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J. Phys. Chem. 1982,
compared to the B for the ArF laser. Extensive spectral studies are now in progress to search for further excited states of Cuz and higher copper clusters. Cluster beams similar to the Cu, beams discussed above have been produced with this nozzle source for Ni, Al, and Fe and we know of no reason why the technique as described should not be applicable to a very wide range of metals. Acknowledgment. We thank D. Cox and A. Kaldor of Exxon Corporate Research for valuable suggestions and
86,2560-2563
encouragement as well as providing the excimer laser used in this work. C. Welborn of Exxon Chemical Co., Baytown, TX provided valuable assistance in analysis of clusters and molten particle ejection from laser vaporized metals. We are also grateful to J. Gole and V. Bondybey for communicating their results prior to publication. This research was supported by the Department of Energy, Division of Chemical Sciences, and by The Robert A. Welch Foundation. Acknowledgment is also made tQ the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Laser Spectroscopy of Cooled Metal Clusters: Copper Dimer J. L. Gale,+ J. H. English, and V. E. Bondybey' Bell Laboratories, Murray Hlll, New Jersey 07974 (Received: April 12, 1982; I n Final Form: May 21, 1982)
The electronic spectroscopy and laser-inducedfluorescence of cluster species formed in copper vapor have been examined both in low temperature matrices and in the gas phase. Vibrationally resolved matrix emission spectra are observed and it is suggested that a combination of homogeneous phonon broadening and inhomogeneous broadening of similar magnitudes are responsible for the appearance of featureless matrix absorption spectra. A long-lived red emission is observed and assigned to a new electronic state, possibly a triplet near 15500 cm-I. Gas-phase spectra of copper species are reexamined by YAG laser vaporization followed by a time-resolved laser-induced fluorescence study of the products of laser vaporization. Laser-induced fluorescence excitation spectra of the B-X band system of Cu2 indicate Trot Tvib 120 K. The identification and assignment of a new band system in the region of the Cuz B-X system, previously observed in the spectra associated with the supersonic expansion of pure copper vapor, indicates the presence of a new Cu2electronic state with origin at 21 848 cm-', we, 221 cm-', w,x,' 2 cm-'. The new state exhibits a lifetime r 1 p s and is efficiently collisionally quenched.
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Introduction In the last several years there has been increasing interest in the spectroscopy and properties of small metal aggregates.' Several diatomic and polyatomic clusters have been studied by a variety of techniques' and it has become apparent that the most fruitful means of cluster characterization may well involve the judicious combination of several approaches. Here, in concert with the report of Smalley et a1.,2 we illustrate this point focusing our attention on cooled copper clusters. Dimeric copper has been the subject of several gas-phase studies3and recently ultraviolet-visible absorption spectra taken in rare gas matrices4 have been assigned to the polymeric clusters Cu3 and Cu,. The diversity of these studies serves to illustrate significant considerations associated with the study of small metal clusters. A drawback of the classical spectroscopic studies of metal clusters in furnaces of various designs3 is usually the complexity of the high-temperature spectra which must be analyzed. The development of low-temperature techniques for studying these high-temperature species has often proved to be the key to unraveling their spectroscopy. Matrix isolation spectroscopy has traditionally been such a technique, providing useful information about a variety of metal clusters. More recently supersonic expansions of metal vapors have been used to cool internal degrees of freedom and produce cold clusters in the gas phase. This technique has 'Chemistry Department and High Temperature Laboratory. Georgia Institute of Technology, Atlanta, GA 30332.
