J. Phys. Chem. 1983,87,3105-3108
be explained as a combination of the CC stretch (1677 cm-l) with a 277-cm-' vibration, which can be assigned to the CF planar bend. The apparent lack of resonance Raman enhancement of the ring breathing mode (not observed in either PBSQ or TFPBSQ) shows that the excited-state ordering of the 2Bluand 2A, states is not affected by the fluorine substitution and the electronic transition in resonance is 2B,,-2Blu. The peak separation 1475 cm-l in the electronic absorption spectrum is assigned to the 8a frequency in the excited (2Blu)electronic state and is, as expected, slightly greater than the separation of 1394 cm-l noted4 for the protonated radical. The present experiments demonstrate that it is possible to use photomultiplier detection methods to carry out time-resolved, resonance-enhanced Raman studies of ra-
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diation-produced radicals at times longer than w milliseconds. For experiments at shorter times one must use some form of a gated detector, such as is available with an OMA, to avoid saturation of the detection system by the Cerekov pulse. Use of a single-channel device allows the use of a double monochromator as the dispersive element and in certain cases has some advantage over the OMA at the expense of multichannel collection of data. In laser flash photolysis experiments it should be possible to use photomultiplier detection methods where the photolytic source can be discriminated against by filters or other methods. Registry NO. C6F4(OH)z1771-63-1; C~H~OZ-., 3225-29-4; C6F402--,42439-31-6; C6F4O2, 527-21-9.
Pulse-Rate Effects in the Photochemistry of Gas-Phase Bromobenzene Ions. A Study of Ion Relaxation Mechanisms Robert C. Dunbar Chemistry Department, Case Western Reserve University, Cleveland, Ohio 44 106 (Received: October 18, 1982)
Two-proton photochemistry of gas-phase bromobenzene ion using a repetitively pulsed light source was studied in the ion cyclotron resonance (ICR) spectrometer and modeled by numerical integration of the kinetic equations. Theoretical modeling and experiment were in excellent agreement, showing as expected that effects of pulse rate depend strongly on the relation of pulse rate and ion relaxation rates. Effects attributable to infrared radiative relaxation of ions were clearly evident, and this relaxation was found to have a rate between 1 and 3 s-1.
Introduction It is easy to contain gas-phase ions in an ion trap for periods of many seconds or longer. This has opened the opportunity of studying photochemical and other reactive processes on a time scale of unprecedented slowness for laboratory study of isolated molecular species. Processes which have been of particular interest in these studies are slow relaxation processes of long-lived excited ions by collisional' or infrared-radiative2 mechanisms. An exceptionally interesting experiment is the sequential two-photon photodissociation of ions by a repetitively pulsed light source. Interpretation of such results is a demanding test of our understanding of the interplay of kinetic processes under isolated-ion conditions. In an earlier exploration of these possibilities, using iodobenzene ion: the fundamental point was made that a periodically pulsed light source is equivalent to a continuous source when the pulse rate is faster than the fastest ion relaxation mechanism but that distinctive pulse-rate-dependent effects are expected for pulse rates slower than relaxation. Described here is a much more comprehensive examination of the photochemistry of bromobenzene ion with a pulsed light source, in which the interplay of periodic excitation, collisional and fluorescent relaxation, and finite ion trapping time is sorted out in some detail. It will be seen that our conceptual model of ion two-photon photochemistry gives an excellent quantitative understanding of these observations, and provides in addition a confirmation that (1) Kim, M.S.;Dunbar, R. C. Chem. Phys. Lett. 1979,60, 247. (2) Dunbar, R. C.Spectrochim. Acta, Part A 1975, 31, 797. (3) Lev, N. B.; Dunbar, R. C. Chem. Phys. Lett. 1981, 84,483. 0022-3654/83/2087-3105$01.50/0
