2004
J. Phys. Chem. 1984,88, 2004-2009
Molecular Beam-Laser Spectroscopy of Ne-CI,: Vibrationally Excited van der Waals Molecule
Observation of a Metastable
David E. Brinza? Colin M. Western,*Dwight D. Evard, Fritz Thommen, Barry A. Swartz,l and Kenneth C. Janda* Arthur Amos Noyes Laboratory of Chemical Physics,Il California Institute of Technology, Pasadena, California 91 125 (Received: December 20, 1983)
Laser-induced fluorescence excitation spectra of expansions containing He, Ne, and C12were obtained with a free-jet molecular beam apparatus and excimer-pumped dye laser system. Spectroscopic features observed -6 cm-’ to the blue of the (6-0) through (15-0), the (8-1) through (12-1), and the (9-2) through (12-2) vibronic origins of the B-X system of C12 have been attributed to the van der Waals molecule Ne-C1,. Fitting of the computed band shape to the observed data indicates that Ne-C12is a T-shaped molecule with an Ne to C1, center-of-massseparation of -3.6 %, in the ground state. Of particular interest is the observation of Ne bound to C12 (X’Zg+,ut’ = 1,2), since the C12 vibrational energy far exceeds the van der Waals bond strength. A lower bound of s has been placed upon the lifetime of this metastable complex, based on time of flight from the nozzle to the laser interaction region. The observation of “hot-band:’ Ne-C1, lends strong support to “energy-gap” and “momentum-gap” theories which predict long-lived vibrationally excited triatomic van der Waals molecules.
I. Introduction The predissociation of van der Waals molecules has become an active area in experimental and theoretical chemical physics. Supersonic molecular beam techniques, coupled with dye lasers, have made possible the preparation and spectroscopic determination of vibrational predissociation rates of van der Waals molecules in a collision-free environment. The wide disparity in energy, hence weak coupling, between the vibrational modes of chemically bound species and the van der Waals vibrational modes in a small van der Waals molecule permits the preparation of a well-defined excited state of the complex by a laser. If vibrational energy in excess of the van der Waals binding energy is deposited into the complex upon excitation, the energy will be redistributed within the complex, eventually leading to dissociation. The unimolecular decomposition generally proceeds on a single potential energy surface. The dissociation energy of the van der Waals bond is generally much less than the energy required to break typical covalent bonds. Therefore, the vibrational predissociation of van der Waals molecules is a comparatively simple process, providing models for detailed theoretical investigations into the dynamics of dissociation and intramolecular energy transfer. The elegant series of laser-excited fluorescence experiments performed by the Levy group’-’ have provided considerable insight into the structure and dynamics of small van der Waals molecules. Iodine, with its strongly fluorescent B-X system and relatively long fluorescence lifetime (-1 ps), has proved to be an ideal molecular substrate for the preparation of excited states of van der Waals species by laser excitation. Under the conditions of a supersonic expansion with approximately 1 ppm of I, in He, the van der Waals molecule He-I2 was identified as the species responsible for bands occurring 3.4-4.0 cm-’ to the blue of the (3-0) through (29-0) I2 vibronic bandheads.’ The rotational structure of the He-12 band was analyzed2 and yielded a T-shaped structure for the complex. Analysis of the line shape of the R-branch head (high-frequency side) of the He12bands by fitting to a Lorentzian function yielded predissociation lifetimes3 ranging from 220 ps for the (12-0) band to 38 ps for the (26-0) band. The vibrational predissociation rate was found to be empirically described by a quadratic plus cubic dependence on upper state vibrational
quantum number. The dispersed fluorescence spectrum of He-I2 (B, u’ = 22) was dominated by transitions from I2 in the u’ = 21 state4 giving rise to the propensity rule Av = -1 for the vibrational predissociation process. The binding energies of the rare-gas I2 series for the ground state and excited complexes were determined’ by using the anharmonicity of the I2 B state to bracket the binding energy in the excited state, then adding the observed band shift to obtain the ground-state binding energy. Morse potential parameters were also reported as determined from van der Waals vibrational progressions noted in the excitation spectra. It is interesting to note that, even though the complex Ar-12 ( X , v” = 1) is energetically stable, that is, the vibrational frequency (21 3.3 cm-’) is exceeded by the Ar-I, (X) bond strength (-235 cm-’), evidence of the “hot-band” complex was not observedS6 The theoretical efforts of Beswick et a1.8-’2have attempted to simulate the results of the Levy experiments. The model first advanced involved the collinear vibrational predissociation of the van der Waals complex X-BC8,9 where BC is a diatomic molecule and X is a rare-gas atom. The absolute rates calculated were not in good agreement with the observed rates for He-12, but qualitatively the model correctly described the propensity rule Au = -1 and the superlinear dependence of rate on the vibrational quantum number. An improvement was made by considering the T-shaped molecule with the rare-gas atom restricted to move perpendicular to the diatomic axis. The calculated vibrational predissociation rate for this model was in better agreement with experiment than the collinear case. The study concluded that vibrational predissociation in He-I, is essentially a pure V T process, since the fraction of available energy going into rotation was found to be very small compared to the final translational energy of the fragments.
