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Dissociative excitation of acetylene by electron impact
CHFMC~
PHYSICS 6 (1974) 291-300. F) NORTH-HOLLANDPUBLISHING COMPANY
DISSOCIATIVE EXCITATION OF ACETYLENE BY ELECTRON IMPACT C.I.M. BEENAKKER* and F.J. DE HEER FOM-Institute
for Atomic and Molecular Physics. Amsterdam,
llre NetherLmds
Received 24 June 1974
The dissociative excitation of acetylene is studied by determining the emission cross sections of the excited frag(CH, C2, H and C) asa function of the electron impact energy (O-1000 eV). From the low-energy results thresholds for Uuorescence are derived. Application of the theory of Bethe IO the high energy data yields information about the symmetry of the relevant dissociative states of the acetylene molecule. A comparison with proton impact
ments
&la is made.
1. xntroduction
2. Experimental
Excitation of molecules to states with sufficiently high energy may result in the formation of excited fragments, of which the light emission can be studied. A relatively simple method to bring molecules into these states is the excitation by electron impact. Only a few systematic studies on the light emission produced by electron impact on aliphatic hydrocarbons have appeared. Of them the work of Vroom and De Heer [ 11, who obtained emission cross sections of excited hydrogen atoms from methane, ethylene and ethane, can be mentioned. Sroka [2] studied the light emission from fragments from methane in the vacuumultraviolet (900- 17OOA). Aarts et al. [3] measured the light emission of fragments from methane and ethylene in the wavelength region 1000-10000 A_ Balmer emission from propane has been investigated by Kurepa et al. [4]. In this paper we present measurements on the dissociative excitation of acetylene. The results are compared with proton impact data [S]. A comparison of the electron impact results of various aliphatic hydrocarbons will be made in refs. [6] and [7].
The apparatus has been described [6] previously. For this study it was slightly modified with respect to the original design [8]. A schematic representation of the apparatus isgiven in fig. 1. It consists of a vacuum chamber containing an electron gun, and of a collision chamber, The electron beam is directed by an axial magnetic field (” 300 gauss) and enters the collision chamber through a collimator consisting of two diaphragms. The intensity of the beam is measured at a Faraday cage. The energy of the electrons can be varied between 0 and 1000 eV; the intensity of the beam is usually kept below 100 PA. A differential pumping system is used in which a diffusion pump is connected to the vacuum chamber. For the evacuation of the apparatus the collision chamber and vacuum chamber can also be COMeCted by a by-pass. Acetylene gas is admitted into the coliision chamber by a variable leak, such that the pressure is in the range 10W4- 10-s torr. The density of the target gas is measured by a capacitance manometer. The light emission from the target gas is detected at right angles to the electron beam by two L.&s monochromaton positioned at both sides of the collision chamber. For the region 1850-5500 A we used a grating of 2160 grooves per mm (reciprocal linear dispersion 15 A/mm) blazed at 2000 A and an EMI 62568 photomultiplier. The grating and mirrors have been coated with magnesium fluoride in order
l
Department of Theoretical Organic Chemistry, University of Leiden, The Netherlands. Present address: Philips’ Research Laboratories, Eindhoven, The Netherlands.
C.LM.
292
pimp
Beenakker
and
EJ.
de Heer.
0
km
L
I
Dissociative
Fig. I. Schematic representationof the apparalur
to obtain a high reflectance below 2300 A. For the region 5500-9000 A we made use of a grating of I200 grooves per mm (reciprocal linear dispersion 27 !&run) blazed at 7500 8, and an RCA 31034A photomultiplier. The photomultipliers are thermoelectrically cooled to below - 15’C. After preamplitication and discrimination againstdark current counts, the photon pulses are converted to standard height and width and fed into a dual counter (SSR model 1110 DigitalSynchronous Computer).
The emission cross sections are evaluated by the relation:
excitation
of acetylene
by electron
impact
For the determination of the quantum yield of the optical equipment we made use of a tungsten nibon lamp as a standard light source. The intensity of the radiation emitted by the tungsten nibon has been measured as a function of ribbon temperature and wavelength by De Vos [IO]. The calibration procedure with this standard has been descnied in ref. [ 1I]. Below 30OOAthis standard cannot be used because of the sharp decrease of the emissionby the hot tungsten fdament towards lower wavelengtband the relative increase of scattered light. In the wavelengthregion 1850-3000~ the relative yield as a function of wavelengthwas determined by means of a deuterium lamp (1850-27OOA) (see ref. [ 121) and a quartziodine lamp with a coiledxoil tungsten filament (2500-44OOA) [ 131.This was fitted to an absolute scale by normalization to the quantum yield determined by the tungsten standard. Thresholds are determined by simultaneous introduction of acetylene and a gas which gives radiation of known threshold energy into the collision chamber using two variable leaks. In this work the 4 3S-23P transition of helium with a known threshold at 23.5 eV [ 141is used for calibrations of the energy scale. In order to avoid space charge effects electron currents below 10 ti are used near threshold. The accuracy of the threshold measurements is limited by the energy spread of the incident electrons (- 0.3 eV).
3. Error discussion In this formulaS(o) represents the Ii&t intensity
measured in the solid angle w,l/e is the number of electrons passingper second through the collision chamber,N is the density of the target gas,L is the observation length Gong the electron beam and k(I) is the quantum yield of the optical equipment at a wavelengthX.The quantum yield k(X) is defined as the signal,in the same units as S(w), per incoming photon per second. The polarization factor P is a correction factor for the cross section, which accounts for an anisotropic distribution of the emitted radiation and for a different sensitivity of the monochromator for the polarized components of the radiation. The degree of polarization is generally found to be small both for molecular radiation and for radiation from dissociativeexcitation [ 1,8,9]. We therefore put P= 1.
The reproducibility of the results is connected to random errors in the measurement of the pressure (3%), the electron beam intensity (2%) and the light signal(1%). The main systematic error results from the absolute intensity call5ration of the monochromators. This error depends on the wavelength and ranges from 10%at3000At04%at9000~[[11]. We estimate the probable error in the absolute quantum yield below 3000 A to be 20%. Additional systematic errors occur in the measurement of the electron beam current (2%) and in the determination of the geometrical factors w and L (2%).l’be neglect of polarization effects introduces an error probably smaller than 5%.The systematic error in the pressure, determined by the Baratron, is estimated to be 2%. This capacitance manometer has been calibrated
CM
Beetwkker and F.J. de Heer. Dissociative excitation of acetylene by eleetmn impuct
against a McLeod manometer, described in ref. [15]. These factors together give probable errors in the emission cross sections ranging from 22% at short wavelengths to 8% at longer wavelengths. The maximum slit width of our monochromators (3 mm) defmes the wavelength region, which can be observed at a fixed position of the grating. In the case of the extensive CH (4314A) and C, (Mull&en) band systems, we divided the relevant wavelength region into parts. The intensity of the radiation in each par! is measured. The emission cross section is determined by adding up the intensities of the different parts. This procedure gives an extra uncertainty of 5%. Larger random errors come into the emission cross sections for weak light signals.
