Penning Ionization of Thiophene, Furan, and Pyrrole by Collision with

May 16, 1996 - Chem. , 1996, 100 (20), pp 8204–8211 ... The Journal of Physical Chemistry A 2012 116 (1), 111-118 ... Study of the Valence Wave Func...
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8204

J. Phys. Chem. 1996, 100, 8204-8211

Penning Ionization of Thiophene, Furan, and Pyrrole by Collision with He*(23S) Metastable Atoms Naoki Kishimoto, Hideo Yamakado, and Koichi Ohno* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aramaki, Aoba-ku, Sendai 980-77, Japan ReceiVed: January 12, 1996X

Penning ionization of thiophene, furan, and pyrrole upon collision with He*(23S) metastable atoms was studied by collision-energy-resolved Penning ionization electron spectroscopy. A strong negative collision energy dependence of the partial ionization cross sections was observed for π bands. This tendency was used to make an assignment for the lowest π orbital in the ultraviolet photoelectron spectrum. In the studied collision energy range, the interaction potentials between He*(23S) and target molecules were found to be anisotropic, which is consistent with calculated interaction potential curves. For the out-of-plane directions, attractive interactions were found around the π orbital region. For the in-plane directions, repulsive interactions were found around the σCH orbital region and the sulfur atom of thiophene, while attractive interactions were found around the oxygen atom of furan. In the Penning ionization electron spectrum of pyrrole and thiophene, a satellite band was found to be associated with a shake-up ionization process from an occupied π orbital.

I. Introduction When a metastable atom A* collides with a target molecule (or atom) M, where A* has a larger excitation energy than the lowest ionization potential (IP) of M, a chemi-ionization process known as Penning ionization1 can occur:

M + A* f M+ + A + e(1) The measurements for the intensity of positive ions or electrons would give the total ionization cross section, whose dependence on the relative collision energy (E) reflects details of interaction potentials.2,3 Total ionization cross sections as functions of collision energies have been measured for various atoms and simple molecules in collision with metastable atoms.3-10 In these studies it is difficult to obtain information on anisotropic interaction potentials because the total ionization cross section, which is the sum of partial ionization cross sections, reflects only averaged characteristics of the interaction potentials. In the Penning ionization process, an electron in a molecular orbital (MO) having large electron densities outside the surface of a target molecule (M) is transferred to the inner-shell orbital of a metastable atom (A*), and the excited electron in A* is ejected.11 And it has been shown from the study of partial ionization cross sections or branching ratios for Penning ionization that the most effective geometrical situations for the collisional ionization are different depending upon the electron distribution of the target MOs.12 Since the electron distribution of individual MOs is more or less localized on a special part of the molecule, the measurement of the collision energy dependence of partial ionization cross sections for ionization from particular MOs enables us to obtain the local information on interaction potentials. In another kind of experiment called Penning ionization electron spectroscopy, a kinetic energy analysis of ejected Penning electrons13 provides Penning ionization electron spectra (PIES).2,14 Band intensities in PIES reflect branching ratios for various ionic states, which would be connected with partial ionization cross sections. In these experiments for total ionization cross section and PIES, the following two techniques, (1) velocity (or collision energy) selection of metastable atoms X

Abstract published in AdVance ACS Abstracts, April 15, 1996.

S0022-3654(96)00144-X CCC: $12.00

and (2) electron kinetic energy analysis, have been employed separately. Recently, coupled techniques including velocity selection and electron kinetic energy analysis have been developed.15-25 Velocity-controlled supersonic metastable beams have been utilized to measure collision-energy-resolved PIES of Ar by collision with He*(21S, 23S).16 In our recent studies,17-25 we have reported (1) collision-energy-resolved PIES and (2) collision energy dependence of partial ionization cross sections (CEDPICS) with the velocity distribution of He*(23S) beam resolved by the time-of-flight (TOF) method, and we have obtained information on the anisotropy of the interaction potentials for some molecules. Strong attractive interactions with He*(23S) were found for local regions around the oxygen atom of the CdO group of formaldehyde and acrolein,23 the hydroxy group of methanol,25 and the oxy group of ethers.25 Interaction potentials with He* around a hydrogen atom of hydrocarbons were found to be repulsive.19,20,23 A study of benzene indicated that an extra band which could not be assigned to a simple ionization band originating from an occupied MO was related to an electron correlation band due to a π - π* excitation associated with ionization from the highest occupied π orbital.20 In this study, we investigate the anisotropic interactions between metastable He*(23S) and five-membered heterocyclic aromatic compounds, thiophene (C4H4S), furan (C4H4O), and pyrrole (C4H5N). Assignments for the ultraviolet photoelectron spectra (UPS) and PIES of these compounds are also discussed. II. Experimental Section The experimental apparatus used in this study has been reported previously.17-20 The metastable atoms of He*(21S, 23S) were produced by a discharge nozzle source with a tantalum hollow cathode. The He*(21S) component was quenched by a water-cooled helium discharge lamp, and the He*(23S) beam flux was about 1 × 1015 atoms s-1 sr-1. He I UPS were measured by using the He I resonance photons (21.22 eV) produced by a discharge in pure helium gas. The kinetic energy of electron ejected by Penning ionization or photoionization was measured by hemispherical electrostatic deflection type analyzer © 1996 American Chemical Society

