J. Phys. Chem. 1993,97, 4930-4935
4930
REMPI-TOF Mass Spectra of e,m, and pChlorofluorobenzene and Their Clusters: Enhancement of the C1 Elimination Reaction by Cluster Formation Yasushi Numata,. Yosbito IshiiJ Masatoshi Watahilri, Isamu Suzuka,,' and Mitsuo Itot Department of Industrial Chemistry, College of Engineering, Nihon University, Koriyama, 963, Japan Received: January 5. 1993; I n Final Form: February 26, 1993
The dissociation of 0-, m-, and p-chlorofluorobenzene (CFB) and their Yan der Waals clusters in supersonic jets were studied by resonance-enhanced multiphoton ionization (REMPI) combined with time of flight (TOF) mass spectrometry. The elimination of the C1 atom to produce C6HdF+ was,found to occur in the dissociative u,?r excited state of the parent molecular ion reached by three-photon absorption via the SIstate of the neutral molecule. The dimers and higher clusters are easily formed for 0- and m-CFB but not for p-CFB. The C1 elimination reaction was found to be greatly enhanced by formation of the clusters for 0- and m-CFB. The enhancement and mechanism of the elimination reaction are discussed.
Introduction The dissociation of halobenzene has been extensively studied by the resonance-enhanced multiphoton ionization (REMPI) method combined with mass spectrometry. The dissociation rate of elimination of C1from chlorobenzenewas determined by Durant et al.,' and the photodissociation mechanism of bromobenzene was discussed by Koplitz et al.2 Szaflaski et al. reported the mechanism of the dissociation of p-dichlorobenzene by using the picosecond laser.3 In the present paper, we report the dissociations of o-,m-, and p-chlorofluorobenzene (CFB) and their clusters (mainly dimers) studied by REMPI combined with time of flight (TOF) mass spectrometry. We are interested in how the dissociation mechanism is different for the different isomers and also how the dissociation process is affected by dimer formation. The elimination of the C1 atom to give C6H4F+ was mainly studied here. It was found that the eliminationmechanism is different between 0-, m-, and pCFB. For 0- and m-CFB, the elimination occurs in the dissociative u,?r excited state of the parent molecular ion reached by three-photonabsorption via SIof the neutral molecule. However, in p-CFB, the elimination seems to occur in a twophoton excited level by a radical dissociation reaction. It was found from the fluorescence excitation spectra that the dimer and higher clusters are easily formed for o- and m-CFB but not for p-CFB. A great enhancement of the C1 elimination reaction to produce C&F+ by dimer formation was found for o- and m-CFB. The enhancement is explained by increased e x w s energy in the dissociative u,?r state due to dimer formation.
Experimental Section The supersonic molecular jet apparatus used in this study is the same as that described in a previous paper.4 Briefly, the samples seeded in 2-4 atm of He were heated to 20-70 "C. The supersonic jet was generated by expandingthe sample vapor into a vacuum chamber at 2 X 10-6 Torr through a pulsed nozzle (General Valve series 9,0.2-mm orifice diameter). A frequencydoubled dye laser pumped by an excimer laser was used as the exciting light. The laser power was about 30 WJlpulse. The laser beam crossed the jet downstream 30 mm, and the molecules were excited to their SIstate. The ions produced by one-color two-photon resonance ionization were led into a TOF mass spectrometer by two stage acceleration and were detected by a
' Present addressYSakuragawa Works, Hitachi Chemical Co., Ltd., Ayukawamachi, Hitachi 316, Japan. t Present address: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. 0022-3654/93/2097-4930$04.00/0
micro-channel plate (Hamamatsu Photonics F222-21s). The ion signals were averaged by a digital oscilloscope (LeCroy 9400A). The mass-selected REMPI spectrum was obtained by scanning the laser wavelength while detecting the mass-selected ions, and the ion signal was averaged with a boxcar integrator (Stanford Research System SR250). Thelaser power dependence of the ion intensity was measured by controlling the laser power with neutral density filters. The fluorescence excitation spectra were obtained by the same method as that described in a previous paper.4 Chlorofluorobenzenes used in this study were purified by vacuum distillation.
