Parity dependence in rotationally inelastic collisions of CaF(A2II, υ′ = 0) with He and Ar

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K&IC)

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fLF2 heated in a mod&d Broida-&c own. G excited by a cw d5e.k into kc1 of the +fn. ckctronic sap% The fluoresan CT siglzzl is zm&5cc.~~

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The .ratio of the popuiation of sa~&te 1cveIs~popuIaLedby colkcio~k uith & or Hc_Lo rhc popu@dn pf_Lh& initially pkped level is de&d from.inL&iLy ~kusurements at cuykg buffer gas prcsures (O-25-2 Torr). CrossXCL~OLLL~~RZ Lhcocx~raaed_ The upcrimcn~~&sults confirm ~hi rheoreLi&ly predicLedpropensiLy.Lousrd conseti~rio~ of Lht e/f paLiLy cross sefxi~ns’qrees nrl! l&d_ In the case of CaF+k~coUisionr ~hhe hiCal a&i f-1 stale dwdentc of the crp&i&~d arilh the zippropriaiate sudden limit scaling &Lion Lcs satisfactory agreemcn~ is f&nd_\\-hen Hc.is !he ~~~i:aing pa17n~:

spa~rornc~cr.

-. _-

ing the pressure ~dependence-of s&ellite I& fluorescence intensities in the- present% qf cw ‘laser excitation of an indi\.idual- A-doublrt le&l in- the There have been rehively~- few experimental excited state. An earl& qualitative observation of studies of rotationally inelastic collisions induced by collisions betueen @atomic mol$cules in a ‘II e/f conservation in-CaF \&repoti&d by Bern-at? and. Field [2] in an- op&&optica? double rqoelectronic state and a buffer -gas [l-9]. Several earlier~studies have involved hydrid& excited with nance study. This w&k is continuing 191and will a pulsed laser [5,6]. For &~olecules with. a &onprovide results complement&y to those reported . gested spectrum a C@z-mode ti dye laser can be here. -. _- . . used to populate indi6idual roiational levels zllowIn -addition to. exploring the validity bf’ th6 ing the study of energy transfer even bet&en the pre&cted e/f propensity rule.- we shall- uie -our lowest rotati&al levels of the e&it&. stat*. This is .. experimental cross sections to examine the appli-of special inipo&nce in view of recent theor&caI z cability of the well-known sudden scaling relations interest [lO-133 in rotationally inelastic collisionG : 1141, which were extended by Alez:an~er_ .[l?] to, of ‘II molecules. in pa@tl~_~~wen~in+6dual cbllisions of inolecules in %I elect~oniti_st+sI -I -. -. ‘>:%_. 1 1 &d?ublet Iekels: Alexander shoyedi$?] that the ~. -~. : spectroscopice/flabeIwillte~dto~econserved_a :-~..:-. 1.: .. ‘- ..~ ---. ‘-,:. -_ 1.. _. :: ‘- prop&sityruIealrea~yobservedjn:s~veralexpe+eZE_xpe~entd. :_me+l+t+s [173,$,8] ._ 1:_...._._ _..; : -. ..~ ~,-: ._ : .- _ . .] :. _~ __ .: _.:_ In. @s paper we ?&dy. indi~~duaI,state-to-state-~ :- I:.-. ~~&iified B&da-type oven [15] has b+_@d,_. to obtain.directly. ga++ CaF by’sublimatiori,of 5 ~&s&ztions in the 4% siat&;of-CaF. by observ1.

Introduction

-

0301_~104/85/~~3.30 8 .&ev&

Worth-Holland Physics +b@kuz

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. impurities cannot be negLected and will be discussed later_ The chopped (900 Hz) focused laser beam enters in the oven verticaIly (OX axis) poIatized Iinearly~&aIIeI to the horizontal Or axis (f&g_ 1). The fhtorescence isspectmIly resolved with a double-pass 15 m spectrometer (Jobin-Yvon THR, dispersion 13 A/mm) in the direction of the 0~ axis and- detected with a cooIed photomuItiRIier (Hamamatsu R928)_ The holographic grating of the spectrometer acts as a pohuizing mirror which reduces the intensity of the light pokuized aIonS the vertical 0.x axis to a few percent of incident light_ In order to detect a signal directiy proportional to the natural population of each rotational Ievei, a half-wave retardation plate has been used in a parallel beam of incident fhtorescent Ii&t_ The axis of this plate from the verticaI axis_ In these is adjusted at 37O _ conditions the intensity of the detected signd is proportional to IB+ 21, and therefore to the popuIation [17] of the rotational level_ In order to identify the CaF Iines, the chopped (360 Hz) Iight of a thorium hollow cathode is sent simuitaneously with the CaF fluorescence through the oven onto the entrance slit of the spectrometer_ The two signals are then recorded on a two channel recorder through two lock-in amplifiers. Since the excited and observed lines fall in a narrow spectra1 region (5 cm-‘), no correction is

