J . Phys. Chem. 1990, 94. 6718-6723
6718
within the bubbles is no longer present to maintain contact between the NVR and the HMX. Summary and Conclusions The temporal behaviors of the rates of gas formation and gas release of the pyrolysis products from HMX show that the mechanisms controlling the identity and release of gas products from the particles are due to complex physicochemical mechanisms. The formation of bubbles within the solid HMX particles is apparent from both the temporal behaviors of the rate of gas release of the different pyrolysis products and the macroscopic and microscopic morphology of the polymeric nonvolatile residue formed during the decomposition. The TEM of the NVR shows that the NVR consists of broken ellipsoidal shells that are probably the remnants of bubbles that had formed within the H M X particles. Simple arguments indicate that the pressure of the gases within the bubbles ranges up to 9 MPa and may range up to 240 MPa depending on the strength of the solid H M X at the decomposition temperature. Comparison of the rate of vaporization of HMX and the rate of release of HMX decomposition products suggests that the decomposition products may be retained in the solid for long periods of time after they are formed and that their rate of release is substantially controlled by the coalescence of the products and the creation of exit channels from within the particles. This implies that previous studies used to determine reaction rate constants by either pressure-time or TGA methods were measuring rates that were substantially controlled by the gas release process and not the decomposition reaction kinetics. The identity of the pyrolysis products and their temporal behaviors indicate that at least two different general reaction schemes are producing products. The first one, which is operational by itself during the induction stage of the decomposition and concurrently during the other stages, produces N 2 0 and C H 2 0 at approximately equal rates and appears to depend on the surface area of the HMX. The second scheme, whose products first appear at the start of the acceleratory stage, produces a wider range of products with varying ratios of gas release rates during the de-
composition. The second reaction scheme is associated with the formation of the bubbles and produces N 2 0 , H 2 0 , C H 2 0 , NO, and C O in larger quantities and HCN, ( C H 3 ) N H C H 0 , (CH,),NNO, ONTNTA, and the NVR in lesser quantities. Thermal decomposition of the NVR indicates that it is a form of a polyamide. The formation of the relatively high concentration of water and the polyamide film early in the acceleratory stage of the decomposition is consistent with the reactions of the second scheme occurring in the high-pressure bubbles. The second reaction scheme emphasizes the complex nature of the physical processes occurring during the decomposition. Most of the products observed during the decomposition are formed in this reaction scheme. Under the high-pressure conditions in which the products are formed, a reaction environment is created that can affect, and perhaps control, the H M X decomposition. This environment consists of a polyamide film at the interface of HMX and high-pressure gaseous products. The relatively large concentration of water indicates that the reactions occur in a hightemperature aqueous environment. Further insight into the decomposition mechanisms requires more studies on which bonds in the products have been formed during the decomposition and which remain from the original molecules. Isotope scrambling experiments may answer some of these questions. In addition, the development of a model to predict the formation and subsequent release of gas products from the particles will allow more conclusive statements about the mechanism and the determination of the temperature dependence of the controlling reactions. Acknowledgment. The author thanks Y. K. Lutz, J. P. Damico, and M. G. Mitchell for assistance in collecting the data, G. Gentry and Dr. G. Thomas for the transmission electron micrographs, and Dr. A. Kumar for technical discussions concerning bubble formation. Research was supported by a joint Memorandum of Understanding between the U S . DOE and the U S . Army. Registry No. HMX, 2691-41-0.
Photoionizatlon Mass Spectroscopic Studies of Ethylene and Acetylene Clusters: Intracluster Excess Energy Dissipation Hisanori Shinohara,* Hiroyasu Sato, Chemistry Department of Resources, Mi'e University, Tsu 51 4, Japan
and Nobuaki Washida* The National Institute f o r Environmental Studies, Tsukuba-Gakuen, Ibaraki 305, Japan (Received: December I , 1989; In Final Form: March 30, 1990)
The intact cluster ions of ethylene, (C2H4),+ ( n = 2-6), and acetylene, (C2H2),+ ( n = 2-4), have been found in addition to normally observed fragmented ions by using the near-threshold photoionization method. These intact ions have either not been observed or detected only very weakly in the (full-collisiontype) conventional ion-molecule reactions of ethylene and acetylene. The intact ions are produced by photoionization (1 1.83 and 11.62 eV) of the mixed neutral clusters with argon, (Ar),(C2H4)mand (Ar),,(C2H2),,,,in the ionization threshold. The excess energies on ionization are randomized within the ionized clusters [(Ar),(C2H4),+],,* and [(Ar),(C,H,),+],,* ("intracluster excess energy dissipation") and finally converted to the decomposition of argon atoms, giving rise to the observed intact cluster ions, where vip represents vertically ionized points. The evidence of some especially stable cluster ions of ethylene and acetylene is also presented.
