J. Phys. Chem. 1903, 87, 4647-4650
decrease in the intensity of the zero phonon line and an increase in the phonon sideband intensity. The net negative area under the TM spectrum of each of the vibronic lines suggests that a small amount of linear coupling may be present. From the preceding discussion we may conlude that the primary effect of electronic excitation (of PCP- to the 'B, state) on the lattice is a net increase in the frequency of the lattice acoustic modes with little change in the equilibrium positions of the PCP- ions in the lattice. Conclusions The first highly structured luminescence from a percyanocarbanion (pentacyanopropenide anion) is reported. This luminescence is assigned as the 'BZto 'Al, T* to T fluorescence. Huckel MO calculations provide a reasonable prediction of the positions of bands in the electronic absorption spectrum of PCP- and of the intensity of the lowest energy allowed absorption. At 2 K, single-crystal CsPCP displays fluorescence having a rich vibronic structure due, primarily but not exclusively, to 0 to 1 transitions in totally symmetric modes. Thus, the sym-
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metry of the electronically excited PCP- is essentially the same as the ground-state ion, and the geometrical change is delocalized over the entire ion. The low temperature dependence of the vibronic line shapes in emission and thermal modulation of emission are indicative of very strong quadratic electronic-lattice coupling. The electronic-vibrational and electronic-lattice coupling present in CsPCP are unusual and exciting. Given a complete normal-coordinate analysis of the ground-state vibrational modes, it will be possible to determine the geometry of the ion in the 'B2 excited state. The normal-coordinate analysis is presently under way in this laboratory. Given the crystal structure of CsPCP, it should be possible to generate quantitative information about the electronic-lattice coupling observed in the fluorescence.
Acknowledgment. We thank the National Science Foundation for providing partial support in the form of grant DMR-8115978. We also thank Mr. Dennis Roberts for his assistance with some of the synthetic chemistry, and Prof. A. H. Francis for a helpful telephone conversation. Registry No. CsPCP, 82085-20-9.
Observation and Characterization of a New Electronic State of Cu, in Solid Neon V. E. Bondybey' and J. H. English Bell Laboratories. Murray Hili, New Jersey 07974 (Received: Msrch 15, 1983)
A new weakly bound electronic state of Cu2with origin near 25 508 cm-' in solid neon is observed and investigated by laser-inducedfluorescence. The state is characterized by a long, 6.5-ps lifetime and large, -2.38A internuclear distance. It is assigned to one of the spin-orbit components of the 311uor 32ustates expected in this region and correlating with the 4s2S + 4s2 2D atomic limit. The atomic 'D512 2S1,z emission is observed following excitation of molecular Cuz in solid neon and its origin and excitation mechanisms are discussed.
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Introduction Rare gas solids containing copper have been studied rather extensively. In several earlier studies numerous absorption bands in the visible and ultraviolet have been observed.'-4 Concentration-dependence studies have permitted one to distinguish atomic absorptions from those of the Cuz dimers and of larger clusters. The molecular absorptions assigned to Cu2 were found to be broad and lack vibrational fine structure. Recently two studies of laser-induced fluorescence of Cuz in matrices have appeared,&' both reporting emission spectra with clearly resolved vibrational structure. There were, however, several puzzling differences between the results of the two studies. Ozin et al.,5 using mercury arc excitation of argon and krypton matrices, have observed a structure emission system with origin near 25 000 cm-'. This was assigned to the A X fluorescence of Cuz, extremely strongly blue shifted in the matrix environment.
