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Negative ion formation in CF2Cl2, CF3Cl and CFCl3 following low energy (0—10 eV) impact with near monoenergetic electrons
Chemical Physics 37 (1979) 21-31 0 North-Holland Publishing Company
AND CFCI,FOLLOWING LOW-ENERGY(0-10eV)lMPACTWITHNEARMONOENERGETICELECTRONS
NEGATIVE ION FORMATION IN CFZC12, CF,Cl
E.
ILLENBERGER,H.-USCHEUNEMANN andH.BAUMG;iRTEL
Instinctfiir PhysikalischeClzemieund Quantenchemieder Freien L’niversitiit Berlin, 1000 Berlin33, WestGermany Received 18 August 1978
Negative ion formation trochoidal monochromator
in CF,Clz, CF,CL and CFCIB under low-energy electron impact has been investigated using a as electron gun (Ae = 0.12 eV, I = 8 X 10S8 A) and a quadrupole mass ftiter for ion detection. Theions observed are F-, Cl-, FCI-, Cl,, CFCl; from CF,Cl,; F-, Cl-, FCl-, CFzCl- from CF,Cl and F-, Cl-, Cl;, Ccl; from CFCIs. Quoting available thermochemical data, it can be shown that most of the observed negative ions arise from dissociative attachment processes. Appearance potentials and for some radicals electron affinities (within the limit of the unknown cxccs: energy) are given. The extremely high yield of Cl- in CFC13, which is observed at E = 0.0 eV, will be discussed with regard to the lifetime of this molecule in the troposphere.
1. Introduction
2. Experimental
Recently the interest in the atmospheric chemistry of the halogenated methanes has grown rapidly since in 1975 Rowland and Molina [I] states that these molecules might destroy the atmospheric ozone layer via a Cl-0 reaction. In the meantime there has been done a wealth of laboratory studies with these molecules such as kinetics of the chemical reactions [2] and photoreactions, i.e. photoelectron, photoion and absorption cross section measurements [3-51. On the other hand, however, relatively little is known about the energy selective interaction of electrons with these molecules in the low-en&y region. In a mass spectrometric investigation with a conventional electron gun (AE = 1 eV), Hickam et al. [6] could detect Cl- ions from CFCl, and CF.-&l, and Buchel’nikova [7] measured negative ion current (but without mass analysis) following electron impact on CF#$. Curran [8] detected Cl-, F- and CC15 ions from CFCl,, employing the RPD-method as an electron gun (Ae = 0.3 eV). In this paper, we report on the energy selective (AC = 0.12 eV) formation of negative ions from CF,Cl,, CF,Cl and CFCl,.
The apparatus used for the present experiment consists of an electron monochromator which produces a beam of nearly monoenergetic electrons, a reaction chamber, a quadrupole mass analyzer, an electron collection system and a He(I)-photon source. The electron monochromator is constructed of molybdenum and bakeable up to 300°C in order to avoid surface’ problems. Nonmagnetic stainless steel is used for the whole vacuum system which is pumped by a turbomolecular pump. The experimental setup is shown schematically in fig. 1. 2.1. Electron monochromator In order to produce an electron beam with sufficient intensity especially in the low-energy region, a trochoidal monochromator was constructed. This instrument was described by Stamatovic and Schulz [9] who used it in the framework of electron transmission spectroscopy [IO] _Electrons, emitted froin a thoria coated tungsten filament are aligned in the z-direction by a homogeneous magnetic field (B =
22
E. Illenberger et al/h’egarive ion /ornm&on in CF&z,
ION
CF$3arrd CFCt3
to Multiplier
Quadrupale Mass Analyzer
TO Differential Pumping Unit (100 tk)
filament
Electron
Monochromator
-_$(70 Gauss)-
Electron Cdlection
PhotonSource
Tarset GQj
Fig. 1. Schematic drawing of the main parts of the appxntus used. 70 G), generated by a pair of Heimholtz coils outside
the vacuum system. The three electrodes following the filament have holes drilled off-center by 3 mm. Thus, electrons enter the deflection region, w!Cch is defined by the crossedmagnetic (z-direction) and electric field @-direction) off-center. In the crossed field region, the electrons describe a trochoidal motion [l I] and their guiding center moves with a constant velocity u = (E XB)/B2 in the x-direction along a plane of constant electric potential, i.e. dispersion of the electrons according to their z-velocity occurs and only electrons which have reached the axis are transmitted through the (axial) hole of the electrode S, . The energy selected electrons are now accelerated by the electrodes S, and S, into the reaction chamber, where they make collisions with the target molecules. The full width Aef at the base of the energy distribution at the whole of electrode St is given by the expression Aef = 24Q1 + c&)/d + e-&q,
(1)
where + = 1 mm and r$2 = 1 mm are the hoIes of the electrodes B, and St. d = 3 mm is the displacement from the axis and E$, the potential drop across the electron beam in the crossed field region. To obtain good energy resolution, one needs to operate the monochromator at low energies. This is achieved by retarding the electrons by the electrode B,. This retarding field also rejects most ofthose electrons whose
velocity vector is not oriented axially i.e. no further ener,v spread occurs from those electrons. 2.2. ReaCtim chamber aild ioll detectiorz Ions, formed in the reaction volume which is defined by the crossing of the electron beam with the target gas beam are extracted by a small electric field (+I.7 V/cm) and focussed onto the entrance hole of a quadrupole mass spectrometer. The extraction voltage is applied in such a way, that the potential on the axis of the electron beam is equal to the potential of the entrance (S3) and exit electrode (D1), i.e. a uniform potential exists along the path of the electrons. After mass analysis the ions are detected by a multiplier. Pulses from the multiplier are counted and stored in a multichannel analyzer. A complete synchronization between channel number and electron energy is achieved by converting the digital channel number to an analogous voltage signal which is then amplified by an operational power supply to the appropriate voltage. The trajectories of the ions through the lens system and the quadrupole are not essentially influenced by the relatively small magnetic field. 2.3. Electrot collection and He(I) photon source The transmitted electrons are accelerated to the
E. Illenberger et aI./Negatise ion formation in CF#2,
CF$l
and CFCI,
23
detecting system and the quadrupole properties, measuring photo-ions of known systems. 2.4. Energy resolution and energv scale calibration
-
Electron
Energy
(eV)
-
Fig. 2. (a) Negative ion yield for the production of SF: from SFs. The width of SF;* is an indication of the electron energy spread. (b) Current of the transmitted electrons on the electron collector C.
electron collector which is connected with an electrometer amplifier. The pressure in the reaction chamber is kept low G10m4 mb) thus, the portion of electrons lost by the process is negligible compared to the incoming beam intensity. Thus, the electron collector acts as a beam monitor. For the lowest energies, however, a small deflection of the electron beam by the ion extraction voltage cannot be neglected. Furthermore, the small pressure in the collision chamber helps to avoid possible ion-moIecule reactions. The differential pumped and water cooled He (584 A) lamp serves as a testing unit for the ion
The electron energy distribution is obtained by measuring the we11known electron attachment process on SF, [ 12,131. Fig. 2 shows the electron energy distribution obtained by this method and the current of the transmitted electrons on the electron collector C. For the present experiment, the operation of the monochromator provides a reasonable compromise between electron energy spread (Ae = 0.1-0.14 eV) and electron intensity (1% (5-10) X 10T8 A). The energy scale is calibrated measuring the SF,/ SFi* and SF6/SFT resonance following 2 procedure suggested by Johnson et al. [13]. This was achieved by adding a small amount of SF, to the gas and for every negative ion studied, both the SF;* and SF; resonance is measured under the same experimental conditions. Considering that our energy calibration is done at low energies (Sq peaks at 0.03 eV and SF; at 0.37 eV), we estimate the accuracy of our energy scale ~~0.2eV in the upper energy region (24 ev). The electron energy scale obtained in this way corresponds to the most probable electron enera.
