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Two-dimensional Penning ionization electron spectroscopy of carbon disulfide: spectral assignments and anisotropic interactions with a He*(23S) metastable atom
Chemical Physics Letters 365 (2002) 40–48 www.elsevier.com/locate/cplett
Two-dimensional Penning ionization electron spectroscopy of carbon disulfide: spectral assignments and anisotropic interactions with a Heð23SÞ metastable atom Shan Xi Tian, Naoki Kishimoto, Koichi Ohno
*
Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan Received 10 June 2002
Abstract Collision-energy-resolved Penning ionization electron spectra are measured for carbon disulfide ðCS2 Þ by collision with a He ð23 SÞ metastable atom. Assignments in Penning ionization electron spectrum are made on the basis of collision energy dependence of the partial ionization cross sections (CEDPICS). CEDPICS for the X2 Pg , A2 Pu , B2 Rþ u and C2 Rþ g states demonstrates that interactions for the He access perpendicular to the molecular axis are more attractive than those for the collinear access, which is supported by the model calculations of the interaction potential energies. It is found that the perpendicular access plays an important role in a formation of the intermediate Heþ þ CS 2 state. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon disulfide ðCS2 Þ has been extensively investigated as an example for studying some fundamental molecular ion interactions. In particular, reaction processes between CS2 and photon or metastable atom attract considerable experimental and theoretical interest. Recently, the photoelectron spectrum (PES) of CS2 was recorded and examined thoroughly by Baltzer et al. [1]. Although the ionization bands of single-hole
*
Corresponding author. Fax: +81-22-217-6580. E-mail address: [email protected] (K. Ohno).
states are resolved well in the PES, strong satellite bands arising from configuration interaction (CI) break down the single particle model. There have been a few reports on Penning ionization of CS2 by collision with metastable atoms such as He ð21 S, or 23 S) [2,3], Ne ð3 P0;2 Þ [3], Ar ð3 P0;1;2 Þ [3,4]. It is well known that Penning ionization process ðA þ M ! Mþ þ A þ e Þ can be explained by the electron exchange model [5]. In this model, an electron of a molecular orbital (MO) of the target M is transferred to the inner-shell orbital of a metastable atom A , and the excited electron of A is ejected as a Penning electron e . Obviously, transition probability to each ionic state in Penning ionization is determined by an overlap of the related orbitals. Moreover, dynamic characteris-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 1 7 - 3
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
tics of Penning ionization can be studied by measurements of collision energy ðEc Þ dependence of partial ionization cross sections (CEDPICS) [6–9]. Namely, slope parameters (m) of CEDPICS are varied from negative for an attractive interaction to less negative or positive for a repulsive one. It is noted that some interesting features in the Penning ionization of CS2 collided by the He ð21 SÞ and He ð23 SÞ atoms were observed by Benz et al. [2], and the dissociation processes were discussed on the basis of the Penning electron-ion coincident spectra and fluorescence spectra. Steric reaction dynamics of the CS2 –Ar ð3 P2;0 Þ system was studied by photodissociation of the aligned CS2 , and it was found that the Penning ionization cross sections 2 for CSþ 2 ðX PÞ state were the largest when the CS2 bond was parallel to the relative velocity vector [10]. In our laboratory, an ionic-state-resolved measurement combined with a time-of-flight (TOF) technique has been developed as two-dimensional (collision-energy- and electron-energy-resolved) Penning ionization electron spectroscopy [11] which can be used to obtain collision energy resolved Penning ionization electron spectra (CERPIES) and CEDPICS for the target molecule. In this Letter, we report the two-dimensional Penning ionization electron spectrum (2D-PIES) of CS2 by collision with the He ð23 SÞ atom. Information on the anisotropic interactions between CS2 and He ð23 SÞ can be obtained from: (i) peak shifts ðDEÞ observed in Penning ionization electron spectra (PIES) with respect to He I ultraviolet electron spectrum (UPS) and (ii) CEDPICS analyses. The DE values reflect the potential energy differences between the entrance and exit channels at the geometry of ionization, and the CEDPICS reflects the interactions localized in a certain molecular region where the respective MO is distributed. More details on assignments in the PIES as well as the anisotropic interactions between CS2 and the He atom are discussed.
2. Experiment and calculation The experimental apparatus for He ð23 SÞ PIES and 2D-PIES was reported previously [6–9,11].
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Metastable atoms He ð21 S; 23 SÞ were produced by a discharged nozzle source with a tantalum hollow cathode. The He ð21 SÞ component was quenched by a water-cooled helium discharge lamp. The energy resolution of electron analyzer was estimated to be 80 meV for the energy-higher-resolution PIES and UPS measurements; the resolution was lowered to 250 meV for the 2D-PIES measurements. The transmission efficiency curves of the electron energy analyzer for these two modes were determined by the reference UPS data [12,13]. For the collision-energy-resolved measurements, the metastable He ð23 SÞ beam was pulsed by a pseudorandom chopper [9] rotating at ca. 400 Hz and introduced into a reaction cell. The time-dependent spectra can be transformed to time-offlight (TOF) spectrum and to the CERPIES normalized by the TOF spectrum of the He beam which was determined by monitoring secondary electrons emitted from an inserted stainless steel plate. Using the linear structure determined by the experiment [14], we plotted the electron contour maps for the outer valence orbitals 2pg , 2pu , 5ru , 6rg and 5rg , by self-consistent filed calculations with 6-311 þ G basis functions. Interaction potentials V ðRÞ of CS2 –He system were calculated for three directions: access to the S or C atom perpendicular to the molecular axis and access to the S atom along the axis. It is well known that there are resemblances between He ð23 SÞ and Lið22 SÞ [15] and interaction potential well depths and the location of the potential wells have been found to be very similar for interactions of various targets with He ð23 SÞ and Lið22 SÞ [16,17]. Therefore, the system CS2 –He can be modeled by the CS2 –Li system for the interaction potential energy calculations. Since we found serious spin-contamination when using the unrestricted second-order Møller–Plesset perturbation method, BeckeÕs three parameter hybrid method using the LYP correlation functional (B3LYP) [18,19] with the 6311 þ G basis set was utilized thoroughly for the interaction potential energy calculations. The basis-set-super-position error (BSSE) was corrected by the standard counterpoise (CP) technique [20]. All the calculations were carried out using GA U S S I A N 94 program [21].
