FIELD DISSOCIATION BY ATOMIC TUNNELING OF COMPOUND IONS : ISOTOPE AND ORIENTATIONAL EFFECTS

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FIELD DISSOCIATION BY ATOMIC TUNNELING OF COMPOUND IONS : ISOTOPE AND

ORIENTATIONAL EFFECTS

T. Tsong

To cite this version:

T. Tsong. FIELD DISSOCIATION BY ATOMIC TUNNELING OF COMPOUND IONS : ISOTOPE AND ORIENTATIONAL EFFECTS. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-81-C7-86.

�10.1051/jphyscol:1986715�. �jpa-00225905�

JOURNAL DE PHYSIQUE

Colloque C7, supplement au n o 11, Tome 47, Novembre 1986

FIELD DISSOCIATION BY ATOMIC TUNNELING OF COMPOUND IONS : ISOTOPE AND ORIENTATIONAL EFFECTS

T.T. TSONG

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

Abstract

-

I. INTRODUCTION

Dissociation of compound ions in high electric field occurs by tunneling of a neutral atom out of the ion, similar to field ionization where an electron tunnels out of an atom. A quantum theory of field dissociation was given by Hiskes/l/ in 1961 and an experimental observation was reported by Riviera and Sweetman/2/ in the same year. In field ionization mass spectroscopy, Beckey and coworkers/3/ inter- preted the low energy tail often observed in mass spectral lines of large organic molecules to be produced by field dissociation. Hanson/4/ interprets a m a l l secondary peak in field ion energy distribution of H+ to be produced by further field ionization of field dissociated H. Many of these earlier works involve with systems such as H2+ or AD+ where the remaining electron in the system revolves around the two nuclei with a period much shorter than the period of the atomic vibration. Thus the concept of orientational effect in field dissociation is vague at best. In field ionization, a low energy tail can also be produced by space field ionization of hopping molecules, as well as further field ionization into 2+

ions followed by Coulomb repulsive dissociation (or Coulomb explosion). The inter- pretation.~£ Hanson, if it were correct, would produce two low energy peaks

instead of only one as observed in the experiment. There are some compound ions in field desorption which are ideally suited for studying kunneling and orientational effects in field dissociation. These are metal helide and metal hydride ions. Our data/5/ on HeRh2+ are particularly well defined and simple to interpret for the following reasons: ( 1 ) The first ionization energy of He is higher than the second ionization energy of Rh by 6.5 eVi thus the He r e i n s neutral and is bound to ~ h ~ + by a polarization force. (2) HeRh2+ ions are always desorbed from the surface in the same orientation. Thus a "phase coherencen is achieved in the rotational motion in the experiment. This orientational coherence cannot be achieved in field ioniza-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986715

C7-82 JOURNAL DE PHYSIQUE

tion mass spectrometry. ( 3 ) The reduced mass of the system is small enough for tunneling to be important. (4) HeRh2+ ions always originate right at the surface.

There is no free space ionization of hopping molecules. Thus no tails or secondary peaks can be produced by free space ionization as can in field ionization.

11. EXPERIMENTAL RESULTS AND DISCUSSIONS

In 10-I Torr vacuum, when a.Rh tip, cooled down to

-

~h': 55K. 9.5ekV 5.0- 4.8 V/A

&, : 0 55L ions

FLIGHT TIME (ns)

Our high resolution pulsed-laser atom-probe is almost canpletely free from noise signals. If the same experiment is done in '1x10-8 Torr of 4 ~ e , then the ToF s p e c trum contains a 4 ~ e + line, a Rh2+ line with a low energy secondary peak, and a 4 ~ e R h 2 + line. These are shown in Figs. 2 (a) and (b)

.

line is identical to,that shown in Fig. 1, and the He+ line is identical to that of pulsed-laser field desorbed E?e+ without field evaporation of the substrate. A flight time difference of 30 ns of Rh2+ in the main peak and the secondary peak cor- responds to an energy difference of -51 eV. With use of the field distribution of a parabolic electrode configuration one finds that ~ h 2 + ion in thp secondary peak is formed in a spatial zone of -150A width which is centered at-220? above the emitter surface (Fig. 3). It will take the ion (790k20) fs to travel 220A under the accel- eration of the field. If the dissociation is induced by further field ionization followed by Coulomb dissociation into a He+ and a then the He+ ions will have an energy -600 eV less than those in the main peak and should show up at the flight time indicated by the arrow in Fig. 2(a). No such peak indicates that HeRh2+ dis- sociates into a neutral He and a Rh2+, exactly what is to be expected £ran field dissociation effect. The neutral He will acquire only 51 eV and cannot be detected in our system.