Present address: Jet Propulsion Lab, California Institute of Technology 4800 Oak Grove Drive. Pasadena. CA 91109. *Present Address: Department of Chemistry, University of Bristol, Bristol, United Kingdom. Present address: Western Research Corp., 9555 Distribution Ave., San Diego, CA 92121. * Alfred P. Sloan Research Fellow, Dreyfus Teacher-Scholar. 11 Contribution No. 6959.
(7) Blazy, J. A,; DeKoven, B. M . Russell, T. D.; Levy, D. H. J . Chem. Phys. 1980, 72, 2439. (8) Beswick, J. A,; Jortner, J. Chem. Phys. Lett. 1977, 49, 13. (9) Beswick, J. A.; Jortner, J. J . Chem. Phys. 1978, 68, 2277. (10) Beswick, J. A.; Jortner, J. J . Chem. Phys. 1978, 69, 512. (1 1) Beswick, J. A.; Delgado-Barrio, G.; Jortner, J. J . Chem. Phys. 1979, 70, 3895. (12) Beswick, J. A.; Delgado-Barrio, G.J . Chem. Phys. 1980, 73, 3653.
0022-3654/84/2088-2004$01.50/0
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(1) 3266. (2) 671. (3) 2719. (4)
Smalley, R. E.; Levy, D. H.; Wharton, L. J . Chem. Phys. 1976, 64, Smalley, R. E.; Wharton, L.; Levy, D. H. J . Chem. Phys. 1978, 68, Johnson, K. E.; Wharton, L.; Levy, D. H. J . Chem. Phys. 1978, 69, Sharfin, W.; Johnson, K. E.; Wharton, L.; Levy, D. H. J . Chem. Phys.
1979, 71, 1292. ( 5 ) Kenny, J. E.; Johnson, K. E.; Wharton, L.; Levy, D. H. J. Chem. Phys. 1980, 72, 1109. (6) Johnson, K. E.; Sharfin, W.; Levy, D. H. J . Chem. Phys. 1981, 74, 163.
0 1984 American Chemical Society
Molecular Beam-Laser Spectroscopy of Ne-C12
dF
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2005
1 2 5 m MONOCHROMATOR
TRIGGER
DATA ACQUISITION FOR T H E SYSTEM
FLUORESCENCE COLLECTION 8 DISPERSION
PULSED MOLECULAR 0 E A Y APPARATUS
PULSED MOLECULAR B E A M APPARATUS
=---.n ,-
TIMING SEQUENCE
Figure 1. Schematic diagram of the pulsed molecular beam apparatus. Experiments were also performed with a CW nozzle in place of the pulsed
nozzle.
VALVE TRIGGER HCSA TRIGGER
.+--4p*
-13~1 LASER TRIGGER
Ewing, in addressing the role of vibrational predissociation of van der Waals molecules in vibrational relaxation processes, apT lifetimes of excited plied the Golden Rule to determine V comp1e~es.l~From the calculated vibrational predissociation rates, the equlibrium constants for several van der Waals species were computed. The study led to the conclusion that van der Waals molecules play a significant role in vibrational relaxation processes at very low temperatures, e.g., T on the order of half the well depth ( T E/2k). In examining the dependence of the lifetime upon the molecular parameters, he recognized an insightful correlation. It was observed that a logarithmic plot of V T lifetimes vs. the quantity (2pAE)1/2/ah, where p is the reduced mass of the complex, AE the kinetic energy of the fragments, and a is the Morse range parameter, yields very nearly a straight line for the species ~onsidered.’~The correlation serves as a guide for estimating the lifetime of vibrationally excited van der Waals molecules, provided the Morse range parameter a and the internal energy distribution of the fragments can be determined. In a previous communication,’s the observation of the metastable vibrationally excited van der Waals molecule Ne-C12 (X, u ” = 1) was reported. The present paper concerns itself with the details of the spectroscopic studies performed upon the Ne-C12 molecule at -0.2-cm-’ resolution. Section I1 of the paper describes in detail the free-jet molecular beam apparatus employed in the spectroscopic measurements of the Ne-C12 molecule. Spectra originating from the ground and vibrationally excited states of Ne-C12 are shown in section 111. The results of fitting the spectra by a nonlinear least-squares program to an asymmetric top model in order to determine band origins and structures is also given in section 111. The significance of the structure and observed band origins for the complex is discussed in section IV. Also included in section IV are estimates of the vibrational predissociation lifetimes for the Ne-C12 (X, u ” = 1) and Ne-C12 (B, u’) complexes as determined by the “energy-gap” collinear harmonic model and the “momentum-gap” correlation diagram.