Introduction A recent important progress in the ,.luster ion chemistry is the finding of the so-called intracluster ion-molecule reactions.I-I2 The intracluster ion-molecule reactions that have been reported 'Authors to whom correspondence should be addressed.
0022-3654/90/2094-67 18$02.50/0
so far are unique as compared with the conventional (full-collision type) ion-molecule reactions. Several distinct features of the ( 1 ) Shinohara, H. J . Chem. Phys. 1983, 79, 1732. (2) Shinohara, H.; Nishi, N.; Washida, N. Chem. Phys. Lelr. 1984, 106, 302. (3) Shinohara, H.; Nishi. N.: Washida. N. J . Chem. Phys. 1985, 83,1939.
0 1990 American Chemical Society
Ethylene and Acetylene Clusters
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6719
intracluster ion-molecule reactions can readily be enumerated: ( 1 ) small impact parameters, (2) zero collision energy, ( 3 ) fixed collisional orientation, and (4) very low vibrational-rotational temperatures of the clusters. Because of these distinct features, the intracluster ion-molecule reactions can produce a different set of complex ions that the corresponding conventional ionmolecule reactions fail to p r o d ~ c e . ~ - ~ Of particular interest is the detection of 'intact" parent complex ions. One of the best examples of this is the observation of unprotonated water cluster ion^.^,'^ Due to the rapid exothermic reaction H 2 0 + H 2 0 H 3 0 + O H (AH = -20 kcal/mol), it has been difficult to observe the ( H 2 0 ) 2 +complex and, in particular, it has not been possible to detect the higher homologues ( H20),,+. However, the unprotonated (intact) cluster ions (H20),+ can be observed, if one photoionizes water-argon binary clusters ~ unprecedented (H20),(Ar)m in the ionization t h r e ~ h o l d .The detection of the intact cluster ions has been ascribed to, what we term, intracluster excess energy dissipation5 where the excess energies are randomized within the binary clusters and finally converted to the decomposition of argon atoms, giving rise to the stable intact (H20),+ ions. The intact (H20),,+ ions have also been observed by using ( H20),(C02), and (H20),(N20), mixed clusters in which C 0 2 and N 2 0act as energy dissipators like argon atoms.I4 A similar result has been reported for ammonia cluster~.~.~ In this report, we will present the first observation of intact ethylene (C2H4),,+ and acetylene (C2H2),+ cluster ions. The ethylene and acetylene ion-molecule reactions have been the subjects of many investigations in the past three decadesi5 These studies show that C3H5+and C4H7+and C4H3+and C4H2' are the major product ions when respectively the C2H4' and C2H2+ reactant ions are formed by electron-impact ionization or photoionizaton:
+
-
+
+ C2H4
+ CH3 C4H7++ H C2H2++ C 2 H 2 C4H3++ H
C2H4"
+
-
C3H5'
-
+ H2 C2H3+ + C2H C4H2'
(1)
(2) (3) (4) (5)
The results of previous investigations support the conclusion that these reactions proceed through long-lived complexes such as [C4Hs+]*and [C4H4+)*for the ethylenei6 and acetylenei7 case, respectively. Ng and co-workers have reported the appearance energies of the [C4H8+]*and [C4H4+]*ions and discussed the formation mechanism of the complex i o n ~ . ~ ~ J ' In different studies, it has been reported that the intact (C2H4),+ and (C2H2),+ cluster ions are either not detected or only weakly observed by electron-impact ionization of the corresponding neutral c l ~ s t e r s , primarily ~ ~ J ~ because of the fact that ion-molecule re(4) Washida, N.; Shinohara, H.; Nagashima, U.;Nishi, N. Chem. Phys. Lett. 1985, 121, 223. (5) Shinohara, H.; Nishi, N.; Washida, N. J . Chem. Phys. 1986,84,5561. ( 6 ) Shinohara, H.; Nishi, N.; Washida, N. Chem. Phys. Lett. 1988, 153,
....
