1188
J . Phys. Chem. 1991, 95, 1188-1 194
and characterized by considering the perturbation of the second H F submolecule on the 1:l complex. Although similar 1:l complexes were observed in samples of WF6 and MoF6 condensed with H F and DF, the relative population of the 1:l complexes was dominated by the anti-hydrogen-bonded complexes, WF,-FH and MoF6-FH. In addition, several HF-perturbed WF6 and MoF, modes that are normally infrared inactive were identified. Furthermore, a different structural arrangement of the 1:2 complexes, WF,-FH-FH and MoF6-FH-FH, was observed. In contrast to the complexity of the metal hexafluorides, SF6 and H F produced only one 1 : l complex that probably has a hydrogen-bonded structure. Even though SF6-HF and SF6-HF-HF complexes exhibited significantly weaker principal interactions than the other
hexafluorides, the band positions of the 1:3 and 1:4 complexes for SF6 were very similar to the metal hexafluoride counterparts, which indicates the structures of these complexes are primarily determined by the cyclic nature of H F trimer and tetramer.
Acknowledgment. This research was sponsored by the Division of Chemical Sciences, US. Department of Energy, under Contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc. Registry No. UF,, 10049-14-6; HF, 7664-39-3; UF6-FH, 131 15314-5; UFb-(HF)z, 131 153-15-6; WFb-FH, 131 153-16-7; WF,-(HF),, 131 153-18-9; MoFb-FH, 131 153-17-8; MoF,-(HF)Z, 131 153-19-0; SF,, 2551-62-4; SF,-FH, 131 153-20-3; SF,-(HF),, 131153-21-4.
Spectroscopy of the Ionic Ground State of Monohalogenated Benzenes K. Walter, K. Scherm, and U. Boesl* Institut fur physikalische und theoretische Chemie, der Technischen Universitat Munchen. Lichtenbergstrasse 4, 0-8046 Garching, Federal Republic of Germany (Received: July 23, 1990; In Final Form: October 4 , 1990)
We present multiphoton ionization photoelectron (PE) spectra of monofluoro-, -chloro-, and -bromobenzene. The population of the vibrational levels in the cation after one-color, two-photon ionization via various vibronic intermediate states of the neutral molecule has been investigated. The observed structure has been assigned, and the frequencies of some vibrations in the ionic X-state were determined, providing new data for these molecular ions. Our results show that the PE spectra reflect Fermi resonances and Duschinsky rotations in the neutral intermediate states, allowing an interpretation of these states. The consequences of our results for neutral as well as ion spectroscopy will be discussed.
1. Introduction Since its first application to polyatomic molecules,' multiphoton ionization photoelectron spectroscopy (MPI-PES) has proven to be a very useful technique for the investigation of the electronic ground state of polyatomic cations. Molecules, excited to a neutral intermediate vibronic level by the absorption of a first photon, are ionized from this level by the absorption of a second photon. The kinetic energy of the outgoing photoelectrons has discrete values, correlated with the internal energy levels of the ion. Especially for nonfluorescing molecular ions such as benzene and many substituted benzenes the only data concerning the vibronic structure of the X-state result from photoelectron spectroscopy; in particular, most highly resolved data are due to MPI-PES. Moreover, with MPI-PES not only can vibrational frequencies be determined but also the population of the vibronic levels in the ion after multiphoton ionization. This information is of great interest for the use of multiphoton ionization as an ion source, especially when used for the spectroscopy of excited ionic state^.^-^ On the one hand, the vibronic structure of these PE spectra is in general not very congested because of symmetryselection rules for the ionization process. On the other hand, these selection rules give access to a variety of vibrational modes in the ion if one uses different vibrational levels as intermediate states for multiphoton ionization. In future, ZEKE-PES (zero kinetic energy PES)5 may deliver much better resolved spectra. Nevertheless, MPI-PES will be useful for fast survey spectra and will be necessary for the investigation of ion ground-state populations after MPI. ( I ) For a review see: Kimura, K. Int. Reo. Phys. Chem. 1987, 6, 195. (2) Ripoche, X.; Dimicoli, 1.; LeCalve, J.; Piuzzi, F.; Botter, R. Chem. Phys. 1988, 124, 305. (3) Walter, K.; Weinkauf, R.; Boesl, U.;Schlag, E. W. Chem. Phys. Lea. 1989, 155. 8. (4) Walter, K.; Boesl, U.;Schlag, E. W . Chem. Phys. Lett. 1989, 162, 261. ( 5 ) Chewter, L. A.; Sander, M.; Muller-Dethlefs, K.; Schlag, E. W . J. Chem. Phys. 1987, 86. 4737.
