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ABSORPTION OF O2, CO2 AND CS2 ; FLUORESCENCE PROM CS2 ; AND
PHOTOIONIZATION OF ATOMIC CARBON
G. Weissler, M. Ogawa, D. Judge
To cite this version:
G. Weissler, M. Ogawa, D. Judge. ABSORPTION OF O2, CO2 AND CS2 ; FLUORESCENCE PROM CS2 ; AND PHOTOIONIZATION OF ATOMIC CARBON. Journal de Physique Colloques, 1971, 32 (C4), pp.C4-154-C4-159. �10.1051/jphyscol:1971428�. �jpa-00214629�
JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 32, Octobre 1971, page C4-154
ABSORPTION OF 0,, CO, AND CS, ; FLUORESCENCE PROM CS, ; AND PHOTOIONIZATION OF ATOMIC CARBON
G. L. WEISSLER, M. OGAWA and D. L. JUDGE
Department of Physics University of Southern California Los Angeles, California 90007 U. S. A.
Rbum6. - Nous prksentons quelques exemples de recherches nouvelles relatives B diffkrents aspects de l'action de la radiation (hv = 6-60 eV ou 2 000-200 A) sur des atomes ou des molecules.
L'un de nous (M. 0.) s'occupe principalement des sections efficaces, obtenues avec une grande pr6- cision, de 0 2 et de CO2 dans la region de Schumann, des determinations des sections efficaces d'absorption de l'etat metastable l A , de 0 2 et des exemples d'analyse spectroscopique moleculaire de CS2 conduisant B des series nouvelles de Rydberg. De plus, nous prksentons des resultats nou- veaux (par D. L. J.) sur Ia fluorescence caracteristique des transitions des etats excites de CS:.
Les 6tats de CS: sont crkes, par bombardement avec des photons U. V. monochromatiques.
Nous demontrons que l'analyse en longueur d'onde de ces rdsultats de fluorescence contribue a des renseignements supplkmentaires sur la molBcule de CS2. Enfin, nous discutons des methodes permettant d'obtenir les forces d'oscillateur de raies dans l'U. V. B vide (par G. L. W.), telles que celles de la serie Lyman de l'hydrogkne atomique, ainsi que des sections efficaces d'absorption (de photoionisation) du carbone atomique, en utilisant les proprietes d'un plasma produit par un arc stabilise.
Abstract. - Examples on new researches dealing with various phases of the interaction of radiation (hv = 6 to 60 eV or 2 000 to 200 A) with atoms and molecules will be presented. Areas of interest by one of us (M. 0.) concern themselves with some accurately determined absorption cross sections of 0 2 and CO, in the near Schumann region, some measurements of absorption cross sections starting from the metastable l A , state of 02, and some examples of molecular spectroscopic analysis of CS2 leading to a number of new Rydberg series. New results (by D. L. J.) will also be presented on the fluorescence, characteristic of transitions from excited states of CS:.
These CS:* states are formed by bombardment with a monochromatic vaccuum U. V. photon beam. The wavelength analysis of these fluorescence results will be shown to contribute additional information on the CS2 molecule. Finally, methods will be presented for measuring in the vacuum U. V. (by G. L. W.) f-values of lines, such as the Lyman series of atomic hydrogen, and absorption (photoionization) cross sections of atomic carbon, using the plasma properties of a wall-stabilized arc.
I. Absorption cross sections of 0, and CO, continua in the near Schuman region. -Absorption cross sections of the 0, continuum in the region from 2 350 A to 1 814 A and of CO, in the region from2 160 A to 1 718 A
have been measured, with photoelectric detection. The monochromator itself was used as an absorption cell whose path length was 618 cm. Maximum pressures were 745 torr for 0, and 596 torr for COz.
Cross sections of the 0, continuum in the Schumann Runge bands region were measured at wavelengths of the deepest minima between rotational lines of indivi- dual bands. Absorption coefficients, k, or cross sections, a, increase linearly with pressures (in contrast to Beer's law) and are given by
respectively. Measured cross sections c,, including previous ones [l, 2, 31, are shown in figure 1. The cross section is oo = 3.8 X 10-24 cmZ at 2 350 W, it increases
slowly toward shorter wavelengths and reaches a weak maximum of 10.5 X 10-24 cm2 in the neigh- borhood of 2 000 A. Below 2 000 A, it increases very rapidly and its values a t 1 814 A is 7.8 X 10-22 cm2.
