Electron impact ionization cross sections of phosphorus and arsenic molecules

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Electron impact ionization cross sections of phosphorus and arsenic molecules

G. Monnom, Ph. Gaucherel, C. Paparoditis

To cite this version:

G. Monnom, Ph. Gaucherel, C. Paparoditis. Electron impact ionization cross sec- tions of phosphorus and arsenic molecules. Journal de Physique, 1984, 45 (1), pp.77-84.

�10.1051/jphys:0198400450107700�. �jpa-00209742�

Electron impact ionization cross sections of phosphorus

and arsenic molecules

G. Monnom, Ph. Gaucherel and C. Paparoditis

Laboratoire de Physique ^{de la} Matière Condensée (*), Université de Nice, ^Parc Valrose, 06034 Nice Cedex, ^France

(Reçu ^le ler juin 1983, accepté ^{le 8} septembre 1983)

Résumé.

Cet article présente des résultats de mesures des sections efficaces totales d’ionisation et d’ionisation dissociative obtenues par bombardement électronique ^des molécules d’arsenic As4 ^et As2 ^{et de} phosphore P4 ^et P2.

Les espèces moléculaires sont obtenues par effusion thermique. Les différents ions résultant du bombardement

électronique ^sont analysés ^par spectrométrie ^{de masse. Le domaine} d’énergie des électrons est : 0-200 eV. Pour

chaque ^réaction d’ionisation, la valeur du seuil, ^le comportement de la section efficace au voisinage ^{du seuil}

ainsi que l’allure générale de celle-ci sont présentés. Les valeurs maximales des sections efficaces d’ionisation directe pour P4, As4, P2 ^et As2 ^sont respectivement ^{17, 23,4,} 7,8 ^et 11,4 03C0a20. ^De même, les sections efficaces d’ioni- sation dissociative menant aux ions X+n (avec ^{X = P et As et n =} 1, 2 ^et 3), ^ont des valeurs maximales comprises

entre 1,4 ^et 3,8 03C0a20. ^{Les erreurs sont} estimées à 16 % ^sur les valeurs des sections efficaces à leur maximum et à 0,5 ^eV

sur l’énergie. L’ensemble des résultats est discuté en fonction de l’énergie des électrons et des processus de formation.

Abstract.

This paper reports ^on measurements of electron impact ^direct ionization and dissociative ionization total cross sections of arsenic As4 ^and As2 molecules and phosphorus P4 ^and P2 molecules. Molecular species ^are produced by ^{thermal effusion.} The various ions resulting from electron impact ^are analysed by ^mass spectrometry.

The electron energy range is 0-200 eV. For each ionization reaction, the threshold value, ^{the behaviour in the} vicinity ^{of the} threshold, ^{as well as} the main features of the curve are given. At the maximum in the ionization

efficiency ^{curve, the} values of the direct ionization cross sections for P4, As4, P2 ^and As2 ^are respectively 17, 23.4,

7.8 and 11.4 03C0a20. ^{In the same} way, for dissociative ionization towards X+n ^{where X = P and As and} n = 1, 2 ^and 3,

these values are in the range 1.4-3.8 03C0a20 ^{with an} accuracy of about 16 % ^on the values of cross sections and of 0.5 eV on those of the energy. Results ^are discussed against electron energy and against formation processes.

Classification

Physics ^Abstracts

34.80G - 35.20G - 35.20V

1. Introduction.

Experimental determination of absolute electron

impact direct ionization as well as of absolute disso- ciative ionization cross sections of arsenic molecules

(As4 ^and As2) ^{and of} phosphorus ^molecules (P4 ^and P2 ) ^is important in the field of physico-chemistry.

Knowledge of these values is a fundamental step ⁱⁿ ion-molecule reactions in the chemical reactors [1, 2].

In addition, it allows the calibration of various

diagnosis methods, particulary ^{in mass} spectrometry.

Also in the field of solid state physics, ^{there is a need}

for doping semiconductors (gallium ^{or indium arse-} nide, gallium ^{or indium} phosphide) by ^ion implan-

tation. A detailed knowledge ^{of ion} production ^rate

as well as the search of best conditions to obtain different species, amply justify ^this study. ^{Almost no}

data are available today in the literature for these cross

sections. The only existing ^{data are the various}

ionization potentials determined by photoelectron

spectroscopy [3-6]. ^{In the case of the} As4 ^tetramer,

studies on electron capture reactions have allowed

knowledge ^of dissociation energy of arsenic [7]. ^The

dissociation energies and the different ionization

potentials ^are also known for P4 ^and P2 [5, ^8, 9].

