J . Phys. Chem. 1991, 95,4250-4254
4250
Penning Ionlzatlon Electron Spectroscopy of Dichlorobenzenes S. Fujisawa, I. Oonishi,+ Departments of Chemistry and Biomolecular Science, Faculty of Science, Toho University, Miyama, Funabashi-shi, Chiba 274, Japan
S.Masuda,K. Ohno, and Y. Harada* Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro- ku, Tokyo 153. Japan (Received: August 20, 1990; In Final Form: November 29, 1990)
He*(Z3S) Penning ionization electron spectra (PIES) and He I ultraviolet photoelectron spectra (UPS)of c-, m-, and p-dichlorobenzenes were measured under the same experimental conditions. The analysis of the relative band intensity of PIES was shown to give useful information on the spatial electron distribution of individual molecular orbitals. On the basis of the characteristics of PIES, all the bands in the UPS of dichlorobenzenes were assigned. Since the Penning ionization process is regarded as an electrophilic reaction of a metastable atom with a molecule, the relative intensity of the bands in PIES is related to the reactivity of the corresponding orbitals. It was confirmed that Penning ionization electron spectroscopy is a useful method for probing the reactivity of n. T , and u orbitals and the steric shielding effect of some orbitals upon the impact of an electrophilic reagent.
Introduction In Penning ionization electron spectroscopy we measure the kinetic energy distribution of electrons emitted following collisions between target molecules, M,and metastable atoms, A*:' M + A* M+ + A eIn this process, an electron in a molecular orbital of M is transferred to the lowest vacant orbital of A*, and the excited electron from A* is ejected.2 The probability of the electron transfer largely depends on the spatial overlap between the orbitals of M and A*; outer molecular orbitals of M exposed outside the repulsive molecular surface (van der Waals surface) interact with A* more effectively than inner ones, so that the former orbitals give larger intensities in the Penning ionization electron spectrum (PIES) than the latter. Thus, the relative band intensity of the spectrum reflects the spatial electron distribution of individual molecular orbitals of Mm3This characteristic of PIES has been used to assign bands in the ultraviolet photoelectron spectra (UPS) of various organic compounds and also to study the stereochemical properties of molecular orbitals.c6 Furthermore, such a specific feature of PIES has been applied to probe the geometrical orientation and electronic state of molecules lying at the outermost layer of solid surface.'** In the Penning ionization process, A* can be regarded as an electrophilic reagent, because A* extracts an electron from a molecular orbital of M. Considered from this viewpoint, the relative band intensity of PIES is closely related to the reactivity of the corresponding orbital. Furthermore, the introduction of a bulky group reduces the band intensity in PIES, because the reactivity of the molecular orbital decreases owing to the steric shielding of the group. This steric effect can also be discussed on the basis of the band intensity in PIES9 Previously, we measured the PIES and UPS of monoha~ogenobenzenes-C6HSF, C~HSCI, C6H5Brrand C6H51-and analyzed the data using the characteristics of PIES described above; we assigned the UPS bands to respective molecular orbitals and also discussed the reactivit and stereochemical property of x and nonbonding (n) orbitals.lg Such a study is of considerable chemical significance because it provides insight into the role of the 17 and n orbitals in the electrophilic reactions of these compounds. In the present work we have extended our study to dichlorobenzenes, which are expected to show various stereochemical properties owing to the presence of three isomers, 0-, m-, and p-dichlorobenzenes.
-
+
Department of Biomolecular Science.
0022-3654/9 1/2095-4250$02.50/0
The UPS of dichlorobenzenes have been previously measured by Streets et al.," Kimura et al.,Iz RuBEif: et al.," and Potts et al." However, there still remain some doubtful points in the band assignments. Therefore, we have reexamined the assignments on the basis of the characteristics of the PIES and also of ab initio MO calculations. Experimental Section The measurements of the PIES and UPS were performed by the apparatus reported before.I5 Helium metastable atoms, He*(2'S, 19.82 eV) and He*(2'S, 20.62 eV), were obtained by cold-cathode discharge with a current of 50 mA and a voltage of 240 V.I6 He*(2lS) atoms were eliminated by a quench lamp, and only He*(2%) atoms were allowed to enter a collision chamber, where Penning ionization took place by collisions between metastable atoms and target molecules. The electron spectra were obtained at an ejection angle of 90' with respect to the metastable beam or the photon beam by means of a 180° hemispherical analyzer. The relative band intensity of the spectra was calibrated by using the transmission efficiency curve of the ~pectrometer.~ The excitation source of the UPS was the He I (1) C€rmiik, V. J. Chem. Phys. 1966, 44, 3781-3186.
