Search for the laser-induced crossed beam reaction of excited I2 (B 3Π) with Hg

Chem~wl Phyztcs 79 (1983) 341-350 North-Holland Pubhshtng Cornpan>

331

SEARCH FOR THE LASER-INDUCED OF EXCITED I, (B “ll) WITH Hg -c M-M.

OPRYSKO

*. F.J.

Dcparmenr 01 Chemrsr~,

AOIZ

CROSSED

*_ R B. BERNSTEIN

Coiutnbra Urmersrr~ New 1 orA .Vm

BEAM REACTION

5 AND Yo0r.L1002 7

MA.

McXlXH?,N

=

L S -1

Recened 25 March 1983

A search for the laser-tnducrd crossed brnm reactmn 1: + Hg - HgI+ I hat hcen condwted at tot.tl cnsrgtss G 1 s\ (translatlonal enrrgtes G 2 5 eV) The 1, molecules are prepared tn the B qll,,) state xtlthtn the wattermg rsgton \,a 513 5 nm >lsld of Hgl m c~ccss of that Ar+ laser excttatton The fractton of 11 molecules tn the I, beam was = 4x 10e3 So sgtflcant from the ground-state reactton was detected o\er the en& range of scattenng angles The expenmsnt blslds an upper hnut for the ratto of the cross sectton for the exctted-state reacuon relattxe to that of the ground state ewmatsd 10 be < 0 5 at a total (rebound) scatterrng for the cxcltcd-state reactmn. thts upper bound IS reduced to energy of 3.9 eV Assummg “backHard 6004

1. Introduction A number of molecular-beam studies =ith laser-excited reagents have been fruttful. They have elucidated the role of vibrational energy (the HFT -k Sr reaction) [I], of rotattonal energy (the HCl( J) + K reaction) [2] and of electronic excttation (reaction of Ca* with HCl) [3] m influencing the reaction dynamics [4] Experiments wtth laser-excited I, have been numerous, partly because of the convemence of electronically exciting Iz with a cw argon-ion laser [5] and because much is bnown about its spectros-

copy [6.7]. Balykin et al. [S] have used the laser excitation I,

technique

by the reaction

to separate

of selecwely

ortho-

from para-

excited 1: wtth

by NSF Grants CHE-Sl-163S6 and CHE-7711384 Present address Gould Laboratories, Rollutg &leddo\\s IL Supported

60008 USA Fulbrtght Fellow of the Mumtry of Unwxsltm and Research of Spam Present address Departamcnto de Qutmtca Ftstca, Utwrrstdad Complutense, Madnd-3. Spat-t Present address- Occidental Research Corp. Irvine. CL\ 92713. USA Present address. Lawrence Ltvermore Laboratop Ltvrrmore. CA 94550. USA

0301-0104/83/0000-0000/$03.00

0 North-Holland

hexene. Similar studies haxe been carried out with ICI [9]_ These experiments hale been camed out under bulk-gas condttions. Curtously. lo\\-density e\perrments on laser-e\reactions ha\e apparently ctted I,. e.g. beam-gas been unsuccessful [ 10.111. These include studies on If +Tl and If -t- in. with I2 the “target gas” [ 12.131. Beam-beam expertments. typically 1~Ith a thermal Iz beam crossed at 90° by the laser beam_ have sho\\n no significant ewdsnce of laser-induced reaction [ 13.141. The present \~oork\\ds mtended to e~amme the I under crossed molecureaction 1: +-HH~+HSIT lar-beam conditions. and compare it with the prewously studied ground-state I2 reaction [15-171 The Hz-l, system \\ds espectally suttable because of its endoergcity (1.15 eV) [17]. The ground-state reaction had been found to proceed via translational ewitation at essentially the thermodynamic threshold of 1.15 eV_ This mahes it possrble to set the colhsion cnersy Just belo\\ the reaction threshold and then irradiate the I, beam \\ith d laser. Any HgI product detected sgnchronousl> \~ith the laser radiation could then be attributed to the 2.41 eV-excited 1; reagent. The center-of-mass angular distributions and relatne reaction cross section for the ground-state

