Electron-impact ionization and dissociative ionization of sulfur in the gas phase

ISSN 1063�7842, Technical Physics, 2014, Vol. 59, No. 7, pp. 951–958. © Pleiades Publishing, Ltd., 2014.Original Russian Text © A.N. Zavilopulo, P.P. Markush, O.B. Shpenik, M.I. Mykyta, 2014, published in Zhurnal Tekhnicheskoi Fiziki, 2014, Vol. 84, No. 7, pp. 8–14.

951

INTRODUCTION

Sulfur is a unique element changing its propertiesdepending on external conditions; under normal con�ditions, sulfur has the form of octa�atomic ring mole�cules in which the rings are ruptured upon an increasein temperature and its form becomes an open chain;i.e., the structure of sulfur molecules depends on tem�perature, which explains the existence of allotropicmodifications. Owing to its diversified properties, sul�fur appears in the composition of some amino acids(cysteine, methionine), vitamins (biotin, thiamine),and ferments [1]; redox reactions of sulfur are theenergy sources in chemosynthesis. It is important toemphasize that sulfur exists in the atmosphere of somespace objects and is an abundant element in the Uni�verse, as well as in the interstellar medium [2]. Thisexplains the keen interest in the study of various phys�ical and chemical properties of sulfur.

Natural sulfur has four stable isotopes [1]:32S (95.084%), 33S (0.74%), 34S (4.16%), and36S (0.016%), which can easily be separated by a massspectrometer. In the ground state, sulfur atoms haveelectron configuration 3s23p4 of the [Ne] type, wheretwo p�electrons remain unpaired; however, molecularorbitals in sulfur compounds do not repeat completelythe electron shells of inert gases, but are only similar tothem. Sulfur molecules with a number of atoms largerthan three can be classified as cluster compounds;since clusters in many respects are the state of matter

that has not been studied comprehensively (somethingin between atoms and molecules on the one hand anda solid on the other), various experiments should obvi�ously be carried out to clarify the nature of this state.Important information on the properties of neutralatomic particles (including clusters) can be gainedusing electron�impact mass spectrometry. The studyof fragmentation of complex sulfur molecules (clus�ters) in the gas phase during ionization may providevast information on the origin of the effects occurringin this case. Therefore, detailed analysis of fragmenta�tion of complex molecules and clusters during theinteraction with electrons is especially valuable.

This study aims at complex investigations of sulfurionization in the gas phase; the mass spectra at varioustemperatures of vapor are studied by mass spectrome�try; the electron impact method is used for measuringthe energy dependences of the formation of ion�frag�ments and total ionization cross sections for both pos�itive and negative ions.

1. EXPERIMENT

The experiments were carried out simultaneouslyusing two experimental setups: a setup with a mono�pole mass spectrometer [3] and a setup with a hypocy�cloidal electron spectrometer [4]. The working sub�stance was 99.00%�grade sulfur, which was addition�ally purified prior to the experiment by distillation in aspecial vacuum chamber.

Electron�Impact Ionization and Dissociative Ionization of Sulfur in the Gas Phase

A. N. Zavilopulo*, P. P. Markush, O. B. Shpenik, and M. I. Mykyta

Institute of Electron Physics, Ukrainian National Academy of Sciences, Universitetskaya ul. 21, Uzhgorod, 88017 Ukraine

*e�mail: [email protected]

Received November 7, 2013

Abstract—We describe the methods and the results of investigation of the yield of positive ions formed as aresult of electron�impact ionization of sulfur. The ionization energy for the basic molecule and the energiescorresponding to the emergence of fragment ions are obtained from the ionization efficiency curves. Thedynamics of formation of molecular sulfur ions in the temperature range 320–700 K is investigated. Theenergy dependences of efficiency Sn of the ion formation for n = 1–6 are analyzed, and their appearanceenergies are determined. The total cross section of sulfur ionization by a monochromatic electron beam isalso investigated. Using the linear approximation method, we marked out features on the ionization functioncurve, which correspond to the ionization and excitation energies for multiply charged ions. The total crosssection of the formation of negative sulfur ions is measured in the energy range 0–9 eV.

