7873
J. Phys. Chem. 1992, 96, 7873-7876 8.5
threshold for dissociative chemisorption, we have shown that there is an antithreshold at higher energies, above which all molecules scatter nondissociatively. More generally, we have also demonstrated the feasibility and utility of analyzing and characterizing such moleculesurface collisions in terms of their inherent fractal behavior.
8 D
+
Acknowledgment. This study was supported in part by a grant from the INDO-US Subcommision and also partly by a grant from the Council of Scientific and Industrial Research, New Delhi.
u)
z C -
6.5
References and Notes
A=. 7."
2.1
3.5
In
4.9
W E )
Figure 5. In N , vs In ( I / < ) with E,,,, varied parametrically, as labeled.
transitions a t the antithreshold), the collision time at high E,,,,, is too short to allow for the translational to vibrational energy flow required to break the intramolecular bond. Consequently the collision results in a scattered, possibly vibrationally excited, but intact molecule.
Summary and Conclusion Using a model moleculesurface potential, we have shown that the extent of chaoticity, as evidenced in action-angle and lifetime plots, decreases dramatically as the translational energy of the diatomic molecule increases. In addition to the existence of a
(1)Balasubramanian, V.;Sathyamurthy, N.; Gadzuk, J. W. Surf. Sci. 1989,221, L741. (2)Gadzuk, J . W. J . Chem. Phys. 1987,86,5196. (3)Mandelbrot, B. B. The Fractal Geometry of Nature; Freeman: San Francisco, 1983. (4)(a) Gadzuk, J. W.; Holloway, S . Chem. Phys. Lett. 1985,114, 314. (b) Holloway, S.; Gadzuk, J. W. J. Chem. Phys. 1985,82, 5203. (c) Sumpter, B. G.; Thompson, D. L.;Noid, D. W. J . Chem. Phys. 1987,87,1012. (5)Cheney, W.; Kincaid, D. Numerical Methods and Computing, 2nd ed.; Brooks-Cole: Monterey, CA, 1985. (6)Slater, N. B. Nature 1957,180, 1352. (7)Eckhardt, B.; Jung, C. J . Phys. A . 1986,19, L829. (8) Baker, G. L.;Gollub, J . P. Chaotic Dynamics; Cambridge University Press: Cambridge, 1990. (9)Bleher, S.; Ott, E.; Grebogi, C. Phys. Rev. Letz. 1989,63,919. (10)Essex, C.; Nerenberg, M. A. H. Am. J . Phys. 1990, 58, 986. (11) Noid, D. W.; Gray, S.K.; Rice, S . A. J. Chem. Phys. 1986,84,2649. (12)Thareja, S.;Sathyamurthy,N. Surf. Sci. 1990, 237, 266. (13)(a) Wright, J. S.;Tan, K. G.;Laidler, K. J.; Hulse, J. E. Chem. Phys. Lett. 1975,30,200. (b) Wright, J. S.; Tan, K. G.; Laidler, K. J. J . Chem. Phys. 1976,64,970.(c) Wright, J. S.;Tan, K. G. J . Chem. Phys. 1977,66, 104. (14)Pechukas, P.; Pollak, E. J . Chem. Phys. 1977,67,5976. (15)Davis, M. J.; Gray, S. K. J . Chem. Phys. 1986,84, 5389. (16)Dove, J. E.;Mandy, M. E.; Mohan, V.; Sathyamurthy, N. J . Chem. Phys. 1990, 92, 7373.
Photodissociation of Size-Selected Benzene Cluster Ions: (C,H,),+
wlth n = 2-8
Yasuhiro Nakai, Kazuhiko Ohashi, and Nobuyuki Nishi* Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Fukuoka 81 2, Japan (Received: March 6, 1992; In Final Form: June 9, 1992)
Photodissociation of size-selected benzene cluster ions, (C&),,' with n = 2-8, was investigated in the wavelength range of 440-960 nm. The average number of neutral monomers ejected from each cluster ion was obtained from the size distribution of photofragment ions. Two 'different electronic excitations (7 7 local excitation and charge resonance transition) exhibited a similar dissociation propensity with respect to the partitioning of the available energy. An upper bound of 0.35 f 0.1 eV was determined for the average binding energy of a neutral monomer to a cluster ion with n = 6-8.
