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Evidence for the fragmentation of clusters upon electron impact ionization from electric deflection experiments
International Journal of Mass Spectrometry and Ion Processes, 66 (1985) 217-222 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
217
EVIDENCE FOR THE FRAGMENTATION OF CLUSTERS UPON ELECTRON IMPACT IONIZATION FROM ELECTRIC DEFLECTION EXPERIMENTS
A.W. CASTLEMAN,
Jr. * and BRUCE
D. KAY *+
Department of Chemistry, 152 Davey Laboratory, The Pennsylvania Park, PA 16802 (U.S.A.) (Received
31 January
State University, University
1985)
ABSTRACT
A molecular beam electric deflection technique was employed to examine the polarity of the mixed cluster species (H20),,,(SOZ),,,. The results indicate that the mixed dimer (H,O). (SO,) is polar, as expected for a charge-transfer complex, and the higher species (N + M > 3) are non-polar. The dependence of focusing behavior upon stagnation conditions provides evidence that extensive fragmentation occurs during electron ionization of the neutral clusters (H,O),(SO,),.
INTRODUCTION
Cluster research is a rapidly growing area which is pervading a number of areas of science and technology [l]. Many of the methods of studying the formation and properties of clusters involve the use of mass spectrometry, frequently employed in combination with electron impact as the source of ionization. Despite the fact that the fragmentation of molecules following electron ionization is a well-known, universally accepted phenomenon [2], many investigators have interpreted their work with clusters by assuming a one-to-one correspondence between the distribution of precursor neutral clusters and the detected ions [3-61. During recent years the issue of cluster fragmentation has become a controversal subject, but one in which there is intense interest [7-201. The
* The experimental phases of this research were undertaken at the Department of Chemistry, Chemical Physics Laboratory, CIRES, University of Colorado, Boulder, CO 30309. U.S.A. + Present address: Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.
0168-1176/85/$03.30
0 1985 Elsevier Science Publishers
B.V.
218
techniques of investigation vary and include study of metastable peaks using magnetic section mass spectrometers, post-acceleration and reflecting fields with time-of-flight mass spectrometers, measurement of angular distributions in crossed molecular beam experiments, and photoion-photoelectron coincidence methods. During the course of electric deflection experiments of clusters comprised of mixed species of H,O and SO,, evidence was found for fragmentation by a different method which does not depend on dissociation within a specific time, as do some of the metastable techniques. The findings are of interest, not only because they provide additional evidence of fragmentation in classes of systems not heretofore reported, but also because the technique is seen to be a useful one for effecting a partial separation of polar dimers from systems having a higher degree of clustering where the larger clusters units are non-polar. EXPERIMENTAL
The technique involves the use of a quadrupole (or hexapole) electric deflection field located along the path of a skimmed and well-collimated molecular beam containing clusters. The apparatus consists of five major pumping regions: the cluster source, nozzle exhaust chamber, differential chamber, deflection chamber housing the electrostatic fields, and the detection chamber in which is housed the electron ionizer and the quadrupole mass spectrometer. The details of the apparatus, the methods of cluster production, the principles behind the operation of the deflection fields, and the data collection system are described elsewhere [21,22]. The potential to the quadrupole field is supplied by a power supply having a maximum voltage of 30 kV. In the quadrupole field, one set of diametrically opposed rods is grounded and the other set is at a desired voltage. A beam obstacle, comprised of a 2.0 mm diameter ceramic rod which can be moved in and out of the beam axis by means of a “push-pull”-type feedthrough, is located at the entrance to the field. The purpose of the beam obstacle is to block on-axis beam particles. Since the electric field and field gradient vanish along the field axis, on-axis species cannot be found and only contribute to unnecessary (and unwanted) background. In the absence of an electric field, the beam obstacle used in the present experiments was found to block more than 95% of the primary beam from the detector. RESULTS
Recent molecular orbital calculations [23] indicate that a stable gas-phase complex H,O . SO, should exist. This complex has a structure which can be
219 I’ABLE 1 Quadrupole
defocusing
of (H,O),(SO,),
Cluster
Detected
H,O.SO, (H,O),(SO,) (H,O),(SO,) (H,O)(SO,)
(H20.S0,)+
at 15 kV ’ ion
Percent defocusing
lW,O),@Wl+ lW,O),W,)l+ z
KH,O)(SO,)
W,O),W,),
W,O),W,),l+
W,O),W,),
KH,%W2)21+
2 I+
b
15.1 19.2 19.1 17.9 15.1 12.6
apso,
= 740 torr, PHsO = 28 torr, T-, = 25°C. ’ Percent defocusing is defined as percent diminution of straight-through when voltage is applied to rods with beam obstacle removed.
