UF6 Relaxation dynamics following near-ultraviolet excitation

Volume

CHEhlICAL

97, number 3

UF6 RELAXATION

DYNAMICS

PHYSICS

FOLLOWING

LlXl-ERS

NEAR-ULTRAVIOLET

20 hln)

1983

EXClTATlON

S. CASTIGLIONE. G. FREDDl and P. MORALES Lab. Teo~ Spec. ENEA 0-e. Gxaccia. P-0. Box 2400 Rome AD, Iralj Rcceivcd 28 November 1982; in final fomt 22

kbNary

1983

Collisional relaxation dynamics of Uf-6 following excitation in the fit electronic sbsorpion bzmd is studied b\ an inwstigarion of the rcd-shiftcd emission. A comparison with dissociation data is made and a simple kineric nlodcl proposrd previously is improved.

I_ lntroductixt

2. Experimental

The avenge density of states of UF6 is very high: estimates based on simple assumptionsare in the range 104-105 states/cm-t [I] _ If one compares this density of states even with the collisional broadening alone, which at room temperature is z-43 MHz/Torr or 1.4 X lo-4cm_t/Torr, one can clearly see that the usual definition of a “statistical limit” molecule ap-

In our apparatus a tunable dye laser dissociates UF, (contained in a stainless-steel cell with CaF, win dows): wavelengths are between 370 and 390 mn, pulse energies are a few hundred microjoules and the repetition rate is 10 Hz. A He-Ne laser is then used to detect dissociation through the scattering of its beam by UF5 particulates [5]; a high-reject, high-lu-

plies [I] _It is usually recognized that in such molecules intramolecular redistribution of the energy among all possible states occurs on a very short time scale and no preferential relaxation path can a priori be defined f3 ] _It is therefore somewhat surprising that relasation to a dissociative state after excitation

minosity double monocllromator positioned at the peak of the emission (421 nm) separates the redshifted fluorescence from elastically scattered light; a boscar integrator with gate apenure of 5 ns used in the scan mode records time-resolved emission spectra; and a miniromputer stores, processes and displays our

to energies above the dissociation limit [4] should not occur as a result of intramolecular relaxation; indeed, it has been shown that dissociation of UFs can only be achieved by means of collisions, at least for excitation in the lowest electronicabsorption band [s] _

data. The laser pulse duration at the esit of the photo. tube is =7ns: the fast peak (=30 11s) that can be observed at short time (see the spectra in fig_ 4) must therefore contain some fast emission feature that has been reported in the past [6,7]. besides a leakage of elastic scattering. The dye-laser pulse energy was monitored before and after the sample cell in order to normalize the emission intensity by the true power in the cell. that tends to decrease with progressive misting of the optical windows due to UF5 deposit _

It is challenging both for its economic implications and for its intrinsic scientific interest to investigate on the relaxation dynamics of UF,. Previous work [S] examined the dissociation dependence of UF, after excitation in the A-z band, on excitation wavelength, pressure and atomic number of a monoatomic co&ion partner. In the present work we analyze the dependence of a second probe of relaxation, namely visible fluorescence, on the same parameters; we also extend the analysis of the pressure dependence of dissociation to lower excitation wavelengths. 0 009-2614/83/0000-0000/S

03.00

0 1983 North-Holland

3_ Results and discussion We first recall briefly some well-established

results 319

\‘l~l~n~r

97.

number 3

CHEMICAL

PHYSICS LETTERS

on 111espsctroscopy of UF,. Excitation in the first efrctronic absorption band yields no emission between 350 and 365 nm: fluorescence is emitted only on exci131ion between ~370 and 420 nm, the masimum yield beiug obr.uued around 390 run; this emission has a > 1r1d rmch smaller than unity. it is red-shifted to 430 nm ( pc~k ) md its distribution has a fwhm of z3O cm- 1 rcg.irtIless ofekkition wavelength [S .7] _Since almost ml tIuort’sccncc is observed for excitation around the pe&. ui 1111:absorption spectrum (370 nm) dissocia1ml .xmid in principle be rhe dominant relasarion product dt such dn ekcitatiun wd\elenglh_ Our previ‘,us \\urk &n\ed however [S] that the dissociation ! 1e1d 1s more or less uruform over the range 360-390 nm. mlp!ving rk presence of other relasation paths. .I‘\\ o clcctroutc b,md he& have been identified at 299.3 and 3SS.4 mii respectively [S]. so that escita11011III the luw- or high-energy part of the spectrum IIIJ~ imply sclcctiw rxcitstton of either of rhe two &clrumc

states

20 May 1983

I

U’6 IN

A-

S-

-

<

I 0

5-

f 0

4_

=: ”