--solid C&F2, heated ia a carbon crucible by a resistant tungsten wire i_nsulated with zirconium cloth_ A singIe-mode cw dye Iaser (Coherent 69921) running with rhodamine 6G pumped by a Coherent CR4 azon-ion laser is employed for excitation of individual rotational lines. A slow continuous flow of Ar or He is added. The pressure is measured with a capacitance manometer (Datametrics 600)_ The fluorescence sipal appears just above the crucibIe when the temperature, measured with an optical pyrometer_ is in the range 1300-1400 K. At this temperature the vapor pressure of CaF, is equal to 5 x 10-’ Torr 1161.Since the nascent CaF molecules wiI1 be rapidiy transhtionally cooled by coliisions with the buffer gas. we assume a room temperature (I= 300 K) relative transIationaI eneqglg distribution for the CaF + Ar (He) collision partners. Two chemical reactions are possible for the dissociation of CaF2: CaF2 --, CaF + F or CaF2 Ca i- F?_ with endothermicities of 247 and 370 kcaI/moIe_ respectiveIy [16]_ Thus the first reaction is more likely to occur than the second one_ The parthI pressure in the absence of buffer gas is = 3 x IO-’ Tot-r. corresponding to a. molecular density of 3 X 10” moIecuIes/cm’_ Since the CaFT and_ consequently. CaF pressures are Iess than 1OT1 Torr #16]_ this observed pressure is due to residua1 permanent gases arising from leaks and outzgrssinS_Collisions between CaF(’ Ii ) and these

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*S @kition oi CaF mjth Ar $zssure of 1.5 T&kc level of the *I& upper state The foIIo&g _,..Mtions

Iaser is tuned tb the Ptxt( J” = 2SjIine attd are illustrated. in the energs Ieve dia~nm: 1.

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to*take

into

account

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the

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spxtrai

3_ Ohse~ation The A ‘fi state of CaF. excited in the A’II-X ‘\‘- transition (pm = 16529.67 cm-*) is well described by Hun&s case (a} [lS]_ In both upper and Iowr states each rotational Ieve is spiit into w-0 fine structure levels Iabelled e and f in spectroscopic notation (19]_ In a doublet electronic state_ for a rovibronic Ievel with rotational quantum number J_ the Ie\-eIs with overail parity ( - I )‘- t,“-‘are designated e and those with overall parity ( - I)f - 1.1:are designated f [19]_ The e and f lab&_ which should not be confused with the + and - or_sraIl parities_ are sometimes termed the rotation-sscluded parity index f19]_ The symmetry properties of the e and f labelled A doubkts have been discussed in detail by Alexander and Dagdigian [ZOl. A ‘n-‘E transition displays six branches for each spin-orbit component (fig 2)Three of these invoice e-labelied levels.of the ‘II upper state. the three others invoke f-labelled Ie~eIs of the same state_ In the CaF spectrum_ at Iow J_ the resolution of the spectrometer is not sufficient to resolve lines which are separ~ed by the smaII spin-rotation spiitting of the ‘X- ground state (-f = 13 X lo-” cm- ’ [X1)_ In excitation spectroscopy esperiments ~:akagrnvtl et al_ [ZlI could not resolve this splitting for f x-alues lower thzn 27-S. So only four hunches are observed: two “single branches-- and two -double branches”_ In the A’II,.,2-X ‘\‘(A’Il:=_.2 - X ‘X&) transition of CaF. ;he laser is tuned &-a rotational Iine (J” + 1) of the “singleL P,2n (P,,) branch in order to excite the level I’ of the upper state_ The corresponding Ruorescence lines are R&J” + 1) and Qsr,(J“) (R,,, and Q-.<,) which ori$nate from the same i’ Ievei_ These lines belong to the same -double brnnch” because they are not xsolved zrs explained above_ They are named Gpnrent linez? since they arise from the Ievei initialiy populated by the Iaser line_ Three kinds of collision-induced transitions can

