J. Phys. Chem. 1883, 87, 1701-1707
of lptl I10 D, required for internal consistency of the model, is a realistic expectation for the dipole moment.12J3 We conclude that the suggested model is consistent with the experiments if, reasonably, lptl is in the range of approximately 2-10 D and u ( t l )- u ( t t ) is less than -0.10 eV. Obviously this model is very crude and does not take into consideration that in a real stack dipolar molecules are not geometrically symmetrical (i.e., a molecule may not fit into the same space on rotating 180° about an axis perpendicular to the dipole), are not necessarily situated exclusively within one plane, and might adopt other than strictly parallel and antiparallel orientations, and that CY may be a function of E. The model gives, therefore, only a qualitative idea of the effects of the electric field. It requires that the interaction between dipoles of different stacks be small compared to the interactions within stacks. In the case of strong three-dimensional molecular interaction a potential barrier for the field-induced reorientation of molecular dipoles must be so high that it is inconceivable that fields of 10-20 kV/cm would produce any significant effect. Estimates of the field-induced reorientation against a force constant typical of molecular crystal librational motions show a shift of equilibrium orientation by less than degree. Presumably, then, the cores must be composed of a more or less ordered assembly of stacks with a structure reminiscent of smectic liquid crystals. Probably this is the cause of the poor X-ray diffraction pattern of the quasi-crystals. Two important aspech of the SHG experiments are not described by this model: observation of a residual SHG
1701
signal prior to field perturbation and the asymmetric SHG response to fields of different polarity. These effects may be due to the fact that dipoles within the stacks are subjected not only to the applied field but also to the fields arising from surrounding molecules, stacks, and globules. While the globules were formed in the presence of an aligning field, removal of that field would leave correlated nonzero interstack field and interglobule fields, preserving the inequality W T> WJ.The asymmetry must be associated with a higher ordering or arrangement of stacks which somehow does not allow for a hysteresis symmetric about zero voltage. The latter point touches on the possibility of a distribution of nonidentical stacks, another factor not treated by our simple model. One final suggestion to explain the electric field dependence of the SHG is presented. It is possible that the stacks themselves are pushed by the electric field against the surrounding stacks and amorphous material. Since the dipole moment of stacks must be relatively large, the torque which they exert against their surroundings may be enough to allow slight twists or perhaps alteration of the strains developed in the material upon removal of the aligning field used in the thread-forming process. Details of why or how an alignment substantially larger than the zero-field residual is created later in comparable fields are unclear, which reduces, in our eyes, the probability that this explanation is appropriate. Registry No. 1-[@-(Methacryloyloxy)ethyl]-3,3-dimethyl-6'nitrospiro[indoline-2,2'- [2H-11benzopyran], 25952-50-5.
Comparison of Multiphoton Ionization-Fragmentation Behavior of Benzene, Fluorobenzene, and Toluene David W. Squlre, Michael P. Barbalas,+ and Richard B. Bernsteln" Department of Chemistry, Coiumbie University, New Yo& New York 10027 (Received: November 2, 1982)
The relative intensities and fragmentation pathways of benzene and fluoro- and methyl-substituted benzene under multiphoton ionization have been compared as a function of both excitation wavelength and laser pulse power. A two-photon excitation resonant with the I L b band was utilized. The results of Goodman and Rava (1981)for the relative total ion intensities are confiied. The fragmentation is found to be laser power dependent but essentially wavelength independent over the range 500-534 nm. The energetics and the fragmentation results imply a standard resonance enhanced multiphoton ionization 2 + 2 process to form the molecular ion, followed by a one-photon absorption and photodissociation of the excited molecular ion to yield the observed fragments.
