- Email: [email protected]
Dissociative excitation of benzene by electron impact
Volume 29, number 1.
CHEMICAL PHYSICS LETT’ERS
DISSOCIATIVE
EXCITATION
OF BENZENE’BY
C.I.M. BEENmRz* FOM-3nstitute
for Atomic
I November 1974
ELECTRON
IMPACT
and F.J. DE HEER
and Molecular
Physics,
Amsterdam,
The Netherlands
Received 8 July 1974
In the wavelcngti~ Iegion 1850-9000 sociative excitation been determined
of bcnzenc
by electron
A radiation from H, C and CH fragments is observed as impact
(O-1000
for the Balmer series of the hydrogen
Impact of electrons’on molecules may result in the formation of excited states which can be both optically allowed or optically forbidden (by symmetry or by spin multiplicity) with respect to the ground state. In the present paper we deal with excited states of benzene which lead to dissociation into excited fragments that are detected by their emission spectrum: We have determined the threshold energies and emission cross sections for Balmer emission of H and A2A X211 emission of CH fragments. The ener,? of the incident electrons is varied between 0 and 1000 eV. The emission c:oss sections of the fragment radiation are proportional to the excitation cross section of the me, lecular states of benzene dissociating into H(n > 3) and CH(A2A). Therefore the dissociative states can be characterized by analysing the emission cross’sections (aim). Both the threshold energy, the threshold behaviour and the energy dependence of the emission cross sections at high energies give information about the intermediate excited states of the benzene molecule. At high impact energies the data are analyzed by means of a Fano plot [I] , which is a plot of a,,E,,/4na~R +rsus In Ed, where Ed is the energy of the incident electrons, a,j is the Bohr radius and R is the Rydberg energy. According to the theory of Bethe [2] such a plot gives a straight line with Presetit
Emission
atom and the A*A
1. Introduction
*.
cV).
,-
address: Philips Research Laboratories, Eindhoven. The Ne’&erlanck *. Depakment of Theo&d Org+iC Chemistry,, University of &eyden, The Netherlands.
2 remlt
cross sections aad threshold - X2rl
emission
of.‘hc diseneigics~kave
of the CH Gqment.
a positive slope when the excited state of benzene is optically allowed with respect to the ground state and a horizontal line when the excited state is symmetry forbidden (see for instance refs. [3-51). Spin forbidden excitation processes reveal their presence by a decreasing behaviour of uemEel [6]. In addition to the radiation froin H artd,CH we could
also detect a weak emission from the carbon fragment (3s IPO - 2p2 l D, 1931 18Land 3s lPa - 2p2 ‘S, 2479 A. The results on the molecular ‘BZu - ‘Alg fhorescence have been presented before [7,8].
2. Experimental The apparatus has been described before [9]. It consists of a vacuum chamber and oFa collision chamber. An electron gun placed in the vacuum chamber produces a quasi mono-energetic electron bean-r. For the measurements. of the thresholds we used a gun with an indirectly heated cathode and at higher energies a gun with a directly heated fiiament. The resulttrig energy spread of the, electrons then amounts to 0.3 and 2 eV, respectively. The electron beam enters tZle collision chamber through a small diaphragm and is detected at a Faraday cage. Beat-n currents from IO 6 at threshold to 100 PA at higher energies have been used. The pressure of the target gas in the collision chamber (p < low3 torr) is measured with a capacttance manometer (h&S Baratron,‘pressure head 77H-1). The photons, which result from the impact process, are detected at right angles to the electron bea&by two konoctiomators
,. ,”
:
89
.’ _
,;-.
._.‘.,
:
,:
v0hme 29,‘kmber’i:’
: ..
,_
‘. ‘T;yi .:.,:_. 1.. ,, 1. &niss~oh erosi sectidns ior Khmer rad&on .cm2.at: 100 eV electron *pnct energy.,
‘i&j&
‘.
‘.
_,
,.
: U~JH~) ‘.
‘,
’ lL6.8.I .’
,, ..
‘. .‘.