be observed be&en levels of a_ particular spin-orbit Fnifoldr (i) Transitions which are inelastic in J iAJ_# 0, but which conserve the e/f label (f 4-f or e ---, e transitions). These give rise to satellite @es which belong to the same branches as the parent lines The intensity of these satellite lines in general decreases as the distance (a J) from the parent Iines increases (indicaied g the transition labelled 2 in fig 2). (ii) Coliisions which -are inelastic in the e/f label but which are elastic in ;I( AJ = O)_ They are ‘observed in P,, and Q,,, overlapped branches in the ‘I’It,2-2E* transition or in Pz,- and Qz,r branches in the ‘II Se- %+ transition (transition 3 in fig 2). (iii) Tmnsfers which are simultaneously inehtstic in J and the e/f label_ They are also observed in Pr (5, ) and Qiz (Q2) branches (transition 4 in fig. 2). Relative intensities of all these lines can be calculated from the appropriate H&xl-London factors [22]. Thus it is not necessary to measure the intensity of a11lines arising from each level to deduce the relative populations of two levels_ This property is veq useful as no intensity measurement has to be performed on the laser-pumped rotational line_ Consequently difficulties associated with scattered laser light are completely eliminated from the lines studiedAll measurements were done on the O-O vibrational band. on I’= 1/2-7/Z levels in the ‘ll,,z manifold and J’ = 3/2 aad S/2 levels in the zl13,2 manifold_ In both spin-orbit manifolds we studied only f levels_ Using the same method it should be possibie to populate e levels by tuning the laser to the R,,(‘II, ,2) or R2rcr(‘IIsr) branches of the O-O band_ Unfortunately, the first Iines_of these branches are overlapped by an intense head of the 1-l band [31I and the spectrometer is not able to resolve the numerous Iines induced by collision in this narrow region_ The tom1 pressure was increased from 025 to 2 Tot-r in steps of 0.25 Torr. At each pressure three scans were recorded and average&to%nimize~the noise and laser fiuctuations. Attempts made. to stud; &&on-induced knfer between the two spin-orbit manifolds

~-

c. ~Dujkir

.-

eiii/

.-. ;

,- :

__~‘-_...._..

is very W&k in spite bf ._ -where a, is the theniialcross’&%ction’for the_i’T-,S- _-:. thZ%tthiSD& -.I- transition (ok G k!&), 5 is the relaiive _G+F+: ’ the fact that the thermal energy
I = NvJqi tf, c”) S(J’;J”) 4, Results It is known that linearly polarized laser light does not equally populate all magnetic sublevels M corresponding to a given J level [17]_ Similarly. detection of the fluorescence by a polarizing detector will measure both the population and alignment of the final state_ However, in our experiments the inclusion of a half-wave retarder associated to the polarizing grating of the spectrometer is equivalent to the “magic angle” [U] so that we are able to deduce the final state populations directly from intensity measurements_ In our experiment involving cw laser excitation the population of the studied satellite levels N_ is governed by the steady-state equation:

Here Aii. N,,. and IV, denote, respectively, the number densities of the initially pumped CaF level. the buffer gas.. and any other_ possible collision partners [Ca. CaF(X ‘Z+), CaF,]. Also k& and kz denote the rate constants for transitions from the initial to satellite levels induced by collisions with, respectivelyY the -buifer g& and ,the other possible partners_ The radiative lifetime of the satellite level is designated rr_ In eq_ (1) we implicitly -neglect collision-induced transitions out of the satellite level. as well as multiple-step (i -b s’ A s) processes_ This assumption will be confirmed o pbiferioti by the linear evolution of the pressure dependence of the ratio NJ& in the experimental range 0.25-2 TorrFrom eq. (1) we ‘see: that the &io~ of ,populations’in the satellite and initial lev&s is given by:

(Vi

1)-r.

(3)

where Y is the. transition frequency, S(J= J”) is .the H&l-London factor (these factors f&r. a ‘II-‘2 transition have been given by Earls [22]). and q( o’, u”) is the Franck-Condon Factor_ As .I. discussed above the presence of a half-wave retarder plate allo& us to ignore the polarization of the emitted light. It then follows that the‘~mtio of the satellite and initial level population is given by:

ivJ _ $Si(

Ji’, Jr) (25,’ + 1) 1,

Ni - v:s,( J:. J:‘) (2J;’ -I- 1) r; -

(4

The Franck-Condon factor is cancelled out in the ratio provided that the emission from both the. satellite and initial lines belongs to the same vibmtional band_ In the kinetic analysis presented above we have explicitly ignored the alignment [27] of the initial state produced by the linearly. polarized pump laser; the. collisional .transfer cross sections uL; refer only to transfer of population_ If the Ca-M relative velocity vectors are distrib_uted isotropitally with respect to the laboratory-fixed coordi: nate system [defined in eq. (l)], then it is well kncnvn [27-301. that the excited state poptdation evolves independently of the higher multipoles t (orientation, alignment) of the initial density -ma-. trix_ .Since our detector. is sensitive only to the population of. the satellite levels, due to the prcsence- of the half-wave retarder. it is sufficient to consider just the .collisional transfer of. population_ The ‘assumption of an isotropic distribution _of relative velocity vectors is certainly justified in the case bf Hq- but perhaps not entirely in the use of ~1 ._:_~-‘.._ :-I..~_ -. Ar_. --~

Fig; 3 shows-the

pressure. dependence, of.- the

80

8i

60

6f

of the relative ncdli~c to initial levci population for tious nteUitc Iina in the Fi~_3_Ekpatd-onrhc buffergas(A In alI cases the initial lewd is J; = 3/2 f and the tanpcraturc is 300 K- (a) displayr tnmsitions for which x’rl ,P_ spin-orbit manifdb chee/flabcIis coclscnwhik (b) di@ays tralxsitioas for ww zhe e/f IabeI cbangcs_

ratio NJ&: for several transitions with or without conservation of the e/f label; in all cases the initial level populated by the laser is Jr = 3/L In fig 3 the linear dependence on pressure confirms our neglect of multicollisional effects in the pressure range sampled (025-2 To+_ The linear fits to the points have a non-zero intercept_ This arises (eq: (2)) from cohisions with partners other than the particular buffer gas_ Taking r,= 22C4 and IS f 4 ns for respectively, the A%,, and A’II, rotationalmanifoIds(31),aada residualgas~number density of 3 x lO’*)cmr (section 2). we deduce that a zeropressure intercept of 0.01-0.02 (fig 3) could be expected if the thermal cross sections for collisions between CaF(A’lT) and residual gases were = 306-600 AZ_ This is certainly consistent with the values obtained by Dagdigian -and Bullman 1321 in their recent experimental study. of collisions of CaCI(X?Z+) with various collision partners. The cross sections for collisions of

CaF(A’II,) with Ar and He derived from our experimental results are tabulated in mbles 1 and 2_ The experimental errors I&d reflect the reproducibility from run to run as well as the statistical error in carrying out a -least-squares fit to the pressure dependence of the ratio NJ&, but do not reflect the un certainty in the CaF excited state lifetimes [31]_ The large difference between the radiative lifetimes of the -12= l/2 and D =-3/2 spin-orbit manifolds may reflect a substantial degree of spin-rotation’ and orbit-rotation mixing between the 52= l/2 manifold and the B’Zt state. Since the lifetime of the B state is kksiderably longer than that of the A state [31]. this mixing would explain the slightly Ionger lifetime of the 52= l/2 manifold.’ compared to the Q=-3/2 manifold_ UItimately~ this mi&ng would have to ‘bc taken into accdtintin a kinetic analysis, in a manner similar to. that of Alexander, and coworkers [351 in their.a&lysis of .a la&r-induced

_

32 45(03) 4-s

f

28 Il.3 (0.6) 10-6 21(02) 2-1 3.9 (0.7) 49 1.7 (02) 1.6 25 (0.6) 3.8

e

f

3/z

e

5/2

f

5r_

c

7/z

f

7f_ 9/z

c f

95 (13) 95 8.0 (O-8) s-0

cross section, while, in contrast, the B = l/2- e/f. changing cross section will be srnajler th& the S2= 3/2 e/f cha+ng cross section_ FiiyT .& -also. expect [13] that the cross se&o& for e/f changing transitions, .eIas?ic -in J, namely transition~across a A doublet, wiII approach zero nine times faster in the S2= l/2 manifold than in the. S = 3/2 manifold. Our results do not clekly con; fii this propensity rule because no experiments have been performed for sufficiently high J val-,

1.6‘ . lo-6 (0.5) 105 1.8 (05) O-7 5.1 (0.8) 4.7

UeS.