DOI: 10.1134/S1063784214070299

ATOMIC AND MOLECULAR PHYSICS

952

TECHNICAL PHYSICS Vol. 59 No. 7 2014

ZAVILOPULO et al.

1.1. Monopole Mass Spectrometer

The setup in which the MX 7304A monopole massspectrometer was used as an analytic instrument witha mass resolution not worse than ΔM = 1 Da isdescribed in detail in [5–7]. The sulfur molecularbeam under investigation was formed using a multi�channel source of effusion type (the concentration ofmolecules in the beam was on the order of 1010–1011 cm–3) and was directed to an ion source with elec�tron ionization, which operated in the regime of stabi�lization of the electron current and made it possible toobtain electron beams with a fixed energy of 5–90 eVfor currents 0.05–0.5 mA and an energy spreadΔE1/2 = 250 meV, where ΔE1/2 is the full width at halfmaximum (FWHM) of the electron energy distribu�tion. The mass scale was calibrated for the isotopes ofAr and Xe atoms, and the energy scales were calibratedfrom the initial region of the ionization cross section ofa Kr atom and N2 molecule. The experiment was per�formed in two stages; at the first stage, the mass spectraof sulfur were analyzed at different temperatures,while at the second stage, the energy dependences ofthe relative cross sections (including dissociative ion�ization cross section in the energy range 5–30 eV ofionizing electrons) were measured. The experimentalresults were recorded and processed automaticallyusing special computer programs.

1.2. Setup with a Hypocycloidal Electron Spectrometer

To analyze the process of formation of positive andnegative sulfur ions in the gas phase in greater detail,we carried out experiments using a hypocycloidal elec�tron spectrometer (HES) with a vapor�filled cell,which makes it possible to take measurements with ahighly monochromatic electron beam. The schematicdiagram of the HES is shown in Fig. 1, and detaileddescription of the design and the principle of its oper�ation can be found in [4]. The electron current in thebeam was 40 and 25 nA for positive and negative ions,respectively, and the monochromaticity of the electronbeam was ΔE1/2 = 0.11 eV. The value of ΔE1/2 wasdetermined by differentiating the initial segment of thecurrent–voltage characteristic of the electron beamto collector F1 (Fig. 1). The complete collection ofions was ensured by applying to ion detector DI apotential of –1.5 V or + 1.5 V relative to the collisionchamber for positive and negative ions, respectively.The electron energy was specified by a potential differ�ence between the cathode and the collision chamber.The electron energy scanning step was chosen at50 meV for positive ions and 20 meV for negative ions.Vacuum in the working chamber was not worse than2 × 10–4 Pa. The HES was placed into a uniform mag�netic field produced by a pair of Helmholtz rings230 mm in diameter. We developed a PC program forsignal recording and control over measurements in theautomatic regime, which allowed us to trace the mea�surements of the energy dependences in real time.

2. RESULTS AND DISCUSSION

2.1. Mass Spectrum

Figure 2 shows the mass spectrum of sulfur vaportaken at temperature T = 450 K of the molecularsource and energy Ee = 70 eV of ionizing electrons. It

F2

DI

T1

B1

A1

A4A3

A5

A6

A7

A8

B3B4

B2

A1

A2

T2

RVFC

EA

EM

K

H

F1

Fig. 1. Hypocycloidal electron spectrometer with a vapor�filled cell: A1–A7—electrodes for electron beam forma�tion; B1–B4—electrodes of cylindrical capacitors; T1 andT2—thermocouples; F1, F2, and A8—collectors of elec�trons from the primary beam and scattered electrons;EA—electron analyzer; EM—electron monochromator;DI—ion detector; VFC—collision chamber (vapor�filledcell); R—reservoir; H—magnetic field direction.

64S+2

32S+

34S+66S+

2

96S+3

98S+3

128S+4

160S+5

130S+4

192S+6 224S+

7

256S+8

10000

8000

6000

4000

2000

030 60 27024021018015012090

m/z

I, arb. units

Fig. 2. Mass spectrum of sulfur for Ue = 70 eV and T = 510 K.


