Experimental auger electron spectrum of ammonia

CHEMICALPHYSICSLETTERS

Volume 46, number 1

15

February 1977

EXPERIMENTAL AUGER ELECTRON SPECTRUM OF AMMONIA *

J.M. WHITE **, R.R. RYE and J.E. HOUSTON Snrrdin Laboratones, Albnquercp, NCw Mexico 87115, USA Received 8 October

1976

The first electron excited Auger electron spectrum for gas-phase ammonia has been obtained and is compared with an existing theoretical prediction. Decomposition of the spectrum using a gaussian approximation rev&s close agreement in both intensity and energy for the higher energy components provided the theoretical data is shifted toward lower energies by = 2.7 eV. As the transitions involve progressively deeper fmal state Ievels there is a general trend of increasing widths and increasing discrepancy towards lower energy with respect to their predicted positions. Such behavior has been seen in other experimental-theoretical comparisons for gas-phase Auger results and whiIe the variation in widths is interpreted in terms of transitions to a manifold of final state vmrational IeveIs, the nature of the differential shift in the Iower energy components is unclear.

1. Introduction Auger electron spectroscopy (AES) of gases, as opposed to its more widely used application to solid surfaces, affords a unique view of molecules. As Brundle has pointed out for the case of CO, the Auger spectrum yields an atomic identification of the carbon and oxygen, a fingerprint of the CO molecule from the detailed Auger line shape, and a means of studying the nature and parentages of the various molecular valence orbitis [ I]. Moreover, the Auger process is highly localized on the atom which contains the primary vacancy [2] and, as a result, Auger transitions involving the vaIence electrons provide a uniquely localized view of bonding. Despite this potential wealth of information and the widespread use of electron excited Auger spectroscopy in surface science, there has been only limited application of AES to gaseous molecules. To our knowledge electron excited Auger spectra have been obtained only for some two dozen molecules (2-61. Recently Okland et al. [7] published a calculated

* This work was supported by the Division of physical Research of the U.S. Energy Research and Development Administration (ERDA) under Contract E(29-1)789. ** Summer Visiting Faculty. Permanent address: Department of Chemistry, University of Texas, Austin, Texas, USA.

146

Auger spectrum for ammonia, ammonia being the one molecule remaining in the isoelectronic series Ne IS], HF [43 , H,O [6] , NH,, CH, [3,6] for which there is no experimental data. In the present paper we report the first electron excited Auger electron spectrum for gas-phase ammonia and demonstrate that excellent agreement is found with the higher energy components of the predicted spectrum in both intensity and energy position if the theoretical spectrum is shifted uniformly downward by = 2.7 eV. For the lower lying structure the peaks are found to increase in width with increasing binding energy and to be progressively higher in energy with respect to their predicted positions. These effects are discussed with respect to the vibrational excitation of the molecule and the results of other valence-level probes.

2. Experimental Fig. 1 shows a schematic drawing of the experimental apparatus used in the present work. The electron energy analyzer consists of a retarding, single-pass cylindrical mirror described eariier by Cerlach and Tipping [9] - The combination of retarding and cylind&l mirror analysis of the electrons allows the data to be obtained with a constant energy resolution. In the present

VoIume 46, number 1

CHEMICAL PHYSICS LETTERS

I

m3Rcl’r CL?‘.

Fig. 1. Schematic drawing of the experimental apparatus.

case the resolution was set at 1.5 eV, which was found to be adequate in view of the large inherent widths found for the ammonia peaks. A critical feature in obtaining gas phase Auger spectra is the maintenance of a high gas density at the focal point of the analyzer and a relatively low pressure within the analyzer itself. The former is required in order to obtain adequate signal strengths and the latter to reduce the background due to electron-molecule scattering within the analyzer. This is especially critical in the present system in that the analyzer is basically designed for solid state work and no provisions were made for internal pumping of the cylindrical mirror analyzer. To localize high gas density at the focal point, the analyzer was modified by replacing the normal coaxial electron gun with a gas nozzie constructed from a #22 hypodermic needle (length, 35 mm; inside diameter, = 0.5 mm). The axis of the needle is co-axial with that of the analyzer and the tip extends slightly past (= 1.5 mm) the analyzer face. Molecules leaving this needle pass through the analyzer focal point and proceed directly into a LN, cooled titanium sublimation pump located approximately 10 cm from the nozzle. In addition to this high gas-load pumping arrangement the system was pumped by a LN, trapped 5 cm oil diffusion pump which was capable of maintaining background pressures of = 1 X IO-’ torr. The subhmation pump was ca able of obtaining ultimate pressures in the low 10-l 1 torr range while in operation the system pressure rose to z 5 X 10e5 torr. No actual measurements were made of the effective pressure at the tip of the needle but it is estimated that the molecular density at this point was about 200 times greater than that of the background. The electron gun (Physical Electronics Industries

