resonance studies of h atoms adsorbed on frozen h2 surfaces

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RESONANCE STUDIES OF H ATOMS ADSORBEDON FROZEN H2 SURFACES

S. Crampton

To cite this version:S. Crampton. RESONANCE STUDIES OF H ATOMS ADSORBED ON FROZEN H2 SURFACES.Journal de Physique Colloques, 1980, 41 (C7), pp.C7-249-C7-255. <10.1051/jphyscol:1980739>.<jpa-00220177>

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ&ment au n o 7, Tome 41, juiZZet 1980, page C7-24 9

RESONANCE S T U D I E S OF H ATOMS ADSORBED ON FROZEN H z SURFACES

S.B. Crampton

WiZZiams College, WiZZiamstown, Massachusetts 01267, USA.

Resum&.- Nous avons observe l a resonance hyperfine dans l l @ t a t fondamental d'atomes d'hydrogene contenus dans une ce l lu le de 5 cm de diametre interieurement recouverte en surface d'hydrogene mol@culaire solide. Le dephasage de l a resonance hyperfine pendant l e temps 00 les atomes sont adsorb& produit des deplacements de frequence qui varient d'un facteur 2 dans l a gamme de tem- perature a l lant de 3,7 a 4,6 K , ainsi que des taux de decroissance radiative qui varient d'un facteur 5 dans ce t t e m6me zone de temperature. Ces deplacements de frequence e t ces taux de de- croissance ont une grandeur e t une dependance en temperature qui s'expliquent bien en supposant une distr ibution non uniforme des energies d'adsorption sur l a surface, caracteris@e par une valeur moyenne de 38(8) K en accord avec l e s estimations theoriques qu'on peut f a i r e pour une surface reguliere. Si 1 'on extrapole l a valeur de 30 nanosec. mesuree a 4,2 K comme duree moyen- ne d'adsorption, on peut predire des durees d'adsorption trPs longues pour H sur H2 au-dessous de 1 K . L1@tude des taux de retour a 1 'Gquilibre des populations des niveaux montre 1 'existence des coll isions d'echange de spin electronique en surface entre atomes adsorbes, l a duree de ces coll isions etant longue devant l a pfriode hyperfine ; ceci suggere que l e s atomes sont par t ie l - lement mobiles sur l a surface. Les taux l e s plus bas observes pour l e retour i 1 'equil ibre des populations f ixent une limite inferieure de l 'ordre de 500 au nombre des coll isions atome-surface non suivies de recombinaison 5 4,2 K.

Abstract.- Observations are reported of the ground s t a t e hyperfine resonance of hydrogen atoms stored in a 5 cm. diameter bott le coated with frozen molecular hydrogen. Dephasing of the hyper- f ine resonance while the atoms are adsorbed produces frequency sh i f t s which vary by a factor of two over the temperature range 3.7 K to 4.6 K and radiative decay rates which vary by a factor of f ive over th i s range. The magnitudes and temperature dependences of the frequency sh i f t s and decay rates are consistent with a non-uniform distr ibution of surface adsorption energies with mean about 38(8) K, in agreement with theoretical estimates for a smooth surface. Extrapolation of the 30 nanosec. mean adsorption times a t 4.2 K predicts very long adsorption times fo r H on H, below 1 K . Studies of level population recovery rates provide evidence for surface electron spin exchange col 1 isions between adsorbed atoms with col 1 ision duration long compared to the hyperfine period, suggesting that the atoms are par t ia l ly mobile on the surface. The lowest rates observed for level population recovery s e t a lower l imit of about 500 atom-surface coll isions a t 4.2 K without recombination.

