Angle-resolved photoelectron measurements on the first two orbitals of acetylene using synchrotron radiation

Journal of Electron Spectroscopy and Related Phenomena, 27 (1982) 223-232 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands

ANGLE-RESOLVED

PHOTOELECTRON

FIRST TWO ORBITALS OF ACETYLENE

MEASUREMENTS

ON THE

USING SYNCHROTRON

RADIATION*

PAUL R. KELLER,

DAVID MEHAFFY

and JAMES W. TAYLOR**

University of Wisconsin, Madison, WI 53706 FREDERICK

(U.S.A.)

A. GRIMM

University of Tennessee,

Knoxville, TN 37916

(U.S.A.)

THOMAS A. CARLSON Oak Ridge National Laboratbry,

Oak Ridge, TN 37830

(U.S.A.)

(Received 30 December 1981; in final form 5 April 1982)

ABSTRACT The angle-resolved photoelectron spectrum of acetylene has been measured using synchrotron radiation over a range of photon energies from 12 to 28 eV. The angular asymmetry parameter 0 has been derived from data corresponding to photoionization of the In, and 30, orbitais. Theoretical calculations of fl based on the multiple-scattering theory have been carried out and comparisons with the experimental data give reasonably good overall agreement. Of particular interest is the dip in fl as a function of photon energy that occurs for photoionization of the In, orbital and is not present in our calculations. This dip in fl is - 1 V wide and occurs at approximately the same photon energy as the previously noted minimum in the partial photoelectron cross-section. Values of /.I have been determined for the individual vibrational bands in the case of lz,, photoionization. The minimum of the dip in 0 for the first and second vibrational bands (corresponding predominantly to the Cq stretching mode) has been found to occur at respectively 14.4 and 14.8 eV. The various possible explanations for this dip consistent with the present experimental data are discussed. * Research sponsored by Wisconsin Alumni Research Foundation and NSF Grants CHE81-21205 and DMR79-24555. The Synchrotron Radiation Center is operated under NSF Grant DMR77-21888. Support at Oak Ridge and at the University of Tennessee is provided by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract W-7405eng-26 with the Union Carbide Corporation and the University of Tennessee. ** Author to whom correspondence should be addressed.

0368-2048/82/0000-0000/$02.75

o 1982

EIsevier Scientific Publishing Company

224 INTRODUCTION

A question has been raised concerning the explanation of a dip in the partial cross-section as a function of photon energy, - 3-4 V above threshold, of the first band (In, - * ) in the photoelectron spectrum of acetylene. In an examination of the photoionization cross-section of this orbital in acetylene, Unwin et al. [l] observed this feature and ascribed it to a resonance without elaborating on the type of resonance involved except that it was to a state embedded in the continuum. In addition to these suggestions we here discuss other possible explanations and how they relate to the experimental data both for cross-sections and for angular asymmetry parameters. In the work reported here, the technique of angle-resolved photoelectron spectroscopy was employed to examine the first two orbitals of acetylene, 17r, and 3a,, by measuring the asymmetry parameter /? as a function of excitation energy using synchrotron radiation over a photon energy range from 12 to 28 eV. The asymmetry parameter is a measure of the angular distribution of photoejected electrons as given by the relationship do/da