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been particularly useful in studies of alkali metal cluster^'^^ and, more recently, laser-induced fluorescence spectra of clusters generated in an adiabatic expansion of pure copper vapor have been reported.6 Although very promising, a drawback of these studies is their experimental difficulty and the complexity of the apparatus which is required. Low-temperature matrix studies, on the other hand, are relatively simple. High concentrations of metal clusters and strong optical signals are usually readily obtained. In addition, matrix studies provide an important complement to the gas-phase work in studies of dissociative or predissociating states. These can be either readily studied in absorption, or monitored by detecting the fluorescence from lower-lying electronic states populated by matrix cage recombination of the fragment atoms.' A disadvantage (1) See, for example, J. L. Gole, G. J. Green, S. A. Pace, and D. R. Preuss, J. Chem. Phys., 76, 2247 (1982); T. G. Dietz, M. A. Duncan, R. E. Powers, and R. E. Smalley, ibid., 74,6511 (1981);V . E. Bondybey and J. H. English, ibid., 67, 3405 (1977), for appropriate references to a number of experimental and theoretical studies. (2) D. E. Powers, S. G. Hansen, M. E. Geusic, A. C. Price, J. B. Hopkins, T. G. Dietz, M. A. Duncan, P. R. R. Langridge-Smith, and R. E. Smalley, J . Phys. Chem., preceding letter in this issue. (3) B. Kleman and S. Lindkvist, Ark. Fys., 8, 333 (1954); R. E. Steel, J. Mol. Spectrosc., 61, 477 (1976); N. Aslund, R. F. Barrow, W. G. Richards, and D. N. Travis, Ark. Fys., 30,171 (1965); A. D. Smirnov, N. E. Kuzmenko, and Yu. Ya. Kuzyakov, Opt. Spectrosc., 47, 149 (1979). (4) M. Moskovits and J. E. Hulse, J. Chem. Phys., 67, 4271 (1977). (5) S. Leutwyler, A. Herrmann, L. Wostz, and E. Schumacher, Chem. Phys., 48, 253 (1980), and references therein. (6) D. R. Preuss, S.A. Pace, and J. L. Gole, J. Chem. Phys., 71, 3553 (1979). (7) V. E. Bondybey and J. H. English, J . Chem. Phys., 72,3113 (1980).
0022-3654/82/2086-2560$01.25/00 1982 American Chemical Society
Letters
of matrix work is that one must contend with perturbations of the guest spectra by the host and overcome difficulties associated with the various mechanisms of homogeneous or inhomogeneous line broadening in the solid matrix environment. It is apparent that an appropriate combination of matrix and gas-phase techniques will serve to elucidate cluster characteristics. In the present manuscript, we combine matrix isolation with two different gas-phase techniques to examine further the spectra of clusters of copper atoms.
Experimental Section The experimental techniques employed in this study have all been rep~rted.~J"Matrix isolation studies were carried out by depositing samples of copper vaporized from a resistively heated Knudsen cell together with the matrix gas (Ne or Ar) on a platinum substrate held at 4 K. Typically, copper atom concentrations of 1:5000 or less were employed. Fluorescence was excited with a tunable nitrogen-pumped dye laser and resolved with a Spex 14018 double monochromator. The signal was then digitized in a Biomation 8100 waveform recorder, averaged, and further processed numerically. In one series of gas-phase experiments, copper was vaporized by using the pulses of a Nd:YAG laser and the vapors were entrained in a flow of cold helium gas which had passed through metal coil cooled by liquid n i t r ~ g e n . ~ The molecular and atomic species present in the vapor were again probed with dye laser-induced fluorescence. Finally, the laser vaporization gas-phase results are compared to those gas-phase studies in ref 6 where pure copper vapor is expanded from an oven at -2400-2600 K and the resulting fluorescence is probed by a tunable cw dye laser. Results and Discussion Matrix Studies. As we have noted, one of the major limitations of matrix isolation studies results from the various homogeneous and inhomogeneous mechanisms which lead to broadening of the guest spectral lines and thus reduce the detail and accuracy of available spectral information. In the limit where no vibrational fine structure is observable, little useful spectral information is obtained. The gas-phase spectra of atoms are characterized by sharp lines whose width is largely determined by their Doppler broadening; however, the corresponding transitions in the matrix may result in features which are several orders of magnitude broader. The extent of homogeneous broadening is a consequence of differences between the guest-host interaction potential for the lower and upper state of the transition. This broadening is particularly severe for alkali atoms but is somewhat lessened for the elements of group 1B. The ground state of the potassium atom consists of a core argon-like shell surrounded by an additional loosely bound 4s electron. The ground electronic state, partially, and readily accessible excited electronic states, in large part, have a Rydberg character. Hence, the electronic wave function is very diffuse and will interact rather strongly with the host in the solid environment of the matrix, the magnitude and shape of this interaction being strongly (8) E. Schumacher, W.H. Gerber, H. P. Hiirri, M. Hofmann, and E. Scholl in 'Metal Bonding and Interactions in High Temperature System with Emphasis on Alkali Metala" ACS Symp. Ser., No. 179,83 (1981). (9) V.E.Bondybey and J. H. English, J.Chem. Phys. 76,2165 (1982); V. E. Bondybey, G. P. Schwartz, and J. E. Griffiths, J. Mol. Spectrosc.,
89, 328 (1981). (10) V. E. Bondybey and J. H. English, J. Chem. Phys., 74, 6978 (1981).