~~emf~~j : ‘_ :
-1.47
.’
: : .
I,
.i&$,~
3.3s .’
:
,:
CHEMI&ALI’tfYSICS LE&& : _, .l’Nq;e&ber 1974 .” .,’ .‘,.. .’ ,..:. ‘, : i ._ :. we grve.in ‘table 1 the.BaIm&emisSion cross sections in &its of.i6!’ ’ at__c& ir$act eqe& and in tabIe 2 the energy depen-‘
0.91
:
a.
-_
:
dence for one Bahner line; namely HPSThe errors in the absolute em&ion cross sections are mainly determined by unckrtainties irl the quantum yield of the optical equ$m&nt; For.the Balmer P, y and. 6 lines the error in’ the::mission cross s&ions iS estimated to be I Cl%.An error of’ 15% arises m. the BaImer Q emission cross sec-
.. .-
;,
&()fij_
.
.,
:’
‘.
the wavelength reglori ~SSC!-9&lO A. The :’ ‘. Cross sections for &lrner emission from benzene procedure to &&rate em&on cross sections and ‘thie,%- .have also been measured by Vroom and de Heer [4]. ... hoId.energjes’is,als.o described in ?ef. .[S] 1 ‘. The ratios of the emission cross &ions for. the different .’ ‘. Balmer lines, as given in table 1, are within 10% in agree,. ,. ._ .’ ‘.., me& -&Li&&i;i-rresuIfs. Rowever, the absolute value, ‘, ‘. 3. Balnrer &&ion ofhydrogen of the em&ion cross secti0ns.i.s lower than measured : .:. .’ ..’ : 7. ‘previously [4] _This difference may be caused by the We four-m&e same energy deperidence of the emisfact hat iri ‘&e latter study a McLeod manometer is ‘. sion bross‘Fcti&s for thk Balmer’cu,~, 7 and Glines as used for the measurement of the pressure. This pro& -‘~a‘functiqn ‘of the electron’~pact energy. Therefore, dure may give too low values for the pressure in the _ ;_ : .’ ,. .dase of condensable vapours and hence too high emis. .. .: . . sion cross se&&s. The Baratron capacitance mano” Table 2 meter, used in the present work, enables a mere Gzcu&&ion c&s s&tic& for Balmer fi radiation from I! and for, ” rate pressure measurement (see ref. [9]). A’A -‘X2n ratition from &fragmcnts in units of 16’gcm2 At high energies the values of uemEel approach a ‘. CH(A*aX%1 H(n=4 - !2) f*,CeV> constznt value {fig. I) as has been found before [4]. This indicates that optically f&bidden e&.itation pro30 ” 4.17 0.23 ce&es dominate in the formation of excited hydrogen. -. po ‘. 5.40 0.63 atoms. At energies above.200 eV the’emission cross &yring
50 ..
80 “,
-;..
100
.:
120 140’
1.43
5.68 .6.35
60
.’
7.01
.,
!70, aa0
~,2q.l - .‘. :3do 350
4QO 45il ~.’ ::;s(Jo :,
,:
.-7(g),
-I
; 800’ -900’ rim0
,’
;
..
6.?4 6.87
3.28
6.09
5,60
3.05 2.68
4.62
Z-24
3.86
1.81
3.28
1.51
‘2.91
1.30
2.61
1.10 ‘_
0.973 0.795
1.41
,.
13.657 0.50’4.-.
1.21 Li7
:
0.492
2.27
._.
px
-_ 600 ’
‘.
i-c3
:
.:
‘.
h&a
2.97 3.35. 3.32
6.8cJ
.‘.
.:
,2.19
.: _, 0.442
4Ra;F
: :
.. ,’
.. .”
.,y; :.
. ‘..;..,”
_,‘. .;
L: ,L’*,. .” y: ,.::
Fig. 1. Eksion trek sections fci Balrrier b radikion presented in the form of 2 Fano.plot: ,_, &Vroom and de He& [4] ; 0, presentwork.. ,.: ,’ :
., _. ,y,‘. ‘.
,.