the radiative relaxation rate of this ion, which has been controversial, is not faster than 3 s-l. The observation of infrared fluorescence from gas-phase molecules has a long h i ~ t o r y . ~But , ~ for gas-phase ions, determination of the rate of radiative decay is not straightforward, and recently the process of two-photon photodissociation has been explored as an indirect route to characterizing radiative relaxation. Freiser and Beauchamp first observed and correctly interpreted two-photon dissociation in benzene ion.6 Subsequent quantitative kinetic analyses of the process in cyanobenzene ion7 and bromobenzene ion2*smade evident the possibility of exploiting such observations to characterize relaxation processes in excited ions. van Velzen and van der Hart9 reported the first serious effort to separate out the radiative contribution to relaxation of excited ions, and calculated several radiative relaxation rates, including a value of 7 f 3 s-l for bromobenzene ion. The very high-quality data of this work showed deviations from simple two-photon kinetics, and a more complicated mechanism was postulated to account for the deviations. Values of 14 s-' for (4) For instance: Perry, D. S.; Polanyi, J. C. Chem. Phys. 1976, 12, 4419. (5) An observation of luminescence in an ion-molecule system is described by: Bierbaum, V. N.; Ellison, G. B.; Futrell, J. H.; Leone, s. R. J. Chem. Phys. 1977,67, 2375. (6) Freiser, B. S.; Beauchamp, J. L. Phys. Chem. Lett. 1974, 35, 35. (7) Orlowski, T.E.;Frieser, B. S.; Beauchamp, J. L. Chem. Phys. 1976, 16, 439. ( 8 ) Dunbar, R. C.; Fu, E.W. J . Phys. Chem. 1977,81, 1531. (9) van Velzen, P. N. T.; van der Hart, W. M. Chem. Phys. 1981,61, 325. van Velzen, P. N. T. Ph.D. Thesis, University of Leiden, Leiden, The Netherlands, 1981.
0 1983 American Chemical Society
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pulse rate is the only variable and the influence of pulse rate on two-photon kinetics should be clearly isolated.
ICR E X P E R I M E N T A L S E Q U E N C E
I n
1
Electron Beam
L Light
n
1
n
ion Detect
n
ton Quench
I
I
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(s)
Figure 1. Diagram of the repetitive ICR detection sequence, showing the chopped optical exckation along with the usual Ion formatlon and
ion detection sequence.
cyanobenzene iong and 1 s-l for iodobenzene ion3 were derived by similar methods. A contradictory observation was that the bromobenzene ion experiments reported from our laboratory gave no evidence of a noncollisional component of relaxation, even at pressures below 2 x lo4 torr, where a radiative rate as high as 7 s-l ought to have led radiative relaxation to be completely dominant over collisional relaxation. Very recently,1° experiments using two visible laser pulses have again indicated that infrared fluorescent relaxation of visible-light-excited ions is not faster than 3 s-l. In related work, finally, our laboratory has studied a two-laser analogue to the two-photon dissociation process, in which the ions are excited by both a visible and an infrared laser. A radiative lifetime of 200 ms was determined by this method for IR-excited bromobenzene ions.” The present experiment was arranged by using a light chopper and a continuous laser in such a way that the time-averaged light flux was the same at all pulse repetition rates, while the repetition rate varied from 1 to 12 s-l. Thus,in contrast to varying the puke rate of a pulsed laser as in our previous work, no corrections need to be made for differences in total fluence at different pulse rates. Reliable comparisons of photodissociation rates at different pulse rates are thus possible. (See Figure 1for a diagram of the repetitive ICR detection sequence.) Experimental Section The use of the ICR ion trap to measure the extent of two-photon dissociation has been described.12 In these experiments an ion trapping time of 3 s was used, with ions produced by a 100-ms electron beam pulse at a nominal electron beam energy of 8-9 eV. The argon ion laser (Coherent CR-12) was run at 5-W power at 515 nm, with the beam spread to 1-2-cm diameter. A chopper was used with chopping rates from 1to 12 s-l. The duty cycle of the light beam was 1/30; that is to say, the width of the light pulses was varied so that the product of pulse width and pulses per second was constant at 1/30. Since the total fluence through the ICR cell, as well as the light beam characteristics, is invariant, (10) Dunbar, R. C.; Chen, J. H., to be submitted for publication. (11) Honovich, J. P.; Dunbar, R. C. J. Am. Chem. SOC.1982,104,6220. (12) Dymerski, P. P.; Fu, E. W.; Dunbar, R. C. J. Am. Chem. SOC. 1974, 96,4109. Freiser, B. S.; Beauchamp, J. L. Ibid. 1974, 96,6260. Dunbar, R. C.; Fu, E. W.; Olah,G . A. Ibid. 1977, 99,7502.
