The Journal of
Physical Chemistry
0 Copyright 1993 by the American Chemical Society
VOLUME 97, NUMBER 37, SEPTEMBER 16, 1993
LETTERS Raman Spectra of Mass-Selected Dihafnium in Argon Matrices Zhendong Hu, Jian-Guo Dong, John R. Lombardi, and D. M. Lindsay' Department of Chemistry and Center for Analysis of Structures and Interfaces (CASZ), The City College of New York (CCNY), New York, New York 10031 Received: June 23. 1993"
The absorption and Raman spectra of hafnium dimers in an argon matrix have been measured. Four weak dimer absorption bands were found between 300 and 700 nm. Resonance Raman spectra (obtained by exciting into a band centered at 620 nm) give w:' = 176.2 (26) cm-l with w a x ,< 1 cm-l. Our results represent the first experimental or theoretical study of the dihafnium molecule.
1. Introduction
The seminal review article' by Morse in 1986 provided not only a summary of experimental and theoretical results for transition metal dimers but has also served as a guide for interpreting these data. Morse's article also underscored (perhaps unintentionally) how little was known about many of these species. Thus, in the case of dimer vibrational frequencies (of particular interest here), experimental data were reported for all but one of the 3d metals, approximately half of the 4d metals, but the only measured frequency for a 5d metal dimer was that for gold.z Subsequent photodetachment (Re2 and Ptz 4, and matrix Raman (Taz,s W2,6 and Rez ') experiments have considerably expanded our knowledge of the third row transition metal dimers. In this paper, we present Raman spectra for dihafnium in argon and so complete (apart from La2) the measurement of vibrational frequencies for the early (groups IIIB-VIIB) 5d metal dimers. To our knowledge, there are no other experimental or theoretical results on dihafnium. 2. Experimental Section
The CCNY cluster deposition source has been described previously.8-9 Clusters were produced by sputtering a hafnium *Abstract published in Advance ACS Abstracts, September 1, 1993.
target (H. Cross, 99.7%) with argon ions, typically 15 mA at 25 keV. It is of interest to note that secondary ( 2 O ) ion currents were significantly enhanced when the sputtering target was composed of several thin sheets of metal. Thus, in the case of Hf, we found that sixteen 0.125-mm sheets gave approximately three times the 2O ion current as two pieces each of thickness 1 mm. Similar results were found for vanadium: a sandwich of 0.125-mm sheets gave 5 times the 2O ion current as a single 2-mm target. The sputtered products consist of a mixture of cluster sizes, both neutral and ionic. Positive cluster ions were extracted with a modified Colutron Model 200-B lens system, mass-selected with a Wien filter (Colutron 600-B, followed by a drift space and aperture) and then bent by loo to separate out the neutral species. Hafnium dimer (or atomic) ions were then deposited with the matrix gas (argon) and excess electrons on a 14K polished aluminum substrate. The ions were simultaneously slowed to about 10 eV by a surrounding "Faraday cage". Ion currents under soft landing conditions could be measured on a Faraday plate in the deposition region and were typically 20 nA for the dimer (but 90 and 15 nA for the atom and trimer, respectively). In contrast to many of our earlier measurements, we used an aluminum substrate (held at the same potential as the Faraday cage) in order to avoid interference from the CaFz Raman line10 at 330 cm-1. Matrices were grown at about 2-3 pfh with an
0022-365419312097-9263%04.00/0 0 1993 American Chemical Society
9264 The Journal of Physical Chemistry, Vol. 97, No. 37, 1993
Letters
TABLE I: Experimental Force Constants (mdyn/A) for Transition Metal Dimer Ground States 3B 4B 5B 6B 7B 8B 8B 8B SCQ Tib P C# Mnk F@ Co“ NiO 0.76
2.35
Y
ZF
4.33 Nbf 4.84
2.51
HP
La
Tug
1.63
4.80
3.51 Mol 6.43
0.094 Tc
w
Re‘
6.14
6.66
1.48 Ru
os
1.51 Rh Ir
1.36
1B
1.38
1.33 AgJ 1.18
Ptq
Aut
Pdp
2.66
2B ZnU 0.013
CU‘
Cdo 0.0 17 HgW 0.020
2.12
Using w< = 238.9 cm-I from ref 14. Using w< = 407.9 cm-1 from ref 15. e Using wc)’= 305.7(35) cm-l from ref 16. This work. *Using w:’ = 536.9(11) cm-I from ref 8; see also ref 15. fusing w< = 420.5(5) cm-l from ref 9; see also ref 17. 8 Using w:’ = 300.2(12) cm-1 from ref 5. Using w< = 479(2) cm-I from ref 18. Using w;’ = 477.1 cm-I from ref 19.1 Using wc)’= 336.8(7) cm-I from ref 6. Using ’0: = 76.4 cm-I from ref 20. I Using wc)’ = 348.4(6) cm-I from ref 7. Using wc)’= 299.5 cm-I from ref 21. Using @. =I)294.8(28) cm-I from ref 22. 0 Using wc)’= 280(20) cm-1 from ref 4. P Using w< = 210( 10) cm-I from ref 23. 4 Using w< = 215( 15) cm-I from ref 4. Using w< = 266.4 cm-I from ref 24. Using we,’= 192.4 cm-I from ref 2. Usinn w.” = 190.9 cm-I from ref 2. ” Using - wrl’- = 25.7(2) cm-1 from ref 25. Using w f l = 22.9(2) cm-I from ref 26. Using wc)’ = 18.5(5) cm-I from rif 27.
