- Email: [email protected]
Anisotropic Auger emission from molecules
s __
__ EB
Nuclear Instruments and Methods in Physics Research B 99 (1995) 68-71
NIOMI B
Beam Interactions with Materials & Atoms
ELSEVIER
Anisotropic Auger emission from molecules U. Becker
*, A. Menzel ’
Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-141 95 Berlin, Germany
Abstract Auger decay of molecules shows normally no or very little anisotropy. However, there are two important exceptions: resonant Auger decay and normal Auger decay in the vicinity of a molecular shape resonance, depending on the symmetry of the resonantly excited state. Because these symmetries are associated with preferred molecular orientations when dipole allowed photoabsorption takes place, the subsequent Auger decay will be effected by this photon induced molecular alignment. This statement holds also in case of K-shell photo-excitation and -ionization as has been demonstrated recently for the case of CO [O. Hemmers, F. Heiser, R. Wehlitz, J. Eiben and U. Becker, Phys. Rev. L&t. 71 (1993) 9871. However, this close relationship between absorption anisotropy and Auger-emission anisotropy is only fulfilled for the lowest bound and continuum resonances, it breaks down when higher and higher Rydbergorbitals are reached. Another source of anisotropic Auger emission is the fast dissociation of the excited molecule before Auger decay takes place. In this case the anisotropy reflects more the symmetry of the atomic components (LCAO) rather than the symmetry of the whole molecular state. The Auger spectroscopy of dissociating molecules allows therefore to examine the fractional intensities of these components in some detail.
Auger electron emission differs from photoelectron emission in two significant ways: i) it takes place at a fixed kinetic energy, and ii) its anisotropy reflects the dynamical properties of both emission processes: photoelectronand Auger emission. It is the second point which gives rize to the smaller electron ejection anisotropy normally observed in Auger decay compared to photoelectrons. This is because the Coulomb interaction governing the Auger decay has no directional preference as in the case of the electric field vector in direct photoionization, the only quantization axis is the alignment induced by the primary photoionization process. If this alignment is small, the anisotropy of the subsequent Auger decay will be reduced correspondingly. This is reflected by the expression for the angular distribution of the emitted Auger electrons after dipole excitation or ionization [l]: w(e)
= (&/4=)[1+
a,A,,Pz(cos
e>],
(1)
where W,, is the total decay intensity, AZ0 the alignment parameter of the intermediate state, which depends on the dynamics of the primary ionization process, CQ the intrin-
* Corresponding author. Tel. +49 30 8413 5694, fax f49 30 8413 5695, e-mail becker_ [email protected].
’ Present adress: Department of Physics, Florida, Orlando, FL 32816-2385, USA.
University
of Central
sic anisotropy coefficient, which contains the decay matrix element and P, the second-order Legendre polynomial. Hence the experimentally observed Auger angular anisotropy, p = A 2. a2 depends critically on the size of the alignment parameter. This result is equally valid for both atomic and molecular Auger decay [l]. However, for molecules, the chance for strongly anisotropic Auger emission is even worse than in case of atomic transitions. This results from the large number of close lying molecular Auger transitions which may have quite different intrinsic anisotropies. The average anisotropy measured by low resolution Auger spectroscopy will therefore show predominantly an isotropic decay emission pattern, a result corroborated by most angle resolved Auger emission studies on molecules. However, because this is not the consequence of a general rule or symmetry argument, rather than the unfavorable combination of several independent factors diminishing the possible anisotropy of the Auger decay, there must be cases where this anisotropy becomes apparent. The key points in order to reveal such anisotropies are large alignment and high resolution. Where the latter point is purely instrumental the first one requires a careful selection of ionization- or excitation energies and processes. There are two ways how to create large molecular alignment in the initial state of the possible Auger transition, one is photoexcitation instead of photoionization and the other is photoionization associated with excitation. In both cases large Auger anisotropies can be expected and have been, in fact, experimentally found [2]. This will be
