J. Phys. Chem. 1988, 92, 5333-5337
5333
Klnetlc Energy of Fragment Ions by Pulsed Laser-Pulsed Extraction Field Technique and the Mechanism of Laser Multiphoton Ionization Dissociation: 2,4-Hexadiyne M. A. El-Sayed* and Tsong-Lin Tai Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 (Received: January 19, 1988; In Final Form: March 28, 1988)
The kinetic energy of the different ionic fragments produced from the laser multiphoton absorption of 2,4-hexadiyne is determined from the decay of their ion signals with the delay time between the pulsed exciting picosecond laser and the pulsed extraction electric field in a laser linear reflectron mass Spectrometer. It is shown that the observed number of decay components for any fragment ion gives the minimum number of the dissociationchannels leading to its formation and/or the minimum number of isomers contained in the same mass peak being monitored. If statistical dissociation dominates, a distinction between these two possibilities could be made. The mechanisms of formation of several ionic fragments in the MPID of hexadiyne are discussed in light of their observed kinetic energies.
Introduction
The use of lasers as a source in mass spectrometry has enabled researchers to study the dynamics of ionic dissociation and the evolution of a mass spectrum in real time. A great deal of work has already been published on using visible and ultraviolet lasers to study multiphoton ionization dissociation (MPID) and mass spectrometric detection.’ Recently, we have developed a new technique using picosecond lasers to determine rates of processes involved in the ionic fragmentation of ions. On the short time domain, we have used two picosecond pulses of different colors. The first is used to produce the mass spectrum. The second laser, of much lower frequency, is used to change the population of the different vibronic levels excited by the first laser, leading to a change in the relative intensity of the mass peaks of the different fragments. Following this change as a function of the delay time between the two pulses changes could, in some cases, be used2 to extract the energy redistribution times in the vibronic levels of the parent ion populated by the first pulse. On the longer time scale, we have developed a t e ~ h n i q u e ~by- ~ which the daughter ion formation times and their kinetic energies are determined by use of a time-of-flight linear reflectron.6 Linear reflection is a modification of the V-shaped reflectron7-12in which the extraction field is pulsed, thus allowing the reflected ion to return on its own path to reach the detector. The ion reflectron mass spectrometer has a very high resolution for ions formed in the extraction region. Ions formed outside this region appear as new mass peaks at fractional m a ~ s e s . ~We J ~ have made use of the fact that linear reflectrod uses pulsed laser and pulsed electric field. The study of the mass spectrum as a function of the delay (1) For a review see: (a) Gedanken, A.; Robin, M. B.; Kuebler, N. A. J . Phys. Chem. 1982,86,4096. (b) Schlag, E. W.; Neusser, H. J. Acc. Chem. Res. 1983, 16, 355. (c) Gobeli, D. A.; Yang, J. J.; El-Sayed, M. A. Chem. Rev. 1985, 85, 529. (d) Syage, J. A.; Wessel, J. Appl. Spectrosc. Rev., in press. (2) Gobeli, D. A.; Morgan, J. R.; St. Pierre, R. J.; El-Sayed, M. A. J. Phys. Chem. 1984,88, 178. Gobeli, D. A.; El-Sayed, M. A. J . Phys. Chem. 1985, 89, 1722. (3) Tai, T.-L.; El-Sayed, M. A. Chem. Phys. Left. 1986, 130, 224. (4) Tai, T.-L.; El-Sayed, M. A. J. Phys. Chem. 1986, 90, 4477. (5) Tai, T.-L. Ph.D. Dissertation, Department of Chemistry, UCLA, 1987. (6) Shmikk, D. V. Zh. Tekh. Fiz. 1981,51, 1024 [Sou. Phys.-Tech. Phys. (Engl. Transl.) 1981, 26, 6151. (7) Lubman, D. M.; Bell, W. E.; Kronick, M. N. Anal. Chem. 1983, 55, 1437. (8) Mamyrin, B. A.; Shmikk, D. V. Zh. Eksp. Teor. Fiz. 1979, 76, 1500 [Sou. Phys.-JETP (Engl. Transl.) 1979, 49, 7621. (9) Boesl, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J. Phys. Chem. 1982,86,4857. (IO) Kuehlewind, H.; Neusser, H. J.; Schlag, E. W. J. Mass Spectrom. Ion Phys. 1983, 51, 25. (11) Kuehlewind, H.; Neusser, H. J.; Schlag, E. W. J. Phys. Chem. 1984, 88. 6104. (12) Kuehlewind, H.; Neusser, H. J.; Schlag, E. W. J. Phys. Chem. 1985, 89, 5600.