ELECTRON�IMPACT IONIZATION AND DISSOCIATIVE IONIZATION 953

can be seen from the figure that peaks (m/z = 64),

S+ (m/z = 32) and (m/z = 256) have the highestintensity at this temperature, the intensity of the peakfor the atomic ion being 43.7% and that of molecular

ion being 32.3%, while the peaks corresponding to

remaining ions ( , , , , and ) do not

exceed 25% of the intensity of molecular ions

(Table 1). The main contribution to the intensity of S+

and peaks comes from the fragmentation of basicsulfur molecule S8 whose concentration dominates invapor at the above temperature. Our experiments haveshown that the relative intensities of the ion peaks inthe mass spectrum strongly depend on the sulfur evap�oration temperature (see below). In Table 1, the rela�tive intensities of the ion peaks obtained by differentauthors using different mass spectrometers [8–11] bythe electron impact method are compared. It can beseen that for a normalization to the ion peak of molec�

ular ion (taken for 100%), our results are in goodagreement with the NIST data [11], but considerablediscrepancies are observed with the results obtained byother authors. The reason for these discrepancies isapparently associated with different temperatureregimes in the collision region and the methods of for�mation of sulfur ions; for example, a mixture of sulfurwith helium (PHe = 2.8 Torr) at T = 490 K was used

S2+

S8+

S8+

S3+

S4+

S5+

S6+

S7+

S2+

S2+

S2+

in [8], while the evaporation temperature in [9] wasT = 459 K.

In the mass spectrum shown in Fig. 2, the isotopicpeaks of the sulfur atom can be seen clearly. The ratioof the intensities of the main peak and the isotopepeaks is worth noting. For example, the height of the34S isotope peak amounts to 4.7% of the main 32S peak,while this ratio for other peaks increases with the num�ber of sulfur atoms in the molecule (see Table 1); for

example, the intensity of the isotope peak

amounts to 9.1% of the peak, which is almost

twice as large as that for 32S+.

2.2. Temperature Dependences

We measured the temperature dependences of for�

mation of the sulfur ions (n = 1–6) upon a changein the temperature of the ampoule with sulfur in therange from 300 to 700 K at a constant temperature ofthe ionization chamber for energy Ee = 70 eV of ioniz�ing electrons (Fig. 3). It can be seen that the intensitiesof individual peaks considerably increase with temper�ature, and the ratio of intensities at different tempera�tures changes. Inset (a) in Fig. 3. shows the tempera�ture dependence of the percentage of sulfur moleculesin vapor in the temperature range from 400 to 700 K[12]. A remarkable feature of these dependences is thefact [13] that the saturated sulfur vapor pressure as a

S66 +

2

S64 +

2

Sn+

Table 1. Relative intensities of the ion peaks and the energies of their emergence for an electron energy of 70 eV

Ion Ion mass,m/z

Relative intensity, % Intensity ratioof the main peak

to the isotopic peak, %

Threshold energies

ourresults

Dudek[9]

Bradt[10]

NIST[11]

ourresults

Rosinger[17]

S 32 33.7 4.7 13.5 41.1 4.7 10.30 ± 0.2 10.4 ± 0.3

10.36 ± 0.1*

32 17.37 ± 0.1*

16.84 [18]

22 29.40 ± 0.1*

29.28 [18]

64 100 100 100 100 9.1 9.55 ± 0.2 9.6 ± 0.2

96 23.4 19.1 4.7 21.1 19.1 10.20 ± 0.2 10.2 ± 0.2

128 32.6 24.2 8.0 52.2 24.2 10.30 ± 0.2 10.2 ± 0.2

160 34.7 31.7 5.2 53.1 31.7 8.70 ± 0.2 8.8 ± 0.2

192 24.9 32.1 3.2 41.0 32.1 9.50 ± 0.2 9.7 ± 0.3

224 9.5 0.5 0.3 18.0 – – –

256 52.6 28.2 5.4 70.0 – – –

Asterisks mark the results obtained with a gas�filled cell.