Introduction The coupling of multiphoton ionization with mass spectrometric detection has been used to elucidate the multiphoton spectroscopy of a variety of molecular systems.1*2 The systematics of the various processes which occur during the production of ionic fragments during multiphoton ionization have been recently reviewed.'b+ The multiphoton spectroscopy of a variety of substituted aromatic compounds has been ~ t u d i e d . ~ ~ Goodman **~-~~ and Rava (GR) have explored the two-photon behavior of a series of monosubstituted benzenes and developed a 'Current address: Hoffman-La Roche, Inc., Nutley, NJ 07110. *Current address: Occidental Research Corp., Imine, CA 92713.
perturbation theory to explain their They treat the shifts in frequency and intensity of the so-called (1)R. B. Bernstein, J. Phys. Chem., 86,1178 (1982). (2)U.Boesl, H.J. Neusser, and E. W. Schlag, J. Chem. Phys., 72,4327 (1980). (3)P.M.Johnson and C. E. Otis, Annu. Reu. Phys. Chem., 32, 139 (1981). (4)M.A.Duncan, T. G. Dietz, and R. E. Smalley, J.Chem. Phys., 75, 2118 (1981). (5) D. A.Lichtin, R. B. Bernstein, and K. R. Newton, J. Chem. Phys., 75,5728 (1981). (6) W. Dietz, H. J. Neusser, U. Boesl, E. W. Schlag, a n i S.H. Lin, Chem. Phys., 66,105 (1982). (7)B. A. Heath, M. B. Robin, N. A. Keubler, G. J. Fisanick, and T. S.Eichelberger, IV,J. Chem. Phys., 72, 5565 (1980). (8)A. Sur, J. Knee, and P. Johnson, J. C h e n . Phys., 77, 654 (1982).
0 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87,No. 10, 1983
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TABLE I : Observed Total Ion Intensities‘ total ion intensity counts pulse-’
500.0 nm
9.2 X l o 6 0.23 X l o 6
0-0 E1 ( 7 0 e V )
3.6
14; (504.4 n m )
x
500.0 nm 0-0 ( 5 2 8 . 8 n m ) E1 ( 7 0 e V )
2.3 X lo6 1.4 X l o 6 0.64 X l o 6 8 . 2 x 104
14; (512.4 n m ) 500.0 nm 0-0 (533.6 n m ) E1 ( 7 0 e V )
Benzene 18.5 x 1014 0.46 x 1014
104
2.2 x lo6 0.32 X l o 6 0.041 X l o 6 4.6 x 104
14; ( 5 0 7 . 6 n m )
re1 signal strength
counts
3.6 X
1 0.025
lo1”
1
0.0039
Fluorobenzene 4 . 5 x 1014 0.65 x 1014 0.082 X 1OI4 4.6 X 10’’ Toluene 4.6 x 2.7 x 1.3 x 8.2 X
GR resultsb
1014
1014 1014 10’”
0.24 0.035 0.0045 0.0050
0.50
0.25 0.15 0.069 0.0089
0.50
0.025
0.25
a Total ion intensity values are scaled t o 1.0 x torr and corrected for ion gauge sensitivity. They are uncorrected for spectrometer transmission. E1 values are a t 2.50-A filament current and 0.20-MAelectron trap current. MPI values are scaled to 10 m J per pulse and averaged f r o m several data sets, taking into account t h e observed quadratic power dependence. Reference 11. Relative signal strengths are calculated from the “per pulse” results.
-
Lb band in the substituted benzenes as arising from inring charge transfer troduction of substituent group character into the ring wave functions, and thereby explain an observed reversal of relative Franck-Condon strengths between the one-photon and two-photon spectra. Their results give a consistent picture of the multiphoton spectroscopy of these systems. The present fragmentation study of the ion products is intended to provide insight into the ion dissociation processes as a function of both excitation wavelength and laser power. Such a study also permits a more accurate determination of relative ion intensities for the monosubstituted benzenes.
14;
TRANSITION
s LOPE 0 BENZENE 503.4nm 2.0 0+F 507.6nm 2.0 A +CHS 512Anm 2.1 2.50
m
J
a z 5? 1.50 z
Apparatus The apparatus used in these studies has been described s J previ~usly.~ The experimental conditions are those of ref 20 5 except as noted. All experimental data were taken with c a 0.25-m focal length spherical lens. Data were averaged 1.00 W for 2000 laser pulses. Exciton C500 dye was used 0 -I throughout. Background pressure was 2 X torr in the ionization region and 1 X loT7torr in the flight tube. The samples of substituted benzenes were vapors taken directly 0.50 off the commercially pure liquids after a degassing procedure. Pressures used were 5.0 X torr in the ionization region (uncorrected for the varying sensitivity of the ionization gauge). All gases were introduced through the molecular leak inlet. Electron ionization (EI) mass spectra 0.00 0.2 0.4 0.6 0.8 1.0 were recorded with pressures of ca. 2 X lo+ torr in the L O G (POWER IN m J ) ionization chamber to verify sample purity was adequate. Pressures quoted in the figure captions are nominal Flgure 1. The logarithm of the total ion intensity for the 14; transition for the I+band plotted against the logarithm of the average laser power readings uncorrected for ion gauge sensitivity. All tabu(expressed in terms of pulse energy in ml hereafter) for ( 0 )benzene lated data, however, utilize corrected pressure. The corat 504.4 nm, (A)toluene at 512.4 nm, and (0)fluorobenzene at 507.6 rection factor was obtained from literature values for ion These results were taken at nomlnal ion gauge pressures of 5.0 gauge sensitivities and electron impact cross s e ~ t i o n s . ’ ~ J ~ nm. X lo-‘ torr with a 0.25-m focal length lens.