;: .;
.. r
.,
‘. .’ .. ..;
.. :.:,.
‘. ._ .’ .. : .,
:,:
-_&-e..-rm..... 5 &U-V!-
‘.
. .‘) ‘.:. ‘.: ,‘I,. ;.,_. .:. . ...,’ _(’ ‘..’ : -:, ,: ,:
,... ,, ,.;.,, ‘:_ ; :,,: .’ ‘.
.., ‘.
.I., ,. ,’
Volume 29, number 1
3,
CHEMICAL PHYSICS LETTERS
1 November i974
of methane, ethylene and acetylene [9,14,15] the benzene cre,Eel values show a maximum at 250 eV , which is not present in the case of aliphatic hydrocarbons (fig. 1); the emission cross sections in the asymptotic region are 50% lower than those for the ahphatic hydrocarbons. Sufficiently accurate experimental and theoretical data on orbital energies [I6] are not available to decide whether these differences can be justified in view of the ideas presented before [9, 15]_
farbilrarytiils)
2-
E.I
(ev)
Fig. 2. Energy dependence below 100 eV of the emission cross sections for Balmer p radiation.
sections have the same energy dependence within 3% as measured by Vroom and de Heer [4]. At lower energies their values of uem become relatively higher than ours, as is also found in the case of Balmer emission from water [9, IO] . The threshold behaviour of the H, emission cross sections is shown in fig. 2. From the absence of structure it is concluded that one or only a few optically forbidden states of the benzene molecule are involved in the formation of excited hydrogen atoms. The onset for Balmer fl emission is found at 19.3 f i eV. From this, the process which is involved in the formation of Balmer radiation may be derived. Taking for the dissociation energy of the C-H bond a value of 4.6 eV [I I] and for the excitation energy of H(n = 4) a value of 12.69 eV [ 121, it follows from the energy balance and the absence of structure in the threshold behaviour that the process CgHs(19.3 ? 1 ev) + C6Hs(%2Al)
t H(n = 4)
ground state and since cascade to the A’A state from higher.states is not possible (because either radiation from these states to the A2A state is optically forbidden or these states are repulsive [I 7]), the emission cross sections are equal to the total cross sections for production of CH(A*A). The emission cross sections are collected in table 2. Besides errors of 1C% due to uncertainties in quantum yield, pressure and electron beam intensity additional errors of 30% arise in the CH cross section data due to a continuous background under the 43 14 8, system and the use of the ‘adding up’ procedure [ 141 The results are also presented in a Fano plot (fig. 3). The zero slope of the straight Iine portion indicates that - contrary to the ahphatic hydro~carbons 19, 14, I.51 - the formation of CH(A’A) mainly proceeds by optically forbidden processes in the benzene molecule.
1
+ 2.0 +- I eV excess energy plays an important role in the formation of excited hydrogen atoms. Another possibility is that the phenyl radical is formed in the x2B state, which has an cxcitation energy of 2.35 eV [13\. However, we cannot obtain evidence for this process, because we did not find emission from ,the phenyl radical. Contrary to the Balmer emission cross section data
J
I.
Fig. 3. Emission cross sections For CH(A”A - X’fl) radiation presented in the form of a Fano plot.
!, . ,: ~.,‘
,.,,-,..’ I., ..-: ,.,.
,: .,.
B?ile$ ana RX. Schumm, Selectkd Values of Chemical : .‘ .‘ 1 ‘flzermndynamic Properties, NBS Tech. Note 2,‘iO-3 (U.S. b;ovt.-Printin@ Office; ~a~~~ton, 1968). ;. ,-[!2) ‘C-E: ?dioqri, &niultiplet t&&of astrophysicai inte+t ,’ ..,I : .‘ (Observatory, P&c+on, 1945): ” :li] G_ H&berg -El&t~onic spectia’of polynto& mole&es :,, 1’ ,I ._,,. ‘. ‘.. .:’ ., .fvan lJbotr;xd, Pri-;ice;ton 1366),
._. _ ...I. ,-.,Ac&noir;&e,erjlent ,‘ ’ ,. ..:..,‘ ._ ,. _ :, ., ‘ : .’‘ :‘. .;. .: .~ ,,‘ ._ .-..’ We:are.iildebre.dto~Prbfessors J. Kistema&
.
