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Spectral characteristics of discharge in gases
Physica 68 (1973) 595-606
SPECTRAL
0 North-Holland Publishing Co.
CHARACTERISTICS
OF DISCHARGE
IN GASES
B.B. LAUD Department
of Physics, Marathwada Aurangabad,
University,
India
and N. MEHENDALE Department
of Physics, University
of Poona,
Poona, India
Received 29 November
1972
Synopsis In this paper we report our observations on the spectral characteristics of CO molecules obtained from different parent molecules and excited in a discharge tube under varying conditions of pressure and voltage. The effects of voltage and pressure on the intensities of the bands are critically discussed. The excitation of CO has been shown mainly to be due to electron impact.
1. Introduction. Molecular spectra are found in a great variety of astronomical sources and hence, whatever factual information that can be gathered in the laboratories about these molecules will be of considerable help in interpreting the data collected from astrophysical studies. To this end, the investigation of the spectral characteristics of the CO molecule was undertaken with emphasis on the study of the relative effects of various operating conditions on the energy and character of the radiation emitted by the molecule. CO bands are usually observed in discharges through a number of compounds containing C and 0 as constituent parts. CO molecules thus formed are, no doubt, the products of the discharge and the conditions prevailing in the discharge are known to influence their activation. It is also evident that the molecules thus obtained from various parent molecules will be in different environments. If, therefore, the characteristics of the bands produced by CO molecules, when pure CO is admitted to the discharge tube, are observed while varying parameters such as pressure, voltage etc., and this is further followed by similar observations on the CO molecules produced by different parent molecules, the data obtained, will be of considerable help in investigating the influence of the environment, if such an influence exists, on the excitation of the molecules. 595
596
B.B. LAUD AND N. MEJSENDALE
2. Experimental. 2.1. The system. An h.f. discharge was found to be suitable for the excitation of CO molecules as various band systems may be emphasized or suppressed in it by controlling the conditions of the discharge. Discharges were generated by means of a push-pull-type oscillator. Pressures were measured with an ‘Auto-vat’ gauge LKB Produckter 3294 B-10. Stepped-up voltages were measured on a cathode-ray oscillograph calibrated with a frequency generator of known output voltage.
2.2. Materials. Four different compounds, viz. formic acid, carbon dioxide, acetone and ethyl alcohol, were used for the production of CO molecules. The compounds were purified by standard processes and then allowed to pass through the discharge tube at a constant rate. 2.3. Spectroscopy. AFuessglassspectrographandaSteinheilquartzspectrograph were used for the measurements. Intensity measurements were made by means of photographic photometry. 3. Results, observations and discussion. 3.1. Colour. The colour of the discharge was initially whitish in all four cases, but changed to green with acetone and formic acid, and to bluish green with ethyl alcohol and carbon dioxide. In the case of acetone, the white colour persisted for a slightly longer time before it changed to green, while in the other cases the change was almost abrupt. 3.2. Deposit on the wall. In the course of the discharges through acetone and ethyl alcohol, solid deposits of a brownish material were formed on the walls of the discharge tube. Formic acid and CO, gave no such deposits. An attempt was made to identify the deposited compound by taking its absorption spectrum with a Perkin Elmer IR spectrophotometer (model 221, NaCl Prism). We summarize in table I the observations made in this regard. TABLEI
Absorption
(cm-1) 1350-1375
1465 1700 2960
spectrum of deposited compound Type of vibration symmetricCHJ deformation which appears when the methyl group is next to thecarbony1 group’) asymmetric CHJ deformation stretching of the C=O bond in carbonyl compounds asymmetric CH3 stretching vibration
SPECTRAL
CHARACTERISTICS
OF DISCHARGE
IN GASES
597
Although no exact structural information regarding the deposited compound could be obtained, the evidence described above clearly indicates the existence of CH3 and C=O in the compound. Such deposits were also observed earlier by Harkins and Jackson*) when an electrical discharge was passed through benzene and by Lind and Glockler3) in butane as well as in other hydrocarbons when subjected to semi-corona electrical discharges. No attempt, however, was made towards their identification. 3.3. Pressure. Although the flow of the gas was adjusted at a constant pressure, some alteration in pressure was observed on passing a discharge through the gas. The change in pressure, at different initial pressures is represented graphically in fig. 1. ETHYL
ALCOHOL
ACETONE co
t
rll -
’ 0.1
’
’
’
0.3
’
05
’
PRESSURE (BEFORE
Fig. 1. Alteration
THE
DISCHARGE
’
0.7 IN
’
’ 0.9
’ 1.1
TORR WAS
PASSED)
in pressure when a discharge is passed.