-
(9) L. Zandee and R. B. Bernstein, J . Chem. Phys., 71, 1359 (1979). (10)L.Zandee and R. B. Bernstein, J . Chem. Phys., 70,2574(1979). (11)L. Goodman and R. P. Rava, J. Chem. Phys., 74,4826 (1981). (12)L.Goodman and R. P. Rava, ‘Advances in Laser Spectroscopy”, Vol. 1, B. A. Garetz and J. R. Lombardi, Ed.,Heyden, Philadelphia, 1982, pp 21-53. (13)L.Goodman and R. P. Rava in “Advances in Chemical Physics”, Vol. 54, I. Prigogine and S. A. Rice, Ed., Wiley, New York, 1982,Chapter 2.
(14)R. P.Rava and L. Goodman, J.Am. Chem. Soc., 104,1014(1982). (15)K.Krogh-Jesperson,R. P. Rava, and L. Goodman, Chem. Phys., 44,295 (1979).
The correction values used (applied to the nominal ion gauge readings) were 5.8, 5.8, and 6.8 for benzene, fluorobenzene, and toluene, respectively.
Results Figures 1-3 are log-log plots of the total ion signal vs. average laser power (actually laser pulse energy in mJ) for (16)R.L. Summers, NASA Tech. Note, TN D-5285(1969). (17)J. A. Beran and L. Kevan, J. Phys. Chem., 73, 3866 (1969).
MPI of Benzene, Fluorobenzene, and Toluene
The Journal of Physical Chemistry, Vol. 87,No. 10, 1983 1703
500.0 nm NON-RESONANT SLOPE 0 BENZENE 1.9 0 4F 1.7 A #CH3 2.(
1.50
a
14;
TRANSITION
0 A 0
12-15 24-28 36-41
3.0
4.0
504.4nm
1 CARBON 2 CARBON 3 CARBON
1.00
J
a z 2 v)
0.50
El J
a
6I-
-
0.00
0
0 -I
-0.50
- 1.00 (
A !
0.4 0.6 0.8 1.0 LOG(P0WER IN m J )
Flgure 2. The logarithm of the total ion intensity off-resonance, Le., at 500.0 nm for (0)benzene, (A)toluene, and (0)fluorobenzene, vs. the logarithm of the average laser power. Conditions as in Figure 1. 0-0 TRANSITION SLOPE
0 +F 0 #CH,
528.8nm 2.0 533.6nm 2.1
1.00
0.50
J
a z
0.00
‘3 v)
2
El -0.50 J
a
I-
0
I(3
-1.00
-1.50 0
- 2.00
u
0.2 0.4 0.6 0.6 1.0 LOG (POWER IN m J )
Figure 3. The logarithm of the total ion intensity of the 0-0 transition of the b band vs. the logarlthm of the average laser power for (0) toluene at 533.4 nm and (0)fluorobenrene at 528.8 nm. Conditions as In Figure 1.
