Reactive and inelastic processes involving I2(D 1Σ+ u ) with the collision partners CH4,CH3Cl, CF3Cl, and CF4Robert J. Donovan, Barry V. O’Grady, Luis Lain, and Constantine Fotakis Citation: The Journal of Chemical Physics 78, 3727 (1983); doi: 10.1063/1.445147 View online: http://dx.doi.org/10.1063/1.445147 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/78/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Comparison of experimental and computed vibrationally inelastic rate constants for Li2 A 1Σ+ u –Ne collisions J. Chem. Phys. 103, 860 (1995); 10.1063/1.470067 Trajectory studies of SN2 nucleophilic substitution. I. Dynamics of Cl−+CH3Cl reactive collisions J. Chem. Phys. 93, 7962 (1990); 10.1063/1.459326 Inelastic collisions of CaCl(X 2Σ+) with Ar: A collaborative theoretical and experimental study J. Chem. Phys. 83, 556 (1985); 10.1063/1.449521 Propensity rules in rotationally inelastic polar molecule collisions involving 2Σ+ molecules: CaCl(X 2Σ+)–CH3Cl J. Chem. Phys. 81, 3347 (1984); 10.1063/1.447999 Molecular beam measurements of inelastic cross sections for transitions between defined rotational states (j,m)of CsF in collisions with He, Ne, Ar, Kr, CH4, CF4, SF6, C2H6, N2, CO, CO2, N2O, CH3Cl, CH3Br, CF3H, andCF3Br J. Chem. Phys. 71, 1722 (1979); 10.1063/1.438524

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.2.176.242 On: Sun, 30 Nov 2014 16:58:47

Reactive and inelastic processes involving 12(0 1.1;:-) with the collision partners CH4, CH3CI, CF 3CI, and CF4

Robert J. Donovan, Barry V. O'Grady,a) Luis Lain,b) and Constantine Fotakisc)

Department of Chemistry. University of Edinburgh. West Mains Road. Edinburgh. EH9 3JJ Scotland. United Kingdom (Received 30 August 1982; accepted 16 September 1982)

Fluorescence from the D '.I.+ state of I" and from other ion-pair states populated as a result of inelastic collisions with the molecules CH., CH3CI, CF3CI, and CF., was studied throughout the 200-510 nm region. I,(D '.I.+) was optically pumped using an ArF laser (A. = 193 nm) and the above spectral region recorded with single laser shots by employing an optical multichannel analyzer. The physical quenching of I,(D '.I.+) was found to involve an electronic cascade, through intermediate states. terminating in the lowest ion-pair state 1,(3n ,g), The presence of reactive channels was identified by the reduction in I, fluorescence following successive laser shots. Fluorescence from I,(D '.I.+) was also studied following excitation with a cw mercury lamp (A. = 185 nm) and Stern-Volmer analysis of these data yielded total removal rate constants (reactive + inelastic channels), viZ; k(CH.) = (7.9±0.3)X 10- 10

, k(CH3CI) = (21.0± 1.0) X 10-10,

k(CF3CI) = (7.1 ±O.4)X 10- 10, k(CF.) = (6.0± 1.0) X 10-'0 (all in em3 molecule-' s-' units). The presence of

reactive channels was confirmed using end-product analysis techniques and it was shown that the lowest ion­pair state 1,(, n ,g) also reacts with CH. and possibly with CF 3CI.

I. INTRODUCTION

The collisional quenching of the low lying (E"" 20 000 cm-1) electronically excited states of the halogens, which are of valence character, 1 has been studied in considerable detail. Particular emphasis has been given2 to the BSnO: state of ~ which is readily populated from the ground state with visible radiation.

In recent work the higher lying states of 12, in the region of - 45 000 cm-1, have also received attention. S-10

These states have a high degree of ion-pair character1

and their collisional behavior may thus be expected to differ Significantly from that of the lower valence states. We have recently shown that 12 (D 1~:) is quenched with essentially unit collision effiCiency by all of the noble gases8 and that the quenching process involves a cas­cade through other close lying ion-pair states. This cascade ends in what is presumed to be the lowest ion­pair state 12(sn~) and provides the basis for the opera­tion of an efficient optically pumped 12 laser, 7 which emits at 340 nm.

Willard and co-workers5 have shown that excitation of 12(D 1~:) at 185 nm leads to efficient reaction with CH4 and CH3CI, with quantum yields of 0.3 and 0.4, respec­ti vely. The high efficiencies for quenching of 12(D 1~:), noted above, suggest that competition between in­elastic and reactive channels could account for the quan­tum yields of less than unity.

