Influence of the charge state of 1.16 MeV/u incident ions on the desorption process

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Submitted on 1 Jan 1987

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Influence of the charge state of 1.16 MeV/u incident ions

on the desorption process

S. Della-Negra, O. Becker, R. Cotter, Y. Le Beyec, B. Monart, K. Standing,

K. Wien

To cite this version:

Influence of the

_charge

state

of 1.16

MeV/u incident ions

on

the

desorption

_process

S.

_Della-Negra,

O.

_Becker(*),

R.

_Cotter(**),

Y. Le

_Beyec,

B.

Monart,

K.

_Standing

_(***)

and K. Wien

_(*)

Institut de

_Physique

_Nucléaire, B.P. N° _1, F-91406 _Orsay, France

(*)

Institut für

_Kernphysik,

_Darmstadt, F.R.G.

(**)

John

_{Hopkins University,}

School of _{Medicine, Baltimore,} U.S.A.

(***) University

of _Manitoba,

_Winnipeg,

Canada

(Reçu

le 9 juin 1986, revise le 3 _{octobre, accept6} le 10 octobre

₁₉₈₆₎

Résumé. 2014 L’accélérateur linéaire d’ions lourds de l’Institut de

Physique

Nucléaire

_d’Orsay

a été _{utilisé pour} étudier l’influence de l’état de

_charge

_{incident qi}

des

_{projectiles (ions}

Ne, Ar, Kr de 1,16

MeV/u)

sur l’émission

ionique

secondaire à

_partir

de films minces solides

_organiques

et

_{inorganiques.}

Le rendement d’émission

_ionique

dépend

fortement de l’état de

_{charge qi}

de l’ion incident mais aussi de la nature du

_projectile

_(numéro

_atomique).

La variation du rendement d’emission entre les trois _types de

_projectiles

est de la

_{forme y ~}

_{q4eq, qeq}

_{étant pour}

_chaque

projectile

la

_{charge d’équilibre}

calculée à l’intérieur du matériau. Abstract. 2014

The

_heavy

ion linear accelerator of the Institut de

_Physique

Nucléaire has been used to

_study

the influence of the

_{projectile charge}

_{state qi} on

_secondary

ion emission. Ions of _Ne, Ar or Kr with a

_velocity

of 1.16 MeV/u bombarded thin films of

_organic

and

_inorganic

solids. The

_experimental

_arrangement is described. The ion emission

_yield

is

_{strongly dependent}

on the

_charge

state of the incident

_{ion qi}

and of its atomic number

_(nature

of the

_projectile).

The emission

_yield

between the three _types of

_projectiles

varies as

q4eq

where _qeq is the

_equilibrium

charge

state within the material for each

_projectile.

Classification

Physics

Abstracts 79.20

When a

_heavy

ion of several MeV/u strikes the

surface and

_penetrates

the interior of an

_insulating

solid,

a _great deal of

_damage

is created

_along

the ion track. The incident

_particle

loses _energy

predomi-nantly

through

its interaction with the electrons of the

_solid,

and much of this _{energy remains} localized. The zone, where

permanent

complete

damage

of the

surrounding

bulk material is

_observed,

has a radius of a few tens of

_Angstroms

_depending

on the

_specific

energy loss and the type of material

_{[1, 2].}

The

region

of small defects

_(«

extended

_{defects »)}

is however much _greater with a radius of several hundred

_Angstroms.

Effects of the bombardment

are

_particularly

noticeable on the

_surface,

where various

_particles

_{may be}

_ejected

such as for instance atoms, atomic clusters or more

_complex

structures like very

large organic

molecules

_containing

several

hundred atoms

_[3-6].

Experimental

studies of the influence of various

entrance channel

_parameters

on the

_desorption

process have

already

been carried out

_using

beams

from

_heavy

ion accelerators

_[7-10].

Measurements

have been made over a

_large

mass _range

_(N

to

_U),

as well as some measurements at

_high

_energy

_(up

to 5

_Mev/u)

for

_projectiles

of intermediate mass. These have

_{yielded fairly complete}

results on the behaviour of the

_secondary

ion

_yields

as a function of the

primary

ion

_{velocity [11].}

The

_desorption

_processes

depend

in a

_complex

_way on the

_dE/dx

of the ion in

the material and thus on the

_charge

of the incident ion. A variation of several orders of

_magnitude

in

yield

between

_nitrogen

and uranium has been ob-served in

_experiments

at G.S.I. Darmstadt

_[10].