5.0 6.0 POWER IN m J
8.0
10
Figure 4. The fraction of the total ion signal appearing as n carbons, regardless of the number of hydrogens, vs. the kgarithm of the average laser power for benzene at 504.4 nm, the 14; transition. “Molecular ions” is taken as all ions containing six carbons. The relative abundance and the slopes of all n carbon “clumps” remains constant as wavelength is altered, excepting only the molecular ion. The data are those of Figure 1.
each of the three molecules, tuned to the peak of the 14; Lb resonance, off-resonance at 500.0 nm, and to the peak of the 0-0resonance of the Lbband, respectively. The 0-0 resonance is parity and symmetry forbidden and thus inactive in benzene.l’ Table I summarizes total ion intensities. Figures 1 and 3, as well as the results shown in Table I, agree well with the GR results.’l Their intriguing result, that the 146 transition is far more intense in benzene than toluene or fluorobenzene (roughly equal), is verified in Figure 1 and Table I and will be considered at length in the Discussion section. The 500.0-nm off-resonance comparison shown in Figure 2 has not been previously reported. The inversion in ion signal strength relative to the 146 transition shown in Figure 1is noteworthy. The high intensity of the ions from toluene relative to fluorobenzene and benzene is due to another toluene two-photon transition lying near 500.0 nm.15 The second-order power dependence of the off-resonance signals is somewhat unexpected but has been observed in previous studies of several molecules in this laboratory. Note that this second-order off-resonance dependence is seen within the envelope of the Lbband. Saturation takes place at lower power off-resonance, as seen in Figure 2. Figure 3 compares the 0-0 two-photon transition (at 530 nm) in toluene and fluorobenzene (inactive in benzene, as noted above). The total ion intensity results are summarized in Table I. Divorced from power dependencies and corrected for ion gauge sensitivity, Table I lists the intensities for each molecule at all three wavelengths. Included also are the E1 data. These results are given as counts per laser or electron beam pulse and as counts per second. Signal strength relative to the benzene 14; transition is then calculated and compared to the GR results.” Figures 4 and 5 show the fragmentation pattern of benzene presented as the fraction of the total ion signal containing a given number n of carbons plotted against
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10, 1983
Squire et al.
BENZENE 100 1
FLUOROBENZENE (
,
J IC+
14;
14;
I
TRANSITION 5 0 4 . 4 n m
TRANSITION 507.6nm 5.2
z
s 1001 !c+ L
, I
2.6 W
m
i
3 Q \
m c
z 3
0
W
a
100 1
c ) 2mJ
1 0.7
I
10
20
30
40
50
60
70
00
90
0.0 W
L loo
1
100
MASS
10
Figure 5. The actual mass fragmentation patterns obtained for benzene on the 14; transition at stated laser pulse energles. Conditions as in Figure 1.
20
30
40
50 60 MASS
70
do
90
100
Figure 7. Actual mass fragmentation pattern obtained for fluorobenzene on the 14; transition at 507.6 nm at stated laser pulse energies. These mass spectra were obtained with a 0.25-m focal length torr. lens at nominal Ion gauge pressures of 5.0 X
TABLE 11: Percentage of Total Ion Intensity Appearing as Molecular Ion (M’ . )”
benzene 500.0 nm
z FLUOROBENZENE T R A N S I T I O N 507.6nm
0
LL
-
I
Y
91 98 MOLECULAR I O N 12-15 1 CARBON 2 CARBON
1 -
0.00
3.0
4.0 5.0 6.0
8.0 10
POWER IN m J
Flgure 6. The fraction of the total ion signal appearing as n carbons, regardless of the number of hydrogens, vs. the logarithm of the average laser power for fluorobenrene at 507.6 nm, the 14; transition. These are pure hydrocarbon ions, containing no fluorine except for the molecular ion “clump”, and the n carbon groups are the same as those plotted in Figures 4 and 9. “Molecular ion” is taken as all ions containing six carbons and a fluorine. Only the molecular ion “clump” percentage varies with wavelength, the other mass groups remaining constant in abundance relative to each other and in power dependence. The data are those of Figure 1.