In the past decade benzene6and many substituted benzenes7-" have been studied by MPI-PES. The ionic ground state of monohalogenated benzenes, however, has not been investigated very thoroughly. No data are available for bromobenzene and iodobenzene. MPI-PE spectra of monochlorobenzene have been published by Anderson et but some of their results are in conflcit with the assignment of the B X transition of this cation investigated by Ripoche et aL2 In a recent work" we have published the MPI-PE spectrum of monofluorobenzenefor the neutral @o(S, So) transition to confirm the assignment of our REMPD (resonance-enhanced multiphoton dissociation) spectrum of the B X transition of this molecular cation. These examples have shown that for an unambiguous assignment of ionic REMPD spectra it is essential to know the vibrational population in the ionic ground state. In this current work we present a detailed study of the ionic X-state of monofluoro-, monochloro-, and monobromobenzene by MPI-PES via a great variety of intermediate states in the neutral molecule. Spectra of the SI So transitions have been recorded by resonance-enhanced multiphoton ionization, and the corresponding assignments have been taken from the literature. lodobenzene has not been investigated because its neutral SIstate is not stable, and a resonant ionization is not possible with laser intensities as used by us.12 We observed richly structured MPI-PE
-
-
-
(6) (a) Long, S.R.; Meek, J. T.; Reilly. J. P. J. Chem. Phys. 1983, 79, 3206. (b) Kuhlewind, H.; Kiermeier, A.; Neusser, H. J. In Resonance Ionizafion Spectroscopy 1986. Inst. Phys. Con/. Ser. 1986, No. 84, p 121. (7) Anderson, S.L.; Rider, D. M.; Zare, R. N . Chem. Phys. Lett. 1982, 93, 11. (8) Meek, J. T.; Long, S. K.;Reilly, J. P. J. Phys. Chem. 1982.86. 2809. (9) Meek, J. R.; Sektreta, E.;Wilson, W.; Viswanathan, K. S.; Reilly, J. P. J. Chem. Phys. 1985.82. 1741. (IO) Sektreta, E.; Viswanathan, K. S.; Reilly, J. P. J. Chem. Phys. 1989, 90, 5349. ( I I ) Walter, K.; Scherm, K.; Boesl, U. Chem. Phys. Leff. 1989,161,473. (12) Dietz, T. G.;Duncan, M. A.; Liverman, M. G.;Smalley, R. E. J. Chem. Phys. 1980, 73, 4816.
0022-3654/9l/2095-1188%02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1189
Ground State of Monohalogenated Benzenes spectra of these compounds and have succeeded in assigning most of the vibrational levels populated in the ionization process. Many vibrational frequencies of the ions were determined and compared between the three molecules. For fluoro- and chlorobenzene, the MPI-PE spectra obtained by ionization via levels mixed by a Fermi resonance or a Duschinsky rotation reflect the structure of this mixing, thus allowing an interpretation of the involved vibrations.
2. Experimental Section The experimental setup has been described in detail elsewhere;" we give here only a brief description. The resonance-enhanced MPI spectra have been generated in a reflection time-of-flight (RETOF) mass spectrometer. The inlet system consists of a pulsed, skimmed supersonic jet with 2 bar of Ar back pressure. The beam of an excimer-pumped, frequency-doubled dye laser crossed thc molecular beam in an electric acceleration field. The generated ions were then mass selected and detected in the RETOF. The MPI-PE spectra have been observed with a time-of-flight photoelectron spectrometer. It consists of an evacuated drift tube surrounded by a magnetic shielding. The gaseous sample (vapor pressure some millibars) could be introduced only without carrier gas but still with a pulsed nozzle. This resulted in a considerable reduction of cooling efficiency. Nevertheless, overlapping of adjacent vibronic bands due to their rotational envelope was no severe problem for all observed bands. Even at room temperature, vibronic bands with a spacing as small as 6 cm-I (see monofluorobenzene and cited references) are very well separated due to a very sharp R branch and an extended P branch with low peak intensity. The molecules were ionized by the absorption of two U V photons of the appropriate wavelength generated by the pulsed, frequency-doubled dye laser. The photoelectrons drifted 52 cm from the ionization region to a microchannel plate detector. Thus the time of flight of the electrons is inversely proportional to the square root of their kinetic energy. The signal was digitized with a sampling rate of 200 MHz and processed in a microcomputer. The time of flight of the electrons was converted to kinetic energy. An ion internal energy scale is known from the energy of the two absorbed photons and the ionization threshold. Absolute energies are correct within about 20 meV.I1 The resolution of our PE spectra is typically 10-1 5 meV; however, the peaks are not always of ideal Gaussian shape. Hence we think that for a cautious interpretation an error margin of 4 meV for relative energies between two peaks is appropriate. The spectra were averaged over 10000-40000 laser pulses with a repetition rate of 20 Hz.
-
3. Results and Discussion 1. Neutral SI So Transition of Monohoalogenated Benzenes. All three molecules belong to the symmetry group CZu; the z axis is collinear with the C2axis; t h e y axis is perpendicular to the molecular plane. The symmetries of the electronic states are as B2. The neutral SI-So transition is therefore follows: So, A,; SI, dipole allowed with the transition moment parallel to t h e y axis. The corresponding spectra for (a) monofluorobenzene, (b) monochlorobenzene, and (c) monobromobenzene are shown in Figure 1. An interesting point in all three spectra is the intensity of the 6b'o transition. The mode u6! (Wilson notation13) is of b2 symmetry in C2,,and the excitation of this mode in the SI state is therefore due to vibronic coupling. The relative intensity of this transition increases from fluoro- to chloro- and bromobenzene. For fluorobenzene,the assignment has been taken from the work of Lipp and Seliscar.I4 A characteristic feature of this molecule is the occurrence of Fermi resonances in the SIstate. The structure of these levels is reflected in the PE spectra as will be shown in the next sections. The assignment of the chlorobenzene SI So
-
( I 3) Wilson, Jr., E. B. Phys. Reu. 1934, 45, 706. (14) (a) Lipp, E. D.; Seliscar, C . J. J . Mol. Spectrosc. 1978, 73, 290. (b) Lipp, E. D.; Seliscar. C . J. J . Mol. Spectrosc. 1981,87, 242. (c) Lipp, E. D.; Seliscar, C . J. J . Mol. Spectroosc. 1981, 87, 255.
