8866
J. Phys. Chem. 1994,98, 8866-8869
Collision-Induced Reactions of (CH30H)J-I+ with Rare-Gas Atoms Shinji Nonose, Hideki Tanaka, Takashi Nagata,' and Tamotsu Kondow. Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received: March 24, 1994; In Final Form: June 20, 1994'
Dissociation of protonated methanol cluster ions, (CH30H),H+ (n = 2-23), in collision with He, Ne, or Kr atoms were studied by means of a tandem mass spectrometer equipped with octapole ion guides. The mass spectra of the product ions show that the dominant process is evaporation of C H 3 0 H molecules from (CH3OH),H+. The absolute cross section for the evaporation was measured as functions of the cluster sizes and the collision energy in the range 0.05-100 eV in the center of mass frame. The branching fractions for the product ions were also determined. It is inferred that the evaporation proceed via the collisional excitation and subsequent unimolecular dissociation.
1. Introduction
2
Collisionalprocesses involving cluster ions have attracted much attention because they provide a unique opportunity to study dynamical properties characteristics of few-body The dynamical properties depend specifically on the collision energy and the size of the cluster ion involved. In this regard, the measurementsshould be performed in a wide range of the collision energy. At a high collision energy, an incident particle "seesnthe cluster ion as if its constituent particles behave independently. Woodward and Stace have investigated collision of AT,+ with a rare-gas atom at a collision energy of =8 keV. It is found that the target rare-gas atom collides mainly with a single constituent Ar atom of Ar,+and theremainder actsas a spectator.' A similar spectator model is successfully applied to collision of (CO2),+ with an Ar atom at a collision energy of 2 keV; a single constituent COz molecule is vibrationally excited by the Ar collision, and finally the whole cluster ion is excited by energy transfer from the excited C02 molecule.2 At a low collision energy, on the other hand, the cluster ion behaves as one particle as a whole, and low-energy collective vibrations play an important role.G11 For example, a particleapproaching slowly to a cluster ion is captured efficiently in it (fusion). In our previous study on the Ar,+ + 36Ar collision system, 36Ar is found to be fused with or captured by Ar,+.4 The fusion is more important in cluster4uster collisions which have been studied theoretically by Schmidt et al., as observed in heavy nuclear colli~ions.5-~ The dynamics of such processes can be elucidated through quantitative measurements of collision parameters, such as absolute reaction cross sections. In the present report, dissociation dynamics of protonated methanol cluster ions, (CH30H),H+, in collision with rare-gas atoms was investigated by measurements of the absolute reaction cross sectionsand the branching fractions for the collision system. The cluster ion, (CHsOH),H+, was chosen because this ion has been explored inten~ively,~'-)~ and much informationis accumulated for the analysisof the collisional dissociation. In addition, the energy of the intermolecular interaction between a CH30H pair in (CHsOH).H+ is comparable with the energy resolution of the present apparatus so that a reliable energy dependence of the cross section can be obtained from the present measurements. A simple collision model was used for interpretation of the absolute reaction cross sections and the branching fractions, which are given as functionsof the cluster size, n, and the collision energy.
,\,
t Present address: Department of Chemistry, The College of Arts and Sciences, The University of Tokyo, Meguro-Ku, Tokyo 153, Japan. e Abstract published in Advance ACS Abstracts, August 1, 1994.
0022-3654/94/2098-8866%04.50/0
1
,
a
, ';6
4,,
1',
?
3
Figure 1. Schematicdiagram of the experimental apparatus. 1, nozzle; 2, electron gun; 3, einzel lens; 4, quadrupole mass selector; 5 , octapole ion guides; 6, inlet of a target gas; 7,collision cell; 8, acceleration lens; 9, electrostatic sector; 10, magnetic sector; 11, detector.