15 February 1977

model 04-015) was mounted perpendicular to the axis of the analyzer such that the electron beam passed parallel to the analyzer face approximately I.5 mxu in front of the nozzle tip. In practice the optimum position was determined experimentahy by maximizingthe signal due to electrons elastically scattered from the gas. In order to reduce electron scattering from other parts of th* system the electron beam was completely enclosed in grounded shields except for the region around the entrance aperture of the anaIyzer_ After passing the tip the beam was collected in a biased Faraday cup. Typically the electron gun was operated at 1.5 kV with a beam intensity of x 30-60 PA. The intensity of the electron beam was chopped by periodically driving the gun’s control grid heaviiy negative with respect to the cathode. This ahowed the N(E) spectrum to be synchronously detected by a tack-in amplifier. In order to mitigate slow variations in the molecular flow rate from the nozzle, data in this mode was accumulated by a Nicolet model 1074 signal averager over many scans each requiring approximately LC s. The voltage range was swept over approximately 100 eV at a rate of 10 V/s and the lock-in ampkfiet time constant was set at 10 ms (6 dB/octave). Ammonia, obtained from Matheson, was used directly from the tank without further purification. Composition of the gas phase was monitored using a quadrupole mass analyzer which indicated ammonia as the major gas phase species. Since the Auger eIecCron is specific to a given atom, the question of purity is not as critical as for other spectroscopies; in this case ody other nitrogen compounds would interfere.

3. Results The experimental Auger spectrum of ammonia is given as the solid curve in the top panel of fig. 3,. This spectrum results from the accumulation of a total of 384 sweeps or a total accumulation tune ofapprosimately 1 h 45 min. The spectrum is actually observable after only a few sweeps with the additional accumulation being used to increase the signal-to-noise ratio. The energy scale has been corrected for the work function of the analyzer by assuming a value of4.5 eV; with this assumption the absolute eneru scale is expected to be accurate to within t I eV. The relative accuracy is considerably better. The experimental data 147

CHEMICAL PHYSICS LETI’ERS

Volume 46, number 1

fLuRON

E1ERcY

iv

Fig. 2. The Auger electron spectrum of gas-phase ammonia. In the top panel the solid curve and the bar graph are, respectively, the experimental and the calcnIated I71 Auger spectra. The latter has been shifted 2.7 eV toward lower energies. The lower panel shows the gaussian peaks used in decomposing the experimental spectrum. The sum of these is the dotted cvrvein the upper portion of this fire.

IS February 1977

iF superimposed on a rising background, an effect probably due to gas scattering of the primary electrons onto the tip of the needle. Thii rising background was removed to yield the curve in the top panel of fig. 2 by subtracting a linear ramp whose slope was equal to the slope of the data on the high energy side of the ammonia structure. ‘l&e bar graph superimposed on the experimental data represents the spectrum calculated by OkIand et al. [7]. We have shifted this calculated spectrum uniformly downwards in energy by 2.7 eV in order to bring the hi& energy structure into closer overaI agreement. The experimental spectrum has been decomposed into component peaks using a gaussian approximation with a low energy tail equal to 114% of the area under each peak. The dotted curve in the top panel of fig. 2 represents the sum of these gaussian components found to most closely approximate the experimental result. The lower panel of fig. 2 shows the individual components. The peak energies, relative intensities (peak areas), and fuE widths at hatf maxima obtained from this decomposition are listed $I table 1 along with the calculated values given by Okland et al. 17) shifted down in energy by 2.7 eV. The experimental spectrum can be synthesized quite well using only 7 peaks while a total of 11 are theoretically predicted. However, the

Table I Auger eIectron transition

energies, relative intensities,

Energy (eV) experimental

Energy a) (eV) caIculated [7]

371.5

(a)b)

366.6 360.2 356.5 352.4 345.1 337*0

(b) (cl (d) (el Q @ 01) (i> 0 (k)

371.0 368.8 366.9 362.7 360.4 358.2 356.1 350.2 350.1 342.4 331.3

and full widths at half maximum

Relative intensity calculated 171

Relative intensity experimental

Width c) (eV)