(2) Hotop. H.; Niehaus, A. Z . Phys. 1969, 228, 68-88. (3) Ohno, K.; Mutoh, H.; Harada, Y. J. Am. Chem. Soc. 1983, 105, 4555-4561. (4) Ohno, K.; Matsumoto, S.;Harada, Y. J. Chem. Phys. 1984. 81, 4441-4454. (5) Ohno, K.; Harada, Y. In Theoretical Models of Chemical Bonding, Part 3; Mabic, 2.B., Ed.;Springer: Berlin, to be published. (6) Harada, Y. Pure Appl. Chem. 1990,62, 451-462. ( 7 ) Harada. Y. S u r . Sci. 1985, 158, 455. (8) Harada, Y.; Ozaki, H. Jpn. J. Appl. Phys. 1987, 26, 1201. (9) Ohno, K.; Fujisawa, S.;Mutoh, H.; Harada, Y.J. Phys. Chem. 1982, 86,440-441. Ohno, K.; Imai, K.; Harada, Y. J . Am. Chem. Soc. 1985,107, 8018-8082. (10) Fujisawa, S.;Ohno, K.; Masuda, S.;Harada, Y. J . Am. Chem. Soc. 1986, 108, 6505-6511. (11) Streets, D. G.; Ceaser, G . P. Mol. Phys. 1973, 26, 1037. (12) Kimura, K.; Katsumata, S.;Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules: New York, 1981. (13) RuZiE, B.; Klasinc, L.;Wolf, A,; Knop, J. V. J. Phys. Chem. 1981, 85, 1486-1489. (14) Potts, A. W.; Lyus, M.L.;Lee, E. P. F.; Fattahallah, G . H. J. Chcm. Soc., Faraday Trans. 2 1980. 76, 556-570. ( I S ) Harada, Y.; Ohno, K.; Mutoh, H. J. Chem. Phys. 1983, 79, 325 1-3255. (16) Aoyama, M.; Masuda, S.;Ohno, K.; Harada, Y. J. Phys. Chem. 1989, 93, 1800-1805.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4251
PIES of Dichlorobenzenes
a;:
biW
45
TABLE I: Observed md Calcul.ted Idutioa Poteatiala for 0-DiChlOrokazcac
band no. 1 2 3 4 5
10
6
8
6 7 8 9 10 11 12 13 14
2
4
4(/eV 12
10
I
I
8 I
6
I
I
4
I
I
I
4 5
h.
He1 UPS
IP,/eV 9.24 9.65 11.26 11.76 11.76 12.37 12.65 13.28 13.70 14.56 14.85 15.70 15.95 16.99
IPaw/eV
MO character"
9.32 9.64 12.05 12.78 12.82 13.27 14.19 14.39 15.68 16.07 16.45 17.63 18.41 19.36
4bi(*3) 3a2(*2) 15b2(q) 3bl(nL) 16adnl) 2a2(nL) 14b2
15a1
2bl(*I) 1481 13b2 12b2
13a1 12a1
'The symbol nl or n, indicates the MO predominantly due to the chlorine 3p A 0 distributed parallel or perpendicular to the benzene ring. TABLE 11: Observed md Cdcubted IOaiutioa Potentials for m-Dicblorobenzew
J
I
I
U
10
,
12
I
1
,
I
I
16
16
14 I P/eV
Figure 1. Hc*(Z3S) PIES and He I UPS of o-dichlorobenzene.
band no. 1 2 3 4 5
6 7 8 9 10 11 12 13 14
IP,/eV 9.28 9.65 1 1.46 11.58 11.70 12.77 12.84 12.98 13.55 14.56 14.95 15.76 16.02 17.03
IPaW/eV 9.36 9.71 12.43 12.45 12.65 13.93 14.18 14.50 15.54 16.22 16.45 17.56 18.45 19.57
MO characteP
'The symbol nr or nL indicates the MO predominantly due to the chlorine 3p A 0 distributed parallel or perpendicular to the benzene ring. 8
10
6
TABLE III: Observed md Cdculrted IOaiuHoa Potentials for
2
4
p-Mchlorobeazew
Ek/eV I
8
10
12 I
I
I
I
6 I
I
2
4 I
I
I
1
3
He1 UPS
I P/eV Figure 2. He*(Z3S) PIES and He I UPS of m-dichlorobenzene.
