Добавил:
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:

Chen The electron capture detector

.pdf
Скачиваний:
17
Добавлен:
15.08.2013
Размер:
3.98 Mб
Скачать

REFERENCES 167

19.Azria, R.; Abouaf, R.; and Tellet-Billy, D. J. Phys. B: At. Mol. Opt. Phys. 1988, 21, L213.

20.Chen, E. C. M. and Wentworth, W. E. J. Phys. Chem. 1985, 89, 4099.

21.Chen, E. S. and Chen, E. C. M. Chem. Phys. Lett. 1998, 293, 491.

22.Ayala, J. A. Chen, E. C. M.; and Wentworth, W. E. J. Phys. Chem. 1981, 85, 768.

23.Maslen, P. E.; Papanikolas. J. M.; Faeder, J.; Parson, R.; and O’Neil, S. V. J. Chem. Phys. 1994, 101, 5731.

24.Zanni, M. T.; Taylor, T. R.; Greenblatt, B. J.; Miller, W. H.; and Neumark, D. M. J. Chem. Phys. 1997, 107, 7613.

25.Zanni, M. T.; Batista, V. S.; Greenblatt, B. J.; Soep, B.; and Neumark, D. M. J. Chem. Phys. 1999, 110, 3748.

26.Eyring, H.; Hirshfelder, J. O.; and Taylor, H. S. J. Chem. Phys. 1936, 4, 479.

27.Dalgarno, A. and McDowell, M. R. C. Proc. Phys. Soc. 1956, A69, 617.

28.Elizer, I.; Taylor, H. S.; and Williams, J. K. J. Chem. Phys. 1967, 47, 2165.

29.Nesbet, R. K. Comm. Atom. Molec. Phys. 1981, 11, 25.

30.Schulz, G. J. Phys. Rev. 1959, 113, 816.

31.Allan, M. J. Phys. B 1985, 18, L451.

32.Ichikawa, T.; Tachikawa, H.; Kumada, T.; Kumagai, J.; and Miyzaki, T. Chem. Phys. Lett. 1999, 307, 283.

33.Mulliken, R. S. J. Chem. Phys. 1934, 2, 782.

34.Pauling, L. The Nature of the Chemical Bond, 3rd ed. Ithaca, NY: Cornell University Press,

1960.

35.Streitweiser, A. S. Molecular Orbital Theory for Organic Chemists. New York: Wiley,

1961.

36.Coulson, C. A.; O’Leary, B.; and Mallion, R. B. Huckel Theory for Organic Chemists. New York: Academic Press, 1978.

37.Hine, J. Physical Organic Chemistry. New York: McGraw-Hill, 1962.

38.Briegleb, G. Angew. Chem. Internat. Edit. 1964, 3, 617.

39.Page, F. M. and Goode, G. C. Negative Ions and the Magnetron. New York: Wiley,

1969.

CHAPTER 8

Selection, Assignment, and

Correlations of Atomic

Electron Affinities

8.1INTRODUCTION

The electron affinities Ea of the main group atoms are the most precisely measured values. Recall that the Ea is the difference in energy between the most stable state of the neutral and a specific state of a negative ion. It was once believed that only one bound anion state of atoms and molecules could exist. However, multiple bound states for atomic and molecular anions have been observed. This makes it necessary to assign the experimental values to the proper state. The random uncertainties of some atomic Ea determined from photodetachment thresholds occur in parts per million. These are confirmed by photoelectron spectroscopy, surface ionization, ion pair formation, and the Born Haber cycle. Atomic electron affinities illustrate the procedure for evaluating experimental Ea.

Random and systematic errors are characteristics of the method, not the values. Random errors can be determined by repeating the experiment. Systematic errors can only be determined by comparisons of values determined by different methods. Uncertainties can be estimated from precision and accuracy plots if it is assumed that there are only random errors. The Ea of the d and f block elements, electronegativities, and work functions of the elements will be evaluated in this chapter using this procedure.

The extrapolation of values in the Periodic Table for the main group of elements can be examined and applied to the transition elements. The evaluated values can be combined with ionization potentials to obtain Mulliken electronegativities. These may be correlated with other electronegativities and the work functions of the elements. This provides another example of the use of the Periodic Table to evaluate and predict values.