the logarithm of the laser power (Figure 4) and the actual fragmentation pattern at three different powers (Figure 5). As Zandee and Bernstein noted in the initial MPI fragmentation studies on ben~ene,~JO ion current from five-carbon fragments were so weak that they do not show up in Figure 4. The strong power dependence of the n-
14; transition 530.0 n m 0-0 transition
3.2 2.0
fluorobenzene
toluene
14 74 74 80
1.5 3.2 17 27
” All values are a t 5.0 m J per pulse, averaged from several data sets. carbon clumps is striking. The data shown here are the results from 504.4 nm, but there is negligible change in the fragmentation pattern with wavelength over the range studied. In fact, if the “clump” corresponding to the molecular ion with no loss or only hydrogen loss is excluded, the branching fraction for each carbon “clump” is essentially constant as wavelength is varied from 500 to 540 nm. Figures 6 and 7 are similar to Figures 4 and 5, but refer to fluorobenzene. Figure 6 shows the molecular ion “clump” and the four largest mass groups in the fragmentation pattern as a fraction of the total ion intensity plotted against the logarithm of the laser power, while Figure 7 shows the actual fragmentation pattern to show its variation with laser power. The data points marked “one carbon“ in Figure 6 are only those that do not contain fluorine (i.e., m/z 12-15). Figure 6, therefore, refers to the same actual masses as do Figure 4 and 9 (for the molecular fragments). The fragmentation of the fluorobenzene molecular ion is less than that of benzene molecular ion, at least at 507.6 nm on the peak of the 14; transition. However, there is much greater variation with wavelength of the fraction of the fluorobenzene molecular ion. A t the 14; transition peak the fraction of molecular ion is approximately half the total ion signal, depending on the laser power (Figure 6). A t 500.0 nm, however, the fraction of
The Journal of Physical Chemistry, Vol. 87, No. 10, 7983
MPI of Benzene, Fluorobenzene, and Toluene 0.60 - 1
0.50
1705
0.60
km.
i 0.50 _I
a z a
0.40
14;
z 4;
TRANSITION 507.6nm-
E
i.
0
1
0 A
0
20
31 - 34(1 CARBON-FIx3 43 - 4 7 (2CAREON-F)X3 55 - 5 9 (3CARBON-FIx3
0.30
I-
A
0
0
!A
I
TRANSITION 512.4nm MASS 84-93 12 -15 24 - 2 8 36-41 48-53
MOLECULAR I O N 1 CARBON 2 CARBON 3CARBON 4 CARBON
0
z
2
0.20
I-
o
a a
LL 0.1 0
-
0.00
1 3.0
4.0
5.0 6.0 POWER IN m J
8.0
0.00
10
Figure 8. The fraction of total ion signals appearing as fluorine containing n carbon groups vs. the logarithm of the average laser power for fluorobenzene on the 14; transition at 507.6 nm. Note that only fluorine-containing ions are plotted. The molecular ion results of Figure 6 are shown to facilitate comparison. (The least-squares fit shown weights the points at higher laser power more heavily; it represents an alternative to that of Figure 6.) Note also that the fragment values have been tripled for clarity. The relative abundance and power dependence of the fragment ions is independent of the wavelength, except for the molecular ion. The data are those of Figure 1.
the total signal represented by the molecular ion has dropped to 0.14, as tabulated in Table 11. As in benzene, if the molecular ion is excluded, the branching fraction for each mass “clump” is wavelength independent. Figure 6 is not complete, though. The fragment groups plotted in Figure 6 are the same as those in Figures 4 and 9, but in the case of fluorobenzene the n-carbon “clump” must also include n-carbon fluorocarbons. The fluorinated fractions of the n-carbon masses are plotted in Figure 8 as total ion fraction vs. the logarithm of the laser power. The branching fraction for each of the mass groups has been tripled (X3), and the same molecular ion mass “clump” as in Figure 6 has been included to facilitate comparison. Note that the empirically fit straight lines have approximately the same slopes as those in Figure 6, and that a combined plot is not very different from Figure 6. Figures 9 and 10 refer to toluene, in a presentation similar to Figures 4 and 5 and Figures 6 and 7. The branching fractions vs. the log of the wavelength for the 14; transition are plotted in Figure 9. Figure 10 shows the fragmentation patterns for three different laser powers at the same wavelength. Note that the same mass groups are plotted in Figure 9 as in Figures 4 and 6, but the molecular ion has almost completely disappeared and the branching fractions do change slightly with laser power, as compared to benzene. Figure 10 shows the elimination of the higher mass fragment ions in toluene, even at the relatively low laser pulse energy of 2 mJ. The fragmentation pattern of toluene does not change greatly between 512.4 (the 14; transition) and 500.0 nm but as wavelength is increased the percentage of total signal due to toluene molecular ion increases. As in fluorobenzene and benzene, the intensities of the fragment ions relative to each other are constant (within experimental error) over the wavelength range studied.
