The Journal of
Physical Chemistry
0 Copyright, 1985, by the American Chemical Society
VOLUME 89, NUMBER 13
JUNE 20, 1985
LETTERS Competitive Fragmentation Processes In Multiphoton Ionization: The Role of Ladder Switching Steven W. Stiller and Murray V. Johnston* Department of Chemistry and CIRESt University of Colorado, Boulder, Colorado 80309 (Received: November 26, 1984; In Final Form: April 10, 1985)
The competitive photofragmentation pathways of cleavage vs. rearrangement in molecules undergoing multiphoton ionization are used to elucidate the role of ladder switching after the absorption of one photon by the parent ions. For diphenyl ether, absorption of one photon at 266 nm provides enough energy to form the rearrangement daughter ion, but two photons are required to form the cleavage ion. Our results indicate that ladder switching does not occur after the absorption of the first photon by the parent ion which places an upper limit on the unimolecular decay rate constant of ca. lo8 s-l. For anisole and benzyl acetate, the parent ion internal energies produced by the absorption of one photon are sufficient to allow efficient switching to both the cleavage and rearrangement daughter ions.
Introduction Resonantly enhanced multiphoton ionization (REMPI) has been shown to be a highly selective and efficient ionization method for mass spectrometry.' Unlike the more comon ionization methods such as electron impact where a large amount of energy is initially deposited in the molecule followed by ionization and fragmentation, energy deposition by MPI occurs in a sequential fashion by the absorption of several 'low"-energy (ca. 3-5 eV) photons. Much theoretical2+ and experimental%" work has been undertaken in the past few years to better understand MPI ionization and fragmentation mechanisms. It is now well established that most organic compounds including those considered in this paper undergo a ladder-switching mechanism whereby the molecular ion is formed as .soon as the parent molecule has a h r b e d enough photons to allow ionization. The molecular ion may then absorb additional photons to provide the required energy for fragmentati~n.~-"Less clear is the mechanism of fragmentation of the molecular ion. Two general pathways are possible: (1) the parent Cooperative Institute for Research in Environmental Sciences.
ion absorbs several photons followed by dissociation to one of several higher energy fragments, and (2) the parent ion absorbs (1) For a recent review, see: Parker, D. H. In 'Ultrasensitive Laser Spectroscopy", Kliger, D. S., Ed.; Academic Prcss: New York, 1983; pp 233-309. (2) Silberstein, J.; Levine, R. D. J . Chem. Phys. 1981,75, 5735. (3) Rebentrost, F.;Ben-Shaul, A. J . Chem. Phys. 1981,74,3255. (4) Dietz, W.; Neusser, H. J.; Boesl, U.; Schlag, E. W.; Lin, S. H. J. Chem. Phys. 1982,66, 105. (5) Boesl, U.; Neusser, H. J.; Schlag, E. W. J. Chem.Phys. 1W, 72,4327. ( 6 ) Meek, J. T.; Jones, R. K.; Reilly, J. P. J. Chem. Phys. 1980,73, 3503. (7) Carney, T.; Bacr, T. J. Chem. Phys. 1981,75,477. (8) Miller, J. C.; Compton, R. N. J . Chem. Phys. 1981,75,2020. (9) Durant, J. L.; Rider, D. M.; Anderson, S.L.; Proch, F. D.; Zarc, R. N. J . Chem. Phys. 1984,80, 1817. (10) Newton, K. R.; Lichtin, D. A.; Bemstein, R. B. J. Phys. Chem. 1981, 85,15. (1 1) Pandolfi, R. S.; Gobeli, D. A.; El-Sayed, M. A. J. Phys. Chem. 1981, 85, 1779. (12) Lichtin, D.A,; Bemstein, R. B.; Newton, K. R. J. Chem. Phys. 1981, 75, 5728. (13) Bocsl, U.;Neusser, H. J.; Schlag, E. W. Chem. Phys. k r t . 1982,87, 1.