The major part of the continuum is probably due t o the 311, (repulsive)-X3 2, transition in the region below 2 000 A and is due to the A 3 ~ : - ~
transition in the region above 2 000 A.
The absorption coefficients or cross sections of CO,, in contrast to O,, follow Beer's law. Numerous discrete bands overlap a weak continuum in the region below 2 000 A, and measured cross sections of the continuum only are shown in figure 2. The wavelength dependence of the continuum is very similar to that of the 0, continuum. The cross section is about 2 X 10-24 cm2 at 2 100 A, gradually increases toward shorter wave- length and reaches about 4 X 1OdZ4 cm2 at 2 000 A.
Below 2 000 A, the cross section increases very rapidly and reaches 1.19 X 10F20 cm2 at 1718 A.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971428
ABSORPTION OF 0 2 , CO2 AND C S ~ ; FLUORESCENCE FROM C S ~ C4-155
X DITCHBURN & YOUNG A HUDSON, CARTER, 8 STEIN
+ SHARDANAND 0 PRESENT
of 0, plus Ar in a region down stream from a micro- wave discharge. Many absorption bands of O,(a l A J were observed in addition to those reported previously [4,5]. By using a 3 m vacuum monochromator, the absorption coefficients of several bands of 02(a 'A,) in the region of 1 408 to 1 486 A have been measured.
The partial pressure of Oz(a 'A,) in the down stream region was estimated from the pressures of Oz(X 3 ~ i )
in the absorption cell before and after the microwave discharge was applied and from the partial pressure of atomic oxygen while the discharge was on. The partial pressure of atomic oxygen was obtained from the increased total pressure by assuming that such an increase was due to dissociation of 0, into atomic oxygen by the discharge. For an example, in one case the pressure of 02(X 3Z:i) was 0.288 torr when the dis- charge was off, and the partial pressures of
Oz(X 32,-) , O2(a Id,) ,
and 0 were 0.240 8, 0.041 4 and 0.001 16 torr, respec- tively, when the discharge was on.
Measured absorption coefficients are listed in Table I.
1800 2000 2200 2400
WAVELENGTH, ( i?t )
FIG. 1. -Absorption cross sections of the 0 2 continuum. Absorption Coefficients of &(a Id,) Absorp. Coef. Relative Absorp. Coef.
K (cm- 1)
Case of 0 2 Case of 0 2 Case of (02 + Ar)
0.01 / I I I I I
1700 1900 2200
WAVELENGTH, (i
FIG. 2. - Absorption cross sections of the CO2 continuum.
11. Absorption coefficients of 0 2 ( a ' A , ) in the wave- length region from 1 325 to 1 650 a. - The absorp- tion spectrum of 0, was photographed in the region from 1 100 t o 1 900 A either in pure 0, or in a mixture
In the case of a mixture of 0, plus Ar, the change of pressure by the microwave discharge was not measured and only relative absorption coefficients were obtained.
These relative values of pure 0, and of a mixture of 0, plus Ar listed in the table agree very well. Since a band at 1 442.0 A is strong and rather broad, this band could be used in other experiments to measure the concentration of 02(a '43.
111. Absorption spectrum of CS, in the region from 600 to 1 015 A. - The absorption spectrum of CS, in the wavelength region from 600 to 1 015 A, the Hopfield helium continuum region, has been reinvesti- gated. A 3 m vacuum spectrograph was used, whose reciprocal linear dispersion was 2.84 A/mm in its first order. In addition to new member bands of the Rydberg series, I11 to VIII, found by Tanaka et al. [6], many new Rydberg series were observed.
We identified in the region from 940 to 1 015 A two new series, A and B, consisting of seven vibrational series with doublet bands, whose separation is about 180 cm. The convergence limits of these series (12.59 to 13.02 eV) correspond probably to the first excited 'If, state of CS;.
