JOURNAL OF CHEMICAL PHYSICS VOLUME 109. NUMBER 19 15 NOVEMBER 1998

Effect of temperature on electron attachment to and negative ion statesof CCI2F2

Yicheng Wang,8)Loucas G. Christophorou,b)and Joel K. VerbruggeNationalInstituteof Standardsand Technology,ElectricityDivision,Electronicsand ElectricalEngineeringLaboratory,Gaithersburg,Maryland20899-0001

(Received 19 June 1998; accepted 14 August 1998)

The effect of temperature on electron attachment to dichlorodiftuoromethane (CCI2F2) has beeninvestigated for temperatures up to 500 K and for mean-electron energies from thermal to 1.0 eVusing an electron swarm method. The measurements were made in mixtures of CCl2F2 withnitrogen. The electron attachment rate constant increases with temperature over the entiretemperature and mean-electron energy range investigated. The variation of the thermal value ofthe electron attachment rate constant with temperature compares well with earlier measurementsof this quantity and shows an increase by a factor of 10 when the temperature is raised from 300to 500 K. From a comparison of published data on the electron affinity, electron attachment usingthe swarm method, electron attachment using the electron beam method, electron scattering,electron transmission, indirect electron scattering, and related calculations, the lowest negative ionstates of CCl2F2have been identified with average positions as follows: al(C-Clu*) at +0.4 eVand -0.9 eV, b2(C-Clu*) at -2.5 eV, a)(C-Fu*) at -3.5 eV, and b1(C-Fu*) at -6.2 eV; anelectron-excited Feshbach resonance is also indicated at -8.9 eVe @ 1998 American Institute ofPhysics. [S0021-9606(98)02043-1]

I. INTRODUCTION

There have been a number of studies dealing with theinteractions of dichlorodiftuoromethane (CCI2F2) with low-energy electrons. These have recently been reviewed byChristophorou et al.) In this paper we report on two aspectsof low-energy (< 10 eV) electron interaction processes inCCI2F2:(I) The measurement of the temperature dependenceof the total electron attachment rate constant for CCl2F2inthe mean electron en~rgy range from 0.04 to 1.00eV and thetemperature range from 300 to 500 K. These measurementswere made t:sh.1ga pulsed Townsend swarm technique2and anew wave form analysis method which improved data qual-ity. (2) The number and energy position of the negative ionstates of the CCI2F2molecule below -10 eVe This knowl-edge is obtained by a synthesis of published data from ex-periments and calculations.

II. EXPERIMENTAL METHOD

The experimental method employed here is a pulsedTownsend technique2 whereby electron swarms are photo-electrically produced using a 5 ns, frequency quadrupled(266 nm) Nd:YAO laser. A 3chematic diagram showing itsoperating principle is given in Fig. 1. The two parallel elec-trodes are circular stainless steel disks with a diameter of 6.2cm. They are separated by 2 distance, d, of 1.664 cm and arecontained in a cubic stainless steel vacuum chamber. The

laser beam enters the chamber through a sapphire windowand is focused through a small hole (-0.6 mm diam) in the

a~lectronic mail: Yicheng.Wang@nist.gov

b~lectronic mail: loucas.christophorou@nist.gov

0021-9606/98/109(19)/8304nI$15.00

center of the anode electrode before striking the cathodeelectrode, generating a photoelectron pulse which quicklyreaches a steady state and drifts toward the anode under theinfluence of the applied uniform electric field, E. The currentinduced by the motion of this electron swarm and the nega-tive ions which are formed by attachment in the drift (inter-electrode) region is integrated by the RC (R = 100 0.0. andC- 50 pF) circuit in front of a high-impedance unity-gainbuffer amplifier with a slew rate of 0.22 VIns. The outputvoltage of the amplifier is then digitized with a LeCroy 9420digital oscilloscope (Certain commercial equipment, instru-ments, or materials are identified in this paper to foster un-derstanding. Such identification does not imply recommen-dation or endorsement by the National Institute of Standardsand Technology, nor does it imply that the materials orequipment identified are necessarily the best available for thepurpose.) which has a resolution of eight bits and a maxi-mum sample rate of 108samples/so To minimize the influ-ence of the alternating current (ac) line noise, laser pulses aresynchronized with the zero crossings of the line frequency.The synchronization scheme allows subtraction of the linenoise and thus improvement in data quality.