Res& and Discussion (a) o.CFB. Figure 1 showsthe fluorescence excitation spectrum of o-CFB in a supersonic jet. The spectrum consists of sharp and broad bands. The longest wavelength sharp band at 37 032 cm-l isassigned to the0,O bandof theS, +SO transitionof this molecule. All the sharp bands appearing on the shorter wavelength side of the 0,Oband are assigned to the vibronic bands, whose frequency differences from the 0,O band are shown in the Figure 1. A strong broad band appears with its peak at 270.8 nm (36 930 cm-I). Other broad bands also appear on the longer wavelength sideofthemainvibronicbandsofthebaremolecule. Theintensity ofthe broad band increases with theincreaseof thevapor pressure. Therefore, the broad bands are considered to be due to a cluster of o-CFB. Figure 2 shows TOF mass spectra of o-CFB obtained by REMPI of the molecule excited at the 0,O band (Figure 2a) and of the cluster excited at the peak maximum of the broad band (Figure 2b). In Figure 2a, the signal intensity of the monomer ion is the strongest in the spectrum. However, the ion signal of C.&F+ which is produced by the elimination reaction of the C1 atom is very weak. When exciting the cluster band (Figure 2b), cluster ions up to hexamers can be observed in the mass spectrum. This proves that the broad band can be assigned to the clusters of various sizes ranging from n = 2 to n = 6. The presence of the monomer ion in this mass spectrumis explained to be produced by dissociation of the higher clusters. It should be noted in Figure 2b that the intensity of C6H4F+relative to the intensities O f C&+ and the monomer ion is very large compared with that shown in Figure 2a. Figure 3 shows the mass-selected REMPI spectra of jet-cooled o-CFB obtained by monitoring (a) the monomer ion, (b) the dimer ion, (c) CbH4F+, and (d) C6H3+. The MPI spectrum obtained by monitoring the monomer ion (Figure 3a) is identical with the sharp component of the fluorescence excitation spectmm assigned to the neutral monomer. The MPI spectrum monitored 0 1993 American Chemical Society
0-,m-,
and p-Chlorofluorobenzene and Their Clusters
The Journal of Physical Chemistry, Vol. 97,No. 19, 1993 4931
*
37032~11104
h I
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WAVELENGTH (nm) Figure 1. Fluorescence excitation spectrum of jet-cooled o-CFB. a) Monomer
Monomer+
(37032cm-1 J
a)
&I
37032cm-I
Monomer'
0-0
I
640
Dimer+
I
I
I
I
0
4
8
12
1
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20
Time-of-flight (us) b) Cluster
(36930cm-1 I
272
271
270
269
268
267
266
26s
264
WAVELENGTH (nm)
Figure 3. Mass-selected REMPI spectra of jet-cooled o-CFB obtained by monitoring (a) the monomer ion, (b) the dimer ion, (c) C6HdF+, and ( 4 Ct.H3+. I
I
I
0
IO
20
I
30
I
40
A 50
Time-of-flight (ps)
Figure 2. TOF mass spectra of o-CFB obtained by REMPI (a) of the molecule excited at the 0-0 band (37 032 cm-I) and (b) of the cluster excited at the peak maximum of the broad band (36 930 cm-I). The spectrum was normalized by the signal intensity of the monomer ion.
