J . Phys. Chem. 1988, 92, 3785-3789
3785
Multiphoton Mass Spectrometry of Clusters: Dissociation Kinetics of the Benzene Cluster Ions A. Kiermeier, B. Ernstberger, H. J. Neusser,* and E. W. Schlag Institut f u r Physikalische und Theoretische Chemie I, Technische Uniuersitat Munchen, Lichtenbergstrasse 4, 0-8046 Garching, West Germany (Received: November 4, 1987; In Final Form: February 2, 1988) Neutral benzene clusters (C,H,), with n up to 40 are produced in a supersonic jet expansion and ionized by two-photon absorption with tunable UV laser light in a reflectron time-of-flight mass spectrometer. Photon energy is chosen so that cluster ions are produced with a small amount of energy (several hundred millielectronvolts) and a slow decay on the microsecond time scale is observed. The reflectron mass spectrometerserves as an instrument for the direct observation of all major dissociation paths of clusters near threshold. The precursor ion mass and the daughter ion mass can be assigned unambiguously by the analysis of kinetic energy. In particular the fragmentation pathways of benzene cluster ions up to n = 7 are displayed in full detail. It was found that close to the dissociation threshold, the elimination of one neutral benzene monomer is the favored process, leading to an ionized cluster of size n - 1. The efficiency of the metastable decay of the benzene dimer ion has been measured as a function of two-photon energy. From the resulting breakdown graph the first purely experimental value of the binding energy of the benzene dimer and a value for the binding energy of the benzene dimer cation have been obtained.
I. Introduction Interest in the properties of neutral clusters and cluster ions has increased considerably during recent years. This is above all due to the fascinating link cluster physics provides between atomic or molecular physics on the one hand and condensed matter or solid-state physics on the other hand. It is not possible to produce selectively a cluster of certain size, but instead a spectrum of clusters is always produced. The most elementary information obtained from spectra of clusters is their abundance as a function of cluster size. One of the persisting problems in all cluster research has been the falsification of the spectrum of clusters in the analysis process. Mass spectrometry, often the method of choice, seldom gives a true picture of the neutral cluster spectrum. We present here a general method that comes closer to a solution to the problem. As such, the kinetics of cluster ion decay assumes added importance. Several groups have focused their interest on the dissociation dynamics of atami@ and m o l e c ~ l a r ~clusters -'~ or cluster ions. A great amount of data concerning the properties of clusters is now a~ai1able.l~ The usual technique for cluster production involves a supersonic jet expansion. Here a variety of clusters of different size is normally produced. For the precise investigation of a particular cluster type of defined size, i.e., dimer, trimer etc., different cluster masses have to be separated. This is normally done by ionizing all clusters and separating the ions in a mass spectrometer ac~ ~mass spectrum cluster cording to their q / m r a t i ~ . ' , * , ~InJ ~the (1) Kappes, M. M.; Schar, M.; Schumacher, E.; Vayloyan, A. Z. Phys. D. 1987,5,359. Kappes, M. M.; Schar, M.; Schumacher, E. J. Phys. Chem. .. 1987, 91, 658. (2) Geusic. M. E.: Jarrold. M. F.: McIlrath. T. J.: Freeman. R. R.: Brown. W.'L. J . Chem. Phys. 1987,86, 3862. (3) Mark, T. D. Int. J. Mass Spectrom. Ion Processes 1987, 79, 1. (4) Brumbaugh, D. V.;Kenny, J. E.; Levy, D. H. J. Chem. Phys. 1983, 78, 3415. (5) Leutwyler, S.; Even, U.; Jortner, J. Chem. Phys. Lett. 1982, 86, 439. (6) Schriver, K. E.; Paguia, A. J.; Hahn, M. Y.; Honea, E. C.; Camarena, A. M.; Whetten, R. L. J. Phys. Chem. 1987, 91, 3131. (7) Alexander, M. L.; Johnson, M. A,; Lineberger, W. C. J. Chem. Phys. 1985.82, 5288. (8) Gray, S . K.; Rice, S. A. Faraday Discuss. Chem. SOC.1986,82, No. 15. (9) Kelley, D. F.; Bernstein, E. R. J. Phys. Chem. 1986, 90, 5164. (10) Kobayashi, T.; Kajimoto, 0. J. Chem. Phys. 1987, 86, 11 18. (1 1) Vernon, M. F.; Lisy, J. M.; Kwok, H. S.; Krajnovich, D. J.; Tramer, A,; Shen, Y.R.; Lee, Y. T. J. Phys. Chem. 1981,85, 3327. (12) Silberstein, J.; Ohmichi, N.; Levine, R. D. J. Phys. Chem. 1984, 88, 4952. (13) Hays, T. R.; Henke, W.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1981, 77, 19. (14) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1987, 87, 115. (15) For a review see: Castleman Jr., A. W.; Keesee, R. G. Annu. Rev. Phys. Chem. 1986, 37, 525. Keesee, R. G.; Castleman Jr., A. W. J. Phys. Chem. ReJ Data 1986, 15, 1011.
