Ultra-violet Photoelectron Data on the Complete Valence
Shells of Molecules Recorded using Filtered
30.4
nm
Radiation
BY
A.W.
Pons,
T.
A. WILLIAMS ANDW.
C.
PRICE
Physics Department, King’s College, London WC2R 2LS Received 27th June, 1972
Ionization energies have been obtained for all the valence orbitals of some simple hydrocarbons and also some of their isoelectronic analogues in which a nitrogen or an oxygen atom replaces CH or
CH2 respectively. This required the use of a helium discharge run under conditions such that a high proportion of 30.4 run radiation was emitted. In order to utilize the full range of the 41 eV photon it was further necessary to use filters, usually polystyrene films about 100 nm thick, selectively to absorb
58.4 nm radiation.
Analysis of the results show that in many molecules the summed binding energies of the molecular orbitals built from 2s atomic orbitals may be divided up into atomic contributions which agree in magnitude with those obtained for the same atoms in other molecules, i.e., the atomic contributions
are additive as is expected from the application of simple theory. A similar additive behaviour was
also found for the orbitals built from 2p and ls(H) atomic orbitals though theory does not directly indicate this for incompletely filled orbital systems of this type.
Certain weak photoelectron bands have been found at high energies which arise from transitions to configurationallv mixed ionized states. The experimental results are compared with theoretical
predictions.
The use of radiation giving 41
eV
photoelectrons permits the determination of the ionization energies of all electrons in the valence orbitals of certain simple hydro- carbons and also their isoelectronic analogues in which an N or an 0 atom replaces a CH or CH2 group. It is of great assistance in assigning the orbitals to be able to start from the bottom of the valence shell where the lowest orbital is the in-phase combination of 2s atomic orbitals and to proceed outward to 2s derived orbitds containing successively increasing numbers of nodal planes. This is followed, and possibly overlapped, by the assignment of orbitals built from 2p and ls(H) combina- tions where, in addition to in-phase and out-of-phase combinations, the possibilities of end-on, broadside-on, in-plane and out-of-plane combinations have also to be considered. These two classes of orbitals, which we shall call s-type and p-type, occur at widely different energies in the simplest molecules, e.g., at 23 and 13 eV respectively in CH4. They spread by in-phase and out-of-phase combinations for the molecules containing many atoms but remain separated, at least in the case of simple hydrocarbons up to C5, all s-type occurring below 16-17 eV and all p-type occurring above this value (see fig. 1.)Photoelectron spectra have been used to assess the extent to which present theoretical models fit experimental data. So far attention has largely been directed to the ordering of the outer orbitals l * and in some cases very good agreement has
been obtained between the i.p. data and semi-empirical models.3* These analyses have dealt mainly with limited groups of orbitals and only in few cases with the com- plete valence shells. Only when data are available for all electrons in the valence shell can the bonding contribution of each orbital to the total binding energy of the
104
A . W . POTTS, T . A . W I L L I A M S A N D W . C . P R I C E 105
FIG. 1.--30.4 nm spectra of the methyl methanes.
molecule be properly evaluated. Once the binding energies of all the valence orbitals are known it is of interest to deduce from the observed data effective atomic orbital contributions to the total molecular binding to see if these are approximately additive, i.e., whether the sum of the assigned appropriate binding energies of the atomic orbitals associated
with
a molecule equals the sum of the binding energies of the molecular orbitals found experimentally. This topic will be dealt with in the second half of this paper.EXTENSION O F THE ENERGY RANG E O F U.P.S.
In the case of simple hydrocarbons it was found possible to fit the spacings of s-type bands to simple Hiickel t h e ~ r y . ~ Fig. 1 shows how, in the methyl methane series, the s-type bands arise from progressive in-phase, out-of-phase combinations. Since the highest i.p. involved for the hydrocarbons is 26eV, all the data can be obtained with unfiltered HeI/HeII radiation. A 26 eV i.p. corresponds to a photo- eIectron energy of 14 eV from a 40 eV photon and no photoelectrons of this high
energy can be obtained from a 21 eV photon since the lowest i.p.
of
the molecule exceeds 21-
14 = 7 eV. Thus no overlapping of the energies of the He11 photo- electrons excited from inner orbitalswill
occur with He1 photoelectrons excited from106 P . E . SPECTRA O F VALENCE S H E L L S
outer orbitals. This is no longer true if N or 0 atoms are incorporated in the mole- cule, since the binding energies of their 2s electrons are in the range 27-35 eV and the associated He11 photoelectrons will have energies 13-5 eV which overlaps the He1 photoelectrons ionized from the outer orbitals of these hetero molecules. It would
Frci.[2.-30.4 nm spectra of molecules containing nitrogen.
be desirable to have sources emitting strong lines in the region of 100 eV to study the valence orbitals of all molecules under good resolution (e.g., molecules containing fluorine), but satisfactory sources have not yet been found. X-ray sources such as
A . W. POTTS, T . A . WILLIAMS A N D W . C. PRICE 107 MgKa and AlKa do not permit high enough resolution to make them adequate for this purpose.
