J. Phys. Chem. 1993,97, 12530-12534
12530
Formation of Protonated Ammonia Clusters Probed by a Femtosecond Laser J. Purnell, S. Wei, S. A. Buzza, and A. W. Castleman, Jr.' Department of Chemistry, Pennsylvania State University. University Park, Pennsylvania 16802 Received: June 16, 1993; In Final Form: September 8, 1993'
Femtosecond pumpprobe techniques combined with a reflectron time-of-flight mass spectrometer are employed to investigate the formation mechanisms of protonated ammonia clusters. P u ~ pu_lses p are employed to excite the ammonia clusters to electronically excited states corresponding to selected A or C' states, while probe pulses with variable delay times are used to ionize the clusters. The results reveal that both the absorption-ionizationdissociation and absorption-dissociation-ionization mechanisms occur in the A state, while the absorptionionization-dissociation mechanism is the sole one operative in the state.
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I. Introduction Ammonia clusters have been studied with various ionization techniques including m~ltiphOton,l-~ single-photon,e and electron impact i0nization.~-9 The protonated clusters, (NH3),,H+, are formed under all experimental conditions, although varying amounts of unprotonated clusters, (NH3),,+, have been reported for experiments made in which single-photon, electron impact, and femtosecond multiphoton ionization techniques have been employed. Two conceivable alternative mechanisms have been proposed to account for the formation dynamics of protonated ammonia cluster ions and the above-mentioned findings: (1) absorption-dissociation-ionizationlo (ADI) and (2) absorptionionization-dissociationl,z,ll (AID). Several observations seem to support the AD1 mechanism. Specifically, work done by Zieglerlz has shown that electronically excited ammonia rapidly predissociates, and studies by Gellene and co-workers13have led to findings that (NHs),,H has a lifetime on the order of microseconds. On the basis of measurements of the lifetime of the intermediate to the formation of (NHp),.H+ ( n L 2) through the A state, Misaizu and co-worker~~~J5 suggested that the AD1 mechanism is operative. To the best of our knowledge, until recent studies conducted in our l a b o r a t ~ r y , ~the ~ J ~mechanism for the formation of protonated ammonia cluster ions through electronically excited states had not been investigated in detail. We have reportedI6J7 preliminary-findings from experimental investigationsof the (v=l) and A(v=2) states made using femtosecond pumpprobe techniques.18 In the present paper, we present data on the formation of potonated clusters studied over several vibrational levels in the A state, and u = 1 for the state, for a range of cluster sizes. In particular, various pumpproje wavelengths are used corresponding to the Rydberg states C'(u=l) and A(v=O,1,2) of ammonia molecules. [It should be noted that the assigned vibrational levels are those of isolated ammonia molecules; the correspondingvibrational levels of a specific ammonia cluster size could be different due to the solvation spectral shifts. Nevertheless, as the results show, we are probing different vibrational levels as we vary the photon energy.] Experimental findings are presented for clusters ranging in size from the dimer to the pentamer and for delay times between the pump and probe extending to as long as 15 ps. The results reveal that the AID mechanism is the only one operative in the e(u-1) state, while both the AID and AD1 mechanisms occur in the A states.
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11. Experimental Section
The apparatus used in these experiments is a reflectron timeof-flight (TOF) massspectrometer~9~~coupled with a femtosecond Abstract published in Advance ACS Absrracrs, November 1, 1993.