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00747-0
U. Becker, A. Menzel/Nucl.
69
Ins@. and Meth. in Phys. Rex B 99 (199.5) 68-71
demonstrated by two examples: the KVV Auger emission of CO and the LVV/LMM Auger emission of HCl. The selectivity of these Auger processes regarding the excitation energy makes synchrotron radiation the excitation source of choice. The case of KVV emission in CO, but also in other molecules, is an even more intriguing problem, because K-shell filling Auger decay is in atoms principally isotropic due to the isotropy of the K-shell hole which allows no alignment. However, molecules are different in this respect; they may show anisotropic behavior due to symmetry arguments based on their molecular structure. For example in a a + n * excitation molecules are preferentially excited with an axis perpendicular to the electric vector whereas in a u + cr * excitation the molecules oriented parallel to electric vector are preferred, hence the name, perpendicular and parallel transitions. This gives the basis for any anisotropy to be expected in KW Auger decay. The show case example for the anisotropy effect in molecular KVV Auger decay was the 7~* and u * C(ls) excitation of CO [3]. Both cases represent photoexcitation rather than photoionization, although the u * resonance is a quasi-bond state excitation in the photoionization continuum yielding normal KVV Auger transition rather than resonant Auger transitions as in the case of 7~*. Fig. la shows two spectra taken under 0” and 54.7” with respect to the electric vector on the m * resonance, whereas Fig. lb shows the corresponding spectra, but in the vicinity of the (T* shape resonance. The bars in the 54.7” spectra show line components used to obtain a reasonable fit of the whole spectrum, whereas the bars underneath the spectra give the p values of these components along with the average /3 values for specific line groups. The resonant Auger spectrum shows quite distinct Auger asymmetries towards negative p values as expected from the results of earlier work [4]. However, this high-resolution measurement reveals that the anisotropy of the different Auger lines, including participator transitions, varies considerably, even changing sign. This may give rise to nearly isotropic electron emission if one considers the unresolved intensity of all transitions. Nevertheless, the observed variation in the intrinsic anisotropies is in good accord with corresponding observations in the decay spectra of core excited atoms (see, e.g., Ref. [2]). A more surprising result were the asymmetries seen recently in the regular C(KVV) Auger spectrum [2], as is shown in Fig. 1b. The observed Auger spectra gave clear evidence for non-isotropic emission behavior as predicted by Dill et al. [s] more than a decade ago. Considering the Auger anisotropies these are much larger on the IT * resonance than on the u * shape resonance due to the substantially lower alignment in the cr * resonance. This is a direct manifestation how strongly the Auger anisotropy depends on the alignment created in the photoabsorption process as mentioned at the beginning. Nevertheless, it demonstrates that anisotropic KW Auger emission is possible, although
240
245
250
255
260
265
270
27:
Kinetic energy (eV)
230
235
240
245
250
255
260
265
Kinetic energy (eV) Fig. 1. C(KVV) Auger spectra of CO in the vicinity of the (a) n* and (b) cr * resonance. The solid curve represents the angle-independent spectrum taken under the “magic angle” of 54.7” in Eq. cl), whereas the dotted line shows for comparison the spectra taken under 0” with respect to the electric vector of the incoming soft X-ray radiation. The bars underneath the spectra represent fitted line components, the angular-distribution asymmetry parameters p derived for these components are shown underneath each figure in the form of a separate bar diagram.
