Mass Spectroscopy

Mass Spectroscopy F. E. SAALFELD, J. J. DECORPO, AND J. R. WYATT Physical Chemistry Branch Naval Research Laboratory Washington, D.C.

I. Introduction ............................................................................... 11. Instrumental Design and Techniques.. .................................................. 111. Surface Studies ............................................................................ A. Introduction ........................................................................... B. Ion Scattering Spectrometry (ISS) ................................................... C. Ionized Neutral Mass Spectrometry (INMS) ....................................... D. Secondary Ion Mass Spectrometry (SIMS) ................................ E. Electron Spectroscopy for Chemical Analysis (ESCA) ............................. F. Auger Electron Spectroscopy (AES) ...................................... IV. Ionization Processes ...................................................................... A. Introduction .....................................................

2 2 5 5 6 7

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C. Chemical Ionization (CI) E. F. G. H. I. J.

Field Ionization (FI) ....................................... Field Desorption Ionization ............................................ Surface Ionization (SI) ............................................................... Penning Ionization ........................................ Other Ionization Processes.. . . . . . . . . . . . Shapes of Ionization Efficiency Curves

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A. Introduction

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C. Flow Systems ......................................................................... 21 .......... 22 D. Low Pressure, Single Collision Techniques E. Other Systems ......................................................................... 23 VI. High Temperatu ........................................... 25 A. Introduction ........................................... 25 B. Knudsen Cel ................................................................. 25 VII. Sampling of Reactive Species ............................................................ 28 A. Introduction. .... .... ...... 28 B. Sampling of Plasmas ................................................................. 28 C. Sampling of Combustion. ....................... 30 D. Sampling of Radicals ................................................................. 33 References ................................................................................. 35 1

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F. E. SAALFELD,

J. J. DECORPO, AND J. R. WYATT

I. INTRODUCTION

Three mass spectroscopy reviews appeared in earlier volumes of Advances in Electronics and Electron Physics. “ Modern Mass Spectrometry ” by Mark Inghram was published in 1948 as part of Volume 1 of this series; “ Mass Spectroscopy by Larkin Kervin appeared in Volume 3, and in 1969 (Volume 27) P. H. Dawson and N. R. Whetten reviewed “Mass Spectroscopy using RF Quadrupole Fields.” In addition, many topics closely related to mass spectrometry have also been covered. For example, Eldon Ferguson reported on “ Thermal Energy Ion-Molecule Reactions ” in Volume 24, and Michael T. Bowers and Timothy Su reviewed the same subject in Volume 34. Since mass spectroscopy has received substantial coverage in this series, the reader may well ask why the Editor would wish to publish another review on the subject. The answer is straightforward; mass spectroscopy has and is undergoing a rapid and remarkable expansion which has not been slowed or stopped by inflation, recession, or other social or scientific upheavals. An indication of mass spectrometry growth is the number of published volumes on mass spectrometry and the number of mass spectrometry meetings being held every year throughout the world. For example, more than 20,000 mass spectroscopy papers were published in 1972 and 1973 (Burlingame et al., 1974). Mass spectrometry has been applied to almost every area of research being pursued today. Studies of diverse subjects such as cancer research, identification of drugs, forensic analysis, atmospheric end water environmental analysis, combustion, and lasers have benefited from mass spectrometry. In some of these studies, the mass spectrometer is used both as a chemical reactor and as an analytical instrument. Because of these diverse applications, no person or group can completely review the field of mass spectroscopy; we must therefore limit the scope of this review. This review covers instrumental designs and techniques, surface studies, ionization processes, ion-molecule reactions, high temperature systems, and sampling of reactive species. The subjects of organic structure, biological studies, and environmental analyses are not covered since these subjects were adequately reviewed by Burlingame et al. (1974). ”

11. INSTRUMENTAL DESIGNAND TECHNIQUES Any mass spectroscope has four basic components: 1. a system by which the sample to be studied is introduced into the instrument; 2. an ion source where ions (either positive or negative) that are characteristic of the sample are produced;

MASS SPECTROSCOPY

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3. an analyzer region where the ion beam is sorted into its various massto-charge ratios (rn/e); 4. a detector system where the separated ion beams are collected and, by some method, rendered observable. There are many different approaches to accomplishing each task outlined in these four steps. We will be concerned only with a general description of the major components. Many variations have been reported in each of these components. These changes are limited only by the experimenter’s ingenuity and the current technology. Some of these approaches are discussed later in this chapter. Some of the methods of sample introduction being used today are the molecular ieak, viscous leak, gas chromatography, solid sample electrodes, direct insertion probes, and heated filaments. Gaseous samples are normally introduced into the mass spectrometer through an inlet system that incorporates a molecular leak. A molecular leak is characterized by molecular flow of the molecules in the system. This means that the rate of flow is limited by wall collisions rather than molecular collisions, i.e., the mean free path of the molecules in the sample reservoir must be greater than the diameter of the leak. This inlet is normally used for quantitative gas analysis and is designed so that the composition of gases in the ion source is the same as that in the sample reservoir. It should be noted, however, that the composition in the reservoir changes with time. A viscous-leak inlet is employed to investigate the properties of a single gas (e.g., isotope ratios). In this inlet, which is usually a long capillary tube, the gas flow is limited by the viscosity of the gas. Therefore, the gas flow will depend on the nature of the mixture in the inlet. With this inlet, the composition of the gas in the reservoir does not change with time, but the composition of the gas in the ion source is not the same as that in the reservoir. A viscous leak is used to sample high pressure systems ( - 1 atm) permitting continuous analyses of various chemical processes. The gas chromatograph inlet, with the various pressure reducing devices, is the most important tool for the identification of the components of complex mixtures such as flavor extracts and enclosed environmental atmospheres. These inlets and the application of gas chromatography have been adequately reviewed (Burlingame, 1970; McFadden, 1973). Spark source mass spectrometers are used for the direct analysis of trace impurities (at the parts-per-billion level) in solids. The sample under study is shaped into a solid electrode and placed in the ion source. A radio-frequency discharge is initiated between the electrodes to vaporize and ionize the sample. This source produces ions that have a considerable energy spread; therefore, this source is used exclusively in double focusing mass spectroscopes.

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

The direct-insertion probe has greatly extended the applications of mass spectrometry in organic chemistry. The sample is placed in a capillary tube, which is then directly inserted into the ion source of the mass spectrometer through a vacuum lock. The sample is heated and vaporizes directly into the ionizing electron beam. Thus, many materials with insufficient vapor pressure to be sampled with a molecular or viscous leak can be investigated. In addition, the direct-insertion inlet may also be operated at cryogenic temperatures, enabling mass spectrometric studies of unstable compounds to be carried out. Several laboratories have used these cryogenic probes with notable success (e.g., McGee et al., 1966). In the heated filament inlet, also known as surface ionization, a solution of the sample is painted on the filament and allowed to dry. The filament is then introduced into the ion source and the source evacuated. The filament is heated to vaporize the sample. A proportion of the sample vaporizes as ions and these ions are mass analyzed. The exact proportion of ions produced in the vaporization process is dependent on the ionization potential of the sample and work function of the filament. Measurement of isotope ratios of the alkali and alkali-earth metals is the chief use of this type of inlet system. There is no universal ion source for all mass spectrometric applications. Accordingly, various sources have been developed for specific applications, and each source has several variations. The most widely used source today is the electron impact type; it is sold by every commercial manufacturer. Ionization in this source is produced by the bombardment of the gaseous molecules with controlled, low energy (0 to 100-V) electrons. There are many other types of sources that are gaining in popularity. These include the photoionization, field ionization, surface ionization, chemical ionization, discharge, and laser sources. A discussion of some of these esoteric sources is given in Section IV. The methods of mass sorting the ion beam produced in the various ion sources are numerous. There are three basic methods to accomplish this mass sorting, or ion focusing: direction focusing, where ions of the same mass and velocity, but different initial direction, are focused on the detector; velocity focusing, where ions of homogeneous velocity and direction are focused; and double focusing, where ions of the same mass, but with varying velocities and directions, are focused. The three measurable quantities of a moving electrical charge are velocity, momentum, and energy. If two of these quantities are specified, the mass-to-charge ratio of the ion can be determined. The more common types of mass analyzers in use today are direction focusing 60°,90",and 180" magnetic fields; velocity focusing linear time-of-flight, radio frequency, quadrupoles, etc. ;and double focusing. This last type of instrument employs an energy seiector (an electrostatic analyzer)

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and a momentum selector (a magnetic field). The two most widely used configurations are the Mattauch-Herzog (a 31"50' energy selector and a 90" momentum selector) and the Nier-Johnson (a 90" energy selector and a 90" or 60" momentum selector). There are presently two principal methods of ion detection: the photoplate (a mass spectrograph) and electrical detection (a mass spectrometer). The photoplate detector has advantages when employed for exact mass measurements and for the integration of the ion signal, such as in a spark source mass spectrograph and quantitative analysis when a direct-insertion probe is used. An electrical detector usually consists either of a Faraday cup and/or an electron multiplier, followed by signal amplification. This is the most widely used method ofdetection and is employed for precision measurement of ion abundances.