3-

ttfGtN

o-

ARGON

h-

I n

n

I

i 0

/ 6

a

21 0 1-

~lFy

n

[7].

0x1 the ~JSIS of s previous model that accounts for the luw-pressure fluorescence lifetimes. we checked Ihe cansisrrnq of dissociation dsra with a simplified three-level 111uclc1[5]. We made the hypothesis that d~ssocidtlou could only be achieved through colliskuidl coupling lo the upper electronic state. The mwiel irnplwd 11m on excitation at h, < 370 nm (Ilur is. on rhe upper electronic sf3te [7] )I ( 1 ) The dissuclarion yield should be almost collision uidcperidcnt or even be decreased by co&siuns_ (2) The fluorescence yield should be enhanced by collk~ons. The first pomt IS .m31yLal in fig. 1 of the present \\~brk. where the pressure dependence ofdissociation IS gven iul A\xC = X0 rind &., = 36s ml for both Ar .md Ye as collisiunJ partners_ These wavelengths may hc Jssucidired with excitation of the two mentioned clrsrronic stxtr’s 171. One can note no qualirarive diff2rsnsc bcr\\ecn rhc trends for the two excitation \i Jvclengths c~rpt. perhaps. for a different mass depcndcnce. Thus ;1dissociative state wn only be achieved ~llrougtl LollMona relaxation from both electronic s[.gIr‘s. wirh more or less the same efficiency_ RelaxJ1wi to tiic dissocistivr state can however occur Ihrr~# several possible mechanisms, Iike vibrational fcl2?h.ulk~n 10 a --gate” 10 rbe predissociative or dissocktive state. or collision-induced intersystem crossing or mtrrual comersion to some repulsive electronic

0-

N E 0 II

Bna

0

300

ml--_-

O

6

0

0

I BUffER

500

600

PtttssUttE/TORR

I‘_. 1. Dissoctirion yield of UI_b dilured in different collisionp.rrrner g.~ses forescitation on different clecrronic sates. Nore rh.tl \\hite a strong rnzss dependence is found a hesc = 360 nm [A). very ilmilar behaviour c.m be observed for the two buffer :g.rses at hcxc = 370 nm (0). UF, pressure I Torr.

surface [9]. Some insight in the processes involved may be obtained by a careful investigation of the mass dependence of the dissociation and fluorescence yields 110,l 11. although the drastic approximations contained in the available theories do not allow an unambiguous identification at present. Point (2) is given an answer by inspection of the total amount of emission as a function of the pressure of different collision partners (fig_ 2). We have included SF6 among our collisional partners because two vibrational degrees of freedom of SF, are ahnost resonant with other vibrations of UFs [ 12]_ The dependence of fluorescence yield on collisions with SF6 may thus give information on the resonant nature of the collisions that lead to relaxation to the fluorescent manifold_

CHEMICAL

Volume 97. number 3

PHYSICS

20 May 1983

LETTERS

2

EXClTATlOlS

A

x

I

&3Mlm

I

-4

B

1

-

Fig- 3. Schematic drawing of the ground and first excited elec tronic states. with the rilte constants pertinent to the model discussed in the text. Wavy arrows represent opticrtl tnnsitions, dashed xrowsnre collision-induced tmnsitiorts, full arrow are radistionless transitions_ The position of the prediioeiative level is not indicative of its actual enersr.

FY=(~p+-X-l)/[B~2t(A~B~C)~ A

DY=(.@~+-X-i)/[flyp~

I

c

/ Ion

2m BUFFER

300 PRESSURE

boo

+k2].

(13)

+(cu+S+y)~+&].