The pressure remains unaltered when formic acid is admitted into the tube. It goes on increasing, however, with the increase in the initial pressure, with the other compounds in the tube. With CO2 , the change is very small-; but with acetone and ethyl alcohol, particularly with the latter, the change is appreciable. The enhancement of the pressure is, obviously, due to the partial pressures exerted by the fragments resulting from the breaking down of the parent molecules on passing the discharge. With increasing initial pressure, more molecules enter the tube giving
598
B. B. LAUD AND N. MEHENDALE
rise to an increased number of fragments, which cause a still larger change in pressure. The complicated processes in electrical discharges are not well understood and hence a rational interpretation of this fragmentation process is not very easy. The following, however, seems to us to be the probable mechanism by which fragmentation of the parent molecules takes place. Formic acid vapourizes at low pressure and, passing over PZ05, decomposes into H,O and CO, due to the chemical reaction taking place there. The H,O formed is absorbed by P,O,. The dehydration thus being complete, only pure CO enters the discharge tube. Although the CO molecules are excited to various levels on passing through the discharge, there is no further decomposition and hence there is no further alteration in the pressure. CO2 decomposes into CO and oxygen and hence the slight alteration of pressure may be attributed to the oxygen formed. The C-C bond energy in acetone being only 0.73 eV it first decomposes into CH, radicals and CO. The methyl group is further decomposed, as is evident from the spectrum, into H, Hz and CH. On the basis of the bond energies, we suggest the following as the probable scheme for the decomposition of ethyl alcohol and the formation of CO: C,H,OH
+ CH, + CH,OH -+ CH~ -t- CH,O + CH, +
co + H,.
The next stage of fragmentation leads to the decomposition of H, and of CH,. It is evident from the decomposition processes suggested above that the number of particles formed due to electron impact in the case of ethyl alcohol is fairly large; next in order being acetone. 3.4. Spectral characteristics. We present in table II some of the features of the spectra recorded in the cases studied. TABLE II
Some features of the recorded spectra Parent molecule Formic acid Carbon dioxide Acetone
Ethyl alcohol
Spectra observed Angstrom, Triplet (weak), Herzberg (weak) and Asundi (weak) systems as above Angstrom, Triplet, Herzberg and Asundi systems. HZ, CH, Hn, HP, and Hy are quite intense as above; however, Hz. CH, Her, HP and Hy are much less intense than the AngStrom bands as compared to their relative intensities in acetone
SPECTRAL CHARACTERISTICSOF DISCHARGE IN GASES
599
Judging from the relative intensities of CO and those of the other bands and lines we may conclude that in the case of ethyl alcohol the number of CO molecules is considerabiy larger than that of other particles and hence one can infer that ethyl alcohol is more conducive to the formation of CO molecules than acetone. Harkins and Jacksonz) in their study of benzene, xylene, mesytelene and methane observed C, and CH (14300, il3900), bands and H and C lines. With the addition of oxygen, CO and OH made their appearance. The spectral characteristics observed by them are somewhat similar to those observed in the present investigation. They found, however, that the intensities of H and CH increase as the ratio H/C in the parent molecule is increased. In the present investigation, although the ratio H/C in ethyl alcohol is higher than that in acetone, the intensities of H and CH are much lower. It may be noted in this connection, that the molecules studied in the present investigation contain oxygen as a constituent part, while the compounds used by Harkins and Jackson were pure hydrocarbons. Another point ofdifference between our observations and those of Harkins and Jackson is that, while our observations indicated the presence of Hz in the electrodeless discharge, they could observe it only in the glow discharge and not in the electrodeless discharge. 3.4.1. ‘Triplet’ system. The Conditions in the discharge tube with acetone and ethyl alcohol seem to be more favourable for the excitation to the triplet state than those with CO, and formic acid. Curiously enough the presence of Hz also coincides with the presence of the ‘triplet’ system, both occurring when the colour of the discharge is whitish and the pressure is low. This suggests that H, probably plays an important role in the formation of the ‘triplet’ system. These findings are in conformity with those made by Merton and Johnson4) and Camerons) as early as in 1923 and 1926, respectively. The former observed that, when a small quantity of hydrogen was admitted through a palladium regulator, the comet-tail bands almost entirely disappeared and were replaced by well-marked triplet bands. The latter found that on admitting hydrogen with neon, the ‘triplet’ system was isolated. 3.4.2. Angstrom system. In all the four cases, the U’ = 0 progression of the CO Angstrom system seems to be well developed. Five bands (0,O; 0,l; 0,2; 0,3; 0,4) of the system appear with appreciable intensity. The o’ = 1 progression was, however, very weak. 3.4.3. Herzberg system. Only one v” progression is observed in the spectrum emitted by the molecules under investigation. 3.4.4. Third positive system. In the ultraviolet system was found to be well developed.
region the third positive
3.5. Influence of voltage and pressure on intensities. Variation of intensity was studied in some 100 exposures, planned so as to distinguish between
600
B.B. LAUD AND N. MEHENDALE
the effects of pressure and power output. The intensity of the 0,O band of the f%ngstrijm system was measured at different voltages keeping the pressure constant. These measurements were made at various settings of pressure. This was followed by the investigation of the variation of the intensity with pressure, the voltage remaining constant. a) Angstrom system. In all the four cases the 3.5.1. Effect of voltage. intensities of the bands were found to increase with voltage. The variations observed in the case of acetone are shown graphically in fig. 2. Observations made with the other compounds follow a similar course.
ACETONE h-4510.9
A
PRESSURE
I
2100
I
1900
IN TORR
I
I
!
I
1700
1500
1300
1100
VOLTAGE
(VOLTS
I
900
I
Fig. 2. Variation of intensity with voltage (0,O band of the Angstriim system).
