- Email: [email protected]
Accurate measurement of the pressure broadening of the ν1 Raman line of CH4 in the 1–50 atm region by inverse Raman spectroscopy
Volume
91, number
4
CHCMICALPHYSICS
17 Scptcmbcr
LETTCRS
1982
ACCURATE MEASUREMENT OF THE PRESSURE BROADENING OF THE vI RAMAN LINE OF CH,
IN THE I-50 atm REGION
BY lNVERSE RAMAN SPECTROSCOPY
y. TMRA, K. IDE * and H. TAKUMA The Inshlutefor Rcccivcd
10 hlay
Laser Science, llle Umersrty
1982; III fin~~Iform
of Electto-conmlunlcanons,
Chofu, Tohyo 182, Japan
2.5 June 1982
The lmcshapc of the VI Raman band of methane IS mcasurcd as a function o~prcssurc bctwccn I and 50 atrn by InverSe Raman spectroscopy using the 488 nm output of a cw Ar+lascr as n probe beam and the output of a pulad dye Iayx as a Stokes pump beam. The lincmdth ckpressed as Au = 0.32+ 0 012p,p
IS found to mcrcasc linearly rn this pressure bcmg the mcthanc prcssurc in atm.
The technique of Rarnan spectroscopy has made great progress through the application of lasers.However, little work has been done on lineshapes of the Raman spectra of high-pressure gases. In fact, there ISa serious lack of such mformation even for a typical Raman aclve molecule such as CH,. May et al. [l] observed the v1 Raman line of CH, at 600 amagat by a classIca method and obtained a linewidth of 7 5 cm-‘. On the other hand, Murray et al. [2] concluded that the width of the same line was constant m the l-30 atm region from their backward Raman measurement carried out to pursue mterest in laser pulse compression by stunulated backward Raman scattering [3,4]. The rotational structure of the u1 Raman band was found to spread out to a wdth of 0 3 cm-l, as studled with high resolution by Owyoung et al. [S] at low pressure. It is of fundamental interest to understand the relationship of the pre. vious works through an accurate measurement. The study of the lineshape is also of practical interest for an accurate analysis of pulse compresston by stlmulated backward Raman scattering. The experunental apparatus is standard to the technique of coherent inverse Raman spectroscopy, as shown in fig. 1, where only the apparatus for forward scattering IS Illustrated. The probe laser is a * Present
address. Research and Development Center, ToshIbaCorporatIon. Kawasak 210, Japmn.
0 009-2614~82/0000_0000/S
02.75 0 1982 North-Holland
rcpon,
and the fwhm
I-I&1. Eupcnmcntal arnngcment
au (In cm-‘)
for mvcrsc
R3man
can bc
spcctro-
scopy (Totward configuratIon). L,, L2 lenses of 800 mm focal length. D,. Dz. PIN photo Lode detectors P- prism. H &chro~c muror wrth hgh rcflcctwlty at 570 nm.
Spectra-Physics 165 argon ion laser, which was op erated in a single axial mode by a temperature stabihzed intracavlty etalon (SP-589) at 488 run. The Stokes pump laser was a Molectron DLISP pulsed dye laser pumped by a frequency-doubled Q-switched YAG laser (Molectron MY35). The pump laser was operated at an output enerm of several mJ at 10 Hz around 570 nm using rhodamine.6G in ethanol. The measured IineHndth of the output was 0.3 and 0.03 cm -’ in broad band and narrow band modes res ectlvely. In both cases a tuning width of 4.85 cm- P was possible by changing the pressure m the vacuum box containing the tuning elements of the oscillator from 0 to 1 atm. The mput pump pulse and dip of 299
Volume 9 I, number 4
CHEMICAL
the probe beam due to Inverse Raman scattering were detected separately by Hamamatsu S 1188 PIN diodes matched to 50 52 cables. The signals from the detectors were recorded by transient digitizers (Tektromx 7912 AD with 7A19 and 7B90P plug-ins). Tne dye laser (pump) was pressure tuned, and then waveform data were recorded on diskettes by an LSI-I 1 microcomputer system. The PIN diode was cahbrated by a Molectron J3-05 energy meter, and the output voltage showed a hnear dependence wtth the input energy under the present expenmental condltlons. The Raman cell was made of a stamless steel pipe 1 m long and 20 mm mner diameter. Fused stica plates 10 mm thick were used for windows, Much were tested at pressures up to 70 atm. The loss of the probe beam mduccd by the pump beam through mverse