3. Results In the electron energy range O-10 eV, we could detect the negative fragments listed in table 1. The numbers in the tabIe represent the energy integrated intensities relative to Cl-/CFCl,. One should notice, however, that these numbers are only 2 very rough indication of the intensity of the ions, considering that .
Table 1 Negative ion formation from CFCls, CFzCl2 and CFaCI. The numbers indicate the relative energy integrated intensities of the negative ion fragments (see text) hlolecule
CFCl, CF,Cla CFsCI
Negative ion fragment F-
cl-
FCi
5 8 0.5
1000 58 2.5
1.3 0.5
cl;
CFaCl-
CFCl;
ccl; 0.4
1.2 1.2
2.5 0.2
E. IIlenberger et al/Negative
24
ion formation in CFzClz,
CF3Cl and CFC13
I
1
3
2 -
Electron
Energy leV)
-
-
Fig. 3. Negative ion yield for the production
of Cl- from
CF2CI2.
our ion counting rate is not corrected on the mass dependent transmission of the quadrupole, and that the extraction optics system discriminates on the (unknown) translational excess energy of the negative fragments. No negative parent molecule could be observed though collision experiments indicate that CFCI; should be a stable ion 1141. Quoting available themro-
Fig. 5. Formation
Electron
4
5
Energy (eV) -
of FCI- from CF2C12 by electron
impact.
chemical data, most of the negative ions can be attributed to dissociative attachment processes. CIfrom CFCl, is by far the most abundant, and CF,CllCF,Cl(which is 0.02% of CFCIJCI-) lies somewhat above our detection limit. For the determination of the appearance potential M of the negative ions one should notice that besides the problem of energy calibration - further uncertainties arise. Since we did not use a deconvolution technique and the functional form of the cross section at the threshold is unknown, the accurate determination of the onset is somewhat criticaL
..: ..
>
I
0
e- l
c EpJcl~
I
3
1
c-+CF,C$/(CFClJ -_z 0
1
e-*C~C12/F-
! :. ‘r 0 -
Electron
2 Energy
5’
‘:k. ‘.-.. ‘T’-._. 4
5
‘.-.. :.. ‘: ‘k.,
f .,,
/ 1
:’ ,: ;y2
:
1 r Q“*Y
4’
...-.... .--.
3 _.-. :>’ L.<
2
q’ fi
3.1. CF2C12
;
‘X_ 3 (eV)
Fig. 4. Negative ion yield for the production CFCL; and (c) F- from CFsCls.
4
For this molecule, five negative fragment ions are observed. Their yields as a function of electron energy are shown in figs. 3-5. Cross section data are available for CF,Ci,/Cl- (table 2) derived from the swarmbeam techn&ue [ 151 using electron swarm data [ 16j and the earlier beam experiment of Buchel’nikova [7] (who determined the negative ion current’without mass analysis) and from swarm data, using the swarm unfolding technique [18]. In the systems CF,CI,/CIand CF~Ci.-JC1~ two appearance potentials are obtained, the second AP is overlapped by the first resonance. The appearence potentials are listed in table 2. 3.2. W&7
5
-
of (a) Cl;, (b)
As may be seen from table 1, the ion yield from this molecule i relatively small. For this compound no exact cross section data are available. The energy
E. Illerlbergeret al.jNegativeion formation in CFzCr,, CF3Cland CFQ3
25
Table 2 Negative ion formation from CFzC12 ___._-.____._Ion
AP (eW _-__-__-_--.
_____.__.._
Peak position (eV1
F-
1.8 f 0.1
3.1 + 0.2
Cl-
0.15 f 0.1 2 2.2 k 0.3
0.55 ? 0.1
0.2
0.65 r 0.1
a; CFCI;
2.8 + 0.1 ______--____-__-
-..--
Relative energy integrated intensity ____-____.I__--
Peak cross section (cm’)
13.8 1.1 x IO-l6 [lS,lSl
I
1
3.55 + 0.2 ---
dependence of the four negative ion fragments is shown in figs. G and 7 and the appearance potentials and peak positions are listed in table 3.
2.1 4.3
-
The finite slope at the onset of the Cl- curve in fig. 8 seems to be purely instrumental. Appearance potentials and peak positions are listed in table 4.
3.3. CFC!, 4. Discussion For this compound, four negative ions could be observed: F- , Cl-, Cl, and Ccl, _Their yield as a function of electron energy is shown in figs. 8 and 9. Compared with the other compounds studied, the Cl- ion is by far the most abundant. This clearly shows that the cross section strongly depends on molecular structure. The peak cross section given in table 4 is obtained by the swarm-beam technique [IS].
4.1. Theoretical corrsiderations A description of dissociative electron attachment to diatomic molecules (AB f e- -+ A + B-) has been developed by O’Mtiey [17], treating the process as a resonance phenomenon. Using the Bom-Oppenheimer separation, the process is understood as an
e-e CF,Cl/F-
I
I
o
t
-
2 Electron
3
4 5 6 Energy (CL’-)
7
Fig. 6. Negative ion yield for the production of (a) Cl- and @) CF2CI- from CF3Cl.
-
Electron
Energy (eV) -
Fig. 7. Negative ion yield for the production of (a) FCl- and (b) F- from CF3Cl.
E. Illenberger et aI.JNegntiw ion fonnation in CF, Clz, CFsCl and CFCls
26
Table 3 NcK3tiveion formation from CF$Zl Reh tive energy integrated intensity
Peak
Ion
position W) __........_ .__-______ 3.0 t 0.2 4.1 ?: 0.2
__ P-
2
Cl-
0.7 c 0.3 3.4 + 0.3
1.3 + 0.2 4.8 r- 0.2
I 10
1X1-
3.0 r 0.3
3.9 + 0.2
2
CF2CI
3.5 +_0.3
4.2 f 0.2
0.8
_---.---.-.-_.
+
potential curve of the molecule and the electron at rest, separated at infinite distance, i.e. the potential
curve for the neutral molecule, V(R)- denotes a negative ion potential curve i.e. a potential curve with the captured electron. For polyatomic systems A and/or B- may be an atom or n radical. According to the Fran&-Condon principle, transitions within the restricted energy range (E, f e < ~2) from the initial to the final state can occur. The compound negative ion AB- may either dissociate into neutral and negative ion fragments or decay via auto-
-
k _.
-
Electran Energy (eV) -
Fig. 9. Negative ion yield for the production of (a) F- and (b) Ccl; from CFQ.. (autodetachment). Thus, the cross section for dissociative attachment, udo, is a product of the cross section for the formation of AB-, IJO,and the probability P that AB- will dissociate, once formed, rather than decay by autoionization. ionization
Ud3
=
ooP.
(2)
For a diatomic molecule AB, initially in the u = 0 level of the ground eIectronic state, O’hlalley [ 171 derived a simple explicit formula for the dissociative attachment cross section as a function of electron energy
e- + CFCI, 1 Cl-
I
‘a
3
____. ___-. .-__.
electronic transition from a continuum state (AB + c-) to a discrete state (AB-, degenerate with the continuumj which then dissociates. The situation is schematically illustrated in fig. 10. V(R) denotes the
‘i
2
ii.
:_ ‘, x la “i,__
!
z
.___ _ o
I
2
3
4
5
G
s z
-_
2 ”
.CFC$ / Cl;
e-
:
s 1:
:‘.._ -__--+.._., ...L_
i-
Fig. 8. Fwxation trun imp&.