by C6H3+is also the same as that of the monomer ion, indicating that C6H3+is produced by photofragmentation of the monomer. On the other hand, a diffuse feature is observed for the MPI spectrum obtained by monitoring the dimer ion. The broad spectrum is identical with the broad component of the fluorescence excitation spectrum. This proves that the broad component of the fluorescence excitation spectrum is assigned to the clusters including the dimer. The MPI spectrum obtained by monitoring C6H4F+shown in Figure 3c is an overlappingof the broad spectrum of Figure 3b (dimer ion) and the sharp spectrum of Figure 3a (monomer ion), the former being much stronger than the latter. This shows that C&F+ is generated by the photodissociations of both the dimer and the monomer. However, the fact that the intensity of the broad component is much stronger than that of the sharp component clearly indicates that C&F+ is mainly produced from the dimer. The result is consistent with that obtained from the comparison of the mass spectra shown in Figure
2. It is concluded from the above that the generation of C6HdF+ by the photodissociation of o-CFB is greatly accelerated by dimer formation. (b) mCFB. The fluorescenceexcitation spectrum (a) and the mass-selected REMPI spectra of jet-cooled m-CFB obtained by monitoring the monomer ion (b), the dimer ion (c), CsHdF+ (d), and C6H3+(e) are shown in Figure 4. The 0,O band of the SI SOtransition of the molecule is located at 37 022 cm-I in the fluorescence excitation spectrum (Figure 4a). The broad band appearing on the longer wavelength side of the 0,O band is due to clusters of m-CFB similar to the case of o-CFB. It is seen from Figure 4b that all the vibronic bands of frequencies lower than 240 cm-I are absent in the mass-selected MPI spectrum monitored by the monomer ion. However, vibronic bands of frequencies larger than 336 cm-1 from the 0,O band appear strongly. This indicates that the ionization potential of m-CFB is larger than twice the frequency of the 0,0+240-cm-1 band and smaller than twice the frequency of the 0,0+336-cm-I band. The ionization potential of this molecule is therefore determined to be between 74 524 cm-1 (9.24 eV) and 74 716 cm-I (9.26 eV). The MPI spectrum monitored by C6H3+is identical with that monitored by the monomer ion. This shows that C6H3+ is produced from the monomer, similar to the case of o-CFB. As can be seen in Figure 4c,the MPI spectrum monitored by the dimer ion exhibits
-
4932
Numata et al.
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
a) Fluorescence excitation spect" 0-0 37022cm.'
1
A I
Monomer+
F
F
a) Monomer (37658 cm-I)
240 366 636 *04 336 428 455 695
/1 c) Dimer*
847
A b) Cluster (36970 cm-l)
4
J
C6H4F+
Monomer+
C6H3'
5
7
9
II
13
Time-of-flight 213
272
271
270
269
268
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266'
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WAVELENGTH (nm)
Figure 4. (a) Fluorescence excitation spectrum of jet-cooled m-CFB and (b-e) its mass-selected REMPI spectra obtained by monitoring (b) the monomer ion, (c) the dimer ion, (d) C6H+tF+, and (e) C&+.
a broad spectral feature, very similar to that of the broad component in the fluorescence excitation spectrum. The MPI spectrum monitored by C6H4F+(Figure 4d) is also identical with that of the dimer ion. It is concluded from these results that C6H4F+is mainly produced from the dimer. Figure 5 displays the TOF mass spectrum obtained by exciting the vibronic band of the monomer (a) and that obtained by exciting the cluster band (b). It is apparent from the figure that C&F+ is produced more efficiently in the cluster excitation than in the monomer excitation. The efficient production of C6H4F+from the cluster is quite similar to the case of o-CFB and seemed to be a general phenomenon. In spite of the absence of a 0,O band of the monomer in the MPI spectrum monitored by the monomer ion (Figure 4b), the dimer band can be observed in the MPI spectrum monitored by the dimer ion (Figure 4c) even in the frequency region below the 0,Oband of the monomer. This can be explained by the decrease in the ionization potential of m-CFB due to dimer formation. The decrease in ionization potential can be estimated from the observed spectra to be at least 0.2 eV. The reduction of the ionization potential is responsible for the efficient production of C&F+ in the dimer as will be discussed later. (c) pCFB. Figure 6 showsthe fluorescenceexcitationspectrum (a) and the mass-selected MPI spectra of jet-cooled p-CFB obtained by monitoring the monomer ion (b), C&F+ (c), and C6H3+(d). It is seen that all the spectra exhibit a sharp spectral feature. This is in contrast to the cases of 0- and m-CFB in which the broad band component was present together with the sharp one. All the sharp bands in the spectra ofp-CFB can be assigned to the vibronic bands of the SI SOtransition of the monomeric molecule. The longest wavelength strong band at 36 272 cm-I in the fluorescence excitation spectrum is the 0,Oband. In all the MPI spectra, the 0,O band is very weak and the band intensity
-
15
17
19
21
23
(LIS)
F i p e 5 . TOF mass spectra of jet-cooled m-CFBobtained (a) by exciting the vibronic band of the monomer (37 658 cm-I) and (b) by exciting the cluster band (36 970 cm-I). The spectrum was normalized by the signal intensity of the monomer ion. F
Q
a) Fluorescence excitation spectrum
36272cm-I
1
b) Monomer'
276
274
272
270
1
268
WAVELENGTH (nm)
Figure 6. (a) Fluorescence excitation spectrum and (b-d) mass selected REMPI spectra of p-CFB obtained by monitoring (b) the monomer ion, (c) C6H4F+, and (d) CsHj+.