-
-
11. Experimental Section
The molecular beam apparatus employed in these measurements is diagrammed in Figure 1. The nozzle assembly is mounted on an xyr translation stage such that the molecular beam is directed toward the pump. During operation, the apparatus is maintained at pressures below torr by a Varian HS-16 diffusion pump capable of a pumping speed of 10 000 L/s. The diffusion pump is backed by a Leybold-Heraeus WS-160 Roots blower in series with a Heraeus-Engelhard DK-20 mechanical pump. Under typical operating conditions, the average gas throughput of the system is about 0.5 torr L/s, below the ultimate system throughput capability of 1 torr L/s.
183, 79,1541.
Figure 2. Schematic of the data acquisition system for the pulsed molecular beam apparatus. The timing sequence is shown on the lower portion of the figure. The pulsed valve driver and valve trigger in the timing sequnce are irrelevant in the CW experiment.
Spectra were obtained with both pulsed and continuous wave (CW) molecular beams. The pulsed beam source is similar in design to a modified commerical solenoid valve (Peter Paul Electronics, no. H22G7DCM) used by the Rice group at the University of Chicago.I6 A 1-ms current pulse, when applied to the solenoid coil, accelerates the soft iron plunger which moves freely until it impacts the lip of the seating pin thus opening the valve. High-pressure gas flows past the valve seat and through a 400-pm electron microscope aperture into the vacuum apparatus to form the molecular beam. The head of the seating pin is in direct contact with the closing spring; therefore, the valve remains open for only a short time. Careful adjustment of mechanical and electrical parameters permits formation of pulsed beams with durations of less than 300 ps. The C W beam was produced by expanding gas mixtures through a 40-pm quartz nozzle into the chamber. The helium, neon, and chlorine gases for the mixtures were used as supplied (Matheson) without further purification. Mixtures of 0.5% Cl,, 25% Ne, and the remainder H e were prepared in a Monel metal cylinder and thoroughly agitated to ensure homogeneity of the gas. The spectra of C12 (X, u f r= 2) and associated Ne-C12 were obtained by expanding the above gas mixture through a 40-pm quartz nozzle heated to 320 OC. Features attributed to Ne-Cl, were observed -6 cm-’ to the blue of the (9-2) through (12-2) vibronic origins of the B-X system of C12. The C12 signal was quite intense, but unfortunately the signal to noise in the Ne-C1, peaks was too low to obtain good fits with the nonlinear leastsquares fitting program. We report the observation of this species, while this paper will deal with Ne-C12 (X, u” = 0,l) in more detail. An excimer-pumped dye laser (Lambda-Physik FL 2002) provides the tunable excitation radiation for these measurements. Pumped by a XeCl excimer laser (Lumonics 861-S) delivering approximately 80 mJ of 308-nm radiation per pulse, the dye laser produces -8 mJ pulses from Coumarin 503 in the 490-540-nm region. The frequency bandwidth of the laser varies from 0.10 to 0.30 cm-’ in this region, depending on grating order and operating wavelength of the laser. The unfocussed dye laser beam is directed by means of kinematically mounted mirrors through the light baffles on the vacuum apparatus. The baffles are equipped with Brewster angle mounted Supracil 2 windows and knife-edge apertures designed to minimize admission of stray light without incurring excessive refractive scattering of the dye laser beam. The total fluorescence collection optical system consists of a 75-mm-diameter aspheric lens cf/0.7), which projects the fluorescence region image onto defining slits approximately 35 (16) Rice, S. A.; Tusa, J. private communication.
2006
Brinza et al.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
4000
35
35
c1
CI
I
1
0 5% CI,
:
25% Ne : BALANCE He
NOZZLE DIAMETER = 40011
IO-
06 06
oc P I 4
04
-
cm-'
Figure 3. Laser-induced fluorescence spectrum in the vicinity of the B-X (1 1-0) vibronic band for CI,. The spectrum was obtained by expanding 0.5% C1,-25% Ne-balance H e at 150 psi through a 400-wm nozzle.
