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J. Phys. Chem. 1989, 93, 6700-6704
Veloclty Distributlons of Iodlde Cations as a Monitor of the Mechanism of Laser Multiphoton Dissociation Ionization of Iodo Compounds Diane M. Szaflarski,+R. van den Berg, and M. A. El-Sayed* Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90024- 1569 (Received: March 7, 1989)
The time-of-flight mass peak line shape, and thus the velocity distribution of the iodide cations produced from the dissociation of CH31and iodobenzene with a nanasecond laser polarized along the flight axis, is examined as a function of the laser wavelength and the delay time between the laser pulse and the extraction electric field pulse. The results suggest that when the laser is in resonance with the two-photon absorption of the iodine atom in its ground (2P3/2)or excited state, the 1' detected is dominantly formed from the one-photon nonstatistical ladder-switching dissociation of the parent molecule followed by a 2 1 resonant ionization of the iodine atoms. For nonresonant wavelengths, an additional mechanism involving ladder statistical dissociation of CH31+and nonstatistical dissociation of the iodobenzene parent ion is proposed to account for the observed results. These results are discussed in terms of the competition between the one-photon dissociation and the second photon absorption in the parent molecules, as well as the difference in the absorption cross sections of the different transitions in the two systems.
+
Introduction The interaction of high-intensity lasers with gaseous molecules leads to the formation of fragment ions that can be detected by mass spectrometers. The mechanisms of formation of those ions have been under both theoretical and experimental investigation over the past decade.'-' The two most likely mechanisms now believed to account for the fragment ions produced after photon absorption are the ladder and ladder-switching mechanism. In the ladder mechanism, the absorption of a few (2-3) ultraviolet photons results in ionization. Higher energy electronically excited states of the parent ion can be populated by further absorption of photons. Each of these excited states could lead to the formation of different fragment ions, whose appearance potential lies below the energy of the states populated. For systems in which the absorption of one or more photons leads to dissociation in a time shorter than the laser pulse width, the ladder-switching mechanism usually dominates. In this mechanism, the fragments can absorb additional photons and become ionized, if they are neutrals, or they can dissociate into smaller ionic fragments, if they are ions. Since most of the pulsed lasers used presently in multiphoton absorption studies have nanosecond pulse widths, the ladder-switching mechanism is expected to dominate in systems for which the dissociation occurs on the picosecond time scale, e.g., on a repulsive or predissociative energy surface. The alkyl iodides from an interesting series of molecules to study from this perspective, because both ionizationldissociation mechanisms can be observed.w The first step in both mechanisms is absorption of one UV photon (-200-300 nm), accessing the A state, which is dissociative along the C-I bond. The neutral iodine radicals are formed in either the ground (zP3/2)or spin-orbit excited (*PI/,) state. The atoms can be ionized either by 2 + 1 resonance-enhanced ionization, which will dominate if a resonant wavelength is used, or by the less probable three-photon simultaneous absorption. Alternatively, the fragment ions can be formed from the parent ion via the ladder mechanism. In this case, the neutral molecule in the A-state absorbs additional photons (when photon absorption competes with dissociation) and can become ionized and dissociate if the parent ion is formed with an internal energy that exceeds the appearance potential of some of the fragment ions. The dissociation of the parent ion can occur either by direct access of a repulsive state, or by dissociation from a bound state that is crossed by a repulsive state, or by undergoing internal conversion to the vibrational levels of the ground state that are above the dissociation energy of the ionic ground state. The fragments formed by each of the above mechanisms (ladder-switching and ladder) are expected to have characteristic 'Present address: University of Colorado, JILA, Campus Box 440, Boulder, CO 80309-0440.