0022-3654/88/2092-3785$01.50/0
ion abundance are falsified due to dissociation of larger precursor cluster ions of size > n which yields fragment ions of size n. Hence, the mass spectrum does not record accurately the neutral cluster concentration; mass peaks almost invariably are contaminated from the breakdown of higher cluster ions. Similarly, ion cluster concentrations are reduced by subsequent ion decomposition. To understand the intensity distribution in the mass spectrum it is necessary to investigate the dissociation dynamics of the cluster ions. Beyond that practical problem, fundamental questions on the kinetic behavior of clusters as a whole may be answered by precise investigation of the cluster ion decay. In this work we would like to present a new method to identify the pathways of cluster ion decay for determination of the dissociation thresholds. This method includes the energy and intensity analysis of metastable ions in a reflectron time-of-flight mass spectrometer. Benzene cluster ions with an energy content of several 100 meV are produced via two-photon ionization. At this low energy slow metastable fragmentation on a microsecond time scale takes place. By energy analysis of the daughter ions produced by a metastable decay in the field-free drift region of a reflectron time-of-flight mass spectrometer (RETOF), we are able to determine all major dissociation channels for each cluster size. Recording the breakdown graph of the benzene dimer cation as a function of two-photon energy (Le., the intensity of the metastable benzene dimer dissociation signal), we are able to determine the binding energies not only of the dimer ion but also of the neutral dimer. 11. Experimental Section
Benzene clusters are produced in a pulsed supersonic jet expansion of benzene gas (20-80 mbar) seeded in noble gas (He) at a pressure of 1-2 bar through a pulsed nozzle (modified Bosch valve, diameter 200 pm) into the expansion chamber. During operation (10 H z repetition rate) the pressure in the expansion to 8 X mbar, depending chamber increases from 8 X on the stagnation pressure and the opening time of the valve. By optimization of the experimental conditions of the expansion, benzene clusters (C,H,), with n up to 40 have been produced. For differing experimental conditions it is also readily possible to produce monomers and dimers exclusively. However, it is not possible to select oligomer ions greater than n = 2. Rotational (16) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Phys. Chem. 1981, 85, 3739.
(17) Fung, K. H.; Selzle, H. L.; Schlag, E. W. J. Phys. Chem. 1983, 87, 5113. (18) Garvey, J. F.; Bernstein, R. B. Chem. Phys. Lett. 1986, 126, 394. (19) Schriver, K. E.; Camarena, A. M.; Hahn, M. Y.;Paguia, A. J.; Whetten, R. L. J. Phys. Chem. 1987, 91, 1786. (20) Echt, 0.;Dao, P. D.; Morgan, S.;Castleman Jr., A. W. J . Chem. Phys. 1985, 82, 4076.
0 1988 American Chemical Society
3786 The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
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Figure 1. Experimental setup for multiphoton mass spectrometry of benzene clusters. The skimmed jet and the flight path of the ions in the reflectron time-of-flight mass spectrometer are collinear. The energy correcting properties of the reflectron TOF are demonstrated by the two trajectories (1 and 2) of ions with mass M but different kinetic energy.