52 (05) 53 26 (0.8) 29

In the energy sudden Iimit [33] the degeneracy.averaged cross sections for rotationAlly inelastic transitions w&in both the D = 1/2_and S+? = 3/2 spin-orbit manifolds can be expressed in -terms of the Ji = l/2, B = l/2, e/f - 3,* S2= l/2. e/f cross :sections. We have [13]:

5.6 (1.0) 5.8 7.8 (0.5) 8.1

%;Q<*- I_Pc.

lO.l(l.3) 2_9

I’ The indicated

uPer;nen~

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5. Discussion

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Our results clearly confirm the ‘qu&Iitative pro-! Ik&tj ales presented by Alexander [13] For a ‘II molaule in _the case (a) lint&- t&&ions which

-.

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where E = + 1 for the e-labekd A doubI&s and - 1 for the f-Iabelled A doublets--For a %I. tiole-. culein-thecase(a)Iimitthee-,e~andfjfcross se+ons are ident& for a given Ji --, ;I,- t&n&ion 11.31,So Fat thi fundamental cross sections Which appear in the rhsof eq 15) can refer to t.hn+ti~* between eith& the e-labelled or. the-,r-labelled -A-dopbIetlp&. -1. ..-. ; I,-:~-:-, _: -T The-derivation of eq. (5); which is an exterkioti ~.

o-7 (0-I) O-7 1.4 (02)

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26 (0-S) 1.6 3.6 (0.5) 3.0

I.1

0.8 (03) 0.9 1.9 (0.5) 23

2.5 (05) 32

=’ The imiiitcd

eqerimcnti unccrtaintia correspond to 20. a’ The theoretical walucs wmc dctcrmiucd from cq_ (5) with the fundamental cross sections tabuhrcd in iable 3 below_

to colhsions of ‘J3 mokcules of the weWmown infinite-order-sudden skiing relation [14]. is based on M-0 assumptions: The first is that the collision time is fast compared to the rotational period of the molecule At the temperature of our experiments the former is typically 5 X lo-l3 s for collisions with weon and 2 x lo-l3 s for collisions with helium. while the rotaticnal period of Cal? in the highest levels studied (J = 7/Z) is = 12 X 10-t’ s. The first assumption is thus well fulfilled

in our experiments The second assumption is that the inelastic change in rotational energy be small compared to the mean kinetic energy_ This is also sati$ted since the changes in rotational energy associated with the transitions fisted in tables 1 and 2 are afl~less than 52 cm-’ while the average relative translational energy at the temperature of our experiments is of the order of.200 cm-~‘_ I. Since! energy sudden conditions cl_early apply to the experiments reported here, we can use the ~.

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the~elrperimental data pointi aS well & a’probe for systematicerrors. .:.z- .;:- -. .J_. ‘:I_ cross.. To extract-the fund&e&l a,,,,~,.~,; -sections from the experimental cross sections-we tied out a linear least-squares fit to’minimize the x2 deviation [3+lJ_In principle, both the $2=: L/2 aitd 9 =:3/2 cross sections should be used in the fit sin& eq_ (5) ~applies equally to both spin-orbit manifolds, for algiven initial translational energy. The maximum number of I values u&d in the fit was adjusted to minim& the resulting x1 subject to several restrictions: (a) that the maximum value-of I included not exceed the maximum- vaIui permitted by -the triangular relation contain&i iq the 3j symbol in eq. (5), and (b) that all of the resulting a, parameters be positive Since increasing -1 valueS correspond, from e+(6), to cross sections for transitions for successively larger AJ transitions out of the J= l/2 level..we expect that the values of arr__, will gradually decrease with increasi ng f, although perhaps somewhat non-monotonically_ This expected behavior is illustrated clearly be a recent fully-quantum study of rotationally inelastic collisions of CaCl(X ‘Z*) with Ar [35]_ With these constraints we fitted th& CaF(A’II) Ar ex$erimental cross sections of table Z to obtain the parameters ~,~_,,r,,,,r listed in table 3_ The errors in the fitted parameters are defined

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-_

12

=’ I+_ (5); \du& obtained by~hst-sqfit tomurpaimmtal cross sectionslisted in tables 1 and 2. The listed any-. tieswzrt ulcutatedusingf+(6). b’ Reduced x square. “f_ j341. .