It seemed

_{particularly interesting}

to

_exploit

the

properties

of the

_{Orsay heavy}

ion linear accelerator

(the injector

for

_ALICE)

in

_continuing

such studies. This machine can accelerate a

_large

_{range of} ions

(from

12C

to

_{109 Ag}

or

134Xe)

and its energy of

262

1.16 Mev/u is close to the value where the

_desorption

yield

is maximum

_{[5, 9,11]. Experiments}

complemen-tary to the

_previous

measurements

_[10]

have therefore been undertaken at

_Orsay,

in collaboration with the

Darmstadt group.

Here we describe the overall

experimental

method,

and

_present

results

_illustrating

the influence

of the incident ion

_charge

state on the

_secondary

ion

emission from the surface. We have examined thin

deposits

of

_organic

molecules as well as

_inorganic

targets,

such as thin films of CSI.

1.

Experimental

method.

1.1 GENERAL DESCRIPTION. - The ion beam from

the linear accelerator is focused onto an area of several

mm2

on a _very thin carbon foil

( -

20

JLg/cm2)

at the

centre of a reaction chamber. In

_{passing through}

the foil the ions lose a certain number of electrons and emerge with a distribution of

_charge

states.

_Figure

1 shows the distributions measured

_by

E. Baron

_[12]

for

the ions used here

_(Ne,

Ar and

_Kr).

Fig.

1. -

Charge

distribution after

_pasing

_through

a

20

_wglcm2

carbon foil and measured after the

_magnetic

analyser

(from

Ref.

_[12]).

At 50 cm from the centre of the chamber there is a slit of variable width

₍₂

to 10

_mm).

It is mounted on a

sliding

window which serves as the entrance _port of a

movable

_magnet

_(radius

=

0.8 m,

Bmax

= 1.5

tesla).

The

_angle

of this

_magnet

entrance slit with

_respect

to the centre of the chamber can be set _very

_precisely,

so it is

_possible

to

_accept

the beam

_elastically

scattered

through

a well-defined

_angle.

The beam

_intensity

is controlled

_by

_varying

the

_{angle (usually}

_5°).

The different

_charge

states of the scattered ion beam

can be

_separated

_by

_varying

the

_magnetic

field. Measurement of the

_magnetic

field

_strength

determines the

_charge

state selected.

The _{energy of the beam scattered}

_through

a

_given

angle

can be calculated

_classically

_{(elastic scattering) ’}

and for the small

_angles

used

_(a

few

_degrees)

_{the energy} of the scattered ions is close to the _{incident energy.}

Likewise the energy loss in the carbon foil is small

( ~

1 MeV for

_84Kr).

Figure

2 shows a schematic

diagram

of the exper-imental _arrangement

_(magnet

+

_{spectrometer).}

At the

exist of the

_magnet

there is a device for

_measuring

and

viewing

the

beam,

then the

_{time-of-flight}

_{spectrometer.}

Figure

3 shows a schematic

_diagram

of _the spec-trometer, which has

_already

been described in re-ference

_[9].

Detector

_Dl,

which

_usually

measures the

time of arrival of the

_primary

ions in order to determine their

_velocity,

is used here to control and

_adjust

the

scattered beam

_intensity

before

_starting

an

experiment.

During

measurements this detector is _swung out of the beam to the

_position

shown.

Fig.

2. - General view of the

experimental equipment.

Fig.

3. - Schematic

diagram

of the _apparatus for

_measuring

secondary

ions and electrons.

Secondary

ions

_ejected

from the

_target

are

acceler-ated

_by

a fine

_grid

at a

_potential

of 6

kV,

then recorded

by

detector

_D4,

a

_pair

of microchannel

plates.

This

provides

the « _{stop »}

signal

for

measuring

their

Detector D3 is a 150 um Si detector which is used to

monitor the incident beam after _{it passes}

_through

the

target. The

_{signal corresponding}

to the arrival of each

primary

ion at the Si detector

_provides

the « start »

signal

for

_measuring

the

_{time-of-flight}

of the

_secondary

ions. Detector D3 also measures the

_primary

ion

energy, so it is

_possible

to set a coincidence window in

the _energy

_spectrum.

