Laser Diagnostics of surface-emitted Na2 molecules

16 A@

CHEMICAL PHYSKS LETTERS

Volume 87. number 6

LASER DIACNOSTKS

OF SURFACE-EMITTED

1981

Naz MOLECULES

G. MIKSCH and H.G. WEBER

Recctved

1 December 1981.m finat form 1 rebrwry

1982

We repott mresrifations on N+ molcculcs emrtted from 3 hqutd sodium surfxe into CIvxuunt Wbr3ttonztl md rotatronal tcmperaturcs of theemitted molccuks .rgee wrth thu temperature of the sodtum surfarc, and Ihe angulx moments oi the cmntud molcculcs arc tsotropszdly dlstrlbuted. Tbi: ukpcriments demonstrate WI apphcation of ktscr dtztgnostlcs for sturhcs of 3 gds-liqutd uttcrface

in the present study we mvesttgate the internal energy and the orientation of the angufar momenta of Na, molecules which ore emitted from 3 hquid soduml su&cc mto .I v~~uun~, We find that the vibrattonal and rotarlonal temperatures of the ermtted molecules are equal IO the temperature ofthe liquid, and that the angular momenta of the rtmtted molecules have an tsotroplc dntnbution Both results are in agreement with the generally assumed mechamsm of evaporation from a liquid surface into a vacuum [I-3] _A first report on our experiments has been given [-%I*. * In ltie pretrmmztr) rcjults ue reported on an zntsotroptc dtstrtbution oi the .mm_euhr montent3 of the emttted Naz tnoleeulfs Wc ztpologtze ior thtsrrrror m ourexly commumGllIOtl

The experimental apparatus is indicated in fig. I. A laser beam IS directed over an approumately plane liquid sodtum surbcr (circular surface wrth 3 dtameter of 20 mm) at a distance of= 10 mm. The laser Ight (I- I5 rn~V~c,l~~)excites Na? molecules whtch are emitted from the sodium surface and the molecular fluorescence is detected. The diameter of the laser beam near the eacLtatton area vanes berween 1 and 5 mm depending on rhe experiment. By moving a plunger into the hquid sodium it IS posstble to regulate the level of the sodmm surface wth respect to the laser beam and also the shape (the curvature) of the surface. The background pressure m the vacuum chamber is = 10v7 Torr and the temperature of the

vacuum Clump5 -

If

II

II

JJ

Ftg. 1. SchematIc view of the experimental appxnus.

544

0 009-2614/81~0000-0000/S

02.75 0 1982 North-Holland

Volume 87. number 6

CHEbllCAL

PHYSICS

hquid sodium is always b2low 500 K. This results in a mean fret path of more than 100 mm for the particles enutted saturated

from

the surface

sodrum

vapor

(assummg

the density

of a

for the parrlcle flow). Colh-

sions or2 therefore neghgible. Nsarly all particlcs leaving rhe sodium surface n1ov2 m straight hnes from the sodium surface to tl1e condensmg surface which ISat hquid mtrog2n temperaturz and which is ~30 mm above the sodmm suriace. Bsckscattenng of particles from other surfaces, for instance_the shield which prorccts the window for the tluorescence detection, is neghglble. It is v2ry unportanr to clean 11~2sodnun surface nnmediately before the experiment. For this purpose the sodium surface can be lifted dloppmg Ihe plunger into the liquid sodmm. A metal plate. actmg as a wiper. IS then carried over the surface. Tl~s wiper cuts away the upper part of the sodium and thus seems to produce

a clean surface.

The laser m the experiment ISa single-mode cw Ar+ laser wluch is tuned continuously over several molecular absorption hnes (u’.J’) c (u”,J”) of 1112 B 1n, - X L1; band system IS]. In order to tune the laser to a particular molecular absorption hne we Split the laser light mto two beams and send one beam through a sodium reference cell, whsrc a fluorcscencc specrrum can be takrn. The same refrrencc cell srrves dso

3s a

reference

for the determination

of the Inter-

n.d energy of the surface-emlttcd n~olcculcs. The other beam ISdlrected over th2 sodium suriacc. In some elperlments this laser beam passesa combmatlon of a crrcular and hnear polarizer before 11emers rhe vacuum chanlber This allows rotation of the poIarlzatlon vector of the light beam. The laser-Induced molecular

fluorescence

multrpher

after

IS detected

by a cooled

photo-

passed a spatial filter, an mlcrierence filter and. in some ehpsrimenrs, a rotatable polarizer.. The photoncounting techmque provides ior further signal processing. The mvestlgatron of the angular momentum ahgnment of surfaceemttted molecules clos2ly follows work on the deterrninatlon of molecular alignment in a supersonic nozzle expansion [G]. To facilitate the dIscussIon we introduce a coordinate system with rh2 x,y plane parallel to the mztal surface and the z a~ bemg perpendicular to the surface. The laser beam IS dlrected along the x axis. It IShnearly polarized with an angle (Ybetween the direction of the electric field vector and the z axis. Molecular fluorescence is detecthwng

