J . Phys. Chem. 1988, 92, 318-324
318
experimental data and the simplicity of the TIP4P potential. The experimental densities of pressurized ice, estimated from the compression curve reported in ref 32, are compared with the N M D results in Figure lb. Experimentally, when ice is compressed, the density increases approximately linearly until about 10 kbar where it undergoes a pseudo-first-order transition into a high-density amorphous phase.32 As evident from the figure, a similar compression curve was obtained from the MD calculations. Below 10 kbar the ice sample compresses elastically, resembling the experimental trend. The bulk modulus of ice Ih at zero pressure, estimated from the initial slope of the density vs pressure curve, is 80 kbar, which compares favorably with the experimental value of 89 kbar.33 However, the calculated densities are systematically larger than experiment by about 3.0%. Between 10 and 17 kbar the ice sample transforms gradually into a higher density amorphous form. The transformation is less abrupt than observed experimentally. On completion of the “transition”, the calculated density of the high-density amorphous phase is 1.41 g cm-) whereas the experimental value is 1.31 g cm13.32 The “sluggish” transition found in the simulations is probably due to the small sample size and/or the length of the M D trajectories. amorphous solid-phase The mechanism of this ordered transformation is discussed in detail e l s e ~ h e r e . ~ ~
-
(32) Mishima, 0.;Calvert, L. D.; Whalley, E. Nature (London)1984, 310, 393. (33) Gammon, P. H.; Kiefte, H.; Clouter, M. J. J . Phys. Chem. 1983, 87, 4025.
Conclusion In summary, we have shown that the TIP4P intermolecular potential for water provides a reasonable description of the structure of liquid water and ice under moderate external pressure. Below 10 kbar, the calculated density for liquid water agrees reasonably well with experimental data. Moreover, the agreement between the calculated and measured neutron radial distribution functions of water at about 10 kbar demonstrates the reliability of the predicted liquid structure. Although the densities were amorphous transition overestimated slightly and the ordered region is broader than observed, the TIP4P potential also reproduces correctly the effect of pressure on crystalline ice. The systematic offset in the calculated solid density is consistent with a previous NMD ~ p t i m i z a t i o nof~ the ~ structure of proton-ordered ice IX where the TIP4P potential model also gave a slightly higher density. The results presented here indicate that the TIP4P effective, pairwise additive, intermolecular potential has a surprisingly wide range of utility for investigating the properties of water. Our calculations complement recent work on the nature of supercooled water which was carried out with this potential.35
-
Acknowledgment. We express our appreciation to Dr. D. D. Klug for many helpful discussions. Registry No. H 2 0 , 7732-18-5. (34) Impey, R. W.; Klein, M. L.; Tse, J. S. J. Chem. Phys. 1984,81,6406. (35) Speedy, R. J.; Madura, J. D.; Jorgensen, W. L. J . Phys. Chem. 1987, 91, 909.
Optical Spectra of Matrix-Isolated Palladium Atoms. 1. The Absorption and Emission Spectra and the Decay Kinetics of the 3D3Metastable State of Pd in Ar Matrices Geoffrey A. Ozin* and Jaime Garcia-Prieto Lash Miller Chemical Laboratories, Chemistry Department, University of Toronto, Toronto, Ontario, Canada M5S I A1 (Received: May 22, 1987)
Using the principle of optical-optical double resonance spectroscopy, the 4d95s’, PD3 metastable state of Pd atoms isolated in an Ar matrix at 12 K has been identified. It is shown that the optical absorption spectra from this state correspond to those previously reported for “transient states” of Pd atoms isolated in Ne and Kr matrices at 5.5 K. On this basis a complete assignment of these spectra is presented for the first time. With this information it has also been possible to assign in a consistent manner the narrow line width emission spectra which result from photoexcitation of Pd atoms in the 200-260-nm absorption region. An explanation of the dynamic effects observed in these spectra is also offered. A lifetime of 2.2 iC 0.2 min for the 3D3metastable state has been determined. As a direct consequence of these discoveries, several conclusions have been drawn. First, the absorption spectra of Pd atoms isolated in Ar matrices consist of the superposition of two spectra originating from the AJ (AJ = 0 iC 1,O .ry~,0) allowed transitions from the gas-phase ‘So ground state and the 3D3metastable state. Second, there are at least two major Pd atom trapping sites which display distinct excited electronic state relaxation behavior. Third, almost all of the observed radiative electronic transitions are allowed under the AJ selection rule and moreover show a good correlation with the gas-phase Pd atomic resonance spectra. Finally the optical spectra of Pd atoms isolated in Ar and Ne matrices show more similarities than previously thought.
