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J . Phys. Chem. 1990, 94. 2226-2229 Following illumination of the coadsorbed layer at 90 K, an intense photoemission peak, very probably due to CH3(a) perturbed by K, appeared at 9.2 eV, at somewhat higher energies than for a clean surface. Further photoemission peaks were registered at 6.5 and 4.7-5.0 eV (Figure 2). The former is assigned to a CH,(a) specie^,^ while the latter is mostly due to adsorbed C1 attached to K. The intensity of the 6.5 eV emission slightly increased between 136 and 150 K, and the peak disappeared at around 220 K. The 9.2-eV peak vanished when the adsorbed layer was heated to 325 K. The emission at 4.7-5.0 eV shifted to 5.2 eV at higher temperatures, and was eliminated above 600 K. With regard to this feature, we assume that another very unstable and/or reactive surface species, possibly C H or C, also contributes to the emission at 4.7-5.0 eV. This species can be formed in the complete decomposition of adsorbed CH,. To explain the influence of the irradiation on the reactivity of the adsorbed layer, several factors have been c o n ~ i d e r e d . ~ ~ ~ l l . l ~ We assume that the main effect in the present case is the photoexcitation of substrate electrons, which attach to an adsorbed CHJ1 molecule, to form a negatively charged species that dissociates more easily. This process, the excitation of substrate electrons, obviously occurs more favorably from metallic potassium on a Pd surface than from clean Pd, which explains the more dramatic effect of irradiation at monolayer K coverage. This explanation is in harmony with the observations that the bombardment of adsorbed methyl halides with electrons facilitates halogen-carbon bond rupture and subsequent secondary reactions on several surfaces.2,’3
This assignment is further supported by the results obtained for CH31 adsorption on a clean Pd( 100) surface.8 In this case the majority of the adsorbed CH31 dissociated even at 100 K, producing an intense photoemission peak at 8.2 eV. In harmony with the results of TD studies, the 8.2-eV peak was eliminated at 210 K. Without illumination, only the photoemission signals due to molecularly adsorbed CH$I (5.5, 8.2, and 9.5 eV attributed to 2e, 3a,, and le orbitals, r e s p e ~ t i v e l y ) ~were ~ ~ J observed, ~ which were eliminated at 189 K without the production of new photoemission signals. The above photoinduced features were not present after irradiation through a cutoff filter which removed all radiation below 280 nm. As mentioned in the Introduction, potassium adatoms exerted a marked influence on the behavior of adsorbed CH,CI, and the details of the surface reaction depended on the potassium coverage.’ At monolayer coverage, the degree of K-induced dissociation of CH3CI during heating of the coadsorbed layer was 37%: the decomposition products were methane, ethane, ethylene, hydrogen, and chloride. As a result of illumination at 90 K, the amount of CH3CI desorbed at monolayer coverage decreased by about 80% and the amount of methane formed was doubled. Surprisingly, the peak temperature of methane formation was shifted to lower temperature, from 293 to 247 K, and the high-temperature methane peak observed in postirradiation TPD for clean Pd was missing (Figure 1 B). (8) Kiss, J.; Rivisz, K.; Solymosi, F. To be submitted for publication. (9) Steinbach, F.; Kiss, J.; Krall, R. Surf. Sci. 157, 401, 1985. (IO) Zhou, X.-L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci. 1989, 219, 294.
( 1 1) Chuang, T.J . Surf. Sci. Rep. 1983, 3, 1. (12) Germer, T.A.; Ho, W. J . Chem. Phys. 1988, 89, 562. ( 1 3) Zhou, X-L.; White, J. M. To be submitted for publication.
