5050
J . Phys. Chem. 1986, 90, 5050-5058
the C, symmetry with the cyano group in the equatorial position ('E), and no axial conformation (,E) is observed.15 The molecule stays in a rather shallow potential well with the relatively low amplitude of ring puckering (25.3').''
mony for providing us with a preprint of N-cyanopyrrolidine (ref 15) and valuable comments and to Professor K. B. Mertes and Mr. P. Franklin for a critical reading of the manuscript. We also thank the University of Kansas for a generous allocation of computer time.
Acknowledgment. We are indebted to Professor M. D. Har-
Registry No. Cyclopentane, 287-92-3;N-cyanopyrrolidine, 1530-88-7.
Study of the Low-Energy Channels in the Multiphoton Ionization-Dissociation of 1,6Dichlorobenzene by Two-Color Picosecond Laser Mass Spectrometry Diane M. Szaflarski, John D. Simon: and M. A. El-Sayed* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 (Received: August 16, 1985; In Final Form: April I , 1986)
The laser multiphoton ionization-dissociation (MPID) dynamics of 1,4-dichlorobenzeneare studied by two-color (266 and 532 nm) picosecond laser mass spectrometry. The total ion current as well as the ion currents of the various fragments and parent ion are examined as a function of the delay between the 266 nm and 532 nm picosecond laser pulses. The dependence of the mass spectrum on the 266 nm laser (and in some cases the 532 nm laser) intensity is studied. A comparison of the low UV intensity multiphoton mass spectrum with electron impact mass spectra shows the dominance of the C6H4C1+,C6H3+, and C4H2+mass fragments, suggesting that these species are produced from the lowest energy channels. The ion currents of the MPID mass fragments C6H3+and C4H2CI+show a characteristic recovery time of 500 f 200 ps. The origin of these fragments is discussed. Mechanisms are proposed for the multiphoton formation of species produced from these lowest energy channels to account for the observed results. Both a ladder mechanism involving the three and four UV photon levels of the parent ion and ladder-switching mechanism at the two UV photon level followed by the absorption of one or two additional photons are proposed to form these low-energy fragment ions. In the ladder-switching mechanism, the C6H4C1' radical is proposed to be formed and ionized within the laser pulse width. Further UV photon absorption by C6H4C1+produces C6H3+ and C4H2CItvia an excited state of C6H4C1+.A qualitative discussion is given regarding the possible competition between dissociation and ionization (Le., ladder-switching) processes near the ionization threshold of polyatomic molecules.
Introduction Multiphoton ionization-dissociation (MPID) mass spectrometry has been widely used to study the fragmentation mechanisms of polyatomic molecules.'-7 Two general mechanisms which have been proposed to explain the mass spectra observed are commonly referred to as ionization-dissociation (ID) mechanism in which ionization precedes dissociation and dissociation-ionization (DI) mechanism in which dissociation precedes ionization.s These categories serve to distinguish molecules where the parent ion is the precursor to all smaller ions (ID) from others in which additional neutral fragments give rise to ions which contribute to the mass spectrum (DI). A detailed molecular understanding of the fragmentation processes of polyatomic molecules by MPID processes requires the determination of the cross sections, kinetic pathways, and rate constants for the energy redistribution and the formation of the various ionic species. A technique was first described by Gobeli et aL9 by which the rate of energy redistribution in the molecular or ionic excited states can be monitored by mass spectrometry. A number of reports then followed from this laboratory in which this technique was NSF Presidential Young Investigator 1985-1990. Present Address: Department of Chemistry B-014, University of California, San Diego, La Jolla. CA 92093.
0022-3654/86/2090-5050$01.50/0
applied to study a number of In this technique a high-intensity UV (266 nm) laser pulse excites the molecule of interest creating a population distribution among the various electronic states of the neutral and ion which can be accessed by the UV light. At a variable time with respect to the UV pulse, a 532 nm (green) laser pulse interacts with the initially prepared (1) Berezhetshaya, N. K.; Veronov, G. S . ; Delone, G. A,; Delone, N . B.; Piskova, G. K. Sou. Phys. JETP 1970, 31, 403. (2) Chin, S. L. Phys. Reu. A 1971, 4, 992. (3) Antonov, V. S . ; Letokhov, V. S . ; Shibanov, A. N. Appl. Phys. 1980, 22, 293. (4) Antonov, V. S.; Letokhov, V. S . Appl. Phys. 1980, 2489. ( 5 ) Boesl, U.; Neusser, H. J.; Schlag, E. W. J. Chem. Phys. 1980, 72, 4327. (6) Lubman, D. M.; Naaman, R.; Zare, R. N. J . Chem. Phys. 1980, 72, 3034. (7) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 71, 1359. (8) Gedanken, A.; Robin, M. B.; Kuebler, N. A. J . Phys. Chem. 1982,86, 4096. .. ..
