Experimental methods and techniques for negative-ion production by surface ionization. Part I. Fundamental aspects of surface ionization

International Journal

ofMass

Spectrumetry

and Ion Physics, SO (1983) 1-33

Elsevier Scientific Publishing Company, Amsterdam

- Printed in The Netherlands

EXPERIMENTAL METHODS AND TECHNIQUES FOR NEGATIVE-ION PRODUCTION BY SURFACE IONIZATION. PART I. FUNDAMENTAL ASPECTS OF SURFACE IONIZATION

HlROYUKl

KAWANO

*

of Chemistry,

Department

Faculty

of Science,

Ehime

University,

2-5 Eunkyo-cho,

Matsuyama

790 (Japan)

FRANCIS

MICHAEL

PAGE

Department of Chemistry, 7ET (Gt. Britain)

(Received

University

of Aston

in Birmingham,

Costa Green, Birmingham

B4

30 July 1981)

ABSTRACT Negative-ion production by surface ionization is surveyed comprehensively and critically in order to determine the best experimental technique to produce strong and stable negativeion beams or alternatively to yield reproducible data under research conditions. The review consists of three parts, and this, the first part, includes the topics: (1) historical background; (2) applicability to various research purposes; (3) the Saha-Langmuir model and its modifications; and (4) theoretical and practical treatment of related subjects such as the work function of an ionizing surface, sticking probability, and the degree of dissociation of incident sample molecules. About 150 references are cited.

1. INTRODUCTION

Surface ionization, often termed thermal ionization or thermal ion emission, is the process by which a part of the material coated on or directed onto a heated metal surface evaporates in a negative or positive ionic state. 4s described below in detail, both experimental and theoretical investigations began with positive surface ionization (PSI) and the methods and techniques thus developed are often useful also for negative surface ionization (NSI), because NSI is similar to PSI in both principles and instrumentation. In addition, the combined use of NSI and PSI by changing the polarity * Author to whom correspondence 0020-738

l/83/$03.00

should be addressed.

0 1983 Elsevier Science Publishers B.V.

2

of the ion-extraction voltage may offset some of the disadvantages inherent in either. From this point of view, PSI is often mentioned or cited in this review paper. 1.1. HistoricaE background According to Duckworth [ 11, positive-ion emission from heated metal filaments was discovered by Elster and Geitel as early as 1889 [2]. A similar phenomenon was observed for heated salts on metals by Beattie in 1899 [3], after which many workers tried to determine the laws of emission and also the nature of the particles carrying the charge [4]. In 1906 1907 Gehrcke and Reichenheim investigated the positive rays originating from a heated anode, and determined the masses of the positive ions emitted from alkali-metal chlorides placed on a heated platinum foil, obtaining values of 8.3-l 4 and 21-23 a.m.u. for LiI and NaI, respectively [5-81. Thomson found in 1908 [9] that the mass of positive ions emitted from a bare platinum wire heated to incandescence was - 27 a.m.u. Also in 1908, Richardson [lo] obtained nearly the same values (25.7 and 27.6 a.m.u.) for the positive ions emitted from bare platinum and carbon filaments heated to - lOOO- 1500 K, and suggested that the ions detected consist of Naf arising from a common impurity occluded in the strips. Later he suggested again [ 1l] that K+ as well as Na+ was emitted from many glowing bare-metal wires. Reliable identification of the positive ions emitted from known sample materials (alkali-metal sulphates or sodium halides deposited onto a platinum ribbon) was first made in 19 10 by Richardson, who found [12,13] that the mass of the ions was nearly equal to the atomic weight of the constituent alkali metal and also was independent of the nature of the constituent anion. Since then such PSI has been investigated both experimentally and theoretically by many workers, especially by the two groups of Langmuir and Ionov. In consequence, PSI is now widely utilized as one of the most convenient techniques to generate ion beams, to detect neutral beams, to analyze chemical or nuclear reaction products, to determine the first ionization potentials of various elements such as the lanthanides and actinides, and to study physico-chemical processes occurring on the surfaces of metals and metalloids. The progress thus achieved has been summarized in many review papers and books [14-241. On the other hand, it was in 1904 that the possibility of negative-ion emission from a heated bare metal wire was first investigated by Owen, who found evidence of the presence in the emission of a small percentage (less than - 5%) of negative ions [25]. He later studied the nature of the carriers of the negative emission currents from bare Pd, Pt and Ir filaments at various temperatures (dull red-white hot), and concluded that the carriers

3

consisted almost entirely of .electrons and that the proportion of negative ions to electrons was probably less than one part in ten thousand even if any of the discharge was carried by ions [26]. In contrast to this rather negative conclusion, a fruitful result was obtained in 1912 by Richardson, who found [27,2S] that negative ions as well as electrons were emitted from alkali-earth iodides and bromides on a platinum ribbon heated to - 600 K or higher and that the masses of the ions were - 120 and 88 a.m.u., corresponding nearly to I- and BY, respectively. Independent of the above investigations, Haber and Just in 1909 began to study the emission of negative electricity from an unheated surface of .either potassium-sodium alloy or alkali-metal amalgam that was exposed to a gas or vapour such as H,O, HCl, I,, 0, or SOCl, [29,30], and in 19 12 they found, by employing a magnetron-like method, that negative-ion emission occurred when Br, impinged upon the surface of Cs-K-Li amalgam at room temperature [31]. After the above investigations only a few workers, such as Schmidt [32-341, Glockler [35,36] and Mayer [37-421, appear to have tried to investigate NSI up to the end of the Second World War, and NSI was for a long time neglected in comparison with PSI; nevertheless, NSI possesses some unique advantages over PSI and probably has comparable usefulness for the investigation of various surface reactions, for the determination of the thermochemical properties of impinging molecules or atoms, for the detection of neutral beams, and for the generation of ion beams. There are, however, only a few papers that specifically review NSI [43-471, although NSI has been partly reviewed together with PSI by Zandberg and Ionov [ 17,22,23] and Kaminsky [20]. I. 2. Negative-ion production methods Recently, ion-beam techniques have been widely employed in many fields of natural science and, in consequence, many investigators have attempted to develop new or improved methods for producing ions, especially negative ions. In order to obtain gaseous negative ions, it is possible to employ methods such as: (1) electron impact on sample gas molecules or atoms; (2) arc or glow discharge in sample gases; (3) cu-ray radiation of sample gases (secondary-electron capture); (4) positive-ion bombardment of solid sample surfaces (sputtering) or of adsorbed sample molecules (positive-ion-induced desorption); (5) electron impact on sample-covered solid surfaces (electronstimulated desorption); (6) collision of ‘neutral sample molecules with others (chemi-ionization); (7) spark or arc discharge between solid sample electrodes or between electrodes coated with sample materials; (8) field ionization of sample gas molecules; (9) resonant or nonresonant charge transfer between negative ions and neutral sample molecules; (10) positive-ion beam

4

incidence upon a glowing metal surface [48] (double electron capture); (11) superthermal neutral-sample beam bombardment at solid surfaces (reflection of fast particles as ions on striking a surface, which is in general called “fast” or “cold” surface ionization) [49]; and (12) superthermal neutral-beam impact at sample-covered solid surfaces (fast-neutral-beam-induced desorption, usually called “stimulated” surface ionization [50]. 1.3. Advantages

of NSI

NSI has the following advantages over the methods of producing negative ions listed above. (1) The ion source can be very small in size and simple in structure. TABLE

1

Atomic-

or molecular-ion

production

by surface

atomic positiveand negativeions havealfeady been respectively;