Electrons play an important part in the spectral excitation in a discharge, the population of the excited molecular and atomic levels being determined by the distribution of energy among the electrons. In the present case, if the excitation of the molecules is assumed as solely due to electron impact, the intensity of the band, which depends upon the population of the upper state, should increase with
SPECTRAL
CHARACTERISTICS
OF DISCHARGE
IN GASES
601
the increase of energy of the free electrons, i.e., should increase with voltage. Our observations conform to this expectation. There are, however, other possibilities open for the molecules to get to the upper state, viz., 1) by being first excited to a higher state from which tliey afterwards fall to the state under consideration, and 2) by collisions of the second kind. As far as the first of these possibilities is concerned, no spectral evidence has so far been obtained, indicating transition from a higher level to the level BIE+. We cannot, however, entirely rule out the other possibility. It has been suggested6) that in discharges the excitation to CIE+ occurs due to the collisions of the second kind. The system resulting from the transition from this level, viz. the Herzberg system, recorded on our spectrograph is extremely weak, and hence, we conclude that, although such an excitation is possible, its probability is very small. Considering the relative positions of the C’Z+ and B’E+ levels on the energy-level scale, we are inclined to believe, that the probability of excitation to the B’X+ state by collisions of the second kind is also small and hence the excitation may be mainly due to the electron impacts. b) ‘Triplet’ system. In order to investigate the effect of voltage on the intensities of the bands of the ‘triplet’ system, the integrated intensity of the 1,0 band (A 6010 A) was measured at different voltages, the pressure remaining con-
VOLTAGE
IN VOLTS
Fig. 3. Variation of intensity with voltage (I,0 band of the triplet system).
602
B.B. LAUD AND N. MEHENDALE
stant. The experiment was done with acetone in the discharge tube. The variation is shown graphically side by side with that of an Angstrom band in fig. 3. It will be seen that the rise in the excitation function of the B’C+ state is quite steep, while that of the triplet state is comparatively slow. The excitation to the singlet state, therefore, seems to be more probable than excitation to the triplet state. 3.5.2. Effect of pressure. a) Angstrom system. The variation of the intensity with pressure is shown in fig. 4. In all the four cases, at 2080 V, the intensities of the bands at first increase with pressure, reach their maxima and then fall. The intensities of the two bands presented in the figures are not correlated. They have been given in arbitrary units, chosen so as to avoid overlapping and to bring out clearly the mode of respective variations. It is seen that the curves for both bands run almost a parallel course and reach their maxima at exactly the same position on the pressure scale. With the lower voltage, i.e., 1880 V, the maximum is not reached (except in the case of formic acid) in the region of pressures investigated. The general trend of the curves shows, however, a tendency to reach the maximum at lower pressures. The position of the maximum is different for different parent molecules. Table III gives the relative positions of the maxima at the two voltages. TABLE III
Relative positions of maxima 2080
Formic acid co2
Acetone Ethyl alcohol
1880
W)
09
0.58 0.32 0.40 0.24
0.38 _
In the following paragraphs we attempt to explain the variation in intensity described above, on the assumption that the excitation is purely due to electron impacts. i) Formic acid. At a given voltage the electrons will have to fall through a certain potential difference without collision, in order that they acquire enough energy for exciting the molecules. This is possible at low pressures, the mean free path being sufficiently large. However, the effective cross section being small, the probability of collision is low. With increasing pressure, the effective cross section increases, giving rise to more collisions, which result into the increase of intensity. There is a limit, however, to this progressive increase of intensity. Above a certain
SPECTRAL
CHARACTERISTICS
OF DISCHARGE
ACETONE
co2 -VOLTAGE
2060
VOLTS
x
0 -VOLTAGE
1660
VOLTS
O-VOLTAGE
I
iNCSTRfH
$“;I
\ ,T )I
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;10.7-
?
:
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SYSTEM
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VOLTS
1720
VOLTS
fNC.ItRb;M
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ii
(1,O)
A-4123.6
f
’ z
’
’ 1 I I )
0’4 -
0.2
-VOLTAGE
tO,U~-4655.3t
uo.10 _#0.9-
IN GASES
;
\
(0.1 I *-
I\ \
4635.1
ii
I \
\ %
\ ’
\
i
‘1
’
flr0) :
\
’
‘+
I 111,,,1I11 o., 0.5
(O,ll
* -aesrsf
(1,O)
L-4123.6f
0.7
PRESSURE
o-9 IN
i-4123.6
ii
1.1 TORR
PRESSURE
ETHYL
co x -VOLTAGE
2060
VOLTXJ
0 -VOLTAGE
1660
VOLTS
i NGSTRbA
x
IN
TOAR
ALCOHOL
-VOLTAGE
2060
VOLTS
SYST E U
fO,ll*-4635.3
f
o-
I 0.1
I 0.3
,
I 0.5
I
I I I O.? 0.9
PRESSURE
IN
,
1 I.1
TORR
, 0.1
o.,
0.5
0.7
PRESSURE
0.9 IN
I.1
TORR
Fig. 4. Variation of intensity with pressure (0,l and 1,O bands of the Angstriim system).