342

M M

Oprrvl, o el a l /

The h t v e r - m d u c e d trox~ed b e a m reattton !*, + l t q --* l l q l + i

reacuon are known from threshold to a collision energy near 4 0 eV [15-17], making possible a comparison of the cross section for the electronically exctted reaction with that for the translat~onaily excited one Antlclpatmg what follows, the present search for the laser-induced reaction has failed to detect statistically significant product (Hgl) signal. F r o m the data It is possible to put an upper hmit on the laser-indut.ed reaction cross section relative to that for the ground state. This is done by assuming various reasonable forms for the center-of-mass (c m.) angular distribution for the excited-state reaction and by taking advantage of prex IOUSx~ork which makes possible the evaluation of the fraction of the 12 molecules in the beam which are excited by the laser [18] For total energies E-to a >_-3 91 eV. it is possible to form Hgl in the electronically excited B state. Its lifetime is a few nanoseconds, with the transition terminating m an u n b o u n d state [19.20] It is therefore conceivable that at high energies the laser-reduced reacuon forms Hgl* which subsequently dissocmtes - a process which goes undetected b~r the present mass spectrometric detection system (described m section 2) This has been Investigated by the fluorescence detection techtuque" a photomultipher tube was placed above the scattering center and the 440 nm emission from HgI* detected synchronously with the laser chopper. The results were negative, howexer, no statisucally sigmficant 440 nm fluorescence signal was observed In what follows the experimental configuration is described, the data reported and the analysis of the results presented winch provides upper bounds on the laser-incuded reaction cross section.

2. Experimental procedure T h e essentml features of the crossed lasermolecular-beam apparatus have been described [15.18]. The first investigations [15-17] were crossed molecular-beam studies of the H g + I~ (ground-state) reaction, the second [18] studied the process of laser excitation of a molecular beam T h e integration of these techniques is described below.

Fig. 1 is a pictorial representation of the essential components of the experiment Fig 2 is a schematic view of the apparatus, consistmg essentially of three separately p u m p e d chambers housed within a large scattering chamber: two source chambers (one for the supersonic beam, the other for the thermal-effusive beam), and the rotatable U H V mass-falter detector chamber. The dinaensions of the apparatus are listed in ref. [15]. only exceptions are noted hereafter. A seeded supersonic beana of 12 is crossed with a therinal beam of Hg under single-collision conditions. The 12 is electromcally excited by a cx~ argon-ion laser (Spectra Physics model 171) crossm g the beana at an angle of 135 ° within the scattering region (0.157 m downstream from the I2 nozzle exit). The typical multimode laser output power at 514.5 nm is 3 W. The resulting 12 excitation is monitored by two means, fluorescence and laser-induced 12 beana loss [18]. The fluorescence is measured by a photomultipher located above the scattering center. Using the method outlined in ref_ [18]. it is possible to determine the fraction of 12 molecules excited by the laser from a combination of the fluorescence data, the 12 beana velocity distributtons as a function of carrier gas pressure and the laser-reduced I 2 beam losses as determined with the T O F mass filter. (12 molecules excited at 514.5 n m not only fluoresce but also suffer direct photodissocmtlon and predissociatlon. Both effects are manifested as a laser-induced loss of 12 from the beam ) Reactive product, HgI, due either to the groundor excited-state reaction ~s monitored with the rotatable, separately p u m p e d (UHV) high-resolution mass-filter detector. This detector can be operated independently whale monitoring the 12 beam fluorescence or beam losses. The experimental procedure for the laser-reduced experiment is different than that for the ground-state experiment. Here the HgI product is detected by gating the detector in phase with the chopped (50 Hz) laser beam. This makes possible the detection of laserinduced product in the presence of product from the ground-state reacuon at translational energies above the threshold for the latter. This capability proves to be important in setting an upper limit on