*
J
I
I
X e x = 609.6nm w
-I
-
\
0
200
400
600
800
(cm-’) Figure 1. Raman spectrum of Hfz in an argon matrix. The dimer content is 90 nA.h.13 R A M A N SHIFT
argoxmetal ratio of 20 000: 1. By comparing the intensities of known11 atomic excitation features in a dimer deposition with those obtained from depositions of the atom under similar conditions, the dimer fragmentation is estimated to be approximately 5%.
3. Results and Discussion The absorption (actually “scattering dep1etion”)lZ spectrum of dihafnium in argon consists of about four broad, relatively featureless transitions centered at approximately 350,450,540, and 620 nm. Resonance Raman spectra were obtained by using a dye laser (Coherent Radiation, Model 599) and Rhodamine 6G pumped by a Spectra Physics Model 2045 argon ion laser. Raman spectra were observed throughout the region 590-620 nm but not upon excitation at wavelengths further to the blue. Further experimentaldetails for our Raman system can be found el~ewhere.~.~ A typical Raman spectrum (7-cm-I resolution) is shown in Figure 1. No more than four Stokes transitions (labeled 0’’ = 0 u”in Figure 1) could be positively assigned, and these were often superimposed on a weak fluorescence background. The average Stokes shifts found for 22 different excitation wavelengths are (1 standard deviation in parentheses): 175.7 (13), 352.5 (20), 525.5 (lo), and 704.8 (17) cm-1 for u ” = 1-4, respectively. The relatively large errors and the limited set of Stokes transitions observed preclude an accurate determination of the dimer anharmonicity, but we estimate og;‘ < 1 cm-1. Averaging the values of AG(u”+~/z)from the Stokes shift data gives w:’ = 176.2 f 2.6 cm-1. Table I (CCNY results in italics) summarizes all known groundstate force constant data (mdyne/A) for the homonuclear transition metal diatomic molecules. Aside from the (mildly) radioactive element technetium, dimer force constants have now been measured for all of the early transition metals. The late (groups VIIIB-IB) transition metal dimers are less well characterized. Force constants have not yet been reported for Ruz, Os?,Rhz, and IrZ, and the vibrational frequencies for Coz, Niz, and Pdz are known only to within 5-10%. Even with these shortcomings, several trends may be discerned. In general, the
-
force constants (and concomitantly bond strengths) for the early transition metal dimers are much larger than those of the later elements. This effect (noted by others)’ arises froma combination of the greater d-orbital contraction (smaller overlap) in the heavier elementsof a particular period and the natural tendency for more antibonding orbitals to be filled in dimers composed of atoms whose d-orbitals are more than half-filled. For those elements with a half-filled or filled d-shell (Mnz and thegroup IIB metals), the bonding is best described as van der.Waals in nature. An interesting exception is dirhenium which, despite a ground-state configuration of 5d56sZfor the atom, has the largest force constant yet reported for any transition metal dimer. Finally, we note the confirmation of a trend discussed by us earlier.5.6 For groups IVB, VB, and VIB the dimer force constant increases in passing from the 3d to the 4d metal, but then decreases for the third row