J. Phys. Chem. 1982, 86, 724-728
724
Infrared Multiphoton Decomposition and Energy-Dependent Absorption Cross Section of Chloroethane Joseph S. Francisco,t Warren D. Lawrance,t Jeffrey I. Steinfeld,' Department of Chemistry, Mssachusetts Institute of Technology, Cambrklge, Massachusetts 02 139
and Robert 0. Glibert Department of Theoretical Chemistty, University of Sydney, N.S.W. 2006,Australia (Received: Ju/y 23, 198 I; In Final Form: September 8, 198 I)
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The results are presented of an investigation of the multiple infrared photon decomposition of chloroethane with radiation of wavelength 10 pm, which is absorbed by the C-C stretching mode of the molecule. The study employed a focussed beam of maximum fluence 190 J/cm2. The fluence dependence of the fractional decomposition with b = 966 cm-I radiation under nearly collision-freeconditions (0.035 torr) was quantitatively interpreted by solution of the appropriate master equation (taking postpulse collisions into account) to obtain the energy-dependent absorption cross section, u(E). It was found that decomposition at high fluence (arising from energies in the range 19000-28000 cm-') and energy absorption at low fluence (mean absorbed energy 150-1000 cm-') could be simultaneously fitted with o(E)/cm2= 2.5 X exp[-3.45(E/h~)~.'~]. Comparison of this dependence with that found from data obtained with b 2700 cm-' suggests that the oscillator strength is strongly dependent on the mode being excited, even when the energy is thoroughly randomized within the molecule.
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Introduction It is generally accepted1 that infrared multiple photon dissociation (mpd) processes may be conveniently, albeit somewhat arbitrarily, divided into two classes: "large'! molecules, in which the quasi-continuum is reached after absorption of one or two quanta, and "small" molecules, in which 4 5 , or more photons must be absorbed, possibly in a coherent manner, before the quasi-continuum is reached. In order to clarify our understanding of this phenomenon, it is clearly interesting to study systems which are intermediate between these two categories. B a ~ e rhas ~ ?pointed ~ out that such intermediate cases are more difficult to analyze than either the normal-mode or ergodic limiting cases, and thus provide a more stringent test of our understanding of such systems. With this general aim in mind, we here present and interpret data on the mpd of chloroethane, using laser radiation of wavelength ca. 10 pm. Our results complement the extensive data obtained by Dai et al.4 employing ca. 3-pm radiation. These authors also report some results with 10-pm radiation, but the latter are less complete than those given here. Our data consist of fractional conversion of chloroethane to ethylene + hydrogen chloride as a function of fluence and wavelength, obtained under conditions of pressure (-0.036 torr; 1torr = 133 Pa) and laser pulse characteristics (-300-ns spike with no appreciable tail) such that collisional effects during the pulse may be neglected. I t has been shown5 that such data may then be fitted by solution of the appropriate master equation6 to give the energy dependence of the laser absorption cross section, u(E). It will be seen that comparison of the a(E) obtained from our data with that obtained from the data of Dai et ala4indicates that the oscillator strength for vibrational excitation retains a strong dependence on the mode being pumped, even at excitation levels at which the ' U S . Department of Health, Education, and Welfare Graduate Fellow, 1978-81. School of Science, Griffith University, Nathan, Qld. 4111, Australia. 0022-3654/82/2086-0724$01.25/0
energy is presumed to be completely randomized within the molecule.