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3. Results and discussions 3.1. Results Fig. 1 exhibits the He I UPS and He ð23 SÞ PIES, where the electron energy scales for PIES are shifted 1.40 eV relative to those for UPS by the difference in the excitation energies between the He I photon (21.22 eV) and the He ð23 SÞ atom (19.82 eV). Fig. 2 shows the CERPIES at various Ec values. Log r vs. log Ec (where r represents the partial ionization cross section) for the CEDPICS
is shown in Figs. 3 and 4 for the outer valence orbitals and satellite bands, respectively. In Fig. 3, the calculated electron density maps of the respective orbitals are also shown with simplified maps indicating component atomic orbitals. The thick solid curves in the density maps indicate the repulsive molecular surface approximated by van der Waals radii [22]. Fig. 5 shows the interaction potential energy curves V ðRÞ and some density maps at particular access positions. Table 1 summarizes band assignments, the vertical ionization potentials (IP), peak shifts DE in the PIES with respect to the nominal energy E0 (E0 ¼ the difference between metastable excitation energy and the IP) and the slope m values of the log r vs. log Ec obtained by a least-square fitting method. 3.2. Assignments in Penning ionization electron spectrum
Fig. 1. He I UPS and He ð23 SÞ PIES of CS2 .
The electron spectra in Fig. 1 are extremely similar to those observed by Benz et al. [2]. Before discussion of the anisotropic interaction, we should start from assignments of the observed PIES. The IP values and relative band intensities (pole strength) predicted by the ADC (3 or 4) (algebraic diagrammatic construction accurate to order 3 or 4 in the electron–electron interaction [23]) calculations [1] are in good agreement with the observations for the ionic states X2 Pg , A2 Pu , 2 þ B2 Rþ u and C Rg . The electron spectra were also studied by 2ph-TDA [24] and SAC-CI [25] methods. Although the main ionic states have been resolved well, there are still some arguments on the satellite bands in the PIES (see Fig. 1). Firstly, it was explained by Benz et al. that bands 1 and 2 were completely due to the intermediate Heþ þ CS 2 state [2] because of a large positive electron affinity of CS2 [26]. The Heþ þ CS 2 system accordingly gives rise to a large negative slope of CEDPICS. However, the partial ionization cross sections of band 2 decrease a little more rapidly than those of band 1 with the increase of Ec . As shown in Fig. 4b and Table 1, the absolute value jmj of band 2 ðm 0:58Þ is much larger than that of band 1 ðm 0:42Þ. On the other hand, vibrational structure of the X2 Pg state was
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
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Fig. 2. CERPIES in the collision energy range ðEc : 95–274 meVÞ.
2 þ Fig. 3. CEDPICS (log r vs. log Ec ) for the X2 Pg , A2 Pu , B2 Rþ u and C Rg bands: arrows represent the effective approaches for Penning ionization.
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S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
Fig. 4. CEDPICS (log r vs. log Ec ) for the satellite bands (a) and bands 1 and 2 (b).
observed in the range up to ca. 10.6 eV in IP of the PES [1]. Obviously, band 1 is out of this Frank– Condon transition region of X state, and it should correspond to the autoionizations from particular superexcited states. The autoionization from superexcited states can be expected to be observed in PIES due to the complex mechanisms including the dipole-forbidden transfers. In the Penning ionization studies, the autoionization for O2 [6], CO [27] and HCl [28] has been observed. In particular, the absolute values of m for the autoionization bands of O2 [6] and HCl [28] were found to be much larger than those for their main bands. Thereby, we assign band 1 as the autoionization band while band 2 is related to the Heþ þ CS 2 interaction, the strong Coulomb (attractive) interaction for the latter leads to the larger negative slope value and the larger negative energy shift ð1:2–1:8 eVÞ with respect to band X (see Fig. 1). Secondly, there are some reaction channels around the energy of the B2 Rþ u band in the PIES.
Fig. 5. Interaction potential energy curves of the Li–CS2 system: (a and c), access perpendicular to the molecular axis; (b) access collinear the molecular axis.