The vacuum chamber is enclosed in a fiberglass heatingmantle. The heating power to the mantle is controlled withan Autotune Temperature Controller (CN9000A) made byOmega, achieving a temperature stability within 1°C. Thetemperature uniformity of the chamber is monitored with

m~ltiple thermocouples around the chamber and is constantto within 1°C. The experiments were performed using N2(quoted purity 99.999%) and CCI2F2(quoted purity 99.9%)purchased from Matheson Oas Products. Before use, the N2gas was cooled to liquid nitrogen temperature to condense

8304 @ 1998 American Institute of Physics

J. Chern. Phys., Vol. 109, No. 19, 15 November 1998

Laser Beam

HV PowerSupply

FIG. I. Schematic diagram of the pulsed Townsend method used in thepresent study (see text).

impurities, especially H20. The N2 vapor was extracted justabove the boiling point. The CCl2F2gas was also purified bya number of freeze-pump-thaw cycles to remove air andother possible impurities. In the experiments conducted inthis work, the N2buffer gas pressure was varied from 75.3 to127 kPa and the partial pressure of CCl2F2was 5.12, 1.63,and 0.449 Pa for data taken at temperatures of 298, 400, and500 K, respectively. The total pressures and the mixture ra-tios were measured using two temperature-controlled, high-accuracy capacitance manometers. The estimated measure-ment uncertainty for the total pressure, the mixture ratio, andE/N is less than :t 1%.

20

(b)o.~

-2 o 2 4 6 8 10 12

Time (JlS)

FIG. 2. Typical voltage wave fonns at room temperature. (a) For a mixture

ofCCI2F2 and N2 (attaching gas to buffer gas density ratio=6.8x 10-5; notethe change in the time scale for Ve and Ve + Vi) and an EI N value of 0.33Td. (b) For pure N2 at EIN=0.29 Td (the signal "peak" at the origin is dueto the laser pulse) (see text).

Wang, Christophorou, and Verbrugge 8305

40 , . . . Io McCOI1cIe(1982)6 W.Wang(1987). . PresentResults--- Averagex Thenna/Valuao Average 01Thenna/ Values

~ 30C')

E(J

~ 20b~.:.:.~ 10

.~oo s~__~_o~

tlt1 0 30 . . ... r..""6 ~ ~_ ~t:" ~I ..".~.....x..

(b)OL.J..

0.2 0.4 0.6 0.8

Mean Electron Energy (eV)

1.0

FIG. 3. Total electron attachment rate constant ka for CCI2F2 measured atroom temperature in mixtures with N2. (a) as a function of EI N (0, Ref. 3;6., Ref. 4; ., present), and (b) as a function of the mean-electron energy.

Also shown in the (b) are values of ka, measured at only thermal energy.

Two typical voltage wave fonns that were acquired atroom temperature are shown in Fig. 2. The wave fonn in Fig.2(a) was acquired in a CCI2F2+N2 mixture (the ratio ofCCl2F2to N2 was 6.8X 10-5, and the density-reduced elec-tric field E/N was 0.33X 10-17 V cm2). The initial voltagestep, Ve, which occurs in t- 1 jLS, is proportional to thenumber of electrons drifting across the gap without attach-ment. The plateau voltage, Ve+ Vi' which is reached at t- 400 jLS, is proportional to the sum of the electrons and thenegative ions crossing the gap. The electron attachment co-efficient, TI.is detennined from

Ve/(Ve+ V;)=[I-exp( -17d)]/( 17d). (1)

TABLE I. Values of the thermal electron attachment rate constant, (ka)Ih'for CC12F2. Average value: (l6.1:!:7.2)X 10-10 cm) S-I.