STUDIES 111. SURFACE A . Introduction

The ability to characterize surfaces has increased dramatically due to combining of mass spectrometry with scattering and sputtering techniques. Several techniques are discussed in this section: ion scattering spectrometry (ISS), ionized neutral mass spectrometry (INMS), secondary ion mass spectrometry (SIMS). In addition, two other surface techniques where mass spectrometry has recently been incorporated, electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES), are discussed. We discuss principles of operation, capabilities, and limitations of these techniques. Except for ISS, these techniques utilize some type of emission (photons, electrons, atoms, molecules, or ions) from the studied surface. This emission results from surface bombardment. In general, the emitted photons, electrons, and ions are analyzed according to their energy. The ions and neutral particles are also mass analyzed. The mechanisms of ion scattering on surfaces were reviewed by Kaminsky (1964) and those of ion sputtering of surfaces by Carter and Colligan (1968). Therefore, our discussions of these mechanisms are brief. The concentration profile below the surface of a particular species is important. The techniques discussed in this section can, with various degrees of success, provide an in-depth concentration profile. This is accomplished by bombarding the surface with energetic particles that sputter away layer after layer, thus exposing new layers deep in the solid. The newly exposed surface layers can then be analyzed.

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B. Ion Scattering Spectrometry (ZSS) ISS uses ion scattering in which the energy of the reflected beam can be used to determine the mass of the interacting surface species. At a scattering angle of 90°, the scattering equation for an elastic collision is given by

( M , - MlMM2 + Ml) (1) where E , and E , are the initial and scattered ion energies, respectively, M , and M, are the masses of the primary ion and scattering atom, respectively. From Eq. (1) it is clear that scattering occurs only when M , > M , . Thus, it is convenient to use the 4He ion as the primary beam. A typical commercial instrument (Goff, 1973) has a primary ion source, a sample holder and manipulator, an electrostatic energy analyzer, and an electron multiplier. These components are all housed under ultrahigh vacuum. The primary ion beam is focused onto a 1-mm diameter area of the sample. The electrostatic analyzer accepts the ions scattered through a 90" angle. Generally, inelastic processes can be neglected if the primary ion beam has an energy less than X keV, where X is the atomic mass of the primary ions. During a typical analysis the primary ion beam (4He+jwith an initial energy of 1-3 keV is kept constant and the voltage on the electrostatic energy analyzer is varied. The instrument thus sweeps through the El / E , ratio in Eq. (1). Therefore, the output signal represents the concentration of the scattering atom on the surface having mass M 2 . ISS has the unique ability to analyze species exclusively from the surface. In comparison, sputtering techniques analyze species between the surface and a depth of 10-60 A. Besides providing a surface analysis, ISS has the advantages of uniform sensitivity to most elements and low sample consumption, which is essential for thin film work. ISS cannot accurately provide a depth concentration profile because some of the ions electrostatically analyzed come from regions near the crater wall that are produced by the primary ion beam. To reduce this problem the entrance slit of the electrostatic analyzer can be made smaller. However, this has the effect of lowering the detection sensitivity. Another limitation of ISS is the dependence of the scattered particle intensity on a scattering cross section, neutralization probability, and a geometric factor. Bingham (1966) has tabulated the scattering cross sections for most elements; however, there is little information on the other two parameters. Therefore, standards are required to determine quantitatively the chemical composition of a surface. A combined ISS and SIMS instrument has recently been marketed. This type of instrumentation is discussed later in Section II1,D on SIMS. More sophisticated ISS instrumentation has been described (Suumeijer and Boers, 1971; Wheatley and Caldwell, 1973). These instruments utilize mass&/EO

=

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analyzed primary beams, and an adjustable scattering angle. Suumeijer and Boers’s instrument also has a second mass filter in series with the electrostatic analyzer, which permits any secondary sputtered ions to be identified. C. Ionized Neutral Mass Spectrometry ( I N M S )

The sputtering of neutral atoms from the surface is the most probable emission process. Primary ions with energies up to 1 keV have a sputtering coefficient (number of atoms emitted per primary ion) of unity (Carter and Colligan, 1968). Other processes have a coefficient of Many experiments use various modfied conventional mass spectrometers (Honig, 1959) to detect the large number of neutrals liberated from the surface. Most modifications are designed to collimate the neutrals through an ionization chamber. The resulting ions are mass analyzed (Bradley and Ruedl, 1962). These initial experiments were severely limited by sensitivity problems. This was due to the fact that the sputtered neutrals have an energy of a few electron volts; thus, their probability for ionization is extremely small. Woodyard and Cooper (1964) developed the first practical INMS. Their apparatus used a conventional Nier-type electron impact ion source with the ion repeller replaced by a copper plate-the material being studied. The ion repeller was negatively biased with respect to the ion source. When argon was added to the ion source to form a low pressure discharge, a fraction of the ions from the discharge bombarded the sample material. The sputtered neutral particles were ionized in the argon discharge. These new ions were then extracted and mass analyzed. The actual mechanism for ionization was not clear. Woodyard and Cooper (1964) claimed that about one atom in lo4 surface neutrals could be detected. Coburn and Kay (1971; Coburn e f al., 1973) refined the above experiment and concluded that the ionization process was of the Penning type (to be discissed in a later section). Coburn and Kay constructed an rf sputtering system with the cathode made of the material of interest. As in the previous experiment when argon was added to the system, a discharge formed and the ions bombarded the cathode material. The sputtered neutrals ionized in the discharge and were subsequently mass analyzed. The rf sputtering system enhanced the ionization of the sputtered neutrals by several orders of magnitude. A lower pressure, but otherwise similar rf discharge arrangement has been reported (Oechsner and Gehard, 1972). Typical operating conditions of INMS produce sputtering rates from 1 to 200 monolayers/min, and the neutrals are representative of the lattice material (Honig, 1973). Penning ionization cross sections of various atoms are known, so that the relative detection sensitivity can be calculated. The primary ions have energies of about 200 eV, causing sputtering to be limited

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

to a few layers. The major limitation of INMS is that the whole sample must be sputtered; thus, there is no opportunity to examine the lateral profile of the surface.

D. Secondary Ion Mass Spectrometry ( S I M S ) SIMS uses sufficiently high energy ions to bombard the surface in order to liberate positive and negative ions, which are characteristic of the chemical composition of a surface. The emission of these secondary ions is well known (Sloan and Press, 1938; Herzog and Viehboek, 1949).The sample is bombarded in the ion source of a mass spectrometer with a beam of primary ions. The resulting secondary ions are mass analyzed and in some cases also energy analyzed. High primary ion densities were used in SIMS during the past decade (e.g., Liebl and Herzog, 1963; Fogel, 1972). It was previously thought that SIMS would be unusable for the analysis of individual monolayers since the surface is rapidly removed during the intense bombardment. Recently, Benninghoven (1973) has used low primary ion currents to analyze individual monolayers. This SIMS approach is known as the “static method.” With static SIMS, the primary ion current is low, so that an individual monolayer may last many hours. The necessary sensitivity is achieved by bombardment of a large area of the sample (0.1 cm2) and the use of ion counting equipment. Secondary ion emission is a complex interaction between the primary ion and a limited area of the solid surface. As the primary ion penetrates the solid it loses energy. Part of this energy apparently returns to the surface and is transmitted to a surface particle. If the transmitted energy is high enough, the particle is liberated. Due to complex ionization processes which are not understood (Benninghoven, 1973), some of these particles leave the surface as ions. The surface particles may be emitted as molecular ions or as fragment ions. Secondary ions formed with a primary beam in the kiloelectron volt region will have several electron volts of energy when they leave the surface. This low energy ensures that the ion originates from the surface. After the primary ion bombards the surface atom, it will be backscattered. The energy analysis of the backscattered primary beam allows the atomic mass of its impact partner to be calculated. Readers interested in a theoretical treatment of ion emission from surfaces are referred to a number of works: Andersen and Hinthorne (1972), Jurela (1970), Beske (1967). SIMS can identify many species at 1-ppm concentrations. However, Honig (1959) and Andersen (1973) showed that the magnitude of the secondary ion signal is a complex function of the state of the surface and the concentration of the particular constituent in the sample. Some typical SIMS investigations are the change of chemical composition of the surface

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monolayers induced by ion bombardment, the changes in catalysts after use, the reactions of metals with gases, and studies of adsorption layers. Evans (1972) reviewed two additional types of SIMS instruments: first, the direct-imaging analyzer which was developed by Castaing and Slodzian (1962) and more recently made available commercially (Morabito and Lewis, 1973). The sample is bombarded with a primary ion beam about 300 pm in diameter. The secondary ions are extracted from the sample chamber and are mass analyzed in a stigmatic magnetic prism. The ions are then reflected and energy analyzed by an electrostatic mirror lens, further mass analyzed, passed through a projection lens, and displayed on a scintillator. The image on the scintillator represents the distribution of a particular element in the sampled area. The second instrument described by Evans (1972) is the secondary ion microprobe mass spectrometer which was originally designed by Liebl and Herzog (1963). This instrument is known as the ion microprobe mass analyzer (IMMA). In this technique, the mass spectrometer is tuned to one particular mass and the primary ion beam of about I-pm diameter scans the surface across an area 300 x 300 pm. The output current of the multiplier modulates the brightness of a synchronized oscilloscope beam. In this way the topographical distribution of a particular element is displayed. Since the direct-imaging analyzer records all information simultaneously, it yields the information in less time than the IMMA system. Liebl(l972, 1974) recently constructed an ion-electron microprobe. The instrument has the acronym UMPA (universal microprobe analyzer) and permits ion and electron bombardment, separately or simultaneously, by use of a new lens design. This instrument is reported to have better resolution and sensitivity than that of the IMMA design (Liebl, 1974). The SIMS technique is useful for qualitative analysis of surface compositions and depth distributions. Using an empirical approach Andersen and Hinthorne (1972) showed that secondary-ion currents can give semiquantitative information. In addition, a complicated calibration technique does exist (Wermer, 1972) for certain systems, such as the oxides. The SIMS approach, like other techniques, has advantages and limitations. For example, the direct-imaging analyzer and IMMA use intense high energy primary beams, which can change the characteristics of the surface. Static SIMS uses a large target area, which gives poor lateral concentration profiles. On the other hand, the direct-imaging analyzer and the ion microprobe can provide lateral elemental distribution (1-2 pm). Static SIMS provides information from a monolayer, whereas the other SIMS techniques provide in-depth concentration profiles. Secondary ion techniques are sensitive (sometimes at the expense of the sample). Benninghoven (1973) claims a SIMS detection limit below 1 ppm