When a hydrogen atom approaches a smooth,

uniform plane of molecular hydrogen surface, i t

f ee l s a t f i r s t a weak van der Waals a t t rac t ion whose

potential energy is--in f i r s t approximation--the

simple sum of pairwise interactions with the

individual surface atoms or molecules. The f i r s t

figure shows schematically the at traction of an H

atom to a (100) surface plane of a molecular hydrogen

surface for the two extremes of minimum and maximum

binding, depending on the la tera l location of the

adatom above the surface plane. (The scales are

based on some numerical calcuTations done by Abel

Weinrib as part of his undergraduate thesis [I] a t

MIT in 1979.) Both potentials and potentials

having intermediate depths a t other la tera l

SURFACE PROBLEMS

Van Der Waals Adsorption of Adatom @

t o Van D er Waals Solid 5559 A. Weak B i n d i n g on (100) Surface Plaoe

B. Stronger Binding on (100) Surface Plane

positions are deep and broad enoughb support a t Figure 1.

leas t one bound s t a t e , shown w i t h binding energy EB.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980739

C7-250 JOURNAL DE PHYSIQUE

On t h i s simple p i c t u r e an atom approaching the

surface i s f i r s t accelerated by t h e van der Waals

a t t r a c t i o n , slams i n t o the sur face w i t h more than

thermal speed, loses most o f t h i s excess energy t o

the l a t t i c e , and i s caught i n the lowest l o c a l bound

state. While bound t o the sur face as a whole, i t

may be almost completely f r e e t o move l a t e r a l l y

along the surface, i f i t s energy i s greater than the

p o t e n t i a l b a r r i e r s i t meets l a t e r a l l y , o r i t may be

bound t o a surface s i t e so t i g h t l y t h a t i t i s ab le

t o migrate only very s lowly by tunnel ing through

l a t e r a l p o t e n t i a l b a r r i e r s t o neighboring s i t e s .

The actual p i c t u r e should be o f course in termediate

between these two extremes, w i t h the adatom moving

i n a p o t e n t i a l t h a t va r ies both w i t h he ight and

p e r i o d i c a l l y along the surface, so t h a t the re i s a

band s t r u c t u r e o f energy l e v e l s , a r e s t r i c t e d

m o b i l i t y depending on the band gap and the dens i t y

of s ta tes, and a work func t ion o r mean adsorpt ion

energy, Ea t h a t must be reacquired by the adatom i n

order t o be again desorbed t o the gas phase a f t e r

some adsorpt ion t ime ta.

According t o Weinrib, the energy f o r adsorpt ion

of H on H2 i s l i k e l y t o be d i f f e r e n t f o r the (100)

and (111) H p surface planes by as much as 5 t o 10 K.

I n add i t i on , the experimental surfaces are not smooth

and uniform. Vacancies, p o l a r i z a t i o n o f the surface

by the adsorbed atoms, d i s l o c a t i o n s and other imper-

f e c t i o n s a l l tend t o increase the b ind ing t o t h e

surface and may produce some number o f t i g h t b ind ing

s i t e s on a sur face on which the adatoms would be

otherwise h i g h l y mobi le.

The r e l a t i o n s h i p between mean adsorpt ion energy

Ea and the leng th o f t ime ta t h a t an atan i s bound

on a s i n g l e encounter i s simply i l l u s t r a t e d by

thermodynamics f o r the simple extremes o f t i g h t

binding, on the one hand, and canp le te ly f r e e

m o b i l i t y , u s u a l l y c a l l e d the two dimensional (2D) gas

on t h e other. We w i l l t r e a t these extremes on ly i n

the approximations o f low surface coverage and

adsorpt ion energy apprec iab ly greater than kT, as i s

reasonable f o r H on H2 around 4 K and f o r H on a

l i q u i d hel ium sur face about 0.2 K. For t h e two

dimensional gas the d e r i v a t i v e o r the f r e e energy

w i t h respect t o the number o f adsorbed atoms i s

w i t h the sur face dens i t y o f adatoms and A =

Jh2/2~mkT the thermal wavelength. S e t t i n g t h i s

equal t o t h e chemical p o t e n t i a l o f the gas above the

surface provides the r e l a t i o n

w i t h n the dens i t y o f atoms i n the gas phase. The

r a t e a t which gas atoms s t r i k e the sur face i s a n7

per u n i t sur face area, so t h a t the sur face dens i t y u

i s a l so r e l a t e d t o the gas dens i t y n by

1 = - n v t 4 aa

w i t h fa the mean sur face adsorpt ion t ime per

c o l l i s i o n and a 2 1 the accomodation c o e f f i c i e n t .