cm from the interaction region. A cutoff filter (e.g., Schott OG-5 15) is used to remove scattered laser light since the bulk of the C12 molecular fluorescence is expected to be red shifted 30 nm or more on the basis of published Franck-Condon factors.17 The data acquisition system for the pulsed molecular beam apparatus is depicted in Figure 2. The system is controlled by an IBM PC microcomputer through a commercially available interface card (Tecmar Labmaster). The microcomputer operates in an interrupt-driven mode under the PC-FORTH system (Laboratory Microsystems, Inc.). A typical acquisition cycle timing sequence is also shown in Figure 2. The computer first triggers the pulsed valve driver which generates the current pulse to the solenoid coil. After 1-2 ms (depending upon the characteristic delay in valve opening), the microcomputer then triggers the multichannel signal averager (MCSA). Approximately 15 ps after initiating the MCSA averaging cycle, the excimer laser is triggered. The transient fluorescence signal, obtained from a cooled photomultiplier tube (RCA 7265), is digitized and summed into the MCSA memory. In the C W experiment only the MCSA and laser need to be triggered. Data are collected at each point for ten laser shots, and then the fluorescence signal is extracted from the proper MCSA time channels. The dye laser grating is stepped to the next frequency point. This process is repeated until the scan is completed. 111. Results
A . Spectroscopy of Ne-Clz (X1Zgf,uN= 0). The laser-induced fluorescence excitation spectrum of a pulsed molecular beam containing natural isotopic abundance molecular chlorine, neon, and helium in the vicinity of the B3110u+-XlZgf(1 1-0) vibronic transitions is shown in Figure 3. The bands with easily discerned rotational structure are due to, in order of increasing intensity, the 37C12,3sCP7C1,and 35C12isotopic variants of molecular chlorine. It should be noted that the 35C12bandhead was of sufficient intensity to exceed the input dynamic range of the MCSA and, based on isotopic abundances, the actual intensity of this feature should be approximately 1000 signal units greater than appears in Figure 3. Still clearly visible, however, are the effects of nuclear spin statistics upon the intensities of the rovibronic transitions for the homonuclear (35C12and 37C12)vs. the heteronuclear (35CP7C1) species. Both isotopes of chlorine are fermions with nuclear spin I = 3/z which leads to a 5:3 alternation of statistical weights for odd and even J levels of the homonuclear diatomic species. The effective rotational temperature and laser bandwidth (assumed to have a Gaussian frequency dispersion) were determined by modeling the (1 1-0) band of W12with a nonlinear least-squares fitting program. If the rotational constants Bd=ll= 0.12616 cm-l (17) Coxon, J. A. J . Quant. Spectrosc. Radiat. Transfer 1971, 1 1 , 1355.
I
I
I
I
40
60
80
'RESSURE
I
I
100
200
(psioi
Figure 4. Pressure dependence of integrated intensity of the Ne-CI, band near the B-X (10-0) vibronic band for C1,. The intensity of the CI2 band is proportional to the pressure, so the pressure dependence exponent for the formation of Ne-CI, is 2.4. This indicates that formation of NeCI, is not via simple two- or three-body collision.
and Bu+O = 0.24327 cm-' as reported by Coxon are used,'* the fit yields Trot= 4.0 K and Avlaser = 0.20 cm-'. The spectroscopic features occurring approximately 5.3 cm-I to higher energy of the 3sC12and 35C137C1bandheads in Figure 3 are observed only in molecular beams formed from mixtures containing N e as well as C12 and He. The integrated intensity of the feature to the blue of the 35C12(10-0) vibronic band, normalized to the C1, integrated intensity, as a function of source pressure is plotted in Figure 4. Since Zclz 0: P and the slope of the plot yields INeC12/IC12 0: P1.4, the pressure dependence for Ne-ClZ formation is INecll P-2.4. A cubic pressure dependence might be expected since the simplest scheme for formation of Ne-C12 involves the three-body collision: Ne
+ Clz + M
-
Ne-C12
+M
where M removes the heat of formation of the van der Waals complex as relative kinetic energy. The actual kinetics of formation of Ne-ClZ in a comparatively rich expansion mixture are certainly more complicated than the above reaction. Therefore, the value of the pressure exponents does not always provide conclusive information concerning the identity of a van der Waals species.Ig Excitation spectra were also obtained in the vicinity of the (6-0) through (15-0) vibronic origins of the 35C12B-X system. Bands of similar relative intensity of the feature to the blue of the (1 1-0) bandhead were observed at nearly constant frequency shifts from each of the uncomplexed 35C12band origins. The positions and shifts of the origins of the bands due t o the complex relative to the 35C12(u'= 0) band origins are given in Table I. The positions of the band origins for the van der Waals bands were determined by fitting an asymmetric top bandshape to the experimental data by the nonlinear least-squares program described more fully in section C. B. Spectroscopy of Ne-C12 (X'Z,', u" = 1, 2). The laserexcited fluorescence spectrum of the B-X (10-1) hot band for 35C12is shown in Figure 5. The observation of the spectrum (18) Coxon, J. A. J . Mol. Spectrosc. 1980, 82, 264. (19) Casassa, M. P.; Western, C. M.; Celii, F. G.; Brinza, D. E.; Janda, K. C. J . Chem. Phys., in press.