A relatively pure 30.4
nm
photon beam was produced by placing a thinf
i
l
m
of polystyrene (approximately 100 run thick) in the photon beam from a low pressure helium discharge emitting both He1 and He11 lines. The photoelectron spectra of the hydrocarbons had suggested that a simple hydrocarbon polymer might prove to be an effective filter since it was likely to have a number of strong Rydberg bands in the 21 eV region capable of absorbing 58.4 nm radiation but no bands in the 40.8 eV region which would absorb 30.4 nm radiation. Polystyrene was also chosen because thin films could be readily formed from it.FIG. 3.-30.4 am spectra of molecules containing oxygen.
The spectra obtained for a number of oxygen and nitrogen containing molecules using filtered 30.4 nm radiation are shown in fig. 2,3, and 4. In all cases the 58.4 nm spectra have been effectively removed. Photoelectrons were accelerated by an average of 6 V prior to analysis to increase the signal for the high i.p. (i.e., low photoelectron
108 P . E . S P E C T R A OF V A L E N C E S H E L L S
energy) bands. This produces a rising background at low electron energies due to the extraction of low energy scattered electrons from the ionization chamber.
A study of the halogeno-methanes was also attempted using filter techniques. It proved less successful than the study of oxygen and nitrogen containing hydrocarbons. This is to be attributed to the low photoelectron cross section of the inner valence
FIG. 4.--30.4 nm spectra of molecules containing oxygen.
shells of halogen containing molecules. The fall in the intensity ratio of s and p type bands for elements going from left to right across the periodic table has already been noted and is no doubt responsible for the difficulty in observing bands associated with halogen valence s orbitals. It is significant that in fig. 2,3, and 4, oxygen valence s bands are, in general, less intense than nitrogen valence s bands in agreement
with
the intensity trend. It is apparent that the observation of inner valence shell orbitals with
U.P.S.
still presents a number of problems largely associated with the low crossA . W. P O T T S , T . A . W I L L I A M S A N D W . C. P R I C E 10 9 sections of these orbitals. It is to be hoped that these will be overcome with improve- ments in filtering technique and the development of more intense and possibly shorter wavelength sources.
A N A L Y S I S OF S P E C T R A (s-TYPE BANDS)
It proved possible to sub-divide the full spectra of the molecules studied into two regions as with the hydrocarbon spectra. In the low ionization energy region the bands could be fully assigned in terms of ionization from p-type orbitals, and in the higher ionization energy range the bands were all attributable to ionization from s-type orbitals. Initial discussion will
be
limited to the simpler s-type orbital region.TABLE l.-\'ALUES OF ax IN
ev
FOR SOME ELEMENTS ( x ) OF GROUPS 111, Iv,v
A N DVI
element
4
H Fc1
Br I 0 S se Te N P As Sb C in CH3 C in CH2 C in CH C Si Ge Sn B-
39.0 25.7 24.4 21.7 32.2 22.2 21.o
18.6 27.0 19.0 19.0 17.3 22.30 21.53 20.25 19.44 18.17 18.41 16.88-
All values except cti and ct; calculated from data in ref. (15).
ct: calculated from unpublished data on the methyl methanes.
4
(7.99) 44.01 34.01 31.26 27.75 29.89 23.29 21.45 18.41 19.69 13.93 12.41 10.32 1 1.09 11.09 1 1.09 1 1.09 6.50 5.42 3.20 5.38aB calculated from data for BC13.2
An examination of the spectra (fig. 1-4) immediately shows that the s-type orbitals of a molecule possess considerable atomic character. Thus in the methyl methanes, the s region of the spectrum consists of groups of bands clustered around 22 eV binding energy. Bands at around this energy are also present whenever there is a carbon atom in the molecule (fig. 2 , 3 and 4) and can be associated with combinations of C 2s atomic orbitals. Introduction of a nitrogen atom into the molecule produces bands at around 28 eV binding energy which are clearly associated with the nitrogen 2s atomic orbital. The presence of oxygen in the molecule gives bands in the 32eV region which can be associated with the oxygen 2s atomic orbital. It is thus apparent that the inner valence shells are beginning to show some of the atomic properties characteristic of X.P.S. core level ~ p e c t r a . ~ Despite the low resolution tolerated in these experiments because of low signal, the s-type bands are shown to be broadened through the bonding interactions characteristic of valence shell orbitals. Where
110 P . E . S P E C T R A OF V A L E N C E S H E L L S
the skeletal symmetry of a molecule (i.e., neglecting the H atoms) is high, s-type bands have been assigned on the basis of the bonding order expected from the number of nodal surfaces in the molecular orbitals.