0
1
-
Electron Energy (eV) -
2
3
4
5
6
of(a) Cl-and (b) Cl, from CFC13 by elec-
where ki is ;he incident electron’suwave number (kf E the electron energy,g a statistical factor, Z. = en + $fiw (e. is the electron energy at R = R,, $ho is the zero point energy), rd the width for dissociation (rd is a function of the slope of V(R)and 2fi/rd is a measure of the time it takes to the final products to move a distance from the turning point to R, where autoionization becomes impossible), r, the partial and ra the total autoionization width. If there is onIy one electronic channel open, r,=r,. T!te survival probability P is given by the expression = (Zm/fi)~),
5. illenbergef et at/NeEa?ive ion formation in CF,C&. CF3Cl and CFi&
27
Table 4
from ClX& .-Negative ion fnrnlation l_..--..-_.--_.---Ion
AP (eV) -p*.---_____ this work
Peak position foV)
ciCl; CCI;
Relative cncrgy integrarcd inmnsity
Curran [S]
_I
F-
,--
I. ---
--.--_-_-_
1.6 k 0.2
1.8 * 0.1
3.0 f 0.2
0.0 c 0.05
0.0 f 0.05
0.0 f 0.0s 1.6 f 0.1
0.6 t 0.2 52.5 t 0.3 2.7 + 0.2
2.75
2
v(R) = 2fL-’ [{.E-, - V@)-)]
1’2,
1.2
1
0.4
is the imaginary part of the complex potential energy of the negative ion state V(R)- - if’& It describes {similar to the contpiex index of refraction in optics) the “leakage” (~utoioni~tion) on F/I/?)-. The autoionization width ra(R) is connected to the auloioni~~tion rate (per unit time) Jlt(R) by the expression W(R) = ra(R)/tt. Christophorou and Stockdate 1151 pointed out that for purely repulsive negative ion states in the Fr~nck-Condon region with il resonance enera beiow the Iowest excited electronic state of the neutral moIecule, the survival probability never differs much from unity. The dissociative ~tt~cllment cross sec-
par-
(5)
-and f the time it takes them to dissociate
.-_ 9.5 x to-‘5 [IS].
1000
(41 v(R) is the cIassicaI velocity of the dissociating ticles
-_.-
5
3.3 f 0.2
0.1
Pcnk cross section (cm”)
to the cross-
ing point R,. in eq. (~),E’K is the asymptotic translational energy of the dissociating particles. t’,(R)/2
A8
..._.
-.-.-.-._._. _ _ A+B+e-
-.O
.I
L
NCQ, 1087Intertsity
-
Internuclear
Separation.
R -
Fig. 10. Potential energy cwves for the dissoci+ve attachment of electrons (with energy G) to motecule AB. The ground state shown is for the neutraI molecule AB plus the electron at rest at infinity.
E. Illenberger et al/Negative ion formation in CF2Ci2. CFJCI and CFCIJ
28 Table 5 Summary of the Iiteraturc wloulations
_-.-
AHy= (CV) --
Ref.
cry& cI:$I CIXTI~ cl-y3
-5.11 -7.34 -2.95 -2.79
t 0.08 h 0.03 + 0.07 +_0.1
Pll WI PII [ZZJ
CKl,
-1.0
0.82 + 0.04
[27l 1511
-4.86 2 0.16 2.08 -2.01 c 0.1 -0.54
CF3
CIXI CFZ I-Cl
-.-
the thermodynamical
Compound ----
CCIS
_____
where rvIf means the heat of formation. data used Tar
4.2. Discussim of the results 4.21. Negariw ion formation from CF2Ciz e- + CF,Ci#‘(jig. 3) For this;on, two appearance potentials (AP, = 0.15 + 0.1 eV, APz 5 2.2 t 0.3 eV) are observed. It is clear that the first resonance arises from the process
[301 v31 I251
I: CI
0.82 1.26
Compound ._____
EA (eV) __._ ___--.-..-_
Cl Cl? F
3.6 1 2.52 i 0.17 3.45
1281 I231 1231 Ref.
e- f CF,Cl, - _ + CF,CI f Cf-. With AfY,(CF,C12) = -5.11
1241 [I61
1241
tion would then be controlled dominantly by 00. When, however, the negative ion state lies at or above known electronically excited states of the neutral molecule, the negative ion state can now decay to an electronically excited neutral molecule in addition to ZI rotationally and/or vibrationaliy excited neutral moIecule. In such cases, the effect ofautoiorhtion is expected ro be large which could lead to wry large isotopic effects [19,20] _ Evaluation of tfle cnergetics of negative ion formation can provide much thermochemical information. From fig. IO. one obtains the expression
E,~AP(B-)=D(A-B)-EA(B)+Ec,
(6)
whew EA is the electron affinity, D(A-B) the dissociation energy and Ee the excess energy, which is the sum of the translational (EK), electronic, vibronic and rotational energy. Using the thermodynamical expression D(A-0,
= M&i\)
f Mt.(B) - M$.(AB)
(7)
and L/II+ U-) = M,.(B) - W(B),
(8)
one obtains q = AP(R-) = &Y&A) + M&B-)
In diatomic molecules,.the absence of vibrational and rotational excitation greatly simplifies the problem, and in most cases electronic excitation can be inferred from the appearance potential.
- M&AB) i-E,,
eV [21], M~CF2Ci) = -2.79 eV [22] ) AI+ = I .X eV [23] and EA(C1) = 3.61 eV [24] one calculates the dissociation energy of the C-Cl bond D(CF,CI-Cl) = 3.58 eV. Expression (9) leads to the excess energy E, = 0.18 eV at the onset E = 0.15 eV. The second resonance may be due to the further fragmentation process e- + CF,Cl, + CF, f Cl + Cl-. With iWf(CF2) = -1.01 eV [25], this would lead to the excess energy E, = 0.3 eV at the onset E = 2.2 eV. This ion curve shows some structure at E = 3.4 eV and the high stability of the CF, fragment (D(CF-F) = 5.8 eV) could lead to an additional excitation at these electron energies. One cannot exclude, however, that this ion yield is due to the first fragmentation process with the corresponding excess energies In the fragments. e- + CF2Ci2/F- (Jig. 4) The yield of this ion comes from the simple fragmentation process c- + CF,Cl, _ _ + CFC$ + F-s Taking the literature data AHt
+ CF, + Cl,_’
with EA(CQ = 2.52 + 0.17 eV [26], the process is (9)
E. INenbergeret al./Negotive ion formationin CF,cl,. CFsCIandCFCs (E = 3.4 -C0.3
eV) but within the uncertainty of the
energetically possible at E > 0.58 eV. The difference to the experimentally determined appearance potential lies within the uncertainties of the values of EA(C$) and AHACF,). For the second resonance, the further fragmentation
Quoting data from tabIe 4 it is clear, that this ion comes from the process
e-+CF,C$+CF+F+Cl;
e- t CF,Cl+
can energetically be excluded. It may be due to excitation of the CF, fragment. [fig. 5) e- f CF,CI,/FCIThe interpretation of this ion curve is somewhat conflicting. With .W&FCl) = -0.54 eV [28], EA(FC1)= 1.5 + 0.4 eV [29] and MACFCI) = 2.08 eV [23] one would expect the onset of the process
This leads to the excess energy ,!?, = 1.28 eV at the onset E = 3.0 eV. From expressions(7) and (8), one obtains the dissociation energy of the C-F bond, D(CF$l-F) = 5.37 eV. e- f CF3Cl/FCl- (fig. 7) Evaluation of the energetics shows, that this negative ion originates from the process
e- + CF,C$ + CFCl + FCl-
e- + CF,Cl -+ CF, + FCI-.