2. Experimental Section Figure 1 shows a schematic drawing of the apparatus; the details of the apparatus have been reported el~ewhere.~ Briefly, ions were produced by impact of 40-eV electrons on a supersonic expansion of Ar gas containing 4 mol % of CH3OH at a stagnation pressure of 3 atm and were mass-selected by a quadrupole mass spectrometer (Extrel 162-8). A mass-selected cluster ion transported through the first and the second octapole ion guides was admitted into a collision cell which surrounds the second octapole ion guide. The mass-selected cluster ion was allowed to collide with a target atom in the collision cell. The collision energy was varied from 2 to 500 eV by changing a dc voltage applied at the second octapole ion guide. Ions produced in the collision cell were transported through the third octapoleion guide and were mass-analyzed and detected by a sector-magnet mass spectrometer (JEOL JMS-D300). A pressure of a target gas in the collision cell was measured by a spinning rotor gauge (MKS, SRG-2).A pressure reading of the spinning rotor gauge was found to agree with the pressure determined by the known cross section for the ion-molecule reaction3s Ar'
+ H,
--*
ArH'
+H
(1)
when an effective path length, L, of the collision cell was set to be 12 f 2 cm. In this measurement, the collision cell was filled with Hz gas and its pressure was measured by the spinning rotor gauge. Then, Ar+ was admitted into the cell and the product ion, ArH+, was detected. The pressure of the H2 gas was calculated from the known cross section for process 1 (see section 3.2 for 0 1994 American Chemical Society
Collision-Induced Reactions of (CH30H),H+
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 8861
the procedure of the cross-section determination). The background pressure in the collision cell was in the order of 10-7 Torr. The intensities of the parent and product ions were measured by varying the pressure of the target gas in the range 10-5-10-6 Torr. In this pressure range, the single-collision condition was fulfilled. The kinetic energy spread of the Ar+ ion beam was estimated by applying a retarding field at the second octapole ion guide; the energy spread was estimated to be less than 8 eV in the laboratory frame. The energy spreads of all the cluster ions, (CHsOH),H+, studied were more or less in the same range. It was confirmed by changing the voltage of the third octapole ion guide that more than 95%of the fragment ions were produced by the collisionaldissociationin the collision region. The fragment ions produced after the collision region were found to be less than 5%. In this experiment, the fragment ions produced in the sectormagnet mass spectrometer cannot be detected, if any.
,
180
, ,
1 , 1 , , ,
I
,
j , , , I ,
I
I
/ / , , I ,
, , ,
, 1 , ,
160
2
140
' .c;,
u m Q,
3
-
I20
-
100
-
80
-
60
-
4020-
5
1
1
3. Results 3.1. Mass Spectra of Product Ions. A typical mass spectrum of cluster ions, (CH30H)pH+, produced by collision of (CH3OH),H+ with a rare-gas atom, Rg (Rg = He, Ne, or Kr), shows that the dominant reaction process is evaporation of CH3OH molecules from the parent cluster ions, (CHsOH),H+. The evaporation process is expressed as (CH30H),H+
+ Rg
-
(CH30H&,H+
-
1
+ (n - p ) C H 3 0 H + Rg
(2)
Even without any target gas in the collision cell, a small amount of (CHsOH),H+ (p < n) were produced through unimolecular dissociation of (CHsOH),H+, because (CHjOH),H+ gains a sizable internal energy, Eintrat electron impact ionization of a neutral cluster, (CH30H)k. The temperatures of the parent cluster ions were estimated from the unimolecular dissociation rates to be 200-250 K, with the aid of RRK theory as argued in section 4.4. 3.2. Reaction Cross Section. The total reaction cross section for process 2 was measured by intensity depletion of a parent cluster ion, (CH3OH),H+, in collision with an Rg atom in the collision region. The total reaction cross section, u,, is given by 6, =
