Field desorption of H3 and field dissociation of H+3

apphed surface sc,ence ELSEVIER

Apphed Surface Science 76/77 (1994) 108-114

Field desorption of

H 3

and field dissociation of H3

M C Reckzugel *, W Drachsel, J H Block Frttz-Haber- lnstltut der Max- Planck-Gesellschafi, Faradayweg 4-6, D-14195 Berhn, Germany

(Recewed 2 August 1993, accepted for pubhcahon 20 October 1993)

Abstract Ab-lnltlO calculations using a non-local spin-density approximation have been done for linear and triangular H + ions in an external homogenous electric field From these calculations it is predicted that linear H~ is not stable above 2 V / A if its molecular axis is parallel to the field vector, whereas triangular H~- resists field dissociation up to at least 3 1 V / A Linear H 3 is formed at kink sites on the surface of the field emitter Laser-stimulated field desorptlon of that H~ could lead to linear H~- In spite of the rotation of the H~- ion, a majority should field-dissociate in fields greater than 2 4 V / A However, if the linear H 3 is bending during laser-stimulated field desorptlon the more stable triangular H~ will be formed upon field Ionization The H~ field dissociation for fields between 2 4 and 3 1 V / , ~ was experimentally investigated using laser pulse correlated Ion pair spectroscopy in combination with a pulsed-laser atom probe During these measurements a total of 605 H~- ions arrived at the time-of-flight detector, but only one event occurred which could be attributed to H~ field dissociation However, H 2, + formed by field ionization of the H3~ field dissociation product H e, could have been field-dissociated also Therefore the H~ field-dissociation probability has been calculated for the case where the H~ molecular axis is parallel to the field vector Taking this maximum dissociation probablhty of H + into account, it followed from processing of the measured yields that the H~- field-dissociation probability is smaller than that of fleld-desorbed H~- for fields up to 3 1 V / , ~ Hence, it is Inferred that linear H~ bends during laser-stimulated field desorptlon, resulting in a more stable triangular H + 3 after field ionization

1. Introduction T h e dissociation of the H ~ ion caused by high electric fields was observed in field ion mass spectroscopy by I n g h r a m a n d G o m e r as early as 1954 [1] T h e field d e p e n d e n c e of this process was later investigated theoretically by H a n s o n , Hlskes a n d G r u n d l e r [2-4] F r o m the observed

* Corresponding author Fax +49 30 8305420

field dissociation of H ~ arises the q u e s t i o n w h e t h e r H ~ , p r o d u c e d from a d s o r b e d h n e a r H~ [5-7] which s u b s e q u e n t l y can be field-desorbed by a laser pulse [6], may field-dissociate T h e goals of this investigatton are, first, to evaluate theoretically the p o t e n t i a l surfaces of H~- dep e n d m g o n the geometrical structure a n d field, second, to e x p e r i m e n t a l l y show field dissociation + of H~- occurs a n d third, to estimate the H~ field-dissociation p r o b a b t h t y from the m e a s u r e d ion yields

0169-4332/94/$07 00 © 1994 Elsevier Science B V All rights reserved SSDI 0169-4332(93)E0247-J

M C Reckzugel et al/Apphed Surface Sctence 76/77 (1994) 108-114

2. Theory

direction At all other configurations higher dissociation barriers up to a few eV were obtained

2 1 FteM dtssoctatzon of hnear H3+

2 3 Bending vtbratton of hnear Hs+

H~- is stable when not influenced by an external electric field [8] To investigate the stability of linear H ~ in a homogeneous static electric field, calculations using a non-local spin-density approximation (NLSDA) have been p e r f o r m e d for the case of the H~- molecular axis being parallel to the field vector, using the computer program deMon [9] The field F is incorporated into the self-consistent field algorithm as an additional external potential Vext( R ) =Fx,

109

The potential-energy curves for the bending vibration of linear H0+ 3 have been calculated for fields up to 3 08 V / A [10] They show that linear H~ can convert into triangular H~ without an activation barrier and with an energy gain of 1 5 eV at 0 V/,~,, 2 4 eV at 2 06 V / . ~ and 2 8 eV at 3 08 V / A