B. B. LAUD AND N. MEHENDALE
604 limiting
pressure,
the mean free path becomes
smaller
and smaller
and hence the
electronic energy may be lost by impact and momentum exchange with gas molecules, which in turn, results into the decay of the excitation function. It must be mentioned here that the energy losses due to momentum exchange between electrons recently,
and gas molecules are generally believed to be small. It has been shown however, by Misakian, Pearl and Mumma7) that the effects of a linear
momentum transfer and gas molecules.
can be significant
in collision
processes
involving
electrons
ii) Carbon dioxide. In the case of carbon dioxide, electrons bombarding the CO, molecules dissociate them into CO and oxygen and hence, part of the electron energy is expended in this process. A major part of the remaining energy is available for the excitation of CO molecules. With the increase of the number of CO, molecules entering the tube, more and more energy is utilized for dissociation; but the number of excited CO molecules also increases until the maximum is reached. Thereafter, as a larger and larger fraction of the energy available is expended in dissociation, the number of excited molecules gradually decreases. The following very qualitative reasoning should make this clear. Let A be the electronic energy for a given field, n the number of molecules entering the tube at a certain pressure, and X the energy of dissociation; then the energy available for excitation of CO molecules is A - nX. Since each molecule gives rise to one CO molecule, the energy required for their excitation will be nY, if Y is the excitation energy of each molecule. A - nX = nY + E, E being the excess energy. As n increases, E tends to zero. Under this limiting condition, A -nX=nY.
(1)
The intensity reaches its maximum at the pressure corresponding to the value of n following from (1). With n further increasing, more energy is utilised for dissociation and the energy available for excitation is not sufficient to excite n molecules. Then the number of excited molecules, and consequently the intensity, gradually decrease. We can see from (1) that, when the intensity is maximum, n = A/(X + Y). In the case of formic acid this number is A/Y, as X = 0; while in the case of CO,, based on the recent work of Wells, Borst and Zipf’), X could be fixed somewhere near 6 eV. The maximum of intensity for COP, therefore, is attained at a much lower pressure than that for formic acid. iii) Acetone. In the The amount of energy of dissociation of CO2 of CO2 on the pressure
case of acetone CO is formed by successive fragmentation. expended for this purpose is much lower than the energy and, hence, the maximum lies on the higher side of that scale.
iv) Ethyl alcohol. In this case also as CO is formed by successive mentation, a large fraction of the energy (about 9 eV, if the decomposition
fragpro-
r-----ACETONE
VOLTAGE
1.8
;
2080
VOLTS
1.4
z 0
1.0
5-I
= 0.6ci 5 of0.4-
TRIPLET (1,OJ
0.2
-
0.0
-
X-
SYSTEM BAND SOlOCMEAN)
ANGSTROM
I
I
I
o.2
0.4
0.6 PRESSURE
SYSTEM
0,OJ
BAND
h-
4123.6
1 0.8
ii
w
I 1,o IN
TORR
Fig. 5. Variation of intensity with pressure (I,0 band of the triplet system).
ACETONE X 0
-VOLTAGE -
20.0
vOLTACE18a0
THIRD t0.2)
co2
POSITIVE
OAND
X-
2050 1800
-VOLTAGE
THIAD
SYSYEM
3134.4
X -VOLTAGE 0 CO,21
i
BAND
POSITIVE X-
SYSTEM 3134.4
i
_ 0.5
1
1
:: -I
4
O.!
z; 04 :,
0.:
5 -
0.;
0.1
> C PE 0.9
0 .!
0 0
0.:
0. 0.
/++ \,
0.
0.
0.
\,
0. I 0.3
.
I
I
I
0.5
0.7
0.9 PRESSURE
IN
TORR
Fig. 6. Variation of intensity with pressure (0,2 band of the third positive system).
606
B.B. LAUD AND N. MEHENDALE
cesses suggested above are assumed to be correct) is utilized for this purpose. X thus being very large, A/(X + Y) is very small, and hence the maximum falls further on the lower-pressure side of the maximum of COZ . 3.5.2. b) ‘Triplet’ system. The variation of the intensity of the 1,0 band of the ‘triplet’ system with pressure is shown in fig. 5. It is observed that the intensity follows the same course of variation as that of the Angstrom bands and reaches a maximum at exactly the same position on the pressure scale. A striking feature of the intensity variation of ‘triplet’ bands is that the rise of intensity is about 6 times that of Angstrom bands for the same change in pressure. After reaching the maximum the intensity begins to fall almost at the same rate. 3.5.2. c) Third positive system. The system was well developed in all the cases studied. Observations were made by exciting the system with acetone and carbon dioxide. The variation in the intensity of the 0,2 band is presented in fig. 6. The band intensity varies with pressure in the same fashion as for the ‘triplet’ band. It reaches a maximum in the case of acetone at 0.40 torr and for carbon dioxide at 0.32 torr, i.e., exactly at the position corresponding to the maximum of the Angstrom band. REFERENCES 1) 2) 3) 4) 5) 6)
Colthup, N.B., J. Opt. Sot. Amer. 40 (1950) 397. Harkins, W. D. and Jackson, J.H., J. them. Phys. 1 (1933) 37. Lind, S.C. and Glockler, G., J. Amer. Chem. Sot. 51(1929) 3655. Merton, T.R. and Johnson, R.C., Proc. Roy. Sot. A103 (1923) 383. Cameron, W.H.B., Phil. Mag. Ser. 7 1 (1926) 405. Herzberg, G., Molecular Spectra and Molecular Structure, Spectra of diatomic D. Van Nostrand Comp. (Toronto, New York, London, 1951) 158. 7) Misakian, M., Pearl, J.C. and Mumma, A.J., J. them. Phys. 57 (1972) 1891. 8) Wells, W. C., Borst, W.L. and Zipf, E. C., J. geophys. Res. 77 (1972) 69.
molecules,
SPECTRAL
0 North-Holland Publishing Co.
CHARACTERISTICS
OF DISCHARGE
IN GASES
B.B. LAUD Department
of Physics, Marathwada Aurangabad,
University,
India
and N. MEHENDALE Department
of Physics, University
of Poona,
Poona, India
Received 29 November
1972
Synopsis In this paper we report our observations on the spectral characteristics of CO molecules obtained from different parent molecules and excited in a discharge tube under varying conditions of pressure and voltage. The effects of voltage and pressure on the intensities of the bands are critically discussed. The excitation of CO has been shown mainly to be due to electron impact.