Fig 1 P~tor~sl rcpresencatlon of the appxatus The supercomc beam I< formed m .I c~p~lltn noulc (XZ) +.mm~ud (Sh) colhms~ed \e\cral tunes (C, C,) cnopped. flagged agam collmxusd (C-b and aacd at 90’ b> A hedm cffuwy from J rhermxl oxen through a mulnchannel array The supersonic beam densIt> and TOF xcioaty dxsrnbucwn I‘ medared \tIth .m rkrron m~p~ct quAdrupoic msic f111er located dxrectly opposite the beam It consists of a colhmdtor (C,). lomzrr (I) and qu.idrupolr rod Js~crnbl? The ourpur I, collected on a Farada> cup (FC) In-plane reactne ScJtIrred product 1s dcrecrsd \\lrh rhs rcv.aabls quJdrupolc rnJ.\ fdrcr (colhmnror C, lomax I) m 11s UHV chamber The mass-selected ions xe ~ollscled on a Ch.mnslrron ehxlron mulr~phcr (CEM) Ths lsssr beam (irr ) IS brought mto (and out of) the mnm ~xuum chnmbcr through a scne.\ of Iassr baffles (LB1 Ths Iacr mwrsccls the supsrsomc 1: fluors~csncr 1~momtorcd \\rlhA phacomultlpher tube hedm dt an angle of 135”. 11 IS chopped for s>nchronous detsct~n purp~s. (PMT) housed m a telescope assembly. located dtrsctl> abo\s thr scatlcrmr cemer perpendxu1.w 10 rhe plaw of the barn<

the reactton cross sectton for the laser-induced reaction Typically. for the ground-state reactton only 5 min of data collectton at a given scattering angle are required to obtam stattsttcally signtftcant results at m/i = 327 (HgI’) [15]. For the laser-induced reactton. the product detection involved counting periods at each angle up to 120 min In addition to the seeded I, beam conftguration descrtbed above. the Hg as the

expenments supersonic

were performed sourct and I,

laser-mduced If fluorescence u’\‘~sclearl? visible to the naked qe_ For both confrgurattons d set of mirrors \vas placed around the scattermg center. m such a uay that the laser beam could be reflected bath through the scattering center up to nine times making is posstble to further ewtte the Iz beam. The monitormg of I, fluorescence was forfeited \xhen using the mirror system because of “laser scattei’: how-

wrth

e\ er.

as

x\eere still posstbk

a

thermal beam. In this conftguratton. by further ratstng the Iz oven temperature it was possible to obtain a jet of I3 and study the laser-excited 15 reaction under “beam-jet” conditions. Here the

laser-induced

I2

beam

loss

measurements

Fmally, modtftcations in the PhlT detection system were made to make posstble the observatton of emission from any Hgi* electrontcally ewtted B state. This

formed tn the state ts at 3.91

344

THERMAL

BEAM

.y SOURCE

REF.

IN

1 AMPLIFIER RECORDER

AMPLIFIER

1 WAVEFORM EOIJCTOR

4 AMPLIFIER

PULSE COUNTING

DIGITAL SYNCH COMPUTER

PULSE HEIGHT ANALYZER

Ftg

1

Plan \leu of the apparzxus

and flo\x sheet of the signal procrssmg

rlectromcs

The components

of fig

mnm ~acuurn chamber. contammg three separateI> pumped chambers the supersomc beam source. thermal UHV rotatnble mass-filter detector chamber. Hatchrng denotes hquld mtrogen cr)oshlrlds The TOF bclm operated m the analog mode. the rotatable mass filter (for scattered signal detectIon) m the pulse-countmg detectIon

IS a\aktble

for use with enher

thr chopped

laser or molecular

1 are assembled

beam

source

m the

snd

the

momtor mass fdtrr 1s mode Phase sensmxe

beams

eV and is therefore energettcally accesstble wtth 1.5 eV of colhston energy (m addition to the 2 41 eV of electromc energy of the If). A set of collectron optics was added to the PMT housing whrch

enough

focused a volume of = 100 mm3 of the scattermg region. The fluorescence background is reduced by using a Schott interference filter centered at 440 nm (wluch transmrts c low4 of the 514.5 nm

ton IS added EToT remains below 4 eV. This 1s done because the ground-state relative reactron cross section is known only up to that energy [15] The upper limit on the product counting rate.

scattered hght) The PMT is operated in the pulse-countmg mode with the product synchronously detected by chopping the Hg beam. Fluorescence due to the 1: becomes a small and nearly constant background which does not interfere with HgI* detection_

II*,, for the laser-induced reaction was 1.5 s- ’ for the conditions as spectfted in table 1. Thus only an