Raman scattermg was measured in three experimental configurations: (1) forward scattenng by a wide band pump beam, (2) backward scattenng by a wide band pump beam and (3) backward scattenng by a narrow band pump beam. Both the pump and probe laser beams were colhmated by an mverted telescope, passed through several irises inserted in a 3 m optical path to remove the transverse mhomogenelty, focused m the cell, separated by a mirror and a pnsm placed behmd the exit wmdow of the cell, deIccted by PIN diodes, and recorded by the transient dlgtlzers. In the backward configuration, a similar setup WIS employed except that the beams were counter-propagating. A computercontrolled optical shutter was used to block the pump beam, and the background signal waveform was subtracted from the measured signal. Although the PIN diode had a risetime of 0.9 “5, tht; fieqt&fl-v response was not sufiiclent because the dye laser output (If 3 ‘IS duration mhented the sharply spiked structure of thz \Ac imp laser. In such a case, a serious power-dependent error 2 %+roduced if one simply calculates the absorption coeffrclLz+ from the observed signal usmg the Lambert-Beer law [6]. Such an effect was eliminated by suppressing the pulse energy to a few mJ so as to keep the absorption of the probe beam under 5%. The lineshape was measured at more than 30 points to coves the whole line profile at a given methane pressure in the l-50 atm region. Response of the probe beam for 192 shots of the pump beam were accumulated at each pomt Typical examples of 300
17 Scptcmbcr 1982
PHYSICS LETTERS
Raman Shift
”
Raman
3
Shift +
rig 7. blcasurcd@IIIproriie ofCH4 at (a) 3 atm and 0) 40 atm. The center frequency is mbn.rsy UItech of (a) and
(b). the lineshape observed m the forward configuration are shown in fig. 2. At pressures less than 10 atm, the line showed some asymmetry reflecting rotational structure, but at higher pressures the lmeshape was well fit to a lorentzian. The asymmetric hneshape agrees well with the calculated lineshape assuming that each component of the rotational structure observed by Owyoung et al. [5] is spread into a lorentzlan lineshape with an appropriate width. The fwhm of the hneshapes measured in the three configurations are plotted as functions of pressure m fig. 3. The line~dth is linearly dependent on pressure (blBackward
La._x lmc wdth (1) 03cni
01
I 0
I
IO 20 30 LO 50 cy Prcuurc (awl)
Fig 3. Pressuredependenceof the lmewldthin forwardconfiiuration unth (I) a wide band pump Lser III(a). and in backward configurabon wth (2) wde and (3) narrow band
pump lasersm (b).
CHEMICAL
Volume 91, number 4
17 Scptembcr 1982
PHYSICS LETTERS
for forward scattering. Since the Doppler broadening UI forward Raman scattering is estimated to be only 0 009 cm-‘, the hnewidth measured in the forward configuration should be caused mainly by rotational structure and pressure broadening. On the other hand, the hnewldth measured in the backward configuration showed a non-hnear dependence m the low-pressure region. This difference between the forward and backward configuratlons
should be caused
by Doppler broadening, which ISestimated to be x0.12 cm-’ for backward scattering. The curves for the narrow and broad band pump beams are parallel within experimental error. This leads us to conclude that the pump beam spectral width mcreases the measured linewldth additively. The same effect should occur for forward scattermg. and the pressure broaderung is gven by subtraction of the laser bandwidth of 0.3 cm-l from the measured hnewidth. The gradents of the line for forward scattering and of the linear parts of the backward scattering at higher pressure are equal to withm experimental error. Thus, we conclude that the pressure broadening of the v1 Raman line of CH, ISa linear function of pressure, in contrast to the conclusions of ref. [2]. The linewidth is given by Au = 0.32 + 0.012~ cm-’
(at 293 K) ,
(1)
where Au is the fwhm in cm -I, and p is the pressure in atm. The linewdth at 600 amagat is estimated to be 8.0 cm-’ by extraporation of the present result, III reasonable agreement with ref. [ 11.