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
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Table 1 Band assignments, vertical ionization potentials (IP, eV), peak shifts (DE, meV), and slope parameters (m) of the electron spectra for CS2 Band assignments
IPobsd
X2 Pg ð2p1 g Þ A2 Pg ð2p1 u Þ B2 Rg ð5r1 u Þ C2 Rg ð6r1 u Þ 1b 2b 3 4 5 6 7 8
10.07 12.85 14.48 16.20 ð10:62–11:12Þc ð11:30–12:30Þc 14.09 (15.17)c (15.55)c 17.07 (18.10)c (19.07)c
9
20.42
IPcalc (pole strength)a
DE
ADC(4)
ADC(3)
This work
[2]
9.51(0.87) 12.56(0.62) 14.27(0.86) 16.12(0.72)
9.75(0.88) 12.68(0.41) 14.57(0.85) 16.28(0.68)
150 20 120 40 90 20 60 20
115 15 125 20 85 15 100 15
13.75(0.077)d 14.92(0.018)d
13.54(0.27)d 14.42(0.072)d
17.17(0.17)d 18.43(0.006)e 19.20(0.047)f
17.57(0.14)d
20.23(0.052 + 0.007)g
18.03(0.012)f 18.84(0.075)f 20.07(0.042)h
m
)0.33 )0.41 )0.17 )0.24 )0.42 )0.58 )0.27 )0.20 )0.24 )0.25 )0.23 )0.20
20.21(0.008)i a
Cited from [1]. Observed in the PIES, the details to see in the text. c Determined by the PIES (see Fig. 1). d A satellite state of 2 Pu ð2p1 u Þ [1]. e 1 A satellite state of 2 Rþ g ð5rg Þ [1]. f 1 A satellite state of 2 Rþ g ð6rg Þ [1]. g 2 1 A combination of a satellite state of 2 Ru (4r1 u with a pole strength of 0.052) and a satellite state of Ru (5ru with a pole strength of 0.007) [1]. h A satellite state of 2 Ru (4r1 u with a pole strength of 0.042) [1]. i A satellite state of 2 Pg (2p1 g with a pole strength of 0.008) [1]. b
A satellite state (band 3) of 2 Pu is resolved well in the UPS, this band becomes diffuse and overlaps with the B2 Rþ u band in the PIES. It is more interesting to assign the lower-electron-energy shoulder of the B2 Rþ u band. Another one satellite state (here noted as band 4) of 2 Pu should be assigned, according to the (Penning) electron-ion coincidence spectra ðe þ CSþ 2 Þ [2]. Band 4 ðIP 15:17 eVÞ as a 2p1 satellite is further supported by the ADC(3) u and ADC(4) calculations [1] (see Table 1), although the energy sequences between the satellites and 2 Rþ u state are a little different for these two calculations. The 2ph-TDA also predicted some 2p1 satellites in this region [24]. The different u slopes for these two 2 Pu satellite bands (band 3, m 0:27; band 4, m 0:20) can be interpreted by their different pole strengths of 2 Pu ð2p1 u Þ as shown in Table 1 as well as seriously overlapping
of bands B, 4 and 5. Namely, the different configurations of other MOs are involved in these two ionic states. The ðe þ Sþ Þ spectra [2] indicated that a dissociation process was expected for the shoulder (here noted as band 5) at 4.3 eV in the PIES. Benz et al. [2] suggested a vibrational predissociation mediate state for this band. Only one dissociation channel is energetically capable for this band. According to the recent high-resolution PES [1], these events may possibly result from the following process: the Penning ionization leads to 2 þ the CSþ 2 ðB Ru Þ cation at the excited vibrational state (2,0,1) (IP ¼ 14.788 eV) [1] or the higher vibrational states which subsequently dissociates into Sþ ð4 Su Þ þ CSð1 Rþ Þ ðlimit 14:787 eVÞ [29]. Thirdly, it has been proved that there are no CSþ 2 signals detected in Penning ionization for the range with the electron-energy less than ca. 3.8 eV
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S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
[2]. The enhanced intensities can be partly explained by the dissociation processes involved in the range of the electron-energy less than ca. 4 eV of the PIES (Fig. 1). Around band C (IP: 16:62–16:02 eV), there are two dissociation chanþ nels energetically close to the C2 Rþ g state: S þ ð2 Du Þ þ CSð1 R Þðlimit 16:628 eVÞ and Sð3 Pg Þ þ CSþ ðX2 Rþ Þðlimit 15:762 eVÞ [2,29]. In the lowelectron-energy range ð0 3 eVÞ, the excitation transfer may be followed by a dissociation and autoionization, He þ CS2 ! CS2 ! CSða3 PÞ þ S ð½4 S nd5 DÞ ðlimit 18:216 eVÞ [29]! CS þ Sþ þ e , based on the fluorescence spectra of the S species [2]. Moreover, one may consider the other energetically capable channels, He þ CS2 ! CS2 ! CS þ S (limit 16:907, 17.209 eV) [29] ! CSþ þ S þ e . However, bands 6, 7 and 8 in this electron-energy range may correspond to the 2 þ satellite states of 2 Pu ð2p1 Rg ð5r1 u Þ, g Þ and 2 þ 1 Rg ð6rg Þ, respectively, as predicted by the ADC(3) and ADC(4) calculations [1]. The existence of particular satellites was also predicted by the 2ph-TDA [24] and SAC-CI calculations [25]. In the PIES of Fig. 1, bands 6 and 8 are much stronger than their main bands (A2 Pu and C2 Rþ g ). Their collision energy dependence is significantly (band 6, m 0:25) or slightly (band 8, m 0:20) weaker than the main band (the A2 Pu band, m 0:41 or the C2 Pþ g band, m 0:24). Therefore, in this part of PIES, the dissociation processes more or less involve in the He –CS2 reactions. On the other hand, the earlier He I and He II UPS studies gave two different explanations of band 6: a satellite state correlated with A state and 2 þ an indirect dissociation from CSþ 2 ðD Ru Þ which þ þ þ leads to the products S2 , CS and S having relative intensities of 0.1, 0.6 and 0.3 [30]. 3.3. Anisotropic interactions between CS2 and He (23 S) As shown in Fig. 2, the partial cross sections decrease with increasing Ec values. Accordingly, the CEDPICS for each band exhibits a negative slope (see Fig. 3 and Table 1). However, the absolute values of slope parameters for the X2 Pg ðm 0:33Þ and A2 Pu ðm 0:41Þ bands are larger than those for the B2 Rþ u ðm 0:17Þ and