Molecular Beam-Laser Spectroscopy of Ne-C12
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2007
TABLE I: Band Shifts for the Ne-Q van der Waals Molecule band shift 6-0 18993.788 18999.14 (18)' 5.35 7-0 19 179.307 19 185.01 (13) 5.70 8-0 19353.976 19359.57 (16) 5.59 9-0 19517.746 19523.39 (14) 5.65 10-0 19670.509 19676.35 (21) 5.84 11-0 19812.366 19817.75 (18) 5.38 12-0 19943.390 19948.95 (13) 5.56 13-0 20063.670 20069.54 (15) 5.87 14-0 20 173.447 20 180.49 (26) 7.05 15-0 20 273.263 20 280.65 (17) 7.38 8-1 9-1 10-1 11-1 12-1
18799.66 18963.43 19 116.210 19258.051 19389.036
18805.34 (18) 18969.10 (19) 19 121.40 (12) 19263.58 (14) 19394.76 (17)
"Band origins reported by Coxon in ref 18. cUncertainty taken as the laser bandwidth.
3000
1
0.5% Clp 35
35
I
5.67 5.67 5.19 5.53 5.72 bThis work.
: 2 5 % Ne :
1
19.816
19,EIe
Cm-i
Figure 6. Model fits to the observed Ne-C1, (1 1-0) band. The points are the experimentally obtained data. The solid line is the best-fit generated spectrum. The dashed line uses the Ne-Ar atom-atom separation as determined experimentally20for the Ne-C1 distance to generate the spectrum.
B a l H4
Nozzle diom = 0.40mm
C I CI
i': n
Cl
8-x (10-11
2000
I
1
is
CI
I,
I
N e - C 1 2 8-X Ill-I1
/I ,I
II
I
Cm-'
Figure 5. Fluorescence excitation spectra showing the pressure dependence of the feature 6 cm-I to the blue of the (10-1) vibronic origin of Cl,. The peak assigned to Ne-C1, was absent in spectra of expansions containing only C1, and He.
arising from vibrationally excited chlorine molecules in a supersonic molecular beam is not surprising since high-frequency vibrations do not relax rapidly under the conditions of expansion. In addition, the Franck-Condon factors17 for the hot-band transitions are an order of magnitude greater than the transitions to the corresponding levels from the (u" = 0) level. The measured intensities of the hot bands relative to bands arising from C1, (u" = 0) indicate an effective vibrational temperature of 200-250 K in the beam. The rotational temperature of 35C12(u" = 1 ) was determined to be 3.7 K by fitting the (10-1) spectrum using the literature constantsis = 0.13018 cm-' and = 0.24165 cm-'. The observation of the feature appearing approximately 5.0 cm-' to higher energy of the (10-1) bandhead in Figure 5 is rather unexpected. The integrated intensity of this feature has been measured as a function of stagnation pressure and is found to vary as Z 0: P-25.Similar bands are found to occur near the (8-1) through the (12-1) and (9-2) through (12-2) hot bands of C1,. The vibrational energy of 35C12(X'B,+) is approximately 560 cm-' while the van der Waals bond strength for Ne-C1, should be less than 75 cm-' (as determined for Ne-I2 (X)).7 In consideration of these energies the observed species, Ne-C12 (XIZ,+, v" = 1, 2), is a metastable vibrationally excited van der Waals molecule. Assuming typical molecular beam velocity (u d lo5 cm/s) and a 1-cm nozzle to laser interaction region distance provides a lower s. bound for the lifetime of this "hot-band" complex of T 2 The direct measurement of the lifetime of the complex has been attempted by increasing the flight path from the nozzle to laser
I
19,262
cm.,
19,264
Figure 7. Model fits to the observed Ne-C12 (11-1) band. See the caption for Figure 6 for a brief explanation of the spectra shown.
interaction region by about 1 cm. If we take into account the diminishing number density of the complex from the l/$ beam intensity decrease, no decay of the complex was observed. C. Structure of the Ne-C12 van der Waals Molecule. With more extensive signal averaging, evidence of rotational structure in the fluorescence bands due to Ne-Cl? became apparent (see Figure 6). Although the laser bandwidth (-0.2 cm-I) prohibits observation of the fully resolved rovibronic spectrum, the band profiles obtained do permit some structural investigation by a nonlinear least-squares fitting program. The program adjusts the molecular structural parameters in order to fit the modeled band shape to the observed spectrum. The model includes the effects of nuclear spin statistics as well as molecular parameters corresponding to the band origin and the effective rotational temperature. Values for the parameters characterizing the dye laser bandwidth and frequency offset are established first by fitting the uncomplexed chlorine band using the literature values for the rotational constants and band origin for the vibronic transition of interest. The rotational temperature of C1, is also determined in this preliminary fitting. The computed band shapes for Ne-C12 near the (1 1-0) and (1 1-1) transitions are illustrated in Figures 6 and 7. The points represent the observed spectra while the solid curves describe the best fit to the data. It should be noted that attempts to fit the
2008
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
I‘
I
6
I
8
IO
I
I
1
12
14
16
v
Figure 8. Plot of bandshift vs. B-state vibrational quantum number for Ne-C12. The large jump in band shift going from u‘ = 13 to u’ = 14 is attributed to a perturbation of the van der Waals potential by the A’IIl, state of CI2.