The Electron Capture Detector and the Study of Reactions with Thermal Electrons by E. C. M. Chen and E. S. D. Chen

ISBN 0-471-32622-4 # 2004 John Wiley & Sons, Inc.

168

EVALUATION OF ATOMIC ELECTRON AFFINITIES

169

An application of the electron affinities of the elements and the experimental work functions involves the prediction of the electron affinities of clusters. The Cn molecules are an important type of cluster studied experimentally and theoretically. With experimental data the CURES-EC method of calculating electron affinities can be evaluated. The READS-TCT procedure can also be used to determine relative electron affinities. The clusters of C, Si, and Ge involve covalent bonds, while the bonds in the Sn and Pb clusters are partially metallic. With available electron affinities the relationship between the electron affinities and work functions of these anion clusters can be investigated.

The precision of a metric is determined by the random uncertainties of a method and the number of replications. The equipment, ability of the investigator, and material investigated affect the random uncertainties. It is important to know the ‘‘best’’ precision that has been attained and the number of replications used to attain that precision. In establishing the precision, it is assumed there are no systematic uncertainties. In the case of atomic electron affinities the largest systematic uncertainty is the state assignment.

The statistical evaluation of experimental data involves the following questions: ‘‘Do the results agree with previous results within the random and systematic uncertainties?’’ and ‘‘Is this value assigned to the correct state?’’ For the main group elements the data are the Ea obtained by different techniques. The largest precise value is assumed to be the ground state Ea. If this value has been confirmed by measurement with another technique, the assumption is validated. The weighted average is the ‘‘best’’ value. If the Ea has not been determined by an alternative technique, but is confirmed by theoretical calculations or the expected variation within the Periodic Table, the assumption may be validated. If there are no validations, the assumption can only remain an assumption.

8.2EVALUATION OF ATOMIC ELECTRON AFFINITIES

As new values were obtained, atomic electron affinities were reviewed periodically beginning in 1953 [1–13]. All the available experimental, extrapolated, and theoretical values were tabulated in 1984 [7]. Presently, experimental values are available at the NIST website [12]. Prior to 1970 the majority of the values for the main group elements were determined by the Born Haber cycle, electron impact, or relative and absolute equilibrium surface ionization techniques. However, values for C, O, and S had been measured by photodetachment [1–3]. By the mid-1970s virtually all the Ea of the main group elements in the first three rows had been measured by photon methods [4–7]. By the early 1980s values were obtained for the transition elements by photon techniques [7, 8]. In the 1990s the values of Ca, Sr, and Ba were measured [9–13]. Recently, experimental values have been reported for Ce, Pr, Tm, and Lu [14–17].

In 1971 all the experimental atomic Ea in the literature were evaluated and compared with extrapolated values [3]. None of the experimental values were eliminated. Considered were 123 values for 23 elements. All but four of the elements

170 SELECTION, ASSIGNMENT, AND CORRELATIONS OF ATOMIC ELECTRON AFFINITIES

TABLE 8.1 Atomic Electron Affinities: Current Best Averages to 1970, Weighted Averages to 1970, and Weighted Average of Photon Values to 1975 [3–5]

 

 

 

 

Ea ðeVÞ

 

A

N

Current

Average

Weighted Average

1975

 

 

 

 

 

 

H

1

0.754

0.77

0.77

0.754

Li

3

0.618

0.50

0.50

0.62

C

6

1.263

1.31

1.31

1.263

O

8

1.462

1.71

1.465

1.462

F

9

3.401

3.53

3.41

3.400

Na

11

0.548

0.3

0.3

0.548

S

16

2.077

2.22

2.1

2.077

Cl

17

3.613

3.75

3.6

3.613

K

19

0.502

0.5

0.5

0.502

Cr

24

0.676

1.2

1.2

0.77

Cu

29

1.236

1.5

1.5

1.226

Br

35

3.364

3.58

3.47

3.365

Rb

37

0.486

0.6

0.6

0.486

Mo

42

0.747

1.0

1.0

1.0

Ag

47

1.303

1.95

1.95

1.303

Sb

51

1.047

1.5

1.5

1.15

I

53

3.059

3.19

3.12

3.07

Cs

55

0.472

0.6

0.5

0.472

W

74

0.815

0.5

0.5

0.5

Re

75

0.2

0.15

0.15

0.15

Au

79

2.309

2.8

2.8

2.309

Tl

81

0.4

1.5

1.5

0.5

Pb

82

1.1

1.1

1.1

1.2

Bi

83

0.942

1.76

1.76

1.0

 

