J. Phys. Chem. 1992, 96, 2931-2932
2931
Photodepletion Spectroscopy on Charge Resonance Band of ( c ~ H ~ and ) ~ +(@&)3+ Kazuhiko Ohashi and Nobuyuki Nishi* Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 81 2, Japan (Received: December 10, 1991; In Final Form: January 22, 1992) Photodepletion spectra of (C&6)2+ and (c6H6)3+ are obtained from the depletion yields of the parent ions as functions of photodissociation wavelengths of 750-970 nm. The spectrum of (c6H6)2+shows a broad and featureless band with a maximum at -920 nm and an (estimated) width of 4 0 0 0 cm-I. The spectrum of (c& , )3: shows a similar absorption to the (c&6)2+ band, suggesting that the (C6H6)3+ band is due to the charge resonance transition in the dimer ion subunit.
Introduction One of the very attractive features of cluster ion spectroscopy is the identification of central core ion in large cluster ions.]J Schriver et al. found that benzene cluster ions are particularly stable with respect to evaporative decay when they contain 14, 20, 24 or 27 molecule^.^ These numbers are each one greater than the corresponding magic numbers of rare-gas clusters and are therefore associated with icosahedral packing about a central dimer ion. Recently Beck and Hecht investigated photofragmentation of mass-selected (C&),,+ cluster^.^ They observed the same strong absorption in the red and near-infrared region for n = 2-1 5 and concluded that the dimer ion is the chromophore in the charge resonance (CR) absorption for each of the cluster ions. The binding energy of the cluster ions, (C&,),,+ - C&, was determined by Krause and c o - ~ o r k e r s . ~They performed a simple Huckel molecular orbital calculation for a sandwichlike trimer ion (and also for tetramer ion) assuming a delocalized charge and obtained good agreement with the experimental results. The dimer core model and the charge delocalized structure for the small cluster ions are inconsistent with each other. In the previous spectroscopic study for (c6H6)2+ and (C6H6)3+ in the visible region, we proposed a triple sandwich structure for ( C ~ H S ) ~ ’ . ~Interest in (C6H6)3+ is promoted by a question whether the charge is localized on the two molecules (dimer core) or is delocalized over three molecules. If a delocalization of the charge occurs within (C6H6)3+ as concluded by Krause et then the C R band of (C&,)3.+ should show a different feature from that of (c&)2+. In this letter we describe the results of a photodepletion spectroscopy on the charge resonance band of (c&)2+ and (C&6)3+ over 750-970-nm wavelength region. Experimental Section The experimental apparatus has been described in detail elsewhere.6 Briefly, neutral benzene clusters were formed in a supersonic beam. Between two acceleration grids of a time-offlight mass spectrometer (Jordan Co., angular type reflectron), a pulsed ionizing laser (wi, 210 nm) irradiated the neutral clusters to produce the cluster ions by resonance-enhanced two-photon ionization via the Sz state. While traveling in the acceleration region, the prepared ions were excited by a second time-delayed dissociation laser (q, 750-970 nm). The size of the parent ions was selected by adjusting the time-delay between oiand wd. After secondary acceleration up to ~ 1 . kV, 3 both the remaining parent ions and the photofragment ions were introduced into an ion reflector. The reflector was employed to separate the fragment and the parent ions. Reflected ions were detected by dual mi(1) Johnson, M. A.; Alexander, M. L.; Lineberger, W. C. Chem. Phys. Leu. 1984, 112, 285. (2) Levinger, N . E.; Ray, D.; Alexander, M. L.; Lineberger, W. C. J . Chem. Phys. 1988,89, 5654. (3) Schriver, K. E.; Paguia, A. J.; Hahn, M. Y.; Honea, E. C.; Camarena, A. M.; Whetten, R. L. J . Phys. Chem. 1987, 91, 3131. (4) Beck, S . M.; Hecht, J. H. J . Chem. Phys. 1992, 96, 1975. (5) Krause, H.; Ernstberger, B.; Neusser, H. J. Chem. Phys. Lett. 1991, 184, 411.
(6) Ohashi, K.; Nishi, N. J . Chem. Phys. 1991, 95, 4002.
0022-365419212096-2931$03.00/0
crochannel plates. Two pulsed dye lasers ( 8 4 s pulse duration) were used. Ionization was achieved by pumping the first dye laser (Lumonics HD-300) with a XeCl excimer laser (Lumonics TE861) and by frequency doubling with a BBO (&Ba2B204)crystal. The second dye laser (Lumonics HD-300) pumped with a XeCl excimer laser (Lumonics EX-400) was used for dissociation. No laser dyes were available at wavelengths longer than 970 nm.
