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R.f.-spark source mass spectrometric studies of molecular ions
International Journal of Mass Spectrometry
and Ion Processes, 91 (1989) 241-260
241
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
R.F.-SPARK SOURCE MASS SPECI’ROMETRIC OF MOLECULAR IONS
STUDIES
B.P. DATTA, V.L. SANT, V.A. RAMAN, C.S. SUBBANNA and H.C. JAIN Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085 (India)
(First received 29 September 1988; in final form 22 February 1989)
ABSTRACT The small molecular ions such as the carbides (MC,‘,*‘), oxides (MO,‘,*‘) and some other matrix-sensitive ions in the beam produced from an r.f.-spark ion source have been studied for a number of elements (M) of varying electronic configurations. The MC: ions of a given element, M, generally follow either of the following two yield distribution patterns, namely, (i) a monotonic decrease in the MC: ion yield as n increases and (ii) a zigzag abundance distribution with the peaks appearing at even n values. The oxide ions, MO:, due to any given element (M) show only the decreasing trend in yields with n and the higher oxides (n >/ 3) are generally rare. The other molecules, some of which are metal or matrix-sensitive and which may be of concern for elemental analysis, include metal hydride (MI-I), hydroxide (MOH), halide @IX), cyanide (MCN), polymeric species such as M,, M,C,, M,O,,, M,O,H,, M,X,, etc. The studies show that, unless the nature of the molecular mass spectrum for a given matrix is predetermined, the elemental analysis may turn out to be erroneous. Factors governing the abundance of molecules in the recorded ion beam and the probable processes of their formation are discussed.
INTRODUCTION
The ion beam produced from an r.f.-spark ion source consists mainly of monoatomic ions of all the constituent elements (major, minor and trace) of the sample and, therefore, spark source mass spectrometry (SSMS) is commonly employed for elemental analysis, particularly for trace analysis. The multi-atomic ions in the beam are generally low in abundance. However, the studies of molecular ions are important for following reasons: (i) for making possible an unbiased trace elemental analysis; (ii) to explore the types of molecules formed from a spark ion source and (iii) for understanding the processes which could be responsible for their formation. Thus an extensive literature [l-11] (and references cited therein) unveiling the various aspects of molecular ions formed in a spark discharge is available. However, the 016%1176/89/$03.50
0 1989 Elsevier Science Publishers B.V.
242
data on molecular ions neither are exhaustive nor appear matrix independent. It was, in these contexts, considered worthwhile to look for the features of molecular ions generated in a spark ion source using different matrices. Moreover, the studies of molecular ions offer an opportunity to compare the behaviour of material in an r.f.-spark ion source with that in the equilibrium vapours at high temperatures, thereby complementing each other. One of the bright features of SSMS is that it renders possible the analysis of non-conducting matrices. Normally this is achieved through the aid of a conducting support material, and particularly for a non-conducting powder sample high purity graphite powder is widely used. But graphite generates several carbon polymers (C,) and carbides (MC,) which may sometimes appear as a source of problems for elemental analysis and hence an a priori knowledge of them is important. This work is devoted to the studies of molecular ions, especially the metal carbides formed from an r.f.-spark ion source, using different matrices and/or elements belonging to different periodic groups. It also undertakes to study the oxides and certain other molecules which are generally matrix specific. EXPERIMENTAL
The investigations were carried out using a Mattauch-Herzog type double focusing r.f.-spark source mass spectrometer [12] operating under a fixed set of experimental conditions. The sparking settings were as follows: power amplifier anode voltage, 2.5 kV; breakdown voltage, 23% spark level of the automatic spark controller; r.f.-pulse width (1 MHz source), 20 ps and pulse repetition frequency, 1 kHz. One of the requirements for studying molecular ions is high mass resolution. With this in view the photoplate system was normally employed for ion detection. Depending upon the sample and the requirements such as highest exposure and working resolution, the widths of the slits such as object slit (S,), divergence angle defining slit (a) and the energy defining slit (p), were varied whenever desired from one experiment to another for a given sample, in the ranges 20-50 pm, 0.4-0.8 mm and 0.4-0.8 mm, respectively. The acceleration voltage was maintained at 30 kV for all experiments. These ensured a mass resolution of - 4000 to a maximum of - 8000 while allowing a reasonable sensitivity for recording 100-300 nC of highest exposure on a photoplate. However, the mass resolution of 8000 or even the maximum achievable working resolution of lo4 is insufficient for the separation of MC, and MO, molecules. Such problems, as discussed later, were resolved indirectly and results are reported only for cases which left no ambiguity. In this context, it could be noted that the investigations were carried out on nuclear grade or spec pure
243
or at least analar grade samples, and the studies were restricted to matrix elements. For studying the carbides, a homogenised mixture (1: 1 by weight) matrix of graphite and a salt (or metal powder) of the element of interest, was used as a sample. Both the left and the right electrodes made from the sample were cylindrical in shape and were used for sparking. The oxides were studied using either metal electrodes or a mixture matrix of silver and an oxide sample of the metal. The details of experimentation, photoplate development and calculation procedures are given in ref. 13. RESULTS AND DISCUSSION
Uranium carbides Figure 1 shows the yield distribution patterns of uranium carbide (UC,‘,2+) ions as a function of n in the recorded ion beam of U,O,-graphite samples.
48-
-3.5
/1 4
44-
40-
0
UCn+
- 3.0
- 2.5
36-
-2.0
8 x
b
x
32-
-1.5
,+ $+I
-
\ --I 28-
-1.0
24-
- 0.5
20
-
16’ 0
-
s
’ 1
(
’ 2
’
’ 3
’
’ 4
’
’ 5
’
n
Fig. 1. Yield distribution of UC:*+
ions as a function of n.
’ 6
0.0
1-0.5
+, v 2
t = Gl _o
244 TABLE 1 Concentrations and ion yields of UC, molecules according to charge in the beam from U,O,-C matrix; yield ratio of UC, to UC,_,; ratio of the integrated concentration of UC,, molecules in the ion beam to the carbon to uranium atom ratio in the sample, and concentrations of C, molecules Mole-
No. of
PPM a
P&,
UC,/
cUC,/
cule (UC,)
photoplates
(SRSD) b
(XRSD) b
UC,_,
(“c/U)
UC
6
322
6
23 2124
(1.5) 99
3
(27) (Z)
(0.4)
UC3 UC4
3
123
6.2
2
(25) $;”
uc6
2
93 6.6
Mole-
No. of
PPM a
de
photoplates
(SRSD)
(C,) C,
3
3538
C,
3
(26) 2569
b
(27)
0.0094 c4
3
(E)
0.065
Cs c,
3 3
(Z) 1.6
0.625
C,
3
(11) 2.1
110.7
(12)
Approximate composition of the recorded ion beam C+ ( - 80%) > U+ ( - 11%) > U2+ ( - 6%) > C2+ ( - 3%) > U3+ ( - 0.5%) a Concentrations (parts per million) of UC,, (3rd column) and C,, (9th column) with reference to U and C, respectively, in the recorded beam. b Relative percent standard deviation.
In Fig. 1, a log scale is used for singly charged ions and a linear scale for doubly charged ions: this is done because of the wide gap in the yields of the two series of ions. Table 1 gives the concentrations, the percentage abundances of the singly charged ions (calculated considering only the available singly and doubly charged ions) of uranium carbide molecules and a rough estimate of the major ion composition of the beam. The concentrations are calculated using the formula
where PPMf refers to parts per million concentration of AX with reference to A in the beam. Ei and E,& are the numbers of singly charged ions of A and AX, respectively, required to produce a fixed degree of blackening on the photoplate. PA+ and Pp5( stand for the percentage abundance of singly charged ions of A and AX respectively.
245
The percentage abundance (Pp) given by Eq. 2 [14]
of ions of charge j + for a species Y is
(2) where EG+ is the number of ions of charge j + of species Y required to produce a fixed degree of blackening on the photoplate. The concentrations of UC,, (n 2 3) molecules are calculated assuming PA equals 100, as the data for doubly charged ions for them are not available. It should be noted that, for the evaluation of results from a photoplate it was assumed that the small molecular ions and the atomic ions of a given mass (m) and charge ( j + ) behave similarly with regard to the photoplate blackening process, and the method of calculations discussed for atomic ions in ref. 13 is used here. For background correction the method due to Franzen et al. [15] was followed. Further details of photoplate evaluation and the calculations of ion abundance according to charge are given in refs. 13 and 14, respectively. Table 1 also shows the trend in the abundances of C, molecules as a function of n in the ion beam from the U,O,--C matrix for comparison. For the calculations of concentrations of C, molecules the percentage abundance of singly charged ions of carbon (P,‘) was experimentally determined, while that for C,, molecules, that is, PA was assumed to be 100. The yield distribution pattern of C,, molecules is in agreement with the observations made by Ramakumar et al. [ll] for ‘*C, type molecules. However, it should be noted that the calculations of E+ (Eiq. 1) or E j+ (Eq. 2) involve abundance corrections for the monitored species [13]. The abundances of C,, molecules for a given value of n were found from the statistical equation
WI
where x,? is the fractional abundance of the ith isotope of element A, and 1 stands for the total number of isotopes of the element. The abundance correction is, however, equally important for UC, or any other molecule. The abundances of compound molecules such as AnBpC4 . . . , are calculated using the following statistical formula (cf. Eq. 3) ($xJ where xi,
(@)’
(&)‘...=(I)(l)(I)...
yi and zi are the fractional
=I isotopic
(4) abundances
of the ith
246
isotopes of elements, A, B, and C, having a total of J, K and L numbers of isotopes, respectively. The trends in the yields of UC22+ ions in the beam from U,O,-C samples are in conformity with the earlier observations by Becker and Dietze [9] who have reported the yield distribution of UC: (n = l-4) and UC:+ (n = 1, 2) ions using a uranyl-graphite matrix. However, it may be of interest to analyse the results as follows. (1) Uranium was present as U,O, in the samples. The formation of different carbide molecules, therefore, supports the proposition [3-5,7,9] that the molecules in a spark ion source are generated, at least partially, by gas phase reactions. (2) The UC,+ ions follow a zigzag abundance distribution pattern as a function of n. Up to n equals 4, the peaks appear at even n values while an inversion in the abundance trend occurs between UC: and UC:. (3) The intensity of C+ ions is highest in the beam and the abundance of U+ ions is second to it. However, the abundances of UC: ions neither maximise at UC+ nor do they follow the abundance distribution pattern of Cz ions. This means that, irrespective of the mechanism of formation, there is no steadfast relationship between the yield distribution pattern of UC,, molecules and the concentrations of the apparent constituent species of the UC, molecules such as U and C, (n >, 1) in the recorded beam. Alternatively, the nature of the observations suggests that, for a given experimental environment the characteristic stability of a molecule is the key factor. This is supported by the fact that the trends in the yields of UC: ions in the present observations are in agreement with the abundance pattern reported for UC: (n = 1-6) ions in the equilibrium vapours at high temperatures, over a uranium carbide-graphite system [17,18]. (4) The ion yield distribution of UC, molecules according to charge differ from one given molecule to another and hence, it is a characteristic property of the molecules. The abundance ratio, = 0.75, of UC2+ to UC+ is more than the ratio, = 0.0101, of UC:+ to UC;. This justifies the statement that, for a given set of sparking conditions and matrix the relative stabilities of the molecular ions control their composition in the beam from a spark ion source. (5) Under the prevailing conditions of an r.f.-spark plasma, the triply and higher charged small molecular ions are highly unstable, rendering difficult even their detection. (6) The sensitivity of formation of molecular ions is much less compared to atomic ions. The carbon to uranium atom ratio in the sample is 22.4 and in the ion beam is approximately 5. That is, the sensitivity factor for carbon with reference to uranium is roughly 0.2, while the sum of concentrations of UC, molecules relative to uranium in the beam is only around 2500 ppm.