= (Dtotall47Q [I+

PP, (cos0

)I

where 8 is the angle between the polarization vector and the direction of the ejected photoelectron and Pz is the second-order Legendre polynomial. This technique has been shown to be a sensitive probe of ionization processes for molecules [ 2-6 3 . It is because of this sensitivity that this technique was chosen to investigate the spectral feature corresponding to the In, orbital of acetylene. In addition, the processes of autoionization and shape resonance can have dramatic and differing influences upon the angular distribution of ‘photoejected electrons. Using line sources, Kreile et al. [7] were able to examine 0 at a few restricted wavelengths. Contemporaneously with the present study, Holland et al. [ 81 are also using synchrotron radiation, and in a similar manner. The orbitals of acetylene are interesting for several reasons in addition to those arising from the 71 orbital photoejection feature. Because acetylene is the simplest molecule containing a carbon-carbon triple bond, it provides a natural extension of previous work [9] on ethylene, in which a carboncarbon double bond was examined. It is expected that the angular asymmetry parameters for the distribution of ejected electrons as a function of photon energy will be highly dependent upon the nature of the orbital from which the electrons are ejected. The possibility that this behavior could be exploited. in molecular orbital assignments has been discussed previously [ 91. In general, there is a significant difference in the experimental 0 values for different orbit&, especially between a and a orbitals. In the case of ‘TTorbitals, a strong energy-dependence is generally observed near threshold, which leads to a relatively high 0 value at higher energies. On the other hand, (I orbitals

225

exhibit a much weaker energy-dependence and a /I value considerably lower in magnitude. Thus, it is important to examine this generalization in terms of the orbitals of acetylene.

EXPERIMENTAL

In this investigation, the 240 MeV Tantalus I electron storage ring at the Wisconsin Synchrotron Radiation Center was utilized as a continuum source of elliptically polarized photons. The light was monochromatized by passage through a l-m Seya monochromator equipped with a 1440 lines mm-’ osmium-coated grating which was blazed at 750 A. Two spectrometers were employed for the acquisition of the photoelectron spectra of acetylene. The initial work was carried out using a retarding-grid spectrometer incorporating energy-dispersive elements. The later work was done using a 180” sphericalsector analyzer having a radius of 3.6 cm. This latter analyzer was initially designed and built by M.O. Krause at Oak Ridge National Laboratory. Details of the instrumentation are given elsewhere [lo]. With the monochromator set for a bandpass of 28 excitation. Utilizing the spherical-sector analyzer and a monochromator bandpass of 1 A, the FWHM for the ground vibrational band of the first orbital of acetylene as measured by the retardinggrid analyzer was - 170 mV for 584 A, the FWHM was 110 mV at 584 A. At each wavelength investigated, spectra were recorded at two different angles, in the plane and perpendicular to the plane of polarization. The asymmetry parameter p can then be determined using the expression p = 4(R - 1)/[3P(R

+ 1) - (R - l)]

(1)

In this expression, R = 1(0")/1(90") is the ratio of photoelectrons collected in the plane (0”) parallel to the polarization vector to those collected perpendicular (90”) to the vector. The intensities at each angle were determined by numerical integration of each band after a smooth background had been subtracted from each band. The factor P in eqn. 1 is determined experimentally by measuring the angular distribution for calibrant gases such as Kr or Xe whose /3 values are given in ref. 8. This factor incorporates effects such as the degree of polarization and pressure-scattering effects, as well as any other systematic errors which cause variations in the angular distributions of molecules similar to those expected from polarization variations. In this work, the factor P ranged from 0.80 to 0.85 when the retarding-grid spectrometer was employed. Using the spherical-sector analyzer, the factor P did not show the same pressure dependence and was found normally to have a value of 0.95. The error bars shown in each of the Figures presented here were arrived at mainly by consideration of the agreement between replicate runs, with counting statistics taken into consideration.