3.0
4.0
5.0 6.0
8.0
10
POWER IN m J
Figure 9. The fraction of total ion signals appearing as fragments containing n carbons, regardless of the number of hydrogens, vs. the logarithm of the average laser power for toluene on the 14; transition at 512.4 nm. “Molecular ion” is taken as all seven-carbon ions. The relative abundance of the fragment ions, as well as their power dependence, is Independent of the exciting wavelength, excepting only the molecular ion. The data are those of Figure 1.
14;
TOLUENE TRANSITION 512.4 n m
100
a1 l O m J
-I
U
2
( 3 0 v)
100 Iv)
W
0
a
a -I
I-
z
Ea o w
c) 2mJ
j 10
20
30
40
50
60
70
80
90
II
o’2
100
MASS
Flgure 10. Mass spectra of the 14; transition of toluene at 512.4 nm at stated laser pulse energies. Spectra were taken WWI a 0.25-m focal length lens at nominally 5.0 X torr.
Table I1 tabulates the percentage of total ion intensity arising from the molecular ion for each of the three molecules at each wavelength/transition. The fragmentation seems to be separable into two parts, the molecular ion, with loss of one or more hydrogens, and the smaller fragments. The distribution within the latter appears to be independent of laser wavelength, while the relative amount
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The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
Squire et al.
of the former depends on both wavelength and power. Discussion The quadratic laser power dependence of the total ion signal intensity observed a t all wavelengths in this study (Figures 1-3) implies that the substituted benzenes are undergoing a two-photon resonance, corroborating GR’s analysis that the molecule is excited into the Lb band of the neutral as the initial rate-determining step in MPI. The other major experimental results can be summarized as follows: (i) the resonance results; on the 14; two-photon resonance the total ion signal intensity of benzene is four times that of toluene and fluorobenzene; at the 0-0 resonance toluene has five times the signal strength of fluorobenzene (Figures 1 and 2); (ii) the offresonance “reversal” of total ion signal intensity (here toluene > fluorobenzene > benzene) (Figure 3); (iii) the wavelength independence of the fragmentation pattern (exclusive of the molecular ion masses) for all three molecules (Figures 4, 6, 8, and 10); (iv) the wavelength dependence of the branching fraction for the molecular ion for fluorobenzene and toluene (Table 11);(v) the similar dependence upon laser power of the fraction of n-carbon fragments in fluorobenzene whether or not they contain fluorine (Figures 6 and 8). On the 14; resonance, the total ion signal intensity results qualitative reproduce those of GR.11J3Js Some differences deserve comments, however. As GR have pointed out, the amplitude of the two-photon 14; transition should be unaffected by substitutent groups, according to simple molecular physics arguments. The fourfold increase in signal strength for benzene relative to toluene and fluorobenzene would therefore be attributed to an increased probability for ionization out of the Lb state, rather than two-photon excitation into it. The presence of higher order resonances for benzene in this region has not been observed.8 The identical signal strengths for toluene and fluorobenzene support the concept that the two-photon 14; resonance transition is not significantly affected by the nature of the substituent on the ring. The ratio of the total ion yield from benzene compared to that of toluene and fluorobenzene was found to be twice as large in this study as reported by GR (Table I). The increased ratio can be tentatively attributed to the higher power of the present Nd:YAG pumped dye laser system vs. the Nz pumped dye laser of GR.l’ The 0-0 transition intensities in this study are in the same ratio relative to each other found by GR, but are lower by a factor of 5 when compared to the 14; transition. But the wavelength region longer than 525 nm is on the shoulder of the dye lasing profile when the Nd:YAG laser system is used, which is not so for the same dye when the nitrogen system is used. In this study, the 0-4 ion yields may include a contribution in the “laser power” from superradiant fluorescence and mode structure leading to reduced ionization area and, consequently, lower signal intensity. Thus, this feature of the present results cannot be regarded as definitive. Point ii, the inversion of total ion signal strengths between the 14; resonance and the 500.0-nm off-resonance wavelengths, is the most striking feature of those results. This inversion can be clearly understood in terms of the GR perturbation m ~ d e l . ’ ~ JFluorobenzene ~J~ is an example of GR weak or intermediate, primarily inductive, coupling while toluene is an example of intermediate resonance coupling. Hence, the off-resonance 500.0-nm signal from toluene is significantly “less forbidden” than that from (18) L. Goodman and R. P. Rava, private communication.
l6
r
1
-io
-1.E.
t
-1.E.