S22+

S23+

S2+

S3+

S4+

S5+

S6+

S7+

S8+

954


ZAVILOPULO et al.

function of temperature is inversely proportional tothe number of atoms (n) in the molecule: the numberof Sn molecules increases with temperature, where n < 8;i.e., the S2 saturated vapor pressure amounts to 0.1%of that of S8 even at T = 500 K. Consequently, theadditional contribution to the intensity of ion peaks

corresponding to the formation of S+– ions (attemperatures below 400 K) comes from the fragmen�tation (dissociation) of molecules as a result of elec�tron impact. Analysis of the behavior of the curves inFig. 3 shows that all curves have peaks in the region of~450 K, which is associated in all probability with thethermal destruction of the sulfur crystal lattice and therupture of S8 rings with the formation of open chainsof atoms. It can be seen in addition that the concentra�tions of S atoms and S2 molecules in the ionizationchamber increase with temperature. Therefore, wecan assume that ions can be formed during the passageof electrons through sulfur vapor as a result of directionization of the sulfur atom or the molecular compo�nents of the vapor as well as due to the fragmentationof molecules with a large number of atoms. In our

opinion, the behavior of the ion upon a change intemperature is peculiar: the intensity of this ion fromthe beginning of heating to 570 K is higher than the

intensity of the and ions, but at 670 K, the

intensities of and become higher than that of

. Such a behavior of these ions can be associatedwith the activation of additional sources of formation

S5+

S6+

S3+

S4+

S3+

S4+

S6+

for and ions due to thermal dissociation of the

main molecule S8 as well as the S6 molecule and also

due to the appearance of and molecular ions.

As noted above, the molecular composition of sul�fur substantially depends on temperature. For exam�ple, at T = 372.5 K, the intermolecular bonds in sulfurcrystals are ruptured, and cyclic molecules S8 arereleased. In other words, beginning from T = 320 K,the percentage of molecules with n = 6–8 (see inset (a)in Fig. 3) is the highest. Upon an increase in tempera�ture to 468 K, two�step polymerization begins; as aresult, homochains are formed, and the contributionfrom molecules with n = 5, 2, and 4 increase; after theattainment of temperature T = 650 K, the contribu�tion of S3 molecules to the total molecular composi�tion becomes significant. Thus, the contributions to

the intensity of S+, , , and ion peaks to the

mass spectrum at T = 510 K (see Fig. 2) due to thermaldestruction process should be minimal. It should benoted that it was shown in [14], where a discharge inthe mixture of sulfur with noble gases was studied, thatat T > 320 K, intense lines belonging to the sulfur atomappear in the optical spectrum; with increasing tem�perature, the intensity of these lines decreases. Thisfact confirms the presence of sulfur atoms in the mix�ture even at low temperatures; upon a further increasein temperature, the number of atoms decreases proba�bly due to clusterization. This is confirmed by the tem�perature dependence obtained in our experiments forthe S+ sulfur ion for which a smoother increase is

S3+

S4+

S3+

S4+

S2+

S4+

S5+

12

10

8

6

4

2

0200 160 120 80 40

400

500

600700

300m/z

I, a

rb. u

nit

s

400 500 600 700T, K

0.1

1

10

100(a)

Mo

l. f

ract

ion

, %

T, KS6

+S5

+S4

+S3

+S2

+S+

S6

S7S5

S4 S2

S3

S8

Fig. 3. Temperature dependences of the formation of Snions (n = 1–6); electron energy is 70 eV. Inset (a): temper�ature dependence of the composition of sulfur vapor [12].

0

I, arb. units

6 8 10 12 14 16 18 20 22E, eV

01

02

10.2011.65

14.44

9.5512.82

17.27

16.59

10.3011.81

13.47

S+

S2+

S3+

Fig. 4. Energy dependences of the relative cross sections ofsulfur ion formation: total cross section (1) and partialcross sections for S+, S2+, and S3+.