3. Results

Two basic beam geometries have been used: supersonic iodine-thermal mercury and thermal

iodine-supersonic mercury. One set of experimental conditions for each beam geometry is hsted in table 1. In each case. the collision energy is low so that when

the energy

of the laser pho-

upper limit on the laser-induced product yield could be deduced. Thus limit was established by consrdering what counting rate would have been necessary for laser-mduced product to be observed. Counting was carried out for the longest possible time (= 120 min) before expenmental drifts and fluctuations outpaced any gams made m reducing the statistical counting error. The results

I 2 thermal bean

Hg supersomc

beam

Iaqer h’

r~,,B(kn~ s- ’ )

L ,z(km \- ’ )

E, 0, cc\ )

#, (S”)(\_‘)“’

err on

16 16

0 I6 0 16

I 52 3.93

500 -z 2 6

I IS 3 24”

IlOD <17

I 2 supersomc lJ!w

*’

off on

beam Hg thermalbeam n,,,(hm

c-l)

024 0 24

ul_(km s-‘) I.4 I 1

;I’ 11 m umts of qnchronousl> detected counts of Hgl + pLr srcond h’ Ldser off Hgl’ detected qnchronousl~ \\1111chopped molecular bean1 Lawr on heam “ Total colhslon energy before laser photon energ IS Jdded IC 0 53 cV

_

I 71

cl7

H_el- dc~rctrd \~nchronou.l\

GROUND

are tabulated in table 1 under the symbol $ (@*). where O* denotes the laboratory angle (mdicated in table 1).

4. Analysis

of the results

The goal of this section is to determme the corresponding limit for the laser-induced reaction cross section. The method for determinmg the relutice reactton cross section from one energy to another usmg an electron-Impact (number-densit!) mass-filter detector has been described previousl>

__280

t\\llh &>pped

STATE

ETOT=l

Irlxr

REACTION

5 e”

vizq~ ,’

-_

’

O”

-13’

[151-

The relatwe reaction cross sectton. uK. can be expressed uR = F/Kn,n,c,.where F is the total product flux (not number denstty). tz, and 11~ are reactant number denstttes. C, the average relatt\e veloctty. and K a constant contammg geometnc and purely instrumental factors. In most of the reported work on If reactions. the total yield of reaction product has been detected by chemtluminescence, where the intensity is proportional to number density [12.14] If the reaction occurs near threshold so that the product velocity is close to that of the centroid. q,,, one can approximate the flux F by nq,, with )I the product number density. In the present work. however. this approwmntton is not applicable because of the large recotl energy available to the products (see fig 3) and. of

LASER INDUCED REACTION E TOT= 3 9 eV

Fig 3 Sommal \c\xron wt.mglcs for the rextmn Hg- I. Hgl - l The upper Klangle 1s for I Colhslon JI J roial ener& of I.5 CV Ths zmgls of the cenrrold 1s R The Hz1 pr conhned berxxsen 25 and - 13 degrees x\Irh rc3pe~1 to the dlrectmn of the Hg beJm The lo\xcr uxmgls .ho\xs the effect of .iddmg 2 II e\ IO the s>swm 1l.a Iacr SWIIXKI~ of I2 Note the much larger mawmum-rccod reloans sphere The producr 1s confmed het\\e?n 79 and -65 degrees The mnsr dashed c&s represems the mm~mum allo\\ JDI~ product recad ~SlCx-11: (>ee refs [ 15 161)

course, because the total product yield was not determmed Thus the procedure used was more comphcated. namely the method outlined prevtously [15] The m-plane number density, detected at dn angle IS given [21]P Z@ II(O) = Kn,n, du,duz ~P,(~,)P,(o,) /J0 0 X

00

J(0

~‘/,l”)aK(E,,)h(lt’)g(B)

n(O),

do’.