AI 7
(7) Shinohara, H.; Nishi, N. Chem. Phys. 1989, 129, 149. (8) Garvey, J. F.; Bernstein, R. B. J . Phys. Chem. 1986, 90, 3577. (9) Garvey, J. F.; Bernstein. R. B. Chem. Phys. Lett. 1986, 126, 394. (IO) Garvey, J. F.; Bernstein, R. B. J. Am. Chem. SOC.1987, 109, 1921. ( 1 1 ) Garvey, J. F.; Bernstein, R. B. Chem. Phys. Lett. 1988, 143, 13. (12) Schriver, K. E.; Camarena, A. M.; Hahn, M. Y.; Paguia, A. J.; Whetten, R. L. J. Phys. Chem. 1987, 91, 1786. (13) Haberland, H.; Langosch, H. Z . Phys. D 1986, 2, 243. (14) Shinohara, H.; Nishi, N.; Washida, N. Unpublished results. ( I 5 ) Futrell, J. H.; Tiernan, T. 0. In Ion-Molecule Reactions; Franklin, J. L., Ed.; Plenum: New York, 1972; p 485, and references therein. (16) Ono, Y.; Linn, S. H.; Tzeng, W. B.; Ng, C. Y. J . Chem. Phys. 1984, 80, 1482. (17) Ono. Y.; Ng, C. Y. J. Chem. Phys. 1982, 77, 2947. (18) Fischer, G.; Miller, R. E.; Watts, R. 0. Chem. Phys. 1983, 80, 147.
actions (1)-(5) proceed effectively also in the cluster ions. In an attempt to observe the 'phantom" (C2H4)"' and (C2H,),,+ cluster ions, we have used the near-threshold vacuum-UV photoionization method2" to softly ionize van der Waals (ethylene),-(Ar), and (acetylene),-(Ar), binary clusters. In seeded beam conditions with relatively high nozzle stagnation pressures, it has been found that the intact (C2H4),+ and (C2H2),+ cluster ions dominate in the mass spectra. The results are interpreted within the framework of the intracluster excess energy dissipation5 to the van der Waals modes of the binary clusters.
Experimental Section The basic experimental apparatus has been described in detail elsewhere.35 Only a brief description is presented in the following. Ethylene and acetylene clusters in neutral molecular beams were ionized by a vacuum-UV photoionization from an Ar resonance lamp with a LiF window (1 1.83 and 11.62 eV). The ethylene and acetylene clusters were formed in a continuous supersonic expansion through a nozzle of 100-pm diameter of either Ar or He at stagnation pressures from 1 to 8 atm. Throughout the experiment the temperature of the nozzle was maintained in the range 298-303 K. Argon-seeded ethylene (14%) and heliumseeded ethylene (14%) were prepared by premixing the gases in a stainless steel vessel. Argon-seeded acetylene (14.9%) and helium-seeded acetylene (15.2%) were prepared by mixing the gases through an URS-100-5 mass flow controller system (Unit Instruments, Inc.) prior to supersonic expansion. The seeded molecular beam was introduced into the ionization region of a quadrupole mass spectrometer (ULVAC M-400). The nozzle skimmer distance was kept constant at 5 mm. At the ionization region, a light beam from a microwave-discharged(2450 MHz) Ar resonance lamp was intersected at 90' relative to the molecular beam axis. In order to increase the light intensity from the lamp, a LiF lens (f= 45.3 and 12.5-mm diameter) was used to focus the resonance lines into the ionization p ~ i n t . ~The ?~ mass-selected ions were detected by a channeltron electron multiplier (Galileo 4816). The greatest care was taken in order not to generate secondary electrons ejected from the ionizer surface. Results and Discussion Intracluster Excess Energy Dissipation. Let us suppose that, in general, an ion-molecule reaction of the type XY+ XY (XY)X+ Y proceeds rapidly so that the related intracluster (XY),IX+ + Y also takes place ion-molecule reaction (XY),+ instantaneously if one ionizes the corresponding neutral (XU),, clusters. Even if the intracluster ion-molecule reaction is so rapid, intact parent cluster ions (XU),,+ can, in