A

and Ha

Ia 1

H

2

*A Li

A

that polyatomic

positive

ionization: reported

0

and 0

indicate

to be produced

and negative

that

ions including

the same

IIIa

IVa

Va

Via

VIIa

VIII

Mn 0

Fe OA

co-

Tc 0

RU 0

Rh 0

Re 0

OS

Ir

A Be

0 0 3

Na CIA

Mg

K

SC

Ti

V

q lA

Ca OA

OA

OA

OA

Cr 0

Rb 0

Sr q A

Y

Zr

OA

OA

Nb OA

MO OA

a CS

Ba

*

Hf

Ta

.A W

cl

DA

OA

OA

7

Fr c7

Ra

* *

*

La

Ce

Nd DA

q

Sm q A

EU

CIA

Pr tlA

Pm

q lA

OA

Gd QA

Tb q iA

AC

Th OA

Pa QA

u

NP OA

Pu OA

Am OA

Cm 0

Bk

4

5

6

0

aA

* *

q A A

OA @A

mon-

by PSI and NSI,

o

.A

or

5

(2) The ionization efficiency can be very high for particular elements and radicals. (3) Background ions due to residual gases are often of only a few species and are relatively small in amount. Consequently, high-vacuum conditions are not always required. (4) The initial energy spread of ions produced by NSI is only -OS--O.6 eV, approximately twice the thermal energy at the ionizing surface [51-531. The practically monochromatic ions thus produced can readily be analyzed using a single-focusing mass spectrometer. (5) Usually NSI does not produce internally excited ions, and hence NSI is suitable for the dynamic study of collisions between slow negative ions and neutral species or photons. different element(s) have been found to be formed by PSI and NSI, respectively; Cl and I that positive and negative ions are readily produced by PSI and NSI, respectively, because the corresponding elements (M and X) satisfy the conditions I(MJS6.0 eV and A(X)>2.0 eV Ib

IIb

IIlb

IVb

Vb

+Ib

VIIb

0

He

Ni 0 A Pd 0

Pt

CU 0 l

Zn 0

‘%

Cd 00 A

0 0 Au

Hg

0

N

C

B A

A

A

lA Si

Al 0 l Ga 0 In 0 0 Tl 0

OA rnA Ge

F

0 A A

P

aA

IA

S

Cl

OA

l

Ar A

As

IA

IA

Se

Br

Kr A

0

l Sn 0 0 Pb

Ne A

A

@A Sb n .A Bi 0 rnA

DY

Ho

Er

CIA

DA

Tm q A

+b

CIA

LIA

Lu q A

Cf

Es

Fm

Md

No

Lw

0

0

0

IA

IA Te

I

I

A IA

PO

At

Xe

Fzn

6

(6) Only a small amount of sample material is required in general. (7) The spatial distribution of neutral atoms or molecules can easily be measured because ion production is usually restricted to a very small surface area. (8) When the chemical or nuclear reaction products coming from a solid sample (target) are to be separated or analyzed mass-spectrometrically, the solid itself can serve as an ionizing surface for NSI. Accordingly, some of the products (e.g., an artificial radioactive nuclide 90Br with a short half-life, 1.4 s) can be readily extracted as negative ions immediately after the reaction and hence directly separated from the solid (target) without any additional physical or chemical processing [54,55]. It is generally accepted that NSI is useful in producing negative ions (X-) from those atoms, molecules and radicals whose electron affinity A(X) is larger than - 1 eV. Under favourable experimental conditions, however, NSI seems to be applicable even to atoms having an electron affinity of 0.5 eV or smaller. According to a handbook [56] and several review papers [57,58], the number of elements satisfying the condition A(X) 2 0.5 eV is - 46. However, to date only 28 elements have been found to be converted into monatomic negative ions by NSI (those marked with a solid circle or square in Table 1). Even including those elements which have been found to form polyatomic negative ions (solid triangles in Table 1) dontaining other or identical element(s), the number reported to be ionized by NSI is only 33. This number is much smaller than the number (79) of elements which have been reported to be converted by PSI into monatomic positive ions (open circles and squares) or molecular positive ion(s) (open triangles) containing other element(s) such as 0, C or halogen. These data for PSI are cited from more than 100 articles, which are not listed in this paper. The sources of the data for NSI, on the other hand, will be tabulated in Section 7 (Part III) together with the chemical compositions of the negative ions observed during various experimental studies in a variety of fields in natural science: By means of PSI it is very difficult or virtually impossible to produce a monatomic ion Mf from those elements M which have a very high ionization potential (I(M) 2 9 eV). Among such elements, however, 11 (Au, As, Sb, and both oxygen and halogen groups) can readily be converted by NSI into monatomic ions, as shown in Table 1, and phosphorus also ( I(P) = 10.5 eV, A(P) = 0.74 eV) may perhaps be included, although the production of Pby NSI has not yet been reported. This is one of the unique merits of NSI over PSI. According to refs. 56-58, A(X) of the following elements seems to be larger than - 0.5 eV: Na, K, V, Cr, Co, Ni, Rb, Zr, Tc, Ru, Rh, Pd, Cs, La, OS, Ir, Pt, PO and At. Judging from the experimental result that NSI can be successfully applied to lithium [59] (A(Li) = 0.62 eV) and also to indium [60] (A(In) = 0.30 ev), appreciable quantities of monatomic negative ions

7

may possibly be obtained from the above 19 elements using NSI by employing favourable conditions such as will be described in Sections 3 and 4 (Part II). On the other hand, it seems to be definitely impossible to produce the following ions by NSI although they can readily be formed by some of the different methods mentioned in Section 1.2: B- , Ti- , Fe-, Ga- and Tll, for which elements A(X) ranges from - 0.2 to 0.3 eV [57]. 1.4. Applicability

of NSI

Since NSI has these advantages over the other negative-ion production methods mentioned in Section 1.2, it has frequently been applied in various fields and is nowadays recognised as one of the important experimental techniques. The .applications of NSI may be divided into five categories, as follows. (A) Investigations of physico-chemical and of surface properties by experimental ties:

processes occurring on solid surfaces measurement of the following quanti-

(a) sticking probability u of incident sample gas molecules; (b) initial velocity distribution of emitted ions; (c) mean adsorption lifetime and/or desorption energy of ions; (d) degree of thermal dissociation of impinging molecules; (e) heat of adsorption of thermal dissociation product(s) from incident molecules; (f) ionization efficiency or relative ion yield for sample atoms or molecules; (g) inhomogeneity and/or change in the work function of a solid surface exposed to incident molecules or atoms; (h) rates of subsequent reactions accompanied by thermal electronic and ionic.. emission, or probability of electron emission caused by incidence of sample gas atoms or molecules; (i) effects of laser irradiation on molecular-beam surface ionization. In these research fields either the ionic species, the emission current, the desorption rate or the initial velocity distribution is studied as a function of the conditions of emission of negative ions {surface material and temperature, chemical composition and flux density of incident sample material, etc.). The results thus achieved may provide the following: (1) valuable information concerning the physico-chemical properties of the solid surface under study and the mechanisms of physical and/or chemical reactions on the surface; (2) fundamental data that may be helpful in applying NSI to the other fields (B}-(E) described below, and also (3) clues to the problem of how to develop or prepare a high-efficiency NSI source suitable for the purpose intended in a given experiment.