(9) Gobeli, D. A.; Morgan, J. R.; St. Pierre, R. J.; El-Sayed, M. A. J . Phys. Chem. 1984,88, 178. (10) Gobeli, D. A.; Simon, J . D.; El-Sayed, M. A. J . Phys. Chem. 11984, 88, 3949. (1 1 ) Gobeli, D. A,; Simon, J. D.; Sensharma, D K.; El-Sayed, M . A. Int. J. Mass Spectrom. Ion Processes 1985, 63, 149. (12) Gobeli, D. A,; El-Sayed, M . A. J . Phys. Chem. 1985, 89, 1722.
0 1986 American Chemical Society
MPID of 1,4-Dichlorobenzene excited-state population distribution. Molecules which absorb green photons are excited to higher electronic states opening new fragmentation pathways at the expense of those originating from the electronic states populated by the UV laser pulse. If the states populated by the UV pulse have energy redistribution times or fragmentation times comparable to the delay between the two pump pulses, then the changes in ion currents of the various mass peaks formed from these states as a function of this delay will reflect these rates. Experimental applications of this technique have focused on achieving two major goals: the determination of the characteristic rates of energy redistribution in the lower excited states of molecular ions, and the mechanism by which fragment ions are formed by multiphoton laser absorption. In these previous studiesg-'* only two mass peaks could be examined simultaneously. This limits the ability to compare the behavior of several mass fragments or a mass fragment with the total ion current under identical experimental conditions. Recently, the technique was modified so that all of the mass peaks could be monitored at the same time. The details of this improvement have been r e p ~ r t e d . ' ~The ability to compare the behavior of the various ion currents under identical experimental conditions leads to a more detailed understanding of the dynamics associated with fragment formation. The previous studies were carried out on 2,4-hexadiyne, a molecule for which a great deal of spectroscopic, threshold energy, and breakdown curve information is known. The studies were done in order to examine the utility of the new technique. In this work a molecule for which little fragmentation information is known, typical of most molecules, is examined. In this manner we hope to realize the full potential and limitations of this technique in elucidating fragmentation mechanisms. In this paper, picosecond studies of energy redistribution and dissociation in 1,4-dichlorobenzene (p-DCB) are reported. Excitation by 193 nm (6.4 eV) ofp-DCB leads to the dissociation of the C-Cl bond.I4-l6 The wavelengths used in our study also aaess high-lying electronic states in the neutral molecule. If bond dissociation also occurs from these states, and is rapid compared to the time duration of the laser pulse, the resulting radical fragments can absorb photons and contribute to the production of the mass spectrum. In a recent letter, the dynamics of formation of C6H4C1+from 1,3-dichlorobenzene were reported." Two different mechanisms were proposed although the data available did not allow unique determination of the states and the dynamic processes responsible for the formation of this ion. With the additional data obtained by simultaneously monitoring several mass peaks of the isomer p-DCB, new conclusions about the possible mechanisms for the formation of C6H4Cl+are discussed in this paper. In addition, a detailed description of the possible mechanisms that explains the two-color delay experimental results for the formation of the most prominent mass fragment, C6H3+,is discussed. Experimental Section A block diagram of the experimental apparatus is shown in Figure 1. The fourth harmonic of an activepassive mode-locked Nd3+:YAG (Quantel International) is used as the primary pump beam. Filters or screens (copper mesh) are used to attenuate the pulse energy to be in the range of 1-20 pJ. The second harmonic, 532 nm (0.1-1.0 mJ/pulse), is used as the secondary pump beam. These two wavelengths are spatially separated by a Pellin-Broca prism. The UV beam travels a fixed path while the green pulses travel a variable length. A stepper motor delay line is adjusted so that the green beam can be scanned (in time) from a negative delay (green precedes UV) of 1 ns to a positive delay (green follows (13) Simon, J. D.; Szaflarski, D. M.; El-Sayed, M. A. Proc. Int. ConJ Lasers. 1984. 84. 176.