1. Introduction. Molecular spectra are found in a great variety of astronomical sources and hence, whatever factual information that can be gathered in the laboratories about these molecules will be of considerable help in interpreting the data collected from astrophysical studies. To this end, the investigation of the spectral characteristics of the CO molecule was undertaken with emphasis on the study of the relative effects of various operating conditions on the energy and character of the radiation emitted by the molecule. CO bands are usually observed in discharges through a number of compounds containing C and 0 as constituent parts. CO molecules thus formed are, no doubt, the products of the discharge and the conditions prevailing in the discharge are known to influence their activation. It is also evident that the molecules thus obtained from various parent molecules will be in different environments. If, therefore, the characteristics of the bands produced by CO molecules, when pure CO is admitted to the discharge tube, are observed while varying parameters such as pressure, voltage etc., and this is further followed by similar observations on the CO molecules produced by different parent molecules, the data obtained, will be of considerable help in investigating the influence of the environment, if such an influence exists, on the excitation of the molecules. 595
596
B.B. LAUD AND N. MEJSENDALE
2. Experimental. 2.1. The system. An h.f. discharge was found to be suitable for the excitation of CO molecules as various band systems may be emphasized or suppressed in it by controlling the conditions of the discharge. Discharges were generated by means of a push-pull-type oscillator. Pressures were measured with an ‘Auto-vat’ gauge LKB Produckter 3294 B-10. Stepped-up voltages were measured on a cathode-ray oscillograph calibrated with a frequency generator of known output voltage.
2.2. Materials. Four different compounds, viz. formic acid, carbon dioxide, acetone and ethyl alcohol, were used for the production of CO molecules. The compounds were purified by standard processes and then allowed to pass through the discharge tube at a constant rate. 2.3. Spectroscopy. AFuessglassspectrographandaSteinheilquartzspectrograph were used for the measurements. Intensity measurements were made by means of photographic photometry. 3. Results, observations and discussion. 3.1. Colour. The colour of the discharge was initially whitish in all four cases, but changed to green with acetone and formic acid, and to bluish green with ethyl alcohol and carbon dioxide. In the case of acetone, the white colour persisted for a slightly longer time before it changed to green, while in the other cases the change was almost abrupt. 3.2. Deposit on the wall. In the course of the discharges through acetone and ethyl alcohol, solid deposits of a brownish material were formed on the walls of the discharge tube. Formic acid and CO, gave no such deposits. An attempt was made to identify the deposited compound by taking its absorption spectrum with a Perkin Elmer IR spectrophotometer (model 221, NaCl Prism). We summarize in table I the observations made in this regard. TABLEI
Absorption
(cm-1) 1350-1375
1465 1700 2960
spectrum of deposited compound Type of vibration symmetricCHJ deformation which appears when the methyl group is next to thecarbony1 group’) asymmetric CHJ deformation stretching of the C=O bond in carbonyl compounds asymmetric CH3 stretching vibration
SPECTRAL
CHARACTERISTICS
OF DISCHARGE
IN GASES
597
Although no exact structural information regarding the deposited compound could be obtained, the evidence described above clearly indicates the existence of CH3 and C=O in the compound. Such deposits were also observed earlier by Harkins and Jackson*) when an electrical discharge was passed through benzene and by Lind and Glockler3) in butane as well as in other hydrocarbons when subjected to semi-corona electrical discharges. No attempt, however, was made towards their identification. 3.3. Pressure. Although the flow of the gas was adjusted at a constant pressure, some alteration in pressure was observed on passing a discharge through the gas. The change in pressure, at different initial pressures is represented graphically in fig. 1. ETHYL
ALCOHOL
ACETONE co
t
rll -
’ 0.1
’
’
’
0.3
’
05
’
PRESSURE (BEFORE
Fig. 1. Alteration
THE
DISCHARGE
’
0.7 IN
’
’ 0.9
’ 1.1
TORR WAS
PASSED)
in pressure when a discharge is passed.
The pressure remains unaltered when formic acid is admitted into the tube. It goes on increasing, however, with the increase in the initial pressure, with the other compounds in the tube. With CO2 , the change is very small-; but with acetone and ethyl alcohol, particularly with the latter, the change is appreciable. The enhancement of the pressure is, obviously, due to the partial pressures exerted by the fragments resulting from the breaking down of the parent molecules on passing the discharge. With increasing initial pressure, more molecules enter the tube giving
598
B. B. LAUD AND N. MEHENDALE
rise to an increased number of fragments, which cause a still larger change in pressure. The complicated processes in electrical discharges are not well understood and hence a rational interpretation of this fragmentation process is not very easy. The following, however, seems to us to be the probable mechanism by which fragmentation of the parent molecules takes place. Formic acid vapourizes at low pressure and, passing over PZ05, decomposes into H,O and CO, due to the chemical reaction taking place there. The H,O formed is absorbed by P,O,. The dehydration thus being complete, only pure CO enters the discharge tube. Although the CO molecules are excited to various levels on passing through the discharge, there is no further decomposition and hence there is no further alteration in the pressure. CO2 decomposes into CO and oxygen and hence the slight alteration of pressure may be attributed to the oxygen formed. The C-C bond energy in acetone being only 0.73 eV it first decomposes into CH, radicals and CO. The methyl group is further decomposed, as is evident from the spectrum, into H, Hz and CH. On the basis of the bond energies, we suggest the following as the probable scheme for the decomposition of ethyl alcohol and the formation of CO: C,H,OH
+ CH, + CH,OH -+ CH~ -t- CH,O + CH, +
co + H,.