200
( k , T / P O ln[I(O)/I(n)l
(3)
where P represents the pressure of Rg in the collision cell, L is an effective path length of the collision region, Z(0) and Z(n) are the ion intensitiesof the parent ion, (CH3OH),H+, at theentrance and the exit of the collision region, respectively.36 The difference, Z(0) - Z(n), is approximated by the sum of the intensities, ZZ,, of the product ions: (4)
The intensity, Z(n), was measured by varying the Rg pressure from 10-5 to 10-6 Torr, and the cross section was estimated from the slope of a linear dependence of ln[Z(O)/Z(n)] on the Rg pressure. The cross section was found to be almost independent of the stagnation pressure (1-5 atm) and the electron impact energy (15-200 eV). This result implies that the cluster temperature does not affect significantly on the reaction cross section and the branching fraction. 3.3. Dependenceof Reaction Cross Sectionon Collision Energy and Cluster Size. The reaction cross section, u,, was measured as a function of the collision energy, E-1, in the range 0.1-100 eV in the center of mass frame. Unless otherwise noted, E-1 is given in the center of mass frame. Figure 2 shows the reaction cross sections for (CHsOH)loH+ as a function of the collision energy in a logarithmic scale. In the He collision, u, starts to rise at -0.3 eV, reaches a broad maximum at 4-20 eV, and decreases,
1 1
2
3
IO
J
20
30
Cluster Size n Figure 3. Reaction cross sections for the collision of (CH,OH),H+ with Kr are plotted against the size of the parent cluster ion, n. The solid line exhibits the n dependence of rb&.
as E-1 increases. The collision-energy dependences for the Ne and Kr collisions resemble that for the He collision, although the cross sections for the He collision are slightly smaller. In Figure 3, the reaction cross section for (Ch30H),H+ in collision with a Kr atom is plotted as a function of the cluster size, n, at the collision energy of 4.0 eV, where a, and n are expressed in logarithmic scales. The cross section follows a nZl3dependence in the n > 7 range and tends to deviate downward from the n213 dependence in the n < 7 range. Similar n dependences were obtained for the He and the Ne collisions. Figure 4 shows typical population distributions for the product ions from (CH30H)loH+ in collisionwith Kr at different collision energies. Below a collision energy of =l eV, at most one CH3OH molecule evaporates by the collision, while evaporation becomes more extensive above -1 eV.
4. Discussion 4.1. Reaction Scheme. It is conceivable in comparison with the results of Ar,+ 4,9-11 that the reaction proceeds via collisional excitation and subsequent evaporation as (CH30H),H+ [(CH30H),H+lt
+ Rg
-
-
+ Rg
(5)
(CH30H)pH+ (n -p)CH,OH
(6)
[(CH3OH),H+It
+
where [(CH30H),H+]t represents a collisionally excited cluster ion. In the frame work of this two-step scheme, the reaction
Nonose et al.
8868 The Journal of Physical Chemistry, Vol. 98, No. 36, 1994
;
:
(a)
1
practically zero when b exceeds a critical value, b,,, expressed as
u,,
is
if ucxdoes not significantly depend on bin the 0 Ib Ib , range. In a collision between two hard spheres, E,, is expressed as
If the reaction always occurs when E,, exceedsa threshold energy, &,a threshold impact parameter, bth, which gives Eth is provided by the relation
Size of Product ion / n' 30
I
Eex(bth) E Eth
(12)
where bth satisfies the relation bth
= bmax(l
-Eth/aEmI)"2
(13)
Then, u, turns out to be Q,
(14)
Comparing eqs 14 with 7, one obtains the efficiency, r, as
Size of Product Ion / n'
Figure 4. Branching fractions of product ions for the (CHoOH)loH+ collision with Kr. Panels (a) and (b) show the branching fractions for 2and 5-eV collisions, respectively. The plots indicated by open diamonds (0)show the branching fractions calculated from RRK theory.
