J . Phys. Chem. 1984, 88, 5172-5176
5172
translational distribution will become invariant when velocitychanging collisions cease, and similarly for rotational and vibrational distributions. The relative degrees of relaxation, in the broadest sense, reflect the ratios of inelastic collision cross sections for the appropriate degrees of freedom. For rotations, the unequal spacing in energy from level to level provides a natural mechanism for non-Boltzmann relaxation, in the sense that collisions affecting any given level will cease at a point in the expansion which is different for each level. This mechanism is, respectively, more or less strongly coupled to the translational relaxation as the local translational energy (Le. the translational energy in a reference frame moving at the flow velocity, or 3kT,,/2) is larger or smaller than rotational level spacings from a given level to neighboring levels. Such coupling can account not only for a non-Boltzmann distribution but also for the observation of Douketis et al.I5 that the terminal rotational temperature of the lowest levels of a molecule is a monotonic (and apparently linear) function of rotational constant at any fixed PODexpansion condition. More to the point of the measurements reported here, however, is the possibility that molecules with velocities closest to so (Le. those which can be said to have undergone the greatest degree of translational relaxation) could be able to display nonequilibrium internal relaxation. Should the cross section for rotational relaxation exceed that for translational relaxation, the rotational distribution in a particular velocity group would continue to relax after the translational temperature has become fixed. This continued relaxation would be most dramatic for a molecule with a small rotational constant (so that inelastic collisions would not
significantly alter the translational distribution), and the effect would be most readily observed if experimental constraints limited the range of interrogated velocities. An equivalent conclusion is reached if one takes the point of view that so is the “goal” velocity of all the molecules in the beam. Those with velocities nearest so in any arrested expansion have achieved that state as the result of experiencing a greater number of relaxing collisions than their neighbors. It does not seem unreasonable to expect these molecules to be the more highly relaxed internally as well. There is no a priori reason why the distribution of velocities and internal energies in a supersonic expansion should follow Boltzmann-like equilibrium forms. An interesting, but difficult, calculation of the terminal distribution function would seem to be possible starting from the Boltzmann transport equation and various assumptions about state-to-state collision cross sections. Such a calculation should show if reasonable assumptions for these cross sections are capable of yielding not only nonequilibrium distributions but also distributions which show coupling among various degrees of freedom.
Acknowledgment. We thank Professor G. Scoles for communicating results on his CH,F/He expansions. This research was supported by the National Science Foundation, by the donors of the Petroleum Research Fund, administered by the America1 Chemical Society, and by the Division of Chemical Sciences, Office of Basic Energy Sciences, US.Department of Energy, under Contract No. DE-AC03-765F00098. Registry No. Ar, 7440-37-1; OCS,463-58-1.
Metal-Support Interactions on Rh and Pt/T102 Model Catalysts D. N. Belton, Y.-M. Sun, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: March 14, 1984)
Model catalysts comprised of Pt and Rh films on oxidized Ti were studied with static secondary ion mass spectrometry to observe their temperature-dependent structural characteristics. Encapsulation of the metal overlayers by the support material is observed and correlated with thermal desorption spectra. The results suggest that encapsulation and electronic interactions occur simultaneously to alter the behavior of these model catalysts.
Introduction Since the report
Of strong metal-support interactions (SMS1) in 1978 by Tauster et a1.’a2 the source of this interesting effect has been an area of active debate in the surface science-catalysis Oxide community. Many studies On a variety Of systems have been carried Out in an attempt to discover the underlying causes for the suppressed H2 and CO chemisorption which is characteristic of these SMSI system^.^-'^ The inherent com-
(1) S.J. Tauster, S. C. Fung, and R. L. Garten, J . Am. Chem. Soc., 100, 170 (1978). (2) S.J. Tauster and S. C. Fung, J . Catal., 55, 29 (1978). (3) “Metal-Support and Metal Additive Effects in Catalysis”, B. Imelik et al., Ed., Elvesier, Amsterdam, 1982, and references cited therein. (4) T. Huizinga, Dissertation, Eindhoven University of Technology, 1983. (5) X.-Z. Jiang, T.F. Hayden, and J. A. Dumesic, J. Catal., 83,68 (1983). (6) D. E. Reasco and G. L. Haller, J . Catal., 82, 279 (1983). (7) M. A. Vannice and C. C. Twu, J. Catal., 82, 213 (1983). (8) D. R. Short, A. N. Mansour, J. W. Cook, Jr., D. E. Sayers, and J. R. Katzen, J . Catal., 82, 299 (1983). (9) S.-M. Fang and J. M. White, J . Catal., 83, 1 (1983).