at E > 5.15 eV which differs from the onset of the experimentally determined FCl- yield by about 3 eV. This discrepancy cannot be explained entirely by the inaccurate value of EA(FC1). It is striking that a similar discrepancy is obtained in the system e- f CFCl$l, where the same neutral fragment CFCLshould arise (see discussion in this section). e- + CF2~~ICFCl~ (fig. 4) It js clear that this ion is due to the process
Within the unknown excess energy E, at the onset (E = 3.0 eV), we calculate the electron affinity of FCI, EA(FC1) = 1.79 eV t E, (c = 3.0 eV). This value can be compared with EA(FCI) = I .5 f 0.4 eV given
e- + CFzC12 -t F + CFCI;. With expressions (8) and (9), we calculate the electron affinity of CFCl2, EA(CFC1,) = 2.13 + Ee (E= 2.8 eV). Ee is the unknown excess energy at the onset of the process. 4.2.2. Negative ion formation from CF@ e- + CF7CIJCl- (fig. 6)
For this ion, two appearance potentials are observed_ With LWi(CF3C1) = -7.34 eV [21] and &YHfiCF,) = -4.86 eV [30], we calculate the excess energy E, = 0.57 eV at the onset E = 0.7 eV of the process e- + CF3CI + CF-, + CI-. Expressions (7) and (8) lead to the dissociation ener= 3.74 eV. For the further fragmentation
gy of the C-CL bond, D(CF+I) e-+CF3Cl-tCF2+F+CI-,
one expects the onset at e = 3.8 eV (E, = 0). This is somewhat above the onset of the second resonance
data used. e- f CF3Cl/F- (fig_ 7)
CF,Cl t F-.
in ref. 1291. e- f CF, Cl/CF2CT (ftg. 6) It is clear that this resonance is due to the process e- + CF,Cl+
F + CF,CI-,
and again within the limit of the unknown excess energy E, at the onset of the process we calculate the electron affinity of CF*Cl, EA(CF2Cl) = 1.87 eV + E, (6 = 3.5 eV). 4.2.3. Negative ion formation from CFC$ Within all systems reported in this paper, this ion is by far the most abundant. From energetics it is clear that it originates from the dissociative attachment process e- + CFC13 + CFCl, f Cl-. With AHf(CFCl$ = -1 .O eV (271 and A&(CFCI,) = -2.95 eV [21], we calculate O(CFCl+I) = 3.21 eV and the excess energy E, = 0.4 eV at the onset t: = 0.0 eV, i.e. the asymptote of the dissociative channel lies below the ground state of the neutral molecule. The vertical onset of the process and the extremely high cross section may be qualitatively exp!ained as follows: The dissociative channel V(R)crosses the Franck-Condon region of the ground state of the neutral molecule (fig. 11). This would
29
E. Illenberger et al./Negativeion formation in CF2Clz.CF3C7and CFCl3
30
Electron Et-erg& E
e- + CFC13 + CC$ + F-. With expressions (8) and (9), one obtains the electron affinity of Ccl,, EA(CC13) = 1.89 eV + 15,. E, is the unknown excess energy at the onset (E = 2.7 eV).
-1nternudeor
Separati0n.R -
Fig. 11. Schematic potential energy curve for the dissociative attachment process c-+ CFC13- CFClz t Cl-. R is the distance C-Cl.
of u&e) at e = 0.0 eV and one would expect a high cross section value. This would mean, however, that even electrons with E <0 could induce the dissociative attachment process. This would occur, if the system V(R) stays at internuclear distances R B R, (R, curve crossing) of the Franck-Condon region and slightly bound electrons (E < 0) (e.g. on surfaces) are present. e- + CFClJ Cl; (fig. 8) Suppgsing the process explain the onset bellaviour
e- t CFCI, + CFCI t Cl;, with Mf(CFCI) = 2.08 eV [23] and EA(Clzj = 2.52 f 0.17 eV [26], one would expect the onset at E = 2.53 eV which differs from the onset of the Cl: curve by about 2 eV. As mentioned above, this situation is similar in the system e- t CF,Cl,jFClwhere the same neutral fragment should arise. This suggests the accurate investigation of the value AIYACFCI) = 2.08 eV given in ref. [23] _On the other hand one should notice, that at the low pressure in the reaction chamber ion-molecule reactions are indeed improbable but not impossible. This ion yield arises from the process
42.4. Conchtding remark The hypothesis of Rowland and Molina [l] concerning the ozone depletion in the stratosphere is essentially based on the infinite lifetime of the halogenated methanes in the troposphere, i.e. these molecules reach the stratosphere without being decomposed. In a recent paper, Ausloos et al. [32] demonstrated, that halogenated methanes (Ccl,, CFCl,), adsorbed on certain surfaces can be broken down by photons at wavelengths (X = 3660 A) well beyond the gas phase photodissociation cross section. From this, one may see some connection to the above mentioned suggestion, that even electrons with E < 0 (e.g. slightly bound electrons on surfaces) may decompose the CFCl, molecule via dissociative attachment, and that those electrons are responsible for the decomposition processes obtained on surfaces. Moreover, one should notice that for the inert molecule CFCI, (the atmospheric lifetime is suggested to be > 10 years [I]) the electron decomposition cross section with low-energy electrons is 10” higher than the photodecomposition cross section at hv = 7.6 eV [33,5]. That slow electrons are far more disruptive in their impacts than are low-energy photons has been clearly pointed out by Johnson et al. [13] studying the fragmentation of aliphatic chlorocarbons under low-energy electron impact.
Acknowledgement Financial support from the Bundesministerium fdr Forschung und Technologie is gratefully acknowledged. Thanks are due to M. El Hadj-Ali for skilful technical assistance during this work.
e- + CFCl, + Ccl, + F-. With ArYdCCl,) = 0.82 eV [3 I] , we calculate the excess energy Ee = 0.46 eV at the onset of the process (E = 1.6 eV). Expression (7) leads to the dissociation energy of the C-F bond, D(CCI,-F) = 4.6 eV. e- + CFC7&CT3~ (fig. 9) It is clear that this resonance is due to the process
References [ 11 F.S. Rowland and H.J. hlolina, Rev. Geophys. Space Phys. 13 (1975) 1. [2] R.G. Green‘and R.P. Wayne, J. Photochem. 6 (1976177) 371.
E. Rlenbergerer &hk?gutiveion formationin CF~Cr,. CFscI and CFCI,
[3] T. CvitaS, H. C&ten and L. Klasinc, J. Chem. Phys. 67 (1977) 2687. [4 j H.W. Jochims, W. Lohr and H. Baumg%rtel. Ber. Bunsenges. Physik. Chem. 80 (1976) 130. [5] C. Hubrich. C. Zetzsch and F. Stuhl, Ber. Bunsenges. Physik. Chem. 81 (1977) 437. [6] W.M. Hickam and D. Berg, J. Chem. Phys. 29 (1958) 517. [7] J.S. Buchel’nikova, Soviet Phys. JETP 8 (1959) 783. [S] R.K. Curran, J. Chem. Phys. 34 (1961) 2007. [Y] A. Stamatovic and G.J. Schulz, Rev. Sci. Instr. 41 (1970) 423. [lo] L. Sanche and C.J. Schulz, Phys. Rev. A5 (1972) 1672. [ll] W. Bleakney and J.A. Hipple Jr., Phys. Rev. 53 (1938) 521. 1121 A. Stamatovicand C.J. Schulz, Rev. Sci. Instr. 39 (1969) 1752. [13] J.P. Johnson, L.G. Christophorou and J.G. Carter, J. Chem. Phys. 67 (1977) 2196. 114) K. Lacmann and H. Dispert, Proceedings of the 9th International Conference on the Physics of Electronic and Atomic Collisions, Seattle 1975, Abstracts, p. 924. [15] L.C. Christophorou and J.A. Stockdale, J. Chem. Phys. 48 (1968) 1956. [16] T.G. Lee, J. Phys. Chem. 67 (1963) 360. [17] T.F. O’MaUey, Phys. Rev. 150 (1966) 150. [IS] LG. Christophorou, D.L. McCorkle and D. Pittman, J. Chem. Phys. 60 (1974) 1183. 1191 G.J. Schulz and R.K. Asundi, Phys. Rev. 158 (1967) 25.
