Cation spectroscopy of aromatic–argon van der Waals complexes by ZEKE photoelectron spectra

Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31–39 www.elsevier.nl / locate / elspec

Review

Cation spectroscopy of aromatic–argon van der Waals complexes by ZEKE photoelectron spectra Katsumi Kimura b

a,b ,

*

a Institute for Molecular Science, Okazaki 444 -8585, Japan Japan Advanced Institute of Science and Technology, Tatsunokuchi 923 -1292, Japan

Abstract This article is concerned with the application of two-color ZEKE (zero kinetic energy) photoelectron spectroscopy to weakly bounded aromatic–argon van der Waals (vdW) complexes in supersonic jets. A two-color ZEKE photoelectron technique gives us a very high resolution ‘cation spectroscopy’. In this article, we describe what kinds of spectroscopic information can be deduced about the cations of vdW complexes from ZEKE photoelectron spectra, by mentioning mainly four examples, namely, (1) aniline–Ar and –Ar 2 , (2) benzonitrile–Ar and –Ar 2 , (3) pyrimidine–Ar and –Ar 2 , and (4) azulene–Ar. We focus our attention mainly on (1) the energy shifts in the first adiabatic ionization energies of the aromatic molecules by complex formation and (2) the low-frequency vdW vibrations. Characteristics of two-color ZEKE photoelectron spectroscopy are also mentioned. The first adiabatic ionization energies (Ia ) of these vdW complexes are summarized (Table 1), together with many other aromatic–argon vdW complexes, that were studied by the author’s group.  2000 Elsevier Science B.V. All rights reserved. Keywords: ZEKE photoelectron spectra; Aromatic–argon van der Waals complexes

1. Introduction Molecular photoelectron spectroscopy associated with the ionization electron transition of a neutral molecule provides direct information on ionization energies corresponding to the energy levels of its cation, as in single-photon He(I) photoelectron spectroscopy [1]. Molecular laser photoelectron spectroscopy based on one- or two-color REMPI (resonantly enhanced multiphoton ionization) was originally developed to study an excited state of a jet-cooled molecular species with pulsed UV/ visible lasers by *Corresponding address: Molecular Photonics and Photoelectron Group, Shidami Science Park, 2268-1 Anagahara, Nagoya 4630003, Japan. E-mail address: [email protected] (K. Kimura).

measuring photoelectron kinetic energy spectra [2,3]. The REMPI-based photoelectron spectroscopy was soon extended to two-color ZEKE (zero-kineticenergy) photoelectron spectroscopy providing veryhigh-resolution cation spectroscopy [4–6,10,11]. A two-color threshold photoelectron spectrum (nearlyZEKE spectrum) showing cation vibrational structure was first observed with jet-cooled aniline by the author’s group [11,14]. The advantages of molecular excited-state photoelectron spectroscopy with pulsed UV/ visible lasers are the following: (1) it is possible to study dynamic behavior of a neutral resonant excited state from a standpoint of photoelectron spectroscopy. (2) Using a gaseous sample of neutral molecular species as a target species, it is possible to carry out cation spectroscopy. (3) It is possible to select a specific

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K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

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Table 1 Adiabatic ionization energies obtained by ZEKE photoelectron spectroscopy. The data shown are those obtained mainly by the author’s group Molecular species

Ia (cm 21 )

Toluene (C 6 H 5 –CH 3 ) Toluene–Ar Ethylbenzene (C 6 H 5 –C 2 H 5 ) Ethylbenzene–Ar n-Propylbenzene (trans) n-Propylbenzene (trans)–Ar Styrene (C 6 H 5 –CH=CH 2 ) Styrene–Ar Phenylacetylene (C 6 H 5 –C≡CH) Phenylacetylene–Ar Aniline (C 6 H 5 –NH 2 ) Aniline–Ar Aniline–Ar 2 Benzonitrile (C 6 H 5 –C≡N) Benzonitrile–Ar Benzonitrile–Ar 2 Anisole (C 6 H 5 –OCH 3 ) Anisole–Ar Anisole–Ar 2 Fluorobenzene (C 6 H 5 –F) Fluorobenzene–Ar Fluorobenzene–Ar 2 Thioanisole (C 6 H 5 –SCH 3 ) Thioanisole–Ar Thioanisole–Ar 2 p-Dimethoxybenzene (cis) (CH 3 CO–C 6 H 4 –OCH 3 ) p-Dimethoxybenzene–Ar (cis) p-Dimethoxybenzene–Ar 2 (cis) p-Dimethoxybenzene (trans) p-Dimethoxybenzene–Ar (trans) p-Dimethoxybenzene–Ar 2 (trans) Pyrimidine (C 4 H 4 N 2 ) Pyrimidine–Ar Pyrimidine–Ar 2 Indole (C 9 H 7 N) Indole–Ar Naphthalene (C 10 H 8 ) Naphthalene–Ar Azulene (C 10 H 8 ) Azulene–Ar Anthracene (C 14 H 10 ) Anthracene–Ar (isomer I) Anthracene–Ar (isomer II) Anthracene–Ar 2 (isomer I) Anthracene–Ar 2 (isomer II) Anthracene–Ar 3 (isomer I) Anthracene–Ar 4 (isomer II) Anthracene–Ar 4 (isomer I) Anthracene–Ar 5 (isomer I)

