- Email: [email protected]
Angle-resolved photoelectron spectroscopy of the chloro-substituted methanes
Chemical Physics 79 (1983) 269-276 North-Holland Publishing Company
269
ANGLE-RESOLVED PHOTOELECTRON SPECTROSCOPY OF THE CHLORO-SUBSTITUTED METHANES * P.R. KELLER, Depamnenf
J.W. TAYLOR
of Chentisny.
Unicersiry
**
of Wisconsin - hludison.
Mudison.
WI 53706.
USA
Thomas A. CARLSON Oak Ridge Narional Laboraror).
Oak Ridge. TN 37830.
US.4
and F.A. GRIMM Deparmrenr Received
of Chentisq.
28 February
Unioersiry of Tennessee.
Kno.wille.
TN 37996 - 1600. USA
1983
The angular distribution parameter, j3. was determined for the valence orbitals (IP c 21.2 eV) of Ccl,. CHCI,. CH,Cl,. and CH,CI in the IO-30 eV photon energy range using dispersed polarized svnchrotron radiation. The energy dependence of ~3 in the photoelectron energy range of 2 to 10 +V for the non-bonding chlorine n(C1) orbit& of thcrc molecules wx found to bs similar for all n(CI) orbitals investigated. The energy dependence of /3 for the o orbit& in these molsculss was similar to that observed previously for orher o orbitals. The experimental Ccl, results were compared wirh theoretical Ccl, results obtained using the Xa multiple scattering formalism. Theory predicts the existence of two strong shape resonances in each of the valence orbilals of Ccl,. The overall agreement between experiment and theory is evaluated along with the experimental evidence concerning the verification of the predicted shape resonances.
1. Introduction Recent studies of molecules containing carbon-carbon multiple bonds [l-5] using the technique of angle-resolved photoelectron spectroscopy have indicated that the angular distribution parameter, p, exhibits behavior which is consistent within a series of orbitals possessing similar character. This characteristic behavior is maniResearch 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 is provided by the Division of Chemical Science, Office of Basic Energy Sciences. U.S. Department of Energy. under Contract W7405-eng-26 with the Union Carbide Corporation . ** To whom correspondence should be addressed. l
0301-0104/83/0000-0000/$03.00
0 North-Holland
fested mainly in the dependence of fl upon the photoelectron energy in the first 10 eV above threshold. In those earlier studies [l-5], orbitals of =. CI and nitrogen non-bonding character were investigated. For the 7 orbitais in unsaturated systems [l-5]. it was found that in the first 10 eV above threshold, the snergy dependence of p was greater on the average than that found for the u orbitals in organic molecules- In the case of the azabcnzene study [4]. the z orbitals also exhibited an energy dependence near threshold that was much greater than that of the nitrogen non-bonding orbitals in these molecules. Originally. it was thought that. due to this strong energy dependence. the 72 orbitals would have high j3 values (> 1.0) at photoelectron energies above 10 eV or so. However. a more extensive examination of m
orbitals did not bear this out, but showed that the overall magnitude of p is also sensitive to the substituents on the double bond. One intent of this work 1s to investigate the possibility that generalizations of this type can be extended to other molecular systems. The chlorosubstituted methanes (Ccl,, CHCI,. CH,CIz and CH,Cl) were selected for the following reasons: (1) the orbital assignments are well known, (2) the individual bands are relatively free of serious spectral overlaps and (3) two distinct types of orbitals are available for investigation_ The delocalized orbitals arising from bonding between C and H or Cl are similar to the (I orbitals studied before. The non-bonding orbitals of Cl, which are essentially localized about the Cl atoms, possess orbital character different from those studied so far. The investigation of these systems should permit examination of the applicability of the generalizations suggested for the bonding orbitals studied previously. Also. it should be possible to evaluate the behavior of fl as a function of energy for the localized non-bonding orbi:c.ls of Cl. Another goal of this work is to carry out a comparison of the experimental angular distribution results for Ccl, with theoretical results obtained by one of us (FAG). The theoretical results were obtained using the continuum multiple scattering Xa (MS Xcu) method and have been discussed in part in ref. [6]. Theory predicts the existence
of
two strong
shape
resonances
in each
of the first four valence orbitals of Ccl,. These comparisons should, therefore, provide an evaluation of the overall theoretical results as well as an experimental check for the existence of shape resonances in the orbitals of Ccl,. In order to carry out this comparison as well as the general energydependence study, angular distribution measurements were made in the photon energy range of 14 to 30 eV.
2. Experimental The 240 MeV Tantalus I electron storage ring at the Wisconsin Synchrotron Radiation Center (SRC) was utilized as a continuum source of elliptically polarized photons for this experiment.
Monochromatic radiation was obtained by passage of the synchrotron radiation through a 1 m Seya-Namioka monochromator equipped with a 1440 lines/mm osmium-coated grating. This configuration provided acceptable photon flyxes in the lo-30 eV range at a bandpass of 2 A. Two electron spectrometers, a 180” spherical sector of radius 3.6 cm and a retarding-grid spectrometer incorporating energy dispersive elements. were employed in the acquisition of the photoelectron spectra. A detailed discussion of the instrumentation will be given elsewhere [7]. The photoelectron intensity resulting from the ionization of particular orbital must be measured at no less than two angles in order to determine its angular distribution parameter. In this experiment, spectra were obtained at two angles, in the plane and perpendicular to the plane of polarization, for each photon energy investigated. The experimental procedures have been discussed in detail previously [4].