0022-3654/93/2091- 12530$04.00/0
laser system. An overview of the laser system is shown in Figure 1. Femtosecond laser pulses are generated by a colliding pulse mode-locked (CPM) ring dye laser. The cavity consists of a gain jet, a saturable absorber jet, and four recompression prisms. The gain dye, rhodamine 590 tetrafluoroborate dissolved in ethylene glycol, is pumped with 5 W, all lines, from an Innova 300 argon ion laser. In order to generate short pulses, passive mode locking is performed with DODCI, also dissolved in ethyleneglycol, acting as the saturable absorber. Four recompression prisms are used to compensate for group velocity dispersion (GVD). The output wavelength, pulse width, and energy are -624 nm, 100 fs, and -200 pJ, respectively. Due to the large photon flux needed for multiphoton ionization (MPI), the laser pulses are amplified through three stages, each pumped with the second harmonic (532 nm) from an injection seeded GCR-5-30 Hz Nd:YAG laser, which is synchronized with the femtosecond laser. The three sequential pump energies for the amplification stages are -33, -100, and -250 mJ. The first stage of amplification is a bowtie amplifier. The gain dye, sulforhodamine 640, is dissolved in a 50/50 mixture of methanol and water. The beam makes six passes through the dye cell, giving a total amplification of 10 pJ. The laser configuration to this point is the same for both the A state and C' state experiments, but the remaining arrangement varies dependingon thestate tobepumped. For thepstateexperiments, a wavelength of 624 nm is used while for the A state experiments, the third harmonics of wavelengths 642, 633, and 624 nm are used for accessing the v = 0, u = 1, and v = 2 vibrational levels of ammonia molecules, respectively. The wavelength corresponding to the v = 2 vibrational level is the third harmonic of the CPM fundamental wavelength; however, the wavelengths for the u = 0 and u = 1 vibrational levels are obtained by generating a white light continuum in a water cell. After continuum generation, the appropriate wavelength is selected with a 10nm-bandwidth interference filter. The second and third stages of amplification are performed with 6-mm bore prism dye cells, termed Bethune cells. For the amplification of 624-nm wavelength light, the gain dye is sulforhodamine 640, which is dissolved in a 50/50 mixture of methanol and water. For amplification of the 633- and 642-nm wavelengths, DCM, dissolved in methanol, is used. The output energy for the 624-nm-wavelength light is -200 pJ after the first dye cell and 1.5 mJ after the second, with 10% amplified spontaneous emission (ASE) and a pulse width of -350 fs. The output energy for the wavelengths 633 and 642 nm is -50 pJ after the first dye cell and 1 mJ after the second, with 10% ASE and -350-fs pulse width. The second stage of amplification for the state is the same as for the A state (v = 2); however, the third stage utilizes a 12." bore instead of another 6-mm bore Bethune cell. The gain dye for the 12-mm bore cell
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0 1993 American Chemical Society
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Formation of Protonated Ammonia Clusters
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12531
F e m t o s e c o n d P u l s e Generation and Amplification Colliding Pulse Mode-Locked Ring Dye Laser (CPM)
Amplification a n d Pump-Probe Delay
Bode Amplifier
Figure 1. Schematic of femtosecond laser system.
is also sulforhodamine_640. The output energy and pulse width are the same as for the A state; however, the effective pulse width involved in the multiphoton process is considerably shorter, depending on the number of photons absorbed in the excitation and ionization steps. After amplificatjon, the beam is split into pump and probe beams. For the A state experiments, the pump beams are frequency-tripled and the probe beams are frequency-doubled. This gives pump wavelengths of 214 (u = 0), 211 (u = l), and 208 nm (u = 2) and probe wavelengths of 321, 316.5, and 312 nm. Using a 45O high reflector coated for the pump wavelength, the beams are separated by reflecting the pump beam and transmitting the probe beam. The probe beam is sent through a delay stage which can be varied from 0.1 pm to 1 nm. Thereafter, the beams are recombined using another 45' high reflector. For the C' state experiments, the laser beam is split into identical pump and probe beams at a wavelength of 624 nm. A Michelson interferometric arrangement is used to set the time delay between the pump and probe beams. After recombination, the laser beams are focused into the interaction region with a 50-cm lens, where they intersect the molecular beam containing the neutral ammonia clusters which are produced via supersonic expansion through a pulsed valve. The ions formed in the multiphoton ionization process are accelerated in a standard Wiley-McLaren double-electric field arrangement to an energy of 2000V. The ions are directed through the first field-free region, which is -1.5 m long, toward a reflectron. Ionsare then reflected, whereupon they travel through a second field-free region which is -0.5 m long. They are thereafter detected by a chevron microchannel plate detector. The signals received by the detector are directed into a digital oscilloscope coupled to a personal computer. 111. Results
A. .&State of Ammonia Clusters. As discussed in detail in the following section, the mechanism by which protonated cluster ions originate upon the ionization of ammonia clusters has been a subject of some debate. A series of femtosecond pumpprobe experiments were performed to determine the mechanisms by which the protonated cluster ions are formed. Figure 2 displays a typical time-of-flight mass spectrum of ammonia clusters acquired at various delay times between the pump laser and the probe laser. The pump wavelength for this spectrum, 21 1 nm, is tuned to the u = 1 level of an ammonia molecule.21 The time given with each spectrum corresponds to the delay time between the pump and probe lasers. Figure 2a ( r = -2 ps) is a spectrum displaying the background from singlecolor ionization (the pump and probe pulses acting alone). A negative time denotes the fact that the probe pulse arrives before the pump pulse. Figure 2b ( r = 0 ps) shows a spectrum obtained with complete overlap of the pump and probe pulses. Notice, the
(b) t = 0 ps
(0) t
18
20
22
24
26
= -2
PI
28
Flight T ~ m e( m c r o 9 e c o n a s )
Figure 2. Time-of-flightmass spectrum of ammonia clusters at efferent delay times between pump (21 1 nm) and probe (3 17 nm)pulses, A(u= 1).