In 1956, J. S. Kane et al. [10] ^have already ^carried

out an experiment ^{similar to ours in order to deter-}

mine with _accuracy the ratio of different species ^{in the}

sublimation of red phosphorus ^{and y} arsenic. The

fragmentation and ionization processes in P4 ^have

been discussed by J. D. Carette and L. Kerwin [11]

in their mass-spectrometric ^{studies of} P4 phosphorus.

These authors only give the relative ratios of the different ions, and the data relative to the ionization

potentials ^{and to the} dissociation energies ^{are some-}

what different from the expected ^values, especially

those of Carette. These results inform ^us only ^{on the} general behaviour of the curves.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198400450107700

78

The _gaseous species ^{which were studied have been}

obtained by ^{means of a} classical Knudsen effusion cell.

Vapour pressure and phase equilibria ^{in the Ga + P} system [12] and in the Ga + As system [13, 14]

allowed us to determine the nature and the production

rate of the species. ^The crucible is loaded with _y arsenic

or red phosphorus ^{for a tetramer} vapour and gallium

arsenide or gallium phosphide for dimer vapour

species.

2. Apparatus ^{and method.}

Figure 1 a shows the schematic diagram of the appa- ratus. The vacuum system ^{has a} pumping speed ^of

15001. s -1. The vacuum limit is 10-6 torr. This pressure rises ^{to 1 or 2 x} 10- 5 torr during evaporation.

Special ^care is taken in the control of partial pressures of residual vapours : these ^are maintained at levels two orders of magnitude lower than the lowest partial

pressure of the examined elements. The evaporation

source is of a Knudsen type. ^The graphite ^crucible (diameter ^{= 10 mm,} height ^{= 20} mm) containing

the load is placed ^{in an oven heated} by Joule effect.

The crucible is mounted with a chimney made in the

same material, ^{with a} great ^ratio length (15 mm) :

diameter (1.5 mm), ^{in order to} get ^a well collimated

molecular beam. ^The _working temperatures regulated

to 1°C, ^{are those at which} the pressure ^over the solid in the crucible is about 10-1 torr. Typically, ^{in the}

case of _y arsenic and red phosphorus, ^{the source is}

heated at about 300 °C in order to obtain the tetramer molecular species (see § 3.1 and § 3. 2). On the other

hand, ^to get ^{the dimer} species, ^from gallium phosphide

and gallium arsenide, higher temperature ^{must be} used, typically ^950°C [12, 13]. ^{At these elevated} temperatures and in order ^to protect ^{the whole}

apparatus, ^the evapouration ^{source is} surrounded by

a water cooled jacket. ^The evapourated species ^then

enters into the ion source through ^an aperture ^{4 mm} in diameter, situated 5 mm above the top ^{of the} chimney. ^{This source, of an electron} impact type, without magnetic field, ^{is 40 x 40 x 45 mm in dimen-}

sions. The _energy of the electrons emitted by ^the tungsten filament varies from 0 to 200 eV.

Various ionization and dissociative ionization _pro-

cesses can then take place :

where ð-E is the threshold _energy at room temperature (parent ^and products being ^{in the} fundamental state).

X = P or As , i = 2 and 4 , j integer from 0 to i -1.

2. 1 DERIVATION OF CROSS SECTIONS FROM EXPERI- MENTAL PARAMETERS. - A first series of experiments

allows the measurements of the branching ^{ratios of}

different reactions.

Knowing ^{that the} phosphorus and arsenic mole- cules have a very low condensation coefficient, they

are bound to bounce around in the ionization source

whose walls have a temperature distribution between 100 °C and 300 °C. This is why ^{in order to} get ^a maximum target density and therefore a maximum ionic current, the extraction aperture (3 ^{mm in dia-} meter) ^is displaced sideways ^{from the} incoming ^beam

axis. After their extraction by ^a voltage ^{of 210} V, the various ions are focalized and analysed by ^means

of a quadrupole ^filter (Balzers QMS 311, ^mass range

1-300) ^whose analysis ^{axis is} brought ^{to 200 V. The}

ionic current Iij ^{is then collected on a} Faraday cylinder.

Two different _ways of measurements are used : one

Fig. ^1.