2 4 Esttmates of the field-dtssoclatton probabthty of H2+

(1)

where R = (x, y, z) and F xs chosen to he along the x-axis For 2 06 V / A the calculation [10] predicts that the dissociation barrier of linear H~ vanishes and, consequently, the H 3÷ should fielddissociate into H 2 and H ÷ The formation of H 2 - H + is favoured over H ~ - - H due to the following two reasons First, If the linear H ~ ion is not aligned perpendicular to the electric field, polarization torces of the external field cause the two electrons to be driven towards the two protons of the H~- which are nearest to the tip Second, in the field-free case, the energy needed to produce H e - H ÷ is 2 87 eV (calculated from the difference of Etotal(lln n ~ - ) = - 3 4 83 eV [8] and Etotal(H 2) = - 3 1 96 eV [8]) compared to 4 83 eV for H - H ~ (calculated with Etotal(lin H ~ - ) = - 3 4 83 eV [8], Etotal(H~-) = - 16 40 eV [11] and Etotal(H) = - 1 3 60 eV [12]) For fields between 2 06 and 3 1 V / . ~ these energies will change only by a few hundred m e V

2 2 Fteld dtssoctatlon of triangular H3+ Calculations of the potential energms of triangular H ~ predict that the dissociation barrier D e decreases from 4 56 eV [8] at zero field to D e > 250 m e V at 3 1 V / A [10] The dissociation barrler of 250 m e V was calculated for a configuration where one side of the H ~ triangular configuration was perpendicular to the field and the opposite tip of the triangle pointed in the field

After the hypothetical field dissociation of linear H~ the formed H 2 may subsequently be field-ionized Hence, the H~- field-dissociation probability r/2F will be calculated, fixmg the molecular axis parallel to the field vector F, using F a n c k - C o n d o n factors [13] and taking into account that H~- dissociation by tunnelling might be possible The H 2 field ionization happens in the ionization zone within a few 10-I6 s [14] Therefore, it can be assumed that the B o r n - O p p e n h e i m e r approximation is valid, which means a charge transition takes place from H 2 to H ~ without movement of the two protons Thus, the wave functions ~ for H 2 and q~ for H~- can each be approxamated by a product of an electronic and a vibrational part The potential-energy curves for H 2 have been acquired by ab-lnltiO N L S D A The derived non-ideal numerical potentials V(r), where r denotes the internuclear distance, are introduced into the stationary Schrodinger equation For zero vibrational states which are only a few hundred m e V above the potential minimum it can be calculated from the J e f f r e y s - W e n t z e l Kramers-Brillouln approximation [15], that the H 2 barriers for fields up to 3 60 V / A are so large that tunnelling through them is negligible At each field strength only the zero vibrational state of H 2 has been determined numerically because the vibrational temperature for the first excited state is greater than 1000 K The corresponding electronic potentials for H~- have also been cal-

110

M C Reckzugel et al /Apphed Surface Sctence 76 / 77 (1994) 108-114

culated [16] with deMon, but using the local-density approximation (LDA) In a stmilar manner to the above dertvatlon of the wave function ~/r°b(r) for H e, the stable vlbraUonal wave functions • v~ib(r) for H~- have been d e t e r m m e d By applymg the F r a n c k - C o n d o n principle [13], the transltlon probablllttes from the H e zero vlbraUonal state to the different stable wbratlonal states of the H~- ton have been calculated [10] for the case where the molecular axis of H 2 was oriented parallel to the field vector Possible H~ dlssoctatlon by tunnelling through the potential barrier has been taken into conslderatton but has been found to be neghglble The calculated H~ fielddissociation probablhtles r/2 F were 73% for 2 1 V / A , 85 6% for 2 6 V / A , 97% for 3 1 V / , ~ and 100% for 3 6 V / A The respectwe H~- field-dissociation barriers were 768, 408, 152 and 10 m e V