(0.0’) + (8.~ ‘) IS one-to-one Thus to determine ~(0). WI eq. (I) one must consider the specific values of 8 dnd IV‘ generated for each v’ and use them to evaluate h( IV’) and g( 19). Assuming that uR( ET) is essentially constant over the spread in E,, around the average collision energy E,,. It can be wrltten: (2)

N here J’@‘=/o~/,

Xdo,dvlv,P,(c,)P2(~~) X

d/d2)h(d)g(0)

Table 2 Calculdted relarne angular dlslrrbunon J(8)

dv’

u’/w”)h(,,‘)g(8)du’

reaction

Here IJ, and u1 xe the respecttve reactant veloctties (speeds). E,, the relative translatlonal energy. L;’ .md W’ the product speed m the lab and m the c m.. h( H ‘) and g(0) the normahzed cm. recoil veloctty and angular distributions. respectively. Aithough the mapping (0.1% ‘) -+ (0. u’) IS not umque.

= II(O)/KJI,UJ(O).

/(om

is evaluated by usmg the values of 6 and I$’ which result from the range of velocittes u’ (0 to a~) for the particular lab angle 0 being consldered. Denotmg with asterisks quantities related to the

(1)

CJ( E,,)

The integral

(3)

&‘. correspondmg

of IT. the ratio of the laser-induced

(4)

For the standard confIguratIon (1: supersonic. thermal beams) II~~/IZ& = 1. The ratio K/K*

assumed c m anaular dlstnbuuon

Off

COll+X

0”

backuard pnor

The ratio of cross secttons [eq. (4)] does not require a knowledge. vta integratton of product flux m- and out-of-plane. of the entire product flux but simply the measurement of product number density at only a single lab angle (Of course this procedure will not be as accurate as a de-

to expenments listed m table

J(5”)

94 075 26

1 J(40”)

0 18 1.7 27

I 2 supersomc beam Hg thermal beam laser

assumed c m angular dlstnbutron

J(87O)

off

complex

140

on

bachuard pnor

a) 8 for laser off, 8‘

for laser on See text for derails

Hg has

been simplified to AV/AV*. the ratto of the scattering volumes for the ground-state experiment to the laser-excited experiment. (The other terms making up K/K* such as the mass-filter detectlon efflclency are assumed the same for both.)

I, thermal beam. Hg supersome beam laser

reac-

tion cross section to that of the ground-state reaction can be expressed in terms of product signals at arbitrary angles. say 0 and O*. and speclflc E,, values-

I8 23

J(92 5”) 28 24

termmatlon carried out wth a fluu detector where an entire in- and out-of-plane integration is used

for the ground-state

PI

action puted

) Thts

method,

however.

demands

knowledge

of

the J(0) Integrals. The mtegratlon over the beam velocity distributions is based on measured beam velocity dlstributlons The cm. functions g( 0) and h(w’) are known for the ground-state reaction from threshold ( 1.15 eV) to = 4 eV [ 151 For the laser-induced reaction these functions xe unknown so some assumptions as to their shape must

used

to cJculate

redctlon J*(O*)

at high energies

for the laser-induced

were re-

dt the same E,,,, _ Each J(0) was comtahing mto account various apparatus fx-

tors such as the fmlte size of the detector

aperture

(37

The quantity 11(O) is the HgI’ counting rdte at 0. Since no slgmflcant laser-mduced HgI- signal was observed. tP(O*) IS t&en to be the upper limit of laser-induced product. Table 1 lists a number of the expermlentnlly observed tr( 0) and

be made. Two types of cm. angular dlstrtbuttons have been assumed. The first is a “prloi distrlbu-

,I*(@*) for the angles

tlon

the Hg atom to the Iz bond [24]. It is therefore presumed that the reaction would proceed through a direct rebound mechanism [The ground-state

of J(0) and J*(O*). In general the J mtegrals are much smaller for the laser-ewlted reactlon. The velocity vector diagrams of fig 3 show that the recoil velocity sphere for the excited-state reaction IS much larger than that for the ground-state reaction. ‘4 detector ‘tt any_ 2 wen angle ~111 therefore sample a much smaller sohd angle of the laser-induced reaction. Thus. gIlen the same cm dlfferen-

Hg + I,

tial cross sectlon.

[23].

g(6)=

scattering-’

constant.

the

second

a ” bacb-

shape.