0022-3654/88/2092-5333$01.50/0
time between the pulsed energizing laser and the pulsed extraction electric field could yield information about the dependence of the intensity of the different ionic fragments on the delay time after exciting the parent molecule by laser multiphoton absorption. From this study, the rate of formation of the different daughter ions can be determined.3 At long times, the formed ions begin to escape the collection region of the spectrometer before the extraction field is pulsed on, thus causing a decrease in the different ion signals at a rate which is determined by the average speeds ofthe different io& and a geometry factor of the collection region. The latter is determined4 from the fact that the parent ion escapes with thermal velocity since the ionized electron, on account of its small mass, carries almost all the kinetic energy released in the ionization process. From this fact, the average speeds, and thus the kinetic energies, of the different fragment ions formed by statistical dissociation can be determined.4 In this paper, we discuss the latter technique and use it to determine the kinetic energy of the fragment ions having intense mass peaks in the 266-nm MPID mass spectrum of 2,4-hexadiyne. We show that the number of components appearing in the signal decay of a certain mass peak at long times could yield information about the number of channels producing a certain fragment ion and/or the number of isomers contained in the mass peak being monitored. Experimental and Sources of Errors in Kinetic Energy Determination
The configuration of the linear reflectron time-of-flight mass spectrometer we built has been discussed in detail p r e v i o ~ s l y . ~ , ~ It has a configuration similar to that designed by Shmil~k.~-’ The size of the spectrometer is 66 cm long, which is about 20 times longer than that used by Shmikk.6 The resolution of this mass spectrometer is limited by the maximum digitizing speed (10 ns) of the waveform digitizer (Biomation 8100). The acceleration region is pulsed with a high-voltage pulse generator (Princeton Applied Research Model 121 1) to extract ions from the ionization region. The maximum the electric field can be generated in this region is 1200 V/cm. This pulsed extraction voltage can be delayed from the ionizing laser pulse by different delay times with a minimum delay of 10 ns. This pulse delay between the laser and the extraction voltage will allow more and more ions to escape the extraction-collection region of the mass spectrometer. The rates of escape of the different ions, and thus the decay of the different ion signals, are determined by their average speeds. The r e ~ o l u t i o nof ~ -this ~ mass spectrometer can be as high as 1000 at mass around 136. An active-passive mode-locked Nd:YAG picosecond laser (Quantel International) is used to ionize the sample molecules with the 266-nm radiation. The power of each laser pulse is monitored by a radiometer (Laser Precision Co. Model RJP-735) which is interfaced to an LSI- 11/23 computer. Only mass spectra produced with a power between 20 and 30 fiJ/pulse are recorded. 0 1988 American Chemical Society
5334 The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 Each TOF spectrum recorded at a particular delay was a result of averaging 100 single laser shot spectra. The vapor of 2,4-hexadiyne was used without further purification. It was leaked into the ionization region by use of a precision leak valve and a needle that directs the vapor above the ionization region. The pressure in the ionization region is not known. However, the sample pressure in the mass spectrometer is kept at (5.0-7.0) X 10” Torr. The mass spectrum of 2,4hexadiyne with zero delay setting in the pulsed high-voltage generator was taken twice, once at the beginning and the other at the end of the experiment. These two spectra showed negligible intensity changes in the parent ion peak, indicating that the sample pressure in the ionization region is steady during the experiment. No ion signals with mass values higher than that of the isotope peaks of the parent ion were observed, indicating the absence of collisions in the ionization region. Furthermore, the relative intensity of the different isotope peaks of the parent ion is that predicted based on the isotope abundance. This suggests the absence of saturation effects. Some practical limitations of this technique might now be mentioned. First, since the average kinetic energy of the ions is estimated by comparing their decay rate with that of the parent ion, it is very difficult to precisely determine the average kinetic energy of different fragments to the 0.001-eV range or to have more than two significant figures in these experiments due to the signalto-noise ratio of the decay curve. The precision of the average kinetic energy can be improved by better maintaining low gas pressure and low laser power and perhaps cooling the parent