3. Experimental method If H 3+ field-dissociates, the products H + and H 2 are formed Field ionization of H 2 leads to the final products H + and H ~ This ion pair could be detected by using the so-called "field ion pair spectroscopy" [17-19] The expected H { dissociation products could be masked by H + and H~ ton pairs which do not originate from H~ field dlssoctatlon These maskmg processes are, first, electron-stimulated field desorptlon (ESFD) [17] and, second, ions created at two surface sites For the definite identification of field dissociation products from H~-, the ton pair spectroscopic set-up was modified by adding a laser pulse coincidence technique The laser heats the tip, stimulating the field desorptlon of adsorbed particles from the tip surface Simultaneously, an elec-

Laserpulse

I

, h:toConstant fraction dlscnmmator

st l

y

7_1

TAC

out

to71 I( " sphtter Beam

,...] Dxgxtaldelay/ v I Pulse generator

i Aperture

135 ns delay

Upper MCP Constant fracUon dlscrmalnator

FIM plate

Lower MCP

C)

Gate ' Start J

I lr

,C

Fig 1 Schematic diagram of the electronic detection set-up The laser pulse starts the time-of-flight (ToF) tlme-to-amphtude converter (TAC) via a fast photocell and a constant-frachon discriminator and simultaneously triggers the Digital delay/Pulse generator The first ion of an ion pair starts the ion pair TAC and the second stops it If the ion pair TAC is m the gated mode the first ion can only start It when it arrives In the ume window of the pulse of the Digital delay/Pulse generator The amphtudes of the TACs are registered by pulse-height analysers (PHAs) and accumulated over many laser shots

M C Reckzugel et a l / A p p h e d Surface Sctence 76 / 77 (1994) 108-114

tronlc time delay is triggered which consequently activates the ion pair detector at the expected H ÷ arrival time If a H + ion reaches the detector during the calculated time window the clock of the ion pair detection is started The second incoming ion stops the clock So only the laserdesorbed time-correlated ion pairs (H ÷, H~-) or (H ÷, H~-) were counted Hence, using the laser to both initiate desorption and trigger the ion detector ensures that Ion pairs produced randomly by electron-stimulated field desorptlon (ESFD) are rejected from detection In order to exclude ions, formed at two surface sites, the detection area was confined by inserting an aperture to monitor a single atomic surface site

4. Experimental equipment The pulsed-laser atom probe has been described elsewhere [17,20] The nttrogen p u m p e d dye laser has been replaced by a Q-switched

Time-of-flight of hydrogen species at the ion pair T~C (lrrespectwe of gating)

_= Dlgttal delay/Pulse generator signal at the gated ion pair TAC Gate opens

~ ~

Time

Nd Y A G laser The wavelength used was 532 nm, the pulse repetition rate 50 Hz and the pulse full width at half maximum ( F W H M ) 11 ns The tlme-of-fhght (ToF) experiments were performed by allowing desorbed Ions to pass through the central probe hole of a field ion microscope (FIM) multi-channel plate (MCP) and an aperture as shown in Fig 1 The F I M image was used to identify the condition of the tungsten surface before and after the experiment and the position of the monitored surface site The electronics [17] for the detection of ion pairs were extended by gating one of the timeto-amplitude converters (TAC) by a Digital delay/Pulse generator (Fig 1) If the T A C lS ungated it samples ion pairs independent of the laser desorption In the gatted mode the T A C could only be started within the pulse width of 20 ns, generated by the Digital d e l a y / P u l s e generator, which began at the expected H + arrival time (Ftgs 1 and 2) The additional delay line in the start channel prevents activation of the stop regtster of the T A C by the start event

5. Experimental results

The mggergenerated by the $20 photocell startsthe time of flight measurement

The tnggergenerated by the $20 photocell alsostartsthe delay timegeneration

111

Gate closes

)

Fig 2 Time correlahon between gate pulse and H + flight time In the gated mode of the ion pair TAC (lower part of the figure) the gate pulse generated by the Digital d e l a y / P u l s e generator was 20 ns long, l e twice the F W H M of the H + ToF peak distribution, and delayed so that the leading edge arrived at the gated 1on pair T A C when the signal from the fastest H + ion after a laser pulse was expected If a H + ion arrived during the 20 ns window, then the gated T A C started its clock and stopped it upon arrival of a H ~ or a H~- ion In the ungated mode the TAC could detect ion pairs any time