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of a monolayer for many elements and compounds. Unfortunately, all secondary ion techniques suffer from a matrix effect,” i.e., certain elements affect the total secondary ion emission. For instance, the secondary ion yield enhancement by oxygen and other electronegative species limits the usefulness of SIMS information by making it difficult to obtain a true concentration profile. On the other hand, hydrogen can be detected, which is difficult by other techniques. It is difficult to interpret secondary ion mass spectra because the ionization and fragmentation mechanisms producing the observed ions are not well known. The interpretation is further complicated by the uncertainty as to whether or not an observed ion corresponds to a similar surface compound. The field of SIMS is still in its infancy. This is evidenced by the rapid development of new instrumentation designed to overcome some of the present deficiencies. For example, Bakale et al. (1975) reported a high mass resolution ion microprobe mass spectrometer. The exact mass measurements enable the identification of the chemical composition of the secondary ions. An instrument that allows simultaneous operation of ion scattering spectrometry (ISS) and SIMS is commercially available. Three spectra are recorded: the ISS spectrum and the positive and negative SIMS spectra. The latter two are complementary in their sensitivity variation; the positive SIMS spectra are extremely sensitive to elements on the left-hand side of the periodic table, while the negative SIMS spectra favor the right-hand side. The three spectra taken together are claimed to represent a complete surface analysis. “

E . Electron Spectroscopy for Chemical Analysis (ESCA) ESCA is a surface analysis technique that is still in its initial stages of development. It uses photoexcitation to stimulate electron emission. The electron energy distribution analysis is used (Siegbahn et al., 1967) to determine chemical structure. Within the past few years virtually all photoexcitation has been accomplished by an X-ray source. ESCA’s most important advantage is its ability to determine the chemical environment of a surface atom. ESCA’s limitations are: it provides data from the first ten monolayers; it has a detection limit of approximately 1% of a monolayer; and it cannot detect hydrogen. A more fundamental problem with ESCA is that the energy of the emitted electron must be corrected for the surface work function, electric charging, and recoil effects. Mass spectrometers have recently been combined with ESCA for mass analysis of any sputtered particle. Evans (1975a) reported the combining of ESCA with ion sputtering to determine depth profiles. For this combination

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one problem that has not been addressed is the possibility of sample damage by the sputtering process which destroys the chemical bonding. In this case ESCA is only an elemental analysis technique. A thorough discussion of ESCA is given in a treatise on electron spectroscopy edited by Shirley (1972). F . Auger Electron Spectroscopy ( A E S )

AES has developed rapidly over the past several years as a powerful method for chemical surface analysis. AES originated when Lander (1953) noted that electron bombardment of a material resulted in the emission of secondary electrons with a certain energy distribution. He was able to relate the distribution to Auger transitions of atoms on the surface. The technique was advanced by Harris (1968) who demonstrated that the Auger peaks were enhanced by differentiating the distribution. AES is accomplished by bombarding the surface with a primary electron beam having an energy of 1-10 keV while an energy analysis is performed on the emitted electrons. A small number of these electrons are due to Ailger transitions. The Auger electron energy is related to the core levels of the parent atom. The Auger transitions are well known and tabulated as a function of atomic number (Shirley, 1972). Therefore, the atomic number of the atom on the surface can be determined. The chief limitations of AES are: it cannot detect hydrogen and helium (they do not have core electrons); it is primarily an elemental technique (identification of a chemical compound is rarely successful); and its electron bombardment may change the surface composition (e.g., desorption). In addition, if the Auger electrons undergo any inelastic collisions, they lose part of their original energy and are not characteristic of the emitting atom. These scattered electrons and other secondary electrons form a severe background. Current instruments employ differentiation to overcome these problems (Evans, 1975b). Holloway (1975) has recently reported combining AES with sputtering techniques. His in-depth profiling approach yields qualitative and quantitative information. Holloway’s AES in-depth profiling has a significant advantage over other techniques using only sputtering since only AES and ISS results reflect the actual concentrations at or near the surface (Honig, 1973) at a given moment. However, as mentioned previously, ISS suffers from geometric and topographic effects and can examine only the surface monolayer. Haque (1973) has successfully combined mass spectrometry with AES in an investigation of metal contact phenomena. The combination was used in an ultrahigh vacuum system with facilities for electron and ion bombardment and gas admission. Komiya et al. (1975) and Narusawa et al. (1975) have also combined AES with mass spectrometry. A more complete discussion of AES is given in review articles by Morabito and Lewis (1973) and Chang (1974).

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TABLE I SUMMARY OF MAJORADVANTAGESAND LIMITATIONS OF SURFACE TECHNIQUES ~

Technique

Limitations

Advantages

ISS

Outer monolayer analysis Uniform sensitivity for most elements

Low sensitivity Poor lateral resolution Slow profiling

INMS

Uniform sensitivity

Low sensitivity Poor lateral resolution

SIMS

PPM detection limits for many elements Microanalysis Good mass resolution

Quantitation and matrix effects Difficult to interpret secondary ion spectra

ESCA

Chemical information

Poor lateral resolution Slow profiling Poor sensitivity

AES

Microanalysis Minimal matrix effects

Quantitation Poor sensitivity, elemental only

Major advantages and limitations of the various surface techniques are summarized in Table I.

IV. IONIZATION PROCESSES A. Introduction Often the success of a mass spectrometric study depends on the source of ions. For instance, a molecular structure determination might require a different method of ion production than the determination of an ionization potential. Therefore, new ionization methods are incorporated into mass spectrometry. In this section we briefly review the most common method, electron impact, and recent ionization methods.

B. Electron Impact ( E l ) The majority of mass spectrometers use an EI ion source. A beam of electrons is produced by heating a filament. The emitted electrons accelerate through an electric field between the filament and ionization chamber. The electrons are directed into the ionization chamber and their energy varied by changing the potential between the filament and the ionization chamber.

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EI results in ionization (positive and negative), excitation, and fragmentation of gas molecules. The amount of fragmentation has been the subject of a great number of theoretical papers (e.g., Wahrhaftig, 1972). However, the theoretical understanding of mass spectra has been successful in only a few cases. Some important advantages of EI are that most molecules have large ionization cross sections, that the ionization produces ions with fragmentation, and that the intensity and type of fragment ions can be changed by varying the electron energy. The EI mass spectra obtained give considerable information on the structure and fragmentation of the bombarded molecules. However, the fragmentation mechanisms may be difficult to interpret (Muccino and Djerassi, 1973; Tomer et al., 1972). The theoretical aspects of ionization by EI have been treated in a number of reviews. An excellent comprehensive treatise covering both the theoretical and experimental aspects of EI ionization was written by Massey and Burhop (1969). An important aspect of EI ionization is the relationship between the ion intensity ( I + ) and the electron energy (E). This relationship is expressed by the equation I + = I,( 1 - exp{ - a,(E)L(E)[M])) At low pressure, Eq. (2) reduces to

(2)

I + = Z,o(E)L(E)[M] (3) The electron beam current I, and the concentration of molecules in the source [MI are independent of energy. The path length of the electrons Land the ionization cross section a,, depend on electron energy (Massey and Burhop, 1969). Because a magnetic field is usually employed to collimate the electron beam, the electrons transverse the ionization region in a helical path. The actual change in L over the usual range of electron energies may be neglected. The dependence of the ionization cross section on energy is zero for energies equal to or less than some critical energy E, and is approximately a linear function of the excess energy above E, . Such a threshold law applies only a few volts greater than E, .The ionization cross section reaches a maximum at 20-50 V greater than E , , then gradually decreases with increasing electron energy. The energy at which E = E, for a given ion is called the appearance potential of the ion. The initial shape of the ionization efficiency curve is of particular importance in determining appearance potentials. Major disadvantages of an EI source are that the energy spread (1-2 eV) of the electron beam prevents accurate appearance potential measurements, and that the heated filament causes decomposition of the gas molecules. Negative ions are formed by resonance capture, dissociative capture, or

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pair production (Melton, 1970). Because of smaller ionization cross sections and experimental difficulties, negative ions are not as valuable as positive ions in analytical studies. However, they are of great interest in ion chemistry. Dillard (1973) comprehensively reviewed this subject. C . Chemical Ionization (CZ)

CI mass spectrometry is an outgrowth of the ion-molecule reaction studies of Munson and Field (1966). In a review of CI Field (1968) defined chemical ionization as a mass spectrometric technique in which the ionization of a molecule is the result of ion-molecule reactions. The ions produced by ion-molecule reactions make up the CI mass spectrum of the compound being studied. Chemical ionization is not to be confused with chemiionization in which a charged species is produced by the reaction of neutral species (Fontijn, 1974). CI requires a reagent gas in the ion source at approximately l-torr pressure. The primary ionization of this reagent gas occurs by electron impact. The primary ions react with other reagent gas molecules. The resulting ions have two important properties: they react with the compound of interest, which has been added to the ion source in a much smaller quantity ( - 0.1%) (this compound is known as the additive); and they do not react further with reagent molecules. Methane and isobutane are the most widely used reagent gases. As an illustration of the CI method, methane undergoes the usual electron impact reactions to form primary ions: e-

+ CH, -+CH:,

CH;, CH;, C H t , C+

(4)

At high ion source pressures ( - 1 torr) these ions undergo the following major secondary reactions: CH; +CH,+CH: CH;

(5)

+CH,

+ CH,-+C,Hi + H,

(6)

The secondary ions, which comprise approximately 90% of the total ionization, are inert to further reactions with methane. However, upon collisions with additive molecules (AH), they react in the following manner: CH: C,H: C,H:

+ AH +AH: + CH, + AH +AH: + C,H, + AH + A + + C,H,

(proton transfer)

(7)

(proton transfer)

(8)

(hydride transfer)

(9)

Therefore, no molecular ion, M’, is expected from CI, but a “quasi-parent ion, (M - H)+ or (M + H)’, is observed. CI ionization is more “gentle” than EI ionization. Hence, this technique is an excellent method of determining the molecular weight of the compound when the EI method fails.