31
[20] L.G. Christophorou, Atomic and molecular radiation physics (Wiley Interscience, New York, 1971) ch, 6. [tl] S.S. Chen, R.C. Wilhoit and BJ. Zwofinski, J. Phys. Chem. Ref. Data 5 (1976) 571. [22] L.M. Leyland, J.R. Majer and J.C. Robb, Trans. Faraday Sot. 66 (1970) 898. [23] J-L. Franklin, J.G. Dillard, H.M. Rosenstock; J.T. Herran, K. Draxl and F.H. Field, Nat. Bur. Stand. NSRDSNBS26 (Washington, 1969). [24] R.S. Berry and C.W. Reimann, J. Chem. Phys. 38 (1963) 1540. 1251 E-N. Okafo and E. Whittle, J. Chem. Sot. Faraday Trans. 170 (1974) 1366. [26] J.J. De Corpo and J.L. Franklin, 1. Chem. Phys. 54 (1971) 1885. 1271 R. Foon and K.B. Tait, J. Chem. Sot. Faraday Trans. I 68 (1972) 104. [28] A.G. Gaydon, Dissociation energies and spectra of diatomic molecules, 3rd Ed. (Chapman and HaB, London, 1968). [29] P-W_ Harland and J.C.J. Thynne, I. Chem. Phys. 73 (1969) 4031. [30] D.M. Golden and S.W. Benson, Chem. Rev. 69 (1969) 125. [31] C.D. Mendenhall, D-M. Golden and S.W. Benson, J. Phys. Chem. 77 (1973) 2707. [32] P. Ausloos, R.E. Rebbert and L. Glasgow, NBS J. Res. 82 (1977) 1. 1331 R.E. Rebbert and P.J. Ausloos, J. Photochem. 4 (1975) 419.
AND CFCI,FOLLOWING LOW-ENERGY(0-10eV)lMPACTWITHNEARMONOENERGETICELECTRONS
NEGATIVE ION FORMATION IN CFZC12, CF,Cl
E.
ILLENBERGER,H.-USCHEUNEMANN andH.BAUMG;iRTEL
Instinctfiir PhysikalischeClzemieund Quantenchemieder Freien L’niversitiit Berlin, 1000 Berlin33, WestGermany Received 18 August 1978
Negative ion formation trochoidal monochromator
in CF,Clz, CF,CL and CFCIB under low-energy electron impact has been investigated using a as electron gun (Ae = 0.12 eV, I = 8 X 10S8 A) and a quadrupole mass ftiter for ion detection. Theions observed are F-, Cl-, FCI-, Cl,, CFCl; from CF,Cl,; F-, Cl-, FCl-, CFzCl- from CF,Cl and F-, Cl-, Cl;, Ccl; from CFCIs. Quoting available thermochemical data, it can be shown that most of the observed negative ions arise from dissociative attachment processes. Appearance potentials and for some radicals electron affinities (within the limit of the unknown cxccs: energy) are given. The extremely high yield of Cl- in CFC13, which is observed at E = 0.0 eV, will be discussed with regard to the lifetime of this molecule in the troposphere.
1. Introduction
2. Experimental
Recently the interest in the atmospheric chemistry of the halogenated methanes has grown rapidly since in 1975 Rowland and Molina [I] states that these molecules might destroy the atmospheric ozone layer via a Cl-0 reaction. In the meantime there has been done a wealth of laboratory studies with these molecules such as kinetics of the chemical reactions [2] and photoreactions, i.e. photoelectron, photoion and absorption cross section measurements [3-51. On the other hand, however, relatively little is known about the energy selective interaction of electrons with these molecules in the low-en&y region. In a mass spectrometric investigation with a conventional electron gun (AE = 1 eV), Hickam et al. [6] could detect Cl- ions from CFCl, and CF.-&l, and Buchel’nikova [7] measured negative ion current (but without mass analysis) following electron impact on CF#$. Curran [8] detected Cl-, F- and CC15 ions from CFCl,, employing the RPD-method as an electron gun (Ae = 0.3 eV). In this paper, we report on the energy selective (AC = 0.12 eV) formation of negative ions from CF,Cl,, CF,Cl and CFCl,.
The apparatus used for the present experiment consists of an electron monochromator which produces a beam of nearly monoenergetic electrons, a reaction chamber, a quadrupole mass analyzer, an electron collection system and a He(I)-photon source. The electron monochromator is constructed of molybdenum and bakeable up to 300°C in order to avoid surface’ problems. Nonmagnetic stainless steel is used for the whole vacuum system which is pumped by a turbomolecular pump. The experimental setup is shown schematically in fig. 1. 2.1. Electron monochromator In order to produce an electron beam with sufficient intensity especially in the low-energy region, a trochoidal monochromator was constructed. This instrument was described by Stamatovic and Schulz [9] who used it in the framework of electron transmission spectroscopy [IO] _Electrons, emitted froin a thoria coated tungsten filament are aligned in the z-direction by a homogeneous magnetic field (B =
22
E. Illenberger et al/h’egarive ion /ornm&on in CF&z,
ION
CF$3arrd CFCt3
to Multiplier
Quadrupale Mass Analyzer
TO Differential Pumping Unit (100 tk)
filament
Electron
Monochromator
-_$(70 Gauss)-
Electron Cdlection
PhotonSource
Tarset GQj
Fig. 1. Schematic drawing of the main parts of the appxntus used. 70 G), generated by a pair of Heimholtz coils outside
the vacuum system. The three electrodes following the filament have holes drilled off-center by 3 mm. Thus, electrons enter the deflection region, w!Cch is defined by the crossedmagnetic (z-direction) and electric field @-direction) off-center. In the crossed field region, the electrons describe a trochoidal motion [l I] and their guiding center moves with a constant velocity u = (E XB)/B2 in the x-direction along a plane of constant electric potential, i.e. dispersion of the electrons according to their z-velocity occurs and only electrons which have reached the axis are transmitted through the (axial) hole of the electrode S, . The energy selected electrons are now accelerated by the electrodes S, and S, into the reaction chamber, where they make collisions with the target molecules. The full width Aef at the base of the energy distribution at the whole of electrode St is given by the expression Aef = 24Q1 + c&)/d + e-&q,
(1)
where + = 1 mm and r$2 = 1 mm are the hoIes of the electrodes B, and St. d = 3 mm is the displacement from the axis and E$, the potential drop across the electron beam in the crossed field region. To obtain good energy resolution, one needs to operate the monochromator at low energies. This is achieved by retarding the electrons by the electrode B,. This retarding field also rejects most ofthose electrons whose
velocity vector is not oriented axially i.e. no further ener,v spread occurs from those electrons. 2.2. ReaCtim chamber aild ioll detectiorz Ions, formed in the reaction volume which is defined by the crossing of the electron beam with the target gas beam are extracted by a small electric field (+I.7 V/cm) and focussed onto the entrance hole of a quadrupole mass spectrometer. The extraction voltage is applied in such a way, that the potential on the axis of the electron beam is equal to the potential of the entrance (S3) and exit electrode (D1), i.e. a uniform potential exists along the path of the electrons. After mass analysis the ions are detected by a multiplier. Pulses from the multiplier are counted and stored in a multichannel analyzer. A complete synchronization between channel number and electron energy is achieved by converting the digital channel number to an analogous voltage signal which is then amplified by an operational power supply to the appropriate voltage. The trajectories of the ions through the lens system and the quadrupole are not essentially influenced by the relatively small magnetic field. 2.3. Electrot collection and He(I) photon source The transmitted electrons are accelerated to the
E. Illenberger et aI./Negatise ion formation in CF#2,
CF$l
and CFCI,
23
detecting system and the quadrupole properties, measuring photo-ions of known systems. 2.4. Energy resolution and energv scale calibration
-
Electron
Energy
(eV)
-
Fig. 2. (a) Negative ion yield for the production of SF: from SFs. The width of SF;* is an indication of the electron energy spread. (b) Current of the transmitted electrons on the electron collector C.