I
I1
-5
0
k
5 10 Delay Time (picoseconds)
I
15
Figure 3. Pumpprobespectrumof (NH3)2H+an_d(NH~)sH+withpump pulses at 214 nm and probe pulses at 321 nm, A(v=O).
largest enhancement is observed a t this time delay due to the resonant-enhanced two-color, two-photon ionization. Figure 2c (t = 15 ps) shows a decrease in the ion intensity for the protonated clusters, while an increase is observed for the unprotonated dimer. As seen in the figure, the dominant species are protonated clusters. However, a second series corresponding to unprotonated clusters (NHp)"+is also observed. Discussion of the origin and behavior of the intact ammonia cluster ions is the subject of another publication.20 Figures 3-5 show typical pumpprobe spectra of protonated clusters, (NH$2H+ and (NH&H+, through different vibrational levels of the A state, i.e., u = 0, 1, 2, respectively. All of these spectra have some features in common; namely, they display a large increase in intensity at t = 0 (maximum temporal overlap between the pump and probe pulses) and thereafter a subsequent
Purnell et al.
12532 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
(vH3)2H* (NH3)5H+
--,..2
r
0
x 4
-5
0
5
10
15
Delay Time (picoseconds)
Fipre4. Pumpprobe spectrum of (NH1)2H+an_d(NH&H+ with pump pulses at 211 nm and probe pulses at 317 nm, A(o=l).
3
(SH3)zH+ (NH3)5H+
I
-2
0
5
10
I 15
Delay Time ( p i c o s e c o n d s ) Fipre5. Pump-probespectrumof (NH&H+an_d(NH3)sH+withpump pulses at 208 nm and probe pulses at 312 nm, A(u=2).
rapid intensity drop. However, the various spectra do display some noticeable differences with regard to the shape of the falloff region following the initial substantial peak. Except possibly for u = 0, when the vibrational energy of the A states increases, the long-time intensity level of all cluster ions increases. More importantly, the difference between (NH3)2H+ and (NH&H+ becom_esevident at higher vibration levels, Le., u = 1 and u = 2. B. C’ State of Ammonk Clusters. The dynamics of ionization of clusters through the C’(u=l) was also investigated using a similar pumpprobe technique at 624 nm. Since ammonia clusters are easily ionized with 1 mJ of light at 624 nm, it is necessary to determine whether the observed signals are from a resonant or nonresonant process. For this determination we carried out studies of the ionization signal versus the laser power dependence. In contrast to the case in typical nanosecond experiments,3 fragmentation is not found to be affected by laser fluence, even when varied over 2 orders of magnitude.. As reported in a preliminary communication16 dealing with this work, a linear relationship between logarithm of laser power and that of ion intensity is maintained for all cluster ions studied and for laser powers ranging from the minimum power for observable ionization up to the maximum power obtained in the current setup. Each measurement revealed a slope of 4 f 0.2, corresponding to a four-photon process involved in the excitation of the clusters to the resonantly excited electronic state which serves as the intermediate in the ionization process. It is known that the ionization potential4 o,f ammonia is 10.17 eV, and the energy22 corresponding to the C’(u= 1) state of the ammonia monomer is 8.04 eV. Since each photon contains an energy of 2 eV, giving a total of 8 eV, a resonant process is involved. This corrzsponds to a 4 1 or 4 2 ionization of the clusters through the C’(u=l) state, depending on cluster size.
+
+
P u m p - P r o b e Delay Time ( f e m t o s e c o n d s ) Figure 6. Pumpprobe spectrum of ammonia clusters with both pump and probe pulses at 624 nm, e(o=l).