ion to sharpen up its kinetic energy value. If this is done, the kinetic energy of the parent molecule needs to be determined. With our laser setup and the laser power range selected, it will take us 8 h for an experiment to determine a decay curve under these conditions. Therefore, we believe that, for a more precise experiment, a laser with more stable output power will be required. Furthermore, a new method of introducing the vapor with a constant flow and temperature needs to be devised. Second, the highest value of the average kinetic energy of the ion that can be obtained in this experiment will be determined by the ability to fit the decay curve of the ions. There are factors that will affect our determination of the ion intensity decay rates. We believe that, for a better estimation of the ion decay, we need at least 10 data points. This means that we should be able to measure the intensity of the ions at 100-ns extraction delay. There is an intrinsic delay in the high-voltage pulse generator of 150 ns. Thus, in order to measure the decay rate, the ion speed has to be such that the decay time of its signal has to be 3250 ns. This limitation can easily be overcome by using a pulse generator with a shorter delay unit (less than 10 ns) or by pretriggering the pulse generator so that the intrinsic delay of the pulse generator can be eliminated. The mass of the daughter ion will also affect the maximum average kinetic value that can be determined in this experiment because the calculation is based on the mass ratio of the daughter ion to the parent ion. In this experiment, the maximum kinetic energy that can be determined is around 5 eV for the lightest (C2H3+)ion. Third, upon calculation of the kinetic energy release, the average kinetic energy value is multiplied by the ratio of the mass of the parent ion to the mass of the neutral fragment. The size of this factor will thus affect the precision of the value of the kinetic energy release estimated in this technique. In the case of the C6HS+ion, this factor is 78. Therefore, the precision of the kinetic energy release value will be no better than 0.08 eV. For smaller ions such as C3H3+,a factor of 2 is used in the calculation, so that the value of the estimated kinetic energy release will be more precise. Other sources of errors for nonstatistical dissociation will be discussed below. Theoretical Discussion
The Ion Signal Decay and the Average Velocity. The ions are formed at the laser focus, which represents a small volume as compared to the total volume of the instrument. The small volume
El-Sayed and Tai of ions initially produced rapidly expands out of the collection volume into the rest of the large volume of the instrument. For statistical dissociation, the decay of the signal of the different ions reflects the average speed, Vi,by which these ions escape a critical volume, V, in the ionization region in which any ion present will be collected and counted. Assume this volume has a boundary of surface area A. Once the ion crosses this surface, it never returns, since Vis much smaller than the volume of the instrument. Thus, the rate of the decrease of the signal intensity of ion i, -dNi/dt, is given by its collision rate with this hypothetical surface, i.e. -dNj/dt = (A/V)UjNi
(1)
which gives Ni(t) = Ni(0) exp(-(A/V)Uit)
= Ni(0) exp(-k,t)
(2) where ki = (A/V)Ui, and U,is the average speed of ion i. If the mass peak i represents ions formed from different channels, j , or itself represents several isomersj, it is possible that ions of the same mass will be produced with different and wellIn this case, the decay resolved values for their average speeds, of the observed ion signal for mass peak i is given by
Ni(r) = CNij(0) exp(-kijt)
(3)
ki, = ( A / V u i j
(4)
I
where Thus, the number of components observed in the current signal decay of any mass peak i gives either the minimum number of channels responsible for the formation of single fragment ion or else the minimum number of isomers contained in the mass peak being monitored. Different statistical channels, leading to different average kinetic energies of a certain fragment ion, could involve different electronic states of the parent ion resulting from the absorption of a different number of laser photons, or else some result from the parent ion and others result from larger fragments. If different isomers are being observed, they could involve the same or different electronic states of the parent ions. The difference in the kinetic energy of isomers formed from the same electronic excited state of the parent ions results from the difference in their appearance potentials. The Ion Kinetic Energy. Since in the ionization process the electron carries most of the kinetic energy released, the decay of the parent ion signal should show only one resolved decay component with a decay constant that is determined by the average speed of the parent molecule prior to its ionization (thermal speed). From eq 2, the average speed can be calculated from the observed decay constant ki as given in the equation Vi = ( V / A ) k , (5) It is clear from this equation that the average speed Vi for any ion i can only be determined if the geometrical factor of the instrument, VIA, is determined. Since the parent ion is expected to have average thermal energies and velocities, the geometric factor of the instrument can be eliminated if the kinetic energies of the fragment ions are determined relative to the parent ion as follows: Ei/Ep