The devialon
from a straight line in the backward
scattenng is caused by Doppler broadening. Doppler broadening of this hne is calculated [3] by taking into account “Dicke
The calculated
lmeshape,
which
by Murray narrowng”
is a convolution
et al. [7]. of
the lorentzian lineshape based on the present work and
the Doppler-broadened lineshape based on the
calculation by Murray et al. and the band structure measured by Owyoung et al. [5] shows a good agreement with the hneshape observed III the backward configuration. The pressure shift was also observed. Although we did not attempt an accurate measurement, a pressure shift coefficient of -0.02 -C0.003 cm-‘/at.m of the line center was observed in the pressure range of l-50 atm. The result is conQstent with the previously measured value of -0.0175 cm-‘/atm by May et
Tg 4 Raman pm of meth3nc (broken hncs) glvcn by the prcscnt work superlmposcd on tig. 3 m ref. [ 21.
al. [ 1 ] wlthm the error caused by the frequency drift of the lasers used m the present experiment. Fmally, the experimental result of Murray ct al. [7_] can be explained by the present result as follows.
The Stokes gain coefficient in the backward Raman scattenng can be expressed as g = KP@J
(3
+ Au,) .
where p is the pressure, K a constant, AV the hnewidth given by (1) and Aup the linewidth of the pump laser beam. The gam coefficient calculated by (2) is shown in fig. 4 by broken lme curves compared to their experunental results. The broken lme curves
agree with expenmental observation better than do the solid lines. where the latter have been drawn by Murray et al. assuming a pressure-Independent linewdth. The authors dxusslons.
W&I
to thank
The present
work
S.S. Kano
was partly
for valuable
supported
by the Crannt in Ald for Fusion Research from the Mmlstry of Educattons, Science and Culture and also by the Science and Technology Grants from the Toray Science Foundation.
References [ 11 AD. May, 1-C. Strylnnd and H.L. Welsh. 1. Chcm. Phys. 30 (1959) 1099. 121 J.R Murray, J. Goldhu Letters 32 (1978) 551
and A. Szokc. Appl. Phys.
301
Volume 91, number 4
CHEMICAL PHYSICS LETTERS
[ 31 J.R. Murray, J Goldhar. D ElmerI and A. Szokc, IEEE J Quantum Electron QE-15 (1979) 342 [4] J.J. Ewng, R.A. Haas, J.C Swmde, E.V. George and W F. Krupke, IEEE J Quantum Electron. QE-15 (1979) 368
302
17 September 1982
[S] A. Owyoung, C.W. Patterson and R.S. h!cDoweU, Chem. Phys. Letters 59 (1978) 156. [6] Y. T;un, K.Ide and H. Takuma, unpublished. 171 R.H. Dxke, Phys. Rev 88 (1953) 472.
91, number
4
CHCMICALPHYSICS
17 Scptcmbcr
LETTCRS
1982
ACCURATE MEASUREMENT OF THE PRESSURE BROADENING OF THE vI RAMAN LINE OF CH,
IN THE I-50 atm REGION
BY lNVERSE RAMAN SPECTROSCOPY
y. TMRA, K. IDE * and H. TAKUMA The Inshlutefor Rcccivcd
10 hlay
Laser Science, llle Umersrty
1982; III fin~~Iform
of Electto-conmlunlcanons,
Chofu, Tohyo 182, Japan
2.5 June 1982
The lmcshapc of the VI Raman band of methane IS mcasurcd as a function o~prcssurc bctwccn I and 50 atrn by InverSe Raman spectroscopy using the 488 nm output of a cw Ar+lascr as n probe beam and the output of a pulad dye Iayx as a Stokes pump beam. The lincmdth ckpressed as Au = 0.32+ 0 012p,p
IS found to mcrcasc linearly rn this pressure bcmg the mcthanc prcssurc in atm.
The technique of Rarnan spectroscopy has made great progress through the application of lasers.However, little work has been done on lineshapes of the Raman spectra of high-pressure gases. In fact, there ISa serious lack of such mformation even for a typical Raman aclve molecule such as CH,. May et al. [l] observed the v1 Raman line of CH, at 600 amagat by a classIca method and obtained a linewidth of 7 5 cm-‘. On the other hand, Murray et al. [2] concluded that the width of the same line was constant m the l-30 atm region from their backward Raman measurement carried out to pursue mterest in laser pulse compression by stunulated backward Raman scattering [3,4]. The rotational structure of the u1 Raman band was found to spread out to a wdth of 0 3 cm-l, as studled with high resolution by Owyoung et al. [S] at low pressure. It is of fundamental interest to understand the relationship of the pre. vious works through an accurate measurement. The study of the lineshape is also of practical interest for an accurate analysis of pulse compresston by stlmulated backward Raman scattering. The experunental apparatus is standard to the technique of coherent inverse Raman spectroscopy, as shown in fig. 1, where only the apparatus for forward scattering IS Illustrated. The probe laser is a * Present
address. Research and Development Center, ToshIbaCorporatIon. Kawasak 210, Japmn.
