Journud of Electron Spectroscopy and Related Pkenomena, 62 (1993) 237-243 0368-2048/93/$06.00 @ 1993 - Elsevier Science Publishers B.V., Amsterdam

‘Photoelectron spectra of acetone and acetone dimer

Kenj i Furuyaa~*, Shunji Katsumatab, Katsumi Kimurac

237

aInstitute for i%kxukr Science, Okazaki 444, Japan; Department of Mo&cular Science and Technology, Kyushu University, Kasuga-shi, Fukuoka 816, Japan bDepartment of Fundamental Science, lwaki Meisei University, Iwaki-ski, Fukushima 970, Japan ‘Institute for Molecular Science, Okazaki 444, Japan; Japan Advanced Institute of Science and Technobgy, Tatsunokuchi. Ishikawa 923-12, Japan

(First received 13 October 1992; in final form 16 November 1992)

Abstract

In order to study the ionic states of acetone produced in a supersonic jet, we have carried out threshold photoelectron spectroscopy (TPES) with synchrotron radiation in the region 122&130.0 nm. Acetone has also been re-investigated by 58.4 nm He(I) photoelectron spec- troscopy (PES). It has been found from a comparison of the PES and TPES spectra that there is an autoionizing state at 9.893 f 0.015 eV. Furthermore, a threshold-photoelectron photoion coincidence (TPEPICO) spectrum due to the acetone dimer has been obtained for the first time in the region 124.0-135.0 nm with an interval of 0.2 nm. The dimer intensity in each spectrum was plotted against the excitation wavelength, giving rise to intensity distri- bution corresponding to a TPES spectrum of the non-dissociative acetone dimer. In contrast to the PES and TPES spectra of the acetone monomer, no vibrational structure appears in the dimer speqtrum. From this spectrum the appearance ionization potential of the acetone dimer has been evaluated as being 9.210 f 0.015 eV. Two bands appear in the dimer spectra, one of which may be assigned to a Rydberg state of the neutral dimer, and the other to the electronic ground state of the dimer cation.

Introduction

Synchrotron radiation is a convenient and efficient light source for carrying out threshold photoelectron spectroscopy (TPES) as well as threshold-photoelectron photoion coincidence (TPEPICO) in the gas phase. It is interesting to apply these photoionization spectroscopic techniques to jet-cooled molecules, including molecular cluster species. In this labora- tory, a molecular-beam photoionization apparatus has been constructed on Beamline BL2B2 ‘at the synchrotron radiation facility (UVSOR) of the Institute for Molecular Science to study the photoionization of molecules

* Corresponding author.

238 K. Furuya et al.lJ. Electron Spectroec. Relat. Pherwm. 6.2 (1993) 237-243

and van der Waals clusters. The molecular species studied so far with this beamline are ethylene and acetylene [l], water clusters [2], and water- containing mixed clusters such as the Ar-water system [3] and the CO,- water system [3]. Recently we have improved this apparatus so as to obtain TPES and TPEPICO spectra, and have applied it to study the energy partitioning problem in the dissociation reaction Ar3+ +Ar,+ + Ar by means of synchrotron radiation [4] _

Acetone is a very familiar molecule, and its properties have been investigated by various types of spectroscopic method [5]. However, with respect to the properties in the ionic state, such as geometric structure, vibrational frequencies, and electronic structure of the acetone dimer cation, there have so far been no detailed reports, although ESR (electron spin resonance) and absorption studies have been reported by Shida et al. [6,7]. In the present work we have further applied our improved synchrotron radiation apparatus to jet-cooled acetone to study acetone and its dimer.

Experimental

The present TPES and TPEPICO measurements were performed by using a molecular-beam photoionization apparatus on Beamline BL2B2 at the UVSOR. This apparatus has been described in detail elsewhere [4,8]. The following improvements were made: (1) a grating with a blaze wavelength of 96nm was used; (2) a LiF window was used to cut off the second order light; and (3) a conical nozzle with a minimum diameter of 0.1 mm was used to increase the molecular-beam intensity.

The room-temperature vapor of acetone (Nacalai Tesque, 99% minimum purity) was expanded without any carrier gas to suppress the production of clusters larger than the dimer and of the acetonerare gas heterodimer. The vacuum in. the main chamber was kept at 2.9 x 1O-5 Torr during our TPES and TPEPICO measurements.