The r e l a t i o n s h i p between mean adsorpt ion energy Ea

and mean sur face dwel l t ime ta i s then found t o be

For -Ea l a r g e compared t o kT, fa i s long, surface

dens i t i es are large, and i n t e r a c t i o n s between

adsorbed atoms become common a t r e l a t i v e l y low gas

dens i t i es .

For example, f o r H adsorbed on H, a t 4.2 K,

-Ea = 38 K, fa = 3x10-*set., and u = 10-3 n. For

-Ea = 0.6 K a t T = 0.2 K, ta = 5x10-9 sec. and

o = l o m 5 n.

For t i g h t b ind ing t o So s i t e s per u n i t area

rmSoh4 -Ea/kT microwake r a d i a t i o n close i n frequency t o t h e H - t = - a ah (6) ground s t a t e hyper f ine frequency s t imu la te t h e atoms

which d i f f e r from t h e 2D case simply by a f a c t o r t o emi t r a d i a t i o n a t the ground s t a t e hyper f ine

frequency. H a l f o f t h e emi t ted r a d i a t i o n i s . S o ~ 2 . If the loca t ions o f maximum b ind ing o f H t o

coupled ou t o f t h e c a v i t y t o a rece ive r which the H2 (110) and (111) surface planes were t i g h t

detects and s to res the atom output i n what i s b ind ing s i tes , S,h2 2 10. But i f t i g h t b ind ing s i t e s

e f f e c t i v e l y a computer o f average transi.ents (CAT). are produced on ly because o f surface heterogeneties,

Soh2 should be much less than one. SCHEMATIC OF APPARATUS

Questions o f adsorpt ion energy Ea, mean sur face

dwel l t ime fa and surface m o b i l i t y are v i t a l t o the

product ion o f H+. Ea i s a v a i l a b l e as an a c t i v a t i o n H, INLET

energy t o d r i v e recombination reac t ions otherwise STAINLESS STEEL

LIQUID NITROGEN DEWAR i n h i b i t e d by the Boltzmann fac to r u,B/kT f o r H+ i n

la rge magnetic f i e l d . If the atoms are mobi le dur ing D 1 s s o c l a ~ o ~ RF COIL

ORIFICE long fa, t h e probabi 1 i t y of i n t e r a c t i o n s between atoms

wh i le adsorbed on the surface becomes h igh a t the gas

phase d e n s i t i e s needed t o produce H4. Our resonance COUPLING LWP

experiment [2,3] on r e l a t i v e l y unpolar ized H adsorbed Pl lcnowav~ CAVITV

on H, i n very low magnetic f i e l d provides an e x p l i c i t CAPACITIVE LOADING

example o f these problems. I t i s important t o b . 6 ~ 1 ~ 7 4 -

quest ions o f producing and observing t1.r f o r a t l e a s t F igure 2.

the f o l l o w i n g reasons: F igure 3 shows the average o f 500 successive

1. Any experimental arrangement t o cool H traces. The s o l i d l i n e i s a f i t o f the f u n c t i o n

atoms below 1 K i s l i k e l y t o inc lude a reg ion

i n which H atoms c o l l i d e w i t h Hz surfaces a t

4.2 K and below. H RESPONSETO 90" PULSE

2. H2 produced by recombinations may soak up A(t)

a d d i t i o n a l H t if n o t somehow removed. c n 3. S i m i l a r problems w i l l e x i s t f o r H+ on

l i q u i d hel ium f i l m s a t 0.2 K t o 0.3 K.