cross section, ur, is expressed as a, = rue,
(7)
where ucxis the cross section for the collisional excitation (process 5 ) and r represents an evaporation efficiencyof CH3OH molecules from [(CH30H),H+]t. The efficiency, I' (0 < I' < l ) , should depend on an energy, E,,, acquired by (CH30H),H+ in the collision. The cross section, u,, is approximated by a,, when a sufficient energy is introduced in the parent cluster ion, since the evaporation is expected to be extensive (I' 1) in this case. In the following section, r is estimated by taking advantage of a classical hard-sphere spectator collision model. 4.2. Spectator Model for Collisional Excitation. Presumably, the collision time, t,, is much shorter than the period of a CH3OH-CH30H vibration of (CHsOH),H+ and the collision is impulsive, on the basis of the fact that the collision energy (0.310 eV) is much higher than the energy of a CHpOH-CH30H bond (-0.5 eV).36 If a target atom interacts mainly with a single constituent CHsOH molecule in the collision and the remainder behaves as a spectator (spectator model), the momentum and the kinetic energy of the two collision partners should be conserved. When the kinetic energy acquired by a particular CH30H molecule which mainly interacts with the target atom is fully transmitted to the whole cluster ion, E, is given by
-
E,, = aEm1 a = [4(n - l)(nm,
= rb,; = a,,( 1 - Eth/aEm,)
+ m1)m11/b2(m, + mb2I
(8) (9)
where mI and m2 represent the masses of the target atom, Rg, and the CH30H molecule, and a (0 Ia I1) is a fraction of the collision energy converted to the internal energy of the cluster ion; the a values for the (CH30H)loH+ collisions with He, Ne, and Kr are 0.32, 0.82, and 0.84, respectively. 4.3. Hard-Sphere Collision. To estimate the reaction cross section, E,, should be expressed in terms of an impact parameter, 6. In the classical hard-sphere collision model, where u,, is
= (l -
(15)
The Eth value is estimated by assuming unimolecular dissociation of a collisionally excited parent cluster ion, as argued in the next section. 4.4. Unimolecular Dissociation. In the present reaction scheme, a collisionally excited parent cluster ion undergoes unimolecular dissociation. The rate constant, ki, for the unimolecular dissociation of a given cluster ion, (CH30H)iH+, is calculated by RRK theory4+ll.37-40 as
ki = A [ ( E i- VJ/EiI6"
(16)
where vi is the binding energy for a CH~OH-(CH30H)I-1H+ bond, A is the vibrational frequency in the transition state along the reaction coordinate, and Ei is the excitation energy shared by the CH30H-CH30H bond which is associated with the dissociation. The A value is estimated to be 4 X 1012 Hz, from the intermolecular stretching vibration which is related to the dissociation.37~38In the calculation of ki, the equation Ei
= Ei+l - K+I- Etrans(i)
(17)
is used, where is the translational energy of a CH3OH molecule leaving from (CH30H)i+lH+and is assumed to be twice as large as the energy shared by one vibrational degree of freedom at the transition state. In addition, V1-V7 are adapted from the reported ~ a l u e s and 3 ~ the values with i 1 8 are assumed to be all equal to V7. The branching fractions calculated from the rate constants, ki, reproduce the measured ones, as shown in Figure 4. The threshold energy, Ethrgiven in section 4.3 can be estimated by use of evaporative ensembles.39.a When a parent cluster ion gains Eth during the reaction time window, t, (-100 ps), at least one CHJOH molecule is released with an efficiency of more than 90%. The threshold energy, Eth, is given as Eth