71,20365 71,03765 70,76264 70,63464 70,26766 70,15566 68,26765 68,15165 71,17565 71,02765 62,26864 62,15764 62,04964 78,49062 78,24164 78,00764 66,39666 66,20066 66,02366 74,23864 74,01164 73,81664 63,90663 63,78963 63,67563

a

21

b

60,77467 60,68767 60,50967 60,56367 60,47967 60,29567 75,26166 75,00066 74,74566 62,59264 62,50466 65,69263 65,60763 59,78165 59,70865 59,87265 59,80765 59,82565 59,75765 59,77465 59,69565 59,60665 59,66065 59,56565 21

DIa 2166 2128 2112 2116 2148 2111 2219 2249 2483 2196 2373 2227 2422 2117 2231

287 2265 284 2268 2261 2516 288 285 273 265 247 2115 298 2177 2212 2266 2307 c

Ref. [35] a [35] b [36] [36] [36] [36] [37] [37] [37] [37] [14] c [14] d [14] e [15] [15] [15] [38] [38] [38] [26] f [26] g [26] [39] [39] [39] [40] [40] [40] [40] [40] [40] [16] [16] [16] [41] [41] [42] [42] [17] [17] [20] [20] [20] [20] [20] [20] [20] [20] [20] 21

71,199 cm [43]. 71,033 cm [43]. 62,281 cm d e f [21]. 62,168 cm 21 [21]. 62,061 cm 21 [21]. 74,229 cm 21 [44]; 74,222 cm 21 [45]. g 74,004 cm 21 [44]; 74,000 cm 21 [45].

molecular species as well as its specific excited state among a mixture of many analogous species in a scheme of resonant-ionization photoelectron spectroscopy. Such laser REMPI photoelectron spectroscopy was earlier applied to the NO–Ar vdW complex [7] as well as to hydrogen-bonded clusters of phenol and 7-azaindole [8]. Very recently, the author wrote a review article providing a cross section of the development of REMPI-based photoelectron spectroscopy in the last two decades, including several applications of two-color ZEKE photoelectron spectroscopy to chemistry [9]. Two-color ZEKE photoelectron spectroscopy based on REMPI photoelectron spectroscopy has an excellent state- and species-selectivity as well as the spectroscopic ability of ‘cation spectroscopy’ providing vibrational and rotational structure of a molecular cation. A compact ZEKE photoelectron analyzer (highbrightness and cm 21 -resolution) with a short flight distance was developed by the author’s group [10,11]. Recently this analyzer was much improved by introducing two-pulse field ionization (2PFI) [12]. Very-low-frequency cation vdW vibrations can be observed by two-color ZEKE photoelectron spectroscopy, as was earlier demonstrated for the vdW cations, NO–Ar [11,13] and aniline–Ar [14]. Recently, further studies on vdW vibrations have been reported for benzonitrile–Ar [15], pyrimidine–Ar [16], azulene–Ar [17], and many others, as listed in Table 1. In this article, we want to focus our attention on spectroscopic information deduced from two-color ZEKE photoelectron spectra of jet-cooled aromatic– argon vdW complexes, by mentioning four examples, which have been studied by the author’s group. Several review articles have so far been published on vdW complexes. However, only a few examples of ZEKE studies of aromatic–argon vdW complexes are included [18,19], since such studies had been limited ¨ at that time. The review article of Muller-Dethlefs et al. [18] concentrates on hydrogen-bonded aromatic complexes.