= (Mi(Ui)*)/(Mp(up)’) = (CiMiui2)/(CpMpu,2)
= (CiMik,Z)/ (CpMpk,Z) or where Ci and Cp are the constants relating the root-mean-square speeds to the average speeds. If the speed distribution of the fragment ions is the same as for the parent ion, Ci/C = 1, which we shall assume to be true for lack of knowledge of such a distribution. If this is not the case (e.g., in nonstatistical dissociation on a repulsive potential energy surface), errors in the calculated energies can be expected. In this case, if a linearly polarized laser is used, the dissociation axis could be parallel or perpendicular
Ionic Fragmentation of 2,4-Hexadiyne Cdi'
The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 5335
21't-HEXAOIYNE 266 NM
I
Ii
N
T
E N
2
S
I
T Y
10 20 TIME OF FLIGHT [ M I C R O SECl
Y
Figure 1. Observed reflection mass spectrum of 2,4-hexadiyne produced by a 266-nm picosecond laser. The ions are extracted with pulsed electric field at different delay times (0, 0.5, 1 .O, 1.5, and 2.0 ps) from the pulsed
laser. D E L I I TIME I U - 1 HlCPO SEC!
to the excitation absorbing axis. The geometrical factor (the decay constant) and the shape of the signal should depend on the direction of the laser polarization axis relative to the extraction field direction. In this case the observed decay characteristics should depend on the relative orientation of the laser to the electric field (laser polarization) with respect to the extraction field. Another possible source of error that can be introduced in determining the kinetic energy release by this technique at high laser intensities or high sample pressure can result from space charge. This would give the parent ion a kinetic energy value which is larger than 3kT/2. In the limit where the kinetic energy given to all the ions from the space charge exceeds the thermal and kinetic energy release in the dissociation process, all the singly charged ions will have the same value of kinetic energy, independent of the identity of the ion. Thus, relative to the parent ion, all the daughter ions will have similar energies. In addition, since the contribution of the space charge to ion kinetic energy depends on the ion density, the ion kinetic energy and thus the ion decay of the different ions should depend on the laser intensity. Thus, the decay of the ion signal should become faster at higher laser intensity or gas pressure. In the observed results section, it is shown that the parent ion signal increases with the laser intensity while its decay is independent of the laser intensity. This, and the fact that some daughter ions showed very different kinetic energy from the parent ion and from each other, strongly suggest that, at the laser energy used in our experiments, space charge does not contribute significantly to the observed kinetic energy of the ions. This is also concluded from the shape of the mass peaks observed in our regular time-of-flight mass spectrometers using the same laser and the same focusing arrangement. At higher laser energies (mJ/pulse), when space charge becomes important, a broadening in the mass peaks is observed and the mass resolution is greatly reduced.
Results The Mass Spectrum and Its Decay. A 266-nm picosecond laser with energy of 25 f 5 pJ/pulse is used to ionize 2,4-hexadiyne. The pulsed extraction electric field in the ion source is turned on upon receiving the signal sent from the picosecond laser. Five TOF mass spectra of 2,4-hexadiyne obtained at 0-, OS-, 1.O-, 1.5-, and 2.0-ms delay settings on the high-voltage pulse generator are shown in Figure 1. The strongest peak in each spectrum is that of the parent ion peak. The C6H,+ ion peaks including two isotope peaks of the parent ion are well-separated and are shown as the weak peaks around the parent ion peak. The group contained six weak ion peaks that have flight times shorter than that for C6H,+. These are for the C5H,+ (x = 0-5) ions. Among these, the C5H3+ peak is the strongest peak. The C4H,+ (x = 1-4) and C3H,+ (x = 0-3) peaks are strong in the spectrum with zero delay setting between laser and pulsed electric field. The strongest peak in the C4H,: group is that for the C4H2+ion(s) while the strongest mass peak in the C3H,+ group is for C3H3+ion(s). The C2H3' peak is the strongest peak amongst C2H,+ (x = 0-5) group. No ions in the C1 group are detected due to the low laser intensity used in these experiments.