3. Results and discussion The experimental results for each of these molecules are presented as plots of the angular distribution parameter as a function of the photon energy, /3(hv). The results for Ccl, are given in figs. 1-4 along with the respective theoretical curve (solid curve)_ The /3(hv) curves are given for CHCl, in figs. 5 and 6, for CH$l, in figs. 7-9 and for CH,Cl in fig. 10. Also plotted in these figures are the He(I) line source (584 A) p values of Carlson et al. [8]. Typical photoelectron spectra for these molecules obtained with this instrumentation are shown in fig. 11. In the systematic study of rr orbitals in unsaturated organic molecules reported earlier [S], the energy dependence of j3 near threshold was expressed in a quantity called A/!. This quantity was defined as the difference in p at a photoelectron energy of 10 eV and 2 eV_ In order to facilitate comparison of energy dependences, this convention has been retained. This photoelectron energy region was chosen because angular distribution measurements carried out in this energy range should be sensitive to the overall nature of
Fig. 1. The angular distribution parameter /3 plotted as a function of photon energy for the 21, orbital of Ccl,. The solid curve represents hlS Xa calculations. The He(I) line source value ( x ) of ref. [S] is also presented.
Fig. 3. The angular distribution parameter p plotted as a function of photon energy for the 2s orbital of Ccl,. The solid cm-xx represents the MS Sa calculations. Ths He(l) line source value ( x ) of ref. [S] is also presented.
the molecular orbital. The investigation of the nature of the molecular orbital by means of angular distribution measurements is one of the primary interests in this work. At higher photoelectron energies (PE > 15-20 eV), one begins to probe more localized or atomic aspects of the molecular orbital such as the Cooper minimum (ref. [6] and references therein). The Afl values for the orbitals of Ccl,, CHCl,. CH,Cl, and CH,Cl are given in table 1 along with a listing of the orbitals and their respective character and ionization potentials.
An examination of the /I curxes and J/3 values for the orbitals of non-bonding Cl character shows that these orbitals have a consistent appearance and energy dependence. The I/? values for these orbitals range from 0.34 to 0.73. Although the average dp value for these n(C1) orbit& (0.56) is lower than that of the z orbitals in unsaturated hydrocarbons (0.78) [4]. there is considerable overlap between the S/3 range for the n(C1) orbitals and that of the z orbitals (A/3: 0.54- 1.10). On the other hand. the LIP values of the n(Ci) orbitals are
mm ;
Fig. 2. The angular distribution parameter B plotted as a function of photon energy for the 7t, orbhal of Ccl,. The solid curve represents MS Xa calculations. Tbs He(I) line sourc‘e value (x) of ref. [8] is also presented.
IS.
20.
30.
32.
Fig. 4. The angular distribution psramcter fl plotted as a function of photon energy for the 61~ orbital of Ccl,. Tbs solid curve represents the MS Xa calculst~ons. The Hdl) line sour~rt value ( X ) of ref. [S] is also presented.
P.R. h*elIer EI al. / Angle-resolvedphoroelecfron
specrroscopy of the chloro-subsfilured nrerhones
,
I
.
..I
.
.
.
.
.
I
.
.
.
.
15.
1
.I
30. PliOTON2oiNERGv
*:.eV
I
Fig. 5. Plot of the angular distribution parameter /? verstt~ photon energy for the first (2az) (x), third (2e”) (0) and fourth (6e’) (A) bands of CHCI,. The larger symbols at 21.2 eV are the respective He(I) values of ref. [S].
Fig. 7. Plot of the angular distribution parameter /I versus photon energy for the first [(7b2+3h,)] (0) and third (6b2) (X) bands of CHaCla. The larger symbols at 21.2 eV are the respective He(l) p values of ref. IS].
considerably
ing similar energy dependences in p must be taken into consideration when using this technique as an aid in molecular-orbital assignment. Second, the energy-dependence comparison of the two different types of non-bonding orbitals indicates that the energy dependence can be sensitive to changes in orbital character brought about by changes in the atomic center from which the orbital arises. The Afl values for the 0 orbitals of these molecules (table 1) range from 0 to 0.62. This range is within the range seen for the CJorbitals previously
different
from those of the non-bond-
ing nitrogen n(N) orbitals in the azabenzenes investigated earlier. In the case of the n(N) orbitals, the A/I range was -0.39 to 0.29 with an average value of 0.07. The n(C1) versus z and n(C1) versus n(N) comparison illustrate two important points. First, as seen in the comparison of the n(C1) and s orbitals, orbitals of different character can have similar energy dependence. Thus, the possibility of orbitals of different character in a given molecule exhibit-
U-I
‘fj
0 J Q
0
0
Q? 0 ,- _
c
i
IIn
. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
25ie”
I
..I.
30. PH&%ERGY
Fig. 6. Plot of the angular distribution parameter /I versus photon energy for the second [@a, +7e’)] (0) and fifth @a,) (X) bands of CHCl,. The larger symbols at 21.2 eV are the respective He(I) p values of ref. [8].
.
15.
Fig. 8. Plot of the angular distribution parameter fl versus photon energy for the second [(9a, +Za,)] (0) and fourth (Eta,) (x) bands of CHaCI,. The larger symbols at 21.2 eV are the respective He(I) /3 values of ref. [S].
Ln
.
,.
..,....I....,.
30.
