J . Phys. Chem. 1988, 92, 1835-1839 ion determined, we can only see aualitativelv that the amounts of EuZ+ion and carbon dioxide gasthrough the reduction (eq 19) are much higher than those obtained from the photoreduction (eq 2) and disproportionation (eq 20) reactions, respectively. Then, detection of Eu2* ion in the irradiated solution seems to be due to its formation through cycle 4, that is, eq 19 for each revolution
1835
of the cycle in scheme a , Registry No. Eu”, 22541-18-0; CH,COOH, 64-19-7; CH,COONa, 127-09-3; HCOOH, 64-18-6; HCOONa, 141-53-7; Eu(OAc),, 16922-
co2,
. 05-7; E~(HCOO),, 3252-55-9; CH,OH, 67-56-1; H ~1333-74-0; 124-38-9; CH,, 74-82-8; C2H6,74-85-1; HCOO-, 71-47-6; CO, 630-08-0; CH,(OH)2, 463-57-0; Eu2(C03),, 5895-48-7.
Effects of Molecular Organization on Photophysical Behavior. Time-Resolved Fluorescence of a Pyrene-Labeled Phosphatidylcholine in Spread Monolayers of Dloleoylphosphatidy lcholine Maria Bohorquez and L. K. Patterson* Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 7, 1986; In Final Form: November 3, 1987) The time-resolved behavior of l-(6-pyreneylhexanoyl)-2-palmitoyl-sn-glycero-3-phosph~ho~ine ( 1-PyPC) has been examined in spread monolayers of dioleoylphosphatidylcholine (DOL)at the N2-water interface as functions of concentration of 1-PyPC and of surface pressure within the layer. Both excited monomer emission at 380 nm and excimer emission at 480 nm were monitored after excitation by a 80-ps pulse at 354 nm. It was found that for first-order processes the rate constants are comparable to those reported for pyrene in cyclohexane. Rate constants for the diffusion-controlledexcimer formation exhibit a small but definite dependence on surface pressure and may be used to obtain diffusion coefficients of the probe of (5-7) x io-’ cm2/s.
Introduction There are several different photophysical and photochemical pathways involving the singlet excited state of pyrene that are sensitive to the immediate environment of the chromophore. The band structure of the fluorescence spectrum itself, for example, responds to the polarity of the surrounding solvent as does the lifetime of the monomeric singlet state.’ The polarity of the surroundings has also been shown to govern exciplex and ion pair formation from the excited singlet state such that reaction with an electron donor becomes diagnostic for the nature of the microenvironment.2 However, the pyrene excited-state phenomenon most widely studied as a function of environment is excimer formation. Pyrene excimers are readily measured spectroscopically and allow one to monitor the diffusion of the chromophore moving through a particular medium. Such measurements have been applied to a wide variety of organizates from micelles and vesicles to cells.3 More recently, excimer kinetics have been examined in spread monolayers at the air-water interfaces4 In studies of molecular organization, this is one of the most fruitful model environments in which to work as it provides for the ready control of interactions among molecules which constitute the monolayer and alqo provides (1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970; Chapter 7. (2) Mataga, N. In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic: New York, 1975; p 113. (3) (a) Galla, H.