0022-365418912093-6700$0 1.50/0
time-of-flight distributions that depend on the type of dissociation that occurs and the amount of energy available above the appearance potentials of the fragments. In the ladder-switching mechanism, the iodide ion formed should have the same kinetic energy as the dissociated atom formed on the one-photon repulsive state of the alkyl halide molecule. The value of the kinetic energy should be large if the dissociation is occurring on a repulsive surface, and the velocity distribution should be highly anisotropic since the dissociation time is shorter than the energy redistribution time and molecular rotation time. If, on the other hand, the dissociation occurs via a ladder mechanism, and the parent ion thus dissociates statistically, the kinetic energy should be thermal and the fragment ion distribution should be isotropic, as the dissociation time is expected to be longer than the energy redistribution time and molecular rotation time. In either case, the time-of-flight distribution is dominated by the transition that is responsible for the dissociation. For the alkyl iodides, in the ladder-switching mechanism, the first photon will determine the distribution, whereas in the ladder mechanism the line shape may be determined by the polarization of the last photon absorbed provided dissociation occurs faster than the ionic rotation. In a previous paper, a comparison was made by this group5 of the dissociation mechanisms that occurred when a nano- and picosecond pulsed laser was used. This was done by studying the kinetic energy of the iodide ion. The pulsed-laser/pulsed-extraction field reflection techniquelo was utilized to monitor the decay of the I+ signal (produced using 266-nm photons) as the delay time between the pulsed-laser and pulsed-extraction field was increased. The decay of the signal was much faster when the iodide ions were produced by a nanosecond laser. This result suggested that the ladder-switching mechanism was dominant when a nanosecond laser was used, whereas a statistical ladder mechanism, leading (1) Gedanken, A,; Robin, M. B.; Kuebler, N. A. J. Phys. Chem. 1982,86, 4096. (2) Gobeli, D. A.; Yang, J. J.; El-Sayed, M. A. Chem. Rev. 1985,85,529. (3) (a) Rebentrost, F.; Kompa, K. L.; Ben-Shad, A. Chem. Phys. Lett. 1981, 77, 394. (b) Rebentrost, F.; Ben-Shad, A. J . Chem. Phys. 1981, 74, 3255. (4) (a) Silberstein, J.; Levine, R. D. Chem. Phys. Lett. 1980, 74, 6. (b) Dietz, W.; Neusser, H. J.; Boesl, U.; Schlag, E. W.; Lin, S.H. Chem. Phys. 1982, 66, 105. ( 5 ) Szaflarski, D. M.; El-Sayed, M. A. J. Phys. Chem. 1988, 92, 2234. (6) Parker, D. H.; Pandolfi, R.; Stannard, P. R.; El-Sayed, M. A. Chem. Phys. 1980, 45, 27. (7) Chupka, W. A.; Colmn, S.D.; Seaver, M. S.; Woodward, A. M. Chem. Phys. Letr. 1983, 95, 171. ( 8 ) Jiang, Y.;Giorgi-Arnazzi, M. R.; Bernstein, R. B. Chem. Phys. 1986, 1 .in6 -", 1 7 .. L
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(9) Kuhlewind, H.; Neusser, H. J.; Schlag, E. W. J . Phys. Chem. 1985, 89, 5593. (IO) Tai, T. L.; El-Sayed, M. A. J . Phys. Chem. 1986, 90, 4477.