A Potential Surface for Ar-OH(?Z) and Ar-OD(?Z): Fitting and Assigning Experimental Data with Rigorous Theory Joel M. Bowman,* Bela Gazdy, Pamela Schafer, and Michael C. Heaven Department of Chemistry, Emory University, Atlanta, Georgia 30322 (Received: November 21, 1989; In Final Form: January 24, 1990)
We report the results of a large-scale, iteration procedure to assign and fit experimental spectra of Ar-OH(?3) and APOD(~Z). The calculations employed a new multiparameter functional form for the global potential. The parameters were varied randomly, and converged vibrational energies were obtained for each “trial” potential. After recognizing an inverse isotope effect, the experimental vibrational/bending energy intervals are accurately reproduced for both Ar-OH and Ar-OD. A preliminary rotational analysis is also in excellent agreement with experiment.
adiabatic approximation with a modification of the MaitlandSmith (MS) potential6 (which is basically a rotated Lennard-Jones potential) to successfully “invert” experimental data to obtain potentials for Ar-HC1. H ~ t s o n recently ~-~ reported fits to experimental spectra of Ne-HCI and to several rare gas-HBr systems using a coupled channel method and a much more sophisticated version of the MS potential. Recently, Nesbitt et al.IO applied an interesting rotational RKR-like inversion method, based on the adiabatic approximation, to Ar-HF.
Introduction With the availability of efficient methods to perform “exact” calculations of vibrational energies of triatomic systems,’ it is becoming feasible to obtain potentials from experimental data for such systems. “Inversions” of experimental data have been reported based on efficient, approximate methods to obtain vibrational energies. LeRoy and c o - w o r k e r ~used ~ * ~ a small directproduct basis in calculations of vibration/rotation energies of rare gas-H2 (D2) complexes to fit experimental spectra. The potential they used was a spherically symmetric interaction plus a Pz-type anisotropy term in the angular variable and multiparameter functions for the radial variable. Hutson and H ~ w a r dused ~ , ~the
(4) Hutson, J. M.; Howard, B. J. Mol. Phys. 1981, 43, 493. (5) Hutson, J. M.; Howard, B. J. Mol. Phys. 1982, 45, 769, 791. (6) Maitland, G. C.; Smith, E. B. Chem. Phys. Lert. 1973, 22, 443. (7) Hutson, J. M. J . Chem. Phys. 1988, 89, 4550. (8) Hutson, J. M. J . Chem. Phys. 1989, 91, 4448. (9) Hutson, J. M. J . Chem. Phys. 1989, 91, 4455. (10) Nesbitt, D. J.; Child, M. S.;Clary, D. C. J . Chem. Phys. 1989, 90,
( I ) For a recent review, see: Bacic, Z.; Light, J. C. Annu. Reo. Phys. Chem. 1989, 40, 469. (2) LeRoy, R. J.; Carley, J. S.Ado. Chem. Phys. 1980, 42, 353. (3) LeRoy, R. J.; Hutson, J. M. J . Chem. Phys. 1987, 86, 837.
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1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2221
Letters An exact, multidimensional RKR-like procedure to invert potentials probably does not exist, except in very favorable, Le., nearly separable, cases. (In such cases the semiclassical self-consistent-field method has been proposed as a multidimensional RKR method.”-I3) Thus, in general, an “inversion” procedure based on an exact calculation of vibrational energies will require a flexible functional form for the potential and a search in multidimensional parameter space to obtain the optimum potential. In this Letter we present results of such a procedure for the Ar-OH(2Z) and Ar-OD(2B) systems. Fluorescence excitation spectra of Ar-OH@) have been reported by Lester and c o - ~ o r k e r s and ~ ~ -Fawzy ~ ~ and Heaven,I7J8 who also reported spectra for Ar-OD(2Z). Both groups reported and assigned energy intervals for vibrational overtone transitions of the van der Waals stretching mode, i.e., Ar vibrating against OH. Fawzy and Heaven also measured the vibrational progression in Ar-OD (with some ambiguity in the assignment). In addition, they reported ”U” bands in both Ar-OH and Ar-OD. Those bands were not assigned; however, they were thought to involve an excited bend. (The Lester group tentatively assigned those bands to higher order van der Waals clusters.) A complete simulation of the fluorescence spectrum requires information about the ground-electronic-state potential(s), the excited-state potential, and the electronic transition moment(s) coupling them. We are in the midst of such a simulation, and in this Letter we report one step in achieving that goal, namely, the semiempirical determination of the excited-state potential.