C2 Rþ g ðm 0:24Þ bands. According to their orbital electron distributions, the most effective accesses for Penning ionization have been represented by arrows in Fig. 3. Both the CEDPICS and DE values of the X2 Pg , A2 Pu , B2 Rþ u and C2 Rþ g bands in Table 1 indicate that the He access perpendicular to the molecular axis is more attractive than the collinear access. This is contrast to the conclusion derived by deViries et al. in which the Ar access parallel to the molecular axis was more effective than the perpendicular access for the X2 Pg state [10]. In Fig. 5a,b, the anisotropic interaction potential curves support strongly the conclusion derived from the present observations. Similarly, the strongly attractive interaction was found with a well depth of ca. 500 meV for the access perpendicular to the C–S double bond in thiourea [31]. However, the well depth of the attractive interaction energy curve in Fig. 5a is significantly large (ca. 700 meV). To take insights into the perpendicular interactions, the density maps of single occupied MO (SOMO) of Li–CS2 system are plotted shown in Fig. 5a,c. The electron distributions of the CS2 part in the SOMO maps at the bottoms of potential wells are extremely similar to that of the lowest unoccupied MO (LUMO) of free CS2 . This demonstrates that the charge transfer occurs when the He atom approaches perpendicularly to the molecule, and the charge transfer plays an important role in a formation of the Heþ þ CS 2 pair. It is interesting to find that the well depth in Fig. 5a is about twice larger than that in Fig. 5c, which further suggests that the charge is mainly transferred from the He 2s orbital to the unoccupied S 3p orbital. The well depth in Fig. 5a is much larger than the DE value of the X2 Pg band, which suggests that the peak shift is not only related to the interaction potential curve of the entrance channel of Penning ionization [32]. In the exit channel for 2 1 the CSþ 2 ðX Pg Þ þ Heð SÞ system, we calculate the interaction energy at the geometry of the minimum Þ on the curve in Fig. 5a, and it is point ðR 2:0 A predicted to be ca. 500 meV. Therefore, by the two-state potential curve model [33], the theoretical value of DE is, 700 (the potential well-depth of the entrance channel) – 500 (the potential well-
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
depth of the exit channel) 200 meV, and it is in good agreement with the observed value jDEj 150 meV. Moreover, the large shift 1:5 eV of band 2 with respect to band X can be explained by the significant structural deformation of CS2 after accepting the electron from the He atom. This deformation may lead to much deeper well (well-depth > 700 meV) of the interaction potential curve. Moreover, one may notice that there are certain differences ð0:07–0:08Þ of the m values between the X2 Pg and A2 Pu , also between the B2 Rþ u and C2 Rþ g bands. They may be caused by anisotropic interactions around CS2 and the orbital characteristics. There are two nodal planes for a 2pg orbital but only one for a 2pu orbital (see Fig. 3). As shown in Fig. 5, the interactions in the direction (arrow) shown for 2pu are more attractive than those for 2pg . This is consistent with the observed slopes in Fig. 3. Similarly, there are three pseudo nodal planes for 5ru but two for 6rg . In Fig. 3, the access to the C atom (a broken-lined arrow) for 6rg may be another effective trajectory of Penning ionization, because there are some electrons distributed outside the molecular surface.
4. Conclusion 2D-PIES have been measured for CS2 by collision with a He ð23 SÞ atom. In the PIES, autoionization and an intermediate Heþ þ CS 2 state were suggested for two bands in the ionization-energy range ð10:62–12:30 eVÞ, according to their distinct slopes of CEDPICS. Some satellite bands predicted by the ADC(3) and ADC(4) calculations [1] were observed in the PIES and their CEDPICS exhibited relatively weak negative collision energy dependence. CEDPICS for the X2 Pg , A2 Pu , B2 Rþ u and C2 Rþ g states demonstrates that interactions for the He access perpendicular to the molecular axis are much more attractive than those of the collinear access. This is further supported by the model calculations of the interaction potential energies. Moreover, it is found that the perpendicular access plays an important role in a formation of the intermediate Heþ þ CS 2 state.
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Acknowledgements This work is partially supported by the Japan Society for the Promotion of Science (JSPS). One of the authors (S.X.T.) is a JSPS Research Fellowship (ID No. 00111). References [1] P. Baltzer, B. Wannberg, M. Lundqvist, L. Karlsson, D.M.P. Holland, M.A. MacDonald, M.A. Hayes, P. Tomasello, W. von Niessen, Chem. Phys. 202 (1996) 185. [2] A. Benz, O. Leisin, H. Morgner, H. Seiberle, J. Stegmaier, Z. Phys. A 320 (1985) 11. [3] M.T. Jones, T.D. Dreiling, D.W. Setser, R.N. McDonald, J. Phys. Chem. 89 (1985) 4501. [4] P.B. Foreman, T.P. Parr, R.M. Martin, J. Chem. Phys. 67 (1977) 5591. [5] H. Hotop, A. Niehaus, Z. Phys. 228 (1969) 68. [6] K. Mitsuke, T. Takami, K. Ohno, J. Chem. Phys. 91 (1989) 1618. [7] K. Ohno, T. Takami, K. Mitsuke, T. Ishida, J. Chem. Phys. 94 (1991) 2675. [8] T. Takami, K. Mitsuke, K. Ohno, J. Chem. Phys. 95 (1991) 918. [9] N. Kishimoto, J. Aizawa, H. Yamakado, K. Ohno, J. Phys. Chem. A 101 (1997) 5038. [10] M.S. deViries, G.W. Tyndall, C.L. Cobb, R.M. Martin, J. Chem. Phys. 86 (1987) 2653. [11] K. Ohno, H. Yamakado, T. Ogawa, T. Yamata, J. Chem. Phys. 105 (1996) 7536. [12] J.L. Gardner, J.A.R. Samson, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 469. [13] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific, Tokyo, 1981 (and references cited therein). [14] G. Blanquet, J. Warland, C.P. Courtoy, Ann. Soc. Sci. Bruxelles 88 (1974) 87. [15] H. Hotop, Radiat. Res. 59 (1974) 379. [16] E. Illenberger, A. Niehaus, A. Z. Phys. B 20 (1975) 33. [17] H. Haberland, Y.T. Lee, P.E. Siska, Adv. Chem. Phys. 45 (1981) 487. [18] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [19] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [20] S.F. Boys, F. Bernardi, Mol. Phys. 19 (8) (1970) 553. [21] M.J. Frisch et al., GA U S S I A N 94, Gaussian Inc., Pittsburgh, PA, 1995. [22] L. Pauling, The Nature of the Chemical Bond, Cornell University, Ithaca, NY, 1960. [23] J. Schirmer, L.S. Cederbaum, O. Walter, Phys. Rev. A 28 (1983) 1237. [24] J. Schirmer, W. Domcke, L.S. Cederbaum, W. von Nissen, sbrink, Chem. Phys. Lett. 61 (1979) 30. L. A [25] H. Nakatsuji, Chem. Phys. 76 (1983) 283.