(lb)

with

YIB

1 TORE

Fis. 2. Emission yield (&em = 421 nm) versus buffer gas pressure for different buffers. UFe pressure: 2 Torr. A = argon; n = SF6 ; p = pure UF6. dsta normalized to the pressure of 2 Torr. (A) also reports the best tits IO eq. (la). to

One can note from fig_ 2A (most easily with Ar) that the emission yield increases with pressure up to a maximum, then decreases slowly. This behaviour can easily be explained by a three-level model similar to the one proposed previously [5], the only difference being that we now prepare optically the upper electronic state rather than the lower manifold_ The scheme of our set of levels with the various rate constants is shown in fig. 3. Note that the same rate equations apply as for the calculation of the dissociation quantum yield on excitation in the lower manifold [S] and therefore,apart from a change in the name of the rate constants the expression for the fluorescence yield (FY) looks much the same as that for the dissociation yield (DY) [S] :

account

for the contribution

of UF6-UF6

colli-

sions to the relaxation process. All measurements were performed at a fLved UF6 pressure (2 Torr). The best fits of eq. (1) to our data are reported in fig_ 2a.Table 1 shows the parameters of our curves for the fluorescence yield at h, =370 nm (state A,) and one can note the ir creasing effect of collisions on the reia_xation steps on

Table 1 Best-fit parameters for fluorescence yield dota refer to cq. ( 1 n)

Ar SF6 UFe

A (Torr-‘1

B (Torr-‘)

C (Torr-t

0.03 0.28 0.41

0.004 0.01 0.01

0.003 0.01 0.50

,I

C/A 10 x 10-Z 3-6 x 10’ 1.22

Vohm~c

3

97. number

CHE>lIC.AL

PHYSICS

changing the colhsion partner from Ar to SF6 to UF6 This suggests that resonance of internal states increases the rdte ofrelaxation to the radiative manifold. On the other hand, one C~II also observe from the ratio C/A =

20 May 1983

LETTERS

i 2

UF&-AR6011

lI\-ARGON

3

100 TORR

k,,l fkl., that relaxation

to the dissociative state is etthdnced by “resonant“ collisions even more than that to the fluorescent state. We do not attempt a more det.ulcd analysis of these parameters due to the rather Ltrge cq~enment.tl error and to the small number of our points. It 111ay bs interesting to note that while our rate constant C for pure UF, is in very good agreemcn[ with the constant c-i in (7). there is very little or no agreement so far as the other constants are concerned_ Figs. ZB Jnd 11C show that on escitation in that portion ofthc spectrum where no collisions are needed to r&x on the radiative state (380-390 run), any increase‘ in the buffer gas pressure decreases the total rmisston yield by increasing the populations of both stJtc A, .md 01’the predissociative state D in the diagr.un ocfig. 3. Again UF6 - -UF6 collisions are the most eftictcnt in quenching the fluorescence and UF,-SF6 collisions .tre more efficient than UF6-Ar. It is intrigumg that while UF,-UF6 collisions do give a strong contribution to the fluorescence yield at zero pressure of 11x buffer g.~s (2 Torr UF,), this contribution is not ohscrved Ior the d~ssoci~tio~i yield at very low buffer gas pressures (1‘1~. 1). despite the fact that the

0

I

~.

_-_._ _.___._____

377 __

I

up\

Ut-p6

30 TORR

300 TORR 1

,A

0

200

% 8j L 1 kw._____.__ 0 200

dO0 ns

i 1.2i

UFs 10 IORR

.B1. ii

.d-

‘\j;+

0

200

ns

1 is:. 4. Tnnc-resolved 370 mn. Resolution ncnrial

400

I

t

.-.

0

200

emission at A,,,

I-or rile hiphcsr

0

10

20

30

40

pressures

rhc numbers

refer to rhc decay

50

100

200

300

400

500

355

310

191

270

215

177 203

250 11s

212 10s

150 92

175 -

160 108 -

113 85 -

85 60 -

80 55 -

75 45 -

‘L’.\C= 3711nm A& SI‘, Lz 6

353 353 353

482 366 745

317 305 160

267 123 93

313 260 70

267 243 -

217 140 -

143 110 -

85 60 -

60 55 -

60 60 -

_.__--_

fit!

cay of the emission curves becomes highly non-exponential at suftkicntly high pressure. This effect is remarkable for UF6-UF6 collisions, that yield a non-

393

- ....-

don

= 421 nm and hex, =

__--

Ill11Ar

ns

is = 10 ns. Dashed lines are single-cxpo(lcfr column) and muiti-cxponential (ridn column)

393 393

c\c = 3so

4OD

ns

30 JORR

Sk, L.“b

h

_...-.

qscs.