D, =
For the cut at y equal to zero a2is not twice a,; however, for the saddle point cut it is, giving the usual Morse potential. a2is also twice a1for the cut at x; however, in this case a,is a polynomial function of R , as discussed below. The functionsfly) and g(y) are switching functions which vary between 0 and 1 in the ranges 0 Iy Iyspand ysp5 y 5 T, respectively. In terms of the variable x , f l x ) and g ( x ) are given by the same expression, i.e.
(6
+
p
-
$5,
0 Ix I1
and &z(YTsp)
x=
Calculations There are two aspects to the calculations. One is the potential function, and the other is the calculation of vibrational energies and rotational constants. We discuss the potential first. The functional form of the potential is a slight modification of one given recently by us.19 In terms of the variables R , the distance of Ar to the center of mass of OH, and y, the angle between R and rOH(y equals zero corresponds to linear ArHO), the potential for rOH fixed at equilibrium is
D 2(1 - d a 2 )
-1
eMvip) - 1
for yspIy IT
In the present case a linear relationship between x and y was used for yspIy Ix. Finally, the total potential is
W C ~ O =Hv(R9.1) ,~) + VM(~OH)
-153. - (16) - .-17.Berry, M. T.; Brustein, M. R.; Lester, M. I. J . Chem. Phys. 1989,90,
where VM(roH)is a Morse potential for OH(2Z) based on experiment.20 In the iteration procedure to fit the experimental data, not all of the potential parameters just described were varied. We fixed Re for the zero-degree cut by using unpublished a b initio calculations of the Ar-OH(Q) potential.2’ Those also guided us in establishing sampling ranges for some of the other parameters. In addition, there was firm experimental evidence that only ArH O configurations, corresponding to 0 5 y Iysp,had substantial Franck-Condon factors with the linear ArHO in the electronic ground state. Thus, the parameters for V(R) for the x-cut were not varied; they were determined based on plots of the a b initio potential. Thus, the parameters varied were D, a,, azrfor the zero-degree cut,f,,(O), XI, ysp,and D, Re, al (a2= 2a1)for the saddle-point cut. The calculation of vibrational energies was done using a method, which has been described previously.22 The body-fixed Hamiltonian in mass-scaled scattering coordinates was diagonalized by using a compact, direct-product basis. Each component of the radial basis was a contraction of a harmonic basis, and the angular basis was a contraction of a Legendre polynominal basis. The contractions were done by diagonalizing one-dimensional, reference Hamiltonians. The potential used in the reference Hamiltonian for R was the radial cut of the full potential for the linear Ar-HO geometry, and with rOHfixed at 1.92 a. (its equilibrium position in the %state.) Thirty harmonic functions were used to obtain 17 basis functions for the final diagonalization. For the bending (y)mode, the reference potential was a cut through the potential with rOHfixed at 1.92 a. and R fixed at 5.8 ao. Twelve bending functions were used in the final diagonalization. The harmonic basis for the O H vibration was contracted by using the Morse potential VM(r0H) as the reference potential. Only the ground vibrational state was used in the variational calculation^.^^ This
5879. (17) Fawzy, W. M.; Heaven, M . C. J . Chem. Phys. 1988,89, 7030. (18) Fawzy, W. M.; Heaven, M. C. Electronic Spectroscopy of the A r O H and Ar-OD complexes. J . Chem. Phys., in press. (19) Bowman, J. M.; Schafer, P. A New Functional Form for Global Potentials of Floppy Molecules. J . Mol. Sfrucr., in press.
(20) Herzberg, G. Spectra of Diaromic Molecules; Van Nostrand Reinhold: New York, 1950; p 560. (21) Degli Esposti, A.; Werner, H.-J. To be published. (22) Gazdy, B.; Bowman, J. M. J . Chem. Phys. 1989, 91, 4615.
