Ionization degree for alkali atoms on a rhenium surface

Vacuum/volume 45/numbers 2/3/pages 289 to 291/1994

0042-207X/94S6 00+ 00 @ 1993 Pergamon Press Ltd

Printed m Great Britain

I o n i z a t i o n d e g r e e f o r alkali a t o m s on a r h e n i u m surface k Gtadyszewski,

Institute of Physics, M CuHe-Sk¢odowska Umverstty. 20-031 Lubhn. M C Sktodowska Sq

No 1, Poland

Ion and atom desorpt/on energies for hve alkah metals on rhemum were determined usmg the ton thermal emtsston nome method The acttvatton energms for the charge transfer process m the adsorbed state were calculated using a specml energetm balance equation, which describes the surface tomzatton and thermal desorptton effects Energies for desorptlon of LI, Na, K, Rb and Cs from Re surfaces have been determined by measurmg the time autocorrelatlon function of the ton thermoemmston current fluctuattons

Introduction In o u r previous works ~ -' we presented results o f the measurements o f the Ion Q, a n d a t o m Q, thermal d e s o r p t m n energies for alkah metals on tungsten In a recent p a p e r ' we q u o t e the results of the calculations o f a t o m - ~ o n transition energies for these a t o m s on polycrystalhne tungsten w~th (001) texture In th~s p a p e r a n ldenUcal m e t h o d was used for five alkah metals o n polycrystalhne r h e n i u m with the work f u n c t m n eq5 = 5 10 eV

Method The m e t h o d as based on the relationship between the n u m b e r n of a t o m s a d s o r b e d on a small region of the emitter a n d the ion t h e r m o e m l s s l o n current t emitted from this region Due to r a n d o m surface diffusion a n d t h e r m a l desorptlon o f a d a t o m s , n undergoes fluctuaUons These fluctuations are reflected in the work function a n d finally in Ion current fluctuations dt, where dr(t) = t ( t ) - t0, 10 = ( l ) The o p e r a n o n of the t h e r m o e m l s s l o n ion source is based on the fact t h a t when the a t o m s are incident on the hot metalhc surface, some o f t h e m are e v a p o r a t e d as positive ~ons The ion current to d u r i n g e v a p o r a t i o n o f an element with Ionization potential V from a surface with the work function eq5 at the t e m p e r a t u r e T is given by the S a h a - L a n g m u l r formula 4

10

=

ane{1 + (gOld+) e x p [e(V-qS)/kT]}

1,

(1)

where a is a geometrical factor of the ion source, e ~s the electron charge, gO~q+ is the ratio of the statlsUca[ we=ghts of the atomic and runic states and k is B o l t z m a n n ' s c o n s t a n t (for alkali metals g+/qO= 1/2)

IS the i n s t a n t a n e o u s value of the ion current a n d r is a delay time of the correlator F o r the whole t e m p e r a t u r e range investigated ( 1 4 0 ~ 2 0 0 0 K) exponential a u t o c o r r e l a n o n f u n c n o n s were determined

R(z) = ( d l 2) e x p ( - - U r 0 )

(3)

By fitting R(z) curves to the experimental d a t a we determine zo In the case of L1 a n d Na. the atomic desorptlon d o m i n a t e s T h a t ~s why zo concerns the atomic state, c o n t r a r y to K. R b a n d Cs where ion desorptlon d o m i n a t e s F o r LI a n d N a the desorptlon energy Q, was estimated from plots o f log (%) as a function o f 1/T z0 = T0o e x p (Qa/kT)

(4)

F o r L~ and N a ions. a s t a n d a r d m e t h o d was used 56 to determine the ion thermal desorptlon energy Q, In this m e t h o d the alkah ions are removed from the emitter surface by a pulsed acceleration p o t e n n a l The exponentially decreasing ion current was amphfied by an i o n - e l e c t r o n m u l t l p h e r ( E M I 9306/2B) here used as a wide-band dc a n d ac electrometer The decay time of this current was measured by an oscilloscope The ~on d e s o r p n o n energies Q, were determined by a c o m p u t e r fit to the e q u a t i o n z0 = z°exp (Q,/kT) The process of the surface Ionization can be characterized by the degree o f l o n l z a U o n ~ 9: = n +/n°, or by the surface Ionization coefficient fl fl = n+/n. where n + and n o are the n u m b e r o f ions and atoms e v a p o r a t i n g in unit time. respectively, a n d n is the n u m b e r of a t o m s incident on the surface n = n + + n o The Ionization coefficient is related to the degree of ionization by