226

RESULTS AND DISCUSSION

Figure 1 shows a typical photoelectron spectrum for the first orbital of acetylene. The first three vibrational bands are clearly resolved. On the basis of the average energy spacings of these bands (- 0.227 eV) they have been assigned [ 111 to the harmonics of v2 , the C=C stretching mode. Higher resolution might show contributions due to other harmonics or overtones. In Figs. 2-4, the asymmetry parameters respectively for the v2 = 0, 1 and 2 vibrational bands of the X2 II, state of C2Hz have been plotted as a function of excitation energy. Also shown are the data of Kreile et al. [7] obtained using line sources. The agreement between their results and those reported here is reasonable except that Kreile et al. found a sharp drop in p for the two photon energies of the Ne(1) doublet (16.89 and 16.67 eV). This might have occurred as a result of the fortuitous overlap with a sharp autoionization level present in their data. The p values for the AZ E:+B state of acetylene have also been plotted and are shown in Fig. 5. Plotted with the experimental values are the corresponding theoretical curves for the v. vibrational band of the In, and 30, orbit& as calculated by Grimm [12] on the basis of multiple-scattering theory. By far the most striking feature present in the entire data set is the broad depression (- 1 V) in the experimental /I curve for the n orbital which occurs - 3-4 V above threshold (see Figs. 2-4 and 6). The minimum of the depression occurs at N 14.4 + 0.1, 14.8 + 0.1 and 14.9 + 0.2 eV respectively for the v2 = 0, 1 and 2 vibrational bands. Other data for 0 derived from the photoionization of n orbitals in unsaturated aliphatic hydrocarbons such as ethylene [ 91 and larger molecules [13] also show dips qualitatively similar to that found for acetylene. A number of possible explanations can be advanced for the feature seen in the cross-section and p curves for the In, orbital of C2 H2 in the region of 14eV photon energy. The behavior of the 6 curve is very similar to that found [ 2, 31 for the 4~7,orbital (fourth band) of CO2 , which has been assigned to a molecular shape resonance. The shape resonance in CO2 was predicted by multiple-scattering calculations [3, 141, which have been successful in predicting the qualitative behavior of resonances in photoionization. In the case of acetylene, multiple-scattering calculations have been made by one of us (F.A.G.) using both the standard nontangent-spheres muffin-tin potential and an overlapping-spheres potential, of which the latter was very successful in the case of ethylene [ 91. The theory of the multiple-scattering formalism used in these calculations is already available in the literature [ 151. The calculations used bond lengths of 1.2076 A for CSC and 1.0594 A for C-H. The atomic sphere radii for the tangent spheres were rc = 0.6038 A and rH = 0.4556 A, with a tangent outer sphere. The overlapping-sphere results as plotted in Figs. 2 and 5 used rc = 0.8302 A and rn = 0.5869 A. In all cases

227

Fig. 1. Photoelectron spectrum of the first band in acetylene as a function of electron kinetic energy (eV) for radiation of x = 584A using synchrotron radiation with 1 A bandpass, showing the first four vibrational bands.

ACETYLENE

mm 9 12.0

16.0

Photon

20.0

Energy

i 1

V, = 0

24 .o

28.0

(eV1

Fig. 2. Plot of experimental fi values versus photon energy for the ~2 = 0 level of the 2&, state of the acetylene ion. The solid curve is the theoretical calculation of Grimm [ 121. Experimental values obtained by Kreile et al. [ 71 are labelled with an asterisk (*).

Photon

Energy

(eV)

Fig. 3. Plot of experimental 6 values versus photon energy for the v2 = 1 level of the ‘I&, state of the acetylene ion. Experimental values obtained by Kreile et al. [7] are labeled with an asterisk (*).

ACETYLENE V2

q

2 1

1

l *

2:

i#&:, * A

0

A

d. I

In

I

d

12.0

*

I

16.0

Photon

I

I

,

I

20 .o

24 .o

Energy

leVl

,

I

28.0

Fig. 4. Plot of experimental fi values versus photon energy for the-v2 = 2 level of the ‘lIu state of the acetylene ion. Experimental values obtained by Kreile et al. [7 J are labeled with an asterisk (*).

229

ACETYENE X,

u-3

0 d. I In 9

I 17.0

I

I

21.0

I

25.0

29.0

PHOTON

ENERGY

Fig. 5. Plot of experimental p values versus photon energy for the * X, state of the acetylene ion. The solid curve is the theoretical calculation of Grimm [ 121.