suddenly increases for vibronic bands in the frequency region higher than the 0,0+35O-cm-I band as seen from the Figure 6.
0-,
The Journal of Physical Chemistry, Vol. 97, NO.19, I993 4933
m-, and p-Chlorofluorobenzene and Their Clusters +
MC
I
'6"3*
6 CI
Dimer*
h ) 1066"'
+532nm(YAG)Ex.
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I
1
6
7
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12
I
I
I
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Time-of-flight (us1
Figure 7. Comparison of TOF mass spectra of p-CFB obtained by (a) one-color REMPI using a vibronic band (0,0+1066 cm-I) and (b) twocolor REMPI with the same UV light and visible light of 532 nm (the second harmonic of a Nd:YAG laser). The spectrum was normalized by the signal intensity of the monomer ion.
exponentially with a delay time of 27 ns. This suggests the existence of an intermediate species of lifetime of 27 ns in the process of the production of C&F+. A candidate for the intermediate species produced by the two UV photon absorption is the parent ion CsH4FCl+. If the parent ion is the intermediate species leading to the production of C.&F+, an increase in the ion signal of C6H4F+induced by the increase in the power of the visible laser must be accompanied by a simultaneous decrease in the signal of the parent ion. However, such a power dependence was not found for the signals of the parent ion and C&F+. It is strongly suggested from this observation that C & t F + is produced via a neutral intermediate species rather than ionic species. The most probablecandidate is C6H4F'radical produced by radical dissociation reaction of the molecule in the two UV photon excited state. Then, the decay time of 27 ns is interpreted to be the lifetime of this radical. Such an aromatic radical usually has its electronic absorption in the visible r e g i ~ n .The ~ C6H4F. radical will be excited by the visible laser light to its excited state. The ionization of the radical in the excited state will occur by further excitation with another visible photon. Therefore, the great enhancement of the ion signal of C&F+ in the two-color excitation is explained by the involvement of visible light in the absorption of the C6H4F. radical produced by two UV photon excitation. (e) Formation of Clusters. It was shown from the fluorescence excitation spectra and TOF mass spectra that o- and m-CFB form their clusters butp-CFB shows no sign of cluster formation. We shall consider relative stabilities of the dimers of these molecules in view of steric hindrance. In the case of benzene, it is generally accepted that the parallel sandwich structure is one of the stable structures of dimer.+I0 Assuming similar sandwich dimer structures for the three isomers of CFB, the most probable dimer structures are
GeF Cl
This clearly shows that the weak 0,O band appears by threephoton ionization and the strong vibronic bands by two-photon ionization. The ionization potential of this molecule is accurately determined from these results to be 9.08 eV. In contrast to the cases of 0- and m-CFB, a broad band such as those found in these two molecules could not be observed in all the spectra of p-CFB even under the jet condition favorable to cluster formation. This means that it is very hard to form p-CFB clusters such as dimers. The reason for this will be discussed later. (a) Elimination of C1 in pCFB. It is seen from Figure 6 that the MPI spectra obtained by monitoring C&F+ and C&+ are similar to that monitored by the monomer ion. Therefore, these fragment ions are produced by the