The pump beam excites the clusters to the @(u= 1) state while the probe beam, at various time delays, ionizes the electronically excited clusters. Figure 6 shows a typical pumpprobe spectrum of ammonia clusters. Data for n = 2 acquired in our earlier study are included for comparison, with more recent measurements extending to n = 4. The pump and probe beams are of identical wavelength, and in all cases the curve is seen to be symmetrical about zero. Also, although not shown, the base line of the leading edge (probe before pump) is at the same level as the trailing edge. In order to ascertain the origin of the ion signal existing at long delay times between the pump and probe laser beams, the ion intensities were carefully measured with and without the laser beams being individually blocked. The nonzero base line signal is established to be due to the sum of the ions arising from the pump and probe lasers acting independently. Importantly, as seen from Figure 6, the data reveal that the response curves for all ions, at least up to the tetramer, are identical. As discussed later, this suggests that the lifetime of the state leading to the formation of both the unprotonated and the protonated clusters is the same. It should be noted that the findings of short-lived intermediates for the @ state are also supported by the observation that the leading edge of the data shown in Figure 5 (Astate studies) does not change with the laser power at a wavelength of 312 nm. With regard to the experiments shown in Figure 5, at low laser power_, ionization of the ammonia clusters is achieved through the A state only. The leading edge should reflect the extent of the temporal overlap between the pump (208 nm) and the probe (3 12 nm) pulses. However, at-high laser power, ionization can also be achieved through the C’ state (two photons of 312-nm laser pulses acting as the pump, 208-nm pulses acting as the probe). In this situation, the leading edge of Figure 5 would reflect the convolution of the laser overlap and the dynamics of the state. The fact that the leading edge does not change with laser power at 312 nm indicates that the dynamics involved in the state is faster than the pulse width limit, in agreement with the 624-nm pumpprobe measurements. This finding establishes the fact that failure to observe a long-time tail in the C’ state is not due to the inability to ionize any possible NH4 residing in the cluster with 624-nm pulses.
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IV. Discussion
Two mechanisms have been proposed to account for the formation of protonated ammonia clusters under multiphoton resonant ionization conditions. They are absorption-ionizationdissociation1JJ I (AID) and absorption-dissociation-ionization lo (ADI). The absorption-ionization-dissociation mechanism is expressed as follows:
Formation of Protonated Ammonia Clusters
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12533
\ (NH3),*
+ hu2-(NH3),,+
+e-
The alternative absorption-dissociation-ionization mechanism is expressed as
The AD1 mechanism was initially proposed10 based on theoretical calculations and supported13 by findings that hydrogenated ammonia clusters can have lifetimes of a few microseconds following neutralization of protonated cluster cations. Recent nanosecond pumpprobe studies by Mizaizu et al.I4 also provided some evidence for the-AD1 mechanism for the case of large clusters ionized through the A state. The fact that protonated ammonia clusters are formed under electron impact and single-photon ionization conditions provides evidence that the AID mechanism must be operative at least in some situations. As discussed in what follows,the femtosecond pumpprobe studies reported herein provide a detailed and complete picture for the formation of protonated ammonia _cluster cations produced by ionization of clusters through the A and states. Consider the implications of the mechanism, first with regard to the state results presented in Figure 6 . The findings that lifetimes for the formation of both unprotonated ammonia clustep and protonated ammonia clusters are very short through the C' state can only be explained by the AID mechanism. In considering the AD1 mechanism, the neutral species (NH3),H, if present, would be formed by the predissociation of ammonia; it would be a long-lived species (microsecond lifetime).l3 Hence, for the AD1 mechanism, the lifetime of the state would be expected to be equivalent to the lifetime of the intermediate (NH3),H. However, we observe a lifetime of less than 100 fs. Direct ionization is certainly responsible for the sharp intensity peak at t = 0, and an initial drop would be expected to be observed irrespective of the mechanism. Predissociation of ammonia would cause the signal intensity to display at least an initially diminishing trend with time. The failure to observe any ionization attributable to that of NH4 incorporated in the cluster via the predissociation of NH3 to NH2 and H, and subsequent reaction of H with NH2, eliminates this as the major predissociation mechanism in the C' state and also as a contrikutor to the formation mechanism of protonated clusters in the C' state. This conclusion is reinforced by observations that there is no ionization attributable to NH4 in the high-fluence studies when the pump and probe delays are reversed Jleadicg edge of Figure 5 ) , establishing a difference between A and C' state mechanisms. In fact, studies by Simons et al.23 suggest the mechanism of predissociation of ammonia in the state occurs via the formation of H2 and N H , although their findings do not totally exclude the other possible channel. Our experimental results point to the probable formation of a species other than H being the dominant mechanism, and failure to observe ionization of NH4 shows the consistency in these two sets of findings from our laboratory and those of Simons. Next consider the results for the A state. The results obtained by pumping through the u = 0 level (Figure 3) at first glance also seem to eliminate the possibility of NH4 coming from the predissociation of ammonia as contributing to the mechanism of multiphoton ionization. Yet, careful examination of Figure 4 suggests a slight enhancement of the signal above the uncorrelated
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Figure 7. Proposed schematic of the potential energy diagram for the ionization of ammonia clusters. The dashed potential curve represents a different reaction coordinate for species formed by predissociation in the A state (see text). The probe beam (dash-dot) is time delayed by a selected value.