I.€.
t
31 w
I
500.0 n m
2 c
0
i
-
+H
4F
+
CH3
Figure 11. Potential energy level diagram for the substituted benzenes. Solid lines are ionization energies (IE) or appearance energies (AE) of the listed fragment. Dashed lines are the relevant states, the b state of the neutral and the lowest excited n-state of the molecuhr ion. IE’s and AE’s are from ref 19, state energies from ref 27 and 29.
fluorobenzene. Toluene should also have a greater density of states than fluorobenzene, arising from progression formation in the rotations and vibrations of the methyl group. Fluorobenzene, in turn, has a larger off-resonance signal than benzene. This explanation is corroborated by the 0-0 band results. In benzene, with a high symmetry, the 0 4 band is nonexistent. In fluorobenzene, with small charge-transfer character, the transition appears but is very weak (a factor of 40 lower in intensity than the benzene 14; mode). In toluene, the intermediate resonance coupling case, the 0-0 band has reasonable signal strength, about five times that of fluorobenzene. The fragmentation behavior of the substituted benzenes can best be understood from Figure 11, showing the ionization energies of the three molecules examined, as well as the appearance energy (AE) for each of the important fragment ions arising from the parent molecules, as a function of energy above ground state.lg The Lb states are also plotted, as dashed lines on the figure, as are the lowest excited states of the molecular ion. The wavelength independence of the relative fragmentation branching fractions (iii above) indicates that, once fragmentation of the molecular ion occurs, only the average laser power is important in determining the fragmentation pattern. It would appear that a single average energy characterizes the fragmentation. This is one of the characteristics expected on the basis of the Silberstein and Levine statistical theory of MPI fragmentation.% Whether the fragmentation is statistical cannot be directly ascertained. Excitation through a different two-photon resonance (e.g., the E, state) would give a different power dependence and fragmentati~n.~ Photoelectron and two-laser results have repeatedly shown that the molecular ion is formed by emission of a low kinetic energy e l e c t r ~ and n ~ fragmentation ~ ~ ~ ~ ~ takes (19) H. M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J . Phys. Chem. Ref. Data, 6, Suppl. No. l(1977). (20) R. S. Pandolfi, D. A. Gobell, and M. A. El-Sayed, J . Phys. Chem.,
85, 1779 (1981).
MPI
of Benzene, Fluorobenzene, and Toluene
The Journal of Physical Chemistty, Vol. 87, No. 10, 1983
1707
to raise the AE of C6H5+(Figure 11). The latter increase place from the molecular ion. The observed wavelength may be the reason the fractional contribution of the 3dependence of the molecular ion fraction (see Table 11) carbon masses rises rather than falls with laser power in would indicate an energy threshold for excitation of the fluorobenzene, unlike toluene and benzene. On the time molecular ion. This threshold implies eventual absorption scale of these experiments, fluoride eliminates from ca. of sufficient energy to reach the lowest fragment AE. 75% of the fragment ions. Ion photodissociation studies The wavelength dependence of the branching fraction do not offer an explanation for this p h e n o m e n ~ n . ~ ’ ? ~ ~ of molecular ion indicates that the threshold occurs at energies equivalent to five or six absorbed photons above Conclusions neutral ground. A one- or two-photon absorption by the The results of Goodman and Rava1’J2for the relative molecular ion reaches a threshold energy whereupon furion intensities of the substituted benzenes have been ther absorption and fragmentation to a “statistical” disqualitatively substantiated. Quantitatively, their results tribution occurs. However, statistical theories of fragon ionization have been confirmed in regions where the m e n t a t i ~ n ,as ~~ well ~ ~as- experimental ~~ confirmations of differences between the lasers used to excite the molecular t h e ~ r yhave , ~ been unable to account for the amount of are not important. The