15. PHDToNZDiNERGV
‘:a”
I
Fig. 9. Plot of the angular distribution parameter p versus photon energy for the fifth band (Zb, ) (0) of CH,Cl,. The larger symbol al 21.2 eV is the He(I) p value of ref. IS].
investigated (0.0 to 0.72). Once again, the u orbitals which Kimura et al. [9] suggest show pseudo 7 character are in the upper regions of the range. Multiple scattering Xa calculations have been carried out on Ccl, with the results given in figs. 1-4. The details of the actual calculations are presented elsewhere [lo]. Numerous shape resonances are predicted in the calculations on Ccl,. Some of these resonances produced much larger increases in the cross sections for photoionization for one molecular orbital than for another. Thus, in table 2 we show the positions of the maximum
U-J ,-
.
.
.
.
15.
.
.
..I....I....I.
P”0&&w?~V “;,“I
30.
Fig. 10. Plot of the angular distribution parameter /3 for rhs first (3e) (0). second (7a,) (A) and third (2s) (X ) bands of CH,CI. The larger symbols at 21.2 eV are the respectiw He(l) p values of ref. [S].
Fig. 11. Phoroelecwon spectra of CCI,. CH,CI ar 2 1.2 eV and an angle of 0”.
CHCI,.
CH,Cl,
and
in the cross sections only for the strong resonances for photoionization from a given molecular orbital. because only these strong resonances have a significant effect on the total cross sections and angular distribution parameters. For convenience in the following discussion. we will refer to the position of the maximum in the cross section for each shape resonance as the predicted position of the shape resonance. In the 3t, orbital of Ccl,. two shape resonances are predicted at a photon energy of 14.0 eV. The experimental P(hvf curve for this orbital (fig. 1) shows some evidence of the existence of these resonances at = 19 eV. Overall. however. the agreement bettveen experiment and theory is poor. One possible explanation for this disagreement and relatively weak evidence for the resonances is that the two resonances may not be at exactly the same energy as predicted. but separated slightly in energy instead. If this were the case. the interference of these two resonances would be expected to alter drastically the theoretical fl curve obtained. This interference could a!so have the result of dampening the effects of these resonances on j3 to the point where they are observed only weakly in the p(1r~) curve. Another possibility is that the assumption used in these calculations that the nuclei can be considered as fixed with respect to one another is responsible for the exaggeration of the shape-resonance effects In calculations where
Table I Ap values for the orbit& of Ccl,, hfolecuie
CHCI,.
CH2C12 and CH,Cl Character [9]
L\jJ”’
NC11 n(CI) n(Cl) o(CCI)
0.70 0.34 0.40
61?
11.69 12.62 13.44 16.5fi
2az
11.4s 11.91 12.85
n(CI) n(C1) n(C0
0.67
9a, i7e’ 2e” 6e’
15.99
o(CCI)
0.45
Sa,
16.96
o(CCI)
0.24
7b, +3b, 9a, -t-z%,-
11.40 12.22
n(C1) n(C1)
0.60 0.62
tib,
15.30
o(CCI)
0.53
Sal
15.94
0 (ccl)
0.20
2b,
16.17
O(%I,)
0.62
3e 7ai 2s
11.29 14.42 15.47
n(CI) u(CCI) a(%,,)
0.73 0.24 0.36
Orbital 191
ionization potential 191
CCI,
211 712 2e
CHCI,
CH,CI,
CH,CI
“’ d/3 = /3(PE = 10 eV)-/3(PE
Table 2
Theoretical positions of the shape resonances predicted to give a strong maximum in the cross section [IO] Orbital
Theoretical position n’ (photoelectron energy in eV)
21,
2.4 (X-e)
712
4.4 (ke) ”
2.4 (x-t,) 6.4(kt,) 2.6 (x-t,) 8.6 (x-t,)
61~
2.4 (ke) 8.4(x-t,)
a) PosiIion of the maximum in the cross section for the continuum channel shown in parentheses. b’ This is the same resonance as seen in the 21, and 6t, orbit& at 2.4 eV.
0.50 0.49
= Z eV).
averaging over several internuclear distances was incorporated, the effects of the shape resonance on p tend to decrease in magnitude. This was found to be the case for the shape resonance found in the fourth band of CO2 [ 1 l- 131. In that instance, the shape resonance was also predicted at a lower energy than it was observed at experimentally_
2e
0.00
The disagreement is also due in large part to the energy at which theory predicts the Cooper minimum in this orbital. The Cooper minima in the orbitals of Ccl, have been studied by Carlson et al. 161. In the 2t, orbital of Ccl, [6] a maximum in p is seen experimentally at = 31 eV, and a minimum in p, corresponding to the Cooper minimum, at = 44 eV. Theoretically, these features are predicted at photon energies of = 22 eV and = 26 eV respectively. Thus, the overall theoretical curve is compressed toward lower energy. When the experimental results are examined over the complete range of energy from 14 to 70 eV and compared with the theoretical curve, there is good qualitative agreement between experiment and theory although the various features in the two curves are offset in energy. The shape resonances predicted at photon energies of 17.0 and 19 eV for the 7tz orbital are not evident in the experimental ~(IzP) curve. Once again, agreement between the experimental and theoretical curves is not good. As was mentioned in the discussion of the shape resonances in the 2t, orbital, the positions of these two resonances may
be different
than theoretically
predicted.