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using an electron collection angle 90° to the incident He*(23S) or photon beam. The energy resolution of the electron energy analyzer was estimated to be 40 meV from the full width at half-maximum (fwhm) of the Ar+(2P3/2) peak in the He I UPS. The transmission efficiency curve of the electron energy analyzer was determined by comparing our UPS data with those by Gardner and Samson26 and Kimura et al.27 In the measurement of collision-energy-resolved He*(23S) PIES for each sample, two spectra with low collision energy of about 95 meV on average (ca. 70-110 meV) and high collision energy of about 250 meV on average (ca. 190-450 meV) were measured for each sample molecule. The resolution of the analyzer was lowered to 250 meV (fwhm for He I UPS of Ar) in order to obtain higher counting rates of Penning electrons. In separate measurements, the metastable beam of He*(23S) was chopped by a mechanical chopper rotating at 400 Hz to produce a pulsed beam and then introduced into a reaction cell located at 504 mm downstream from the chopper disk. Velocity distributions of the pulsed He* beam in the reaction cell with sample molecules were recorded as a time-of-flight spectrum IHe*(VHe*) by monitoring time-dependent counts of secondary electrons emitted from a stainless steel plate inserted at the center of the reaction cell. The background pressure in the reaction chamber was on the order of 10-7 Torr, and the experiments were performed under a sample pressure of ca. 2 × 10-5 Torr. In order to determine CEDPICS, the time-dependent signals of Penning electrons from sample molecules for a given ionic state were measured using the energy-fixed mode of the electron analyzer (fwhm for He I UPS of Ar was 250 meV) after removing the stainless steel plate. Since the time-dependent spectrum gives the electron intensity Ie as a function of the velocity of He*(23S), the partial ionization cross section σi(E) for the ith ionic state can be determined by the equations

σi(Vr) ) c{Ie(VHe*)/IHe*(VHe*)}(VHe*/Vr)

(2)

Vr ) [VHe*2 + 3kT/M]1/2

(3)

where c is a constant, Vr is the relative velocity averaged over the velocity of the target molecule, k is the Boltzmann constant, and T and M are the gas temperature and the mass of the target molecule, respectively. Finally, σi(Vr) is converted to σi(E) by the relation

E ) (1/2) µVr2

(4)

where µ is the reduced mass of He* and the target molecule. III. Calculations In order to discuss assignments of UPS and PIES bands, ab initio SCF calculations were performed with standard 4-31G basis functions. The geometries of neutral target molecules were selected from a microwave spectroscopic study.28 Schematic diagrams of MOs with circles and ellipses were used as in a previous study.22 In-plane p type orbitals were shown by couples of ellipses. The out-of-plane component of p orbitals was shown by a dashed circle. Valence s orbitals were shown by solid circles. Signs of orbital coefficients were indicated by the thickness of the lines. In electron density contour maps, thick solid curves indicate the repulsive molecular surface approximated by van der Waals radii.29 Interaction potentials between He* and the sample molecule in various directions were also calculated on the basis of the well-known resemblance between He*(23S) and Li(22S); the shape of the velocity dependence of the total scattering cross

Figure 1. He I UPS and He*(23S) PIES of thiophene.