1



observed as compared to molecular ions , , and

(see Fig. 3)

2.3. Energy Dependences

We measured the relative cross sections of the for�mation of sulfur ions in the energy range from the pro�cess threshold up to 60 eV at T = 510 K. Figure 4 showsthe threshold regions of the energy dependences of

formation of the S+, , and ions. The most prob�able channels of the sulfur monocation formation aredirect ionization of sulfur atoms by electrons,

(1)

as well as the dissociative ionization of the Sn mole�cule,

(2)

According to the data obtained in [13], sulfuratoms appear as a result of thermal destruction at atemperature above 700 K (at T = 700 K, Ps = 1.08 ×10–6 Pa); in other words, the S+ ions are formed in ourexperimental conditions mainly due to dissociativeprocesses occurring in accordance with reaction (2).The S+ fragment ion can mainly be formed as a resultof dissociation of the S8 and S6 molecules and, to alesser extent, due to dissociation of S2 molecules dur�ing their interaction with electrons. The threshold seg�ments depicted in Fig. 4 were used for determining theenergy values corresponding to the formation of S+,

, and ions by the least squares method [15]; theresults are given in Table 1. The behavior of the curvedescribing the efficiency of ionization for the S+ ionsreflects a monotonic increase from the process thresh�old (10.36 eV) to 17 eV, and the special feature at E =13.47 eV can be attributed either to the excitation ofthe 3s23p3 ionic state with the detachment of thep electron of the excited core (according to the resultsobtained in [11], the energy of this electron is 13.42 eV)or due to the autoionization process during the disso�ciation in the course of formation of this ion. The S2

molecule is most stable among molecules with a smallnumber of sulfur atoms, which is confirmed, on one

hand, by the maximal intensity of the ion in themass spectrum and, on the other hand, by approxi�mate coincidence with the initial segment of the totalcross section of sulfur ion formation (see Fig. 4). Theionization function shows that the sources for the for�mation of these ions can be direct ionization of S2

molecules produced as a result of thermal fragmenta�tion as well as dissociative ionization of S6 and S8 mol�ecules induced by electron impacts. In the effective

ionization curve for (see Fig. 4) at E = 12.94 eV, weobserve a special feature corresponding to the forma�

S2+

S4+

S5+

S2+

S3+

S e–+ S+ 2e–

,+=

Sn e–+ S+

Sn 1– 2e–.+ +=

S2+

S3+

S2+

S2+

tion of the ion as a result of dissociative ionizationof the S6 molecule according to the scheme

(3)

The slope of the curve at an energy of 17 eV is probablyassociated with the formation of excited states of the

ion [10, 16]. The main channels for the formation

of excited states of the ion are the dissociative ion�ization of the S6 and S8 molecules as well as direct ion�ization of the S3 molecule formed as a result of thermaldissociation. The special feature on this curve (see

Fig. 4) at 11.69 eV indicates the formation of the iondue to dissociative ionization of the S8 molecules [17]:

(4)

and a small peak on the curve at an energy of 14.38 eV

corresponds to the formation of the ion as a resultof fragmentation of the S6 molecule.

2.4. Total Ionization Cross Section

Figure 5 shows the energy dependence of the totalcross section of the formation of positive sulfur ions inthe energy range 8–36 eV with an energy scanning stepof 0.05 eV. The curve has a number of special featuresassociated with the unique composition of sulfurvapor, viz., the presence of the molecular and atomiccomponents in it. As mentioned above, the emergenceof these features is probably associated with electron�

S2+

S6 e–+ S2+

S4 2e–.+ +=

S2+

S3+

S3+

S8 e–+ S3+

S5 2e–,+ +=

S3+

31.15

32.0432.64

33.1133.60

34.16(a)

(b)

8 12 16 20 24 28 32 36

E, eV

I, a

rb. u

nit

s

30 31 32 33 34 35 36

9.0 9.5 10.0 10.5 11.0

9.45

10.36

Fig. 5. Energy dependence of the total cross section of for�mation of positive sulfur ions; inset (a): segment of thecurve in the energy range 30–36 eV; arrows mark thethreshold energies of the formation of multiply charged

and ions and their excited states. Inset (b):

example of approximation of linear segments of the curve.

