Gas phase photodissociation spectrum of Cl−2

. Volume 48, number 2

CIlEhiICAL

GAS PHASE PHOTODlSSOCIATlON S.A. SULLIVAN,

B.S. FREISER

1 Jane 1977

PIIYSICS Lrx-rEKS

SPECTRUM OF Cl;

* and J.L. BEAUCIIAMP

Arthur Amos Noye,- Loboratory of Cltcrtttcaf Pitystcs. Caltforttta Ittstttutr of TccItaolo~y **. Pasadct:a, Caltjorttia 9112.5, USA

Rcccwcd

15 I cbruary 1977

Photoinduced rcxtions of Cl: have been cx,nnincd in tho gas ph.rsc using ion cyclotron wsonrlncc tcchniqucs. The molccu1Jr ion is obwrvcd LOundergo phorodissocl.rtlon in prcfcrcncc to photodctnchrncnt. The photoclis\ocidtion opcctrum CYhIbit\ one broad peak in the rc.~vcIcnglh rcclon 220-700 nm, wltb a m.lGnum hma, = 350 t 10 nm, which can be .ittritransition, Tlw, rpcctrum IS compared to nbsorptwn spectra of Cl; obtained in solid state cnvironbulcd t~thc~X+u-*~X;

incnts.

1. Introduction The optlcal spectrum of Cl, has been studred cxtensively in KC1 crystals [ 1,2], KC1 alkali borate glasses [3] and recently in argon matrlccs as alkali metal salts, hl+Cl~ [4]. Two absorption maxima are observed; one in tllc region it,,,, = 338-3G5 nm rtssigned to the 1x+ + “Xi tmusition, and a second less Intense peak, nm, which is asqned to the wit: XI,:, = 700-750 ‘z+ 11 --L211 transition. Calculations have been pcrforms cd to detcrrninc internrlclear distance, bond energy and transrtion cnergics in the isolated ion [5,6]. Available spectra cannot be compared directly to thcorctical calculations, however, smce absorptron maxima may be strongly affcctcd by cliaoging the counter 101-1 or crystal envlronmcnt. Thcsc effects cau be &tnbutcd to the char:gc in internculcar dlstancc when the Isolated Ion IS subjcctcd to a crystal or matrix er~vJronmcnt. Further it has been suggested [3,5 ] that valcncc bandy

of solids will depress excited lcvcls relative to the ground

state, causing pe,lk slufts to lower energy.

In

order to quantify poss~blc effects of surroundmgs on the absorptwn spectrum ol_CL~ and to provide a cornpiIriS

to

theoretical

cc~lculrftilOns, we have investi-

* Prcwnt address’ Wpxtmcnt ot Chemistry. Chemistry Bullding, Purdue University. West Lafclycttc. Indian.1 47907, USA. ** Contrlbutlon No. 5506.

294

geted the photoinduccd reactions of Cl? m the gas phase usmg ion cyclotron resonance spectroscopy. Ions arc formed at low pressures of neutral precursors and trapped for periods up to 2 s, during which time they may be irradiated. Abundances of reactant and possible productions may be observed as a function of wavclength of irradiation, lrradiatlon time and pressure. lntcractiori of light with Cl: may result in either photodetachmcnt, eq. (I), or photodissociation, ccl. (2). (1) + Cl + cl-

.

(2)

Thcsc processes can be drscussed with reference to the potential energy curves shown in fig. 1 for the ground states of Cl,(‘Z~) and Cl,(2Xz) and the excited states of Cl~<‘E~ and 2H&_I~l fig. I, r&IF) has been taken as 2.48 8, estnnnterl using Badger’s equatwn [7] and w,(Cl~) from Ar matrix studies [8l_ Photodetachment,

cq. (I), should be observed from Cl, (22z, v = 0) at a rninunurn cncrgy corrcspondmg to EA(C12) = 2.36 + 0.1 c\’ [9] or 525 nm. The cross section for process (1) depends on the threshold law for photodetachment 1IO] as well as the Franck-Condon fattors which describe the overlap of the u = 0 level of Clt(2Zi) with vibrational levels of the C12(iZ,‘) ground state. VIbrational excitation in the molecular ion will also have a pronounced effect on the probabil-