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plications of powder catalysts make application of most modern surface analisis techniquks very diffi<. Hence, information gained from studies of this type have been unable to clearly demonstrate the of SMSI, In order to separate and characterize the possible effects, thin film models of the actual catalysts are helpful,13-18 Someof the first studies concentrated on photoelectron spectroscopy, both X-ray (XPS) and ultraviolet (UPS), in an attempt to observe changes in the electronic structure of metals deposited on oxide (10) K. Tanaka and J. M. White, J. Catal., 79, 81 (1983). (11) R. T. K. Baker, J . Catal., 63, 523 (1980). (12) S.C. Fung, J . Catal., 76, 225 (1982). (13) M. K. Bahl, S. C. Tsai, and Y. W. Chung. Phys. Rev.B, 21, 1344
,---,.
( 1 9xm
(14) J. E. E. Baglin, G. J. Clark, J. F. Ziegler, and J. A. Cairns, J . Mol. Catal., 20, 299 (1983). (15) B. R. Powell and S.E. Whittington, J . Catal., 81, 382 (1983). (16) Y.-W. Chung, G. Siong, and C.-C. Kao, J . Catal., 85,237 (1984). (17) J. A. Schriefels, D. N. Belton, J. M. White, and R. L. Hance, Chem. Phys. Lett., 90, 261 (1982). (18) D. N. Belton, Y.-M. Sun, and J. M. White, J P h y s . Chem., 88, 1690 (1984).
0 1984 American Chemical Society
Metal-Support Interactions of Rh and Pt/TiO,
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5173
18
I'
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Figure 1. AES spectra of oxidized Ti(0001) (top) and oxidized Ti with 30-A Rh (bottom).
substrates. Conflicting results were obtained, with some studies concluding charge was transfered from the oxide to the meta112-13 and others concluding there was n ~ n e . These ~ ' ~ ~contradictory results stem largely from the difficulties of accounting for changes in measured binding energies due to changes in metal cluster size and final state relaxation effects. A study of the chemisorption of Hz and C O on a Pt/TiOz thin film model catalyst17J8 showed that the model indeed possessed the characteristics of powdered Pt/TiO, catalysts. It was concluded that reduced Ti0, interacted electronically with thin overlayers of Pt to suppress chemisorption. In this work we present evidence for the migration of TiO, to the surface of metal overlayers upon heating thin films of Rh and Pt on TiO, to 625 K. This migration/encapsulation is shown by both Auger electron spectroscopy (AES) and temperature-programmed static secondary ion mass spectroscopy (TPSSIMS). Decreased chemisorption capacity of the metal, as studied by thermal desorption spectroscopy (TDS) of both H, and CO, is correlated with the encapsulation. Experimental Section
The experiments were conducted in an ultrahigh vacuum chamber equipped with a PHI AES system, Extranuclear quadrupole mass spectrometer, and 3M minibeam ion gun. Both Rh and Pt were deposited from metal evaporation sources. The Ti(0001) single crystal was mounted on a liquid nitrogen cooled mainpulator. Pressures were routinely 3 X torr during the experiments. The experiments discussed in this paper involve three basic steps: (1) oxidation of Ti, (2) deposition of the metal, and (3) adsorption/desorption of CO or Hz. First, the Ti sample was oxidized at 775 K in 5 X lo-' torr of 0, to prepare the support. The oxidation time was long enough (Typically 30 min) to produce Ti AES line shapes (see arrow in Figure 1) consistent with fully oxidized Ti02.19 Once the support was prepared and charaterized, the metal to be studied was vapor deposited onto the oxide support held at 130 K. Samples were cooled to minimize radiative heating effects by the evaporation source. The thickness of the metal overlayer and its cleanliness were then characterized by AES. Both Rh and Pt were consistently deposited free of C or other impurities once the doser was properly outgassed. After metal deposition the SSIMS, AES, and TDS experiments were performed.