spectra with a collinear structure failed to qualitatively describe the observed band shape, in particular the structure on the redshaded portion of the band, over a large variation in Ne-C1, separation. For the band shapes shown in Figures 6 and 7, the complex was assumed to be a T-shaped molecule with the Cl, internuclear separation essentially unperturbed by the presence of the N e atom. The dashed line corresponds to the computed band shape obtained for the Ne-Cl atom-atom separation taken to be the same as that determined for N e A r by crossed molecular beam scattering measurements.m The best-fit structure indicates that the potential minimum for ground-state Ne-Cl, occurs at 3.61 f 0.08 A.
IV. Discussion The absolute value for the frequency shift of the bands due to a van der Waals molecule relative to the corresponding free constituent provides only a measure of the comparative binding energy in the upper and lower states. A series of measurements, as reported in Table I, originating from a common ground level does provide information concerning the nature of the van der Waals interaction in the vertically excited state. The band shifts of the B-X (u’ = 0) transitions for Ne-C12 are plotted in Figure 8. In contrast to the Izvan der Waals molecules: the band shift is not a smooth, monotonically increasing function of the upper state vibrational quantum number. Rather, there is a considerable (- 1 cm-’) break in the band shift between the (13-0) and (14-0) transitions. Presumably this abrupt increase in destabilization of the complex at higher vibrational excitation is due to a perturbation of the van der Waals potential surface arising from the repulsive limb of the A3111,state of Clz. The A3111,state, which has been predicted to provide the dissociative channel for the B-X system of Clz above the (u’= 12) level, correlates to ground-state chlorine atoms.,’ The predissociation rate was found to have a J(J 1) dependence?, indicating a strong heterogeneous (An
+
(20) Parsons, J. M.; Schafer, T. P.; Tully, F. P.; Siska, P. E.; Wong, Y. C.; Lee, Y . T. J . Chem. Phys. 1970,53, 2123. (211 Heaven, M. C.; Clyne, M. A. A. J. Chem. SOC.,Faraday Trans. 2 1982, 78, 1339. (22) Clyne, M. A. A,; McDermid, I. S . J . Chem. SOC.,Faraday Trans. 2 1979, 75, 1677.
Brinza et al.
= f l ) interaction between the B3111,+ state and the repulsive part of the A3111,state. The effect of this perturbation became evident in our Cl, spectra for (u’> 13) where the P(1) line is the strongest feature rather than the line corresponding to the blended R(0) and R ( l ) transitions, which dominates the spectra for u’ 6 12. Since above the (u’ = 12) electronic predissociation threshold, the quantum yield of fluorescence drops by several orders of magnitude, the quality of data suffers in this interesting region. Unfortunately, data obtained with the present resolution do not permit further discussion of the influence of the perturbing state on the van der Waals interaction nor the effect that a van der Waals partner may have on such a perturbation. The structure of the Ne-C12 molecule, like He-I,,, is T-shaped as indicated from fits to the band profiles shown in Figures 6 and 7. The T-shaped structure is that expected if the intermolecular interaction is described by a pairwise additive potential. As pointed out by Levy,Sthe donor-acceptor would predict a linear graund-state structure since the acceptor orbital would be the q,* orbital on the halogen. Upon excitation the rg*orbital becomes the lowest unoccupied orbital, indicating a bent structure. There is no indication of a large geometry change in the spectra such as a van der Waals vibrational progression. The relatively small frequency shift observed for the complex implies that there is little difference between the ground- and excited-state potentials. A first guess for the Ne-C12 pairwise model potential might be constructed by replacing the C1 atoms with Ar atoms and using The N&12 the N e A r parameters (rmin = 3.48 A, t = 52 separation calculated by this model (3.33 A) is approximately 0.3 A shorter than the fit indicates. Also, the well depth based on additivity of the Ne-Ar potentials (- 104 cm-’) is considerably larger than that for Ne-I2 (70 cm-’). Intuitively one might expect a shallower well for Ne-Cl, than for Ne-I, according to the relative polarizabilities of Cl, and I,. The Ne-C1, distance, as determined from the fit (3.61 f 0.08 A), is approximately 0.1 A shorter than the estimated collinear potential minimum reported by Secrest and E a ~ t e s , ,obtained ~ by averaging the C12-C12 and Ne-Ne intermolecular potential^.