(ka)\h(lO-IO cm) S-I) T (K) Method Reference

16.69.6

138.37

181912.3223219

298 Electron swarm295 Electron swarm

298 Microwave conductivity300 Electron cyclotron resonance298 Electron cyclotron resonance293 Electron cyclotron resonance298 Electron swarm298 Electronswarm298 Electron swarm300 Flowing afterftow293 Flowing afterglow

Present56789103 and 111213 and 1415

150(a)

100

I V.+50I- .i!-

1-04- V

o.

t......... 0

<1>-2 0 2 2000) 400 600 800

co

gJ I t. ' !

25 ,...---.-----..--.- r--.---, I I I I

0 McCorkle (1982)- A W. Wang (1987)... . PresentResults'f/) o 0

C') 20 o A 06

§ % 66 6 0Q

00 o ......... 0 6-0 6. · · .boo ;15

.:.:.o. 6,

(a)10' . . . . . I . . . I

0 1 2 3 4 5 6 7 8 9 10

E/N (10-21Vm2)

6306 J. Chern. Phys., Vol. 109, No. 19, 15 November 1996

100

-.(/)

C?e()

~"0~.:.:,-

6666

6A 500KIII.

66

66

aoo400K 66DaD 66

~aaD- '66--u a 66

DD 666

298K ~ODQ;JOoo~6A6620 I-: qJoaa666666

o 0 00 0 0 00000 00000t::JlJ(1:D~000 o oOOOOQ:0088 a

80 (a)

60

40

o0.1 1

EIN (10.21 Vm2)

10

100'\

6

OO~ \ 00~ SOOK

60 ~ 660' 66

66

40 I 400K 666~ ~ 666

~a 6 6 6

oaOaoaaaa 66666

20 ~ 298 K a a 0 a aoaocc:6666~oooooooooo 00000000 0 000 0 OO~~~

o0.0 0.2 0.4 0.6 . 0.8

Mean Energy (eV)

1.0

FIG. 4. Variation of the ka of CCI2F2 with temperature. (a) ka(EIN), (b)ka«£».

The digital oscilloscope used in the present work has arecord length of 50 000 samples for each laser pulse, whichallows recording of wave fonn details in multiple timescales. This capability of the present experiment of acquiringdetailed data in multiple time scales allows more accuratedetermination of Ve and Ve+ Vi and is an improvement overearlier data acquisition procedures which use the pulsedTownsend method.

The wave fonn shown in Fig. 2(b) was taken in pure N2at an EIN of 0.29X 10-17 V cm2 with the sample rate of thedigital oscilloscope set to the maximum. The distinct breakat t =5.5 J.Lsis associated with the electron swarm transittime, te' The response time of the electronic system isshorter than 20 ns. The electron drift velocities, W= dl te' aredetermined from the occurrence time of this break, and have

TABLE II. (ka)d\ of CCI2F2 as a function of temperature.

Reference

Present work

13 and 14

15

-- --

Wang, Christophorou, and Verbrugge

...-.(/)

C')e()

~b~

-=~

~

o200

00 T.

o200 300 400 500 600 700 800

Temperature (K)

FIG. 5. Variation of the thennal value, (ka) 11\, of the total electron attach-ment rate constant for CCl2F2 with temperature. ., present; 0, Refs. 13 and

14; 0, Ref. 15. The arrow shown for one of the data points indicates that thevalue of (ka)d\ is lower than shown.

an estimated uncertainty of :!:5%.The uncertainty is prima-rily due to determining the position of the break point and isdetermined from the standard deviation of the scatter ob-served in multiple runs for identical experimental conditions.