_Secondary

ions are thus recorded

only

when the

_{corresponding primary}

ion has the

correct _energy. This is

_{particularly important}

when the selected ion beam is rather

_impure,

as shown in

figure

4. It was found

_possible

to reduce the

_parasitic

peaks

greatly

by adjusting

the

_operating

_parameters of the

_accelerator,

_{but in any} case the coincidence window selected

_only

those events

_{produced by}

_primaries

of the correct _energy.

Fig.

4. -

Ion energy spectrum measured with a silicon detector. The _peak

_{corresponding}

to Kr5 + is indicated in the

figure together

with the limits of the coincidence window.

The

_following

table shows the ions and the selected

charge

states for the first

_experiments

carried out with

this

_experimental

_arrangement.

Thin

_deposits

of caesium iodide and

_{phenylalanine}

(C9H11O2N)

were

prepared by

vacuum

_evaporation

onto aluminized

_mylar.

The thickness of the

_deposits

was - 2 000

A

so as to obtain a

_homogeneous

_target.

The ions emitted

_by

bombardment with

_primary

ions of several MeV/u are well known and exhibit

characteris-tic

_peaks

in the

_{time-of-flight}

_spectrum

_[9,10].

Positive or

_negative

ions are detected when the acceleration

grid

is at a

_negative

or

_{positive potential respectively.}

Figure

5 shows an

_example

of a time of

_flight

_spectrum

obtained from the

_{phenylalanine}

_target.

The molecular

JOURNAL DE PHYSIQUE.-T. 48, N* 2, FTVRIER 1987

Fig.

5. -

Example

of a time of

_flight

mass _spectrum for an

organic

solid -

phenylalanine

-

the molecular ion

( M - H ) -

is

_clearly

visible. This spectrum is obtained from

only

about 5 000 Kr ions incident on the _target.

ion

_{(M -}

_{H) -}

at mass 164 u is one of the dominant

peaks.

1.2 MEASUREMENTS OF SECONDARY ION YIELDS. -The collimators shown in

_figure

3 were

_carefully

_aligned

to ensure that _every

_primary

ion

_{passing through}

the

target struck the Si detector. The number of

_primary

ions with a selected

_charge

state was obtained

_by

integrating

the selected

_peak

in the _energy

_spectrum

of this detector. The number of

_secondary

ions of mass M was obtained

_by

_integration

of the

_{corresponding}

_peak

in the time-of

_flight

_spectrum, coincident with events in

the

_primary

_{energy window.} The

_yield

is the ratio of the number of

_secondary

ions to the number of

_primary

ions.

The

_primary

ion beam is

_pulsed,

with a

_repetition

frequency

of 24 MHz. In order to avoid a second

primary

ion

_striking

the _target after the start of a time interval

_{triggered by}

a

_given

_primary

ion,

it was necessary to limit the number of

_primary

ions to

1000 per second or less.

The

_{time-of-flight}

measurements of the

_secondary

ions were carried out with a time

_digitizer

_(TDC)

which can _accept 255 _stop

_signals

for each start

_[13].

_However,

if several ions of the same mass are

_ejected

_by

a

_given

primary

ion,

their times of

_flight

are the same,

and

the TDC delivers

_only

a

_single

_output. The measured

_yields

have been corrected for this effect

_{by assuming}

a Poisson distribution for the emitted ions. The

_following

relation then

_gives

the total

_{yield Y,}

for an ion of

_given

264

Here p

is the overall detection

_efficiency

for ions of the

_given

mass. For measurements of the relative

_yields

as a function of the incident ion

_charge

_state, it is

necessary

only

that p

is

_independent

of this _parameter. For a measured

_yield

_ym =

0.3,

for

instance,

the total

yield yt

reaches

0.36,

or for _ym = 0.9 it is 2.3.

2.

_Experimental

results and discussion.

During

all the

_experiments

the

_targets

were

_kept

in

place

in the

_spectrometer

and a _{pressure of}

10- 6

torr

was maintained with a turbomolecular _pump.

2.1 CAESIUM IODIDE TARGET. - The variation of the emission

_yield

of the

_secondary

ions Cs+ and

( CsI )

Cs+ as a function of the

_primary

_charge,

state is shown in

_figure

6 for the three

_types

of

_primary

ions

used,

neon, argon and

krypton.

Since

H+ ,

ions are

always

desorbed

_abundantly,

their emission

_yield

is also

given

in the

_figure.

The

_yields

for different incident ions are

_{directly comparable.}

Fig.