Results S I M S and AES. Figure 1 shows AES data for the oxidized Ti(0001) before and after deposition of 30 8, of Rh at 130 K. The Rh thickness was calculated from the attenuation of the 0 AES signal, assuming layered growth of the metal.20 Due to nonuniformity of the metal layers and the very small substrate signals,
0
c
I
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800
400
800 Tirndsec)
1000
Figure 2. TPSSIMS for 30-A Rh on oxidized Ti(0001). I
I
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80 mle
=
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Figure 3. SSIMS broad scans for 30-A Rh on (a) oxidized Ti(0001)and (b) the same after anneal to 760 K.
some uncertainty in these thicknesses (of the order of 20%) is inevitable. Other studies on thicker Rh layers shown that oxygen is not being deposited with Rh, indicating that the oxygen signals is coming from the substrate. The 30-A thickness was chosen to model the particle sizes of actual catalysts. Furthermore, the Rh thickness can be detd. with AES of the substrate 0 but the layer is thick enough to fully attenuate the Ti from the substrate. Figure 2 shows the TPSSIMS results from the Rh/Ti02 model catalyst. The SSIMS data was taken with a 500-eV, 1-nA Ar+ beam rastered over the entire 1-cm2sample. This sputtering rate is low enough that less than one monolayer is removed during the course of the experiment. The sample temperature was ramped at 1 K/s from 130 to 760 K and held at 760 K for approximately 8 min. The Ti+ intensity rises sharply beginning at 625 K; concurrently there is a decline in the Rh" intensity. The drop in Rh" intensity at 450 K, before the increase in Ti', corresponds roughly to the desorption temperature of CO. This follows as the SIMS ion yields for clean metal surfaces are often quite low as compared to surfaces with adsorbed gases.21s22 A similar effect accounts for the increase in Rh" intensity at high temperatures. As the surface concentration of 0 increase, the SSIMS cross section for Rh' increases. Qualitatively, the migration of Ti and 0 shown in Figure 2 is reproducible. However, different metal film thicknesses lead to changes in the detailed shapes of the SSIMS curves. Moreover, comparing reduced (prior to metal deposition) and oxidized titania substrates shows that TiO, migration sets in about 150 K lower on the reduced form. Examination of ions other than Ti+ and Rh" is also helpful. SSIMS broadscans of the sample before and after annealing are shown in Figure 3. The clean sample (not annealed) shows large Rh+, Rh(H20)", Rho+, and Rh(H,O),+ signals. (Without small amounts of adsorbed gases such as water, the Rh+ signal is extremely small). Before annealing there are only trace amounts of Ti+ and TiO+ as well as some low mass impurities. As expected
(19) G. D. Davis, M. Natan, and K. A. Anderson, Appl. Surf. Sci., 15, 321 (1983).
( 2 0 ) M. P. Seah and W. A. Tench, Surf. Interface Anal., 1, 2 (1979).
(21) K. Wittmaack, Surf.Sci., 112, 168 (1981). (22) J. R. Creighton, Dissertation, University of Texas at Austin, 1983.
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Belton et al.
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 I
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Figure 4. AES of 30-A Rh on oxidized Ti(0001) after anneal to 760 K.
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c
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Timebed
Figure 6. TPSSIMS of 30-A Pt on oxidized Ti(0001).