The latter form is plausible based on conslderations which suggested that the laser-induced reaction would be favored

reactlon

by a colhnear

exhibited

such

approach

behawor

of

at high

( ETOT > 3 0 eV) ] Thus.

the same g( 6) energies (cf. fig. 8 and eq. (9) of ref. [15]) and h( 1~‘) used

Table

for

2 presents

mdmted. the results

the lab sIgna

the laser-induced

order of (r/c*)’

of the calculations

reactlon

should

be smaller

by a factor

This contributes

of

the

to the difficulty

Table 3

Upper hmlt esumates on the laser-Inducedreacnon cross secuon 0; I 2 thermalbeam HS suprrsomc beam AG AY’

n*(40°) II

(400)

bachm 3rd

0;;(393)

J(40’=) ~ J-(40”)

3jX10--A’

0 11

<20x10-’

s sx

206

lo-’

c’

3jx10-‘~’

0 066

pr,0r

--

/

s sx

lo-’

0;1(391)

0,(3%)

E 1:
< 12

<77 b’

r’

OK(l 52)

<3

I

d’

g52 <77
1 2 supersomc beam. Hg thermal beam n’(92

So)

,I (870)

AV Al”

bachward

50 61 5x1o-3

pnor

J(S7O) J-(92

172

59

So)

i 1 jxlO-“’

45-t

3Sxlo-‘*’

<’

1 jx

lo-ad’

Q 9s

:sx

lo-‘+’

< 39

60 -l

11

<00-t


11) Here/Is the fracllon of I, molecules excned dssummg d smgle Idscr p.13~(bJscd on rsf [ lS]) h, Fracnon of I1 molecules cxclted ulth multIpass gstrm estmlduon based on ref [lS] and beam lo\w~ (we reu) ‘) Comparison of laser-induced IO ground-stare rexnon cross sscnons at su,ne rrunrlur~orrulsnug\ s,, Sumbcrh m pxemhrses enerples m eV.

xe

In obserwng remion product. is the fraction of I, rnoleThe rdrio f= I$/u,_ CIJ~S. excited by the hr. a function of the I2 bwrn vrloc~ty. speed ratlo. and laser power. The full description of the procedure for determiningf IS outlined in ref [IS] Table 3 lists the f calcuI,lted for the thermal I, beam and the supersomc I, be,mi for a single laser pass and multiple laser passcq (= 9). In the case of a single laser pass, the overall percent of 1, molecuies exerted IS calcuI~cd 10 be 0.035% for the thermal beam and 0 15% for [he supersonic beam. The estimation off for J multipass configuration is based upon the observed increase in Iz beam loss. a factor of = 2 5 for the multipass system used *. Thus the o\orall percent of 1, molecules excited in the mult~pass system IS 0.088% for the thermal beam and 0.388 for the supersonic beam. The separate terms contributmg to eq (4) are gwen in table 3. Listed first is an estimate of the upper hmit on the laser-induced cross section at the same translational energy as the ground-state reactton. To compare the cross sections at the use IS made of the known same total energy. E,,,. ewitauon funcuon for the ground-state reaction. Fig. 12 of ref. [15] shows that ran (at 1.52 eV) = uK (at 3 93 eV) and that uK (1.18 eV)= 1.25 X lo-’ aK (3.24 eV) From table 3. the upper lmm on the ratio of the reaction cross sections for the laser-induced reaction rrlatwe lo the ground-state reactlon at E,,, = 3 24 eV 1s 0.04 for the assumed backward c m. angular distribution and 0 5 for the pnor dlstribunon. Thus If the laser-induced reaction IS Indeed of the rebound type. its cross section is < 4% of that for the ground-state reaction at the same total energy of 3.24 eV.

5 Discussion Despite unfavorable factors such as the relatl\ely small fraction (j3.8 X 10e3) of 1, mole* C~lculat:ons of ref [IS] tttdtcate that an mcreasc of a factor of Z 5 m the fracuon of I, molecules exctted was obtamed uwtg the multtpass laser qstem. eren though rune nommal Passes were used

cules excited by the laser, the small solid angle of laser-Induced reaction sampled by the detector. and the small binding energy of the product (0.39 eV) **. the present results show that the cross section for the laser-induced reaction is smaller than that of the ground-state reaction. Since the cross section for the latter is believed to be moderately large (e g. S-10 A’), this does not preclude the possibility of a laser-induced reaction cross section of several A2. The chemiluminescence channel: Hg + 1; --, Hgl* + I 1s open at ETOT > 3.91 eV, but appears to be unimportant [25] ‘. Finally no information on the cross section for the reaction of wbrationally excited I2 (I;) can be mferred from the present experiments because of the small 1; number density m the scattering volume =_ To date no sigmflcant product yields have been observed for I; reactions in crossed molecularbeam expenments Rettner et al. [13] set an upper lmut of 2 5 2 for the In -I- 1: reaction cross section The corresponding I, ground-state reactlon cross section is = 100 2 [26]. Kahler et al. [14] have also confirmed that there 1s no significant reactton due to If with FL_ Beam-gas experiments