(14) Ichimura, T.; Mori, Y.; Shinohara, H.; Nishi, N. Chem. Phys. Lett. 1985, 122, 55.
(15) Shimoda, A.; Hikida, T.; Mori, Y. J. Phys. Chem. 1979, 83, 1309. (16) Shimoda, A.; Kohso, Y.; Hikida, T.; Ichimura, T.; Mori, Y. Chem. Phys. Lett. 1979, 64, 348. (17) Yang, J. J.; Simon, J. D.; El-Sayed, M. A. J . Phys. Chem. 1984,88, 6091.
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5051 13
Picosecond Nd
\
I Y f f i Laser
532 nm ( I rnJ T r i ager Pul se5 t o Laser Pwer 1
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I
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Sueplr
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u Variable Delay
Computer and
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I
Time of F l i h t
DRY-I1 Interfaces
Mass SPectrometer
I
C.E.M. xi00
Amp1 I f i e r
To Pumps
"
c
Figure 1. Block diagram of the two-color picosecond mass spectrometry
experiment. UV) of 8 ns. The two beams are recombined and focused (fl = 12.5 cm) into the ionization region of a time-of-flight mass spectrometer. The resulting mass spectrum is sensitive to the laser intensity and focusing conditions. The time-of-flight mass spectrometer consists of a two-stage acceleration region. The electric field in the first stage (where ionization occurs) is less than 1000 V/cm. At this electric field ions will remain in the focal volume of the laser beams for a minimum of 50 ns. This ensures that the ion current intensity changes observed as a function of delay between the two pump pulses results from molecular photophysics and not from a decrease of ion concentration in the focal volume. Due to the wavelength-dependent index of refraction of air and quartz, the two beams will focus at different points in the ionization region. The lens which focuses these beams into the mass spectrometer is positioned so that the green beam produces no ions and is spatially larger than the primary UV pump beam. The overlap of the two laser beams is optimized by maximizing the difference in mass peak intensities between a spectrum produced by the UV beam alone and that produced by the UV and green beams together. The sample effuses into the ionization region. The ionization region consists of a stainless steel box which houses the accelerating electrodes. The sample pressure inside the box is >1 X Torr. Mass peaks larger than the parent ion are not observed, confirming the absence of collisions and ion-molecule reactions at these pressures. The background pressure in the detection region is typically ( 5 X lo-' Torr. The ions are detected by a channeltron electron multiplier (Galileo Electrooptics 4800). The electron multiplier voltage is kept between -2500 and -2900 V. It is important to point out that the voltages in the mass spectrometer are optimized before any dynamics studies are performed. Once determined these voltages are not changed during an experiment. The output of the electron multiplier is amplified by a video amplifier (Pacific Instruments, Model 2A44) and fed directly into a waveform digitizer (Biomation 8100). The waveform digitizer is interfaced and controlled by an LSI-l1/23 computer system (D.E.C. Micro-1 1). At a digitizing rate of 100 MHz, a large part of the mass spectrum is recorded with each laser shot. At slower rates (Le., 50 MHz) the entire mass spectrum can be obtained. By recording the entire mass spectrum as a function of delay between the two pump beams, the dynamics of all the mass peaks can be simultaneously monitored. The computer system is used to externally control the laser and stepper motor delay line. All connections are optically isolated and the stepper motor is deactivated between data sets to prevent RF noise from walking the delay line. The delay between successive mass spectra can be set in multiples of 1.2 ps which is the distance corresponding to one step of the stepper motor. Usually 100-200 laser shots are averaged for each stop along the delay line. Two approaches are used for data collection. If a large number of data points are desired on several mass peaks, the computer system averages three digitized points around the maximum of a given mass peak and this value is then used to
5052 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Szaflarski et al.
I50
a!