To justify further the separation of the spectrum into “ s ” and “ p ” regions, an
attempt was made to fit the vertical i.p.’s of s-type bands to the simple Huckel theory which had proved useful for the hydrocarbons. According to Huckel theory for a complete set of orbitals neglecting overlap, the summed orbital energies should equal the summed Coulomb integrals. Applying this to photoelectron spectra it should be possible, ignoring interactions between s and p-type orbitals, to break down the sum- med vertical ionization potentials for the s-type bands of a molecule into atomic contributions, i.e., the coulomb integrals of Huckel theory. This was tested by plotting EEs the summed vertical i.p.’s (E,) of the molecules studied against Xxa% for the molecule, where a& is the vertical i.p. of the s-type band of the simple hydride XH, of the element X (table 1.) It was found that the agreement between L?Zs and Xxak could be improved if different values of
ark
were used depending on the number of hydrogen atoms attached to the carbon atom. XEs plotted as a function ofXxak
in fig. 5 shows reasonable agreement between experimental data and simple theory. Ideally, deviations should arise from the variation of as( with chemical environment, i.e., from chemical shifts,’ however, they are more probably due to breakdown in the theory that is to the neglect of overlap and of interactions between s and p orbitals. The correlation between theory and the experimental data justifies the division of the100
>
2 W @ 5 0 0 = HYDROCARBONS. o = OXYGEN CONTAINING HYDROCARBONS, a = NITROGEN CONTAINING HYDROCARBONS. 0 = MISCELLANEOUS MOLECULES 5 0 100 CxaS /eVFIG. 5.--CE3 plotted as a function of Cxcts(.
A . W . POTTS, T . A . WILLIAMS A N D W . C. P R I C E 1 1 1
spectra into two regions and provides an interesting example of an additive electronic property-
A N A L Y S I S O F S P E C T R A (p-TYPE B A N D S )
The p-type bands of molecules, in general, fall below 21 eV binding energy and are hence readily studied by use of 58.4nm radiation. For many of the spectra presented here, a full analysis of the 58.4 nm spectra already exists in the literature [ref. (I), references contained therein and ref. (S)], and it would not be relevant here to analyze in detail the p-region of the few remaining cases. Since complete vertical i.p. data were available for the valence orbitals of a number of molecules, it was decided to investigate ZE the summed ionization potentials of the valence shell of a molecule to see whether it could be expressed as the sum of contributions character- istic of the constituent atoms. Changes in this parameter between the two sides of a chemical reaction or between similar molecules might be expected to show averaged changes in the bonding of the valence shell. Such a parameter, however, has no
I 5 0
\%
3
100
F-
H2O2-A/
SbBr, THAN FLUORIDES. o = OXYGEN CONTAIN IN G MOLECULES. A -NITROGEN CONTAINING MOLECULES. )Q = HYDROCARBONS.SO
I60
Iso
&a$/eVFIG. 6 . 4 4 ZE’ plotted as a function of &ag.
112 P . E . SPECTRA OF VALENCE S H E L L S
immediate theoretical significance, being associated with an incomplete group of orbitals and not, as was the case with the s-type orbitals, with a complete group. In order to examine the change in ZE for a chemical reaction, it was assumed that the
additivity property of ZEs was general for all molecules. The change in A E should
2 0 0
150
-
2
@
W100
SO
CF4--+
SIF4-
+
+ -
SbF3+
31 FLUORIDES. x = MOLECULES CONTAINING A CN GROUP. o = DIATOMIC AND TRIATOM IC MOLECULES WITH MULTIPLE BONDS..
5 0
100
I 5 0&ag/eV
FIG. 6 . 4 6 ) XEp plotted as a function of Exas.
Note.-For the calculation of CE,, data were taken from ref. (l), (2), (9) and unpublished results of this group. Where a broad photoelectron band is associated with several orbitals the centre of gravity of the band has been taken as the vertical i.p. and associated with the appropriate number of orbitals
.
then equal that for ZEp the summed vertical i.p.s of all p-type orbitals. On this assumption it was possible to investigate the changes in ZE for a far wider range of molecules than would have been possible had the investigation been restricted to those molecules for which complete valence shell data were availabie.