Experimental set-up. a) Branching ^ratio measurements ; b) Absolute values determination ; c) Scaling ^factor

determination, 1. Oven heated by ^Joule effect, ^2. Evaporation cell, ^3. Chimney, ^{4. Ion source, 5.} Tungstdn wire, ^6. Qua-

drupole analyser, 7. Detector.

with a constant sensitivity ^{and a minimal} resolving

power of the quadrupole, the other with a constant

resolving power (with ^{AM - 1 for M =} 300). ^In

the latter _case, and for each _mass, the sensitivity ^is

corrected by ^{means of a} calibration curve (sensitivity

vs. resolving power). ^{The same results are obtained}

with these two methods, the difference in mass of dissociated species (31 ^{amu for} phosphorus, ^{75 amu}

for arsenic) being ^far greater ^{than the} spectrometer resolution. The branching ratios for the different reactions are then obtained :

A second series of experiments is carried out in order to obtain the absolute value of cross sections. In this case, the ion source is modified (Fig. lb) : the upper wall is removed and the total collected ionic current

Ii(E) is measured. Verification is made that electrons emitted by the filament do not influence the measu- rement when the potential drop between the source

and the detector is 210 V. In our case, ^a direct measu- rement of the target pressure (phosphorus ^or arsenic)

is impossible. Nevertheless, the absolute values of the

cross sections are determined by systematic

measurements of the mass flow bmi (g. s-1) ^{in each} experiment. ^The experimental set-up employed during

this second series of experiments (Fig. lb) ^{allows us to}

break free from possible bounces of molecules on the walls of the ion source and from their thermalisation to wall temperature (which ^{is not} measurable with

accuracy). ^{The mean} speed of the molecules in the ion

source is then the same as at the chimney ^exit.

The characteristic dimension of the chimney being

1.5 mm and the _pressure in it having ^{a maximum}

value of 0.1 torr, ^{we assume} that the flow is molecular

[15]. ^{This statement is confirmed} by the linear variation,

in our temperature range, of the density of the ionic current Ii(E) ^versus P/ft, within the experimental

errors.

Taking ^{as the mean molecular} speed vi ⁼

(8 kTinmi)1/2 (cm . s-1 ) ^{and as the} equivalent ^molecu-

lar current Ioi ⁼ N.ðmiM (s-’), ^we obtain the

expression ^{of the} particle density effusing ^{from the}

crucible :

The mass conservation principle yields :

1 = interaction length (cm)

ni ⁼ target density (cm-3 )

so ⁼ chimney ^area (cm2)

si(z) ⁼ molecular beam area in the ion source (cm’)

z ⁼ beam axis.

Then, ^{the cross section} aij(E) ^{is given by the expres-}

with r : scaling factor and le : electronic current (mA).

In the single collision conditions (see below), ^we

have :

We deduce the cross section :

A third series of experiments ^is made in order to measure the scaling ^{factor r} of the whole apparatus.

In this case (Fig. lc), ^{the oven} is removed and an

orifice is made in the wall of the crucible through ^which

a gas ^can be fed. The above two first series of experi-

ments are then repeated by feeding successively ^the

ion source with _argon and nitrogen. ^{In this case mass}

flow measurements are substituted by ^volumetric

flow measurements. The scaling factor is then deter- mined with the help of the well established data of Ar and N2 ionization cross sections [16-18] :

where fij represents ^{here the} branching ratio between

single and double ionization cross sections.

The flow being ^molecular during ^our experiments,

the geometric ^factor so/si(z) ^{is the same} in thv second and in the third series of experiments. The results obtained with Ar and N2 give ^an identical calibration factor rl(so/si(z)),

3. Results and discussion.

The impurity ^{content of all} evaporant materials, ^made by CERAC/PURE, ^{is below 10-4.} In every case, ^an

inspection of the entire mass spectrum justifies ^our

assertion that the rate of ionized impurity ^never

exceeds 10-3. The ion source conductances are such

that, during experiments, the pressure in the gas- electron interaction zone does not rise above 10-5 _torr;

we are thus confident to be in presence of single

collision conditions. At this pressure and in view of the interaction length ^{in the source,} possible ion-molecule reactions are negligible. ^{In the most} unfavourable case

of an exothermic reaction (e.g. As) + AS4 ---+As: +As2,

AE ⁼ 1.2 eV) that would have a high ^rate, say

10- 9 cm3 . .s-1, ^the _density ^of product ions would then be only ^0.5 % that of the parent ^ions.