100
J
50
E
I
A
0
28
90
b0
88
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I40
lb0
mss Figure 2. Low-energy UV mass spectra of p-dichlorobenzene. At low UV, A (1 pJ/pulse), only the parent ion is observed. At slightly higher UV energy, B (3 pJ/pulse), the low-energy fragments C4H2+, C4H3+, C6H3', C4H2C1+,and C,H4C1' appear. B is displaced from A along the mass coordinate.
represent the intensity of that peak. The software is designed to rind the maximum channel number before performing the sum, thus accounting for any jitter in the Biomation trigger which could occur on a shot-to-shot basis. The reason for this approach is the limitation in storage space available on our disk system. If total ion current studies or a small number of delay points are desired (50%), positive and negative spikes are seen for the C6H4C1+and parent ion signals, respectively, when the two pump pulses overlap. For positive delays the ion currents remain constant at a decreased level. At lower green pump energy (Figure 7B, parent ion decreases -30%), spikes are observed at zero delay. For positive delays, C6H4C1+shows no net change in signal, while the parent ion again shows a net decrease. Further lowering of the green intensity (Figure 7C, parent ion decreases 15%) results in a spike at zero time for C6H4Cl+but the ion current for positive delay is greater than that for negative delay. The parent ion still shows a net decrease, but no spike is observed. Similar behavior was found for 1,3-dichlorobenzeneI7 and 1,2-dichlorobenzene (these results are still preliminary).
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The Journal of Physical Chemistry, Vol. 90, No. 21, I986 682
1 522
6 7T
362
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20 1
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7T
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752
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100
DELAY TIME IPS1 Figure 5. Ion currents of (A) C6H4C1+,(B) C6H4CI2+, and (C) total are plotted as a function of delay between the two laser pulses. These data correspond to the mass spectra shown in Figure 3.
300
i
I 200
!
i Y
100
1
Ill1
Il1 I, I
13.24 eV, indicating that formation from the three-UV-photon state is thermodynamically feasible. B. From Comparison of Multiphoton and Electron Impact Mass Spectra. Multiphoton absorption mass spectra recorded at low UV power are shown in Figure 2. At low UV intensity, essentially only the parent ion appears. On increasing the UV intensity, the mass fragments C,H2+, C,H,+, C6H3+,C4HzC1+, and C6H4C1+appear (Figure 2B). This fragmentation pattern is similar to that obtained by electron impact ionizationzzmass spectrometry (EI) (Figure 9). Since both methods of ionization show the same low-energy fragments, it is reasonable to conclude that these fragments are produced from the lowest channels. In the case of UV multiphoton absorption, the lowest channels are at the three (14.01 eV)- and four (18.68 eV)-UV-photon levels. In E1 mass spectrometry, where ionization is expected to occur as a first step, all fragment ions are formed from excited electronic states of the parent ion, Le., via an ionic ladder mechanism. If the fragments formed by multiphoton absorption also arise from the ionic ladder mechanism, then similar ion intensities would be expected.23 However, the multiphoton spectrum shows that the C6H4Cl+fragment is less abundant than C6H3+,in contrast to the E1 mass spectrum. This difference in intensities might indicate that a ladder-switching mechanism is at least partially responsible for the formation of these ions upon multiphoton absorption of the UV pulse. C. From the Dependence of Mass Peaks on Delay Time. In Figure 4, the ion currents of various mass fragments are presented as a function of delay time between the two laser pulses. Some of these fragments show a time dependence at t > 0 while others remain constant. If the quasiequilibrium theory is assumed to be valid, the time dependence of these fragments reflects the energy redistribution rate occurring either in the parent ion or in one of its fragment ions (if the fragment is formed within the UV laser pulse width). It is reasonable to assume that lower levels (the three- and four-photon levels) have the longest lifetime. Thus, the ionic species produced from these levels would show a change in their ion current as the green pulse is delayed from the UV pulse. With the above assumptions, the C6H3+and C4H2C1+ fragments could be produced from a low-energy state populated by UV photon absorption. One should be cautious, however, with assigning the decay or rise times in the delay experiments solely to energy redistribution times. It is quite possible, as will be discussed below, that the observed times could result from isomerization or even dissociation times. Mechanisms for the Formation of C6H4CI+
0 100
120
1Li0
160
MASS
Figure 6. Mass spectra in the range of m / e 100-170 are shown, highlighting the region around t = 0. These data show that the C6H4CI+ signal begins to increase prior to a decrease in parent signal.
and cross sections for green absorption. Similarly, if fragmentation is occurring on the time scale of the delay between the two pump beams, the resulting fragments could be ionized by absorbing additional UV or green photons and contribute to the mass spectral signal. The remainder of the paper is organized as follows. First, the low-energy-channel mass fragments are identified. Then mechanisms for the formation of C6H4CI+,C6H3+,and C4HzCI+are examined. The possible role of dissociation in the neutral manifold leading to the formation of these ion fragments is discussed in detail. Identification of Low-Energy Ionic Dissociative Channels A . From Appearance Potential Data. Appearance potential data are only available for C6H4CI+. The measured value" is (21) Brown, P. Org. Mass Spectrom. 1970, 3, 639.