electron collector which is connected with an electrometer amplifier. The pressure in the reaction chamber is kept low G10m4 mb) thus, the portion of electrons lost by the process is negligible compared to the incoming beam intensity. Thus, the electron collector acts as a beam monitor. For the lowest energies, however, a small deflection of the electron beam by the ion extraction voltage cannot be neglected. Furthermore, the small pressure in the collision chamber helps to avoid possible ion-moIecule reactions. The differential pumped and water cooled He (584 A) lamp serves as a testing unit for the ion
The electron energy distribution is obtained by measuring the we11known electron attachment process on SF, [ 12,131. Fig. 2 shows the electron energy distribution obtained by this method and the current of the transmitted electrons on the electron collector C. For the present experiment, the operation of the monochromator provides a reasonable compromise between electron energy spread (Ae = 0.1-0.14 eV) and electron intensity (1% (5-10) X 10T8 A). The energy scale is calibrated measuring the SF,/ SFi* and SF6/SFT resonance following 2 procedure suggested by Johnson et al. [13]. This was achieved by adding a small amount of SF, to the gas and for every negative ion studied, both the SF;* and SF; resonance is measured under the same experimental conditions. Considering that our energy calibration is done at low energies (Sq peaks at 0.03 eV and SF; at 0.37 eV), we estimate the accuracy of our energy scale ~~0.2eV in the upper energy region (24 ev). The electron energy scale obtained in this way corresponds to the most probable electron enera.
3. Results In the electron energy range O-10 eV, we could detect the negative fragments listed in table 1. The numbers in the tabIe represent the energy integrated intensities relative to Cl-/CFCl,. One should notice, however, that these numbers are only 2 very rough indication of the intensity of the ions, considering that .
Table 1 Negative ion formation from CFCls, CFzCl2 and CFaCI. The numbers indicate the relative energy integrated intensities of the negative ion fragments (see text) hlolecule
CFCl, CF,Cla CFsCI
Negative ion fragment F-
cl-
FCi
5 8 0.5
1000 58 2.5
1.3 0.5
cl;
CFaCl-
CFCl;
ccl; 0.4
1.2 1.2
2.5 0.2
E. IIlenberger et al/Negative
24
ion formation in CFzClz,
CF3Cl and CFC13
I
1
3
2 -
Electron
Energy leV)
-
-
Fig. 3. Negative ion yield for the production
of Cl- from
CF2CI2.
our ion counting rate is not corrected on the mass dependent transmission of the quadrupole, and that the extraction optics system discriminates on the (unknown) translational excess energy of the negative fragments. No negative parent molecule could be observed though collision experiments indicate that CFCI; should be a stable ion 1141. Quoting available themro-
Fig. 5. Formation
Electron
4
5
Energy (eV) -
of FCI- from CF2C12 by electron
impact.
chemical data, most of the negative ions can be attributed to dissociative attachment processes. CIfrom CFCl, is by far the most abundant, and CF,CllCF,Cl(which is 0.02% of CFCIJCI-) lies somewhat above our detection limit. For the determination of the appearance potential M of the negative ions one should notice that besides the problem of energy calibration - further uncertainties arise. Since we did not use a deconvolution technique and the functional form of the cross section at the threshold is unknown, the accurate determination of the onset is somewhat criticaL
..: ..
>
I
0
e- l
c EpJcl~
I
3
1
c-+CF,C$/(CFClJ -_z 0
1
e-*C~C12/F-
! :. ‘r 0 -
Electron
2 Energy
5’
‘:k. ‘.-.. ‘T’-._. 4
5
‘.-.. :.. ‘: ‘k.,
f .,,
/ 1
:’ ,: ;y2
:
1 r Q“*Y
4’
...-.... .--.
3 _.-. :>’ L.<
2
q’ fi
3.1. CF2C12
;
‘X_ 3 (eV)
Fig. 4. Negative ion yield for the production CFCL; and (c) F- from CFsCls.
4
For this molecule, five negative fragment ions are observed. Their yields as a function of electron energy are shown in figs. 3-5. Cross section data are available for CF,Ci,/Cl- (table 2) derived from the swarmbeam techn&ue [ 151 using electron swarm data [ 16j and the earlier beam experiment of Buchel’nikova [7] (who determined the negative ion current’without mass analysis) and from swarm data, using the swarm unfolding technique [18]. In the systems CF,CI,/CIand CF~Ci.-JC1~ two appearance potentials are obtained, the second AP is overlapped by the first resonance. The appearence potentials are listed in table 2. 3.2. W&7
5
-
of (a) Cl;, (b)
As may be seen from table 1, the ion yield from this molecule i relatively small. For this compound no exact cross section data are available. The energy
E. Illerlbergeret al.jNegativeion formation in CFzCr,, CF3Cland CFQ3
25
Table 2 Negative ion formation from CFzC12 ___._-.____._Ion
AP (eW _-__-__-_--.
_____.__.._
Peak position (eV1
F-
1.8 f 0.1
3.1 + 0.2
Cl-
0.15 f 0.1 2 2.2 k 0.3
0.55 ? 0.1
0.2
0.65 r 0.1
a; CFCI;
2.8 + 0.1 ______--____-__-
-..--
Relative energy integrated intensity ____-____.I__--
Peak cross section (cm’)
13.8 1.1 x IO-l6 [lS,lSl
I
1
3.55 + 0.2 ---
dependence of the four negative ion fragments is shown in figs. G and 7 and the appearance potentials and peak positions are listed in table 3.
2.1 4.3
-
The finite slope at the onset of the Cl- curve in fig. 8 seems to be purely instrumental. Appearance potentials and peak positions are listed in table 4.
3.3. CFC!, 4. Discussion For this compound, four negative ions could be observed: F- , Cl-, Cl, and Ccl, _Their yield as a function of electron energy is shown in figs. 8 and 9. Compared with the other compounds studied, the Cl- ion is by far the most abundant. This clearly shows that the cross section strongly depends on molecular structure. The peak cross section given in table 4 is obtained by the swarm-beam technique [IS].
4.1. Theoretical corrsiderations A description of dissociative electron attachment to diatomic molecules (AB f e- -+ A + B-) has been developed by O’Mtiey [17], treating the process as a resonance phenomenon. Using the Bom-Oppenheimer separation, the process is understood as an
e-e CF,Cl/F-
I
I
o
t
-
2 Electron
3
4 5 6 Energy (CL’-)
7
Fig. 6. Negative ion yield for the production of (a) Cl- and @) CF2CI- from CF3Cl.
-
Electron
Energy (eV) -
Fig. 7. Negative ion yield for the production of (a) FCl- and (b) F- from CF3Cl.
E. Illenberger et aI.JNegntiw ion fonnation in CF, Clz, CFsCl and CFCls
26
Table 3 NcK3tiveion formation from CF$Zl Reh tive energy integrated intensity
Peak
Ion
position W) __........_ .__-______ 3.0 t 0.2 4.1 ?: 0.2
__ P-
2
Cl-
0.7 c 0.3 3.4 + 0.3
1.3 + 0.2 4.8 r- 0.2
I 10
1X1-
3.0 r 0.3
3.9 + 0.2
2
CF2CI
3.5 +_0.3
4.2 f 0.2
0.8
_---.---.-.-_.