In the work presented here we examine the collisional behavior of ~(D 1~:) with the molecules CH4, CH3CI,

alpermanent address: Department of Chemistry, University of Tasmania, Box 252C, G.P.O .• Hobart, Tasmania 7001, Australia.

b)Permanent address: Department of Chemical Physics, Uni­versity of Bilbao, Apartado 644, Bilbao, Spain.

·)Permanent address: Department of PhYSics, University of Crete, Iraklion, P.O. Box 470, Crete, Greece.

CF sCI, and CF4, A preliminary report of work on CHsCI has been given elsewhere. 8

II. EXPERIMENTAL

Two experimental arrangements were used for the work presented here. In the first, the unfocused output of an ArF laser (Lambda Physik EMG201), emitting at 193 nm (~X "" O. 5 nm) and capable of delivering 500 mJ in 14 ns, was passed through a fluorescence cell, simi­lar to that described in previous work. s Fluorescence from 12(D 1~u) and from other states populated as a re­sult of inelastic processes was viewed at 90° to the laser beam by means of an EG and G optical multichan­nel analyzer (OMA2 with silicon intensified vidicon) fitted to a Jobin-Yvon 0.3 m monochromator. Spectra were recorded over the range 200-510 nm, at a resolu­tion of - 0.6 nm, following single laser shots. Mixtures containing a fixed initial partial pressure of ~(40 N m -2) and varying partial pressures of methane or halomethane (P = 0-10 kN m-2) were introduced into the fluorescence cell. In general, each mixture was subjected to only a single laser shot but some mixtures were exposed to several shots, with spectra being recorded after each shot, so that the extent of reaction (i. e., removal of 12) could be monitored.

In the second experimental arrangement a radio fre­quency low pressure mercury lamp (Thermal Syndicate) was used as a continuous wave excitation source. Flu­orescence was observed at right angles to the excitation source using a conventional arrangement. A Monospek 1000 monochromator, fitted with a holographic grating having 1200 lines mm-1 on a ruled area 102 x 102 mm, was used to resolve the emission from excited 12 and spectra were recorded over the range of 310-350 nm. With this particular arrangement, the fluorescence cell contained solid 12 to maintain a constant vapor pressure of 12 and to replace the small amounts removed by re­action.

J. Chem. Phys. 78(6). Part 11.15 March 1983 0021-9606/83/063727-05$2.10 © 1983 American I nstitute of Physics 3727

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.2.176.242 On: Sun, 30 Nov 2014 16:58:47

3728 Donovan, O'Grady, Lain, and Fotakis: Reactive and inelastic processes

(d)

(e)

(b)

(a)

200 300

-In both experimental arrangements the fluorescence

cells were attached to a greaseless vacuum line for the introduction of known pressures of gases.

The course of the reaction of excited Iz with CH4 was also monitored using a Perkin Elmer 554 spectropho­tometer (in conjunction with an arrangement employing the cw mercury lamp) by observing the formation of CHsI via its 201 nm Rydberg band5 and the loss of Iz via its visible absorption system. In these experiments a fixed initial pressure of Iz(40 Nm-z) was added to the cell while the pressure of the quenching gases was var­ied. End product analysis for CF sCI, CH4, and CHsCl/ ~ mixtures (excited at 185 nm) were performed by gas chromatography using a 3 m silicon oil column and a flame ionization detector.

Spectroscopic grade CH4 (BOC Special Gases) was used directly. CH3CI (BDH), CF3CI (Matheson Gas Products), CF4 (BOC), and 12 (BDH) were thoroughly degassed before use.

III. RESULTS

Observations of the emission spectrum from pure Ia and from mixtures of Ia with methane and halomethanes were made over the range 200-510 nm. The charac­teristic fluorescence from pure Iz was readily observed as described in some detail previously. 3,4 With in­creasing pressure of foreign gas the intensity of the fluo­rescence from the D 1~: state (200-330 and 350-430 nm) decreased as a result of inelastic and reactive colli­sions and new emission systems appeared (see Fig. 1). The most prominent feature in these new emission sys­tems was the band at 340 nm which arises from the slI2I state and has been assigned to the transition s1I2I - 3IIau.