6. - Variation of

desorption yield

as a function of the

charge

state for the ions H+ Cs+,

( CsI )

Cs+ from a caesium iodide target. All

_yields

_{may be}

_{compared directly}

but the

_yields

_{per incident}

_particle

are

_only

relative. The lines serve as

_guide

to _{the eyes.}

Figure

7 shows similar results for

_{negative secondary}

ions,

including

the ion of mass

24 (C2 ),

which is

always

present in the

_negative

_spectrum. We note that

the qi dependence

for

_Ci

_{is very} different from the _qui

dependence

for the ions characteristic of the thin film

itself.

The results shows in

_figures

6 and 7 for bombardment

of a CsI

_target

_{by primary}

ions of well defined

_charge

qi show

clearly

the

importance

of this parameter in the

desorption

process.

Fig.

7. - Variation of

desorption yield

as a function of the

charge

state for the ions

_C2’

I- and

_{( CsI )}

J- from a caesium iodide _target. See _caption to

_figure

6.

2.2 PHENYLALANINE TARGET. - This molecule was chosen since it had

_already

been used in

_experiments

studying

the effect of incident ion

_velocity

on

desorp-tion

_[10].

Moreover,

thin films of this

_compound

can be

prepared

without

_altering

its structure, in contrast to most other

_organic

molecules.

Figure

8 shows the variation of emission

_yield

for the

negative

molecular ion

_{(M - H) -}

as well as for the

ions H- and 26-

_(C2H2

or

_{CN- ).}

_Figure

9 shows similar results for the

_positive

ions

H+,

C+ and

_NH4

ejected

from the same _target. Yields for the

_positive

molecular ion

_{( M + H ) +}

and for

_{(M - C20H) +}

Fig.

8. - Variation of

desorption yield

as a function of the

Fig.

9. - Variation of

desorption

yield as a function of the

charge

state for the ions H+, C+ and

_NHl

from a film of

phenylalanine.

See

_caption

to

_figure

6.

were measured

recently using

a different

phenylalanine

target. For all these ions

_(and

also

_{Cs+ ... )}

there is _{a qui}

dependence

which is different with

Ne,

Ar and Kr

projectiles.

Our _{results agree with those} obtained earlier

_by

other authors

_[14-16]

for low

_charge

states

( qi

13)

and

_{relatively light projectiles.}

Here

_{the q}

dependence

extends to

_larger

i values and heavier

projectiles.

The

_comparison

of the

_yield

curves between the three different

_projectiles

shows a

_larger

variation of

_slope

for low

_charge

states than for

_high

ones.

_Also,

at the same incident

_charge

_{state, the}

_yields

increase with the atomic number of the

_projectile.

However certain ions like

_{H+, C+,}

_NH4

_{(a fragment}

from the

_{phenylalanine molecule)}

behave

_differently

and have

_yields

_{ya qi n}

with n - 4 whatever the incident

ion. A similar observation was made in the

_experiments

at GSI Darmstadt

_[17].

It

_probably

indicates that these ions

_originate

close to the

_point

of

_impact

of the

primary

ion,

where the radiation

_damage

is

_large.

On the other

_hand,

the atomic ions I- or

Cs+,

and the

organic

molecular ions _may come also from

_regions

far

from the

_point

of

impact.

The relation between the energy

deposited

in the medium and the emission from

the surface is then less

_direct,

but the effect of the

incident ion

_charge

is still

_clearly

_present.

2.3 CHARGE STATES OF THE PRIMARY ION. - The

average

charge

states of

_heavy

ions after

_passing

through

thin carbon foils can be

_{predicted fairly}

well

with the use of

_{semi-empirical}

formulas

[12,18-20].

The

_charge

distributions,

on the other

hand,

are more difficult to

_predict

since

_they

can

_depend

on the atomic shell structure of the

_{projectile [20].}

Although

such distributions can be measured

_easily

after the ion has

_{passed through}

the

foil,

they

give

little information on the distribution function in the interior of the

_material,

which is still

_poorly

known.

Indeed,

according

to Betz and Grodzins

_[22],

the

_charge

state of

an ion increases

_rapidly

while

_leaving

the

material,

since the excitation of the

_projectile

leads to the

ejection

of several

_Auger

electrons. In this

_picture,

the mean

_charge

and the

_{experimental charge}

distribution are the results of

_charge

_exchanges

in the interior of the material followed

_by

de-excitation outside. There will

therefore be

_only

a

_relatively

small variation of the

charge

inside the material.