A . W . POTTS, T . A . WILLIAMS A N D W . C . PRICE 113
An investigation of the changes in XEp occurring for a number of simple reactions such as the chlorination of CH4 in the gas phase :
CH,+Cl, = CH3Cl+HCl
indicated that in many cases changes were small or insignificant. This suggested that, although there were exceptions, for many molecules XEp depended only upon the atoms present and hence could be broken down into atomic contributions. As with the s-type orbitals the contributions ah were deduced from the known i.p.s of the simple hydrides of the elements. Thus af; = 15.98/2eV, where 15.98 eV is the vertical i.p. of the (1uJ-l band of H;,l and a& = 2 x 12.78+ 16.44-a& = 34.01 eV where 12.78 eV is the vertical i.p. of the (1-71)-l band of HC1+ and 16.44 eV is the vertical i.p. of the (24-l band of HCl.15 In this treatment, the hydrogen 1s atomic orbital is treated as ap-type orbital. From the values of a; found in this way [table 11
&ag can be calculated for any molecule and compared with the value of ZEp. This has been done in fig. 6(a) and 6(b) where values of Zxaf; and CE, have been plotted against one another for a large number of molecules. The agreement in fig. 6(a) can be seen to be relatively good but that in fig. 6(b) is less satisfactory. From the division of molecules between the two figures it appears that agreement between Z& and
ZED is best in molecules not containing fluorine and without strong multiple bonding, It should be noted, however, that agreement is in general good for unsaturated hydro- carbons including the few benzene derivatives investigated and that large deviations only occur where three or four fluorine atoms surrounded a central atom. Thus, for CH3F and CH2F2, A is small (fig. 6(b)) where A = ZEP-Zx&. It therefore appears that ZEp for a molecule may be broken down into atomic contributions.
D E V I A T I O N S B E T W E E N ZEp A N D Exas ( M U L T I P L E B O N D I N G )
As has already been noted, A = ZEP--Zx& is large where strong multiple bonding exists in a molecule (see fig. 6(b)). Thus for
N2,
A =+
10.12 and for C 0 2 A =+
9.99, i.e., for both these cases A is large and positive. For CS2 where multiple bonding might be expected to be weaker, A = +2.67 and for many unsaturated hydrocarbons it is also small. Thus for the series CzH6, C2H4 and C2HZ, A has values of -0.82, -0.44 and+
1.14 respectively. Predictably, with the increase in bond multiplicity along the series, A becomes progressively more positive. The negative value of A for C2H6 seems to be characteristic of most saturated hydrocar- bons.The large positive value of A observed for molecules with strong multiple bonding is presumably to be associated with the large increase in electronic stability (i.e., greater effective shielding of positive framework by negative cloud) compared with the more extended situation that Z,,af; represents. The anomalously small values of A observed for unsaturated hydrocarbons indicates an approximate balance between the loss in bonding nature of the cr bond and the gain in bonding nature of the n bond over the range of interatomic distances involved. For smaller atomic distances the gain of " electronic stability " of the n bond more than compensates for the loss of
that of the cr bond, cf. N2.
N O N - C L A S S I C A L n B O N D I N G A N D d--71 B O N D I N G
Besides being characteristic of classical multiple bonding, positive values of A also appear to be associated with any molecule where a single bond is strengthened. For many fluorides, positive vafues of A occur, i.e., for the series CH3F, CH2F2, CHF3, CF4 [A =
-
1.81,+
1.49, +7.12, and+
16.95 respectively. The low values for114 P.E. S P E C T R A OF V A L E N C E SHELLS
CH3F and CHzFz suggest that the large positive values for CHF3 and CF, are not due to an inductive effect but are due to bonding between the shell of fluorine atoms i.e., through-space bonding. This is supported by the drop in the value of A to +2.16 for CC14 being a result of the increased interhalogen distance and consequent loss of interhalogen bonding.
A further important effect that can be noticed from the study of halides is that accompanying d--n bonding. Values of A for the molecules CF,, CCl, and NF, are all anomalously low by comparison with the trends shown by the series SiF, and GeF, ; SiC14, GeCl, and SnCl, ; PF,, AsF, and SbF,. In each case, the low value can be ascribed to the absence of d--n bonding in the molecules
XY, (X
being a first row atom), and its presence in other members of the series. Similar anomalies also occur for the series XH3Y and XH2Y2 where X = C, Si, Ge. For the heavier members of the series, the smaller effects of d--n bonding due to increased bond lengths are reflected in the low values of A. Anomalies arising from d--n bonding have previously been noted in the U.P.S. analysis of many of thesemolecule^.^-^^
FIG. 7.-30.4 nm spectrum of Nz showing configuration interaction bands.