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15

CI advances have resulted from the use of different reagent gases. For example, charge exchange followed by fragmentation occurs when helium or argon, which have high ionization potentials, are used as the reagent gas. Hydrogen as a reagent gas produces both the quasi-parent ion and fragment ions. Use of these reagent gases allows the CI technique to be used for structural analysis (Foltz et al., 1973), kinetic measurements (Field, 1969), and equilibrium studies (Field, 1972). Negative chemical ionization was studied by Dougherty et al. (1972, 1973) and is a technique of adding nonreactive gases to enhance negative ion production. CI is important in the study of compounds that do not have an EI molecular ion. Often CI and EI data supplement each other in chemical analysis (Fales, 1971). One caveat concerning CI is that, unlike low pressure ionization, the ions produced are not isolated. Thus, collisions between the CI ions and other molecules affect the mass spectra. D. Photoionization ( P I )

The energy necessary to excite or ionize an atom or molecule can be obtained from a collision between the molecule with a photon as well as an electron. The design of a photon impact ion source is similar to EI except that the photon beam replaces the electron beam. Experimentally, the chief problems with PI are the generation and transmission of the photon beam. Since vacuum ultraviolet energies are needed to ionize molecules, there are few “windows” suitable for PI (LiF and aluminum windows have limited use; Gorden et al., 1969).Therefore, windowless systems are used to transmit the photon beam into the ion source. The windowless systems consist of a vacuum monochromator, a differentially pumped photon source, and ac ion source (e.g., Rowe et al., 1973; Dibeler, 1970; Chupka, 1972). Two common types of photon sources used are the low pressure spark and rare gas discharge (Marr, 1967; Cairns et al., 1973).In the low pressure spark photons are produced repetitively in a ceramic capillary. The radiation has intense emission lines between 1350 and 400 di (9.2-31 eV). The radiation is passed through a vacuum monochromator where the desired energy is selected. Typical resolution for this source is 0.02 eV. The rare-gas discharge method has the advantage that it is easy to construct a rare-gas discharge lamp. The resonance radiation produced in the discharge lamp depends on the particular gas. The usefulness of this type of photon source is limited because it produces photons at few energies. Another less commonly used photon source is synchrotron radiation (Parr and Taylor, 1973).In a synchrotron an electron storage ring accelerates electrons to relativistic velocities in a circular orbit. These orbiting electrons emit an intense continuum of radiation. A vacuum monochromator is used to select the desired energy. The sychro-

16

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

tron’s high photon flux and available energy range make it an ideal photon source. PI sources operate at lower temperatures than EI sources because they do not use hot filaments. This is clearly an advantage when studying thermally sensitive compounds. The well-known and controlled amount of energy of the photon beam is an important characteristic of photoionization. This permits very accurate energetic measurements, but PI sensitivity is orders of magnitude lower than that of EI. PI is principally of interest in research problems (e.g., Berkowitz et al., 1973; Diebler, 1970). Danby and Eland (1972) showed that photoelectron and PI coincidence studies can be used to study ions having known internal energies. Sieck and Ausloos (1972) use PI to study ion-molecule reactions and to determine the structure of reactant ions (Sieck and Gorden, 1973).PI was used to test quasi-equilibrium theory (Rosenstock et al., 1973).

E . Field Ionization ( F I ) Atoms or molecules can be ionized by high electric fields of 107-108 V/cm. This type of ionization has been termed field ionization (FI). The high potential is generated between a surface and a sharp point or edge (known as the FI tip). FI occurs by the tunneling of an electron from a molecule to the surface. FI and its applications in mass spectrometry originated with the field-emission work of Muller and Bahadur (1956) and Inghram and Gomer (1955).Reviews covering the physical theory of FI have been published (Beckey, 1971 ; Robertson, 1972). Field ions are formed with very little internal energy. Therefore, there is a high probability of their being detected as molecular ions. In addition, at low field strengths, reactions take place at the surface of the thin film surrounding the FI tip. Here, intermolecular processes are observed and the spectra resemble those obtained from CI. The feature of producing an intense molecular mass + 1 ion makes FI a powerful analytical tool. In theory, FI provides mass spectra with a minimum of fragmentation and isotope scrambling. This information is critical for the analysis of complex mixtures and isotope dilution analysis. FI has been combined with conventional mass spectrometers in such a fashion that EI and FI techniques can be applied sequentially. Comparison of the spectra obtained from the use of such a dual ion source was used in structural elucidation of organic compounds (Fales et al., 1975). A FI source has focusing lenses common to all mass spectrometers. However, the design and the materials used in the construction of the FI tip are very critical. The earlier FI tips were fragile and subject to arching. A recent design (Brown et al., 1973; Anbar and Aberth, 1974) of a multipoint

MASS SPECTROSCOPY

17

-

FI source appears to overcome these drawbacks. Their FI source consists of lo00 metal points spaced 25 pm apart over a 2-mm2 area. an array of FI is a valuable tool for the determination of the molecular weight but has been limited by a number of problems. For instance, understanding of the FI fragmentation and correlating it with molecular structure are more difficult than with EI since catalytic and ion-molecule processes are involved. FI appears to be an excellent technique for studying catalytic effects of the tip material; however, it is difficult to extrapolate to field-free conditions. Instrumental problems have also limited the usefulness of FI. Even though Beckey (1971) increased the intensity of field ions by careful preparation of the tip, the FI sensitivity was an order of magnitude lower than that of EI. Commercially available FI instruments are not satisfactory as received from the manufacturer (Fales, 1971), and it is left to the researcher to make the necessary improvements.

F . Field Desorption Ionization Beckey (1971) showed that a FI tip coated with a sample and placed in a high field desorbs sample ions. Presumably the ions are formed on the surface of the emitter. The mass spectra produced by field desorption consist of molecular ions and molecular mass plus one ions. This technique permits generation of mass spectra of thermally unstable nonvolatile compounds. The key to the success of this technique is the preparation of the tip. The larger the surface of the tip, the greater the amount of sample ions desorbed. Field desorption has great utility for molecular weight determinations, and, when used with other mass spectral data, is valuable for structure determination. G . Surface Ionization (SZ) At high temperatures a fraction of atoms or molecules adsorbed on a metal surface vaporize as ions. The ratio of the number of neutrals (NO) to ions ( N + ) vaporized is given by the Saha-Langmuir equation:

N + / N 0 = exp[(lP - q5)/kT]

(10)

where IP is the ionization potential of the neutral species, T the surface temperature, and q5 the work function of the metal. For efficient surface ionization, the metal should have a high work function, the neutral species a low ionization potential, and the surface a high temperature. Tungsten oxide ribbon, which has a high work function, has been successfully used in surface ionization. Investigations of negative ion formation by surface ionization have been reported (Zandberg and Paleev, 1972). Surface ionization has the advantage of selectivity for the species ad-

18

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

sorbed on the surface; thus background gases present little problem. In addition, the energy spread of the resulting ion beam is small because the ions are formed in a field-free region. It is a sensitive technique-as little as 10-l4 gm can be detected. The principal use of SI is the determination of isotope ratios of elements with IP less than approximately 9 eV.

H . Penning Ionization Penning ionization can b e understood in terms of the following: A + Bf + e Penning ionization A*

(11)

+- B< +e

(12) If the energy of A*, frequently helium metastable, exceeds the ionization potential of B, then Penning ionization can occur. The cross section at thermal energies is larger than a gas-kinetic cross section. Important uses of Penning ionization are the reduction of starting AC discharge potentials, a probe to the study fundamental chemical processes, efficient ionization of a gas, and a source of lasing action (Silfvast, 1971). Penning ionization has been used in INMS (see Section 111). ABf

associative Penning ionization

I . Other Ionization Processes

New ionization methods are continually being developed. Two recent examples are discussed. An electrojet method (Dole et al., 1973) overcomes the inability to volatize high molecular weight species. Dole et al. showed that when a compound dissolved in a solvent is sprayed from a negatively charged jet, negative charge accumulates on the droplets. If the concentration is adjusted correctly, then each droplet contains only one solute molecule. These solvated ions are analyzed by a mass spectrometer. Cold electron sources such as 63Ni, '08Po, and 'H have been used in the construction of durable ion sources. (There is no risk of burning out a filament at high pressures.) The plasma chromatograph (Cohen and Karesek, 1970) and other instruments operating at atmospheric pressure (McKeown and Siegel, 1975) employ such an ion source. A disadvantage of this ion source is the reduction of ionization caused by the accumulation of any organic film on the radioactive material. J . Shapes of Ionization Eficiency Curves The ionization efficiency curves of the processes discussed in this section have different shapes. Some of these shapes are shown in Fig. 1. A theoretical treatment of ionization has been discussed by several authors (Massey

19

MASS SPECTROSCOPY

tt 0

1

2

1 1:::

Theory

Experimental

Typical process

iq;;Ti:::e He + hs + He*

(1) Photoionization He hv He+ + e - (+hv') (2) Electron excitation He e - +He* + e ( 3 ) Associative ionization He* e He e He* He -+ He: e(4) Penning ionization He $. eHe* + e He* A r - + A r +t He + e -

ii f/

+ +

-+

+ + +

-+

+

+

-+

( 1 ) Electron ionization He + e - + He+ + 2e-

FIG.1. Dependence of threshold law on the type of process.

and Burhop, 1969; Reed, 1962). The probability of ionization ( P ( E ) )is approximated by the expression P ( E ) tl ! 6 ( E - E,) dF

(13)

where 6 is a delta function centered on E , . If q = 1, then P ( E ) = ( E - EC)'-'/(q - l ) !