band no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14
IPnkd/eV 8.95 9.84 11.37 11.46 11.60 12.65 12.77 12.96 13.46 14.55 15.03 15.75 15.95 16.96
IPmM/eV 9.20 9.96 12.29 12.46 12.56 14.04 14.09 14.66 15.49 15.85 16.85 17.86 18.48 19.13
MO characteP 3b3&*3) lbig(*2) 3b~u(nl) 6b3u(n1) 5b~,(q) 2b3&n~ 1 1Oa, 4ba %(*i)
9b1u
5b3u
8b1u 4th 9%
'The symbol n or n, indicates the MO predominantly due to the chlorine 3p A 0 distributed parallel or perpendicular to the benzene ring.
Calculations The molecular orbitals of e, m-, and pdichloro&nzna were ~ using calculated by the ab initio method with a 4 - 3 1 basis the library p r a m GSCR at the &mputer center, m e Univeffity of Tokyo.'
R€Sults Figures 1-3 show the transmission-corrected He*(2%, 19.82 eV) PIES and He I (21.22 eV) UPS of 0-, m-,and p-dichlorobenzenes. The electron energy Scales for the PI= are shifted relative to those for the UPS by the difference in the excitation energy. (21 2 2 - 19.82) eV = 1.40 eV, so that the correspondence between the PIES and UPS bands may be easily seen. Tables
(17) Ditchfield, R.;Hchrc, W.J.; Pople, A. J. Chcm. Phys. 1971, 51, 724-728.
(18) Kosugi, N. Program GSCFZ; Program Library, The Computer Center: The University of Tokyo, Tokyo, 1981.
resonance line produced by dc discharge in helium gas.
Fujisawa et al.
4252 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 bzu(n,J b3u(n,) b z g W
3
4
5
4
3A5
He1 UPS 7
I P/eV Figure 3, He*(23S) PIES and He I UPS of p-dichlorobenzene.
1-111 list the vertical ionization potentials (IPS) obtained from the UPS together with the calculated IP via Koopmans’ theorem. Figures 4-6 show the shapes of the occupied molecular orbitals of dichlorobenzenes. In the figures, the atomic orbitals are r e p resented by circles or pairs of ellipses, and their areas are proportional to the square of the orbital coefficients. The solid and broken circles indicate the s and out-of-plane p orbitals, respec- , tively, while the pairs of the ellipses designate the in-plane p orbitals. The thick and thin curves denote the lobes with the positive and negative signs, respectively.
Discussion A . Assignments of the UPS Bands. The UPS bands for dichlorobenzenes are assigned on the basis of the feature of PIES and the results of the MO calculation. The characteristics of the PIES of monochlorobenzene are also utilized for reference and summarized as follows:1o (1) The u bands and the n bands due to the chlorine 3p orbitals are generally enhanced relative to the u bands, because the u and n orbitals are exposed outside the repulsive molecular surface and hence interact with metastable atoms more effectively than the u orbitals, yielding stronger bands
in PIES. (2) The nIIband due to the C1 3p orbital distributed parallel to the benzene ring is weak in intensity compared to the nI distributed perpendicular to the ring, since the nr orbital is shielded by the benzene ring from the attack of a metastable atom. 1 . o-Dichlorobenzene. In dichlorobenzenes, there are three u orbitals (al, u2,and u3)derived mainly from the benzene u orbitals and four n orbitals (two nu and two nl). Being similar to the case of monochlorobenzene, these orbitals have large electron densities outside the molecular surface and are expected to give stronger bands in the PIES. As is seen in the PIES of o-dichlorobenzene (Figure l), bands 1-6 and 9 appear as prominent peaks in the higher & region (6-11 ev). Using this feature together with the results of the MO calculation, we can assign bands 1, 2, and 9 to the 4b1(uj), 3a2(u2),and 2bl(ul) orbitals, respectively. The remaining bands 3-6 are attributed to the n orbitals, i.e., 15bz(ni), 3bl(n,), 16al(nll),and 2a2(nl) orbitals. Owing to the shielding effect of the benzene ring, band 3 assigned to the nIIorbital is much weaker than band 6 assigned to the nI orbital in the PIES, in