After cleaning the emitter by heating to 650 K and field evaporation at 80 K hydrogen was continuously supplied at a pressure of 1 1 × 10 -2 Pa and the tip held at 80 K to attain high ion emission yields In the first step of the experiment the aperture for the ion beam (Fig 1) was adjusted in such a way that only one kink site, which was situated at the edge of the Inner most ring of the W(110) plane, could be seen at the upper part of the T o F - M C P Furthermore, the absence of (H +, H a ) ion pairs in the gated Ion pair spectra gave definite proof that only a single surface site was investigated The ion pair spectra are presented in Fig 3 and were taken at local field strengths from 2 48 to 3 10 V / , ~ The field strength was calibrated by takmg 2 2 V / . ~ for best image field for hydrogen [21] Each upper spectrum displays the ungated, and each lower spectrum the gated mode The peaks in the ungated spectra can be assigned to ion pairs of (H~-, H~'), (U +, H~-) and (H +, H~-)

M C Reckzugel et al /Apphed Surface Science 76 / 77 (1994) 108-114

112

+

!

0 hh,,,~l,,~,.~.........~,h I,..,

(i

I |

(+

--~ 15

H

0

o

I] ~,l,l.,lla.. l i n t .

~1~ l . . . . .

I

+-

.i~]m~,. ~..=,a~ . . . .

~,,13,Lgd,],.

......

II

c

H2 )t

Jh~lel

(H+H/-)

. ~ l...i.., ak. ~ ° ~ , ~ , ~ ,ul~,l~k° I,

15

0

"t+H[) 1,

'~

d

o ik,~hL,~.,.~,~a~,~...... ~ u~d,~-,.~,,, h~,,,~,aa,&..... ~.a..,,a,, 15

0 0

100

260

360

460

560

600

700

zxToF / channels Fig 3 Ion pair spectra depending on field in gated (only H ~ field dlssoclaUon) and ungated mode (ESFD and possible H ~ dissociation) The T o F difference A T O F was m e a s u r e d in channels (1 channel = 0488 ns), T = 80 K, p = 1 1 × 10 2 Pa and the laser r e p e t m o n rate was 50 Hz Spectra (a) 2 48 V / , ~ - top 1500 s acqulsltmn time, ungated, bottom 1500 s acqmsition time, gated (b) 2 6 4 V / ~ , - top 1700 s acqmsltion time, ungated, bottom 2000 s acqulsltmn time, gated (c) 2 87 V/dK - top 1500 s acquisition time, ungated, bottom 2000 s acqmsition time, gated (d) 3 10 V / A - top 1000 s acquismon time, ungated, bottom 1500 s a c q m s m o n time, gated Only m case (c) one smgle (H +, H~- ) ion pair responsible for H~- field dissociation was detected

However, they do not correlate with the laser pulse and are due to ESFD In the gated mode there is only one single event (at 2 87 V / A ) which could be assigned to a (H +, H~-) ion pair The respective ESFD rate of (H +, H~-) ton pairs in the ungated spectrum at 2 87 V / / ~ was 2 3 ×

10 -2 S 1 after subtraction of statistical background This rate is eight orders of magnitude too small to explain the occurrence of the single event in the gated ion pair spectrum by a colnmdental ESFD event A diagram, as depicted in Fig 4, can be drawn to determine the relationship between the dissociation probability for the H~- Ion r/2(F) and for the H~ ion r/3(F) from the observed ion yields by the ToF detection, o(°F(F), oT°F(F) and oT°F(F), for H +, H~ and H ~ , respectively In 0 v°v is taken into account that the ToF TAC, activated by a laser-desorbed H +, cannot register a succeeding H~ As can be seen in Fig 4 the dlssoclatlon probablhty of H~- which originates from H~ is distinguished by the index 2F The species H + as an ion generated at the critical distance from laser-desorbed H ts not included in the diagram According to our observations from previous experiments and Fig 30 of Ref [22], observed H + is always a field-dissociation product at fields below 4 V / A Four independent equations for the ToF yields of the three ions can be extracted from Fig 4, presuming the same detection probablhty p for H +, H)- and H ~ , with four unknown vartables, l e the ion yields 02(F) for H~ and O~(F) for