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(1)M. Moskovits and J. E. Hulse, J. Chem. Phys., 67,4271 (1977). (2)C. A. Ozin, H. Huber, D. F. McIntosh, S. A. Mitchell, J. C. Nor101,3504 (1979). man, and L. Noodleman, J . Am. Chem. SOC., (3)R. Grinter, S. Armstrong, V. A. Jagasooria, J. McCombie, D. Norris, and J. P. Springall, Faraday Symp. Chem. SOC.,94 (1980). (4)L.Brewer and B. King, J. Chem. Phys., 53, 3981 (1970). (5)G. A. Ozin, S. A. Mitchell, and J. Garcia-Prieto, J.Phys. Chem., 86, 473 (1982). (6)V. E. Bondybey, J. Chem. Phys., 77, 3771 (1982). (7)J. L.Gole, J. H. English, and V. E. Bondybey, J . Phys. Chem., 86, 2560 (1982). O022-3654/83/2O87-4647$01.50/0
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Furthermore, it was found that excitation in the region of the X B absorption produced no molecular fluorescence but resulted in a rather intense Cu 'D lS atomic emission. This was attributed to dissociation of the B state, followed by emission of the resulting electronically excited atomic fragments. On the other hand, a neon matrix study employing tunable pulsed laser excitation6 resulted in observation of X and A X emission systems, each of both the B them very close to their gas-phase positions. In each case, the intensity distribution in the fluorescence was very close to that predicted by Franck-Condon factors calculated by using the gas-phase spectroscopic constants, suggesting very little perturbation by the solid medium. Furthermore, it was found that excitation of either the B'Z:,+ or of the A state results in efficient predissociation and formation of ground-state Cu atoms. The cage recombination of these atoms was found to populate the previously unknown a 32u+ state which was detected and characterized by its long-lived, spin-forbidden phosphorescence into the X'Z,+ ground state.6 In order to resolve the differences between the two studies, it appeared necessary to reexamine the rare gas matrices containing copper. In the present manuscript we undertake such a study of time-resolved, laser-induced fluorescence of Cup We focus our attention in particular on neon matrices doped with copper, and at UV excitation wavelengths above the range of the previous LIF study.
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0 1983 American Chemical Society
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Bondybey and English
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
12
Cu2 I N NEON I
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Figure 2. Fluoresence of Cu, in solid neon obtained with excitation at 26 000 cm-'. The left-hand panel shows the featureless fluorescence excitation spectrum obtained by scanning the laser over the range of the Pilot 386 dye.
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Figure 1. Fluorescence observed following excitation of Cu, in soli neon: (a) B'Z,', excitation at 21 150 cm-I; (b) A excitatlon'at 22700 cm-I. The bars shown under the individual vibronic bands represent relative intensities expected based on the Franck-Condon factors calculated by using gas-phase spectroscopic constants.
('nu?),
A small discrepancy is observed in the sense that the intensities in both of the experimental spectra are slightly skewed in favor of higher ut' levels. Thus, for example, while the calculations for the B X systems correctly predict the 0' 1" transition to be most intense, they suggest the 0-2 band should be only slightly more intense than 0-0 transition, while experimentally the former band is quite considerably stronger. This may be partially due to the fact that the higher bands me superimposed upon a background due to underlying phonon sidebands of the lower u'levels. On the other hand, the observed discrepancies may reflect a small but real increase in the re of the excited electronic states in the matrix (or, less probably, shortening of the ground-state re, since the matrix data give information only about the relative re values). This would be consistent with the observation of a slight blue shift of the electronic transitions, suggesting that the excited states are in the solid neon slightly destabilized relative to the ground electronic state. In the recent Ar matrix study of Ozin et al.,5 a progression of bands wjth w,, 252 cm-' and origin near 24 990 cm-' was observed following excitation near 2800 A and assigned to the A X transition of Cu2,extremely strongly blue shifted from its gas-phase position at 20433 cm-'. The subsequent observation of the A-X transition in solid neon very near to its gas-phase wavelength casts a considerable doubt upon this assignment. In order to investigate this point further, we have examined Cu, in neon with IJV excitation a t higher energies than in our previous study. A typical spectrum with excitation near 26 OOO cm-', shown in Figure 2, exhibits two separate progressions, each with -260-cm-' spacing characteristic of the X'Z,: ground state of Cu2. The lower energy emission with origin near 22 140 cm-' is identical with the B'Z,+ X'Z,+ fluorescence discussed above and shown in Figure la. The higher energy progression with origin at 25 508 cm-' is then clearly due to the same transition as the 24990-cm-' spectrum observed by Ozin et al.5 in argon. It is only moderately shifted from the argon matrix position and also shows an almost identical intensity distribution. This is further evidence against assignment to the A state, since large gas-matrix shifts are invariably accompanied by comparably large shifts from matrix to matrix. The left-hand panel of Figure 2 shows an excitation spectrum of the fluorescence obtained by monitoring the intensity of the 4"-0' band near 24 430 cm-I and scanning the laser over the range of the Pilot 386 dye. It shows no vibrational fine structure, but a gradual increase over the range of this dye. The onset occurs close above the 25508-cm-' origin of the emission, suggesting that the emitting state is in this wavelength range excited directly, rather than being populated by nonradiative relaxation processes from some higher-lying electronic states. The