0 009-2614~82/0000_0000/S
02.75 0 1982 North-Holland
rcpon,
and the fwhm
I-I&1. Eupcnmcntal arnngcment
au (In cm-‘)
for mvcrsc
R3man
can bc
spcctro-
scopy (Totward configuratIon). L,, L2 lenses of 800 mm focal length. D,. Dz. PIN photo Lode detectors P- prism. H &chro~c muror wrth hgh rcflcctwlty at 570 nm.
Spectra-Physics 165 argon ion laser, which was op erated in a single axial mode by a temperature stabihzed intracavlty etalon (SP-589) at 488 run. The Stokes pump laser was a Molectron DLISP pulsed dye laser pumped by a frequency-doubled Q-switched YAG laser (Molectron MY35). The pump laser was operated at an output enerm of several mJ at 10 Hz around 570 nm using rhodamine.6G in ethanol. The measured IineHndth of the output was 0.3 and 0.03 cm -’ in broad band and narrow band modes res ectlvely. In both cases a tuning width of 4.85 cm- P was possible by changing the pressure m the vacuum box containing the tuning elements of the oscillator from 0 to 1 atm. The mput pump pulse and dip of 299
Volume 9 I, number 4
CHEMICAL
the probe beam due to Inverse Raman scattering were detected separately by Hamamatsu S 1188 PIN diodes matched to 50 52 cables. The signals from the detectors were recorded by transient digitizers (Tektromx 7912 AD with 7A19 and 7B90P plug-ins). Tne dye laser (pump) was pressure tuned, and then waveform data were recorded on diskettes by an LSI-I 1 microcomputer system. The PIN diode was cahbrated by a Molectron J3-05 energy meter, and the output voltage showed a hnear dependence wtth the input energy under the present expenmental condltlons. The Raman cell was made of a stamless steel pipe 1 m long and 20 mm mner diameter. Fused stica plates 10 mm thick were used for windows, Much were tested at pressures up to 70 atm. The loss of the probe beam mduccd by the pump beam through mverse Raman scattermg was measured in three experimental configurations: (1) forward scattenng by a wide band pump beam, (2) backward scattenng by a wide band pump beam and (3) backward scattenng by a narrow band pump beam. Both the pump and probe laser beams were colhmated by an mverted telescope, passed through several irises inserted in a 3 m optical path to remove the transverse mhomogenelty, focused m the cell, separated by a mirror and a pnsm placed behmd the exit wmdow of the cell, deIccted by PIN diodes, and recorded by the transient dlgtlzers. In the backward configuration, a similar setup WIS employed except that the beams were counter-propagating. A computercontrolled optical shutter was used to block the pump beam, and the background signal waveform was subtracted from the measured signal. Although the PIN diode had a risetime of 0.9 “5, tht; fieqt&fl-v response was not sufiiclent because the dye laser output (If 3 ‘IS duration mhented the sharply spiked structure of thz \Ac imp laser. In such a case, a serious power-dependent error 2 %+roduced if one simply calculates the absorption coeffrclLz+ from the observed signal usmg the Lambert-Beer law [6]. Such an effect was eliminated by suppressing the pulse energy to a few mJ so as to keep the absorption of the probe beam under 5%. The lineshape was measured at more than 30 points to coves the whole line profile at a given methane pressure in the l-50 atm region. Response of the probe beam for 192 shots of the pump beam were accumulated at each pomt Typical examples of 300
17 Scptcmbcr 1982
PHYSICS LETTERS
Raman Shift
”
Raman
3
Shift +
rig 7. blcasurcd@IIIproriie ofCH4 at (a) 3 atm and 0) 40 atm. The center frequency is mbn.rsy UItech of (a) and
(b). the lineshape observed m the forward configuration are shown in fig. 2. At pressures less than 10 atm, the line showed some asymmetry reflecting rotational structure, but at higher pressures the lmeshape was well fit to a lorentzian. The asymmetric hneshape agrees well with the calculated lineshape assuming that each component of the rotational structure observed by Owyoung et al. [5] is spread into a lorentzlan lineshape with an appropriate width. The fwhm of the hneshapes measured in the three configurations are plotted as functions of pressure m fig. 3. The line~dth is linearly dependent on pressure (blBackward
La._x lmc wdth (1) 03cni
01
I 0
I
IO 20 30 LO 50 cy Prcuurc (awl)
Fig 3. Pressuredependenceof the lmewldthin forwardconfiiuration unth (I) a wide band pump Lser III(a). and in backward configurabon wth (2) wde and (3) narrow band
pump lasersm (b).