The total electron attachment rate constant, ka, is ob-tained from

ko=WTJINa, (2)

where No is the number density of the electron attaching gas.The combined experimental uncertainty for ka is approxi-mately :!:10% for data acquired at 298 and 400 K. For thedata acquired at 500 K, the uncertainty is approximately:!:20%. The larger uncertainty at this higher temperature isrelated to possible thermal decomposition of CCI2F2.

III.TEMPERATURE DEPENDENCE OF THE ELECTRONAIT ACHMENT RATE CONSTANT FOR CCI2F2

We measured the total electron attachment rate constant,ka, for CCl2F2 as a function of EIN in nitrogen at roomtemperature (298), 400, and 500 K. In Fig. 3(a) are shownour room temperature measurements of ka (EI N) along withtwo earlier room temperature measurements.3.4The variationbetween the data of the various groups is barely within thecombined uncertainties which are about:!: 10% for each ex-

periment. The data in Fig. 3(a) are plotted in Fig. 3(b) as afunction of the mean-electron energy, (E), calculated fromthe electron energy distribution functions in N2 using a Bolt-zmann code. Also, there have been a number of earlier mea-surements of only the room temperature thermal value,(ka)th, of ko. These are listed in Table I and are plottedin Fig. 3 along with their average, ( 16.1:!:7.2)X 10-10 cm3S-I.

Figure 4(a) shows the measured dependence of ko (EIN)on,temperature, and Fig. 4(b) shows the same data plotted asa function of mean-electron energy (E). The rate constant isseen to increase considerably with increasing temperatureover the entire energy range from 0.04 to 1.0 eV. However,the lower the value of (E)the larger the temperature enhance-ment. This behavior is consistent with earlier findings onother compoundsl6 and with the results of two recent elec-

(ka)d\(10-IO cm) S-I) T(K)

16.6 29860 400

<140 500

<10 20532 300

160 455530 590

19 293140 467240 579420 777

600

00 SniIh (1984)C Bums(1996). Y. Wang (1997) C

400'

J. Chern. Phys., Vol. 109, No. 19, 15 November 1998

TABLE 1lI. Negative ion states of CCI2P2.

Wang, Christophorou, and Verbrugge 8307

Energy (e V) ReferenceMethod

0.4:t0.30.40.67-0.0<-0.1--0.18 (shoulder)-1.05-0.07-0.30-0.93-0-0.8-3.8-0.0--0.7--3.5-0.0--0.6-3.5-0.55 (CI-)-0.65 (CI;)-2.85 (FCI-)-3.1 (P-)-3.55 (CFCI;)The energy dependence of the sum of all negative ions givespeaks at -0.6 eV and at -3.2 eV (Ref. I)-0.0-0.3-0.95-3.6-0.7 (CI-)- 3.2 (P-)- 3.7 (CFCI;)-1.0-2.6-4.0-5.9-1.2-3.4-4.6-6.4-0.8-3.1-5.1-6.7-0.98-2.35-3.88-1.0-2.5-4.0-6.0-9;0

Potassium-atom beam techniqueMultiple scattering Xa calculationQuantum mechanical calculation

Kr photoionization electron attachment techniqueSwarm-unfolded total attachment cross

section using N2 as the buffer gas

Swarm-unfolded total attachment cross

section using N2 as the buffer gas

Swarm-unfolded total attachment cross

section using N2 and Ar as buffer gases

Energies where the total attachment rate

constant measured in N2 and Ar buffer gasesshows maxima

Electron beam measurement of the totalattachment cross section

Mass spectrometric study of dissociativeattachment using a trochoidal monochromator

Mass spectrometric study of dissociative

attachment using a trochoidal monochromator

2021 and 22232412

3

25

4

26

27-29

30

Ion cyclotron resonance study of dissociative attachment 31

Total electron scattering cross-sectionmeasurements

Estimates. of resonance energies detenninedfrom electron scattering experiments

Quantum mechanical calculation.