From the examples discussed, it is clear that deviations from the additivity pro- perty of the summed orbital energies can be used to comment on anomalies in bonding between particular molecules. The additivity property may also, in certain circum- stances, be useful in identifying complete groups of bands and hence assist in spectral analysis. As mentioned, the empirical parameter al; has no basis in existing molecular orbital theories and doubtless the actual division of valence shells into s and p-type orbitals is an oversimplification. However, the results presented do possess the merit that they appear consistent with the empirical data and would seem to suggest that
A . W . POTTS, T . A . WILLIAMS A N D W . C . PRICE. 115
for most molecules the contribution to " electronic stability " ZCE of a molecule arising
from a given atom is a constant and is an additive property. In XEp for a molecule, it appears that we have a molecular parameter somewhat analogous to other molecular properties such as molecular refractivity, total molecular bond energy etc. which can be expressed approximately as the sum of characteristic atomic (or bond) contributions.
CONFIGURATION INTERACTION B A N DS
Finally, we should like to report the application of filtered discharges to the study of complex ionized states which arise from the interaction of simple states of the same symmetry but having different configurations, i.e., from configuration interaction. 2 *
These states correspond to ionization plus excitation of the ion and are generally more readily observed by X.P.S. than U.P.S. However, the extension of the range of U.P.S. to 41 eV has made it possible to observe a number of these states for various molecules. Fig. 7 shows the photoelectron spectrum of N2. The lowest of the configuration interaction bands corresponds to the well-known C state of N; (prob- able valence shell structure (la,)2 (10,)~ ( 1 7 ~ ~ ) ~ ( 2 0 ~ ) ~ (17~~)').'~ The two bands to higher energies appear to coincide with those observed for N2 by Siegbahn et a l l 4 using A1
Ka
radiation. The first of these, with a maximum at 28.9 eV, appears to be the convergence limit of the bands found by Codling l 7 in the region 500-400&which are the strongest absorption systems observed for N2 in the region below 500
A.
There are interesting variations in the intensities of the X.P.S. and U.P.S. bands and it is to be hoped that further work of the type recently carried out by Lorquet 12* l 3will help to elucidate the nature of these bands and the states from which they are derived.
We are grateful for financial assistance from the Science Research Council, the Institute of Petroleum and Imperial Chemical Industries, in carrying out this work.
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A. W. Potts, H. J. Lempka, D. G. Streets and W. C. Price, Phil. Trans. A, 1970,268, 59. J. H. D. Eland, Int. J. Mass. Spectr. Ion Phys., 1969, 2,471.
F. Brogli, E. Heilbronner and T. Kobayashi, Helv. Chim. Acta, 1972,55,274.
A. W. Potts, D. G. Streets and W. C. Price, Inst. Physics Conf. Photoionization Phenomena and
Photoelectron Spectroscopy (Oxford 1970), p. 22.
W. C. Price, A. W. Potts and D. G. Streets, Electron Spectroscopy, ed. D. A. Shirley (North
Holland Pub. Co., 1972), p. 187.
'
K. Siegbahn, et al., Electron Spectroscopy for Chemical Analysis. Nova Acta Reg. SOC. Sci.Uppsala, 1967, Ser. IV, 20.
A. B. Cornford, D. C. Frost, F. G. Herring and C. A. McDowell, C u d . J. Chem., 1971,49,
1135.
S . Cradock and R. A. Whiteford, Trans. Faraday SOC., 1971, 67, 3425.
l o D. C. Frost, et al., Canad. J . Chem., 1971, 49, 4033.
''
J. C. Green, et al., Phil. Trans. A , 1970,268, 111.l 2 J. C. Lorquet and C. Cadet, Chem. Phys. Letters, 1970, 6, 198.
l 3 J. C. Lorquet and M. Desouter, to be published.
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(a) H. J. Lempka, T. R. Passmore and W. C. Price, Proc. Roy. SOC. A , 1968,304,53 ; (6) A. W. Potts and W. C. Price, Proc. Roy. SOC. A , 1972,326, 165; (c) A. W. Potts and W. C. Price,
Proc. Roy. SOC. A, 1972,326, 181 ; ( d ) A. W. Potts, unpublished results.
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