8

(B) Determinatiun

of thermochemical properties

as fuilows:

(a) electron affinities of atoms, molecules and radicals; (b) bond dissociation energies of polyatomic molecules. Since adsorbed sample molecules usually attain both dissociation and ionization equilibria on a heated metal surface, measurement of the negativeion current as a function pf surface temperature makes it possible to determine the value of the electron affinity or bond dissociation energy. In connection with this determination, the determinations (d)-(h) described in (A) above are also made in many cases. (C) Production of negative-ion beams for the following research purposes: (a) measurement of the total scattering cross-sections of negative ions in collision with neutral atoms or molecules and also determination of the interaction potentials between the two; (b) dynamic study of chemical reactions due to inelastic collision between low-energy negative ions and neutral molecules or atoms; (c) measurement of the spectral dependence of photodetachment crosssections for negative ions; (d) preparation and control of adlayers on a solid surface by a negative-ion incidence method. In this research area, NSI has been utilized to obtain a stable, strong and monoenergetic negative-ion beam. An abundance of negative ions of various species may be readily produced, for example, by a hot-cathode gaseous -discharge method. This method, however, is likely to produce ions which have a wide spread of kinetic energy and/or which are highly excited in internal energy. NSI, however, is usually free from such defects and hence suitable for studies in the field (b). In general, NSI itself is not investigated in this field. However, when, for example, NSI is effected by the injection of target-&as molecules, interpretation may still be possible by considering some of the data obtained in field (a). Until recently it was generally believed that very high-energy neutral hydrogen beams for heating thermonuclear plasmas must be produced not from positive but from negative hydrogen-ion beams [61,62]. The production of H- on a heated surface of either W, Ba-SrOcovered Ni, or CsO-covered v was examined about 30 years ago, and the results obtained were not convincing [63]. However, it seems worthwhile to investigate the possibility thqt NSI alone or a combination of NSI with other ionization method(s) might be applicable to negative hydrogen-ion production with high efficiency_ As a negative-ion beam generator an NSI source may be expected to have practical applicability to many other research purposes in addition to (a)-(d) described above. (0)

Defection

of neutral beams in the following research fields:

(a) study of the dynamics of elementary chemical reactions by a molecular-beam method;

9

(b) determination of nuclear magnetic moment by an atomic-beam method; (c) investigation of sputtering. When a beam of potassium atoms is detected, for example, the detection efficiency using PSI is - 3000 times higher than that of an electron-impact ionization method [64]. When a beam under study consists of molecules (or atoms) including electronegative element(s), a similar superiority may probably be established for NSI also, because NSI has the advantages (2) and (7) described in Section 1.3. To date, however, beam detection using NSI has been limited to a few cases, in contrast to that using PSI. Development of a new or improved ion source having a higher ionization efficiency would probably promote the application of NSI to those elements having a smaller value of A(X) and also extend such application to many other research fields. (E) Analysis or separation of the following substances:

(a) stable or unstable isotopes; (b) surface reaction and/or thermal decomposition products; (c) nuclear reaction products; (d) surface contaminants and/or trace impurities contained in sample materials; (e) high-temperature vapours. When samples containing electronegative atoms, molecules or radicals are submitted for mass-spectrometric analysis, an NSI source is often employed to produce ions characteristic of the samples, For example, NSI is utilized as a routine technique to monitor air pollutants [65,66], to determine trace amounts of the chlorides included in snow [67], in blood serum [68] or in silicate rocks [69], and also to separate radioactive halogen isotopes from miscellaneous nuclear fission products [54,55,70-721. Similarly, as in PSI, isotopic dilution analysis can be successfully performed in NSI [67-691. These applications are due to the advantages (2), (3), (4), (6) and (8) described in Section 1.3. Recently, Schmick and Wassmuth [73] have reported that a combination of flash desorption with PSI makes it possible to separate isobaric nuclides (e.g., g7Rb and *7Sr) without using a very high-resolution mass spectrometer_ Application of this new technique to NSI may possibly be useful for separating some electronegative isobaric substances. Numerous examples of NSI applied in the above research fields (A)-(E) are compiled later, together with experimental NSI data. 1.5. Disadvantages

of NSI

Compared with the other ionization methods of Section 1.2, NSI has disadvantages as follows. (1) The number of elements which may be converted into negative ions is

10

relatively small. As already mentioned in Section 1.3, it is usually very difficult to ionize those atoms, molecules or radicals whose electron affinities are lower than - 1 eV. (2) At high temperatures with a strong thermal electron emission spurious NSI is liable to occur, since negative ions can also be formed by other mechanisms such as (l), (4) and/or (10) mentioned in Section 1.2. In contrast to the case for PSI, such spurious NSI is often serious. (3) The ionization efficiency p-(X-) for the atom or radical X is liable to depend upon the chemical composition of sample materials containing X even if the same ionizing surface is employed to produce the same ion X- . (4) According to both the nature and extent of coverage by surface contaminants, fi- (X- ) is liable to change markedly. When the surface temperature is not very high and/or the residual gas pressure around the ionizing surface is not very low, it is usually difficult to avoid surface contamination and hence to keep the work function constant throughout the course of a run. Disadvantage (1) may be overcome by, for example, employing a highly sensitive ion-detection system, which has made it possible to measure a quite weak negative-ion current of lithium [59]. Another way of overcoming this disadvantage, which would also increase p-(X-) itself, would be to develop new surface materials having low work functions; this will be discussed in Section 3 (Part II). Disadvantage (l), however, is the converse of advantage (2), and hence can enhance the usefulness of NSI in those special cases where a particular molecule alone is to be detected in a crossed-molecularbeam experiment [74] or where only some of the trace impurities contained in polluted air are to be analyzed mass-spectrometricalIy [65]. In this latter case, element(s) characteristic of the molecule or impurities of interest are converted into ions with high efficiency, thereby yielding either an essentially single-component ion beam or a simple mass spectrum. Disadvantage (2), on the other hand, can hardly be overlooked; an experimental study [75] of the subject (h) in field (A) mentioned in Section 1.4 suggests that disadvantage (2) can cause large error when an electron affinity or bond dissociation energy is determined using an NSI method. Observation of the temperature dependence of the ion/electron current ratio may be useful for making a distinction between true and spurious NSI [75]. This method is very simple and convenient, because a special expejmental system such as an energy analyzer or a very high-resolution‘mass spectrometer is not required. In the research fields (D) and (E) mentioned in Section 1.4, on the other hand, disadvantage (2) can be neglected because possible production of spuriouS thermal negative ions has little effect upon any research purpose intended in these fields. As a typical example of disadvantage (3) we may cite the result that

11

p-(SiO,) decreases in the order Cs,SiO,, K,SiO,, Na,SiO, when alkalimetal silicate vapor impinges upon a tungsten surface [76]. This is probably because the effects of each of the following factors (a>-(c) upon NH are significantly different among the three silicates: (a) the sticking probability u of each silicate molecule on the ionizing surface; (b) the degree of dissociation, depending upon the binding energy between alkali-metal and silicate ions; the energy probably decreases in the order Cs, K, Na; (c) work function changes resulting from adsorption of alkali-metal atoms produced by thermal dissociation on the surface. Among the three factors, (b) seems to be the most important. Much more work, however, must be done to elucidate the mechanism of the above differences in p- (SiO, ). The application of NSI is therefore usually accompanied by the old but perennially recurrent problem of how to overcome the disadvantages, without disturbing either research purposes or experimental conditions. I.6

Intention and contents of this paper

It should be noted that the applications described in Section 1.4 are fewer and narrower than those of PSI. This may be partly because NSI has not yet received such universal recognition of potential utility as has PSI, and partly because fundamental investigation of NSI has not been pursued as actively as for PSI. Consequently, for example, an NSI source having as high an ionization efficiency as a PSI source has not yet been developed. This, however, seems to suggest that proper understanding of the characteristics of NSI as a technique to produce negative ions would extend its applicability to wider fields in natural science, and that further investigation would make possible great progress in NSI. From this viewpoint, the authors here try to show the successes achieved to date in the field of NSI, to make a critical comparison between NSI and PSI and between NSI and other negative-ion production methods, and also to point out some problems to be resolved by further investigations_ This review paper is divided into three parts, and a complete list of the contents of Parts II and III is given below. Part II. Instrumentation

and operation

Section 3. Choice of surface material Section 4. Selection of operating conditions Section 5. Structure of ion source Section 6. Separation of charged particles