(14)

where n is a freedom factor expressing the number of electrons leaving the collision complex. Thus, for EI, tj = 2, and P ( E ) is a first-order function of excess energy. For PI, 9 = 1, and P ( E ) is a zero-order function (i.e., a step function). V. ION-MOLECULE STUDIES A . Introduction

The study of ion-molecule reactions is an intimate part of mass spectrometry. All mass spectrometers convert a fraction of the neutral molecules to ions. These ions can react with neutral molecules. In most mass spectrometers these reactions are eliminated by operating at low pressures. However, for certain applications it is desirable to have a large number of

20

F. E. SAALFELD, J. J. DECORPO, A N D J. R. W Y A ' l T

ion-molecule reactions occurring. Several commercially available mass spectrometers have a high-pressure ion source for this purpose. Ion-molecule reactions play an important role in many chemical processes and are the subject of much investigation. They can be studied under a variety of conditions, from single collisions to equilibrium conditions requiring thousands of collisions.The ionic products of these reactions are mass analyzed. One can truly state that the study of mass spectrometry and ion-molecule reactions are symbiotically related. Virtually all aspects of ion-molecule reaction studies have been extensively reviewed previously, most recently by Ferguson (1975).A two-volume series edited by Franklin (1972) covers thoroughly the entire field. McDaniel et al. (1970) wrote a book discussing ion-molecule reactions from a physicist's viewpoint, providing a good theoretical treatment of the subject. A large amount of data about ion-molecule reactions at thermal collision energies has been measured using the ion cyclotron resonance technique (ICR). Baldeschwieder and Woodgate (197 1) reviewed the technique. Bowers and Su (1973) discussed in detail thermal energy rate constants obtained using ICR. Another technique for studying ion-molecule reactions, ion kinetic energy spectroscopy, is discussed in an article by two of the principal investigators in this new area (Beynon and Cooks, 1974). Ferguson (1973) compiled a review of ion-molecule reaction rates at thermal energies. A broader collection of rate data was compiled by Sinnott (1973). This section concentrates upon ion-molecule studies reported over the years 1970-1975. Particular emphasis is placed upon areas that the authors feel are evolving rapidly. Studies of ion-molecule reactions can be divided into several areas based upon the experimental conditions employed. Each area can provide particular types of information, such as kinematics, rates, and thermochemistry. B. Static Systems In static systems ions are produced and undergo reactions at pressures greater than about 1W3 torr. They range from simple, closed ion sources to elaborate sampling systems operating at atmospheric pressure. Much of the work studying static systems involves the use of a pulsed ion source which can operate at pressures up to several torr (Franklin, 1972). Although this technique has several drawbacks, such as the lack of selectivity in ion formation, it still is widely used. Much work continues in this area (Beggs and Field, 1971; Rhyne and Dillard, 1971; Chong and Franklin, 1973; Lifschitz and Tassa, 1973; Cheng and Lampe, 1973; Schnitzer and Klein, 1975). An outgrowth of such studies has been the development of a different ionization technique for mass spectrometry. The technique of chemical ionization is

MASS SPECTROSCOPY

21

now widely available as an option to some commercial mass spectrometers as an aid for the analysis of high molecular weight compounds; see Section IV. Another approach is to use photons instead of electrons to produce ionization. Most workers have used resonance lamps as photon sources since they are simple and yield adequate photon fluxes. The lamps cannot be rapidly pulsed, therefore it is not possible to vary the time between ion formation and extraction. A description of this technique is given by Sieck et al. (1971). In order to study ion-molecule reactions that become significant at pressures higher than a few torr, a somewhat different ion source is required. In general, electrons from a radioactive material undergoing beta decay are used to produce ionization. After passing through several stages of pressure reduction, the ions are mass analyzed. Such an instrument was constructed by Kebarle and Haynes (1967). This technique is particularly applicable to clustering reactions, for example, H,O+(H,O),

+ H20$H,0+(H,0),+,

Recently ion sources operating at atmospheric pressure have been investigated as an analytical tool (McKeown and Siegel, 1975). Ionization is produced using electrons from a radioactive source. Due to the large number of collisions, the ionization is transferred to species having lower ionization potentials, for example, water clusters. Such a technique was shown to give detectability limits for certain compounds in air of less the one part per trillion (French and Reid, 1975). Interpretation of the mass spectra from such a system is different from conventional mass spectral interpretation and requires an understanding of ion-molecule chemistry. C. Flow Systems

Flow systems have been used for many years to study kinetics. The application of flow systems to the study of ion-molecule reactions is more recent. Flow systems used to study ion-molecule reactions operate at pressures on the order of 1 torr and flow rates of 10-100 m/sec, hence high speed pumping is required. Using electron impact or electrical discharge, ionization is produced in the carrier gas at the upstream end of the flow tube. Various gases are added along the tube to produce ions via charge exchange. These ions can be titrated with another gas, yielding information about the kinetics of the reaction. In some cases equilibrium occurs between the various species allowing direct determination of the heats of formation of the ions.

22

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

Thermal equilibrium data of ion-molecule chemistry can be obtained using only a flow system. This is the only system in which ion-molecule reactions can be studied in a field-free region and at pressures sufficient to obtain equilibration. Flow reactors can operate at temperatures approaching 1000°K (Fehsenfeld, 1975). In order to study ion-molecule reactions at higher energies in a flow system, electric fields have been imposed parallel to the direction of flow. The following interesting result has been obtained using such a “ flowing drift ” technique (Fehsenfeld, 1975).The observed rate of an ion-molecule reaction depends upon the buffer gas. Although the ion kinetic energy was the same for both helium and argon buffer gases, the rate of translational to vibrational energy transfer with helium was slower. The rate was sufficiently slow that the vibrational temperature of the ion depends upon the buffer gas. This experiment emphasizes the importance of knowing the temperature of a system to obtain true rate constants. The major limitation of the flow technique is the inability to study systems at kinetic energies higher than 1-2 eV. In order to obtain such ion kinetic energies the pressure must be reduced. Therefore, the mean free path of the ions approaches the size of the tube diameter where they are neutralized by wall collisions. Methods for studying higher energy systems are discussed in the following section.

D. Low Pressure, Single Collision Techniques Over the past decade a large amount of information concerning ionmolecule reactions in the 1-100 eV energy range has been obtained using “beam” instruments. In this type of instrument a monoenergetic ion beam is formed. The beam collides with target molecules, yielding ionic products. The products are formed as the result of a single collision. The energy and angular dependence of the product molecules can be measured. This information is used to understand the mechanism of the individual reaction on a molecular level. Beam machines can be divided into two classes: crossed beam and collision chamber. In the crossed-beam apparatus the ion beam collides with a beam of neutral molecules. An example of such an instrument was reported by Herman et al. (1969b). Since both the direction and speed of the ion and target molecule can be defined, studies with this instrument can yield more detail about the kinematics of the collision than with other types of instruments. Additionally, reactions involving little momentum transfer, such as proton transfer from the reactant ion to the neutral molecule, can be studied because the product ion has the velocity of the neutral precursor. The initial velocity of the neutral is necessary to propel the product ion to the detector. The power of the technique is demonstrated by a study of Herman et a/.

MASS SPECTROSCOPY

23

(1969a) of the reaction of ethylene ion with ethylene. The authors showed how the ratio of various products strongly depends upon the relative energy of the reactants. A similar type of instrument is one using a collision chamber to contain the target gas, for example, the instrument reported by Hied et al. (1973). There are several advantages to using a collision chamber containing target gas at a pressure of about torr vs. a beam of target gas. The number density of the target gas is easily measured, permitting the determination of absolute reaction cross sections. The apparatus is simpler than the crossedbeam apparatus. The consumption of the target gas is orders of magnitude smaller, permitting the use of an expensive target gas, e.g., CH,D, . The major beam technique limitation is that the lower energy limit for an ion beam having a usable intensity is about 1 eV. Only for reactions between a heavy ion and a light molecule, for example, Ar+

+ H, -,ArH' + H

(16)

does the center-of-mass energy approach thermal energies. Another difficulty is that the ions are not in the ground state since they are formed using electron impact. This problem has been attacked by producing the primary ions in an ion source at pressures of about 1 torr. This technique reduces the excited ions to their ground state by collisions (e.g., Leventhal and Friedman, 1969).