The next stage of fragmentation leads to the decomposition of H, and of CH,. It is evident from the decomposition processes suggested above that the number of particles formed due to electron impact in the case of ethyl alcohol is fairly large; next in order being acetone. 3.4. Spectral characteristics. We present in table II some of the features of the spectra recorded in the cases studied. TABLE II
Some features of the recorded spectra Parent molecule Formic acid Carbon dioxide Acetone
Ethyl alcohol
Spectra observed Angstrom, Triplet (weak), Herzberg (weak) and Asundi (weak) systems as above Angstrom, Triplet, Herzberg and Asundi systems. HZ, CH, Hn, HP, and Hy are quite intense as above; however, Hz. CH, Her, HP and Hy are much less intense than the AngStrom bands as compared to their relative intensities in acetone
SPECTRAL CHARACTERISTICSOF DISCHARGE IN GASES
599
Judging from the relative intensities of CO and those of the other bands and lines we may conclude that in the case of ethyl alcohol the number of CO molecules is considerabiy larger than that of other particles and hence one can infer that ethyl alcohol is more conducive to the formation of CO molecules than acetone. Harkins and Jacksonz) in their study of benzene, xylene, mesytelene and methane observed C, and CH (14300, il3900), bands and H and C lines. With the addition of oxygen, CO and OH made their appearance. The spectral characteristics observed by them are somewhat similar to those observed in the present investigation. They found, however, that the intensities of H and CH increase as the ratio H/C in the parent molecule is increased. In the present investigation, although the ratio H/C in ethyl alcohol is higher than that in acetone, the intensities of H and CH are much lower. It may be noted in this connection, that the molecules studied in the present investigation contain oxygen as a constituent part, while the compounds used by Harkins and Jackson were pure hydrocarbons. Another point ofdifference between our observations and those of Harkins and Jackson is that, while our observations indicated the presence of Hz in the electrodeless discharge, they could observe it only in the glow discharge and not in the electrodeless discharge. 3.4.1. ‘Triplet’ system. The Conditions in the discharge tube with acetone and ethyl alcohol seem to be more favourable for the excitation to the triplet state than those with CO, and formic acid. Curiously enough the presence of Hz also coincides with the presence of the ‘triplet’ system, both occurring when the colour of the discharge is whitish and the pressure is low. This suggests that H, probably plays an important role in the formation of the ‘triplet’ system. These findings are in conformity with those made by Merton and Johnson4) and Camerons) as early as in 1923 and 1926, respectively. The former observed that, when a small quantity of hydrogen was admitted through a palladium regulator, the comet-tail bands almost entirely disappeared and were replaced by well-marked triplet bands. The latter found that on admitting hydrogen with neon, the ‘triplet’ system was isolated. 3.4.2. Angstrom system. In all the four cases, the U’ = 0 progression of the CO Angstrom system seems to be well developed. Five bands (0,O; 0,l; 0,2; 0,3; 0,4) of the system appear with appreciable intensity. The o’ = 1 progression was, however, very weak. 3.4.3. Herzberg system. Only one v” progression is observed in the spectrum emitted by the molecules under investigation. 3.4.4. Third positive system. In the ultraviolet system was found to be well developed.
region the third positive
3.5. Influence of voltage and pressure on intensities. Variation of intensity was studied in some 100 exposures, planned so as to distinguish between
600
B.B. LAUD AND N. MEHENDALE
the effects of pressure and power output. The intensity of the 0,O band of the f%ngstrijm system was measured at different voltages keeping the pressure constant. These measurements were made at various settings of pressure. This was followed by the investigation of the variation of the intensity with pressure, the voltage remaining constant. a) Angstrom system. In all the four cases the 3.5.1. Effect of voltage. intensities of the bands were found to increase with voltage. The variations observed in the case of acetone are shown graphically in fig. 2. Observations made with the other compounds follow a similar course.
ACETONE h-4510.9
A
PRESSURE
I
2100
I
1900
IN TORR
I
I
!
I
1700
1500
1300
1100
VOLTAGE
(VOLTS
I
900
I
Fig. 2. Variation of intensity with voltage (0,O band of the Angstriim system).