The pos-
sible effects of different shape-resonance positioning have been discussed above along with the effects of assuming fixed nuclei in the calculations. The agreement between experiment and theory suffers from the same compression of the theoretical curve as was seen in the 2t, orbital. A maximum and minimum (Cooper minimum) in fl were seen experimentally [6] at = 34 eV and = 44 eV respectively. These features were predicted at = 24 eV and = 38 eV respectively_ If the low- and high[6] energy results in combination are compared with the theoretical results. good qualitative agreement is obtained. The theoretical calculations also predict the existence of two shape resonances (16 and 22 eV) in the 2e orbital of Ccl,. However, in this case the resonances are well separated in energy and thus the interference effects should be minimal_ The shapes of the theoretical and experimental p(!zv) curves are very similar and although the theoretical curve lies slightly below the experimental. the overall agreement is good. The agreement at higher energies [6] is also reasonably good. The effects of the two resonances are clearly evident in the experimental curve. although the depth of the minimum at = 20 eV is not as large as expected_ The agreement in this case suggests that lack of evidence for shape resonances in the first two orbitals is due mainly to interference effects of shape resonances separated slightly in energy. The effects of assuming fixed nuclei in the calculations should not be ignored, however, since this is still a likely explanation for the difference in the minimum depth at = 20 eV for the 2e orbital. In the 6tz orbital. shape resonances are predicted at 19 and 25 eV. From the argument put fonvard above for the 2e orbital. the effects of the resonances as predicted by the theoretical results should be evident in the experimental /3( hv) curve. This is indeed the case. The overall shapes of the curves are the same. Once again. there is some disagreement in the magnitude of the curves. but in the opposite sense of that seen in the 2e orbital. Although MS Xa calculations were not carried out on the other molecules investigated. it would seem Iogical that these molecules possess shape resonances as well. Experimentally. there is some
evidence
for
the
existence
of shape
resonances.
The /3(/rrl) curve of the 2e” orbital of CHCl, has a shape remarkably similar to that seen for the 2e orbital of Ccl,_ The Sa, orbit& of CHCl, and CH,Cl z both show broad depressions in their experimental fl(hv) curves lvhich are similar in some respects to P(hz~) curve of the 6t, orbital of Ccl,. Although the assignment of these features to be shape-resonance effects appears promising. further judgement should be withheld until theoretical calculations have been carried out on each of these molecules.
4. Conclusions The angular distribution data presented in paper have demonstrated the applicability of generalizations made earlier ]I-5] concerning energy dependence of /3 near threshold. to molecular systems. The CJorbitals examined in
this the the new this
work exhibited energy depsndences quite similar to those observed for the u orbitals investigated previously. The non-bonding chlorine. n(CI). orbitals were also found to possess an energ!. dependence tvhich 1~~9s similar for all the n(C1) orbitals investigated. On the average. the snergy dependence of p for the n(U) orbitals (0.56) was found to be slightly xveaker. than that found for the 5 orbitals (0.7s) although there is considerable overlap betlveen the _I/3 ranges of the t\vo orbital classifications_ The comparison of experimental and theoretical angular distribution results for the first four orbitals of Ccl, indicated that the agreement hetween experiment and theory was highly depcndent upon the relative spacings or the two strong shape resonances xv-hose existence is predicted in each of the four orbitals. In the Zt, and 7t, orbitals of Ccl,. qualitative agreement was obtained only when the combined lo\v- and high- [ 131 energy experimental results were compared with theory_ This lack of agreement at low energy was attributed to the incorrect predictions of the relative positions of the two shape resonances and thus. to a miscalculation of the interferences of one resonance with the other. When the predicted shape
resonances
were spaced
relatively
far apart.
276
as was the case in the 2e and 6t, orbitals of Ccl,. good agreement between experiment and theory was found even at low energy. The results of these comparisons could then be used to make some preliminary judgments on the possible existence of shape resonances in the other chloromethanes for which theoretical calculations have yet to be carried out.
Acknowledgement One of the authors (FAG) gratefully acknowledges the support of the University of Tennessee Computing Center and of the Synchroton Radiation Center for the Visiting Investigator appointment.
References [I] D. Mehaffy. P.R. Keller. J.W. Taylor. T.A. Cartson, M.O. Krause. F.A. Grimm and J.D. Allen Jr.. J. Electron Speclry. 26 (1982) 213.
[Z] P.R. Keller. D. Mehaffy. J.W. Taylor. F.A. Grimm and T-A. Carlson. J. Electron Spectry. 27 (1982) 223. 131 D. Mehaffy. P.R. Keller. J.W. Taylor, T.A. Carlson and F.A. Grimm. J. Electron Spectry. 28 (1983) 239. (41 M.N. Pianustelli. P.R. Keller. J.W. Taylor. F.A. Grimm and T.A. Carlson, J. Am. Chem. Sot. 105 (1983) 4235. [5] P.R. Keller. J.W. Taylor. T.A. Carlson and F.A. Grimm. J. Electron Spectry.. to be published. [6] T.A. Carlson. M.O. Krause. F.A. Grimm. P.R. Keller and J.W. Taylor. J. Chem. Phys. 77 (1982) 5340. 17) D. Mehaffy. P.R. Keller. J.W. Taylor and J.D. Allen Jr.. to be published. IS] T.A. Carlson and R.M. White. Faraday Discussions Chem. Sot. 54 (1972) 285. [9] K. Kimura. S. Karsumara. Y. Achiba. T. Yamazaki and S. Iwata. Handbook of He(l) photoelectron spectra of fundamental organic molecules (Halsted Press, New York, 1981). [IO] F.A. Grimm. to be published. [ 1 I] J.R. Swanson, D. Dill and J.L. Dehmer. J. Phys. B13 (1980) L231: B14 (1981) L207. 1121 T.A. Carlson. M.O. Krause. F.A. Grimm. J.D. Allen Jr.. D. Mehaffy, P.R. Keller and J.W. Taylor. Phys. Rev. A23 (1981) 3316. 1131 F-A. Grimm, J.D. Allen Jr.. T.A. Carlson, M.O. Krause. D. Mehaffy. P.R. Keller and J.W. Taylor. J. Chem. Phys. 75 (1981)
92.