0022-365418512089-2717S01SO10 Q 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 13, 1985
one photon, dissociates, and the daughter ion absorbs additional photons to create higher-energy fragments. Recent studies involving theoretical m ~ d e l i n gand ~ , ~ laser power dependences of fragment ion intensities" have suggested that the second, ladder-switching mechanism plays an important if not dominant role. In this paper, we present a study of the competitive fragmentation processes of rearrangement vs. cleavage in diphenyl ether and other simple aromatic molecules undergoing MPI. If fragmentation occurs exclusively after the absorption of one photon by the parent ion, then the ratio of the cleavage to rearrangement ion intensities observed will be different than the ratio observed if two or more photons can be absorbed by the parent ion. This difference arises primarily from the parent ion internal energy dependence of the unimolecular decay rate constants.lS In this way, the cleavage to rearrangement ion abundance ratio can act as a sensitive probe of the number of photons absorbed by the parent ion prior to fragmentation.
Experimental Section MPI spectra were generated by using the fourth harmonic of a Quanta-Ray DCR-2A Nd:YAG laser a t 266 nm. The laser pulse (nominal pulse length of 5 ns) was focused to a ca. 0.25" beam diameter. Relative laser powers were measured by splitting off a portion of the beam into a photodiode. Due to the difficulty in measuring the beam diameter, absolute laser fluxes could only be approximated. For the data given in Figure 3, the laser intensity ranged from ca. 10 to 250 MW/cm2. Mass spectra were obtained with a modified CVC MA-002 time-of-flight mass spectrometer having a 1-m flight tube and an enlarged ionization region for laser ionization. During each laser pulse, the voltages of the ion drawout grid and the backing plate to the ionization region were kept at ground. After a 1 - 2 - ~delay, the drawout grid was pulsed negative to extract the laser produced ions into the flight tube. The delay time and potential of the drawout grid pulse were adjusted for maximum resolution. This procedure allowed unit mass resolution (10% valley criterion) to be achieved across the entire mass range studied. The ion current vs. time profile from the electron multiplier was sampled by a LeCroy 3500SA signal averager. The 8K histogramming memory of this device was sufficient to digitize and store the entire mass spectrum at 10-11s intervals for each laser pulse. The data collection process caused a slight drop in the effective resolution of the mass spectrometer, but unit mass resolution could still be achieved. MPI mass spectra were obtained by averaging 500 laser shots. Relative ion abundances were measured by comparing either peak heights or peak areas. Both methods gave essentially identical results. Collisionally activated dissociation studies were performed on a VG Analytical, Ltd. tandem mass spectrometer (Model 7070EQ-HF). The ions subjected to CAD were generated by 70-eV electron impact ionization of either diphenyl ether or 2-methylnaphthalene. CAD spectra were recorded with a 6-kV accelerating potential and a collision gas (air) pressure of 1 X torr. These conditions typically corresponded to a 30% collisional activation as measured by the loss of the parent ion signal. Diphenyl ether, anisole, benzyl acetate, and 2-methylnaphthalene (Aldrich, 99+% pure) were used without further purification. Results The first allowed electronic absorption transition of diphenyl ether consists of a broad band from 260 to 285 nm at elevated temperatures. Sequential absorption of two photons at 266 nm through this intermediate state yields a total internal energy (14) -1, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J. Phys. Chem. 1982, 86, 4857. (1 5) Kuhlewind, H.; Neusser, H. J.; Schlag, E. W. Inr. J . Mass Spectrom, Ion Phys. 1983, 51, 255. (16) Newton, K. R.; Bernstein, R. B. J. Phys. Chem. 1983, 87, 2246. (17) Kopfitz, B. D.; McVey, J. K. J. Chem. Phys. 1984, 80, 2271. (18) Brown, P. Org. Muss. Specrrom. 1970, 3, 1175.
00
.-;
(C6Hj)
5b-: E 40 h
:~ ~
a 0 -
Letters
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2719
Letters
ether to determine the specific fragmentation pathways of this molecule. The results are given below:
C12H90’
L
- c o c,
H + -CzHz II 9
C9H;/
1
65 etc.
I
P-
Figure 2. Multiphoton Ionization mass spectra of diphenyl ether at (a) 66 pJ/pulse (X15 scale) and (b) 200 pJ/pulse ( X I scale).