Electrons play an important part in the spectral excitation in a discharge, the population of the excited molecular and atomic levels being determined by the distribution of energy among the electrons. In the present case, if the excitation of the molecules is assumed as solely due to electron impact, the intensity of the band, which depends upon the population of the upper state, should increase with
SPECTRAL
CHARACTERISTICS
OF DISCHARGE
IN GASES
601
the increase of energy of the free electrons, i.e., should increase with voltage. Our observations conform to this expectation. There are, however, other possibilities open for the molecules to get to the upper state, viz., 1) by being first excited to a higher state from which tliey afterwards fall to the state under consideration, and 2) by collisions of the second kind. As far as the first of these possibilities is concerned, no spectral evidence has so far been obtained, indicating transition from a higher level to the level BIE+. We cannot, however, entirely rule out the other possibility. It has been suggested6) that in discharges the excitation to CIE+ occurs due to the collisions of the second kind. The system resulting from the transition from this level, viz. the Herzberg system, recorded on our spectrograph is extremely weak, and hence, we conclude that, although such an excitation is possible, its probability is very small. Considering the relative positions of the C’Z+ and B’E+ levels on the energy-level scale, we are inclined to believe, that the probability of excitation to the B’X+ state by collisions of the second kind is also small and hence the excitation may be mainly due to the electron impacts. b) ‘Triplet’ system. In order to investigate the effect of voltage on the intensities of the bands of the ‘triplet’ system, the integrated intensity of the 1,0 band (A 6010 A) was measured at different voltages, the pressure remaining con-
VOLTAGE
IN VOLTS
Fig. 3. Variation of intensity with voltage (I,0 band of the triplet system).
602
B.B. LAUD AND N. MEHENDALE
stant. The experiment was done with acetone in the discharge tube. The variation is shown graphically side by side with that of an Angstrom band in fig. 3. It will be seen that the rise in the excitation function of the B’C+ state is quite steep, while that of the triplet state is comparatively slow. The excitation to the singlet state, therefore, seems to be more probable than excitation to the triplet state. 3.5.2. Effect of pressure. a) Angstrom system. The variation of the intensity with pressure is shown in fig. 4. In all the four cases, at 2080 V, the intensities of the bands at first increase with pressure, reach their maxima and then fall. The intensities of the two bands presented in the figures are not correlated. They have been given in arbitrary units, chosen so as to avoid overlapping and to bring out clearly the mode of respective variations. It is seen that the curves for both bands run almost a parallel course and reach their maxima at exactly the same position on the pressure scale. With the lower voltage, i.e., 1880 V, the maximum is not reached (except in the case of formic acid) in the region of pressures investigated. The general trend of the curves shows, however, a tendency to reach the maximum at lower pressures. The position of the maximum is different for different parent molecules. Table III gives the relative positions of the maxima at the two voltages. TABLE III
Relative positions of maxima 2080
Formic acid co2
Acetone Ethyl alcohol
1880
W)
09
0.58 0.32 0.40 0.24
0.38 _
In the following paragraphs we attempt to explain the variation in intensity described above, on the assumption that the excitation is purely due to electron impacts. i) Formic acid. At a given voltage the electrons will have to fall through a certain potential difference without collision, in order that they acquire enough energy for exciting the molecules. This is possible at low pressures, the mean free path being sufficiently large. However, the effective cross section being small, the probability of collision is low. With increasing pressure, the effective cross section increases, giving rise to more collisions, which result into the increase of intensity. There is a limit, however, to this progressive increase of intensity. Above a certain
SPECTRAL
CHARACTERISTICS
OF DISCHARGE
ACETONE
co2 -VOLTAGE
2060
VOLTS
x
0 -VOLTAGE
1660
VOLTS
O-VOLTAGE
I
iNCSTRfH
$“;I
\ ,T )I
2 0.6
:/p, *,
;10.7-
?
:
O-6 -
z
0.5 -
O’S_ -
0.1
_
0o.,
‘.
SYSTEM
2080
VOLTS
1720
VOLTS
fNC.ItRb;M
\ \
I’ 15 I’
‘,
SVSTEM
10.1)
& -4615,3
ii
(1,O)
A-4123.6
f
’ z
’
’ 1 I I )
0’4 -
0.2
-VOLTAGE
tO,U~-4655.3t
uo.10 _#0.9-
IN GASES
;
\
(0.1 I *-
I\ \
4635.1
ii
I \
\ %
\ ’
\
i
‘1
’
flr0) :
\
’
‘+
I 111,,,1I11 o., 0.5
(O,ll
* -aesrsf
(1,O)
L-4123.6f
0.7
PRESSURE
o-9 IN
i-4123.6
ii
1.1 TORR
PRESSURE
ETHYL
co x -VOLTAGE
2060
VOLTXJ
0 -VOLTAGE
1660
VOLTS
i NGSTRbA
x
IN
TOAR
ALCOHOL
-VOLTAGE
2060
VOLTS
SYST E U
fO,ll*-4635.3
f
o-
I 0.1
I 0.3
,
I 0.5
I
I I I O.? 0.9
PRESSURE
IN
,
1 I.1
TORR
, 0.1
o.,
0.5
0.7
PRESSURE
0.9 IN
I.1
TORR
Fig. 4. Variation of intensity with pressure (0,l and 1,O bands of the Angstriim system).