photon behavior, a t long times. This is clearly seen to be the case for u = 2, with an even larger contribution for the case of n = 5 compared to n = 2; see Figure 5. It should be noted that, due to the pulse width used in these experiments, the early time response of the pulse profiles may be instrumentally limited. However, the important point is that the presence/absence of the long tail is indicative of the ADI/AID mechanism. It is known12that ammonia in the A state predissociates via formation of the NHz and H. Verification of the predissociation of NH3 to NH2 and H as being a contributor to the mechanism of protonated cluster formation through the A state is seen from the data at v = 2 (Figure 5 ) . Here, the channel due to predissociation and subsequent ionization becomes readily observable. In the case of v = 0, predissociation in the cluster may be endothermic as revealed by the appearance potential measurements reported in the 1iterat~re.l~ Based on the general time response features observed for the A state, the following dynamical processes are considered (see Figure 7). 1. The neutral clusters are excited to the A state through absorption of the first photon (NH3),+ hu1- (NH3),* The excited clusters undergo intracluster reactions as follows: 2. Predissociation of the excited ammonia moiety (NH3),*
-
(NH3),,.H3N*(H-.NH2)
3. The intermediate species can lead to the loss of H or NH2 or reaction of the H to form NH4.
(NH3),,-H3N.(H**-NH,)
-
(NH3),,*NH4
+ NH,
4. Ionization of either (NH3),* or radicals (NH3),rNH4 leads to formation of protonated cluster ions,
a. (NH3),*
+ hu, -(NH3);
+ e-(NH,),,NH,+
+ NH2 + e-
It should be noted that the rapid intensity drop observed for all protonated cluster ions when n Z 2 is attributed to reaction 2, where theNH2- or H-containing species cannot be readily ionized. Reaction 3 leads to formation of long-lived radicals in accordance with the findings of nonzero ion intensity Yalues a t long p u m p probe delays observed in the data for the A state. The relative
Purnell et al.
12534 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
importanceof the ionization of the NH4 for different clpter sizes in the overall ionization of ammonia through the A state at different vibrational levels is seen by comparing the data for the trimer and hexamer, detected as the protonated dimer and pentamer cluster ions; see Figures 3-5. The overall dependence of thedecaying signal intensity on the vibrational levels is indicative of the influence of the energetics on the predissociation and reaction forming NH4, while the trend in the long-time tail reflects effects due to solvation and retainment of NH,. Work is in progress to quantitatively model the reaction dynamics involved in the ionization processes of ammonia clusters. Our results, which show a rapid decay and leveling off to a nonzero valueof intenscy, suggest that two processes are operating simultaneously in the A state. Since it is known12 that ammonia clusters rapidly predissociateinto NH2 + H, the rapid decay that we observe would suggest that a similar predissociation is taking place for the clusters. It is also known” that the radicals (NH3),,NH4have long lifetimes (greater than 1 ps), and evidence suggests that formation of these radicals is taking place through intracluster reactions between H and NH3. This is seen in the leveling off to a nonzero value which persist for longer than 1 ns. Unlike the state-which follows only the AID mechanism, it is evident that the A state competes between both the AID and AD1 mechanisms. The AID, which is the dominant process, is seen when the pump and probe pulses are overlapped ( t = 0), while the AD1 occurs when the probe photon is absorbed at long time delays. V. Conclusion We employed femtosecond pumpprobe techniquts to investigate the reaction dynamics involving the and A states of ammonia clusters. Varying the pump-probe time delay up to 12 ps for the state revealed lifetimes of 1100 fs for the species (NH3),+ and (NH3),H+ (n = 1-5). These obsfrvations support AID as the sole operative mechanism in the C’ state. Since it is known that (NH3),H has a lifetime on the order of a microsecond, the observed lifetime of 1100 fs is too short for the AD1 mechanism to be operative. For the A state pumpprobe experiments we observe two distinct features with respect to the pumpprobe delays: a fast decay process, followed by a leveling off to a nonzerovalue of ion intensity. These observations support a competing process between the AID and AD1 mechanisms. The first process, a maximum peak at t = 0 followed by a rapid decay, supports the AID mechanism. These observations are consistent with thoseobserved for the e s t a t e . The second process, leveling off to a nonzero value persisting for longer than 1 ns,
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supports the AD1 mechanism. This is consistent with the predissociation of ammonia to NH2 and H, with subsequent intracluster reactions forming (NH3).H, which has a lifetime on the order of 1 ps and its ionization contributing to the protonated cluster distribution detected at long times.
