2246
J. Phys. Chem. 1983, 87, 2246-2255
system like Dabco where the center of charge is located in the middle of the cage. The mechanism for observing these multiphoton multicolor absorptions is in itself interesting, paralleling earlier work on atomic v a p ~ r s . ’ ~These J ~ results will be reported in a companion paper which discusses pressure, laser intensity, and electric field effects.13 Finally, extrapolation of these Av = 0 only selection rules to direct optical excitation above the ionization limit presents the possibility of producing sources of vibrationally selected ions of large molecules. Alternatively, such ions might be prepared by two-color absorption to a highly excited Rydberg followed by pulsed field ionization. Such experiments would allow direct measurement of autoionization lifetimes in the nanosecond regime. These studies are currently underway in our laboratory.
rules to the high Rydbergs. Such calculations are presently underway in our laboratory. Summary The present two-color studies on the Rydberg states of the 20-atom molecule Dabco demonstrate that useful information on very highly excited large molecules may be obtained at unprecedented resolution. The key feature of the experiment is the use of the first two-photon absorption to a low-lying Rydberg state to provide vibrational selection which is preserved upon subsequent excitation. The vibrationally selected Rydberg series obtained are remarkably unperturbed, and may be used to obtain best fit ionization potentials to vibrationally selected ions to within 5 cm-l. Several different Rydberg series are found which converge to the same ionization limit. The variation of quantum defects for these series provide a unique test case for extension of concepts developed in the understanding of electron-core interactions in highly excited atoms. This is especially true for a model
Acknowledgment. It is a pleasure to acknowledge useful discussions with W. A. Chupka and P. M. Dehmer. Registry No. Abco, 100-76-5;Dabco, 280-57-9.
Multiphoton Ionization and Fragmentation Pathways of Benzene, Fluorobenzene, Iodobenzene, and terf-Butylbenzene Kenneth R. Newtont and Richard B. Bernsteln’ Deparfment of Chemistry, Columbia Unlversity, New York, New York 10027 (Received: December 17, 1982)
Multiphoton ionization (MPI) and fragmentation pathways of benzene and several substituted benzenes have been studied via a modified laser ionization time-of-flightmass spectrometer. To study the ion fragmentation pathways we used electron impact ionization or laser-induced ionization to provide a known ensemble of ions; a second, tunable laser (whose pulse is suitably delayed) photodissociates these “primary”ions and the resulting distribution of fragments is measured. Information is thus gained on the fragmentation pathways of a number of the principal primary ions from benzene and the substituted benzenes. Experiments have been carried out with a Nd:YAG pumped dye laser as well as a flashlamp pumped dye laser, allowing comparisons to be made of the effect of laser pulse length and spectral bandwidth on the fragmentation process. Extensive fragmentation of the parent molecular ions has been observed (with either laser) in one, two-, and even three-photondissociation steps. The present results are interpreted in terms of known ion photodissociation spectroscopywhere relevant ICR data are available.