beam
intensity
described as a Lewis acid-base pair, analogous to that of the SO,. N(CH,), complex [24]. The H,O . SO, adduct is fairly strongly bound (- 10 kcal mall’) and is expected to have a large dipole moment ( - 7 Debye) [23]. In order to confirm the existence of a stable adduct H,O . SO,, as well as examine the possibility for the existence of high clusters, (H,O),(SO,),, mixed clusters of SO, and H,O were produced as follows: gaseous SO, at a pressure of 740 torr was bubbled through liquid water at 25°C and subsequently expanded through a 100 pm diameter nozzle. The resultant cluster distribution contained several heteromolecular clusters having the stoichiometry (H20)M(S02)N. Table 1 lists the observed clusters and the results of the quadrupole electric deflection experiments. The results of the first series of electric deflection experiments indicated that all observed heteromolecular species (H,O) M( SO,) N, are non-polar, in contradiction to the theoretical prediction [23] that the species H,O . SO, should be very polar. It is interesting to note that the detected cluster ions have stoichiometry similar to the neutral precursors, namely, (H,O) M(SO,) n;. This suggests strongly that multi-channel cluster fragmentation may play a dominant role in the ionization of these species as it does in atomic clusters [25,26]. The findings would be consistent with predictions if a dominant ionization channel as (H,0),,z(S0,)Na2
+ e-4
(HZ0 . SO,)++
neutral fragments
(1) such that the detected ion (HZ0 . SO,)+ arose predominantly from neutral clusters other than the mixed dimer species H,O . SO,. On this basis, it was suspected that defocusing of H,P. SO: might arise from non-polar higher clusters and not from the (probably) polar species H,O . SO,. In order to check this hypothesis experimentally, the partial pressure of SO, behind the nozzle was reduced to inhibit formation of clusters other than H,O . SO,.
220
F
(H 030
)+ (~UAORUPOLE)
APPLIED
VOLTAGE
(1.c.V)
Fig. 1. Quadrupole focusing of (H,O.SOz)‘. The increased focusing with decreasing SO2 partial pressure is discussed in the text. PHzo, 28 torr. Psoz: A, 740 torr; B, 300 tom; C, 140 torr.
Figure 1 displays the quadrupole focusing of the detected ion (H,O . SO,)+ as a function of applied voltage for various partial pressures of SO,. As seen in the figure, the species (SO,. H,O)’ displays strong refocusing when the SO, partial pressure in the expansion source is reduced. At the lowest SO, partial pressure shown in the figure ( Pso,= 140 torr), the highest observed mass heteromolecular ion was (H,O . SO,)+. This strongly suggests that heteromolecular neutral clusters larger than SO,. H,O are not formed in appreciable quantities under these mild expansion conditions (Psoz= 140 torr, PHzO= 28 torr, T, = 25°C). The strong refocusing of (H,O * S02)+ under condition (C) in Fig. 1 establishes that the species H,O . SO, is very polar as predicted [23] and recently observed [27]. The binding in this complex is believed to occur via the relatively strong Lewis acid-base pair interaction between the oxygen (in H,O) “lone pair” and the electropositive sulfur atom.