on a much reduced level compared to resonant Auger decay. In order to look for strong alignment in non-resonant Auger emission, satellite transitions may be a good candidate because the additional excitation along with ionization can potentially create strong alignment if this excitation is a dipole excitation corresponding to a conjugate shake up transition [6]. The C(ls) satellite spectrum of CO shows some satellites which have been designated as conjugate shake up satellites. Auger decay of such an excited ionic state results in an Auger satellite line shifted to higher kinetic energies with respect to the diagram lines. Our studies showed that the Auger satellite lines associated with conjugate shake up transitions displayed indeed a non-isotropic angular distribution outside of the CT* shape
I. ATOMIC/MOLECULAR
PHYSICS
V. Becker, A. Menzel /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 68-71
70
resonance. However, most of the Auger satellite lines are very weak and, therefore, not easy to study. A last point of anisotropic Auger emission concerns on resonant Auger decay, but on atomic-like transitions following fast dissociation of the photoexcited molecule [7]. A show case example for the competition between atomic and molecular decay processes is the 2p excitation of HCl. The 2p subshell of HCl is predominantly an atomic-like orbital whereas the unoccupied u * orbital, to which the excitation takes place, is an antibonding molecular orbital. Whether one describes the atomic-like orbital in terms of molecular ones of rr and u symmetry, or vice versa, in both cases parallel and perpendicular transitions are allowed, because no selection rule applies in the same way to both orbitals. Consequently, the molecular alignment created in this photoabsorption process is relatively small. On the other hand, atomic-like p-electrons are subject to strong alignment in terms of non-uniform magnetic sub-
resonant
‘s I
‘D
‘P
I
1
HCl
1
h
spectrum -11’
j 176
hu=200.8eV
i
’
’
176
Kinetic
’
160
j
energy
’
’
162
’
164
’
(eV)
Fig. 3. Resonant 2p,,* + 60 * spectra taken under two different angles along with the anisotropy parameter /? of the fitted atomic-like components. The thin dotted lines represent the nonresonant 50-t intensity as seen in the original spectra before substraction of a non-resonant reference spectrum.
difference
10
15 BirZing
spectrum
e2nsergy3PeV) 35
J-l
1
Fig. 2. (a) Non-resonant, (b) resonant and (c) difference spectrum representing the pure decay spectrum of the HCI 2p,,, + 6a * excitation at hv = 200.8 eV. The dotted line shows the atomic-like spectrum generated by a fitting procedure of the upper part of the individual lines with Gaussian profiles.
level population of the excited atomic system. Such aligned excited systems may decay via strongly anisotropic Auger decay. The question is whether the Auger transition is fast enough that the decay occurs outside the Franck-Condon regime. Fig. 2 shows how the resonant Auger spectrum of HCl, obtained after 2p,,, -+ 6a * excitation, is derived by subtraction of a non-resonant spectrum from a resonant spectrum. The spectrum consists of two parts: a molecular and atomic-like. Because 2p-excited Cl * is isoelectronic to Arc the atomic-like Auger transitions should have therefore a similar Auger spectrum as Ar LMM. The mechanism governing the intensity distribution between these two decay modes was subject of different studies [7,8] and will not be discussed in the present context. Rather it is
U. Becker, A. Menzel/
71
Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 68-71
taken as an experimental fact that atomic-like Auger transitions appear in the dissociative ionization of photoexcited molecules. The question raised in the present context is whether or not are the atomic-like transitions isotropic or non-isotropic regarding their angular distribution. We show here that the angular distribution is in fact anisotropic. Fig. 3 shows two atomic-like Auger spectra derived by a fitting procedure of Argon-like structures in the difference spectrum. The raw spectra were taken under 54.7” and 0” with respect to the electric vector of the exciting synchrotron radiation of the undulator Ul of the Berlin Synchrotron Radiation Laboratory BESSY [9]. The energy of the monochromatized radiation was chosen to excite the Cl resonance at 200.8 eV. The bottom part of 2p,,, -+ 6~ * . _ this figure shows the anisotropy parameter j3 of the emitted Auger electrons. The emission pattern is clearly anisotropic showing the strongest anisotropy for the ‘S line. Because the corresponding Auger transitions have only one outgoing partial wave its intrinsic anisotropy is a geometrical factor, in this case - 1 [I]. The observed /3 parameter of 0.68 is therefore a direct measure for the atomic alignment of the system being A,, = -0.68. The question comes about in what terms this alignment may be interpreted. We suggested the following interpretation. The excited 6a * orbital consists, besides p-like orbitals, of s-like and d-like atomic orbitals due to the polarization of the excited molecule [lo]. In terms of dipole transition amplitudes between atomic-like components both components, s and d, are excited with a relative intensity being reflected in the alignment of the excited atomic-like system. After fast dissociation of the excited molecular system the created alignment remains with the excited chlorine atom, causing the anisotropy of the subsequent Auger decay. Following this interpretation one may consider this Auger anisotropy as a monitor of atomic-like components in molecular orbitals which are otherwise not accessible.