17000
- F R E Q U E N C Y icm-11
-
3eooo
184
31000
Figure 1. ( 1
- FREQUENCY
3nooo
1cm.11-
+ 1) photon ionization spectra of (a) monofluorobenzene,
-
(b) monochlorobenzene, and (c) monobromobenzene. The spectra show So transition. the vibrational structure of the S ,
transition is due to the work of Bist et aI.l5 For bromobenzene we used the assignment of Dietz et a1.;I2 however, we interchanged the notation of uI and v l 2 to be consistent with the other halobenzenes. It should be noted that for benzene derivatives belonging to C, symmetry group the notation especially of the u I / u 1 2and u , ~ / Y ,modes ~ is often interchanged throughout the literature. Another point is that our MPI-PE spectra raise some doubt concerning the assignment of the 9alo(SI So) transition in bromobenzene. This will be discussed in detail in section 3.4. Table I gives the frequencies of the SI So transitions used for our MPI-PE spectra and the corresponding excess energies in the ion for the three molecules. 2. MPI-PE Spectra of Monojluorobenrene. The observed MPI-PE spectra due to ionization via the SIstates Oo, 6b1, 1 I, 12', 18b2, 18a1,and 9aI are shown in Figures 2 and 3. The corresponding excess energies are given in Table la. From these spectra we determined the ionization potential to be 1P = 9.18 f 0.02 eV, in good agreement with values obtained from He(1) PE ~pectra.,~,~' According to Rabalais18 there exists the following symmetry selection rule for a one-photon ionization process: In the case where the point group of the molecule is conserved and for nondegenerate electronic states, the vibrational symmetry is conserved independent of electronic symmetries (pure Franck-Condon transition). That means if the ionization takes place from a
-
-
(IS) Bist, H. D.;Sarin, V. N.; Ojha, A,; Jain, Y. S. Appl. Spectrosc. 1970, 24, 292. (b) Jain, Y. S.; Bist, H. D. J . Mol. Spectrosc. 1973, 47, 126. (16) Kimura, K.; Katsuma, S.;Achiba, Y.; Yamazaki, T.; Iwata, S. In Handbook of He1 photoelectron spectra of fundamental organic molecules; Halsted Press: New York, 1981.
( 1 7) Turner, D. W.; Baker, C.;Baker, A. D.; Brundle, C . R. In Molecular Photoelectron Spectroscopy; Wiley-lnterscience: London, 1970. (18) Rabalais, J. W. In Principles oJultraviolet photoelectron spectroscopy; Wiley: New York, 1977; p 69.
Walter et al.
1190 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE I: Excess Energies of Two UV Photons above the Ionization Threshold
(a) Monofluorobenzene transition one-photon energy, cm-' excess energy, meV
6b'o 38 336
O00
37818
210
I 8b20 38 598 400
18a'o 38 734 440
1 IO
1 8a20
37 984 360
38017 370
7a10 38 118
1 210
38 582 400
340
gato 38 740 440
1IO
38 786 450
IP: 9.18 f 0.02 eV
(b) Monochlorobenzene transition
O00
one-photon energy, cm-' excess energy, meV
37 054
6b'o
37 574
I30
260
390
1P: 9.06 f 0.02 eV
transition
(c) Monobromobenzene 6b'o 1 IO 1 8a20 37 829 37 958 37514 3 20 400 430
O00
36 997 I90
one-photon energy, cm-' excess cnergy, meV
9a10 38017 450
1P: 8.98 f 0.02 eV
80
'I
0
ZOO
- ION INTERNAL
0
I
w
-
-wh
4b0
ENERGY lmeV1-
Figure 2. (1 + 1) photon ionization photoelectron spectra of monofluorobenzene. The spectra are due to ionization via the intermediate SI levels (a) Oo, (b) 6b1,and (c) 1'.