Kinetic Analysis Sequential two-photon dissociation has frequently been discussed in terms of the kinetic scheme *t
Io1
-
*+*
\w/
dissociation products
(1 )
The excited state A+* is believed to be a vibrationally excited ion in its ground electronic state. Visible light of intensity I populates this state with rate Io1 and photodissociates it with rate l a 2 . The excited state can decay to the unexcited ground state by collisional processes having rate k3p at pressure P, or by radiative relaxation, whose rate is symbolized by kr& In the case of bromobenzene ion, no fluorescence is observed from excited electronic states: l3 the picture of initial optical excitation into one of several electronic states accessible at 514 nm, followed by radiationless internal conversion in less than lo-” s, is a reasonable one. If the four processes making up eq 1 are treated as simple firsborder rate processes (the “rate-process”model), the kinetic scheme is readily solved for continuous irradiation. With pulsed radiation, an analytical solution will be less easy; and if the pulses are of finite duration, as in the present case, it is doubtful if a solution is possible. It seemed much more feasible to integrate the equations describing eq 1 by numerical simulation, which was accordingly used to produce all of the theoretical results described. It seems likely that infrared radiative relaxation of a highly excited ion is not well described as a first-order rate process. The intially photoexcited ion contains about 2.5 eV of energy, while the ion can be considered as “relaxed” (in the sense that the photon of the Io2 process cannot dissociate it) when it contains less than 0.4 eV of internal energy. Relaxation thus corresponds to emission of a number of infrared photons (of the order of 20) and should clearly be better described by a picture (call it the “cascade” picture) in which an initially photoexcited population of ions radiates infrared photons for a fairly well-defined time, at which time the whole population becomes relaxed within a relatively brief time span. Numerical integration of eq l including a cascade version of radiative relaxation is straightforward, which allows comparison with the rate-process picture. Figure 3A displays the calculated extent of photodissociation as a function of pressure for several pulse rates. The rate-process model was calculated by using a radiative relaxation time krad-’ = 0.7 s; the cascade model calculations are an average of two sets of numbers using 0.7 and 0.4 s for the cascade time. (This average is desirable to smooth out unrealistic discontinuities occurring when the time between light pulses is close to the chosen cascade time.) It can be seen that pulse rate has a very strong effect on the kinetics of photodissociation. A t high pulse rates and low pressures, there is little predicted pulse-rate, dependence, since excited A+* ions are extensively carried over from one pulse to the next, and the extent of dissociation D is determined by total fluence, while at high pressure and low pulse rates the extent of dissociation D becomes approximately proportional to the inverse of the (13) Maier, J. P.;Marthaler,,0.; Mohraz, M.; Shiley, R. H. Chem. Phys. 1980,47, 307.
Photochemistry of Gas-Phase Bromobenzene Ions
The Journal of Physical Chemism, Vol. 87, No. 16, 1983 3107
DISSOCIATION vs PULSE RATE
DISSOCIATION v s P R E S S U R E A . Theory
1
Bromobenzene Ion
I 5
,
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,
I , 10
,
,
,
i
I
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I
,
l
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I
,
l
.