LETTERS

16 Aptd I982

cd along iheJ As IV2 detect rhc tluorcscencc intenslty I,(a) of fluorescsncc hncarly polarized parallel to the .r axis as well as the Intensity /,(a) oi fluorescence linearly polarlzcd pdrallcl to the -_awls. Thcs2 measuremsnts g1v2the degr22 of polarization of the molrcular fluorescence

as a function of the angle Q In lh2 cvpcrim2nt we induce the transitlon (6.43) + (3.43) of the B 1 llu .Y ‘Xl band system and we m2asure P(a) ior dlffcrent Iassr hght mt2nUtl2s. To calculare the tluoracencs mtcnsl[lr’s I,(a) and f,(a) th2 transition moment of 1112molecule IS replaced by a classical dipole oscdlator. Lrt ii,?, Z and .c denote umt vectors Jlong the dipole oscdlator. the eketrlc field of the mcldcnt hght beam, the I 311sand 1112 x 3~5, r2spectlv2lp. Th2 lluorcsccnc2 intensity f,(a) can then bc wrlttcn as

The same e\presslon holds also for /.,(a) If i IS rcplaccd by .k. In this equation (zs fi)’ 1sthe probdbihty of dbsorptlon oil&t by the molecules, (b- 5)’ is the prob&dlty that the m0l2cul~s 2nut hght which 1s polJrlz2d parallel to tl12 z a\is. and K ISa constant n(6. p) repmscnts the dlstrlbutlon of th2 dipole oscdIJtors of the ground-stare moleculss. ~11212it and p Jr2 polar angles m th2x.j1,: coordmat2 system. Brcaus2 rhe e\perimentd snuat1on has a~ul symmetry with respect to the z L\S,II(I~, 9) is mdrpendcnt of 9 and nldy be replaced by ~(8) We will furrher assume that r~(0) 15synunctrlc about any plane normal to 1112 : ak15. An expansion of ~(9) m a complete set of Ixgend_rc polynomlalsP,(cos 19)d2p2nds therl’iore only on the 2ven polynomials. Usmg rl(8) = 2 b,,P,,(cos 8) with I = 0, I, 2,3. we cvalu.~te lhc fl&rescsncc mtensitiesI,(ol) and f-(a) Only the first rhr22 terms of thusexpmsion contrIbute to I,(a) Jnd I,(o) because of the transformation properties of the electric dipole moment under rotations. Furthermore we may set b0 = I due to the normahzarlon ofr~(if). The degree of polanzation P(a) depends thcrefor2 only on thr cocfficienk b, and b, and a fit of th2 iunctlon P(a) to the expeknental results allows to determme both coefficients. For an isotropic disrribunon of rhe dipolc oscdlators w2 elpcct bl = b4 = 0. 545

Volume 87. number 6

16 Apnl 1981

CHEMICAI., PHYSICS LETfERS

i

I‘$. 1. The degree of polarization P(o) ~crsos the an& a. The dots rcprcxnt measurements urrth 25 rn\Vlcm~ laser light tntcnstty. The soltd CUNCSrc‘prcscnt cakuhted vducs. Ime 1 iorb: = bq = 0 (an isotroprc dernburlon) andbile 1 n fir to ll~ci2up~r~mcnl3l results

The distributton of the anguhtr momentum vectors of the molecules follows from n(9) tfwe know how the transttion moment in the chosen laser-mduced transitton :s attached to the molecular frame. Because we have chosen a Z -L fi 4 S tranntlon tn which both the absorptton and emisston involve a AJ = 0 transi. t~on, the dtpole oscillator hes along the angular momentumJ. Therefore n(S) is also the distrtbution of the angular momentum vectors. Fig. 1 shows a plot of the espcrtmentally deternutted degree of polartzatton P(o) when the laser light intensity m the expenment IS 15 ~n~V~crn~ The two solid lines in fig. 1 represent (hne I) a ca~curatton of Pfo) correspondmg to an tsotroptc distnbution [b, = b4 = 0) and lme 2 a tit of P(o) to the eapertmentti results wtth coeitictents bz and b4 which both dtffer from zero. Varymg the laser light mtenstty, we find a dependence of 62 and 6, on the hght intenstty which indicates an optical pumpmg effect [7) With decreasing

mtenstry

the coefficrenrs

approach

Fzg. 3. Dependence of the degree of polmuatron tight mtensity.