269
ANGLE-RESOLVED PHOTOELECTRON SPECTROSCOPY OF THE CHLORO-SUBSTITUTED METHANES * P.R. KELLER, Depamnenf
J.W. TAYLOR
of Chentisny.
Unicersiry
**
of Wisconsin - hludison.
Mudison.
WI 53706.
USA
Thomas A. CARLSON Oak Ridge Narional Laboraror).
Oak Ridge. TN 37830.
US.4
and F.A. GRIMM Deparmrenr Received
of Chentisq.
28 February
Unioersiry of Tennessee.
Kno.wille.
TN 37996 - 1600. USA
1983
The angular distribution parameter, j3. was determined for the valence orbitals (IP c 21.2 eV) of Ccl,. CHCI,. CH,Cl,. and CH,CI in the IO-30 eV photon energy range using dispersed polarized svnchrotron radiation. The energy dependence of ~3 in the photoelectron energy range of 2 to 10 +V for the non-bonding chlorine n(C1) orbit& of thcrc molecules wx found to bs similar for all n(CI) orbitals investigated. The energy dependence of /3 for the o orbit& in these molsculss was similar to that observed previously for orher o orbitals. The experimental Ccl, results were compared wirh theoretical Ccl, results obtained using the Xa multiple scattering formalism. Theory predicts the existence of two strong shape resonances in each of the valence orbilals of Ccl,. The overall agreement between experiment and theory is evaluated along with the experimental evidence concerning the verification of the predicted shape resonances.
1. Introduction Recent studies of molecules containing carbon-carbon multiple bonds [l-5] using the technique of angle-resolved photoelectron spectroscopy have indicated that the angular distribution parameter, p, exhibits behavior which is consistent within a series of orbitals possessing similar character. This characteristic behavior is maniResearch 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 is provided by the Division of Chemical Science, Office of Basic Energy Sciences. U.S. Department of Energy. under Contract W7405-eng-26 with the Union Carbide Corporation . ** To whom correspondence should be addressed. l
0301-0104/83/0000-0000/$03.00
0 North-Holland
fested mainly in the dependence of fl upon the photoelectron energy in the first 10 eV above threshold. In those earlier studies [l-5], orbitals of =. CI and nitrogen non-bonding character were investigated. For the 7 orbitais in unsaturated systems [l-5]. it was found that in the first 10 eV above threshold, the snergy dependence of p was greater on the average than that found for the u orbitals in organic molecules- In the case of the azabcnzene study [4]. the z orbitals also exhibited an energy dependence near threshold that was much greater than that of the nitrogen non-bonding orbitals in these molecules. Originally. it was thought that. due to this strong energy dependence. the 72 orbitals would have high j3 values (> 1.0) at photoelectron energies above 10 eV or so. However. a more extensive examination of m
orbitals did not bear this out, but showed that the overall magnitude of p is also sensitive to the substituents on the double bond. One intent of this work 1s to investigate the possibility that generalizations of this type can be extended to other molecular systems. The chlorosubstituted methanes (Ccl,, CHCI,. CH,CIz and CH,Cl) were selected for the following reasons: (1) the orbital assignments are well known, (2) the individual bands are relatively free of serious spectral overlaps and (3) two distinct types of orbitals are available for investigation_ The delocalized orbitals arising from bonding between C and H or Cl are similar to the (I orbitals studied before. The non-bonding orbitals of Cl, which are essentially localized about the Cl atoms, possess orbital character different from those studied so far. The investigation of these systems should permit examination of the applicability of the generalizations suggested for the bonding orbitals studied previously. Also. it should be possible to evaluate the behavior of fl as a function of energy for the localized non-bonding orbi:c.ls of Cl. Another goal of this work is to carry out a comparison of the experimental angular distribution results for Ccl, with theoretical results obtained by one of us (FAG). The theoretical results were obtained using the continuum multiple scattering Xa (MS Xcu) method and have been discussed in part in ref. [6]. Theory predicts the existence
of
two strong
shape
resonances
in each
of the first four valence orbitals of Ccl,. These comparisons should, therefore, provide an evaluation of the overall theoretical results as well as an experimental check for the existence of shape resonances in the orbitals of Ccl,. In order to carry out this comparison as well as the general energydependence study, angular distribution measurements were made in the photon energy range of 14 to 30 eV.
2. Experimental The 240 MeV Tantalus I electron storage ring at the Wisconsin Synchrotron Radiation Center (SRC) was utilized as a continuum source of elliptically polarized photons for this experiment.
Monochromatic radiation was obtained by passage of the synchrotron radiation through a 1 m Seya-Namioka monochromator equipped with a 1440 lines/mm osmium-coated grating. This configuration provided acceptable photon flyxes in the lo-30 eV range at a bandpass of 2 A. Two electron spectrometers, a 180” spherical sector of radius 3.6 cm and a retarding-grid spectrometer incorporating energy dispersive elements. were employed in the acquisition of the photoelectron spectra. A detailed discussion of the instrumentation will be given elsewhere [7]. The photoelectron intensity resulting from the ionization of particular orbital must be measured at no less than two angles in order to determine its angular distribution parameter. In this experiment, spectra were obtained at two angles, in the plane and perpendicular to the plane of polarization, for each photon energy investigated. The experimental procedures have been discussed in detail previously [4].
