J. P h p . Chem. 1983,87,3370-3372
3370
The particle sizes for the colloidal samples were on the order of 750 A in radius whereas the sizes for the dispersion averaged 10000 A in radius. Several other procedures of colloid preparation were utilized, among them, precipitation in the presence of a perfluoro surfactant and carboxymethylcellulose. All of these colloids exhibited similar properties to CdS colloids prepared at room temperature.
Conclusions The luminescence properties, as well as the X-ray data, exhibit significant differences between the CdS crystalline powder dispersion and colloidal samples indicating that the colloids are structurally modified. Numerous colloids prepared by precipitation from various cadmium salts and several sulfides in the presence and absence of stabilizers do not show “pure” crystalline behavior. Intercalation of ions (e.g., the counterion of the cadmium salt, Cd2+and/or Ss) into the CdS structure produces defect sites which lead to red shifts in the emission maxima and longer-lived luminescence.
The efficiency of electron transfer from the irradiated CdS samples is dependent on the particle sizes, Le., surface area/g of CdS. Preliminary results indicate that the electron transfer efficiency increases as the surface area is increased. Further research is necessary in order to understand the correlation between bulk structure and surface properties of colloidal particles with the observed photochemical behavior. Particle size distribution, excitation spectra, adsorption isotherms of various electron acceptors, and surface analysis of the CdS particles, as well as other related work, are presently in progress.
Acknowledgment. J. K. thanks the donors of the Petroleum Research Foundation, administered by the American Chemical Society, for support of this work. B.H.M. acknowledges partial support from the National Science Foundation (CHE 82-01226) and the Army Research Office (DAA 6 29-80-K-0007). Registry No. CdS, 1306-23-6; Na2S, 1313-82-2; CdCl,, 10108-64-2;SDS, 2386-53-0.
Reactive Infinite Order Sudden Rate Constants for F
+ H2(v=OJ=O)
-
H
+ HF(v’)
J. Jelllnek,+ Department of Chemlcal Physics. Weizmann Instltute of Sciences, Rehovot, Israel 76100
M. Baer, Applied Mathemetlcs, Soreq Nuclear Research Center, Yavne, Israel 70600
and D. J. Kourl’ Department of Chemistry and D e p a m n t of Physlcs, Unlverslty of Houston-Unlverslty (Recelvd: June 10, 1983)
Park, Houston, Texas 77004
-
State-selected and total reactive rate constants for the F + H2(u=0,j=O) HF(u=1,2,3) + H,reaction have been recalculated from the most recent reactive infinite order sudden (RIOS) approximation cross sections obtained with the l-average choice of 1. Comparisons with classical trajectory and experimental results are made. The RIOS results, although larger than any others obtained with the Muckerman 5 (M5) potential, are still smaller than any of the experimental results.
-
Recently, we presented results of calculations of stateselected rate constants for the F + H2(u=Oj=O) HF(u=1,2,3) + H reaction using cross sections obtained with the 1-average version of the reactive infinite order sudden (RIOS) appr0ximation.l However, subsequently, it was found that a minor error had caused the cross sections we used to be somewhat smaller than in fact the l-average RIOS predicts., The state-selected rate constants obtained with the earlier values of the cross sections were somewhat low compared to classical trajectory results. In addition, an estimate of the thermally averated rate constant was well below experimental results. The present paper contains the F + H2 rate constants obtained with the correct l-average RIOS cross sections. Chaim Weizmann Fellow. Present address: Department of Chemistry and James Franck Institute, University of Chicago, Chicago, IL 60637.
The theoretical expression for the rate constant for the reaction F + H2(uo,j,) HF(u) + H (1) -+
is given by
(2)
In this expression, f,( T ) is the fraction of F + H2 collisions (1) V. Khare, D. J. Kouri, J. Jellinek, and M. Baer in ‘Potential Energy Surfaces and Dynamica Calculations”,D. G. Truhlar, Plenum, New York, 1981, pp 475-93. (2) The corrected integral and differential cross sections were published in M. Baer, J. Jellinek, and D. J. Kouri, J. Chem. Phys., 78,2962, (1983).
0022~3654/83/208~-337~~0~.50/0 0 1983 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983
3371
I
2
3
4
-
lO'/T
Flgue 1. The rate constant for reactkn of F w b Hdv=Oj=O) to yield H,(v,, The dashed curve is the 00 2 rate obtained with the earlier incorrect cross sections (ref 1). The units are cm3/(molecuie
E,,).