section of He*(23S) by He, Ar, and Kr is very similar to that of Li(22S),30 and the location of the interaction potenital well and its depth are very similar for He*(23S) and Li(22S) with various targets.3,4,31,32 Because of these findings and difficulties associated with calculations for excited states, the Li(22S) atom was used in this study in place of He*(23S). A standard 6-31+G* basis set was used, and the correlation energy correction was partially taken into account by using secondorder Møller-Plesset perturbation theory (MP2). All the calculations in this work were carried out by using a quantum chemistry program.33 IV. Results Figures 1-3 show the He I UPS and PIES of thiophene, furan, and pyrrole. The electron energy scales for PIES are shifted relative to those for UPS by the difference in the excitation energies, 21.22 - 19.82 ) 1.40 eV. Figures 4-6 show the collision-energy-resolved PIES of thiophene, furan, and pyrrole, respectively. In each figure, the low-collision-energy spectrum is shown by a solid curve and the high-collision-energy spectrum is shown by a dashed curve. The relative intensities of the two spectra are normalized in the figures using the data of log σ vs log E plots given below. Tables 1-3 list the vertical ionization potentials (IP determined from the He I UPS) and the assignment of the observed bands for thiophene, furan, and pyrrole, respectively. The peak energy shifts (∆E) in PIES measured with respect to the “nominal” energy E0 (E0 ) the difference between the metastable excitation energy and the target ionization potential) are also shown in the tables. The peak energy shifts of some diffuse bands or shoulders were not determined. Valence IP values by some earlier calculations are also shown. Values of the slope parameter m for the log σ vs log E plots estimated in a collision energy range 70-400 meV by a linear least-squares method were also listed. Figures 7-9 show the log σ vs log E plots of CEDPICS for thiophene, furan, and pyrrole, respectively. The calculated electron density maps of the molecular orbitals are also shown in

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Figure 4. Collision-energy-resolved He*(23S) Penning ionization electron spectra of thiophene: dashed curve at 195-447 meV, average 248 meV; solid curve at 70-111 meV, average 95 meV.

Figure 2. He I UPS and He*(23S) PIES of furan.

Figure 5. Collision-energy-resolved He*(23S) Penning ionization electron spectra of furan: dashed curve at 196-451 meV, average 250 meV; solid curve at 71-112 meV, average 96 meV.

Figure 3. He I UPS and He*(23S) PIES of pyrrole.

the figures with simplified diagrams indicating component atomic orbitals. Electron density contour maps for the σ orbitals are shown on the molecular plane, and those for the π orbitals are shown on a plane at a height of 1.7 Å from the molecular plane. Figure 10 shows potential energy curves V*(R) as functions of the distance R between Li and the molecule, which is measured from the heteroatom or the molecular plane. V. Discussion Photoelectron spectra of thiophene, furan, and pyrrole have been extensively investigated.27,34-49 The lowest two ionization

Figure 6. Collision-energy-resolved He*(23S) Penning ionization electron spectra of pyrrole: dashed curve at 194-448 meV, average 247 meV; solid curve at 71-112 meV, average 96 meV.

bands for each molecule have been assigned to π3 and π2 bands, but the assignments for π1 bands are somewhat different by different experimental27,34-40 and theoretical studies.41-49 In this

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TABLE 1: Band Assignments, Ionization Potential (IP), Peak Energy Shift (∆E), Obtained Slope Parameter (m), and Calculated Ionization Potential for Thiophene (See Text) obs band IP/eV ∆E/meV 1 2 3 4 5 6 7 S 8 9 10

8.96 9.58 12.04 12.49 13.15 13.71 14.26 15.66 16.52 17.62 18.3

-150 40 160 -120 130 50 0 250 170

m -0.26 -0.40 -0.15 -0.33 -0.04 -0.05 -0.13 -0.19 -0.13 -0.08

orbital Koopmans character IP/eV

Green functiona,b IP/eV

1a2(π3) 2b1(π2) 6a1(ns) 1b1(π1) 4b2 5a1 3b2

8.94 9.46 12.71 14.31 14.17 14.96 15.78

8.77 8.98 12.01 12.48b 13.29 13.44 14.52

4a1 2b2 3a1

19.08 20.04 20.80

16.88b 18.06b 17.90,b 18.22b

a Reference 41. b 2ph-TDA calculation; IPs with pole strength more than 0.20 are listed (ref 46).

connection, it is important to note that π bands of heterocyclic aromatic molecules in PIES were enhanced with respect to σ bands.39 A. Thiophene. For the assignments of UPS (Figure 1), experimental studies have supported the assignments of band 4 to the ionization from the 1b1(π1) orbital,38-40 although there were disagreements about the position of the π1 band in some literature.36,37,45 Theoretically, von Niessen et al. suggested the ordering in IPs shown below with the Green function (GF) method41 and electron correlation bands above 15 eV in IP by a two-particle-hole Tamm-Dancoff approximation (2ph-TDA) for ionization in the inner valence shell:46