Final sh te [71

0.50

0.55

1.8 (0,YY)

0.02 1.0 0.02

0.89

2,O (1.32)

1.0 0.14

3.2 (2.83) I.5 (0.00)

0.45

2.5 (1.00)

0.29

3.0 (2.60)

3a;2 le’3a; le-‘3ai’ le-’ 1;’ 1G-= 2a;‘3a;’ 2a;‘3a;’ 2a-&? Zg;le-’

0.17

3.5 (3.16)

2&e-’

1.0

0.27 0.06 0.14 0.12 0.29 0.17

a)Peak position from ref. [ 71 shifted down in energy by 2.7 eV. b, Letters here identify the individual theoretical lines in the top panel of f’ii. 2. c) Numbers in parentheses are the true widths of the transition.

148

IAt 3E ‘E ‘A2 ‘E IA1 ‘At ‘AL 3E ‘E ‘At

Volume 46, number 1

CHEMICALPHYSICSLETTERS

peaks not observed in the experimental spectrum correspond to components which theory predicts to have relative intensities of less than lo%, and these would be within the noise level of the experimental data. Considering the level of sophistication of the relative intensity calculation, the degree of agreement with experiment is excellent_ The ‘Al and 3E lines, which the theory predicts to be separated by only 0.1 eV, are both contained in the experimental peak at 352.4 eV. The peak in the decomposition which appears at 356.5 eV shows up in tlie data as a shoulder on the 360.2 eV component. Compared to theory, its position is 2.7 eV lower while the general trend seems to favor the theoretical peaks being increasingly lower as deeper levels are involved in the transitions. In addition, the width of this peak also appears to be counter to the apparent general trend toward peaks of increasing width for transitions involving deeper levels. The source of this apparently anomalous behavior is most likely due to the fact that the signal-to-noise of the experimental data does not permit a closer specification of these particular components. The half width values given first in column five of table 1 were determined from the decomposition of the experimental spectrum, and contain the contribution due to instrument broadening (1.5 ev). Estimates of the true width valdes (given in parentheses in column five) were calculated under the assumption that the peak widths accumulate as the square root of the sum of the squares of the bdividual components [lo]. This is strictly true only of peaks having a gaussian shape and these values should only be taken as approximate.

4. Discussion In general there is rather good agreement between the experimental results and theoretical predictions shown in the top panel of fig. 2. In fact, the absolute energies of the various peaks are predicted to an accuracy of less than 1% and there is no difficulty in assigning experimental peaks to specific electronic transitions in the molecule. The differences in relative intensities appear to be random with major variations occurring only for the 356.5 and 352.4 eV components (the former case undoubtedly the result of inaccuracies in the spectral decomposition). This overall level of

I.5 Febnrary 1977

agreement in the relative intensities is especially pleasing in view of the fact that the theoretical values were obtained from an empirical comparison with the gas-phase Auger results from neon. Viewed in greater detail, however, the data of fig. 2 reveals several additional pieces of information. First, the closest overall agreement is obtained only after a uniform shift of the theoretical spectrum toward lower energies by 2.7 eV. Second, there appears to be a general trend in the data with respect to the widths of the individual peaks. The components near the high energy end are narrowest with the widths increassing toward the low energy end of the spectrum. Third, the positions of the peaks appear to increasingly deviate toward higher energies with respect to the predicted positions for transitions involving deeper valence levels. Finally, note should be taken of the fact that the multipeaked structure appears to be superimposed on a step extending toward lower energies. With respect to the absolute energy comparison, a similar uniform downward shift was also required to bring the calculated transition energies for methane into agreement with experiment. In this case it was pointed out that the shift resulted from a poor optimization of the initial core state [l l] . This is apparently also the case for the ammonia calculations in that the 1s binding energy obtained from the total energies given in ref. [7] is 1.6 eV larger than the ESCA measurement

PIAs for the variation in line widths, Shaw and Thomas [4] point out following Turner et al. [12] that the loss of valence electrons strongly involved in bonding, in contrast to non-bonding valence electrons, can lead to vibrational excitation, the extreme limit of which is dissociation. In agreement with their experimental data for HF, they expect the linewidths of Auger transitions to depend on the vacancy configuration and to become increasingly broad as the number of bonding electrons involved increases. An identical effect is seen in the ESCA spectra of valence electrons for gas-phase molecules (see, for example, refs. [8,12]). In the case of ESCA this effect is due to vibrational broadening with numerous examples of resolved vibrational structure [8,12,13] _In fact vibrational!;r resolved spectra for ammonia have been reported [12]. ln the case of ammonia, the 3a, electrons are non-bonding while the rest are bonding. With the exception of the 356.5 eV peak, which apparently is anomalously narrow due to 149