0.5 V steps ^{are taken in} experimental measurements.

The electrons delivered by ^the tungsten ^{filament are}

not monoenergetic. ^{A 3 eV} spreading ^{due to the}

potential drop between the two ends of the filament

80

is to be taken into account. In order to perform ^a

deconvolution procedure ^{on the} measurement ionic current, the distribution of the electronic emission is assumed constant all along the filament :

where I(V) ^{is the measured current and} I(E) ^{is the}

deconvoluted current, ^{0.5 eV} being ^the step ^{value of}

our deconvolution procedure. ^{In this} procedure, ^we

have neglected ^the energy distribution function of the electrons (e - Avl" - ^0.05 where A V is the voltage step and T the filament temperature), because the main

source of error is to omit to take into account the cold ends of the filament.

The uncertainty in the value of the measured thres- hold _energy is estimated to be around 0.5 eV, ^which is the step ^{value of our} experimental measurements.

In addition, ^{the error due to the} scattering ^of experi-

mental measurements is about 15 % ^for points ^{at the}

lowest _{energy ;} it then decreases to a nearly ^constant

value of about 3 % ^for points ^at 0.75 (Jrnax and above.

Indeed, data used for the calibration can introduce another systematic ^error in absolute values of about

10 %. ^For example, ^we have chosen a maximum ionization cross section of 4.2 7ra 2 ^for argon and 3.3 naõ ^for nitrogen [18], knowing ^{that the} uncertainty

in these values is of the order of 10 %. Therefore, ^{we can}

estimate our error on the cross sections at about 16 %

for values near the maximum and our accuracy ^at 0.5 eV on the electron _energy.

3.1 P4 PHOSPHORUS MOLECULE.

With red phos- phorus, ^at 350°C for the crucible temperature, ^the thermodynamic equilibrium being well described by : P(s) ⁼ 1/4 P4(g), only P4 molecules effuse (P4/?2 -

2 ^x 105, P4/P - 1021 ; [19]).

We have obtained the cross sections which are

represented ⁱⁿ figure ^2.

Figure ^2a gives the variation of the direct ionization

cross section

The threshold of this reaction could be measured as

equal ^to 9.5 eV. This result is in agreement ^with pre-

We have given ^{all the} possible ways of dissociative ionization together with the threshold of their _pro- duction (Table I).

Fig. ^2.

Ionization cross sections of the P 4 phosphorus

molecule.

vious photoelectron spectroscopic measurements [3, 4]

which pointed ^{out a} vertical ionization potential ^of

9.54 eV for P4 ^{molecule to the ion} ground ^state X(2E).

However, ^our result is somewhat different from the values of the adiabatic ionization potentials : ^{9.2 eV} [4] ^{or 9.34 eV} [9].

Near this threshold value, ^{the cross} section beha- viour shows a smooth curvature, a ^oc (E - Eth)a.,

with a ⁼ 1.5 _up to 11.8 eV. The cross section then becomes linear _up to 25 eV with a slope ^{= 4.9 x}

10-2 eV-1 :

In figure 2b, the dissociative ionization cross sections of the P4 ^{molecule are shown :}

For the (2B1) processes, the measured threshold is 18.3 eV. The (2B12) process does not occur at this value;

only ^the (2B10) process is observed at the threshold

Table I.

Dissociation energies ^and ionization poten- tials of phosphorus ^molecules [8, 9].

together ^{with a} possible contribution of the (2B11)

process where p+ ion is in the 1 S excited state : AE ⁼ 17.93 eV.

For the production ^of P’ ions, only ^the (2B20)

reaction occurs at the threshold, but the 13.6 eV observed threshold is situated higher than that of the process :

Besides the single observed vibrational level of the P’

peak corresponding ^{to the} Â(2 l’ g+) ^{state, found by}

D. K. Bulgin ^{et al.} [5] ^{in their} photoelectron spectrum, is more important than each of the three vibrational levels corresponding ^{to the} R(2ff.) fundamental state

of Pi . Nevertheless the total intensity of the first band

X(2 n u) ^is greater ⁱⁿ photoelectron spectroscopy ^than the second Â(2Eg+). ^{In our case, in the observed}

dissociation, the better agreement between the obser- ved 13.6 ± 0.5 eV and the calculated 13.19 eV suggests that the Â(2 l’ g+) ^{excited state is liable to contribute}

in the _process.