Of the quenching gases studied CF4 was expected to show the simplest behavior as previous work by Willard and co-workers5 had demonstrated that the quantum yield for reaction is very low (<I> = 2 x 10-3). Our results

, 500

FIG. 1. Fluorescence from 12, fol­lowing a single ArF laser shot, In the presence of CF4, The quenching of the D 12:: state (200-330 and 350-430 nm) and the formation of the 3n2£'

state (340 nm) is clearly seen. The formation of intermediate states (450-490 nm, see the text) is also shown. Note that the 340 nm band extends to shorter wavelengths, under the bands between 300-330 nm. Conditions PI2 = 27 N m-2; P CF

4 = (a)

0.67 kNm-2, (b) 1.3 kNm-2, (c) 2.3 kNm-2, (d) 3.6 kNm-2. Spectra have not been corrected for the response of the detection system.

using ArF laser excitation clearly show that physical quenching of Iz(D 1~:) is the dominant channel and that a rapid electronic cascade into the slI2I state takes place (see Fig. 1). Evidence for the population of states in­termediate between the D 1~: and slI2I states can be seen from the weak emission between 450-490 nm. The in­tenSity of emission from these intermediate states at first increases with increaSing pressure of CF4 and then declines for P'> 4 kN m -z, as they are collisionally transferred into the slI2I state.

Quenching of the slIa, state by CF4 appears to be neg­ligible as the intensity of the 340 nm band continues to increase up to the highest pressures studied (80 kN m-2

).

The only other emission which appears over the pres­sure range studied is a structureless band at 510 nm which is also associated with the slIa, state. 4

The colliSional behavior of CF sCI with Iz(D 1~:) was found to be very Similar to that observed for CF4, and efficient cascading into the slIa, state was again ob­served. The presence of a reactive channel was es­tablished by the identification, using gas chromatogra­phy, of CFsI in the end products. The quantum yield for this reaction has not been established but observa­tions on the yield of I2(sII2I) suggest that physical quench­ing dominates the removal of ~(D 1~:) by CF sCI. It would be difficult to quantitatively assess the importance of the processes leading to Ia('II2I), as this state is it­self quenched by CF sCI (unlike CF4), Thus, the inten­sity of the fluorescence at 340 nm was found to pass through a broad maximum with increasing pressure of CFsCI at PCFSCl'" 13 kNm-z. This observation raises the interesting possibility that it is the slIa, state which reacts with CF sCI rather than, or in addition to, the D 1~: state. (This point will be discussed at further length below. )

Both CH4 and CHsCl were found to induce some physi­cal relaxation of Iz(D 1~:), following ArF laser excita-

J. Chern. Phys., Vol. 78, Part II, No.6, 15 March 1983

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.2.176.242 On: Sun, 30 Nov 2014 16:58:47

Donovan, O'Grady, Lain, and Fotakis: Reactive and inelastic processes 3729

TABLE I. Rate of CHsl formation from CH,/12 mixtures in the presence of SFs.

A. P CH , = 1.4 kNm-2 , P 12 =24 Nm-2 B. P CH,=0.25kNm-2, P 12 =24Nm-2

SFs pressure d[CHsI]/dt SFs pressure d[CHsI]/dt (kNm-2) (moleculecm-3 S-I x 10-12) (kN m-2) (molecule cm-3 S-1 x 10-12)

0 7.3 0 1.1 7.2 1.0 6.9 4.6 2.9

18.1 3.9 29.0 35.9 3.8 63.0 57.5 3.1 88.7 2.9

tion, however, the presence of reactive channels was also clearly evidenced by a reduction in fluorescence following successive laser shots (shown for CH~ in Fig. 2: see Ref. 6 for similar spectra obtained with CHsCI). The presence of reactive channels, confirming earlier

(C)

(b)

(8)

200 300 400 A/nm -

500

FIG. 2. Fluorescence from ~ following excitation at 193 nm (ArF laser) in the presence of CH,. The upper trace of each pair shows the fluorescence from the first laser shot while the lower trace shows the fluorescence after several laser shots. The quenching of the D I~ state (200-330 and 350-430 nm) and the formation and subsequent quenching of the 3n21 state can be seen with increasing pressure of CH, (a)-(c). Conditions: P I2 = 27 Nm-2; PCH4 = (a) 293 Nm-2 (lower trace 104 shots), (b) 4.4 kNm-2 (lower trace 46 shots), (c) 11.6 kNm-2 (lower trace 33 shots). Spectra have not been corrected for the response of the detection system.

3.6 2.5 1.8 1.2 1.2

work by Willard et al. 5 was also demonstrated using gas chromatographic analysis of end products; CHsI was ob­served from CH~ and both CHsI and CH21CI from CHsCl.