However, this interpretation has to be proven by a more profound theoretical analysis. In summary, we have shown, that molecular Auger emission may be anisotropic if the alignment of the photoionized or photoexcited molecular system is large. This condition is primarily fulfilled in cases of photoexcitation rather than for non-resonant photoionization. Given this, large Auger anisotropy can be expected, and has been in fact observed, even in case of K-shell excitation.
Acknowledgement This work was supported Forschung und Technologie.
by the Bundesminister
fur
References (11 N.M. Kabachnik and 1.P. Sazhina, J. Phys. B 17 (1984) 1335. [2] U Hergenhahn and U. Becker, J. Electr. Spectrosc. Rel. Phenom., in press. 131 0. Hemmers, F. Heiser, J. Eiben. R. Wehlitz and U. Becker, Phys. Rev. Lett. 71 (1993) 987. [4] U. Becker, R. Holzel, H.-G. Kerkhoff, B. Langer, D. Szostak and R. Wehlitz, Phys. Rev. Len. 56 (1986) 1455. [5] D. Dill, J.R. Swanson. S. Wallace and J.L. Dehmer, Phys. Rev. Lett. 45 (1980) 1393. [6] M.O. Krause and C.D. Caldwell, Phys. Rev. Lett. 59 (1987) 2736. [7] P. Morin and 1. Nenner, Phys. Rev. Lett. 56 (1986) 1913. [8] Z.F. Liu. G.M. Bancroft, K.H. Tan and M. Schachter, Phys. Rev. A 48 (1993) 4019. [9] W. Peatman, C. Carbone, W. Gudat, W. Heinen, P. Kuske, J. Pfliiger, F. Schlfers and T. Schroeter, Rev. Sci. Instr. 60 (1989) 1445. [lo] W.H.E. Schwarz. Chem. Phys. 11 (1975) 217.
I. ATOMIC/MOLECULAR
PHYSICS
__ EB
Nuclear Instruments and Methods in Physics Research B 99 (1995) 68-71
NIOMI B
Beam Interactions with Materials & Atoms
ELSEVIER
Anisotropic Auger emission from molecules U. Becker
*, A. Menzel ’
Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-141 95 Berlin, Germany
Abstract Auger decay of molecules shows normally no or very little anisotropy. However, there are two important exceptions: resonant Auger decay and normal Auger decay in the vicinity of a molecular shape resonance, depending on the symmetry of the resonantly excited state. Because these symmetries are associated with preferred molecular orientations when dipole allowed photoabsorption takes place, the subsequent Auger decay will be effected by this photon induced molecular alignment. This statement holds also in case of K-shell photo-excitation and -ionization as has been demonstrated recently for the case of CO [O. Hemmers, F. Heiser, R. Wehlitz, J. Eiben and U. Becker, Phys. Rev. L&t. 71 (1993) 9871. However, this close relationship between absorption anisotropy and Auger-emission anisotropy is only fulfilled for the lowest bound and continuum resonances, it breaks down when higher and higher Rydbergorbitals are reached. Another source of anisotropic Auger emission is the fast dissociation of the excited molecule before Auger decay takes place. In this case the anisotropy reflects more the symmetry of the atomic components (LCAO) rather than the symmetry of the whole molecular state. The Auger spectroscopy of dissociating molecules allows therefore to examine the fractional intensities of these components in some detail.
Auger electron emission differs from photoelectron emission in two significant ways: i) it takes place at a fixed kinetic energy, and ii) its anisotropy reflects the dynamical properties of both emission processes: photoelectronand Auger emission. It is the second point which gives rize to the smaller electron ejection anisotropy normally observed in Auger decay compared to photoelectrons. This is because the Coulomb interaction governing the Auger decay has no directional preference as in the case of the electric field vector in direct photoionization, the only quantization axis is the alignment induced by the primary photoionization process. If this alignment is small, the anisotropy of the subsequent Auger decay will be reduced correspondingly. This is reflected by the expression for the angular distribution of the emitted Auger electrons after dipole excitation or ionization [l]: w(e)
= (&/4=)[1+
a,A,,Pz(cos
e>],
(1)
where W,, is the total decay intensity, AZ0 the alignment parameter of the intermediate state, which depends on the dynamics of the primary ionization process, CQ the intrin-
* Corresponding author. Tel. +49 30 8413 5694, fax f49 30 8413 5695, e-mail becker_ [email protected].