vibrationless level or from another level with a vibrational symmetry of a,, then only totally symmetric vibrational levels can be populated in the ion. On the other hand, by starting from a level with another symmetry (e.&, from the 6b1 and b2 symmetry) the vibrationless origin should not be populated. Another, less stringent, rule is the "Au = 0 propensity rule". It means that in general the most intense signal of a MPI-PE spectrum corresponds to a transition without changes in vibrational quantum numbers. This is, of course, due to Franck-Condon factors for transitions with only small changes in molecular geometry and applies well for photoelectron spectra recorded for excited states of aromatic molecules. Figure 2a shows the MPI-PE spectrum recorded via the Ooo(S1 So) transition. Most of the ions are prepared in the vibrationless ground state. The second most intense peak in this spectrum corresponds to the excitation of a vibrational state with an energy of 63 meV ( 410 c d ) ; two less intense peaks correspond to states with energies of 101 (=810 cm-I) and 126 meV (=I020 cm-I). This progression with a spacing of 63 meV can be observed in all of our fluorobenzene PE spectra. Because of the selection rules it should be due to a progression of a totally symmetric vibration. Considering the frequency of 63 meV ( 4 1 0 cm-I), we assign these signals to a progression of the V6a mode (frequency in the so,51 7 cm-' 14). From the PE spectrum observed via the 6bIo(S, -So) transition thc frequency of the 4,b mode can be determined to be also 63 mcV (frequency in the So, 615 cm-'I9). +-
photon ionization photoelectron spectra of mono-
-
i
'
200
ION INTERNAL ENERGY ImeVl
&io
Figure 3. (1 + 1) fluorobenzene. The spectra are due to ionization via the intermediate SI levels (a) 12l, (b) 18b2,(c) 18a1,and (d) 9al.
The question may arise whether, although in conflict with selection rules, the observed 63-meV progressions could be due to the Y6b mode. Indeed, for toluene (also a substituted benzene with C , symmetry) Reilly et ale8assigned an observed structure to a Y6b progression for exactly the same reason. We have now compared these results for fluorobenzenewith our data of the other halogenated benzenes. Both vibrational modes are in-plane modes. For the uh mode, however, the substituent is heavily involved in the vibrational motion, whereas in the u6b mode this is not the case. This results in a nearly constant Y6b frequency for all three neutral molecules; on the other hand, the frequency of Y6a depends very strongly on the mass of the s u b s t i t ~ e n t . ' ~ ~The ' ~ ,same ' ~ ~ ~effect ~ can now be seen for the cations in our PE spectra. The frequency of 4,b is nearly constant for the three investigated molecular ions, whereas the spacing between the observed progression bands shows ( I 9) Varsanyi, G.In Assignments for vibrational spectra of seven hundred benzene derivatives; Adam Hilger: London 1974.
The Journal of Physical Chemistry, Voi. 95, No. 3, 1991 1191
Ground State of Monohalogenated Benzenes
TABLE 11: Vibrational Frequencies in the Ionic X-State of Monofluorobenzene
mode freq. meV
VI
I22
y6s
"6b
"8.
%a
VI 2
63
63
145
VI81
200
101
I20
51
%a
VI2
VtSa
Vx
89
I I9
89
Vl8b
TABLE 111: Vibrational Freauencies in the Ionic X-State of Monochlorobenzene
mode freq, meV
VI
y 6 ~
1 I8
52
Y6b
63
V7a
I37
146
TABLE IV: Vibrational Frequencies in the Ionic X-State of Monobromobenzene
mode freq, meV
VI
y6s
40
1 I8
the mass dependence expected for the 4.a mode. These results indicate very clearly that our assignment is correct. For fluorobenzene the frequencies of the V6a and V6b modes in the ionic X-state are accidentally equal within our resolution. As has been shown in a previous paper of our group? this assignment allows a consistent interpretation of the B t X spectrum in the fluorobenzene cation obtained by multiphoton dissociation spectroscopy. In that work we determined the frequency of the v6a mode in the X-state via a hot band to be 500 f 2 cm-I, which is in excellent agreement with our PE value. According to the Au = 0 propensity rule, the most intense peak of the PE spectrum recorded via the I1o(S, So) transition (Figure 2c) is due to the excitation of the 1 I level in the ion. The frequency of uI can therefore be determined to be 122 meV (e980 cm-l; frequency in the So, 1009 cm-' 14). Besides the progressions of u6a, based on the origin and on the 1 ' level as well, another prominent feature is the excitation of a level about 201 meV (=I620 cm-I) above the 1 ' level. This 200-meV spacing is a characteristic feature of nearly all of our PE spectra of fluorobenzene. We tentatively assign it to the totally symmetric mode uga because of energetical considerations (frequency in the So, 1604 cm-I 14). Of particular interest are the MPI-PE spectra recorded via neutral transitions that include either anharmonic coupling (Fermi resonance) or Duschinsky rotation.20 A Fermi resonant level can be understood no longer as pure normal-mode excitation but as a linear combination of all involved vibrational modes. A Duschinsky rotation also can be understood as mode mixing by using a single normal-mode basis set to describe the vibrations in all electronic states. That means by ionizing via such an intermediate level all mixed vibrational modes will be excited in the neutral intermediate level, and this vibrational population will be transferred into the ion because of the Au = 0 propensity rule. This effect of the Duschinsky rotation can be seen in the PE spectra where fluorobenzene has been ionized via the levels 18a' (Figure 3c) and 9a1 (Figure 3d). These two levels have been interpreted by Lipp and SeliscarI4to be Fermi resonant. We prefer to explain the effects, observed in their SI So spectrum and in our PE spectra, by a Duschinsky rotation of the SI-statewith respect to the So- and X-state, because single quantum excitations should not interact.21 For both transitions the 18a' level as well as the 9a' level in the ionic ground state is populated with great intensity; however, in the first case the signal due to the I8a' level is more intense, in the second case the signal due to the 9al level. This result may indicate that for the SIlevel labeled as 18a' indeed the 18a' character (using the vibrational modes of the ionic X-state as a basis set) dominates and analogously for the 9a' level the 9a1character. The frequency found for ugain the ion is 145 meV ( = I 170 cm-I; frequency in the So, 1156 cm-I 14) that for u I a a is 120 meV (-970 cm-I; So. 1023 cm-Il4). The usa mode also appears in other PE spectra as a progression forming mode (Figures 2b,c and 3a,b). In both spectra (Figure 3c,d) the progressions of +,a and ul(I are built up on both base peaks leading to characteristic double peaks. The effects, mentioned above, also should appear in the PE spectra recorded via the SI levels 12l and 18b2, but now these
-
-
(20) Duschinsky, F. Acta Physiochim. 1937, 7, 551. (21) Wilson, E. 8.: Decius. J. C.; Cross, P. C. In Moleculor Vibrations; McGraw-Hill: New York, 1955; pp 193ff.