Figure 2. Observed decay of the parent ion signal produced at different laser intensities (from top to bottom): 50, 25, 15, 10, and 5 pJ/pulse. The decrease in the ion signal at t = 0 as the laser energy decreases indicates that we are not in the saturation regime. The fact that the slopes are independent of the laser (ion) intensity strongly indicates that the contribution of space charge to the kinetic energy of the ions is
negligible. Figure 1 also demonstrates that all the ion intensities decrease with increasing delay time. Ions with lower mass decay faster , inthan those with higher mass. At a delay time of 1 ~ s the tensities of the C2H,+ ions diminished while those of the C3H,+ ions are moderately weak. The intensity of the C4H2+ion at 2 - k ~ delay is less than 10% of its intensity at the zero delay. The intensity of the parent ion at 2-ks delay is more than 30% of its intensity at zero delay. The decay of these ions could be fitted to one or more exponentials as predicted in the theoretical discussion above. Examining the Importance of Space Charge Effects. The escape of these ions from the collection volume results from the kinetic energy that the ions acquired from both the thermal energy of the parent ion and the kinetic energy released during the ion dissociation process. The kinetic energy determined for these ions, however, may also be a result of the potential energy resulting from the repulsion with other ions (space charge effect) if the ion density is high. In order to detect such a contribution, the decay of the parent ion signal is examined as a function of the laser intensity. The space charge effect should increase as the total number of ions in the ionization region increases by increasing the laser intensity. If the observed escape rate is a result of the space charge, the escaping rate, and thus the decay rate, of the parent ion signal (as well as that of all the other ions) should increase with increasing the laser intensity. Figure 2 shows a semilogarithm plot of the intensity of the parent ion as the delay time increases for different laser intensities. The slopes of these straight lines give the escaping rate constant at the different laser intensity. Since these lines are almost parallel to one another, one may conclude that either space charge effects are not dominant in determining the kinetic energy of these ions or else we are in the saturation regime where all the molecules in ionization volume are ionized at the lowest laser intensity used. If this were the case, the straight lines in Figure 2 should fall on the top of one another. Since the parent ion mass signal at zero time or at any delay time increases as the laser intensity increases suggests that we have not reached saturation at the laser intensities used. This is also supported by the fact that the observed relative intensity of the isotopic peaks agrees well with what one calculates from statistics and the relative abundance of the carbon isotopes. Thus, the observed independence of the decay of the parent ion signal of the laser intensity strongly suggests that space charge effects do not contribute significantly to the observed kinetic energy determined by this method. This is further supported by the observation that the mass resolution in our regular (nonreflectron) type time-of-flight mass spectrometer is high under similar laser intensity, gas pressure, and focusing arrangement. If the laser energies are increased to the mJ/pulse range, broadening of the
El-Sayed and Tai
5336 The Journal of Physical Chemistry, Vol, 92, No. 19, 1988
0
1 0
I U
DELAV TIHE ( 0 - 1 MICRO S I X !
Figure 3. Decay [log (signal) vs time] of the signals for the different ions of some of the mass peaks in the mass spectrum of 2,4-hexadiyne produced by a 266-nm picosecond laser.