some cases, be detected by using the near-threshold photoionization method. Figure 1 illustrates the general scheme of the appearance of (otherwise missing) intact cluster ions. The neutral mixed clusters containing Ar atoms, (XY),(Ar),,,, produced in a supersoic jet are photoionized in the ionization threshold. At the vertically ionized point (vip), the corresponding ion clusters [(XU),,+(Ar),liip* have excess energies and might be subjected to the rapid intracluster ion-molecule reactions which produce only fragmented cluster ions (Ar),(XY),IX+.5 However, if the excess energies on photoionization are randomized in vibrational modes of the ionized cluster and, in particular, converted to the excitation of the van der Waals modes (i.e., XY-Ar and Ar-Ar modes) of the clusters, then the intact (XU),+cluster ions can be stabilized by evaporating argon atoms, for such a period that the intact ions are actually observed by a mass spectrometer. Such phenomena have been observed in water5J3and ammonia2s3clusters not only with Ar atoms but also with such molecule^'^ as C 0 2 and N 2 0 which act as excess energy dissipators. In the following examples of ethylene and acetylene clusters, we will show how the excess energy dissipation plays a crucial role in producing, otherwise unstable, intact cluster ions by using
+
-
+
-
( 1 9 ) Fischer, G.; Miller, R. E.; Vohralik, P. F.; Watts, R. 0. J . Chem.
Phys. 1985, 83, 1471.
6720
Shinohara et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
Figure 1. General scheme of intracluster excess energy dissipation to the van der Waals modes of clusters: the appearance of otherwise unstable (XU),,' intact cluster ions. The intact (XY); cluster ions are detected as a result of the evaporation of solvated argon atoms due to the energy dissipation to the van der Waals modes (Ar-XY and Ar-Ar) of the clusters. Almost all excess energies are transferred to translational energies of evaporating argon atoms. Note that, in the corresponding conventional ion-molecule reaction, the reaction XY' + XY (XY)Xt + Y proceeds instantaneously (essentially zero potential barrier) so that the intact (XU),' complex ion has never been observed. All but He rare gases can play similar roles as an energy dissipator. vip stands for vertically ionized points.
-
TABLE I: Observed Ions by Vacuum-UV Photoionization of Ethylene Clusters" mle assignment* neutral species*
I
CiHb
Ar Lamp (LiF) 11.83. 11.62 eV CzH4 seeded in Ar (14%) a) 1.5 atm
55 56
68
x 10
A
69
70 ( w ) 83 84
97 ( w ) 1 1 1 (w) 1 I2
I40
168 (w) "C2H4seeded in Ar (14.0%) at a stagnation pressure of 8 atm; Ar lamp (LiF) 11.83, 11.62 eV; room-temperature nozzle. *See text. the near-threshold photoionization method. Photoionization Mass Spectra of Ethylene Clusters. Figure 2 shows typical photoionization mass spectra of ethylene clusters with the Ar resonance lines at 11.83 and 11.62 eV at four different stagnation pressures. I n a stagnation pressure of 1.5 atm, the intense peaks are due to the fragmented ions such as C3H5+and C4H7+and the intensity of the dimer intact ion is relatively weak, which is consistent with the normal ion-molecule reactions (1) and (2). At 3 atm the peak due to the intact dimer (C2H4)2+ becomes as intense as that of the fragmented ion. At the higher stagnation pressures (4.5 and 6.0 atm), a series of the intact cluster ions are observed along with the fragmented cluster ions. The observed ions are summarized in Table I . It should be noted here that the mixed Ar-C2H4+ ion is also observed. In Figure 3 ( P = 8 atm), the presence of the mixed cluster ions, such as Ar(C2H4)' and Ar(C2H4)2+,is much clearer. The fragmented cluster ions are produced as a result of the intracluster ion-molecule reactions'6.20