0 1989 American Chemical Society
Velocity Distribution of Iodide Cations as a Monitor to the formation of thermal I+ resulting from the ionic dissociation of the CH31+ parent ion, dominated when a picosecond laser was used. In this paper, we attempt to extract information about the multiphoton dissociation mechanism from the observed velocity distribution of the photoproducts at different laser wavelengths for C H J and iodobenzene (C6H51). This is done by examining the difference in the spatial distribution of I+ produced by laser photodissociation. For this, we use a slightly modified version of one-dimensional photofragment s p e c t r o ~ c o p y . ~ ~ In - ' ~this experiment, the molecules are irradiated in a field-free region of a time-of-flight mass spectrometer, after which they dissociate and/or ionize. The ions are allowed to travel in this region for 0.5-4.0 ps, after which a pulsed electric field accelerates the ions to the detector. The I+ time-of-flight mass peak line shape is directly related to the spatial and energy distribution of the ions at the time the electric field was pulsed. The latter are determined by a number of parameters, e.g., the rate of dissociation, whether it is faster or slower than the rotational time; the laser polarization direction with respect to the flight axis; and the electronic-vibrational coupling in case of rapid, nonstatistical dissociation. Using the above technique, we have examined the laser multiphoton ionization/dissociation mechanism of CHJ and CsH51 as a function of similar laser wavelengths. The wavelengths have been chosen to be either in resonance with the two-photon absorption of the ground (2P3/2)or the first excited (2P1,2)state of the iodine atoms on- or off-resonance of these absorptions. The use of the on-resonance wavelengths is expected to enable us to observe the ladder switching dissociation on the one-photon repulsive surface of the iodide compounds. When off-resonance wavelengths are used in order to detect this channel, simultaneous three-photon absorption by the iodine atoms will be required for ionization. The low probability of this process might allow us to detect I+ produced directly from the parent ion via a ladder dissociation, resulting from sequential (not simultaneous) absorption of two or more photons. The line-shape analysis of the I+ signal as a function of delay time and laser polarization direction is consistent with these expectations. The difference in the offresonance results of CH31and C6H51is discussed in tems of the difference in the absorption cross section.
Experimental Section The details and methodology of the experiment have been described elsewhere." Briefly, a nanosecond Nd3+:YAGpumped dye laser (Quanta-Ray DCRlA/PDLl/WEX) is used for ionization. For these studies, the output of a rhodamine 640 dye laser was doubled to produce light in the wavelength region from 300 to 310 nm. The ions are formed in a field-free ionization region of the mass spectrometer by absorption of polarized laser light. The time-of-flight mass spectrometer is oriented such that the drift tube axis is parallel with horizontally polarized light from the laser. The ionization region consists of three plates: a, b, and c. Plates b and c are spaced 0.5 and 1.O in. from plate a, respectively. When the laser pulse ionizes the ions, all three plates are grounded. A 0.02-0.6 mJ/pulse of horizontally polarized light is focused with a 12-cm lens into the region between plates a and b. Since the absorption is predominantly parallel for the compounds discussed here, the I+ ions recoil along the drift tube (horizontal) axis. After a delay time of 1-4 ps, the ions are accelerated out of this region by applying a negative voltage pulse to plates b and c. The delay time is chosen so that a maximum separation is achieved without losing ion signal or creating distortion that will occur if the ions are too close to the plates. The -1.5-kV pulse (rise time, 40 ns) is left on until both 1' components (Le., the ones traveling toward and away from the detector) are accelerated into the region between plates b and c. The pulsed field however is turned off before
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( 1 1) Szaflarski, D. M.; El-Sayed, M. A. J . Phys. Chem., to be submitted for publication. (12) Riley, S. J.; Wilson, K. R. Faraday Discuss. Chem. SOC.1972, 53, 132. (1 3) Ogorzalek Loo,R.; Hall, G. E.; Haerri, H.-P.; Houston, P. L. J . Phys. Chem. 1988, 92, 5.
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6701 on-resonance I (ZPd
off-resonance
304.67 nm
305.0 nm
I+
I+
CH,I+
'I
I+
CH31+
Figure 1. Effect of laser wavelength (left vs right) and delay time (top vs bottom) on the observed laser mass spectra of CH& On the left are the spectra resulting from 304.67-nm excitation, which could ionize ground-state iodine atoms via a resonant 2 + 1 photon ionization, whereas on the right are the spectra from the absorption of 305.0-nm photons, which are not resonant with either I or I*. The top spectra are those obtained at zero delay time between the ionizing laser and the pulsed electric field, whereas in the lower spectra a delay of -0.5 ps is introduced. The laser at the resonance wavelength has an energy of 20 pJ/pulse and is polarized parallel to the flight axis. In the resonant spectra, at delay times greater than zero, a doublet is observed, indicating that I+ are formed with an anisotropic distribution indicating that dissociation is faster than electronic energy randomization and molecular rotation. In the corresponding nonresonant spectra, an additional central peak is observed, indicating that the'1 ions may be produced from an additional channel, for example, by a statistical dissociation of the parent ion, giving rise to I+ with an isotropic distribution.
the fastest ions leave this region. Plates b and c are then grounded, and the ions travel to the detector in a field-free environment. This design allows us to have a grounded flight tube. The ion signal is amplified (X100) and digitized with a transient recorder (Biomation Model 8 100). The transient recorder is interfaced via a parallel board (Data Translation Model 2817) to an IBM/AT computer.