 

 

 

 

 

were from the main group. Ninety-seven of the measurements were for the halogens and oxygen. The data are summarized in Table 8.1. The first column is the current ‘‘best’’ value. The simple average is given in the second column and the weighted average in the third column. Given in the fourth column are the weighted average of the values determined by photon methods up to 1975 [1–6]. Figure 8.1 is a precision and accuracy (P and A) plot for the Ea values given in Table 8.1. The ‘‘simple average’’ values have a slightly larger zero intercept slope than do the weighted averages primarily because of the Ea of the halogens. The weighted average of the values determined up to 1975 is equal to the current best values within the experimental errors. The outliers in the earlier values are indicated by the large deviations from the unit slope line. For subsequent averages these values will automatically be weighted out by their uncertainties.

EVALUATION OF ATOMIC ELECTRON AFFINITIES

171

Figure 8.1 Precision and accuracy plot of atomic electron affinities measured up to 1971. The simple average and weighted average are shown. The simple average has a zero intercept slope of 1.08, indicating systematic errors. The random uncertainties are large, indicating possible outliers. The weighted average values have a slope of 1.04 because some photodetachment values are included. The weighted average values reported up to 1975 indicate no systematic or significant random uncertainties.

Table 8.2 lists the best confirming Ea for the main group elements determined by the photon and surface ionization methods for selected atoms. These are the weighted averages of the values determined by the method up until that time [1–13, 18–29]. Many of the current ‘‘best’’ accurate and precise values have been determined from photodetachment thresholds. The random uncertainties range from 50 parts per billion (ppb) for 0 to 10 parts per million (ppm) for Ir. The AEa of N, Be, and Mg and the rare gases are slightly positive and have not been determined by photon methods. A complete list of the Ea and random uncertainties for the elements is given in the appendix.

The photodetachment threshold values for Ga, As, and Pb (0.30(15), 0.81(3), and 1.10(5) eV, respectively) were determined by an older photodetachment technique [30]. The PES values for Ga, 0.43(3), and As, 0.814(8), are more precise, but are supported by the PD values [31, 32]. The PES value for Pb, 0.364(8), is significantly lower than the PD value, 1.10(5) eV. The photodetachment values for the majority of the other main group elements have also been confirmed by photoelectron spectroscopy. The values for the halogens have been determined by most methods: photodetachment thresholds, photoabsorption, ion pair photodissociation, relative and absolute surface ionization methods, and the Born Haber cycle. Values for H, Li, C, F, Cl, Cu, Ge, Br, Nb, Ag, Sn, I, W, Re, Au, and Pb have also been determined by relative and absolute surface ionization methods, as shown in Table 8.2.

172 SELECTION, ASSIGNMENT, AND CORRELATIONS OF ATOMIC ELECTRON AFFINITIES

TABLE 8.2 Atomic Electron Affinities (in eV) Determined by Photodetachment, Photoelectron Spectroscopy and Surface Ionization Techniques [4, 7–12]

 

 

PD

PES

SI

 

 

 

 

 

H

1

0.7542

0.754

0.8(1)

Li

3

0.6182

0.620(7)

0.8(2)

B

5

0.2797

0.28(1)

C

6

1.2621

1.268(5)

1.24(2)

O

8

1.4611

1.462(3)

F

9

3.4012

3.399(3)

3.40(2)

Na

11

0.5479

0.546(5)

Al

13

0.4328

0.46(3)

Si

14

1.3895

1.385(5)

P

15

0.7465

0.743(10)

S

16

2.0771

2.077

2.09(7)

Cl

17

3.6127

3.615(4)

3.67(4)

K

19

0.5015

0.5015(5)

Ca

20

0.0245

Cu

29

1.2358

1.226(10)

1.18(6)

Ga

31

0.30(15)

0.43(3)

Ge

32

1.2327

1.2(1)