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Introduction From the view of its structural simplicity and aromatic property, benzene has been a subject of numerous cluster works. Identification of the geometric and electronic structures of the benzene cluster ions is receiving much attention. Examination of the dependence of intermolecular binding force on cluster size is of interest to give the insight into the connection between condensed matter and isolated molecules. Photodissociation study can successfully provide such information. Snodgrass et al. measured kinetic energy distribution of charged photoproducts of (c6&)2+ at a photon energy of 2.71 eV.' From the observed partitioning of the available energy, they concluded that the upper state in the photoabsorption process is a bound state. They estimated the binding energy of the upper state from the observed onset of photodissociation. Ohashi and Nishi reported
photodissociation spectra of gas-phase (c,&),=2,3+ in the wavlength range of 41Q-970 nm.L3 Recently, Beck and Hecht studied photodissociation of larger benzene cluster ions, (CsH6),+ with n = 7-15.4 They extracted an upper limit to the average binding energy from the dependence of the size of dominant fragments on the excitation wavelengths and supported a charge-localized (core ion) model proposed by Schriver et al.5 On the other hand, Krause et al. presented a charge-delocalized model based on a precise measurement of the ionization potentials and the appearance potentials of the monomer evaporation from (c&),+ ( n = 2-4).6 In this article, we present the size distribution of the photofragment ions from (C6H6),,+with n = 2-8 in the wavelength range nm. The average number of neutral benzene molecules of 44Q9-60 ejected from each parent cluster was obtained from the ratios of
0022-365419212096-7 873$03.00/ 0 0 1992 American Chemical Society
Nakai et al.
7874 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
FRAGMENT SIZE n-m 1
2
3
4
5 880nm
(cawn++( c ~ H ~ ) ~ - ~ + + ~ ( c ~ H ~ )
n=4
i
+4
n=3
‘0.0
FLIGHT TIME
/
pus
-
Figure 1. Difference TOF mass spectra of fragment ions from benzene clusters (C6H6),+(n = 2-8) photodissociated at 880 nm. The negative peaks at the n - 1 positions are due to the depletion of metastable (n n - 1) process induced by wd.
the photofragment ion intensities as a function of the photon energy. Photon energy dependence of the number loss was examined for inferring the photodissociation mechanism after two different types of photoexcitation ( A A local excitation and charge resonance transition).
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Experimental Section A detailed explanation of the apparatus has been described elsewhere.2 Briefly, neutral benzene clusters were generated in a pulsed supersonic beam. These clusters were ionized by q (210 nm) from a pulsed dye laser (Lumonics HD-300) pumped with a XeCl excimer laser (LumonicsTE-861). After traveling through a flight path of 7.3 mm for size selection, the parent ions were excited by wd (440-960 nm) from a second pulsed dye laser (Lumonics HD-300) pumped with a XeCl excimer laser (Lumonics EX-400)in the acceleration region of a time-of-flight mass spectrometer (TOFMS). The size selection was achieved by adjusting the delay time between wi and a d with a digital pulse generator (Stanford Research DG-535). Product ions were mass-analyzed by a second reflectron-type TOFMS (Jordan Co.). In the photodissociation experiments performed in the field-free region, photofragment signals appear at the same arrival times as the metastable decay signals of the parent ions which are not excited by a d . Thus a photodissociation spectrum can be obtained after the subtraction of the metastable signals.’ However, if such metastable peaks swamp the photofragment ion signals, it is difficult to attain reliable intensity ratios of the photofragment ions. In the present experiment, photodissociation was carried out in the acceleration region, and thus the photofragment ions were secondarily accelerated before leaving the ion source region. With this configuration, photofragment signals were separated from metastable fragment signals by suitably tuning the reflectron potential.2 The ability to control the relative position of the photofragment signal to the metastable one is one of the main advantages of the method of selective photodissociation in the acceleration region.