agreement with the case of monochlorobenzene mentioned above. Further, we have correlated the peak at IP = 11.76 eV in the UPS to two orbitals, 3bl(n,) and 16aI(nll), because it shows strong intensity and broad width in both the UPS and PIES. This is also supported by a slight difference in the calculated IP between these orbitals (see Table I). After the assignment of the u and n bands, we can relate bands 7 and 8 to the 14b2and 15al orbitals. These bands are clearly observed in the UPS, while the corresponding bands are almost missing in the PIES. This is partly because bands 7 and 8 are overlapped with strong band 6. However, the main reason probably lies in the fact that the 14b2and 15al orbitals have C-C bonding character distributed along the benzene frame (Figure 4) and do not interact with metastable atoms effectively. This is especially true for the 13al orbital, which is completely C-C bonding in character (Figure 4) and gives a much weaker band 13 in the PIES. In contrast to band 13, bands 10, 12, and 14 are not so weak in the PIES. We assigned these bands to the 14a,, 12b2, and 12al orbitals, respectively. The 14a, orbital has a character of the chlorine 3p orbital exposed outside the molecular surface along the C-Cl bond (Figure 4). The 12b2 orbital is related to the hydrogen 1s and the chlorine 3s and 3p orbitals, which are distributed outside the molecule. Further, the 12al orbital protrudes outside along the C-H bond of the benzene ring. Owing to these characters of the orbitals, they are considered to interact easily with metastable atoms to give strong bands in the PIES. Finally, a weak band S appears near Ek 4.5 eV in the PIES. The appearance of this band cannot be interpreted by a simple independent particle model of Penning ionization based on the electron transfer from the target molecule to the metastable atom. This band is probably a satellite one arising from the
-
w
H\ H’
A
iOI
\c,
I
15al
2bl(lrl)
14al
13b2
12 b2
Figure 4. Shapes of the occupied molecular orbitals of o-dichlorobenzene listed in Table I.
13al
12al
The Journal of Physical Chemistry, Vol. 95, No. I I, I991 4253
PIES of Dichlorobenzenes
H
2 blOrl)
11 b2 15al 14al Figure 5. Shapes of the occupied molecular orbitals of m-dichlorobenzene listed in Table 11. 16al
Figure 6. Shapes of the occupied molecular orbitals of pdichlorobenzenc listed in Table 111.
many-body effect.I9 The appearance of such a satellite is observed more clearly in the PIES of m-dichlorobenzene (Figure 2). The assignments of bands 1-4 and 7 in Table I are in agreement with those by RuEiE et al., except that they assigned bands 6 and 8 to the nI and ?rl orbital, respecti~ely.'~Further, they assigned band 9 to a u orbital. Since the band due to the n, orbital generally gives a strong intensity in the PIES, band 6 should be assigned to the 2a2(n,) orbital, as is supported by the result of the MO calculation. As for band 9 enhanced in the PIES, it is natural to assign it to the 2 b l ( r I )orbital, which is distributed outside the surface. On the other hand, band 8 showing a weak intensity in the PIES is due to the 15al(u) orbital. In the 14-18-eV region of the UPS, our assignments of u bands agree with those by RuSEiE et al. 2. m-Dichlorobenzene. Figure 2 shows the transmissioncorrected He*(2%) PIES and He I UPS of m-dichlorobenzene. As shown in Table 11, all the bands in the UPS were assigned from a comparison between the PIES and UPS and also from the results of the calculation. In the table bands 1-6 and band 9 were related to the 3a~(*3),W * Z ) , 13b2(nH),3bl(nL), 17al(nll),2a2(nL), and 2 b l ( ~ Iorbitals, ) respectively, as in the case of dichlorobenzene. The assignments of the r and n bands are in agreement with those by RuSEie et al.I3 However, they did not refer to band 7 at IP (19) Masuda, S.;Aoyama, M.; Ohno, K.; Harada, Y. Phys. Reo. Lerr.