Acknowledgment. Financial support by the U.S.Department of Energy, Grant DE-FG02-88ER60648, is gratefully acknowledged. References and Notes (1) (a) Echt, 0.; Morgan, S.;Dao, P. D.; Stanley, R. J.; Castleman, A. W., Jr. Bunsen-Ges. Phys. Chem. 1984,88, 217. (b) Echt, 0.;Dao, P. D.; Morgan, S.;Castleman, A. W., Jr.J. Chem. Phys. 1985, 82, 4076. (2) Shinohara, H.; Nishi, N. Chem. Phys. Lett. 1987, 141, 292. (3) Wei, S.;Tzeng, W. B.;Castleman, A. W., Jr. J. Chem. Phys. 1990,
93..~ 2506. (4) Ceyer, S.T.; Tiedemann, P. W.; Mahan, B. H.; Lee, Y. T. J . Chem. Phys. 1979, 70, 14. (5) Kaiser, E.: de Vries, J.; Stener, - H.: Menzel. C.: Kamke. W.: Hertel, I. V.’Z. Phys. D 1991, 20. 193. (6) Shinohara, H.; Nishi, N.; Washida, N. J . Chem. Phys. 1985, 83, 1939. (7) Stephan, K.; Futrell, J. H.; Peterson, K. I.; Castleman, A. W., Jr.; Wagner, H. E.; Djuric, N.; MBrk, T. D. Int. J. Mass Spectrom. Ion Phys. 1982. 44. 167. (8) Buck, U.;Meyer, H.; Nelson, D.; Fraser, G.; Klemperer, W. J. Chem. Phys. 1988,88, 3028. (9) Peifer, W. R.; Coolbaugh, M. T.; Garvey, J. F. J . Chem. Phys. 1989, 91, 6684. (IO) Cao, H.; Evleth, E. M.; Kassab, E. J . Chem. Phys. 1984.81, 1512. (11) Tomcda, S.Chem. Phys. 1986, 110, 431. (12) Ziegler, L. D. J. Chem. Phys. 1985, 82, 664. (13) Gellene, G. I.; Porter, R. F. J . Phys. Chem. 1984, 88, 6680. (14) Misaizu, F.; Houston, P. L.;Nishi, N.; Shinohara, H.; Kondow, T.; Kinoshita, M. J . Chem. Phys. 1993, 98, 336. (15) Misaizu, F.; Houston, P. L.;Nishi, N.; Shinohara, H.; Kondow, T.; Kinoshita, M. J. Phys. Chem. 1989, 93, 7041. (16) Wei, S.;Purnell, J.; B u m , S.A.; Stanley, R. J.; Castleman, A. W., Jr. J. Chem. Phys. 1992,97,9480. (17) Wei, S.;Purnell, J.; Buzza, S.A.; Cajtleman, A. W., Jr. Ultrafast
Reaction Dynamics of Electronically Excited A State of Ammonia Clusters. J . Chem. Phys., in press. (18) Raker, M.J.; Dantus, M.; Zewail, A. H. J. Chem. Phys. 1988,89, 61 13. Khundkar, L. R.; Zewail, A. H. Annu. Rev. Phys. Chem. 1990,41,15. Dantus, M.; Janssen, M. H. M.; Zewail, A. H. Chem. Phys. Lett. 1991,181, 281. (19) Stanley, R. J.; Castleman, A. W., Jr. J . Chem. Phys. 1991,94,7874. (20) Buzza,S.A.; Wei, S.;Purnell, J.; Castleman, A. W., Jr. Origin and
Dissociationof Unprotonatcd Ammonia Clusters. J. Chem. Phys., sbumitted. (21) Herzberg, G. Molecular Spectra and Molecular Structures; Van Nostrand Reinhold: New York, 1960; Vol. 3, pp 463-466. (22) Glownia, J. H.; Riley, S.J.; Colson, S.D.; Nieman, G. C. J . Chem. Phvs. 1980. 72, 5998. i 2 3 ) Quinton, A. M.; Simons, J. P. J. Chem. SOC.,Faraday Trans. 2, 1982, 78, 1261.