mass spectrum and deterioration of the mass resolution is observed, suggesting the presence of space charge effects at these high energies. The Decay of Ion Signals and the Kinetic Energy of the Ions Produced. Figure 3 shows the semilogarithm ion signal decay plots for seven major daughter ions. The diamond points are the experimental data while the solid lines give the best fit of the data to a single-exponential decay. At least four of the eight ions have an intensity change that fits well to a single-exponential decay. They are the parent ion, C6HS+,C4H4+,and C2H3'. These ions have a single dominant channel for their formation, and their mass peaks each represent either one dominant isomer or several isomers with similar appearance potentials. The others, C4H3+,C4H2+, and C3H3+,might have at least one other component of much reduced amplitude and smaller decay constants, and thus smaller kinetic energy release. From Figure 2, the average escaping rate constant of the parent ion obtained with different laser intensities is found to be 0.64 X lo6 s-'. By use of this value and the average speed of the parent molecule at 300 K (285.36 m/s), the geometric factor (AIL') of the collection volume is estimated to be 2242.8 m-l. The average kinetic energy for different daughter ions can thus be calculated from their signal decay constant by using this geometric factor; as long as they are studied under the same laser focusing condition and the same gas pressure, the dissociation is statistical and the fragment ions produced all have the same Maxwell-Boltzmann distribution function for their energies. In Tab!e I, a summary is given for the average kinetic energy calculated from fitting the observed decays of the different strong mass peaks to one or two decay components. Given also in the table are the calculated average kinetic energies of the fragment ions calculated from the statistical equation of Franklin13 and assuming the excited states of the parent ion produced by three-, four-, or five-photon absorption as precursors for the different fragment ions if they are above their known appearance potent i a l ~ . ' ~ , The ' ~ agreements and disagreements for the different fragment ions are discussed below. Discussion In Table I, a summary of the results obtained in our decay experiments is given in the second column. In cases where the decay curves are fitted to two decays, the percent of each com(13) Haney, M. A.; Franklin, J. L. J . Chem. Phys. 1968, 48, 4093. (14) Dannacher, J. Chem. Phys. 1978, 29, 339. (15) Baer,T.; Willet, G. D.; Smith, D.;Phillips, J. S. J . Chem. Phys. 1979, 70,4076.
TABLE I: Comparison between the Observed Average Kinetic Energies of the Fragment IOM of Z,+Hexadiyne with Those Calculated by an Approximate Statistical Method') Assuming a Ladder Mechanism of MPID species
C6H5' CSH,+ C4H4+ C4H*+ C,H,+ C2H,+
av kinetic energy, eV this expt theory" 0.030 0.029 (3hv) 0.026 (45%) 0.026 (3hv) 0.10 (55%) 0.089 (4hv) 0.064 0.075 (3hv) 0.025 (6%) - (3hv) 0.38 (94%) 0.17 (4hv) 0.025 (5%) - (3hv) 0.36 (95%) 0.17 (4hv) 0.033 (15%) 0.047 (3hv) 0.33 (85%) 0.25 (4hv) 0.41 0.26 (5hv)
Estimates based on the Franklin approximate equation" and the known appearance potential.'4*15The precursors of the different fragments are assumed to be the electronic excited states of the parent ion resulting from three-, four-, or five-photon absorption by the parent molecule. (A number of recent observations support the ladder mechanism of the MPID of this molecule.I6) By use of the Franklin equation. the dissociation is assumed to be statistical.
ponent is also given in parentheses. In the third column, the results of a qualitative statistical calculation, using the Franklin equat i o n ~are , ~ given ~ for three, four, or five 266-nm photon absorptions. It is assumed that since we are using a picosecond laser, the parent ion is the precursor of all ionic fragments a t these relatively low laser intensities. There are recent observations on the MPID of 2,4-hexadiyne16 that support this proposal. Furthermore, dissociation into a fragment ion and only one neutral species is assumed. The appearance potentials for the different fragments are taken from the l i t e r a t ~ r e . ' ~ . From '~ Table I, it can be concluded that the formation of the large species, e.g., C6HS+,CSH3+,and C4&+, can be described by the simple statistical dissociation with very small barriers. For the other smaller species, the Franklin equation gives a smaller kinetic energy than that observed. C6HS+,C4H4+,and C2H3' have only one resolvable decay component. This suggests the involvement of a dominant single channel (or more channels with very near similar energetics) and one single dominant isomer (or more isomers with similar appearance potentials and dynamics). The approximate Franklin (16) Szaflarski, D. M.; Chronister, E. L.; El-Sayed, M. A. J. Phys. Chem. 1987, 91, 3259.