: { 141 J.F.M. +arfs,‘C.I.hf.
F.$ee&ker and F. j. I% Heer, Physica. ‘_ ;
&d ‘f.,i:.. ,.’ .53 (1971)32. ,‘.’ ,: “L ,, .. “-. (70SteIhdff for continuous interest. This w&k is pGt:of. -,:.: [IS] d.l& Beenakk& an@ F.J.‘de’Heer, (N& 1 mj, i0 b&published. .’ -‘, ‘.&~~r&arch’p+qpun of the Stichtinivoor Funda.rne~$ ,. : I:. ,$ge?+i~pk :&xzo&%-I
Matter} .md the’S,tichting Sc$ik&un~g OnNederland (~e~er~~~ Fq.u-&ti& for
:: Ch&rCcaj R&ea‘rch),and‘was tiade:po,ss.ibi$by financial’.. .“‘ .z; _, : ; _‘ _ :..’ .. ..’ _, :. .:: : .:, :_’ ‘ ._ ,.-_. .‘ ,,, ,: ,,‘.. ..:,. I.. .( .’: ,. . . ,,.’ ,,.-‘., ., _, .‘ .‘,_: ‘ .’ ,‘, : _.,‘ :‘: .,:. .: : .‘ ..:_ :’ ..‘ -. ,., _‘ : ,’:‘.‘ _.‘ .:‘ :_ : _. ._ ._-;‘ ., :
: .;_,’
hioifk~~larph&delectron
.. New Yo;k 1970j’. :
?I+
:
5p+ctroscopy
_
Phys. 6.
:.
(Wiley-lntcrs$ence,
,..
[ 171 G:Herzb&, S&?&of diatom& &IO&C&~ gan N&and, ’ ‘_ :’ .‘ . . Princecon, mioj::. .. ,‘ ;. ; ,..,. : .‘ ,_ ., ,,. ‘ -, ,’ ,,.: . . .‘, ; ., ._ .-. ..; ; ; : : -_ .‘ ,‘ .‘ .,’
:.
:
.
CHEMICAL PHYSICS LETT’ERS
DISSOCIATIVE
EXCITATION
OF BENZENE’BY
C.I.M. BEENmRz* FOM-3nstitute
for Atomic
I November 1974
ELECTRON
IMPACT
and F.J. DE HEER
and Molecular
Physics,
Amsterdam,
The Netherlands
Received 8 July 1974
In the wavelcngti~ Iegion 1850-9000 sociative excitation been determined
of bcnzenc
by electron
A radiation from H, C and CH fragments is observed as impact
(O-1000
for the Balmer series of the hydrogen
Impact of electrons’on molecules may result in the formation of excited states which can be both optically allowed or optically forbidden (by symmetry or by spin multiplicity) with respect to the ground state. In the present paper we deal with excited states of benzene which lead to dissociation into excited fragments that are detected by their emission spectrum: We have determined the threshold energies and emission cross sections for Balmer emission of H and A2A X211 emission of CH fragments. The ener,? of the incident electrons is varied between 0 and 1000 eV. The emission c:oss sections of the fragment radiation are proportional to the excitation cross section of the me, lecular states of benzene dissociating into H(n > 3) and CH(A2A). Therefore the dissociative states can be characterized by analysing the emission cross’sections (aim). Both the threshold energy, the threshold behaviour and the energy dependence of the emission cross sections at high energies give information about the intermediate excited states of the benzene molecule. At high impact energies the data are analyzed by means of a Fano plot [I] , which is a plot of a,,E,,/4na~R +rsus In Ed, where Ed is the energy of the incident electrons, a,j is the Bohr radius and R is the Rydberg energy. According to the theory of Bethe [2] such a plot gives a straight line with Presetit
Emission
atom and the A*A
1. Introduction
*.
cV).