Experimental Section The chloroethane irradiations were carried out with a Tachisto 215G TEA-COZ laser and the sample handling and transfer system described else where.'^^ Researchgrade chloroethane from Matheson was subjected to several freeze-pumpthaw cycles prior to use, following which no impurities-in particular, no ethylene or acetylenecould be detected by gas chromatography. Infrared photolysis was carried out on each sample with from 3000 to 9000 laser pulses at the specified pulse energy and line frequency. The beam was focussed into the sample with a 30-cm focal-length Ge lens. A spatial filter consisting of a lens system and pinhole was inserted into the beam path to ensure that the beam would have a Gaussian profile, which is necessary in order for the convolution procedure described below to be valid.g Since the stainless-steel pinhole previously used was found to melt in the focal zone of the COz laser beam, it was replaced with a Ta-foil pinhole, which eliminated this difficulty. The products were analyzed with a Perkin-Elmer Model Sigma-3B gas chromatograph with a flame ionization detector connected to a Spectra-Physics Minigrator inte(1) W. C. Danen and J. C. Jang In 'Laser-Induced Chemical Processes", J. I. Steinfeld, Ed., Plenum, New York, 1981, pp 45-164. (2) S. H. Bauer and K.-R. Chien, Chem. Phys. Lett., 45, 529 (1977). (3) S. H. Bauer, Chem. Rev., 78, 147 (1978). (4) H-L. Dai, A. H. Kung, and C. B. Moore, J . Chem. Phys., 73,6124 (1980); H-L. Dai, Ph.D. Thesis, Lawrence Berkeley Laboratory, University of California, 1981. (5) J. E. Eberhardt, R.B. Knott, A. J. Pryor, and R. G. Gilbert, Chem. Phys., in press. (6)W. D. Lawrance, A. E. W. Knight, R. G. Gilbert, and K. D. King, Chem. Phys., 56, 343 (1981). (7) J. S. Francisco, M. Findeis, and J. I. Steinfeld, Znt. J. Chem. Kinet., 13, 627 (1981). (8) J. S. Francisco and J. I. Steinfeld, Int. J . Chem. Kinet., 13, 615 (1981). ' (9) J. S. Francisco, J. I. Steinfeld, and R. G. Gilbert, Chem. Phys. Lett., 82, 311 (1981).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982 725
I R Multiphoton Decomposition of Chloroethane
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00
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0)
c e
.E
1
c 0
c 30 $?
,
1000 -56
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I
10
18
P, Torr
Flgure 1. Dependence of chloroethane fractional decomposition on pressure, at R1(6) C02 laser line and 0.32 Jlpulse. . -,
I
I
-60'
L 04
0
01
02
0 3
E (JI
Figure 2. Dependence of chloroethane fractional decomposition on pulse energy at R1(6) COPlaser line and 0.035 torr pressure. The soli curve is the prediction of the master-equation model, with parameters p = 0.14, x = -3.45 in eq 3.
grating recorder. The GC was equipped with a Poropak column and operated at an injection temperature of 69 "C, an oven temperature of 100 "C, and a detector temperature of 230 "C. The analysis system was calibrated for retention times and response factors with chloroethaneJethylene mixtures of known composition. The observed products (in addition to chloroethane) were only ethylene, resulting from elimination of HC1, and acetylene, arising from secondary mpd of the ethylene product.
Results The yield of ethylene ( Y K )after K laser pulses was measured as a function of chloroethane pressure, laser pulse energy, and laser line frequency. Since the yield generally did not exceed 2-3%, the fractional decomposition per pulse (f) can be well approximated as f = YK/K. Figure 1 shows the dependence of f on chloroethane pressure at the Ri(6) line [966.25 cm-'1 and a pulse energy of 0.32 f 0.01 J. The ethylene yield increases with chloroethane pressure over the range 0.036-1.60 torr, which is typical behavior for small molecules undergoing sequential excitation. Figure 2 shows the dependence off on infrared laser pulse energy when the laser is tuned to the R,(6) line. The chloroethane pressure is 0.035 f 0.001 torr, so that the molecules are nearly collision-free during the 300-ns laser pulse. The solid curve is the prediction of the master-equation model described in the following section. The dependence of Y on laser frequency is shown in Figure 3. The P(8)-R(40) lines of the 10-pm COPlaser
940
980 Y,
70
cm-'
Figure 3. Comparison of frequencydependent chloroethane decomposltion yield with low-resolution, low-intensity infrared absorption spectrum. The right ordinate gives the transmittance through 6.3 torr of chloroethane in a 1 k m absorption cell. The left ordinate gives the net decomposition of chloroathane after 3000 pulses at the Pj(8), R1(6), Rl(lO), R,(18), and R1(26) COPlaser lines at -0.2 J/pulse and chloroethane pressure of 0.040 torr.