Results In Figure 1, the time-of-flight spectra resulting from methyl iodide in the mass region 100-150 amu are shown for on- (left) and off-resonance (right) wavelengths of the atomic iodine twophoton absorption. On the left, the on-resonance wavelength spectra, ionizing I (2P32), are shown at 0.0- (top) and 0.5-ps (bottom) delay time. The delay time refers to the time between the ionizing laser pulse and the accelerating field pulse. In the lower spectrum, a doublet results from the high recoil velocities of the iodine photofragments. On the right, the off-resonance wavelength spectra are shown at 0.0 (top) and 0.5 ps (bottom). In these spectra, two peaks are observed, I+ and CH31+. At zero delay time, the peaks are sharp. As the delay time is increased,
6702 The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
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Szaflarski et al.
Iodobenzene
lodobenzene off-resonance spectrum
250
I+
200
150
E
CHx+
100
50
0
0
500 1000 CHANNEL NUMBER
Figure 3. Time-of-flight mass spectrum of iodobenzene when a nonresonant laser wavelength (303.7 nm with 0.6 mJ/pulse) is used for excitation. In this spectrum,the only fragments observed are I+, C2H,+, CH,', and H'. This observation can be explained by the high laser intensity and the strong absorption of the C6HSI,leading to "hot" I+ and C6H5radicals. Strong absorption of the latter could lead to further absorption of a number of photons, leading to the formation of the energy-expensive species observed.
0
500
lo00
CHANNELNUMBER Figure 2. Line shapes of I*, resulting from laser excitation at resonant wavelengths for I (304.67 nm; energy, 0.05-0.1 mJ/pulse; top) and for I* (305.56 nm; energy, 0.05-0.1 mJ/pulse; middle) and also a nonresonant wavelength (304.0 nm; energy, 0.5-0.7 mJ/pulse; bottom). All three spectra are recorded at a 4.0-ps delay time. Also plotted are the zero delay time spectra, which appear as sharp peaks. The peak for the I+ and the lower energy peak in the I* spectrum could result from the ladder-switching mechanism on the one-photon level. The off-resonance peak is located at the same position as the high-energy mass peak of the I*+ spectrum, suggesting the presence of an additional mechanism (e.g., ladder) that produces anisotropically distributed I+. The strong absorption of C6HSIand its ion, as well as the high intensity of the laser used in this experiment (0.6 mJ/pulse), might lead to the absorption of a large number of photons that might access a repulsive state in the parent ion.