Series 111, IV, and 111' (new series), converging to
C4-156 G . L. WEISSLER, M. OGAWA AND D. L. JUDGE 14.47 eV, are accompanied by weak bands at the
longer wavelength side, with a separation of about 40 cm-'. These weak series are probably due to the (100-100) or (01 '0-01 '0) transition. Absence of the (000-100) bands is probably due to their very small Franck-Condon factors. The fact that C-S distances of the % '2; ground state of CS, and the g 'z: state of CS: at the series limit are 1.554 5 A and 1.554 2 A, respectively, which are practically the same (see Herzberg [7], 1966), may support this explanation.
Series V also is accompanied by a weak band 40 cm- l on the longer wavelength side of its own individual bands. This series of weak bands is probably due to the same type of transition just mentioned above for the weak series. A new series, V', converging to the same limit as series V (16.19 eV), was also identified.
There are two further series converging to the limit at 19.38 eV, one of which is a new one, IX, and this value of the limit is smaller by 0.12 eV (1 070 cm-') than the previous value reported by Tanaka et al. [6].
Values of the series limits are summarized in Table 11.
TABLE I1 Rydberg Series of CS, Series Limit
(cm- l)
Present Tanaka Series
et al. [6]
101 515"
102 120 102 720
103 305 A and B
103 880 104 440 104 990
116 751 116 760 111, IV and Ill' 130 567 130 540 V, VI, V11 and V' 156 320 157 390 V111 and IX
* Listed are series limits converging to the 'lit.
state of CS;. A coupling constant of the 'lZ state is about - 184 cm-', estimated from the separations of the doublet bands of both series A and B.
IV. Absorption and fluorescent processes in CS,. - The fluorescence spectrum of CS;" and CS, photo- dissociation fragments has been investigated between 2 000 and 8 000 A with a McPherson 218 monochro- mator (0.3 meter), coupled to a McPherson 225 mono- chromator (1.0 meter) which dispersed the primary extreme ultraviolet radiation (XUV). The system is shown schematically in figure 3 together with a repre- sentation of the electronic pulse counting system.
One purpose of the present research was to determine which states were involved in producing fluorescence when CS2 absorbed photons having an energy greater than about 12 eV. A further purpose was to investigate
p] pG-HzlzJ ELECTRONICS
COOLED DETECTOR IPIVIT)
ter rnonochrornatorl
F I ~ . 3. - Schematic diagram of the fluorescence measuring apparatus.
possible photodissociation processes and, if possible, determine spectroscopic constants and Franck-Condon factors of the states involved. This type of work is complimentary to the work by one of us (M. 0.) on the absorption spectrum of CS,. For example, it is suggested from an analysis of the absorption spec- trum that the series limit for the 'II, state is at 101,335 cm-' (Section 110. This limit should also correspond to the onset of the fluorescence transition + g Cook and Ogawa [8] have previously found the fluorescence onset at 976 A,
about 10 A less than the threshold suggested by the absorption data. Recognizing that the true threshold could be missed because of a low Franck-Condon factor for absorption into the 'L!, (v' = 0) level [g], two improvements over the pr&ious work were employed. A line emission souice having an intensity of at least ten times that of the helium Hopfield conti- nuum provided the background radiation, and the signal to noise ratio was increased further with synchro- nous detection and a cooled photomultiplier. Using a bright source line at 980 A a feeble fluorescence was observed for an incident wavelength of 980 A. It was not possible, however, to determine the precise thres- hold since the light source consisted only of line emission features and there were no other lines avai- lable between the onset suggested by Ogawa's work (Section 111-986.8 A) and 980 A. Although the source line at 980 A should yield a simple spectrum, the low values of the Franck-Condon factors were such that the statistics were not adequate for detailed analysis of the bands, as is evident i n figure 4. Even with the improved statistics with 955 A incident, the resolution of-the fluorescence spectrum was not sufficient to resolve all the features of interest. It is clear, however, that the fluorescence at wavelengths greater than 4 000 A is that of the CS:(~ 'Ua, + -) g 211+, *) system. The identification of the bands is based on Ogawa's identification of the v, levels of the state, and correspond to the various series limits (A & B ) observed by Ogawa (Section 111). The dispersed fluorescence results are shown in figure 4 (for 955 A
incident), where some of the dominant v , progression members have been indicated. Although contributions from v, and v, transitions both in absorption and
ABSORPTION OF 0 2 , COz AND (2 ;2 ; FLUORESCENCE FROM Cs2 C4-157
FIG. 4. - Fluorescence spectrum of the CS:& 217 + z l l system resulting from simultaneous photoionization and exci- tation with 955 A incident. The resolution is approximately 20 A.