~
~~~
~~
(20) Tzeng, W B , Ono, Y , Linn, S H , Ng, C Y J Chem Phys 1985, 83. 2813
t
c) 4.5 atm
n
CeHf
x 1
h
A I
5
50
75
100
Moss Number
Figure 2. Vacuum-UV photoionization mass spectrum of ethylene clusters at I 1.83 and 1 I .62 eV (Ar resonance lamp; LiF window) with unit mass resolution: (a) 1.5 atm stagnation pressure seeded in Ar (14%); (b) 3.0 atm in Ar (14%); (c) 4.5 atm in Ar (14%); (d) 6.0 atm in Ar (14%); room-temperature nozzle. Intact cluster ions (C2H4),,+become stronger with increasing stagnation pressure. which correspond to the conventional ion-molecule reactions of ethylene (reactions 1 and 2,15 respectively). According to the previous studies by Ng and co-workers, the photoionization efficiency (PIE) curves of (C2H4)2+I6v2Iand (c2H4)3+20showvery
Ethylene and Acetylene Clusters AT Lamp (LIF) 11.83 and 11.62
CeH) Seeded
in
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6721 Ar tamp (LIF)
ev
CzHf
Ar (14%) 8 otm
C4H;
h
&HI
seeded in He (14%)
a) 1.6 atm x 100
Jl
.
'W3
1
X l
b) 3 o t m
I
1 50
150
100
200
Mass NUmbOr
Figure 3. Photoionization (1 1.83 and 11.62 eV) mass spectrum of
ethylene clusters with unit mass resolution. Ethylene is seeded in Ar (14%) at a stagnation pressure of 8.0 atm. The dominant peaks are due to intact cluster ions, (C2H4)"+. Note that there is a sudden intensity drop between (C2H4)4+and (C2HJS+. slow rises in the ionization threshold, indicating geometrical changes of the complexes upon ionization. Furthermore, previous electron-impact mass spectra, where the excess energies on ionization are very large, of the ethylene clusters show that the spectra are dominated by fragmented ions and the intensities of the intact ions are very meager.'* Under these circumstances, it is inferred that the Franck-Condon excitations from the neutral ethylene clusters (C2H4),, to the corresponding ionic states cannot produce the intact cluster ions (C,H4),,+: There exists no effective Franck-Condon excitation below the fragmentation limit.5 Then a question arises as to the origin of the (C,H4),,+ ions as observed in Figure 2. What are the neutral precursors of the such cluster ions? At the highest stagnation pressure (Figure 2d), the intact cluster ions increase markedly to such an extent that the signal intensities of (C2H4),,+ exceed those of the fragmented cluster ions. The appearance of the (C,H4),,' ions is quite sensitive to the Ar carrier gas pressure in the stagnation, and the enhancements of the ions are seen always concomitant with the observation of the ethylene-Ar complex. This evidence suggests that the binary mixed clusters of the type (Ar),,(C2H4),,,are the neutral precursors of the intact ions. The intact ethylene cluster ions are produced as a result of the intracluster excess energy dissipationS in which some portions of the excess energies are liberated as a translational energy of the evaporating argon atoms via the excitation of van der Waals modes of the clusters:
x 10
A
I J1 I
x 1
.
I J'
A
1 Ji,
A
25
x 1
*
X I
A
50
75
100
Mass Number
Figure 4. Photoionization (1 1.83 and 1 1.62 eV) mass spectrum of ethylene clusters with unit mass resolution. Ethylene is seeded in He (14%) at several stagnation pressures. Note that even at a stagnation pressure of 8 atm the intensity of the fragmented cluster ion, such as C3H5+,is much stronger than those of the intact parent ions, indicating less effective excess energy dissipation in He than in the Ar case (see text).