transition coincide closely with the v8 [C-C stretching] fundamental of chloroethane, a low-resolution IR scan of which is also shown in Figure 3. For the photolysis runs, the pressure of chloroethanewas kept at 0.040 f 0.005 torr, the COz laser was adjusted to give -0.2 J/pulse on each line, and a total of 3000 laser pulses was used for each run. A strong line-to-line variation of Y with Iis evident, which can be represented roughly as the PQR structure of the absorption band shifted by 5 or 6 cm-' to lower frequencies.
Master Equation Interpretation of Data A complete interpretation of mpd data in principle requires proper inclusion of dynamics in individual states below the quasi-continuum, with account taken of anharmonicity and the interaction with the laser field,1° in addition to an energy-grainedmaster equation description of dynamics in the quasi-continuum. However, extensive theoretical and experimental studies of mpd in a large molecule, viz., ethyl acetate, have shown5that a quantitatively accurate interpretation of data such as fractional decomposition and average absorption cross section can be obtained through the use of an energy-grained master equation alone. This is in spite of the fact that, even at moderately high fluences and hence at high average internal energies, well into the quasi-continuum, the transient absorption spectrum of ethyl acetate shows a line width of ca. 50 cm-l, showing that some coherence effects are present despite the enormous density of states. This indicates (at least for a molecule as large as ethyl acetate) that an energy-grained master equation is a valid means of interpreting such multiple photon absorption observables which sample only high energies (e.g., fractional decomposition, etc.) despite the persistance of coherence effects. The molecular parameters, such as energy-dependent absorption cross sections, gained from this data fitting are of course energy-grained smoothings of the equivalent microscopic (state-to-state) quantities. This indicates (at least for a molecule as large as ethyl acetate) that an energy-grained master equation is a valid means of interpreting such multiple photon absorption observables which sample only high energies (e.g., fractional de(10) For a recent review, see H. w. Galbraith and J. R. Ackerhalt in 'Laser-Induced Chemical Processes", J. I. Steinfeld, Ed., Plenum, New York, 1981, pp 1-44.
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Francisco et ai.
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
composition, etc.) despite the persistence of coherence effects. The molecular parameters, such as energy-dependent absorption cross sections, gained from this data fitting are of course energy-grained smoothings of the equivalent microscopic (state-to-state) quantities. Chloroethane is appreciably smaller than ethyl acetate, and hence coherence effects in the former are expected to be greater. Nevertheless, coherence effects in ethyl acetate occur to a significant extent, and the success of a master equation interpretation for multiple photon absorption data for this molecule indicates that a similar treatment should also be applicable for interpreting similar data in chloroethane. The fully energy-grained master equation for mpd for reactant moderately dilute in a bath gas is as follows: = w J q P ( E E 9 g(E9 - P(E’E) g(E)1 dE’-
k(E) g(E)- [ L A @ )
+ LS(E)] g(E)+ LA(E - hv) g(E hv) + LS(E + hv) g(E + hv) (1)
Here g(E,t) is the time-varying population of molecules with internal energy E , w a reference (e.g., gas kinetic) collision rate, P(EJ9 is a suitably normalized probability for collisional energy transfer from E’ to E , k ( E ) is the microscopic reaction rate, LA@) and Ls(E)are respectively the rates for absorption and stimulated emission of radiation, and v is the laser frequency. For accurate data interpretation, eq 1 must also be coupled with the appropriate equation for the evolution of translational temperature in the systema5 An accurate solution of these equations is in general a computationally demanding task.5 However, if (as in the present case) the pressure is such that the effect of collisions during the laser pulse is negligible, in which case laser-driven and collisional events are nonoverlapping, eq 1 may be accurately and rapidly solved by using an eigenvalue procedure.6 Comparison with complete exact solutions shows that the u ( E ) obtained from such lowpressure data by the eigenvalue procedure (and by invoking the assumptions that temperature rise effects can be neglected and that reactant/reactant collisions may be treated as reactant/bath gas collisions) can be