/? = ~/(1 + ~ )

(5)

The m e a n square (dr z) of the Ion current fluctuations is gwen by~

The value e ( q S - V ) defines the degree of lOnlzatmn in the desorbed flux

(dr") = to(Ttett/eokTl)2nDzo

c~ = 1/2 e x p [ e ( 4 ~ - V)/kT]

(2)

The following symbols were i n t r o d u c e d here to is the dc ion current intensity,/~ the dipole m o m e n t o f the a d s o r b e d a t o m s , e0 the pcrmlttlvlty o f v a c u u m , l the geometrical factor o f the emitting ribbon, D the surface diffusion coefficient a n d r0 is the m e a n residence time o f a d s o r b e d a t o m s on r h e n i u m To describe the m n current fluctuations the time autoc o r r e l a n o n function was used R(r) = (Kt)'t(t+z)), where t(t)

(6)

F o r the a d a t o m s o n the surface the value of E determines the degree of ionization 7 a n d m a y be calculated using the following formula c~ = 1 / 2 e x p [E/kT]

(7)

where E is the activation energy for charge transfer from the metal to the Ion 289

L G,ladyszewskt Re surface We examine the energetic balance for the following cycle atomic desorption with energy Q., lOnlzauon o f the a t o m in free space with energy eV. replacing o f the m n and its electron o n t o the surface with recovering o f the energy - Q , and -ec~, and electron transfer from the metal to the ~on with energy E ~ ~ ~ ~0 F o r a cycle process the sum o f the energy changes must be zero

(Q~-Q,)+e(V-c))+E

= 0

(a) 08 0 LI/Re

o

(8)

This energetic balance was used to determine E

Experimental

•

01

The measurements were p e r f o r m e d in a stainless steel vacuum chamber, evacuated by an 1on p u m p with a high p u m p i n g speed, allowing a final pressure o f 10 9 h Pa The a n o d e o f the thermal emission ion source was a rhenium b a n d o f size 10 x 0 8 x 0 02 mm The procedure o f depositing atoms on the Re emitter was described earlier ~ by the a u t h o r and details concerning the ion source construction can be also found there The F a r a d a y collector and m e a s u r e m e n t system was described in ref 12 The autocorrelatlon functions were determined using a stochastic analyser ( K F K I N S A 1000)

Results

0

*

o8

0 06 0

o4 o

0 O2

0 01

~-

20

40

I

~60

'

' 80

i

~ ' 100

? (-r)

Work function of the rhenium ribbon. The work function o f the polycrystalhne rhenium emitter was determined from equation (1) during an additional experiment, in which a small c o n s t a n t atom beam was directed o n t o the investigated emitter surface In Figure l, log (h.) is plotted for cerium and europium as a function o f the reciprocal temperature F o r temperatures above 1400 K we have straight lines as predicted by equation (1) F r o m their slopes we obtain values o f the work function o f e~b = 5 1 I eVforCe(V=557V) ande~b=5 10eVforEu(V=565V) 4

Autocorrelation functions and energy of desorption. The time autocorrelation f u n c u o n can be a p p r o x i m a t e d by an exponenual one. R(T) = (dl ~)exp (--T/T0). where T is a delay time o f the correlator and r,, is a characteristic relaxauon time (see Figure 2)

--

T[ps]

(b)

E

08 06

No/Re

o4

\

0081

/

oo' t

\1460K

~

oo\o

°°'F ,i \° \ 1520

=O=101°[A]

100 .

J ~o

.-

001'

Ce+/Re

00"%.

' 20

i\

'%" ' 40

' ~e 60

'

' 80

'

"

' 100

= T[)Js]

Figure 2. Normahzed autocorrelatlon functions xersus delay time T tor

•

(a) L1 on Re, (b) Na on Re

• ~ e ~ ~ e

10

'

"

.