the peaks broaden. This width reflects the kinetic energy distribution of the ions. Of special interest is the I+ peak in the onand off-resonance spectra. In the off-resonance spectrum, the I+ peak is broadened but a resolved doublet is not observed, indicating that either the kinetic energy is lower than that in the on-resonance case and/or that the lifetime of the state producing I+ is considerably longer than the rotation time of the dissociating parent. In Figure 2, a comparison is made of the time-of-flight spectra of I+ from iodobenzene at resonant wavelengths, 304.67 and 305.56 nm, which ionize the I and I*, respectively (top and middle), and at 304.0 nm, which is off-resonance (bottom). In this figure, the spectra obtained for zero and 4.0-ps delay time are shown for each wavelength. The zero delay spectra appear as a single sharp ion peak. The broadened line shapes result after the 4.0-ps delay. If only one channel were responsible for the production of I+, a doublet should appear for delay times larger than zero, resulting from the groups of ions that travel toward and away from the detector prior to the electric field pulse. These two groups will have a longer and shorter time-of-flight, respectively, than the ions accelerated with zero delay. The doublet can be seen in the top spectrum. The broad peak on the left results from the ions that initially travel away from the detector and therefore expe-
rience a stronger electric field pulse and thus, a higher acceleration and shorter time of flight. The long time-of-flight component is decreased in signal amplitude because of its low acceleration and the relative importance of the off-axis velocity components. The difference in the splitting for the upper and middle spectra corresponds to the difference of the two spin-orbit states of I. The splitting in the off-resonance spectrum will be addressed in the Discussion. A substantially longer decay time than shown for methyl iodide has been used for this figure, in order to differentiate between the splittings of the doublet observed in the on- and off-resonance spectra. The reflectron time-of-flight mass spectrum of iodobenzene is shown in Figure 3. In this spectrum, the dominant ions are labeled I+, C2Hx+,CH,', and H+. The fact that no other C,H,+ ions are observed might suggest that a relatively high laser flux was used. The I+ peak is the most dominant mass peak in the spectrum. Approximately 0.6 mJ of 303.7-nm light, focused with a 12-cm lens, is used to generate this spectrum. This wavelength is not resonant with the two-photon absorption of either the I or I*. An intensity dependence of the I+ signal found from C6H51was carried out at X = 304.56 nm (in resonance with I*) and at 304.38 nm (off-resonance). Interestingly enough, a log-log plot showed a three-photon process at both wavelengths.
Discussion a. Methyl Zodide. In Figure 1, the results obtained with laser excitation parallel to the flight axis with an on-resonance (left) and off-resonance (right) wavelength at zero (top) and at 0.5-gs delay time (bottom) are shown. The important observations and conclusions can be summarized as follows: (1) For the on-resonance excitation, the 1' mass peak is dominant with no parent ion visible. This strongly suggests that the ladder-switching mechanism is responsible for most of the I+ signal observed. In this mechanism, there is a high probability of forming I+, since the laser used for dissociation is in resonance with the two-photon absorption of the iodine atom. (2) As a delay is introduced, the distance between the I+ that are moving away from the detector and those moving toward it increases. Upon application of the extraction field, two mass peaks for those two distributions are observed, as can be seen in Figure 1 bottom left. (3) The introduction of delay for the off-resonance excitation causes a broadening in the mass peak (bottom left). The peak actually
Velocity Distribution of Iodide Cations as a Monitor looks like it is composed of the doublet seen for the resonance case and an additional broad peak at the center. The doublet results from the production of hot iodine atoms via a ladder-switching mechanism with nonresonant ionization or hot I' ions from a high-energy ladder absorption mechanism. The central peak suggests the presence of isotropically distributed I+ probably resulting from statistical dissociation processes, e.g., those resulting from the dissociation of the excited state of the parent ion, as was observed for picosecond excitations of CH31. This is also supported by results that showed that the CH$ peak appears in the offresonance excitation mass spectrum at zero delay time (Figure 1, top right). In summary, when a laser in resonance with the two-photon absorption of the iodine atom is used for multiphoton dissociation/ionization, the I+ observed are formed via a ladder-switching mechanism in which the one-photon dissociation of CH31 is followed by two-photon resonance, three-photon ionization of the iodine atoms. On the other hand, when