3. Results and discussion The experimental results for each of these molecules are presented as plots of the angular distribution parameter as a function of the photon energy, /3(hv). The results for Ccl, are given in figs. 1-4 along with the respective theoretical curve (solid curve)_ The /3(hv) curves are given for CHCl, in figs. 5 and 6, for CH$l, in figs. 7-9 and for CH,Cl in fig. 10. Also plotted in these figures are the He(I) line source (584 A) p values of Carlson et al. [8]. Typical photoelectron spectra for these molecules obtained with this instrumentation are shown in fig. 11. In the systematic study of rr orbitals in unsaturated organic molecules reported earlier [S], the energy dependence of j3 near threshold was expressed in a quantity called A/!. This quantity was defined as the difference in p at a photoelectron energy of 10 eV and 2 eV_ In order to facilitate comparison of energy dependences, this convention has been retained. This photoelectron energy region was chosen because angular distribution measurements carried out in this energy range should be sensitive to the overall nature of
Fig. 1. The angular distribution parameter /3 plotted as a function of photon energy for the 21, orbital of Ccl,. The solid curve represents hlS Xa calculations. The He(I) line source value ( x ) of ref. [S] is also presented.
Fig. 3. The angular distribution parameter p plotted as a function of photon energy for the 2s orbital of Ccl,. The solid cm-xx represents the MS Sa calculations. Ths He(l) line source value ( x ) of ref. [S] is also presented.
the molecular orbital. The investigation of the nature of the molecular orbital by means of angular distribution measurements is one of the primary interests in this work. At higher photoelectron energies (PE > 15-20 eV), one begins to probe more localized or atomic aspects of the molecular orbital such as the Cooper minimum (ref. [6] and references therein). The Afl values for the orbitals of Ccl,, CHCl,. CH,Cl, and CH,Cl are given in table 1 along with a listing of the orbitals and their respective character and ionization potentials.
An examination of the /I curxes and J/3 values for the orbitals of non-bonding Cl character shows that these orbitals have a consistent appearance and energy dependence. The I/? values for these orbitals range from 0.34 to 0.73. Although the average dp value for these n(C1) orbit& (0.56) is lower than that of the z orbitals in unsaturated hydrocarbons (0.78) [4]. there is considerable overlap between the S/3 range for the n(C1) orbitals and that of the z orbitals (A/3: 0.54- 1.10). On the other hand. the LIP values of the n(Ci) orbitals are
mm ;
Fig. 2. The angular distribution parameter B plotted as a function of photon energy for the 7t, orbhal of Ccl,. The solid curve represents MS Xa calculations. Tbs He(I) line sourc‘e value (x) of ref. [8] is also presented.
IS.
20.
30.
32.
Fig. 4. The angular distribution psramcter fl plotted as a function of photon energy for the 61~ orbital of Ccl,. Tbs solid curve represents the MS Xa calculst~ons. The Hdl) line sour~rt value ( X ) of ref. [S] is also presented.
P.R. h*elIer EI al. / Angle-resolvedphoroelecfron
specrroscopy of the chloro-subsfilured nrerhones
,
I
.
..I
.
.
.
.
.
I
.
.
.
.
15.
1
.I
30. PliOTON2oiNERGv
*:.eV
I
Fig. 5. Plot of the angular distribution parameter /? verstt~ photon energy for the first (2az) (x), third (2e”) (0) and fourth (6e’) (A) bands of CHCI,. The larger symbols at 21.2 eV are the respective He(I) values of ref. [S].
Fig. 7. Plot of the angular distribution parameter /I versus photon energy for the first [(7b2+3h,)] (0) and third (6b2) (X) bands of CHaCla. The larger symbols at 21.2 eV are the respective He(l) p values of ref. IS].
considerably
ing similar energy dependences in p must be taken into consideration when using this technique as an aid in molecular-orbital assignment. Second, the energy-dependence comparison of the two different types of non-bonding orbitals indicates that the energy dependence can be sensitive to changes in orbital character brought about by changes in the atomic center from which the orbital arises. The Afl values for the 0 orbitals of these molecules (table 1) range from 0 to 0.62. This range is within the range seen for the CJorbitals previously
different
from those of the non-bond-
ing nitrogen n(N) orbitals in the azabenzenes investigated earlier. In the case of the n(N) orbitals, the A/I range was -0.39 to 0.29 with an average value of 0.07. The n(C1) versus z and n(C1) versus n(N) comparison illustrate two important points. First, as seen in the comparison of the n(C1) and s orbitals, orbitals of different character can have similar energy dependence. Thus, the possibility of orbitals of different character in a given molecule exhibit-
U-I
‘fj
0 J Q
0
0
Q? 0 ,- _
c
i
IIn
. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
25ie”
I
..I.
30. PH&%ERGY
Fig. 6. Plot of the angular distribution parameter /I versus photon energy for the second [@a, +7e’)] (0) and fifth @a,) (X) bands of CHCl,. The larger symbols at 21.2 eV are the respective He(I) p values of ref. [8].
.
15.
Fig. 8. Plot of the angular distribution parameter fl versus photon energy for the second [(9a, +Za,)] (0) and fourth (Eta,) (x) bands of CHaCI,. The larger symbols at 21.2 eV are the respective He(I) /3 values of ref. [S].
Ln
.
,.
..,....I....,.
30.
15. PHDToNZDiNERGV
‘:a”
I
Fig. 9. Plot of the angular distribution parameter p versus photon energy for the fifth band (Zb, ) (0) of CH,Cl,. The larger symbol al 21.2 eV is the He(I) p value of ref. IS].
investigated (0.0 to 0.72). Once again, the u orbitals which Kimura et al. [9] suggest show pseudo 7 character are in the upper regions of the range. Multiple scattering Xa calculations have been carried out on Ccl, with the results given in figs. 1-4. The details of the actual calculations are presented elsewhere [lo]. Numerous shape resonances are predicted in the calculations on Ccl,. Some of these resonances produced much larger increases in the cross sections for photoionization for one molecular orbital than for another. Thus, in table 2 we show the positions of the maximum
U-J ,-
.