Results and Mscursion Benzene dimer ions or trimer ions showed a sharp absorption band centered at 440 nm and a broad one with a peak around 920-930 nm. These bands were assigned to a r A local excitation band and a charge resonance band, respectively.2v3 Size dependence was observed for the position of a weak band around 600 nm that was assigned to a r u local excitation band.2
-
1.0
1.5
2.0
2.5
3.0
3.
PHOTON ENERGY / eV Figure 2. Photon energy dependence of the average number loss of neutral monomers from the parent cluster ions. The solid lines are the least-squares fittings of the data in the range of 1.29-1.94 eV.
60
40
20
0.5
-
Figure 1 shows the difference TOF mass spectra of the fragment ions obtained in the photodissociation at 880 nm. The background spectra due to metastable decay signals were measured without introducing the dissociation photons ( a d ) and these metastable signals were subtracted from the spectra with a d , although the metastable peaks did not overlap with the photofragment ion peaks. The negative peaks that appeared in these spectra were due to this subtraction and originated from the depletion of the metastable decay process (n n - 1) in the field-free region of the TOFMS. In order to avoid any two-photon excitation of the size-selected cluster ions, incident laser energies were weakened as low as possible. The applied laser power was normally of the order of ~ 1 0 pJ/pulse. 0 The laser beam was mildly focused on the ion beam through a quartz lens withf = 250 mm. In fact, there is no peak which corresponds to the loss of more than four benzene molecules (m > 4). Such peaks were seen in the mass spectra obtained by Beck and Hecht and attributed to photofragments produced by multiphoton absorption: Alexander et al. introduced the ‘average number of neutrals” ejected following the photoabsorption,N,,, in order to elucidate the binding energy of (CO,),* cluster ions.’~* We employed a similar method to analyze the size distribution of the photofragments. Here N,, was determined from the ratios of the fragment ion intensities. Figure 2 shows the plots of the N,, values obtained for (C6H6),+ (n = 4-8) against the applied photon energies. The N,, values for the parent (C6H6)4+ and (C6H6),+ are approximately constant over the photon energies 1.29-1.94 eV (640-960 nm) and they are approximately 2 and 3, respectively. Since the stability of (C6H6)2+ is much greater than those of the larger the obtained N,, value of such a small cluster is prone to become smaller. For the clusters with n = 6-8,Nay increases almost linearly with increasing photon energy in the range of 1.29-1.94 eV. The lines drawn through the data points are the least-squares fitting to the experimental points in the range of 1.29-1.94eV. Extrapolation of the lines to the photon energy of 2.81 eV (440nm) provides Nayvalues larger than the experimental values. Photoexcitation of (C6&)2+ at the charge resonance band (129-1.94 eV) promotes the ions to a repulsive potential, whereas the upper state at 2.81 eV (r r local excitation band) is a bound state. Ohashi and Nishi compared the dynamia of the d d t i o n of (C6H6)f from the charge resonance state with that from the (A,*) statelo and found that the dissociation occurs in a statistical way after the complete randomization of the available energy in the case of the local excitation, whereas the complete energy randomization is not achieved for the charge resonance transition. One can expect a similar dissociation propensity for the larger clusters. On the basis of the observed long lifetime of the dissociating state,1° it is reasonable to assume that the cluster ions +
Photodissociation of Benzene Cluster Ions
%
* E
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4
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440nm 640nm 720nm 800nm 880nm
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-
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Charge-delocalized model Core ion model Thermochemical
ii 5 w
10
CLUSTER SIZE n Figure 3. Average binding energy (Eav)as functions of cluster size n. The results with 440-nm excitation are not in agreement with those with the other wavelengths, because dissociation occurs via the bound excited state after the 440-nm excitation.