-J.; Sackmann, E. Ber. Bunsen-Ges. Phys. Chem. 1974, 78,949-953. (b) Sackman, E. Z . Phys. Chem. (Munich) 1976,101,391-416. (c) Yonei, S.; Todo, T.; Kato, M. Radiat. Res. 1979, 80, 484-493. (d) Loughran, T.; Hatlee, M. D.; Patterson, L. K.;Kozak, J. J. J . Chem. Phys. 1980, 72, 5791-5797. (e) Donner, M.; Andre, J.-C.; Bouchy, M. Biochem. Biophys. Res. Commun. 1980, 97, 1183-1 191. (f) Sunamoto, J.; Nomura, T.;Okamoto, H. Bull. Chem. SOC.Jpn. 1980,53, 2768-2772. (g) Liu, B. M.; Cheung, H. C.; Chen, K-H.; Habercom, M. S. Biophys. Chem. 1980, 22, 341-355. (h) Georgescauld, D.; Desmasez, J. P.; Lapouyade, R.; B a k u , A,; Richard, H.; Winnik, M. Photochem. Photobiol. 1980, 31, 539-545. (i) Melnick, R. L.; Haspel, H. C.; Goldenberg, M.; Greenbaum, L. M.; Weinstein, S . Biophys. J. 1981,34,499-515. (j) Lukac, S. Photochem. Photobiol. 1982, 36, 13-20. (k) Luedi, H.; Hasselbach, W. 2. Naturforsch., C Biosci. 1982, 37C, 1170-1 179. (I) Viriot, M. L.; Guillard, R.; Kauffmann, I.; Andre, J. C.; Siest, G. Biochim. Biophys. Acta 1983, 733, 34-38. (4) (a) Subramanian, R.; Patterson, L. K. J. Am. Chem. SOC.1985, 107, 5820. (b) Subramanian, R.; Patterson, L. K. J. Phys. Chem. 1985, 89,
for monitoring those interactions on a continuous basis. Those initial measurements involved time-resolved fluorescence of a pyrene probe, 10-pyrenedecanoic acid, PyDeca, in matrices of phosphatidylcholine with distearoyl, dioleoyl, or dilinoleoyl hydrocarbon chains.4a It was found that the apparent decay of excimer fluorescence was concentration dependent and could be interpreted in terms of a variation of Birk’s treatment of excimer kinetics a t high chromophore concentrations.’ From these data it was possible to derive values for diffusion coefficients in these lipids which agreed quite well with values obtained by other techniques. However, the mechanism for excimer formation was not unambiguous in that system and may be clarified by the present work. The behavior of those systems may be seen in light of the present study, and a comparison of the two is given at the end of the discussion. The behavior of the system described above suggested that one may expect the geometry of the carrier molecule on which the probe is located to play a significant role in the excimer kinetics. Both lateral diffusion as well as molecular alignment of the carrier for proper chromophore contact can be affected. Additionally, the miscibility of the probe in the monolayer matrix can be expected to depend on the character of the carrier as well as the lipid making up the monolayer. To explore the role of the carrier molecule, excimer behavior of the probe, 1-(6-pyreneylhexanoyl)-2-palmitoyl-sn-glycero-3-phosphocholine( 1-PyPC) has been measured in spread monolayers of dioleoylphosphatidylcholine (DOL).The structure of these two lipids is
1202-1 205.
0022-3654/88/2092-1835$01.50/0
0 1988 American Chemical Society
1836 The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988
Bohorquez and Patterson
X(nm) 02
(A/molrculr)
Figure 1. Force-area isotherms for (a) probe (I-PyPC); (b) host lipid (DOL);(c) a 1:l mixture of (a) and (b).
This host lipid bears singly cis-unsaturated hydrocarbon chains. The host and probe are, in this case, of comparable size and head-group structure. Further, the DOL matrix system may be expected to remain in the fluid state over all surface pressures below the collapse point, avoiding the complications of phase transitions which can impose additional constraints on the system in addition to those of changing surface pressure.