.
.
.
15.
.
.
..I....I....I.
P”0&&w?~V “;,“I
30.
Fig. 10. Plot of the angular distribution parameter /3 for rhs first (3e) (0). second (7a,) (A) and third (2s) (X ) bands of CH,CI. The larger symbols at 21.2 eV are the respectiw He(l) p values of ref. [S].
Fig. 11. Phoroelecwon spectra of CCI,. CH,CI ar 2 1.2 eV and an angle of 0”.
CHCI,.
CH,Cl,
and
in the cross sections only for the strong resonances for photoionization from a given molecular orbital. because only these strong resonances have a significant effect on the total cross sections and angular distribution parameters. For convenience in the following discussion. we will refer to the position of the maximum in the cross section for each shape resonance as the predicted position of the shape resonance. In the 3t, orbital of Ccl,. two shape resonances are predicted at a photon energy of 14.0 eV. The experimental P(hvf curve for this orbital (fig. 1) shows some evidence of the existence of these resonances at = 19 eV. Overall. however. the agreement bettveen experiment and theory is poor. One possible explanation for this disagreement and relatively weak evidence for the resonances is that the two resonances may not be at exactly the same energy as predicted. but separated slightly in energy instead. If this were the case. the interference of these two resonances would be expected to alter drastically the theoretical fl curve obtained. This interference could a!so have the result of dampening the effects of these resonances on j3 to the point where they are observed only weakly in the p(1r~) curve. Another possibility is that the assumption used in these calculations that the nuclei can be considered as fixed with respect to one another is responsible for the exaggeration of the shape-resonance effects In calculations where
Table I Ap values for the orbit& of Ccl,, hfolecuie
CHCI,.
CH2C12 and CH,Cl Character [9]
L\jJ”’
NC11 n(CI) n(Cl) o(CCI)
0.70 0.34 0.40
61?
11.69 12.62 13.44 16.5fi
2az
11.4s 11.91 12.85
n(CI) n(C1) n(C0
0.67
9a, i7e’ 2e” 6e’
15.99
o(CCI)
0.45
Sa,
16.96
o(CCI)
0.24
7b, +3b, 9a, -t-z%,-
11.40 12.22
n(C1) n(C1)
0.60 0.62
tib,
15.30
o(CCI)
0.53
Sal
15.94
0 (ccl)
0.20
2b,
16.17
O(%I,)
0.62
3e 7ai 2s
11.29 14.42 15.47
n(CI) u(CCI) a(%,,)
0.73 0.24 0.36
Orbital 191
ionization potential 191
CCI,
211 712 2e
CHCI,
CH,CI,
CH,CI
“’ d/3 = /3(PE = 10 eV)-/3(PE
Table 2
Theoretical positions of the shape resonances predicted to give a strong maximum in the cross section [IO] Orbital
Theoretical position n’ (photoelectron energy in eV)
21,
2.4 (X-e)
712
4.4 (ke) ”
2.4 (x-t,) 6.4(kt,) 2.6 (x-t,) 8.6 (x-t,)
61~
2.4 (ke) 8.4(x-t,)
a) PosiIion of the maximum in the cross section for the continuum channel shown in parentheses. b’ This is the same resonance as seen in the 21, and 6t, orbit& at 2.4 eV.
0.50 0.49
= Z eV).
averaging over several internuclear distances was incorporated, the effects of the shape resonance on p tend to decrease in magnitude. This was found to be the case for the shape resonance found in the fourth band of CO2 [ 1 l- 131. In that instance, the shape resonance was also predicted at a lower energy than it was observed at experimentally_
2e
0.00
The disagreement is also due in large part to the energy at which theory predicts the Cooper minimum in this orbital. The Cooper minima in the orbitals of Ccl, have been studied by Carlson et al. 161. In the 2t, orbital of Ccl, [6] a maximum in p is seen experimentally at = 31 eV, and a minimum in p, corresponding to the Cooper minimum, at = 44 eV. Theoretically, these features are predicted at photon energies of = 22 eV and = 26 eV respectively. Thus, the overall theoretical curve is compressed toward lower energy. When the experimental results are examined over the complete range of energy from 14 to 70 eV and compared with the theoretical curve, there is good qualitative agreement between experiment and theory although the various features in the two curves are offset in energy. The shape resonances predicted at photon energies of 17.0 and 19 eV for the 7tz orbital are not evident in the experimental ~(IzP) curve. Once again, agreement between the experimental and theoretical curves is not good. As was mentioned in the discussion of the shape resonances in the 2t, orbital, the positions of these two resonances may
be different
than theoretically
predicted.