S22+

S23+

956


ZAVILOPULO et al.

impact ionization of molecules (M) as a result ofwhich positive ions are formed:

(5)

The dissociative ionization can be described as

(6)

The fragmentation of an excited molecular ion hasthe form

(7)

An increase or a decrease in the slope of the ioniza�tion function at the points of inflection of the curveindicates the activation of new ionization processes.For example, the slope of ionization functionincreases for reactions (5) and (6) and decreases forreaction (7). It should be noted that the excitation ofelectron levels of the positive ions formed in the reac�tions also contributes to the ionization function; thisfollows from the increase or the decrease in the slope.Using the special approximating procedure for the lin�ear segments of the measured curve, we singled outthese kinks, whose points of intersection give the ener�gies of the emergence of new ionization channels. Fig�ure 5 (inset (b)) shows an example of the linearapproximation of the segments of the curve, whosepoints of intersection give the energies of activation ofthe ionization channels. In this way, we singled out thesingularities on the curve depicted in Fig. 5 at energies of11.91, 12.48, 13.23, 17.37, 22.84, 24.20, and 29.40 eV,as well as singularities in the vicinity of 30 eV, whichappear due to direct ionization and the fragmentationof the parent state of the sulfur ion, as well as due to theexcitation of the energy states of the formed ions.

M e M+ 2e.+ +

M e A+ B 2e.+ + +

M e M+* 2e.+ +

A+ + B.

In the near�threshold energy range, we detectedtwo clearly manifested kinks in total ionization crosssection at energies of 9.45 and 10.36 eV (Fig. 5b),which correspond to the potentials of the formation of

the S+ ion and the molecular ion. These data onthe appearance energies are in good agreement withthe results obtained from the dissociative ionizationcurves (see Fig. 4 and Table 1). For energies higherthan 11 eV, the total ionization cross section increasessharply; at energies of 11.91, 12.48, and 13.23 eV, wedetected special features associated in all probability

with the excitation of the 4Πu, 2Πu, and states of

the molecular ions. It should be noted that theenergy dependence of dissociative ionization for the

ion also exhibits special features in the energyrange 11.5–14.0 eV (see Fig. 4); however, these fea�tures are poorly resolved because of a larger energyscanning step (0.2 eV) and the larger electron energyspread. Our results for the electron state excitation

energies for are in satisfactory agreement with thedata obtained in [17]. The ionization functionincreases at an energy of 17.37 eV, indicating the acti�vation of a new ionization channel; as a result, theintensity of the useful signal increases. In our opinion,

this is associated with the double ionization of the molecular ion. Our assumption is confirmed by thetheoretical results of calculation of the double ioniza�

tion energy for [18], which is equal to 16.8 eV. Thecurve measured in our experiments (see Fig. 5) has twokinks at energies of 22.84 and 24.20 eV, which differfrom the previous features and show a decrease in theslope of the curve. In all probability, this can beexplained by dissociative ionization in accordancewith the reaction

(8)

We also observe a change in the intensity of the use�ful signal at 29.40 eV, which corresponds to the tripleionization potential of the diatomic sulfur molecule,which was calculated theoretically in [18] and equals29.28 eV. At energies higher than 30 eV, the increase inthe intensity of the useful signal considerably slowsdown, and a number of special features are manifestedclearly (see inset (a) in Fig. 5). We can assume that thedecrease in the signal intensity growth rate is associ�ated with the dissociation of the ions formed in accor�dance with reaction (7). We attribute the emergence ofspecial features at energies of 31.15, 32.04, 32.64,33.11, 33.60, and 34.16 eV to the excitation followed

by the decay of the autoionization states of the and

S+ ions.

S2+

Σ4 –

g

S2+

S2+

S2+

S22+

S22+

S2 e–+ S2+* e–+ S

+S 2e–

.+ += =

S2+

0

I, a

rb. u

nit

s

1 2 3 4 5 6 7 8 9 10

E, eV

*15

Fig. 6. Energy dependence of the total cross section of for�mation of negative sulfur ions.