I
7
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1000
Tim4sec) Figure 5. Ti+ SIMS depth profile for 30-A Rh on oxidized Ti(0001) annealed to 760 K.
from the TPSSIMS measurements, the annealed sample shows a large increase in the Ti/Rh ratio. Several high-mass Ti, ( x > 1) and Rh-Ti species also appeared. The presence of Rh-Ti species may suggest a direct (Le., chemical bond) interaction between Rh and Ti. However, further SSIMS measurements of the secondary ion energy distribution are necessary in order to rule out its formation above the surface during the SSIMS process. After the temperature ramp, AES data were taken to confirm the changes measured by SSIMS. Figure 4 shows the spectrum of the annealed sample. As expected a large increase in the Ti and 0 signals is observed. The Ti line shapes, particularly around 425 eV, and O/Ti ratios indicate a reduced Ti oxide which we will refer to as Ti0,.20 The oxygen cannot be removed by exposure to H, at T = 500 K, supporting the notion that it is bonded to Ti (or a Rh/Ti species) as opposed to chemisorbed on the Rh. A SIMS depth profile of the annealed Rh sample (Figure 5) confirms the formation of TiO, on top of the Rh layer. As is apparent from the Ti+ profile shown, the surface is rich in Ti. Beneath this outer layer is a region with trace amounts of Ti corresponding to the original Rh layer. Deeper into the sample, Ti from the substrate is detected. This information elminates islanding of the Rh overlayer as a possible explanation for the increases in the Ti/Rh ratio observed during the temperature ramp experiment. Regardless of sensitivity changes and/or preferential sputtering of 0 and/or Ti, the depth profile of Figure 5 can only be explained as Rh covered with an overlayer of TiO,. To further demonstrate that sensitivity effects were not important in the depth profile experiment, we performed separate AES measurements on another annealed Rh/Ti02 sample. In this experiment the sample was depth profiled continuously until the minimum Ti+ intensity was reached. At that point AES data were taken and confirmed that only trace amounts of Ti and 0 were present. In fact, if a freshly prepared sample was flashed to 760 K and then quickly cooled to 130 K, subsequent sputtering completely removed Ti and 0 leaving only Rh. Similar experiments were performed on Pt/Ti02 model catalysts. In all aspects the results paralleled those obtained for the
Timetsac)
Figure 7. Ti+ SIMS depth profile of 30-A Pt on oxidized Ti(0001) annealed to 760 K.
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800
time
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Figure 8. TPSSIMS of the oxidized Ti(0001) substrate without a metal overlayer.
Rh system. TPSSIMS annealing and SIMS depth profiles for the Pt model catalyst are given in Figures 6 and 7, respectively. The experimental conditions were identical with those of the Rh experiments except for the length of time the sample was held at 760 K during the anneal. The Pt data in both figures follows the same qualitative trends as the Rh/Ti02 system. Finally, changes occurring in the substrate during the anneal were characterized. Figure 8 presents the TPSSIMS of the TiOz substrate without a metal overlayer. During the temperature ramp both Ti+ and TiO+ signals were monitored simultaneously. The ratio Ti+/TiO+ was a sensitive indicator of the extent of surface oxidation. The purpose of the experiment was to confirm that the oxide layer was not being totally depleted of oxygen during the course of the experiment. This ratio remains constant up to about 760 K, well beyond the temperature (625 K) at which the onset of migration into the metal overlayer is observed. If the sample is heated to 825 K, then a decrease in the TiO+ and an increase in the Ti+ signals are observed. This indicates that diffusion of 0 into the Ti(0001) is proceeding, and the oxide layer is being destroyed. Holding the temperature constant at 825 K causes both signals to decrease simultaneously due to loss of 0
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5175
Metal-Support Interactions of Rh and Pt/Ti02 TABLE I: Thermal Desorption Spectroscopy
oxidized/ reduced
thickness,
0
30
Rh/Ti02
0
30
Pt/Ti02
0
30
Pt/Ti02
0
30
Pt/Ti02
0
2.4
Pt/Ti02
R
2.4
no. 1A
system Rh/Ti02
1B 1c 2A 2B 2 3A 3B 4A 4B 5A 5B 6A 6B
A
anneal temp, K
adsorbate
525 760 760" 525 760 760"
co
525 760 525 760
co
130 370 130 370
H2
H2
H2
H2