Ionic Fragmentation of 2,CHexadiyne statistical calculation is carried out for the formation of these ions. The results of the calculation using the three-photon level as a precursor agree well with those observed for C6HS+and C4H4+. This might suggest statistical formation of these two ions. This agreement together with the fact that their kinetic energies are not far from thermal suggests that the formation of these ions does not involve large kinetic barriers. The appearance potential of C2H3' is only a fraction of an electronvolt below the four-photon energy. The calculated kinetic energy, using the four-photon level, is only -0.01-0.02 eV, much smaller than that observed. Calculation using the five-photon absorption level (with -5-eV excess energy above threshold) gives a kinetic energy of 0.26 eV, which is also smaller than that observed (0.41 eV). The disagreement might be blamed on a partially nonstatistical dissociation, the presenceof a large kinetic barrier, or a nonladder mechanism for the formation of C2H3'. The decay of the C5H3+signal shown in Figure 3 is very nonexponential, suggesting more than one channel for its formation and/or the presence of more than one isomer. Possible channels for the formation of C5H3+are
The Journal of Physical Chemistry, Vol. 92, No. 19, 1988 5337
agree with either of the observed values. Possible channels for the formation of C4H3+ in the energy range between the threeand four-photon level are1'
+ CzH3 C4H3++ C2H2+ H
C6H.5'
-
+
C4H3'
(VI
Channel IV should dominate in the energy range around the four-photon level. The statistical calculation for channel IV gives a kinetic energy for C4H3+of 0.17 eV. The observed value is 0.38 eV, i.e., over twice the calculated value. This large deviation could result from either a dominance of another channel, e.g., ladder switching mechanism (dissociation to C4H3followed by the ionization of this radical), nonstatistical dissociation, or the presence of a barrier to dissociation. The first possibility can be eliminated by our previous observation that the C4H3+/C4H4+ratio obtained in the picosecond MPID of hexadiyne in the energy range corresponding to the three-photon absorption of the laser is the same as that obtained from the one-photon PEPICO results." Similar discussion could be given for the formation of C4H2+. The dissociation channels are
C4H2' A good fit of the observed decay is formed for two components with equal contributions. The kinetic energies observed are found to agree with the statistical calculation for channels I and 11. If this agreement is not accidental, it would suggest that molecules absorbing three photons and those absorbing four photons contribute equally to the production of C5H3+. This fact might result from two factors. First, the electronically excited parent ions produced from the absorption of four photons are expected to have much less number density than those resulting from the absorption of only three photons. However, the rate constant for the formation of CSH3+from the excited parent level resulting from four photons is expected to be larger than that from the three-photon excited parent level. These two factors could balance one another in this case to yield a comparable rate for the formation of C5H3+ from the three-photon and four-photon levels. The decay of C4H3+can be fitted to two components, with the ions formed via the minor mechanism (6%) having kinetic energy of 0.025 eV while those produced via the dominant mechanism (94%) have 0.38 eV. The appearance potential of C4H3+is above the three-photon level but below the four-photon level. The result of the statistical calculation, using the four-photon level, does not
(IV)
+ C2H2 + H2
(VW
Channel VI represents the dominant one. Again, the observed kinetic energy is larger than that calculated from the Franklin equation." This might be due to the presence of a kinetic barrier. For C3H3+,the dominant channel (or isomer) has higher value for the kinetic energy than that calculated from the Franklin equation. This could easily be due to the presence of a barrier for the dissociation. The minor component could be due to the formation of another isomer. It is clear from the above that more detailed calculations are required, on both the stable structures and the energy of formation of the different isomeric species as well as the potential surfaces, before a detailed understanding of the above results can be accomplished. In any case, this simple technique is giving us a great deal of valuable results and thus opening the door for more detailed understanding of the processes involved in ionic dissociation. Acknowledgment. The authors thank the National Science Foundation for its support of this work (Grant CHE84-12265). Registry No. 2,4-Hexadiyne, 2809-69-0. (17) Chronister, E. L.; Szaflarski, D. M.; El-Sayed, M. A.; Silberstein, J.; Salman, I.; Levine, R. D. J . Phys. Chem., in press.