Vertical electron affinity values detennined

by electron transmission spectroscopy

Maxima in the total indirect inelastic electron

scattering cross section

32

33

33

34

Analysis byChristophorou el al. (Ref. I)based on the work of

Mann and Linder (Ref. 35)

8prom fig. 5 of Ref. 33.

tron beam studiesl7.1Son the temperature dependence of theproduction of CI- from CCI2F2. No other measurementshave been made at temperatures higher than ambient at en-ergies higher than thermal. However, there have been twomeasurementsl4.ISof the thermal value, (ka)th, of the totalelectron attachment rate constant of CCl2F2as a function ofgas temperature. These data are listed in Table n and arecompared with the present results in Fig. 5. While there is

considerable disagreement in the absolute value of (ka)th asm~asured by the various groups, the variation of (ka)th withtemperature is rather consistent. The strong effect of internalvibrational energy on the dissociative attachment of thermalelectrons to the CCl2F2molecule is manifested at even lowertemperatures than shown in Table n as recent measurementsof the (ka)th for supersonically cooled CCl2F2 in the tem-perature range from 48 to 170 K have shown.19

8308 J. Chern. Phys., Vol. 109, No. 19, 15 November 1998

-10

-9

-8

-7

-65".!. -5

CI)Z

w -4

-3

-2

-1

o

FIG. 6. Energy positions. ENIS' of the negative ion states of CCI2F2 below-10 eV as obtainedby variousmethods.ColumnI: - - lthreshold at-tachmenttechnique(Ref.24)];_ [potassium-atomcharge-exchangecollision technique (Ref. 20). calculation (Refs. 21 and 22); calculation(Ref. 23)]. Column 2: Electron swarm attachment techniques: - - -, Ref.25; Ref. 4; - - -; Ref. 12; _' Ref. 3. present. Column 3;Electron beam attachment techniques: - - -. Ref.26;_. Ref.27(peaks in the sum of all negative ions. see Table III); Ref. 30. Column 4:Electron scattering: Ref. 32; - - -, Ref. 33. Column 5: Electrontransmission: - - -. Ref. 34. Column 6: Calculation: - - -. Ref.33.Column 7: Indirect inelastic electron scattering: - - -. Ref.I. Column8:Possible negative ion states and their energies and assignments. The arrowsshown in columns I, 2, and 3 indicate that the energies of these resonancesas determined by the corresponding methods are lower than shown in thefigure.

IV. NEGATIVEION STATES OF CCI2F2

In an effort to identify the number of negative ion statesof the CCl2F2molecule below -10 eV and their energies, wesearched for information from various experimental and the-

C~F2 \ l ca2F2-'

r---~--'().geV

o--:~.t~ \JY :::::io.!8-evcF2a. a-:t ~_~ t

FIG. 7. Schematic potential-energy curves for CF2CI-CI and the lowestnegative ion state of CCl2F;* consistent with the (adiabatic) positive (+0.4eV) electron affinity of CCI2F2. the vertical electron affinity (-0.9 eV) ofthe lowest negative ion state of CCI2F2. and the observation that the disso-ciative electron attachment cross section rises steeply as the electron energy

decreases towards zero (Ref. I). The asymptotic limit CF2Cl+CI- lies 0.28eV below the 0.0 eV level taken to be at the u =0 level of the CCIF2-CI

symmetric stretch vibration J.l3. using a value of 3.33 eV for the CClF2-CIdissociation energy and a value of 3.61 eV for the electron affinity of the CIatom (see text).

Wang, Christophorou. and Verbrugge

oretical sources. Table III summarizes the infonnation foundin the literature from six different types of experimentalmethods: The heavy-atom collision technique,20 the Krphotoionization electron attachment technique,24 the swarmelectron attachment technique,3.4.12,25electron beam/massspectrometric techniques for electron attachment,26-30elec-tron scattering,31-33and electron transmission.34 Addition-ally, infonnation obtained from a number ofcalculations,21-23.33.34and the recent analysis by Christo-phorou et at.I is also listed in Table III. The data in Table IIIare compared in Fig. 6. The adiabatic electron affinity, E.A.,of CCl2F2has been detennined by Dispert and Lacmann20tobe 0.4 eV:to.3 eV using a potassium-atom beam to createCCI2F2"via electron transfer in binary potassium-CCl2F2collisions. A multiple-scattering Xa (MSXa) calculation21.22also gave a positive value for the E.A. of the CCl2F2 mol-ecule equal to 0.4 eV. Similarly, a more recent quantummechanical calculation23also gave a positive value for theadiabatic electron affinity of the CCl2F2 molecule equal to+0.67 eVe