247
Carbides due to different metals and matrix effect With a view to examining the differences, if any, in the abundance patterns of MC, molecules caused by different metallic systems (M), investigations were carried out for a number of elements (M) belonging to different periodic groups. This includes K and Cs (Group Ia), Cu and Ag (Group Ib), Sr and Ba (Group IIa), Zn (Group IIb), Sb (Group Va), Cr, MO and W (Group VIb), Ni (Group VIII), and Th, U and Pu among the actinides. Table 2 furnishes a few of the typical results. The concentrations of the ThC, molecules could be biased roughly by a constant factor. This is because, except for a few very low exposures the monoisotopic Th+ and Th2’ lines on a photoplate are saturated. Similar biasing in the concentration values (cf. ThC,) could also be true for AgC, molecules. With molecules for which the data for doubly charged ions are not available, the concentrations are calculated assuming PAs( (Eq. 1) equals 100. The results show that the carbide ions of a given element (M) exhibit either of the following two yield distribution patterns: (i) a monotonic decrease in the intensity of MC: ions with n, and (ii) an alternating abundance distribution of MC: ions as a function of n with the peaks appearing at even n values. The CuCc ions, for which the yield data are not provided, were studied in a mixture matrix of copper and molybdenum salts. However, the CuCT (n = 1-3) ions display a decreasing trend in yields with respect to n. For a few of the systems studied the data for MC, molecules are limited up to n equals 2 or 3. For the WO,-C system (Table 2), high exposures could not be taken. However, in the beam from a mixture matrix consisting mainly of tungsten and nickel salts, the monotonically decreasing yield distribution pattern as n increases was observed for WC: (n = l-5) ions. The antimony carbides (see Table 2), follow roughly the alternating abundance distribution pattern, but the more appropriate description is that the SbCJ ions display the decreasing yield distribution pattern with increasing values of even n. That is, the successive odd-even pairs, e.g., SbC and SbC,, SbC, and SbC,, etc., are roughly equally stable at the prevailing spark discharge environment. The thorium penta- and hexacarbides are definitely formed in a spark ion source, but their yields are low and higher exposures are required to ascertain the exact abundance relationship between the two. Some of these metal carbide (MC,) systems such as those due to Cr, Ni, Cu, MO, Ag, W and U were also studied by Becker and Dietze [9] and the observations are generally in agreement. For MoCJ and AgCz systems Becker and Dietze [9] observed a decreasing abundance trend up to n = 3 and then a sudden increase in MC: ion yield. Using a mixture matrix of spec pure silver and graphite (1: 1 by weight), we however, observed only the decreasing abundance pattern for AgCz ions as a function of n (see Table 2), because, while
248 TABLE 2 Concentrations and charge distributions of MC, molecules due to different elements M; yield ratio of MC, to MC,_,; ratio of the integrated concentration of MC,, molecules of a given element M in the ion beam to the carbon to metal atom ratio in the sample, and concentrations of C, molecules Matrix
Molecule
PPM a
P&,,>
WC,, 1 PUO, -c
(UPu)oxidegraphite
ThO, -C
Ba(OH),-C
Ag-C
Ammonium molybdate-C was-c
PUC PUC, PUC, PUC, PUC, PUC, PUC, PUC, PUC PUC, PUC, PUC, UC UC, UC, UC, ThC ThC, ThC, ThC, ThC, ThC, BaC BaC, BaC, BaC, BaC, BaC, BaC, BaC, AgC AgC, AgC, MoC MoC, MoC, MoC, WC WC2
Sb-C
SbC SbC, SbC, SbC, SbC, SbC,
193 2605 23 166 10 18 1.6 4.6 81 511 6 24 239 1440 16 82 406 2172 20 14 -1 -1 14 984 30 125 2.5 8 0.06 0.2 15.3 0.85 0.34 1353 147 10 7 866 341 112 118 3.5 5.4 1.4 1.3
95.6 99.8
95 99.4
88.2 99
91 98.4
MC,/
cW,/
W-I
(nC/M)
Molecule
- (C,) c2
13.5 0.009 1.22 0.06 1.8 0.09 2.9
C3 C4 G C6 C, G
133.7
C9 c2
7.1 0.01 4
C3 C4 G
C2
6.83 0.0072 3.7
C3 C4 G G C, C2
13.3 0.03 4.2 0.02 3.2 0.088 3.33
c3 C4 C5 C6 C,
c3
1.83
99.6
C4 c2
0.11 0.07 0.7
103.2
0.39
62.5
c3 c4 G c2 C3 c2
1.05 0.3 1.54 0.26 0.93
1227 891 16.4 11.4 0.5 0.64 4833 4915 110 12 5 6
60.3 c2
0.056 0.4
99.9 99.9
8585 10755 476 728 103 172 41 37 1616 938 218 23
6.03 0.011 5.12
148.7 92.4 99.8
PPM ’
c3 c4 C5 G
23.8
C,
1894 1552 46 1416 1071 41 35 1773 1496 1275 1227 65 119 16 40
249 TABLE 2 (continued) Matrix
Molecule
PPM a
P&-,
(MC, ) ZnO-C NiO-C
Cs,Cr,O,-C
ZnC ZnC, NiC NiC, NiC, NiC,, CrC CrC,
KNO,-C
csc csc, csc, KC
KCl-C
KC
srco,
KG SrC SrC,
-c
SrSO, -C
SG SG SrC, SrC, SrC, SrCs SrC SrC, SrC, SrC, SrC, SrC, SrC, SrC,
13 6.5 128 38 2 0.3 113 89
MC,/
cMC,,/
Molecule
MC,-,
k/M)
(C,)
0.5
2.9
0.30 0.05 0.15
24
C2 C3 C2 C3 C4 C5 c2
0.79
9.8
C3 C4
2.2 0.08 0.05 2 5 0.3 115 870 18 105 0.8 4 < 0.1 b 0.2 95 1030 30 167 3 32 < 0.4 b 3
0.04 0.63
C5 G
0.11 0.24
C, c2 C3 c2
0.6 95 99.9
0.85
C3 C2
7.57 0.021 5.8 0.08 5.0
c3 C4 G C6
PPM a
1078 1187 2339 3171 81 100 1.4x 104 1.8 x lo4 553 651 71 106 3375 1517 4141 6872 3506 2933 48 26 2.1
90.5 94.6 99.9
c2
10.8 0.03 5.57 0.02 10.7
c3 C4 C5 G C,
2016 2783 60 63 4.2 4.3
88.9
a Concentrations (parts per million) of MC,, (3rd column) and C, (8th column) with reference to M and C, respectively, in the recorded beam. b SrC,, data not corrected for Sr, interference; the interference correction reduces the values, i.e., 0.1 and 0.4 to - 0.03 and - 0.2, respectively.
the lines due to AgCT ions for a given exposure were recorded on the photoplate, the AgCc lines were missing. This was confirmed by duplicate analysis of the Ag-C system. Similarly, under the conditions of our experiment a decreasing trend in yields with n is confirmed for MoCT (n = l-4) ions.