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Experimental Section The experimental techniques employed were identical with those used The copper was vaporized from an alumina crucible and codeposited with the matrix gas on a metal substrate held near 5 K. The rate of metal deposition could be estimated by monitoring the rate of decrease in the light transmitted through a quartz slide exposed to the vaporization crucible. The matrix thickness was determined by observing a 6328-A laser beam reflected from the matrix and counting the interference fringes. All the samples appeared colorless to the eye and the concentrations were believed to lie in the range of 1:2000-1:lOOOO. Following deposition the matrices were usually annealed (at 9-10 K for -5 min for neon matrices) to enhance cluster formation. Fluorescence was excited by using a pulsed, tunable dye laser pumped by a nitrogen laser. The sample emission was dispersed in a Spex 14018double monochromator and detected, and the signal time resolved in a Biomation 8100 transient digitizer. The signal was then averaged and processed by a minicomputer which also controlled the laser or monochromator scanning. Results and Discussion Cu, Fluorescence Spectra. The appearance of the emission spectrum depends strongly upon the excitation wavelength, as noted in our previous studies.6 For excitation between 20 500 and 21 500, the spectrum shown in Figure l a is produced. Excitation in the range of 22 000-24 OOO cm-l, on the other hand, yields the spectrum shown in Figure Ib. We have noted in our previous communication that, based on their wavelengths and on the intensity distributions, the two progressions are readily assigned to the vibrationally relaxed A'II, XIZg+and B1&+ X'Zg+ fluorescence systems, respectively. This can be seen in Figure 1 by comparing the band intensities with the stick diagram representing the values predicted by Morse Franck-Condon overlap integrals calculated by using the gas-phase values of re and of the spectroscopic c o n s t a n t ~ . ~ The J ~ observed patterns are in fairly good agreement with the calculation and identify rather clearly the two transitions, each of which undergoes a small, 300-cm--' blue shift in the matrix.
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(8) V. E. Bondybey and J. H.English, J . Chem. Phys.,73,42 (1980). (9) N. Aslund, R. F. Barrow, W. G . Richards, and D. N. Travis, Ark. Fys., 30, 171 (1965). (10) J. Lochet. J. Phys., R, 11, L65 (1978).
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The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
New Electronic State of Cu, in Solid Neon
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time-resolved profile of the emission shows no rise time and an exponential decay of 6.5 f 0.5 ps, as shown in Figure 3. This confirms that the emitting state cannot be the A state, whose u' = 0 level exhibits in the gas phase a much shorter lifetime of 115 f 10 ns, probably controlled by predissociation." In view of all the evidence discussed above we have to conclude that the 25 508-cm-' emission is due to a previously unobserved state of Cu2 which we will denote D in the following discussion. Properties of the D State and Assignment of the Cu2 Electronic States. The broader intensity distribution with maximum in the 0-4 band suggests that the new state is considerably less strongly bound than either the A'n or the BIZ; states and has a considerably longer bond length. Both the A and B states are believed to correlate with the second dissociation limitlo of Cu2, 2S1/2+ 2D5/2. The excitation energy of Cu 2D5/2(11 202.6 cm-') combined with the dissociation energy12of the X'Z; ground state of Cu2 ( 16 400 cm-') and with the known T,values suggests dissociation energies of -7100 and 5800 cm-' for the A and B states, respectively. By similar arguments, one can conclude that the new state is somewhat more strongly bound than the lowest a 3Zu+ state which exhibits an even broader intensity distribution envelope and for which we have deduced a De -11000-1500 cm-'. One can estimate that the dissociation energy of the new state should be in the 2000-4000-~m-~ range, and it should hence dissociate into atomic products with 27 500-29 500 cm-I of energy with respect to the Cu, X'Z,+ ground state. Only the 2Slj2 +. TI5/? (-27600 cm-') and 2S1/2+ 2D3/2 ( 29 600 cm- ) dissociation limits are close to this range, and the new state must correlate with one of these limits with the lower one appearing more likely. By fitting the observed emission intensities of the 0' u " progression to the calculated Franck-Condon factors, one can estimate re -2.38 f 0.03 A and w,' -160 cm-' for the upper state, which we will denote D in the following
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Figure 4. Schematic potential diagram summarizing the known lowlying electronic states of Cu,. The wiggly arrows show the nonradiative processes observed in neon matrix.