Experimental Section The Langmuir trough and associated spectrofluorometer equipment used for steady-state and time-resolved measurements are described in detail e l ~ e w h e r e . ~ ~While ’ a Photochemical Research Associates (PRA) LNlOO pulsed nitrogen laser was used for a portion of these measurements, it has been replaced for the remainder-especially those at very low probe concentration-by a mode-locked, Q-switched Quantronix 416 Nd:YAG system which provides an 80-ps pulse with a frequency of 5 kHz at the tripled line, 354 nm. Integrated power at this frequency is about 10 mW. Dioleoylphosphatidylcholine,DOL, was purchased from P L Biochemicals. The probe, 1-(6-pyreneylhexanoyl)-2-palmitoyl-sn-glycero-3-phosphocholine( 1-PyPC) was obtained from KSV Chemicals. Results Force-Area Measurements. During each of the emission measurements described below, the thermodynamic behavior of the PyPC:DOL monolayer was monitored by means of a Wilhelmy plate as a function of surface area to produce a force-area isotherm. From these data one may establish the “encounter” conditions (onset of positive surface pressure) and the collapse points of the system. Curves for pure probe, DOL, and a 1:l mixture are given in Figure 1. It may be seen that there are no evident phase transitions which would indicate immiscibility of the two monolayer components, and the shape of the curve suggests that the liquid-expanded character for all three systems persists until the surface pressure exceeds 35 dyn/cm. The area per molecule of the probe lipid ( 1-PyPC) may be seen to exceed by very little the size of the host DOL. Steady-State Emission Measurements. Emission spectra for 1-PyPC in DOL monolayers have been determined at the N2( 5 ) Agrawal, M. L.; Chauvet, J.-P.;Patterson, L. K. J . Phys. Chem. 1985, 89, 2979-2982. ( 6 ) Vaidyanathan, S.; Patterson, L. K.; Modius, D.; Gruniger, H.-R. J . Am. Chem. Sac. 1985, 89, 491-497. (7) Federici, J.; Heman, W. P.; Hug, G. L.; Kane, C.; Patterson, L. K. Compur. Chem. 1985, 9, 171-177.
Emission spectra of 1-PyPC in DOL monolayers: (a) probe:lipid ratio of 1:7; (b) probe:lipid ratio of 1:3; (c) 5 X M 1-PyPC in benzene (bulk phase). Figure 2.
water interface, and those corresponding to 1ipid:probe ratios of 1:3 and 1:7 are given in Figure 2. Each was determined at a surface pressure of 30 dyn/cm. Included is a spectrum of 5 X 10” M probe in benzene solution for reference. It may be seen that the dominant monolayer fluorescence is that from the 1-PyPC excimer which emits in the characteristic broad band centered around 480 nm. This is expected as a consequence for high local concentration of 1-PyPC used; a comparison of spectra at the two concentrations shows the increase in the long-wavelength band with the expected increasing proportion of probe. However, the vibrational structures of the monomer spectra are clearly observable in the region 380-400 nm. The solution spectrum of the probe provides essential agreement between monolayer and solution spectra with regard to peak positions and relative intensities of the monomer bands. Time-Resolved Emission. Kinetics of the probe behavior in DOL were determined from lifetime measurements of pyrene emission from both the monomer (380-ilm) and excimer (480-nm) bands. Because of the data treatment given below, all data were fit by an equation of the form
with
G(t) = aIe-AI1 + a2e-A2f I ( ? ) is the system response function to the laser pulse, and G ( t ) is a double exponential to describe fluorescence behavior. Figure 3 provides typical time-resolved data from the two wavelengths showing the expected complementary behavior of A, and A, (see below). In all the cases, the values of a2 for growth of pyrene excimer at 480 nm are negative. The constants A, and X2 as functions of 1-PyPC concentration in molecules/cm2 at surface pressures of 2.5, 10 and 30 dyn/cm are plotted in Figures 4 and 5. The data for 2.5, 10, and 30 dyn/cm are presented numerically in Table I. It may be noted from Figures 4 and 5 that the data at 2.5 dyn/cm exhibit significant scatter for XI vs concentration and are rather better behaved for X2.