The emission from 12(,nz,) at 340 nm was found to pass through a maximum with increasing pressures of both CH, and CHsCI, showing that this state is in turn quenched by these gases. As noted above, this raises the possibility that the reactions with CH~ and CHsCI, reported by Willard et al. and attributed to 12(D 1~:), could involve the snz' state. To test this point experi­ments were carried out in which increasing pressures of SF 6 (knownB to effiCiently populate the I2(Snar) state) were added to CHJI::! mixtures and the formation of CHsI was monitored by spectrophotometry at 201 nm. The results are presented in Table I. At pressures of SF 8

> 60 kN m-2 it woulcibe expectedB that only a very small fraction of 12(Dl~:) would remain, however, the rate of CHsI formation has only been reduced to apprOXimately 40% of its original value suggesting that some other state produced in the quenching of la(D l:E:) by SF 8, can aiso react with CH4• The most likely candidate, based on our present knowledge of this system, is the snar state.

Rate constants for the total removal (i. e., reactive + inelastic channels) of I2(D l:E:) , by all four collision partners, were obtained by Stern-Volmer analysis of the intenSity of fluorescence at 320 nm (excitation at 185 nm with the cw Hg lamp) as a function of colliSion gas pressure (see Fig. 3). Absolute rate constants were derived using a radiative lifetime ll of 15.5 ns for the D l:E: state and these are listed in Table II.

IV. DISCUSSION

A simple mechanism which accounts for the above observations can be written as follows

TABLE II. Rate constants for the deac­tivation of ~(D I~:). a

Collision partner

CF, CFsCI CHsCI CH,

kDx 1010

(cm3 molecule-I S-I)

6.0 ±1.0 7.1±0.4

21. 0 ± 1. 0 7.9±0.3

"-Errors correspond to two standard deviations.

J. Chern. Phys., Vol. 78, Part II, No.6, 15 March 1983 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.2.176.242 On: Sun, 30 Nov 2014 16:58:47

3730 Donovan, O'Grady, Lain, and Fotakis: Reactive and inelastic processes

CH,CI

ro

5·0

CH, •

FIG. 3. Stern-Volmer plot of fluorescence from 12(D Il:!) at 320 nm, (lo/I), against pressure of added gas: & CHaCl, • CH4, • CF4, Data for CFaCl have not been included as they lie in the same region as those for CF4 and would obscure the plot.

Excitation Iz + hv - Iz(D 1:E:) la,

Fluorescence la(D 1:E:) - Iz + hv(320 and 420 nm) k1'

Quenching Iz(D1:E:)+M-:rt+M kz,

Reaction Iz(D 1:E:) + M - products ka,

Fluorescence 1;-lz+hv(470 nm) k4'

Quenching It + M - Iz(31121') + M k5'

Fluorescence Iz(sII2I') - Ia + hv(340 nm) ks ,

Reaction Izellag) + M - products k7 ,

Quenching Izellag) + M - Ia + M ks .

(I; represents all of the intermediate states involved in the cascade process. )

The quenching rate constants measured from Stern­Volmer plots of the fluorescence at 320 nm (Table II) are equal to the sum of the physical and chemical quenching rate constants kz + ks in the above mechanism. The values of (ka + ks) observed for CF4, CHh and CF 3CI are approximately equal and are an order of magnitude larger than those observed for the monatomic gases. s The value for CH3CI is even larger. We would expect the values of ka to be similar for the four gases studied so that the large total rate constant (ka + k3) for CHsCl most probably reflects a larger value for k3 •

As has already been noted, the reactivity of CF4 with Iz(D 1:E:) is low and physical quenching is the major chan­nel for the removal of this state. The presence of inter­mediate states It is clearly shown from the increase in emission at 450-490 nm in Fig. 1. Although the states associated with this emission have not been assigned

we have observed them previously (at higher intensity) following the quenching of Ia(D 1:E:) by Ar. s Tellinghuisen et al. 9 have suggested that the emission originates from three states which lie close to the E 1:E: state. Emission from the E state may also be present in our spectra but would be obscured by the strong band from Ia(D 1:E:) at 430 nm. From the low intensity of these emissions and the time scale for the formation of Izell2l') we may con­clude that (i) the efficiency of quenching of the states I; is very high, or (ii) there is a direct quenching channel available from Iz(D 1:E:) to Izellag), or (iii) there are ad­ditional intermediate "dark" states. If there is a direct quenching channel then it can be shown that

(/S40 = intensity of 340 nm band) .