._

:-

.z

;

-.

experimental values lie virttially.within expkimental error_ Better_agr&qenfparticularly in -the l&t case, can be obtained be exteudiwthz summatiok in eq. (5) to include I= 6. However, doing so results in a value of 4.4 -+ 1.1 AZ-for &I/2C/f--13/2C/~ which, by the argument at the end of the preceding paragraph, seems unphysically large in comparison to the l/2 -_, 912 and l/2 d-11/2 cross sections_ This_ residual discrepancy_ suggejts that- future experimentai studies be directed at themore precise determination of cross Sections which are sensitive to the 1=_6 (and higher) terms in eq_ (5). The l/2 e/f -, 3/Z-e/f cross section is consid~erably larger than the succeeding values, entirely_ similar to what was found for then&iH_He [36j and C&l-Ar [35] system%: The dominance of this by WI: : term most likely reflects the polar character of the CaF(4’II) mole&& with the result. that -the ft:=CA:(go,r_-,/ao$, (6) strongest anisotropy in the CaF+r potential vnries i ~. linearly with the cosine of the angle between the CnF bond &is and _thecvector connecting the & where the sum ru” over all the- experimentally Atom with the centei of ma& of the CaF molecule_ determined cross sections _,oi,with assodated exT&is term,. which depends on this angle .as : a perimental -error% A, -The quality. of ‘the’ .fit is Qzgendre polynomial. of. ,order c1. : will ixsgri&i illustrated -by the comparison, of; the .fitted and only to t&e e-e b&e crag sections_wit& J =. I [35]. : experimental cross sections’shown ii tab1e.EWith _ the exce$ion of.&e 3/2 f 7 9j2.f and 5/? f-S 7/I .. j &-athe case of co@sions with H< the q&men: _td cross Sections for transit&s within the f+3fl e transitions in the 9 = lj2:;iianifold~~and .esl& manifold are & much larger: than those for,traus& -T ‘- cially~ the-50 f + 9/2 .f trapsition in -the,D L-.3/2 ’ tions within-the. D = l/2 manifold. that both set!---manifold, the differences between the fitted :and .:. t

carmot be fit well with the same set of base cross sections_ This- despite the fact- that .the--sudden s&&g relatien shortId_be even more rtppropriate for cohisions of the mass&+ slowly rotating CaP with the light He projectile The reason for this inconsistency is not yet clear, and is too huge to be expIained by the uncertainty in the radiative Iifetimes of the CaF A’Il spin-orbit_manifolds_ By fitting the D = l/2 and D = 3/2 manifolds separately we obtain the parameters shown in table 3. The agreement between the. fitted and experimental cross sections is excehent. as ilIustrated in table 2 and confirmed by the low reduced xz in- table 3. 6. concIusion

We have reported experimental cross sections for rotationally inelastic cross sections within both spin-orbit manifoIds of the A’H electronic state of CaF. By concentrating on the lowest rotationaI 1eveI.swe were able to probe unambiguously the dependence of the inelastic cross sections on the parity label of the initial and fmaI rotational levels. Clear vahdation of severaI theoretical propensity rules [13,24] was seen. The experimental CaF(A”H)-Ar cross sections were found to be consistent with the energy sudden scaling relation (131. with the exception of 2 few transitious. These few discrepancies can provide a fm for further experiments. In principle the sudden scaIing &ation allows the interreIation between the degeneracy-averaged cross sections, %etermined h& and M-dependent cross sections, which can be determined in experiments involving variation of the optical poIarization, such as those now being carried out by Norman et al. 191.Thus the theoretical scaling relation can facihtate an eventuaI comparison between these latter experiments and those reported in the present article. In the case of CaF(A’H)-He cohisions the present expetimentsd rcsu1t.sare inconsistent ~5th the sudden scaIing relation in that the cross.sections for transitious within the 9 = 3/2 spin-orbit manifold are much larger than for transitions within the B = l/2 manifold. Clearly, future experimental work shouid be aimed at an explanaGon of*-&screpauq_ -. ..~

.. mm UItimateIy it &ouId be’useful-to

deveIop.arrab

initio CaF~AzIi)% He poten&.I ‘surface, _&5 that accurate quantum. cross-’ sections_ could be -de+. termined and compared with e.+eriment. simiIarIy~ to the comparison being carried out by. Davis-et 91. [35] For CaCI(X ‘Z) + Ar coIli.%ions..This.:would provide. additional ir&ght -into the dynamics of inelastic processes in highly polar open-shell mole.. cules. ‘.

The authors wish to thank’ PauI Dagdigian for fruitful suggestions and~comments about key experimental details and for encouragement of this work_ Ml-IA is g&efuI to the US National Science Foundatiort(Grant CHEf34.05828) for support, to Robert Field, Jeff Norman, and Stewart Cameron for communicating preliminary detaiis of their experimental study of -inelastic collisions of CaF(A’H), and to -Andrew DePristo for he!pfuI comments about the use of scaling relations.

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