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[26] G.L. Gutsev, R.J. Bartlett, R.N. Compton, J. Chem. Phys. 108 (1998) 6756. [27] B. Lescop, M. Ben Arfa, M. Cherid, G. Le Coz, G. Sinou, A. Le Nadan, F. Tuffin, J. Phys. B 30 (1997) 1241. [28] K. Imura, N. Kishimoto, K. Ohno, J. Phys. Chem. A 106 (2002) 3759. [29] J.H.D. Eland, J. Berkowitz, J. Chem. Phys. 70 (1979) 5151.
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Two-dimensional Penning ionization electron spectroscopy of carbon disulfide: spectral assignments and anisotropic interactions with a Heð23SÞ metastable atom Shan Xi Tian, Naoki Kishimoto, Koichi Ohno
*
Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan Received 10 June 2002
Abstract Collision-energy-resolved Penning ionization electron spectra are measured for carbon disulfide ðCS2 Þ by collision with a He ð23 SÞ metastable atom. Assignments in Penning ionization electron spectrum are made on the basis of collision energy dependence of the partial ionization cross sections (CEDPICS). CEDPICS for the X2 Pg , A2 Pu , B2 Rþ u and C2 Rþ g states demonstrates that interactions for the He access perpendicular to the molecular axis are more attractive than those for the collinear access, which is supported by the model calculations of the interaction potential energies. It is found that the perpendicular access plays an important role in a formation of the intermediate Heþ þ CS 2 state. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon disulfide ðCS2 Þ has been extensively investigated as an example for studying some fundamental molecular ion interactions. In particular, reaction processes between CS2 and photon or metastable atom attract considerable experimental and theoretical interest. Recently, the photoelectron spectrum (PES) of CS2 was recorded and examined thoroughly by Baltzer et al. [1]. Although the ionization bands of single-hole
*
Corresponding author. Fax: +81-22-217-6580. E-mail address: [email protected] (K. Ohno).
states are resolved well in the PES, strong satellite bands arising from configuration interaction (CI) break down the single particle model. There have been a few reports on Penning ionization of CS2 by collision with metastable atoms such as He ð21 S, or 23 S) [2,3], Ne ð3 P0;2 Þ [3], Ar ð3 P0;1;2 Þ [3,4]. It is well known that Penning ionization process ðA þ M ! Mþ þ A þ e Þ can be explained by the electron exchange model [5]. In this model, an electron of a molecular orbital (MO) of the target M is transferred to the inner-shell orbital of a metastable atom A , and the excited electron of A is ejected as a Penning electron e . Obviously, transition probability to each ionic state in Penning ionization is determined by an overlap of the related orbitals. Moreover, dynamic characteris-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 1 7 - 3
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
tics of Penning ionization can be studied by measurements of collision energy ðEc Þ dependence of partial ionization cross sections (CEDPICS) [6–9]. Namely, slope parameters (m) of CEDPICS are varied from negative for an attractive interaction to less negative or positive for a repulsive one. It is noted that some interesting features in the Penning ionization of CS2 collided by the He ð21 SÞ and He ð23 SÞ atoms were observed by Benz et al. [2], and the dissociation processes were discussed on the basis of the Penning electron-ion coincident spectra and fluorescence spectra. Steric reaction dynamics of the CS2 –Ar ð3 P2;0 Þ system was studied by photodissociation of the aligned CS2 , and it was found that the Penning ionization cross sections 2 for CSþ 2 ðX PÞ state were the largest when the CS2 bond was parallel to the relative velocity vector [10]. In our laboratory, an ionic-state-resolved measurement combined with a time-of-flight (TOF) technique has been developed as two-dimensional (collision-energy- and electron-energy-resolved) Penning ionization electron spectroscopy [11] which can be used to obtain collision energy resolved Penning ionization electron spectra (CERPIES) and CEDPICS for the target molecule. In this Letter, we report the two-dimensional Penning ionization electron spectrum (2D-PIES) of CS2 by collision with the He ð23 SÞ atom. Information on the anisotropic interactions between CS2 and He ð23 SÞ can be obtained from: (i) peak shifts ðDEÞ observed in Penning ionization electron spectra (PIES) with respect to He I ultraviolet electron spectrum (UPS) and (ii) CEDPICS analyses. The DE values reflect the potential energy differences between the entrance and exit channels at the geometry of ionization, and the CEDPICS reflects the interactions localized in a certain molecular region where the respective MO is distributed. More details on assignments in the PIES as well as the anisotropic interactions between CS2 and the He atom are discussed.
2. Experiment and calculation The experimental apparatus for He ð23 SÞ PIES and 2D-PIES was reported previously [6–9,11].
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Metastable atoms He ð21 S; 23 SÞ were produced by a discharged nozzle source with a tantalum hollow cathode. The He ð21 SÞ component was quenched by a water-cooled helium discharge lamp. The energy resolution of electron analyzer was estimated to be 80 meV for the energy-higher-resolution PIES and UPS measurements; the resolution was lowered to 250 meV for the 2D-PIES measurements. The transmission efficiency curves of the electron energy analyzer for these two modes were determined by the reference UPS data [12,13]. For the collision-energy-resolved measurements, the metastable He ð23 SÞ beam was pulsed by a pseudorandom chopper [9] rotating at ca. 400 Hz and introduced into a reaction cell. The time-dependent spectra can be transformed to time-offlight (TOF) spectrum and to the CERPIES normalized by the TOF spectrum of the He beam which was determined by monitoring secondary electrons emitted from an inserted stainless steel plate. Using the linear structure determined by the experiment [14], we plotted the electron contour maps for the outer valence orbitals 2pg , 2pu , 5ru , 6rg and 5rg , by self-consistent filed calculations with 6-311 þ G basis functions. Interaction potentials V ðRÞ of CS2 –He system were calculated for three directions: access to the S or C atom perpendicular to the molecular axis and access to the S atom along the axis. It is well known that there are resemblances between He ð23 SÞ and Lið22 SÞ [15] and interaction potential well depths and the location of the potential wells have been found to be very similar for interactions of various targets with He ð23 SÞ and Lið22 SÞ [16,17]. Therefore, the system CS2 –He can be modeled by the CS2 –Li system for the interaction potential energy calculations. Since we found serious spin-contamination when using the unrestricted second-order Møller–Plesset perturbation method, BeckeÕs three parameter hybrid method using the LYP correlation functional (B3LYP) [18,19] with the 6311 þ G basis set was utilized thoroughly for the interaction potential energy calculations. The basis-set-super-position error (BSSE) was corrected by the standard counterpoise (CP) technique [20]. All the calculations were carried out using GA U S S I A N 94 program [21].