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temperatures vary from 4 to 10 K under above-mentioned conditions and are determined by fitting measured rotational envelopes (two-photon ionization spectra) of the 610 transition of the benzene monomer with computer simulated spectra.21 The central region of the molecular beam is selected with a skimmer (1 mm open diameter) placed 4 cm downstream of the nozzle. The molecular beam enters the mass spectrometer in a collinear configuration (see Figure 1). Five centimeters downstream of the skimmer the benzene clusters are ionized in the acceleration region of the RETOF instrument in a two-photon absorption process. The light of a frequency-doubled tunable dye laser (Lambda Physik, FL 2002E, UV output 500 pJ/pulse), pumped by an Excimer laser (Lambda Physik, EMG 150), is slightly focused onto the center of the molecular beam. Resonance enhancement of particular cluster ion signals with n = 2-5 is possible when tuning the laser frequency exactly to the ~ e l l - k n o w n SI ~ ~ . ~So ~ transition of the corresponding neutral clusters. However, in order to record mass spectra of all cluster ions simultaneousely we normally operated the laser frequency off resonance. Ion signals are then weaker by only one order of magnitude. Probably there is still a resonance enhancement, however, by the structureless broad background due to the low-frequency van der Waals modes. The RETOF instrument24has been described in great detail in our previous ~ o r k Briefly, . ~ ~ it ~consists ~ ~ of an acceleration region, a field-free drift region, two reflecting fields (reflector), and an ion detector (see Figure 1). During operation the pressure in the apparatus is less than 8 X mbar. The acceleration region consists of two plates spaced by 3 cm. The repeller plate typically 1100 V. The reflector consists is at the potential Urepr of a deceleration region (2 cm long) leading to a rapid deceleration in the potential U, and a reflecting field region (13.5 cm long) leading to a soft reflection of the ions in the potential Urep Two trajectories typical for ions with mass M and slightly different kinetic energies are given in Figure 1. Differences in flight times due to different kinetic energies (trajectories 1 and 2 in Figure 1) are corrected to second order if appropriate reflector voltages are chosen (full correcting mode). The mass resolution of the RETOF is then increased by more than a factor of 20 with respect to a conventional linear TOF. However, we emphasize that in order to detect metastable ions we normally avoid complete time focusing for ions with different kinetic energies (partial correcting mode) (see section IIIB).
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(21) Kiermeir, A,; Kiihlewind, H.; Neusser, H. J.; Schlag, E. W.; Lin. S.
H.J. Chem. Phys., in press. (22) Law, K. S.; Schauer, M.; Bernstein, E. R. J . Chem. Phys. 1984.81, 487 1. (23) Bornsen, K. 0.;Selzle, H. L.; Schlag, E. W. Z . Naturforsch. 1984, 39a, 1255. (24) Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Zh. Eksp. Theor. Fiz. 1973, 64, 82. (25) Neusser, H. J.; Boesl, U.; Weinkauf, R.; Schlag, E. W. Int. J . Mass Spectrom. Ion Processes 1984, 60, 141. (26) Boesl, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . Phys. Chem. 1982, 86, 4857.
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Figure 2. Multiphoton mass spectrum of benzene clusters (C6H6), ionized with UV laser light of X = 259.4 nm in the reflectron T O F shown in Figure 1. The high mass resolution achieved for complete correction is demonstrated on an extended mass scale by the isotopic structure in the benzene hexamer ion peak. For larger cluster ions (n > 11) an additional mass spectrum with increased sensitivity is given.
Ions arriving at the detector are monitored with two channel plates (40 mm diameter). The signal output is fed into a %bit, 200-MHz transient recorder system (LeCroy 3500 SA), externally triggered by a photodiode. In order to achieve good signal-to-noise ratios, time-of-flight spectra are sampled by integration of 500 laser shots. 111. Results and Discussion A . Excitation Conditions. Accurate values for the ionization
potentials have been measured only for the smallest benzene clusters. The ionization potential of the benzene dimer was found to be 8.86 eV27328and for the trimer 8.85 eV,27respectively. The ionization potential for the benzene mononer is 9.243 eV.29 A comparison of all three values shows that a substantial stabilization is achieved for the dimer, whereas no further stabilization is observed for the trimer. For that reason the ionization potential is presumed not to change drastically with increasing cluster size. For two-photon ionization with X = 259.4 nm the excess energy above the ionization potential is therefore roughly 700 meV for all cluster ions with n 1 2. Most likely this excess energy is partitioned between the kinetic energy of the electrons and the internal energy of the ions and we may assume that cluster ions are originally produced with an energy content between 0 and 700 meV. If another photon is absorbed by the ion clusters produced with this small energy content, the energy is more than 4.7 eV and a fast dissociation process may occur. This point is discussed in more detail below. B. Muss Spectra. We observed benzene cluster