221 CONCLUSIONS
The observed change in the extent of refocusing of the observed ion (SO, . H,O) + with decreasing expansion conditions is clear evidence for the polarity of the mixed dimer. Furthermore, the findings show conclusively that large clusters fragment upon electron ionization. Under high expansion conditions, the fragment contribution to (SO, . H,O) + from higher-order clusters is sufficient to mask completely the detection of the l-1 complex which does display rather strong focusing and is polar. It is expected that with a carefully designed experiment employing either spatial [28] or electric field modulation, it would be possible to deconvolute results for dimers from those arising from larger clusters that may be fragmenting under the experimental conditions employed. ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support of the U.S. Army Research Office, Grant No. DAAG-29-79-0133. The experimental phases of this research were performed at the University of Colorado. The authors thank the U.S. Army Research Office, Grant No. DAAG29-82-K0160 which enabled this work to be completed and published. REFERENCES
4 5 6
7
8 9 10
T.D. Mark and A.W. Castleman, Jr., Adv. At. Mol. Phys., 20 (1984) 65. J.H. Beynon, A.G. Brenton and F.M. Harris, Int. J. Mass Spectrom. Ion Phys., 45 (1982) 5. (a) 0. Echt, K. Sattler and E. Recknagel, Phys. Rev. Lett., 47 (1981) 1121. (b) 0. Echt, A. Reyes Flotte, M. Knapp, K. Sattler and E. Recknagel, Ber. Bunsenges. Phys. Chem., 86 (1982) 860. (c) J.M. Soler, N. Garcia, 0. Echt, K. Sattler and E. Recknagel, Phys. Rev. Lett., 49 (1982) 1857. A. Ding and J. Hesslich, Chem. Phys. Lett., 94 (1983) 54. E.E. Polymeropoulos and J. Brickmann, Chem. Phys. Lett., 96 (1983) 273. (a) V. Hermann, B.D. Kay and A.W. Castleman, Jr., Chem. Phys., 72 (1982) 185. (It should be noted that the authors did take account of fragmentation in the case of large clusters; the assumption with regard to smaller sizes implied that if fragmentation occurred, it was similar for adjacent sizes and the features were merely translated.) (b) P.M. Holland and A.W. Castleman, Jr., J. Chem. Phys.. 72 (1980) 5984. (a) H. Haberland, in J. Eichler, 1.V. Hertel and N. Stolerfoht (Eds.), Electronic and Atomic Collisions, North-Holland, Amsterdam, 1984, pp. 597-605. (b) K. Sattler, in J. Eichler, I.V. Hertel and N. Stolerfoht (Eds.), Electronic and Atomic Collisions, NorthHolland, Amsterdam, 1984, pp. 607-616. H. Haberland (Ed.), IXth International Symposium on Molecular Beams, Cluster Session, Freiburg, June 1983, pp. 15-157. Ber. Bunsenges. Phys. Chem., 88 (3) (1984). N. Lee and J.B. Fenn, Rev. Sci. Instrum., 49 (1978) 1269; 53 (1982) 1492.
222 11 K. Stephan and T.D. Mark, Chem. Phys. Lett., 90 (1982) 51. 12 J.H. Futrell, K. Stephan, A.W. Castleman, Jr. and T.D. Mark, Proc. lnt. Symp. Molecular Beams, Cannes, 1981, pp. 262-265. J.H. Futrell, K. Stephan, T.D. Mark, K.I. Peterson, A.W. Castleman, Jr. and N. Djuric, 29th Annu. Conf. Mass Spectrom. Allied Top., Minneapolis, 1981, pp. 150-151. J.H. Futrell, K. Stephan and T.D. Mark, J. Chem. Phys., 76 (1982) 5893. K. Stephan and T.D. Mark, Chem. Phys. Lett., 87 (1982) 226. K. Stephan and T.D. Mark, Chem. Phys. Lett., 90 (1982) 51. K. Stephan and T.D. Mark, Proc. Xlth SPIG, Dubrovnik, 1982. K. Stephan, J.H. Futrell, K.I. Peterson, A.W. Castleman, Jr., N. Djuric and T.D. Mark, Proc. 8th lnt. Symp. Molecular Beams, Cannes, 1981, pp. 211-215. K. Stephan, J.H. Futrell, T.D. Mark and A.W. Castleman, Jr., Vat. Technol. Appl. Ion Phys., 33 (1983) 77. K. Stephan, T.D. Mark and A.W. Castleman, Jr., J. Chem. Phys., 78 (1983) 2953. K. Stephan, T.D. Mark, E. Mark, A. Stamatovic, N. Djuric and A.W. Castleman, Jr., Beitr. Plasmaphysik, 23 (1983) 369. K. Stephan, A. Stamatovic and T.D. Mark, Phys. Rev. A, 28 (1983) 3105. T.D. Mark, 4th Symp. Elementary Processes and Chemical Reactions in LTP, Stara Lesna, 1982, pp. 55-73. T.D. Mark, Europhys. Conf. Abstr. 