For the (2B30) reaction, the threshold _energy is 12.55 eV with P in the 4S fundamental state. A photoionization

threshold with a very weak intensity ^{has been observed} by J. Smets et al. [9] ^at 12.54 eV for the _{process :}

This intensity ^{does not} grow above 13.95 eV, ^value

at which the (2Du) ^{state of P} should have appeared :

As our signal ^{is at 13.2} eV, ^the process (3B20) ^{must be :}

with a greater degree ^of uncertainty ^on the threshold value in the present measurements. The strong slope

observed in the linear region ^{of the cross section} beginning ^{at 16.0 eV was also obtained} by ^{J. S. Kane}

et al. [10] ^and by ^{J. D.} Carette et al. [11], ^near that value.

Our observation is to be compared with the rather

pronounced rise in the P’ intensity ^{at 15.3 eV, follo-}

wed by ^a broad band near 17 eV in the photoelectron spectra ^observed by J. Smets et al. [9]. ^This behaviour,

observed by us, ^comes in support ^{of Smet’s} hypothesis

about the part played by ^the predissociation ^{of the}

è2F 2 ^{state of} P/ .

Salient features of the cross sections are summarized in table II and the variations near the threshold are

shown in figure ^2c.

3.2 As4 ARSENIC MOLECULE.

When the crucible

containing y arsenic is heated at 300 OC, ^the predo-

minant effusing species ^is As4, ^the thermodynamic equilibrium being well described by As(s) = 1/4 As4(g). The concentration of other species ^{is excee-} dingly ^small (As4/As2 - 10’; AS4/As - 1014, [13,19]).

In figure 3a, results for arsenic cross sections are

represented ^{as a} function of the electron _energy in

7ra2unit.

From the results shown in figure 3a, ^{we can observe}

that the maximum amplitude of the direct ionization

cross section is 1.4 time that of phosphorus.

Contrary ^to phosphorus, ^{the cross section} linearity begins ^at the threshold (f3 ^{= 2.9 x 10-2} e V-I). ^The

threshold value (Eth ^{= 9.00} eV) ^{is in} agreement ^with

photoelectron spectroscopic ^results [6] ^which give

an ionization potential ^{of 8.92 eV for the} As4 ^molecule

to the ion ground ^state X(2E). The maximum value of the cross section is _{(J Max} ⁼ 23.4 naõ.

In figure 3b, ^the dissociative ionization cross sections of As4 ^{molecule are} presented.

Table II.

Characteristics of ^the ionization cross sections of ^the P4 phosphorus ^molecule.

I I I I

82

Fig. ^3.

Ionization cross sections of the As, ^arsenic

molecule.

Considering the dissociation enthalpies ð.Hf98 ^{of the}

As4 ^molecule [7] and the ionization potentials (I.P.)

of the arsenic molecules As4 ^and As2 [6, 7] ^{and of the}

arsenic atom [20], ^the following interpretation ^is presented.

By inspection ^{of the} (3Bl) group reaction energies,

the only ^{one that occurs at} the threshold is the (3B10)

process. For As’ ^{formation reaction, we} will retain

only ^the (3B20) ^one. Concerning ^the (3B30) process, it is possible ^to estimate the As3 ionization potential

around 7.5 eV, by analogy ^with phosphorus. ^This

would give ^{a reaction} energy of 11.4 eV, ^{with an As}

atom in the 4S ground ^{state. As with} P+3 ^a strong slope

is observed in the linear region, ^{also obtained} by

J. S. Kane et al. [10]. ^{Thus, it can} reasonably be assumed.

that, above 14.2 eV, ^{when the} linearity ^{starts, a} pre- dissociated state of As: ^{must exist.}

The characteristic ^cross sections parameters ^for these reactions are summarized in table IV.

Table III.

Dissociation energies ^and ionization poten-

tials of arsenic ^molecules [7].

Fig. ^4.

Ionization cross sections of the P2 phosphorus

molecule.

Table IV.

Characteristics of ^the ionization cross sections of ^the AS4 ^{arsenic molecule.}

3.3 P2 PHOSPHORUS MOLECULE.

In order to obtain

a dimer _vapour, we have evapourated gallium phos- phide. ^{In this} case, with a crucible temperature ^of

950 OC, P2, P4 ^{and Ga molecules are the} effusing species [12, 14]. The reaction GaP(s) = Ga(l) + 1/4 P4(g) ^is

very weak : P4/P2 ^{= 10-4.}

However, ^{in our} experiments, this ratio is far more

important (about 10-2), ^{the reason} being ^{that there}

is an association reaction 2 P2(g) -+ P4(g) ^which

occurs essentially ^on the wall of the source [14]. ^This

reaction is taken into account by ^{the use of the} pre-

viously ^{obtained cross sections} (§ 3.1).