4. Analysis of the hi& energy results The relation between high energy electron impact data and optical data is given by the Bethe theory [ 161. This theory has recently been revisited by Inokuti 1171. According to this theory, in the case of an optically allowed transition leading to the formation of a particular excited fragment, the emission Cross sections at sutTiciently high energies (2 20 times threshold energy) of the incident electrons can be expressed as:
where a0 is the first Bohr radius, R the Rydberg energy, Eel the kinetic energy of the incident electrons corrected for relativistic effects and c, is a constant dependent on the properties of the excited state from which the excited fragment is formed. ,%f& is related to the sum of the optical oscillator strengths for all dipole transitions to the intermediate molecular states giving rise to the excited fmgment emitting the photons under consideration. For symmetry forbidden excitation processes the emission cross sections are inversely proportional to the energy of the incident electrons: Ue, Q&l. Plotting aemEe1/4rr@ versus In& - a so called Fano plot [ lg] - will give a straight line at sufficient-
193 ,
ly high energies with a positive slope in the case of an optically allowed excitation process. Just as in this work, in previous experiments a straight line is often also found at relatively low electron impact energies, where the Bethe approximation is not valid. From the slope of the high energy part of the curve in the Fano plot we derive Af&, , whereas the intercept with the abcissa of the extrapolated linear portion gives cm. A horizontal line in the Fano plot will be found in the case of a symmetry forbidden excitation process. In this work the values ofM&, and cem are determined by a least squares analysis.
5. Spectrum In the emission spectrum between 1850 and 9000 A only radiation from fragments is observed (table I) [ 19-2 1I. For reasons of intensity and overlap of band systems the threshold and cross section measurements were confined to the 4300 A systems of CH. the Mulliken system of C,, the Bahner series of hydrogen and CI radiation.
6. Emission
of the CH radical
Radiation is observed from CH in the three lowest excited doublet states A*A, B’Z- and CzZ+, which decay by radiation only to the X211 ground state.The A2A-X2n emission shows its presence by intense bands in the 4 150-4400 A region, which are attributed to the O-O, I- 1 and 2-2 vibrational trwsitions [ 191. In this experiment we did not observe emission from other vibrational transitions. CaIcuIations on Franck-Condon factors (221 show that these contribute less than 1% to the total A*A-X*fx emission. Cascede from higher states to the A*A state has not been found [23]. The emission cross section, evaluated from the light signal between 4150 and 44OOA, is therefore equal to the cross section for formation of CH in the A2A state. The emission cross sections have been determined by means of the same procedure as described in ref. [3]. This procedure excludes errors in the emission cross section by possible contamination from other excited species like CH+ and H, which also radiate in the 4 ISO-44OOll region, The CH(A*A-X*n) emis-
Cf.U. Becnakker and F.I. de Hew. DimcLzGvc~cifarlon ofacefylene by electron impact
294
Table 1 Emission observedby electron impact on acetylene Name
WA)
TCUlSitiOIl
1931 2325 2479 3143
CI 3s’P”+ 2~’ ‘D C2 D’z+, +X’S; CI 3s ‘PO+ 2p2 ‘s CH C2Z* +X211 CH+ ‘A + ‘l-l CH+ 3Z *?I
A'Il,
C2 C’“9
-’
CH B%H n=7 H n=6
-+X+‘J +n=2 -+n=2
4861
CH H Cz H
A20 n=5 d3”B n=4
-+X’n -rn=2 +a3nu -cn=2
6563
H
n=3
+n=2
3900 3970 4101 4314 4340
Mull&ensystem 3100 A system
kslmdres- d’Azmbuja system 3900 A system Balmere Balmer6 4300 A system Balmer 7 Swan system Bdmer 0 Balmer II
sion cross sections are collected in table 2. The error in them is estimated to be 12%. In fq. 2 the emission cross sections are presented in a Fano plot. According to Bethe’s theory it follows from the straight line portion at the high energy side that M&, = 0.049 and cem = 1.25. The positive slope indicates that optically allowed excitation processesare of major importance in the formation of CH(A2A). Emissioncross sections for CH(A2A-X211) radiation for proton impact on acetylene have been measured by Carri [S]. In the theory of Bethe [ 16, 171 these should become equal to the electron impact cross sections at sufficiently high and equal velocities of the incident particles. A direct comparison between electron and proton impact can therefore be made by scalingthe proton energiesby the rati. of the electron and proton rest masses.Fig. 2 shows that, within the framework of the Bethe theory, the agreement between the proton impact data of Carti IS] and the present electron impact data is rather poor.
Tab!e 2 Emissioncross sections in units of lO-‘9 cm2 for electron impact on acetylene
&I W-9
Cl ‘PO+ ‘s
(1931 A)
-
20 40
60 80 100 120 140 170 200 250 300 350 400 450 500 600 700 800 900 1000
Hn=4-+2 (4861 A)
4.35
8.43 10.0 10.3 10.1 9.31 8.23 7.34 5.90 5.04 4.28 3.73 3.25 2.99 2.50 217 1.83 1.66 1.44
3.20 5.71 6.45 6.68 6.34 5.74 5.03 4.64 3.67 3.05 2.67 2.38 2.09 1.81 lS2 1.30 1.09 1.02 0.908
M&,, a)
C2 D’Z: -c X*X; (2325 A)
30.5 44.4 49.4 48.8 46.6 43.8 40.6 37.1
I .07 1.33 1.67 1.65 1.55 1.44 1.35 1.22 1.10 0.932 0.826 0.708 0.642 0594 0541 0.470 0.409 0.381 0.336 0.304
34.2 29.8 25.7 22.9 20.9 19.0 17.8 15.6 13.7 12.3 11.5 10.7 0.049 f 0.002 1.25
%m a) hn
CHA2A*X2D (4300 A)
refer to standarddeviation in slope only.
0.00114 f 0.00005 3.97
Ci.M. Becnakkerand F.J. de Heel, Dissmiariveexciraricmof acewlene by elecmn impucr
295
E&kcV)
Fig. 2. Emission cros! sectionsfor CH and C2 radiation presented in the form of a Fano plot for electron and proton impact on acetylene; solid lines refer to electrons;dashed line refers to protons (5 1.The emission cross sections for C2 radiationhave been multiplied by 20.
has also been shown to be the case for CH(A2P-X2il) emission from methane and ethylene [3]. The B2C--X211 and C2C+-X211 band systems of CH are contaminated by radiation from background gas in the collision chamber, namely by the fist negative band system of N$(B2Z+-XZZ?) and by the second positive group of N2(C~flU-B~ll ). For that reason the B-X emission cross sectionsgof CH could not be measured. In the case of C-X emission the contributions of nitrogen radiation to the total light signalat 100 eV could be estimated from the light signalat 14 eV, at which energy the radiation can only be due to the strong emission of the second positive group of nitrogen, and from the ratio of the emission cross sections for the second positive group at 14 eV and at 100 eV 1281.We determined u,(CH, C2Zt-X21T) = (8.9 + 4.5) X 10-2Ocm2 at 100 eV. Hence u,(C-X)/a,(A-X) = 0.02 + 0.01. Carri [S] found for this ratio 0.01. His value could be more accurate, because in the case of proton impact triplet states cannot be excited when the effect of secondary electrons is avoided. Consequently radiation from the second positive group of nitrogen may not be present.