photoionization of the monomeric molecule. To see the mechanism of the production of C&F+, we measured the laser power dependence on the ion signal of C&F+ produced by REMPI. The ion signal was found to be proportional to the third power of the laser intensity, showing that three photons are needed for the production of C&F+. Figure 7a shows the TOF mass spectrum of p-CFB obtained by one-color REMPI using the vibronic level at 1066 cm-I (at 267.8 nm) above the zero-point level in SIas a resonance state. The ion signal of C,&F+ which is produced by three-photon excitation of the molecule is very weak compared with those of the parent ion and CsH3+. Next, we observed the TOF mass spectrum ofp-CFB obtained by two-color REMPI with the same UV light as that used in the one-color REMPI (267.8 nm) and visible light of 532 nm (the second harmonic of a Nd:YAG laser). As can be seen from Figure 7b, a great enhancement was observed for the ion signal of C6H4F+. It was also found from the measurement of the laser power dependencethat two UV photons are needed in the two-color REMPI. We also measured the ion signal of C6H4F+ by changing the delay time between the UV and visible laser pulses. The ion signal was found to decrease
CI"
CI
F&
p-CFB
m-CFB
Cl *F
F.&F
0-CFB
where the two overlapping phenyl rings are represented by a single ring. The halogen atoms belonging to the upper molecule are connected by solid lines to the phenyl ring and those belonging to the lower molecule by dashed lines. The dimer structure shown above for each molecule is thought to be the most stable one among many possible structures in view of the assumed criterion that the sum of pairwise intermolecular repulsive interactions between the halogen atoms of one molecule and those of another molecule is minimum for the most stable structure. Next, weshallconsider therelativestabilitiesof the threedimer structures given above with the same notion that the relative stability is simply determined by the intermolecular repulsive interactions between the halogen atoms. We have three types of pair interactions: Cl-Cl, C1-F, and F-F. For CI-CI, there are two different pairs: a meta-like pair (m C1-C1) and para-like pair (p Cl-Cl), the repulsive interaction being much stronger for the former than the latter. For Cl-F, we have ( 0 CI-F) and (m Cl-F). For F-F, we have (m F-F) and (p F-F). Judging from the kinds of pairs and their interatomic distances, the order of the repulsive interaction will be ( 0 C1-F) (m Cl-Cl) >> (m Cl-F) (p Cl-Cl) >> (m F-F) > (p F-F). We have 2 ( 0 Cl-F) + (m Cl-Cl) (m F-F), 2[(0 CI-F) + (p Cl-Cl) + (p F-F)], and 2[(m Cl-F) + (p Cl-Cl) + (p F-F)], respectively, for the dimers of p-, m-, and o-CFB. Taking into account the above order of the interaction, the repulsive interaction is the largest forp-CFB, intermediate for m-CFB, and the smallest for o-CFB. This is a very rough estimation,but the largest repulsiveinteraction is apparent for p-CFB. This explains the absence of the dimer
-
-
+
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
4934
IO0 50
100
a) Monomer
-
50
.-x
-
.-x
vi
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vi
,3 .--
b) Cluster
.-
-
IO -
0
."
IO
-
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".l/
2
d
I -
f I
I
I
/
/
I I
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I
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10
50
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Laser intensity
Figure 8. Laser power dependence of the ion signal of CbH4F+ for both (a) the monomer and (b) the cluster of o-CFB.