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3. Results and discussions 3.1. Results Fig. 1 exhibits the He I UPS and He ð23 SÞ PIES, where the electron energy scales for PIES are shifted 1.40 eV relative to those for UPS by the difference in the excitation energies between the He I photon (21.22 eV) and the He ð23 SÞ atom (19.82 eV). Fig. 2 shows the CERPIES at various Ec values. Log r vs. log Ec (where r represents the partial ionization cross section) for the CEDPICS
is shown in Figs. 3 and 4 for the outer valence orbitals and satellite bands, respectively. In Fig. 3, the calculated electron density maps of the respective orbitals are also shown with simplified maps indicating component atomic orbitals. The thick solid curves in the density maps indicate the repulsive molecular surface approximated by van der Waals radii [22]. Fig. 5 shows the interaction potential energy curves V ðRÞ and some density maps at particular access positions. Table 1 summarizes band assignments, the vertical ionization potentials (IP), peak shifts DE in the PIES with respect to the nominal energy E0 (E0 ¼ the difference between metastable excitation energy and the IP) and the slope m values of the log r vs. log Ec obtained by a least-square fitting method. 3.2. Assignments in Penning ionization electron spectrum
Fig. 1. He I UPS and He ð23 SÞ PIES of CS2 .
The electron spectra in Fig. 1 are extremely similar to those observed by Benz et al. [2]. Before discussion of the anisotropic interaction, we should start from assignments of the observed PIES. The IP values and relative band intensities (pole strength) predicted by the ADC (3 or 4) (algebraic diagrammatic construction accurate to order 3 or 4 in the electron–electron interaction [23]) calculations [1] are in good agreement with the observations for the ionic states X2 Pg , A2 Pu , 2 þ B2 Rþ u and C Rg . The electron spectra were also studied by 2ph-TDA [24] and SAC-CI [25] methods. Although the main ionic states have been resolved well, there are still some arguments on the satellite bands in the PIES (see Fig. 1). Firstly, it was explained by Benz et al. that bands 1 and 2 were completely due to the intermediate Heþ þ CS 2 state [2] because of a large positive electron affinity of CS2 [26]. The Heþ þ CS 2 system accordingly gives rise to a large negative slope of CEDPICS. However, the partial ionization cross sections of band 2 decrease a little more rapidly than those of band 1 with the increase of Ec . As shown in Fig. 4b and Table 1, the absolute value jmj of band 2 ðm 0:58Þ is much larger than that of band 1 ðm 0:42Þ. On the other hand, vibrational structure of the X2 Pg state was
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
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Fig. 2. CERPIES in the collision energy range ðEc : 95–274 meVÞ.
2 þ Fig. 3. CEDPICS (log r vs. log Ec ) for the X2 Pg , A2 Pu , B2 Rþ u and C Rg bands: arrows represent the effective approaches for Penning ionization.
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S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
Fig. 4. CEDPICS (log r vs. log Ec ) for the satellite bands (a) and bands 1 and 2 (b).
observed in the range up to ca. 10.6 eV in IP of the PES [1]. Obviously, band 1 is out of this Frank– Condon transition region of X state, and it should correspond to the autoionizations from particular superexcited states. The autoionization from superexcited states can be expected to be observed in PIES due to the complex mechanisms including the dipole-forbidden transfers. In the Penning ionization studies, the autoionization for O2 [6], CO [27] and HCl [28] has been observed. In particular, the absolute values of m for the autoionization bands of O2 [6] and HCl [28] were found to be much larger than those for their main bands. Thereby, we assign band 1 as the autoionization band while band 2 is related to the Heþ þ CS 2 interaction, the strong Coulomb (attractive) interaction for the latter leads to the larger negative slope value and the larger negative energy shift ð1:2–1:8 eVÞ with respect to band X (see Fig. 1). Secondly, there are some reaction channels around the energy of the B2 Rþ u band in the PIES.
Fig. 5. Interaction potential energy curves of the Li–CS2 system: (a and c), access perpendicular to the molecular axis; (b) access collinear the molecular axis.