The pos-
sible effects of different shape-resonance positioning have been discussed above along with the effects of assuming fixed nuclei in the calculations. The agreement between experiment and theory suffers from the same compression of the theoretical curve as was seen in the 2t, orbital. A maximum and minimum (Cooper minimum) in fl were seen experimentally [6] at = 34 eV and = 44 eV respectively. These features were predicted at = 24 eV and = 38 eV respectively_ If the low- and high[6] energy results in combination are compared with the theoretical results. good qualitative agreement is obtained. The theoretical calculations also predict the existence of two shape resonances (16 and 22 eV) in the 2e orbital of Ccl,. However, in this case the resonances are well separated in energy and thus the interference effects should be minimal_ The shapes of the theoretical and experimental p(!zv) curves are very similar and although the theoretical curve lies slightly below the experimental. the overall agreement is good. The agreement at higher energies [6] is also reasonably good. The effects of the two resonances are clearly evident in the experimental curve. although the depth of the minimum at = 20 eV is not as large as expected_ The agreement in this case suggests that lack of evidence for shape resonances in the first two orbitals is due mainly to interference effects of shape resonances separated slightly in energy. The effects of assuming fixed nuclei in the calculations should not be ignored, however, since this is still a likely explanation for the difference in the minimum depth at = 20 eV for the 2e orbital. In the 6tz orbital. shape resonances are predicted at 19 and 25 eV. From the argument put fonvard above for the 2e orbital. the effects of the resonances as predicted by the theoretical results should be evident in the experimental /3( hv) curve. This is indeed the case. The overall shapes of the curves are the same. Once again. there is some disagreement in the magnitude of the curves. but in the opposite sense of that seen in the 2e orbital. Although MS Xa calculations were not carried out on the other molecules investigated. it would seem Iogical that these molecules possess shape resonances as well. Experimentally. there is some
evidence
for
the
existence
of shape
resonances.
The /3(/rrl) curve of the 2e” orbital of CHCl, has a shape remarkably similar to that seen for the 2e orbital of Ccl,_ The Sa, orbit& of CHCl, and CH,Cl z both show broad depressions in their experimental fl(hv) curves lvhich are similar in some respects to P(hz~) curve of the 6t, orbital of Ccl,. Although the assignment of these features to be shape-resonance effects appears promising. further judgement should be withheld until theoretical calculations have been carried out on each of these molecules.
4. Conclusions The angular distribution data presented in paper have demonstrated the applicability of generalizations made earlier ]I-5] concerning energy dependence of /3 near threshold. to molecular systems. The CJorbitals examined in
this the the new this
work exhibited energy depsndences quite similar to those observed for the u orbitals investigated previously. The non-bonding chlorine. n(CI). orbitals were also found to possess an energ!. dependence tvhich 1~~9s similar for all the n(C1) orbitals investigated. On the average. the snergy dependence of p for the n(U) orbitals (0.56) was found to be slightly xveaker. than that found for the 5 orbitals (0.7s) although there is considerable overlap betlveen the _I/3 ranges of the t\vo orbital classifications_ The comparison of experimental and theoretical angular distribution results for the first four orbitals of Ccl, indicated that the agreement hetween experiment and theory was highly depcndent upon the relative spacings or the two strong shape resonances xv-hose existence is predicted in each of the four orbitals. In the Zt, and 7t, orbitals of Ccl,. qualitative agreement was obtained only when the combined lo\v- and high- [ 131 energy experimental results were compared with theory_ This lack of agreement at low energy was attributed to the incorrect predictions of the relative positions of the two shape resonances and thus. to a miscalculation of the interferences of one resonance with the other. When the predicted shape
resonances
were spaced
relatively
far apart.
276
as was the case in the 2e and 6t, orbitals of Ccl,. good agreement between experiment and theory was found even at low energy. The results of these comparisons could then be used to make some preliminary judgments on the possible existence of shape resonances in the other chloromethanes for which theoretical calculations have yet to be carried out.
Acknowledgement One of the authors (FAG) gratefully acknowledges the support of the University of Tennessee Computing Center and of the Synchroton Radiation Center for the Visiting Investigator appointment.
References [I] D. Mehaffy. P.R. Keller. J.W. Taylor. T.A. Cartson, M.O. Krause. F.A. Grimm and J.D. Allen Jr.. J. Electron Speclry. 26 (1982) 213.
[Z] P.R. Keller. D. Mehaffy. J.W. Taylor. F.A. Grimm and T-A. Carlson. J. Electron Spectry. 27 (1982) 223. 131 D. Mehaffy. P.R. Keller. J.W. Taylor, T.A. Carlson and F.A. Grimm. J. Electron Spectry. 28 (1983) 239. (41 M.N. Pianustelli. P.R. Keller. J.W. Taylor. F.A. Grimm and T.A. Carlson, J. Am. Chem. Sot. 105 (1983) 4235. [5] P.R. Keller. J.W. Taylor. T.A. Carlson and F.A. Grimm. J. Electron Spectry.. to be published. [6] T.A. Carlson. M.O. Krause. F.A. Grimm. P.R. Keller and J.W. Taylor. J. Chem. Phys. 77 (1982) 5340. 17) D. Mehaffy. P.R. Keller. J.W. Taylor and J.D. Allen Jr.. to be published. IS] T.A. Carlson and R.M. White. Faraday Discussions Chem. Sot. 54 (1972) 285. [9] K. Kimura. S. Karsumara. Y. Achiba. T. Yamazaki and S. Iwata. Handbook of He(l) photoelectron spectra of fundamental organic molecules (Halsted Press, New York, 1981). [IO] F.A. Grimm. to be published. [ 1 I] J.R. Swanson, D. Dill and J.L. Dehmer. J. Phys. B13 (1980) L231: B14 (1981) L207. 1121 T.A. Carlson. M.O. Krause. F.A. Grimm. J.D. Allen Jr.. D. Mehaffy, P.R. Keller and J.W. Taylor. Phys. Rev. A23 (1981) 3316. 1131 F-A. Grimm, J.D. Allen Jr.. T.A. Carlson, M.O. Krause. D. Mehaffy. P.R. Keller and J.W. Taylor. J. Chem. Phys. 75 (1981)
92.