J . Phys. Chem. 1988, 92, 5561-5562
5561
Infrared Photodissociation Spectra of Size-Selected (CH,OH), Clusters from n = 2 to n=a Udo Buck,* Xijia Gu, Christian Lauenstein, and Andreas Rudolph Max-Planck-Institut f u r Stromungsforschung, Bunsenstrasse 10, 0-3400 Gottingen, Federal Republic of Germany (Received: July 25, 1988) The infrared photodissociation spectra of (CH30H), clusters from n = 2 to n = 8 were measured near the absorption band of the C-0 stretching frequency of the monomer. The different clusters are generated in a continuous molecular beam, selected according to their size by a scattering process with He and dissociated by a CW C02laser collinear to the scattered cluster beam. The spectral features vary from the dimer to the octamer. A special transition is observed going from the one-peak spectrum of the pentamer to the two-peak structure of the hexamer which is attributed to the existence of additional isomers.
Introduction The photodissociation of weakly bound complexes following the excitation with one infrared photon has attracted much interest in recent years. From the measured spectra useful information on the lifetime and the structure of small clusters can be obtained.' The method has also been applied to identify moleculesz or reaction products3 in a large cluster environment. In all these experiments it is difficult to get cluster specific information. Cluster beams are usually generated as a distribution of different cluster sizes, and a bolometer or mass spectrometer, which is used as a detector, does not provide this information, in the latter case because of the extensive fragmentation during the ionization p r o ~ e s s . ~A method to solve this problem has recently been developed in this laboratory.s The cluster beam is size selected by momentum transfer in a scattering experiment with He atoms. This method has been applied successfully to the measurement of the vibrational predissociation of small ethylene clusters with 1ine-tunable6*'and high-resolution waveguide CO, lasers.* The spectra from dimer to hexamer are, aside from the effects due to internal excitation, quite similar. For hydrogen bound systems, however, different spectra are expected for different cluster sizes. Methanol serves as a good candidate for such a photodissociation experiment. From an experimental point of view, methanol clusters can be scattered into a reasonably large angle region for size selection since it is a ''light'' molecule. Secondary, the CO stretch band of methanol at Y = 1034 cm-' matches the COz laser spectrum. From a theoretical point of view, methanol clusters have attracted much interest for a long time. The intermolecular potential for its dimer has been calculated by different groups. The binding energies for both cyclic and open-chain structures of trimer and higher clusters were also calculated.9-" In this Letter we report photodissociation experiments with size-selected internally excited (CH30H), clusters. The spectral features vary from dimer to octamer due to the different cluster configurations. A special transition is observed between pentamer and hexamer which we attribute to the existence of isomers for the larger clusters. A similar study has been carried out for cold methanol clusters for n = 2,3, and 4.Iz Earlier studies without size selection exhibited an unstructured spectrum.13 (1) Miller, R. M. J . Phys. Chem. 1986, 90, 3301.
(2)Gough, T.E.;Mengel, M.; Rowntree, P. A,; Scoies, G. J . Chem. Phys. 1985, 83, 4958.
(3) Levandier, D.J.; McCombie, J.; Pursel, R.; Scoles, G. J . Chem. Phys. 1987,86, 7239.
(4)Buck, U . J . Phys. Chem. 1988, 92, 1023. ( 5 ) Buck, U.; Meyer, M. Phys. Rev. Lett. 1984, 52, 109. (6)Buck, U.;Huisken, F.; Lauenstein, Ch.; Meyer, H.; Sroka, R. J . Chem. Phys. 1987, 87, 6276. (7)Huisken, F.;Pertsch, T. J . Chem. Phys. 1987, 86, 106. (8) Buck, U.;Lauenstein, Ch.; Rudolph, A,; Heijmen, B.; Stoke, S.; Reuss, J. Chem. Phys. Lett. 1988, 144, 396. (9)Del Bene, J. E. J . Chem. Phys. 1971, 55, 4633. (10)Curtiss, L.A. J . Chem. Phys. 1977, 67, 1144. (11) Jorgensen, W.J. J . Chem. Phys. 1979, 71, 5034. (12)Huisken, F.;Stemmler, M. Chem. Phys. Lett. 1988, 144, 391.
0022-3654/88/2092-556lSO1.50/0
TABLE I: Peak Frequencies u0 in the Dissociation Spectra of Methanol Clusters from R = 2 to n = 8" n
2 3 4
un.
cm-'
1027.5,1047.0 1041.5 1046.5
n
5 6 7, 8
yo,
cm-'
1047.3 1041.0,1052.0 1040.0, 1052.0
"The accuracy is about fl cm-'.