12

Part III. Compilation and criticism of experimental data on negative surface ionization Section 7. Operating conditions and performance of ion sources developed

to date Section 8. Concluding

remarks

At the end of each part the references cited therein are listed in order of appearance in the text. Their total number is - 400. On the basis of - 160 reports published by various research workers during the last half century, experimental data concerning the production of negative ions (- 170 species) by NSI from miscellaneous sample materials (- 200 species) are tabulated in Section 7 together with the corresponding experimental purposes and/or research subjects among the 23 fields (A)-(a)(E)-(e) conveniently classified in Section 1.4. This survey may be helpful as a guide to those research workers who are interested in experimental methods and techniques of producing gaseous negative ions. 2. PRINCIPLES

OF SURFACE

IONIZATION

Theoretical studies of surface ionization have long been devoted principally to positive-ion production from incident atoms having thermal energies. With appropriate modifications, however, the results thus obtained can generally be applied to both positive- and negative-ion production from incident molecules having kinetic energies much less than 0.1 eV. Neutral atoms or molecules striking a glowing metal surface will stick to it for a time much longer than a microsecond, depending upon both the surface temperature and the ion or neutral-species desorption energy. An adsorption lifetime of the above order is usually regarded as being sufficiently long to establish thermal, chemical and ionization equilibrium between the atoms or molecules and the substrate metal surface. In this Section a theoretical treatment of both surface ionization and related subjects is briefly described. 2.1. Atomic-beam

surface ionization

When a beam of atoms M having a small value of the first ionization energy 1(M) impinges upon a positively biased metal surface at a high temperature, a positive ion M + is emitted after the following equilibrium is attained on the surface: M = M+ +e-

(in the substrate metal)

In this case, the ionization bY

coefficient

0) or degree of ionization

of M is given

13

CY+(M+),

= n(M+)/n(M)

[l -

= {

r(M+

>I/[1 - r(M)])

x exp{ [ 6’ +e( c+‘)“~

[w(M+

)/w(M)1

- I(M)],‘kT)

(2)

where n(M+) and n(M) are the numbers of M’ and M, respectively, evaporating from unit surface area per unit time; w(M+)/w(M) is the statistical‘ weight ratio of M+ to M; u(M+) and r(M) are the internal reflection coefficients for M+ and M, respectively, at the potential barrier on the emitter surface; 6’ is the average work function effective for producing M+ on the polycrystalline patchy metal surface; T is the absolute temperature of the surface; k is the Boltzmann constant; e is the elementary electric charge; and F is the externally applied electric field at the emitter surface used to extract M+ into the vacuum. When an ionizing filament (radius Ri) is located in the centre of a cylindrical plate (radius R,), F is given by F=

V,/[Riln(Rc/Ri)]

(3)

where V, is the ion-extraction voltage applied between the filament and plate, Since the Schottky term e(eF)“* is less than 0.012 eV for F-C 1 kV cm-‘, the term can be neglected in neutral PSI. In NSI, however, in general V, must be much larger than that in PSI in order to overcome the space charge effect due to thermal electrons emitted simultaneously with the negative ions under study, and hence it is often necessary to take account of the term even in normal NSI. When the following equilibrium holds for an atom X incident upon a negatively biased glowing metal surface: X + e- (in the substrate metal) = Xthe following 6(x-)x

(4)

equation is obtained, analogously to the case for PSI:

= n(x-)/n(X) = {[l

-~WW[~

x exp( [ -q-

-Gmrwwv~(X~1

+e( e/F)“2

+A(X)]/kT)

(5)

where A(X) is the electron affinity of atom X, $ is the average work function effective for producing the negative ion X- on the inhomogeneous metal surface, and a, n, Y and w have the same significances as in eqn. (2). The internal reflection coefficient r(Y) for the ionic or atomic species of Y having attained equilibrium (1) or (4) represents the fraction of those Y particles which do not possess thermal energy sufficient to pass the potential barrier at the solid-vacuum interface. The values of r(M+) and r(M) have been discussed by several authors [77-801. When Bi or Tl is ionized on Ir( 111) at 1250-2200 K, for example, both r(M+) and r(M) are concluded to

14

be independent of T and also low in value [81]. Nowadays, in general r(M+> and P(M) are considered to be zero in the temperature range ( TZ 1000 K) that is covered in normal surface ionization. To the best of the present authors’ knowledge, neither a theoretical nor an experimental study of the internal reflection coefficients for negative ions seems to have been made in detail up to the present, and both r(X-) and r(X) are assumed to be zero by analogy with PSI. This is mainly because the precision of no experiment has yet been sufficiently high that the ratio [ 1 - r(Y *)I,/[ 1 - r(Y)] may be determined accurately. The statistical weight w(Y) for the ion or atom Y is given [82] by W(Y)

=

j$[24(Y) + +xp[--E,(W/kT]

(6)

q=o

where J,(Y) is the inner quantum number for the electronic state 4, and E,(Y) is the qth excitation energy above the ground state (E,(Y) = 0). The contribution of excited states to the ratio w(M+))/w(M) has been investigated for uranium [83]. In the temperature range (Ts 2500 K) usually used in PSI or NSI, it is often sufficient to take into account only the ground states of the atoms or ions, or the first excited states at most. For negative ions, even more so, only the ground state need be taken into consideration, because any excited states are of low probability. On the basis of data compiled by Moore [84], the weight ratio w(Y ‘)/w(Y) may be evaluated to be - OS for alkali metals and - 0.25 for halogens. If in eqn. (2) or (5) it is assumed that all the reflection coefficients are zero, that the Schottky term is negligibly small compared with [$’ -I(M)] or [A(X) - Cl, respectively, and that the substrate metal surface is homageneous with respect to the work function (namely, &’ = &- = +), then the well-known Saha-Langmuir equations for PSI and NSI are obtained: ~‘(M+),=Ew(Mi)/w(M)lexp{[~-~(M)/k~l}

(7)

~-Ix->,=[w(X-)/w(X)lexp{[A(X)-_/kTl}

(8)

Moon [SS] seems to have been the first to give -the derivation of eqn. (7) on the basis of the results achieved by Saha [86] and Langmuir and Kingdon

l371.

The ionization efficiencies fi’ and p- for positive- and negative-ion production respectively from incident atoms M and X are given by p+(M+),

= n(M+),‘N(M)

~-(x-),=n(X-)/N(X)=u(X)~a-(X-)X/[l

= u(M) - a+(M+)&[l

+ a+(M+),] +a+(x-),]

(9) 00)

where N(Y) and u(Y) are respectively the incident flux per unit time per unit surface area, and the sticking probability of Y. This probability represents

15

the fraction of the impinging atoms which adsorb and reach the thermochetical equiiibrium- (1) or (4) on the ionizing surface under study. 2.2. Molectilai=b&im

surface

ionization

Wh& a bean-i df polyatomic molecules- MX including an atom or radical X having a- large value of A(X) is directed onto a negatively biased metal surface at a high temperature, X - is usually emitted together with e- after the equilibrium (4) and the following equilibrium have been attained on the silrface: MX=M+X Provided satisfied, n(MX) n(X)

-’

01)

that’ the steady-state conditions (C-l)-(C-3) the following equations may be written: = n(M)

+ n(X-)

+ n(X) + n(X-)

= a(MX)N(MX)

= [n(MX)

+ n(X)

below

are

02)

= a(MX)y(MX)N(MX)

03)

where y(MX) is the degree of dissociation sticking probability of MX is expressed by a(MX)

described

of MX

into M and X, and the

+ n(X-)],‘N(MX)

04)

The conditions are: (C-l) both the atom X and the ion X- thus formed do not combine with any other atoms, molecules or ions present on the surface; (C-2) the surface coverage B by both MX and its dissociation products is so small that the probability of self-recombination such as 2X + X, is sufficiently low that it may be neglected; (C-3) all the undissociated molecules MX, the amount of which is equal to N(MX)[ 1 - a(MX> y(MX)I, leave the surface as a result of surface reflection (N(MX)[ 1 - a(MX)]) or thermal desorption (a(MX)N(MX)[ 1 - y(MX)]), and both X- and X evaporate from the surface after a short adsorption lifetime, which seems to range in general from lo-* to 1O-5 s at T- 15002000 K according to refs. 88-90. Under the above conditions the ionization coefficient and the ionization efficiency for molecular-beam NSI are given respectively by the equations (Y-(x-),,

= n(x-)/n(X) = [w(X-)/w(x)]exp{

= u(MX)y(MX)

[ -$-

.