emission are possible, they are weak in a symmetric and linear molecule such as CS,. The effect of the available energy levels v, and v, is primarily, as in CO, [10], to shift the v, series such that progressions of the type (m, 0,O) 4 (n, a, b) are dominant. No serious attempt has been made here to analyze these bands since the resolution is presently inadequate.
The second result of the present work is the observa- tion of the photodissociation of CS, between 1 032 and 955 A. Since the ground state of CS, is a singlet state we expect that it will dissociate into either singlet or triplet states. The most likely products to be formed would be CS(A 'l7) + S('D, 'S) or CS(311) + S(3P).
The fluorescence spectrum has a peak at about 2 800 A,
as may be seen in figure 5, and evidently this short
FIG. 5. -Fluorescence spectrum resulting from CS2 absorp- tion of 980 A photons. Here the dominant emission is from CS(A 117 -r X 'C+). Emission from the CS+(L 2 1 7 + ? 217) system is seen to be extremely weak at this incident energy.
The resolution is set at about 50 A.
wavelength peak corresponds to the formation of CS(A 'H), resulting in the bands CS(A 'l7 + X 'Z).
The observation of this system in a photodissociation process has not been previously reported and is some- what surprising since the corresponding photodissocia- tion in CO, has not been reported.
If one accepts the identification of the A state of CS as a singlet [ll], then it is possible to set an upper limit on the dissociation energy of CS,. The lowest excited singlet states of S are 'D (9,238 cm-') and 'S (22,181 cm- l) and the minimum excitation energy of CS is 38,797 cm-l. Thus, the maximum dissociation energy is D(CS, -+ CS + S) = 6.059 eV or 4.454 eV depending, respectively, upon whether the unobserved fragment was 'D or 'S. Here the dissociation limits were obtained by subtracting the excitation energy of the fragments from the lowest incident energy which gave rise to the CS bands. In the present work the lowest energy available was 103 1.012 A (96,907 cm- l).
Dissociative ionization of CS, has also been observed by Dibeler and Walker [12], but the processes which they observed,
csz(fi l$) + hv (< 837 A) -t S+ + CS + e
and
CS,(% 'C,+) + hv (< 767 A) 4 CS' + S + e ,
both occur at wavelengths shorter than that of the presently observed photodissociation threshold. They do, however, give a value of 4.454 eV
(102.7 kcal. mol-l)
for the dissociation limit, which they obtain by sub- tracting the ionization energy of S from their observed onset at 837 A.
One of the possible dissociation limits (4.454 eV) found in the present work is the same as Dibeler and Walker's value but this agreement is completely fortuitous. Further study at longer incident wave- lengths is presently in progress to investigate the true onset of the photodissociation process
CS, + hv -+ CS(A lH) + S('D, 'S).
V. Vacuum W properties of arc plasmas. - High-
temperature plasmas [l31 are known to produce spec- tra of their constituent particles, which are not present under normal conditions and at room temperature.
There exist many problems which make it desirable t o investigate the optical properties of atoms, ions, and perhaps some radicals. For instance, photon-atom interactions are often more amenable to theoretical calculations [l41 than molecules. In addition, this type of work is obviously intimately involved in our understanding of stellar and planetary atmospheres and of plasma physics in general, quite aside from contributing to atomic physics per se [15]. Methods for producing such plasmas are usually found in the various types of electrical discharges through gases, either dc, ac, or disruptive (sparks), or by electron and ion beams, or finally by photons (flash photolysis).
C4-158 G. L. WEISSLER, M. OGAWA AND D. L. JUDGE Among them, the low-pressure shock tube and the
high-pressure wall-stabilized high-current arc [l61 can be made to operate under conditions of local thermo- dynamic equilibrium (LTE), which permits the calculation of both the temperature and therefore the particle densities, and which then in turn allows the determination of oscillator strengths (f-values) of spectral lines and/or photoionization cross sections of such particles. The following will present, in outline form, two examples, namely preliminary measure- ments of f-values [l71 of H-Ly (P, y, 6) and of the photoionization cross section of atomic carbon [l 81, using such a wall-stabilized arc.