Introduction A knowledge of matrix perturbation effects on the ground and excited electronic states of a metal atom isolated in a solid matrix is a fundamental prerequisite for understanding, controlling, and predicting reaction pathway(s) with other coisolated atoms or molecules. In the past 5 years the understanding of these effects has been greatly enhanced by the application of sophisticated spectroscopic techniques in the matrix isolation field.’ An example (1) (a) Ozin, G . A.; Mitchell, S . A. Angew. Chem., Int. Ed. Engl. 1983, 22, 674. (b) Matrix Isolation Spectroscopy; Barnes, A. J., Orville-Thomas, W. J., Muller, A., Gaufres, R., Eds.; Reidel: Dordrecht, 1981.
0022-3654/88/2092-0318$01 S O / O
of this is the study of the electronic spectra of matrix-isolated Ni atoms by UPS? MCD,3 UV and vacuum-UV absorption? LIF,S and lifetime measurements.6 All these spectroscopic approches have unequivocally established that Ni atoms coexist in both 3F4 (2) Jacobi, K.; Schmeisser, D.; Kolb, D. M. Chem. Phys. Lett. 1980, 69, 113.
( 3 ) Barret, C. P.; Graham, R.G.; Grinter, R. Chem. Phys. 1984,86, 199. (4) Schroeder, W.; Grinter, R.; Schrittenlacher. W.; Rotqmund, H. H.; Kolb, D. M. J . Chem. Phys. 1985,82, 1623. (5) (a) Celluci, T. A.; Nixon, E.R. J . Chem. Phys. 1984,81, 1174. (b) Rasaneu, M.; Heimbrook, L. A,; Bondybey, V. E. J . Mol. Struct. 1987, 157, 129. (6) Celluci, T. A.; Nixon, E. R. J . Phys. Chem. 1985, 89, 1991
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 319
Optical Spectra of Matrix-Isolated Pd Atoms
Experimental Section and 3D3electronic states in Ar, Kr, and Xe matrices. A similar level of comprehension has been achieved in the appreciation of The experimental configuration and sample preparation method the matrix effects respansible for the reknowned three-fold splitting are essentially the same as those described previously.21-zz The of the P S resonance transition of matrix-isolated metal atoms Pd metal (99.99%, supplied by McKay Inc., NY) was resistively of the groups Ia,7 Ib,8 and IIb9 (groups 1, 11, and 12).46 From evaporated from Pd strips as well as Pd wire wound at the center these kinds of studies a clearer understanding of the nature of the of a Ta filament. Research grade Ar (99.99%) and N 2 (99.99%) interaction between the matrix and the isolated metal atoms in were supplied by Matheson of Canada. The rate of Pd atom their ground and excited states has been achieved and as a result deposition was continuously monitored by using a quartz crystal one is better able to resolve the effects of these interactions on microbalance built into the furnace-cryostat assembly.z3 Matrices the mechanisms involved in bimolecular reactions of metal atoms were deposited onto a NaCl optical window cooled to 12 K by in low-temperature means of an Air Products Displex closed-cyclehelium refrigerator, In spite of these impressive developments, the knowledge of with the ratio of metal to matrix gas larger than l/104. These matrix effects of the excited states of transition-metal atoms is conditions favor the isolation of atomic Pd. Typical vacuum still in its infancy. Substantial evidence indicates that efficient Torr. conditions prior to cryopumping were intersystem crossing occurs for matrix-isolated atoms thereby Some samples were annealed in the range of 12-40 K and some populating metastable states with relatively long l i f e t i m e ~ . ~ * ~ , ' ~ *were ' ~ irradiated by using monochromatic UV light for periods of Such observations raise the question about the role of metastable time longer than 1 h. Control experiments