An additional error in these experiments ^{is intro-}

duced in the mass flow measurements, due ^to the

non-negligible gallium pressure compared ^{to that of} P2 : P(Ga)/P(P2) = ^’3 % [12].

Only ^{two reactions are} possible, ionization (4A) ^and

dissociative ionization (4B). The results are summa-

rized in table V.

For the (4A) reaction, ^our threshold value is in good agreement ^{with the} expected ^{reaction energy. This is}

not the case however with the (4B) reaction where a

0.85 eV discrepancy ^is observed between expected

and measured threshold, ^thus suggesting ^a possible

contribution of the 1 D excited state of the P+ ion

(AE ^{= 16.6} eV).

One can note that P4 ionization cross section (2A)

is 2.13 times that of P2 (4A). This result is in good

agreement, within the experimental ^{errors, with the}

atomic cross sections additivity ^{rule in molecules} proposed by J. W. Otvos and D. P. Stevenson [21].

3.4 As2 ARSENIC MOLECULE.

When gallium ^arse-

nide is evapourated, As2 is the main _vapour species

in equilibrium with the solid : GaAs(s) ⁼ Ga(l) ⁺ 1/2 As2(g).

At the crucible temperature ^{of 900} OC, As4, As2 ^and

Ga gaseous molecules ^are obtained [13, 14, 22, 23]. ^As

with phosphorus, ^a correction in the mass flow must be introduced due to a 3 % contribution from the eva-

pouration of Ga. Results obtained for both reactions

(5A) ^and (5B) ^are summarized in figure ^{5 and table VI.}

Here, ^the agreement between the measured thresholds and the reaction energies ^is good.

As with phosphorus, ^the As4 (3A) ionization cross

section is twice that of As2 (5A). ^{In the same} way, ^as with As4 ^{tetramer, the} linearity ^{of the} As2 ^direct

ionization cross section starts at the threshold.

Fig. ^5.

Ionization cross sections of the As2 ^arsenic

molecule.

Table V.

Characteristics of ^the ionization cross sections of ^the P2 phosphorus ^molecule.

Table VI.

Characteristics of ^the ionization cross sections of ^the AS2 ^{arsenic molecule.}

84

Completely different results have been given by

different authors [12,13, 22-26] ^{for the} vapour pressure above GaAs. Experiments made until 1958 pointed

out a tetramer pressure greater than that of the dimer.

New experiments ^{carried out first} by ^{J. R. Arthur} [13], using ^a liquid nitrogen cooled shield surrounding

the ionizer, ^{and then} by ^C. T. Foxon et al. [14, 23], using ^a flux modulation method with ^a phase ^sensitive

detection over a wide frequency range, show a dimer concentration higher ^{than that of the tetramer. These}

two methods enabled the distinction between the

signal produced by ^the direct flux and the background species. ^{It is now well} established that the As2 pressure

over GaAs is predominant and that the observed As4

molecules result from the recombination ^on the walls

of As2(g) ^followed by spurious reevapouration [14, 27, 28]. ^Our experimental configuration ^{« a »} being ^basi- cally ^{the same as} that of J. Drowart [24], ^{we have used}

an evapouration temperature corresponding ^{to a}

pressure of a few 10-6 torr in the electron-molecule

interaction _zone, so as to reduce as much as possible

the tetramer concentration (proportional ^to the square of that of the dimer). ^In spite ^of this, the reactions like

(3B1) ^and (3B2) increase the As’ ^{and As+} production

rates. In our case, the currents due to the evapourated As2 ^{are 8.2 times greater than those due to the} As4 pro- duced by the association of As2 species ^{on the walls.}

Previous results obtained in § ^{3.2 enable us to correct}

for this contribution.

4. Conclusion.

Results on the cross sections of direct and dissociative’

ionizations obtained by ^electron impact ^{have been} reported ^for phosphorus and arsenic molecules. Such data are very important ^{for the} doping of semiconduc- tors. These experiments should also have an impor-

tance within a larger framework, ^{in the} determination of the cross sections by ^electron impact ^{of certain}

volatile II, ^{V and VI elements.}

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