A . Ladder Mechanisms. 1 . Formation from the Three-UVPhoton Level. Absorption of three UV photons (14.01 eV) exceeds the appearance potential of C6H4CI+ (13.24 eV) by 0.77 eV (Figure 8). Thus upon absorbing three UV photons p-DCB possesses enough internal energy to form C6H4Cl+. The C6H4C1+ signal remains constant for positive delay (within 5%). This behavior is characteristic of a fragment whose precursor state does not undergo energy redistribution during the delay between the two laser pulses. Consequently the lifetime of the initially populated state must either be longer (-20 ns) or shorter (35 ps) than the resolution of this delay experiment. In comparison with lifetime measurements for excited states of substituted benzene ions,24a lifetime longer than 20 ns is not expected at this excitation energy. Thus the lifetime of this state must be shorter than the pulse width of the laser, 35 ps. If the three-UV-photon level is the major channel of C6H4Clf formation the green power dependence shown in Figure 7 can be explained as follows. At low green laser intensity the increase in C6H4CI+ion current is produced by absorption of two green photons by the ground-state parent ion. In this case no time delay dependence is expected. At high green power the depletion of (22) C R C Atlas of Spectral Data and Physical Constants for Organic Compounds; Graeselli, J. G., Ritchey, W. M., Eds.; CRC Press: Cleveland, OH, 1975. (23) Rosenstock, H. M. Ado. Mass Spectrom. 1968, 4, 523. (24) Maier, J. P.; Thommen, F. Chem. Phys. 1981, 57, 319.
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5055
MPID of 1,4-Dichlorobenzene
N I
-30
t
I
L 0
R
5b0 1000 1&0
0
OELAY TIM IPS1
s
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-90
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0 m n y TIM IPS)
B
500 1000 1588
DELAY TIM IPS1
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Figure 7. C6H4CI+(top) and C6H4CI2' (bottom) ion currents are plotted as a function of delay between the two laser pulses for various green powers. The UV power for these studies was 14 (*2) GJ/pulse. From left to right, the green intensities were 0.8,0.35,and 0.2 mJ/pulse, respectively. These data show the relative behavior of the parent and C6H4CI' fragments. As the level of the C6H&1+ ion current increases for t > 0, the magnitude of the spike and the percentage change in the parent ion signal (relative to negative delay) decrease. The intensity scale is labeled in absolute counts, relative to the negative time signal. For each green intensity, the two ion current plots were simultaneouslyrecorded.
15.92
7 -
13.44
0
5
Y
>
Q
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K
8.98
w z w
1
0
Figure 8. The known states of the neutral and cation of p-dichlorobenzene. The appearance potential of C6H4CIt is also shown.
C6H4Cl+can only be explained if energy redistribution from the three-UV-photon level is rapid (
nm
nm
cm-’
e
kJ mol-’
377 368 372 374 371 360 377 390
520 496 490 487 499 497 500 510
7294 7013 6474 6204 6914 7657 6525 6033
78.3 32.6 24.3 20.3 36.7 31.5 42.5
264 232 211 212 183 192 238
shift,
“See text.
bound fluorophore are not at present understood. The aim of this work was the elucidation of the fundamental photophysics of ABF with a view to understanding the reason for the fluorescence enhancement in the presence of (1+3)-@-D-glucans. Experimental Section ABF was synthesized as described earlier.* All solvents were checked for fluorescing impurities and redistilled as required. Absorption spectra were recorded on a Cary 17 spectrophotometer. Uncorrected emission spectra were recorded on a Perkin-Elmer MPF-44A spectrofluorimeter and then corrected by using a previously determined correction curve.1° Fluorescence quantum yield measurements were made on solutions of absorbance less than 0.05 with recrystallized quinine bisulfate in 0.5 M H2S04 as reference.” The sample and reference solutions had approximately equal absorbances at the excitation wavelength in order to minimize errors due to differences in optical geometry. No evidence of sample deterioration with the time was found. Steady-state fluorescence measurements below room temperature were made using an Oxford Instruments DN 704 cryostat. Above (10) Ghiggino, K. P.; Skilton, P. F.; Thistlethwaite, P. J. J . Phorochem. 1985, 31, 111.
(11) Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75, 991.
0 1986 American Chemical Society