On the other

hand,

the

_theory

of Bohr and Lin-dhart

_[23]

assumes a variation of the

_charge

state in the

material as a result of successive collisions between the

projectile

and the electrons of the

_target

material. An average

equilibrium charge

(qeq)

is

expected

to be reached after a certain distance of

_penetration.

Bohr

_[24]

has

_suggested

a relation between the

_charge

q

(x )

and the thickness traversed x :

qeq

is the final

equilibrium charge

for the ion

considered, qi

its initial

_charge,

and K is a

_parameter

which

_depends

on the

_projectile

and the

_{target material,}

and which can be determined

experimentally.

However,

experimental

values of K refer

_always

to the

_change

after

_{passing through}

a thickness x of material.

The evolution of the

_charge

state inside the material

is at _present the

_object

of theoretical calculations

_[25],

which should soon be

_applicable

to the case of

desorp-tion in

_simple

materials.

In

_fact,

the influence of the

_charge

state of the ion as it _passes

_through

the material is also felt on the surface

and the total

_{yield depends}

in a

_complex

_way on the

energy loss

dE (x )

/dx

and therefore _{on q}

_{(x )}

as a function of the distance x of

_penetration.

As Nieschler

et al. have

_already

_pointed

out

_[14],

the initial

_charge

state acts

_only

as a fixed

_parameter,

and the exper-imental values of

_{desorption yields depend}

also on the cumulative effect of the

_{charge exchange}

_{processes in} the interior of the medium. The real

_dependence

on the incident

_{charge state qi}

is therefore hidden. For

_light

ion

_projectiles,

Nieschler et al. concluded from their

measurements that the

_depth

of

_penetration

below

which the effect at the surface was no

_longer

_apparent

was smaller than the

_{depth required}

to attain

_charge

equilibrium. According

to our

_{preliminary comparisons}

with

_theory

for

_primary

ions of

_high

_charge

state, the conclusions of Nieschler et al. are

_mainly

valid for

_light

ions,

for which the _{energy losses}

_dE/dx

are smaller and

for which the variation of the

_charge

state as a function

of the thickness traversed is more

_rapid.

The

_equilibrium

values of the

_charge

state in the CsI

and

_{phenylalanine}

_targets

can be estimated

_according

266

these three incident

_ions,

the variation of

_desorption

yied

Y

_(at

1.16

_{MeV/u) for q}

_{= qeg}

deduced from the

experimental

curves of

_figures

_6,

7 and 9 is

_given

_by

a

relation of the form

_{ya q q}

when the

_yield

is treated as a

function of the

_equilibrium

_charge

of each incident

projectile.

3. Conclusion.

We have

_presented

new

_experimental

results on the

secondary

ion emission from surfaces under the

_impact

of - 1 MeV/u ions. It has been shown that the variation

of the

_{desorption yield}

with the incident

_charge

state

depends

also on the

_type

of

_{projectiles. Charge}

chang-ing

in a medium as a function of the

_depth

of

penetration

varies from one

_projectile

to another one

_[25]

and the

_knowledge

of the variation of the

charge

inside the material is

_certainly

needed to

_explain

the different

_shapes

of the

_yield

curves. Calculation

using

the results of these authors

_[25]

will be

_published

shortly.

The

ions,

neutral atoms, and electrons emitted

from the surface are observables which

_give

information

about the internal

_{perturbations produced}

in the

ma-terial

_by

the incident ions of the beam. It should also be

possible

to measure in the near future the

_charge

states of ions

_leaving

the surface of the

material,

since the

desorption yield

is very sensitive to the

_charge

state.

The

_experimental

set _up

_together

with the data

acquisition

system for this _type of

_experiment

consti-tutes a

_{unique facility}

for the _present

_study

of the

interactions between MeV ions and

_materials,

particu-larly

material surfaces.

Apart

from the fundamental _aspects it is clear that an

analytic

program

using

highly charged

ions of energy

- 1 MeV/u is _a

particularly

useful

_probe

for

identifica-tion of molecules

_deposited

on a surface.

Acknowledgment.

We are _very

_grateful

for the efficient _assistance

pro-vided

_by

the technical staff of the linear accelerator

(G.

Coeur

_Joly,

J. C.

_Dubois,

P.

_Nael,

J. C.

_Potier).

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