This
The energy dependence of u,(A2A-X211) below 100 eV is depicted in fig. 3. The threshold for CH(A2A) radiation is found at 13.0 f 1.5 eV. Another onset is observed at 28 C_2 eV. Table 3 shows minimum excitation energies for formation of excited fragments formed by dissociativeexcitation of acetylene, calculated from known dissociation energies and excitation energiesof the fragments. it follows from a comparison of the observed threshold and the calculated energies, that, near threshold, CH(A2A) is formed by C$f;(!3.0
,+ 1.5 eV) -, CH(A2d) f CH(X2ff)
t excess energy (< I .7 ev). This excess energy is released as internal and kinetic energy of the fragments. From the energy balance it follows that at the first observed threshold for CH(A2A) the second fragment cannot be an ion (table 3). This indicates that these fragments are formed via super-excited states of acetylene [29,30]. At the second threshold CH(A2A) can also be forrned from ion states.
C.I.M. Beenakker and El. de freer, DinocLtive excimtion of acetylene by electron impact
296
Fig. 3. Energydcpcndcnce
below
mc ordinate of fit CH(A*A-X*n)
CIOS sections for CH and C2 radiation for electron impncf on acetylene. emission crow sections has been displaced to avoid confusion.
100 eV of the etion
Table 3 Calculatedminimum energiesa. b) for formation of excited fragmentsby dissociativeexcitation of acetylene Dissociationpwducts
EN)
CzH2 + CH(A*A) + CH(X2n)
12.8
+CH(A’A) -. CH(A’A) -c CH(A2a) +(d3ns) -. C2(d3ttg) -+C2[C’n,) -rC2(C’ng) -C2(D’z;) -c Ca(Dk:) -LCaHiR) -C2(X’Zi) +C2H+(% +C(‘P) 4 C(‘P) * C(‘P)
15.7
+ CH(A’A) + C(‘P)
+ H(n = 1)
16.2
+ CH+(X12+) + H2(X’C;) + H(n = 1) tH(n= 1) + H2(XLC;) + H(n = 1) + H(n = 1) + H#‘x;) + H(n = 1) +H(n= 1) + H(n =4) +H(n=4) + H(n = 1)
23.9 8.6 13.1
+H(n=4) + C(“P) + Hz&;) + CH f_x*rD +H(r:= 1) + C(“P) + H(n = l)+ +H(n= 1)
10.4
14.9
11.5 16.0 18.1 23.3 30.0 20.0 21.0 24.5
a) D(HC-CH) = 9.89 eV [I!SI.DK-H) = 3.47 eV 1261,
D(C-C) = 6.2 2 0.2 eV (211, D(H-H) = 4.48 eV (261,
D(HCC-H) = 5.38 f 0.05 eV 1271,IPKH) = 11.1 eV (261, lP(C2H) = 11.96 t 0.0s eV (271. b) Excitation energieshave been taken from rek [ 14,21,24].
7. Emission of the C2 molecule
Electron impact on acetylene produces the Swan, Deslandres-d’Azambuja and Mulliken systems of C2. These systems correspond respectively to the d311,-a3U,, (?I$--A*&, and D1?Z;-XIBg+ transitions. The emission cross sections for Mulliken radiation are collected in table 2. The enor in oem is estimated to be 25%. Since cascade from higher states to the D state, has not been observed and only emission from this state to the ground state is known 1211, the cross section for C2 Mu&ken radiation is equal to the cross section for formation of C, in the DIZi state by dissociative excitation of acetylene. In fig. 2 these cross sections are presented in the form of a Fano plot. From the slope and the intercept of this plot one derives M&, = 0.00114 and c, = 3.97 respectively. This indicates that, like CH(A2A), an optically allowed excitation process or processes contribute to the formation of C2(D1Zz). The energy dependence of the emission cross sections below 100 eV is depicted in fig. 3. The threshold is found at 18.0 f 0.8 eV. It follows from a comparison with table 3 that part of the radiation is formed by the process(es): C2H;(18.0f0.8eV)+C2(D1C~)+H(n=l)+H(n=l) + 2.0 f 0.8 eV excess energy,
CIA
Beenakker
and
El.
de Heer.
Diswclativr
and/or C2Hi (18.0 * 0.8 ev) + C2(D’Z*,) + Hz(XtCi) t 6.5 k 0.8 eV excess energy. A second onset for Mulliken radiation is observed at 33.5 f 1.5 eV. At this energy various processes can be operative in the formation of C, (DtZt).
8. Bahner emission from the hydrogen atom One of the stronger features in the emission spectrum obtained by electron impact on acetylene is the Balmer series of hydrogen. The emission cross sections for BaImer fl radiation are presented in table 2 and in the form of a Fano plot in fig. 4. The same energy dependence is found for Balmer LI,7 and 6 radiation within 4%. We therefore give for these Balmer lines otdy
the
emission cross sections at 100 eV (table 4).
Due to the weak light signal, the emission cross section for Balmer E radiation could only be measured at 100 eV. The accuracy of the emission cross sections is estimated to be 10% for Balmer 0, y and 6 radiation and 20% for Balmer Q and e radiation. For the Balmer
0
.,
I .
50
,.‘.
of acetylene
excitation
by electron
297
hpact
Table 4 Balmcr emission cross sections at 100 eV in units of 1Oa’9 cm’ for electron impact on acetylene Baimer =
Babner 0
Balmer y
Balmcr 6
Babner E
29.8
6.68
2.42
1.34
0.63
6 line an additional error of 10% may arise from contamination with C, radiation. In fig. 4 we also compare our Balmer fl data with proton impact data of Cart6 [S]. As is the case for CH radiation and Baher p radiation from methane and ethylene [3] the agreement is rather poor. in the asymptotic region the electron impact cross sections are 46% higher than the proton impact cross sections. This is within the stated errors, which zutz 10% in this experiment and 40% in that of Carrb. ln addition to Balmer fi radiation Carri measured dso BZIIITICX7 The ratio of the emission cross sections of these two lines is 0.34 in agreement with our value of 0.36. lotion.
The low energy cross section data for Balmer fi radiation are shown in fig. 5. The threshold for production of H(n = 4) from acetylene is found at
5&l’
l&l
“‘doll fL&V)
’
lb0
I
%I0
.
.
.-I
loo0
Fig. 4. Emission cross sections for H Balmer 0 and CI radiation presented in the form of a Fano plot for electron and proton intpact on acetylene; solid lines refer to electrons; dashed line refers to protons IS]. The emission cross sections for czuboa radiation have been multiplied by l/2.