250
The results show that for certain matrices the ion yield distribution patterns of MC: ions and Cz ions are parallel and for some other cases this is not so (Tables 1 and 2). Such similarity and dissimilarity in the abundance patterns of C,’ and MC: ions could be interpreted as follows. The structures and the type of bonding involved in the C, and MC,_, types of molecules are nearly similar [17,18] such that in certain cases the substitution of a carbon atom from a C,, molecule by a metal atom (M) may lead to enhance the stability of the molecule and in some other cases it is not so. As a result, in a given experimental and/or reaction environment which may help produce kinetically both the C, and MC,_ 1 types of molecules, the composition of the reaction products, that is, the ion beam which is being extracted out of the reaction chamber (ion source), will be governed by the thermodynamic parameters of the molecules. This is so, irrespective of whether the molecule is of the C, or MC, or even the M,C, type. Thus the deviation in the trends of yields of C,, molecules from matrix to matrix and in particular between salt-graphite type matrices and the matrix of graphite alone [ll], could be accounted for by the competitive reactions involving different M and which are naturally absent in the matrix of graphite alone. It is redundant to mention that yields or yield distributions of any other types of component molecules of the ion beam from a spark ion source are controlled by the thermodynamic parameters of the respective species and reactions. Nevertheless, such an explanation invokes the criteria of thermodynamic equilibrium while it is certain that the true thermodynamic equilibrium picture is inconceivable in the case of a spark discharge. This is because throughout the period between spark initiation and termination of the discharge all conceivable parameters, such as the temperature, go on changing. Thus, Ramendik et al. [4] place emphasis on kinetics rather than on thermodynamic parameters for the production of molecules in a spark ion source. However, the present observations compel us to think of a local environment in the processes of formation of the ion beam where thermodynamics play the crucial role. We return to this point below. The conclusion is supported by the following literature reports: (i) the control of the spark discharge parameters leads to a reduction of the scatter of results [19], (ii) the trend in the yields of MC: ions of a given M, such as UC:, ThCz, MoC: ions, etc., in the present investigations is in conformity with the trend in the abundances of MC: ions in the equilibrium vapours at high temperatures [17,18,20-221; and (iii) the C, yield distribution pattern in the beam produced by spark ionisation of graphite is in agreement with the abundance pattern of C, molecules generated thermally [ll, and refs. cited therein]. It may be noted that, for the elements of a particular periodic group such as Sr and Ba or K and Cs or Cr, MO and W, etc. the ion yield distribution
251
pattern of MC, molecules remains unchanged. This reflects that the stabilities of MC,, molecules and the electronic configurations of elements, M, are correlated. In other words, the studies reveal that, for elements belonging to a particular periodic group the stability patterns of MC,, molecules are similar. However, the reverse is not always true, as the elements belonging to different periodic groups also show similar abundance patterns of MC, molecules. Furthermore, Tables 1 and 2 show that a change in matrix composition does not generally cause a change in the trend of yields among the different MC: ions of a given M. Moreover, the variation in the yields (Table 2, column 3) of the different MC, molecules of a given element M and the slight variations in their yield ratios (Table 2, column 5) from one matrix to another give a measure of the probable variations caused by a change of matrix composition, in the environment responsible for the production of molecular ions. Column 6 in Tables 1 and 2 lists the ratio, xMC,,/(C/M), of the integrated concentration of the MC,, molecules of a &ven element (M) to the carbon-to-metal atom ratio in the sample. If the matrix effect on the formation of molecules is ignored, then c MC,,/(C/M) could be taken as a measure of sensitivity for the element M t”oform a carbide. The results show that the actinide elements from their respective matrix top the list while the first periodic group of elements appears to be least sensitive. The rough order of sensitivity is as follows: actinides > MO = Sr = W = Ba > Sb = Ni >Cr>Zn>Ag= K = Cs. Becker and Dietze [9] observed the first group element (silver) among the elements studied, as having the lowest sensitivity towards carbide formation. Oxides (MO,) Irrespective of the nature of the matrix, a SSMS spectrum of a sample generally contains lines due to the oxide ions (MO:). However, without exception, for all the elements M studied the MO: ion intensity decreases monotonically as n increases. In fact, their abundances fall off drastically at n >, 3 such that under the conditions of our experiment for no element studied could the formation of MO, and higher oxides be confirmed. For elements such as K, Cs and Ag from the first periodic group, Sr and Zn from the second group, it could not be ascertained as to whether MO, molecules are formed in a spark ion source. This is because the MO: lines are generally missing from photoplates even for an exposure as high as 3 X lop7 coulomb. It should be noted that under the SSMS operating conditions (maximum achievable resolution is 104), the MO, and MC, species are not resolvable. Hence, the search for MO, molecules was made using metal oxide-silver matrices. Table 3 furnishes a few typical results.
252 TABLE 3 Concentrations Matrix
and charge distributions Molecule
of MO, molecules, and yield ratio of MO,, to MO,_, PPM a
P+MO,
1.98X105 1.6~10~ 19 1514 176 2331 125 951 23 936 8 2398 228 3 114 30 1849 65 1325 74 1.4 18
99.89 99.97
MQ/M%,
Won > U3Q3
-4
b
(3 : 1 by wt.)
uo uo2 uo3
u,o, -c
uo uo2
(UPu) oxide-C
uo uo2
PUO ho2
The,-c
ThO no2
MOO, -Ag (1:l by wt.) MO metal b Sb metal &Co, -c Ba(OH),-C KNO, -C
MOO MOO, MOO, MOO MOO, SbO SbO, SrO BaO BaO, KO
0.08 0.0012
95 0.12 97 0.054 98.5 0.024 98 0.009 99.9 0.095 0.013 0.26 99.6 0.035 99.8 98.6 0.02
a Concentration of MO, with reference to M in the recorded beam. b Matrix sparked using g power amplifier anode voltage of 4 kV.
Molecules other than the MC,, and MO,, The other molecules detected are metal hydrides, hydroxides, metal clusters (M,), intermetallic compounds and other polymeric species of the type M,X, (X = C, 0, OH, halogen, etc.). In general, for a given set of experimental conditions the intensity of a particular type of molecule depends upon the nature of the metal and/or matrix composition. However, for molecules such as hydrides, oxides, hydroxides, etc., the instrumental vacuum could also be a controlling factor. Table 4 lists some of the metal- or matrix-sensitive molecular ions recorded using an r.f.-spark ion source. Apart from the actinides and tungsten, M, molecules were observed for all other elements studied. The search for the dimers of the actinide elements was not carried out, and the difficulty of taking high exposures, perhaps prevented the detection of W, molecules. (Certain specific behaviour of M, molecules has been discussed in ref. 16.) Similarly, the metal hydride is also
253 TABLE 4 Concentrations of certain typical matrix-specific molecules (other than MC, and MO,) Matrix ISNO, -C
Molecule KOH
5 57 765 290 26
K2 K2O
K,CN K202
K,O,H (KOH) 2 K3C2
PPM a
Matrix
Molecule
PPM =
KCl-C
KOH
2.5 37 1 248 2089 14 2 5 0.06
K2 K2O
KC1 K,Cl
b
K3Cl2
2 11 8 41 3.5
K3C2
K3G K3C8
Cs,Cr,O,-C
CsOH CsCr cs2 cs,o
Cs,OH
0.04 1 4 0.3 0.5
SrCO, -C
SrH SrOH Sr2 Sr,O Sr,OH Sr202
Sr,O, H (SrOH) Sr2C2
Ba(OH),-C
BaOH Bar Ba,O Ba202 BaG
0.07 0.7 111 207 3.4 2 0.2 0.3 0.4
2
4
42 1 11 0.4 15
a Concentration (parts per million) of the molecule in question with respect to the element M. b K202H is also formed but in substantially less abundance with respect to (KOH) 2, white the reverse is true for Sr, i.e., Sr202H+ > (SrOH): .
a very common species. The metal hydroxide ions (MOH+) are generally significantly abundant in the beam from a matrix of alkali or alkaline earth metals. Group I and Group II elements also generate polymeric oxides such as M,O, M,O, and hydroxides of the types M,OH, M,O,H and (MOH) *. The polymeric alkali and alkali metal oxides are also identified in the Knudsen cells effusate from respective matrices [18]. Moreover, it is worth mentioning that Bacon and Ure [23] have reported the formation of many complex and matrix-specific molecules, particularly oxides, in the beams from a spark ion source. The types of species generated by a matrix of an alkali metal such as KNO,-C, generally outnumber all other metal-graphite systems studied.
254
Furthermore, the behaviour of MX, and M,X, types of molecule is not similar. Thus, the yields of KC: ions for which only the KC+ and KC: ions were recorded, follow a decreasing pattern with n, while the K&z series displays a complex yield distribution pattern with respect to n (see results for KNO,-C matrix). For the K3Ci series of ions only those ions with n equal to 2, 4, 5 and 6 were observed to a measurable extent while the formation of K,C+ and K,Cz ions of the K3Cn series and the parent Kz ions could not be confirmed. In other words, the r.f.-spark mass spectrum of KNO,-C matrix contains, among many, four pairs of prominent lines at m/z values 141 and 143, 165 and 167, 177 and 179, and 189 and 191. Equation 2 predicts seven K, molecules having mass numbers 117-123. However, only two or three of them are significantly abundant. These K, molecules are m = 117, 119 and 121 and have abundances 81.107%, 17.56% and 1.267% respectively. The above mass lines are identified as the carbides of m = 117 and m = 119. For some cases the lines due to the carbide K&, of K,(m = 121) were also detectable but were much too weak to be considered for measurement. It may be mentioned that, for a multi-isotopic element M, it was rather the conformity between the estimated intensities owing to the different probable M,X, molecules of given M, p, X and 4, and the abundance values predicted for them by Eq. 4, which was used as the yardstick for identification of the M,X, types of molecules reported here. Faint peaks were also observed at nominal positions of m/z 117, 129 and 153. They are certainly not atomic peaks, but as the K, abundance pattern is of no avail for ascertaining their identity, it is not confirmed whether they are due to the parent K,(m = 117) and the corresponding mono and tri carbide species, K,C and K,C,, respectively. However, if one is allowed to extrapolate the results, then it appears that in the beam from the KNO,-C matrix the K3Cz series of ions obeys an alternating abundance distribution up to n equals 4 while a reversal of abundance trend occurs at K,C,. This is confirmed by triplicate analysis of the KNO,-C system. However the KC1 system, which was also sparked for reasons stated below, does not produce a maxima at K3C5, and the K&l ions, if formed, are masked by rather intense and non-resolvable peaks due to K&l; ions. The yield distribution of K&J ions in the beam from a KCl-C matrix closely represents a zigzag pattern. The most abundant multi-atomic ion in the beam from the KCl-C system is the K&l+ ion, followed by KCl+ ion, and this yield distribution pattern of K&l: ions parallels the abundance trend among them in the molecular ion beam generated from equilibrium vapour over liquid KC1 [24]. The KCl-C matrix, however, does not help us to study the K, molecules, as they are non-resolvable from rather intensely abundant K&l molecules. A number of mass lines were observed at mass numbers corresponding to the following 39K,X and 39K3X types of mole-
255