’ Present adress: Department of Physics, Florida, Orlando, FL 32816-2385, USA.
University
of Central
sic anisotropy coefficient, which contains the decay matrix element and P, the second-order Legendre polynomial. Hence the experimentally observed Auger angular anisotropy, p = A 2. a2 depends critically on the size of the alignment parameter. This result is equally valid for both atomic and molecular Auger decay [l]. However, for molecules, the chance for strongly anisotropic Auger emission is even worse than in case of atomic transitions. This results from the large number of close lying molecular Auger transitions which may have quite different intrinsic anisotropies. The average anisotropy measured by low resolution Auger spectroscopy will therefore show predominantly an isotropic decay emission pattern, a result corroborated by most angle resolved Auger emission studies on molecules. However, because this is not the consequence of a general rule or symmetry argument, rather than the unfavorable combination of several independent factors diminishing the possible anisotropy of the Auger decay, there must be cases where this anisotropy becomes apparent. The key points in order to reveal such anisotropies are large alignment and high resolution. Where the latter point is purely instrumental the first one requires a careful selection of ionization- or excitation energies and processes. There are two ways how to create large molecular alignment in the initial state of the possible Auger transition, one is photoexcitation instead of photoionization and the other is photoionization associated with excitation. In both cases large Auger anisotropies can be expected and have been, in fact, experimentally found [2]. This will be
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00747-0
U. Becker, A. Menzel/Nucl.
69
Ins@. and Meth. in Phys. Rex B 99 (199.5) 68-71
demonstrated by two examples: the KVV Auger emission of CO and the LVV/LMM Auger emission of HCl. The selectivity of these Auger processes regarding the excitation energy makes synchrotron radiation the excitation source of choice. The case of KVV emission in CO, but also in other molecules, is an even more intriguing problem, because K-shell filling Auger decay is in atoms principally isotropic due to the isotropy of the K-shell hole which allows no alignment. However, molecules are different in this respect; they may show anisotropic behavior due to symmetry arguments based on their molecular structure. For example in a a + n * excitation molecules are preferentially excited with an axis perpendicular to the electric vector whereas in a u + cr * excitation the molecules oriented parallel to electric vector are preferred, hence the name, perpendicular and parallel transitions. This gives the basis for any anisotropy to be expected in KW Auger decay. The show case example for the anisotropy effect in molecular KVV Auger decay was the 7~* and u * C(ls) excitation of CO [3]. Both cases represent photoexcitation rather than photoionization, although the u * resonance is a quasi-bond state excitation in the photoionization continuum yielding normal KVV Auger transition rather than resonant Auger transitions as in the case of 7~*. Fig. la shows two spectra taken under 0” and 54.7” with respect to the electric vector on the m * resonance, whereas Fig. lb shows the corresponding spectra, but in the vicinity of the (T* shape resonance. The bars in the 54.7” spectra show line components used to obtain a reasonable fit of the whole spectrum, whereas the bars underneath the spectra give the p values of these components along with the average /3 values for specific line groups. The resonant Auger spectrum shows quite distinct Auger asymmetries towards negative p values as expected from the results of earlier work [4]. However, this high-resolution measurement reveals that the anisotropy of the different Auger lines, including participator transitions, varies considerably, even changing sign. This may give rise to nearly isotropic electron emission if one considers the unresolved intensity of all transitions. Nevertheless, the observed variation in the intrinsic anisotropies is in good accord with corresponding observations in the decay spectra of core excited atoms (see, e.g., Ref. [2]). A more surprising result were the asymmetries seen recently in the regular C(KVV) Auger spectrum [2], as is shown in Fig. 1b. The observed Auger spectra gave clear evidence for non-isotropic emission behavior as predicted by Dill et al. [s] more than a decade ago. Considering the Auger anisotropies these are much larger on the IT * resonance than on the u * shape resonance due to the substantially lower alignment in the cr * resonance. This is a direct manifestation how strongly the Auger anisotropy depends on the alignment created in the photoabsorption process as mentioned at the beginning. Nevertheless, it demonstrates that anisotropic KW Auger emission is possible, although
240
245
250
255
260
265
270
27:
Kinetic energy (eV)
230
235
240
245
250
255
260
265
Kinetic energy (eV) Fig. 1. C(KVV) Auger spectra of CO in the vicinity of the (a) n* and (b) cr * resonance. The solid curve represents the angle-independent spectrum taken under the “magic angle” of 54.7” in Eq. cl), whereas the dotted line shows for comparison the spectra taken under 0” with respect to the electric vector of the incoming soft X-ray radiation. The bars underneath the spectra represent fitted line components, the angular-distribution asymmetry parameters p derived for these components are shown underneath each figure in the form of a separate bar diagram.