In this case, the corresponding neutral clusters are pure ethylene clusters. The mechanism, which is supported by the results of
Ng and co-workers,'6,20becomes important in neat ethylene expansions. Effect ofSeed Gas. The primary effect of the seeding in Ar (or other rare gases) is to cool the beam and thus the clusters,22 which obviously enhances the formation of both (C2H4)mand ( A T ) , , ( C ~ H ~The ) ~ . presence of weakly bounded complex ions, Ar(C2H4)+,in Figures Id and 3 confirms that the clusters are indeed cold. In contrast to this, helium (which is less polarizable than argon) has much less ability to form the van der Waals clusters of the type (He)n(C2H4)m,because helium has the fewest cluster-cluster collisions in a seeded molecular beam, the lowest thermal accommodation coefficient, and the weakest van der Waals (vdW) interaction among the rare gases.'.22 Figure 4 shows photoionization mass spectra of ethylene clusters seeded in He. The peaks due to the fragmented cluster ions such as C3H5+and (C2H4)C3H5+are salient even at higher stagnation pressures. The intensity ratio I , = (C2H4)2+/C3H5+ (which is a measure of the intracluster energy dissipation to the vdW modes of the clusters) in Figure 4b ( I , = 0.023) for the He-seeded case, for example, is considerably smaller than that in Figure 2b ( I , = 1.32) for the Ar-seeded case at the same stagnation pressure. Even at the highest stagnation pressure of 8 atm, the signals due to the fragmented ions are intense and the larger intact cluster ions ( n 2 4 ) are not observed, which is in marked contrast to the corresponding Ar case in Figure 3. The primary reason of this
(21) Ceyer, S.T.; Tiedemann, P. W.; Ng, C. Y.; Mahan, B. H.; Lee, Y. T.J . Chem. Phys. 1919, 70,2138.
(22) Hagena, 0. F. In Molecular Beams and Low Density Gasdynamics; Wegener, P. P., Ed.: Marcel Dekker: New York, 1974: p 93.
A further experimental evidence to support the above mechanism is that the signals due to the intact cluster ions become progressively greater with increasing argon pressure, because the number of argon atoms attached to an ethylene cluster increases as the nozzle stagnation pressure increases. The observation of the ethylene-Ar mixed cluster ions in Figure 3 is also an indication of the above evaporation mechanism. Comparison of Figures 2 and 3 clearly demonstrates this point. At a stagnation pressure as high as 8.0 atm, the photoionization mass spectrum is dominated by the intense peaks originating from a series of the intact cluster ions, and the intensities of the fragmented ions are much less intense. Another probable energy dissipation process of importance is an evaporation of ethylene molecules:
Shinohara et al.
6722 The Journal of Physical Chemistry, Vol. 94, No. 17, I990
*
A i lamp(LiF) 11 83,1162 eV C2H2 seeded in Ar (14 9%)
C4Hi
TABLE 11: Observed Ions by Vacuum-UV Photoionization of Acetylene Clustersa
77 (w) 78 103 (vw) 104 ( v w ) "C2H2seeded in Ar (14.9%) at a stagnation pressure of 5.0 atm; Ar lamp (LiF) 11.83, 11.62 eV; 301 K nozzle temperature. bSee text. isotopes of the adjacent species. 'Overlapped with
I
I
25
50
1
IS Mass Number
I
100
Figure 5. Vacuum-UV photoionization (1 1.83 and 1 1.62 eV) mass spectrum of acetylene clusters with unit mass resolution. Acetylene was seeded in Ar (14.9%) at stagnation pressures of (a) 3.1 atm and (b) 5.0 atm. Besides fragmented cluster ions (such as C4H3'), intact cluster ions (C2HZ)ntare detected. Impurity peaks due to acetone are not presented in the spectra.
will be that the formation of relatively large neutral (C2H,),(He), clusters is not effective as compared with that of (C2H4)n(Ar)m clusters, because of the very small polarizability of He. This experimental evidence is completely consistent with the proposed mechanism for the appearance of the intact ions. Photoionization of Acetylene Clusters. Acetylene clusters seeded in Ar are also photoionized in the near threshold. Figure 5 shows the photoionization mass spectra of the acetylene clusters at two different stagnation pressures. Generally, the observed tendency is similar to that of the ethylene clusters. The observation of the fragmented ions C4H3+,C4H2+, C2H3+, and their cluster homologues is consistent with the conventional ion-molecule reactions (3)-(5). The origin of the fragment ion C3H3+,however, is somewhat different. The C3H3+ion is not produced by the above ion-molecule reaction^.'^.^^ It is proposed by Ono and Ng that the precursor ion of the fragment ion might be (CzH2)3+ rather than (C2H2)2+.23I n a different set of experiments, we have measured the stagnation pressure dependence on the signal intensities of the various ions and found that the intensities of the C3H3+and (CZH2)3+ ions exhibit the same pressure dependence, which suggests that the C3H3+ion is produced by decomposition of (C,H2)3+. Furthermore, the C3H3+ion is shown to be one of the major fragment ions in the multiphoton ionization fragmentation of Table I1 summarizes the observed cluster ions.