E . Other Systems Several groups have studied ion-molecule reactions at low pressures using other techniques, a prominent example being the ion cyclotron resonance technique ICR (Bowers and Su, 1973). Ions are produced in the ICR cell and their disappearance is measured along with the appearance of product ions. Since ions can be contained in an ICR cell for several minutes, reactions having extremely small cross sections can be studied. Because the ICR cell operates at pressures < lo-' torr, the reactant ions are in various excited states. Smith and Futrell(l974) have attacked this problem by producing ions external to the ICR cell in a higher pressure source. These ions are mass analyzed and then injected into the ICR cell. The effect of varying the reactant ion internal energy was studied by varying the pressure in the region of ion formation. ICR is useful for the study of the interaction of photons with ions (Richardson, 1975). Because of the long residence time in the ICR cell, there is a greater probability of the ions interacting with a given photon flux than in other techniques. Kramer and Dunbar (1972) reported the effect of photoexcitation of the reactant ion using ICR.

24

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

The effect of reactant internal energy upon the rate of an ion-molecule reaction was studied by Chupka and Russell (1969). They used a photoionization mass spectrometer having a tunable photon source. Hydrogen ions could be selectively prepared in various excited vibrational states. They found that the vibrational state of the hydrogen molecular ion had a significant effect upon the rate of the ion-molecule reaction Hl

+ H e - + H e H ++ H

(17) The quadrupole trap is a new technique for studying ion-molecule reactions. Ions are trapped in a three-dimensional quadrupole field. Such traps can contain ions for periods of days (Walls and Dunn, 1974). This requires the use of ultrahigh vacuum torr). Thus, this technique has potential for studying extremely slow ion-molecule reactions. So far we have discussed techniques for studying ion-molecule reactions over the 0.02-100-eV kinetic energy region. This region is of most interest because it covers the energy range of chemical bonds. Surprisingly much can be learned about the decomposition of ions by studying ion-molecule reactions at higher energies. Of particular note is the technique of ion kinetic energy spectroscopy (IKES) (Beynon and Cooks, 1974), which uses ion beams of several kiloelectron volts. Initial IKES experiments used a conventional double-focusing mass spectrometer with a detector placed at the focus of the electric sector. The electric sector was used to energy analyze ions that decomposed in the field-free region after they were accelerated. Recently Ast et al. (1972) placed a small collision chamber at the focus of the electric sector of a doublefocusing mass spectrometer. The magnetic sector was used in conjunction with the electric sector to examine a doubly charged ion produced by the reaction M+ + N + M + +

+ N +e-

(18)

This type of reaction was used to deduce second ionization potentials (Ast et al., 1972). Another interesting reaction studied by IKES is M + ++ N + M +

+ N'

When M + + is a rare-gas ion having known single and double ionization potentials, the primary and excited ionization potentials of N can be determined from the decrease in kinetic energy of the ions M. This type of reaction was recently observed at lower energies by Hierl and Cole (1975). A variation of this technique is to reverse the position of the electric and magnetic fields. In this way kinetic energy analyses of the reaction products of mass-selected ions can be made. Such an instrument has been constructed

MASS SPECTROSCOPY

25

by Beynon et al. (1973). In some respects this instrument is similar to the “beam machines discussed earlier, except it operates at much higher energies. It is quite useful for elucidating ion decomposition pathways, yielding information on molecular structure from fragmentation patterns. ”

VI. HIGHTEMPERATURE STUDIES A . Introduction

Mass spectrometry is a powerful tool for the study of high temperature systems. By high temperature systems we mean systems at equilibrium above 1000°K. No other technique provides qualitative and quantitative information about high temperature species. High temperature mass spectrometry has been reviewed previously (Inghram and Drowart, 1960; Grimley, 1967; Margrave, 1968; Rapp, 1970; Gingerich, 1972). Therefore, only areas of high temperature mass spectrometry research emphasizing advances that utilize current technology will be covered. B. Knudsen Cell Studies

The principal technique for studying high temperature systems mass spectrometrically is a Knudsen effusion cell. This technique was developed by Chupka and Inghram (1953). A Knudsen cell is placed such that effusing species, presumably from an equilibrium condition, pass directly into the mass spectrometer ion source. The species form a molecular beam which is usually mechanically chopped to distinguish beam species from the background. The most studied system over the past 20 years is the vaporization of carbon. The thermodynamic properties of the vapor species C,-C, were determined by Drowart et al. (1959). The pressures were measured over a range of five orders of magnitude. Due to the importance of graphite as a high temperature material, studies continue of its vaporization. Groups that have recently studied the vaporization of carbon include Wachi and Gilmartin (1972), Meyer and Lynch (1973), Zavitsanos and Carlson (1973), Milne et al. (1973), and Steele and Bourgelas (1973). Since oxides are an important class of refractory materials, their vaporization has been studied by many mass spectrometry groups (Balducci et al., 1971; Wu and Wahlbeck, 1972; Trevisan and Depaus, 1973; Akerman and Rauh, 1973; Bennett et al., 1974; Hildebrand and Murad, 1974). Gingerich and co-workers have concentrated on systems producing various binary and tertiary mixed-metal vapor species and carbides (e.g., Cocke

26

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

et al., 1973). Carbides also have been studied by Stearns and Kohl (1973, 1974). High temperature species that produce negative ions have been studied (Franklin et al., 1974; Petty et al., 1973). These ions are produced via electron impact in the ion source. Negative ions produced directly from an alumina system have been studied (Srivastava et al., 1972). The Knudsen cell technique can be modified by adding a gas inlet to the cell. In this way various pressures of a reacting gas can be added to the system. If the gas-solid reactions are sufficiently fast compared to effusion of species from the cell, equilibrium is obtained. This technique is necessary when there is no convenient solid source of the element required, such as various nitrides which can be used as a source for nitrogen. Using a gas-inlet Knudsen cell, Wyatt and Stafford (1972) measured the thermodynamic properties of various carbon-hydrogen and carbon-nitrogen species. Often various factors limit the accuracy of vapor pressure determinations to the extent that thermodynamic values are useless. This is particularly true for determining alloy activities. One method for correcting this problem is to incorporate an internal standard into the system. Because of possible reactions between the sample and the standard, it is often not desirable to add a standard to the sample; therefore a Knudsen cell having two or more compartments is used (Hackworth et al., 1971). The sample to be measured is placed in one compartment and the standard in another. Such a technique was used by Jones e t a / .(1970)to obtain activities accurate to better than 2%. An alternative method is to measure the ratios of ion intensities for all the components throughout a composition range and convert these to activities using the Gibbs-Duhem equation (Sodeck et al., 1970). The principal goal of the research discussed in the above sections was to obtain thermodynamic data about high temperature species. To do this, it is necessary to sample a system at equilibrium. In many instances it is difficult to determine whether equilibrium exists. There are many other factors that limit the reliability of the data; these are discussed later. However, thermodynamic data also can be obtained by measuring the bond energy of the vapor species. The use of photoelectron and photoionization spectrometry is particularly promising in this regard (Berkowitz, 197 1). Bond energies can be determined to 0.01 eV by this technique under the appropriate conditions. An example of this work for gallium and indium halides is given by Dehmer et al. (1974). There are several limitations to using photon techniques. The ionization produced is orders of magnitude less than with electron impact sources. Therefore, only the most abundant species can be studied. At higher temperatures a larger fraction of the rotational, vibrational, and low-lying electronic

MASS SPECTROSCOPY

27

states of the molecules are populated. These cause a smearing of the spectra making assignment of the appearance potentials more difficult. Finally, tunable photon sources are expensive and complicated. Although mass spectrometry is a powerful tool for the study of high temperature systems, it has limitations (Stafford, 1971). The most difficult problem is the lack of cross-section data for ionization of a given species. The cross section is the most uncertain factor when ion intensity is converted to number density of the parent molecule in the molecular beam. Other uncertainties such as instrumental ion transmission efficiency and multiplier gain must also be determined. Molecular cross sections can be crudely estimated by summing the cross sections of the atoms. Atomic cross sections were calculated by Lin and Stafford (1968) and by Mann (1970).Stafford (1971) believes these values are likely to be accurate within a factor of two. Unfortunately an estimate of the ionization cross section is useless unless the fragmentation pattern is known. Often various fragment ions correspond to parent ions of other species in the system adding more uncertainty. Using lower energy electrons (15 vs. 70 eV) usually reduces the amount of fragmentation, but also lowers the sensitivity. Additionally, calculated atomic cross sections are for 70-eV electrons, and therefore cannot be used with confidence at lower energies. One problem that has plagued those studying high temperature systems is the lack of satisfactory container materials. Often the crucible reacts with the system under study. Additionally, the melting point of the material often limits the highest temperature that can be used. A solution is to use the material being studied as its own container. An example of this technique is the work of Lincoln and Covington (1975) using high intensity laser pulses to produce rapid local heating. They studied the vaporization of graphite and alumina using irradiation levels of 100-600 kW cm-2. With this system they obtained readily temperatures of 3000-4000"K. They tested several models to obtain a value for the surface temperature of the material. The model assuming that the vapor species leave the material with velocities corresponding to an adiabatic free-jet expansion gave the best results. In their instrument they are able to measure the velocity of the vaporizing species as well as the mass. Despite the fact that the quadrupole mass spectrometer mass discriminates more than does a sector instrument, more researchers are using quadrupoles for high temperature research. This is due to the high ion transmission efficiency and relatively small size of quadrupole. Vasile er al. (1975) discuss an analyzer system that uses a quadrupole mass spectrometer and incorporates several ciever design features. The mass spectrometer is placed in a differentially pumped chamber between two inlet systems. One inlet system contains a Knudsen cell oven assembly close to

28

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

the ion source. This is for high sensitivity. The second inlet system contains an identical Knudsen cell oven assembly that is separated from the ion source by a distance of 75 cm. The region through which the beam passes contains a beam modulator and a set of electrostatic quadrupole deflection rods. A stopwire can be placed in the system between the oven and the deflection area such that beam species cannot enter directly into the ion source. When a suitable electric potential is placed on the rods, species having a dipole moment are refocused into the ion source. In this way, information can be obtained about the structure of the effusing species. Additionally, by measuring the phase shift in the ion signal, the flight time of the neutral progenitor of the ions can be determined. If the species are effusing at low pressure and a known temperature, their approximate mass can be calculated. Vasile et al. (1975) found at source pressures greater than about 0.1 torr the velocity of the effusing species was pressure dependent, implying noneffusive flow. By examining the phase shift and the effect of electrostatic deflection of various ions, insight can be obtained about their neutral progenitors. This system, we believe, represents the state of the art in high temperature beam sampling mass spectrometry.