Volume 48, number 2

-

50

-

_- -_-_-

-r-

IGO

CHCBIICAL PIIYSICS Li:?-KRS

I

Cl + Cl

enera and duration. and trapped for up to 2 s. Typical pressures range from 5 X 1W7 to 2 X 1W6 torr. A Hanovia 2.5 kW mercury-xenon arc lamp IS used in conjunctlon with a 0.25 rn Bausch and Lomb mcmuchromator (200-700 nm) set at 13 nm resoWion in these experiments. A Spectra Physzcs model 170 argon ion laser was also employed between 454.5 nm and 5 L45 nm to corroborate the results obtained with the conventional hght source. The release of appreciable kinetic energy on dissociation of ions parallel or perpendicular to the magnetic field can be distinguished by differcntial trapping of the product ions as described by Dunbar [ 14]_ Therefore, it is possible to determine symmetries of the repulsive states of ions utilizing polarifed light. There are a number of sources of Cl: [I 51 in&d-

ing electron impact in COCI, and electron attachment to Cl,. Durmg an exammation of the reactions ofncg-

IO-

W.

00.

I June 1977

=

247cm-I

’ I 0

20

30

INTERNUCLEAR

40

DISTANCE

l’ls. 1. blorsc potcntl~l cncrgy ulrvcs for the CI2(‘Xi) nnd Cli(*\*+ -u). calcuhtcd us~n:: wc froul rcl. [22], .ind wc for Cl; frown ref. [8] cctimatcd a\ noted in the te\t. D,(Clz) = 2.5 D&I;) = 1.26 CV IS]. The rcpulwc ewitcd *I$ of Cl< arc cstinuted Iron1 the absorption given In ref. [ 11.

50

(ill

ground state of and fL’ for Cl1 with re(Cl;) 143 cV and strltcs * C; and

spectra of Cl:

ity of photodetachment. f3ecause of the large difference in bond length of Cl, and Cl,, photodetachment will occur to high vibrational levels of Cl2 with low probability. Photodnsociation. eq. (2), IS expected to result from excitation of Cl,(2Z~) into either of the repulsive excited states. The total photodestruction cross sections, obtained by monitoring the ciisappcar-

ante of Cl,, will represent the combined processes (1) and (2).

2. Experimental The ICR instrumentation and experimental techniques for studying photochcmlcal processes mvolving ions have been previously described [ 1 l- 131. Ions are produced during an electron beam pulse of variable

ativc ions with fluorinated oleflns [ 161, we observed the production of approxrmatcly equal abundances of Cl- and Cl, in CF,CFCI at 1 X 10d6 torr and 7.0 cV. Surprisingly, double resonance experiments indicate no precursor ions to Cl,. Variation of ion intensities of Cl- and Cl: as a function of electron encr_w indicate no reactive coupling of the two ions. Further the ratio of ion intensrtlcs of Cl- to Cl: observed in the ICR drift mode does not change as a function of increased CF,CFCI pressure. The inelastic excitation spectrum of N2 [ I71 in a mixture with CF,CFCI, indicates that Cl: is produced by zero energy clcctron attachment. These results suggest a trace constiluent in the sample which leads to the formation of CIT. [Towever, mass spectra1 analysis of the sample using 8 duPont 492B double focussmg mass spectrometer fdilcd to uncover any polychlorinatcd impurity @-I%) up to m/c 3000. In addrtion, the possibility of impurities being generated in the ion pump or on either the electron or Schulz - Phclp ion gauge fikments was eIiminat-

ed. Several chlorinated olctins and alkanes irlchdrrrg CF2CCl7, CC12CC12 and CCIZC~ 12 were exJmincd for production of Cl? at both 70 eV dnd low clectrorr energy. In only two cases, ch- and ~rans-CCII ECCIEI, were small amounts of Cl? observed with Cl,/CI= 0.25. The absence of Brg in the negative ion spectrum of CF,CFBr indicates that the mechanism of dihalide anion production is unique to CF+FCl. It is suspected that reactions on the metal surfaces within the anaIyzer 295

investigated by addition of SF,, which can be used to scavenge electrons [IS] _ The amount of SF, that could be added was limited, since it competes favorably for electrons, greatly decreasing the Cl, signal. It was concluded that a small fraction (<5%) of the Cl, reacts by photodetachment, eq. (1). The large geometry change between Cl, and Cl2 (fig. 1) is apparently responsible for the predominmce of photodissociation. ‘Ihe photodissociation of Cl, formed in a Penning discharge has prevrously been reported by Rackwrtz et al. [ 19]_ They report a photodissociation cross section which increases linearly with increasing photon energy, beginning well below the thermodynamic limit of 1.25 eV (1032 nm) [S] . This result indicates the presence of a significant amount of vibrationnlly hot Cl,. Recently Brauman [20] has observed the photodissociation of Cl, which is produced from either Cl, or , CO,CI, by electron impact. The maJor feature of this ’ spectrum is a brsad band, A,, = 360 nm, with a long, low cncrgy tail extending below 1.2 eV. The observation of formally endothermic Cl- transfers from Cl, to a variety of compounds confirms the presence of vrbrationally excited Cl,. Comparison of the relative

may result in production of small amounts of a species which has a large cross section for electron attachment, producmg the observed Cl,. The CF,CFCl used in these experiments was obtamed from PCR and used without further purification except for removal of noncondenstble impurities by several freeze-pump-thaw cycles.