are promoted to a bound state at 2.81 eV and dissociate after the complete energy randomization. Then the value of Nayat 2.81 eV is considered to arise from statistical energy partitioning. The fact that the extrapolation of the lines to 2.81 eV (in Figure 2) provides Nayvalues larger than the observed ones indicates that the fraction of the available energy used to break the intermolecular bonds upon the charge resonance band excitation is larger than that expected from the statistical prediction, although the deviation of the observed N,, at 2.81 eV is not so pronounced. Reverse extrapolation of the lines for N = 6-8 to the photon energy (hv) of 0 eV results in N,, (hv = 0 eV) = 1, showing that the parent clusters can release one neutral monomer without absorbing @d. This is consistent with the observation of the n n - 1 metastable decay process and indicates that the parent clusters contain intemal energies of the order of the dissociation energy for the weakest bond. No direct information is available about the internal energy distribution of the benzene cluster ions created by one-color REZPI with q = 210 nm. An initial internal energy of less than 0.1 eV was estimated for (C6H6)2+ by a statistical phase space calculation for the photoexcitation to the (?r,r) state.lo Although internal energies of the same order (or a little bit higher energies) can be expected for the larger cluster ions, the initial internal energy of the parent ions is not taken into account in the following analysis. The average binding energy of a neutral monomer to a cluster ion can be defined as73 E , = hv/" (1) Figure 3 shows the obtained average binding energies for the parent cluster size with n = 2-8. As already mentioned, the determination of the average binding energies by using eq 1 is unreasonable for the small clusters with n < 5, because the present value of Nayis too small for the small clusters. Secondly, the binding energy should not depend on the laser wavelengths. This is indeed the case only for the large clusters with n > 5 in the range of 640-960 nm. Therefore experimental points for n < 5 are of no significance. As shown in Figure 3, the average binding energies smoothly decrease with increasing parent cluster size and almost converge at n > 5. From the asymptotic value of the energies, we estimate an upper bound of 0.35 f 0.1 eV for the average binding energy with n = 6-8. The main assumption in the present analpis is that all of the photon energy is used to break the intermolecular bonds. This analysis overestimates the binding energy because some parts of the available energy can be partitioned into the intemal energy of the products. So that the value given by eq 1 is just an upper limit of the average binding energy. Beck and Hecht determined the average binding energy from the photon energy dependence of the dominant fragments. Their values were in the range 0.37-0.34 eV for (C6H&+ with n =
o.oo
2
4
6
8
10
CLUSTER SIZE n Figure 4. Comparison of the experimental average binding energy with the two kinds of model calculations: charge-delocalized model, a model
in which the charge is delocalized over all molecules in a cluster (from ref 6 ) ; core ion model, a model with a point charge in the center of a continuousmedium characterized by a bulk dielectric constant (from ref 4). The thermochemical binding energies obtained from the high pressure mass spectrometry for the dimer and the timer cations are also shown with the reported experimental uncertainties (from ref 9). 7-1 5.4 The present results for n = 6-8 are consistent with those by Beck and Hecht for n = 7-15 within the experimental uncertainties. Krause et al. obtained the binding energy of (C6H6)@1+- C6H6 (with n = 2-4) from the measurement of the ionization potentials and the appearance potentials of the monomer evaporation from the respective cluster ions? They found a good agreement of their experimental binding energies with the theoretical values calculated by assuming a charge delocalization in the cluster ions. They concluded that the charge fesonance interaction mainly contributes to the binding energies in dimer, trimer, and tetramer ions. A core ion model is also proposed for the larger benzene cluster ions. Schriver et al. found that benzene