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
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Table 1 Band assignments, vertical ionization potentials (IP, eV), peak shifts (DE, meV), and slope parameters (m) of the electron spectra for CS2 Band assignments
IPobsd
X2 Pg ð2p1 g Þ A2 Pg ð2p1 u Þ B2 Rg ð5r1 u Þ C2 Rg ð6r1 u Þ 1b 2b 3 4 5 6 7 8
10.07 12.85 14.48 16.20 ð10:62–11:12Þc ð11:30–12:30Þc 14.09 (15.17)c (15.55)c 17.07 (18.10)c (19.07)c
9
20.42
IPcalc (pole strength)a
DE
ADC(4)
ADC(3)
This work
[2]
9.51(0.87) 12.56(0.62) 14.27(0.86) 16.12(0.72)
9.75(0.88) 12.68(0.41) 14.57(0.85) 16.28(0.68)
150 20 120 40 90 20 60 20
115 15 125 20 85 15 100 15
13.75(0.077)d 14.92(0.018)d
13.54(0.27)d 14.42(0.072)d
17.17(0.17)d 18.43(0.006)e 19.20(0.047)f
17.57(0.14)d
20.23(0.052 + 0.007)g
18.03(0.012)f 18.84(0.075)f 20.07(0.042)h
m
)0.33 )0.41 )0.17 )0.24 )0.42 )0.58 )0.27 )0.20 )0.24 )0.25 )0.23 )0.20
20.21(0.008)i a
Cited from [1]. Observed in the PIES, the details to see in the text. c Determined by the PIES (see Fig. 1). d A satellite state of 2 Pu ð2p1 u Þ [1]. e 1 A satellite state of 2 Rþ g ð5rg Þ [1]. f 1 A satellite state of 2 Rþ g ð6rg Þ [1]. g 2 1 A combination of a satellite state of 2 Ru (4r1 u with a pole strength of 0.052) and a satellite state of Ru (5ru with a pole strength of 0.007) [1]. h A satellite state of 2 Ru (4r1 u with a pole strength of 0.042) [1]. i A satellite state of 2 Pg (2p1 g with a pole strength of 0.008) [1]. b
A satellite state (band 3) of 2 Pu is resolved well in the UPS, this band becomes diffuse and overlaps with the B2 Rþ u band in the PIES. It is more interesting to assign the lower-electron-energy shoulder of the B2 Rþ u band. Another one satellite state (here noted as band 4) of 2 Pu should be assigned, according to the (Penning) electron-ion coincidence spectra ðe þ CSþ 2 Þ [2]. Band 4 ðIP 15:17 eVÞ as a 2p1 satellite is further supported by the ADC(3) u and ADC(4) calculations [1] (see Table 1), although the energy sequences between the satellites and 2 Rþ u state are a little different for these two calculations. The 2ph-TDA also predicted some 2p1 satellites in this region [24]. The different u slopes for these two 2 Pu satellite bands (band 3, m 0:27; band 4, m 0:20) can be interpreted by their different pole strengths of 2 Pu ð2p1 u Þ as shown in Table 1 as well as seriously overlapping
of bands B, 4 and 5. Namely, the different configurations of other MOs are involved in these two ionic states. The ðe þ Sþ Þ spectra [2] indicated that a dissociation process was expected for the shoulder (here noted as band 5) at 4.3 eV in the PIES. Benz et al. [2] suggested a vibrational predissociation mediate state for this band. Only one dissociation channel is energetically capable for this band. According to the recent high-resolution PES [1], these events may possibly result from the following process: the Penning ionization leads to 2 þ the CSþ 2 ðB Ru Þ cation at the excited vibrational state (2,0,1) (IP ¼ 14.788 eV) [1] or the higher vibrational states which subsequently dissociates into Sþ ð4 Su Þ þ CSð1 Rþ Þ ðlimit 14:787 eVÞ [29]. Thirdly, it has been proved that there are no CSþ 2 signals detected in Penning ionization for the range with the electron-energy less than ca. 3.8 eV
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[2]. The enhanced intensities can be partly explained by the dissociation processes involved in the range of the electron-energy less than ca. 4 eV of the PIES (Fig. 1). Around band C (IP: 16:62–16:02 eV), there are two dissociation chanþ nels energetically close to the C2 Rþ g state: S þ ð2 Du Þ þ CSð1 R Þðlimit 16:628 eVÞ and Sð3 Pg Þ þ CSþ ðX2 Rþ Þðlimit 15:762 eVÞ [2,29]. In the lowelectron-energy range ð0 3 eVÞ, the excitation transfer may be followed by a dissociation and autoionization, He þ CS2 ! CS2 ! CSða3 PÞ þ S ð½4 S nd5 DÞ ðlimit 18:216 eVÞ [29]! CS þ Sþ þ e , based on the fluorescence spectra of the S species [2]. Moreover, one may consider the other energetically capable channels, He þ CS2 ! CS2 ! CS þ S (limit 16:907, 17.209 eV) [29] ! CSþ þ S þ e . However, bands 6, 7 and 8 in this electron-energy range may correspond to the 2 þ satellite states of 2 Pu ð2p1 Rg ð5r1 u Þ, g Þ and 2 þ 1 Rg ð6rg Þ, respectively, as predicted by the ADC(3) and ADC(4) calculations [1]. The existence of particular satellites was also predicted by the 2ph-TDA [24] and SAC-CI calculations [25]. In the PIES of Fig. 1, bands 6 and 8 are much stronger than their main bands (A2 Pu and C2 Rþ g ). Their collision energy dependence is significantly (band 6, m 0:25) or slightly (band 8, m 0:20) weaker than the main band (the A2 Pu band, m 0:41 or the C2 Pþ g band, m 0:24). Therefore, in this part of PIES, the dissociation processes more or less involve in the He –CS2 reactions. On the other hand, the earlier He I and He II UPS studies gave two different explanations of band 6: a satellite state correlated with A state and 2 þ an indirect dissociation from CSþ 2 ðD Ru Þ which þ þ þ leads to the products S2 , CS and S having relative intensities of 0.1, 0.6 and 0.3 [30]. 3.3. Anisotropic interactions between CS2 and He (23 S) As shown in Fig. 2, the partial cross sections decrease with increasing Ec values. Accordingly, the CEDPICS for each band exhibits a negative slope (see Fig. 3 and Table 1). However, the absolute values of slope parameters for the X2 Pg ðm 0:33Þ and A2 Pu ðm 0:41Þ bands are larger than those for the B2 Rþ u ðm 0:17Þ and