the laser is tuned slightly off-resonance, the I+ ions observed are formed dominantly from the ladder mechanism, Le., from absorption of two or more extra photons by CH31 that leads to ionization and slow dissociation. The dissociation time has to be much longer than the rotation time and the energy redistribution time of CH31in order to give a broad isotropic spatial distribution for the I+. If the same state gives rise to the observed I+ at this wavelength as the one found by the absorption of the 266-nm picosecond photons,s the I+ found will have near-thermal energies, Le., statistical dissociation will be occurring. b. lodobenzene. A power dependence of the I+ signal at resonant and nonresonant wavelengths gave for both cases a n = 3 dependence. Thus, from this point of view, the different mechanisms responsible for the I+ signal cannot be distinguished. A time-of-flight line-shape analysis as described below can, however, be used to outline the different mechanisms. The time-of-flight mass spectra obtained for I+ from iodobenzene are shown in Figure 2. In this figure, the iodide ion signal is generated by absorption at three different wavelengths. The top and middle spectra are obtained when the laser is tuned into resonance with a two-photon electronic state I+ (304.67 nm) and I*+ (305.56 nm), respectively. The splitting of the doublets observed in these spectra indicates that the iodide ion is formed from the neutral nonstatistical channel in which the dissociation is faster than molecular rotation and energy redistribution and which produces I and I*, and thus I+ and I*+, with relatively high kinetic energy. The I*+ signal showed two peaks on the high-energy (short time-of-flight) side whereas I+ showed only one strong peak. The fact that the I+ produced from I* resonance has two peaks might suggest two mechanisms for the observed I*+ signal or two sources of the iodine atoms preselected (background halides). The relatively high kinetic energy for the neutral channel is expected because in this wavelength region (-300 nm) iodobenzene is prediss~ciative.'~In this region, the dissociation is thought to occur via a curve crossing of the initially populated T-T* state (which is stable with respect to C-I dissociation) to a T U * state, which is repulsive along the C-I coordinate.I4 The dissociation lifetime has been given an upper limit of 0.7 ps.I4 Thus, a ladder-switching mechanism at the one-photon level could account for the formation of the I+ mass peak at the I resonant wavelength and low-energy mass peak at the I* resonant wavelength. The bottom spectrum in Figure 2 is produced when the laser is tuned off-resonance of the neutral I and I* two-photon states. The peak at short time-of-flight is broadened compared to the I-peak, and its maximum is slightly shifted to the left (higher kinetic energy) with respect to the latter. However, it is at the same position as the higher energy peak of the I*+. This might rule out that this peak at the I* resonant wavelength is due to an impurity, as it is unlikely that an iodide impurity will show an I+ peak while iodobenzene will not. It is more likely that the two peaks result from two different mechanisms, producing I*+ (14) Dzvonik, M. J.; Yang, 4408.
S.C.; Bersohn, R.J . Chem. Phys. 1974, 61,
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6703 from C6HsI at 305.56 nm. Both yield hot iodide ions or iodine atoms that are ionized. One is dominant at the I* resonant excitation wavelength (mechanism I), while the other is operative at the nonresonant wavelength (mechanism 11). Both mechanisms, however, are operative at the I* resonant wavelength. There are three possible candidates for mechanism I1 mentioned above. In the first, a ladder-switching occurs at the one-photon level of the C6H51molecule, followed by three-photon simultaneous absorption by I*, resulting in ionization. If this were the case, the off-resonant I* signal observed should have its maximum in between the maxima for the I+ found with resonant photons for the I and low-energy mass peak of the I*. This is because if I is ionized by three-photon nonresonant absorption, there will be no selectivity between the spin-orbit states. Both components will be ionized and their relative signal intensities will depend on the populations in the spin-orbit states as well as the relative efficiencies for ionizing the atoms in each state. The ion signal observed should then be a sum of the signals obtained when ionizing either I or I* individually with on-resonance wavelengths and its maximum would always occur between the maximum of the I and I* signals. As can be seen in Figure 2, this is not the case. The nonresonant signal maximizes slightly outside the range of the I and the low-energy peak of the I* ion signal maxima. Additionally, the nonresonant signal is broader on the left side of the curve, and this broadening cannot be accounted for by simple addition of the resonant signals. For these reasons, the neutral dissociation and nonresonant ionization is ruled