Introduction Much of the previous work in multiphoton ionization mass spectrometry (MPIMS) has been confined to the study of the multiple photon excitation and ionization process of the neutral molecule, with less attention to the sequential photofragmentation of the “primary“ ions. For a review and discussion, see ref 1. The present work deals with the fate of these primary ions and the role of their spectroscopy (and photodissociation) upon the pattern of ion fragments from conventional MPIMS experiments. Somewhat related experiments have been reported by Schlag? Baer? Danby: Compton,SEl-Sayed? Wittig,’ and their respective co-workers. The present method makes use of either electron impact ionization (EI) or laser-induced ionization to provide a Present address: Cordis Research Corp., P.O.Box 525700,Miami, FL 33152. *Present address: Occidental Research Corp., P.O. Box 19601, Irvine, CA 92713. 0022-3654/83/2087-2246$01.50/0
known ensemble of ions; a second tunable laser (pulse suitably delayed) photodissociates these primary ions and the resulting fragmentation pattern is recorded. Two different lasers have been used, one a Nd:YAG pumped narrow-band dye laser (5-11s pulse length), the other a flashlamp-pumped dye laser (FPDL),broad-band, pulse length ca. 300 ns. Thus the same fluence or pulse energies could be achieved with the two lasers, but with very different radiation densities. (1)R. B. Bernstein, J. Phys. Chem., 86, 1178 (1982). (2)(a) U.Boesl, H. J. Neusser, and E. W. Schlag, J. Chem. Phys., 72, 4327 (1980);(b) Chem. Phys., 55, 193 (1981);( c ) Chem. Phys. Lett., 87, l(1982). (3)T.Carney and T. Baer, J. Chem. Phys., 75, 477 (1981). (4)I. Powis and C. J. Danby, Znt. J. Mass Spectrom. Zon Phys., 32, 27 (1979). (5) J. C. Miller and R. N. Compton, J. Chem. Phys., 75, 2020 (1981). (6) R. S.Pandolfi, D. A. Gobell, and M. A. El-Sayed,J. Phys. Chem., 86, 1779 (1981). (7)Y.Haas, H. Reisler, and C. Wittig, J.Chem. Phys., 77,5527 (1982).
0 1983 American Chemical Society
MPI Fragmentation Pathways of Benzene
The Journal of Physical Chemistty, Vol. 87, No. 12, 1983 2247
T 7
1
1 - Butylbenzene
LENS
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’
0.05mJ; 5 6 0 ”
9.5mJ
IONIZATION
I
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Figure 1. Schematic diagram of the apparatus. The PhassR dye laser pulse or electron ionization pulse could be triggered before or after the Quanta-Ray laser pulse.
The four molecules investigated (benzene, fluorobenzene, iodobenzene, and tert-butylbenzene) had been the subject of previous ICR photodissociation s t u d i e ~ . ~ ? ~ They cover the range from one-photon to three-photon dissociation of the parent molecular ion at the wavelengths (570-600 nm) conveniently provided by the FPDL system. Since the chosen compounds are closely related to each other,1° many of the fragment ions are common to two or more of the compounds.
Experimental Section The apparatus (Figure 1)consists of a modification of the laser time-of-flight mass spectrometer described previously,’Jl namely, the addition of a second laser and associated timing as well as the capability of dual EI-laser and two-laser experiments. The first laser is a Quanta-Ray NdYAG pumped dye laser (DCR-lA, PDL-1) with additional crystals for generation of second through fourth harmonics. For certain experiments, two colors were used, Le., one of the harmonics in addition to the dye laser output. The second laser is a Phase-R (DL2100D with high-rate adapter) flashlamp-pumped dye laser (FPDL), upgraded from a DL2100B, with a DP-60 prism tuning element. It was operated with rhodamine 590 dye to give pulses of up to 100-mJ energy with a pulse duration of about 300 ns and repetition rate of 1.1 Hz. The two lasers (as well as the E1 electron beam pulse of 1-ps duration) could each be triggered with appropriate delays relative to the TOF MS drawout pulse by using either the MINC/11 computer or the laser synchronous output. Delays in the range of 0-5 p s were interposed between either the E1 pulse and the laser pulse or between the two laser pulses. The DEC MINC/11 computer controls the experiment; it has been programmed to alternate between a combined E1 plus laser experiment and an E1 mass spectrum (or the same for a two-laser experiment). The program allowed averaging of the E1 data as well as the subtracted (E1 + laser - EI) data for display on any of the media available (8) (a) P. P. Dymerski, E. Fu,and R. C. Dunbar, J.Am. Chem. SOC., 96,4109(1974);(b) R.C.Dunbar, H. H. Teng, and E. W. Fu, ibid., 101,
____
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(9)B. S. Freiser and J. L. Beauchamp, Chem. Phys. Lett., 35, 35 (1975). (10)D. W.Squire, M. P. Barbalas, and R. B. Bernstein, J. Phys. Chem., in press. (11)(a) D. A. Lichtin, S. Datta-Ghosh, K. R. Newton, and R. B. Bernstein, Chem. Phys. Lett., 75, 214 (1980);(b) K.R. Newton, D. A. Lichtin, and R. B. Bernstein, J. Phys. Chem., 85, 15 (1981);(c) D. A. Lichtin, R. B. Bernstein, and K. R. Newton, J. Chem. Phys., 76,5728 (1981).