A He(I) photoelectron spectrum of acetone was measured with a conven- tional photoelectron spectrometer (JASCO Model PE-lA), which has been described in detail elsewhere [9]. Briefly, the photoelectron spectrometer contains a He d.c. discharge lamp, a hemispherical electrostatic analyzer (10 cm in diameter), and an electron channel multiplier (Mullard B318/01). The measurements and analysis of the He(I) photoelectron spectrum were performed with a microcomputer system (NEC PC9801) equipped with a homebuilt multichannel scaler [lo]. The energy resolution of the spec- trometer was about 21 meV full width at the half maximum of the Ar peak (the P3,2 state) at an analyzer voltage of f 0.15 V.

K. Furuya et a&J. Electron Spectrosc. Relat. Phenon. 62 (1993) 237-242 239

IONiZATION ENERGY (eV) r ’ I ’

WAVELENGTH (nm)

Fig. 1. (a) A He(I) photoelectron spectrum and (b) and (c) threshold photoelectron spectra of acetone. The energy resolutions are 21,30, and 42 meV for spectra (a), (b), and (c), respectively.

Results and discussion

A. Acetone munmner

The He(I) PES spectrum between 9.52 and 10.16 eV and the TPES spectra of acetone in the region 122.O-130.0nm are shown in Fig. l(a) and in Fig. l(b) and (c), respectively. The PES spectrum is the same as that reported by McDiarmid [ll], but the signal-to-noise ratio of the present PES spectrum is better. The TPES spectra in Fig. 1 are the first reported, to the best of our knowledge. The observed vibrational wavenumbers are summarized in Table 1.

These cation wavenumbers have been tentatively assigned by McDiarmid [ll] on the basis of the vibrational assignments of the neutral acetone molecule. For neutral acetone, the assignments of the 385 cm- ’ vibration to the CO bending (b,) and the 488 cm-l vibration to the CCC deformation (a,) given in ref. 11 are based on the conclusions reported by Harris and Levin [12]. However, McDiarmid and Sabljic [13] have reversed these assignments from their polarization-selected multiphoton ionization experiments. The reversed assignments were supported by Zuckermann et al. [14]. We adopt the assignments of the 385 and 488 cm-’ bands of neutral acetone to the CCC deformation and the CO bending vibration, respectively, in accord- ance with the conclusion of McDiarmid and Sabljic [13].

240

Table 1

K. Furuy~ et al./& Electron Spectrosc. Relat. Phenom. 62 (1993) 237-243

Cation vibrational wavenumbers (cm-‘) observed by He(I) PES and TPES

Band PES A

TPES ref. 11 A A

1 313 313 363 317 383

2 630 746 419 384

3 1111 1049 1130 221 239 242

4 1332 1288 1372 325 260 342

5 1657 1548 1714

According to the assignments mentioned above, band 1 observed in the present cation work can be assigned to the CCC deformation (al). In addition, band 2 should be assigned to the second harmonic of the CCC deformation, because (1) the frequency of band 2 is almost twice that of band 1, and (2) in general, totally symmetric vibrational progressions appear strongly in photoelectron spectra. The large frequency difference of band 5 between the PES and the TPES spectra probably arises because autoionization contributes to the TPES spectrum, as will be discussed below.

Electrons ejected by autoionization should have kinetic energies defined by the energy separating the Rydberg and the ionic states. If the kinetic energy is comparable with the resolution of our TPES analyzer, any band corresponding to autoionizing states may be expected to appear in the TPES spectrum and, moreover, the intensity of the band resulting from autoionization depends on the resolution of the TPES analyzer. In Fig. 1, the TPES spectra (b) and (c) were observed at resolutions of 30 and 42 meV, respectively. It has been found that the intensity of band 5 becomes stronger on lowering the resolution of the TPES analyzer. This fact cannot be explained in terms of band broadening due to the lower resolution. Thus, we may conclude that there is an autoionizing state at 125.3nm (9.893 f0.015 eV). This band has not been observed in any absorption spectra [l&17]. The reason is that the absorption band intensity depends on the absorption cross-section, whereas the TPES band intensity depends on the kinetic energy of the electron ejected by autoionization as well as on the absorption cross-section of the autoionizing state.

B. Acetone dimer

The TPEPICO spectra of acetone and its clusters were observed in the region 124.0-135.0 nm with an interval of 0.2nm. A typical TPEPICO

K. Furuyu et al./J. Electron Spectrosc. Relat. Phenom. 62 (1993) 237-242 241

0 ‘. ‘.’ ” , &” ; l ._ .;...:

20 40 60 80 FLIGHT TIME (ps)

Fig. 2. A threshold-photoelectron photoion coincidence (TPEPICO) spectrum of acetone clusters observed at 131.0 nm.

spectrum is shown in Fig. 2, observed at 131.0 nm (9.465 eV), which lies 0.235 eV below the adiabatic ionization potential of the acetone monomer (9.70 eV). The monomer cation peak in Fig. 2 is attributed to scattered light arising from some imperfection of the grating.