The Journal of Physical Chemistry, Vol. 86, No. 14, 7982 2561 Cu2 IN NEON 4
2
5
6
n
n
I5 0
4
IV ,
,/,
7
,
,
14 0
\,
I3 0
F [~ o ~ c r n - l ]
IO
, 12 0
Flgure 1. Laser-induced fluorescence of a neon matrix deposit containing copper. The emission is asslgned to Cu,; the vibrational numbering shown is tentative. The wavelength of the exciting laser was -22 500 cm-'.
state specific. Because electronic excitation requires substantial rearrangement of the solvent around the guest atom, the lowest resonant transition of alkali atoms in rare gas solids is several thousand cm-l broad. Unfortunately, these strong interactions are carried, although to a lesser extent, to the spectra of the corresponding molecular species and we find, in agreement with previous investigations, that the absorption spectra of Cuz in solid neon and argon lack any vibrational structure. Least perturbed spectra are usually obtained in neon matrices and the present fluorescence studies therefore focused mainly on solid neon. When neon matrix samples containing copper are excited by a h e r tuned to the strong absorption in the 4000-4400-A region, a structured, short-lived emission is observed. We found that depending on the exact excitation wavelength the emission originates between 21900 and 22150 cm-', consisting of a short progression of 4-5 broad bands with a spacing of -260 cm-l. The spectrum does not correlate with any expected matrix impurities and appears at low guest concentrations when Cuz must be the dominant molecular species present in the matrix. The intensity distribution in the pregression shows excellent agreement with the Franck-Condon factors calculated for vibrationally relaxed B-X fluorescence of Cuz, leaving little doubt that u ''= 0 B Xu+is the emitting level. The vibrational spacing indicates that the ground state of Cuz is not strongly perturbed in the solid matrix and the electronic transition exhibits a moderate 200300-cm-' blue shift from its gas-phase position at 21 748 cm-'. Shifts of the emission progression with the exact excitation wavelength suggest that the inhomogeneous broadening, -250 cm-', is superimposed upon the -200cm-l homogeneous phonon broadening of the bands. The combination of homogeneous and inhomogeneous broadening results in the observed continuous and structureless appearance of the absorption spectrum. The lifetime of the B-X emission is too short to be measured with our -10-11s resolution; however, it is clearly shorter than the -40-11s gas-phase radiative lifetime of the B state. Excitation at somewhat lower energies near 20 50021 500 cm-' produces a similar emission progression with origin near 20 500 cm-' and different intensity distribution. This is clearly due to the A X fluorescence, again slightly blue shifted in the matrix from the gas-phase frequency of urn = 20396 cm-'. The lifetime of this emission is also short, suggesting that the emission quantum yield is considerably less than unity. Excitation of either the A state or the B lZU+state produces, in addition to the fluorescence of the directly
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The Journal of physical Chemlstry, Vol. 86, No. 14, 1982