6 . Experiment A A
A
l 15
Repetition Rate (s") Figure 2. Photodissociationof bromobenzene ion as a function of pulse rate at a pressure of 2 X lo-' torr. D = -In ((lighton signal)/(lightoff signal)). The solid line is the theoretical curve assuming a radiative relaxation rate of 1.5 s-' and a colllsional relaxation rate of 1 X lo-' cm3/(mo~ecute 5).
pulse rate, because there is little A+* carryover. For pulse rates slower than radiative relaxation, pressure dependence is small; the pressure dependence becomes substantial for pulse rates near radiative relaxation, and by the time the pulse rate increases to twice the radiative relaxation rate, the radiative process has little effect on the pressure dependence. The rate-process and cascade models of radiative relaxation are not drastically different, although in the region where the pulse rate and radiative relaxation rate are comparable, the two models give significantly different predictions for the pressure dependence and pulse-rate dependence of dissociation. Either model is adequate to fit the present results.
Results As an illustration of the data, Figure 2 shows one series of dissociation measurements as a function of repetition rate at a pressure of 2 X lo-' to^. As throughout this work, the dissociation parameter D is defined as D = -In ((light-on signal)/(light-off signal)). The solid line is the calculated curve for a radiative relaxation rate of 1.5 s-'. In Figure 3B are shown data for dissociation as a function of pressure for repetition rates of 1,3, and 9 s-l. (Data for repetition rates of 2,4,6,and 12 s-l are not shown: These may be summarized briefly by noting that these data sets show scatter and trends similar to those plotted. The 2-s-I points fall between the 1- and 3-s-I sets, while the 4-,6-, and 1 2 d s e t s are similar within data scatter to the 9-s-l set.)
Discussion The kinetic considerations described above give an entirely satisfactory understanding of the observed kinetic behavior. Figure 2 suggests that the pulse-rate dependence follows theory well. For a more demanding comparison, compare Figure 3, A and B. Theory predicts that dissociation will show very little pressure dependence for repetition rates less than the radiative relaxation rate.
I
2
.
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4
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6
I
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8
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I
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0
P r e s s u r e ( 3 ~ 1 0 -T~o r r )
Flgure 3. Dissociation as a function of pulse rate and pressure: (A) Theoretical calculation using the rataprocess model (dashed curves) and the cascade model (solM curves). (B) Experimental results, normallzed to the dissociation at 3 X lo-' torr, and with the pressure axis plotted in units of 3 x lo-' torr.
Pressure dependence appears more or less rapidly as the repetition rate exceeds the relaxation rate. The data Figure 3B fit this pattern excellently: The 3-s-' series shows pressure dependence which is substantial, but is definitely less strong than at 9 s-l, while the 1-s-' series shows no pressure increase (the small apparent dip at medium pressure being apparently an artifact associated with light inhomogeneity). The fit of theory and experiment is satisfactory within data scatter at all repetition rates, assuming the radiative relaxation rate to be in the vicinity of 2 s-l, The lack of pressure dependence at 1s-l provides the strongest evidence that radiative relaxation is not slower than 1 s-l. A similarly satisfactory fit of theory and experiment is seen in Figure 4. Since the radiative rate is believed to lie in the 1-3-s-' range, the ratio D(l)/D(3) of dissociation at repetition rates of 1 and 3 s-l is particularly sensitive to radiative relaxation effects. This ratio is shown in Figure 4 as a function of pressure. Theoretical lines are also plotted for assumed raditive relaxation rates (rateprocess model) of 0,1.5, and 3 s-l; shown in addition is the pressure-independent line predicted by the cascade model for radiation times less than 0.33 s. This comparison demonstrates particularly strongly that the radiative relaxation rate is less than 3 s-l, since both theoretical pictures (particularly the cascade picture) predict a much higher ratio of D(1) to D(3) at low pressures if a relaxation rate faster than 3 s-l is assumed. The results described here confirm that bromobenzene ions require between 0.3 and 1s to relax radiatively from an initial excitation of 2.5 eV. This is in accord with the measurements by two-pulse photodissociation.1° It is slower than the IR radiative rate derived from IR ab-
J. Phys. Chem. 1083, 87, 3108-3113
3108
REPETITION RATE EFFECT
0
I
3
/
/
(14) Rosmus, P.; Werner,
H.-J. Mol. Phys. 1982, 11, 1.