VII.

SAMPLING OF

REACTIVESPECIES

A . Introduction

Mass spectrometers are frequently used for chemical analysis because of their exceptional versatility, sensitivity, and universal detectivity. For any successful analysis a representative sample must be introduced into the instrument. This requirement is not difficult to meet for routine gas analyses ; however, for the analyses of combustion, lasers, spark discharges, radicals, and certain solid materials special sampling procedures must be developed. Frequently, the development of such procedures constitutes difficult research programs which are either totally ignored or inadequately described when the research results are reported. In an effort to correct this situation, the American Society for Mass Spectrometry held a symposium at its 1974 annual meeting in Philadelphia on sampling reactive systems. The papers in this symposium were published in the January, 1975 issue of Volume 16 of the International Journal of Mass Spectrometry and Ion Physics. B. Sampling of Plasmas

Hasted (1975) thoroughly reviewed the mass spectrometric monitoring of ions in plasmas and swarms. He points out that the purpose of such a monitoring system is to produce an ion signal that is proportional to the ion

MASS SPECTROSCOPY

29

species density in the plasma. Since this goal is difficult to prove experimentally, an understanding of the basic physics is needed in order to assure correct sampling. Hasted also points out that although the discharge plasma can be sampled in a manner such that the observed ion count is proportional to the plasma’s ion density, the proportionality constant is not known. In addition, he notes that the ion count obtained in flame sampling is not proportional to the ion concentration in the flame because the flame cools during expansion into the mass spectrometer. This expansion freezes the ion population at temperatures lower than in the flame and allows collisions to occur during the sampling. Hasted (1974) stated that a system suitable for plasma monitoring must have minimal ion discrimination, high source pressure capability, large dynamic range, and an absence of critical injection geometry. Most plasma analyzer systems use a metal sampling orifice, the potential of which can be varied and across which there is a large pressure differential. Ion optics are usually installed behind the orifice to focus the extracted ions into the mass spectrometer. Normally high capacity pumping is employed both on the lens system and mass spectrometer. This pumping permits the system to operate at a pressure low enough so that the ion mean free path is greater than the dimension of the mass spectrometer. Actual pumping capacity varies depending on the pressure of the plasma and orifice size. Hasted (1974) reviewed the theory of ion optics and the characteristics of biased monitoring orifices. He concludes that, because of collisional effects, biased orifices should be avoided. And even with unbiased orifices the ion accelerating system should always be tuned for maximum signal. Vasile and co-workers (1974; Smolinsky and Vasile, 1975), described in detail their plasma monitor which incorporates the four basic requirements cited by Hasted (1974). Since Vasile et al. used a quadrupole, their system has some dependence on the ion injection geometry; however, this type of mass spectrometer is currently being accepted by most workers for plasma monitoring. An important aspect of Vasile’s apparatus is the sampling orifice. This orifice is a 12-pm diameter hole drilled with a laser in an aluminum plate (Vasile and Smolinsky, 1974).The laser-drilling details were not reported. The potentials of the outer support cylinder and the sampling orifice were electrically floated so that they attained the wall potential. The potentials on the cylinder lenses were adjusted for maximum signal in agreement with Hasted’s recommended procedure (Hasted, 1974). The species produced in a rf discharge in methane have been reported (Smolinsky and Vasile, 1975).These authors identified a variety ofspecies in a CH, discharge. They concluded that the higher homologous hydrocarbon ions were produced by condensation reactions of C: with C , and C ; with C, and C,. They suggest that radical processes are responsible for the production of the neutral species.

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Another interesting type of plasma sampling, reported by Lincoln and his co-workers (Lincoln and Covington, 1975), is the dynamic sampling of laser-induced vapor clouds during the submillisecond expansion after a material has been exposed to laser radiation. This method of sampling makes it possible to use a high power laser as a convenient tool for heating refractory materials to very high temperatures (see the previous section). Typical laser conditions were bursts of 1.06-pm radiation for approximately 0.5 msec producing surface radiation levels of 100-600 kW ern-,. A major problem is the temperature existing at the solid-vapor interface. This problem is compounded by the irregular exposure time-intensity distribution of the laser energy. This complication might be eliminated by operating the laser in the Q-switched mode. However, Lincoln preferred his approach to obtain heating of the bulk material to simulate the heating of a planetary probe. Lincoln’s system consists of a large vacuum chamber containing a nude ion source for a Bendix Model MA-2 T-O-F mass spectrometer. This instrument takes 50 spectra per millisecond and can time-resolve the species produced by a single laser “shot.” The laser beam is split; one portion is directed to a detector which measures an amount of energy proportional to the energy that irradiates the sample. The remaining portion of the beam radiates the sample normal to the surface. The surface is 45” to the center axis of the ion source. Only vapors leaving the surface along this axis enter the system. To measure ions generated from the surface, Lincoln turns off the mass spectrometer filament; to measure neutrals, an ion deflector in front of the ion source and the mass spectrometer filament are turned on. Using this instrumentation, Lincoln and Covington (1975) investigated laser plumes from graphites, alumina, and silica. They identified both the stable and transient species in these plumes. By sampling graphite plumes, they showed that triatomic carbon is the predominant species. Hydrogen is not produced; acetylene and carbon monoxide always have small, but equal concentrations. The major vapor components observed from alumina were Al’, Al, 0, , 0, and A1,O. From silica, the major species observed were O,, SiO,, and traces of Si. No Si’ was observed.

C . Sampling of Combustion The chemistry occurring in combustion is extremely complex and reactive. For example, Fristrom (1975) reported that reactive species such as radicals, ions, and excited molecules (and atoms) exist in permixed laminar flames in mole fraction concentrations ranging from lo-’ to lo-”. Analysis of these species, especially the radicals, is critical to understanding combustion chemistry. However, the quantitative detection of these species is ex-

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tremely difficult. Therefore, considerable effort has been devoted to the development of sophisticated techniques for sampling and analyzing combustion species. Methods used for the detection of stable molecules are well covered in standard mass spectrometry textbooks, such as the one by Kiser (1965) and therefore will not be described here. Our main concern in this section is the analysis of the unstable species in combustion. In general to analyze a combustion system, three important steps must be carried out: the combustion must be probed by some method; the material removed from the combustion must be transferred to the analytical instrument; and the analysis must be made. Each of these steps is discussed below. A fundamental problem in probing combustion is obtaining a representative sample without disturbing the chemistry. While flames can be probed many ways, sampling probes are usually divided into two general categories: isokinetic and sonic. Isokinetic probes remove a combustion sample at the velocity of the gas stream. The quenching of the reactive species is not effective by this approach. Therefore, this method of sampling is used for the analyses of nonreactive species. The principal advantage of this probing method is that disturbance of the flow is minimized. Sonic probes, on the other hand, disturb the combustion in the probe area. However, the use of sonic probes permits the combustion to be rapidly quenched by adiabatic expansion. This expansion lowers both the temperature and the pressure of the reacting system. Fristrom and Westenberg (1965) have published an excellent description and theory of these probes; thus only a brief description will be presented. Most sonic probes are made of quartz with tapers between 15" and 45". It is the taper of the probe, not the tube size, that determines the amount of combustion disturbance caused by the sampling. The orifice of the probe is designed to give a five- to tenfold pressure reduction, and usually has a 10-100-pm diameter. The tubing that connects the probe to the rest of the analysis system is not critical to the probing procedure. But the tubing may cause adsorption problems in the sample transfer. Because adsorption is a serious transfer problem for polar molecules such as water, several approaches have been used to eliminate this problem. A simple method is a flow system. Here, the combustion is sampled for a time sufficient for the adsorbed species to equilibrate with the transfer line walls. The time required for this process varies according to the wall material: (for water vapor) 30 min on stainless steel, 5 min on glass, and f min on Teflon (Fristrom, 1975). There are several types of flows including molecular and sonic. The results obtained from a molecular flow system are the easiest to interpret. This method was described by Pertel (1975). With the molecular