1.14(3)

As

33

0.814

0.80(5)

Se

34

2.0207

2.0206(3)

Br

35

3.364

3.364(4)

3.49(2)

Rb

37

0.4859

0.486

Sr

38

0.0521

Ag

47

1.3045

1.303

1.38(10)

In

49

0.404

0.3

Sn

50

1.1121

1.15

1.16(5)

Sb

51

1.0474

1.07

Te

52

1.9709

1.9708

I

53

3.0590

3.059

3.07(2)

Cs

55

0.4716

0.4716

Ba

56

0.1446

Au

79

2.3086

2.309

2.34(10)

Tl

81

0.38(1)

0.3(2)

Pb

82

1.10(5)

0.364(8)

1.05(8)

Bi

83

0.9424

1.1(2)

Fr

87

0.491(5)

0.46

 

 

 

 

 

In 1957 a thermal charge transfer reaction showed that the Ea(Pb) > Ea(Sb). Since it is now known that the AEaðSbÞ ¼ 1:05 eV, the value for Pb is 1.1(1) eV. In 1971 a surface ionization procedure established that EaðAgÞ EaðPbÞ ¼ 0:1ð2Þ eV so EaðPbÞ ¼ 1:2ð2Þ eV. In 1981 an electron beam dissociative electron attachment experiment gave a value of 1.2(1) eV. These three determinations confirm the

EVALUATION OF ATOMIC ELECTRON AFFINITIES

173

photodetachment value of 1.10(5) [24, 33, 34]. An Ea for Pb was determined to be 0.364(8) eV by photoelectron spectrometry in 1981 [35]. The weighted average of

the four higher Ea is: ðA ¼

½a=s2&=N; N ¼

½1=s2&; S2 ¼ 1=NÞ : A ¼ 1:1=0:01þ

 

:

= :

þ

: =

:

 

þ

 

:

 

Þ

¼ ð

2

þ þ þ Þ ¼

 

 

1

 

04

 

1

 

P

P

100

100 25 400

1:10

 

2

0 01

 

1 2 0

 

 

 

10=:0025 =N and N

 

 

0:04, where the weights are given by wi ¼ ð1=siÞ . The weighted average of the three thermodynamic values is 1.10(5) eV. The ground-state electron affinity for lead is assigned the value 1.10(5) eV. The PES value does not overlap at the 14s level n ¼ d=s ¼ 0:7=0:04 ¼ 18. There are two possible explanations. The PES value may be in error by more than normal for photoelectron spectroscopy data, or it could be for an excited state. Because there are four independent determinations of the higher value that agree within the random uncertainty, the possibility of two states must be considered. This is especially likely since the other Group IV elements have low-lying bound excited states.

Bound excited-state Ea have been measured for Al, P, C, Si, Ge, and Sn. The ground state for all the anions of the Group IVA elements is the 4S3=2 state. For C(-) the 2Dm state is observed with an Ea of 0.035 eV. Two excited states are observed for Si(-): The 2D state Ea for is 0.523(5) eV and the 2P state is 0.029(5) eV. For Ge(-) and Sn(-) the 2Dm state Ea is about 0.4 eV. Thus, for Pb(-) it is reasonable that the Ea of the 4S3=2 state is 1.10(5) eV and the Ea of the 2Dm state is 0.381(8) eV. The ground state for Al(-) is 3P0 with an Ea of 0.428 eV and the excited state 1D2 (m) has an Ea of 0.12(3) eV. The Ea for P(-) are a ground state of 0.7465(3) eV and a 1D2 of about 0 eV. Many other possible excited valence states with negative Ea have been observed. These have been discussed extensively in the literature [11, 36].

Other reported values of atomic Ea are larger than the selected values. For indium the 1998 laser PD value is 0.404(9) eV, an earlier PD value is 0.3(2) eV,

and an electron impact

value is

0.85(30). The

three values overlap at the 2s

level.2 The

weighted

average

is

 

A ¼ ½0:

4040

0

:

009

2

þ 0:30:2Þ

2

þ 0:85=

2

 

2

 

 

 

Þ 2

 

ð0:3Þ &=ðNÞ,

where N ¼ 10:009Þ

 

þ 10:2Þ

 

þ 10:3Þ

and A ¼ 0:4042

0:009 or 0.404(9). None of the data are discarded, but the ‘‘best’’ estimate of the Ea is the properly weighted average [8–13, 39, 40].