It is thought that the dimer cation signal appearing in the TPEPICO spectrum arises from the dissociation reaction of cluster cations larger than the dimer cation as well as from the ionization of the neutral dimer. Contributions from any larger clusters should be removed from the total dimer cation signal in order to discuss the wavelength dependence of the dimer cation intensity produced only from the neutral dimer.

In the present experiments, the trimer and the tetramer cation intensities were much weaker than the dimer cation intensity (almost noise level) in all the observed TPEPICO spectra, as shown in Fig. 2. In addition, the line width and line shape of the dimer cation signal were independent of the excitation wavelength, implying that the dimer cation in the TPEPICO spectra has no translational energy associated with a dissociation reaction producing a dimer cation. Thus, the contribution of larger clusters to the dimer cation peak observed in all the TPEPICO spectra is negligibly small. The solid curve in Fig. 3 was obtained by plotting the total dimer cation counts against the excitation wavelength. The plotted curve may be called a “TPEPICO excitation spectrum”. The ordinate was normalized to the photon intensity. The error bars indicate the standard deviation estimated from three separate measurements at each point. The photoionization efficiency (PIE) curve of the dimer cation observed by Trott et al. 1181 is shown in Fig. 3 by a dashed curve for comparison. From the TPEPICO excitation spectrum, the appearance ionization potential of the dimer cation has been evaluated as being 9.210 _+ 0.015 eV. This value is slightly

K. FUFUYU et aE.IJ. Ekctron Spectrosc. Relat. Phenom. 62 (1993) 237-243

11 I I 11 1 I t 11 11 125 130 135 WAVELENGTH (nm)

Fig. 3. TPEPICO excitation (solid curve) and PIE (dashed curve) spectra of the acetone dimer. The PIE spectrum was take from ref. 18.

lower than that (9.26 + 0.03 eV) evaluated by Trott et al. [18]. In several papers [19-231 the observation of the protonated acetone dimer cation has been reported. In our experiments, we could not observe any signal due to m/e = 117 (the mass of the protonated dimer) stronger than that expected from the natural abundance of 13C under our experimental conditions.

In the TPEPICO excitation spectrum shown in Fig. 3 there are two bands; one is a broad band with a peak at 129.6 run, and the other is a satellite band with a peak at 127.2nm. The latter peak position is identical to the PIE peak position reported by Trott et al. [18]. In contrast to the sharp vibra- tional bands of the monomer, the 129.6nm band shows long tails to either side of the band maximum, implying that the potential minimum of the dimer cation is considerably different from that of the neutral dimer, and that the Franck-Condon factor of direct ionization is not zero for a very wide energy region.

The second electronic state of the acetone monomer cation is located at 12.59 eV (98.47 run) [9], much higher than the maximum positions, 9.747 eV (127.2 nm) and 9.567 eV (129.6 nm), observed in the present TPEPICO excita- tion spectrum. The difference is too large to explain the dimer production. Thus it is not expected that the two observed bands are related to the second electronic state of the acetone monomer. Since the upper state of the charge resonance states produced from the interaction between the two kinds of acetone dimer cations, acetone-(acetone)+ and (acetone)+ - acetone, in the electronic ground states has a repulsive potential well, the upper state should not be observed in the present dimer TPEPICO excita- tion spectrum. The band shape observed at 127.2nm in the PIE spectrum implies that this band cannot be due to the dimer cation. We can therefore conclude that the bands observed at 127.2 and 129.6nm in the TPEPICO

K. Furuya et al./& Electron Spectrosc. Relat. Phenom. 62 (1993) 237-242 243

excitation spectrum may be assigned to a Rydberg state of the neutral dimer and the electronic ground state of the dimer cation, respectively.

Acknowledgments

We thank the staff of the UVSOR Facility for their kind cooperation, We are also grateful to Dr. K. Mitsuke, Dr. H. Yoshida, and Mr. H. Hattori of the Institute for Molecular Science for supporting our experimental prep- aration. Furthermore, we are grateful to Dr. M. C. R. Cockett for his careful reading of the manuscript and for his valuable suggestions. This work was supported by the Joint Studies Program (1992-1993) of the Institute for Molecular Science.

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