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Newton and Bernstein
The Journal of Physical Chemistty, Vol. 87, No. 12, 1983
2248
* 5
1
t - Butylbenzene
4 ( a ) 266nm
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I PHOTODISSOCIATION CROSS SECTION
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Figure 4. (a) MPI mass spectrum of tert-butylbenzene at 266 nm and 0.2 d,using the Nd:YAG laser. (b) Difference mass spectrum taken by subtracting from the twdaser (Nd:YAG followed by the FPDL) MPI spectrum the single laser spectrum (a). The negative peak demonstrates the loss of parent ion through photodissociation to produce the positive peak, indicating loss of a methyl group. The fraction of parent ions involved in this process in a single laser shot is nearly unity although the average is about 0.7 due to laser misfires. The second laser in this case was the FPDL using rhodamine 590 at a 1-Hz repetition rate, prism tuned to 580 nm; pulse energy ca. 2 mJ.
-
t Butylbenzene
"
Figure 3. Energy level diagram for fert-butylbenzene showing appearance potentials for the most abundant fragment ions compared with the present photon energies. Also shown is the photodlssoclation cross-section spectrum obtained from ICR measurements.@
The compounds benzene, iodobenzene, fluorobenzene, and tert-butylbenzene were chemically pure, degassed before use. The mass spectra revealed no significant impurities.
Results tert-Butylbenzene. Figure 2 shows the results of a two-color MPI experiment with tert-butylbenzene. The one-color MPI mass spectrum shown in (d) was obtained by using the 266-nm YAG fourth harmonic focused with a 0.5-m cylindrical lens at low laser power. This is the condition for soft ionization. The single pulse ion intensities in this case are about 1000 times that of a typical E1 spectrum. This is the case even though the pulse lengths are different by a factor of 200 (laser, 5 ns; 50-pA electron pulse, 1ps). This is compensated for only by the relative repetition rates (laser, 10 Hz; EI, 200 kHz). Parts a-c of Figure 2 show the effect of adding a 560-nm dye laser pulse (partially overlapped in time) at different power levels. The effect is to dissociate virtually the entire ensemble of parent ions, via the ejection of a methyl group. Figure 3 offers a simple interpretation for this effect. Dunbar et al." reported the photodissociation spectrum of the tert-butylbenzene ion and identified the process as CloH14+ C9H11++ CH3. Since this is a one-photon absorption it does not require the high power levels of the YAG laser. Thus one expects essentially the same results when using the FPDL since it produces pulses with the same pulse energy but at much lower peak power density and wider bandwidth. Figure 4 shows this result in a two-laser experiment with the YAG 266-nm photons producing the MPI mass spectrum (a), and the difference mass spectrum (b) obtained by subtracting (a) from the two-laser spectrum (not shown). This demonstrates strong conversion from parent
-
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576nm : 2 5 m J
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110
60
80
100
120
111
MRSS Flgure 5. Wavelength dependence (second photon). The pulse from the Nd:YAG at 266 nm precedes the dye laser pulse by 5 ns at the indicated wavelengths and energies.
ion (Cl0HI4+)to CgHll+with loss of a methyl radical. This two-laser experiment is an indication that the relatively lower radiation density of the FPDL (due to wide bandwidth and long pulse length) has a negligible effect on the extent of fragmentation of the parent ion. This is expected for a one-photon dissociation since the power law and the dependence upon pulse length is linear. Thus low power densities suffice to prepare large ensembles of specific ions (such as CgHll+seen here) for study by other means.