desorption peak, K
total peak area
490 490 490 270 270 270
1 0.32 0.98 1 0.20 0.85
400 355 270 240
1 0.33 1 0.36
264 234, 298 263 230
1 0.75 1 0.30
"After heating to 760 K, the sample was sputtered and annealed to 525 K. from the surface which lowers all the ion yields. The data indicate that in the temperature region of the encapsulation experiments the oxide layer remains intact. The possibility exists that the oxide layer behaves differently when a noble metal layer is present. Enhanced oxygen diffusion into the underlying Ti due to formation of stable titanium-noble metal compounds cannot be totally excluded. However, we fell that the results obtained for the substrate with no metal overlayer are consistent with the depth profiling results obtained for samples with metal overlayers suggesting that the integrity of the model catalyst is maintained throughout the experiment. Thermal Desorption Spectroscopy. TDS was performed on a series of model catalysts to correlate the encapsulation process with chemisorption. The results are summarized in Table I. Each numbered set represents a separate experiment. The experiments were conducted according to the following sequence: (1) A metal was deposited and characterized with AES as described above. (2) The clean surface (not encapsulated) was annealed at a temperature given in Table I. (3) TDS of C O or H2 was performed following a saturation dose of the respective gas. The peak temperatures and normalized peak areas are given in Table I. (4) The sample was annealed to encapsulate the metal overlayer. (5) TDS, following another dosing, was performed. The surface was then sputtered (S) with 1.4-kV, 150-nA Ar+ while monitoring the Ti' SIMS; and when the minimum Ti' signal was reached, the surface was annealed to 525 K and TDS performed. For Rh/Ti02 ( 1 in Table I), encapsulation decreases the CO peak area by 68%, but there is no shift of the peak temperature. Nearly all the chemisorption capacity was regained by the removal of surface TiO, (1C). Examination of desorption peak widths indicates that increased surface roughness does not account for the observed increases in chemisorption after sputtering. The results of the H2chemisorption experiment are similar (2A-C). TDS experiments on Pt/Ti02, while similar, show one important difference. There is a downward shift of 45 K for the C O and 30 K for the H2 desorption peaks. (Compare 3A to 3B and 4A to 4B in Table I). This shift is taken to indicate an electronic interaction between the Pt and the encapsulating TiO,. This result is in agreement with peak temperature shifts observed in our previous work for Pt on oxidized Ti.'* To further explore this proposed electronic interaction, 1 monolayer of Pt was deposited on both fully oxidized and partially reduced TiO, (5 and 6). The reduced sample was prepared by Ar+ sputtering of the Ti0, substrate so that the reduced Ti centers were at the Pt-Ti interface. H, TDS was used as the highest H2 desorption temperatures (370 K) occur below the temperature regime of encapsulation. For the reduced sample there was a 70% decrease in H, chemisorption and a 33 K temperature shift when the unannealed sample (6A) was compared to the sample annealed at 370 K (the 370 K anneal is the maximum temperature reached during the first desorption experiment). No change in the AES was observed after either the first or second TDS, showing that
the Pt overlayer does not island or encapsulate. The reduced sample does not island because its interaction with the reduced Ti centers is strong enough to restrict Pt mobility at these relatively low temperatures. We take these low Pt coverage experiments to indicate an electronic interaction (bond formation) between Pt and reduced Ti species that is activated at about 370 K. For the fully oxidized sample (5A, 5B) the results were somewhat different. After annealing at 370 K (5B) there was 25% less Hz adsorption, and the peak split into two peaks, one shifted higher and one lower, each by about 30 K. There were also small changes in the AES Pt/Ti ratio after the first TDS. Since TiO, migrates at lower temperatures for reduced, as compared to fully oxidized, titania, we do not favor TiO, migration as the explanation. Rather we suggest that the changes observed after the 370 K anneal of the oxidized sample are due to small changes in the morphology of the Pt overlayer, possibly due to islanding. These alter the number and kind