3. Results and discussion The disappearance of Cl, was monitored as a function of irrzdiatmg wavelength (200-700 nm). The spectrum obtained (fig. 2) contains a broad peak, with an = 350 A 10 nm. Destruction onset =.SOO nm with A,, of Cl, is accompanied by an increase in intensity of Cl-, indrcating photodissocration of Cl:. The production of Cl- reproduces the wavelength dependence of the destruction of Cl, above z-340 nm, the threshold for photodetachment from Cl-. Delayed cjectlon of electrons from the analyzer did not affect the abundance of Cl,. Possible concomitant photodetachment of Cl? was

200

300

400

500

PHOTON

ENERGY

600

700

WAVELENGTH

I-Q 2. romparison c

of the rcldtive phctodl\socidtion

(B) Rackwitz et ai. [ 191, and (c) lhuman

296

I June 1977

CHEMICAL PHYSICS LETTERS

Volume 48, number 2

(eV)

800

900

1000

Inm)

cross sectlons of Cl, as J function of wavelength from (A) the prcscnt study, at 1.26 cV.

ct al. [20]. The arrow denotes .&(CI~

Volume

48, number

CIIEMICAL

2

PIIYSICS

tion of trapping voltage and light polarization, cithcr parailcl or perpendicular to the trapping ptace, the polariration of transitions may be determined. mle USCof these techniques in this case indicates that Clis produced only with light polarircd parallel to the Cl--Cl- bond, consistent with the polarimtion of the 2.9 + ’ TZi transition in Cl, absorption spectra [ II_ yncluded for comparison to the photodhsocration spectrum of Cl, in fig. 3 are absorption spcccra ofCI_, obtained in irradiated KC1 and in an .4r matrix as K+ClF with h,nak for the 2Xt + ‘Zz transition of 365 nm and 343 nm, rcspcctively. The weaker 22i + 211, transition has been observed in KC1 cry~= 750 run. A summary of the abtal spectra at A,, sorption maxima for Cl, observed in a variety of soIid state cnvironmcnts is given III tahlc I_ Although within cxpcrimental error the gas phxe ‘IX; --f “Eg peak can be considered cquivalcnt to cithcr of the s&d state spectra compared to it m fig. 3, the agreement of the irradiated KC1 spectrum may be fortuitous considermg the wide variation of A,,,, with counter ion. The discrepancy between the crystal spectra and the caIc&ted value of 321 nm [5] for the UV transition could he justified as the result of perturbation of the

photodissociation cross sections obtained in the different experiments (fig. 2) suggests that Cl, generated in the present study contains little or no excess vibrational energy. The Cl, produced from CF,CFCl was was found to be nonreactive with alcohols, acid chlorides, and halogenated alksnes and alkenes, also consistent with vibrationally cold Cl, _ No structure indicating the presence of the low energy 2 Z: + 211s transition was observed in the previous photodissociation spectra. Low light intensity prevcntcd examination of the region ahove 700 nm m this study. When Cl, photodlssociatcs, the repulsive energy released (hv - 1.25 eV) appears as kmctic energy of the fragments Cl and Cl-. Motions of the ions parallel to the magnetic field in the ICR spectrometer is constrained only be the apphcatlon of voltages to the trapping plates [ 11,131. Molecular ions oriented perpcndicular to the trapping plates (parallel to magnetic field) may produce Cl- possessing enough kinetic energy to escape the potential well. Smce ionic products of dissociation parallel to the trappmg plates will be unaffected by variation of trapping voltages, the orientation of dissociating molecules is distinguishable 1141. By observing product ion abundance as a funcPHOTON 50

40

30

ENERGY

25

_.__-

-

- -

K+ Cl,, -

KCI

3. Comparison

absorption

of the rclatlve photodissociation spectra of Cl; observed in an Ar-matrix

15

Ar

Xmox = 343

Motrl x

Crystal

Amax =

Photodlssouatlon

WAVELENGTH rig.

feV) I 75

20

-

1 June 1977

LEI-TLRS

nm

365nm A III(IL=

350

*IO

nm

(nm)

cross %-xtion of Cl; as B function .md in an irmdiatcd KCI crystal

of wavelength

from the present

study

tti the

297

Volume 48. number 2

Table 1 Abborptlon --_

CHEhfICAL

References and plmtodissocutmn -- --._ --

mnvima for Cl, __ 2x;