The experimental arrangement is shown in figure 6, in which the high-temperature arc plasma may be
Capillary
I
To Pumps
FIG. 6. - Optical arrangement for arc plasma spectroscopy.
viewed longitudinally by two spectrometers : a Seya-type covering the long wavelength region up to 8 000 A, and a grazing-incidence vacuum UV instru- ment for short wavelengths between 100 A and
3 000 A. Differential pumping separated the region of atmospheric pressure in the arc chamber from the vacuum spectrograph. By avoiding window materials, the optical range for argon as an arc carrier gas exten- ded from about 800 A, the short wavelength limit due to photoionization of argon, towards longer wave- lengths. The arc channel was about 6 mm in diameter and 70 mm in length, and the arc current was generally of the order of 100 A, stabilized by an ohmic resistance and an inductance. For work on f-values of H-Ly (p, y, 6) the argon carrier gas was diluted with H, of known flow-rate, while for the carbon photoioni- zation research it was necessary to add CO, to argon.
The basic equations which describe the arc plasma in LTE are Dalton's law of partial pressures, the Saha equation for the ratio of the number densities of plasma species, the condition of quasineutrality of charges, and the determination of the electron densities from Stark-broadening of H-lines :
n, + no + ni = p/kT (Dalton) , (1) where the subscripts indicate the number densities of
electrons, neutral atoms, and singly ionized atoms, respectively ;
where Zi and Zo are, respectively, the partition func- tion of the ion and of the neutral atom, m is the elec- tronic mass, Ei the ionization energy of the isolated atom, and AEi is the lowering of E, due to neighboring charges (of the order of AEi
-
0.15 eV) ;n, = ni (neutrality), (3) and
n, = (const.) (Stark broadening), (4) where the half width A 1 is usually measured for the Hp line, when small amounts of H, were added. Making use of these relationships, the oscillator strengths,
fmn, of H-Ly G, y, 6) could be determined experimen- tally from
where the integral extends over the contour over the spec- tral line, I, is the emitted intensity a t A, B, is the Planck function which connects through KirchhofF's law the emissivity with the absorptivity (E = Bz), is the center of the line and L the length of the radiating plasma. Table I11 compares the experimental f-values to theory. Deviations are primarily due to our uncer- tainty in the sensitivity required to measure B, in the vacuum UV.
Finally, the carbon photoionization cross section o, described by the reaction
was obtained at its threshold, 1 100 A, and at some shorter wavelengths, as shown in figure 7. In this region of measurement, the spectrum was free from emission and absorption lines, which could have interfered with the determination of the carbon
Wave- Oscillator Strength Ratio length fmn( X 10-'> -
(A) Present Theory . (S . . Exu./Theor.
Exp.
- - -
LY-P 1025.72 3.5 7.9 0.44
LY-Y 972.54 1.4 2.9 0.48
Ly-6 949.74 1.1 1.4 0.79
Continuum (*) 910 4.6 6.3 0.73
(*) Photoionization cross section ( X 10-18 cmz) at ionization limit, 912 A.
(a) Atomic Transition Probabilities D, Vol. 1, U. S. Dep't of Commerce, National Bureau of Standards, 1966.
ABSORPTION OF 02, C02 AND CSZ ; FLUORESCENCE FROM Csz C4-159 resonance continuum. Because of the complexity of the
experimental procedures, the reader is referred else- where for details. It must suffice here to indicate that these cross sections were measured to within a total error of about 30 %. This was accomplished by
FIG. 7. - Photoionization cross section of neutral carbon.
adding varying amounts of CO, to the argon carrier gas of the arc, operating a t a temperature of 12,550 OK which was too high for any chemical bond to exist.
Thus, the plasma contained, in addition to electrons, ions and neutral atoms of argon, carbon and oxygen.