,-
address: Philips Research Laboratories, Eindhoven. The Ne’&erlanck *. Depakment of Theo&d Org+iC Chemistry,, University of &eyden, The Netherlands.
2 remlt
cross sections aad threshold - X2rl
emission
of.‘hc diseneigics~kave
of the CH Gqment.
a positive slope when the excited state of benzene is optically allowed with respect to the ground state and a horizontal line when the excited state is symmetry forbidden (see for instance refs. [3-51). Spin forbidden excitation processes reveal their presence by a decreasing behaviour of uemEel [6]. In addition to the radiation froin H artd,CH we could
also detect a weak emission from the carbon fragment (3s IPO - 2p2 l D, 1931 18Land 3s lPa - 2p2 ‘S, 2479 A. The results on the molecular ‘BZu - ‘Alg fhorescence have been presented before [7,8].
2. Experimental The apparatus has been described before [9]. It consists of a vacuum chamber and oFa collision chamber. An electron gun placed in the vacuum chamber produces a quasi mono-energetic electron bean-r. For the measurements. of the thresholds we used a gun with an indirectly heated cathode and at higher energies a gun with a directly heated fiiament. The resulttrig energy spread of the, electrons then amounts to 0.3 and 2 eV, respectively. The electron beam enters tZle collision chamber through a small diaphragm and is detected at a Faraday cage. Beat-n currents from IO 6 at threshold to 100 PA at higher energies have been used. The pressure of the target gas in the collision chamber (p < low3 torr) is measured with a capacttance manometer (h&S Baratron,‘pressure head 77H-1). The photons, which result from the impact process, are detected at right angles to the electron bea&by two konoctiomators
,. ,”
:
89
.’ _
,;-.
._.‘.,
:
,:
v0hme 29,‘kmber’i:’
: ..
,_
‘. ‘T;yi .:.,:_. 1.. ,, 1. &niss~oh erosi sectidns ior Khmer rad&on .cm2.at: 100 eV electron *pnct energy.,
‘i&j&
‘.
‘.
_,
,.
: U~JH~) ‘.
‘,
’ lL6.8.I .’
,, ..
‘. .‘.
~~emf~~j : ‘_ :
-1.47
.’
: : .
I,
.i&$,~
3.3s .’
:
,:
CHEMI&ALI’tfYSICS LE&& : _, .l’Nq;e&ber 1974 .” .,’ .‘,.. .’ ,..:. ‘, : i ._ :. we grve.in ‘table 1 the.BaIm&emisSion cross sections in &its of.i6!’ ’ at__c& ir$act eqe& and in tabIe 2 the energy depen-‘
0.91
:
a.
-_
:
dence for one Bahner line; namely HPSThe errors in the absolute em&ion cross sections are mainly determined by unckrtainties irl the quantum yield of the optical equ$m&nt; For.the Balmer P, y and. 6 lines the error in’ the::mission cross s&ions iS estimated to be I Cl%.An error of’ 15% arises m. the BaImer Q emission cross sec-
.. .-
;,
&()fij_
.
.,
:’
‘.