1a2(π3) < 2b1(π2) < 6a1(nS) < 1b1(π1) < 4b2 < 5a1 < 3b2 < 4a1 In this ordering, the order of the 1b1(π1) orbital and the 4b2 orbital is reversed in comparison with that by Koopmans’ theorem. In PIES by Ne*(3P2), Munakata et al. have assigned band 4 to the π1 orbital, which was enhanced as π3 and π2 bands.39 In this study, we observed a strong negative dependence of the log σ vs log E plot (Figure 7) for band 4 (m ) -0.33) as the π3 band (m ) -0.26) and the π2 band (m ) -0.40). These negative m values for the π bands indicate that the potential energy surface is attractive around the π orbital region. When the long-range attractive part of the interaction potential V*(R) plays a dominant role and its form is the type

V*(R) ∝ R-S

(5)

the collision energy dependence of σ(E) can be expressed as

log σ(E) ∝ (-2/s) log E

(6)

This equation gives a relationship between the slope parameter m and the potential parameter s (m ) -2/s).2,3,8 The m values then yield s values of ca. 6 for the π bands (s ) 7.7, 5.0, 6.1), which can be connected with van der Waals force. Indeed, an attractive potential well has been found in the calculated potential curve for the out-of-plane direction (Figure 10). In the collision-energy-resolved PIES (Figure 4), π bands are markedly enhanced at the lower collision energies. In addition, the negative peak shift (∆E ) -120 meV) for band 4 indicates the existence of an attractive potential well of this order, and the estimated value is in good agreement with the calculated well depth of ca. 140 meV. Furthermore, a satellite band labeled S was observed at an IP of 15.66 eV in both the UPS and PIES,

which has been proposed to be a shake-up band by Masuda.50 The valence π-π* excitation51 can be associated with the ionization from the π3 orbital for this shake-up band because the IP value of the satellite band agrees with the sum of the π3 band (9.1 eV) and the π-π* excitation energy (ca. 6.6 or 7.1 eV). The partial ionization cross section for the satellite band (m ) -0.19) shows a collision energy dependence similar to that of the π3 band (m ) -0.26) rather than that of the π2 band (m ) -0.40). A similar shake-up band has been found for the PIES of benzene.20 The presence of a metastable atom upon ionization has been considered to enhance electron correlation effects. On the other hand, the small negative dependence (m ) -0.15) and the positive peak shift (∆E ) 160 meV) for band 3, which originates from the sulfur lone pair orbital, can be ascribed to the scarecely attractive potential energy surface around the sulfur atom. The calculated potential curve for the in-plane access to the sulfur atom (Figure 10) shows a repulsive potential energy curve. As discussed in the literature,18 when the repulsive term is dominant in the interaction, the slope parameter m is related to the parameters d and b by the equation m ) (b/d) - 1/2, where d is the effective decay parameter for the interaction potential between the target molecule and the metastable atom (V(R) ) B exp(-dR); R is the distance) and b is the effective parameter of the transition probability (W(R) ) C exp(-bR)) related to the lowest IP (I(M)) of the molecule (b ) 2{2I(M)}1/2). A steep repulsive wall corresponding to a large d value results in a (b/d) value smaller than 1/2 and a negative dependence. A very small negative collision energy dependence for band 5 (m ) -0.04) and band 6 (m ) -0.05), which are related to the ionization from the σCH orbitals, is also considered to be due to the repulsive potential energy surface around the σCH orbital region. A negative collision energy dependence with a positive peak shift for band 7 (m ) -0.13, ∆E ) 50 meV) and band 8 (m ) -0.13, ∆E ) 250 meV) is thought to reflect the influence of both the repulsive potential around the σCH orbital region and the attractive potential spreading along the upper and lower sides of the ring on average. The strong enhancement of band 8 can be ascribed to the 4a1 orbital being extending in-phase outside the molecular surface, which facilitates ionization because of large overlaps between interaction orbitals. A very small negative collision energy dependence with the large positive peak shift for band 9 (m ) -0.08, ∆E ) 170 meV) is thought to be due to the σCH orbital as band 5 (m ) -0.04) and band 6 (m ) -0.05). B. Furan. In the UPS of furan (Figure 2), four separated peaks were observed in the IP region of 12-16 eV. Concerning the assignments of these bands, there have been discrepancies about the position of the π1 band both experimentally36-40 and theoretically.42-48 Assignments proposed by theoretical calculations beyond the Hartree-Fock approximation are different by different methods (see Table 2). The GF calculation41 and a transition operator method (TOM)43 suggested that the valence IP of the 1b1 orbital is at the 7th position in the ordering I (see below), which is the same ordering as in Koopmans’ IP.