out as a possible explanation for the signal observed at off-resonance wavelengths. A second possible candidate for mechanism I1 is dissociation from the two-photon level of C6HJ followed by a 2 + 1 ionization if resonant wavelengths of I* are used or simultaneous threephoton absorption for nonresonant excitation. This mechanism can account for a rapid nonstatistical dissociation and the observed mass peaks, indicating higher velocities than those in mechanism I. In order to account for the fact that I+ is not observed from this mechanism, one has to assume that dissociation from the two-photon level gives only I*. This might be difficult to explain as the density of states is much higher at the two-photon level than at the one-photon level and crossing from one surface to the other is expected to take place with a higher probability. A final possible mechanism to be considered is a ladder mechanism involving the dissociation of the parent iodobenzene cation. The parent ion will dissociate statistically if the state populated by three UV photons is bound. Dissociation can occur from a bound state if it is either crossed by a repulsive energy level or if the bound state is energetically above the dissociation limit of the electronic ground state of the ion. Statistical dissociation is expected to be relatively slow, and the energy initially deposited as electronic energy will be redistributed amongst all the degrees of freedom in the molecule prior to dissociation. The energy will be randomized statistically among the translational and internal degrees of freedom of the departing fragments. A statistical energy distribution to translational energy results in relatively low recoil velocities. If this t y p of dissociation occurred, a doublet would not be observed in the time-of-flight spectrum of the I+ fragment, but rather a central mass peak, the width of which depends on the amount of translational energy of the I+ fragment produced. This type of behavior is observed for I' from methyl iodide at the nonresonant wavelength. The fact that the mass peak of the I+ produced at off-resonance wavelengths in iodobenzene seems to have a doublet (and is not just a broadened center peak) with apparent larger splitting than the I and I* suggests that the I+ produced have slightly larger kinetic energy. This suggests a ladder mechanism in which the parent ion dissociation is faster than molecular rotation and energy redistribution. This can occur when the excited state of the parent ion giving rise to I* is repulsive. For the reasons outlined above, it is believed that this third possibility is most likely to account for the nonresonant signal observed for I+ from iodobenzene. The reflectron time-of-flight spectrum of iodobenzene supports this conclusion. In the spectrum, the only fragments observed
J. Phys. Chem. 1989, 93, 6704-6710
6704
are I+, C2Hx+,CIHx+,and H+. This type of spectrum can be rationalized if it is assumed that with off-resonant wavelengths the intensity of the laser is sufficiently high that the parent ion formed absorbs a large number of photons to produce the energy-expensive small ionic fragments. In this case, iodobenzene is ionized by two-photon absorption. The third or fourth photon absorbed could access a dissociative or predissociative electronic state, producing I+ as described above. It is possible that a ladder-switching occurs a t this step, i.e., further absorption by the C6H5radicals formed could lead to the small energy-expensive fragments. The strong absorption of the iodobenzene and the phenyl radical could lead to a high probability of forming the observed small fragments by multiphoton absorption in this system.
Conclusions The results show that this technique can be used to identify the dissociation mechanisms resulting from laser multiphoton absorption of organic iodides. By selecting laser wavelengths that are resonant with the two-photon electronic states of the iodine
atoms, a ladder-switching mechanism on the neutral manifold dominates in which dissociation occurs faster than energy randomization and molecular rotation. The I+ produced are expected to have the same anisotropic spatial velocity distribution as the iodine atoms. At laser wavelengths that are off-resonance of the iodine atom two-photon absorption, channels forming I+ from the dissociation of the parent ions via a ladder mechanism are observed. For weakly absorbing molecules, e.g., CH31, the ladder mechanism seems to produce only lower excited states that form I+ statistically. This leads to broadened central mass peaks. For strongly absorbing molecules and their ions, e.g., C6H51,or for very high laser intensities, the ladder mechanism could lead to higher excited states of the ion that are dissociative, giving rise to an anisotropic velocity distribution of the I+. Acknowledgment. The National Science Foundation is acknowledged for the financial support of this work. R.v.d.B. acknowledges The Netherlands Organization for Scientific Research (N.W.O.) for a research fellowship.