the wavelength reglori ~SSC!-9&lO A. The :’ ‘. Cross sections for &lrner emission from benzene procedure to &&rate em&on cross sections and ‘thie,%- .have also been measured by Vroom and de Heer [4]. ... hoId.energjes’is,als.o described in ?ef. .[S] 1 ‘. The ratios of the emission cross &ions for. the different .’ ‘. Balmer lines, as given in table 1, are within 10% in agree,. ,. ._ .’ ‘.., me& -&Li&&i;i-rresuIfs. Rowever, the absolute value, ‘, ‘. 3. Balnrer &&ion ofhydrogen of the em&ion cross secti0ns.i.s lower than measured : .:. .’ ..’ : 7. ‘previously [4] _This difference may be caused by the We four-m&e same energy deperidence of the emisfact hat iri ‘&e latter study a McLeod manometer is ‘. sion bross‘Fcti&s for thk Balmer’cu,~, 7 and Glines as used for the measurement of the pressure. This pro& -‘~a‘functiqn ‘of the electron’~pact energy. Therefore, dure may give too low values for the pressure in the _ ;_ : .’ ,. .dase of condensable vapours and hence too high emis. .. .: . . sion cross se&&s. The Baratron capacitance mano” Table 2 meter, used in the present work, enables a mere Gzcu&&ion c&s s&tic& for Balmer fi radiation from I! and for, ” rate pressure measurement (see ref. [9]). A’A -‘X2n ratition from &fragmcnts in units of 16’gcm2 At high energies the values of uemEel approach a ‘. CH(A*aX%1 H(n=4 - !2) f*,CeV> constznt value {fig. I) as has been found before [4]. This indicates that optically f&bidden e&.itation pro30 ” 4.17 0.23 ce&es dominate in the formation of excited hydrogen. -. po ‘. 5.40 0.63 atoms. At energies above.200 eV the’emission cross &yring
50 ..
80 “,
-;..
100
.:
120 140’
1.43
5.68 .6.35
60
.’
7.01
.,
!70, aa0
~,2q.l - .‘. :3do 350
4QO 45il ~.’ ::;s(Jo :,
,:
.-7(g),
-I
; 800’ -900’ rim0
,’
;
..
6.?4 6.87
3.28
6.09
5,60
3.05 2.68
4.62
Z-24
3.86
1.81
3.28
1.51
‘2.91
1.30
2.61
1.10 ‘_
0.973 0.795
1.41
,.
13.657 0.50’4.-.
1.21 Li7
:
0.492
2.27
._.
px
-_ 600 ’
‘.
i-c3
:
.:
‘.
h&a
2.97 3.35. 3.32
6.8cJ
.‘.
.:
,2.19
.: _, 0.442
4Ra;F
: :
.. ,’
.. .”
.,y; :.
. ‘..;..,”
_,‘. .;
L: ,L’*,. .” y: ,.::
Fig. 1. Eksion trek sections fci Balrrier b radikion presented in the form of 2 Fano.plot: ,_, &Vroom and de He& [4] ; 0, presentwork.. ,.: ,’ :
., _. ,y,‘. ‘.
,.
;: .;
.. r
.,
‘. .’ .. ..;
.. :.:,.
‘. ._ .’ .. : .,
:,:
-_&-e..-rm..... 5 &U-V!-
‘.
. .‘) ‘.:. ‘.: ,‘I,. ;.,_. .:. . ...,’ _(’ ‘..’ : -:, ,: ,:
,... ,, ,.;.,, ‘:_ ; :,,: .’ ‘.
.., ‘.
.I., ,. ,’
Volume 29, number 1
3,
CHEMICAL PHYSICS LETTERS
1 November i974
of methane, ethylene and acetylene [9,14,15] the benzene cre,Eel values show a maximum at 250 eV , which is not present in the case of aliphatic hydrocarbons (fig. 1); the emission cross sections in the asymptotic region are 50% lower than those for the ahphatic hydrocarbons. Sufficiently accurate experimental and theoretical data on orbital energies [I6] are not available to decide whether these differences can be justified in view of the ideas presented before [9, 15]_
farbilrarytiils)
2-
E.I
(ev)
Fig. 2. Energy dependence below 100 eV of the emission cross sections for Balmer p radiation.