6D, 1982, pp. 29-31. 13 A.J. State and A.K. Shukla, lnt. J. Mass Spectrom. Ion Phys., 36 (1980) 119; Chem. Phys. Lett., 85 (1982) 157. A.J. State and C. Moore, J. Phys. Chem., 86 (1982) 3681; Chem. Phys. Lett., 96 (1982) 80. A.J. State, J. Phys. Chem., 87 (1983) 2286. 14 P.M. Dehmer and S.T. Pratt, J. Chem. Phys., 76 (1982) 843. 15 E.D. Poliakoff, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys., 76 (1982) 5214. 1983. 16 A. Ding, private communication, 17 U. Buch and H. Meyer, Phys. Rev. Lett., 52 (1984) 109. 18 0. Echt, S. Morgan, P.D. Dao, R.J. Stanley and A.W. Castleman, Jr., Ber. Bunsenges. Phys. Chem., 88 (1984) 217. 19 0. Echt, P.D. Dao, S. Morgan and A.W. Castleman, Jr., J. Chem. Phys., in press. 20 0. Echt, D. Kreisle, M. Knapp and E. Recknagel, Chem. Phys. Lett., 108 (1984) 401. and A.W. Castleman, Jr., Chem. Phys., submitted for 21 B.D. Kay, R. Hofmann-Sievert publication. B.D. Kay, Ph.D. Thesis, University of Colorado, 1982. 22 B.D. Kay and A.W. Castleman, Jr., J. Phys. Chem., submitted for publication. 23 P. Holland and A.W. Castleman, Jr., J. Photochem., 16 (1981) 347. New 24 F. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley-Interscience, York, 3rd edn., 1972, p. 443. 25 N. Lee, Ph.D. Thesis, Yale University, 1976. 26 A. Hermann, E. Schumacher and L. Waste, J. Chem. Phys., 68 (1978) 2327. 27 R.L. DeLeon and J.S. Muenter, Atmos. Environ., in press. 28 D. Van den Ende and S. Stolte, Chem. Phys. Lett., 76 (1980) 13.
217
EVIDENCE FOR THE FRAGMENTATION OF CLUSTERS UPON ELECTRON IMPACT IONIZATION FROM ELECTRIC DEFLECTION EXPERIMENTS
A.W. CASTLEMAN,
Jr. * and BRUCE
D. KAY *+
Department of Chemistry, 152 Davey Laboratory, The Pennsylvania Park, PA 16802 (U.S.A.) (Received
31 January
State University, University
1985)
ABSTRACT
A molecular beam electric deflection technique was employed to examine the polarity of the mixed cluster species (H20),,,(SOZ),,,. The results indicate that the mixed dimer (H,O). (SO,) is polar, as expected for a charge-transfer complex, and the higher species (N + M > 3) are non-polar. The dependence of focusing behavior upon stagnation conditions provides evidence that extensive fragmentation occurs during electron ionization of the neutral clusters (H,O),(SO,),.
INTRODUCTION
Cluster research is a rapidly growing area which is pervading a number of areas of science and technology [l]. Many of the methods of studying the formation and properties of clusters involve the use of mass spectrometry, frequently employed in combination with electron impact as the source of ionization. Despite the fact that the fragmentation of molecules following electron ionization is a well-known, universally accepted phenomenon [2], many investigators have interpreted their work with clusters by assuming a one-to-one correspondence between the distribution of precursor neutral clusters and the detected ions [3-61. During recent years the issue of cluster fragmentation has become a controversal subject, but one in which there is intense interest [7-201. The
* The experimental phases of this research were undertaken at the Department of Chemistry, Chemical Physics Laboratory, CIRES, University of Colorado, Boulder, CO 30309. U.S.A. + Present address: Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.
0168-1176/85/$03.30
0 1985 Elsevier Science Publishers
B.V.