on a much reduced level compared to resonant Auger decay. In order to look for strong alignment in non-resonant Auger emission, satellite transitions may be a good candidate because the additional excitation along with ionization can potentially create strong alignment if this excitation is a dipole excitation corresponding to a conjugate shake up transition [6]. The C(ls) satellite spectrum of CO shows some satellites which have been designated as conjugate shake up satellites. Auger decay of such an excited ionic state results in an Auger satellite line shifted to higher kinetic energies with respect to the diagram lines. Our studies showed that the Auger satellite lines associated with conjugate shake up transitions displayed indeed a non-isotropic angular distribution outside of the CT* shape
I. ATOMIC/MOLECULAR
PHYSICS
V. Becker, A. Menzel /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 68-71
70
resonance. However, most of the Auger satellite lines are very weak and, therefore, not easy to study. A last point of anisotropic Auger emission concerns on resonant Auger decay, but on atomic-like transitions following fast dissociation of the photoexcited molecule [7]. A show case example for the competition between atomic and molecular decay processes is the 2p excitation of HCl. The 2p subshell of HCl is predominantly an atomic-like orbital whereas the unoccupied u * orbital, to which the excitation takes place, is an antibonding molecular orbital. Whether one describes the atomic-like orbital in terms of molecular ones of rr and u symmetry, or vice versa, in both cases parallel and perpendicular transitions are allowed, because no selection rule applies in the same way to both orbitals. Consequently, the molecular alignment created in this photoabsorption process is relatively small. On the other hand, atomic-like p-electrons are subject to strong alignment in terms of non-uniform magnetic sub-
resonant
‘s I
‘D
‘P
I
1
HCl
1
h
spectrum -11’
j 176
hu=200.8eV
i
’
’
176
Kinetic
’
160
j
energy
’
’
162
’
164
’
(eV)
Fig. 3. Resonant 2p,,* + 60 * spectra taken under two different angles along with the anisotropy parameter /? of the fitted atomic-like components. The thin dotted lines represent the nonresonant 50-t intensity as seen in the original spectra before substraction of a non-resonant reference spectrum.
difference
10
15 BirZing
spectrum
e2nsergy3PeV) 35
J-l
1
Fig. 2. (a) Non-resonant, (b) resonant and (c) difference spectrum representing the pure decay spectrum of the HCI 2p,,, + 6a * excitation at hv = 200.8 eV. The dotted line shows the atomic-like spectrum generated by a fitting procedure of the upper part of the individual lines with Gaussian profiles.