2. Characteristics of two-color ZEKE photoelectron spectroscopy The two-color (1119) photoionization process of an aromatic–argon vdW complex in two-color ZEKE

K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

photoelectron spectroscopy with PFI (pulsed-field ionization) can be expressed by M–Ar n (n51, 2,

h n1 ... )

h n2 1PFI

→(M–Ar n )* → (M–Ar n )1 1 e2 ZEKE .

The characteristics of two-color (1119) ZEKE photoelectron spectroscopy in studying vdW complexes are the following: (1) a specific vdW complex can be selected from a mixture of analogous species produced in a supersonic jet, by tuning the first laser to a resonant excited state. (2) The excited-state vdW complex thus selected is further excited to a high lying Rydberg state just several wavenumbers below an ionization threshold, by scanning the wavelength of the second laser pulse. The excitation to a veryhigh-lying Rydberg state before ionization is a technical requirement to improve the spectroscopic resolution. (3) ZEKE photoelectrons can be detected as a function of the second laser wavelength by applying a delayed pulsed electric field, so that the energy axis of the spectrum can be determined in a resolution of the laser wavelength. (4) For each vdW complex, many vibronic levels of an intermediate resonant state can be selected. Therefore, different kinds of Franck–Condon allowed vibrational progressions are observable for the resonant excited states.

3. Two-color ZEKE photoelectron detection Detection of ZEKE photoelectrons provides very high resolutions (1 cm 21 or less), as first reported by Schlag et al. [5]. In an early stage, considerable effort had been devoted to detection of pulsed threshold photoelectrons (or nearly zero kinetic energy electrons), but soon attention was focused on the field ionization of very high-lying Rydberg states to detect ZEKE electrons. In this section, a series of the ZEKE photoelectron analyzers used in our studies is briefly described. The first attempt in the author’s group was to use an electrostatic analyzer with a set of three electrodes with which threshold photoelectrons are collected through an off-center hole toward a detector, and some vibrationally resolved ZEKE photoelectron spectra were obtained for aniline and benzene [6].

33

Then a compact (short flight distance) ZEKE electron analyzer was developed with two kinds of pulsed electric fields, one of which is to remove energetic photoelectrons, the other to extract nearly zero-kinetic-energy electrons at 500 ns after each laser shot [10,11,20]. The idea of such a compact analyzer was to collect ZEKE electrons as quickly as possible with the shortest possible flight distance, because the brightness of the analyzer is especially important from an experimental point of view. A resolution of 1–2 cm 21 was obtained in our earlier work [10,11,13,14,20]. Recently the compact ZEKE analyzer was improved in brightness as well as in resolution by using a two-pulse field ionization method (2PFI) [12], and applied to study several vdW aromatic–argon complexes [15–17].

4. Typical examples

4.1. Aniline– Ar and – Ar2 Progressions due to vdW vibrations in ZEKE photoelectron spectra were observed first for aniline– Ar and –Ar 2 by Takahashi et al. [14] by using two-color resonant ionization via the S 1 origins. Fig. 1a–c shows the ZEKE photoelectron spectra of aniline, aniline–Ar, and aniline–Ar 2 . The ZEKE vibrational bands of aniline–Ar and –Ar 2 are each divided into several sub-bands due to vdW vibrations. This means that molecular vibrations of the bare aniline cation are slightly perturbed by the attachment of the argon atom. The vdW vibrational progressions appearing in the first ZEKE bands of aniline–Ar and –Ar 2 are also shown on an expanded scale in Fig. 1d and g. Other progressions due to the cation bending (b 1 x ) of aniline–Ar have been obtained via the higher S 1 vibrational levels (b 1x and b x2 ) as shown in Fig. 1e and f. Here, x is the long axis of aniline. Similar ZEKE photoelectron spectra have also been obtained by Zhang et al. [21], using picosecond and nanosecond lasers. From the cation origin bands (D 0 0 10 ), the adiabatic ionization energies are Ia (aniline)5 62,26864 cm 21 , Ia (aniline–Ar)562,15764 cm 21 , and Ia (aniline–Ar 2 )562,04964 cm 21 [14]. The successive attachment of the argon atom gives rise to a reduction in the adiabatic ionization energy. The decreases in Ia are 111 and 219 cm 21 for aniline–Ar

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K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