Fig. G.‘Combixied plot of experimental fl values versus photon energy for the v2 = b (o), = 1 (0) and ~2 = 2 (A) levels of the * n, state of the acetylene ion.

v2

230

the cr value was taken as 0.75. For the *lIu state the theoretical calculation was compared with the v2 = 0 vibrational band (Fig. 2) because this is the dominant vibrational transition. Neither of these potentials predicted the presence of a shape resonance for the In, orbital of acetylene. The absence of a resonance agrees with the calculations of Kreile et al. [7]. In addition to a dip in the 0 curve, a molecular shape resonance would show a maximum in the cross-section in the vicinity of the minimum in 0. The cross-section results of Unwin et al. [l] show a minimum in the cross-section very close to the minimum in the fl curve. On the basis both of the calculations and of the cross-section data the presence of a molecular shape resonance around 14.4 eV is not indicated. However, the possibility of a shape resonance should not be ruled out only on the basis of multiple-scattering calculations, which assume fixed nuclei. Another explanation for the behavior of the cross-section has been advanced by Unwin et al. [l]. In their model the cross-section is calculated assuming an autoionization process. The state embedded in the continuum responsible for this resonance is assumed to be at 14.7 eV and to have a width of 1.4eV, based on comparisons with experimental data. Although these values may not be optimal, as explained by the authors, it would be expected that a width in the region of 1.4 eV would be necessary. This width would correspond to a very short-lived state, and would hence be consistent with the observation that the dip in the p curve is quite broad. A shift in the cross-section maximum as a function of vibrational state was also predicted in the calculations of Unwin et al. and this might explain the observed shifts in the minimum of the /3 dependence curves. As noted by Kreile et al. [7], not enough is known about the autoionization states of acetylene to confirm or reject an explanation hypothesizing a very short-lived autoionization state, and we would have to conclude on the basis of the present data that this would be a reasonable explanation. For example, considering the photoionization cross-section data available for C2H2 [ 16-181, there is the broad 30, + 30, transition which might be considered as the possible autoionization state, whose energy is close to that of the minimum in the /3 curves. The work of Hayaishi et al. [18] provides another possible state that would fit the available data. These authors calculate a lx, + 20, transition having an oscillator strength of 0.429. This broad resonance would also be in the vicinity necessary for the autoionization state. Neither of the autoionization states mentioned above may alone account for the observed dip in p, and some other behavior or interaction between these two states may have to be evoked to give a definitive explanation of the experimental results. Aside from the special structural features discussed previously, the agreement between experiment and theory for the dependence of p on photon energy is generally good. In Figs. 2 and 5 it can be seen that the shape of the calculated energy dependence is quite reasonable for both orbit&, although

231

the magnitude shows slight discrepancies in that the values are shifted lower than experiment for the n orbital and slightly higher for the u orbital. (Calculations of /3 by Kreile et al. [7] for the lr, orbital gave values slightly higher than experiment). The results for acetylene are in general agreement with the data for ethylene as to generalizations concerning the behavior of C-C u and B orbit& for unsaturated hydrocarbons, in that the 0 value for photoionization of a R orbital as a function of photon energy rises fairly rapidly from a low value near the ionization threshold to an asymptotic value of about one or greater, whereas the p value for a u orbital remains fairly constant at a lower value.

CONCLUSIONS

Using angle-resolved photoelectron spectroscopy, it has been found possible to investigate the spectral feature corresponding to the la, orbital of acetylene which Unwin et al. [l] had described previously. The technique proved to be sufficiently sensitive to allow the effects of this phenomenon to be observed in all three observable vibrational bands. Although an unambiguous assignment for this feature remains to be found, valuable data which can be used to evaluate the phenomena have been obtained. It may be that the feature found for acetylene is not unique but may occur frequently for unsaturated hydrocarbons [ 9, 131. In addition, it appears from this work that the study of the angular distribution of molecular orbitals can be used as an aid in molecular orbital assignment, as the behavior of the z and u orbitals followed the patterns noted previously [9]. The value of the multiplescattering method as a tool for predicting the overall energy dependence of the asymmetry parameter has also been demonstrated. It is apparent that angle-resolved photoelectron spectroscopy will be a sensitive and powerful tool for the investigation of other organic molecules and the ionization processes which are relevant to those systems.

ACKNOWLEDGEMENT

We thank M.O. Krause of Oak Ridge National Laboratory for making available the spherical-sector analyzer in this work.

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