ions (C6H&+ with n up to 40. A typical mass spectrum obtained with complete correction in the RETOF is shown in Figure 2, for n = 1-27. The wavelength of the ionizing laser was X = 259.4 nm, and the light pulse e n e r g y was 500 pJ. Each mass peak of Figure 2 consists of several isotope masses, ((C6H6),+, [('3CCsH6)(C6H6),,]+, etc.). The isotopic peaks cannot be distinguished in the compressed mass scaling of Figure 2. However, the extended mass spectrum of the hexamer ion clearly reveals its isotopic constitution and demonstrates the high mass resolution achieved for complete correction of the RETOF. The experimentally determined relative peak heights are 47%, 34%, 16%, and 3% for the masses 468, 469, 470, (27) Bornsen, K. 0. Ph.D. Dissertation, Technische Universitat Miinchen, 1987. (28) Riihl, E.; Biding, P. G. F.; Brutschy, B.; Baumgartel, H. Chem. Phys. Lett. 1986, 126, 232. (29) Grubb, S. G.; Whetten, R. L.; Albrecht, A. C.; Grant, E. R. Chem. Phvs. Lett. 1984, 108,420. Chewter, L. A,; Sander, M.; Muller-Dethlefs, K.: Schlag, E. W J . Chem. Phys. 1987, 86, 4737
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3181
Multiphoton Mass Spectrometry of Clusters and 47 1 amu, respectively. They are in line with the theoretically expected isotopic distribution of 44.4%, 35.8%, 14.2%, and 3.7%, respectively, expected for a non-isotope-selective ionization present for the nonresonant excitation conditions of our experiment (see section 11). The strong monomer and dimer peaks are not to scale. For cluster ions with n > 11 the mass spectrum increased by a factor of 20 is shown. We observe an almost overall monotonic decrease of ion signals with increasing cluster size. However, there are indications of abnormally high signals at n = 14 and 27. Very recently Schriver et ale6found in the mass spectrum of benzene clusters that ion peaks for cluster sizes of n = 14, 20, 24, and 27 are somewhat larger than other cluster ion peaks. This effect was attributed to the stability of certain benzene cluster ions due to icosahedral packing about a central dimer ion. Though our photon energy is less (4.68 eV) compared to the 5.0 and 6.4 eV in ref 6, our measurements seem to confirm their observations. C. Metastable Decay. If a metastable cluster decay M+ m+ n occurs in the drift region, the resulting daughter ion m+ has the kinetic energy
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where eUo is the energy of "stable" ions, Le., ions produced by ionization or rapid fragmentation in the laser focus. In the partial correcting modem of the RETOF instrument, signals of metastable daughter ions are not moved to their corresponding correct mass m by the energy correcting procedure but appear between the stable ion peaks of mass M and m. In order to learn about the daughter and precursor ion mass of a metastable drift signal, we proceed as follows: (i) Identification of m f M . The reflector potential ud 4- Uref is reduced stepwise. For u,+ Urer5 U, all stable ions disappear from the spectrum since they are no longer reflected and hit the end plate of the reflector. In that way eUo, the energy of stable ions, is determined. Then the reflector potential is further reduced. For a certain value U,+ the metastable signal under consideration disappears because the metastable daughter ions are no longer reflected. eU,+ corresponds to the kinetic energy E,+ of the metastable daughter ion. From E,+ and U, we determine the ratio m f M according to eq 1. ( i i ) Identification of M . The deceleration potential u d is ind > Uoall ions are reflected and no energy correction creased. At u takes place ("hard reflection"); Le., the RETOF operates like a linear TOF. During that procedure the observed metastable drift signal moves from its partially corrected position to the signal of d > Uo.Together its parent mass M+ and finally overlaps at u with the result of (i) m+ and M+ can be identified. Figure 3a shows a RETOF mass spectrum of benzene cluster ions under partial correction obtained for nonselective two-photon ionization at X = 259.4 nm. Strong metastable peaks are observed between the signals of stable ions (C,H,),,+ for n = 1-8. Figure 3b shows the corresponding pure metastable spectrum. Here the reflection potential was smaller by 4 V than Uo(1056 V). Under these conditions all stable ions produced in the laser focus are no longer reflected and have disappeared from the mass spectrum. As described p r e v i o ~ s l ythe , ~ ~remaining ~~~ signals which consist of one strong sharp peak and a smaller asymmetric peak result from decays in the drift and the acceleration region, respectively. The height of the metastable drift peaks with respect to that of their stable parent ions increases with increasing cluster size. For example, for n = 8 the signal of ions decaying in the drift region is greater than the number of stable n = 8 ions reaching the detector. This cannot be due only to the longer residence times in the drift region for greater masses. More likely this reflects a decrease of the dissociation thresholds for higher oligomer ions, leading to a more efficient metastable dissociation for larger cluster ions. (30) Kiihlewind, H.; Neusser, H. J.; Schlag, E. W. Int. J. Muss Specrrom. Ion Phys. 1983, 51, 255. (31) Kiihlewind, H.; Kiermeier, A,; Neusser, H. J. J . Chem. Phys. 1986, 85, 4427.