Because of this large number of plasma constituents, it was necessary to keep very careful control of the temperature by adding in steps to the pure argon plasma gradually one or more constituent gas, by deter- mining accurately that length of the arc containing carbon, and finally by averaging over a wide range of flow rates of CO,, in order to obtain the most reliable values of densities of atomic carbon. All of this was achieved by the repeated application of the basic plasma relationships indicated above, yielding a photoionization cross section of carbon at threshold of o = 19 X 10-l8 cm2.
References 111 DITCHBURN (R. W.) and YOUNG (P. A.), J . Afmosph.
Terrest. Phys., 1962,24,127.
[2] HUDSON (R. D.), CARTER (V. L.) and STEIN (J. A.), J . Geophys. Res., 1966,71,2295.
[3] SHARDANAND, Phys. Rev., 1969,186,5.
[4] TANAKA (Y.), JURSA (A. S.), LEBLANC (F. J.) and INN (C. Y.), Planet. Space Sci., 1959,1,7.
[5] ALBERTI (F.), ASHBY (R. A.) and DOUGLAS (A. E.), Can. J. Phys., 1967,46,2213.
[6] TANAKA (Y.), JURSA (A. S.) and LEBLANC (F. J.), J. Chem. Phys., 1960,32,1205.
171 HERZBERG (G.), Electronic Spectra of Polyatomic Molecules (D. Van Nostrand Company, Inc., New York), pp. 594 and 601.
[8] COOK (G. R.) and OGAWA (M.), J. Chem. Phys., 1969, 51, 2419.
[g] TURNER (D. W.) and MAY (D. P.), J. Chem. Phys., 1967,46,1156.
[l01 JUDGE (D. L.), BLOOM (G. S.) and MORSE (A. L.), J. Chem. Phys., 1969,47,459.
1111 HERZBERG (G.), Molecular Spectra and Molecular
Structure - I. Spectra of Diatomic Molecules (D. Van Nostrand Company, Inc., N. J., 1950), 2nd Edition, p. 523.
[l21 DIBELER (V. H.) and WALKER (J. A.), J. Opt. SOC.
Am., 1967,57,1007.
[l31 GRIEM (H. R.), Plasma Spectroscopy (McGraw-Hill Book Co., New York, 1964).
[l41 PEACH (G.), Mem. Roy. Astr. Soc., 1967, 71, 29.
1151 WIESE (W. L.) et al., (( Atomic Transition Puobabili- ties D, Vol. I and 11, National Standard Reference Data Series - Nat'l Bur. Stds. - 4 and - 22 ; U. S. Gov't Printing Office, Washington 1966 and 1969.
[l61 MAECKER (H.) and STEINBERGER (S.), 2. Angew. Phys., 1967,23,456 ; also 2. Naturforsch., 1956, l l a , 457.
[l71 OGAWA (S.), Univ. of South. Calif., private commu- nication; see also Tech. Report No. USC-Vac UV-122, april 15, 1970 by Ogawa (S.) and Weissler (G. L.).
1181 HOFMANN (W.) and WEISSLER (G. L.), J. Opt. SOC.
Am. 1971, 61, 223.
DISCUSSION S. LEACH. - In 1963, Moranj, Rostas and I reported
the observation of an emission spectrum in the visible on bombardment of a beam of CS, molecules with a crosses beam of low energy electrons. We attributed this spectrum to the A2 nu -+ X2 zg transition of CS;.
This spectrum differs from that observed in the same spectral region by callomin in a discharge through CS,. We beleive that the latter spectrum could be due to a triplet -+ singlet transition of the neutral CS, molecule.
I would like to know whether the visible spectrum you have observed with CS, is the same as that which we observed or that observed by Callomin, or is ano-
two regions of fluorescence seem clearly indentified at this time :
1. CS, + hy (955 A) -+ z ~ / ~ , ~ / ~ ) + e-
-1
CS;(X~ ~1/2,3/2) -I- (hy)fluorescence 2. CS, + hy (- 1 000 A)
,/
ther spectrum. Is your spectral resolution sufficient for I- cS(X1 X) + (hy)~uorescence analysis to be carried out ?
More extensive an'alysis at a later time should yield among other things better threshold values for the WEISSLER. - The analysis of the fluorescence from primary photoionization and may cause a revision of CS, has at this time not yet been completed. However, the states involved in the observed fluorescence.