Discussion The accepted scheme for singlet pyrene behavior developed by Birks et a1.* may be written in terms of the processes PyPC*
+
PyPC* PyPC*
-
PyPC
+ hu
PyPC
+ PyPC
-
Ex*
kf,
(a)
kdm
(c)
k,,
Time-Resolved Fluorescence of 1-PyPC
---
EX* EX*
PyPC*
PYPC
EX*
+ PYPC
+ PYPC + hv
PyPc
+ PyPc
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1837 kmd
id)
kfd
(e)
kgd
(f)
where, for analysis, one may group first-order terms by the expressions kM = kf, + k,, and kD = kfd + kgd and may introduce the expressions X = kM + k,j,[PyPC] and Y = kD kmd. FOllowing the development of Birks provides a description of the time-resolved-fluorescence behavior in terms of the equations = 0.5[X + Y - ( ( Y - a 2+ 4kdmkmd[PyPC]]1/2] ( 2 )
+
A2
= 0.5[X + Y + {(Y-X)’ + 4kd,k,d[PYPC])1/2]
(3)
where 1/7’ = XI, 1/72 = X2 (4) For the scheme to hold, cy, and a2taken for excimer fluorescence should be of the same magnitude but opposite sign due to the growth (negative sign) and decay of excimer fluorescence.8 In our measurements, this ratio a1/cy2was observed to have an average value of close to 0.8 with no systematic deviation due to pressure or concentration except that at 2.5 dyn/cm this value was found to approach 0.75. Such deviations from unity have been suggested by de Mayo et al. to indicate chromophore agg r e g a t i ~ n .However, ~ in that work involving pyrene photophysics on decanol-covered silica, values of X, and X2 obtained at monomer and excimer wavelengths varied widely, whereas in the present work agreements are quite reasonable (better for XI at low concentration and for X2 at high concentration) though dependent on scatter of the data. When values of XI and X2 are plotted against probe concentration in molecules/cm2, Figures 4 and 5 , it may be seen that the data follow a pattern similar to that observed in bulk pyrene solutions.’ This means that the curve corresponding to X2 exhibits rate constants for the growth of the excimer fluorescence which increase slowly from an intercept of 4 X lo7 s-l at low concentrations of 1-PyPC and then rise rapidly at concentrations above 2 X 1013molecules/cm2. The curve for XI grows from an intercept of 4 X lo6 s-l before becoming flat a t concentrations above 2 X 1013molecules/cm2. The systems, bulk and monolayer, differ in two ways. First, in the monolayer there are vertical displacements of both intercepts by a factor of 2. These intercepts, where [I-PyPC] 0, correspond to kD and k,, respectively. Second, the concentration range at which the curves flatten is about 50 times higher for the monolayer if one compares the concentrations on the basis of fractional weight of pyrene in the system. This may suggest significant stereochemical restraints on formation of an excimer in the monolayer from the labeled lipids. With the data available and the assumption that the monolayer system follow the scheme above, we can determine all the rate parameters k,, kd, kdmand kmd. One method proposed by Birks for evaluation of those parameters involves measurements at differing concentrations and the relationshipslo = (AX2 + 1)/(A 1) (5)
-
20
c
. .
..
+
x
kd, [PYPC] = x - k, Y = XI
+ A, - x
kmd = ( X - X I ) ( X ~-X)/kdm[PYPcI
M o l r / cma
(6) (7)
(8)
-
kd = Y - kmd (9) As mentioned above, X1 may be taken as k, for [ 1-PyPC] 0. In addition to determining kdmfrom these equations, it was also obtained from the relationship d(Xl+X2)/d[M] = kdm,where [MI is the concentration of ground-state probe. The plot of data cast in this manner is given in Figure 6 . A for eq 5 can be obtained exclusively from the fluorescence response function of the mo~~~
TIME ( n s )
Figure 3. Time-resolved fluorescence of 1-PyPC in a DOL monolayer at molecular ratios of 1:240 under 2.5 dyn/cm surface pressure. (a) Fluorescence was monitored at 480 nm;(b) fluorescence was monitored at 380 nm. In both cases, r2 = 1 / X 2 = 40 ns and r1 = l/Xl = 100ns.
~~~
~~
~
(8) Birks, J. B.; Dyson, D. J.; Munro, I H Proc. R. SOC.London, A 1963,
275 ., -575 ..