for p-CFB. It is expected from the above that o-CFB can most easily form the dimer. The fact that the ions of (o-CFB),+ with n = 1-6 were observed in the TOF mass spectrum (Figure 2) supports the expectation. The larger repulsive interaction for the dimer of m-CFB than that for o-CFB is in accord with the observation that only the cluster ion of n = 2 is present and the higher cluster is absent in the TOF mass spectrum of m-CFB (Figure 5 ) . The order of the stability of the dimer may also be explained by dipole-dipole interactions. The dipole moments" are -0, 1.49,and 2.41 D, respectively, forp-, m-, and o-CFB. Therefore, the stabilization energy due to the dipole-dipole interaction will increase in this order. (f) Enhancement of c1 Elimination by Dimer Formation. It was found from the measurements of REMPI and TOF mass spectra that the elimination reaction of the C1 atom to produce C6H4F+is greatly enhanced by the cluster formation for 0- and m-CFB. To elucidate the reaction mechanism, we measured the laser power dependence of the ion signal of C&F+ for both the monomer and cluster of o-CFB. The ions of C&F+ were produced by REMPI using the 0,O band (37 032 cm-I) in SIas a resonance state and the intensity maximum of the cluster band (36 930 cm-I), respectively, for the monomer and the cluster. Since the dimer is dominant among the clusters as seen from Figure 2b, we call the cluster the dimer hereafter. It was found for both cases that the ion signal is proportional to the third power of the laser intensity as shown in Figure 8. This indicates that three photons are needed to produce C6H4F+ for both the monomer and the dimer. The three-photon energy is about 111 000 cm-I (1 3.7 eV) for both cases. The exact value of the ionization potential of o-CFB is not known, but it should be less than 2(37032) = 74064 cm-I (9.18 eV) because the 0,O band (37 032 cm-I) of the SI SOtransition appears in the resonance enhanced two-photon ionization spectrum. The ionization potentialsl* of chlorobenzene and fluorobenzene are 9.07 and 9.20 eV, respectively. It can be assumed from these values that the ionization potential of o-CFB is about 9.1 eV. Assuming this value, the three-photon energy (13.7 eV) corresponds to an excess energy of 13.7 - 9.1 = 4.6 eV of the parent ion C6H4FC1+in its electronic ground state. In the caseof benzene cation, there exist several dissociative excited states assigned to U,T in the energy region from 4.4 to 5.3 eV.I3 Similar dissociative states are also expected in a similarenergy region for C&I4FCl+. The dissociative energy region is the same energy region as that reached by the three-photon absorption. It is concluded therefore that C&4F+
-
Numata et al. is produced by the elimination of the C1 atom from CsHdFCI+ in the dissociative U,T state excited by the three-photon absorption of 0-CFB. Now, we have to explain why the C1 elimination reaction is greatly enhanced by the dimer formation. The electronic ground state of the dimer ion is a charge resonance state which can be expressed by resonance between two structures with the positive charge being localized on one of the two molecules and with that on another molecule. Such a charge resonance state is generally greatly stabilized in energy compared with the ground state of a monomer ion. In the case of the benzene dimer ion, the stabilization energy amounts to as large as 0.67 eV.I4 Such a stabilization is reflected as a decrease in the ionization potential of a neutral molecule by its dimer formation. As mentioned in a previous section, such a decrease in ionizationpotential by dimer formation was actually observed for m-CFB, for which the decrease was larger than 0.2 eV. Although we do not know the stabilization energy for the o-CFB ion, it will be certainly larger than 0.2 eV, similar to m-CFB, and it will be probably comparable to the stabilization energy of the benzene dimer ion of 0.67 eV. It is expected that there exist dissociative U,T states of the dimer ion in the same energy region above the ground state as that for the monomer ion (4.4-5.3 eV). Since the three-photon energy put into the neutral dimer is nearly the same as that for the monomer, the dimer ions in the dissociativestate reached by the three-photon absorption will have a large excess energy in this dissociative state compared with that for the monomer ion. The difference in the excess energy between the monomer ion and the dimer ion is equal to the difference in the ionization potential between the neutral monomer and dimer, it being larger than 0.2 eV, probably, about 0.5 eV. This large excess energy acquired by dimer formation is responsible for the enhancement of the C1 elimination reaction leading to the production of abundant C&F+. For the process to produce C6H4F+from the dimer ion in the excited state, two different evaporation processes can be considered.