** HgI molecules uhtch are tntttall> formed “hot’ can fall apart Ptthm a \tbrahonal penod Indeed the formatton of a complex at rhese htgh c m energtes is ruled out because the rotational energes assoctated wtth the orbttal angutar momentum of the colhsion udl be so htgh that no slgmficant well m the potenual energ) surface ~111 remam to form a bound complex (see ref [161) However. a -‘superewtted complex once formed, may yteld a stable HgI producr by colhstons wth a thtrd body Influence of buffer gases on the reacuon rate can elucidate thts effect Expenmcnts 10 detect Hgl’ chemtlumtnescence jtelded negatwe results Indtrect mass spectrometric detectIon was not posstble due to the htgh background levels of I‘ and Hg’ Studtes ha\e shown that the branching fracuon for the formation of the chemthtminescent product 1s near unity for the Hg(3P-.)+X2 (where X = F, Cl. Br, I) reactton, see ref I251 The present expenmental condmons are not tdeal for studymg the role of wbrattonally exctted I, The long 11 radtattte hfettme together mtth the relatnely fast I,beam veloctttes result m a poor overlap of If molecules =tth the Hg beam (I e very few 1; molecules m the scattenng volume) In addttion, 1; molecules also decay ~a spontaneous prechssoctatton so that the number densny of 1: molecules IS even less than the number density of If molecules

with I, at pressures

-z 10e3 Torr

ha\e also shown

no stgmftcant product yreld. Zare and co-workers [IO-131 have concluded this for the reactions of 1: with In. Tl. and Fz_ In contrast. bulk-gas expertments (reactant pressures of several Torr) have been successful. The study of the redctton of 1: with 2-hexene IS .m approprtate example [S]. Wrth 40 mTorr of Iz and 2 Torr of 2-hexene reaction WAS detected after lo-12 mm of Arf laser trradtatton (at 514.5 nm). No mechanism was proposed for the laser-mduced reactton The mitial reactton rate was sho\\n to be lmrar wth laser po\\er_ independent of I, pressure. and dependent on the square root of the 2-heuene pressure_ These fmdmgs together ~tth the small reactton rate suggest that the reation does not proceed via an elementary bimolecular collision process and that the initial 11 reaction step has a small cross section. The related redcttons. ICI* + halogenated olefms also appear not to be bimolecular [9] Common to all expertments involving 1; is the posstbhty of colhston-enhanced predtssociation Kurzel and Steinfeld [27] have found that the quenching of 1: fluorescence is direct11 proportional to buffer gas concentration_ Such a process would lead to higher-order bulk kinetics and 101~er rates (if the rate of the initial step depends on the 1; concentratton). The reason for the success of “bulk” ments and fatlure of “beam” experrments been ascertained. The laser-induced cross may simply be too small for detection wa

experthas not sectrons present

molecular-beam

may

techniques

or

reactton

6. Concluding

renmrbs.

Nn stgmftcnnt HpI product from the laser-mduced reacttonHg + 1; - HgI + 1 has been obarr\ed. The study has been hampered by several unfavorable condttions. the small fraction of I, molecules e\crtrd in the beam (only 0.3ScC). the large recoil energy a\atl~ble to the product (rssulting m the samplmg of d small 4oltd angle of the recoil \rloctt> sphere). And the small bmdmg merg> of HgI (whtch can 1eJd to subctantrdI collision-Induced dtbsoct.ttton 1~hen high colhston energies are mvohed for both the laser-mduced and translattonall> exctted reacttons). Xc\ rrthcless. tt has been estimated that u;F. < 0.5 Us_ These results add to the gro\\ing hterature ELIdence [Xl 1.131 for lath of observation of ewitsdstate I2 reactions undsr stngls-colhsion condrtronx. Hov. ever. bulk-gas e\pcrmients v. rth long laser trradtatton periods hur e bsrn successful m demonThe present results taken m strating 1: reactions the hght of these past s~psrunsnts suggest that multiple colhs~ons ma! pla? an important rols in the reactions of I;. Addrtronal bulh-gas phabc kinetic studrrs \\oouldbe useful so that the clcmcntar! reactron stsps. thctr rdtc constants. gas effects can be drrermmsd