=
- Eint
(18)
where Eintis the internal energy of the parent cluster ion concerned and Etho is the threshold energy for dissociation when the parent cluster ion has no internal energy. The Ethovalue must be several
Collision-Induced Reactions of (CH3OH),H+
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 8869
times larger than the CH3OH - (CHsOH),-lH+ bond energy, V,; otherwise, dissociation is unlikely to occur because the energy is more or less shared by many internal degrees of freedom. If at least one CH30H molecule is released during the reaction time window, t,, with an efficiency, P (more than 90%), the lower limit of the rate constant, kl, is given by the equation
P
1 - exp(-k+,)
> 0.9
(19)
Here kl is given by
k1 = A[(Et;-
Vl)/Eth
0 614
1
(20)
By taking advantage of eqs 19 and 20, Etho is estimated to be 1.7 eV for (CH3OH)loH+. Similarly, Einlis calculated to be 1.5 eV for (CH30H)loH+ by using the efficiency of unimolecular dissociation of the parent cluster ion without collision. Putting all these values together, one obtains Etb for (CH30H)loH+ to be 0.2 eV. 4.5. Measured vs Calculated Cross Sections. In the hardsphere collision model, b, is given by bmax
= Rn + Rr,
(21)
where R, and R,, represent effective diameters of (CHsOH),H+ and Rg, respectively. The volume, V,, of (CH30H),H+
V, = 4 / 3 ~ R , 3
(22)
is approximated from the molar volume of solid CH3OH. By Ab,,? is using R, obtained from eq 22 and R,, rep~rted,~' estimated to be 149, 153, and 171 A2 for the collision of (CH3OH)loH+ with He, Ne, and Kr, respectively. Interaction between a parent cluster ion and an Rg atom is slightly attractive at a large separation between the ion and the atom, whereas is strongly repulsive at a separation shorter than R, R,,. In the b < R, R , (=b,,) range, the repulsive short-range interaction causes efficient collisional excitation of the cluster ion through deeply inelastic scattering. Equation 21 may be justified because the steep repulsive wall of the interaction potential is located just inside the well of the van der Waals potential. The collision energy dependence of the cross section calculated from eqs 14 and 23 are shown in Figure 2 as solid curves in comparison with the experimentalcross sections. The calculated curves predict fairly well the experimental cross sections. Slight deviations of the calculated cross sections from the experimental ones are not liable to this simple reaction model. It is noted that these cross sections are insensitive to the change of the collision energy above 4.0 eV. This indicates that E,, is much larger than E,h/a in eq 15 and hence r is roughly equal to unity at a higher collision energy. The gradual decrease of the cross section in much higher collision energies is related to a less interaction time for the collisional excitation of the parent ion. In Figure 3, the reaction cross sections calculated for the collision of (CHsOH),H+ with Kr at the collision energy of 4 eV are shown in comparison with the experimental data. As shown in Figure 3, the calculation predicts the n dependence of the experimental cross sections.
+
+