GR perturbation theory ademolecular ion found in experimental results. quately explains the off-resonance results observed. Fluorobenzene appears to reach threshold at about 507.6 An energy threshold was found for MPI excitation of nm, on the 14; resonance (Table 11). Five photons abthree substituted benzenes in the Lb band. If excited above sorbed at this wavelength would be 12.2 eV above neutral that threshold, a molecule’s fragmentation will have no ground energy. Dunbar et al.27report this energy to be wavelength dependence. This threshold was tentatively that of the first excited rr state of the molecular cation. identified as excitation into the lowest excited state of the Similarly, the lowest excited *rr states of b e n ~ e n and e~~~~ ~ molecular cation. t01uene~~ cation J ~ ~ are ~ ~ reported at 11.6 eV. In toluene, The fragmentation of the three molecules was without this energy corresponds, for five photons, to the 0-0 ressurprises except for fluorobenzene. In that molecule, the onance. The rise in toluene molecular ion branching results are most easily explained if fluorine is a spectator fraction at that resonance (Table 11) allows identification atom in the fragmentation. of the lowest excited state of the molecular ion with the MPI energy threshold level. Acknowledgment. The authors greatly appreciate useful The fluorobenzene fragmentation results are puzzling. suggestions from and discussions with Professor L. In benzene and toluene, the fragmentations are similar, Goodman and Dr. R. P. Rava. Preliminary work on the with more dissociation occurring in toluene (with a fragpresent experiments was carried out by Dr. D. A. Lichtin, mentation “tree” through C7H7+as well as C6H5+)than in whose ongoing contributions are hereby acknowledged with benzene, with only the C6H5+tree. But in fluorobenzene, thanks. This research was supported by NSF Grants CHE the sole difference between the n-carbon fragments with 78-25187 and CHE 81-16368. and without fluorine is a factor of three in the branching ratios. This behavior suggests that fluorine is virtually a Appendix: Sensitivity of MPI Mass Spectrometry spectator atom in the fragmentation, its major contribution The extreme advantage in efficiency of MPI over E1 is demonstrated in Table I. Despite some difficulty in com(21)J. T. Meek, R. K. Jones, and J. P. Reilly, J. Chem. Phys., 73,3503 paring MPI data quantitatively with different lasers, MPI (1980). is, at its worst, the equal of E1 in ionization efficiency and, (22)J. H.Glownia, S. J. Riley, S. D. Colson, J. C. Miller, and R. N. Compton, J. Chem. Phys., 77,68 (1982). at its best, two orders of magnitude more efficient at a per (23)J. T. Meek, S. R. Long, and J. P. Reilly, J. Phys. Chem., 86,2809 “pulse”,either electron or laser, level. At a per second level, (1982). MPI shows a further multiplicative advantage factor of (24)F. Rebentrost and A. Ben-Shad, J.Chem. Phys., 74,3255(1981). (25)F. Rebentrost, K. L. Kompa, and A. Ben-Shaul, Chem. Phys. 200. In addition, the wavelength sensitivity of MPI, Lett.. 77. 394 (1981). lacking with EI, enhances the sensitivity of multiphoton (26)(a) J. Silberstein and R. D. Levine, Chem. Phys. Lett., 74, 6 ionization mass spectrometry enormously as an analytical (1980);(b) J. Chem. Phys., 75,5735 (1981). (27)R. C. Dunbar, H. Ho-I. Teng, and E. W. Fu, J.Am. Chem. Soc., 101,6506 (1979). Registry No. Benzene, 71-43-2; fluorobenzene, 462-06-6; (28)B. S. Freiser and J. L. Beauchamp, Chem. Phys. Lett., 35, 35 toluene, 108-88-3. (1975). (29)T.A. Carlson and C. P. Anderson, Chem. Phys. Lett., 10, 561 (1971). (30)P. P. Dymerski, E. Fu, and R. C. Dunbar, J. Am. Chem. SOC.,96, 4109 (1974). (31)R. C. Dunbar, J.Am. Chem. SOC.,95,472 (1973);R. C.Dunbar in ”Gas Phase Ion Chemistry”, Vol. 2,M. T. Bowers, Ed., Academic Press, New York, 1979,pp 181-220.
(32)K. R. Newton and R. B. Bernstein, J. Phys. Chem., in press; K. R. Newton, Ph.D. Thesis, Columbia University, 1981. (33)D. A. Lichtin, L. Zandee, and R. B. Bernstein in “Lasers in Chemical Analysis”, G. Hieftje, J. Travis, and F. Lytle, Ed., Humana, Clifton, NJ, 1981,Chapter 6,p 125.