V R y ) = VO(R)[l-f(r)I + V S P ( R ) f ( 7 ) 0> Iy IYsp V(’(R9-i)= V s p ( R ) [ l g(y)l
+ V A N g(y),
Ysp
5 7 Ix
Vo,Vsp, and V, are radial cuts of the potential for y equal to zero, the saddle point value, and T,respectively. These radial potentials are all of the form
V ( R ) = D[e-@z((R-&) - ~~-uI(R-R’~)] The minimum of this potential is at Re, and R’e is given by = Re
2a1 + -1 In a2
a2
The dissociation energy (measured from the potential minimum) is given by ( 1 1) Roth, R. M.; Gerber, R. B.; Ratner, M. A. Phys. Rev. Lerr. 1984, 52, 1288. (12) Romanowski, H.; Gerber, R. B.; Ratner, M. A. J . Chem. Phys. 1988, 88, 6157. (13) Romanowski, H.; Ratner, M. A,; Gerber, R. B. Compur. Phys. Commun. (special issue on Molecular Vibrations, Bowman, J. M., Ed.) 1988, 51. 161. (14) Berry, M. T.; Brustein, M. R.; Adamo, J. R.; Lester, M. I . J . Phys. Chem. 1988, 92, 5551. (15) Berry, M. T.; Brustein, M. R.; Lester, M. I. Chem. Phys. Lett. 1988,
~
~~
2228
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
TABLE I: Comparison of Experimental and Calculated Energy Intervals (in cm-') for Ar-OH us experiment theory
Letters TABLE 111: Comparison of Experimental and Calculated Rotational Constants (in cm-') for Ar-OH and Ar-OD
Ar-OH
Ground Bend 1
133.8,' 134.1b 115.88 115.6 97.5, 97.5 79.6, 80.0 61.6, 61.6
2 3 4 5 6
2 3 4 5 6
experiment Ground Bend 0.153," 0.155b 0.144, 0.147 0.135, 0.137 0.125, 0.126 0.1 14
3 4 5
First Excited Bend 0.139' 0.119 0.107
L'
147.2 130.5 113.8 97.2 80.2 62.2
First Excited Bend 1 2 3 4
5
0 1 2 3 4
5
63.3' 37.5
126.1 107.3 87.3 63.7 36.8
exDeriment'
theorv
0.138 0.120 0.105
experiment
0
6
0.150 0.145 0.130 0.120
4 5
First Excited Bend 0.136" 0.125
0.138 0.125
5
'From
theorv
Ground Bend 0.145' 0.136 0.127 0.118
3 4
'From ref 18. bFrom ref 15.
L'.
0.155 0. I50 0.135 0.125 0.115
Ar-OD
Ground Bend to First Excited Bend 280.5 259.4 236.2 206.8' 209.7 172.7 176.2 131.1 132.8
TABLE 11: Comparison of Experimental and Calculated Energy Intervals (in cm-') for Ar-OD
theory
ref 18.
From ref 15.
TABLE I V Potential Parameters for Ar-OH(*Z)
Ground Bend I 2 3 4
5 6
105.1 88.5 72.7
149.3 133.6 118.2 103.0 87.7 72.0
First Excited Bend 1 2 3 4
5 6
0 1 2 3 4 5
74.5 52.4b
135.8 119.3 102.8 85.8 68.0 45.7
Ground Bend to First Excited Bend 196.3 182.8 168.5 153.1 121.6 135.9 107.6 116.2
'From ref 18. bLin, Y . ; Kulkarni, S. K.; Heaven, M. C . Unpublished results.