298
C1.M. Beenakker and F.J. de Heer: Dissociative excitation of acetylene by elecnun impact
I 0
20
/
,
40
6
60
60
loo
E, kv) Fig. 5. Energy dependence below 100 ek of the emission aoss sections for H and Cl radiation for electron impact on acetylene. The ordinale of the H Balmer fl emission cross sections has been displaced to avoid confusion.
20.6 f 1 eV. From table 3 then follows that near threshold H(n = 4) is formed by the process: C2H; (20.6 f 1.0 eV) + CzH(X) + H(n = 4) t 2.5 *- 1.0 eV excess energy.
Becauseof the absence of structure in the energy dependence of the emissioncross sections near threshold (fig. 5) and the zero or nearly zero slope at the high energy side in the Fano plot (fig. 4) we conclude that one or only a few optically forbidden processes are relevant in the formation of excited hydrogen
30% at 193 1 &. Weiss1311has calculated oscillator
strengths for several transitions in Cl and CII by the method of superposition of configurations. It is shown that the calculatedj&i.luesare generally, although not always,obtainable with an accuracy of about 25%. From the calculated oscillator strengths we derive a theoretical value of u,(2479 A)/ oem(193I A) of 0.10 1. This showsthat the values obtained from theory and experiment agreewith each other within the quoted errors. From the energy dependence of the emission cross section (fig. 4) it follows that excitedcarbonatoms
atoms.
in the 3s *POstate are formed, like excited hydrogen
9. Emission from the carbon atom
atoms, mainly by optically forbidden excitation processes.The threshold for carbon (3s rP”) radiation is found at 24.4 + 1 eV (fig. 5). This value allows for a
At 1931A and 2479 A we observedCl emission from the 3s rP” state to the 2p2 ID and the 2p2 tS states respectively.The same energy dependence of uem for both transitions is found, as is expected because the emissionoriginatesfrom the same upper level. Therefore, only the emissioncross sections for 1931A radiation are presented in table 2. The ratio of a,(2479 A)/u,( 193 1 A) was determined to be 0.068 f 0.024. The error is mainly dce to the uncertainty in the quantum sensitivity of the optical equip-
ment, which is estimated to be 20% at 2479 A and
number of processes to be operative in the formation of carbon (3s ‘PO)(see table 3). A second onset is
observed at 45 eV.
10. Further discussion
In the case of optically allowed excitation processes yielding excited fragments the present results can be related to photofluorescence measurements.For acetylene such a study has been undertaken by Metzger and Cook [32]. They measured the relative yield of the
C.I.M.Bcemkker und F.J. de /fee?, Dissoctitiveexcitationof acetyleneby electrvniinpact
undispersed fluorescence(x- 30004000 A) after excitation of acetylene by monochromatic photons in the wavelength region 1000-6000 A (12-2 1 eV).
299
(I 974) 347. Their cross sections for CH raCiation are about 30% lower and for H radiation about 30% higher than those of the present work.
From the electron impact threshold behaviour of the emission cross-sectionsfor C2 Mulliken and CH(A2A-X2fI) radiation (see figs. 3 and 5) it follows that also states of acetylene with a higher excitation energy are involved in the dissociative excitation process. On the other hand the fluorescence measurements of Metzger and Cook give the total radiation of all kinds of transitions (see tables 1 and 3). A comparison of our results with those of ref. [32] is therefore difficult. In addition theoretical and other experimental data on supepexcited states of acetylene are not available. It might be hoped that a comparison of the results of electron impact and photofluorescence experiments in the case of simple aliphatic hydrocarbons may elucidate part of the dissociative excitation mechanism. It is our intention to present such a study in a forthcoming publication 17,301.
Acknowledgement The continuous interest of Professor J. Kistemaker and the late Professor LJ. Oosterhoff is gratefully acknowledged. We wish to thank Drs. J.F.M. Aarts and JJ.C. Mulder for critical comments on this manuscript. We are indebted to Mr. J.W. Bakker for valuable technical assistance. This work is part of the research program of the Stichtingvoor Fundamenteel Ondenoek der Materie (Foundation for Fundamental Research on Matter) and the Stichting Scheikundig Onderzoek in Nederland (Netherlands Foundation for Chemical Research) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research).
Note added in proof Recently V.T. Koppe, N.P. Danilevskii, A.C. Koval and D.V. Pilipenko measured emission cross sections of CH(A2&X211), Ho, H, and C2 Swan radiation for 0.4-6 keV electrons incident on acetylene (and methane), see Opt. Spectry. (USSR) English transl. 36
References 11) D.A. Vroom and F.J. de Heer, 1. Chem. Phyr 50 (1969) 573. [2] W. Sroka, 2. Naturfonch. 24a (1969) 1724. [3j J.F.M. Aarts,C.I.M. 6eenakker and FJ. de Hem. Physic? 53 (1971) 32. 141 1.M. Kurepa.M.D.Tasic and V.V. Urosevic,Abstracts ofPapers,Vlll ICPEAC,Beogad, 1973,eds. B.C.Cobid and M.V. Kurepa(Institute of Physics, Beograd)p_ 373. [S] M. Car& Thesis, Universityof Lyon, France (1967). 161 C.I.hl. Beenakker,Thesis, Universityof Leiden (1974). 171 C.I.M. Beenakkerand F-l. de Heer, to be published. [S] J.F.M. Aarls. Thesis, Universityof Leiden (1970). [9] R.J. van Bruntand R.N. Zare,I. Chcm. Phys. 48 (1968) 4304. [lOI J.C. deVos,Thesis, Universityof Amsterdam(1953); Physim 20 (!954) 690. (111 J. van den Bos, Thesis. Universityof Amsterdam(1967); J. van den Bos, C.J. Winterand F.J. de Hea. Physics 40 (1968) 357. 1121 J.F.M. Aarts and FJ. de Heer, Physic;l49 (1970) 425. [ 131 R. Stair, W.E. Schneiderand J.K. Jackson. AppLOpt. 2 (1963) 1151. [ 141 C.E. hfoore, A multiplet table of astrophysicalinterest (Observatory.Princeton, 1945). [IS] J.C. Bannenbergand A. Tip, Proc. 4th Inten Vat. Congr..Manchester. [ 16j H.A. Bethe, AM. Physik 5 (1930) 325. [ 171 M. Inokuti. Rev. hlod. Phys. 43 (1971) 297. [ 18I U. Fano. Phyr Rev. 95 (1954) 1198. [ 191 R.W.Peafst!and A.G. Gaydon, The Identiacion of molecularspectraChapman. London, 1965). [20) A.R. Striganovand N.S. Sventitskii,Tables of spectral
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alo
CLM Beenakker and F.J. de Heer. Dissociativeexcitationof acetyleneby electron impact
[28] J.F.M. Am and FJ. de HNI, Chcm. Phys Letters 4 (1969) 116. [291 R.L. Phtzmnn, The Vortex 23 (1962) 372. (301 F.I. de Heer, 1. Rad. Phys Chem. (1974), to be publislxd.