HeRh2+ field dissociates in a well-defined spatial zone for the following reasons.

The relative motion of He and Rh2+ in a HeRh2+ ion in a field

3

Fig. 2 ( b ) .

Field Evap. o f Rh at 9.5 kV 1 x Torr ' ~ e

#

I

64CO 66M) 6800

FLIGHT TIME

Rh, 55K, 9.5 kV 4.9

-

^VIA

' ~ e : 2 x 10' torr

22LL4 ns 22878 ns

FL[GHT TIME (ns)

where

? ,

fn

If it is not dissociatfl it has to wait for an additional rotation of 360'. By this time the ion is -1400 A away from the surface and the field is too low to cause field dissociation. Thus (790f20) fs is the time for the ion to rotate 180'. With use of a semiclassical estimate, this time is given by

JOURNAL DE PHYSIQUE

Fig. 3 .

for large j, wheze w is the angular speed when the rotational quantum number is j.

Taking rn = 2 .O A and p - 6 . 3 9 ~ 1 0 - ~ ~ kg, one finds that t10=7.3x10-13 s. This agrees best with our measured time. This state has maximum population at 290 X. The tip temperature was 55 K but laser pulses heated the surface to -200 X or slightly high- er for a few nanoseconds. This calculation, however, does not account for the torque of the applied field. The angular acceleration time and the question of how the value of j increases as a result of the applied electric torque are fundamental problems which should be of interest to quantum theorists.

When 4 ~ e is replaced with 3 ~ e , the secondary ~ h peak disappears as shown in Fig. ~ + 4. This dramatic isotope effect is due to the potential-barrielcreduction tern, -2eFzn/( l+M/m): its magnitude is greatly reduced by the replacement of 4 ~ e with 3 ~ e . When the potential barrier of ~ e ~ is shown in Fig. 5 (a) and (b) h ~ + for both 3 ~ e and 4 ~ e at different field strength. The interparticle potential U(r) is based on a calculation of Cole and ~song/6/. Although naively one would expect 3 ~ e R h 2 + to dissociate more readily because of the smaller reduced mass, the oppo-

Fig. 4.

Rh, 55K, 9.5 kV 1x10' Torr He

4.9 -4.6

VI A

: : : : " '

22joo ' ;j,8;ns ^{' 22650} ( ' ' ' I 230w 22878 ns

FLIGHT T I M E (ns)

DISTANCE <A> DISTANCE

<A>

Fig. 5 ( a ) . Fig. 5(b).

site is found in the experiment. This unexpected behavior can be explained with a WKB calculation of barrier penetration probabilities using the potentials shown in Fig. 5(a) and (b). The calculated barrier penetration probabilities as a function of field for 4~e.Rh2+ and 3HeRh2+ are shown in Fig. 6. The isotope effect observed can be qualitatively explained by this calculation. The barrier penetration prob- ability for 3 ~ e R h 2 + is a few order of magnitude smaller than that for 411eRh2+, in qualitative agreement with the experimental observation.

FIELD

(v/A)

Fig. 6 .

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There are at least three reasons for field dissociation of IieRh2+ to be fundamental- ly interesting. ( 1 ) It involves with tunneling of a mass 4 particle, thus is very similar to a-delay. ( 2 ) The potential barrier depends not only on an experimental- ly adjustable parameter F, but also on the ratio of the masses of the two particles.

This mass ratio dependence can'be studied by observing isotope effects. (3) This tunneling effect is ion orientation dependent. Thus the rotational time of the compound ions in high electric field can be directly studied. The dissociation time measured, (790f 21 fs, which is also the rotation time of 4 ~ ~ h 2 + for 180', is believed t o be one of the fastest reaction times ever being successfully measured, especially with completely resolved peaks. Such a fast reaction can occur only by a tunneling effect.

This w o r k w a s supported by NSF. The help of Y. Liou in some of the experimental preparations is gratefully acknciwledged.

REFERENCES

1 . J . R. Hiskes, Phys. Rev. 122, 1207 ( 1 9 6 1 ) .

2 . A. C. Riviera and D. R. Sweetman, Phys. Rev. Lett. 2, 560 ( 1 9 6 0 ) . 3. H. D. Beckey and 8 . Knoppel, 2 ; . Naturforschq.

*,

5 . T. T. Tsong and Y. Liou, Phys. R w . Lett. 55, 2180 ( 1 9 8 5 ) ; T. T. Tsong, Phys. Rev. Lett. 55, 2826 ( 1 9 8 5 ) .

6 . T. T. Tsong and M. Cole, to be published.