3. CROSS SECTION OF FORMATIONOF NEGATIVE SULFUR IONS

HES makes it possible to study the formation ofboth positive and negative ions during the interactionof electrons with molecules. For detecting negativeions, a small positive potential of 1.5 V relative to thecollision chamber is applied to detector DI (see Fig. 1).The presence of a longitudinal magnetic field and ofan additional grid in front of the ion detector preventsthe electrons from reaching the DI. Control experi�ments show that in the primary beam energy range 0–10 eV, there is no electron current to the detector. Wemeasured the energy dependence of the cross section

of the formation of negative sulfur ions in theenergy range 0–10 eV, which is shown in Fig. 6. It canbe seen that there are three clearly manifested peaks atenergies of 0, 3.5, and 7.2 eV. The identification ofthese features encounters difficulties associated withthe complex composition of sulfur vapor in the colli�sion chamber and the diversity of the processes leadingto the formation of negative ions. The intensity of thefirst feature in the form of a narrow peak in the rangeof nearly zero energies (0 eV) is higher than the inten�sity of the second and third peaks by more than anorder of magnitude. In addition, the energy width ofthis peak, which amounts to ~0.1 eV, almost coincideswith the electron energy spread in the beam, indicat�ing the resonant capture of electrons with such low

energies. The formation of the negative ions at avapor temperature of 388 K was studied in [19] by themass spectrometry in the energy range 0–11 eV. It wasshown that the largest contribution to the total crosssection of the formation of negative sulfur ions comesfrom the S2, S3, and S4 molecules, while the contribu�tion from the S5 molecules is insignificant (Table 2).The energy dependence of the total cross section of theformation of negative sulfur ions measured in ourexperiments is similar in the shape and the positions ofthe peaks to the data on the total intensity of the curves

for , , and [19]; this leads to the conclusionthat the main contribution to the total cross section of

the formation of negative ions comes precisely

Sn–

Sn–

S2–

S3–

S4–

Sn–

from the , , and molecular ions. It should benoted that we succeeded in determining exactly theenergy at which special features in the total cross sec�tion of negative sulfur ions appear (see Fig. 6 and Table2).

CONCLUSIONS

Our experiments on the interaction between elec�trons with a controllable energy and sulfur vapor leadto the following conclusions. The elemental composi�tion of sulfur vapor has been studied for the first timeusing mass spectrometry in the temperature range300–700 K; it is shown that the S2 and S8 moleculesdominate in the mass spectrum; S atoms and Sn mole�cules (n = 3–7) are also present in appreciableamounts. The peaks observed in the energy depen�

dences of the cross section of formation of S+, , and

ions near the threshold are due to fragmentation ofthe initial molecule. Detailed analysis of the completeionization function for sulfur vapor for collisions withmonoenergetic electrons has made it possible to deter�mine the potentials corresponding to the emergence offragment ions, and the values of these potentials are ingood agreement with the mass�spectrometric data; theenergy of triple ionization of S2 molecule (29.4 eV) wasdetermined for the first time. Finally, we were the firstto measure the energy dependence of the total crosssection of the formation of negative sulfur ions; thesecurves exhibit clearly manifested peaks whose posi�tions are in good agreement with the special features inthe cross sections of dissociative attachment of elec�trons to sulfur.

REFERENCES

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S2–

S3–

S4–

S2+

S3+

Table 2. Threshold energy and relative intensity of formation of negative ions

Energy position of peaks inthe total cross section, eV Relative intensity of formation of negative ions [19], %

[19] our data S2 S3 S4 S5 S6

0 0 0.07 37.4 18.7 7.5 65.5

~4 3.5 11.3 11.3 5.6 0.37 28.6

~7.5 7.2 1.9 1.15 3.9

Total intensity 13.2 49.8 24.3 7.8 ~100

958


ZAVILOPULO et al.

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Translated by N. Wadhwa

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Electron-impact ionization and dissociative ionization of sulfur in the gas phase