Results and Discussion Figure 1 displays the TOF mass spectra of small benzene clusters (C&6),,+ (n = 1-4). The spectrum obtained with wi (210 nm) alone is shown in Figure la. Selective excitation of (C&)3+ by wd (935 nm) gives the spectrum shown in Figure lb. Approximately 40% of the parent ions disappeared as indicated by “dip” in the figure. The insets show an expanded view of the monomer (left) and the dimer (right) regions, where the photofragment ions appeared along side the primary (C&,),,2+ produced only by 2wi. The notation y m 3” above each secondary peak indicates the fragment (C,H,),+ from (C,&)3+. In the previous work, we measured yields of photofragment to monitor photoabsorption of (c6H6)2+ and (CpH6)3+, because C6H,‘+ is the single predominant fragment in the wible wavelength region.6 The same technique was employed to record the photodissociation spectrum of (C6&)2+. Figure 2 indicates the present spectrum together with the one in the visible region. For the photodissociationof (C6H6)3+ in the 750-97O-nm region, formation of both (C&)2+ and c6&+was observed. When there exist more than one dissociation channels, photodissociation spectroscopy requires accurate information on the branching ratio of the products at each wavelength in order to convert the fragment yield into absorption cross section. Photodepletion spectroscopy of the parent (C6H&+ is an alternative approach to measure the absorption spectra, although the measurement of depletion is less sensitive and more difficult than the measurement of photofrag ment yield. Figures 3 and 4 show the photodepletion spectra of (C6H6)2+and (C6H6)3+,respectively. During the measurements, the power of the dissociation laser was carefully controlled to ensure that the photodissociation remained in the linear region. The observed depletion was normalized according to the dissociation laser power. Fluctuations in the cluster beam intensity and the ionization laser power were also taken into account. The photodissociation spectrum of (C&)2+ in Figure 2 shows a broad band with a maximum at ~ 9 2 nm. 0 This band has been assigned to an intervalence transition band, or in other words, a charge resonance (CR) band in condensed-phase The C R band arises from the transition between bound-state/repulSiVeState pairs COlTelated to C6H6+(X2Elg)4- C6H6(X’Alg). These f \k1\k2+,where qj+and qi two states are expressed as are the electronic wave functions of C6H6+(X) and C6H,(X), respectively. The absorption cross section of the CR band is almost 1 order of magnitude larger than that of the local excitation (LE) +
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(7) Shida, T.; Hamill, W. H. J . Chem. Phys. 1966, 44, 4372. (8) Badger, B.; Broklehurst, B. Trans. Faraday SOC.1969, 65, 2582. (9) Miller. J. H.; Andrews, L.; Lund, P. A,; Schatz, P. N. J . Chem. Phys. 1980, 73, 4932.
0 1992 American Chemical Society
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2932 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Letters
Cluster Size
I
I
I
I
I
40
50
60
70
80
Flight Time
/
Wavelength
o,2,+1. '".
I
ps
Figure 1. TOF mass spectra of small benzene clusters (C6H6)"' ( n = 1-4). (a) Spectrum obtained with wi (210 nm) alone. (b) Spectrum obtained with the introduction of wd (935 nm), in which (c6H6)3' was
/ nm
I
0.0
1.3
1.2
1.1
Wavenumber x / cm-' Figure 3. Photodepletion spectrum of (C6Ha)2' in the charge resonance
band region. The error bars indicate statistical uncertainty determined from 4-6 repeated dye laser scans.
,
selectively excited.
1.0
Wavelength 7:o
8YO
/ nm
870
9 o:
0 : 9
-
4
$
0.6-
La
a L(
0
0.6-
8
-3
4
2a
0.4-
,t)
n
Wavelength
/ nm
Photodissociation spectrum of (C6H6)2+in the visible and near-IR wavelength region. Local excitation bands and charge resonance band are indicated by LE and CR, respectively. Figure 2.
bands. The existence of such an intense CR absorption suggests that two benzene molecules should be equivalent to each other in (c6H6)2+. The CR energy is defined as t = (Ql+\k21V121P,Q2+), where VI, describes the intermolecular interaction.1° The transition energy for the C R band is equal to 2t. We determined as t = 0.67 eV from the observed absorption maximum of ~ 9 2 nm 0 for (C6H6)2+. Meot-Ner measured dissociation energies for dimer ions and protonated dimer ions of polycyclic aromatics." The resonance energy of ~ 0 . 2 6eV for benzene was reported by assuming that interactions in the protonated dimer ions were purely electrostatic. This value, however, is not consistent with those obtained from the condensed-phase absorption spectrumI2 nor with the present value for t. For (C&6)3+, the question arises whether the charge is localized on the two molecules (dimer core) or is delocalized over three molecules. Different spectroscopic features of the CR absorption (IO) Kasha, M. Spectroscopy of the Excited State; Bartolo, B. D., Ed.; Plenum Press: New York, 1976. (11) Meot-Ner, M. J . Phys. Chem. 1980, 84, 2724. (12) Badger, B.; Broklehurst, B.; Russell, R.D. Chem. Phys. Lett. 1967, I , 122.
0.0
t
I
I
I
1.3
1.2
1.1
Wavenumber x
/ cm-'
Photodepletion spectrum of (C6H6)3+in the charge resonance band region.
Figure 4.
are expected for the two electronic structures. Spectroscopically, Beck and Hecht observed the same strong absorption in the 695-900-nm wavelength region and an additional point at 1064 nm for n = 2-15.4 They concluded that (C&)2+ is the chromophoric subunit in the C R band region for each of the cluster ions, although there was no description about the comparison of spectroscopic features of (c6H6)3+ with those of (c6H6)~'. We extended the detailed measurement to the region of the absorption maximum of (C&)2+. As shown in Figure 4,the spectrum of (C6H6)3+ shows a similar absorption to the (c&6)2+ band, although the band of (C6H6)3+ seems to peak at longer wavelength than 970 nm. No large differences were found between the photodissociation cross sections of (c6&)2+ and (C,H6)3+. From the observation, it is tempting to speculate that (C6H6)3+ has a charge-localized structure, i.e., the dimer ion plus a weakly bound neutral molecule. Ascertainment of the position and width of the (C.&)3+ peak will be helpful to discuss the degree of the charge delocalization and the structure of (C6&)3+.