Our apparatus i s shown schemat ica l ly i n F igure

2. Hydrogen atoms produced by a l i qu id -n i t rogen- 1 cooled RF discharge pass through a 2 mm diameter I

- t n 2 s - AOC c o s w @)

o r i f i c e , then through a 1 cm I .D., 20 cm long pyrex

tube t o a fused quar tz storage b o t t l e . The tube and VARY -rlwuuL

storage b o t t l e a re cooled t o 3.5 t o 4.5 K and coated TEMPERATURE A,J n/Ts

(3.5K To 4.5K)

on t h e i n s i d e w i t h f rozen Hz. The storage b o t t l e $ E,. i,. R e c o r n b l n a t l ~ n

FLUX

i s i n a microwave c a v i t y i n which p e r i o d i c pulses o f w E,. i,. Recomb4nat!oo

Figure 3. 17

C7-252 JOURNAL DE PHYSIQUE

t o t h e experimental po ints . The res idua ls a re

t y p i c a l l y 0.1% o f Ao. The f i t t e d frequency i s the

di ' f ference between the pulse frequency and the

frequency o f the radi.at ing atoms; i t i s ad justed t o

some value convenient f o r d i s p l a y and f i . t t i ng , by

ad jus t ing the frequency o f the pulse. From a l i n e

l i k e t h i s one we would f i t a frequency t o about 1 Hz

p rec is ion and T, t o about 1%.

T, and the dev ia t ion o f I,I from the f r e e space

ground s t a t e H hyper f ine frequency wo are dominated

by d i s t o r t i o n o f the h y p e r f i ne resonance wh i le the

atoms are adsorbed. The e f f e c t s are la rge and

prov ide sens i t i ve , if only approximate, probes o f

Ea and fa. The advance o f phase o f the precessing

atomic e lec t ron s p i n i s a t the H ground s t a t e hyper-

f i ne frequency wo w h i l e corss ing the storage b o t t l e ,

b u t i t i s appreciably slower w h i l e adsorbed on the

surface. The n e t phase o f a p a r t i c u l a r atom a t t ime

t a f t e r a pulse i s re tarded i n p ropor t ion t o the

n e t t ime spent on the sur face a f t e r a pu lse up t o

t ime t. The aggregate r a d i a t i o n from a l l atoms

averages the cosines of the indivi.dua1 phases t o

g ive a t ime dependence t h a t i s very c l o s e l y approxi-

mated b y an exponen t ia l l y damped cosine. The

dev ia t ion o f the hyper f ine frequency w from i t s

f r e e space value var ies approximately as the mean

phase s h i f t per sur face c o l l i s i o n d iv ided by the

mean t ime f between surface c o l l i s i o n s , f o r small C

values o f g. As e increases t o the order of 1

radian, t h a t p r o p o r t i o n a l i t y drops o f f , because

the atoms w i t h l a r g e phase s h i f t s wash ou t o f the

average. 1/T2 var ies a t ;2 d i v ided by fc, and i t

a lso f a l l s o f f f o r l a r g e e u n t i l i t saturates a t

about l/fc,

An undergraduate t h e s i s s tudent and I have

done a Monte Car10 s imu la t ion o f var ious d i s t r i h u -

t i o n s o f i n t e r a r r i v a l t imes tc and adsorpt ion t imes

ta. We f i n d t h a t the expected frequency s h i f t s and

GZLay ra tes do scale w i t h the r a t e l / f c w i t h which

atoms make t r i p s a17 the way across t h e storage

b o t t l e , so t h a t we can scale t h e experimental

r e s u l t s from a range o f temperatures t o 4.2 K, i n

order t o compare them t o t h e Monte Car10 r e s u l t s

f o r a s i n g l e fc.

On t h e 2D gas model we i n f e r from our

comparison o f ca lcu la t ions t o experimental r e s u l t s

t h a t Ea 2 4 0 f 5 K from both the o v e r a l l s i z e o f fa

and i t s temperature dependence. The temperature

dependence i s n o t very s e n s i t i v e t o t h e assumptions

o f sur face smoothness and CY = 1, w h i l e t h e o v e r a l l

s i z e o f fa i s d i r e c t l y a f fected. We can conclude

t h a t these a re f a i r l y good assumptions, or , t h a t

t h e i r e f f e c t s counterbalance.