+e(eF)l”

+A(X)]/kT)~

4X-hix/~~ +4x-Ll

(15)

06)

16

In a similar way, the following

equations are derived for molecular-beam

PSI: a+ (M+),x

= [w(~+)/w(~)]exp(

[$+ +c(~F)‘/*

-I(M)]/~T)

(17)

P+(M+),,=n(M+),‘IL’(MX) = a(MX)y(MX)

*“+(M+h4x/ll+ a+ w+ km1

(18)

In eqns. (15) and (17) all the reflection coefficients are regarded as unity for the reason described in Section 2.1. When F and $- (or $‘) are identical between the atomic- and molecular-beam surface ionizations, c~-(X-)~x [or CY+(M+)~~] is identical to a-(X-)x [or ar+(M+),] given by eqn. (5) [or eqn. (2)] without depending upon the nature of the constituent atom M (or X). On the other hand, even if $’ , F, IVand CTare the same between the two, are usually smaller than p-(X-), and P-(X->,, and P’(M+),, respectively, because y(MX) is generally less than unity except in P+(M+),, a high-temperature range, as described in Section 2.4. It should be noted that eqns. ( 15) and ( 16) do not hold for vibrationally excited molecules. Beterov et al. [91] have found that the ion current due to NSI of an electronegative molecule SF, is decreased considerably when the inside of a magnetron-mode ion source is irradiated by a CO, laser beam with a wavelength of 10.6 pm (or a photon energy of 0.12 eV). 2.3. Work function It is well known that a metal wire usually has a polycrystalline structure consisting of patches exposing various crystal faces. When the ionizing surface is composed of a large number (i) of patches having work functions +i (ranging from a minimum value #min to a maximum +,,,) and fractional areas f;(Zjf;. = 1), the collected current of e- emitted together with X- is given by the modified Richardson-Dushman equation _j-(e-)

= C,S-q(e-)T*x

[l - ri(e-)]Jexp([

-Qi + e(eF)“2]/kT)

I =

CRSq(e-)[l

- r(e-)]

T’exp(

[ -@

+ r(rF)“‘2]/kT)

(1%

where C, is the Richardson constant, S is the surface area of the emitter, r(e-) is the collection efficiency for e-, J(e-) is the mean value of the reflection coefficients r,(e-) for e- emitted from various patches, and @ is the mean effective work function for thermal electron emission (TEE), given bY @ = --kTln(

2 { [ 1 - rj(eP)]/[l i

- r(e-)]}f,exp(

-+,/kT)}

(20)

17

As is well known, $ can readily be determined from the slope of a Richardson plot (log[j-(e-)/T21 versus l/T). It is usually assumed for a metallic emitter that ?(e- ) is negligibly small compared with unity. On the above patchy surface, therefore, the negative- and positive-ion ionization coefficients for atomic- or molecular-beam surface ionization in a weak electric field (F = 1 kV cm-‘) are given [92] respectively by

(21) and ‘Y+(M+)=

(22)

where u(Y -c ) and r(Y) have been assumed to be zero. If i = 1 and hence Xif;-cpi = $ = 6’ = $- = +, then eqns. (21) and (22) reduce to eqns. (8) and (7), respectively. Zemel [92] has shown that the above patch theory reasonably explains the result [93] that at certain temperatures CX+(M+) for sodium halide/tungsten systems may show a maximum and a minimum which are not predicted by eqn. (7) assuming surface homogeneity in work function_ Similar results observed by other workers [94-961 may probably also be explained on the basis of the theory. In the usual case of PSI occurring on a patchy surface, however, it seems sufficient to employ eqn. (2) or (17) instead of eqn. (22). It should be noted that $+ effective for PSI is usually different from @ and is given by

1

(23)

i

1

To the best of our knowledge, no data for NSI have been reported that cannot reasonably be interpreted without employing eqn. (21). It usually seems sufficient to employ eqn. (5) or (15), where $- is given by $-

= --kT

ln[ xi.exp(

-+,/kT)]

i

It may be noted that the experimental values of 6’) ij-, and $ determined on the same surface are usually different from the simple average work function defined as

The

main

contribution

to both

NSI

and

TEE

from

a polycrystalline

18

surface is made by..faces having, a low work function, and hence, both $and @ usually tend. towards qmin_On the other hand, .$+ tends to approach Qmaxbecause P~SI -occurs principally on faces having a high work function_ For a polycrystalline surface, therefore, it is evident in general that +,,,,, > 6’ > $ > ~min and also +,,.,= >&+ > &- > ~min- Equations (20) and (24) show that @ is equal to $- when r,(e-> is substantially constant over the paichy faces under study. The fact that ;i;+ and (p- are not always the same as @ should be taken into consideration when thermochemical values such as A(X) and I(M) are to be determined by means of NSI .or PSI. Attention should be drawn to the fact that the value of @ in eqn. (19) is often different from that of g0 effective for normal TEE, where no sample molecule is directed onto the emitter surface. under study. The difference between the two is usually dependent upon the nature of the incident atoms or molecules, upon the sample beam flux N or the pressure p of the sample gas introduced, and vpon the surface temperature. When a surface of Hf (G = 3.17 eV) at T= 1720-1890 K is surrounded with chlorine gas molecules at_ p = 8.5 X lop6 torr, for example, little change is found, namely $ = 3.20 eV 1971. Wh en a W surface ( p0 = 4.48 eV) at 1942-2072 IS is exposed to iodine vapour at 1.3.X 10p4- 1.84 X lop3 torr, on the other hand, ;3;”is found .to be as high as 5.06-5.46 eV [ 381. Similarly, an increase (by - 0.5 eV) is also observed when chlorine molecules at 4.7 X 1O-6 torr impinge upon W [97]. In the presence of 7 X 10e3 torr of C,(CN),, on the other hand, @ for W is decreased to 2.19 eV: more than 2 eV less than $$ [98]. In the case of LaB, (G = 2.70 eV), which has been expected to be one of the most desirable materials for a thermionic cathode (see Section 3.5), the eV in the temperature range 1200work function is increased to -2.9-3.3 1350 K by the introduction of iodine gas molecules at 2.2 X lop2 torr [99], as in another similar case [ 1001. In the case of GdB, at 1400 -C 50 K, @ is found to increase from - 3.3 to 4.0 eV as ~(1,) increases from 10e6 to 10e2 torr [ 1011. Consequently, employment of the approximation $- = p0 instead of $- = $ may possibly lead to a large error in the calculation of a-(X-) or p-(X-) using eqn. (15) or (16). Even when the ionizing filaments of interest are the same in chemical composition, 5’ , &- and @ all vary depending upon the crystal structure of each filament and also possibly upon the experimental conditions such as the nature of the sample molecule MX, or the values of N(MX) and T, as mentioned above. Some of the data published for &‘+ = $- , and @, are compiled in Table 2, which leads to the following suggestions: (1) for a single-crystal surface, the relation &’ = G- = g0 holds so long as N{MX) or N(X) is so small that 8 is much smaller than a monolayer and hence the surface may be regarded as essentially clean;

19 TABLE

2

Thermionic contrast observed for single-crystal and polycrystalline surfaces: 6’ and $- (first row) are the average effective work functions for thermal positive and negative ion emissions, respectively; $e is that for thermal electron emission in the absence of sample-gas incidence; 0= W, C=Ir, ThO,(Ta) and La&(W) (first column) denote oxygenated tungsten, carburized iridium, thoria-coated tantalum, and lanthanum hexaboride-covered tungsten, respectively (the value with an asterisk was assessed by the present authors assuming that A(Li)=0.62 eV) Surface

5’ (ev>

Mo( 100) MO Mo

5.02* 0.05 4.38~0.01

3.97 -c 0.05 *

Ta Ta

4.38*0.05 4.64t0.03

3.96 * 0.03

W(lOQ W(ll1) W W W o=w

4.50 * 0.03 5.2-5.3 5.14-to.03 5.05 kO.05 6.76-6.74

Ret000 1) Re Re

&i (eV)

Ref.