The dominant cluster ions in both spectra consist of a series of the intact cluster ions (C2H2),,+which have never been observed by electron-impact ionization of the acetylene clusters.19 The detection of the, otherwise normally missing, intact ions of the acetylene clusters is also ascribed to the intracluster excess energy dissipation within the vertically ionized clusters. In the present study, neutral mixed clusters (Ar),,(C2H2), are photoionized in the near theshold, which is followed by the excess energy dissi(23) Ono, Y.; Ng, C. Y. J. Am. Chem. Soc. 1984, 104, 4752. (24) Zandee, L.; Bernstein. R . B. J . Chem. Phys. 1979, 71, 1359. (25) Boesl. U.; Neusser, H. J.; Schlag, E. W . J. Chem. Phys. 1980, 72, 4321. (26) Reilly, J . P.;Kompa, K . L. J . Chem. Phys. 1980, 73, 5468.
c .v) C
0,
c
C
JI 25
A
x1
50 75 Mass Number
Figure 6. Vacuum-UV photoionization (1 1.83 and 1 I .62 eV) mass spectrum of acetylene clusters with unit mass resolution. Acetylene was seeded in He (15.2%) at a stagnation pressure of 4.1 atm. In contrast to the Ar case as in Figure 5, only fragmented cluster ions are observed and higher clusters are not detected with the present nozzle condition. Obvious impurity peaks due to acetone are not shown in the spectrum.
pation to the vdW modes (Ar-C2H2 and Ar-Ar) of the cluster ions. A local heating of some of the vdW modes may result in evaporation of the argon atom(^)^-^^ attached to acetylene clusters, which results in the production of the intact acetylene cluster ions. As in the ethylene cluster case, a mixed cluster Ar(C2H2)+is also seen in the same spectra, suggesting the presence of the neutral (Ar)n(C2H2)m clusters. The overall reaction scheme for the appearance of the intact acetylene clusters will be as follows
where vip represents a vertically ionized point on the ionic potential energy,surface. However, higher mixed clusters than Ar(C2H2)+ are not observed, probably because the photoionization efficiency of the mixed clusters of the type (Ar),(C2H2)" is very small as compared with that of the pure (C2H2)nclusters. Such an observation has recently been reported by Shiromaru et aL2' for the photoionization of Ar-H20 binary clusters via synchrotron radiation, where the photoionization efficiency of the binary clusters is found to be much smaller than that of neat water clusters at the ionization threshold. As mentioned in the production of the intact ethylene cluster ions, the following evaporation scheme also becomes important in neat acetylene expansions: Figure 6 exhibits a photoionization mass spectrum of the acetylene clusters seeded in He. One may immediately notice that (27) Shiromaru, H.; Suzuki, H.; Sato, H.; Nagaoka, S . : Kimura, K . J. Phys. Chem. 1989, 93, 1832.