I ° : A e x p ( - 0 45eV/kT)

By fitting R(T) to the experimental data we determine ro The desorptlon energy was estimated from plots o f log (%) as a function o f 1/T (Figure 3)

Eo'/R

- - ° ° ~ ° ~ o

~-//-

I o = B exp (-0 54 e V / k T )

Transition energy (activation energy for charge transfer from the metal to the adsorbed ion). The energies E were calculated using

I

. . . . I 21 2400 K

i ' 3t ' ' i ' '

2000 1800

1600 K

4

5o,0 - --r--IN-'

1/.00 K

Figure I Dc Ion current mtenslt~ for Ce + and Eu ~ as a function ot the reciprocal temperature

290

equation (8) The desorptlon (Q,. Q,) and transition energies (E) are summarlzed in Table 1 (The error limit for desorptlon energies is _+0 05 eV. only for Cs it is +_0 08 eV) It is interesting to show the dependence o f E vs e(~b--~ ). (Figure 4) F o r the straight hnes the fit procedure was performed

L Gtadyszewsk~ Re surface To

;~s]

Table 1 The desorptlon (Q., Q,), transition energms (E) and ionization

100'

coefficients /3. (on surface). ,6 (in desorblng beam) for alkali metals on r h e m u m Work function e~b = 5 10 eV Values/.¢,. [3 are calculated for T = 1800 K using equations (5), (6) and (7)

10O=1210"exp{23 ~ /

/

//' O

1

.

.

.

.

.

.

.

.

50z,O [ K - 1 ]

2&00K20001800K1600K 1400K Figure 3 Mean residence time % of LI and Na atoms on the r h e n m m surface as a function of I / T

[ev]

I

E=070 el~-V).,-016

0

A+Re

Q, [eV]

L~ Na K Rb Cs

2 37 1 92 1 74 1 62 1 57

2 34 2 10 I 65 I 52 1 38

e(~b- V) - 0 29 - 0 04 076 0 92 1 21

E [eV] 0 32 0 14 067 0 82 1 02

/~,

fl

0 06 0 55 097 0 99 1

0 06 0 28 098 1 I

C ~ W ~/c~

02 / , . * : ~ %

T h i s w o r k w a s s u p p o r t e d by t h e K B N 2-1332-92-01 R e s e a r c h Program

\

References

~:o7~ ~l~-v)

02 0&06 08 10 12 14

t u n g s t e n 3 ~6 a n d t a n t a l u m ~7 also give s i m i l a r r e l a t i o n s b e t w e e n t h e v a l u e s E a n d e ( 4 ~ - V) F o r E < 0 t h e a t o m , c s t a t e o f L~ d o m i n a t e s T h e i o m z a t l o n coefficient fl, xs very s m a l l , h o w e v e r ~t s t r o n g l y d e p e n d s o n t e m p e r a t u r e [see e q u a n o n s (5) a n d (7)] T h e fact t h a t s o m e o f t h e a l k a h n e m e t a l s reveal s u r p r i s i n g l y low m m z a u o n c o e f f i c m n t s h a s b e e n r e p o r t e d e a r h e r ' ~ ~s ~9 m spite o f t h e o r e u c a l c a l c u l a t i o n s 6 ~4 ~s O u r p r e w o u s ~ ~6 a n d t h e p r e s e n t r e s u l t s s h o w th~s p r o b l e m a g a i n F o r K , R b a n d C s . E > 0 a n d t h e l o m c s t a t e ~s m o r e f a v o r a b l e

Acknowledgement

O8 O6 o~

Q~ [eV]

/~TO=31 10-1Zexp (1 g2 eV/kT ) .

12

Element

- • (~-V)

-lO

Figure 4 Dependence of the t r a n s m o n energy E on the value e(~b- 1. ) for alkah atoms on Re (fit procedure performed with four points, value E for Ll IS rejected)

w~th f o u r p o i n t s , t h e v a l u e E f o r l l t h m m w a s rejected, b e c a u s e t h e s i n g u l a r b e h a w o u r o f LI a t o m s a n d i o n s m t h e p r o c e s s e s a d s o r p t i o n , d e s o r p t l o n , m ~ g r a t l o n a n d I o n i z a t i o n ~s k n o w n

Conclusions F o r L1 t h e t r a n s m o n e n e r g y E is n e g a U v e a n d ~s close to t h e e ( ~ b - V) v a l u e Recently carried out measurements for

L Gtadyszewskl. Sur/ace S~1. 213. 481 (1989) ZL Gtad~szewsM. 5ur/a~ e S~I. 231. 120 (1990) L Glad2rszewskl and G Gtadyszewskl. Surla~e & i . 247, 274 ( 1991 ) 4 L Valyt. Atom and hm £our. Phvs Rer B42, 5564 (1990)

291