MPI Fragmentation Pathways of Benzene
1
I-Bufylbenzene
The Journal of Physical Chemisfty, Vol. 87, No. 12, 1983
2249
( 2 6 6 n m : 0.05mJ,5 6 0 n m : 9 . 5 m J ) - 2 6 6 n m
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Figure 6. TCIF difference mass spectrum for fert-butylbenzene from Figure 2a magnified ten times to reveal the smaller ion fragments.
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Figure 7. MPI mass spectrum at 266 nm, 1.2 mJ/pulse, shown on a normal scale and ten times magnified.
Figure 5 shows the results of varying the wavelength and power of the dye laser. As expected from the broad ICR photodissociation spectrum, there is little wavelength dependence over the range used and the laser power affects only the degree of saturation of the dissociation process. A tenfold magnification of the higher power two-color results of Figure 2a is shown in Figure 6. This shows fragments down to C3H3+and C3H5+from the 560-nm photons. Figure 7 shows results of a MPI experiment with higher power at 266 nm, revealing most of the same fragment ions despite the large difference in photon energy. Higher power levels used for MPI at 355 and 532 nm yield much greater fragmentation, with C+ as the most intense peak, as reported earlier for tert-butylbenzenellc and benzene.12 Iodobenzene. The ions for the photodissociation experiments have been produced by electron impact (EI) as well as by soft MPI. Figure 8 shows E1 and EILF difference mass spectra for iodobenzene at electron energies of 70 and 13 eV, respectively, using the Nd:YAG dye laser at 564 nm. The low energy electron ionization is used to produce only parent ions. This identifies a very simple process, namely, the loss of an iodine atom from the iodobenzene parent ion to give C6H5+(and lesser amounts of C4H3+). As for tert-butylbenzene, at the powers used this effect is essentially saturated (all the parent ions destroyed). The explanation can again be found in an ICR (12)(a) L. Zandee and R. B. Bernstein, J. Chem. Phys., 70, 2574 (1979);(b) Zbid., 71, 1359 (1979).
Figure 8. (a) E1 mass spectrum of lodobenzene at 70 eV. (b) Difference mass spectrum showing the photodissociation of the EI-produced ions by the laser at 564 nm, 18 mJ/pulse. Clearly shown is the absorption of the 564-nm photons by the parent ion to dissociate to CBH,+ and C,H,+. (c) E1 mass spectrum at 13 eV. (d) Difference spectrum from the 13-eV spectrum with the same laser conditions as (b), demonstrating that the C,H3+ ion is produced in a single pulse from the lodobenzene parent rather than from the other ions that are formed by the 70-eV E1 fragmentation.
resultab (reproduced in Figure 9), except in this case the process is a two-photon absorption. One expects that by using a lower power density at the same pulse energy saturation would no longer occur and the smaller, more energetically "expensive" fragments would disappear. Figure 10 shows two EILF difference mass spectra taken with the FPDL at two laser bandwidths (prism tuned, AA 0.2 nm, and untuned, Ah 4 nm). The extent of dissociation remains nearly the same in both cases and is again near 100%. In addition, smaller fragments are still visible in both spectra. The lower mass spectrum shows loss of mass 74 (C&2+) and gains of 49 (C,H+) and 37 (C3H+). The saturation with power of the main dissociation process is evident in Figures 11and 12 at 580 and 584 nm, respectively. In Figure 12, the C4 fragment ions are seen to appear as the first process becomes saturated. Figure 13 shows the wavelength dependence over the range 580-596 nm. The lower ion intensities in (b) are due to the lower absorption cross section in that region (cf. Figure 9). Benzene. A set of EI-laser difference mass spectra (EILF) at different electron energies is shown in Figure 14 for benzene. Contrary to the results for tert-butylbenzene and iodobenzene, the fragmentation does not saturate at the laser pulse energies used. The losses of parent ion amount to