In the last column of Fig. 6 are given the "average"positions of the negative ion states of CCl2F2 based on thedata given in the figure. Clearly, on the basis of the presentcomparison of all the various data, there exist at least fournegative states below the electronic excitation threshold ofthe CCl2F2 molecule whose average energy positions are:+0.4 and -0.9 eV (these two belong to the same state, seebelow), -2.5, - 3.5. and -6.2 eVe Another resonance at-8.9 eV lies in the region of electronic excitation and mostlikely is associated with excited electronic states. The addi-tional state indicated by some experiments at -0.25 eV isquestionable; its existence is shown by two electron swarmstudies and one electron beam study, but is absent from thecross section of another similar beam study. It may be asso-ciated with vibrationaBy excited molecules. The calculationof Underwood-Lemons et at.33 located a state at -4.9 eVbut most likely this should be associated with the - 3.5 eVresonance since no experimental evidence exists for a reso-nance at this energy from any other source and since thecalculation predicts four negative ion states which can berationalized with other studies and molecular orbital assign-ments.

In an attempt to rationalize the positions of the negativeion states of CCl2F2in the last column of Fig. 6 we note thefollowing: (1) The calculations of Tosse1l23(see also Ref. 33)have shown profound geometrical changes between CCl2F2and CCI2F2"_ (2) According to an ab initio SCF (self-consistent field) calculation on the neutral CCI2F2moleculeby Burrow et aL.,34four valence-type resonances are ex-pected below about 5 eV which can be ascribed, in increas-ing energetic order, to the unoccupied orbitals a I(C-Clo-*),b2(C-Clo-*), al(C-Fo-*), and bl(C-Fo-*). Burrow et at.h*e ascribed the resonances they detected in an electrontransmission experiment at 1.0, 2.5, and 4.0 eV to the lowestthree of these molecular orbitals. (3) Mann and Linde~5measured the energy-loss spectra of CCl2F2 for electronshaving initial energies equal to 1.0, 2.4, 4.0, and 6.0 eV, i.e.,roughly equal to the energy positions of the observed fournegative ion resonances of CCI2F2.Their results have shown

EJoc8oon S-... - EIocIoonEIocIoonC8aAa1ion-... -NW;y;-- -.. Sca1l8MgT,1ftImisIion ..- NISa8M"nnoIIoIcr eoc.on e,..... _'i .-.e

.------..------ b,IC.fo")...--.......... ------- -4.2

-------1------------- .------ .------..------

",IC.fo").-.............. ------- ..------- .------ oJ.5- ..------ b,(COo")..------ -Il.5 .------- .....----- ------- ',(COo")...........-......-.-----.. .-------.............- .().I

..._.......................... -1

---f--- --r- .().2$--... .0..

J. Chern. Phys., Vol. 109, No. 19, 15 November 1998

that for 1.0 eV, the most prominent energy loss is the exci-tation of the V3 vibration. This is consistent with the(C-Clu*) character of the a 1(C-Clu*) resonance at 1.0eV. Consistent with this assignment of the 1.0 eV resonanceare also the data on dissociative electron attachment around