^^ At 0.2-cm-’ resolution we have no evidence for vibrational predissooiation broadening of any of the NeC1, transitions. It is interesting to attempt to predict the magnitude of this effect. The vibrational predissociation lifetime for a particular initial state of Ne-C1, may be estimated by using Beswick and Jortner’s “energy-gap” or Ewing’s “momentum-gap” approach. The lifetime of the Ne-C1, (B, u’ = 12) state is predicted by use of the “energy-gap” theory to be between 2.0 X lo-’ and 3.0 X lo-* s, while the “momentum-gap” method brackets the lifetime between and 3.2 X s. The longer lifetime in each case 9.8 X corresponds to DNdI2 of 55 cm-’ while the shorter lifetime is calculated by using DNtCll = 80 cm-’. The Morse range parameter was chosen to be 1.3 A-1, a bit smaller than the experimental value of 1.4 A-‘ for Ne-I,. Both models predict a very long-lived Ne-C12 (X, V” = 1) complex and comparatively long-lived N&12 (B, v’) species. These lifetime estimates indicate that the lifetime broadening of the spectral lines of the Ne-C12 molecule may be sufficiently small to permit resolution of individual rovibronic lines. Such measurements can potentially yield very accurate structural and lifetime information. In summary, the van der Waals molecule Ne-C1, has been investigated by laser molecular beam spectroscopy. The molecule appears to be T-shaped with a Ne to Cl, center-of-mass distance of -3.6 A. The band shift data indicate that there may be some interaction with the repulsive limb of the bound A3111, state perturbing the C12 B3110u+state. The metastable vibrationally excited molecule Ne-C1, (X, u” = 1) was determined to have a s. The Ne-Cl, molecule may be the first trialifetime of > tomic van der Waals complex to yield detailed intermolecular potential data and precise lifetime information in its visible (23) Harris, S . J.; Novick, S. E.; Klemperer, W.; Falconer, W. E. J . Chem. Phys. 1974, 61, 193. (24) Secrest, D.;Eastes, W. J . Chem. Phys. 1972, 56, 2502. (25) Hirschfelder,J. 0.; Curtiss, C. F.; Bird, R. B. “Molecular Theory of Gases and Liquids”; Wiley: New York, 1954.
J . Phys. Chem. 1984,88, 2009-2017 spectrum, providing an exacting test for theoretical vibrational predissociation models.
Acknowledgment. This work was supported by the National Science Foundation Grant No. CHE-8202408and by the Atlantic Richfield Corporation. Acknowledgment is made to the donors
2009
of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. F.T. gratefully acknowledges the support of the Swiss National Science Foundation. Registry No. Ne, 7440-01-9; C12, 7782-50-5; He, 7440-59-7.
Effect of Charge Transport in Electrode-Confined N,N’-Dialkyl-4,4’-bipyridinium Polymers on the Current-Potential Response for Mediated, Outer-Sphere Electron-Transfer Reactions Nathan S. Lewis and Mark S. Wrighton* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: May 11, 1983)
Mediated outer-sphere redox processes have been examined at rotating disk Pt/[ (PQ2+/+),],f electrodes. The [(PQ2+/+),],,f is a redox polymer anchored to the surface and is formed from N,N’-bis[(trimethoxysilyl)propyl]-4,4’-bipyridinium,I. The polymer coverages for the electrodes selected for study are sufficiently great that F e ( ~ h e n ) , ~ + /Eo’ ~ + ,= +1.03 V vs. SCE, shows no electrochemical response near its E O ’ . The mediated reduction of Fe(phen)?+ and a number of other outer-sphere oxidants is mass-transport limited when the Pt/[(PQ2+/+),],,f electrode is held 100 mV more negative than Eo’[(PQ2+/+),],,f = -0.45 V vs. SCE in CH3CN/0.1 M [n-Bu,N]C1O4. However, contrary to theoretical expectations based only on the rate constant for reaction of F e ( ~ h e n ) ~with ~ + a surface PQ+, the onset of current for the mediated reduction is at the onset for [ (PQ2+),],,,f [(PQ+),],,,f reduction; in fact, the mediated reduction current in the onset region is directly proportional to the concentration of PQ+ in the surface-confined polymer. Data for Pt/[(PQ2+-~Fe(CN)~-~4-),,]surf electrodes show directly that charge transport in the polymer can be a limitation to the maximum steady-state mediation current in aqueous electrolyte solution at the coverages of [(PQ2+/+),],,,f that have been employed. The charge-transport properties of the polymer are concluded to control the current-potential profile, as has been reported previously for other surface-modified electrodes, for the large polymer coverages employed in these studies.