used to fit accurately a wide range of data, specificallythe fluence and pressure dependence of both fractional decomposition and average absorption cross section, obtained with reactant well diluted in various bath gases.5 We may therefore reliably employ the eigenvalue method for interpreting the present data on chloroethane. It is important to note that this method takes into account all postpulse events, Le., both decomposition and collisional relaxation. The calculation requires that the functions u(E),k ( E ) , and P(EJ9 be specified. The collisional term P(E,E? is taken to be an appropriately normalized exponential.” The data fitting is insensitive to the functional form of P(E,E9,depending almost entirely on the average energy transferred downward, this being estimated as 2000 cm-’. The microscopic reaction rate, k(E), was obtained in the usual manner from an RRKM fit to the extensive data on the thermal decomposition of chloroethane; the RRKM parameters of King et al.I2 were chosen for this purpose, these parameters being consistent with all recent experimental data on the thermal decomposition of chloroethane and its deuterated analogues. The laser absorption and stimulated emission rates are related by the usual microscopic reversibility relation,13 ~
~~~
(11) R. G. Gilbert and K. D. King, Chem. Phys., 49, 367 (1980).
(12) K. D. King, T. T. Nguyen, and R. G. Gilbert, Chem. Phys, 61,221 (1981)
and LA(E)is expressed in terms of the energy-dependent absorption cross section, u(E),as
L*(E) = J u ( E ) / h
(2)
where J ( t ) is the time-varying laser intensity. A flexible functional form was chosen for a(E),viz.
a(E) =
uo exp[-x(E/hv)@]
(3)
where uo is the observed ground-state absorption cross section (2.5 X cm2 in the present case) and x and P are parameters which are adjusted to fit the data. The data consist of fractional decomposition,f,per pulse for a focussed Gaussian beam of total energy, ET,ranging up to -0.4 J. Comparison with experiment may be made by computing the fractional conversion as a function of fluence (4) and then either deconvoluting the observed f(ET)data or convoluting the computed f($)data. Although the deconvolution can be readily carried out, the results may be prone to error (depending on experimental noiseg),and it is more accurate to convolute the computed f ( 4 ) . An exact analytical procedure for this convolution (for a Gaussian beam) has recently been given,gand was employed in the present case. The computation of f ( 4 )requires the time dependence of J ( t ) ,the laser intensity. The computed f(4)is in fact moderately insensitive to the pulse shape provided the fluence is well represented.‘Q* J ( t ) was therefore taken to be a step function of width 300 ns with height chosen to match the fluence. It was found that a fit to the f ( E T )data could be obtained by using x = 3.45, P = 0.14 in eq 3; the data fit is shown in Figure 2. These parameter values could be varied by no more than i l % and still reproduce the data in Figure 2. It should be emphasized, however, that the particular functional form given by eq 3 is by no means a unique way of representing a(E);rather, the curve obtained from the data fit should be considered a good representation of u(E) over a specified energy range. The rapid falloff of u ( E ) with E does suggest that resonant excitation is important at the lower levels, and is much less so as the molecule is excited into its “quasi-continuum”. This point is discussed further below. In the experiments reported here, in which focussed C 0 2 laser beams are employed, the total beam energy range examined (0.08-0.4 J) approximately corresponds to the fluence range 100-190 J cm-2. The average energy achieved by the molecule at the end of the laser pulse of duration t ’, defined as
for this fluence range is found to range from ca. 10000 to 15000 cm-’. One notes that in actual fact our o(E) will be valid over a considerably higher energy range, viz., those energies a t which extensive decomposition occurs. These energies are simply those at which g(E,t)k(E)assumes appeciable values over the range of fluences studied. Results from the master equation calculation show that the maximum energy contributing significantly to the data given here is ca. 28000 cm-l. The lower bound for the range of validity of u(E)is found to be close to the critical energy, viz., 228 k J mol-’ (ca. 19000 cm-I). (13) M. F. Goodman, J. Stone, and D. A. Dows, J . Chem. Phys., 65, 5052 (1976); J. Stone, E. Thiele, and M. F. Goodman, ibid., 59, 2909 (1973). (14) J. G. Black, E. Yablonovitch, N. Bloembergen, and S.Mukamel, Phys. Reu. Lett., 38, 1131 (1977).