of exposed Pt sites. Correction of the H2 desorption peak area for surface area changes are very difficult to calculate in this case. We can say, qualitatively, that the roughly 10% decrease in the Pt/Ti ratio observed with AES after the sample is annealed to 370 K corresponds to a decrease in number of exposed Pt atoms. This suggests that the decrease in H2 chemisorption observed after the sample is annealed could possibly be accounted for by loss of Pt surface area. If this is the case then the interaction between this fully oxidized substrate and the metal overlayer is very weak and the metal is essentially unaltered by the support as far as chemisorption of H2 is concerned. Discussion The results presented above give conclusive evidence that encapsulation of metal by the support is occurring in this model catalyst system. We propose that a TiO, specie(s) migrates to the surface via imperfections in the Rh(Pt) overlayer; however, the data do not exclude the possibility of diffusion through the metal with surface segregation. Appreciable islanding of the Rh does not occur during the anneal as confirmed by the near zero value of the Ti signal via both AES and SSIMS, during the depth profile. This is further supported by TDS data which shows that the H2 and C O uptake of the metal can be restored by removing the surface TiO, by sputtering. The chemisorption studies clearly indicate a correlation between the migration of TiO, to the surface and the decreased chemisorption ability for both C O and Hz. Decreased chemisorption is the original identifying characteristic for SMSI.'* Thus our results show that encapsulation is at least one effect leading to these metal-support interactions. The TDS results show that Pt encapsulation is accompanied by a 40 K downward shift in both C O and H2 desorption temperatures. This result, obtained on 30-A thicknesses of Pt, parallels those observed on low coverages of metal where encapsulation was not allowed to occur (compare 4, 5, and 6 of Table I). On the
5176
J. Phys. Chem. 1984, 88, 5176-5180 the generation of a mobiel reduced Ti species, TiO,. This species segregates to the surface of the metal overlayer (encapsulation), blocking adsorption sites and interacting electronically with the metal. Site blocking accounts for part of the reduction in chemisorption observed for the encapsulated catalysts, but there is a significant electronic interaction of the reduced support with the metal which also lowers the chemisorption ability of the metal. Both factors probably contribute to the changes in selectivity observed for certain reactions with SMSI catalysts. As for the morphological changes which have been proposed in the past,10J1J8 our results allow us to do no more than speculate that the encapsulating oxide may preferentially block one kind of site (step, kink, etc.) so that the effective morphology of the metal particles changes. To summarize, our results show that both encapsulation and electronic effects contribute to SMSI. The two generally occur simultaneously, and, alone, neither can explain all of the characteristics of an SMSI catalyst.
basis of this evidence we conclude that the SMSI character is not due simply to site blocking by the encapsulating oxide. Concurrent with encapsulation there is an electronic interaction between the reduced oxide and the metal. This interaction works to decrease desorption temperatures and lower the chemisorption capacity of the metal. Earlier we reported no difference in the photoelectron spectroscopy (XPS, UPS) of Pt deposited on reduced and oxidized titania.I8 One possible reason is suggested by our TDS work. For Pt deposited on the sputter-reduced titania, the reduction in chemisorption capacity is not observed until after the sample is heated to 370 K. This temperature is necessary to activate the interaction between the metal and the reduced support. After the activation, reduced chemisorption is observed for all subsequent desorption experiments. Since in our previous work all the XPS and UPS data were taken at or below room temperature with no annealing, the surface interaction leading to suppressed chemisorption had not occurred. This suggests further work using XPS and UPS. We propose the following description of these thin film systems. Annealing the catalyst under vacuum (or reducting it in H2) causes
Acknowledgment. This work was supported in part by the Office of Naval Research.