Y6b
"Sa
V9b
V I sa
67
190
134
121
effects are expected due to a Fermi re~0nance.l~The corresponding spectra (Figure 3a,b), however, are not as clear to interpret. The two spectra are very similar to each other as can be expected for Fermi resonant intermediate states. They are dominated by two signals at IO1 meV (=8 IO cm-I) and 125 meV (=lo10 cm-I). Besides the levels Oo and 6a', a signal at an ion internal energy of 51 meV ( ~ 4 1 cm-') 0 is observed. This lowenergy excitation is unique to these two spectra and is more intense in the one recorded via the 18b20(SI So) transition. The interpretation of these two spectra is based on the following arguments: The totally symmetric modes of fluorobenzene and their frequencies (in cm-I) in the So areI4 u I (1009), u2 (3080), (1604), V9a (1156), V I 2 (809), V I 3 (30611, V6a (517), Y7a (1232), uIaa(1023), ~ 1 (1500), 9 ~ and u20a(3094). The modes u , , V6a, us,, uga,and VISa are already assigned in the ion. An excitation of 101 meV ( ~ 8 1 cm-l) 0 appears also as a progression in many of our spectra (Figures 2, and 3a,b). The only totally symmetric mode of appropriate energy is the u12,progressions of n2ux (with u, being a not symmetric vibration) are not very likely. For this reason we assign the 101-meV excitations to excitations of u12. An energy of 51 meV ( ~ 4 1 cm-I) 0 is too low a frequency for any totally symmetric vibration; however, it corresponds very well to the frequency of the tq8b mode in the neutral (so,400 ~ m - 1 ; ' ~ b2 symmetry). The energy of the level 18b2 in the ion, which should be excited by ionizing via the 12I0 and 18bZo(SI So) transitions, would then be about 102 meV and within our resolution equal to the 12) level. The reason for the excitation of the 18b1 level, in conflict with symmetry selection rules, may perhaps be found in the complex structure of the Fermi resonant neutral states involving two quanta of VISb. It should again be noted that the 51-meV excitation appears only when the ionization takes place from a neutral state with ul8b excited. Thus, we tentatively assign the frequency of 51 meV to u I s b The peak at 101-meV ion internal energy is due to both the 12l and the 18b2 levels. We have, however, no explanation for the intense peak at 125 meV. The energy corresponds to the 6a2 level, but the high intensity obviously canqot be explained by this excitation. The attempt to assign either the 12' or the 1 8b2 level to 125 meV fails because, in the first case for the 101-meV progressions and in the latter case for the 5 1 -meV excitation, no reasonable explanation can be given. In addition, in both cases the vibrational frequency in the X-state would be unusually high compared to the So- and SI-states. The frequencies of the assigned vibrational modes of the X-state of monofluorobenzene are collected in Table 11. A complete set of all known vibrational frequencies of monofluoro-, -chloro-, and -bromobenzene for the lowest two electronic states of the neutral and ionic molecule is given in Table V; ; their symmetries are given in Table VI. 3. MPI-PE Spectra of Monochlorobenzene. MPI-PE spectra of monochlorobenzene have already been published by Anderson et a].;' however, their results are in part inconsistent with the assignment of the B X transition given by Ripoche et aL2 for this molecular ion. For the interpretation of their spectra Ripoche and co-workers claimed an intense population of the 6a1 level in the ionic X-state after ionization via the Ooo(Sl So) transition, but not such excitation can be found in the corresponding MPI-PE spectrum of Anderson et al. We have now also investigated monochlorobenzene to clear up this conflict and to present an investigation of the three monohalobenzenes as a whole. +
-
-
Walter et al.