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flow system, the partial pressure of the species in the analytical instrument is proportional to the species concentration in the combustion. Unlike molecular flow, sonic flow uses a continuous input, and there are significant variations between the species concentration and their concentration in the analytical instrument. The chief advantages of sonic flow systems are large samples (thereby permitting the detection of small concentrations) and rapid equilibrium with the walls. Neither of the methods described above successfully transfers reactive species, such as radicals. In order to detect radicals, “collisionless inlet systems such as that discussed by Foner (1966) must be used. Pertel (1975) discussed the theory of these inlet systems. There are two major types of molecular beam inlets: effusive and supersonic. The effusive molecular beam system samples the boundary layer of the combustion. In low temperature combustion studies such as those reported by Wyatt et al. (1975) this type of system is used effectively. Wyatt et al. showed that the combustion being sampled depended on the wall temperature for its existence; the products in the boundary layer were the same as those in the continuum. This observation is not true for “hot ” flames. Wyatt et al. passed their reaction tube directly through the ion source of a mass spectrometer, and the combustion products effused into the ion source, which was less than 1 cm from the reactor. To sample the continuum of a flame requires a supersonic molecular beam inlet. With this inlet, many of the reactive species are frozen in excited states (Pertel, 1975). A problem using mass spectrometry is that the excited species frequently have unusual fragmentation patterns. It is possible, although difficult, to assume an effective temperature and calculate the change in the fragmentation pattern. These spectra are then compared to the literature pattern. Another problem frequently encountered in supersonic molecular beam inlet systems is the formation of cluster species such as water and argon. There are many methods for analyzing the combustion species. The principal purpose of this report is to discuss mass spectrometric methods, but the reader should be aware of other useful analytical methods. The most promising ones are optical methods which eliminate sampling. One of these is coherent, anti-Stokes, Raman spectroscopy (CARS) (Begley et al., 1974). The chief advantages of this method are high sensitivity (105-109 times the sensitivity of commercial laser Raman methods) and the laserlike” CARS beam. Laser-induced fluorescence spectroscopy is another promising method, but is useful only for selected species that fluoresce. Many older techniques such as Raman and infrared spectroscopy, gas chromatography, and the Orsat (pressure-volume) analysis have been used with varying degrees of success to analyze combustion (Fristrom and Westenberg, 1965). Mass spectrometry is the most useful of all the analytical techniques ”

“

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because of its versatility. Mass spectrometry can, in principle, detect all molecules, radicals, and atoms. It thus has no “blind spots.” Mass spectrometry is extremely sensitive and has a large dynamic range. Therefore, the instrument can simultaneously detect the trace constituents and the major products of combustion. Finally, most mass spectrometers have extremely rapid response times and can be computer controlled. Thus, the analyses can be carried out in “real time.”

D. Sampling of Radicals The study of radicals with mass spectrometers has evolved in two directions. The first deals with the study of the properties of the radicals themselves, such as ionization potential, heat of formation, and bond energies. The second focuses on a reacting system; the purposes of these studies are to identify the radicals present and to define the influence of the radicals on the system. Simple inlets are employed in the first type of radical study. These inlets usually consist of a heated quartz tube positioned near the ionization chamber. In the second, the important conditions discussed in the combustion section must be satisfied. Thus, these inlets are more complex. Both types of radical studies evolved from the classic work of Eltenton (1942, 1947, 1948). He employed three different types of reactors attached to a mass spectrometer to study low pressure (-4 torr) thermal decomposition, low pressure flames, and higher pressure ( - 100 torr) thermal decompositions. In all cases, Eltenton’s reactors were separated from the ion source by a 0.008-in. thick gold diaphragm. The gold diaphragm contained a hole with a diameter less than 0.001 in. This hole permitted the decomposition products to effuse into the ion source. Eltenton also showed that by oscillation of a flame on the high pressure side of the diaphragm, he could determine not only the origin of a given species but also study its chemical sequence. Lossing (1971, 1972; Lossing and Semeluk, 1970) extended Eltenton’s method of determining radical properties to an amazing degree. He improved not only the sampling method, but also the mass spectrometry. Others have applied the approaches of Lossing and Eltenton (e.g., Fisher and Henderson, 1967) to various systems. In these studies minor modifications have been incorporated, but the basic concepts employed are not fundamentally different from the approach used by Eltenton and Lossing. A type of radical study that differs from the classic work of Eltenton is the study of radicals produced by electron impact. In these studies a molecule ionizes and fragments in a conventional mass spectrometer ion

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source. The ions are removed and the neutral fragments (radicals) diffuse into a second ion source, where they are studied by mass spectrometric techniques. Beck and Osberghaus (1960) pioneered these radical studies which have been continued by other workers (Niehaus, 1967; Lampe and Niehaus, 1968; Melton, 1966, 1968; Saunders et al., 1969; Preston et al., 1969). The technique was further refined by Reeher (1974) who employed a quadrupole mass spectrometer with a dual ion source in which the operation of the source and the recording of the radical spectra and appearance potential were automatic. Reeher was able to correlate the radical spectra with the positive ion spectra. Eltenton’s classical flame studies were extended and greatly improved by Foner and Hudson (1953, 1962). These workers developed a collision-free molecular beam sampling technique such as the system described in the preceeding section. In order to prevent loss of the radicals at the walls and to reduce the background in their mass spectrometer, they improved the Eltenton oscillation technique by employing a three-stage chopped molecular beam system. The molecular beam was chopped at 170 Hz by a vibrating reed, and phase-sensitive detection was employed to eliminate background signals. This approach was refined further by Fite (1975). In fact, the phasesensitive detection method is now offered commercially. The normal instrumentation used for phase-sensitive detection consists of a system to reduce the pressure and to provide a collision-free beam. This consists of one or more differentially pumped chambers each containing an skimmer orifice. The size of the orifice is determined by the pressure reduction needed. The collimated beam is mechanically chopped and then ionized and mass analyzed. Only the mass spectra signals in phase with the chopper are detected. Another method of radical study is the shock tube experiments performed by Bradley and Kistiakowsky (1961) using the time-of-flight (TOF) mass spectrometer constructed by Kistiakowsky and Kydd (1957). In these experiments, the shock wave chamber is attached directly to the backing plate of the mass spectrometer ion source. The backing plate, which is only 0.3 cm from the ionizing electron beam, is 0.001 in. thick and contains a 0.005-in. diameter orifice. Bradley and Kistiakowsky used the TOF because of its good time resolution (50 psec) and its ability to follow a large number of ions simultaneously. For example, in their N,O studies, they detected the presence of oxygen atoms. Finally, several studies of radicals produced by heterogeneous reactions have been reported. For example, Martin and Rummel (1964) designed a means for producing radicals from a palladium catalyst to study hydrogenation-dehydrogenation processes. A palladium tube (0.25-cm i.d. with wall thickness of 0.008 cm) is inserted into the ion source directly over,

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and parallel to, the ionizing electron beam. The gas to be studied is passed directly over the catalyst. The catalyst temperature was 40°C due to radiation from the mass spectrometer filament. However, Martin and Rummel had provisions for heating the catalyst to 450°C. They observed methyl radicals from methane. In these studies Martin and Rummel reported ionmolecule reactions in the gas phase. Their data, however, did not prove that these homogeneous reactions were independent of the catalyst position. DeCorpo et al. (1974) carried out a similar catalytic study on various hydrocarbons using a platinum catalyst. These workers, who used an instrument design similar to that used by Martin and Rummel, found that the chemical composition of the ions produced was temperature and pressure dependent. At temperatures less than 750°C the radicals formed polymers or clusters; at higher temperatures, the radicals produced were similar to the ones produced in flames. These two studies show that mass spectrometry can be used to study radicals produced by a hetergeneous mechanism. REFERENCES Akerman, R. J., and Rauh, E. G. (1973). High Temp. Sci. 5, 463. Anbar. M., and Aberth, W. H. (1974). Anal. Chem. 46, 59A. Andersen, C. A., and Hinthorne, J. R. (1972).Science 175, 853. Ast, T., Beynon, J. H., and Cooks, R. G . (1972). J. Am. Chem. SOC.94, 661 1. Bakale, D. K., Colby, B. N., and Evans, C. A., Jr. (1975). Anal. Chem. 47, 1532. Baldeschwieder, J . D., and Woodgate, S. S. (1971). Acc. Chem. Res. 4, 114. Balducci, G., &Maria, G., Guido, M., and Piacente, V. (1971).J. Chem. Phys. 55, 2596. Beck, D., and Osberghaus, 0. (1960). Z. Phys. 160, 406. Beckey, H. D. (1971). Field ionization mass spectrometry, I n “International Series of Monographs in Analytical Chemistry,’’ Vol. 42. Pergamon Press, Oxford, New York, Toronto, and Sydney. Beggs. D. P., and Field, F. H. (1971). J . Am. Chem. Sor. 93, 1567. Begley, R. F.. Harvey, A. B., Byer, R. L., and Hudson, B. S. (1974). Am. Lab. 6, 1 1 . Bennett, S. L., Lin, S. S., and Gilles, P. G. (1974). J . Phys. Chem. 78, 266. Benninghoven, A. (1973). SurJ Sci. 35, 427. Berkowitz, J. (1971). Ado. High Temp. Chem. 3, 126. Berkowitz, J., Appleman, E. H., and Chupka, W. A. (1973). J . Chem. Phys. 58, 1950. Beske, H. E.. (1967). Z. Narurforsch.. Teil A 22, 459. Beynon, J. H., and Cooks, R. G. (1974). J . Phys. E 7, 10. Beynon, J. H., Amy, J. W.. Baitinger, W. E., Ridley, T., and Cooks, R. G. (1973). Anal. Chem. 45, 1023A. Bingham. F. W. (1966).Sandia Research Report SC-RR-66-506, TID-4500 Physics. “Tabulation of Atomic Scattering Parameters Calculated Classically from a Screened Coulomb Potential,” Sandia Research Report SC-RR-66-506, TID-4500 Physics. Sandia Lab., Albuquerque, New Mexico. Bowers, M. T., and Su, T. (1973). Adv. Electron. Electron Phys. 34, 223-279. Bradley, J. N., and Kistiakowsky, G. B. (1961). J . Chem. Phys. 35, 256. Bradley, R . C., and Ruedl, E. (1962). Proc. Inr. Con/: Ioniz. Phenom. Gases, Srh, 1961 Vol. 1, p. 150.

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