A STUDY OF ULTRA-FAST ION REACTIONS WITH FEMTOSECOND TIME RESOLUTION : FIELD DISSOCIATION OF METAL HELIDE IONS BY ATOMIC TUNNELING

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A STUDY OF ULTRA-FAST ION REACTIONS WITH FEMTOSECOND TIME RESOLUTION : FIELD DISSOCIATION OF METAL HELIDE IONS BY

ATOMIC TUNNELING

T. Tsong

To cite this version:

T. Tsong. A STUDY OF ULTRA-FAST ION REACTIONS WITH FEMTOSECOND TIME RESO- LUTION : FIELD DISSOCIATION OF METAL HELIDE IONS BY ATOMIC TUNNELING. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-151-C2-155. �10.1051/jphyscol:1986222�. �jpa-00225655�

A STUDY OF ULTRA-FAST ION REACTIONS WITH FEMTOSECOND TIME RESOLUTION : FIELD DISSOCIATION OF METAL HELIDE IONS BY ATOMIC TUNNELING

T.T. TSONG

Physics Department, The Pennsylvania State University, University Park, PA 16802, U.S.A.

A b s t r a c t - An i o n r e a c t i o n time a m p l i f i c a t i o n scheme f o r t h e t i m e - o f - f l i g h t s p e c t r o m e t e r i s proposed. With t h i s scheme i o n r e a c t i o n s can be s t u d i e d w i t h a time r e s o l u t i o n a s good a s 10-14 s e c . We have s t u d i e d f i e l d d i s s o c i a t i o n by atomic t u n n e l i n g of m e t a l h e l i d e i o n s such a s ~ h ~ e 2 + u s i n g t h i s scheme.

F i e l d d i s s o c i a t i o n of ~ h ~ e 2 + can occur by atomic t u n n e l i n g w i t h i n a few 10-l3 s e c i n a f i e l d of -4.8 VIA.

1. INTRODUCTION

The t i m e - o f - f l i g h t (ToF) mass and e n e r g y a n a l y s i s h a s r e c e n t l y g a i n e d c o n s i d e r a b l e p o p u l a r i t y because of i t s s i m p l i c i t y i n o p e r a t i o n , and t h e r a p i d p r o g r e s s i n t h e measurement devices.1-3 E l e c t r o n i c t i m e r s and o s c i l l o s c o p e s of subnanosecond t i m e r e s o l u t i o n a r e now commercially a v a i l a b l e , and i o n r e a c t i o n t i m e s i n t h e subnanosecond range can now b e measured w i t h o u t much d i f f i c u l t y . I n t h i s b r i e f r e p o r t we p r e s e n t a s i m p l e b u t v e r y e f f e c t i v e method f o r a m p l i f y i n g t h e i o n r e a c t i o n t h e , t h u s f u r t h e r improving t h e t i m e r e s o l u t i o n t o t h e subpicosecond range. With t h i s scheme t h o s e i o n r e a c t i o n e v e n t s t a k i n g p l a c e i n a time p e r i o d s 7 w i l l have t h e i r d e t e c t i o n s t r e t c h e d o v e r a much l o n g e r p e r i o d 6 t . T h i s method i s used t o s t u d y f i e l d d i s s o c i a t i o n by atomic t u n n e l i n g of m e t a l h e l i d e i o n s w i t h a time r e s o l u t i o n of 1 0 t o 20 femtoseconds.

II. PRINCIPLE OF I O N REACTION TIME AMPLIFICATION

The b a s i c p r i n c i p l e of t h e method c a n be b e s t i l l u s t r a t e d w i t h a s i m p l e and i d e a l i z e d example. L e t u s c o n s i d e r a p h o t o - d i s s o c i a t i o n r e a c t i o n a s r e p r e s e n t e d by

photon

(M + ^m)"+ absorption +M"+

+

+

.

The (fi)"+ i o n s a r e assumed t o be formed r i g h t a t t h e s u r f a c e , e.g. by p u l s e d i o n bombardment of a s u r f a c e , and t h e y a r e s u b s e q u e n t l y d i s s o c i a t e d by a b s o r p t i o n of pho- t o n s from a c o n t i n u o u s l a s e r s o u r c e o r a p u l s e d - l a s e r w i t h t h e p u l s e w i d t h much l a r g e r t h a n t h e d i s s o c i a t i o n time. The ToF system i s designed t o have two w e l l s e p a r a t e d s e c t i o n s as shown i n F i g . 1. The a c c e l e r a t i o n - r e a c t i o n s e c t i o n of l e n g t h 9. i s w e l l i s o l a t e d from t h e f i e l d f r e e f l i g h t s e c t i o n of l e n g t h L by a s m a l l a p e r t u r e . I f t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986222

JOURNAL DE PHYSIQUE

Acceleration Free flight s e c t i o n s e c t i o n

1

~ ~ of a time-of-flight spectrometer. t ~ ~ t ~ ~

width of the incident ion pulse is 6tp and the resolution of the time measurement de- vice is 6tt, then the overall time resolution of the system is 6t = (6tp2