Fig. 1. Spectra (a), (b), and (c) show ZEKE photoelectron spectra of aniline, aniline–Ar, and aniline–Ar 2 , respectively, obtained by two-color (1119) ionization via the S 1 origins. Several very low frequency vibrational progressions due to the cation vdW bending modes are observed by the complex formation. Spectra (d), (e), and (f) are the cation bending (b 1 x ) progressions of aniline–Ar, obtained via the S 1 origin, S 1 b 1x , and S 1 b 2x , respectively. 1 Spectrum (g) is the bending b xs progression of aniline–Ar 2 , obtained via the S 1 origin (from Takahashi et al. [10]).

and –Ar 2 , respectively, with respect to the bare aniline. In the neutral S 1 origins, on the other hand, the energy decreases of aniline–Ar and –Ar 2 are 54 and 109 cm 21 , respectively, with respect to the bare

aniline [14]. In the aniline–Ar 1,2 complexes, the Ia shifts are almost twice as large as the S 1 energy shifts. These values are essentially the same as those reported in the literature [23,24]. The spectral shift can be written as the sum of the energy shifts due to (a) dipole-induced dipole interaction and (b) dispersion interaction. Both interactions are expected to be proportional to the polarizability of the attached rare gas atom [22]. In the neutral vdW complexes, the dominant contribution is due to the dispersion interaction, while the dipoleinduced dipole interaction contributes about 5% of the total spectral shift [23]. In the vdW cations, however, both are considered to be comparable, since the ionization energy shifts are much larger compared to those in the neutral states. The shift of aniline–Ar 2 in adiabatic ionization energy (Ia ) is almost twice that of aniline–Ar. This fact strongly suggests that the aniline moiety is planar in the cations of aniline–Ar and –Ar 2 . If it were nonplanar, the interaction of the aniline moiety with the first argon atom should be more or less different from that with the second argon atom; in other words, a breakdown of the additivity law in Ia should occur. The vibrational progressions with frequencies of 16 and 11 cm 21 are observed in the ZEKE spectra of aniline–Ar and –Ar 2 , respectively, as shown in Fig. 1d and g [14]. The 16 cm 21 frequency of the aniline–Ar cation has been assigned to the symmetric vdW-bending mode (b 1 x ). This assignment may be supported by the fact that the progressions with the same frequency (16 cm 21 ) are obtained via the higher S 1 vibrational levels (b x1 and b x2 ), as shown in Fig. 1e and f. On the other hand, the frequency of 11 cm 21 obtained in the ZEKE spectrum of the aniline– Ar 2 cation has been assigned to the in-phase vdWbending mode (b 1 xs ), as shown in Fig. 1g. These vibrational progressions have been reproduced by Franck–Condon calculation [14]. According to Bieske et al. [23,24], the vdW vibration frequencies of aniline–Ar in the neutral S 1 state are 49 cm 21 (s z ), 22 cm 21 (b x ) and 19 cm 21 (b y ), and those of aniline–Ar 2 in the S 1 state are 36.4 cm 21 (s zs ) and 13.9 cm 21 (b xs ). It has well been established that the typical frequencies of the vdW bending modes are in the range of about 15–25 cm 21 and those of the vdW stretching mode are about 40–50 cm 21 in the neutral S 1 state [23].

K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

4.2. Benzonitrile– Ar and – Ar2 Benzonitrile is well known as an electron acceptor in organic charge-transfer complexes, and has significantly higher vertical ionization energy (Iv 59.71 eV) [25] in comparison with other substituted benzenes. ZEKE photoelectron spectra of benzonitrile– Ar and –Ar 2 have been observed by Araki et al. [15], by using two-color resonant ionization via the S 1 origins and several higher vibronic levels. The ZEKE photoelectron spectra of benzonitrile, benzonitrile–Ar, and benzonitrile–Ar 2 , are shown in Fig. 2 by spectra (a), (b), and (c), respectively, obtained by two-color (1119) ionization via the S 1 origins. The cation vdW bending progressions ob-