this energy,29-31 both for the production of CI- involving the

breaking of the C-CI bond and for the production of CI2"involving the C-CI2 dissociation. Additionally, the study ofMann and Linder showed that in the a,(C-Fa*) resonanceat 4.0 eV, the excitation of v 1.6is the dominant process. Thisis consistent with the (C-Fu*) character of this resonance.The a 1(C-Fu*) resonance at 4.0 eV is also the appropriateprecursor of the group of the fragment negative ionsobserved29-31 at 3.5 eV (the position of the resonance appar-ently is shifted downward by about 0.5 eV in the dissociativeattachment channel in comparison to the scattering channeldue to the competition between dissociation and autoioniza-tion). The dominant fragment anion at this energy is F- inaccordance with the (C-Fu*) character of this resonance.Interestingly, the b2(C-Clu*) resonance at 2.5 eV does notdecay via dissociative attachment since none of the electronattachment studies has shown a peak at this energy. Pecu-liarly, this resonance shows up only in the vibrational exci-tation channel.35 (4) The data in Figs. 3 and 4 clearly showthat the CCI2F2 molecule attaches electrons with energiesless than I eV, and that below ~0.3 eV the attachment rate

constant increases with decreasing energy toward thermalenergies for all temperatures investigated.

In view of the findings mentioned above. it may be con-cluded that the potential-energy surface of the lowest nega-tive ion state of CCl2F2 has a minimum at about 0.4 eVbelow that of the neutral molecule, and that it rises steeply inthe Franck-Condon region to account for the vertical attach-ment energy at -0.9 eV. The potential-energy curve for thisstate is shown schematically in Fig. 7. This state, then, ac-counts for both the positive E. A. (+0.4 e V) and for the firstobserved vertical attachment energy at - 0.9 eV and is as-signed to a 1(C-Clu*). The electron attachment reactionsbelow about I eV predominantly lead to dissociation produc-ing CI-. Since the CF2CI-CI bond dissociation energy[3.3 eV:t0.2 eV (Ref. 20); 3.58 eV (Ref. 27); 3.1 eV (Ref.36)] is smaller than the electron affinity (3.61 eV, Ref. 37) ofthe CI atom, the reaction

CCI2F2+ e -+ CCIF2 + CI- ,

is exoergic. The energy position of this negative ion statewould make the dissociative attachment process highly tem-perature dependent i.e., the rate constant is expected to in-crease with increasing temperature, thus accounting for theobserved behavior. The higher negative ion states at - 2.5,-3.5, and -6.2 eV, are identified with the assignments ofBurrow eta/., namely, the b2(C-Cla*), a,(C-Fa*), andbl(C-Fu*) unoccupied orbitals, respectively.

V. CONCLUSIONS

The main conclusions of the present work are:(1) The rate constant for electron attachment to the

CCl2F2 molecule increases with increasing temperature from300 to 500 K over the entire mean-electron energy range

Wang, Christophorou, and Verbrugge 8309

from 0.04 to 1.0 eV investigated. The lower the value of themean-electron energy, the larger the temperature enhance-ment. At thermal energies the electron attachment rate con-stant shows an increase by a factor of 10 when the tempera-ture is raised from 300 to 500 K. The measurements alsoshow that the attachment rate constant increases below about0.3 eV as the mean-electron energy approaches its thermalvalue indicating that the potential-energy surface for the low-est CCI2F2negative ion state crosses that of the neutral mol-ecule near its minimum.

(2) The lowest negative ion states of CCl2F2have beenidentified with average positions as follows: a I(C-Clu*) at+0.4 eV and -0.9 eV, b2(C-Clu*) at -2.5 eV,al(C-Fu*) at -3.5 eV, and b1(C-Fu*) at -6.2 eV.

(3) In view of (1) and (2), it may be concluded also thatthe potential-energy surface of the lowest [a, (C-Cla*) ]negative ion state of CCl2F2has a minimum at about 0.4 eVbelow that of the neutral molecule, and that it rises steeply inthe Franck-Condon region to account for the vertical attach-ment energy at -0.9 eV. This state, then, accounts for boththe positive E. A. (+0.4 eV) and for the first observed ver-tical attachment energy at -0.9 eV.

(3)

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