congener despite the relatively greater diffusiveness (i.e. better overlap) of the atomic Sd orbitals. As elaborated on in ref 5, this phenomenon apparently arises from the large s-d promotion (“atom preparation”) energies of Hf, Ta, and W, which in turn may be traced to the fact that thelanthanideelements immediately precede Hf in the periodic table. Acknowledgment. This work was supported by the National Science Foundation under Cooperative Agreement No. RII8802964andGrant No. CHE-9112897and bythecity University of New York PSC-BHE Faculty Research Award Program. References and Notes (1) Morae, M. Chem. Reu. 1986, 86, 1049. (2) Huber, K. P.; Herzberg, G. Constants ofDiatomic Molecules; Van Nostrand New York, 1979. (3) The gas-phase frequency ia 340 15 cm-l for Rq. See: h p o l d , D. G.; Miller, T. M.; Lineberger, W . C. J. Am. Chem. Soc. 1986,108, 178. (4) Ho, J. Ph.D. Thesis, University of Colorado, Boulder, 1991. ( 5 ) Hu, 2.;Shen, B.;Lombardi, JI R.; Lindsay, D. M. J. Chem. Phys. 1992, 96, 8757. (6) Hu, 2.; Dong, J.-G.; Lombardi, J. R.; Lindsay, D. M. J. Chem.Phys. 1992, 97, 8811. (7) Hu,.Z.; Dong, J.-G.; Lombardi, J. R.; Lindsay, D. M.; Harbich, W. Manuscnpt in preparation. (8) Hu, 2.; Shen, E.;Zhou, Q.;Deosaran, S.;Lombardi, J. R.; Lindsay, D. M.; Harbich, W. J. Chem. Phys. 1991,95, 2206. (9) Hu, 2.; Shen, B.;Zhou,Q.; Deosaran, S.;Lombardi, J. R.; Lindsay, D. M. Proc. SPIE 1992,1599,65. (10) Gee, A. R.; O’Shea, D. C.; Cummina, H. 2.SolidStute Commun. 1965, 4, 43.
(11) Klotzbiicher, W . E.; Ozin, G. A. Inorg. Chem. 1980, 19, 3767. (12) The technique of ‘scattering depletion spectroscopy”is deacrihcd in ref 9. (! 3) Product of current and deposition time in hours; 1 nA.h = 2.25 X 1013 particles. (14) Moskovits, M.; DiLella, D. P.; Limm, W. J. Chem. Phys. 1984,80, 626. (1 5 ) .Cod, C.; Fouaaaier, M.; Mejean, T.; Tranquille, M.; DiLella, D. P.; Moskovlts, M. J . Chem. Phys. 1980, 73, 6076.
Letters (16) Hu,Z.; Zhou, Q.;Lombardi, J. R.; Lindsay, D. M. In Physics on$ Chemistry of Finite Systems: from Clusrers to Crystals; Jena, P., Khanna, S. N., Rao, B. K., Us.; Kluwer Academic: Dordrecht, The Netherlands, 1992; p 969. (17) Moskovits, M.; Limm, W. Ultramicroscopy 1986, 20, 83. (18) Casey, S. P.; Villalta, P. W.; Bengali, A. A,; Cheng, C.-L.; Dick, J. P.; Fenn, P. T.; Leopold, D. G. J . Am. Chem. Soc. 1991, 113,6688. (19) Efremov, Y.M.; Samoilova, A. N.; Kozhukhovsky, V. B.; Gurvich, L. V. J. Mol. Spectrosc. 1978, 73, 430. (20) Bier, K. D.; Haslett, T. L.; Kirkwood, A. D.; Moskovits, M. J . Chem. Phys. 1988,89, 6. (21) Haslett, T. L.; Moskovits, M. J . Mol. Spectrosc. 1989, 135, 259.
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9265 (22) Hu, Z.; Dong, J.-G.; Lombardi, J. R.; Lindsay, D. M. Manuscript in preparation. The gas-phase frequency is 280 & 20 cm-1. Set: Leopold, D. G.; Lineberger, W . C. J. Chem. Phys. 1986,85, 51. (23) Ho,J.; Ervin, K. M.; Polak, M. L.; Gilles, M. K.; Lineberger, W. C. J . Chem. Phys. 1991, 95,4845. (24) Rohlfing, E. A.; Valentini, J. J. J. Chem. Phys. 1986, 84, 6560. (25) Czajkowski, M.; Bobkowski, R.; Krause, L. Phys. Reo. 1990, A41,
277. (26) Czajkowski, M.; Bobkowski, R.; Krause, L. Phys. Reu. 1989, A40, 4338. (27) van Zee, R. D.; Blankespoor, S.C.; Zwier, T. S . J . Chem. Phys. 19%8, 88, 4650.