-----._ ---Irr.ukxtcd crystal

b

2,_+

-tI-

211g

[ 1,2.5,6] 393 378 365 364 374

RbCi NI14CI Ar-rnatrru [4]

Li+Cl; Na’CI;

338

K+CI;-

343 343

750 789 a700

1201 prcwnt study c.llculatcd [S]

. _~__.._

338 ~360 350 t 10 311 3.58

.-.---

663 696

Cl, bond length in the solid state. IIowevcr, as Gilbert and WnhI [S] point out. the change in A,,,,,

for the UV transition ns a function of counter ion is the reverse of that expected from calculation of the effect of the various alkali crystals on the bond length of Cl, [2 1] . Simple bond length changes do not explain the variation of A,,,,, in thcsc spectra. The recent calculation by Tnsker et al. [G] obtains A,,, = 358 run for the transition ‘Zz + 2XL, in good agreement with the gas phase spectrum.

Acknowledgement This work was supported Rcsearch

and Dcvclopment

Grant No. E(04-3)767-g.

298

(1958) 1235; C.J. Delbccq. W. llayncs and P.11. Yuster, Phys. Rev. 121 (1958) 1043. 121 W. Kanzig. Phys. Rev. 99 (I 955) 1890; T. Cactner and W. Kanzig. J. Phyn Chem. Soltdc 3 (1957) 178. I31 D.L. Grlscom. J. Chcm. Pbys. 5 1 (1969) 5 186. L. Andrcws. J. Am.Chcm. Sot. 98 (1976) 2147. 1:; T.L. Gdbcrt and A.C. Wahl. J. Chem. Phys. 55 (197 1) 5247.

bond, 3rd Ed. (Cornell Univ. Press. Ithaca, 1960) p. 231. I81 WI. iloward Jr. and L. Andrew, J. Am. Chcm. Sot. 95 (1973) 2056. I91 WA. Chupka. J. Bcrkowltz and D. Gutman. J. Chem. Phys. 55 (197 1) 2724. 1101 K.J. Reed, A-11. Zimmerman. ILC. Anderson and J.1. Bmuman, J. Cbem. Phyc. 64 (1976) 1368. 1111 J.L. Beaucbamp, Ann. Rev. Phys. Cbern. 22 (1971) 527. 1121 B S. I’reiscr and J.L. Beauchamp, J. Am. Chem. Sot. 96 (197a) 6260. [ 131 T-B. Whfahon and J.L. Beauchanp, Rev. Sci. Instr. 43 (1972) 509. [ 141 R-C. Dunbar and J.hf. Kramer, J. Cbem. Pbys. 58 (1973) 1266. [ 15 1 J-G. Dullard, Chcm. Rev. 73 (1973) 64 1. [ IG] S.A. Sulhvan and J.L. Beauchamp. subnutted for publication. [ 17 ] D.P. Ridge and J-1.. Bcauchnmp, J. Chcm. Phys. 5 1 (1969) 470. [ 181 R.N. Compton, R.11. IIucbncr, P.W. Rembardt and L.C. Chrl\tophorou, J. Cbcm. Pbys. 48 (1968) 901. [ 191 R. Rackwitz, D. l’cldmnnn. E. Ilcinicbc and H.J. Knlscr, Z. Naturforsch. 291 (1974) 1797. [20] ($1. AsubloJo, II. L. hlcPctcrs, W.N. Olmsic.rd .md J.I. Urdunidn, Chcm. Phys. Lcttcrb 48 (1977) 127. [ 2 1 1 A N. Jcttc,

in part by the Eneigy Administration

G G. Balmt-Kurt! and R N. D&on, Mol.

PIlys 32 (1976) 165 1. 171 L. P:~ulmg. Tbc nature of the chemical

344

[3 ]

111 C.J. Ilclbccq, B. Smnllcr and PM Yuster. Phys. Rev. 111

161 P.W. Txker,

345

Kb+CI; cs+a; alkali borate @FF (KCI) pborodssociation

--~-

-._2s:

_ ___ &&nIn) __.____hIn=(nm)

LlCl NaCl KC1

161

1 June 1977

PIIYSICS LETTERS

under

T.L. Cdbcri

.md T.P. Dns, I’hyc. Rev. 184

(1969) 884. [22] G. IIcrzberg, hlolcculdr Fpcctrn and molecular structure, Vol. 1. Spectra of dlatomic molcculcs (Van Nostrand. Princeton. 1950).