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982 727
I R Multiphoton Decomposition of Chloroethane 1000
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i
750-
o
0%
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c 6
E
t
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i
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IO0 1
001
002 005 01 02 LASER FLUENCE ( J ~ m - ~ )
Flgure 4. Experimental points [ref 41 and computed values (solid line) for the average energy absorbed, ( E ) , as a function of infrared laser fluence. Note that the experimental values of ( E ) are relative rather than absolute, and these experimental results have been assigned an absolute scale by normalizing mean measured and computed ( E ) values at I$ = 0.02 J cm-’ (filled point).
The form of eq 3 was chosen to ensure that the correct limiting value of a(E) is obtained as E approaches zero, although of course one recognizes that the quasi-continuum approximation will be invalid for low energies. A useful probe of the validity of the energy range for a(E) is to compare predicted values for ( E ) from eq 4 with those obtained for chloroethane from photoacoustic measurements at low fluences. Such measurements have been reported by Dai et al.; employing 966-cm-’ excitation with an unfocussed beam, over the fluence range ca. 0.02-0.2 J cm-’. We have modeled these results by calculating ( E ) from eq 4, with populations g(E,t) obtained from solution of the master equation (eq 1)over this range of fluences. We used the pumping rate LA(E)given by eq 2 and a(E) given by eq 3, even though both of these functional dependences may not necessarily be valid at the low fluences encountered in the photoacoustic experiments. Nevertheless, it can be seen from Figure 4 that the agreement between measured and predicted dependence of ( E )upon fluence is quite good. It would appear therefore that the a(E) obtained by fitting the high fluence f(ET),valid over the energy range 19000-28000 cm-’, is also valid over the energy range contributing to the low fluence ( E ) measurements of Dai et al.,4 viz., ca. 150-1000 cm-’. One may therefore reasonably conclude that the a(E) given here is valid for all energies below 28000 cm-l, at least down to several hundred cm-’. I t must be borne in mind that this a(E) is smoothed over fine fluctuations in energy, since variables such as f and E are insensitive to fine structure on the scale of ca. 100 cm-‘ or less. The comparison of this a(E) with that found at other frequencies will be discussed in the concluding section. We have now given a full quantitative interpretation of the fractional decomposition/energy data of Figure 2. We next consider the data of Figures 1and 3. Figure 1 shows the pressure dependence of fractional decomposition of undiluted reactant. The increase in f with increasing pressure, which is typical of the behavior of “small” or “intermediate” molecules, can be ascribed to hole filling. Quantitative interpretation of such data requires that, at higher pressures, details of energy redistribution in reactant/reactant collisions be taken into account as must also coherence effects. The current lack of knowledge in these areas, together with the inadequacy of current methods of solving the resulting Boltzmann equation describing the population evolution, makes a precise quantitative interpretation of the data of Figure 1 an unviable proposition at present.