C2 Rþ g ðm 0:24Þ bands. According to their orbital electron distributions, the most effective accesses for Penning ionization have been represented by arrows in Fig. 3. Both the CEDPICS and DE values of the X2 Pg , A2 Pu , B2 Rþ u and C2 Rþ g bands in Table 1 indicate that the He access perpendicular to the molecular axis is more attractive than the collinear access. This is contrast to the conclusion derived by deViries et al. in which the Ar access parallel to the molecular axis was more effective than the perpendicular access for the X2 Pg state [10]. In Fig. 5a,b, the anisotropic interaction potential curves support strongly the conclusion derived from the present observations. Similarly, the strongly attractive interaction was found with a well depth of ca. 500 meV for the access perpendicular to the C–S double bond in thiourea [31]. However, the well depth of the attractive interaction energy curve in Fig. 5a is significantly large (ca. 700 meV). To take insights into the perpendicular interactions, the density maps of single occupied MO (SOMO) of Li–CS2 system are plotted shown in Fig. 5a,c. The electron distributions of the CS2 part in the SOMO maps at the bottoms of potential wells are extremely similar to that of the lowest unoccupied MO (LUMO) of free CS2 . This demonstrates that the charge transfer occurs when the He atom approaches perpendicularly to the molecule, and the charge transfer plays an important role in a formation of the Heþ þ CS 2 pair. It is interesting to find that the well depth in Fig. 5a is about twice larger than that in Fig. 5c, which further suggests that the charge is mainly transferred from the He 2s orbital to the unoccupied S 3p orbital. The well depth in Fig. 5a is much larger than the DE value of the X2 Pg band, which suggests that the peak shift is not only related to the interaction potential curve of the entrance channel of Penning ionization [32]. In the exit channel for 2 1 the CSþ 2 ðX Pg Þ þ Heð SÞ system, we calculate the interaction energy at the geometry of the minimum Þ on the curve in Fig. 5a, and it is point ðR 2:0 A predicted to be ca. 500 meV. Therefore, by the two-state potential curve model [33], the theoretical value of DE is, 700 (the potential well-depth of the entrance channel) – 500 (the potential well-
S.X. Tian et al. / Chemical Physics Letters 365 (2002) 40–48
depth of the exit channel) 200 meV, and it is in good agreement with the observed value jDEj 150 meV. Moreover, the large shift 1:5 eV of band 2 with respect to band X can be explained by the significant structural deformation of CS2 after accepting the electron from the He atom. This deformation may lead to much deeper well (well-depth > 700 meV) of the interaction potential curve. Moreover, one may notice that there are certain differences ð0:07–0:08Þ of the m values between the X2 Pg and A2 Pu , also between the B2 Rþ u and C2 Rþ g bands. They may be caused by anisotropic interactions around CS2 and the orbital characteristics. There are two nodal planes for a 2pg orbital but only one for a 2pu orbital (see Fig. 3). As shown in Fig. 5, the interactions in the direction (arrow) shown for 2pu are more attractive than those for 2pg . This is consistent with the observed slopes in Fig. 3. Similarly, there are three pseudo nodal planes for 5ru but two for 6rg . In Fig. 3, the access to the C atom (a broken-lined arrow) for 6rg may be another effective trajectory of Penning ionization, because there are some electrons distributed outside the molecular surface.
4. Conclusion 2D-PIES have been measured for CS2 by collision with a He ð23 SÞ atom. In the PIES, autoionization and an intermediate Heþ þ CS 2 state were suggested for two bands in the ionization-energy range ð10:62–12:30 eVÞ, according to their distinct slopes of CEDPICS. Some satellite bands predicted by the ADC(3) and ADC(4) calculations [1] were observed in the PIES and their CEDPICS exhibited relatively weak negative collision energy dependence. CEDPICS for the X2 Pg , A2 Pu , B2 Rþ u and C2 Rþ g states demonstrates that interactions for the He access perpendicular to the molecular axis are much more attractive than those of the collinear access. This is further supported by the model calculations of the interaction potential energies. Moreover, it is found that the perpendicular access plays an important role in a formation of the intermediate Heþ þ CS 2 state.
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Acknowledgements This work is partially supported by the Japan Society for the Promotion of Science (JSPS). One of the authors (S.X.T.) is a JSPS Research Fellowship (ID No. 00111). References [1] P. Baltzer, B. Wannberg, M. Lundqvist, L. Karlsson, D.M.P. Holland, M.A. MacDonald, M.A. Hayes, P. Tomasello, W. von Niessen, Chem. Phys. 202 (1996) 185. [2] A. Benz, O. Leisin, H. Morgner, H. Seiberle, J. Stegmaier, Z. Phys. A 320 (1985) 11. [3] M.T. Jones, T.D. Dreiling, D.W. Setser, R.N. McDonald, J. Phys. Chem. 89 (1985) 4501. [4] P.B. Foreman, T.P. Parr, R.M. Martin, J. Chem. Phys. 67 (1977) 5591. [5] H. Hotop, A. Niehaus, Z. Phys. 228 (1969) 68. [6] K. Mitsuke, T. Takami, K. Ohno, J. Chem. Phys. 91 (1989) 1618. [7] K. Ohno, T. Takami, K. Mitsuke, T. Ishida, J. Chem. Phys. 94 (1991) 2675. [8] T. Takami, K. Mitsuke, K. Ohno, J. Chem. Phys. 95 (1991) 918. [9] N. Kishimoto, J. Aizawa, H. Yamakado, K. Ohno, J. Phys. Chem. A 101 (1997) 5038. [10] M.S. deViries, G.W. Tyndall, C.L. Cobb, R.M. Martin, J. Chem. Phys. 86 (1987) 2653. [11] K. Ohno, H. Yamakado, T. Ogawa, T. Yamata, J. Chem. Phys. 105 (1996) 7536. [12] J.L. Gardner, J.A.R. Samson, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 469. [13] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific, Tokyo, 1981 (and references cited therein). [14] G. Blanquet, J. Warland, C.P. Courtoy, Ann. Soc. Sci. Bruxelles 88 (1974) 87. [15] H. Hotop, Radiat. Res. 59 (1974) 379. [16] E. Illenberger, A. Niehaus, A. Z. Phys. B 20 (1975) 33. [17] H. Haberland, Y.T. Lee, P.E. Siska, Adv. Chem. Phys. 45 (1981) 487. [18] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [19] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [20] S.F. Boys, F. Bernardi, Mol. Phys. 19 (8) (1970) 553. [21] M.J. Frisch et al., GA U S S I A N 94, Gaussian Inc., Pittsburgh, PA, 1995. [22] L. Pauling, The Nature of the Chemical Bond, Cornell University, Ithaca, NY, 1960. [23] J. Schirmer, L.S. Cederbaum, O. Walter, Phys. Rev. A 28 (1983) 1237. [24] J. Schirmer, W. Domcke, L.S. Cederbaum, W. von Nissen, sbrink, Chem. Phys. Lett. 61 (1979) 30. L. A [25] H. Nakatsuji, Chem. Phys. 76 (1983) 283.
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