1192 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE V Vibrational Freauencies of Halobenzenes (cm-I); Frequencies Obtained by MUPI-PES Rounded to 10 cm-'
fluorobcnzene electronic state I
2 3 4 5 6a 6b 7a 7b 8a 8b 9a 9b 1Oa 10b 11
12 13 14 15 16a 16b 17a 17b 18a 18b 19a 19b 20a 20b
1009 3080 1301 687 978 517 6156 1232 3069 1604 15976 1156 1 I28 818 754 249 809 3061 1326b 1066 414 498 957 895 1023 400 1500 1460 3094 3053b
968
890
980
460 520 1220
chlorobenzene electronic state
460
500" 510
1620 922
1170
1118
~550
'/2.364 765
810
1590 977 '/2*413
~267" =632
916 387
970 410
295
ODo: 37861O
1003 3054 I272 685 98 1 417 615 1093 3067 I586 1598 1153 1 I67 832 74 1 197 706 3031 1327 1068 403 467 96 1 902 1026 287 1482 1447 3082 3096
931 31 I9 I194 422 699 378 521 1065 3056 1564 1584 98 1 1 I49 616 556 138 67 1 3084 15648 1090 203 320 729 655 966 266' 1489 1483 3158 3076 ODo: 37056
950
422h 510 1100
bromobenzene electronic state 869
1001
93 1
950
387
3065 1264 68 1 989 314 614 1070 3056 1578
294 519
320
1176O
1019O
943
1180
540 1530
1I58O
~761" 720
~313"
960
832 736 181 67 1 3029 1321 1068 409 459 963 904 1020
10800
620 ~1590"' I/2*240 960
980
260 1472 1443 3067 ODo: -17900k
ODo: 36992'
OReference 14. bReference 19. 'This work. dReference 4. 'Reference 17. /Reference 15. gMurakami, J.-I.; Kaya, K.; Ito, M. J. Chem. Phys. 1980, 72, 3263. hReference 2. 'Reference 7. &Reference16. 'Reference 12. '"Rava, R. P.; Goodman, L. J. Am. Chem. SOC.1982, 104, 3815. "These are the inducing modes of the dipole-forbidden B X transition in the ion. The given frequencies correspond to the @,(B X) transition
-
-
energies listed, which have been extracted from Hel-PE spectra;16," see also ref 4. "The notation of the vibrational modes 9a, 9b may be interchanged in ref 12; see text, section 3.4. TABLE VI: Symmetries of the Vibrational Modes 1-208 of Monohalogenated Benzenes Due to the Point Group C,, sYm vibr mode I , 2, 6a, 7% 8a, 9a, 12, 13, 18a, 19a. 20a ill
a2 bl b2
loa, 16a, 17a I . 5 , lob. 1 I . 16b, 17b 3, 6b, 7b, 8b. 9b, 14. 15, 18b, 19b, 20b
'k 5 6bb
6b'l'
0
-ION
ZOO
I N T E R N A L E N E R G Y l m e V' I
-
Figure 4. ( I + I ) photon ionization photoelectron spectra of monochlorobenzene. The spectra are due to ionization via the intermediate SI levels (a) Oo and (b) b6'.
Our observed MPI-PE spectra due to ionization via the S,-states Oo, 6b', I ' , 18a', and 7a' are shown in Figures 4 and 5. The corrcsponding excess energies are given in Table Ib. From these spectra thc ionization potential has been determined to be 9.06 f 0.02 cV, in fair agreement with literature values of about 9.10 eV.Ih
0
-ION
Figure 5. ( I
+
ZOO
I N T E R N A L E N E R Y ImeVl
-
I ) photon ionization photoelectron spectra of mono-
chlorobenzene. The spectra are due to ionization via the intermediate SIlevels (a) 1 1 , and (b) 18a1,and (c) 7a'.
-
As can be seen from Figure 4a, the ionization via the Ooo(SI So) transition populates not only the vibrationless origin but
also with great intensity a vibrational level of 52-meV (-420 cm-I) energy. No indication for such an excitation can be seen in the PE spectrum measured by Anderson et al.' Progressions with a spacing of 52 meV can also be observed in all other MPI-PE spectra of chlorobenzene. Because of energetical and symmetry
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1193
Ground State of Monohalogenated Benzenes reasons we assign these signals to excitations of v ~thus , confirming the assignment of R i p h e et ale2They give an energy of 422 cm-I for the 6al level in the X-state, which is in excellent agreement with our value. Considering the Ao = 0 propensity rule and the vibrational frequencies in the neutral, the intense peaks of the PE spectra recorded via the 1 I (Figure sa), 18a' (Figure 5b), and 7a1 (Figure 5c) intermediate levels can be assigned to excitations in the ion and v~~ and their combinations with V6a of the modes u , , excitations. The observed frequencies are as follows: V , 118 meV ( ~ 9 5 cm-l 0 so,1003 cm-I); V18a 119 meV ( e 9 6 0 cm-I; so,1026 cm-I); qa137 meV (e1 I 0 0 cm-I; So, 1098 cm-I). The frequencies in the So have been taken from ref 15. Besides uba two other progression-forming modes can be observed with frequencies of 89 and 146 meV. These modes have to be of a , symmetry, a comparison with frequencies in So leads to the assignments uIz 89 meV (=720 cm-'; So, 706 cm-I) and u9, 146 meV (.=I 180 cm-l; So, 1 153 cm-I). Our frequencies for Vba and u9, are in good agreement and our frequency of v I Zis in fair agreement with the values of Anderson et al.' Our interpretation of the PE spectrum observed by ionizing via the 6bIo(S, So) transition (Figure 4b) differs from that given by Anderson. In common is the assignment of the first and most intense peak to the 6b' level. We determined a frequency of 63 meV ( 4 1 0 cm-I) for this vibration (So, 615 cm-' Is). The second peak, denoted X in Figure 4b, is due to an excitation with an ion internal energy of 89 meV ( ~ 7 2 0cm-I), which is exactly the energy of one u 1 2quantum. Indeed, Anderson et al. assigned this signal to the 12, level (a, symmetry in C2").This interpretation requires that chlorobenzene in its ionic X-state belongs to another point group than in the neutral SI-state, because the Y6b mode excited in the neutral intermediate SI-state is of b2 symmetry. Therefore, only levels of b2 symmetry should be excited in the ion with the point group of the X- and SI-statesbeing the same. Our spectra, however, show no clear indication for a symmetry change; in particular the extremely small population of the Oo level in the X-state by ionizing via the 6bIo(SI So) transition suggests that the point group is conserved and the selection rules are valid. For this reason we prefer another interpretation, which is based on the observation of a Fermi interaction in the SI state of chlor~benzene.'