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Flgure 3. Fluorescence decay of the D cm-'.
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discussion. The potential energy diagram in Figure 4 summarizes the known low-lying electronic states of Cu2. In drawing this diagram we assumed that the gas neon matrix shifts for the a and D states, for which no gas-phase data are available, are similar to those experienced by the other electronic states in this energy range-A and B. Identity of the Known Electronic States of Cu2. Combination of two ground-state 2S1/2copper atoms can give only two distinct electronic states, Z'; and 3Zu+. These are easily identified with the XIZ,+ ground state of Cu2 and with the long-lived a state recently observed in neon m a t r i ~ . ~The ~ ' next two dissociation limits correspond to interaction of a ground-state Cu atom with one electronically excited into one of the components of the metastable '9 state. The next higher group of dissociation limits arises from two 4s22D copper atoms and lies additional 11 OOO cm-' higher in energy. It seems therefore very likely that all the four electronic states observed in the 2000025 000-cm-' range correlate with the 4s 2S + 4s2 2D atomic products and the higher lying dissociation limits need not be considered here. Combination of a 2S and a 2D atom will yield, in the A, S coupling scheme, 12 distinct elecronic E , 'nu, 'A,, 3Zu+, 311u,3Au, as well as the correstates: ' sponding states of even parity. The g 46 g parity selection rule which is quite rigorous for electric dipole transitions in gas-phasehomonuclear diatomics usually holds to a very good approximation for optical transitions in solid matrices also, and only u states therefore need to be considered. The intense X B spectrum carries most of the oscillator strength in this region and is therefore quite easily identified as the fully allowed lZg+ lZu+ transition. This is fully confirmed by the rotational analysis of several bands of this spectrum. Also the A state is characterized by a relatively short, 110-ns lifetime," and the corresponding spectrum was tentatively assigned13to the second allowed electronic transition expected in this range X'Z,+ A'II. It should, however, be noted that in the rotationally resolved spectrum apparently only P and R branches are present and no evidence of the Q branch is found,l0casting some doubt upon this assignment. The only alternative would of course be an = 0 component
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(11)V. E. Bondybey, G. P. Schwartz, and J. H. English, J. Chem. Phys. 78, 11 (1983). (12)S. Smoes, F. Mandy, A. Vander Auwera-Mahieu,and J. Drowart, Bull. SOC.Chim. Belg., 81,45 (1972).
(13)D.E.Pesic and S. Weniger, C.R. Hebd. Seances Acad. Sci., Ser. B , 273,602 (1971).