cules, but their identities are subject to verification: (39K,)‘2C, (39K2)14N, ( 39K2)12C2, ( 39K3)12C3, ( 39K2)12C4, ( 39K3)‘60, ( 39K3)‘602 and also the oxide K3O2 of K,(m = 119). It is, however, certain that they are not due to atomic ions. A pair of relatively intense lines was recorded at mass numbers 169 and 171. Their intensity ratio roughly approaches the K, abundance pattern but could not be identified. The KNO,-C system produces very intense lines at m/z values 104, 106 and 108, (104 being the most intense), and clearly these are not due to atomic ions. Neither are they due to K,C, and K,N, because the molecules should have produced their most abundant lines at m/z 102 and 106 respectively (Eq. 4). However, the intensities of the three lines agree reasonably well with the predicted abundance pattern (Eq. 3) of K, molecules of mass numbers 78, 80 and 82. We ascribe the lines to K2CN+ ions. The difference in the abundance patterns for K, and K,CN molecules is really insignificant because for both carbon and nitrogen a single isotope of each is nearly 100% abundant. The identification is further supported by the fact that the intensities of these lines are drastically suppressed in the beam from the KCl-C matrix, which was sparked precisely for this purpose. For the alkaline earth metals such as strontium and barium, only the M,C, molecules of the M2C,, series are definitely identified. Ba,C could not be detected, but Sr,C appears to have been formed as a definite peak was recorded at m/z 188 using the SrCO,-C matrix. The Sr,C, and Ba,C, molecules are more abundant than Sr, and Ba, molecules respectively (cf. K, and K3Cn molecules). These could be loosely interpreted in terms of the stability of molecular ions increasing with increase in size. DISCUSSION OF THE PROCESSES OF FORMATION OF MOLECULAR IONS It is considered pertinent to analyse the results in the context of the following questions. (1) How are the molecular ions produced? Are they ejected directly from the solid substrate or formed by gas phase reactions or by both these processes? (2) Are the molecular ions and the atomic ions produced simultaneously and/or by similar processes? (3) Is there any relationship between the recorded molecular ion abundances and the abundances of the corresponding neutral molecules? The formation of species (such as UC, from U,Os-C matrix, K,$,, from KCl-C samples, Sr,O, from the SrCO,-C system, etc.) which are not the original molecular constituents of the electrodes being sparked, shows that some molecular ions could only be formed by gas phase reactions. Moreover, it is a general observation by almost all the workers in the field that the
256
concentrations of molecular ions such as oxide ions in the beam from matrices of an oxide-graphite type is low compared to the concentrations of the atomic ions of the constituent elements of the molecules. Such an observation could, perhaps, be taken as an indication that the molecular ions which reach the detector are not ejected directly from the electrode surfaces, and their formation processes are also different from that for atomic ions. By time resolved measurements, Viczian et al. [8] have shown that the multiply charged atomic ions produced from an r.f.-spark ion source reach the detector first, and with respect to the doubly charged atomic ions the singly charged atomic ions are delayed by 150 + 100 ns whereas the molecular ions are delayed to a greater extent ( - 700 ns or even more). They also observed that the spreads in the average delay times for molecular ions are also higher. The ion beam, particularly the ion beam due to the atomic ions reaching the detector, is believed to be formed by the following series of processes: vacuum spark breakdown by electron current and partial transfer of energy of the high velocity electrons to the electrode surfaces + erosion of electrode material forming plasma cloud (evaporation and atomization of solid) + avalanche ionization of atoms by the energetic electrons to the different charge states of the elements (plasma development) + formation of the ion beam for mass spectrometric analysis following intense charge recombination, or rather element-sensitive recombination of the multiply charged ions during the plasma expansion into the vacuum [14,25]. The average time delay of detection [8] of the singly charged atomic ions with reference to the doubly or higher charged atomic ions indicates that the singly charged atomic ions are formed mostly by the recombination of multiply charged ions. However, it is highly improbable that the molecular ions which constitute the ion beam for analysis are also generated through the processes of avalanche ionization followed by intense charge recombination of molecular species which could be assumed to have been ejected directly from the solid electrodes or which might have formed by neutral-neutral reactions in the plasma cloud. This is because the multiply charged ions, especially the small molecular ions, are chemically unstable. This is supported by the report [lo] that only a few triply charged molecular ions, and none with still higher charge states, could be detected in the beam from an r.f.-spark ion source. The studies of energy distribution of ions indicate that the energy of the main bulk of any given molecular ions is low compared to the atomic ions, whereas the molecules produced in a high temperature plasma should have an energy comparable to the atomic ions. The exact data on the energy distribution of ions could not be established due to certain unresolved problems. However, the study by Vos and Van Grieken [26] also supports the idea that the molecules generated in an r.f.-spark ion source have lower relative energy of formation.
257
If the above arguments are correct, and if the molecular ions constituting the ion beam for mass analysis, are formed by gas phase reactions, then it is very logical that they are generated after the plasma has relatively cooled down, that is, after the formation of the main bulk of the singly charged atomic ions and neutrals during plasma dispersal into the vacuum. Moreover, such a process can account well for the delay time as well as the lower abundance of molecular ions with reference to atomic ions. The plasma temperature (both the electron and ion temperatures), is relatively low, so inter-neutral collisions may yield a molecule which subsequently may suffer ionization without undergoing dissociation by the cooled electrons. Similarly, it is equally possible that a molecular ion is produced by attachment of an ion to a neutral. The second process could be more important than the former, as an ion by virtue of its charge can polarise the colliding neutral. The plasma is relatively cool, but the temperature is sufficient to make possible reactions which do not occur in the low temperature range. The plasma reactions may thus be expected to produce many kinds of molecules, but only those which have certain stability under the prevailing plasma conditions are expected to survive and reach the detector. Nevertheless, the temperature gradients at the dispersal stage are less pronounced than at the time of plasma development. As a result, the role of the thermodynamic parameters of species in governing their composition in the beam becomes increasingly significant. The studies of charge distribution of elements (ref. 14) have shown that, even for atomic ions, the element-sensitive thermodynamic parameters play a key role in governing their composition in the beam. Finally, it should be added that there is no sufficient reason to ignore the contributions of the molecular species which could be ejected directly from the solid surfaces as neutrals during this penultimate stage of the discharge, owing to sputtering of ions on the electrode surfaces. Naturally, the relative contributions of the different processes will be governed by the matrix and sparking parameters. In view of the observations and discussion presented here, it follows that the recorded molecular ion abundances are sound for the ions only, because it does not appear correct to assume that the recorded ions are still generated first as neutrals and subsequently suffer ionization by electrons. Even in such a case the ionization efficiency curves could be significantly different from one given molecule to another. The second point to be noted is that the chemical form (such as salt or elemental mixture) of the matrix has little to do with the types and abundances of molecular ions recorded. The point is further clarified by the fact that, even in the case of an arbitrarily chosen element, no two charge states of it are equally stable in any given environment. However, the abundances of the singly charged ions could be taken as the best representative of the abundances of the neutrals in a comparable environment.
CONCLUSIONS
The small molecular ions, especially the carbide (MC,‘,2+) ions, generated in an r.f.-spark ion source and which reach the detector are studied for a number of elements M of varying electronic configurations. By and large the MC: ion abundance pattern due to different metals M falls into two groups, namely, (1) monotonic decrease in the yields of MC: ions as n increases, and (2) a zigzag abundance distribution of MC: ions as a function of n with the maxima appearing at even values of n. For some elements such as antimony, the successive odd-even pairs, e.g., SbC+ and SbCl, SbC: and SbCl, appear to be roughly equally stable. The observed abundance pattern of molecular ions could be adequately explained from the consideration of given ion (M,X’,+) stabilities towards the various possible reactions under the prevailing experimental conditions. However, the investigations show that, for elements M belonging to a particular periodic group the stabilities of the MC, molecules are similar. The observations on MC,, and C, molecules suggest that, for a given set of experimental conditions and matrix, the abundances of MC, and C, molecules in the beam are governed by the different competitive reactions involving both the metal and carbon species. This also explains the variations in the yield distribution patterns of C, molecules as a function of n from one given matrix to another. Irrespective of its constituents, the ion beam reaching the detector is formed following plasma expansion and recombination. However, it appears that there is a difference between the main bulk of atomic ions and the molecular ions reaching the detector. The recorded atomic ions are the same as those which were produced at. the high temperature plasma but suffered intense element-sensitive charge recombination before becoming part of the ion beam, whereas the molecular ions of the beam are generated mainly by gas phase reactions after the production of the bulk of neutrals and singly charged ions following plasma dispersal and charge recombination. This offers an explanation for the higher delay time reported in the literature for molecular ions with respect to the singly charged atomic ions. During the plasma dispersal molecular ions are generated by the following comparable and complementary processes, namely, (i) neutral-neutral collisions followed by ionization by cooled electrons; (ii) ion-neutral reactions; and (iii) ejection of neutral molecules from solid surfaces as a result of sputtering of ions on electrode surfaces and subsequent ionization by low energy electrons. The manifold ways and hence the different possible times of their formation could be a reason for the higher spreads in their average formation times. Compared to the atomic ions, the sensitivity of production of molecular
259
ions is generally low, and could be of little significance if the elements concerned are traces. However, the formation of different matrix-specific molecules suggests that for an unbiased trace elemental analysis of a sample, it will be advisable to establish the nature of the molecular spectrum a priori for the given matrix and experimental conditions. The transfer of material from solid electrodes to the gaseous plasma in a spark ion source is basically a non-thermal process, but it resembles what is commonly known as high temperature mass spectrometry (HTMS) in two ways: (i) a spark ion source generates many species which are not formed in the low temperature range, and thus the former serves as a device for crossing over the energy barrier for reaction; and (ii) the abundance distribution of molecules such as UC: ions in SSMS parallels the abundance trend among the same series of molecules in HTMS, thereby indicating the importance of thermodynamic parameters of molecules in SSMS. ACKNOWLEDGEMENTS
The authors are grateful to Dr. D.D. Sood, Head Fuel Chemistry Division for his interest in this work. One of the authors (B.P.D.) sincerely thanks Dr. S.K. Ghosh and Dr. A. Dhara for their kind encouragement and help in many ways. REFERENCES 1 H. Hintenberger, J. Franzen and K.D. Schuy, Z. Naturforsch., TeiI A, 18 (1963) 1236. 2 J. Luck, P. MoIIer and W. Szacki, Int. J. Mass Spectrom. Ion Phys., 13 (1974) 25. 3 C.E. Rechsteiner, Jr., T.L. Youngless, M.M. Bursey and R.P. Buck, Int. J. Mass Spectrom. Ion Phys., 28 (1978) 401. 4 G.I. Ramendik, 0.1. Kryuchkova, V.I. Derzhiev, N.S. Stronganova and E.B. Strel’nikova, DokI. Phys. Chem., 245 (1979) 336. 5 I. Cornides and T. Gal, High Temp. Sci., 10 (1978) 171. 6 G. Mathieu, L. Lamberts, M. Thomas and G. Demortier, Int. J. Mass Spectrom. Ion Phys., 38 (1981) 35. 7 I. Cornides, Int. J. Mass Spectrom. Ion Phys., 45 (1982) 219. 8 M. Viczian, I. Cornides, J. Van Puymbroeck and R. Gijbels, Int. J. Mass Spectrom. Ion Phys., 51 (1983) 77. 9 S. Becker and H.J. Dietze, Int. J. Mass Spectrom. Ion Phys., 51 (1983) 325. 10 L. Morvay and I. Cornides, Int. J. Mass Spectrom. Ion Processes, 62 (1984) 263. 11 K.L. Ramakumar, V.A. Raman, V.L. Sam, V.D. Kavimandan and H.C. Jain, Int. J. Mass Spectrom. Ion Processes, 75 (1987) 171. 12 Jeol Ltd., Tokyo, Japan, model JMS~lBM-2. 13 B.P. Datta, V.A. Raman, V.L. Sam, P.A. Ramasubramanian, P.M. Shah, K.L. Ramakumar, V.D. Kavimandan, S.K. Aggarwal and H.C. Jain, Int. J. Mass Spectrom. Ion Processes, 64 (1985) 139.