1990,65, 3251-3260.
lob2
= 12.84 eV in the UPS. As described above, the UPS of o-dichlorobenzene exhibits two u bands between bands 6 and 9. Thus, we assigned the shoulder at IP = 12.84 eV to the 12b2orbital, which is supported by the results of the calculation. Our assignments of the other bands agree with those by RuSEiE et al. 3. pDichlorobenzene. The assignments of photoelectron bands for p-dichlorobenzene are listed in Table 111. In the PIES shown in Figure 3, bands 1-5,7, and 9 are observed as prominent peaks in the higher Ek region (6-11 eV). As in the cases of 0- and m-dichlorobenzenes, we assigned bands 1,2, and 9 to the 3bs(r3), 1blg(T2),and 2b2,(mI) orbitals, respectively. Since three n bands are overlapped in the PIES owing to the spread of the bands, the results of the calculation were used for their assignments; bands 3-5 are related to the 3b2,(n,), 6b3,(nu), and 5bz8(nli)orbitals, respectively. The remaining band 7 at IP = 12.77 eV is assigned to the 2b3,(n,) orbital. This assignment leads to the inversion of the calculated ordering of bands 6 and 7. In Figure 3 band 7 is much stronger than band 6. Accordingly, we assigned band 7 to the b3 (n,) orbital extending outside the molecule and the weak shoufder at IP = 12.65 eV (band 6) to the 10a,(u) orbital. The assignments of bands 1-4 and bands 11-14 by RuSEiE et ala" are in agreement with ours. However, they assigned bands 9 and 10 to the u and orbitals, respectively. Since the PIES band 9 has strong intensity as in the cases of 0- and m-dichlorobenzenes, this band originates from the 2b2,(r1) orbital. Further, RuSEiE et al. assigned band 8 to the n orbital of the chlorine atom. As is seen in Figure 3, band 8 is not observed clearly in the PIES, while the corresponding band appears as a shoulder in the UPS. This is probably because band 8 is related to the u orbital, which usually gives a weak intensity in the PIES. Moreover, they did not pick up bands 5 and 6 in the UPS. We assigned band 5 to the 5bz8(ni,)orbital from the results of the calculation and band 6 to the loa, orbital, because two u bands are expected to appear in the 12-14-eV region as in the cases of 0- and m-dichlorobenzenes. Potts et al.14have also reported the assignments of bands 1-10, which are in agreement with ours. B. Relative Reactivity of the Orbital with the Metastable Atom. As was described in the introductory section, in the Penning ionization process the metastable atom behaves as an electrophilic reagent extracting an electron from a molecular orbital. In this section, we will discuss the relative reactivities and stereochemical properties of the T , n, and u orbitals of dichlorobenzenes from the analysis of the relative band intensities of the PIES. 1 . Reactivity of n Orbitals. In general, the n bands of monoand dichlorobenzenes show stronger band intensity in the PIES than the T and u bands. This is because the n orbitals mainly
4254 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
TABLE I V Relative PIES Intensity of the Bands in Dicblorokazcacs compound o-dichlorobenzene m-dichloroknzene p-dichlorobenzene
-I(n)lI(r) 1.1 1.7
1.6
due to the chlorine 3p orbital extend further outside the molecular surface than the r and u orbitals predominantly related to the carbon 2p orbitals. Further, the intensity of the n bands is weaker than that of the n, because the nIIorbital is effectively shielded by the benzene ring from the attack of a metastable atom. The above two effects of the n orbitals have also been observed in monochlorobenzene.I0 Next we will discuss the effect of a chloro substituent that shields the nIlorbitals of the other chloro substituent. In the PIES the 15b2(nll)band (band 3 in Figure 1) in o-dichlorobenzene is much weaker than the nIIband in monoch1orobenzene.I0 This is explained by the fact that the nIIorbital of o-dichlorobenzene is shielded by both the benzene ring and the chloro substituent, showing a markedly weak reactivity with a metastable atom. A similar effect has been found in the PIES of o-chlorotoluene; the nIl band is significantly reduced owing to the shielding of the methyl group.20 This shielding effect cannot be examined in the cases of m- and pdichlorobenzenes, because both ny bands of these compounds overlap with each other and with an n, band. Therefore, in order to compare the relative intensity of the n bands -in these compounds, the ratio I(n)/I(r) was evaluated. Here, I(n) means the average intensity of the three n bands (two nybands and one n, band) having higher Ek value, I(n) = [2Z(nr) + I(n,)]/3, and I(*)means that of bands r 3and r2,I(*)= [ I ( r 3 ) + I(r2)]/2. The intensities of the n band having lower Ekvalue because these and also of the rl band are not taken into account, -bands are overlapped by u bands. The values of I(n)/f(r) are tabulated in Table IV. In the table we find that the value for o-dichlorobenzene is much smaller than those for m- and p-dichlorobenzenes. This indicates that in the case of o-dichlorobenzene the "1 orbital of a chloro substituent is shielded from the attack of metastables by the other substituent as well as by the benzene ring owing to the close Droximitv of the two substituents. 