B. B. LAUD AND N. MEHENDALE
604 limiting
pressure,
the mean free path becomes
smaller
and smaller
and hence the
electronic energy may be lost by impact and momentum exchange with gas molecules, which in turn, results into the decay of the excitation function. It must be mentioned here that the energy losses due to momentum exchange between electrons recently,
and gas molecules are generally believed to be small. It has been shown however, by Misakian, Pearl and Mumma7) that the effects of a linear
momentum transfer and gas molecules.
can be significant
in collision
processes
involving
electrons
ii) Carbon dioxide. In the case of carbon dioxide, electrons bombarding the CO, molecules dissociate them into CO and oxygen and hence, part of the electron energy is expended in this process. A major part of the remaining energy is available for the excitation of CO molecules. With the increase of the number of CO, molecules entering the tube, more and more energy is utilized for dissociation; but the number of excited CO molecules also increases until the maximum is reached. Thereafter, as a larger and larger fraction of the energy available is expended in dissociation, the number of excited molecules gradually decreases. The following very qualitative reasoning should make this clear. Let A be the electronic energy for a given field, n the number of molecules entering the tube at a certain pressure, and X the energy of dissociation; then the energy available for excitation of CO molecules is A - nX. Since each molecule gives rise to one CO molecule, the energy required for their excitation will be nY, if Y is the excitation energy of each molecule. A - nX = nY + E, E being the excess energy. As n increases, E tends to zero. Under this limiting condition, A -nX=nY.
(1)
The intensity reaches its maximum at the pressure corresponding to the value of n following from (1). With n further increasing, more energy is utilised for dissociation and the energy available for excitation is not sufficient to excite n molecules. Then the number of excited molecules, and consequently the intensity, gradually decrease. We can see from (1) that, when the intensity is maximum, n = A/(X + Y). In the case of formic acid this number is A/Y, as X = 0; while in the case of CO,, based on the recent work of Wells, Borst and Zipf’), X could be fixed somewhere near 6 eV. The maximum of intensity for COP, therefore, is attained at a much lower pressure than that for formic acid. iii) Acetone. In the The amount of energy of dissociation of CO2 of CO2 on the pressure
case of acetone CO is formed by successive fragmentation. expended for this purpose is much lower than the energy and, hence, the maximum lies on the higher side of that scale.
iv) Ethyl alcohol. In this case also as CO is formed by successive mentation, a large fraction of the energy (about 9 eV, if the decomposition
fragpro-
r-----ACETONE
VOLTAGE
1.8
;
2080
VOLTS
1.4
z 0
1.0
5-I
= 0.6ci 5 of0.4-
TRIPLET (1,OJ
0.2
-
0.0
-
X-
SYSTEM BAND SOlOCMEAN)
ANGSTROM
I
I
I
o.2
0.4
0.6 PRESSURE
SYSTEM
0,OJ
BAND
h-
4123.6
1 0.8
ii
w
I 1,o IN
TORR
Fig. 5. Variation of intensity with pressure (I,0 band of the triplet system).
ACETONE X 0
-VOLTAGE -
20.0
vOLTACE18a0
THIRD t0.2)
co2
POSITIVE
OAND
X-
2050 1800
-VOLTAGE
THIAD
SYSYEM
3134.4
X -VOLTAGE 0 CO,21
i
BAND
POSITIVE X-
SYSTEM 3134.4
i
_ 0.5
1
1
:: -I
4
O.!
z; 04 :,
0.:
5 -
0.;
0.1
> C PE 0.9
0 .!
0 0
0.:
0. 0.
/++ \,
0.
0.
0.
\,
0. I 0.3
.
I
I
I
0.5
0.7
0.9 PRESSURE
IN
TORR
Fig. 6. Variation of intensity with pressure (0,2 band of the third positive system).
606
B.B. LAUD AND N. MEHENDALE
cesses suggested above are assumed to be correct) is utilized for this purpose. X thus being very large, A/(X + Y) is very small, and hence the maximum falls further on the lower-pressure side of the maximum of COZ . 3.5.2. b) ‘Triplet’ system. The variation of the intensity of the 1,0 band of the ‘triplet’ system with pressure is shown in fig. 5. It is observed that the intensity follows the same course of variation as that of the Angstrom bands and reaches a maximum at exactly the same position on the pressure scale. A striking feature of the intensity variation of ‘triplet’ bands is that the rise of intensity is about 6 times that of Angstrom bands for the same change in pressure. After reaching the maximum the intensity begins to fall almost at the same rate. 3.5.2. c) Third positive system. The system was well developed in all the cases studied. Observations were made by exciting the system with acetone and carbon dioxide. The variation in the intensity of the 0,2 band is presented in fig. 6. The band intensity varies with pressure in the same fashion as for the ‘triplet’ band. It reaches a maximum in the case of acetone at 0.40 torr and for carbon dioxide at 0.32 torr, i.e., exactly at the position corresponding to the maximum of the Angstrom band. REFERENCES 1) 2) 3) 4) 5) 6)
Colthup, N.B., J. Opt. Sot. Amer. 40 (1950) 397. Harkins, W. D. and Jackson, J.H., J. them. Phys. 1 (1933) 37. Lind, S.C. and Glockler, G., J. Amer. Chem. Sot. 51(1929) 3655. Merton, T.R. and Johnson, R.C., Proc. Roy. Sot. A103 (1923) 383. Cameron, W.H.B., Phil. Mag. Ser. 7 1 (1926) 405. Herzberg, G., Molecular Spectra and Molecular Structure, Spectra of diatomic D. Van Nostrand Comp. (Toronto, New York, London, 1951) 158. 7) Misakian, M., Pearl, J.C. and Mumma, A.J., J. them. Phys. 57 (1972) 1891. 8) Wells, W. C., Borst, W.L. and Zipf, E. C., J. geophys. Res. 77 (1972) 69.
molecules,