14 B.P. Datta, V.A. Raman, V.L. Sant, V.D. Kavimandan and H.C. Jam, Int. J. Mass Spectrom. Ion Processes, 73 (1986) 13. 15 J. Franzen, K.H. Maurer and K.D. Schuy, Z. Naturforsch., Teil A, 21 (1966) 37. 16 B.P. Datta and H.C. Jain, Int. J. Mass Spectrom. Ion Processes, MASSPEC 2866. 17 S.K. Gupta and K.A. Gingerich, J. Chem. Phys., 71 (1979) 3072. 18 K.A. Gingerich, Cm-r. Top. Mater. Sci., 6 (1980) 345. 19 G.I. Ramendik, Adv. Mass Spectrom. A, 8 (1980) 408. 20 F.J. Kohl and C.A. Stearns, High Temp. Sci., 6 (1974) 284. 21 S.K. Gupta and K.A. Gingerich, J. Chem. Phys., 74 (1981) 3584. 22 K.A. Gingerich and I. Shim, Adv. Mass Spectrom. A, 10 (1985) 1051. 23 J.R. Bacon and A.M. Ure, Adv. Mass Spectrom., 8 (1980) 362. 24 J.W. Hastie, K.E. Zmbov and D.W. Bonnell, High Temp. Sci., 17 (1984) 333. 25 G.I. Ramendik, 0.1. Kryuchkova, M.Sh. Kaviladze, T.R. Mchedlidze and D.A. Tyurin, Int. J. Mass Spectrom. Ion Processes, 63 (1985) 1. 26 L. Vos and R. Van Grieken, Int. J. Mass Spectrom. Ion Processes, 51 (1983) 63.
and Ion Processes, 91 (1989) 241-260
241
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
R.F.-SPARK SOURCE MASS SPECI’ROMETRIC OF MOLECULAR IONS
STUDIES
B.P. DATTA, V.L. SANT, V.A. RAMAN, C.S. SUBBANNA and H.C. JAIN Fuel Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085 (India)
(First received 29 September 1988; in final form 22 February 1989)
ABSTRACT The small molecular ions such as the carbides (MC,‘,*‘), oxides (MO,‘,*‘) and some other matrix-sensitive ions in the beam produced from an r.f.-spark ion source have been studied for a number of elements (M) of varying electronic configurations. The MC: ions of a given element, M, generally follow either of the following two yield distribution patterns, namely, (i) a monotonic decrease in the MC: ion yield as n increases and (ii) a zigzag abundance distribution with the peaks appearing at even n values. The oxide ions, MO:, due to any given element (M) show only the decreasing trend in yields with n and the higher oxides (n >/ 3) are generally rare. The other molecules, some of which are metal or matrix-sensitive and which may be of concern for elemental analysis, include metal hydride (MI-I), hydroxide (MOH), halide @IX), cyanide (MCN), polymeric species such as M,, M,C,, M,O,,, M,O,H,, M,X,, etc. The studies show that, unless the nature of the molecular mass spectrum for a given matrix is predetermined, the elemental analysis may turn out to be erroneous. Factors governing the abundance of molecules in the recorded ion beam and the probable processes of their formation are discussed.
INTRODUCTION
The ion beam produced from an r.f.-spark ion source consists mainly of monoatomic ions of all the constituent elements (major, minor and trace) of the sample and, therefore, spark source mass spectrometry (SSMS) is commonly employed for elemental analysis, particularly for trace analysis. The multi-atomic ions in the beam are generally low in abundance. However, the studies of molecular ions are important for following reasons: (i) for making possible an unbiased trace elemental analysis; (ii) to explore the types of molecules formed from a spark ion source and (iii) for understanding the processes which could be responsible for their formation. Thus an extensive literature [l-11] (and references cited therein) unveiling the various aspects of molecular ions formed in a spark discharge is available. However, the 016%1176/89/$03.50
0 1989 Elsevier Science Publishers B.V.
242
data on molecular ions neither are exhaustive nor appear matrix independent. It was, in these contexts, considered worthwhile to look for the features of molecular ions generated in a spark ion source using different matrices. Moreover, the studies of molecular ions offer an opportunity to compare the behaviour of material in an r.f.-spark ion source with that in the equilibrium vapours at high temperatures, thereby complementing each other. One of the bright features of SSMS is that it renders possible the analysis of non-conducting matrices. Normally this is achieved through the aid of a conducting support material, and particularly for a non-conducting powder sample high purity graphite powder is widely used. But graphite generates several carbon polymers (C,) and carbides (MC,) which may sometimes appear as a source of problems for elemental analysis and hence an a priori knowledge of them is important. This work is devoted to the studies of molecular ions, especially the metal carbides formed from an r.f.-spark ion source, using different matrices and/or elements belonging to different periodic groups. It also undertakes to study the oxides and certain other molecules which are generally matrix specific. EXPERIMENTAL
The investigations were carried out using a Mattauch-Herzog type double focusing r.f.-spark source mass spectrometer [12] operating under a fixed set of experimental conditions. The sparking settings were as follows: power amplifier anode voltage, 2.5 kV; breakdown voltage, 23% spark level of the automatic spark controller; r.f.-pulse width (1 MHz source), 20 ps and pulse repetition frequency, 1 kHz. One of the requirements for studying molecular ions is high mass resolution. With this in view the photoplate system was normally employed for ion detection. Depending upon the sample and the requirements such as highest exposure and working resolution, the widths of the slits such as object slit (S,), divergence angle defining slit (a) and the energy defining slit (p), were varied whenever desired from one experiment to another for a given sample, in the ranges 20-50 pm, 0.4-0.8 mm and 0.4-0.8 mm, respectively. The acceleration voltage was maintained at 30 kV for all experiments. These ensured a mass resolution of - 4000 to a maximum of - 8000 while allowing a reasonable sensitivity for recording 100-300 nC of highest exposure on a photoplate. However, the mass resolution of 8000 or even the maximum achievable working resolution of lo4 is insufficient for the separation of MC, and MO, molecules. Such problems, as discussed later, were resolved indirectly and results are reported only for cases which left no ambiguity. In this context, it could be noted that the investigations were carried out on nuclear grade or spec pure
243
or at least analar grade samples, and the studies were restricted to matrix elements. For studying the carbides, a homogenised mixture (1: 1 by weight) matrix of graphite and a salt (or metal powder) of the element of interest, was used as a sample. Both the left and the right electrodes made from the sample were cylindrical in shape and were used for sparking. The oxides were studied using either metal electrodes or a mixture matrix of silver and an oxide sample of the metal. The details of experimentation, photoplate development and calculation procedures are given in ref. 13. RESULTS AND DISCUSSION
Uranium carbides Figure 1 shows the yield distribution patterns of uranium carbide (UC,‘,2+) ions as a function of n in the recorded ion beam of U,O,-graphite samples.
48-
-3.5
/1 4
44-
40-
0
UCn+
- 3.0
- 2.5
36-
-2.0
8 x
b
x
32-
-1.5
,+ $+I
-
\ --I 28-
-1.0
24-
- 0.5
20
-
16’ 0
-
s
’ 1
(
’ 2
’
’ 3
’
’ 4
’
’ 5
’
n
Fig. 1. Yield distribution of UC:*+
ions as a function of n.
’ 6
0.0
1-0.5
+, v 2
t = Gl _o
244 TABLE 1 Concentrations and ion yields of UC, molecules according to charge in the beam from U,O,-C matrix; yield ratio of UC, to UC,_,; ratio of the integrated concentration of UC,, molecules in the ion beam to the carbon to uranium atom ratio in the sample, and concentrations of C, molecules Mole-
No. of
PPM a
P&,
UC,/
cUC,/
cule (UC,)
photoplates
(SRSD) b
(XRSD) b
UC,_,
(“c/U)
UC
6
322
6
23 2124
(1.5) 99
3
(27) (Z)
(0.4)
UC3 UC4
3
123
6.2
2
(25) $;”
uc6
2
93 6.6
Mole-
No. of
PPM a
de
photoplates
(SRSD)
(C,) C,
3
3538
C,
3
(26) 2569
b
(27)
0.0094 c4
3
(E)
0.065
Cs c,
3 3
(Z) 1.6
0.625
C,
3
(11) 2.1
110.7
(12)
Approximate composition of the recorded ion beam C+ ( - 80%) > U+ ( - 11%) > U2+ ( - 6%) > C2+ ( - 3%) > U3+ ( - 0.5%) a Concentrations (parts per million) of UC,, (3rd column) and C,, (9th column) with reference to U and C, respectively, in the recorded beam. b Relative percent standard deviation.
In Fig. 1, a log scale is used for singly charged ions and a linear scale for doubly charged ions: this is done because of the wide gap in the yields of the two series of ions. Table 1 gives the concentrations, the percentage abundances of the singly charged ions (calculated considering only the available singly and doubly charged ions) of uranium carbide molecules and a rough estimate of the major ion composition of the beam. The concentrations are calculated using the formula
where PPMf refers to parts per million concentration of AX with reference to A in the beam. Ei and E,& are the numbers of singly charged ions of A and AX, respectively, required to produce a fixed degree of blackening on the photoplate. PA+ and Pp5( stand for the percentage abundance of singly charged ions of A and AX respectively.