+

Other resolution limiting factors will be omitted here to simplify the discussion of the principle of ion qeaction time amplification. An amplification factor will be defined as

where 6~ is the resolution in the measurement of the ion reaction time. We will de- sign the system with R<<L, so that the total flight time of the M ~ + ion is determined essentially by the final velocity vf(x) of the ion when it leaves the acceleration section. The total flight time is then given approximately by

where x is the location where the reaction, Eq. (l), has occurred. An uncertainty of 6t in the flight time measurement will result in an uncertainty of 6x in locating the position where the reaction has occurred, which will in turn cause an uncertainty of

in the reaction time measurement. They are related by neV0 L

6t = 6x

{TE- ^p~~jj}~'~

and

where

is the velocity of the (m+~)~+ ion just before dissociation. By combining Eqs. (3) to (6) one finds

112 A(x) =

ea

[F

4

₌

Thus the amplification factor is proportional to L/R, and is also dependent on the location where the reaction occurs. Maximum amplification occurs if x z R, or when the dissociation occurs after the parent ion has already gained the full energy of the acceleration voltage.

smaller mass whenever possible. An amplification of 103 to 105 should not be diffi- cult to achieve in many experimentç.

III. FIELD DISSOCIATION OF ~ h ~ e ~ + IONS

Field dissociation is a well known phenomenon in field ion emission. Hickes recog- nizes it to be a quantum mechanical tunneling phenomenon, and has developed a compre- hensive theory for it.4 Experimental studies using the magnetic sector mass spectro- meter have been reported by Beckey and coworkers.5 We report here observation of field dissociation of metal helide ions. We have been studying metal helide ion formation for some time using the pulsed-laser ToF atom-probe. In general metal hel- ide ions gan be formed by field evaporation in helium only if the field is above -4.2 to 4.5 V/A. When there are ions of more than one charge state, metal helide ions are found only for the highest charge state provided that the ionization energy of the metal atom of this charge state is less than the first ionization energy of helium.

For example, in field evaporation of W in helium, ions of 2+, 3+ and 4+ can be simul- taneously present in the mass spectrum, but only the 3+ ions contain helide ions.

For Mo, 2+ and 3+ ions are field evaporated but only the 3+ ions contain helide ions.

No metal helide ions are formed for non-refractory metals such as Au, Cu, Ni, Fe etc.

From these experimental observations, we can conclude that a metal helide ion is really a neutral helium atom bounded to a metal ion by a polarization force. The

---

binding energy of this ion-neutral atom interaction is estimated to be 0.2 to 0.4 eV.

If metal helide ions can be formed, some of them can also be field dissociated before they leave the high field region (in field free region, the law of conservation of energy prohibits dissociation of a two-atom ion unless a third body is provided to interact with the ion). This field dissociation is especially obvious for ~ h ~ e 2 + as can be seen in Figs. 2 and 3. Fig. 2 is a mass spectrum and an ion energy distribu-

~h': 55K, 9.2kV 5.0-4.8 V/A pH.

'

554 ions

FLIGHT T I M E (ns)

FIG. 2. A time-of-flight mass spectrum in pulsed-laser stimulated field desorption of Rh tip at 60 K in vacuum of 10-lO Torr range.

JOURNAL DE PHYSIQUE

t i o n of Rh i o n s formed by f i e l d e v a p o r a t i o n i n vacuum a t low temperature w i t h t h e s t i m u l a t i o n of l a s e r p u l s e s . The energy d i s t r i b u t i o n i s v e r y narrow w i t h a FWHM of only 3 t o 5 eV, corresponding t o an i o n i z a t i o n zone of width 0.3 t o 0.5 A. Fig. 3 i s a mass spectrum t a k e n i n 1 x 10-8 Torr of He. Beside t h e r e g u l a r Rh2+ mass l i n u , a mass l i n e of ~ h ~ e ~ + of s i m i l a r FWHM and a secondary Rh2+ peak appear. This second- a r y ~ h peak ~ +i s w e l l s e p a r a t e d from t h e r e g u l a r mass peak of Rh2+. The c r i t i c a l energy d e f i c i t of RhHe2+, c a l c u l a t e d from t h e o n s e t f l i g h t t i m e , i s almost i d e n t i c a l t o Rh*+ i o n s , i n d i c a t i n t h a t t h e s e i o n s a r e formed r i g h t a t t h e m e t a l s u r f a c e and t h a t t h e y a r e indeed Rh3+ i o n s bound w i t h He atoms by a p o l a r i r a t i o n force.6

The secondary peak i n ~ h 2 + i s due t o those Rh i o n s w i t h a much l a r g e r energy d e f i c i t , i.e. t h e y a r e formed a t a l a r g e distanc$+away £rom t h e m e t a l s u r f a c e . They can only be formed by f i e l d d i s s o c i a t i o n of RhHe i o n s v i a

atomic

~ h H e ~ + tunneling > ^Rh2+ + ^He

.

The energy d i f f e r e n c e between Rh2+ i o n s i n t h e main peak and t h e secondary peak AE

R h , 55K, 9.5 kV 4.9

-

^v/,&

PH^

1088 ions

FIG. 3. A t i m e - o f - f l i g h t mass spectrum i n pulsed-laser s t i m u l a t e d f i e l d d e s o r p t i o n of Rh t i p a t 60 K i n 1 t o 2 x 10-8 Torr of helium. The shaded p a r t i s obtained i n t h e e a r l y p a r t of t h e experiment.