Volume 46, number 1

CHEMiCAL PHYSICS LETTERS

problems in resolving this peak, the ammonia data does show this trend of sharper peaks for non-boning transitions. The peak at 371.5 eV, resulting from a final state missing two non-bonding electrons, is the sharpest peak in the spectrum; the next two in width, 366.6 eV and 352.4 eV, invoIve one bonding and one non-bonding electron; and the three broadest peaks, 360.2 eV, 345. I eV and 377.0 eV, involve final states missing two bonding electrons. In the case of methane f3,6], where all electrons are involved in bonding, the peaks where found to be equally broad. Thus, it would appear that the broadening involved in Auger transitions is consistent with vibrational excitation in the final state. It also seems reasonable to associate the low energy tail on the experimental spectrum with transitions to states which resuft ir molecular dissociation. W& respect to the fact that there is an increasing discrepancy between the theory and experiment for the tighter bound valence levels, similar effects are observed in other comparisons between experiment and theory. Provided +&he theoretical transition energies are shifted uniformly for agreement with the high energy peaks, as in the case of ammonia, a differential shift is observed for HF [14], H,O [ 14-161 and CO [ 141 with the shift being in the same direction as found for ammonia. The case of methane is an apparent exception with a uniform shift in the calculated values leading to good agreement with experiment [I 1,171. The methane spectrum, however, is composed of quite broad, poorly resolved peaks [3,6] and does not include levels with binding energies as large as these other examples. At this point, it is unclear as to whether the origin of this discrepancy is a finaI state relaxation effect or whether it is a result of the fact that the calculations are done on a rigid molecule, which are forced to maintain the geometric configuration of the initial state. However, the fact that the widths of the peaks and their deviation from the calculated position both

150

15 February 1977

increase as the transitions involve progressively deeper vafence levels might suggest that they share a common source.

References [ 1] C.R. Brundie and A.D. Baker, in: Electronic states of inorganic compounds new experimental techniques, ed. P. Day (Reidel, Dordrecht, 197.5). f2J L. Karisson, L-O. Werme, T. Bergmark and K. Siegbahn, 3. Electron Spectsy. 3 fi974) 181. [31 R. Spohr, T. Bergmark, N. Magnusson,L.O. Wetme, C. Nordling and K. Siegbahn, Physica Scripta 2 (1970) 31. [4] R,W. Shaw Jr. and T.D. Thomas, Phys. Rev. All (1975) 1491. [Sj WE. ~loddem~, T.A. Carlson, M-0. Krause, B.P. Pullen, W.E. Bull and G.K. Schweitzer, J. Chem. Phys. 55 (1971) 2317. [6] W.E. Moddeman, ORNL Report No. ORNL-TM-3012 (1970). f7J M.T. Okland, K. FaegriJr. and R. Marine,Cfiem. P&s. Letters 40 f1976) 185. [8] K. Siegbahn,C. Nordling,G. Johansson, J. Hedman, P-F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L.O. Werme, R. Marme and Y. Baer, ESCA applied to free molecules (North-Holland, Amsterdam, 1969). [9] R.L. Gerla& and D.W. Tipping, Rev. Sci. Instr. 42 (1971) 1519. [lOI R. Hosemann and S.N. Bagchi, Direct analysis ofdiffraction by matter (North-Holland, Amsterdam, 1962) p- 62. [ll] K. FxgriJr. and R. Marine,Mol. Phys. 31(1976) 1037. [12j D.W. Tumer,C. Baker, A-D. Baker and CR. Bmndie, Molecular photoelectron spectroscopy (Wiley, New York, 1970). [13] T.A. Carlson, Arm. Rev. Pbys. Chem. 26 (1975) 211. [ 141 I.H. Hillier and J. Kenrick, Mol. Phys. 31 (1976) 849. [ 151 H. Siegbahn, L. Asplund and P. Kelfve, Chem. Phys. Letters 35 (1975) 330. [ 161 H. Agren, S. Svensson and U.I. Wablgren, C&em. Phys. Letters 35 (1975) 336. [17] LB. Ortenburger and P.S. Bagus, Phys. Rev. All (1975) 1501.