with the polarizer (50% transmission) in place. The most important result of this study is the determination of the F-atom spontaneous Raman scattering cross section. In order to determine the cross section, we related the average atomic-fluorine signal at 845 K to that measured for vibrational Raman scatterhag from molecular fluorine. The gas temperature of 845 K, determined from the F 2 rotational Raman spectra, was used to calculate the equilibrium concentration of atomic fluorine. Finally, the relative response of the entire optical system was determined as a function of frequency and polarization by means of a calibrated tungsten strip lamp and the emissivity data of Latyev et al. [9]. The relationship between F 2 and F cross sections can be expressed as
aoF aOF2FI F I F N ~ r l F ~ ' I F " ~ = I a~- a~--L/-~-2JLNFJLvF2JLr/FJ' where acr/a~2 is the cross section, I the measured signal intensity, N the concentration, v the frequency, r/the relative response of the optical system, and the subscripts refer to either the atomic or molecular form of fluorine. The cross section of F 2 relative to N 2 has been previously measured [10] and hence our value for the cross section of atomic fluorine can be related to that of N 2: ao F aON2 -~-(measured) = (I.0 -+ 0.5) ~ • This value is very close to that calculated by Schlossberg [1], ao F aON2 a~2 (calculated)= 0.6 a ~ with an error bound that is approximately 50% larger. As indicated previously, the uncertainty of the measured cross section is large due to the large uncertainty in the atomic fluorine concentration. A valuable check on our interpretation of the 404 cm -1 signal is provided by observing its dependence on gas temperature. Using the measured Raman signal to calculate atomic fluorine concentration, and taking account o f the population distribution between
1018
° ~
'
I
'
1
'
!
017
1016
1015 600
I
[ 700
I
[ 800
I 900
TEMPERATURE(OK)
Fig. 3. Atomic-fluorine concentration as a function of temperature. The solid curve was computed for conditions of thermodynamic/chemical equilibrium. Data symbols represent monochromator slit widths of 1.0 mm (o) and 0.2 mm (o). Symbols with arrows represent upperbound values. Error bars indicate estimated uncertainties from all sources. the two electronic states, our results can be compared to those from the thermodynamic/chemical equilibrium computation as a function o f gas temperature. This comparison is shown in fig. 3. Since the question of spectral interference due to impurities was raised in the experiment on infrared absorption of atomic fluorine [3], it is important to note that here the data exhibit the proper temperature dependence. Unfortunately it was not possible to investigate higher temperatures than those indicated in fig. 3 due to the marked increase in reaction rate of fluorine with the monel walls above ~ 8 5 0 K. Although our results do not conclusively rule out the possibility that the 404 cm -1 spectral feature is due to an impurity, the data are consistent with the assumption that it is due to atomic fluorine. The observed Raman-shift frequency agrees well with the previous result determined from vuv emission spectroscopy [8]. The measured cross section agrees with the calculated one, although both values have large margins o f error. Finally, the growth of the 404 cm -1 signal with temperature agrees with the equilibrium calculations for atomic fluorine concentration. Our results indicate that Raman scattering can be used as 167
Volume 31, number 2
OPTICS COMMUNICATIONS
a diagnostic technique to detect and measure atomicfluorine concentration. We would like to acknowledge the valuable assistance provided by discussions with our colleagues F.K. Truby, M.E. Riley, and R.A. Hill. R.W. Willey provided technical assistance in constructing the experiment.
References [ 1] H. Schlossberg, Air Force Cambridge Research Laboratories Report #AFCRL-TR-75-0522 (Oct. 1, 1975); also J. Appl. Phys. 47 (1976) 2044. [2] J.P. Moran and R.B. Doak, Initiation phenomena in pulsed chemical lasers, Aerodyne Research Inc. Report #ARI-RR-139 (Oct. 1978), U.S. Navy Contract #00173-7842-1088.
168
November 1979
[3] B.D. Crane, Ph.D. thesis, Univ. of Arizona (1979). [4] R.J. Jensen and K. Boyer, LASL Report L-JUMP-75; 179 (Nov. 1975), U.S. Air Force Project # AFWL-75254. [5] J.C. Cummings, D.P. Aeschliman and A.J. Mulac, JQSRT 19 (1978) 493. [6] D.P. Aeschliman, J.C. Cummings and R.A. Hill, JQSRT 12 (1979) 293. [7] C.A. Powars and R.M. Kendall, User's Manual Aerotherm Chemical Equilibrium (ACE) Computer Program, Aerotherm Corp., Mountain View, CA (May 1969). [8] K. Liden, Ark. Phys. 1 (1949) 229. [9] L.N. Latyev, V.Ya. Chekhovskoi and E.N. Shestakov, High Temperatures-High Pressures, Vol. 2 (1970) pp. 175-181. [10] J.M. Hoell, Jr., F. Allario, O. Jarrett, Jr. and R.K. Seals Jr., J. Chem. Phys. 58 (1973) 2896.