and buffer

Achnowledgement The authors appreciate the mrportant carlb conrrtbutions of Dr. Shigeo H+aashi to this XX&..

re-

quire multrple or sequential collusions (U hether for collisronal deactivatton of imttally formed -_hot” complexes or for the propagation of a chain mechamsm). Further bulk-gas phase kinetic studies of the reactions of 15 \\oould be important Elucidatton of the elementary reaction mechanisms including buffer gas effects which allow medsurcment of the bimolecular 14 rate constant \\ould provide the necessary background for future molecular-beam 1~ork

References

[I] X Gupu

D S Psrr\ .md R 1 2~s. J Chcm Ph\. 72 (19RO) 6150 [I] H H D~\pcrr. 5111 Gel> and P R Broo)l\. J Chsm Ph\s 70(1980)5;li 131 CT Rs~~nsr 2nd RX_ Zsre J Chsm Ph\s ‘7 (1952) 1316

[5] S Ezch~sl and R 1\ cl>< Ph\ > Ret Lc~rsrs '0 (196s) [S] R S Xluihhen J Chum Ph+ 55 (1971) 2SS 171 J C Lehnxmn Conrcmp Ph>* 1Y (197s) 449

9I

[X] V I B.d~Lm V S LetoLho~. V I Mlshm and VA Semchlshen Chrm Phys 17 (1976) 111 191 D D-S LIU S D.III.I .md RN 2.1re. J Am Chem Sot 97 (1975) 2257 S Dat1.1 Ph D Theus Department of Chemistry Colurnbu Umerrq Net\ Yorh (1978) ] IO] F Engelke. J C Whitehead and R N Z.we. Dlscuwon\ F.rradq Sot 62 ( 1977) 222 [I I] R C Euler. D Lubman .md R N Z.~re Dwxwons Fxad.ly sot 67 (1977) 317. 1121 KC E\~lcr.md RN Zae J 4m Chcm Sot 100 (1978) 1323 [ 121CT Rrttner. L Worse and R N Zarr Chem Ph\s 5b (1981) 371 [ 141 CC K.thIer .md Y T Lee J Chem Phys 73 (19hO) 5122 IlSJ M hl Op~sko F J 401r hl A Xlchlahan and R B Bernstem. J. Chsm Ph\c 78 (1983) 3X16 ] lh] r Xl hl.t\er. B E Wdcomb .md R B Bunstem J Chem Phjs 67(1977)3507 1171 B E \\‘tlcontb T hl hl,t)rr. R B Bermtem and R W Belle\ Jr J Am Chcm Sot 9S (1976) 4676

[ 181 F J AOIZ. M M Opryho and R B Bernswm. Chcm Phys 79(19R3)321 [ 191 R W Waynant and J G Eden Appl Phys Letters 33 (1976) 70g [20] K Wleland. Z Elektrochem 64 (1960) 762 1211 TT Warnoch .md R B Bernstem J Chem Ph\c 49 (196s) 1678 [22] .F J AOIZ. J-l Herrero and A G. Urena. Chem Phys 59 (1981) 61 [?3] MB Farst R D Lexme and R B Bermtan. J Chrm Phjs 66(1977)511 1241 TM Mqer, JT hiuckerman. BE. Wdcomb and R B Bernstem J Chem Phys 67 (1977) 3522 125.1 T.D. Dredrng and D.W. Setser. J Chem Phxs R6 (19x2) 2276 1261 A Gedeon S Edelstem and P Daxldo\lts J Chem Phys 55 (1971) 5171. [27] R B Kutzel and J-1 Stetnfeid J Chem Phys 53 (1970) 3293