I 1a2(π3) < 2b1(π2) < 6a1 < 5a1 < 4b2 < 3b2 < 1b1(π1) < 4a1 < 3a1 On the other hand, calculated IP values for the 1b1 orbital by the GF method with 2ph-TDA were 13.77 and 15.75 eV with pole strengths of 0.33 and 0.50, respectively.46 Recent calculations of valence IPs suggested the ordering II, taking shake-up ionization processes into consideration on the basis of the symmetry-adapted cluster (SAC) expansion and SAC-CI theory47

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TABLE 2: Band Assignments, Ionization Potential (IP), Peak Energy Shift (∆E), Obtained Slope Parameter (m), and Calculated Ionization Potential for Furan (See Text) band

IP/eV

1 2 3 4 5 6 7 8 9

9.11 10.48 13.10 13.74 14.46 15.18 15.44 17.49 18.68

a

obs ∆E/meV -40 -50 -130 -170 30

m

orbital character

Koopmans IP/eV

Green functiona,b IP/eV

SAC-CIc IP/eV

MRD-CId IP/eV

1a2(π3) 2b1(π2) 6a1 5a1 4b2 3b2 1b1(π1) 4a1 3a1

8.77 10.81 14.52 15.37 15.75 16.43 17.16 20.12 21.17

8.87 10.36 13.30 14.10 14.69 15.17 13.77b, 15.75b 17.77b 18.60b

8.90 10.15 12.55 13.60 14.24 14.85 14.76 17.81 18.80

8.40 9.70 13.03 13.16 13.97 14.92 14.40 15.60 16.48

-0.25 -0.32 -0.17 -0.13 -0.10 -0.22 -0.30 -0.11

-40 -10

Reference 41. b 2ph-TDA calculation; IPs with pole strength more than 0.20 are listed (ref 46). c Reference 47. d Reference 48.

TABLE 3: Band Assignments, Ionization Potential (IP), Peak Energy Shift (∆E), Obtained Slope Parameter (m), and Calculated Ionization Potential for Pyrrole (See Text)

a

band

IP/eV

1 2 3 4 5 6 7 S 8 9 10

8.28 9.26 12.74 12.94 13.48 14.29 14.76 (16.62) 17.44 18.00 18.8

obs ∆E/meV -250 -200 -250 -110 -160 -100

m -0.36 -0.39 -0.24 -0.30 -0.13 -0.26 -0.32 -0.44 -0.17

orbital character

Koopmans IP/eV

Green functiona,b IP/eV

SAC-CIc IP/eV

1a2(π3) 2b1(π2) 6a1 1b1(π1) 4b2 3b2 5a1

7.90 9.27 14.17 15.30 14.67 15.68 16.01

8.17 8.92 12.98 12.76b, 14.91b 13.39 14.37 14.86

7.95 8.73 12.68 13.62 13.18 14.12 14.14

4a1 2b2 3a1

19.93 20.07 21.05

17.50b 18.26b 18.90b

17.94 18.65 19.41

Reference 42. b 2ph-TDA calculation; IPs with pole strength more than 0.20 are listed (ref 46). c Reference 47.

Figure 8. Collision energy dependence of partial ionization cross sections for furan with He*(23S).

Figure 7. Collision energy dependence of partial ionization cross sections for thiophene with He*(23S).

and by the multireference multiroot CI (MRD-CI) method.48

II 1a2(π3) < 2b1(π2) < 6a1 < 5a1 < 4b2 < 1b1(π1) < 3b2 < 4a1 < 3a1 Valence IP values for the 1b1 orbital by the latter calculations shifted about 1 eV smaller than those (∼15.5 eV) by GF and TOM calculations. This shift of the π1 band is thought to be caused by the different way of introducing the effect of electron correlation. The effects of electron correlation and overlapping

with another band are thought to cause difficulties in assigning the π1 band. Experimentally, Klasinc et al.40 confirmed the IP ordering (1a2(π3) < 2b1(π2) < 6a1 < 5a1 < 4b2 < 3b2, 1b1(π1)) by observing the changes of the ionization cross section ratio of He II and He I UPS, and Munakata et al.39 measured PIES by He*(23S), but both failed to determine the position of the π1 band. In this study, we observed a similar collision energy dependence of the partial ionization cross section (Figure 8) of the π3 band (m ) -0.25) and the π2 band (m ) -0.32) on band 7 (m ) -0.30) more than on band 6 (m ) -0.22). An obtained s value for band 7 (s ) 6.7) is in the range of those for the π3 and π2 bands (s ) 8.0, 6.5), and one can expect a less pulled-down potential curve for the interaction between He*

Penning Ionization

Figure 9. Collision energy dependence of partial ionization cross sections for pyrrole with He*(23S).