10-la 2 a 4 1031~ Flgwe 2. The rate constant for reaction of F with H&=OJ=O) to yleM
E,,).
H,E:,,, The soiM curve is the classical trajectory result of Muckerman (ref 4). Units are those in Figure 1.
8).
taking place on the reactive Born-Oppenheimer s u ~ f a c e , ~ p is the reduced mass for F colliding with H2, IzB is the Boltzmann constant, e is the initial relative translational energy, and a(u&,+u~t) is the integral reactive cross section for the reaction, eq 1, at initial relative kinetic energy e. These cross sections have been obtained with the RIOS method on the Muckerman 5 potential surface4and have been reported previously.2 The integration in eq 2 has been evaluated numerically. In the case of thermally averaged total rate constants for comparison with experiment, we have summed eq 2 over the final HF vibrational state and assumed the rate to be independent of initial rotational state. It has been argued earlier that this leads to an estimate of the thermal rate constant which is an upper limit to what one can expect to obtain with the assumed potential surface.' In Figure 1 we present the present RIOS results and compare with the earlier (incorrect) ui = 0, ji = 0 uf = 2, summed over j rate constant. It is seen that there is a substantial difference with the present rates being larger by about a factor of 1.5. Next, in Figure 2 we compare the present 1-averaged RIOS final state summed rate constant with the classical trajectory (CT) results. The initial state in both cases is ui = 0, ji = 0. It is seen that the new RIOS results are larger than the CT ones with the largest difference occurring at low temperature (as would be expected). The earlier incorrect RIOS results were smaller than the CT ones. Finally, in Figure 3 we give the rate constant thermally averaged over initial states and summed over final states. In the case of the RIOS results, we have made the assumption that the rate constant k,,,, (7')is independent of the initial rotor level ji. Thus, in hgure 3, the RIOS k ( T ) is identical with k,JT) in Figure 2. This is not true of the CT results. The CT k J T ) do depend on ji and, in fact, the rate constants for higher ji decrease as ji is
-
(3)D.G.Truhlar, J. Chem. Phys., 56,3189(1972);61,44O(E)(1974); J. T.Muckerman and M. D. Newton, ibid., 56, 3191 (1972). (4) J. T. Muckerman in 'Theoretical Chemistry: Advances and Perspectives",D. Henderson and H. Eyring, Vol. 6A, Academic Press, New York, 1981,pp 1-77.
h 10-l1
c4
c
Y
Y
10-l2I 2
I
I
3
4
10 3 / ~
Figure 3. The rate constant averaged over initial states and summed over final states. The experimental resuit is that of Wurzberg and Houston, ref 5. All other experimental results are higher than these. The variational transition state results (with a Marcus-Coitrin tunneling path correctlon) are from ref 6. Units are those in Figure 1.
increased. We believe the same may be true for exact quantal rate constants. In addition, the RIOS cross sections are almost certainly too large in the threshold energy regime. Since the rate constant in the 200-500 K range is strongly influenced by the threshold energy region, we expect that the RIOS result for k ( T ) is an upper limit so far as what can be obtained with the M5 surface. The experimental results are those of Wurzberg and H O U S ~ O ~ ~ and are the lowest of all available experiments. It is seen (5) E. Wurzberg and P. L. Houston, J. Chem. Phys. 72,4811(1980).
3372
J. Phys. Chem. 1983, 87. 3372-3374
that they are still significantly larger than even the RIOS results. It's difficult to see how one may obtain larger quantal cross sections using the M5 surface. This is also supported by the ICVT/MCPSAG variational transition state with tunneling correction results of Garrett et a1.6 Their studies for collinear reactions indicated differences with exact quantum results which are, on average, smaller (6) B. C. Garrett, D. G. Trublar, R. S. Grev, and A. W. Magnuson, J. Phys. Chem., 84,1730 (1980).
than the discrepancy with experiment for this F + H2 system. Thus, it appears that there is still a substantial discrepancy between experiment and theory for this system.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Registry No. Fluorine atom,14762-94-8;hydrogen, 1333-74-0.