~I n the region of the 6blo(S1 So) transition Bist et al.Is observed two strong bands with a spacing of 5 cm-I. These bands are not resolved in our MPI spectrum (Figure 1b). Bist et al. assigned these bands to a Fermi resonance of the 6b' level with the combination level 16a116b'. With the same arguments as given for fluorobenzene, in the ionic X-state the level 6bI as well as the combination level 16a'16b1should be populated with considerable intensity. The energy of 89 meV (e720 cm-I) also is not unlikely for this combination mode, considering the sum of the corresponding So freq~encies'~ of 403 cm-l + 467 cm-' = 870 cm-I. One may consider other posible explanations for the X peak, for example, a combination mode involving V6b. This is not very likely because the energy difference of 26 meV ( ~ 2 1 0cm-I) between the 6b' and X signal is much too low for any totally symmetric mode. For a single-mode excitation the only vibrational modes of b, symmetry in the appropriate energy region are u9b and u l S with a frequency in Sois of 1167 and 1086 cm-I, respectively, which is obviously too high. Additionally no obvious explanation for the great intensity can be given. The third and fourth peak in Figure 4b can obviously be assigned to the combination modes of X and u6b with one quantum of the u6& mode. With the resolution obtained in our spectra it is possible to exclude a progression of the u6b mode as assigned by Anderson and coworkers. Table I l l gives a list of the vibrational frequencies observed by us for the X-state of chlorobenzene. A complete set of vibrational frequencies for all three monohalobenzenes is given in Table V. 4 . MPI-PE Spectra of Monobromobenzene. The MPI-PE So spectra observed by ionizing bromobenzene via the S I transitions Ole, 6b'o, 1 lo, I8alo, and 9alo are shown in Figures 6 and 7. The excess energies are given in Table IC. These spectra
0
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INTERNAL ENERGY ImeVI-
Figure 6. ( 1 + 1) photon ionization photoelectron spectra of monobromobenzene. The spectra are due to ionization via the intermediate SI levels (a) Oo and (b) 6b'.
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,
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200
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INTERNAL ENERGY ImeVl
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Figure 7. (1 + I ) photon ionization photoelectron spectra of monobromobenzene. The spectra are due to ionization via the intermediate S, levels (a) I t , (b) 18a'. and (c) 9al.
provide the first vibrational data for the bromobenzene cation. We determined an ionization potential of IP = 8.98 f 0.02 eV in good agreement with literature values of 9.02 eV.I6 The dominating feature in all the PE spectra is a long progression with a spacing of 40 meV ( ~ 3 2 cm-I), 0 which again can be assigned to the 4, mode (So, 314 cm-' 19), the a l mode with the lowest frequency. Besides these progressions in most of our spectra, a vibrational excitation with an energy of 190 meV ( = I 530 cm-l) can be observed, however, with less intensity. We assign these signals tentatively to excitations of the uga mode (So, 1578 cm-I 19). According to symmetry selection rules and the AD = 0 propSo) ensity rule in the PE spectra recorded via the 6blo(S, transition (Figure 6b), the first and most intense peak can be assigned to the 6b' level in the ion. The frequency of the V6b mode is then 67 meV ( ~ 5 4 cm-I; 0 So, 614 cm-';I9 note the different notation). The large intensity of the fourth peak at an ion internal energy of 190 meV is probably due to the superposition of two signals: one can be assigned to the 6b'6a3 level, and the other to the 6bI I I level. The frequency of u , can be derived from the PE spectrum recorded via the I l0(SI So) transition (Figure 7a). Here again the most intense signal is due to the Au = 0 transition from the intermediate state and therefore corresponds to the ionic level 1 I . Thus, we determined the frequency of uI to be 1 18 meV ( ~ 9 5 0
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J . Phys. Chem. 1991, 95, 1194-1200
1194
cm-'; So, 1001 cm-l 19). In the same way the frequency of vIga can be determined from the PE spectrum recorded via the 18a1 intermediate level (Figure 7b) to be 121 meV (=980 cm-'; 2,,, 1020 cm-' 19). In both spectra, a relatively intense population of the Oo level and other totally symmetric levels (6a1, 6a2) below the Av = 0 transition can be observed, as is typical for PE spectra recorded via intermediate levels of vibrational a l symmetry. In contrast to this, no excitation below the Au = 0 transition can be observed in the PE spectrum recorded via the SI So transition labeled 9aIo. This is a very uncommon feature for a MPI-PE spectrum recorded via an intermediate level of a l symmetry (as is 9aI) but a typical indication for a non-totally symmetric intermediate level. On the other hand, all three of the investigated halogenated benzene cations behave very similarly concerning the population of totally symmetric vibrational levels (including the 00 level); the only exception is the 9a1, spectrum. For this reason we think that the assignment of the 9aIo(S1 So) transition in the neutral molecule by Dietz et al.I2 may be incorrect, and the excited level in the SI may instead be the 9b' level because v9b is, like v6b, a mode of b2 symmetry and the frequencies of v9, and vgb are very similar in So. Therefore, we tentatively assign the observed signals of this PE spectrum to an excitation of the vgb mode and its combinations with a l modes. The frequency of v9b is then 134 meV (=1 180 cm-'; So, 1 158 cm-l 19).