245
The percentage abundance (Pp) given by Eq. 2 [14]
of ions of charge j + for a species Y is
(2) where EG+ is the number of ions of charge j + of species Y required to produce a fixed degree of blackening on the photoplate. The concentrations of UC,, (n 2 3) molecules are calculated assuming PA equals 100, as the data for doubly charged ions for them are not available. It should be noted that, for the evaluation of results from a photoplate it was assumed that the small molecular ions and the atomic ions of a given mass (m) and charge ( j + ) behave similarly with regard to the photoplate blackening process, and the method of calculations discussed for atomic ions in ref. 13 is used here. For background correction the method due to Franzen et al. [15] was followed. Further details of photoplate evaluation and the calculations of ion abundance according to charge are given in refs. 13 and 14, respectively. Table 1 also shows the trend in the abundances of C, molecules as a function of n in the ion beam from the U,O,--C matrix for comparison. For the calculations of concentrations of C, molecules the percentage abundance of singly charged ions of carbon (P,‘) was experimentally determined, while that for C,, molecules, that is, PA was assumed to be 100. The yield distribution pattern of C,, molecules is in agreement with the observations made by Ramakumar et al. [ll] for ‘*C, type molecules. However, it should be noted that the calculations of E+ (Eiq. 1) or E j+ (Eq. 2) involve abundance corrections for the monitored species [13]. The abundances of C,, molecules for a given value of n were found from the statistical equation
WI
where x,? is the fractional abundance of the ith isotope of element A, and 1 stands for the total number of isotopes of the element. The abundance correction is, however, equally important for UC, or any other molecule. The abundances of compound molecules such as AnBpC4 . . . , are calculated using the following statistical formula (cf. Eq. 3) ($xJ where xi,
(@)’
(&)‘...=(I)(l)(I)...
yi and zi are the fractional
=I isotopic
(4) abundances
of the ith
246
isotopes of elements, A, B, and C, having a total of J, K and L numbers of isotopes, respectively. The trends in the yields of UC22+ ions in the beam from U,O,-C samples are in conformity with the earlier observations by Becker and Dietze [9] who have reported the yield distribution of UC: (n = l-4) and UC:+ (n = 1, 2) ions using a uranyl-graphite matrix. However, it may be of interest to analyse the results as follows. (1) Uranium was present as U,O, in the samples. The formation of different carbide molecules, therefore, supports the proposition [3-5,7,9] that the molecules in a spark ion source are generated, at least partially, by gas phase reactions. (2) The UC,+ ions follow a zigzag abundance distribution pattern as a function of n. Up to n equals 4, the peaks appear at even n values while an inversion in the abundance trend occurs between UC: and UC:. (3) The intensity of C+ ions is highest in the beam and the abundance of U+ ions is second to it. However, the abundances of UC: ions neither maximise at UC+ nor do they follow the abundance distribution pattern of Cz ions. This means that, irrespective of the mechanism of formation, there is no steadfast relationship between the yield distribution pattern of UC,, molecules and the concentrations of the apparent constituent species of the UC, molecules such as U and C, (n >, 1) in the recorded beam. Alternatively, the nature of the observations suggests that, for a given experimental environment the characteristic stability of a molecule is the key factor. This is supported by the fact that the trends in the yields of UC: ions in the present observations are in agreement with the abundance pattern reported for UC: (n = 1-6) ions in the equilibrium vapours at high temperatures, over a uranium carbide-graphite system [17,18]. (4) The ion yield distribution of UC, molecules according to charge differ from one given molecule to another and hence, it is a characteristic property of the molecules. The abundance ratio, = 0.75, of UC2+ to UC+ is more than the ratio, = 0.0101, of UC:+ to UC;. This justifies the statement that, for a given set of sparking conditions and matrix the relative stabilities of the molecular ions control their composition in the beam from a spark ion source. (5) Under the prevailing conditions of an r.f.-spark plasma, the triply and higher charged small molecular ions are highly unstable, rendering difficult even their detection. (6) The sensitivity of formation of molecular ions is much less compared to atomic ions. The carbon to uranium atom ratio in the sample is 22.4 and in the ion beam is approximately 5. That is, the sensitivity factor for carbon with reference to uranium is roughly 0.2, while the sum of concentrations of UC, molecules relative to uranium in the beam is only around 2500 ppm.
247
Carbides due to different metals and matrix effect With a view to examining the differences, if any, in the abundance patterns of MC, molecules caused by different metallic systems (M), investigations were carried out for a number of elements (M) belonging to different periodic groups. This includes K and Cs (Group Ia), Cu and Ag (Group Ib), Sr and Ba (Group IIa), Zn (Group IIb), Sb (Group Va), Cr, MO and W (Group VIb), Ni (Group VIII), and Th, U and Pu among the actinides. Table 2 furnishes a few of the typical results. The concentrations of the ThC, molecules could be biased roughly by a constant factor. This is because, except for a few very low exposures the monoisotopic Th+ and Th2’ lines on a photoplate are saturated. Similar biasing in the concentration values (cf. ThC,) could also be true for AgC, molecules. With molecules for which the data for doubly charged ions are not available, the concentrations are calculated assuming PAs( (Eq. 1) equals 100. The results show that the carbide ions of a given element (M) exhibit either of the following two yield distribution patterns: (i) a monotonic decrease in the intensity of MC: ions with n, and (ii) an alternating abundance distribution of MC: ions as a function of n with the peaks appearing at even n values. The CuCc ions, for which the yield data are not provided, were studied in a mixture matrix of copper and molybdenum salts. However, the CuCT (n = 1-3) ions display a decreasing trend in yields with respect to n. For a few of the systems studied the data for MC, molecules are limited up to n equals 2 or 3. For the WO,-C system (Table 2), high exposures could not be taken. However, in the beam from a mixture matrix consisting mainly of tungsten and nickel salts, the monotonically decreasing yield distribution pattern as n increases was observed for WC: (n = l-5) ions. The antimony carbides (see Table 2), follow roughly the alternating abundance distribution pattern, but the more appropriate description is that the SbCJ ions display the decreasing yield distribution pattern with increasing values of even n. That is, the successive odd-even pairs, e.g., SbC and SbC,, SbC, and SbC,, etc., are roughly equally stable at the prevailing spark discharge environment. The thorium penta- and hexacarbides are definitely formed in a spark ion source, but their yields are low and higher exposures are required to ascertain the exact abundance relationship between the two. Some of these metal carbide (MC,) systems such as those due to Cr, Ni, Cu, MO, Ag, W and U were also studied by Becker and Dietze [9] and the observations are generally in agreement. For MoCJ and AgCz systems Becker and Dietze [9] observed a decreasing abundance trend up to n = 3 and then a sudden increase in MC: ion yield. Using a mixture matrix of spec pure silver and graphite (1: 1 by weight), we however, observed only the decreasing abundance pattern for AgCz ions as a function of n (see Table 2), because, while
248 TABLE 2 Concentrations and charge distributions of MC, molecules due to different elements M; yield ratio of MC, to MC,_,; ratio of the integrated concentration of MC,, molecules of a given element M in the ion beam to the carbon to metal atom ratio in the sample, and concentrations of C, molecules Matrix
Molecule
PPM a
P&,,>
WC,, 1 PUO, -c
(UPu)oxidegraphite
ThO, -C
Ba(OH),-C
Ag-C
Ammonium molybdate-C was-c
PUC PUC, PUC, PUC, PUC, PUC, PUC, PUC, PUC PUC, PUC, PUC, UC UC, UC, UC, ThC ThC, ThC, ThC, ThC, ThC, BaC BaC, BaC, BaC, BaC, BaC, BaC, BaC, AgC AgC, AgC, MoC MoC, MoC, MoC, WC WC2
Sb-C
SbC SbC, SbC, SbC, SbC, SbC,
193 2605 23 166 10 18 1.6 4.6 81 511 6 24 239 1440 16 82 406 2172 20 14 -1 -1 14 984 30 125 2.5 8 0.06 0.2 15.3 0.85 0.34 1353 147 10 7 866 341 112 118 3.5 5.4 1.4 1.3
95.6 99.8
95 99.4
88.2 99
91 98.4
MC,/
cW,/
W-I
(nC/M)
Molecule
- (C,) c2
13.5 0.009 1.22 0.06 1.8 0.09 2.9
C3 C4 G C6 C, G
133.7
C9 c2
7.1 0.01 4
C3 C4 G
C2
6.83 0.0072 3.7
C3 C4 G G C, C2
13.3 0.03 4.2 0.02 3.2 0.088 3.33
c3 C4 C5 C6 C,
c3
1.83
99.6
C4 c2
0.11 0.07 0.7
103.2
0.39
62.5
c3 c4 G c2 C3 c2
1.05 0.3 1.54 0.26 0.93
1227 891 16.4 11.4 0.5 0.64 4833 4915 110 12 5 6
60.3 c2
0.056 0.4
99.9 99.9
8585 10755 476 728 103 172 41 37 1616 938 218 23
6.03 0.011 5.12
148.7 92.4 99.8
PPM ’
c3 c4 C5 G
23.8
C,
1894 1552 46 1416 1071 41 35 1773 1496 1275 1227 65 119 16 40
249 TABLE 2 (continued) Matrix
Molecule
PPM a
P&-,
(MC, ) ZnO-C NiO-C
Cs,Cr,O,-C
ZnC ZnC, NiC NiC, NiC, NiC,, CrC CrC,
KNO,-C
csc csc, csc, KC
KCl-C
KC
srco,
KG SrC SrC,
-c
SrSO, -C
SG SG SrC, SrC, SrC, SrCs SrC SrC, SrC, SrC, SrC, SrC, SrC, SrC,
13 6.5 128 38 2 0.3 113 89
MC,/
cMC,,/
Molecule
MC,-,
k/M)
(C,)
0.5
2.9
0.30 0.05 0.15
24
C2 C3 C2 C3 C4 C5 c2
0.79
9.8
C3 C4
2.2 0.08 0.05 2 5 0.3 115 870 18 105 0.8 4 < 0.1 b 0.2 95 1030 30 167 3 32 < 0.4 b 3
0.04 0.63
C5 G
0.11 0.24
C, c2 C3 c2
0.6 95 99.9
0.85
C3 C2
7.57 0.021 5.8 0.08 5.0
c3 C4 G C6
PPM a
1078 1187 2339 3171 81 100 1.4x 104 1.8 x lo4 553 651 71 106 3375 1517 4141 6872 3506 2933 48 26 2.1
90.5 94.6 99.9
c2
10.8 0.03 5.57 0.02 10.7
c3 C4 C5 G C,
2016 2783 60 63 4.2 4.3
88.9
a Concentrations (parts per million) of MC,, (3rd column) and C, (8th column) with reference to M and C, respectively, in the recorded beam. b SrC,, data not corrected for Sr, interference; the interference correction reduces the values, i.e., 0.1 and 0.4 to - 0.03 and - 0.2, respectively.
the lines due to AgCT ions for a given exposure were recorded on the photoplate, the AgCc lines were missing. This was confirmed by duplicate analysis of the Ag-C system. Similarly, under the conditions of our experiment a decreasing trend in yields with n is confirmed for MoCT (n = l-4) ions.