Acknowledgment. The authors thank Dr. T. Tahara for his contribution to the design and constructionof the apparatus. The authors would like to thank Professors S.L. Anderson and K. Okuno for their valuable advices on the design of the apparatus. The present work was supported by Grant-in-Aid for Scientific Research in Priority Areas by the Ministry of Education, Science and Culture of Japan. References and Notes (1) Woodward, C. A.; Stace, A. J. J. Chem. Phys. 1991, 94, 4234. (2) Campbell, E. E. B.; Schneider,R. R.; Hielscher, A.;Tittes, A.; Ehlich, R.; Hertel, I. V. 2.Phys. D 1992,22, 521. (3) Buck, U.; Meyer, H. J. Chem. Phys. 1986,84,4854. (4) Ichihashi, M.; Nonose, S.;Nagata, T.; Kondow, T. J . Chem. Phys. 1994,100,6458. ( 5 ) Schmidt, R.; Lutz, H. 0. Phys. Rev. A 1992, 45, 7981. (6) Seifert, G.; Schmidt, R.; Lutz, H. 0. Phys. Lett. A 1991, 158,231. (7) Schmidt, R.; Seifert, G.; Lutz, H. 0. Phys. Lett. A 1991, 158,237. (8) Campbell, E. E. B.; Schyja, V.; Ehlich, R.; Hertel, I. V. Phys. Rev. Lett. 1993. 70. 263. (9) Ichihashi, M.; Hirokawa, J.; Nonose, S.;Nagata, T.; Kondow, T. Chem. Phys. Lett. 1993,204, 219. (10) Hirokawa, J.; Ichihashi, M.; Nonose, S.;Kondow, T. Z . Phys. D, in press. (1 1) Nonose, S.; Hirokawa, J.; Ichihashi, M.; Sakamoto, M.; Tanaka, H.; Kondow, T. Z . Phys. D 1993,26,223. (12) Parent, D. C.; Anderson, S. L. Chem. Rev. 1992,92, 1541. (13) Jarrold, M. F.;Bower, J. E.; Kraus, J. S.J . Chem. Phys. 1987,86, 3876. (14) Hanley, L.; Ruatta, S.A.; Anderson, S.L. J . Chem. Phys. 1987,87, 260. (15) Hanley, L.; Whitten, J. L.; Anderson, S.L. J . Phys. Chem. 1988,92, 5803. (16) Jarrold, M. F.; Honea, E. C. J. Phys. Chem. 1991, 95,9181. (17) Lian, L.; Su,C.-X.; Armentrout, P. B. J . Chem. Phys. 1992,97, 4084. (18) Su,C.-X.; Hales, D. A.; Armentrout, P. B. J. Chem. Phys. 1993,99, 6613. (19) Su,C.-X.; Armentrout, P. B. J . Chem. Phys. 1993,99,6506. (20) Lian, L.; Su, C.-X.; Armentrout, P. B. J . Chem. Phys. 1992, 97, 4072. . (21) Loh, S.K.; Hales, D. A.; Lian, L.; Armentrout, P. B. J . Chem. Phys. 1989,90,5466. (22) Jarrold, M. F.;Bower, J. E. J . Am. Chem. Soc. 1988, 110, 70. (23) Ruatta, S.A.; Hanley, L.; Anderson, S.L. J. Chem. Phys. 1989.91, 226. (24) Ruatta, S.A.; Hanley, L.; Anderson, S.L. J . Chem. Phys. 1988,89, 273. (25) Jarrold, M. F.;Bower, J. E. J . Chem. Phys. 1987, 87, 5278. (26) Jarrold, M. F.;Bower, J. E. Chem. Phys. Lett. 1988, 149,433. (27) Morgan, S.;Castleman, Jr., A. W. J. Am. Chem. SOC.1987,109, 2867. (28) Morgan, S.;Keesee, R. G.; Castleman, Jr., A. W. J. Am. Chem. SOC. 1989,111, 3841. (29) Morgan, S.;Castleman, Jr., A. W. J . Phys. Chem. 1989,93,4544. (30)Castleman, Jr., A. W.; Tzeng, W. B.; Wei, S.;Morgan, S.J . Chem. Soc., Faraday Trans. 1990,86,2417. (31) Zhang, X.; Yang, X.; Castleman, Jr., A. W. Chem. Phys. Lett. 1992, 185,298. (32) Bowers. M. T.: Su.T.: Anicich. V. G.J. Chem. Phvs. 1973.58.5175. (33j Bass, L: M.;Cat&, R.D.; Jarrold, M. F.; Kirchier, N. J.; Bowers, M. T. J. Am. Chem. SOC.1983, 105,7024. (34) Grimsrud, E. P.; Kebarle, P. J . Am. Chem. SOC.1973, 95,7939. (35) Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1985,83, 166. (36) Levine, R. D.; Bernstein, R. B. Molecular Reaction Dynamics. 2nd ed.: Oxford Universitv Press: Oxford. 1987. (37) Draves, J. A.,I ,uthey-Schulten, Z.; Liu, W.-L.; Lisy, J . M. J. Chem. phys. 1990,93,4589. (38) Selegue, T. J.; Moe, N.; Draves, J . A.; Lisy, J. M. J . Chem. Phys. 1992,96,7268. (39) Klots, C. E.J. Phys. Chem. 1988,92,5864. (40) Klots. C. E.J . Chem. Phvs. 1985.83, 5854. (41) Pauling, L. The Natureof ChemicaiBohd, 3rd ed.;Cornell University Press: Ithaca, NY, 1960.