1311 A.W. Weiss, Phys. Rev. 162 (1969) 71. 1321 P.H. Metzgcr and G.R. Cook, J. Chem. Phys. 41(1964) 642.
PHYSICS 6 (1974) 291-300. F) NORTH-HOLLANDPUBLISHING COMPANY
DISSOCIATIVE EXCITATION OF ACETYLENE BY ELECTRON IMPACT C.I.M. BEENAKKER* and F.J. DE HEER FOM-Institute
for Atomic and Molecular Physics. Amsterdam,
llre NetherLmds
Received 24 June 1974
The dissociative excitation of acetylene is studied by determining the emission cross sections of the excited frag(CH, C2, H and C) asa function of the electron impact energy (O-1000 eV). From the low-energy results thresholds for Uuorescence are derived. Application of the theory of Bethe IO the high energy data yields information about the symmetry of the relevant dissociative states of the acetylene molecule. A comparison with proton impact
ments
&la is made.
1. xntroduction
2. Experimental
Excitation of molecules to states with sufficiently high energy may result in the formation of excited fragments, of which the light emission can be studied. A relatively simple method to bring molecules into these states is the excitation by electron impact. Only a few systematic studies on the light emission produced by electron impact on aliphatic hydrocarbons have appeared. Of them the work of Vroom and De Heer [ 11, who obtained emission cross sections of excited hydrogen atoms from methane, ethylene and ethane, can be mentioned. Sroka [2] studied the light emission from fragments from methane in the vacuumultraviolet (900- 17OOA). Aarts et al. [3] measured the light emission of fragments from methane and ethylene in the wavelength region 1000-10000 A_ Balmer emission from propane has been investigated by Kurepa et al. [4]. In this paper we present measurements on the dissociative excitation of acetylene. The results are compared with proton impact data [S]. A comparison of the electron impact results of various aliphatic hydrocarbons will be made in refs. [6] and [7].
The apparatus has been described [6] previously. For this study it was slightly modified with respect to the original design [8]. A schematic representation of the apparatus isgiven in fig. 1. It consists of a vacuum chamber containing an electron gun, and of a collision chamber, The electron beam is directed by an axial magnetic field (” 300 gauss) and enters the collision chamber through a collimator consisting of two diaphragms. The intensity of the beam is measured at a Faraday cage. The energy of the electrons can be varied between 0 and 1000 eV; the intensity of the beam is usually kept below 100 PA. A differential pumping system is used in which a diffusion pump is connected to the vacuum chamber. For the evacuation of the apparatus the collision chamber and vacuum chamber can also be COMeCted by a by-pass. Acetylene gas is admitted into the coliision chamber by a variable leak, such that the pressure is in the range 10W4- 10-s torr. The density of the target gas is measured by a capacitance manometer. The light emission from the target gas is detected at right angles to the electron beam by two L.&s monochromaton positioned at both sides of the collision chamber. For the region 1850-5500 A we used a grating of 2160 grooves per mm (reciprocal linear dispersion 15 A/mm) blazed at 2000 A and an EMI 62568 photomultiplier. The grating and mirrors have been coated with magnesium fluoride in order
l
Department of Theoretical Organic Chemistry, University of Leiden, The Netherlands. Present address: Philips’ Research Laboratories, Eindhoven, The Netherlands.
C.LM.
292
pimp
Beenakker
and
EJ.
de Heer.
0
km
L
I
Dissociative
Fig. I. Schematic representationof the apparalur
to obtain a high reflectance below 2300 A. For the region 5500-9000 A we made use of a grating of I200 grooves per mm (reciprocal linear dispersion 27 !&run) blazed at 7500 8, and an RCA 31034A photomultiplier. The photomultipliers are thermoelectrically cooled to below - 15’C. After preamplitication and discrimination againstdark current counts, the photon pulses are converted to standard height and width and fed into a dual counter (SSR model 1110 DigitalSynchronous Computer).
The emission cross sections are evaluated by the relation:
excitation
of acetylene
by electron
impact
For the determination of the quantum yield of the optical equipment we made use of a tungsten nibon lamp as a standard light source. The intensity of the radiation emitted by the tungsten nibon has been measured as a function of ribbon temperature and wavelength by De Vos [IO]. The calibration procedure with this standard has been descnied in ref. [ 1I]. Below 30OOAthis standard cannot be used because of the sharp decrease of the emissionby the hot tungsten fdament towards lower wavelengtband the relative increase of scattered light. In the wavelengthregion 1850-3000~ the relative yield as a function of wavelengthwas determined by means of a deuterium lamp (1850-27OOA) (see ref. [ 121) and a quartziodine lamp with a coiledxoil tungsten filament (2500-44OOA) [ 131.This was fitted to an absolute scale by normalization to the quantum yield determined by the tungsten standard. Thresholds are determined by simultaneous introduction of acetylene and a gas which gives radiation of known threshold energy into the collision chamber using two variable leaks. In this work the 4 3S-23P transition of helium with a known threshold at 23.5 eV [ 141is used for calibrations of the energy scale. In order to avoid space charge effects electron currents below 10 ti are used near threshold. The accuracy of the threshold measurements is limited by the energy spread of the incident electrons (- 0.3 eV).
3. Error discussion In this formulaS(o) represents the Ii&t intensity
measured in the solid angle w,l/e is the number of electrons passingper second through the collision chamber,N is the density of the target gas,L is the observation length Gong the electron beam and k(I) is the quantum yield of the optical equipment at a wavelengthX.The quantum yield k(X) is defined as the signal,in the same units as S(w), per incoming photon per second. The polarization factor P is a correction factor for the cross section, which accounts for an anisotropic distribution of the emitted radiation and for a different sensitivity of the monochromator for the polarized components of the radiation. The degree of polarization is generally found to be small both for molecular radiation and for radiation from dissociativeexcitation [ 1,8,9]. We therefore put P= 1.
The reproducibility of the results is connected to random errors in the measurement of the pressure (3%), the electron beam intensity (2%) and the light signal(1%). The main systematic error results from the absolute intensity call5ration of the monochromators. This error depends on the wavelength and ranges from 10%at3000At04%at9000~[[11]. We estimate the probable error in the absolute quantum yield below 3000 A to be 20%. Additional systematic errors occur in the measurement of the electron beam current (2%) and in the determination of the geometrical factors w and L (2%).l’be neglect of polarization effects introduces an error probably smaller than 5%.The systematic error in the pressure, determined by the Baratron, is estimated to be 2%. This capacitance manometer has been calibrated
CM
Beetwkker and F.J. de Heer. Dissociative excitation of acetylene by eleetmn impuct
against a McLeod manometer, described in ref. [15]. These factors together give probable errors in the emission cross sections ranging from 22% at short wavelengths to 8% at longer wavelengths. The maximum slit width of our monochromators (3 mm) defmes the wavelength region, which can be observed at a fixed position of the grating. In the case of the extensive CH (4314A) and C, (Mull&en) band systems, we divided the relevant wavelength region into parts. The intensity of the radiation in each par! is measured. The emission cross section is determined by adding up the intensities of the different parts. This procedure gives an extra uncertainty of 5%. Larger random errors come into the emission cross sections for weak light signals.