2. Reaci;'uity $the u Orbithls. In Cigures 1-3 we find that the relative intensity of the rIband is stronger than those of the r2 and r 3 in the This is due to the that the *I is more widely distributed outside the molecule, because it is mixed with the Chhine 3P orbital having a large distribution (see Figures 4-6). A similar effect has also been observed in the case of monochlorobenzene.I0 Next, we will consider the relative intensity of the 7r2 and r3 bands among the For and m-dichlorobenzenes, it is not easy to estimate the relative intensities between the =3 and =2 bands owingto their overlaP@g* It be noted, however, that the intensity of the *3 band is weaker than that of (20) Fujisawa, S.;et el. To be published.
Fujisawa et al. the r2 band in p-dichlorobenzene [ I ( r 3 / I ( r 2 ) = 0.851. This observation can be explained in terms of the steric shielding effect of the chloro substituents. As is seen from Figure 6, the r 3orbital of p-dichlorobenzene has large electron distributions at the positions of the substitution and is effectively shielded by the chlorine atoms from incoming metastables. On the other hand, the r2 orbital is scarcely shielded by the substituents, giving - - the stronger band in the PIES. 3. Reactivity of the u Orbitals. As a common feature of dichlorobenzenes, the PIES band 13 is considerably weak compared to the other u bands (Figures 1-3). This is because the orbital corresponding to band 13 is of a pure C-C bonding type whose electron distribution is localized along the benzene skeleton (Figures 4-6). This type of orbital scarcely reacts with a metastable atom to yield a weak band in PIES. In contrast to band 13, bands 10,12, and 14 are not so weak in the PIES. As is seen from Figures 4-6, band 10 corresponds to the u orbital having the electron distribution exposed along the C-CI bond. The orbital due to band 12 has the character derived from the hydrogen 1s and the chlorine 3s and 3p orbitals. Further, band 14 corresponds to the orbital with the C-H bonding character in the benzene ring. Since these three orbitals are exposed outside the molecular surface, they readily react with metastable atoms and give larger band intensities in the PIES. Next, we will discuss the differences in the relative intensity of band 14 in the PIES among the dichlorobenzenes. In order to compare the relative intensity, the ratio I(band 14)/I(total) is estimated, where I(tota1) is total band intensity in the PIES. The values of I(band 14)/I(total) are 0.065,0.047, and 0.044for 0-, m-,and p-dichlorobenzenes, respectively. This difference of the relative intensity of the band 14 can be ascribed to the shielding effect of the chlorine atoms among the compounds. In o-dichlorobenzene the shielding areas of the two chlorine atoms are partly overlapped because the positions of substitution are adjacent. The two atoms, therefore, more or less counteract their shielding effect on each other. On the other hand, in m- and p-dichlorobenzenes this counteracting effect is smaller than the case of o-dichlorobenzene; u orbitals are effectively protected by the chlorine atoms from the attack of metastables. Thus, the ordering of the relative intensity of the bands, 0- > m- H p-, can be explained in terms of the shielding effect of the substituents.
Conclusion In the present work, all the bands in the He I spectra of dichlorobenzenes have been assigned on the basis of the feature of In Penning ionization, an electron in a molecular orbital is transferred into the hole of a metastable atom and its excited electron is ejected. The relative intensity of the bands in thus, reflects the relative reactivity of the orbital upon the electrophilic attack, which gives information about the spatial distribution of the orbital density, By use of this character of PIES, the relative reactivities of the n, *, and orbitals of &&Iorobenzenes have been studied, The present method will be useful for the studies of the reactivity and stereochemical property of various molecular orbitals, especially those shielded by the substituents.