A plot of 1/1340 VS 1/ M for CF4 is linear over the pres­sure range 0-1 kNm-z, however, the value of kz calcu­lated from the slope is approximately 30 times smaller than that given in Table II (see also Ref. 10). This re­sult strongly suggests that direct quenching of Iz(D 1:E:) is not a dominant pathway for the formation of Iz(SIl2l') and that intermediate dark states are unimportant unless they lose energy by an efficient nonradiative process. Although no absolute quantum balance has been done for CF 4 we would expect little energy loss through these processes in view of the observed effiCiency of the Ia laser7 (with SFs as quenching gas). We therefore con­clude that Iz(D 1:E:) is effiCiently quenched by CF4 to in­termediate states which are in turn effiCiently quenched to Iz(3I1z,,). Our observations also suggest that similar electronic cascade processes occur with CF 3Cl, CH3Cl, and CH4, although other competing processes take place simultaneously with these molecules.

Whereas with CF4 no quenching of the 31121' state was observed at the highest pressures used, the spectra with CF 3CI, CHh and CHsCl as quenching gases showed that the intensity of the 340 nm band passed through a maximum. It is unlikely that these gases would be more efficient in their physical quenching of Izell2l') than CF4 so that the decrease in 340 nm emission must be caused by the loss of IzellZg) and/or It by reaction. The addi­tion of a large excess of SF 6, an efficient quencher of Iz(D 1:E:), to CHJlz mixtures reduced the rate of CHsI formation by - 60%. This is approximately the change expected if Iz(D 1L:) and Iz(31121') react with CH, with a Similar rate constant, the decrease in rate being then determined by the difference in the radiative lifetimes of the two states (15. 5 and 6.7 ns, respectively). 11,12

Reactive channels are clearly important with CH4 and CHsCl, however, the dynamics of these reactions re­main obscure. Willard et al. 5 have suggested a molecu­lar pathway for the formation of the monoiodides, e. g., for CH4,

CH4 + Iz(D 1:E:) - CH4I; - CH31 + HI .

On the other hand, a sensitized decomposition of the parent molecule was invoked to explain di-iodide forma­tion in the reaction of Iz(D 1L:) with CHsF and CF sH

J. Chern. Phys., Vol. 78, Patt II, No.6, 15 March 1983

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.2.176.242 On: Sun, 30 Nov 2014 16:58:47

Donovan, O'Grady, Lain, and Fotakis: Reactive and inelastic processes 3731

CHsF + Ia(D 1~:\) - CHsF* + Ia ,

CHsF* - CHz + HF ,

CHa + 12 - CHal2 •

Both of these mechanisms were based on results ob­tained by end-product analysis. The present work serves to emphasize that more than one electronic state undergoes reaction with methane and thus conclusions based on end-product analysis can be very misleading. Further work to establish the detailed mechanism of the reactive channels is clearly required.

DEDICATION

This paper is dedicated to the memory of W. H. Fly­gare who through his example did so much to stimulate work in the fields of spectroscopy and collisional energy transfer.

ACKNOWLEDGMENTS

We thank Dr. A. Gibson and Dr. M. C. Gower, and the S. E. R. C. for providing laser facilities at the Ruth­erford-Appleton Laboratory. We also thank the Mini­sterio de Universidades (Spain) for providing a fellow-

ship for L. L. and the British Council for travel support for C.F.

l R • S. Mulliken, J. Chem. Phys. 55, 288 (1971). 2R. B. Kurzel, J. I. Steinfeld, D. A. Hatzenbuhler, and G. E.

Leroi, J. Chem. Phys. 55, 4822 (1971), and references therein.

3M. Martin, C. Fotakis, R. J. Donovan, and M. J. Shaw, Nuovo Cimento B 63, 300 (1981).

'H. Hemmati and G. J. Collins, Chem. Phys. Lett. 75, 488 (1980).

5L. C. Glasgow and J. E. Willard, J. Phys. Chem. 77, 1585 (1973), and references therein.

6B. V. O'Grady, L. Lain, M. C. Gower, and R. J. Donovan, Chem. Phys. Lett. 91, 491 (1982).

7M. J. Shaw, C. B. Edwards, F. O'Neil, C. Fotakis, andR. J. Donovan, Appl. Phys. Lett. 37, 346 (1980).

8L • Lain, C. Fotakis, andR. J. Donovan (to be published). 9 A. L. Guy, K. S. Viswanathan, A. Sur, and J. Tellingbuisen,

Chem. Phys. Lett. 73, 582 (1980). lOA. B. Callear,and M. P. Metcalfe, Chem. Phys. Lett. 43,

197 (1976). IIA. B. Callear, P. Erman, and J. Kurepa, Chem. Phys.

Lett. 44, 599 (1976). 12M. C. Sauer, W. A. Mulac, R. Cooper, and F. Grieser, J.

Chem. Phys. 64, 4587 (1976).

J. Chern. Phys., Vol. 78, Part II, No.6, 15 March 1983

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.2.176.242 On: Sun, 30 Nov 2014 16:58:47