On the t i g h t b ind ing model we i n f e r t h a t Ea =

36 t 5 K from t h e o v e r a l l s i z e o f fa and i t s

temperature dependence, i f we assume t h a t each

minimum o f t h e sur face p o t e n t i a l provides t i g h t

b ind ing. These values overlap, and t h e actual

b ind ing made must be in termediate between these

extremes, so t h a t 38+ 5 K-seems reasonahle a t t h i s

time. This value i s cons is ten t wi th , al though

somewhat h igher than, t h e value expected f o r smooth,

un i form surfaces. I t suggests t h a t imper fect ions

i n the surface p l a y some r o l e b u t n o t an o v e r r i d i n g

r o l e . The imp l i ca t ions of t h i s r e s u l t f o r H+ work

are a lso n o t unexpected. Hz surfaces a t temperatures

below 1 K must be avoided. Once adsorbed, H atoms

w i l l remain adsorbed e f f e c t i v e l y forever . O f course,

i t may be poss ib le t o p a c i f y a reg ion o f H, sur face

by covering i t w i t h H4. We are n o t a b l e t o t e s t t h i s

idea i n our experiment, because the unpolar ized

atoms o f our experiment can recombine when they

c o l l i d e w h i l e adsorbed on t h e surface, making i t

d i f f i c u l t t o b u i l d up a monolayer.

In fo rmat ion about i n t e r a c t i o n s between H atoms

w h i l e adsorbed on H p surfaces has been obta ined by

moni tor ing the recovery o f the s igna l ampli tude

f o l l o w i n g a 180" pulse. A 180" pulse i n v e r t s t h e

l e v e l popu la t ion d i f fe rence on the hyper f ine F igure 4 shows the f a s t and slow l e v e l popul a-

t r a n s i t i o n . The response o f the atoms t o a 90" t i o n recovery ra tes p l o t t e d against s igna l amplitude

pulse some t ime tD l a t e r ind ica tes the degree t o f o r two recent runs a t two d i f f e r e n t temperatures,

which the l e v e l populat ion d i f fe rence has recovered 4.19 K and 3.85 K. The s c a t t e r i s considerably

t o i t s e q u i l i b r i u m value. I n p r a c t i c e we use a l a r g e r than the f i t t i n g e r r o r s o f i n d i v i d u a l po ints ,

sweep o f 8 such 180" pulses fo l lowed a t vary ing due main ly t o d r i f t o f the microwave c a v i t y frequency

delays by 90" pulses. The recovery r a t e i s n o t a and t h e d i f f i c u l t y o f f i t t i n g the two ra tes w i t h 8

simple exponential . We ob ta in good f i t s t o t h e data po in ts . The dependence i s approximately l i n e a r ,

form and o ther data from e a r l i e r runs a lso supports t h i s

l i n e a r i t y . The sur face coverage by atoms i s too low 2 - t ~ / T l s 1 -%/TI

A = A-11 - 2 ~ ( . ~ e + - e 3 ) ] (8) f o r the re t o be any appreciable th ree body c o l l i s i o n

ra te , so 1/T, must be l i n e a r i n t h e atom densi ty .

We have shown t h a t t h i s behavior i s p red ic ted by Consequently, we i n f e r t h a t t h e s igna l ampli tude i s

the theory of e l e c t r o n s p i n exchange c o l l i s i o n s l i n e a r i n the densi ty , and from t h a t p r o p o r t i o n a l i t y ,

between hydrogen atoms fo r the case of small l e v e l we i n f e r t h a t t h e s p i n temperature TS t h a t character-

populat ion d i f fe rences and a c o l l i s i o n t ime i zes the e q u i l i b r i u m hyper f ine t r a n s i t i o n l e v e l

comparable t o the 10-10 sec per iod o f the hyper- populat ion d i f f e r e n c e i s independent o f densi ty . As

f i n e i n t e r a c t i o n . It i s a lso p red ic ted by the t h e dens i t y changes, i n t e r a c t i o n s t h a t thermal ize

theory o f Marie Bouchiat f o r i n t e r a c t i o n s w i t h the l e v e l populat ions must therefore increase a t the

randomly o r i e n t e d spins o r equiva lent magnetic f i e lds , same r a t e as recombination increases.