4. Analysis of the hi& energy results The relation between high energy electron impact data and optical data is given by the Bethe theory [ 161. This theory has recently been revisited by Inokuti 1171. According to this theory, in the case of an optically allowed transition leading to the formation of a particular excited fragment, the emission Cross sections at sutTiciently high energies (2 20 times threshold energy) of the incident electrons can be expressed as:
where a0 is the first Bohr radius, R the Rydberg energy, Eel the kinetic energy of the incident electrons corrected for relativistic effects and c, is a constant dependent on the properties of the excited state from which the excited fragment is formed. ,%f& is related to the sum of the optical oscillator strengths for all dipole transitions to the intermediate molecular states giving rise to the excited fmgment emitting the photons under consideration. For symmetry forbidden excitation processes the emission cross sections are inversely proportional to the energy of the incident electrons: Ue, Q&l. Plotting aemEe1/4rr@ versus In& - a so called Fano plot [ lg] - will give a straight line at sufficient-
193 ,
ly high energies with a positive slope in the case of an optically allowed excitation process. Just as in this work, in previous experiments a straight line is often also found at relatively low electron impact energies, where the Bethe approximation is not valid. From the slope of the high energy part of the curve in the Fano plot we derive Af&, , whereas the intercept with the abcissa of the extrapolated linear portion gives cm. A horizontal line in the Fano plot will be found in the case of a symmetry forbidden excitation process. In this work the values ofM&, and cem are determined by a least squares analysis.
5. Spectrum In the emission spectrum between 1850 and 9000 A only radiation from fragments is observed (table I) [ 19-2 1I. For reasons of intensity and overlap of band systems the threshold and cross section measurements were confined to the 4300 A systems of CH. the Mulliken system of C,, the Bahner series of hydrogen and CI radiation.
6. Emission
of the CH radical
Radiation is observed from CH in the three lowest excited doublet states A*A, B’Z- and CzZ+, which decay by radiation only to the X211 ground state.The A2A-X2n emission shows its presence by intense bands in the 4 150-4400 A region, which are attributed to the O-O, I- 1 and 2-2 vibrational trwsitions [ 191. In this experiment we did not observe emission from other vibrational transitions. CaIcuIations on Franck-Condon factors (221 show that these contribute less than 1% to the total A*A-X*fx emission. Cascede from higher states to the A*A state has not been found [23]. The emission cross section, evaluated from the light signal between 4150 and 44OOA, is therefore equal to the cross section for formation of CH in the A2A state. The emission cross sections have been determined by means of the same procedure as described in ref. [3]. This procedure excludes errors in the emission cross section by possible contamination from other excited species like CH+ and H, which also radiate in the 4 ISO-44OOll region, The CH(A*A-X*n) emis-
Cf.U. Becnakker and F.I. de Hew. DimcLzGvc~cifarlon ofacefylene by electron impact
294
Table 1 Emission observedby electron impact on acetylene Name
WA)
TCUlSitiOIl
1931 2325 2479 3143
CI 3s’P”+ 2~’ ‘D C2 D’z+, +X’S; CI 3s ‘PO+ 2p2 ‘s CH C2Z* +X211 CH+ ‘A + ‘l-l CH+ 3Z *?I
A'Il,
C2 C’“9
-’
CH B%H n=7 H n=6
-+X+‘J +n=2 -+n=2
4861
CH H Cz H
A20 n=5 d3”B n=4
-+X’n -rn=2 +a3nu -cn=2
6563
H
n=3
+n=2
3900 3970 4101 4314 4340
Mull&ensystem 3100 A system
kslmdres- d’Azmbuja system 3900 A system Balmere Balmer6 4300 A system Balmer 7 Swan system Bdmer 0 Balmer II
sion cross sections are collected in table 2. The error in them is estimated to be 12%. In fq. 2 the emission cross sections are presented in a Fano plot. According to Bethe’s theory it follows from the straight line portion at the high energy side that M&, = 0.049 and cem = 1.25. The positive slope indicates that optically allowed excitation processesare of major importance in the formation of CH(A2A). Emissioncross sections for CH(A2A-X211) radiation for proton impact on acetylene have been measured by Carri [S]. In the theory of Bethe [ 16, 171 these should become equal to the electron impact cross sections at sufficiently high and equal velocities of the incident particles. A direct comparison between electron and proton impact can therefore be made by scalingthe proton energiesby the rati. of the electron and proton rest masses.Fig. 2 shows that, within the framework of the Bethe theory, the agreement between the proton impact data of Carti IS] and the present electron impact data is rather poor.
Tab!e 2 Emissioncross sections in units of lO-‘9 cm2 for electron impact on acetylene
&I W-9
Cl ‘PO+ ‘s
(1931 A)
-
20 40
60 80 100 120 140 170 200 250 300 350 400 450 500 600 700 800 900 1000
Hn=4-+2 (4861 A)
4.35
8.43 10.0 10.3 10.1 9.31 8.23 7.34 5.90 5.04 4.28 3.73 3.25 2.99 2.50 217 1.83 1.66 1.44
3.20 5.71 6.45 6.68 6.34 5.74 5.03 4.64 3.67 3.05 2.67 2.38 2.09 1.81 lS2 1.30 1.09 1.02 0.908
M&,, a)
C2 D’Z: -c X*X; (2325 A)
30.5 44.4 49.4 48.8 46.6 43.8 40.6 37.1
I .07 1.33 1.67 1.65 1.55 1.44 1.35 1.22 1.10 0.932 0.826 0.708 0.642 0594 0541 0.470 0.409 0.381 0.336 0.304
34.2 29.8 25.7 22.9 20.9 19.0 17.8 15.6 13.7 12.3 11.5 10.7 0.049 f 0.002 1.25
%m a) hn
CHA2A*X2D (4300 A)
refer to standarddeviation in slope only.
0.00114 f 0.00005 3.97
Ci.M. Becnakkerand F.J. de Heel, Dissmiariveexciraricmof acewlene by elecmn impucr
295
E&kcV)
Fig. 2. Emission cros! sectionsfor CH and C2 radiation presented in the form of a Fano plot for electron and proton impact on acetylene; solid lines refer to electrons;dashed line refers to protons (5 1.The emission cross sections for C2 radiationhave been multiplied by 20.
has also been shown to be the case for CH(A2P-X2il) emission from methane and ethylene [3]. The B2C--X211 and C2C+-X211 band systems of CH are contaminated by radiation from background gas in the collision chamber, namely by the fist negative band system of N$(B2Z+-XZZ?) and by the second positive group of N2(C~flU-B~ll ). For that reason the B-X emission cross sectionsgof CH could not be measured. In the case of C-X emission the contributions of nitrogen radiation to the total light signalat 100 eV could be estimated from the light signalat 14 eV, at which energy the radiation can only be due to the strong emission of the second positive group of nitrogen, and from the ratio of the emission cross sections for the second positive group at 14 eV and at 100 eV 1281.We determined u,(CH, C2Zt-X21T) = (8.9 + 4.5) X 10-2Ocm2 at 100 eV. Hence u,(C-X)/a,(A-X) = 0.02 + 0.01. Carri [S] found for this ratio 0.01. His value could be more accurate, because in the case of proton impact triplet states cannot be excited when the effect of secondary electrons is avoided. Consequently radiation from the second positive group of nitrogen may not be present.