excited level, a second much stronger and long-lived emission in the red and near-infrared. Its spectrum, depicted in Figure l , consists of a progression of 10-12 broad bands, again exhibiting a spacing of -260 cm-'. In the absence of additional information one cannot determine the vibrational numbering with confidence; however, it appears that the emission originates from a previously unobserved state of copper dimer and terminates in the X 'Z,+ ground state. The new electronic state is apparently populated following excitation of either the B state or the A state by a nonradiative relaxation process. This must involve predissociation of the directly populated level followed by recombination of the Cu atoms within the matrix cage. The origin of this new spectrum appears to be between 15500 and 16OOO cm-'. The emission exhibits a long, -30 f 5 ms lifetime. The transition involved must clearly be strongly forbidden. In the absence of any additional data or high quality ab-initio calculations, it is rather difficult to assign this transition to a specific state; however, in view of ita forbidden nature one might speculate that the upper level corresponds to a triplet state. The broad Franck-Condon intensity distribution suggests that the Cu2 internuclear distance increases substantially in this excited electronic state. The differences between Cu2spectroscopy in solid neon reported here and ita behavior in argon and the heavier rare gases observed both by Ozin and co-workers" and in our laboratory are not fully understood at this time. Their elucidation will require additional studies. Gas-Phase Studies. In a recent study Goie et reported laser excitation spectra for molecular species formed in the supersonic expansion of pure copper vapor. The strongest spectral features observed corresponded to the B-X transition of Cu2 characterized by a rotational temperature, T& = 800 f 50 K, and a vibrational temperature, Tvib= 950 f 100 K. In addition, unidentified features were observed between 21 745 and 21 845 cm-l. I t was speculated that these features correspond either to emission from a long-lived excited state of copper dimer, quenched in previous gas-phase studies, or to a polyatomic copper compound, possibly Cu3. Since the establishment of this point is of considerable interest, we have reexamined this spectral region in our present work. A low-resolution laser excitation spectrum for copper vapor produced by laser vaporization is shown in Figure 2a. The spectrum is characterized by a progression of bands corresponding to the Cu2 B-X band system. In comparison with previous work, the spectra exhibit a appear considerablylower temperature. Both Tmtand T ~ b to be close to 120 K. Also the feature reported previously at 21 848 cm-' and not associated with the Cu2B-X system is clearly visible as well as two other similar but weaker bands at 22 065 and 22 278 cm-'. These features appear with the same relative intensity with respect to the Cu2 B-X system for several copper samples with different degrees of purity, including a high-purity (99.9999%, Aldrich) copper metal. This suggests that an impurity intrinsic to copper is not involved. Figure 2b shows the effect of introducing an air leak into the helium carrier gas flow. While several new features, probably due to copper oxide, appear in the spectrum, the band at 21 848 cm-' and its related features are not enhanced again suggesting that they must be assigned t o a Cu, species and do not involve an impurity. The relative intensities of the unknown bands and of the B-X system show no variability with YAG laser power and hence with (11) G.A. Ozin, S. A. Mitchell, and J. Garcia-Pricto, J.Phys. Chem., 86, 473 (1982).
Letters
(b)
B
(C)
216
220
22 4
22 8
P[io3cm-1]
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Flgure 2. Laser excltatbn spectra of vapors produced by YAG laser vaporization of copper. (a) The progression of strong bands is due to the X B (0'' V') transitions. Two weaker bands labeled A0 and A 1 are due to the longer-llved B' state. (b) The same spectral region as in scan a, with a small alr leak intentionally introduced. Fluorescence In the 0-10thne window is integated. (c) The same scan as in b, but the longer lived 100-300-ns emission is monitored.