prov ided t h a t t h e corre la t i .on t ime fo r the i n t e r -

a c t i o n i s comparable t o o r greater than t h e per iod

o f the hyper f ine i n t e r a c t i o n . We suspect t h a t t h i s

form o f the recovery r a t e w i l l f o l l o w f o r any i n t e r -

a c t i o n which mixes the upper th ree F=l ground s t a t e 1 -

hyper f ine l e v e l s a t a f a s t e r r a t e than i t couples

them t o the F=O ground s ta te . I n t h e f i t s E = .85, 625 1

LEVEL POPULATION RECOVERY RATES

due, we th ink , t o incomplete averaging o f the 180" 500..

pulse ampli tude by t h e atoms. l/TIF 2 1/Tls,

i n d i c a t i n g a c o r r e l a t i o n t i m e o f order 10-lO r e c o r

longer, depending on the con t r ibu t ions t o both ra tes

from mechanisms t h a t d r i v e a l l t r a n s i t i o n s w i t h equal

r a t e o r s imply recombine atoms. We are able t o f i t 0 120 240 360

-+-- a,?" - -

t he 2/3 c o e f f i c i e n t w i t h p rec is ion o f ahout 1% f o r SIGNAL A H P L I T U D L A _

our best signal-to-noise, b u t i n o t h e r cases we Figure 4.

s imply assume 2/3. We have confirmed the assumption Note t h a t t h e slopes increase markedly as the

t h a t t h e f a s t r a t e 1/TIF i s due t o m ix ing the F=l temperature i s lowered and the mean surface adsorp-

l e v e l s by app ly ing a d d i t i o n a l RF resonant a t t h e two t i o n t ime increases, increasing the frequency of two

F=l t r a n s i t i o n frequencies, in between t h e 780" and body i n t e r a c t i o n s on t h e surface.

90" pulses.

C7-254 JOURNAL DE PHYSIQUE

The difference between the f a s t and slow rates

we take to be equal to the spin exchange coll ision

ra te on the surface, mu1 tip1 ied by the fraction of

time H / f c the atoms spend on the surface. The

slow rate can include contributions from recombina-

tion and spin exchange coll isions on the surface

and from spin exchange coll isions in the gas phase.

The Figure 5 showssane evidence that places a lower

l imit on the amount of recombination. As the density

and the signal amp1 i tude increase, recombination

heats up the temperature of the molecular hydrogen

surface, lowering the mean adsorption time and the

frequency sh i f t s due to surface adsroption. The

frequency changes approximately quadratically with

density, as it should. Assuming tha t the tempera-

ture r i se s inferred from the frequency s h i f t change

are due simply to driving the heat of recombination

across 140 square centimeters of storage bot t le

envelope 1.5 mm thick places an upper l imit on the

amount of recombination. A t signal amplitude 750

the density i s about 1013 TS and the heat of

recombination i s about 1/8 watt, leading to an

upper l imit on the recombination ra te of

TS can be no lower than 4.2 K , so tha t l/TIR 25

sec -I. That i s only 1/10 of l/TIS a t signal

amplitude 750, which means that 9/10 of l/TIS must

be due to interactions such as spin exchange

col l i s ions , which do thermalize the spins, and TS

must be about 5 K o r less.

The gas phase spin exchange cross section has

been calculated [4] to be about 3x10-l7 cm2 a t 4.2 K ,

so that a t signal amplitude 750 about 40 sec-I out

of 250 sec-1 could be due to gas phase spin exchange

coll isions. The remaining 175 sec-I o r so must be

due to spin exchange coll isions on the surface.

Consequently, t he correlation time which character-

izes these coll isions must be jus t long enough to

RECOMBINATION RATES

VARIATION OF WALL SHIFT WITH DENSITY

RECOPlBINATION HEATS THE SURFACE AND LOWERS WALL SHIFT W - W o T I N PROPORTION TO THE RECOfUJIMTION M T E

425 1 f.. * '-. '.J.

reduce the slow surface coll ision ra te by about a

325

300

factor of two re la t ive to the f a s t surface coll ision

..

r a t e , or , j u s t about equal to the 10-lo sec inverse

o loo zbo 300 ioo sdo 600 loo soo

RECO~BINAT~ON RATESF AT A*= 750

Figure 5.

of the hyperf ine frequency . The density, spin temperature, and coll ision

ra te can be combined to give an estimate of the

range a of the exchange interaction on the surface

times the speed vm with which atoms migrate about

on the surface. From th i s data

I f the correlation time i s taken to be 2a/vm, V,

turns out to be about 1/50 of the f ree space thermal 0

velocity. & = 6 A , which i s the r ight order of

magnitude for the range of such slow coll isions.