400

i,

> ield increases with pressure more rapidly on us~n_c UF, .ts colhsionsl partner than on using other gases 11.: J_This sould be expl.Cned perhaps with s~mw rcson;LIit energy-transfer process involving several consecut~vc colhs~ons before the prcdissociative state IS meshed. and therefore high PO\\ ers of the pressure_ Ancttht’~ intcrestin~ obscrvJtton is that the time de-

&VA> liux. (m n\) for I’Iv6 diluted in diffcrcn: c \:kmcnIiA c~mrriburwn of the drs~j --- ___I tIufIcr &wffcr (.I-c,rr)

ns

2

c11ssoc1.1~o1~

I.mi\wrn 111~.uwn -_I--.-._

200

time of

Volume 97, number 3

CHEMICAL PHYSICS LE-ITERS

exponential behaviour below 30 Torr, whereas nonexponential curves are noted above 200 Torr if Ar is used as collision partner. Fig. 4 shows examples of this behaviour together with low-pressure simple exponential behaviour. Due to the experimental uncertainty it is not possible to assign a quantitative irnportance to the parameters of our multi-exponential fits, reported in the graphs of fig. 4. We can however say that on increasing the pressure a short-time rising exponential is observed and also, at the highest pressures, a very long-lived and weak emission_ It is also interesting to note that the decay time of the main contribution to our emission curves decreases monotonically with increasing pressure (following a Stern-Vohner relationship within experimental error) even for excitation at 370 nm, where the total emission yield is not monotonic versus pressure (see table I?‘)_Such a non-monotonic trend of the FY [ = AT, if lem =A exp(--t/r)] is thus to be connected with an increase of the zero-time intensity A, on increasing the pressure_ This means that on excitation in the upper electronic manifold (370 nm) relaxation to the fluorescent state occurs in times shorter than we can observe (~30 ns). Once molecules have relaxed on this state any further collision quenches the emission.

20

May 1963

References [l] S. DC Silvcstri, 0. Svclto and F. Zara,cl. Proceedings of the International School “Enrico Fermi”, Varennl, lull (1978). [%j K.F. Freed, Advan. Chcm. Phys. 47 (1981) 291. [3] A. Ben Shaul, Y. Haas. K.L. Kompa and R.D. Levine. Lasers and chemical chansc (Springer, Berlin 1951) ch. : [4] D-L. Hildenbrand, J. Chem. Phys. 66 (1977) 4786. [S] F. Catoni, hi. Cavtioii. E.D’Errico. G. Freddi and P. hlorales. Chcm. Phys. Leriers 92 (1982) 292_ [6 1 0. de Witte, R. Dumanchin and hl. hlichon. Chcm. Ph\s. Lerrea48 (1977) 505. 171 R-C. Oldcnbog, W_W. Rice and F.B. Wamplcr, J. Chem. Phys. 69 (1978) 2181; W-W- Rice, P-C. Oldenborg, P-l. Wantuck. J.J. Tiee and F-B. WarnpIer, J. Chem. Phys. (1980) 3560. IS] W.B. Lewis, L.B. Asprey, L.H. Jones, R.S. McDowell. SW_ Rabid-u, A-H. Zeltmann and R-T. PGne. J. f&em. Phys. 65 (1976) 2707. [9] A. Tramer and A. Nirzan, Advan. Chem. Phys. 47 (2) (1961) 291. [IO] J-D. Lambert, Vibrxionai and rorarional relJlarion in wes (Clxendon Press, Oxford, 1977). [ 111 C_A.Thayerand J.T.Yard1cy.J. Chem.Phys.57 (19’2) 3992. (121 D. Jackson, Los Alamos Report 60%MS (1975). [ 131 E.D’Errico, private communicarion.

323