the concentration of copper in the vapor. Using laser-induced fluorescence, we have also monitored the concentration of copper atoms in the vapor. Both ground state 2S1/2and metastable 2D6/2copper atoms are observed in our gaseous flow. The latter are monitored in the 2D6/2-2P3/2transition at 19 581 cm-'. The Cu 2P3 atoms fluoresce with a lifetime of 69 f 5 ns relativeiy independent of buffer gas pressure. As the YAG power used to vaporize the sample is increased, the Cu2fluorescence intensity gains relative to the atomic fluorescence. On the other hand, the relatiue intensities of the unknown band system and the bands of the B-X progression show no dependence on YAG laser power and hence on the concentration of copper atoms. Similarly, one finds no dependence on the delay between the vaporizing YAG pulse and the probe pulse exciting the fluorescence. While relatively more copper atoms are seen at shorter delays, and at longer delay times the Cu2bands gain in intensity, the ratio of the unknown bands to the known Cu2 bands appears constant. Using this information, we conclude that the 21 848-crn-l band and ita related features must be assigned to Cup This conclusion has been reached independently by Smalley and co-workers using two-photon ionization spectroscopy.2 Although the relative intensities of the new Cu2system and the Cu2 B-X system parallel each other, the new features exhibit a considerably longer lifetime than the bands of the B-X transition. This can be seen from Figure 2c which originates from the same scan as Figure 2b except that, instead of a 0-100-ns time interval, the 100-300-ns window was digitally integrated. It is clear that, while the bands of the short-lived B-X transitions have greatly diminished, the unknown bands are enhanced. The features of the apparently long-lived state of copper oxide show even greater enhancement. The evidence presented
The Journal of phvsical Chemistry, Vol. 86, No. 14, 1982 2583
Letters
-I
2
2200Qcm-
21900cm
___il80Ocrr
2l700cm-
2l600cm-
Figure 3. (a) Excbtkm spectrum for supersonically expanded copper showing 2,O sequence for the Cu2 B1 2,+-X1 BP+band system and much weaker bands (bold face) for Cu2 B'-X band system. See text for discussions. (b) Excttatbn spectrum for supersonically expand copper showing 1,0 and 0,O sequences for Cu2 B 'XU+-X1 2; band system and much weaker bands (Wface) for Cu, B'-X band system. See text for discussion.
thus far suggests strongly that the unknown bands should be assigned to a new electronic state of Cu2with the band a t 21 848 cm-I being the 0-0 origin. The lifetime of the new emission shows a strong dependence on pressure. A Stern-Volmer extrapolation suggests a lifetime of 1 f 0.2 ps for the new state in agreement with the measurements of Smalley and coworkers? This relatively long lifetime as well as the efficient collisional quenching of the excited electronic state may explain the failure to observe these bands in previous studies. With a reasonable 1-0 assignment for the second band at 22 065 cm-I and the 2-0 band at 22 278 cm-I one obtains o,' = 221 cm-l and oex,' 2 cm-' for the excited B' state of Cu2. I t is interesting to note that, with this assignment, one might predict a broader Franck-Condon intensity distribution for the new system vs. that for the B-X transition since the vibrational frequency o,' = 221 cm-l is lower than that of the B state and much lower than that of the ground state, o,,"= 264 cm-'. Experimentally the opposite is observed. While the u' = 0-5 levels are readily monitored for the B-X transition, only the u' = 0, 1 , 2 levels of the B' state are clearly observed. A possible explanation would be an increasingly efficient predissociation on collisional quenching of the higher vibrational levels of the B' state. Alternatively, it is possible that the internuclear distance of the B' state is similar to that of the ground state in spite of the decreased vibrational frequency.
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On the basis of the assignments made in the laser vaporization experiments, it has been possible to return to the spectrum generated by employing the supersonic expansion of pure copper vapor6 and assign several additional bands to the B' system. These assignments are indicated in Figure 3. In addition to the O,O, 1,0, and 2,O bands, the hot band structure associated with the u" = 1 level of the Cu2ground state is indicated. These assignmentshave also been reached independently by Smalley and co-workers.2 I t is interesting to note that the bands associated with u' = 2 (Figure 3) display an apparent intensity alternation. This might result from selective and alternating rotational predissociation; however, this is only a speculation which awaits further experimental verification. In this regard it is also interesting to note that the ratio of the integrated fluorescence intensities for the B-X and B'-X band systems appears to be less than the -3O:l ratio of the observed lifetimes (1 ps X 30 ns). This may indicate that the B state lifetime is shortened as a result of its predissociation,2 a fact which is not inconsistent with the known binding energy12of Cu2 (1.9 eV = 15327 cm-'1. Acknowledgment. V.E.B. acknowledges useful discussions with G. Ozin. We also thank R. E. Smalley for providing us with a preprint of his study (ref 2) prior to publication. (12) A. G. Gaydon, 'Dissociation Energies and Spectra of Diatomic Molecules", Chapman and Hall,London, 1968.