Emission and Laser Excitation Spectra of the Cooled Triacetylene Cation
-
i2n,
k2nU Transition of Rotationally
Dieter Klapstein, Robert Kuhn, John P. Maier,* Martin Ochsner, and Werner Zambach Physikalisch- Chemisches Institut der Universitat Basel, CH-4056 Basel, Switzerland (Received: March 16, 1984)
-
Emission and laser excitation spectra of the %’IT, transition of the rotationally and vibrationally cooled triacetylene cation in the gas phase have been obtained. The emission was excited by electron impact of a seeded supersonic free jet and the fluorescence by laser excitation of the cations formed by Penning ionization in a liquid nitrogen cooled environment. From the vibronic analyses of the spectra, the vibration@ frequencjes of the totally symmetric fundamentals could be inferred to within f 2 cm-’ for the triacetylene cation in the X2n, and AZn, states.
1. Introduction In recent years several new approaches have been successfully devised to obtain spectroscopic information on polyatomic cations at much higher resolution than had become available on open-shell ones by photoelectron spectroscopy.’ The highest resolution studies have involved mass-selected fast ion beams using visible2 and IR laser^,^ and the latest developments are the direct absorption measurements of IR laser radiation using modulated technique^.^ In the case of the larger open-shell organic cations, the gas-phase methods have relied either on their radiative decay from excited electronic states in conjunction with electron beam excitation5or laser-induced fluorescence,6or on their fragmentation by laser photodiss~ciation.~
The most recent improvement in the study of the vibrational details in the emission and laser excitation spectra have come about by preparing the cations rotationally cold.8 Thus one benefits from the spectral simplifications and narrowing of vibronic bands, as one has become accustomed to with supersonically expanded molecular specie^.^ To this end, the emission spectra are obtained with rotationally supercooled cations produced by electron impact ionization of seeded helium supersonic free jets,1° and laser excitation spectra of cations rotationally and vibrationally cooled to liquid nitrogen temperature by collisional relaxation following Penning ionization.’ The aim of these measurements is to determine accurately the vibrational frequencies of organic cations in their ground and lowest excited electronic states, since these are hitherto unavailable by other means.” Whereas in the earlier
(1) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. “Molecular Photoelectron Spectroscopy”; Wiley-Interscience: New York, 1970. (2) Carrington, A. Proc. R. SOC.London,Ser. A 1979, 367, 433, and references therein. (3) Carrington, A.; Softley, T. P. In “Molecular Ions: Spectroscopy, Structure and Chemistry”, Miller, T. A., Bondybey, V. E., Eds.; NorthHolland: New York, 1983; p 49, and references therein. (4) Oka, T. Phys. Reu. Lett. 1980, 45, 531. Gudeman, C. S.; Begemann, M. H.; Pfaff, J.; Saykally, R. J. Phys. Reu. Lett. 1983, 50, 727. Amano, T. J . Chem. Phys. 1983, 79, 3595. ( 5 ) Maier, J. P. Acc. Chem. Res. 1982, IS, 18, and references therein. (6) Miller, T. A.; Bondybey, V. E. J. Chim. Phys. Phys.-Chim. Biol. 1980, 77, 695.
(7) Dunbar, R. In “Molecular Ions: Spectroscopy, Structure and Chemistry” Miller, T. A., Bondybey, V.E., Eds.; North-Holland: New York, 1983; p 231. (8) Miller, T. A.; Bondybey, V. E. Appl. Spectrosc. Rev. 1982, 18, 105. Philos. Trans. R. SOC.London, Ser. A 1982, 307, 617, and references therein. (9) See for example: Levy, D. Annu. Reu. Phys. Chem. 1980, 31, 197. (10) Carrington, A,; Tuckett, R. P. Chem. Phys. Left. 1980, 74, 19. Miller, T. A,; Zegarski, B. R.; Sears, T. J.; Bondybey, V. E. J . Phys. Chem. 1980, 84, 3154. Klapstein, D.; Leutwyler, S . ; Maier, J. P. Chem. Phys. Lert. 1981, 84, 534. (1 1) Klapstein, D.; Maier, J. P.; Misev, L. In “Molecular Ions: Spectroscopy, Structure and Chemistry”, Miller, T. A., Bondybey, V. E., Eds.; North-Holland: New York, 1983; p 175.
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0 1984 American Chemical Society