J. Phys. Chem., Vol. 100, No. 20, 1996 8209 tion, the m values for band 3 (m ) -0.17) and band 4 (m ) -0.13) are small, which can be ascribed to the effect of repulsive potential around the σCH orbital region. The small negative m value (m ) -0.10) with a positive peak shift (∆E ) 300 meV) for band 5 on the shoulder of band 4 is thought to reflect a repulsive interaction potential between He* and σCH bond region. Band 8 is strongly enhanced because of the in-phase 4a1 orbital outside the molecular surface. The negative m value of band 8 (m ) -0.11) similar to that of band 4 (m ) -0.13) can be ascribed to the interaction potential for the in-plane direction around the attractive oxygen region and the repulsive σCH orbital region. C. Pyrrole. In the UPS of pyrrole (Figure 3), several bands are overlapping in an IP region of 12-16 eV. Theoretically, Tanaka et al. calculated the excited states of π electron ionization for 12A2 (8.10 eV), 12B1 (8.9 eV), and 22B1 (13.33 eV) states on the basis of the ab initio SCF CI calculation.52 As was shwon by the multireference configuration interaction (MR-CI) calculation,49 the wavefunction for the 22B1 state has a low weight of Koopmans’ type ionization from the 1b1 orbital (∼50%) and considerable contributions of shake-up configurations (1a2-2 f 3b1 ∞, 2b1-2 f 3b1 ∞), which probably cause difficulties for the assignment of the π1 band. Sell et al. investigated the photoelectron angular distributions, but no typical asymmetry parameter β for the π band was observed.38 Similarly, only small HeII/HeI intensity changes were observerd by Klasinc et al.,40 but they concluded that the ordering I (see below) was most favored by comparing with various calculated orderings.

I 1a2(π3) < 2b1(π2) < {6a1, 1b1(π1)} < 4b2 < 5a1 < 3b2 On the other hand, the different ordering II was proposed by the GF method42 and the SAC-CI calculation.47 The ordering II is the same as that of Koopmans’ IP.

II 1a2(π3) < 2b1(π2) < 6a1 < 4b2 < 1b1(π1) < 3b2 < 5a1 < 4a1 The MR-CI calculation,49 however, indicated the ordering III, including some shake-up states.

III 1a2(π3) < 2b1(π2) < 1b1(π1) < 6a1 < 4b2 < 3b2 < 5a1(+ shake-up) < 4a1(+ shake-up) Figure 10. Interaction potential curves V*(R) for thiophene-Li, furanLi, and pyrrole-Li: (b) in-plane access to the S atom; (O) out-ofplane access to the central point of the pentagon; (2) in-plane access to the O atom; (4) out-of-plane access to the central point of the pentagon; (1) in-plane access to the N atom along the N-H axis, where the distance R is measured from the N atom; (3) out-of-plane access to the central point of the pentagon.

and the π orbital region because of larger s values for the π3 band and the π2 band than those of thiophene. The calculated potential for the out-of-plane direction (Figure 10) shows a steep potential form and a shallow well with a depth of ca. 80 meV, which is on the order of the estimated values (ca. 40-50 meV) by the negative peak shifts for these π bands. In the collisionenergy-resolved PIES (Figure 5), π bands are enhanced at the lower collision energies. On the other hand, the relatively large negative peak shifts of band 3 (∆E ) -130 meV) and band 4 (∆E ) -170 meV) suggest a deep interaction potential well for in-plane access to the oxygen atom, which is consistent with the result (ca. 200 meV) of the calculated potential curve for the in-plane direction. In spite of the strong attractive interac-

The same ordering was also proposed by the GF calculation with 2ph-TDA.46 In PIES by Ne*(3P2), Munakata et al. have assigned band 3 or band 4 to the π1 band because the third peak, which is composed of these two bands, was enhanced as π1 and π2 bands.39 Different slopes of the log σ vs log E plot were observed for band 3 (m ) -0.24) and band 4 (m ) -0.30), which were observed in UPS at IPs of 12.74 and 12.94 eV in this study and of 12.60 and 13.0 eV by Derrick et al.,36 respectively. We assigned band 4 to the π1 band because of the similar negative dependence of band 4 to π3 band (m ) -0.36) and π2 band (m ) -0.39). Collision-energy-resolved PIES (Figure 6) also show band 4 to be particularly enhanced at the lower collision energies on its lower-electron-energy shoulder. The smallest |m| value for band 4 among the three π bands should be due to overlapping with the σ bands. A deep potential well with a depth of ca. 320 meV for the out-of-plane direction has been obtained from the calculated potential curve (Figure 10), which is on the order of the negative peak shifts for the π3 band (∆E ) -250 meV), the π2 band (∆E ) -200 meV), and the π1 band (∆E ) -250 meV). Obtained s values for the π3 band (s ) 5.6), the π2 band