sections have the same energy dependence within 3% as measured by Vroom and de Heer [4]. At lower energies their values of uem become relatively higher than ours, as is also found in the case of Balmer emission from water [9, IO] . The threshold behaviour of the H, emission cross sections is shown in fig. 2. From the absence of structure it is concluded that one or only a few optically forbidden states of the benzene molecule are involved in the formation of excited hydrogen atoms. The onset for Balmer fl emission is found at 19.3 f i eV. From this, the process which is involved in the formation of Balmer radiation may be derived. Taking for the dissociation energy of the C-H bond a value of 4.6 eV [I I] and for the excitation energy of H(n = 4) a value of 12.69 eV [ 121, it follows from the energy balance and the absence of structure in the threshold behaviour that the process CgHs(19.3 ? 1 ev) + C6Hs(%2Al)
t H(n = 4)
ground state and since cascade to the A’A state from higher.states is not possible (because either radiation from these states to the A2A state is optically forbidden or these states are repulsive [I 7]), the emission cross sections are equal to the total cross sections for production of CH(A*A). The emission cross sections are collected in table 2. Besides errors of 1C% due to uncertainties in quantum yield, pressure and electron beam intensity additional errors of 30% arise in the CH cross section data due to a continuous background under the 43 14 8, system and the use of the ‘adding up’ procedure [ 141 The results are also presented in a Fano plot (fig. 3). The zero slope of the straight Iine portion indicates that - contrary to the ahphatic hydro~carbons 19, 14, I.51 - the formation of CH(A’A) mainly proceeds by optically forbidden processes in the benzene molecule.
1
+ 2.0 +- I eV excess energy plays an important role in the formation of excited hydrogen atoms. Another possibility is that the phenyl radical is formed in the x2B state, which has an cxcitation energy of 2.35 eV [13\. However, we cannot obtain evidence for this process, because we did not find emission from ,the phenyl radical. Contrary to the Balmer emission cross section data
J
I.
Fig. 3. Emission cross sections For CH(A”A - X’fl) radiation presented in the form of a Fano plot.
!, . ,: ~.,‘
,.,,-,..’ I., ..-: ,.,.
,: .,.
B?ile$ ana RX. Schumm, Selectkd Values of Chemical : .‘ .‘ 1 ‘flzermndynamic Properties, NBS Tech. Note 2,‘iO-3 (U.S. b;ovt.-Printin@ Office; ~a~~~ton, 1968). ;. ,-[!2) ‘C-E: ?dioqri, &niultiplet t&&of astrophysicai inte+t ,’ ..,I : .‘ (Observatory, P&c+on, 1945): ” :li] G_ H&berg -El&t~onic spectia’of polynto& mole&es :,, 1’ ,I ._,,. ‘. ‘.. .:’ ., .fvan lJbotr;xd, Pri-;ice;ton 1366),
._. _ ...I. ,-.,Ac&noir;&e,erjlent ,‘ ’ ,. ..:..,‘ ._ ,. _ :, ., ‘ : .’‘ :‘. .;. .: .~ ,,‘ ._ .-..’ We:are.iildebre.dto~Prbfessors J. Kistema&
.
: { 141 J.F.M. +arfs,‘C.I.hf.
F.$ee&ker and F. j. I% Heer, Physica. ‘_ ;
&d ‘f.,i:.. ,.’ .53 (1971)32. ,‘.’ ,: “L ,, .. “-. (70SteIhdff for continuous interest. This w&k is pGt:of. -,:.: [IS] d.l& Beenakk& an@ F.J.‘de’Heer, (N& 1 mj, i0 b&published. .’ -‘, ‘.&~~r&arch’p+qpun of the Stichtinivoor Funda.rne~$ ,. : I:. ,$ge?+i~pk :&xzo&%-I
Matter} .md the’S,tichting Sc$ik&un~g OnNederland (~e~er~~~ Fq.u-&ti& for
:: Ch&rCcaj R&ea‘rch),and‘was tiade:po,ss.ibi$by financial’.. .“‘ .z; _, : ; _‘ _ :..’ .. ..’ _, :. .:: : .:, :_’ ‘ ._ ,.-_. .‘ ,,, ,: ,,‘.. ..:,. I.. .( .’: ,. . . ,,.’ ,,.-‘., ., _, .‘ .‘,_: ‘ .’ ,‘, : _.,‘ :‘: .,:. .: : .‘ ..:_ :’ ..‘ -. ,., _‘ : ,’:‘.‘ _.‘ .:‘ :_ : _. ._ ._-;‘ ., :
: .;_,’
hioifk~~larph&delectron
.. New Yo;k 1970j’. :
?I+
:
5p+ctroscopy
_
Phys. 6.
:.
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