+
potential curve of the molecule and the electron at rest, separated at infinite distance, i.e. the potential
curve for the neutral molecule, V(R)- denotes a negative ion potential curve i.e. a potential curve with the captured electron. For polyatomic systems A and/or B- may be an atom or n radical. According to the Fran&-Condon principle, transitions within the restricted energy range (E, f e < ~2) from the initial to the final state can occur. The compound negative ion AB- may either dissociate into neutral and negative ion fragments or decay via auto-
-
k _.
-
Electran Energy (eV) -
Fig. 9. Negative ion yield for the production of (a) F- and (b) Ccl; from CFQ.. (autodetachment). Thus, the cross section for dissociative attachment, udo, is a product of the cross section for the formation of AB-, IJO,and the probability P that AB- will dissociate, once formed, rather than decay by autoionization. ionization
Ud3
=
ooP.
(2)
For a diatomic molecule AB, initially in the u = 0 level of the ground eIectronic state, O’hlalley [ 171 derived a simple explicit formula for the dissociative attachment cross section as a function of electron energy
e- + CFCI, 1 Cl-
I
‘a
3
____. ___-. .-__.
electronic transition from a continuum state (AB + c-) to a discrete state (AB-, degenerate with the continuumj which then dissociates. The situation is schematically illustrated in fig. 10. V(R) denotes the
‘i
2
ii.
:_ ‘, x la “i,__
!
z
.___ _ o
I
2
3
4
5
G
s z
-_
2 ”
.CFC$ / Cl;
e-
:
s 1:
:‘.._ -__--+.._., ...L_
i-
Fig. 8. Fwxation trun imp&.
0
1
-
Electron Energy (eV) -
2
3
4
5
6
of(a) Cl-and (b) Cl, from CFC13 by elec-
where ki is ;he incident electron’suwave number (kf E the electron energy,g a statistical factor, Z. = en + $fiw (e. is the electron energy at R = R,, $ho is the zero point energy), rd the width for dissociation (rd is a function of the slope of V(R)and 2fi/rd is a measure of the time it takes to the final products to move a distance from the turning point to R, where autoionization becomes impossible), r, the partial and ra the total autoionization width. If there is onIy one electronic channel open, r,=r,. T!te survival probability P is given by the expression = (Zm/fi)~),
5. illenbergef et at/NeEa?ive ion formation in CF,C&. CF3Cl and CFi&
27
Table 4
from ClX& .-Negative ion fnrnlation l_..--..-_.--_.---Ion
AP (eV) -p*.---_____ this work
Peak position foV)
ciCl; CCI;
Relative cncrgy integrarcd inmnsity
Curran [S]
_I
F-
,--
I. ---
--.--_-_-_
1.6 k 0.2
1.8 * 0.1
3.0 f 0.2
0.0 c 0.05
0.0 f 0.05
0.0 f 0.0s 1.6 f 0.1
0.6 t 0.2 52.5 t 0.3 2.7 + 0.2
2.75
2
v(R) = 2fL-’ [{.E-, - V@)-)]
1’2,
1.2
1
0.4
is the imaginary part of the complex potential energy of the negative ion state V(R)- - if’& It describes {similar to the contpiex index of refraction in optics) the “leakage” (~utoioni~tion) on F/I/?)-. The autoionization width ra(R) is connected to the auloioni~~tion rate (per unit time) Jlt(R) by the expression W(R) = ra(R)/tt. Christophorou and Stockdate 1151 pointed out that for purely repulsive negative ion states in the Fr~nck-Condon region with il resonance enera beiow the Iowest excited electronic state of the neutral moIecule, the survival probability never differs much from unity. The dissociative ~tt~cllment cross sec-
par-
(5)
-and f the time it takes them to dissociate
.-_ 9.5 x to-‘5 [IS].
1000
(41 v(R) is the cIassicaI velocity of the dissociating ticles
-_.-
5
3.3 f 0.2
0.1
Pcnk cross section (cm”)
to the cross-
ing point R,. in eq. (~),E’K is the asymptotic translational energy of the dissociating particles. t’,(R)/2
A8
..._.
-.-.-.-._._. _ _ A+B+e-
-.O
.I
L
NCQ, 1087Intertsity
-
Internuclear
Separation.
R -
Fig. 10. Potential energy cwves for the dissoci+ve attachment of electrons (with energy G) to motecule AB. The ground state shown is for the neutraI molecule AB plus the electron at rest at infinity.
E. Illenberger et al/Negative ion formation in CF2Ci2. CFJCI and CFCIJ
28 Table 5 Summary of the Iiteraturc wloulations
_-.-
AHy= (CV) --
Ref.
cry& cI:$I CIXTI~ cl-y3
-5.11 -7.34 -2.95 -2.79
t 0.08 h 0.03 + 0.07 +_0.1
Pll WI PII [ZZJ
CKl,
-1.0
0.82 + 0.04
[27l 1511
-4.86 2 0.16 2.08 -2.01 c 0.1 -0.54
CF3
CIXI CFZ I-Cl
-.-
the thermodynamical
Compound ----
CCIS
_____
where rvIf means the heat of formation. data used Tar
4.2. Discussim of the results 4.21. Negariw ion formation from CF2Ciz e- + CF,Ci#‘(jig. 3) For this;on, two appearance potentials (AP, = 0.15 + 0.1 eV, APz 5 2.2 t 0.3 eV) are observed. It is clear that the first resonance arises from the process
[301 v31 I251
I: CI
0.82 1.26
Compound ._____
EA (eV) __._ ___--.-..-_
Cl Cl? F
3.6 1 2.52 i 0.17 3.45
1281 I231 1231 Ref.
e- f CF,Cl, - _ + CF,CI f Cf-. With AfY,(CF,C12) = -5.11
1241 [I61
1241
tion would then be controlled dominantly by 00. When, however, the negative ion state lies at or above known electronically excited states of the neutral molecule, the negative ion state can now decay to an electronically excited neutral molecule in addition to ZI rotationally and/or vibrationaliy excited neutral moIecule. In such cases, the effect ofautoiorhtion is expected ro be large which could lead to wry large isotopic effects [19,20] _ Evaluation of tfle cnergetics of negative ion formation can provide much thermochemical information. From fig. IO. one obtains the expression
E,~AP(B-)=D(A-B)-EA(B)+Ec,
(6)
whew EA is the electron affinity, D(A-B) the dissociation energy and Ee the excess energy, which is the sum of the translational (EK), electronic, vibronic and rotational energy. Using the thermodynamical expression D(A-0,
= M&i\)
f Mt.(B) - M$.(AB)
(7)
and L/II+ U-) = M,.(B) - W(B),
(8)
one obtains q = AP(R-) = &Y&A) + M&B-)
In diatomic molecules,.the absence of vibrational and rotational excitation greatly simplifies the problem, and in most cases electronic excitation can be inferred from the appearance potential.