employing high constates on photoinduced metal atom matrix reactions, as well as centrations of Pd were conducted with ratios of metal to matrix their contribution to matrix-phase optical absorption spectra which gas lower than l/103. Impurity effects were investigated using can be modified by the UV-visible radiation used for their desamples containing about 1% of air in the Ar matrix gas source. tection. For example, it has been proposedll,'z that the metastable Absorption spectra were recorded on a Perkin-Elmer 330 3d94s2,2D state of matrix-isolated Cu atoms with a lifetime of spectrophotometer in the range 195-900 nm, whereas broad band millisecond^,^^ and an excess energy of about 50 kcal mol-' as a emission and excitation studies were performed on a Perkin-Elmer result of the nonradiative 2D 2Prelaxation, is partially or totally MFP-44B fluorescence spectrometer in the range 200-900 nm, responsible for the very efficient photoinduced reactions of Cu selecting both the excitation wavelength (Aex) and emission atoms with H2I8and O2.I2 wavelength (b,,,) with scanning monochromators. The excitation In the same vein, the discovery of the efficient population of beam was defocused by removing a lens in the sample comthe metastable 3P state of matrix-isolated Be atoms having a partment and the scattered light was minimized by the use of 'So lifetime of seconds and formed as a result of the 'P, cutoff filters. Control spectra were recorded for Ar matrices in photoexcitation from the light source of the UV-visible specthe absence of Pd atoms in order to monitor background scatter trometer employed in this studyI6 has signalled that some weak from the cell itself or from any contributions associated with absorption bands could be assigned to transitions originating from impurity emission. These effects proved to be small and in general the 3P metastable level in the triplet-triplet manifold.I6 comprised less than 1% of the observed emission signal. Excitation Studies of the absorption spectra originating from "transient spectra were obtained by observing the emission intensity at a given states" of Pd atoms isolated in Ar and KrzOand Ne19 have been A, and scanning the excitation wavelength, &. The fluorescence published. Even though their a b s ~ r p t i o n and ' ~ MCDzOspectra excitation and emission spectra were not corrected for instrumental have been reported, the electronic configuration of these metastable factors arising from the spectral intensity distribution of the 150-W states is still under discussion. In this context, a more complete Xe arc lamp and the response characteristics of the Hamamatsu assignment of the absorption spectra from the a5S metastable state R928 photomultiplier tube. The output of our Xe lamp behaves of Cr atoms (with a lifetime of about 6 s) isolated in Ar and Kr approximately linearly in the wavelength range 400 and 250 nm. matrices has been a c ~ o m p l i s h e d . ~ ~ The intensity near 250 nm is approximately 30% of that at 400 In this paper we present evidence that the UV-visible absorption nm, but below 250 nm it decays much more rapidly and at 225 spectra of Pd atoms isolated in Ar matrices have substantial nm it is about 15%. At 205 nm it is less than 2%. Estimates of contributions from triplet-triplet transitions as a result of the the relative detector response in the wavelength regions of direct photoinduced population of the 4d95s1,3D3metastable state. In importance in the present work are as follows: 200 (20%), 250 papers 2 and 3 in this series we will discuss details of the matrix (65%), 300 (85%), 400 (loo%), 550 (80%), 650 (60%), 750 (35%), effects responsible for the stabilization of this metastable state and 850 nm (15%). of atomic Pd; in part 4 we present its chemistry. The rate of decay of the 3D3 metastable state of Pd atoms isolated in an Ar matrix was estimated by Linschitz's methodz4 of monitoring the intensity of triplet-triplet absorption following (7) Rose, J.; Smith, D.; Williamson, B. E.; Schatz, P. N.; OBrien, M. C. excitation at 240 nm into the 3P10 'So transition. It is noteM. J . Phys. Chem. 1986, 90, 2608. worthy that a similar method has been used by Gruen and co(8) Weinert. C. M.: Forstmann, F.: Grinter, R.; Kolb, D. M. Chem. Phys. w o r k e r ~ 'in ~ their optical-optical double resonance (OODR) 1983,' 80, 95. experiments designed to estimate the lifetime of "trap levels" of (9) Mowery, R. L.; Miller, J. C.; Krausz, R. E.; Schatz, P. N.; Jacobs, S.; Cr atoms isolated in Ar and Kr matrices. In the present study, Andrews, L. J . Chem. Phys. 1979, 70, 3920. (10) McCaffrey, J. G. Ph.D. Thesis, University of Toronto, 1987. the decay kinetics of the 3D3metastable state was measured by (1 1) Garcia-Prieto, J.; Ruiz, M. E.; Poulain, E.; Ozin, G. A,; Novaro, 0. using the fluorescence spectrometer in the excitation mode. An J . Chem. Phvs. 1984. 81. 5920. external source of monochromatic light was used in order to (12) Ozil, G. A.;'Mitchell, S . A,; Garcia-Prieto, J. J. Am. Chem. SOC. populate this state and its decay, and after that the external 1983, 105, 6399. (13) Ozin, G. A.; Mitchell, S. A.; Mattar, S. A.; Garcia-Prieto, J. J. Phys. excitation light was mechanically cut off, followed by recording Chem.. 1983, 87, 4666. the time decay of the intensity of one of the absorption (excitation) (14) Parnis, J. M.; Mitchell, S . A,; Garcia-Prieto, J.; Ozin, G. A. J . Am. bands originating from this state. The external source of Chem. SOC.1985, 107, 8169. monochromatic light consisted of an Osram XBO 450-W Xe arc (15) Pellin, M. J.; Gruen, D. M.; Fisher, T.; Foosnaes, T. J. Chem. Phys. 1983, 79, 5871 and references therein. lamp in an Oriel Corp. lamp housing and coupled by a quartz (16) Brom, J. M.; Hewett, W. D.; Weltner, W. J . Chem. Phys. 1975, 62, focusing lens to a water-cooled cell. This was followed by an Oriel 3122. Model 7240 monochromator with a band width of 8 nm (fwhm), (17) Schrittenlacher, W.; Kolb, D. M. Ber. Bunsenges. Phys. Chem. 1984,
-
-
-
-
88, 492.
(18) Ozin, G. A.; Mitchell, S . A,; Garcia-Prieto, J. Angew. Chem. Suppl. 1982, 785.
(19) Schrittenlacher, W.; Rotermund, H . H.; Kolb, D. M. 1.Chem. Phys. 1985.83, 6145. (20) (a) Grinter, R.; Stern, D. R. J. Chem. Soc., Chem. Commun. 1982, 40. (b) Grinter, R.; Stern, D. R. J . Mol. Srrucr. 1982, 80, 147.
(21) Klotzbiicher, W.; Ozin, G. A. Inorg. Chem. 1976, 15, 292. (22) Ozin, G. A.; Mitchell, S. A.; McIntosh, D. F.; Mattar, S. A,; Garcia-Prieto, J. J . Phys. Chem. 1983, 87, 4651. (23) Moskovits, M.; Ozin, G . A. Appl. Spectrosc. 1972, 26, 487. (24) Linschitz, H.; Sarkanen, K. J . Am. Chem. SOC.1958, 80, 4826.
320 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
\
Ozin and Garcia-Prieto
I"
Pd IAr
PdlAr x3
aJ U c
\
I
i \
m
n
L
W
U
c
0 LA
W
U
n
cn
4
2!0 3
c
I
?OO
,
,
,
,
I
250
,
,
.
.
300
/
,
,
1
1
350
,
,
#
I
LOO
nm
Figure 1. Absorption spectra of Pd atoms isolated in an Ar matrix (Pd/Ar = 1/104) at 12 K. Spectral scanning is from long to short
wavelengths.
coupled to an Oriel Model 6451-1 quartz fiber optic arranged to direct the light to the matrix sample located in the observation compartment of the fluorescence spectrometer.