Figure 3 shows the fractional conversion as a function of the frequency of exciting radiation, with the zero-influence absorption spectrum shown for comparison. It can be seen that the f(?)curve follows the absorption spectrum approximately,while exhibiting the red shift that has been observed in a number of other “small molecules” systems. Note that, in “large” molecules, the frequency dependence off is virtually superimposable on the low-power absorption spectrum.’ Conclusions The foregoing interpretation of the 966-cm-’ multiple photon absorption data of chloroethane shows behavior which is sometimes characteristic of a “large” molecule and sometimes of a “small” one. Thus the successful fitting of f ( E T )and ( E ) data over a particularly large range of fluence with a single a(E) function shows that an incoherent energy-grained master equation treatment is applicable, which is the behavior expected for a “large” molecule. The red shift in the f(2) curve (Figure 3) and the hole-fitting behavior exhibited in Figure 1are, on the other hand, characteristic of “small” molecule mpd. It is interesting in this context to compare the a(E) for chloroethane determined here for 966-cm-’ excitation with the a(E) given by Dai et ala4for mpd using 2977-cm-’ excitation. The a, for both frequencies are similar (2.5 X and 1.5 X cm2 for 966 and 2977 cm-l, respectively). However, the a(E) at energies at which extensive decomposition occurs for these wavelengths are very different, e.g., at 20000 cm-’ (ca. 1000 cm-’ above the critical energy) a(E) cm2for ‘v = 2977 cm-’, while u(E) cm2 for ’v = 966 cm-’. This reflects the fact that, in the mpd process, the nature of the excitation mode (a C-C stretch for 966 cm-’ vs. a C-H stretch for 2977 cm-’) is an important factor in determining the kinetics. On the other hand, it is well established that energy is essentially completely randomized on the mpd timescale. The data presented here thus illustrate that, at least for a molecule that is intermediate between “small” and “large” in its mpd behavior, the efficiency of absorption of radiation at very high densities of states still reflects the nature of the excitation mode, i.e., that at such energies, the oscillator strength is strongly dependent on the mode being excited, even under circumstancesin which the energy is presumed to be completely randomized within the molecule. In the present case, the oscillator strength appears to be much less strongly “diluted” in the C-H stretching modes, dhich are weakly coupled to the lower-frequencyvibrations and retain a great deal of local-mode character even at high excitation levels,15 than in the C-C (10 wm) stretching region. These results suggest several avenues for further investigation. The extent to which structure may be present in the quasi-continuum absorption can be explored by “pump-probe”experiments (similar to those performed on ethyl acetate5 and SF61621), which would permit the transient absorption spectrum of the energized chloroethane molecule to be directly observed. Experiments on several deuterated analogues of chloroethane, in which the
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(15)H.L. Fang and R. L. Swofford, J . Chem. Phys., 72,6382(1980); 73,2607 (1980). (16)H.S.Kwok and E. Yablonovitch,Phys. Reu. Lett., 41,745(1978). (17)T.F. Deutsch and S. R. J. Brueck, J. Chem. Phys., 70, 2063 (1979). (18)V. N. Bagratashvili, V. S. Dolzhikov, and V. S.Letokhov, Sou. Phys. J E W ,49,8 (1979). (19)W. Fuss and J. Hartmann, J. Chem. Phys., 70,5468 (1979). (20)W.Fuss, Chem. Phys. Lett., 71,77 (1980). (21)J. L. Lyman, L. J. Radziemski, Jr., and A. C. Nilsson, IEEE J. Quantum Electron., QE-16, 1174 (1980).
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identity and oscillator strength of the excitation mode accessed by the COz laser differ from those of the parent, would also be valuable, and are currently under way in our laboratory.
Acknowledgment. This research was supported, in part, by U.S. Air Force Office of Scientific Research Grant No. 78-3725. Helpful discussion with Dr. H-L. Dai, and comments by Dr. A.W. Pryor, are gratefully acknowledged.
Photodissociation of a Molecule with Two Chromophores. CH,IBr S. J. Lee and R. Bersohn' Department of Chemistry, Columbia University, New York, New York 10027 (Received: Ju& 31, 1981; In Final Form: October 9, 1981)
When CH,IBr is dissociated with light in its first absorption band, which peaks at 258 nm, it is found that 86% of the fragmentations yield I atoms and 14% Br atoms. The anisotropy parameter, (3, in the angular distribution is 1.42. This fact leads to two conclusions: (1) the dissociation process is direct, and (2) the Br atoms are formed as a result of a weaker absorption to a different upper state than that reached in the main absorption.