Ion yaeld upon field desorptmn

Intermediate ion yields

Ion ymld at To~ detector ~ToF .....-t~ u 3

1- t13 13

Fig 4 Diagram to determine the dissociation probabllitms of H ~ and H ~ from the T o F rates of H +, H ~ and H ~ H ÷ generated by laser-stimulated field desorptlon of H is omitted because the field strength was never sufficient for H desorptlon m th~s experiment

M C Reckzugel et al/Apphed Surface Sctence 76/77 (1994) 108-114

113

Table 1 Ion yields of laser-stimulated fleld-desorbed H +, H~- and H~ and gated (H +, H~-) ion pairs at different field strengths

from gated ion pair measurements Using p = 50% [23], elementary transformatton gives

F(V/A.)

"02 =

H+ counts H~counts O~°V(F) 02X°F(F)

H~-counts O3T°F(F)

(H+,H~) counts

'07 °F

350 851 709 199

398 588 217 50

282 247 73 3

0 0 1 0

20gated

1 - r/2F

,0gated(F)

2 48 2 64 2 87 3 10

5"0 gatedr/2F

× / 1 5 ( O ~ ° F - - 0 ~at°d) + O1ToF

[ 3 519gatedr/2F

2ogated ] -1

1 - r/2F

(3a)

]

2ogated

H~- generated upon field desorptlon, and the d i s s o c i a t i o n probabilities r / 2 ( F ) and r / 3 ( F )

r/3 = o]oF(1 _ r/ZF) + 2ogated

oToF= p03(1 - rl3),

(2a)

O2x°F =p[O2( 1 - r/z ) +pO3r/3(1 - r/ZF)],

(2b)

Inserting the respective yields given in Table 1, r/z(F) and r/3(F) are obtained and plotted in Fig 5 It shows that the dissociation probability r/z(F) is at least three times greater than r/3(F) for fields between 2 5 and 3 1 V / A

O~r°~ = (1 -pZ)O2r/2

+

L,q3r/3

× [(1 - p3)r/ZF + p ( 1 -- r/ZF)], og"t~d =PZd3r/3(l -- r/ZF)

(2C) (2d)

The last equation represents the yields obtained

1

theor 112~

08 ~ o6 e., O

N

04

~

o2

>/ •

o 5

2

25

/ 1111

6. Discussion

Calculations in Section 2 predict that linear H~- will be less stable than H~- for F > 2 V / A ( e g for F = 2 0 6 V/A De(H~-)=768 meV, De(H ~-) = 0 meV) This IS opposite to the experimental result in Fig 5 Therefore, it is mferred that linear H 3 bends during laser-stimulated field desorption prior to field ionization When it ~s ionized triangular H~- is formed which (as can be extracted from Section 2) is energetically more favourable and more stable than linear H~- in fields up to 3 1 V / ~ ,

7. Conclusion

•

3

(3b)

35

Field strength F / (V/A) F,g 5 Dxssoclatlon probabditles of H~- and H~- (calculated from Eqs (3a) and (3b) using the values from Table 1) The crosses denote rt2F(F) from theory (Sechon 2) which have been included for comparison As expected, "qzv(F) is greater than the experimental -qE(F), which is marked by solid squares, because H~- rotates after field desorptlon The sohd orcles denote the dlssocmtmn probabdlty "o3(F) Note that H~- ~s more than three times less stable than H~

For the H~- field dissociation a novel experimental set-up and theoretical treatment has been described The field dependence of the H ~- dissociation has been theoretically studied From the calculations tt can be said that linear H + should field-dissociate in fields greater than 2 ~¢/A because the energy barrier vanishes when its molecular axis ts parallel to the field vector However, experimentally only a single (H +, H~-) ion pair