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The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
of a Hund's case "c" 311uor 3Zustate. Finally, the "C" state observed by Gole et al.,7in a recent gas-phasework, as well as the "D" state characterized in the present study exhibit relatively long lifetimes, of -0.8 and 6.5 ps, respectively. These are probably best consistent with assignment to the Q = 0 or Q = 1 components of the 3Zu and 311u states expected in this region which undoubtedly tend toward the Hund's case "c" in the relatively heavy diatomic Cu2. Unfortunately, the matrix fluorescence is depolarized, probably due to nearly free rotation of the CU, species in the 4 K solid. A more definitive assignment of these states will therefore have to await rotationally resolved gas-phase studies. D-State Radiative Lifetime and Relaxation. We noted in the proceeding paragraph that the D state exhibits in solid neon a lifetime of 6.5 f 1ps. As can be seen in Figure X emission is accompanied by a somewhat 3, the D X stronger B-state fluorescence, as well as by the a phosphorescence in the near-infrared. While following direct excitation the B'Z,+ state has a lifetime shorter than our experimental resolution (> 7B, the decay profile will be governed by the much longer D-state lifetime, with the D B internal conversion being the rate-determining step. X Noteworthy is the absence of an observable A emission in spite of the fact that both the B and a3Zu+ states are popclated. This shows that the A state is bypassed in the BIZ,+ state nonradiative relaxation, consistent with our conclusion that the a3Zu+triplet is populated by the predissociation-recombination sequence. The presence of emission from lower lying electronic states following D-state excitation shows clearly that the D-state fluorescence quantum yield is less than unity and its lifetime is shortened by nonradiative relaxation. We estimate that the true D-state radiative lifetime is at least a factor of 3-4 longer than the experimentally observed 6.5-ps value. Atomic Cu 2D3j2,512 2S1,2 Emission. Ozin and coworkers5 have observed two rather strong emission bands near 7560 and 8940 A following excitation of Ar matrices containing copper with h -3800-4200 A, in the region of the strong X B Cu2 absorption. The same two bands in both solid neon and argon were also detectedI4 several
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Bondybey and English
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years ago in a study of matrix-isolated CuO. They are readily assigned to the %3/2,5/2 2Slj2emission of atomic Cu. Ozin and co-workersexplain the appearance of these bands by a two-step process, consisting of resonant excitation of the B2Z,+ state of Cu2,followed by it dissociation and emission of the electronically excited fragments. These preceding observations are confirmed by the laser excitation experiments in neon matrix, where the 2D312 2S112 emission is clearly seen superimposed over the 0' 8" band of the molecular Cu2 a3Zu+ X'Zg+ emission. Our results demonstrating that the CU, molecular spectra are not strongly perturbed by the solid medium, however, preclude using the interpretation of Ozin et al. to explain the excitation mechanism in solid neon. The atomic Cu emission at 13220 cm-' in solid neon is observed whenever the molecular a3Zo+phosphorescence is present, for excitation energies as low as 20 500 cm-l. On the other hand, a minimum energy of -29 600 cm-' is needed to produce the Cu 'D5/2 atom from ground-state Cu, (Deof X'Z,+(Cu2) -16400 cm-' the -13 220-cm-' excitation energy of the 2D512state in solid Ne). Clearly the energy deficit of more than 9000 cm-' cannot be reasonably attributed to matrix shifts and effects. Furthermore, we find qualitatively that the relative intensities of the atomic and molecular phosphorescenceare dependent upon overall matrix concentration, with higher concentrations favoring the atomic emission. We propose that the Cu 2D5/2level is populated by a "Forster type" energy-transfer process from a nearby CU, molecule in the a3Zu+state. It must, of course, be noted that, since neither Cu2 nor unbound Cu atoms have a dipole moment, the transfer kinetics should be governed by solvation effects and higher order mutipole coupling. While the atomic 2D61zemission is superimposed upon the molecular Cu, phosphorescence, and we have not studied in detail its time-resolved behavior, qualitatively the decay appears nonexponential, consistent with the energy-transfer model discussed above.
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Summary A new electronic state of Cu, is observed in emission in solid neon. Its fluorescence is characterized by a -6.5-ps lifetime and it is interpreted in terms of a component of one of the triplet states correlating with the ,D + 2Satomic products. The possible assignments of other low-lying electronic states are discussed. It is proposed that the occurrence of 2D Cu atomic emission following excitation of molecular Cu2is due to energy transfer from Cu, a3Zu+ to a nearby Cu atom, rather than to a dissociative process leading to electronically excited atoms as recently proposed. Registry No. Cu,, 12190-70-4; Ne, 7440-01-9. (14) V. E. Bondybey and C. Albiston, unpublished data.