2 .o
1
-0
0.2
0:4 mJ/pulse
0:6
0.0
Figure 3. Ion abundance ratio C/R vs. laser power for diphenyl ether at 266 nm (C = C,+-Ist, R = CIIHlot).
molecular decay rate constant must be greater than ca. lo8 s-l if fragmentation is to precede absorption of the second photon. The laser flux dependence of the ion abundance ratio C/R is shown qualitatively by the mass spectra in Figure 2. A plot of the ion ratio for several laser fluxes is also given in Figure 3. The amount of C produced increases rapidly relative to R as the laser flux is increased. The initial rise is linear (laser power < 0.15 mJ/pulse), indicating that one additional photon is needed to produce C relative to the number of photons required to produce R. These results can be explained in the following way. At a low laser flux, P absorbs at most one photon. Those ions which have absorbed the photon are able to dissociate to R. At higher fluxes, P can absorb two photons prior to dissociation (i.e., no ladder switching). As the laser flux is increased, the number of parent ions absorbing two photons increases. Since the formation of C requires two photons while R requires only one, the abundance of C increases relative to R. The absence of ladder switching implies that the distribution of internal energies in P produced by the absorption of one photon yield a rate constant for the dissociation of P to R less than ca. lo8 s-l. At laser powers above 0.15 mJ/pulse, extensive fragmentation is observed which corresponds to the absorption of a t least three photons by P and its fragments. An alternative explanation for the trends observed could involve a mechanism in which d is produced by a one-photon dissociation of R. To test this possibility, we obtained collisionally activated dissociation (CAD) spectra of the prominent fragments of diphenyl
Only the molecular ion, CI2HloO+,was found to produce significant amounts of C6H5+(C). Neither CIIHlo+(R) nor its primary daughter C9H8+gave detectable CAD ion currents a t m / q 77. (It is interesting to note that the CI2H90+ion produced by 70-eV electron impact ionization dissociated (via CAD) only by the loss of CO. C12H90+was not observed in the CAD spectrum of CI2HloO+,nor was CllH9+observed in the CAD spectrum of CIIHlo+.)The CAD results are consistent with the multiphoton ionization mass spectrum of diphenyl ether shown in Figure 2b. The”most abundant fragment in the m / q range between R and C is at m / q 116 which corresponds to the primary daughter of R. Even at this fairly high laser flux, the m / q 116 fragment is approximately ten times less abundant than R which suggests than photodissociation of R is not very efficient. To further confirm that C is not efficiently produced by a one-photon dissociation of R, we studied the laser flux dependence of the multiphoton ionization mass spectrum of 2-methylnaphthalene. The molecular ion of this compound is isomeric with R and apparently has a similar structure since both of these ions were found to give identical CAD spectra. N o laser flux could be found where the molecular ion of 2-methylnaphthalene yielded significant amounts of C6H5+.
Discussion Our results indicate that ladder switching does not occur upon the absorption of one photon by the diphenyl ether parent ion. This observation places an upper limit on the unimolecular decay rate constant of ca. lo8 s-’ within the distribution of internal energies produced. For ladder switching to be suppressed, the unimolecular decay rate must be too small to allow appreciable amounts of the parent ion to dissociate during the laser pulse. This situation can be enhanced when as in the present case, the dissociation is intrinsically slow with a “tight” transition state and ion internal energies close to threshold, or alternatively when the laser pulse is very short.19 We have also studied the laser flux dependence of the rearrangement to cleavage ion ratios for anisole and benzyl acetate. The anisole parent ion can lose either H 2 C 0 by rearrangement to form C6H6+ or lose CH, by cleavage to form C6H50+. The benzyl acetate parent ion can lose either C H 2 C 0 to form C7H80+or CH3C02to form C7H7+. In each case, the cleavage and rearrangement fragments are always observed together and their ratio is independent of laser power. In contrast to diphenyl ether, the parent ion internal energy after the absorption of one photon is a t least 1.15 eV greater than the appearance potentials for both processes in anisole and at least 1.65 eV greater in benzyl acetate.I8 Apparently, the excess internal energies above the appearance potentials are large enough to cause prompt dissociation of the molecular ions upon the absorption of one photon. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. Acknowledgment is also made to the National Science Foundation for partial support of this research under Grant No. CHE-8308049. The authors gratefully acknowledge Robert Barkley and J. Ronald Sadecky for obtaining the CAD spectra. Registry No. Diphenyl ether, 101-84-8; anisole, 100-66-3; benzyl acetate, 140-11-4; 2-methylnaphthalene, 91-57-6. (19) Gobeli, D. A.; Simon, J. D.; El-Sayed, M. A. J . Phys. Chem. 1984, 88, 3949.