served for benzonitrile–Ar and –Ar 2 are also shown on an expanded scale by spectra (d) and (e) in Fig. 2, respectively, essentially the same as spectra (b) and (c), respectively. The adiabatic ionization energies of benzonitrile– Ar and –Ar 2 have been determined from the cation origin bands of the ZEKE spectra as follows: Ia (benzonitrile–Ar)578,24164 cm 21 and Ia (benzonitrile–Ar 2 )578,00764 cm 21 . These values are 249 and 483 cm 21 lower than that of bare benzonitrile (Ia 578,490 cm 21 ). As shown in Fig. 2d and e, well-resolved vibrational progressions with frequencies of 12 and 9 cm 21 have been observed in the ZEKE photoelectron spectra of benzonitrile–Ar and –Ar 2 , respectively, 1 assigned to the bending modes, b 1 x and b xs , respectively [15]. The vibrational assignments of b x1 and b1 xs have further been supported by the following relationship given by Bieske et al. [23], assuming 1 that the force constants of the bending b 1 x and b xs vibrations are the same 1 21 / 2 n1 . bxs / n bx 5 ( mbxs / mbx )

Fig. 2. Spectra (a), (b), and (c) show ZEKE photoelectron spectra of benzonitrile, benzonitrile–Ar, and benzonitrile–Ar 2 , respectively, obtained by two-color (1119) ionization via the S 1 origins. Spectra (d) and (e) show the cation vdW bending progressions of benzonitrile–Ar and –Ar 2 , respectively, essentially the same as spectra (b) and (c), respectively (from Araki et al. [15]).

35

(1)

Here, mbx and mbxs are reduced masses for the 1 vibrational modes of b x1 and b xs , respectively. The 1 1 left term n bxs /n bx in Eq. (1) is 0.8 from the experimental frequencies (12 and 9 cm 21 ), in good agreement with the right term ( mbxs /mbx )21 / 2 , which is calculated to be 0.8 [15]. This result suggests that both force constants are essentially the same. A similar situation has also been found in the pair of the fluorobenzene–Ar and –Ar 2 cations [26]. According to a rotational contour analysis of the LIF spectra of benzonitrile–Ar, it has been indicated by Kobayashi et al. [27] that in the S 1 state the argon ˚ approxiatom is situated at a distance of R53.5 A, mately above the center-of-mass of the benzonitrile moiety. Similar Franck–Condon calculations have been carried out to deduce structural parameters from the ZEKE photoelectron spectra due to the benzonitrile–Ar and –Ar 2 cations [15]. As mentioned before, the vibrational progressions show separations of 12 21 cm 21 (b 1 (b 1 x ) and 9 cm xs ), respectively. For the benzonitrile–Ar cation, it has been indicated that a calculated Franck–Condon pattern which well reproduces the observed ZEKE vibrational pattern gives rise to a change of Df (b x ) 578 in the angle

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K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

[15]. For the benzonitrile–Ar 2 cation, on the other hand, a calculated Franck–Condon pattern which is close to the ZEKE vibrational gives rise to an angle change of Df (b xs )568.

4.3. Pyrimidine– Ar and – Ar2 ZEKE photoelectron spectra of pyrimidine–Ar and Ar 2 have been observed by Sato et al. [16] by two-photon resonant ionization via their S 1 origins. Fig. 3a–c shows ZEKE photoelectron spectra of a series of pyrimidine, pyrimidine–Ar, and pyrimi-

Fig. 3. Spectra (a), (b), and (c) show ZEKE photoelectron spectra of pyrimidine, pyrimidine–Ar, and pyrimidine–Ar 2 , obtained by two-color (1119) ionization via the S 1 origin. Spectrum (d) shows the cation vdW vibrational progressions observed for pyrimidine– Ar, essentially the same as spectrum (b) (from Sato et al. [16]).