Ethylene and Acetylene Clusters
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6123
TABLE Ill: Examples of the Observed Intact (XU),' Cluster Ions as a Result of Intracluster Excess Energy Dissipation
(XY).' (H20),+ (NH3),++ (C2H4), (C2H2)n'
evaDorated species Ar atoms' H 2 0 moleculesB CO>,CN,Oc Ar atomsd Ar atomsc C 2 H 4moleculesf Ar atomse C 2 H 2moleculesg
XY' H20'
+ XY + H20
----
NH,' C2H4'
+ NH3
C2H2'
+ C2H2
+ C2H4
(XYN'
+Y
+ OH NH,' + NH, C3H5' + CH3 C4H,' + H C4H3' + H C4H2' + H,
H30t
'Reference 5 . bReference 13. CReference 14. dReferences 2 and 3. eThis work. /References 16, 20, and 21. 8References 17 and 23. the overall spectral pattern looks quite different from that of the ethylene clusters in helium as shown in Figure 4. Only a trace amount of the intact dimer ion can be seen, and no higher intact cluster ions are observed even in the near-threshold photoionization. The distinct peaks are due to the fragment ions as observed in the normal ion-molecule reactions (3)-(5). A very similar result has been reported in an electron-impact ionization (30 eV) of the acetylene clusters seeded in H e at a stagnation pressure as high as 10.8 atm.I9 The inability to produce the intact cluster ions of acetylene might be associated with extremely weak van der Waals forces between acetylene and helium. In light of the experiment performed by Bomse et the formation of the mixed (He),(C2H2), clusters is not so effective, at least under the present nozzle conditions. The absence of the intact trimer ion, (C2H2)3+, in the same spectrum is puzzling. According to Ono and Ng,23the intact trimer ion may rearrange to form a stable benzene molecular ion C6&,+. The present observation might support this conclusion, in that there are sudden intensity drops between (CzH2)3+ and (CZH2)4+ in the mass spectra of acetylene (Figure 5a,b), indicating a special stability of the intact trimer ion (cf. the following section). Even so, the intact acetylene trimer ion is not seen in the present helium-seeded case. One of the probable rationales of this is that the excess energy dissipation within the cluster after ionization is much less efficient in the He case than in Ar, so that the intact trimer ion is subjected to dissociation. Fischer et aLi9have reported on the basis of pressure dependence experiments that in the He-seeded condition the intact trimer ion is most likely to dissociate into the C4H3+ion. The intense peak originating from the C4H3+ (28) Bomse, D. S.; Cross, J. B.; Valentini, J. J. J . Chem. Phys. 1983, 78, 7175.
ion in Figure 6 is very likely an indication of this fact. Table 111 summarizes the so far observed intracluster excess energy dissipation along with the intact cluster ions, the corresponding (full-collision type) ion-molecule reactions, and the evaporating atoms or molecules. Magic Number Ethylene and Acetylene Cluster Ions? Ng and co-workers have discussed the energetics and dissociation dynamics of the complex ions such as (C4H8)+from (CzH4)21'6 (C6H1?)+ from (C2H4)3,20and (C6H6)' from ( C Z H ~ ) ~Their . ' ~ studies indicate that these complex ions (generated as a result of intracluster rearrangements) are stable with respect to the corresponding cluster ion form. Figure 3 shows that there are intensity drops between (c&)4+ and (C2H4)5+ and more distinct ones between (c2H4)5+ and (C2H4)6+. An electron-impact spectrum of the ethylene clustersi6also exhibits the same "magic number" feature of the (C2H4)5+ ion. A similar magic number feature is also discernible in the acetylene spectra as in Figure 5 . In this case the intensity break is observed between (C2H2)3+ and (C2H2)4+. As suggested by Ono and Ng,23the most probable stable structure of the (C2H2.3' ion is that of the benzene ion C6H6+ which obviously has a particular stability. On the other hand, the structure of the magic number ethylene clusters (whether, in particular, they have linear chain structures or cyclic structures like cyclohexane ion) is not known in the present stage of investigation. The study of the appearance of especially stable ions rearranged from some van der Waals cluster ions is of extreme interest and importance. Such investigationsare now in progress.29
Conclusion Using the near-threshold photoionization mass spectroscopic method, we have observed intact cluster ions of ethylene, (C2H4),+, and acetylene, (C2H2),+, in abundance. These intact ions have never been reported by the conventional ion-molecule reactions of ethylene and acetylene. The observation reveals that such intact cluster ions are indeed stable at least in the time scale of several tens of microseconds (the time scale of mass spectroscopic measurements). The appearance of such cluster ions is discussed in terms of the intracluster excess energy dissipation: an important process of intracluster energy redistribution which determines the stabilities and energetics of various van der Waals and hydrogen-bonded cluster ions. Acknowledgment. The authors thank Hiroo Yokoyama (The University of Tokyo) for his assistance. (29) Shinohara, H.;Washida, N. To be published.