I nfrared-Optical Double Resonance in Organic Solids Jagdish Prasad and Paras N. Prasad' Department of Chemistry, State Unlverslfy of New York at Buffalo, Buffalo, New York 14214 (Received; June 2, 1983)
We report the first observation of IR-optical double resonance in an organic solid. This double resonance is observed for the singletriplet transition of naphthalene in the 1:2 weak crystalline complex naphthalene: 1,4-diiodotetrafluorobenzene.The 10P14 line of COzlaser excites the -950-cm-' ring-breathingungerade mode from where the 4880-A line of an argon ion laser connecta to the T1level to produce naphthalene phosphorescence. In order to separate the thermal effects from a true IR-optical double resonance, the local temperature is monitored by the anti-Stokes Raman intensities of various vibrations in the same experimental arrangement. Our result indicates that the observed effect is due to a nonthermal IR-optical double resonance. The study of anti-Stokes Raman intensities of various vibrations also reveals that the IR-optical double resonance involves an ungerade vibration and a Franck-Condon-type vibronic interaction.
Introduction IR-optical double resonance spectroscopy, with a submillimeter wave laser, provides a novel approach for the investigation of the vibronic interactions, the dynamics of phonon-assisted energy transfer, and the vibrational relaxations in organic solids. The use of a submillimeter wave laser offers the prospect of investigating the effect of selective excitation of phonons and of intramolecular vibrations on the dynamics of electronic excitation and on vibrational relaxations in the ground and the excited electronic states. By using the technique of IR-optical double resonance, one can conveniently study the Herzberg-Teller-type interaction between an ungerade vibration and electronic excitation in a centrosymmetric crystal. The interaction of millimeter and infrared waves with molecular crystals is an area which has remained virtually unexplored even though laser sources in UV and visible regions are now widely used for various spectroscopic applications. The main reason is the numerous difficulties in working in this spectral region. One of the major problems is the lack of availability of fast and sensitive detectors for this region. But then precisely for this reason, IR-optical double resonance studies can be quite useful. Bloembergen' has proposed an IR quantum counter based on IR-optical double resonance which generates a signal in the visibleUV region by the up conversion of the IR photon (due to double resonance). Most studies of this type of quantum counter, however, have remained limited to transitionmetal ions.2 In this paper, we report, to the best of our knowledge, the first observation of IR-optical double resonance in an organic solid.3 Specifically, the crystalline complex (1)N.Bloembergen,Phys. Reu. Lett., 2,84, (1969). H.Lengfellner and K. F. Renk, IEEE J . Quantum Electron., QE13,421 (1977). (2)
0022-365418312087-3372$0 1.5010
naphthalene-1,4-diiodotetrafluorobenzeneis investigated. The formation of this complex was reported, for the first time, by this research group.' It is a weak crystalline complex in which no low-lying charge-transfer band of either singlet or triplet multiplicity was found. The lowest lying triplet is that of naphthalene and it is enhanced nearly two orders of magnitude in ita strength because of the external heavy-atom effect? The energy level scheme of the double resonance is shown in Figure 1. The triplet level being at 21 164 cm-' is not accessible from the zero point level of So by the 48804 line (20492 cm-l) of the argon ion laser. However, if the 950-cm-' ungerade vibration of naphthalene is populated by an appropriate line of the C02laser, then the 4880-A line can be absorbed from this vibrational level to T1 to produce phosphorescence. We have studied this double resonance and have successfully separated the nonthermal effect. Possible routes for this double resonance are also discussed. Experimental Section Zone-refined naphthalene and 1,4-diiodotetrafluorobenzene were mixed in stoichiometric amounts and single crystals were grown from the melt as described in detail earlier.' A single crystal was mounted in a Janis Supervaritemp dewar. Experiments were performed a t several
-
(3) IR-optical double resonance has previously been reported for So SItransition of nile blue A oxazone in CCl, solution by using picose-
cond laser pulses. See A. Seilmeier, W. Kaiser, A. Lauberau, and S. F. Fischer, Chem. Phyu. Lett., 58, 225 (1978). In their experimenta they generated both IR and visible pulses via three-photon parametric processes using LiNbOs crystals. Our system is in the crystalline state, and it consists of a centrosymmetricmolecule (naphthalene),whereas their choice is a large noncentroeymmetricmolecule. "he IR frequency chosen by them is around --3ooo cm-I where the vibrational densit of states is high. In contrast, we focus our attention on - 1 W c m - Pregion of a relatively smaller molecule (naphthalene), for which the vibrational density of s t a h at this frequency is not so large. (4)K. S. Law and P. N. Prasad, J. Chem. Phys., 77, 1107 (1982).
0 1983 American Chemical Society