Temperature Dependence of Fluorescence Decays of Isolated Rhodamine B Molecules Adsorbed on Semiconductor Single Crystals Klaus Kemnitz,* Nobuaki Nakashima,? Keitaro Yoshihara,* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
and Hiroyuki Matsunami Department of Electric Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan (Received: July 25, 1988; In Final Form: January 24, 1989)
Fast nonexponential fluorescence decays of rhodamine B monomers adsorbed on single crystals of a-Sic, &Sic,and GaP have been observed. The decays are fitted with three exponentials and are interpreted by adsorption sites of differing electron-transfer capability. A thermal equilibrium is postulated to exist among the ground-state populations of these sites. The electron-transfer rate is only weakly dependent on temperature but is considerably slowed down by addition of water and by aging of the semiconductor surface.
(I) Introduction From electrochemical measurements of the sensitization effect of dye molecules adsorbed on semiconductors, it is known that electron or hole injection occurs, depending on the relative position of conduction and valence band edge with respect to the energy levels of the dye. In the case of GaP and a-Sic, a cathodic current is observed that is interpreted as electron transfer from the valence band of the semiconductor to the excited, adsorbed dye.I In the case of ZnOl-3 and Sn02?v5 an anodic current indicates electron transfer from the dye to the conduction band of the semiconductor. Energy transfer from the excited dye to the semiconductor, which is an alternative relaxation mechanism, was shown to play a role in the case of eosin adsorbed on In this case, the spectral overlap is sufficient for energy transfer to compete with electron transfer. In the system of rhodamine B (RhB) adsorbed on Gap, energy transfer apparently occurs from the vibrationally nonrelaxed SI state as soon as the excitation energy surpasses 570 nm.6 Recently, detailed studies were performed on the energy transfer of cresyl violet adsorbed on Ti02,' rhodamine 640 adsorbed on Zn0,8 and pyrazine adsorbed on G a A s 9 In the case of single crystals of GaP'OJI and a-SiC,12electron transfer can occur also from surface states that are located about 1.6 eV above the valence band edge in Gap" and in the middle of the forbidden zone in a-SiC.'2 Electron transfer of RhB adsorbed on polycrystalline Present address: Institute of Laser Engineering, Osaka University, Suita, Osaka 565, Japan.
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SnOz5was also shown to involve surface states. Surface states introduced by the creation of dislocations were shown to enhance electron transfer into the conduction band of ZnO from excited RhBa3 The above results indicate that both mechanisms of electron and energy transfer between adsorbed dye and substrate are operative in semiconductor adsorption systems, depending on the individual semiconductor/dye combination, and it is one of the goals of this paper to try to discriminate between both mechanisms. Few direct determinations of the rate of electron and/or energy transfer exist so far, utilizing the fluorescence quenching of the adsorbed m ~ l e c u l e s . ~ ~In~ particular, ~ ' ~ - ~ ~ only a few temperature ( 1 ) Memming, R. Photochem. Photobiol. 1972, 16, 325. (2) Daltrozzo, E.; Tributsch, H. Photogr. Sci. Eng. 1975, 19, 308. (3) Li, B.; Morrison, S. R. J . Phys. Chem. 1985, 89, 5442. (4) Memming, R. Prog. Surf. Sci. 1984, 17, 7. ( 5 ) Kim, H.; Laitinen, H. A. J. Electrochem. Soc. 1975, 122, 53. (6) Memming, R.; Tributsch, H. J. Phys. Chem. 1971, 75, 562. (7) Crackel, R. L.; Struve, W. S. Chem. Phys. Lett. 1985. 120, 473. (8) Anfinrud, P. A,; Caugrove, T. P.; Struve, W. S. J. Phys. Chem. 1986, 90,5887. (9) Whitmore, P. M.; Alivisatos, A. P.; Hams, C. B. Phys. Reu. Left. 1983, 50, 1092. (10) Memming, R.; Schwandt, G. Electrochim. Acta 1968, 13, 1299. (1 1) Beckmann, K. H.; Memming, R. J. Electrochem. Soc. 1969,116,368. (12) Gleria, M.; Memming, R. J. Electroanal. Chem. Interfacial Electrorhem. 1975, 65, 163. (13) Liang, Y.;Ponte Goncalves, A. M.; Negus, D. K. J . Phys. Chem. 1983, 87, 1
0 1989 American Chemical Society