100

-

90 ~ h 2 +

can be found from

Y) 8 0 - K

4 70

\ 60 5 0 - O

$4

40 LL

where n i s t h e charge s t a t e , t 0 i s t h e o n s e t f l i g h t time of t h e main peak. I f one goes through t h e e n e r g e t i c s of i o n formation, and using t h e p a r a b o l i c approximation f o r t h e e l e c t r i c f i e l d s t r e n g t h n e a r t h e t i p s u r f a c e , one f i n d s t h a t & i s r e l a t e d t o t h e l o c a t i o n x where t h e f i e l d d i s s o c i a t i o n o c c u r s by

- -

-

O 30 -

367%

493%

8 i

z 2 0 - At=30ns=51eV

22550 22700

22878 ns

FLIGHT TIME ( n s )

~ h ~ e ~ +

Use t h e e x p e r i m e n t a l d a t a A t = 30 n s , t o ⁼ 22444 n s , M = 1 0 3 u , m = 4 u , r o = 420

1

and k = 5 one o b t a i n s x = 220 A. The f l i g h t time of t h e ~ h ~ e ~ + o v e r t h i s d i s t a n c e can be shown t o be g i v e n by

t ( 2 2 0 1 ) = 7.9 x 10-13 s e c

.

Thus a s i g z i f i c a n t f r a c t i o n o f $hHe2+ i o n s i s f i e l d d i s s o c i a t e d i n a s p a t i a l zone of a b o u t 150 A w i d t h which i s 220 A above t h e t i p s u r f a c e . The f i e l d d i s s o c i a t i o n o c c u r s i n 8 x 10-l3 s e c , o r w i t h i n a few a t o m i c v i b r a t i o n s . Once a ~ h ~ e ~ + p a s s e s through t h i s f i e l d d i s s o c i a t i o n zone, i t becomes s t a b l e s i n c e t h e f i e l d becomes t o o low f o r t h e d i s s o c i a t i o n t o o c c u r .

The i o n r e a c t i o n t i m e a m p l i f i c a t i o n f a c t o r can be shown t o be g i v e n by

Using t h e e x p e r i m e n t a l d a t a one f i n d s t h e i o n r e a c t i o n time a m p l i f i c a t i o n f a c t o r a t x = 220 A i s 4.8 x 104. The time r e s o l u t i o n i n t h e r e a c t i o n time measurement i s 1 n s l 4 . 8 x 104 , o r 2 1 femtoseconds.

F i e l d d i s s o c i a t i o n i s a quantum mechanical t u n n e l i n g phenomenon of atoms which h a s no c l a s s i c a l

anal^^.^

where p = m / ( l + m/M) i s t h e reduced mass of t h e compound i o n , -+ rn i s t h e d i s t a n c e be- tween t h e two atoms and zn i t s component along t h e e l e c t r i c f i e l d d i r e c t i o n , and u(?,) i s t h e i n t e r a c t i o n between t h e two atoms. Atomic t u n n e l i n g i s p o s s i b l e because o f d e f o r m a t i o n of u(?,) by t h e f i e l d F a s r e p r e s e n t e d by - 2 e F z n / ( l + M/mf.

We have a l s o s t u d i e d f o r m a t i o n and f i e l d d i s s o c i a t i o n of ~ h H e 2 + w i t h 3He i s o t o p e . We f i n d t h a t t h i s h e l i d e i o n i s more s t a b l e and no pronounced secondary peak c a n be found. The r e a s o n i s n o t y e t c l e a r , and should be f u r t h e r i n v e s t i g a t e d .

The a u t h o r acknowledges h e l p from Y. L i u i n some of t h e e x p e r i m e n t a l p r e p a r a t i o n s . T h i s work h a s been s u p p o r t e d by NSF.

REFERENCE S

/1/ E. W. ~ G l l e r , J. A. P a n i t z and S. B. McLane, Rev. S c i . Instrum. 39 (1968) 83;

T. T. Tsong, S. B. McLane and T. J. Kinkus, Rev. S c i . Instrum. (1982) 1442.

/ 2 / K. S a t t l e r , J. ~ Ü h l b a c h , E. Recknagel and A. Reyes F l o t t e , J. Phys. (1980) 673; P. P f a u , K. S a t t l e r , R. Pflaum and E. Recknagel, Phys. L e t t e r s *, (1984) 262.

/3/ A. Benninghoven, I n t . J. Mass S p e c t r o m e t r y Ion Phys. 53 (1983) 85; W. Ens, R.

Beavis and K. G. S t a n d i n g , Phys. Rev. L e t t s . (1984) 2 7 T / 4 / J. R. HiskeS, Phys. Rev.

122

/ 5 / See f o r example, H. D. Beckey, F i e l d I o n i z a t i o n Mass S p e c t r o m e t r y , Pergamon P r e s s , 1971.

161 T. T. Tsong, Phys. Rev. (1984) 4946.