(9) de Mayo, P.; Natarajan, L. V.; Ware, W. R. J . Phys. Chem. 1985,89, 3526-3530. (IO) Birks, J. B.; Dyson, D. J.; King, T. A. Proc. R . Soc. London, A 1964, A272, 270.
I
10’’
Figure 4. Plots of XI as functions of surface pressure versus concentration in molecules/cm2. 0 , data taken at 2.5 dyn/cm; 0,data taken at 10 dyn/cm; 0,data taken at 30 dyn/cm.
nomer. Table I shows the fluorescence X values for each concentration and pressure as well as the values of A used in the calculations based on the scheme eq 5-9. The results of these calculations are given in Table I1 along with corresponding parameters for pyrene in cyclohexane bulk solution.I0 It may be seen that, for those processes which are first order, there is good agreement between results in cyclohexane and the probe in the monolayer environment. The parameter kdmcannot be so readily compared in the two environments. However, because it describes a second-order process that depends on the character of the medium through which the probe moves, kd, can be interpreted to give the coefficient of lateral diffusion, D,of the probe in the monolayer; the relationship given kd,/4, has been applied to kdm.3aFrom Table by Sackman, D
-
1838
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
Bohorquez and Patterson
TABLE I
surf press., dyn/cm
molec/ area X
30 30 30 30 30 30 30 30 30 30 30 30 10 10 10 10 10 IO IO 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
A =
x1 x
ai2/(~1
10-7, s-1
(at 380 nm)
at 480 nm
at 380 nm
68.58
1.34 1.34 1.29 1.27 1.17 I .04 0.89 0.74 0.7 1 0.49 0.49 0.48 1.45 1.45 1.38 1.38 1.22 1.05 0.8 I 1.72 1.56 2.09 1.43 1.98 1.51 1.51 1.15
1.32
6.58 7.03 3.59 2.63 2.53 1.85 1.16 0.69 0.72 0.23 0.10 0.06 5.32 2.67 2.21 1.88 1.40 0.85 0.52 4.48 4.47 2.41 1.84 1.52 1.20 0.7 1 0.40
4.34 1.46
0.45 0.27 0.42 0.28 3.96 0.63
2.64 2.16
0.71 0.46 0.47 0.45 3.62 3.24
h2 x 10-7, s-1 at 480 nm at 380 nm
24.69 15.66 7.67 6.27 5.48 3.12 3.25 3.05 2.13 3.50 15.37 5.89 19.54 9.174 6.39 5.35 3.95 3.81 3.38 33.55 21.62 11.65 10.53 9.51 5.15 6.34 4.23
19.03 8.32 5.46
5.93 2.81 4.42 4.52 10.51 8.34
012/011
at 480 nm
-1.178 -0.62 -0.743 -1.02 -0.871 -0.76 -0.84 -0.78 -0.65 -0.83 -0.85 -0.946 -0.73 -0.8044 -0.666 -0.806 -0.8 1 -0.86 -0.81 -1.46 -0.73 -0.73 -0.83 -0.71 -0.75 -0.72 -0.77
3 t
3-
2-
I
,
2
I
I
3
4 MOIK
/ c m p I:
I
I
1
I
5
6
7
B MOIW /
10-l~
Figure 5. Plots of A, as functions of surface pressure versus concentration in molecules/cm2. 0 , data taken at 2.5 dyn/cm; 0,data taken at 10
dyn/cm; 0,data taken at 30 dyn/cm. TABLE I1
system
k,"
kD"
kmda
kdmb
2.66 at 10 dyn/cm
PyPc in DOL
0.45
pyrene in cyclohexane' 0.23
1.66
1.54
1.60 0.65
2.15 at 30 dyn/cm 6.70 X lo9 M-' s-'
"X10-7 s-l. b X 1 0 6 s-I cm2 mol-'. 'Reference 10.