250
The results show that for certain matrices the ion yield distribution patterns of MC: ions and Cz ions are parallel and for some other cases this is not so (Tables 1 and 2). Such similarity and dissimilarity in the abundance patterns of C,’ and MC: ions could be interpreted as follows. The structures and the type of bonding involved in the C, and MC,_, types of molecules are nearly similar [17,18] such that in certain cases the substitution of a carbon atom from a C,, molecule by a metal atom (M) may lead to enhance the stability of the molecule and in some other cases it is not so. As a result, in a given experimental and/or reaction environment which may help produce kinetically both the C, and MC,_ 1 types of molecules, the composition of the reaction products, that is, the ion beam which is being extracted out of the reaction chamber (ion source), will be governed by the thermodynamic parameters of the molecules. This is so, irrespective of whether the molecule is of the C, or MC, or even the M,C, type. Thus the deviation in the trends of yields of C,, molecules from matrix to matrix and in particular between salt-graphite type matrices and the matrix of graphite alone [ll], could be accounted for by the competitive reactions involving different M and which are naturally absent in the matrix of graphite alone. It is redundant to mention that yields or yield distributions of any other types of component molecules of the ion beam from a spark ion source are controlled by the thermodynamic parameters of the respective species and reactions. Nevertheless, such an explanation invokes the criteria of thermodynamic equilibrium while it is certain that the true thermodynamic equilibrium picture is inconceivable in the case of a spark discharge. This is because throughout the period between spark initiation and termination of the discharge all conceivable parameters, such as the temperature, go on changing. Thus, Ramendik et al. [4] place emphasis on kinetics rather than on thermodynamic parameters for the production of molecules in a spark ion source. However, the present observations compel us to think of a local environment in the processes of formation of the ion beam where thermodynamics play the crucial role. We return to this point below. The conclusion is supported by the following literature reports: (i) the control of the spark discharge parameters leads to a reduction of the scatter of results [19], (ii) the trend in the yields of MC: ions of a given M, such as UC:, ThCz, MoC: ions, etc., in the present investigations is in conformity with the trend in the abundances of MC: ions in the equilibrium vapours at high temperatures [17,18,20-221; and (iii) the C, yield distribution pattern in the beam produced by spark ionisation of graphite is in agreement with the abundance pattern of C, molecules generated thermally [ll, and refs. cited therein]. It may be noted that, for the elements of a particular periodic group such as Sr and Ba or K and Cs or Cr, MO and W, etc. the ion yield distribution
251
pattern of MC, molecules remains unchanged. This reflects that the stabilities of MC,, molecules and the electronic configurations of elements, M, are correlated. In other words, the studies reveal that, for elements belonging to a particular periodic group the stability patterns of MC,, molecules are similar. However, the reverse is not always true, as the elements belonging to different periodic groups also show similar abundance patterns of MC, molecules. Furthermore, Tables 1 and 2 show that a change in matrix composition does not generally cause a change in the trend of yields among the different MC: ions of a given M. Moreover, the variation in the yields (Table 2, column 3) of the different MC, molecules of a given element M and the slight variations in their yield ratios (Table 2, column 5) from one matrix to another give a measure of the probable variations caused by a change of matrix composition, in the environment responsible for the production of molecular ions. Column 6 in Tables 1 and 2 lists the ratio, xMC,,/(C/M), of the integrated concentration of the MC,, molecules of a &ven element (M) to the carbon-to-metal atom ratio in the sample. If the matrix effect on the formation of molecules is ignored, then c MC,,/(C/M) could be taken as a measure of sensitivity for the element M t”oform a carbide. The results show that the actinide elements from their respective matrix top the list while the first periodic group of elements appears to be least sensitive. The rough order of sensitivity is as follows: actinides > MO = Sr = W = Ba > Sb = Ni >Cr>Zn>Ag= K = Cs. Becker and Dietze [9] observed the first group element (silver) among the elements studied, as having the lowest sensitivity towards carbide formation. Oxides (MO,) Irrespective of the nature of the matrix, a SSMS spectrum of a sample generally contains lines due to the oxide ions (MO:). However, without exception, for all the elements M studied the MO: ion intensity decreases monotonically as n increases. In fact, their abundances fall off drastically at n >, 3 such that under the conditions of our experiment for no element studied could the formation of MO, and higher oxides be confirmed. For elements such as K, Cs and Ag from the first periodic group, Sr and Zn from the second group, it could not be ascertained as to whether MO, molecules are formed in a spark ion source. This is because the MO: lines are generally missing from photoplates even for an exposure as high as 3 X lop7 coulomb. It should be noted that under the SSMS operating conditions (maximum achievable resolution is 104), the MO, and MC, species are not resolvable. Hence, the search for MO, molecules was made using metal oxide-silver matrices. Table 3 furnishes a few typical results.
252 TABLE 3 Concentrations Matrix
and charge distributions Molecule
of MO, molecules, and yield ratio of MO,, to MO,_, PPM a
P+MO,
1.98X105 1.6~10~ 19 1514 176 2331 125 951 23 936 8 2398 228 3 114 30 1849 65 1325 74 1.4 18
99.89 99.97
MQ/M%,
Won > U3Q3
-4
b
(3 : 1 by wt.)
uo uo2 uo3
u,o, -c
uo uo2
(UPu) oxide-C
uo uo2
PUO ho2
The,-c
ThO no2
MOO, -Ag (1:l by wt.) MO metal b Sb metal &Co, -c Ba(OH),-C KNO, -C
MOO MOO, MOO, MOO MOO, SbO SbO, SrO BaO BaO, KO
0.08 0.0012
95 0.12 97 0.054 98.5 0.024 98 0.009 99.9 0.095 0.013 0.26 99.6 0.035 99.8 98.6 0.02
a Concentration of MO, with reference to M in the recorded beam. b Matrix sparked using g power amplifier anode voltage of 4 kV.
Molecules other than the MC,, and MO,, The other molecules detected are metal hydrides, hydroxides, metal clusters (M,), intermetallic compounds and other polymeric species of the type M,X, (X = C, 0, OH, halogen, etc.). In general, for a given set of experimental conditions the intensity of a particular type of molecule depends upon the nature of the metal and/or matrix composition. However, for molecules such as hydrides, oxides, hydroxides, etc., the instrumental vacuum could also be a controlling factor. Table 4 lists some of the metal- or matrix-sensitive molecular ions recorded using an r.f.-spark ion source. Apart from the actinides and tungsten, M, molecules were observed for all other elements studied. The search for the dimers of the actinide elements was not carried out, and the difficulty of taking high exposures, perhaps prevented the detection of W, molecules. (Certain specific behaviour of M, molecules has been discussed in ref. 16.) Similarly, the metal hydride is also
253 TABLE 4 Concentrations of certain typical matrix-specific molecules (other than MC, and MO,) Matrix ISNO, -C
Molecule KOH
5 57 765 290 26
K2 K2O
K,CN K202
K,O,H (KOH) 2 K3C2
PPM a
Matrix
Molecule
PPM =
KCl-C
KOH
2.5 37 1 248 2089 14 2 5 0.06
K2 K2O
KC1 K,Cl
b
K3Cl2
2 11 8 41 3.5
K3C2
K3G K3C8
Cs,Cr,O,-C
CsOH CsCr cs2 cs,o
Cs,OH
0.04 1 4 0.3 0.5
SrCO, -C
SrH SrOH Sr2 Sr,O Sr,OH Sr202
Sr,O, H (SrOH) Sr2C2
Ba(OH),-C
BaOH Bar Ba,O Ba202 BaG
0.07 0.7 111 207 3.4 2 0.2 0.3 0.4
2
4
42 1 11 0.4 15
a Concentration (parts per million) of the molecule in question with respect to the element M. b K202H is also formed but in substantially less abundance with respect to (KOH) 2, white the reverse is true for Sr, i.e., Sr202H+ > (SrOH): .
a very common species. The metal hydroxide ions (MOH+) are generally significantly abundant in the beam from a matrix of alkali or alkaline earth metals. Group I and Group II elements also generate polymeric oxides such as M,O, M,O, and hydroxides of the types M,OH, M,O,H and (MOH) *. The polymeric alkali and alkali metal oxides are also identified in the Knudsen cells effusate from respective matrices [18]. Moreover, it is worth mentioning that Bacon and Ure [23] have reported the formation of many complex and matrix-specific molecules, particularly oxides, in the beams from a spark ion source. The types of species generated by a matrix of an alkali metal such as KNO,-C, generally outnumber all other metal-graphite systems studied.