4.35 * 0.05 4.33 * 0.07

136 137, 138 59

4.33 10.03 4.3OkO.02

137, 138 139

4.55 -e 0.05 -

4.53 * 0.05 4.38-4.40 4.58 -t 0.03 4.58*0.05 5.7 -5.8

136 140 141 137, 138 142 143

5.15*0.10 5.43 * 0.03 5.21* 0.01

-

5.15*Oo.10 4.93 2 0.04 4.98 kO.03

144 137, 138 145

Ir(ll1) Ir Ir C=Ir(lll)

5.75 * 0.10 5.80% 0.05 5_34f-0.07 4.8

-

5.75kO.10 5.4oio.05 5.25 * 0.05 4.8

146 147 148 149

PS 73%&W-27WRe

5.77 * 0.05 4.92 * 0.07 3.7 2.77 3.2 2.96

-

5.13kO.05 4.68 * 0.07 2.7 2.60 2.7 2.76

147 142 150 151 150 151

ThO,(W) ThO,(Ta) LaB, (W) LaB,(W)

5-

(eV)

4.36 k 0.05

-

(2) much more work must be done to check the accuracy of the assumption that &- = gO, or &- = @, that has usually been made for NSI from a polycrystalline surface. Thermionic emission studies of single-crystal and polycrystalline surfaces of W, MO and Ta have suggested that the following relations hold for metals having a body-centred cubic lattice; g0 = C#B~ [ 1001 and q+,,,, - $D~,_, = 0.9- 1.O eV [ 1021. The latter figure is usually called the true contrast in the work function for a polycrystalline surface, and has an important effect upon the differences among &+, if- and &$ (or p).

20

Generally, values of j-(e-) at various temperatures are required in the research field (B) described in Section 1.4. In the other fields, too, jj(e-) is very important, as one of the pieces of fundamental data that may be helpful in studying the possibility both of space charge effects and of secondary reactions due to thermal electrons. In any case, therefore, it is desirable to measure bothj-(e-) and the ion current i-(X-) under the same experimental conditions. Whenever the value of the work function is required for a study of NSI, it is not the value in vacua ( pO) that should be used, but that ($ or &-) in an atmosphere of the sample gas molecules, determined directly for the surface under study, rather than being cited from a handbook. Strictly speaking, the work function is not independent of temperature, and many experimental results suggest that, for any emitter {within the limits of experimental error), @CT) =+(O)

+ 0

(26)

The value of the average temperature coefficient Z determined for TEE from various polycrystalline emitters is found to range from - + lo- 3 to * 10e6 eV_’ K [103,104]. From both PSI and NSI data for NaCl on W(110) and from TEE data for W in the absence of any sample beam, Fine et al. [IO51 obtained the results that a+ = a- = - 1.3 * 0.2 X 10e4 eV K-’ and ae= -1.6%0.1 X lop4 eV K-r. Wh en eqn. (26) is taken into consideration, eqns. (15) and ( 17) are modified as follows: rx- (x-

)Mx = [w(X-

v4v1

xexp(-~-/k)exp([-~L(0)+e(eF)“2+~(X)]/kT]

a+w+hx

(27)

= bw+ Vw(Wl X

exp(&/k)exp(

[q+(O)

+ e(eF)“‘-

I(M)]/kT)

,(28)

Since the average value ai has not yet been measured for any polycrystalline surface, there is no clue to the general relationships among 7i-, .i? and a”. Simply on the basis of Z’, therefore, it is very difficult in general to estimate H’ for a given polycrystalline emitter. Much more work on these coefficients must be done for a variety of systems. As discussed in ,Section 2.7, the temperature dependence of $* may be a very important problem in a special case such that w(Y =)/w(Y) or [l - r(Y ‘)I/[ I - r(Y)] is to be determined by PSI or NSI. In usual cases, however, &’ may be regarded as essentially constant over a narrow range, such as 1500- 1800 K. 2.4. Degree of dissociation In NSI, however, a-(X-)MX

is usually much smaller than unity, and the -

21

value of y(MX)

is essentially independent of +- and can be evaluated using eqn. (29) [106,107]. In PSI y(MX) is not always independent of 6’ and is given by three different equations according to the value of a+(M+),, [1(x3]:

Y(MX)~/‘[ 1 - y(MX)]

= 1.013 X 106[ N,/o(MX)N(MX)]

[c(M)c(X)/@X)]

X [M(MX)/2~M(M)M(X)RT]“*

Xexp[AS’(MX)/R]exp[-D(MX)/RT] =[2.669

x lo*s/N(~X)]

X exp[AS’(MX)/R]exp[

[ M(MX)/M(M)M(X)T] --D(MX)/RT]

l/2 (29)

where NA is Avogadro’s number, M(Y) the molecular or atomic weight of the molecule or atom Y, and R the gas constant_ D{MX) is the dissociation energy of the bond M-X, and its values for simple molecules (especially diatomic molecules) are readily available from thermochemical tables [1091121. AhSO is the standard entropy change due to dissociation and may easily be evaluated from data compiled by Prophet and co-workers [ 113- 1151. The factor 1.013 X lo6 is necessary because AS0 (MX) refers to the standard gaseous state of 1 atm. The evaporation coefficient E(Y) changes over a wide The range between 1 and lo- ‘, depending upon the nature of Y [ 116,117]. coefficients for the alkali-metal halides, for example, are found to vary from 1 to lo-’ [ 1181. In the case of dissociative evaporation, however, no -data for C(M) and c(X) have been obtained for any dissociation products M and X. It is to be emphasized that eqns. (7) and (8) are derived under the implicit assumptions that the evaporation coefficients for both the atom (M or X) of and the ion (M+ or X-) are unity, and also that the sticking probability the incident atom (M or X) is unity. These assumptions, however, are generally accepted as not contradicting explicitly any experimental results obtained to date. In a similar way, therefore, both c and 0 in molecular-beam NSI may be assumed to be unity, and y(MX) may then be evaluated using eqn; (29) [46,106,107]. The temperature dependence of y(LiF), for example, is illustrated in Fig. 1, where the upper and lower horizontal broken lines [ y( LiF) = 0.95 and 0.0981 indicate the boundaries corresponding to the conditions y(LiF) = 1 and y(LiF) K 1, respectively, because the measurement of an emission current of X- is usually accompanied by an experimental error of at least *5% [106]. When N(LiF) = 1Or2 molecules cme2 sL1, the boundary temperatures r,.,, and To.098above and below which y(LiF) 2 0.95 and y(LiF) 5 0.098 may be read off in Fig. 1, being 1890 and 1570 K, respectively. In a similar way, both To.95 and To.098 can be evaluated for various molecules without depending upon the value of & so long as ~C(x-)~x +C1 [107].

.O_.

0 .5 .z. e

0 098 ~_~__~~~~_..__~___~_____.

I 1500

2000

I

t

1

I

2500

T/K

Fig. 1. Temperature dependence of degree of dissociation y(LiF) (29). The curves (l)-(3) correspond to an incident sample-beam lOI

molecules

cm-*

s-l,

respectively

calculated for LiF using eqn. flux N(LiF)= lOlo, 10” and

1106].