dine–Ar 2 , obtained via the S 1 origin. Spectrum (d) in Fig. 3 shows the cation vdW vibrational progressions of pyrimidine–Ar, essentially the same as spectrum (b), but shown on an expanded scale. The adiabatic ionization energies obtained for pyrimidine–Ar and –Ar 2 from their ZEKE spectra are 75,000 and 74,745 cm 21 , respectively, which are lowered by 261 and 516 cm 21 compared to bare pyrimidine (Ia 575,261 cm 21 ). The shift of pyrimidine–Ar 2 in Ia is almost twice as much as that of pyrimidine–Ar. This result implies that the pyrimidine–Ar cation has a planar structure, and that the two argon atoms are attached to both sides of the pyrimidine ring. As seen from Fig. 3d, four low-frequency bands are observed for the pyrimidine–Ar cation at 36, 50, 72 and 80 cm 21 . These are considered to be due to the vdW vibrations. The lowest frequency of bare pyrimidine is 432 cm 21 , due to the out-of-plane vibration (16a 1 ). For pyrimidine–Ar in the S 1 state, it has been confirmed by Sugahara et al. [28] that the argon atom is located on the center of the pyrimidine ring (therefore the vdW complex has C s symmetry). If this structure is assumed for the pyrimidine–Ar cation, the three vdW normal modes should be the totally symmetric stretching s z (a9), the totally symmetric bending b x (a9), and the non-totally symmetric bending b y (a0). The two peaks at 36 and 72 cm 21 are assigned to 11 b x and b 12 x , respectively. The vibrational frequency of b y11 is similar to that of b x11 , and it is forbidden in symmetry, so that the band observed at 80 cm 21 is 21 assigned to b 12 is assigned to y . The band at 50 cm 1 the s z mode. Similar frequencies (about 50 cm 21 ) have been reported for the s z mode for several mono-substituted benzene–Ar complexes in the S 1 state, and they do not change much upon ionization [14,15]. From their ab initio calculations (MP2 / 3211G**) of pyrimidine–Ar in the S 0 state, Sato et al. [16] have obtained frequencies of 52 cm 21 (b x ), 43 cm 21 (b y ), and 65 cm 21 (s z ). The ZEKE vibrational progressions of pyrimidine–Ar have been reproduced by Franck–Condon calculations. From calculated potential displacements in the normal coordinates, it is indicated that the difference in the pyrimidine–Ar distance (R) is ˚ between the neutral S 1 state and the cation |0.10 A ground state (D 0 ) [16]. The attractive force between

K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

the argon atom and the pyrimidine moiety will be increased by ionization because the pyrimidine cation has a positive charge, leading to charge and charge-induced-dipole interaction. Furthermore, from the Franck–Condon analysis of the b x mode, it has been suggested that the angle (f ) between the pyrimidine ring and the argon atom is changed by 58 [16]. The optimized distances of pyrimidine–Ar are R ˚ and R (D 0 )53.34 A ˚ [16]. This shows (S 0 )53.35 A that the distance R tends to decrease upon photoionization from S 1 to D 0 . Since it is reported that ˚ in the S 1 state from a study of R53.4560.05 A rotationally resolved excitation spectra [28], it is reasonable to conclude that R (D 0 )53.4520.105 ˚ by considering the above-mentioned decrease 3.35 A ˚ This is in good agreement with the of 0.10 A. ˚ optimized distance R (D 0 )53.34 A.

4.4. Azulene– Ar Zero-kinetic-energy (ZEKE) electron spectra of azulene–Ar in supersonic jets have been obtained by Tanaka et al. [17] by two-color (1119) resonant ionization via the S 2 origin and its higher vibrational levels. Fig. 4a and b show ZEKE photoelectron spectra of azulene–Ar obtained via the S 2 origin and its b x1 level. The solid and broken lines in Fig. 4c show the Lennard–Jones potential energy curves of azulene–Ar, calculated along the x-axis both in the neutral S 0 state and in the cation D 0 state, respectively. The resulting adiabatic ionization energy of azulene–Ar is Ia 559,70865 cm 21 , that is 72 cm 21 lower than that of bare azulene. The Ia value of azulene–Ar is much lower than that of naphthalene– 21 Ar (Ia 565,607 cm ) [29], but fairly close to that of anthracene–Ar (Ia 559,807 cm 21 ) [20]. However, the shifts (DIa ) due to complex formation are not very different: namely, DIa (azulene–Ar)5 273 cm 21 ; DIa (naphthalene–Ar)5 285 cm 21 ; and DIa (anthracene–Ar)5 265 cm 21 . A low-frequency vibrational progression with a spacing of about 10 cm 21 is observed in the ZEKE photoelectron spectrum obtained via the S 2 b x level (located at 16 cm 21 ). The vibrational mode responsible for this progression should be the vdW bending

37

Fig. 4. Spectra (a) and (b) show ZEKE photoelectron spectra of azulene–Ar, obtained by two-color (1119) ionization via the S 2 origin and the S 2 b 1x level, respectively. The solid and broken lines show the Lennard–Jones potential energy curves of azulene–Ar, calculated along the x-axis both in the neutral S 0 state and in the cation D 0 state, respectively (from Tanaka et al. [17]).