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A major topic of current interest is the use of chemically modified electrode surfaces to effect electrocatalytic reactions. Modification of surfaces with designed molecular catalysts could be of use in accelerating the rate of irreversible electrode reactions such as oxygen reduction to water,2 hydrogen evolution from aqueous solutions at semiconductor ~ u r f a c e sand , ~ metalloprotein redox reaction^.^ Key issues of current study are factors that control the interfacial kinetics of electron transport at polymercoated electrode surface^.^ For cases in which multielectron transfers are required, or where the heterogeneous rate values of the particular substrate are anomolously sluggish, it is clear that substantial kinetic improvement should be possible by appropriate electrode modification. However, for reactions of outer-sphere, one-electron-transfer agents whose rate behavior would follow the (1) (a) Murray, R. W. Ace. Chem. Res. 1980,12, 135. (b) Snell, K. D.; Keenan, A. G. Chem. Soc. Reu. 1979, 8, 259. (c) Albery, W. J.; Hillman, A. R. “Annual Reports C”; The Royal Society of Chemistry: London, 198 1; pp 377-437. (2) Collman, J. P.;Marroco, M.; Denisevich, P.; Koval, C.; Anson, F. C. J. Electroanal. Chem. 1979, 101, 117. (3) (a) Bookbinder, D. C.; Bruce, J. A.; Dominey, R. N.; Lewis, N. S.; Wrighton, M. S. Proc. Natl. Acad. Sci., U.S.A. 1980, 77,6280. (b) Dominey, R. N.; Lewis, N. S.;Bruce,J. A.; Bookbinder, D. C.; Wrighton, M. S.J. Am. Chem. SOC.1982, 104, 467. (c) Bruce, J. A.; Murahashi, T.; Wrighton, M. S.J. Phys. Chem. 1982, 86, 1552. (4) (a) Lewis, N. S.; Wrighton, M. S.Science 1981, 211, 944. (b) Eddowes, M. J.; Hill, H. A. 0.;Uosaki, K. J. Am. Chem. SOC.1979, 101, 71 13. (c) Chao, S.; Robbins, J. L.; Wrighton, M. S.Ibid. 1983, 105, 181. (5) (a) Anson, F. C. J. Phys. Chem. 1980,84, 3336. (b) Murray, R. W. Philos. Trans. R. SOC. London, Ser. A 1981,302, 253. (c) Andrieux, C. P.; Saveant, J. M. J. Electroanal. Chem. 1982, 134, 163. (d) Andrieux, C. P.; Dumas-Bouchiat, J. M.; Saveant, J. M. Ibid. 1982, 131, 1. (e) Ikeda, T.; Leidner, C. R.; Murray, R. W. Ibid. 1982, 138, 343. ( f ) Pickup, P. G.; Murray, R. W. J. Am. Chem. Soc. 1983, 105, 4510.
predictions of Marcus’ electron-transfer theory,6 it is not clear if derivatization of the electrode surface will possess any advantages over electrolysis at the naked electrode surface or over electron transfer to the substrate mediated by a dissolved molecular catalyst. We have investigated in detail the mediated redox reaction of a number of outer-sphere electron-transfer reagents at electrodes modified with the N,N’-dialkyl-4,4’-bipyridinium reagent Is7 We [(Me013Si i
C
H
~
)
~
N
~
N
t ) 3i S ~C( O MH e ) 3~8‘2 ]
I use the abbreviation [ (PQ2+/+),],,,f for the surface-confined polymer derived from I. Our interest in mediated outer-sphere processes at electrodes derivatized with I stems from our results that show that the [(PQ2+/+),Isd system will catalyze the reduction *~~ of horse heart ferricytochrome c at a variety of s u r f a ~ e s . ~In such systems we are interested in both the maximum rate and potential dependence of the rate, and in this work we show that both can be controlled by the charge-transport properties of the [(PQ2+/+),],,f system at coverages of [(PQ2+/+),],,f in the vicinity of mol/cm2. Similarly, research in this laboratory has established that charge transport through polymer films from an N,N’-dibenzyL4,4/-bipyridinium reagent can limit the maximum (6) (a) Marcus, R. A. Annu. Reu. Phys. Chem. 1964,15, 155. (b) Marcus, R. A. Electrochim. Acta 1968, 13, 995. (c) Marcus, R. A. J. Phys. Chem. 1963, 67, 853. (7) Bookbinder, D. C.; Wrighton, M. S.J. Am. Chem. SOC.1980, 102, 5123; J. Electrochem. SOC.,1983, 130, 1080. ( 8 ) Bookbinder, D. C.; Lewis, N. S.;Wrighton, M. S.J. Am. Chem. Soc. 1981, €03, 7656.
0022-3654/84/2088-2009$01.50/00 1984 American Chemical Society