The relationship between D(MX) and T& or To,o98evaluated for some alkali-metal halides is illustrated in Fig. 2, where N(MX) is lOI* molecules cmW2 s-r and D(MX) is the value at 298 K. As may be seen, good linearity with a standard deviation of - t20 K is observed for both of the temperatures, giving [ 1191 the relations To.95 = 280 D(MX) To.098= 240 D(MX)

+ 220 + 150

(30) (31)

A similar result is obtained for another value of N(MX) such as 10” or 1Or4 for certain molecules cmb2 s- ’ [119]: Since we have no data for ASO alkali-metal halides (such as RbX, CsBr and CsI), it is impossible to evaluate y(MX) for the halides from eqn. (29). Both To,95 and To_098,however, may readily be evaluated from eqns. (30) and (3 I), respectively (within an error of

23

500

I 3

I

I

4

I 5

I 6

D(MX)/eV

Fig. 2. Dependence of boundary temperatures To.95 (solid circles) and To.,,98(open circles) upon dissociation energy D(MX) [ 1191. The numbers (I)-(13) correspond respectively to LiF, KF, CsF, NaF, LiCl, CsCl, LiBr, KCl, NaCI, KBr, NaBr, LiI and KI, for each of which the incident flux is lo’* molecules cm-* s-l.

- &20 K), because D(MX) for each halide is well known from refs. 109-l 12. With respect to other molecules such as Ccl,-Cl, NO-O and Cl,, on the other hand, a larger deviation (- * 50 K) is observed from the linear relationship illustrated for the halides alone in Fig. 2 [ 1071. Such a deviation, however, may be neglected in comparison with the experimental errors in the determination of both T and N(MX) and also with the error due to uncertainty in the reported values of D(MX) [ 1191. It is not surprising that data for D(CsCl), for example, are scattered over a range as wide as 4.4-4.8 eV [ 1 10,l 111. This scatter may perhaps be the reason why the data points (6) in Fig. 2 have the largest deviation from the linear relation expressed by eqn. (30) or (31). In addition, it should be emphasized that the boundary temperatures inherently do not need to be determined exactly. This is the main reason why the temperature dependence of D(MX) is ignored in any of the above calculations of y(MX). When no data for AS”( MX) are available for the molecule MX under study, in consequence, it may be recommended, using N(MX) = lOI molecules cmY2 s-‘, for example, that approximate values of T,.,, and T0.098should be estimated using eqns. (30) and (31), respectively [ 1191.

24

In the case of molecular-beam surface ionization, it is very important to know the approximate values of the boundary temperatures described above, because the quantitative expression for the ion current as a function of surface temperature T changes according to whether T is higher than T0_95, lower than T0,098,or intermediate between the two, as described in the next Section. In general it is not easy to estimate theoretically or experimentally the degree of dissociation of polyatomic molecules such as organic compounds, which are very often studied by NSI. Therefore, it is most desirable to find a practical method by which the degree(s) of single (or multiple) dissociation(s) can be determined exactly for compounds consisting of more than three atoms. 2.5. ion emission current The collected ion current of Xi-(X-),,

= e%(X-

)dX-

produced from MX is given by

)N(MX)P-

(X’ )MX

= eSg(X-)q(X-)a(MX)y(MX)N(MX) -a-(X-)lw/w

+~-wh4,1

(32)

where S is the area of the ionizing surface, q(X-) is the transmission efficiency for X- travelling from the emitter to the ion collector, which can be determined experimentally, and g(X-) is the secondary-electron multiplier gain for X- , which is dependent upon the kinetic energy of X- striking the first dynode of the secondary-electron multiplier employed. The energy dependence of g(X-) is found to be quite different from that of g(M)) [ 1203. Some examples in molecular-beam surface ionization have been reported for g(K+)/g(Cl-), g(Rb+)/g(Br-) and g(Cs+))/g(I-) [121] and for and g(I-) [122]. Needless to say, g(X-) is unity when g(Cl-), g(Br-) i-(X-),, is measured using a d.c. amplifier alone. In a similar way, the following equation may be obtained for PSI of MX: i+ (M+ JMX= eSg(M+)rl(M+)a(MX)y(MX)N(MX) Xa+

@f+),x,‘b

+ ~+(M+frvd

. (33)

Except for some special cases [71,123,124] that will be considered in Section 7 (Part III), both /3-(X-), and p-(X-),, are usually less than low2 because the inequality A(X) + e( eF)‘/* -= $- normally holds, thereby giving the result that both a:- (X-)x and a-(X-),, are much smaller than in the denominator of eqn. (32) can be neunity. Therefore, cw-(X-),x glected in comparison with unity, and substitution of eqn. (15) into (32) yields

25

i-(X-),,

-eSg(X-)q(X-)u(MX)y(MX)N(MX)[w(X-)/w(X)] Xexp(

[ -$-

+e(eF)“2

+A(X)]/kt)

(34)

Consequently, i- (X- ) MX is given by a different equation (i.e., eqn. (35), (37) or (39) below) according to the value of y(MX) [46,106,107].

This condition i-(x-

is satisfied

)MX = C,exp(

[ -$-

for TZ

7&:

+e(eF)t’2

then

+A(X)]/kT}

(35)

. w(X-)/w(X)

(36)

where C, is given by C, = eSg(X-)7(X-)a(MX}N(MX)

Irrespective of both surface temperature and. incident flux, eqn. (35) is applicable also to atomic-beam surface ionization where X instead of MX impinges upon the surface. Then i- (X-) MX is equal to i-(X)x when a(MX)N(MX) is the same as o(X)N(X) and when the values of $- are identical between the atomic- and molecular-beam surface ionizations.

This condition

holds for Ts

= C,T -‘/4exp(

i- (X-j,,

[ -&-

T0_098: then +e(eF)1’2

+ A(X)

- D(MX)/2]/kTj

(37)

where C, is C, = eSg(X-)~(X-)[w(X-)/w(X)][N,a(MX)N(MX)~”2 x [M(MX)/~VM(M)M(X)R]“~~~~[AS~(MX)/~R]

(C) Neither

y(MX) = 1 nor

This condition i- (X-

JMXN C,T-

(38)

y(MX) <
is fulfilled ‘/‘F(T)exp(

in the range T0.95 > T> [ -$-

+e(eF)“2

To.098: then

+ A(X)

- D(MX)]/kT)

(39) where c, =feSg(X-)q(X-)iV,[w(X-)/w(X)] xexp[

A$‘(MX)/R]

[M(MX)/27iM(M)M(X)R]“* (401

26 F(T)

=

(1 +

xexp[

4[a(MX)N(MX)/N,I[2~M(M)M(X)R/M(MX)]”2T’/Z -As”(MX)/N,]exp[D(MX)/kT])1’2-

1

(41)

Any c(Y) to be included naturally in eqns. (36), (38), (40) and (41) has been taken as unity. When N(LiF) is lOI molecules cmm2 s-’ (see Fig. l), for example, i- (X-),, is expressed by eqns. (35), (37) and (39) in the ranges T? 1890 K, Ts 1570 K and 1890 > T> 1570 K, respectively. Since N(MX) is usually kept constant in a run, the factors C-C, extemperature range. pressed above are essentially cbnstant over a -narrow However, F is a function of T, as shown by eqn. (41). In case (C), therefore, T’/‘] versus l/T never yields a straight line so long a plot of log[i-(X-),, as the experimental error in a given run remains within - + 5%. In cases (A) or (B), on the other hand, the gradients of plots of log[r-(X-),,I versus TLj4] versus l/T are essentially constant in the l/T or of log[i-(X-),, temperature range above T0,95or below Tosog8,thereby allowing A(X), &- or D(MX) to be determined. Such a change in the quantitative expression for emitted ion current is found also for PSI, where i+(M+)M, is expressed as a function of T by seven different equations according to the values of both y(MX) and “+(M+)MX [log]. As may readily be understood, the-above results achieved theoretically for impinging neutral atoms or molecules are not available for the evaluation of when a sample deposition method such as that described a-(X-) or i-(X-) in Section 5.1 (Part II) is employed. On the other hand, negative-ion production from positive ions impinging upon a heated metal surface is found to be described by the Saha-Langmuir equation [48]. This result is very interesting from the viewpoint of fundamental surface-science and also provides a new technique applicable, for example, to the direct determination of A(X). 2.6. Sticking probability When the incident beam flux N(MX) is to be determined by an NSI or PSI technique under the experimental condition _for example that both y(MX) and (Y‘/( 1 + (Yi. ) are essentially unity, one of the important values that should be known is the sticking probability a(MX). If o(MX) is unknown, it is not the true value of.N(MX) but only the apparent value a(MX)N(MX) that may be evaluated using eqn. (32) or (33). Theoretical investigations of sticking probability have been made by many workers over a long period of time, but it is very difficult in general to

