Volume 15, number 2 CHEMICAL PHYSICS LETTERS 1 August 1972

X-RAY PHOTOELECTRON SPECTRA OF THE ALKALI AZIDES

J. SHARMA, Thaddeus GORA, J.D. RIMSTIDT and Ralph STALEY* Solid Sfate Braizch. Feltnmn Research Laborarov, Picatirwy Arsenal, Dover, New Jersey, USA

Received 6 .4pril 1972

Core and valence electronic levels in the alkali azides were imestigated by X-ray photoelectron spectroscopy with particu!ar emphasis on the levels associated with ‘he azide anion. Comparisons are mde to existing molecular orbital calculations, and assignments suggested.

The electronic energy structure of an alkali azide is,

to a first approximation, simply a superposition of the energy levels of the alkali cation and of the molecular orbital (MO) levels of the azide anion. This results from the ionic nature of the alkali azides. The azide

ion itself is a singly negatively charged, linear tri-atomic nitrogen mclecular entity (NT) [I]. A study of the

core and valence levels in the alkali azides accessible to X-ray photoelectron spectroscopy (XI’S) is reported here. This study allows a comparison of the X-ray

photoelectron spectra in the energy range of the azide ion’s MO levels (in an ionic environment) to reported Mb leve! calculations fol the free ion [2-61. It tests the sensitivity of these MO levels to the particular identity of the alkali metal cation and to different ;;rystal structures. And, it permits the cataloguing of

the various cationic levels in the alkali azides. This may aid in understanding the nhiure of chemical shifts and of charging effects in ionic systems [7]. Finally, ihe study serve; as a test of the value of the XPS experiment for probing the valence electron struc- ture of molecular ions.

Experiments were performed using a Varian IEE-1.5 spectrometer. Commercially available LiN,, NaN, , KN,, RbN3, and CsN3 were mounted for study either by coating on Scotch tape attached to the surface of the sample cylinder or by precipitating salts directly onto a gold sample cylinder from a methanolic solu- tion. Samples of NaN, purified by rect-y;:aUization

* Present address: Department of Chemistry, California in- stitute of Technology, Pasadena, California.

and ion exchange chromatography were also studied. Spectra were recorded using an Mg or Al anode.

Preliminary spectra indicated no differences between room temperature runs and runs in which the sample holder was cooled by liquid nitrogen, and all further data was taken at room temperature. Although the samples were visibly darkened after exposure to X rays, the spectra changed only slowly with time: the

principal effects being a uniform shifting of all levels by as much as 1 eV, arid a uniform loss in intensity in

the sample lines with respect to the contaminant lines (Cls, 01s). More serious but less frequently obsewed effects included changes in relative intensities and positions and/or broadening of some sample lines. The data reported here have been corrected to short ex- posure times to eliminate these effects.

The XPS data on cationic levels and on the azide’s core levels is summarized in table 1. Both peak posi- tions and half-widths (full width at half maximum) are displayed. All levels are reported with respect to the carbon contaminant line position which was ZZS-

signed to be 285.0 eV. In each alkali azide, two peaks are observed in the binding energy range appropriate to the N 1s level. These pea’ks’ positions, half-widths and separation show only slight variations from one alkali azide to another. The deeper binding energy peak corresponds to the N 1s level of the centml nitro-

gen atom in the linear azide ion (the ion’s lap MO level). The other peak of the pair is always present at twice the intensity of the first. It corresponds to the Nls levels of the symmetrically equivalent outer two nitrogen atoms in the azide ion (the lo, and Zag MO.