(b 1 x ) along the long axis (x) of the azulene moiety. The three vdW vibrational modes in the azulene–Ar cation with C s symmetry are the totally symmetric bending b 1 x (a9), the non-totally symmetric bending b y1 (a0), and the symmetric stretching s z1 (a9). Normal mode calculations with Lennard–Jones (LJ) potential energy curves have been carried out in both the neutral S 0 and the cation D 0 state. The following frequencies have been obtained under the harmonic oscillator approximation: namely, 11 cm 21 (b x ), 5 cm 21 (b y ) and 33 cm 21 (s z ) in the neutral S 0 state, 21 21 and 12 cm 21 (b 1 (b 1 (s 1 x ), 6 cm y ) and 32 cm z ) in the cation D 0 state. Potential energy curves have also been calculated as a function of the distance between Ar and the azulene C and H atoms, by using the atom–atom Lennard–Jones (LJ) potential. The atomic charges of azulene obtained from ab initio geometry optimizations in the S 0 and D 0 states have been used in the LJ calculations of vdW interactions. A charge-induced dipole interaction has been taken into account to explain the vdW interaction in the azulene–Ar cation. Fig. 4c shows the Lennard–Jones potential energy curves of azulene–Ar, calculated along the

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K. Kimura / Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 31 – 39

x-axis both in the neutral S 0 state (solid line) and in the cation D 0 state (broken line). From the potential energy minima, it has been indicated that the argon atom in the S 0 and D 0 states is located at 0.59 and ˚ respectively, from the center of mass of the 0.69 A, azulene moiety along the long axis. In other words, ˚ the position of Ar in the D 0 state is shifted by 0.10 A along the long axis of the azulene moiety from that in the S 0 state. The observed vibrational progressions have been reproduced by Franck–Condon calculations for the vibrational transition D 0 (b x1n )←S 2 (b xn : n50), by considering that the argon atom in the cation D 0 state is shifted by Dfbx 528 with respect to the neutral S 2 state.

5. Concluding remarks The ZEKE photoelectron spectroscopy studying vdW complexes is based on the two-color REMPI process with a mass-resolved technique. With this method it is possible to identify analogous vdW components among a mixture of vdW complexes in a supersonic jet. The advantage of this method is to determine adiabatic ionization potentials with a very high resolution as well as to observe cation vdW vibrations. This technique is applicable to any type of vdW complexes when an appropriate laser system is available. Its application to aromatic–argon vdW complexes has so far been most successful. Although many examples of ZEKE studies for aromatic vdW complexes have so far been published, it would be valuable to accumulate more ZEKE spectroscopic data systematically. The adiabatic ionization energy is the most direct data for connecting a ground-state molecular species with its ground-state cation, playing an important role in providing the entrance to the world of the cation. It should also be mentioned that the adiabatic ionization energy provides important information about the dissociation energies in the case of vdW complexes. In other words, the shift in adiabatic ionization energy upon complex formation provides the difference in dissociation energies of both states. Determination of dissociation energies both in the neutral and the cation ground state are important tasks. Only the

difference in dissociation energy between the neutral and the cationic vdW complex can be determined from adiabatic ionization potentials. More direct determination of the dissociation energies of vdW cations from the measurements of threshold of producing daughter cations by a MATI technique is quite important, as already demonstrated in several studies [30–34]. A vdW frequency provides important information on the potential surface of vdW interaction. Structural information can be deduced from vibrational progressions through Franck–Condon calculations associated with the ionization electron transitions. Therefore, more effort should be invested in observing cation vdW vibrational progressions in the very low frequency region. Extension of the work to many other aromatic vdW complexes (M–R n ) would be interesting. In future work, many substituted benzenes and naphthalenes and many higher polycyclic aromatics will be selected as interesting aromatic moieties (M). The effect of substituted groups in substituted benzenes and naphthalenes on vdW interactions would be important in understanding the geometrical structure of aromatic vdW complexes. Furthermore, the effect of the size of the aromatic molecule on vdW interactions is also interesting from a photoelectron spectroscopic point of view. It is promising to study dynamic behavior of aromatic vdW complexes in the neutral S 1 state and higher excited states by two-color ZEKE spectroscopy. An excited-state species can be studied by REMPI excitation spectroscopy as well as by REMPI photoelectron spectroscopy including two-color ZEKE spectroscopy.

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