cluster ions are particularly stable at n = 14, 20, 24, or 27 and attributed this stability to a certain stable structure due to icosahedral packhg around a central dimer c a t i ~ n . ~Beck . and Hecht found that (C6H6),,+ actually exhibits an anomalously high binding energy? They calculated the binding energies using a model where the cluster ion was represented by a point charge in the center of a continuous medium characterized by a bulk dielectric constant. The model calculations were reproduced here according to the procedum described in detail in the respective references." Figure 4 shows the comparison of the two calculations with our experimental results for n = 6-9. The observed binding energy is expressed by filled circles with error bars. The thermochemical values of 0.893 f 0.043 eV for (C&),+ and 0.34 f 0.02 eV for (C&)3+ by Hiraoka et al.9 are also plotted in the figure. Although the charge delocalized model (dotted line) can reasonably explain the size dependence of the binding energy for the small clusters with n < 4, it predicts much smaller binding energies than the experimental values for the clusters with n = 6-9. On the other hand, the calculated energies based on the core ion model (solid line) agree with the experimental values with n = 6-9. The core ion model calculation failed to predict the size dependence of the binding energy for the clusters with n < 4, probably because it is too simplistic and ignores many details particularly for the small clusters. The average binding energies for n = 6-9 of the present work as well as those obtained by Beck and Hecht are consistent with the values predicted by the core ion model. The recent spectroscopic study showed that (C6H6)3+ has a similar absorption band to the charge resonance band of (c6&)2+? This spectral similarity suggests that the (c6H6)3+ band is due to the charge resonance transition in the dimer ion subunit and that (c6H6)2+ is the core ion in the larger clusters. However, the degree of the charge
J. Phys. Chem. 1992, 96, 7876-7881
7876
delocalization in the small clusters with n = 3-5 is still in question. Spectroscopic study of (C6H6),,+with n = 4-8 is in progress and will provide much detailed information about the cluster structure.
References and Notes (1) Snodgrass, J. T.; Dunbar, R. C.; Bowers, M. T.J. Phys. Chem. 1990, 94, 3648.
(2) Ohashi, K.; Nishi, N. J . Chem. Phys. 1991, 95, 4002. (3) Ohashi, K.;Nishi, N. J . Phys. Chem. 1992, 96, 2931. (4) Beck, S. M.; Hecht, J. H. J. Chem. Phys. 1992, 96, 1975.
(5) Schriver, K. E.; Paguia, A. J.; Hahn, M. Y.;Honea, E. C.; Camarena, A. M.; Whetten. R. L. J . Chem. Phys. 1987, 91, 31 31. (6) Krause, H.; Ernstberger, B.; Neusser, H. J. Chem. Phys. Lert. 1991, 184, 41 1, (7) Alexander, M. L.; Johnson, M. A.; Lineberger, W. C. J . Chem. Phys. 1985, 82. 5288. (8) Alexander, M. L; Johnson, M. A.; Levinger. N. E.; Lineberger, W. C. Phys. Rev. Lerr. 1986, 57, 976. (9) Hiraoka, K.; Fujimaki, S.; Aruga, K.; Yamabe, S.J. Chem. Phys. 1 9 9 1 , 95, 8413. (10) Ohashi, K.; Nishi, N. Unpublished results.
Infrared Spectrum of Matrlx- Isolated Naphthalene Radical Cation Jan Szczepanski, Dennis Roser, William Personette, Marc Eyring, Robert Pellow, and Martin Vala* Department of Chemistry and Center for Chemical Physics, University of Florida, Gainesville, Florida 3261 1-2046 (Received: March 24, 1992; In Final Form: June 8, 1992)
Radical cations of naphthalene (N) have been formed by electron bombardment of a vapor-phase mixture of naphthalene, carbon tetrachloride, and argon and trapped in a matrix (Ar) at 12 K. Infrared spectral scans of this matrix compared to ones containing neutral N alone or electron-bombarded CC14 reveal new vibrational bands at 1525, 1519, 1401, 1218, 1215, 1023, and 1016 cm-I. These bands are attributable to the N cation by their positive correlation with the known N cation X2A, (Do) band system at 675 nm. The observed IR band frequencies and relative intensities compare well 2B,,(D,) with a recent ab initio calculation by Pauzat, Talbi, Miller, DeFrees, and Ellinger. The addition of CC14 to the mixture subjected to electron bombardment ionzation is considered and the probable role of CCll (and its products) as ionization enhancers and matrix charge stabilizers is discussed.