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4. Summary In this work we have investigated the ionic ground state of the benzene derivatives monofluoro-, monochloro-, and monobrombenzene by multiphoton ionization photoelectron spectroscopy. We could determine new-for bromobenzene even the firstvibration data for the molecular cations. Most of the prominent signals have been assigned, and the frequencies of many vibrations have been determined. The population of vibrational levels in the ionic ground state has fundamental consequences for spectra of molecular ions
starting at the ionic ground state. Therefore, it is important to investigate the starting conditions for ion spectroscopy (i.e., dissociation spectroscopy) by MPI-PES when using a multiphoton source of ions. Our results clearly confirm the assignments given for the B X transition of f l ~ o r o and - ~ chlorobenzene,2 which in the case of chlorobenzene were in contrast to other measurements.' The spectra show that in most cases the Au = 0 transition from the intermediate state to the ion is favored but never appears exclusively. With increasing mass of the substituent and with increasing excess energy, other transitions, in particular progressions of the +,a mode, gain in intensity. Of particular interest are the PE spectra recorded via intermediate levels that show mode mixing. These spectra reflect in a very direct manner the situation in the neutral states. Assuming similar Franck-Condon factors for the ionization, the relative intensities of all involved vibrations are directly transferred into the ion, which may help to determine the complex structure of such levels. In addition our results indicate that, for Fermi resonant levels involving a two-quantum excitation of an asymmetric mode, the simple symmetry considerations for the ionization process, as given in this paper, are no longer valid. A further interesting feature of MPI-PES is the difference between spectra recorded via totally symmetric and non-totally symmetric vibrational levels, which at least indicates the vibrational symmetry of the intermediate state. For bromobenzene our results suggest a partially new assignment of the vibration structure of the SI +- So transition. Our results demonstrate that MPI-PES not only is useful for investigations of the ionic ground state and delivers valuable information for further spectroscopy of ionic states but can also be of interest in resolving complex spectroscopic problems of neutral molecules. Acknowledgment. We thank Professor H. J. Neusser for the generous loan of the photoelectron spectrometer, Professor S. D. Colson for the critical reading of the manuscript, and Professor E. W. Schlag for his continuous interest and helpful discussions. + -
Localized and Delocalized Excited States of the Fluorene Dimers Hiroyuki Saigusa* and Edward C. Limt Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 (Received: August 14, 1990)
Excited-state structures and dynamics of isotopically substituted van der Waals dimers of fluorene have been investigated by a hole-burning technique combined with supersonic jet fluorescence spectroscopy. The initially excited state of the mixed dimer (C,3Hlo/C13Dlo)is described as an locally excited van der Waals dimer state in which the initial excitation is localized and (C13D10)2. exhibit exciton splittings due to resonance on either half of the dimer, while those of the pure dimers, (C,3H10)2 interactions. These pure and mixed dimer states are strongly coupled to their intermolecular vibrational modes, leading to rapid formation of delocalized excimer and exciplex states, respectively. The fluorescence lifetimes of the exciplexes produced by exciting the mixed dimers are compared with those of excimers. Unlike the localized nature of the initially prepared excited states of the dimers, the excitation is no longer localized in the excimer and exciplex states.
Introduction The formation and deactivation processes of excimers of aromatic molecules have been extensively studied in solutions and in since the initial discovery of the fluorescent excimer of pyrene by Forster and Kasper.' The term excimer has been introduced4 to describe an excited-state dimer that is dissociative in the ground state, to distinguish it from an excited-state van der Waals (vdW) dimer. The stabilization of the excimer state is 'The inaugural holder of the Goodyear Chair in Chemistry at The University of Akron.
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explained by a configurational mixing of exciton resonance states and charge resonance states. It is also well established that the excimer formation can be described satisfactorily as a diffusioncontrolled process in which an excited-state molecule A* encounters its ground-state partner A, Le., A* + A (AA)*.
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( I ) Birks, J. B. Photophysics of Aromoric Molecules; Wiley: New York, 1 9 7 0 pp 301-371. (2) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Speclra; Marcel Dekker: New York, 1970; pp 41 1-484. ( 3 ) Farster, Th.; Kasper, K. Z . Phys. Chem. 1954, I , 19. (4) Stevens, B.; Hutlon, E. Nature 1960, 186, 1045.
0 1991 American Chemical Society