232

Volume 15, number 2 CHEMICAL PHYSICS LETTERS 1 Augwt 1972

Table 1 Experimental XPS binding erlcrgics and half-widths”)

for cationic levels and core nitrogen levels in the alkali azides (in eV)

Sample Atom Level Peak Hzlf- width

Li LiN3

N

NaN3

Na IS Nn 2s

NCl 2P

N IS

K K

KG K K

N

RbN3

Rb 3~112 Rb 3P3, Rb 3d Rb 4s Rb 4P

N IS

CS cs

C-3 CS CS

N

IS

IS

2s

3P 3s

ZQ

IS

%/z 4d 5s

5P

IS

55.44 1.73 398.9 1.73 403.28 1.67

1071.00 2.04 62.95 2.00 30.17 1.97

398.7 1.78 403.12 1.70

377.39 3.2 292.74 1.80

32.38 1.74 16.21 2.0

398.66 1.74 403.05 1.67

246.60 2.55 237.66 2.64 110.01 2.94

‘X8.9 1.94 13.95 2.14

399.30 1.72 402.6 1 1.57

723.83 75.79 23.78 10.48

398.12 402.40

2.05 3.81

-

1.75 1.57

a) Full width at half peak height.

levels). This separation is mainly due to the large dif- ference in effective charge on the inequivalent nitro- gen atoms. Our experimental value for the separation, averaged over the alkali azides, is 4.36 eV. This is within 0.1 eV of other reported values in particular alkaii azides [5,&g].

To compare calculated values of the separation in the free azide ion to experiment, those values must be corrected for the effects of the crystalline environ-

ment. The simplest approach is to correct for the fact

that central and end nitrogen atoms sit at different crystal sites, with differect Madelung potentials. For

all the alkali tides, this correction approach decreases the calculated separations. The magnitude cf the cor-

rection is 0.4 F 0.2 eV, and depends on the particular lattice, and on the choice of inter-nitrogen distance

and of the azide ion’s charge distribution. The most extensive of the MO calculations on the free azide ion (in the sense of yielding the best total energy so far reported) is the ab initio LCAO MO SCF calculation by Archibald and Sabin [6]. That work predicts a separation of 5.8 eV (5.4 eV corrected), and a calcula- tion by Wyatt et al. [5] predicts 5.1 eY (4.7 eV cor-

rected); while the earlier calculations yield considerably larger separations [Z-4]. Thus only one of the MO separation values falls within 1 eV of experiment. This might be expIained by noting that total energies converge more rapidly than particular orbital energies in MO calculations. The discrepancy between theory and experiment might also he due to other crystalline effects than the simple point-ion correction we have

made, e.g., those involving slight covalent bonding between cation and anion and readjustment of the azide ion structure in the crystal field. The five alkali azides investigated, however, span a large range of cationic radii. One would thus expect that the effects of slight covalency on the separation, if relatively large, would be different from one alkali azide to another. This is contra@ to our observations.

For a detailed study of the azide ion’s valence struc- ture, the lithium salt has two marked advantages. First, only the lithium salt has no interfering cation lines in the low binding ener,y region (see table 1). Second, the more electron-dense cations of the heavier salts

produce more scattering of the primary photoelectrons

and thus a lesser average sampling depth. This results in lower counting rates for any given region of the spectrum and also a higher background of secondary

electrons. In otir experiments, these effects substantial-

ly obscure the azide ion’s valence structure even in NaN3, and practically obliterate it for the case of CsN3.

The valence region spectrum in LiN3 was studied by scanning the binding energy range O-SO eV. Runs totalling up to 30 minutes per point (five points per eV) were made for this region, with corrections for long-term drift made by repeated scanning of a selected

core level line. The experimental spectrum was then corrected for X-ray satellite lines and for stnicture in the background produced by scattering of the primary photoelectrons. These types of corrections have been previously discussed [ 10, 1 l] : the shape of the scat-

‘33

x) 20 IO 0 BINDING EXERGY kc’)

iZig, 1. Corrected sp~c~mnl of valence molecular orbital Ievefs of the uzide ion (1\13) in lithium azide.