basis, of order 204, is well-suited to a description of the bending and excited stretching motion of the Ar-HO configuration. It was tested for convergence against a much larger basis and was found to be converged to less than or equal to several tenths of a wavenumber for all the states measured experimentally. The calculations for Ar-OH and Ar-OD were done in parallel on a two-processor TITAN computer. Vibrational energies and wave functions for 15 states for both systems took roughly 2.5-min elapsed time (with no other jobs running) per trial potential. Approximately 2500 trials were done, and the energy intervals were compared to experiment after batches of several hundred trials were done at one time. In order to make the comparison, a set of assignments had to be assumed. Initially, we made the (23) A one-vibration calculation is equivalent to a rigid-rotor calculation in which the term l/rOH2 in the angular kinetic energy operator is replaced by ( l / r & ) , where the average is over the ground-vibrational-state wave function. Strictly speaking, this is not the same as replacing ( I /roH2) by I / ( ~ O ~ ) ~ ,which is what is usually done in rigid-rotor calculations.
Re, a0 a , , ao-' a2*ao-' D,cm-l fxx gxx
0
1.3771
7T
5.290 1.1205 2.7040 1061.6 9.697
7.173 0.890 1.780 65.6 -1 .o 6.75
4.103 a
a 938.6 -7.1 1
A, = 1.3952
+
u a 2 = 2aI and 01, = 1.85 - 7 . 9 5 4 3 1 2 5 ~ ~ 13.9978125~~ 9 . 0 7 0 1 8 7 5 ~ ~ 2.0391875x5, where x = ( R - 3.0) for 3 5 R 5 10 ao, and a , = 0.85 for R 2 10 a. and 1.85 and R 5 3 ao.
+
reasonable assumption that the van der Waals stretch intervals for Ar-DO would be smaller than the corresponding ones for Ar-HO. In fact, that turned out to be an incorrect assignment, as we discuss next.
Results and Discussion The experimental and theoretical energy intervals for Ar-HO and Ar-DO are given in Tables I and 11, respectively. The energy interval is the difference in energy between the vibrational state u and u - 1. The first two sets of intervals are for the van der Waals stretch progression, us, in the ground and first excited bend, ub = 0 and 1, respectively, and the last set are the energy intervals between the ub = 1 and 0 for the indicated stretching states. As noted above, in order to secure agreement with experiment, a spectral assignment had to be made and then a potential obtained by the search procedure described earlier. After failing to obtain a good fit based on our initial, reasonable set of assignments (see above), we did succeed in making correct assignments and obtaining a quantitative fit to the experimental energy intervals. In the process we uncovered an inverse isotope effect. That is, the energy intervals in the stretch progression are larger for Ar-DO than for Ar-HO, and the differences are bigger for the excited bend than for the ground bend. (Note that the bend excitation exhibits a "normal" isotope effect.) Thus, neither of the two tentative assignments for the Ar-DO stretch progression in ref 18 was correct. Space does not permit a detailed explanation of the inverse isotope effect (one will be given elsewhere); however, we can say that it is due to substantial bend/stretch coupling which leads to a reduced binding of the effective Ar-HO radial potential
J . Phys. Chem. 1990, 94, 2229-2232
'3
s er
91
0
46
90
136
180
y (deg) Figure 1. Equipotential contour plot (in cm-l) of the semiempirically determined potential for Ar-OH(*X). Contour intervals are 100 cm-l. relative to Ar-DO. This is because Ar-HO executes wider angular motion than does Ar-DO. Rotational constants for each of the assigned vibrational/ bending bands were calculated by taking energy differences for different values of the total angular momentum quantum number, J , and its projection, K , on the body-fixed z axis. Preliminary calculations were done neglecting Coriolis coupling. This should be an excellent approximation because the bending energy difference between the ground and first excited bend states is roughly 1000 times larger than the rotation constants (see below). In addition, this approximation is fully supported by experiment and is also consistent with the experimental rotational analysis. For ground bend states, the so-called M e n d , K = 0, the rotation constants are equal to half the energy differences for states with J = 1, K = 0 and corresponding states with J = K = 0. For states with one quantum of bend, the II-bend, I q = 1 and the rotation