This
The energy dependence of u,(A2A-X211) below 100 eV is depicted in fig. 3. The threshold for CH(A2A) radiation is found at 13.0 f 1.5 eV. Another onset is observed at 28 C_2 eV. Table 3 shows minimum excitation energies for formation of excited fragments formed by dissociativeexcitation of acetylene, calculated from known dissociation energies and excitation energiesof the fragments. it follows from a comparison of the observed threshold and the calculated energies, that, near threshold, CH(A2A) is formed by C$f;(!3.0
,+ 1.5 eV) -, CH(A2d) f CH(X2ff)
t excess energy (< I .7 ev). This excess energy is released as internal and kinetic energy of the fragments. From the energy balance it follows that at the first observed threshold for CH(A2A) the second fragment cannot be an ion (table 3). This indicates that these fragments are formed via super-excited states of acetylene [29,30]. At the second threshold CH(A2A) can also be forrned from ion states.
C.I.M. Beenakker and El. de freer, DinocLtive excimtion of acetylene by electron impact
296
Fig. 3. Energydcpcndcnce
below
mc ordinate of fit CH(A*A-X*n)
CIOS sections for CH and C2 radiation for electron impncf on acetylene. emission crow sections has been displaced to avoid confusion.
100 eV of the etion
Table 3 Calculatedminimum energiesa. b) for formation of excited fragmentsby dissociativeexcitation of acetylene Dissociationpwducts
EN)
CzH2 + CH(A*A) + CH(X2n)
12.8
+CH(A’A) -. CH(A’A) -c CH(A2a) +(d3ns) -. C2(d3ttg) -+C2[C’n,) -rC2(C’ng) -C2(D’z;) -c Ca(Dk:) -LCaHiR) -C2(X’Zi) +C2H+(% +C(‘P) 4 C(‘P) * C(‘P)
15.7
+ CH(A’A) + C(‘P)
+ H(n = 1)
16.2
+ CH+(X12+) + H2(X’C;) + H(n = 1) tH(n= 1) + H2(XLC;) + H(n = 1) + H(n = 1) + H#‘x;) + H(n = 1) +H(n= 1) + H(n =4) +H(n=4) + H(n = 1)
23.9 8.6 13.1
+H(n=4) + C(“P) + Hz&;) + CH f_x*rD +H(r:= 1) + C(“P) + H(n = l)+ +H(n= 1)
10.4
14.9
11.5 16.0 18.1 23.3 30.0 20.0 21.0 24.5
a) D(HC-CH) = 9.89 eV [I!SI.DK-H) = 3.47 eV 1261,
D(C-C) = 6.2 2 0.2 eV (211, D(H-H) = 4.48 eV (261,
D(HCC-H) = 5.38 f 0.05 eV 1271,IPKH) = 11.1 eV (261, lP(C2H) = 11.96 t 0.0s eV (271. b) Excitation energieshave been taken from rek [ 14,21,24].
7. Emission of the C2 molecule
Electron impact on acetylene produces the Swan, Deslandres-d’Azambuja and Mulliken systems of C2. These systems correspond respectively to the d311,-a3U,, (?I$--A*&, and D1?Z;-XIBg+ transitions. The emission cross sections for Mulliken radiation are collected in table 2. The enor in oem is estimated to be 25%. Since cascade from higher states to the D state, has not been observed and only emission from this state to the ground state is known 1211, the cross section for C2 Mu&ken radiation is equal to the cross section for formation of C, in the DIZi state by dissociative excitation of acetylene. In fig. 2 these cross sections are presented in the form of a Fano plot. From the slope and the intercept of this plot one derives M&, = 0.00114 and c, = 3.97 respectively. This indicates that, like CH(A2A), an optically allowed excitation process or processes contribute to the formation of C2(D1Zz). The energy dependence of the emission cross sections below 100 eV is depicted in fig. 3. The threshold is found at 18.0 f 0.8 eV. It follows from a comparison with table 3 that part of the radiation is formed by the process(es): C2H;(18.0f0.8eV)+C2(D1C~)+H(n=l)+H(n=l) + 2.0 f 0.8 eV excess energy,
CIA
Beenakker
and
El.
de Heer.
Diswclativr
and/or C2Hi (18.0 * 0.8 ev) + C2(D’Z*,) + Hz(XtCi) t 6.5 k 0.8 eV excess energy. A second onset for Mulliken radiation is observed at 33.5 f 1.5 eV. At this energy various processes can be operative in the formation of C, (DtZt).
8. Bahner emission from the hydrogen atom One of the stronger features in the emission spectrum obtained by electron impact on acetylene is the Balmer series of hydrogen. The emission cross sections for BaImer fl radiation are presented in table 2 and in the form of a Fano plot in fig. 4. The same energy dependence is found for Balmer LI,7 and 6 radiation within 4%. We therefore give for these Balmer lines otdy
the
emission cross sections at 100 eV (table 4).
Due to the weak light signal, the emission cross section for Balmer E radiation could only be measured at 100 eV. The accuracy of the emission cross sections is estimated to be 10% for Balmer 0, y and 6 radiation and 20% for Balmer Q and e radiation. For the Balmer
0
.,
I .
50
,.‘.
of acetylene
excitation
by electron
297
hpact
Table 4 Balmcr emission cross sections at 100 eV in units of 1Oa’9 cm’ for electron impact on acetylene Baimer =
Babner 0
Balmer y
Balmcr 6
Babner E
29.8
6.68
2.42
1.34
0.63
6 line an additional error of 10% may arise from contamination with C, radiation. In fig. 4 we also compare our Balmer fl data with proton impact data of Cart6 [S]. As is the case for CH radiation and Baher p radiation from methane and ethylene [3] the agreement is rather poor. in the asymptotic region the electron impact cross sections are 46% higher than the proton impact cross sections. This is within the stated errors, which zutz 10% in this experiment and 40% in that of Carrb. ln addition to Balmer fi radiation Carri measured dso BZIIITICX7 The ratio of the emission cross sections of these two lines is 0.34 in agreement with our value of 0.36. lotion.
The low energy cross section data for Balmer fi radiation are shown in fig. 5. The threshold for production of H(n = 4) from acetylene is found at
5&l’
l&l
“‘doll fL&V)
’
lb0
I
%I0
.
.
.-I
loo0
Fig. 4. Emission cross sections for H Balmer 0 and CI radiation presented in the form of a Fano plot for electron and proton intpact on acetylene; solid lines refer to electrons; dashed line refers to protons IS]. The emission cross sections for czuboa radiation have been multiplied by l/2.