Results The absorption spectrum of Pd atoms isolated in an Ar matrix at 12 K is shown in Figure 1. Through a limitation of our UV-visible-near-IR absorption spectrometer, the scans were necessarily from long to short wavelengths. As expected this spectrum is virtually identical with that recently reported by Kolb and co-w~rkers'~ at 5.5 K and also with the data published earlier from our laboratory at 6-8 K.21925 With the exception of the two very weak features around 265 nm and 300 nm one finds that high ordinate expansion (X17) in the 260-900-nm region does not reveal any other detectable absorption bands. Annealing experiments at different temperatures show substantial changes in intensity as well as in the profile of the spectra. As reported previously, annealing in the 12-25 K temperature range causes changes in the 200-280-nm spectral range indicative of a redistribution of Pd trapping sites25and Pd agglomeration at higher temperature^.'^ In contrast, photolysis at different absorption regions for periods of 1 h does not induce any detectable changes in the spectra. The emission spectra of Pd atoms isolated in an Ar matrix resulting from excitation at different absorption wavelengths are shown in Figure 2. From these emission spectra one can distinguish two sets of bands, one consisting of several relatively narrow lines in the 300-600-nm region and the other consisting of only one broad band at 486 nm. The excitation spectra of the most prominent emission bands are shown in Figure 3. Note that through limitations of our fluorescence spectrometer the scans were always from short to long wavelengths. At first glance the spectra consist of two regions, one between 200 and 270 nm which corresponds very well with the absorption spectra of matrix-isolated Pd atoms and the second between 290 and 350 nm, which, with the exception of the very weak band around 300 nm, do not have similar detectable bands in the Pd atomic absorption spectra of Figure 1. It is also noticeable that the excitation spectra associated with the broad 486-nm emission band is the only one which does not show any excitation band(s) in the 290-350-nm spectral range. ( 2 5 ) Klotzbiicher, W. E.; Ozin, G . A. Inorg. Chem. 1980, 19, 3767.
x10
-
1
'
Lr 215
L' I
I
300
400
I
I
500
600
nm Figure 2. Emission spectra of Pd atoms isolated in an Ar matrix (Pd/Ar N 1/104) at 12 K, excited at the wavelengths indicated.
200
I
I
300
400
nm
Figure 3. Excitation spectra of Pd atoms isolated in an Ar matrix (Pd/Ar = 1/104) at 12 K, with the emission monochromator set at the wavelengths indicated. Spectral scanning is from short to long wavelengths.
A comparison of our excitation spectra with previous reports on the optical spectra of Pd atoms isolated in neat rare g a ~ e s ' ~ ~ ~ ~ and mixtures with N225-27reveals interesting similarities. Firstly,
Optical Spectra of Matrix-Isolated Pd Atoms
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 32 1
Pd IAr
aJ U
C
a
U v)
a, L 0 3 d
250
300
350 nm
LL
Figure 4. Excitation spectra of Pd atoms isolated in an Ar matrix (Pd/Ar 1/10') at 12 K, with the emission monochromator set at 526 nm.
=
Spectral scanning is from short to long wavelengths starting at (a) 290 nm; (b) 250 nm; (c) 200 nm; and (d) 290 nm. The sample was kept in the dark for 15 min between each scan. the MCD20 and UV-visible a b s ~ r p t i o n 'spectra ~ of Pd atoms isolated in Ar, Kr and N e matrices, respectively, reveal the existence of metastable states for this metal atom, and their respective MCD and UV absorption spectra show bands in the 290-350-nm 650 500 550 region. It is also noteworthy that, with an appropriate frequency nm shift, our bands in this spectral region fit very well with those Figure 5. Emission spectra of Pd atoms isolated in an Ar matrix (Pd/Ar observed in N e and Kr matrices in the above studies. Matrices = 1/104) at 12 K, excited at 340 nm: (a) after 30 min in the dark; (b) containing Nzalso show absorption bands in this r e g i ~ n . ~At~ - ~ ~ immediately after 240-nm photolysis for 5 min; (c) after 15 min in the dark. For comparison purposes the emission spectra excited by 215-nm low concentrations of N z (