CH,IBr is a prototype of a molecule with two possible dissociation pathways: CH21Br CH2Br + I (1)
-
CHJBr
-
CHJ
+ Br
(2)
On thermally dissociating such a molecule, one would expect a high degree of selectivity based on the fact that the C-I bond energy is substantially less than the C-Br bond energy (47 vs. 69 kcal/mol). Unimolecular decomposition on the ground-state surface should in principle be very selective for such large differences in bond energy. What determines selectivity, i.e., branching ratio, in the upper states? Clearly the nuclear potential surface of each electronic state must extend to regions of both dissociations. However, the vertical transition of the ground-state molecule to the upper state brings it to a region in which the potential may be repulsive in a specific coordinate, e.g., C-I or C-Br. Nevertheless, crossings are possible in which a system may cross from an excited surface repulsive in one coordinate to one repulsive in another coordinate. Thus, the dissociation of even such a small molecule as CH2fBr may be surprisingly complex. Here we make a simple beginning by investigating the branching ratio and the fragment anisotropy when exciting in the first absorption band.
Experimental Section The photofragment spectrometer which is used to measure the photofragment mass spectrum and the angular distribution of photofragments is described in detail elsewhere.' A molecular beam is crossed by light from a 1-kW high-pressure Hg-Xe lamp which passes through a filter solution (a mixture 0.3 M in NiCl, and 0.1 M in CoSO,) to cut off the shorter wavelength region and the infrared. The photofragment mass spectrum is obtained by using a quadrupole mass spectrometer. The angular distribution is measured by inserting a chopper and a polarizer in the light path. The signal from the mass spectrometer is amplified by a lock-in amplifier tuned to the chopping frequency and measured as a function of the laboratory angle 0 between the direction of detection and the photon polarization direction. The Polacoat polarizer (1) M. J. Dzvonik and S. Yang, Reu. Sci. Instrum., 45, 750 (1974).
0022-3654/82/2086-0728$0 1.2510
produces a degree of polarization p = 0.83 at 280 nm. The CHzIBr sample reservoir was kept at -33 "C but the molecular beam was warmed to 60 OC to produce a pressure in the reaction chamber around 5 X lo4 torr. Methylene bromoiodide2 was synthesized by refluxing an equimolar mixture of CH212and CH,Br2 under nitrogen at 140 "C (bp of CH,IBr a t 1 atm) for 1 week. The resulting mixture was separated by spinning band vacuum distillation and the desired distillate was collected at 71 "C at 128 torr. The purity of CHJBr was checked by VPC (5% SE-20 column) and NMR (6 = 4.56 ppm for CHJBr, 3.9 ppm for CH212,and 4.9 ppm for CH2Br2). The NMR spectrum was a single line a t 6 = 4.56 ppm.
Results Ultraviolet Absorption Spectrum and the Electronic Configurations of CH21Br. The spectrum of a 2 X lo4 M solution of CH21Brin hexane is shown in Figure 1. There are two bands with maxima at 268 nm (e = 0.93 X lo4 M-l) and at 213 nm (e = 2.88 X lo4 M-' cm-'1. Recalling that the first absorption maxima of CH31and CH3Br are at 258 and 202 nm, respectively, we see that the lower energy band is mainly due to the C-I chromophore and the second band mainly to the C-Br chromophore. The red shift and intensification of both bands as compared to the corresponding bands of the monohalides reminds us that there is moderate coupling between the two chromophores. The n u* nature of the first optical transitions of the hydrogen and alkyl halides, in which a nonbonding electron mostly localized on a a-type "lone pair" orbital of the halogen is promoted to an antibonding u* orbital of the C-X bond, has been extensively discussed3 and experimentally substantiated. In this model the electronic configurations of the ground and first two sets of excited states of CH21Br are
-
N: 3JQ:
U&B,U&-~T&T~ u&-B,u&-~T~,*~u&I 3 4 1 +-BP&-ITB~TI U C - B ~
(3) (4)
The Q notation, due to Mulliken,, means that each of the (2) G. S. Forbes and H. H. Anderson, J. Am. Chem. SOC.,67, 1911 (1945). (3) R. S. Mulliken, J. Chem. Phys., 8, 382 (1940).
0 1982 American Chemical Society