The probability of a spin f l i p i s about one when

the integral over the interaction time of the

difference between the % u and lc molecular inter- 9

action energies i s about equal to h . Using these

numbers implies AE = .03 cm-1, which i s the molecular 0

energy difference a t about 8 A.

Recombination of unpolarized atoms on the

surface i s only 1/10 as frequent as spin exchange, 0

corresponding to an interaction length of about .5 A

o r less, presumably due t o the l a c k o f phase space

f o r t r a n f e r r r i n g the c o l l i s i o n momentum t o the

molecular hydrogen l a t t i c e .

The a1 t e r n a t i v e extreme o f t i g h t b ind ing t o a

simgle s i t e throughout the 4x10-*sec adsorpt ion

t ime i s n o t cons is ten t w i t h o u t data. The slow r a t e

would then have t o be due almost e n t i r e l y t o gas

phase sp in exchange c o l l i s i o n s , whose r a t e should

decrease as t h e temperature decreases, ins tead of

increasing as i n our data. The distances over which

atoms would have t o be able t o exchange e l e c t r o n

sp ins o r recombine would have t o be unreasonably

la rge . We conclude t h a t the atoms are able t o

migrate over the sur face a t speeds much lower than

the f r e e space thermal speeds.

To summarize our r e s u l t s and t h e i r imp l i ca t ions

f o r H research:

1. The energy w i t h which H atoms a r e adsrobed

on H2 surfaces i s o f t h e o rder o f o r on ly s l i g h t l y

l a r g e r than what i s t o be expected f o r a smooth,

uniform molecular hydrogen surface.

2. The r e s u l t i n g adsorpt ion times a re pro-

h i b i t i v e l y long f o r storage o f H above H, a t

temperatures below 1 K.

3. The atoms are mobi le on the surfaces a t

4.2 K and exchange e l e c t r o n spins w i t h each other

a t about t h e r a t e t o be expected from the exchange

i n t e r a c t i o n .

4. C o l l i s i o n s on t h e sur face lead t o re-

combination a t a r a t e much less than the e lec t ron

sp in exchange r a t e b u t nonetheless h igh enough t o

l i m i t the dens i t y o f H atoms t h a t can be s to red

o r t ranspor ted above a molecular hydrogen surface.

A t 4.2 K the p r o b a b i l i t y o f recombining per

c o l i i s i o n w i t h the sur face i s about 4x10-l6 t imes

the gas phase density, and i t should increase

exponential l y as the temperature i s lowered unless

the re i s a l a r g e Boltzmann f a c t o r t o i n h i b i t it.

5. I t would be i n t e r e s t i n g t o study o ther

surfaces us ing these techniques. Our own very

p re l im inary measurements i n d i c a t e t h a t D, surfaces

are somewhat worse than H, a t 4.2 K, bu t t h a t neon

surfaces a re somewhat b e t t e r . L i q u i d He3 and He4

surfaces seem p a r t i c u l a r l y promising, b u t they

r e q u i r e temperatures lower than those c u r r e n t l y

a v a i l a b l e t o us. -

1 . Abel Weinrib, Undergraduate Thesis, MIT,

1979 ( unpublished) . 2. S .B. Crampton, T.J.Greytak, D.Kleppner,

W.D.Phillips, D.A.Smith and H.Weinrib,

Phys. Rev. L e t t . 42, 1039 (1979).

3. G.H.Zimmerman 111, S.B.Crampton, J.S.French,

W.J.Hurlin and J.J.Krupczak, B u l l . Am. Phys.

SOC. 5, 14 (1980).

4. A.C.Allison, Phys. Rev. E, 2695 (1972).