27

evaluate the probability for a given gas-solid system under specified experimental conditions because we have little knowledge of the fundamental thermochemical data. Consequently, the assumption that cr(MX) is unity under the conditions employed is tentatively adopted, and then accepted as reasonable within the limits of experimental error so long as the result thus obtained is not unreasonable. Otherwise, a(MX) is assumed to be constant during the course of measurements, and the temperature dependence of the ion current is investigated. This method makes it possible to determine $-, without knowledge of the absolute value of o(MX). Up to A(X) or D(MX) the present, these methods have been employed widely in NSI as well as PSI. It is usually under the assumption of o(MX) = 1 that y(MX) is evaluated theoretically, as already mentioned in Section 2.4. The values thus evaluated as a function of T may be sufficiently useful to estimate the boundary temperatures T0,,-,5 and T0,098. The experimental result a(MX) = 1 has been reported for many systems, for example, Cs/W [77], Ag/W and Ag/O = W [125], Cs/Pt and CsCl/Pt [ 1261, and CsCl/Ir and Cs/C = Ir [ 1271. On the other hand, Datz and Taylor [ 128,129] have reported that a(MX) is much smaller than unity for some alkali-metal atoms and alkali-metal halide molecules impinging upon the surface of Pt or 92% Pt-8%W alloy, although their explanation is far from convincing [ 126,130,13 I]. Using the NSI technique, both the value of a(MX) and its temperature dependence have been measured for several systems, such as Cl,/W, Cl,/Hf and HCl/Hf [97]. It is quite unlikely that a(MX) is nearly equal to unity for every gas-solid system. Therefore, exact evaluation of a(MX) using a suitable method is very important especially when a true beam intensity is to be determined from the measurement of ion current due to surface ionization. 2.7. General remarks It must be kept in mind that the theory described above is applicable to “equilibrium surface ionization” in which the thermodynamic equilibria (l), (4) and/or (11) are established on an ionizing surface. On the other hand, Bakulina et al. [ 1321 have reported that CHZ and OH- are produced by “non-equilibrium surface ionization” of cyclotriacetone peroxide mainly because ar+(CH; ) and a-(OH-) measured experi((CH,),CO*),, mentally are very much larger than those evaluated from equations similar to eqns. (7) and (8), respectively. Data for the effective work function $’ or &and for the desorption flux n(CH,) or n(OH) in addition to those for n(CHT ) or n(OH-) are required when LY+(CH~ ) or a-(OH-) respectively is to be determined theoretically and experimentally. Unfortunately, ref. 132 does not give a definite answer to the question as to how the above four

28

values were actually determined for the sample-surface

system under study.

Persky et al. [ 1331have found for a KCl/Re system that the experimental value of fi- (Cl-) kc1 obtained using an equation similar to P’(Y*)MX

= i ‘(Y

‘),,/eSr(Y

‘)g(Y

?)N(MX)

(42)

where Y * is either M + or X is in fair agreement with the theoretical value calculated using an equation similar to eqn. (16) if it is assumed that both a(KC1) and y(KC1) are unity. Consequently, the above result may suggest that the simplifying assumption is reasonable under the experimental conditions employed, and also that Re as well as W and Nb can hardly undergo a surface reaction with alkali-metal halides and/or their dissociation products below - 2100 K 11331.When a beam of KX (X = Cl, Br or I) is directed onto thoria ted tungsten, however, the experimental values of p-(X-),, and obtained from eqn. (42) are found to be quite different from the P+(K+& theoretical ones evaluated from eqns. (16) and (1X), respectively, on the assumption that a(KX) = y(KX) = 1 [ 1331. A suggested reason for these discrepancies is the formation of thorium halide on the emitting surface [ 1331. However, such formation of halide is by no means confirmed_ It is also necessary to consider whether the error due to the assumptions of both $‘=$-=@anda(KX)=y(KX)= 1 is negligibly small compared with that due to other causes. On the basis of the experimental data for p-(X-),, and /3’(IS+ )kx obtained for the three different potassium halide/thoriated tungsten systems mentioned above, the statistical weight ratio w(K+)/w(K) is evaluated to be - 30, which is much larger than the theoretical value (- 0.5) [ 1331. On the other hand, w(X-)/w(X) is found to lie in the range 10-4-10-5, which is much smaller than the theoretical value (- 0.25) [ 1331.According to an early study [78] of a K/W system at 1600-2500 K, w(K+)/w(K) would be evaluated as about twice the theoretical value if both r(K+ ) and r(K) in eqn. (2) were taken as zero. An additional example of w(Ba+)/w(Ba) investigated for a Ba/W system at T= 2000-2600 K shows that the experimental value obtained is 3.0 f 0.5, in contrast to the theoretical value of 2.00-2.28 [ 1341. A similar discrepancy has been found for several rare-earth/W( 112) systems [ 1351. It is well known that in thermal electron emission any experimental value determined for the Richardson constant (.C, in eqn. (19)) seldom agrees with the theoretical value { 120 A cmp2 Kp2)_ In PSI or NSI, however, it is generally very difficult or almost impossible to confirm the value of w(M+),‘w(M) or w(X-)/w(X) experimentally, because experimental determination of the other pre-exponential factors in eqn. (2) or (5) cannot be achieved independently of that of the weight ratio. It would be of no use to try to determine w(M+)/w(M) or w(X-)/w(X) experimentally without

29

confirming by some means that all of the errors due to the following assumptions are negligibly small: (9a) both r(Y * > and r(Y) are zero; (b) 6’ is independent of T in the temperature range investigated; (c) the sticking probability u(MX) defined by eqn. (14) is unity; (d) the roughness factor for the surface area, which will be explained in Section 4.4 (Part II), is unity; (e) @ is equal to 6’ ; and (f) the ion under study is produced only by true PSI or NSI. When the weight ratio is to be determined experimentally, eqns. (28) and (27) should be employed instead of eqns. (2) and (5), respectively. As already mentioned in Section 2.3, a seems to be in the range - +O.Ol to k 10 k, where k = 8.6 X 10e5 eV K-l. Therefore, exp (*a/k) is not always in the neighbourhood of unity and can change widely (from lo4 to 10p5) according to the nature of the ionizing surface employed. Within this range are the figures obtained for w(K+))/w(K) or w(X-)/w(X) by Persky et al. [133]. Among many possible errors, an error in the work function may have the most serious effect upon the experimental determination of any of the pre-exponential factors in eqn. (2) or (5). In conclusion, it is more important to determine not g0 but $’ or $- precisely, rather than to discuss possible differences between the theoretical and experimental values of w(Y *)/w(Y), when considering the problem of whether eqn. (2) or (5) is applicable to experimental results obtained for cy* (Y ‘) or fl* (Y * )_ ACKNOWLEDGEMENT

One of the authors (H.K.) would like to thank the Ministry of Education of Japan for a Resident Research Fellowship from March 1976 to February 1977 during which he made much of this survey while carrying out research at the University of Aston in Birmingham. REFERENCES 1 H.E. Duckworth, Mass Spectroscopy, Cambridge University 35. 2 J. Elster and H. Geitel, Ann. Phys. Chem., 37 (1889) 315.

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