In (l), the evaporation occurs first and is followed by the dissociation of the monomer ion produced by the evaporation. On the other hand, the dissociation takes place first from the dimer ion in (2) and is followed by the evaporation. In the case of (l), the excess energy of the dimer ion acquired by the three-photon absorption is partly consumed for the evaporation and the remaining excess energy will be used for the C1 elimination of the monomer ion. Since the excess energy remaining in the monomer ion is rather small, the C1elimination reaction to produce CsH.J+ will not be efficient. As a result, we will have abundant nondissociative monomer ions (CsH4FCl)+. The presence of the abundant monomer ion was actually observed in the TOF mass spectrum (Figure 2b) obtained by excitingonly thedimer. Under our experimental condition of the TOF mass spectrum shown in Figure 2b, the monomer is not excited and there is no possibility to produce the monomer ion directly from the monomer. Therefore, all the monomer ions should come from the dimer ions. The strong ion signal of the monomer ion which is comparable to that of the dimer ion (Figure 2b) supports that process 1 occurs with a high probability. On the other hand, the great enhancement of the C1elimination reaction by the dimer formation can be reasonably explained by (2) as discussed before. Therefore, this dissociation mechanism
o-, m-, and p-Chlorofluorobenzene and Their Clusters
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4935 C1 eliminated dimer (trimer) ion indicates that the evaporation of this ion proceeds very efficiently. As a rault, the strong ion signal of CbHdF+ is observed as seen in Figure 2b. It is concluded from the above that the two dissociation mechanisms (1) and (2) simultaneously occur in o-CFB. The discussion given above for o-CFB can also be applied to m-CFB.
Dimer'
Trimer'
)imer+
.I:
[ Trimer'
Acknowledgment. We thank Mr. Y.Ito for construction of the TOF mass spectrometer. This work was partially supported by a grant from Nihon University. Tetramer+
A 4 -
References md Notes
Pentamer'
Hemmer* ~
16
18
20
22
24
26
28
Time-of-flight
30
32
34
36
(ps)
Figure 9. TOF mass spectrum of o-CFB obtained by exciting the dimer with an increased sensitivity. Other condition is the same as that in Figure 2.
certainly does exist. Then, we expect the presence of (C6H4F)+(C&FCl), which does not have enough excess energy for the evaporation after consuming the excess energy of the dimer ion for the C1 elimination. Figure 9 shows the TOF mass spectrum of o-CFB obtained by exciting the dimer. The spectrum is the same as that of Figure 2b but was obtained with an increased sensitivity. As seen in the figure, the mass peak corresponding to (C6H4F)+(C6H4FCl) is clearly observed. The presence of C1 eliminated trimer ion was also found. The weakion signal of the
(1) Durapt, J. D.; Rider, D. M.; Anderson, S.L.; Proch, F. D.; Zare, R. N. J. Chem. Phys. 1984,80, 1817. (2) Koplitz, B. D.; McVey, J. K. J. Chem. Phys. 1984,80, 2271. (3) Szaflarski, D. M.; Smith, J. D.; El-Sayed, M. A. J. Phys. Chem. 1986, 90, 5050. (4) Suzuka, I.; Tomioka, T.; Ito, Y.Chem. Phys. Lett. 1990, 172, 409. (5) Fukushima, M.;Obi, K. J . Chem. Phys. 1990,93, 8488. (6) Karlstam, G.; Linse, P.; Wallqvist, A.; Jhsson, B. J. Am. Chem. Soc. 1983, 105, 3777. (7) Law, K. S.;Schauer, M.; Bernstein, E. R. J . Chem. Phys. 1984,81, 4871. (8) Schauer, M.;Bernstein, E. R. J . Chem. Phys. 1985, 82, 3722. (9) Hobza, P.; Selzle, H. L.; Schlag, E. W. J . Chem. Phys. 1990, 93, 5893.
(IO) Ebata,T.; Hamakado,M.; Moriyama,S.; Morioka, Y.;It0,M. Chem. Phys. Lett. 1992, 199, 33. (1 1) Riehn, C.; Lahmann, C.; Brutschy, B. J . Phys. Chem. 1992,96,3626. (12) Rosenstock, H. M.;Draxl, K.;Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref. Data 1977, (Suppl. 1 ) . (1 3) Neusser, H. J. Int. J. Mass.Spectrom. Ion. Processes 1987,79, 141. (14) Ohashi, K.; Nishi, N. J. Phys. Chem. 1992, 96, 2931.