!ering spectrum observed for a core 1iF.e is used in cor- recting the valence spectrum for scatfering. In the present case: the N 1 s scattering shape (a deconvolu-

tion of the pair of lines discussed earlier) was used. A corrected valence spectrum is displayed in fig. I. The five peaks labciled I t.hrou& V are the major features. Another corrected spectrum (obtained from a different sample) differed somewhat in shape in the vicinity of peak III, and in the relative intensity of peak I. These

differences could be due to different extrinsic features of the samples used, or perhaps to failures in the as- sumptions used in our correction procedure. Basically

similar SpkXti3 of the LiN, valence region were recent- ly presented by us f 121 and by Barber et al. [ 131.

Barber’s spectrum [ 131 (uncorrected) differs from the present one mainly in the location of peak III. His work places that peak only 4.5 eV to deeper binding energy of peak II.

Assignments for the ob$k!rved peaks are aided by the knowledge that the cross section for X-ray photo- electron emission is substantially different for states of different symmetry. We have calculated the cross section ratio @er electron) for *he 2s and 2p levels of the ground state nitrogen atom to be = 13 (for Mg Kct X rays) using the Born approximation, Siegbahn’s work 1141 yields an experimental figure for the ratio of ~8. To the extent that pi orbit& are composed of piike states and sigma orbit& of s-like states, the

V&me 15, number 2 CHEhSICAL PHYSICS LETTERS 1 August 1972

Table 2 Calculated volt-nce motecuhr orbital cncr_gies for the

free aide ion (in eV)

Peyerimhoff Wyatt Archibald

Orbital znd Buenker [?j et al. [4] and &bin I c1

3Qg 32.5 34.0 32.9

2au 26.3 28.0 26.9

Isru 10.9 12.2 11.2

4af! 9.5 10.0 10.4

3au 7.6 8.0 s.5

lrg 1-i) 1.8 l.B

azide ion’s pi orbitals are then expected to produce XI’S lines 4 to 6 times less intense than its sigma or- bitals.

Table 2 summa&es the valence level energies pre- dicted by the MO calculations yielding the best total energies 13, 5,6] . If only the relative positions of these levels are compared to the experimentaI results, probiems with the interpretation of the zero of binding energy fl5] can be avoided. One possibIe assignment is that peak I corresponds to the ITT, orbital, peak II

contains both the 30, and 40% orbi&, and psaks IiI through V correspond to the In,, 2a, and 30, orbit&, respectively. This assignment agrees reasonably with the calculated fcvel separations, save for peak Iii. And it is supported by an examination of the line intensi- ties. With two of the sigma states contained in peak II at 10 eV, the four sigma states are seen to be ofap- proximately equal intensity. The two pi states are also equal and are about one-fifth as intense as the sigma

states, as expected. An alternative assignment is that peak fIf arises from oxygen contamination or other

extrinsic states and that the lzU level is also contained within peak II. This assignment is in better agreement with the MO &culation spacings, and is also consis- tent with the line intensities. It offers a possible ex- planation for the discrepancy be%veen our results and those of Barber [ 131 on the relative position of peak Iii, since extrinsic structure might well be sensitive to method of sample preparation or even to the XPS in- strumer.t used. And finally, focalized valence region structure possibly due to oxygen contamination has been observed in other systems (around 25 eV binding energy) El 11.

The c,@ity of the spectra obtained in this study was adversely affected by contamination of the saaples.

Volume 15, number 2 CHEMICAL PHYSICS LETTERS 1 August 1972

Future experiments clearly should include more care- ful study of the dependence of the spectra on factors influencing the physical and chemical condition of the sample surface. Considering the shallow sampling depth (X 12 A), X-ray flux, and presence of contaminants, however, it is significant that the internal electronic states of a chemically unstable molecular ion can be so readily observed. While there is as yet some question as to the details of the assignments of these levels, it is apparent that the spectrum obtained does represent the azide ion’s electronic structure. We thus may look forward with expectation to improvements in the technolo~ of sample preparation as well as instrument performance which may lead to sufficient resolution to permit the observation of s&d state effects on this electronic structure_

We gratefully acknowledge helpful comments and suggestions from Drs. R.H. Bartram, P.J. Kemmey, Z. Iqbal and H.D. Fair Jr..

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