P(O0)on the

energy of Na7 molecules escapmg from the sodtum surfxe follo&g closely work on the tnternal temperarure of Na, molecules in a supersonic nozzle expansion [8]. The laser excites a particular molecutar transtion ali’ ={(+.I~) + (U~J~)) stmul taneously in the reference ceil and over the so&urn surface. The mofecuiar fluorescence IS detected vta two tdenticai mrerference fdtersand grves the fluorescence mt~nstty fief at the reference cell andIs,, at the sodtum surface. The ratio (frer/lsU)~ is determined This ratlo depends of course on the dtfferent detection geometries However the ratlo

of two dtfferent

laser-Induced transttions Q and o, is

the

values b2 = b, = 0. This IS demonstrated

in fig. 3 for P(O), the degree of polanzation for a = 0. With decreastng light inrenaty P(0) approaches the value P(0) = 0.50 which means b, = b, = 0. We conclude from these results that tie angular momenta of the surfaceemitted molecules have an isotropic angular distnbubon. We measure also P(O) versus the temperature of the ltqutd sodtum m the temperature range 450400 I; No influence of the tentperature onP(0) ts observed. In the next e~pe~ntent we investi~te the tntemal 546

Fig. 4. The ntios T,&7and

Trot/r versus the temperature ?-.*here T,qb and r,t are the vrbrarionai and rotzttonaf temperature of the emttted Na2 mokeules and T ts the tempcnNre oi the itquid sodmm.

CHEMICAL

Volume 87. number 6

16 Apnl 1982

PHYSICS LETTERS

independent of the detection geometry and is solely a

ments with ttus discnmmatlon

functuxt of the relative occupation

cannot exclude

the two levels

(IJ~, Sk) and

probabilities

of

(uy, Jy) of the molecules

m the reference cell and of the molecules which are emitted from the surface. In the expcnment

may giv2 a non-Isotropic bution.

The cxpenments

tlon oi laser dlagnostlcs

the laser is tuned at the laser

hqutd Interface.

us2d by Woerdman

= 5 GHz at constant

the surface-catalyzed

At 488 nm WC

cxclte molecules m the ground-state (0,X),

(3,13) and (0,98)

levels

(v”,J”)

=

and at 476.5 nm we ehctte

molecules m the ground-state levels (v”,J”)

= (3,43)

studies

angular momentum

dlstrt-

demonstrate

an apphca-

for studlcs of

the gas-

L3ser diagnostics

lines 488 and 476.5 nm over a frequency range of light mtennty..

arc’ possible and we

that these direction-selecttvc

ivas recently

and Eskddsen [9] for studlrs of 7-Na * Na, equdlbrtum.

These

authors conclude front their results In conqxulson wtth our results, and using det.ulcd-balance that a glass surface \nth

arguments.

3 few Na monolayers on top

and (5,55). These measurements give 5 ratiosR(uK/ui).

of it seems to be not so dlficrent from a hquld sodtum

In order to get information

surface. Further

on the internal energy dls-

trlbutlon of the surfaceemltted

molecules from these

studies on the sodnun gas-liqutd

In-

tcriace are m progress.

ratios we assume that the molecules m the resonance are m thermal equdlbrium. The occupatton prob-

c2U

abdity

of the vtbnttonal

molecules IS obtained the resonance

and rotattonal

levels of these

cell and the well-known

rotatlonalconsrantsof

Na2.

In

the case

vIbrational and of the surfacr-

ermtted molecules we assume 3 temperature tlon with

Thr aulhors

thank Dr J.P Woerdman ior scndmg

them IIIS manuscript

pnor to pubhc3t~on.

dtstrtbu-

the temperature Tvlb oi the nbrational

levels and a temperature The expenmental

Trot of the rotational

levels.

References

results yield therefore a system of

algebraic equations

for the temperdturcs Tvlb and

Trot of the surhce-emirtcd the ratios T,,b/Tand

molecules. Rg. 4 shows

Trot/T

for dtfierent

that the vlbratlonal and rotational temperatures of the emltted molecules agree well \nth the surface

temperature. In conclusion, the experunental

Na, molecules whtch are emitted

results show that from a lrquld so-

surface into a vacuum have the same Internal

energy and angular momentum

dlstnbution

as do

molecules in a saturated vapor which is m equlhbrtum with chc liquid surface. However m tie present expefiment we did not select molecules leaving the surface direction (havmg a well-defined angle

in s welldefiied

between the velocity

vector and the z a~).

II]

PD. Croup. J M~rlt. Phys. 15 (1930)

[?I yj;

Clddmgs and P.D.Crout.

1

J. Math. Phys I5 (1930)

tempera-

tures T of the hquid sodtum The results demonstrate

d&n

Acknowledgement

from the known tsmperorure 4’

[3] S H. Algae, J. Chcm Phys. 69 (1978) 538. snd rckrcnccr thcrcm [A] G. Mksch and H.C. Webcr. Curop~~n Confercncc on Atomic Physics, Heldclberg, Aprd 6-10,1981, paprr W6. 15 ] M. Kompuw, K Eolux and 1I.G Wber, Clsm. Phys. 55 (1981) 221. [6] M P. Smha, C.D. CatdaeU and R N. Zarc, J Chcm Phys. 61 (197_0491. 171 R E. Druhgcr and R N. Zarc. J. Chcm. Phys 5 1 (1969) 553,. IS] M.P. Smha. A. Schultz and R N. Zare, J. Chcm Ph)s 58 (1973) ia9 [9] J P \koerdman and S S Eskddscn,Chcm Phys. 65 (198’)

83.

Measure-

547