218
techniques of investigation vary and include study of metastable peaks using magnetic section mass spectrometers, post-acceleration and reflecting fields with time-of-flight mass spectrometers, measurement of angular distributions in crossed molecular beam experiments, and photoion-photoelectron coincidence methods. During the course of electric deflection experiments of clusters comprised of mixed species of H,O and SO,, evidence was found for fragmentation by a different method which does not depend on dissociation within a specific time, as do some of the metastable techniques. The findings are of interest, not only because they provide additional evidence of fragmentation in classes of systems not heretofore reported, but also because the technique is seen to be a useful one for effecting a partial separation of polar dimers from systems having a higher degree of clustering where the larger clusters units are non-polar. EXPERIMENTAL
The technique involves the use of a quadrupole (or hexapole) electric deflection field located along the path of a skimmed and well-collimated molecular beam containing clusters. The apparatus consists of five major pumping regions: the cluster source, nozzle exhaust chamber, differential chamber, deflection chamber housing the electrostatic fields, and the detection chamber in which is housed the electron ionizer and the quadrupole mass spectrometer. The details of the apparatus, the methods of cluster production, the principles behind the operation of the deflection fields, and the data collection system are described elsewhere [21,22]. The potential to the quadrupole field is supplied by a power supply having a maximum voltage of 30 kV. In the quadrupole field, one set of diametrically opposed rods is grounded and the other set is at a desired voltage. A beam obstacle, comprised of a 2.0 mm diameter ceramic rod which can be moved in and out of the beam axis by means of a “push-pull”-type feedthrough, is located at the entrance to the field. The purpose of the beam obstacle is to block on-axis beam particles. Since the electric field and field gradient vanish along the field axis, on-axis species cannot be found and only contribute to unnecessary (and unwanted) background. In the absence of an electric field, the beam obstacle used in the present experiments was found to block more than 95% of the primary beam from the detector. RESULTS
Recent molecular orbital calculations [23] indicate that a stable gas-phase complex H,O . SO, should exist. This complex has a structure which can be
219 I’ABLE 1 Quadrupole
defocusing
of (H,O),(SO,),
Cluster
Detected
H,O.SO, (H,O),(SO,) (H,O),(SO,) (H,O)(SO,)
(H20.S0,)+
at 15 kV ’ ion
Percent defocusing
lW,O),@Wl+ lW,O),W,)l+ z
KH,O)(SO,)
W,O),W,),
W,O),W,),l+
W,O),W,),
KH,%W2)21+
2 I+
b
15.1 19.2 19.1 17.9 15.1 12.6
apso,
= 740 torr, PHsO = 28 torr, T-, = 25°C. ’ Percent defocusing is defined as percent diminution of straight-through when voltage is applied to rods with beam obstacle removed.
beam
intensity
described as a Lewis acid-base pair, analogous to that of the SO,. N(CH,), complex [24]. The H,O . SO, adduct is fairly strongly bound (- 10 kcal mall’) and is expected to have a large dipole moment ( - 7 Debye) [23]. In order to confirm the existence of a stable adduct H,O . SO,, as well as examine the possibility for the existence of high clusters, (H,O),(SO,),, mixed clusters of SO, and H,O were produced as follows: gaseous SO, at a pressure of 740 torr was bubbled through liquid water at 25°C and subsequently expanded through a 100 pm diameter nozzle. The resultant cluster distribution contained several heteromolecular clusters having the stoichiometry (H20)M(S02)N. Table 1 lists the observed clusters and the results of the quadrupole electric deflection experiments. The results of the first series of electric deflection experiments indicated that all observed heteromolecular species (H,O) M( SO,) N, are non-polar, in contradiction to the theoretical prediction [23] that the species H,O . SO, should be very polar. It is interesting to note that the detected cluster ions have stoichiometry similar to the neutral precursors, namely, (H,O) M(SO,) n;. This suggests strongly that multi-channel cluster fragmentation may play a dominant role in the ionization of these species as it does in atomic clusters [25,26]. The findings would be consistent with predictions if a dominant ionization channel as (H,0),,z(S0,)Na2
+ e-4
(HZ0 . SO,)++
neutral fragments
(1) such that the detected ion (HZ0 . SO,)+ arose predominantly from neutral clusters other than the mixed dimer species H,O . SO,. On this basis, it was suspected that defocusing of H,P. SO: might arise from non-polar higher clusters and not from the (probably) polar species H,O . SO,. In order to check this hypothesis experimentally, the partial pressure of SO, behind the nozzle was reduced to inhibit formation of clusters other than H,O . SO,.
220
F
(H 030
)+ (~UAORUPOLE)
APPLIED
VOLTAGE
(1.c.V)
Fig. 1. Quadrupole focusing of (H,O.SOz)‘. The increased focusing with decreasing SO2 partial pressure is discussed in the text. PHzo, 28 torr. Psoz: A, 740 torr; B, 300 tom; C, 140 torr.