I1 one obtained D = 5.4 X cm2/s for 30 dyn/cm and 6.7 X lo-' cm2/s at 10 dyn/cm. These numbers approach in magnitude those reported by Sackman for PyDeca in lipid bilayers.3b It may be seen in Table I1 that the value of kdm (and consequently, D) decreases slightly as the pressure increases from 10 to 30 dyn/cm. This is also implicit in the slopes of Figure 5 at high concentrations. From the force-area curves given in Figure 1 it may be seen that a change in pressure from 10 dyn/cm to 30 dyn/cm represents a change in molecular density at the surface by about a factor of 2. This limited effect of surface pressure on microviscosity is not unexpected; in earlier work with self-quenching of chlorophyll in a DOL matrix no effect of surface pressure on chlorophyll-lipid interactions was ~ b s e r v e d .It~ is evident from that study and the
cm' x 16"
Figure 6. Plots of (A, + h2) as functions of surface pressure versus concentration in molecules/cm2. 0,data taken at 10 dyn/cm; 0,data taken at 30 dyn/cm.
present work that the cis double bonds which characterize the hydrocarbon moiety of DOL prevent significant changes in lipid packing with compression. By contrast, use of a lipid that undergoes a true phase transition upon compression did significantly alter the chlorophyll self-quenching mechanism and would be expected to have a much more significant effect on probe diffusion than simple compression." Studies are presently under way to characterize the effects of lipid phase transitions on 1-PyPC photophysics in spread monolayers. One may also note the low degree of precision to be found in data gathered as a function of concentration at 2.5 dyn/cm. This suggests a significant lack of reproducibility in layer organization in the onset region of positive pressure perhaps arising from patches of material that coalesce slowly at the low pressures. This experience would suggest that care should be exercised with measurements in this region. All our experimental findings, save some deviation in the ratios of a l / a zat 480 nm, are consistent with the scheme outlined above (eq a-f), and the excimer growth data suggest that preexisting ( 1 1) Chauvet, J -P, Agrawal, M L , Patterson, L K J Phys Chem , in press
J. Phys. Chem. 1988, 92, 1839-1846 aggregates as precursors to emission in the long-wavelength region play only a minor role. By contrast, in earlier work using the simple pyrene-labeled fatty acid PyDeca, in lipid monolayers, the fluorescence behavior indicated a high degree of ground-state association among the chromophore^.^^ With the lipid data at hand, a comparison between kinetic behavior in this system and behavior in the parallel study with PyDeca in DOL is instructive. By contrast with 1-PyPC, which exhibited expected growth of the excimer fluorescence with time, no significant growth of excimer was observed with PyDeca; the excimer fluorescence growth followed the exciting pulse. This would suggest a predominate ground-state arrangement of PyDeca molecules, most likely involving some pairing or clustering of the labeled fatty acids, which immediately form excimers upon excitation. With systems following the scheme outlined (eq a-f), plots of the relationship d(Al+A2)/d[M] (see Figure 6) or dX2/d[M] 0 give values of kdm. Yet, for PyDeca, only dA,/d[M] I/[M] gave a linear relationship at high concentrations from which a reasonable value of kdmcould be obtained. This behavior at high concentrations of PyDeca is consistent with a kinetic scheme which would include the steps (PyDeca), + hv (PyDeca),* (g)
-
(PyDeca),* (PyDeca),*
-
-
(PyDeca),
+ (PyDeca),
+ hu'
?(PyDeca)
-
(i)
If indeed there is pairing of (PyDeca) then x 2. Step is is suggested since we have observed very short lifetime components for aggregates of pyrene fatty acids above the surface
1839
collapse point. Encounters between (PyDeca),* and (PyDeca),, even if momentary, would-in this scheme-give quenching and account for the Stern-Volmer type of behavior observed where (PyDeca), >> (PyDeca),*. The dimensions for (PyDeca)* and for PyPC would be expected to be roughly equal, and hence, the diffusion coefficients should, as observed, be of the same order of magnitude. The intercept of a Stern-Volmer type plot (A, vs [PyDeca]) would give ~ / T Dfor the excimer or kD. The value taken from PyDeca measurements of -1 X lo7 s-l (assuming Stern-volmer behavior), approaches the value 1.66 X IO7 s-l for k , obtained in the present work. Additional comparisons of these two types of probes will be required to verify these suggestions. The present comparison of the two probes strongly suggests that the lipid presents far less complicated behavior and, in the study of lateral diffusion in monolayers, is the carrier of choice. These data above lend further weight to the concept that spread monolayers provide an excellent means with which to examine the environment provided by the hydrophobically driven lipid organization. For 1-PyPC there appear to be no significant effects of the environment on first-order processes when compared to bulk solution, and lateral diffusion of the probe here follows closely that observed in bilayer structures. In examining the nature of processes in bilayers, then, information derived from the more readily controlled monolayer can be great value.
Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2909. Registry No. 1-PyPC, 110419-98-2;DOL, 4235-95-4.
Intramolecular Rearrangement in the Infrared Multiple-Photon Dissociation of Dichlorodifluoroethylene Graham Hancock* and Kenneth G. McKendrick Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3Q2,U.K. (Received: June 19, 1987: In Final Form: October 16. 1987)
The infrared multiple-photon dissociation (IRMPD) of l,l-dichloro-2,2-difluoroethylene, CF2=CCI2, ha: been investigated. Under essentially collisionlessphotolysis conditions, four electronic ground-state radical fragments, CF2(X1AI),CC12(X1AI), CFCI(%'A'), and CCI(X*n), were detected directly by laser-induced fluorescence (LIF). Production of atomic CI was also inferred by an indirect infrared chemiluminescence technique. The ratio of products CF2:CFCI:C1was established, within limiting assumptions, by subsidiary experiments involving IRMPD of CF2=CFCI and CF2C12. Yields of the various fragments were measured as a function of the photolysis laser energy, allowing conclusions to be drawn on the involvement of alternative modes of decomposition and the extent of possible sequential fragmentation. Arguments are presented for the likelihood of atom migration in the excited parent molecule, prior to bond fission, involving both CI and F atom shifts. The production of CC1 is thought to be most probably the result of secondary photodissociation of the halovinyl radical formed by primary CI atom loss.
Introduction In recent years, a very considerable volume of literature has accumulated concerning the processes induced in gas-phase molecules by the multiple-photon absorption of infrared laser A wide range of experimental conditions has been accessed in these studies from the isolated, truly unimolecular conditions of molecular beam and very low pressure systems to the highly collisional regime of high-pressure bulk gases. In those investigations where the aim has been to observe the consequences (1) Schulz, P. A.; Sudbo, Aa. S.; Krajnovich, D. J.; Kwok, H.S.; Shen,
Y.R.; Lee, Y.T.Annu. Reu. Phys. Chem. 1919, 30, 319.
(2) Ashfold, M. N. R.; Hancock, G. In Gas Kinet. Energy Transfer 1981, 4, 73.
(3) King, D . S. Ado. Chem. Phys. 1982, 50, 105.
0022-3654/88/2092-1839$01.50/0
of absorption of sufficient radiation to induced chemical change, such as isomerization or dissociation, an equally broad range of detection techniques has been employed, from conventional end-product analysis, where considerable extrapolation may often be required to deduce the primary dissociation mechanism from the observed stable products, to direct, relatively unequivocal real-time spectroscopic or mass spectrometric detection, where not only the identity of the fragments but also details of energy partitioning may be e~tablished.'-~ The advantages of infrared multiple-photon excitation (IRMPE) and dissociation (IRMPD) for the study of unimolecular processes in isolated molecules are well-known; the more obvious of these include the preparation of a sample of highly internally excited molecules in the absence of bimolecular (assuming a sufficiently low density and short observation time) and heterogeneous pro0 1988 American Chemical Society