Experimental Section The experimental apparatus consists of two crossed molecular beams which are rotatable with respect to their scattering center and a fixed quadrupole mass spectrometer. A detailed description of the machine is given elsewhere.6 A (CH30H), cluster beam is generated by an expansion of a mixture of 2.8% CH30Hin Ne through a 80-pm-diameter nozzle at room temperature and a stagnation pressure of 1.0 bar. These beam conditions were chosen in favor of dimer formation since the photodissociation signals from the dimer were relatively weaker than that from higher clusters. The cluster beam is crossed by a H e beam expanded through a 30-pm-diameter nozzle at a pressure of 30 bar. To separate a cluster of size n, the momentum transfer in the scattering process and the mass selectivity of the quadrupole mass spectrometer are combined. By setting the detector at a certain scattering angle, the larger clusters are excluded by kinematical constraints. To discriminate against the smaller clusters, the mass spectrometer is tuned to the (CH30H),lH+ ion mass. This mass is favored in the fragmentation process of (CH30H), during the electron impact ionization, and smaller clusters cannot be detected at this mass. Keeping these settings, a dissociation spectrum for a single cluster size is observed by measuring the decrease in cluster signal at the quadrupole mass spectrometer after irradiation of the scattered beam by a line-tuned C 0 2 laser. It is noted that a certain amount of translational energy is transferred into the cluster by the scattering process. Therefore, the dissociation spectra are carried out for internally excited clusters. The CO, laser used is a homemade CW line-tunable flowing gas laser. The resonant cavity consists of a ZnSe window with a 90% reflecting coating as a output coupler and a curved grating of 150 lines/mm. The laser beam is aligned, after refocusing, to be in a collinear position with the size-selected cluster beam to ensure a long interaction time between them. The photodissociation data are acquired as follows. The He beam is chopped by a chopper at 29 Hz. The mass spectrometer detects both scattered cluster signals and background periodically. The CO, laser beam is also chopped at, however, doubled frequency. Thus, in one cycle of measurement, the mass spectrometer detects the signals from four phases, namely, laser on, cluster beam on and off; laser off, cluster beam on and off. These signals were fed into four channels which are triggered by signals from pho(13)Hoffbauer, M. A,; Giese, C. F.; Gentry, W. R. J . Phys. Chem. 1984, 88. 181.
0 1988 American Chemical Societv
5562 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
Letters
todiodes installed on the two choppers. From these data the percentage of dissociated clusters, AZ, is determined and converted into a relative dissociation cross section u via u a [In (1 - Al)] for a fixed laser fluence.
Results and Discussion The photodissociation spectra are measured from the dimer to the octamer in a frequency range from 1000 to 1100 cm-I. The characteristic features are given in Table I. Similar results for the smaller clusters up to the tetramer have been measured for cold clusters by using a pulsed C 0 2 laser.12 Both experiments reveal a two-peak structure for the dimer which is attributed to two nonequivalent CH,OH molecules in the dimer. As already observed for ethylene dimers, :he peak positions for both hot and cold dimers are nearly identical, while the line width is much broader in the former case.6 The clusters from n = 3 to n = 5 show single peaks which are continuously shifted to the blue compared with the monomer absorption frequency. This is a clear indication of equivalent CH30H molecules which form a planar ring structure in complete agreement with theoretical predict i o n ~ ' ~and * ' ~ experimental observations by deflection in inhomogeneous electric fields.I5J6 A very interesting feature arises for the spectrum of the hexamer. A two-peak structure is observed again with one peak slightly shifted to the blue compared to that of pentamer and another peak shifted to the red by about 10 cm-I. Figure 1 shows the transition from the pentamer to hexamer. The upper spectrum measured at a scattering angle of 4.5' and a mass of 161 amu was produced by hexamers. The lower spectrum measured at 5.5O and a mass of 97 amu was mainly caused by pentamers with a small contribution from hexamers, as indicated by the lower dashed lines. This double-peak structure then continues up to the octamer. The spectrum of the hexamer obviously infers a structural change. Martin et al. calculated the structures of methanol clusters by examination of multidimensional total energy surfaces.14 They found that, in addition to the most stable forms, Le., the planar rings, isomers can also be found. The hexamer exhibits, at least, two very different isomers, and a strong deformation from a planar ring occurs at the octomers. It is therefore reasonable to assign the two peaks that first appear for the hexamer to two different isomers; one of them is close to a planar ring and the other to a distorted ring structure. It is interesting to notice that this shift to the red is in the direction toward the absorption of liquid methanol at u = 1029 cm-I. However, this value is not yet reached. Experiments with a bolometer detector and a direct beam of similar mixtures display a similar two-peak structure as that in (14) Martin, T. P.; Bergmann, T.; Wassermann, B. In Finite Systems; Pullmann, B., Jortner, J., Eds.; Reidel: Dordrecht, 1987. (15) Kay, B. D.; Castleman, Jr., A. W. J . Phys. Chem. 1985,89, 4867. (16) Odutola, J. A,; Viswanathan, R.; Dyke, T. R. J . Am. Chem. SOC. 1919, 101, 4781.
0.0
1000
1050 Frequency / cm"
1100
Figure 1. Photodissociation spectra of methanol clusters. The upper figure is for hexamers, and the lower figure is for pentamers with a small fraction of hexamers. The dashed lines are Lorentzian fits to the data.
the upper part of Figure 1, with nearly the same width. Since these experiments without size selection are carried out with a cold cluster beam, the results indicate that the internal excitation caused by the size selection has no influence on those cluster sizes. However, a further increase of the stagnation pressure leads to positive absorbing signals at the bolometer. Since increasing pressure corresponds to increasing cluster size, this result indicates that clusters do not dissociate anymore from a certain cluster size onward. The results of this work clearly demonstrate that size selection of clusters by the scattering from an atomic beam is a powerful tool in obtaining photodissociation spectra for hydrogen-bonded systems. For methanol the spectra differ from the dimer to the octomer and exhibit a very interesting transition between the pentamer and the hexamer probably due to the appearance of two distinct isomer structures. A direct comparison of the measured spectra with simulations will hopefully reveal many details about the structures of these clusters.