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I. Introduction The field of interstellar chemistry is relatively young, tracing its beginnings to the detection in 1968 of the first polyatomic molecule, ammonia, in interstellar space.’ Four diatomics, CN, CH, CH+, and OH, had been observed earlier, the first in 1937. Since the discovery of ammonia, many molecules have been discovered in space, most by radioastronomical observations. In much of this work there has been a close interplay between laboratory measurements, theoretical calculations, and interstellar observations. One of the most perplexing problem^^,^ in astrochemistry to the present has been the origin of the so-called “unidentified infrared (UIR) emission features”, a series of bands observed at 3.3,6.2,7.8,8.7, and 11.3 pm (3030, 1613, 1282, 1149, and 885 cm-I). In 1984, Leger and Puget proposed4that the UIR bands arise from the polycyclic aromatic hydrocarbons (PAH’s) which are excited by absorption of single ultraviolet photons which thermally heat the molecule and thereby provide excitation of the IR vibrational modes. These authors demonstrated the striking correspondence between the observed interstellar IR emission bands and the main IR absorption bands in a moderate-sized PAH (coronene): 3.3, 6.2, 7.8, 8.7, and 11.3 p m in space vs 3.3, 6.2, 7.6,8.8, and 11.9 pm in the molecule. Allamandola, Tielens, and Barker then proposed5that ionized, partially hydrogenated PAH’s were responsible for the IR emission features. These authors suggested that the PAH’s probably exist as monocations since their first ionization potentials are considerably below the energy obtainable from starlight. Other arguments regarding the probable stability and abundance of the PAH’s in the harsh interstellar environment, their absorption in the visible range, their possible hydrogen content, and state of ionization, though not as compelling as the IR spectral comparison, are sufficientlyappealing to warrant further study. It is worth pointing out, as Allamandola has done: that if the hypothesis of PAH’s in interstellar space is shown to be valid (1) these molecules will be the first interstellar organic ring molecules
known, (2) they will be as abundant as the m a t abundant simpler interstellar polyatomic molecules known, (3) because of their complexity and abundance they may provide a link between interstellar gas and grains, and (4) their stability will imply lifetimes of the order of the age of the clouds in which they are found. Questions on the origin of interstellar and stellar matter, i.e., their astrochemistry, may then be probed on a more stable foundation. One of the major impediments to the conclusive identification of the PAH cations as the carrier(s) of the UIR bands is the lack of vibrational data on these species in low-temperature, isolated environments. In this paper we report on the infrared spectrum of the cation of the smallest PAH, naphthalene, isolated in an argon matrix at 12 K. The cationic species were produced by electron bombardment. By correlation of the new IR bands with the known naphthalene cation visible absorption band system at 675 nm, the IR bands could be assigned with certainty. The IR bands attributable to the naphthalene cation match reasonably well with the bands predicted by Pauzat et al.’ In the following paper, these authors report a b initio theoretical calculations of the infrared spectra of the cationic and neutral forms of naphthalene. They find the unexpected result that ionization effects the intensities of most vibrations very strongly. The intensities of the CC and in-plane C H vibrations increase while the CH stretching vibrations decrease. Experimentally, the question of the effect of ionization on the vibrational mode intensities is problematic, however, and is discussed in detail below. 11. Experimental Section
In the present study a specially-constructed electron bombardment source, sketched in Figure 1, was employed to ionize the naphthalene prior to deposition. The tungsten filament (0.1 mm diameter), was heated by a current of 1.20-1.45 A (at 6-8 V) with resultant electron emission. The anode was held a t a potential (U,)of +20 to +50 V, while the cathode potential (LIB) was maintained at -50 to -200 V. The electron beam intersected the vapor-phase mixture of argon, CCl,, and naphthalene just in
0022-365419212096-7876%03.00/0 0 1992 American Chemical Society