It is generally agreed that nitrogen and the rare gases will not form bound valence-state anions because of their closed-shell configurations. The ‘‘best’’ adiabatic electron affinity for the elements for which Ea is a slightly positive value due to the polarization attractions is 0þ. This is consistent with the precise definition of adiabatic electron affinity. It is more accurate than the statements ‘‘less than zero,’’ ‘‘does not exist,’’ or ‘‘is unstable.’’ It was once believed that the Group IIA and IIB elements did not possess bound anion states. Subsequently, small positive Ea were measured for Ca, Sr, and Ba.

The electron affinities of atoms are presented in Figure 8.2 in the form of a Periodic Table. This format will be used to concisely present the data, whereas the complete values will be given in the appendix. The experimental values are given with the proper number of significant figures, or with the random error in the last figure specified in parentheses. Because chemical accuracy and precision are often considered to be 1 meV, the values are only given to within one-tenth

174 SELECTION, ASSIGNMENT, AND CORRELATIONS OF ATOMIC ELECTRON AFFINITIES

of meV. The values are offered with the date of their determination or the date of their selection as the ‘‘best’’ value. Figure 8.3 plots the Ea for a given family versus the period. The values for the members of a given family are different by about 25 to 30% except for the Group VA elements. The variation is systematic, with the only major discontinuity occurring between the first and second period. Figure 8.4 illustrates the trends in the rows. It is easier to visualize the relative constancy of

Figure 8.2 Electron affinities of the elements in the form of a Periodic Table. The values in parentheses are the uncertainties in the last figure. The other statistics are given with their proper number of significant figures. The dates are those of the determination or selection as the evaluated values [10–17].

EVALUATION OF ATOMIC ELECTRON AFFINITIES

175

Figure 8.3 Plots of the Ea of the main group elements versus the period number to illustrate the consistency of the electron affinities of a given family.

Figure 8.4 Plot of the Ea of the main group elements versus the column number to illustrate the consistency of the electron affinities of a given family and across a period.

176 SELECTION, ASSIGNMENT, AND CORRELATIONS OF ATOMIC ELECTRON AFFINITIES

Figure 8.5 Plot of the Ea of the elements versus the atomic number to illustrate the consistency of the electron affinities of a given family and across a period.

the values for a given family in this plot. The only unusual data point is the very low AEa of the nitrogen atom. This can be explained by its small size. Figure 8.5 is a plot of the selected values versus atomic number for all the elements. This shows the consistency in horizontal variations. The importance of using the value of 0þ for the closed-shell elements is emphasized.

A complete review of the theoretical calculations for the electron affinities of atoms is beyond the scope of this book. The quantum mechanically calculated electron affinities of the first and second row elements—the alkali metals, Ca, Ba, and Sr—support experimental results within their mutual uncertainties. 5 meV has been determined to be the best precision and accuracy of theoretical methods for atoms [13]. For example, the calculated values for Li, Na, K, Rb, and Cs agree with the experimental values to within 5 meV. Thus, the AEa of Fr is 0.491(5) eV calculated theoretically. By the same method, the predicted value for eka-francium (element 119) is 0.663(5) eV [41]. The predicted Ea for Ra is also larger than the experimental value for Ba, 0.145 eV.

The higher value for the AEa of Pb plotted in Figures 8.3 through 8.5 fits the expected trends. The average Ea for C, Si, Ge, and Sn is 1.25 eV so that a change in the Ea of 0.7 eV from Sn to Pb is unexpected. The horizontal trend from Tl to Pb to Bi is the same as for In, Sn, Sb and Ga, Ge, As. This is clearly shown in Figures 8.4 and 8.5. The electronegativities and adiabatic ionization potentials of Pb, Sn, and Bi are virtually the same so the adiabatic electron affinities should also be the same. Finally, the higher value gives a relative bond order of 1.3 for Pb2 to Pb2 (-), the same as predicted by simple molecular orbital theory (i.e., 5/4),

Соседние файлы в предмете Химия