level population of the excited atomic system. Such aligned excited systems may decay via strongly anisotropic Auger decay. The question is whether the Auger transition is fast enough that the decay occurs outside the Franck-Condon regime. Fig. 2 shows how the resonant Auger spectrum of HCl, obtained after 2p,,, -+ 6a * excitation, is derived by subtraction of a non-resonant spectrum from a resonant spectrum. The spectrum consists of two parts: a molecular and atomic-like. Because 2p-excited Cl * is isoelectronic to Arc the atomic-like Auger transitions should have therefore a similar Auger spectrum as Ar LMM. The mechanism governing the intensity distribution between these two decay modes was subject of different studies [7,8] and will not be discussed in the present context. Rather it is
U. Becker, A. Menzel/
71
Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 68-71
taken as an experimental fact that atomic-like Auger transitions appear in the dissociative ionization of photoexcited molecules. The question raised in the present context is whether or not are the atomic-like transitions isotropic or non-isotropic regarding their angular distribution. We show here that the angular distribution is in fact anisotropic. Fig. 3 shows two atomic-like Auger spectra derived by a fitting procedure of Argon-like structures in the difference spectrum. The raw spectra were taken under 54.7” and 0” with respect to the electric vector of the exciting synchrotron radiation of the undulator Ul of the Berlin Synchrotron Radiation Laboratory BESSY [9]. The energy of the monochromatized radiation was chosen to excite the Cl resonance at 200.8 eV. The bottom part of 2p,,, -+ 6~ * . _ this figure shows the anisotropy parameter j3 of the emitted Auger electrons. The emission pattern is clearly anisotropic showing the strongest anisotropy for the ‘S line. Because the corresponding Auger transitions have only one outgoing partial wave its intrinsic anisotropy is a geometrical factor, in this case - 1 [I]. The observed /3 parameter of 0.68 is therefore a direct measure for the atomic alignment of the system being A,, = -0.68. The question comes about in what terms this alignment may be interpreted. We suggested the following interpretation. The excited 6a * orbital consists, besides p-like orbitals, of s-like and d-like atomic orbitals due to the polarization of the excited molecule [lo]. In terms of dipole transition amplitudes between atomic-like components both components, s and d, are excited with a relative intensity being reflected in the alignment of the excited atomic-like system. After fast dissociation of the excited molecular system the created alignment remains with the excited chlorine atom, causing the anisotropy of the subsequent Auger decay. Following this interpretation one may consider this Auger anisotropy as a monitor of atomic-like components in molecular orbitals which are otherwise not accessible.
However, this interpretation has to be proven by a more profound theoretical analysis. In summary, we have shown, that molecular Auger emission may be anisotropic if the alignment of the photoionized or photoexcited molecular system is large. This condition is primarily fulfilled in cases of photoexcitation rather than for non-resonant photoionization. Given this, large Auger anisotropy can be expected, and has been in fact observed, even in case of K-shell excitation.
Acknowledgement This work was supported Forschung und Technologie.
by the Bundesminister
fur
References (11 N.M. Kabachnik and 1.P. Sazhina, J. Phys. B 17 (1984) 1335. [2] U Hergenhahn and U. Becker, J. Electr. Spectrosc. Rel. Phenom., in press. 131 0. Hemmers, F. Heiser, J. Eiben. R. Wehlitz and U. Becker, Phys. Rev. Lett. 71 (1993) 987. [4] U. Becker, R. Holzel, H.-G. Kerkhoff, B. Langer, D. Szostak and R. Wehlitz, Phys. Rev. Len. 56 (1986) 1455. [5] D. Dill, J.R. Swanson. S. Wallace and J.L. Dehmer, Phys. Rev. Lett. 45 (1980) 1393. [6] M.O. Krause and C.D. Caldwell, Phys. Rev. Lett. 59 (1987) 2736. [7] P. Morin and 1. Nenner, Phys. Rev. Lett. 56 (1986) 1913. [8] Z.F. Liu. G.M. Bancroft, K.H. Tan and M. Schachter, Phys. Rev. A 48 (1993) 4019. [9] W. Peatman, C. Carbone, W. Gudat, W. Heinen, P. Kuske, J. Pfliiger, F. Schlfers and T. Schroeter, Rev. Sci. Instr. 60 (1989) 1445. [lo] W.H.E. Schwarz. Chem. Phys. 11 (1975) 217.
I. ATOMIC/MOLECULAR
PHYSICS