- M&AB) i-E,,
eV [21], M~CF2Ci) = -2.79 eV [22] ) AI+ = I .X eV [23] and EA(C1) = 3.61 eV [24] one calculates the dissociation energy of the C-Cl bond D(CF,CI-Cl) = 3.58 eV. Expression (9) leads to the excess energy E, = 0.18 eV at the onset E = 0.15 eV. The second resonance may be due to the further fragmentation process e- + CF,Cl, + CF, f Cl + Cl-. With iWf(CF2) = -1.01 eV [25], this would lead to the excess energy E, = 0.3 eV at the onset E = 2.2 eV. This ion curve shows some structure at E = 3.4 eV and the high stability of the CF, fragment (D(CF-F) = 5.8 eV) could lead to an additional excitation at these electron energies. One cannot exclude, however, that this ion yield is due to the first fragmentation process with the corresponding excess energies In the fragments. e- + CF2Ci2/F- (Jig. 4) The yield of this ion comes from the simple fragmentation process c- + CF,Cl, _ _ + CFC$ + F-s Taking the literature data AHt
+ CF, + Cl,_’
with EA(CQ = 2.52 + 0.17 eV [26], the process is (9)
E. INenbergeret al./Negotive ion formationin CF,cl,. CFsCIandCFCs (E = 3.4 -C0.3
eV) but within the uncertainty of the
energetically possible at E > 0.58 eV. The difference to the experimentally determined appearance potential lies within the uncertainties of the values of EA(C$) and AHACF,). For the second resonance, the further fragmentation
Quoting data from tabIe 4 it is clear, that this ion comes from the process
e-+CF,C$+CF+F+Cl;
e- t CF,Cl+
can energetically be excluded. It may be due to excitation of the CF, fragment. [fig. 5) e- f CF,CI,/FCIThe interpretation of this ion curve is somewhat conflicting. With .W&FCl) = -0.54 eV [28], EA(FC1)= 1.5 + 0.4 eV [29] and MACFCI) = 2.08 eV [23] one would expect the onset of the process
This leads to the excess energy ,!?, = 1.28 eV at the onset E = 3.0 eV. From expressions(7) and (8), one obtains the dissociation energy of the C-F bond, D(CF$l-F) = 5.37 eV. e- f CF3Cl/FCl- (fig. 7) Evaluation of the energetics shows, that this negative ion originates from the process
e- + CF,C$ + CFCl + FCl-
e- + CF,Cl -+ CF, + FCI-.
at E > 5.15 eV which differs from the onset of the experimentally determined FCl- yield by about 3 eV. This discrepancy cannot be explained entirely by the inaccurate value of EA(FC1). It is striking that a similar discrepancy is obtained in the system e- f CFCl$l, where the same neutral fragment CFCLshould arise (see discussion in this section). e- + CF2~~ICFCl~ (fig. 4) It js clear that this ion is due to the process
Within the unknown excess energy E, at the onset (E = 3.0 eV), we calculate the electron affinity of FCI, EA(FC1) = 1.79 eV t E, (c = 3.0 eV). This value can be compared with EA(FCI) = I .5 f 0.4 eV given
e- + CFzC12 -t F + CFCI;. With expressions (8) and (9), we calculate the electron affinity of CFCl2, EA(CFC1,) = 2.13 + Ee (E= 2.8 eV). Ee is the unknown excess energy at the onset of the process. 4.2.2. Negative ion formation from CF@ e- + CF7CIJCl- (fig. 6)
For this ion, two appearance potentials are observed_ With LWi(CF3C1) = -7.34 eV [21] and &YHfiCF,) = -4.86 eV [30], we calculate the excess energy E, = 0.57 eV at the onset E = 0.7 eV of the process e- + CF3CI + CF-, + CI-. Expressions (7) and (8) lead to the dissociation ener= 3.74 eV. For the further fragmentation
gy of the C-CL bond, D(CF+I) e-+CF3Cl-tCF2+F+CI-,
one expects the onset at e = 3.8 eV (E, = 0). This is somewhat above the onset of the second resonance
data used. e- f CF3Cl/F- (fig_ 7)
CF,Cl t F-.
in ref. 1291. e- f CF, Cl/CF2CT (ftg. 6) It is clear that this resonance is due to the process e- + CF,Cl+
F + CF,CI-,
and again within the limit of the unknown excess energy E, at the onset of the process we calculate the electron affinity of CF*Cl, EA(CF2Cl) = 1.87 eV + E, (6 = 3.5 eV). 4.2.3. Negative ion formation from CFC$ Within all systems reported in this paper, this ion is by far the most abundant. From energetics it is clear that it originates from the dissociative attachment process e- + CFC13 + CFCl, f Cl-. With AHf(CFCl$ = -1 .O eV (271 and A&(CFCI,) = -2.95 eV [21], we calculate O(CFCl+I) = 3.21 eV and the excess energy E, = 0.4 eV at the onset t: = 0.0 eV, i.e. the asymptote of the dissociative channel lies below the ground state of the neutral molecule. The vertical onset of the process and the extremely high cross section may be qualitatively exp!ained as follows: The dissociative channel V(R)crosses the Franck-Condon region of the ground state of the neutral molecule (fig. 11). This would
29
E. Illenberger et al./Negativeion formation in CF2Clz.CF3C7and CFCl3
30
Electron Et-erg& E
e- + CFC13 + CC$ + F-. With expressions (8) and (9), one obtains the electron affinity of Ccl,, EA(CC13) = 1.89 eV + 15,. E, is the unknown excess energy at the onset (E = 2.7 eV).
-1nternudeor
Separati0n.R -
Fig. 11. Schematic potential energy curve for the dissociative attachment process c-+ CFC13- CFClz t Cl-. R is the distance C-Cl.
of u&e) at e = 0.0 eV and one would expect a high cross section value. This would mean, however, that even electrons with E <0 could induce the dissociative attachment process. This would occur, if the system V(R) stays at internuclear distances R B R, (R, curve crossing) of the Franck-Condon region and slightly bound electrons (E < 0) (e.g. on surfaces) are present. e- + CFClJ Cl; (fig. 8) Suppgsing the process explain the onset bellaviour
e- t CFCI, + CFCI t Cl;, with Mf(CFCI) = 2.08 eV [23] and EA(Clzj = 2.52 f 0.17 eV [26], one would expect the onset at E = 2.53 eV which differs from the onset of the Cl: curve by about 2 eV. As mentioned above, this situation is similar in the system e- t CF,Cl,jFClwhere the same neutral fragment should arise. This suggests the accurate investigation of the value AIYACFCI) = 2.08 eV given in ref. [23] _On the other hand one should notice, that at the low pressure in the reaction chamber ion-molecule reactions are indeed improbable but not impossible. This ion yield arises from the process
42.4. Conchtding remark The hypothesis of Rowland and Molina [l] concerning the ozone depletion in the stratosphere is essentially based on the infinite lifetime of the halogenated methanes in the troposphere, i.e. these molecules reach the stratosphere without being decomposed. In a recent paper, Ausloos et al. [32] demonstrated, that halogenated methanes (Ccl,, CFCl,), adsorbed on certain surfaces can be broken down by photons at wavelengths (X = 3660 A) well beyond the gas phase photodissociation cross section. From this, one may see some connection to the above mentioned suggestion, that even electrons with E < 0 (e.g. slightly bound electrons on surfaces) may decompose the CFCl, molecule via dissociative attachment, and that those electrons are responsible for the decomposition processes obtained on surfaces. Moreover, one should notice that for the inert molecule CFCI, (the atmospheric lifetime is suggested to be > 10 years [I]) the electron decomposition cross section with low-energy electrons is 10” higher than the photodecomposition cross section at hv = 7.6 eV [33,5]. That slow electrons are far more disruptive in their impacts than are low-energy photons has been clearly pointed out by Johnson et al. [13] studying the fragmentation of aliphatic chlorocarbons under low-energy electron impact.
Acknowledgement Financial support from the Bundesministerium fdr Forschung und Technologie is gratefully acknowledged. Thanks are due to M. El Hadj-Ali for skilful technical assistance during this work.
e- + CFCl, + Ccl, + F-. With ArYdCCl,) = 0.82 eV [3 I] , we calculate the excess energy Ee = 0.46 eV at the onset of the process (E = 1.6 eV). Expression (7) leads to the dissociation energy of the C-F bond, D(CCI,-F) = 4.6 eV. e- + CFC7&CT3~ (fig. 9) It is clear that this resonance is due to the process
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31
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