254
Furthermore, the behaviour of MX, and M,X, types of molecule is not similar. Thus, the yields of KC: ions for which only the KC+ and KC: ions were recorded, follow a decreasing pattern with n, while the K&z series displays a complex yield distribution pattern with respect to n (see results for KNO,-C matrix). For the K3Ci series of ions only those ions with n equal to 2, 4, 5 and 6 were observed to a measurable extent while the formation of K,C+ and K,Cz ions of the K3Cn series and the parent Kz ions could not be confirmed. In other words, the r.f.-spark mass spectrum of KNO,-C matrix contains, among many, four pairs of prominent lines at m/z values 141 and 143, 165 and 167, 177 and 179, and 189 and 191. Equation 2 predicts seven K, molecules having mass numbers 117-123. However, only two or three of them are significantly abundant. These K, molecules are m = 117, 119 and 121 and have abundances 81.107%, 17.56% and 1.267% respectively. The above mass lines are identified as the carbides of m = 117 and m = 119. For some cases the lines due to the carbide K&, of K,(m = 121) were also detectable but were much too weak to be considered for measurement. It may be mentioned that, for a multi-isotopic element M, it was rather the conformity between the estimated intensities owing to the different probable M,X, molecules of given M, p, X and 4, and the abundance values predicted for them by Eq. 4, which was used as the yardstick for identification of the M,X, types of molecules reported here. Faint peaks were also observed at nominal positions of m/z 117, 129 and 153. They are certainly not atomic peaks, but as the K, abundance pattern is of no avail for ascertaining their identity, it is not confirmed whether they are due to the parent K,(m = 117) and the corresponding mono and tri carbide species, K,C and K,C,, respectively. However, if one is allowed to extrapolate the results, then it appears that in the beam from the KNO,-C matrix the K3Cz series of ions obeys an alternating abundance distribution up to n equals 4 while a reversal of abundance trend occurs at K,C,. This is confirmed by triplicate analysis of the KNO,-C system. However the KC1 system, which was also sparked for reasons stated below, does not produce a maxima at K3C5, and the K&l ions, if formed, are masked by rather intense and non-resolvable peaks due to K&l; ions. The yield distribution of K&J ions in the beam from a KCl-C matrix closely represents a zigzag pattern. The most abundant multi-atomic ion in the beam from the KCl-C system is the K&l+ ion, followed by KCl+ ion, and this yield distribution pattern of K&l: ions parallels the abundance trend among them in the molecular ion beam generated from equilibrium vapour over liquid KC1 [24]. The KCl-C matrix, however, does not help us to study the K, molecules, as they are non-resolvable from rather intensely abundant K&l molecules. A number of mass lines were observed at mass numbers corresponding to the following 39K,X and 39K3X types of mole-
255
cules, but their identities are subject to verification: (39K,)‘2C, (39K2)14N, ( 39K2)12C2, ( 39K3)12C3, ( 39K2)12C4, ( 39K3)‘60, ( 39K3)‘602 and also the oxide K3O2 of K,(m = 119). It is, however, certain that they are not due to atomic ions. A pair of relatively intense lines was recorded at mass numbers 169 and 171. Their intensity ratio roughly approaches the K, abundance pattern but could not be identified. The KNO,-C system produces very intense lines at m/z values 104, 106 and 108, (104 being the most intense), and clearly these are not due to atomic ions. Neither are they due to K,C, and K,N, because the molecules should have produced their most abundant lines at m/z 102 and 106 respectively (Eq. 4). However, the intensities of the three lines agree reasonably well with the predicted abundance pattern (Eq. 3) of K, molecules of mass numbers 78, 80 and 82. We ascribe the lines to K2CN+ ions. The difference in the abundance patterns for K, and K,CN molecules is really insignificant because for both carbon and nitrogen a single isotope of each is nearly 100% abundant. The identification is further supported by the fact that the intensities of these lines are drastically suppressed in the beam from the KCl-C matrix, which was sparked precisely for this purpose. For the alkaline earth metals such as strontium and barium, only the M,C, molecules of the M2C,, series are definitely identified. Ba,C could not be detected, but Sr,C appears to have been formed as a definite peak was recorded at m/z 188 using the SrCO,-C matrix. The Sr,C, and Ba,C, molecules are more abundant than Sr, and Ba, molecules respectively (cf. K, and K3Cn molecules). These could be loosely interpreted in terms of the stability of molecular ions increasing with increase in size. DISCUSSION OF THE PROCESSES OF FORMATION OF MOLECULAR IONS It is considered pertinent to analyse the results in the context of the following questions. (1) How are the molecular ions produced? Are they ejected directly from the solid substrate or formed by gas phase reactions or by both these processes? (2) Are the molecular ions and the atomic ions produced simultaneously and/or by similar processes? (3) Is there any relationship between the recorded molecular ion abundances and the abundances of the corresponding neutral molecules? The formation of species (such as UC, from U,Os-C matrix, K,$,, from KCl-C samples, Sr,O, from the SrCO,-C system, etc.) which are not the original molecular constituents of the electrodes being sparked, shows that some molecular ions could only be formed by gas phase reactions. Moreover, it is a general observation by almost all the workers in the field that the
256
concentrations of molecular ions such as oxide ions in the beam from matrices of an oxide-graphite type is low compared to the concentrations of the atomic ions of the constituent elements of the molecules. Such an observation could, perhaps, be taken as an indication that the molecular ions which reach the detector are not ejected directly from the electrode surfaces, and their formation processes are also different from that for atomic ions. By time resolved measurements, Viczian et al. [8] have shown that the multiply charged atomic ions produced from an r.f.-spark ion source reach the detector first, and with respect to the doubly charged atomic ions the singly charged atomic ions are delayed by 150 + 100 ns whereas the molecular ions are delayed to a greater extent ( - 700 ns or even more). They also observed that the spreads in the average delay times for molecular ions are also higher. The ion beam, particularly the ion beam due to the atomic ions reaching the detector, is believed to be formed by the following series of processes: vacuum spark breakdown by electron current and partial transfer of energy of the high velocity electrons to the electrode surfaces + erosion of electrode material forming plasma cloud (evaporation and atomization of solid) + avalanche ionization of atoms by the energetic electrons to the different charge states of the elements (plasma development) + formation of the ion beam for mass spectrometric analysis following intense charge recombination, or rather element-sensitive recombination of the multiply charged ions during the plasma expansion into the vacuum [14,25]. The average time delay of detection [8] of the singly charged atomic ions with reference to the doubly or higher charged atomic ions indicates that the singly charged atomic ions are formed mostly by the recombination of multiply charged ions. However, it is highly improbable that the molecular ions which constitute the ion beam for analysis are also generated through the processes of avalanche ionization followed by intense charge recombination of molecular species which could be assumed to have been ejected directly from the solid electrodes or which might have formed by neutral-neutral reactions in the plasma cloud. This is because the multiply charged ions, especially the small molecular ions, are chemically unstable. This is supported by the report [lo] that only a few triply charged molecular ions, and none with still higher charge states, could be detected in the beam from an r.f.-spark ion source. The studies of energy distribution of ions indicate that the energy of the main bulk of any given molecular ions is low compared to the atomic ions, whereas the molecules produced in a high temperature plasma should have an energy comparable to the atomic ions. The exact data on the energy distribution of ions could not be established due to certain unresolved problems. However, the study by Vos and Van Grieken [26] also supports the idea that the molecules generated in an r.f.-spark ion source have lower relative energy of formation.
257
If the above arguments are correct, and if the molecular ions constituting the ion beam for mass analysis, are formed by gas phase reactions, then it is very logical that they are generated after the plasma has relatively cooled down, that is, after the formation of the main bulk of the singly charged atomic ions and neutrals during plasma dispersal into the vacuum. Moreover, such a process can account well for the delay time as well as the lower abundance of molecular ions with reference to atomic ions. The plasma temperature (both the electron and ion temperatures), is relatively low, so inter-neutral collisions may yield a molecule which subsequently may suffer ionization without undergoing dissociation by the cooled electrons. Similarly, it is equally possible that a molecular ion is produced by attachment of an ion to a neutral. The second process could be more important than the former, as an ion by virtue of its charge can polarise the colliding neutral. The plasma is relatively cool, but the temperature is sufficient to make possible reactions which do not occur in the low temperature range. The plasma reactions may thus be expected to produce many kinds of molecules, but only those which have certain stability under the prevailing plasma conditions are expected to survive and reach the detector. Nevertheless, the temperature gradients at the dispersal stage are less pronounced than at the time of plasma development. As a result, the role of the thermodynamic parameters of species in governing their composition in the beam becomes increasingly significant. The studies of charge distribution of elements (ref. 14) have shown that, even for atomic ions, the element-sensitive thermodynamic parameters play a key role in governing their composition in the beam. Finally, it should be added that there is no sufficient reason to ignore the contributions of the molecular species which could be ejected directly from the solid surfaces as neutrals during this penultimate stage of the discharge, owing to sputtering of ions on the electrode surfaces. Naturally, the relative contributions of the different processes will be governed by the matrix and sparking parameters. In view of the observations and discussion presented here, it follows that the recorded molecular ion abundances are sound for the ions only, because it does not appear correct to assume that the recorded ions are still generated first as neutrals and subsequently suffer ionization by electrons. Even in such a case the ionization efficiency curves could be significantly different from one given molecule to another. The second point to be noted is that the chemical form (such as salt or elemental mixture) of the matrix has little to do with the types and abundances of molecular ions recorded. The point is further clarified by the fact that, even in the case of an arbitrarily chosen element, no two charge states of it are equally stable in any given environment. However, the abundances of the singly charged ions could be taken as the best representative of the abundances of the neutrals in a comparable environment.
CONCLUSIONS
The small molecular ions, especially the carbide (MC,‘,2+) ions, generated in an r.f.-spark ion source and which reach the detector are studied for a number of elements M of varying electronic configurations. By and large the MC: ion abundance pattern due to different metals M falls into two groups, namely, (1) monotonic decrease in the yields of MC: ions as n increases, and (2) a zigzag abundance distribution of MC: ions as a function of n with the maxima appearing at even values of n. For some elements such as antimony, the successive odd-even pairs, e.g., SbC+ and SbCl, SbC: and SbCl, appear to be roughly equally stable. The observed abundance pattern of molecular ions could be adequately explained from the consideration of given ion (M,X’,+) stabilities towards the various possible reactions under the prevailing experimental conditions. However, the investigations show that, for elements M belonging to a particular periodic group the stabilities of the MC, molecules are similar. The observations on MC,, and C, molecules suggest that, for a given set of experimental conditions and matrix, the abundances of MC, and C, molecules in the beam are governed by the different competitive reactions involving both the metal and carbon species. This also explains the variations in the yield distribution patterns of C, molecules as a function of n from one given matrix to another. Irrespective of its constituents, the ion beam reaching the detector is formed following plasma expansion and recombination. However, it appears that there is a difference between the main bulk of atomic ions and the molecular ions reaching the detector. The recorded atomic ions are the same as those which were produced at. the high temperature plasma but suffered intense element-sensitive charge recombination before becoming part of the ion beam, whereas the molecular ions of the beam are generated mainly by gas phase reactions after the production of the bulk of neutrals and singly charged ions following plasma dispersal and charge recombination. This offers an explanation for the higher delay time reported in the literature for molecular ions with respect to the singly charged atomic ions. During the plasma dispersal molecular ions are generated by the following comparable and complementary processes, namely, (i) neutral-neutral collisions followed by ionization by cooled electrons; (ii) ion-neutral reactions; and (iii) ejection of neutral molecules from solid surfaces as a result of sputtering of ions on electrode surfaces and subsequent ionization by low energy electrons. The manifold ways and hence the different possible times of their formation could be a reason for the higher spreads in their average formation times. Compared to the atomic ions, the sensitivity of production of molecular
259
ions is generally low, and could be of little significance if the elements concerned are traces. However, the formation of different matrix-specific molecules suggests that for an unbiased trace elemental analysis of a sample, it will be advisable to establish the nature of the molecular spectrum a priori for the given matrix and experimental conditions. The transfer of material from solid electrodes to the gaseous plasma in a spark ion source is basically a non-thermal process, but it resembles what is commonly known as high temperature mass spectrometry (HTMS) in two ways: (i) a spark ion source generates many species which are not formed in the low temperature range, and thus the former serves as a device for crossing over the energy barrier for reaction; and (ii) the abundance distribution of molecules such as UC: ions in SSMS parallels the abundance trend among the same series of molecules in HTMS, thereby indicating the importance of thermodynamic parameters of molecules in SSMS. ACKNOWLEDGEMENTS
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