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Figure 3. (a) Partially corrected multiphoton mass spectrum of benzene clusters (C6H6)" for nonresonant two-photon ionization with X = 259.4 nm. Stable ion peaks are indicated by the numbers n. The arrows point from the stable ions to the peaks due to a metastable decay of the corresponding parent ion in the drift region. (b) Corresponding pure metastable mass spectrum achieved by the procedure described in the text. No stable ion peaks are seen. The sharp strong peaks are due to a metastable decay in the drift region, whereas the small asymmetric peaks are due to a decay in the acceleration region of the reflectron TOF. The horizontal arrows indicate the origin of the metastable peak found by the procedure described in the text.
D. Fragmentation Pathway. The strongest metastable signals result from the elimination reaction (arrows in Figure 3): This "boiling off" of one neutral benzene in order to decrease the energy of the remaining n - 1 ion was also reported in ref 6 at even higher excitation energies. The same process is also known to be the dominant reaction in neutral benzene cluster dissociation." Cluster decays of the type n+ n - 1+ were also found for N20 clusters32when the background pressure was sufficiently small to avoid collision-induced dissociation processes. Figure 4 shows a magnified portion of the pure metastable time-of-flight spectrum in the region from n = 1 to 5. Stable ions are suppressed. Only the asymmetric peaks due to decay events in the acceleration region at position n = 1-5 remain at the original position of the stable ions. The experimental parameters are similar to those pertaining to Figure 3. The dominant features are the signals resulting from the loss of one neutral benzene in the drift region (eq 2, see above). A closer inspection reveals some smaller peaks which are ascribed to a dissociation process that leads to smaller daughter cluster ions. The energy analysis of small signals using the procedure described in section IIIC is difficult due to the large energy change of these fragments. For this reason we calculated the time of flight of metastable ions for all possible dissociation processes including the ejection of neutral benzene monomers, dimers, and trimers. A computer program33 was developed in order to simulate the metastable time-of-flight spectra. Input parameters are the decays
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(32) Kamke, W.; Kamke, B.; K i d , H. U.; Hertel, I. V. J . Chem. Phys.
1986,84, 1325.
(33) Golling, R. Diplomarbeit, Technische Universitat Miinchen, 1987.
3788 The Journal of Physical Chemistry, Vol, 92, No. 13, 1988
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Figure 5. Dissociation pathways for benzene cluster ions (C6H6),,+found from the analysis of the metastable spectra of Figure 4.
under consideration and the applied voltages. The decay channels so determined are indicated in Figure 4 above the arrows. A comparison with the experimental mass spectra reveals, for instance, that even under the low-pressure conditions of our experiment (p < 8 X lo-' mbar) the trimer ion can also decay to the monomer ion (indicated as 3' 1' in Figure 4), but with a much smaller probability (branching ratio) than to the dimer ion. Four small mass peaks at 66, 75, 86, and 102 ~s remain unassigned. They result from fragment ions smaller than benzene. These are probably due to decays of the cluster ions from energy levels higher than the two-photon energy and yield a negligible contribution to the branching ratios shown in Figure 5 . In Figure 5 the resulting branching ratios of the observed metastable decay channels are shown. The height of the bars represents the normalized signal area resulting from decays of parent cluster ions (c6&),+ in the drift region. For our excitation conditions we found that for benzene ion clusters from n = 2 to 8 the loss of a neutral benzene monomer is the most probable dissociation process. The loss of larger neutral fragments is less probable and less favorable for larger ionized oligomers. This is in line with the higher threshold energy expected for the latter decay process. E. Binding Energies. An important question is how the amount of metastable fragmentation varies with decreasing photon energy and thus for decreasing excess energy above the ionization potential. From these "breakdown graphs" information about the
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Figure 6. Breakdown of metastable drift peak intensity for decreasing two-photon energy. The dissociation process of the benzene dimer cation is marked in the figure. The points represent experimental results for the intensity when normalized to the stable parent ion intensity. The solid line is the result of a linear least-squares fit to the experimental points. The threshold energy (2hu), is found by the intercept of this solid line with the base line.
threshold energies for the individual decays is expected. Before we consider this question in detail we must establish that the metastable decay occurs at the two-photon level rather than at the three-photon level. Further energy take-up of the ions in a three-photon absorption process includes ladder switching34and the absorption of a third photon by a cluster ion. This would lead to an internal energy of the cluster of more than 4.7 eV. For the rather small dissociation energies of