114

M C Reckzugel et al /Apphed Surface Sctence 7 6 / 7 7 (1994) 108-114

c o u l d h a v e b e e n a t t r i b u t e d to t h e f i e l d d i s s o c i a t i o n o f H~- e v e n t h o u g h a t o t a l o f o v e r 600 H~i o n s w e r e d e t e c t e d at t h e t l m e - o f - f h g h t d e t e c t o r durmg the measurements These results can only b e e x p l a m e d ff t h e l i n e a r l y a d s o r b e d H ~ b e n d s during pulsed-laser field desorptlon Upon field x o m z a t l o n , t h e r e s u l t a n g t r i a n g u l a r H ~ is p r e d i c t e d f r o m c a l c u l a t i o n to h a v e h ~ g h - e n o u g h d~sSOClat~on b a r r i e r s so t h a t it r e m a i n s m o r e s t a b l e t h a n l i n e a r H ~ m f i e l d s u p to at l e a s t 3 1 V / A

[5] [6] [7] [8] [9]

[10] [11] [12]

8. Acknowledgements W e w o u l d h k e to t h a n k X Y e f o r p l o t t i n g e l e c t r o n d e n s i t y c o n t o u r s o f H 3~ a n d f o r p e r f o r m m g H~- L D A c a l c u l a t i o n s , H J K r e u z e r f o r fruitful d i s c u s s i o n s , a n d T T e s s n e r for c r i t i c a l r e a d i n g o f t h e m a n u s c r i p t F l n a n c m l s u p p o r t by V o l k s w a g e n F o u n d a t t o n 1 / 6 6 117 is g r a t e f u l l y a c k n o w l edged

[13] [14] [15] [16] [17] [18] [19]

9. References

[20] [21]

[1] M Inghram and R Gomer, J Chem Phys 22 (1954) 1279 [2] G R Hanson, J Chem Phys 62 (1975)1161 [3] J R Hlskes, Phys Rev 122 (1961) 1207 [4] W Grundler, J Comput Chem 11 (1990)548

[22] [23]

N Ernst and J H Block, Surf Scl 126 (1983) 397 T T Tsong and T J Kmkus, Phys Rev B 29 (1984) 529 N Ernst and J H Block, Phys Rev B 29 (1984) 7092 I Cslzmadla, R Karl, J Polanyi, A Roach and M Robb, J Chem Phys 52 (1970)6205 D R Salahub, R Fournler, P Mlynarskl, I Papal, A St-Amant and J Ushlo, in Density Functional Methods in Chemistry, Eds J K Labanowskl and J W Andzelm (Springer, Berlin, 1991) p 77 M C Reckzugel, W Drachsel and J H Block, to be pubhshed A G Turner, Methods in Molecular Orbital Theory (Prentice-Hall, Englewood Chffs, NJ, 1974) p 51 American Institute of Physics Handbook, 2nd ed (McGraw-Hill, London, 1963) pp 7-14 A S Coolidge, H M James and R D Present, J Chem Phys 4 (1935) 193 R Gomer, Field Emission and Field Ionlsatlon (Harvard University Press, Cambridge, MA, 1961) p 70 E C Kemble, The Fundamental Principles of Quantum Mechanics (McGraw-Hill, London, 1937) p 111 X Ye, unpublished calculation J Dirks, W Drachsel and J H Block, Surt Sci 246 (1991) 150 J Dirks, W Drachsel and J H Block, Surf Scl 266 (1992) 62 J Dirks, W Drachsel and J H Block, Appl Surf Scl 67 (1993) 118 W Drachsel, T Jentsch and J H Block, Int J Mass Spectrom Ion Phys 46 (1983) 293 E W Muller and T T Tsong, Field Ion Microscopy (Elsevier, New York, 1969) p 12 X Ye, PhD Thesis, Dalhousle University, Halifax, Canada (1992) T Hashlzume and T Sakural, J Phys (Paris) C2 (1985) 425