8210 J. Phys. Chem., Vol. 100, No. 20, 1996 (s ) 5.1), and the π1 band (s ) 6.7) are near 6. Relatively strong negative dependence and large negative peak shifts for the investigated bands are thought to be affected by the strong attractive potential. The smallest dependence of band 5 (m ) -0.13) can be related to the repulsive potential energy surface around the σCH orbital region far from nitrogen atom and the π electron. For bands 6 and 7, the MR-CI calculation indicated shakeup π states of 22A2, 32B1, and 32A2 that lie near the 22A1 state singly ionized from the 8a1 orbital. Another calculation by the 2ph-TDA method also indicated an electron correlation band ionized from the 1b1(π1) orbital at 14.91 eV. Taking a steep repulsive potential of access to the σNH bonding region (Figure 10) into consideration, the relative strong negative dependence for band 6 (m ) -0.26) and band 7 (m ) -0.32) can be explained by an overlap with the shake-up bands, which all have a relation to the π orbitals. In addition, an electron correlation band by the shake-up state of 42A2 (2b1-11a2-1 f 3b1 ∞) can be expected in an IP value of 17 eV because the sum of the observed valence π-π* excitation energy (7.5 eV), which has been assigned to the 1B2 state (1a2-1 f 3b1)47,53 and the valence IP value of the π2 band (9.5 eV), leads to 17 eV. In PIES, a satellite band labeled S was observed at an IP of ca. 16.6 eV. The slope of the log σ vs log E plot (m ) -0.44) shows a similar collision energy dependence to the π2 band (m ) -0.39). Band 8 was suggested to be ionized from the 4a1 orbital, which is in-plane outside the molecular surface. This band is also enhanced, but is not as enhanced as the band ionized from the in-phase 4a1 orbital of thiophene or furan, because of the small electron density around the σCH orbital region far from the nitrogen atom. The small negative collision energy dependence of band 8 (m ) -0.17), similar to band 5 (m ) -0.13), can be ascribed to a repulsive potential energy surface around the σCH orbital region. VI. Conclusions By collision-energy-resolved Penning ionization electron spectroscopy, the position of π1 band for thiophene, furan, and pyrrole in UPS was indicated to be at IP values of 12.49, 15.44, and 12.94 eV, respectively. Intensities of the π bands decrease with the increase of the collision energy. This indicates that the interaction potential energy surface is attractive around the π orbital region and is in good agreement with the calculated potential curve for the out-of-plane direction. As for in-plane directions, a small negative collision energy dependence of the partial ionization cross sections with positive peak shifts in PIES was observed for the σCH orbitals of thiophene or furan. This indicates that the interaction potential energy surface is repulsive around the σCH orbital region. In contrast, relatively large negative peak shifts in PIES and a small negative collision energy dependence of the partial ionization cross sections were observed for the orbitals localized around the oxygen atom and the σCH orbital region of furan. The small negative collision energy dependence of the partial ionization cross sections with positive peak shift in PIES was observed for the sulfur lone pair orbital of thiophene. These results are ascribed to the attractive interaction potential energy surface around the oxygen atom and the repulsive interaction potential energy surface around the sulfur atom, which are in good agreement with the calculated potential curves for in-plane directions. Although the interaction potential energy surface can be repulsive around the σCH orbital region, a large negative collision energy dependence and negative peak shifts for the investigated bands of pyrrole were thought to be related to the strong attractive interaction potential for the π orbital region.