diatomic ions radiation is likely to be much faster than in the neutral, but it is not clear whether this expectation will carry over to large, charge-delocalized polyatomic ions like this one. Determination of this rate in previous photodissociation work with continuous light sources seems to have been unreliable. Some problems are inherent in these kinetic analyses. The analysis is sensitive to deviations from pure two-photon kinetics as described by eq 1, and use of more elaborate kinetics, as in ref 9, carries the danger that the assumed complex mechanism may be incorrect. A similar uncertainty is whether the rate-process picture of relaxation is correct: assuming a different relaxation picture can significantly affect the plots, extrapolations, and fits involved in these analyses. The alternative cascade-model kinetics explored above illustrate, but do not exhaust, these possibilities. Other kinetic complications are easily imagined but not easily modeled: for instance, the FranckCondon factors and hot-band intensities involved in the q photon absorption presumably change as the ion relaxes, so that treating the u2 process as a rate process may not be accurate; or again, excited electronic states may participate in ways similar to, or more complicated than, van Velzen and van der Hart's mechanism for bromobenzene ionsg In general, there are enough possibilities for complications and deviations that such analyses should be somewhat circumspect.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the National Science Foundation for additional support. Registry No. Bromobenzene, 108-86-1.
Photochemistry of [Co( NH3),Br]*+: Wavelength, Pressure, and Medium Dependence of Redox and Aquation A. D. Kirk,'+ C. Namarlvayam,t Gerald 8. Porter,t M. A. Rampl-Scandola,I and A. Simmons$ Department of Chemistry, UnbersHy of Victor&, Vbtorls, British Columbia. Canada V8W 2Y2, Depertment of Chemistry, University of British Columbia, British Columbia, Canada VBT lY8, and Cenm di Studio sulk Fotochimica e Reectlvita degli Stati, Istituto Chimico deiilJniversita.Ferrara, Ita& (Received: October 28, 1982)
The ratio of redox to aquation of [ C O ( N H ~ ) ~ Bhas ~ ] been ~ + shown to be constant at about 2 for irradiation wavelengths (nm) of 254,275,313, and 365, falling close to zero at 514.5. With increasing pressure, quantum yields for redox and aquation decrease, the former somewhat faster, and in a wavelength-dependent fashion, so that the apparent volumes of activation (mL mol-') for redox and aquation are respectively 6.0 f 0.6 and -0.4 f 3.2 at 365 nm and 4.8 f 0.3 and 2.5 f 2.7 at 313 nm. At ordinary pressure the redox yield falls %fold and the aquation yield increases 50% in 40 w t % glycerol/water. The equations relating volumes of activation for individual reaction steps to the overall apparent volumes of activation for the lifetime and quantum yield are derived. It is shown that all of the above observations are supportive of a mechanism involving generation of a caged radical pair which has no "memory" of the excitation energy of its progenitor. Introduction In a recent article on mechanism and high pressure, Swaddle' remarked on the "curiously refractory problem" of the assignment of mechanism in the substitution reac-
* Author to whom correspondence should be addressed. 'Department of Chemistry, University of Victoria.
* Department of Chemistry, University of British Columbia.
Centro di Studio sulla Fotochimica e Reactivita degli Stati, Istituto Chimico dell'Universita. 0022-3654/83/2087-3108$01.50/0
tions of octahedral transition-metal complexes in solution. Even more refractory has proved to be that of the corresponding photochemical reaction mechanisms, where only rarely has the electronicallyexcited species responsible for reaction been identified. One useful approach to the assignment of mechanism has been the study of the pressure dependence of the rate (1)Swaddle, T. W. Rev. Phys. Chem. Jpn. 1980,50, 230.
0 1903 American Chemical Society