298
C1.M. Beenakker and F.J. de Heer: Dissociative excitation of acetylene by elecnun impact
I 0
20
/
,
40
6
60
60
loo
E, kv) Fig. 5. Energy dependence below 100 ek of the emission aoss sections for H and Cl radiation for electron impact on acetylene. The ordinale of the H Balmer fl emission cross sections has been displaced to avoid confusion.
20.6 f 1 eV. From table 3 then follows that near threshold H(n = 4) is formed by the process: C2H; (20.6 f 1.0 eV) + CzH(X) + H(n = 4) t 2.5 *- 1.0 eV excess energy.
Becauseof the absence of structure in the energy dependence of the emissioncross sections near threshold (fig. 5) and the zero or nearly zero slope at the high energy side in the Fano plot (fig. 4) we conclude that one or only a few optically forbidden processes are relevant in the formation of excited hydrogen
30% at 193 1 &. Weiss1311has calculated oscillator
strengths for several transitions in Cl and CII by the method of superposition of configurations. It is shown that the calculatedj&i.luesare generally, although not always,obtainable with an accuracy of about 25%. From the calculated oscillator strengths we derive a theoretical value of u,(2479 A)/ oem(193I A) of 0.10 1. This showsthat the values obtained from theory and experiment agreewith each other within the quoted errors. From the energy dependence of the emission cross section (fig. 4) it follows that excitedcarbonatoms
atoms.
in the 3s *POstate are formed, like excited hydrogen
9. Emission from the carbon atom
atoms, mainly by optically forbidden excitation processes.The threshold for carbon (3s rP”) radiation is found at 24.4 + 1 eV (fig. 5). This value allows for a
At 1931A and 2479 A we observedCl emission from the 3s rP” state to the 2p2 ID and the 2p2 tS states respectively.The same energy dependence of uem for both transitions is found, as is expected because the emissionoriginatesfrom the same upper level. Therefore, only the emissioncross sections for 1931A radiation are presented in table 2. The ratio of a,(2479 A)/u,( 193 1 A) was determined to be 0.068 f 0.024. The error is mainly dce to the uncertainty in the quantum sensitivity of the optical equip-
ment, which is estimated to be 20% at 2479 A and
number of processes to be operative in the formation of carbon (3s ‘PO)(see table 3). A second onset is
observed at 45 eV.
10. Further discussion
In the case of optically allowed excitation processes yielding excited fragments the present results can be related to photofluorescence measurements.For acetylene such a study has been undertaken by Metzger and Cook [32]. They measured the relative yield of the
C.I.M.Bcemkker und F.J. de /fee?, Dissoctitiveexcitationof acetyleneby electrvniinpact
undispersed fluorescence(x- 30004000 A) after excitation of acetylene by monochromatic photons in the wavelength region 1000-6000 A (12-2 1 eV).
299
(I 974) 347. Their cross sections for CH raCiation are about 30% lower and for H radiation about 30% higher than those of the present work.
From the electron impact threshold behaviour of the emission cross-sectionsfor C2 Mulliken and CH(A2A-X2fI) radiation (see figs. 3 and 5) it follows that also states of acetylene with a higher excitation energy are involved in the dissociative excitation process. On the other hand the fluorescence measurements of Metzger and Cook give the total radiation of all kinds of transitions (see tables 1 and 3). A comparison of our results with those of ref. [32] is therefore difficult. In addition theoretical and other experimental data on supepexcited states of acetylene are not available. It might be hoped that a comparison of the results of electron impact and photofluorescence experiments in the case of simple aliphatic hydrocarbons may elucidate part of the dissociative excitation mechanism. It is our intention to present such a study in a forthcoming publication 17,301.
Acknowledgement The continuous interest of Professor J. Kistemaker and the late Professor LJ. Oosterhoff is gratefully acknowledged. We wish to thank Drs. J.F.M. Aarts and JJ.C. Mulder for critical comments on this manuscript. We are indebted to Mr. J.W. Bakker for valuable technical assistance. This work is part of the research program of the Stichtingvoor Fundamenteel Ondenoek der Materie (Foundation for Fundamental Research on Matter) and the Stichting Scheikundig Onderzoek in Nederland (Netherlands Foundation for Chemical Research) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research).
Note added in proof Recently V.T. Koppe, N.P. Danilevskii, A.C. Koval and D.V. Pilipenko measured emission cross sections of CH(A2&X211), Ho, H, and C2 Swan radiation for 0.4-6 keV electrons incident on acetylene (and methane), see Opt. Spectry. (USSR) English transl. 36
References 11) D.A. Vroom and F.J. de Heer, 1. Chem. Phyr 50 (1969) 573. [2] W. Sroka, 2. Naturfonch. 24a (1969) 1724. [3j J.F.M. Aarts,C.I.M. 6eenakker and FJ. de Hem. Physic? 53 (1971) 32. 141 1.M. Kurepa.M.D.Tasic and V.V. Urosevic,Abstracts ofPapers,Vlll ICPEAC,Beogad, 1973,eds. B.C.Cobid and M.V. Kurepa(Institute of Physics, Beograd)p_ 373. [S] M. Car& Thesis, Universityof Lyon, France (1967). 161 C.I.hl. Beenakker,Thesis, Universityof Leiden (1974). 171 C.I.M. Beenakkerand F-l. de Heer, to be published. [S] J.F.M. Aarls. Thesis, Universityof Leiden (1970). [9] R.J. van Bruntand R.N. Zare,I. Chcm. Phys. 48 (1968) 4304. [lOI J.C. deVos,Thesis, Universityof Amsterdam(1953); Physim 20 (!954) 690. (111 J. van den Bos, Thesis. Universityof Amsterdam(1967); J. van den Bos, C.J. Winterand F.J. de Hea. Physics 40 (1968) 357. 1121 J.F.M. Aarts and FJ. de Heer, Physic;l49 (1970) 425. [ 131 R. Stair, W.E. Schneiderand J.K. Jackson. AppLOpt. 2 (1963) 1151. [ 141 C.E. hfoore, A multiplet table of astrophysicalinterest (Observatory.Princeton, 1945). [IS] J.C. Bannenbergand A. Tip, Proc. 4th Inten Vat. Congr..Manchester. [ 16j H.A. Bethe, AM. Physik 5 (1930) 325. [ 171 M. Inokuti. Rev. hlod. Phys. 43 (1971) 297. [ 18I U. Fano. Phyr Rev. 95 (1954) 1198. [ 191 R.W.Peafst!and A.G. Gaydon, The Identiacion of molecularspectraChapman. London, 1965). [20) A.R. Striganovand N.S. Sventitskii,Tables of spectral
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alo
CLM Beenakker and F.J. de Heer. Dissociativeexcitationof acetyleneby electron impact
[28] J.F.M. Am and FJ. de HNI, Chcm. Phys Letters 4 (1969) 116. [291 R.L. Phtzmnn, The Vortex 23 (1962) 372. (301 F.I. de Heer, 1. Rad. Phys Chem. (1974), to be publislxd.
1311 A.W. Weiss, Phys. Rev. 162 (1969) 71. 1321 P.H. Metzgcr and G.R. Cook, J. Chem. Phys. 41(1964) 642.