2229
constants are equal to 1/4 of the energy differences between levels with J = 2, K = 1 and those with J = 1, K = 1. The results of these calculations are compared with experiment in Table 111. Theory and experiment agree for each line (with one exception) within the experimental error of f0.005 cm-I. Finally, the potential energy surface is shown in Figure 1 and the values of the parameters are given in Table IV. As already noted, the calculated energies and rotation constants are not sensitive to the part of the potential for y > ysp,and that part of the potential is based on plots of the ab initio potential. We defer a detailed discussion of the potential for a subsequent paper; however, we note that the dissociation energy (from the potential minimum) for linear Ar-HO is 1061 cm-I. This appears to be in fairly good agreement with the a b initio result of 1095 cm-I (read off a plot of the potential cut). The 1061-cm-l binding energy is not in good agreement with values of 800-830 cm-I which we infer from the reported values of the binding energy of Ar-OH, relative to the zero-point level, of 718 cm-I (ref 13) and 747 cm-l (ref 18). Those binding energies were based on a simplified one-dimensional Birge-Sponer analysis of an effective one-dimensional potential, e.g., the Ar-HO potential averaged over the ground bend, not the potential at the zero-degree cut. Additional results, including Franck-Condon factors, and a more detailed discussion of the present results will be given in a later paper. Also, we have begun a similar analysis on Ne-OH (D), for which experiments have recently been completed.24 Acknowledgment. J.M.B. thanks the National Science Foundation (Grant CHE-8723042) for partial support of this work, P.S. thanks Emory University for a Woodruff Fellowship, and M.C.H. thanks the Air Force Office of Scientific Research under Grant AFORSR-88-0249. (24) Lin, Y.; Kulkarni, S. K.; Heaven, M. C. Rotationally Resolved Electronic Spectra for the Ne-OH and N A D van der Waals Complexes. J . Phys. Chem., accepted for publication.
Solvent- Induced Excited-State Quenching In a Chromophore-Quencher Complex Thomas A. Perkins,+William Humer,t Thomas L. Netzel,*-sand Kirk S. Schanze*Tt Department of Chemistry, University of Florida, Gainesville. Florida 3261 1 , and Amoco Technology Corporation, Naperville, Illinois 60566 (Received: November 30, 1989)
The emission properties, electrochemical potentials, and picosecond transient absorption spectra of the chromophore-quencher complex, [ (bpy)Re'(C0)3DMABN]+ (where bpy = 2,2'-bipyridine and DMABN = 4-(dimethy1amino)benzonitrile) were studied in CH2C12,CH3CN, and in mixtures of the two solvents. Strong dx(Re) ?r*(bpy) MLCT emission is observed in CH2C12,but not in CH3CN. The MLCT quenching process is attributed to the presence of a ligand-to-ligandcharge-transfer state in the excited-state manifold.
-
There has been a recent surge of interest in the photophysical properties of transition-metal complexes that possess an energetically low-lying ligand-to-ligand charge-transfer (LLCT) excited state.lJ In complexes that contain both electron donor (D) and acceptor (A) ligands, a LLCT excited state, D+-M-A-, can be produced directly or indirectly by net transfer of an electron from a u or ?r orbital localized on the donor ligand into a r* orbital
localized on the acceptor ligand.1,2 Our interest lies in the photophysics of d6 transition-metal complexes that contain both ?r-acceptor and *-donor 1igands.l In these chromophorequencher complexes, the possibility exists for population of states that have either metal-to-ligand charge transfer (MLCT) or LLCT character. In the present Letter, we report preliminary findings concerning the photophysics of fac- [ (bpy) Re( CO) 3DMABN]+ and far- [(bpy) Re(CO)3AN]+
'University of Florida. *AmmoTechnology Corporation. Present address: Department of Chemistry, Georgia State University, Atlanta, GA 30303.
(1) Perkins, T. A.; Pourreau, D. B.; Netzel, T. L.; Schanze, K. S . J. Phys. Chem. 1989, 93, 511. (2) For a review of LLCT states see: Vogier, A,; Kunkely, H. Comm. Inorg. Chem., in press.
Introduction
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