CHEMICAL
Volume 91, number 4
17 Scptembcr 1982
PHYSICS LETTERS
for forward scattering. Since the Doppler broadening UI forward Raman scattering is estimated to be only 0 009 cm-‘, the hnewidth measured in the forward configuration should be caused mainly by rotational structure and pressure broadening. On the other hand, the hnewldth measured in the backward configuration showed a non-hnear dependence m the low-pressure region. This difference between the forward and backward configuratlons
should be caused
by Doppler broadening, which ISestimated to be x0.12 cm-’ for backward scattering. The curves for the narrow and broad band pump beams are parallel within experimental error. This leads us to conclude that the pump beam spectral width mcreases the measured linewldth additively. The same effect should occur for forward scattermg. and the pressure broaderung is gven by subtraction of the laser bandwidth of 0.3 cm-l from the measured hnewidth. The gradents of the line for forward scattering and of the linear parts of the backward scattering at higher pressure are equal to withm experimental error. Thus, we conclude that the pressure broadening of the v1 Raman line of CH, ISa linear function of pressure, in contrast to the conclusions of ref. [2]. The linewidth is given by Au = 0.32 + 0.012~ cm-’
(at 293 K) ,
(1)
where Au is the fwhm in cm -I, and p is the pressure in atm. The linewdth at 600 amagat is estimated to be 8.0 cm-’ by extraporation of the present result, III reasonable agreement with ref. [ 11.
The devialon
from a straight line in the backward
scattenng is caused by Doppler broadening. Doppler broadening of this hne is calculated [3] by taking into account “Dicke
The calculated
lmeshape,
which
by Murray narrowng”
is a convolution
et al. [7]. of
the lorentzian lineshape based on the present work and
the Doppler-broadened lineshape based on the
calculation by Murray et al. and the band structure measured by Owyoung et al. [5] shows a good agreement with the hneshape observed III the backward configuration. The pressure shift was also observed. Although we did not attempt an accurate measurement, a pressure shift coefficient of -0.02 -C0.003 cm-‘/at.m of the line center was observed in the pressure range of l-50 atm. The result is conQstent with the previously measured value of -0.0175 cm-‘/atm by May et
Tg 4 Raman pm of meth3nc (broken hncs) glvcn by the prcscnt work superlmposcd on tig. 3 m ref. [ 21.
al. [ 1 ] wlthm the error caused by the frequency drift of the lasers used m the present experiment. Fmally, the experimental result of Murray ct al. [7_] can be explained by the present result as follows.
The Stokes gain coefficient in the backward Raman scattenng can be expressed as g = KP@J
(3
+ Au,) .
where p is the pressure, K a constant, AV the hnewidth given by (1) and Aup the linewidth of the pump laser beam. The gam coefficient calculated by (2) is shown in fig. 4 by broken lme curves compared to their experunental results. The broken lme curves
agree with expenmental observation better than do the solid lines. where the latter have been drawn by Murray et al. assuming a pressure-Independent linewdth. The authors dxusslons.
W&I
to thank
The present
work
S.S. Kano
was partly
for valuable
supported
by the Crannt in Ald for Fusion Research from the Mmlstry of Educattons, Science and Culture and also by the Science and Technology Grants from the Toray Science Foundation.
References [ 11 AD. May, 1-C. Strylnnd and H.L. Welsh. 1. Chcm. Phys. 30 (1959) 1099. 121 J.R Murray, J. Goldhu Letters 32 (1978) 551
and A. Szokc. Appl. Phys.
301
Volume 91, number 4
CHEMICAL PHYSICS LETTERS
[ 31 J.R. Murray, J Goldhar. D ElmerI and A. Szokc, IEEE J Quantum Electron QE-15 (1979) 342 [4] J.J. Ewng, R.A. Haas, J.C Swmde, E.V. George and W F. Krupke, IEEE J Quantum Electron. QE-15 (1979) 368
302
17 September 1982
[S] A. Owyoung, C.W. Patterson and R.S. h!cDoweU, Chem. Phys. Letters 59 (1978) 156. [6] Y. T;un, K.Ide and H. Takuma, unpublished. 171 R.H. Dxke, Phys. Rev 88 (1953) 472.