Kishimoto et al. In PIES, the enhancement of band intensities was observed for in-phase 4a1 orbitals outside the molecular surface. Satellite bands of pyrrole and thiophene were observed at IPs of 15.66 and 16.62 eV, respectively, and can be ascribed to shake-up ionization processes from an occupied π orbital. Acknowledgment. This work has been supported by a Grant in Aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture. References and Notes (1) Penning, F. M. Naturwissenshaften 1927, 15, 818. (2) Niehaus, A. AdV. Chem. Phys. 1981, 45, 399. (3) Illenberger, E.; Niehaus, A. Z. Phys. B 1975, 20, 33. (4) Parr, T. P.; Parr, D. M.; Martin, R. M. J. Chem. Phys. 1982, 76, 316. (5) Pesnelle, A.; Watel, G.; Manus, C. J. Chem. Phys. 1975, 62, 3590. (6) Woodard, M. R.; Sharp, R. C.; Seely, M.; Muschlitz, E. E., Jr. J. Chem. Phys. 1978, 69, 2978. (7) Appoloni, L.; Brunetti, B.; Hermanussen, J.; Vecchiocativi, F.; Volpi, G. G. J. Chem. Phys. 1987, 87, 3804. (8) Allison, W.; Mushlitz, E. E., Jr. J. Electron Spectrosc. Relat. Phenom. 1981, 23, 339. (9) Riola, J. P.; Howard, J. S.; Rundel, R. D.; Stebbings, R. F. J. Phys. B 1974, 7, 376. (10) Lindinger, W.; Schmeltekopf, A. L.; Fehsenfelt, F. C. J. Chem. Phys. 1974, 61, 2890. (11) Hotop, H.; Niehaus, A. Z. Phys. 1969, 228, 68. (12) Ohno, K.; Mutoh, H.; Harada, Y. J. Am. Chem. Soc. 1983, 105, 4555. (13) C ˇ erma´k, V. J. Chem. Phys. 1966, 44, 3781. (14) Yencha, A. J. Electron Spectroscopy: Theory, Technique, and Applications; Brundle, C. R., Baker, A. D., Eds.; Academic: New York, 1984; Vol. 5. (15) Dunlavy, D. C.; Martin, D. W.; Siska, P. E. J. Chem. Phys. 1990, 93, 5347. (16) Longley, E. J.; Dunlavy, D. C.; Falcetta, M. F.; Bevsek, H. M.; Siska, P. E. J. Phys. Chem. 1993, 97, 2097. (17) Mitsuke, K.; Takami, T.; Ohno, K. J. Chem. Phys. 1989, 91, 1618. (18) Ohno, K.; Takami, T.; Mitsuke, K.; Ishida, T. J. Chem. Phys. 1991, 94, 2675. (19) Takami, T.; Mitsuke, K.; Ohno, K. J. Chem. Phys. 1991, 95, 918. (20) Takami, T.; Ohno, K. J. Chem. Phys. 1992, 96, 6523. (21) Pasinszki, T.; Yamakado, H.; Ohno, K. J. Phys. Chem. 1993, 97, 12718. (22) Ohno, K.; Kishimoto, N.; Yamakado, H. J. Phys. Chem. 1995, 99, 9687. (23) Ohno, K.; Okamura, K.; Yamakado, H.; Hoshino, S.; Takami, T.; Yamauchi, M. J. Phys. Chem. 1995, 99, 14247. (24) Pasinszki, T.; Yamakado, H.; Ohno, K. J. Phys. Chem. 1995, 99, 14678. (25) Yamakado, H.; Yamauchi, M.; Hoshino, S.; Ohno, K. J. Phys. Chem. 1995, 99, 17093. (26) Gardner, J. L.; Samson, J. A. R. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 469. (27) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific: Tokyo, 1981. (28) Nygaard, L.; Nielsen, J. T.; Kirchheiner, J.; Maltesen, G.; Andersen, J. R.; Sorensen, G. O. J. Mol. Struct. 1969, 3, 491. (29) Pauling, L. The Nature of the Chemical Bond; Cornell University: Ithaca, NY, 1960. (30) Rothe, E. W.; Neynaber, R. H.; Trujiro, S. M. J. Chem. Phys. 1965, 42, 3310. (31) Hotop, H. Radiat. Res. 1974, 59, 379. (32) Haberland, H.; Lee, Y. T.; Siska, P. E. AdV. Chem. Phys. 1981, 45, 487. (33) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92; Gaussian, Inc.: Pittsburgh, PA, 1992. (34) Eland, J. H. D. Int. J. Mass Spectrom. Ion Phys. 1969, 2, 471. (35) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley-Interscience: London, 1970. (36) Derrick, P. J.; A° sbrink, L.; Edqvist, O.; Jonsson, B. O ¨ .; Lindholm, E. Int. J. Mass Spectrom. Ion Phys. 1971, 6, 161, 177, 191. (37) Gelius, U.; Allan, C. J.; Johansson, G.; Siegbahn, H.; Allison, D. A.; Siegbahn, K. Phys. Scr. 1971, 3, 237.

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