Figure 1 displays the quadrupole focusing of the detected ion (H,O . SO,)+ as a function of applied voltage for various partial pressures of SO,. As seen in the figure, the species (SO,. H,O)’ displays strong refocusing when the SO, partial pressure in the expansion source is reduced. At the lowest SO, partial pressure shown in the figure ( Pso,= 140 torr), the highest observed mass heteromolecular ion was (H,O . SO,)+. This strongly suggests that heteromolecular neutral clusters larger than SO,. H,O are not formed in appreciable quantities under these mild expansion conditions (Psoz= 140 torr, PHzO= 28 torr, T, = 25°C). The strong refocusing of (H,O * S02)+ under condition (C) in Fig. 1 establishes that the species H,O . SO, is very polar as predicted [23] and recently observed [27]. The binding in this complex is believed to occur via the relatively strong Lewis acid-base pair interaction between the oxygen (in H,O) “lone pair” and the electropositive sulfur atom.
221 CONCLUSIONS
The observed change in the extent of refocusing of the observed ion (SO, . H,O) + with decreasing expansion conditions is clear evidence for the polarity of the mixed dimer. Furthermore, the findings show conclusively that large clusters fragment upon electron ionization. Under high expansion conditions, the fragment contribution to (SO, . H,O) + from higher-order clusters is sufficient to mask completely the detection of the l-1 complex which does display rather strong focusing and is polar. It is expected that with a carefully designed experiment employing either spatial [28] or electric field modulation, it would be possible to deconvolute results for dimers from those arising from larger clusters that may be fragmenting under the experimental conditions employed. ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support of the U.S. Army Research Office, Grant No. DAAG-29-79-0133. The experimental phases of this research were performed at the University of Colorado. The authors thank the U.S. Army Research Office, Grant No. DAAG29-82-K0160 which enabled this work to be completed and published. REFERENCES
4 5 6
7
8 9 10
T.D. Mark and A.W. Castleman, Jr., Adv. At. Mol. Phys., 20 (1984) 65. J.H. Beynon, A.G. Brenton and F.M. Harris, Int. J. Mass Spectrom. Ion Phys., 45 (1982) 5. (a) 0. Echt, K. Sattler and E. Recknagel, Phys. Rev. Lett., 47 (1981) 1121. (b) 0. Echt, A. Reyes Flotte, M. Knapp, K. Sattler and E. Recknagel, Ber. Bunsenges. Phys. Chem., 86 (1982) 860. (c) J.M. Soler, N. Garcia, 0. Echt, K. Sattler and E. Recknagel, Phys. Rev. Lett., 49 (1982) 1857. A. Ding and J. Hesslich, Chem. Phys. Lett., 94 (1983) 54. E.E. Polymeropoulos and J. Brickmann, Chem. Phys. Lett., 96 (1983) 273. (a) V. Hermann, B.D. Kay and A.W. Castleman, Jr., Chem. Phys., 72 (1982) 185. (It should be noted that the authors did take account of fragmentation in the case of large clusters; the assumption with regard to smaller sizes implied that if fragmentation occurred, it was similar for adjacent sizes and the features were merely translated.) (b) P.M. Holland and A.W. Castleman, Jr., J. Chem. Phys.. 72 (1980) 5984. (a) H. Haberland, in J. Eichler, 1.V. Hertel and N. Stolerfoht (Eds.), Electronic and Atomic Collisions, North-Holland, Amsterdam, 1984, pp. 597-605. (b) K. Sattler, in J. Eichler, I.V. Hertel and N. Stolerfoht (Eds.), Electronic and Atomic Collisions, NorthHolland, Amsterdam, 1984, pp. 607-616. H. Haberland (Ed.), IXth International Symposium on Molecular Beams, Cluster Session, Freiburg, June 1983, pp. 15-157. Ber. Bunsenges. Phys. Chem., 88 (3) (1984). N. Lee and J.B. Fenn, Rev. Sci. Instrum., 49 (1978) 1269; 53 (1982) 1492.
222 11 K. Stephan and T.D. Mark, Chem. Phys. Lett., 90 (1982) 51. 12 J.H. Futrell, K. Stephan, A.W. Castleman, Jr. and T.D. Mark, Proc. lnt. Symp. Molecular Beams, Cannes, 1981, pp. 262-265. J.H. Futrell, K. Stephan, T.D. Mark, K.I. Peterson, A.W. Castleman, Jr. and N. Djuric, 29th Annu. Conf. Mass Spectrom. Allied Top., Minneapolis, 1981, pp. 150-151. J.H. Futrell, K. Stephan and T.D. Mark, J. Chem. Phys., 76 (1982) 5893. K. Stephan and T.D. Mark, Chem. Phys. Lett., 87 (1982) 226. K. Stephan and T.D. Mark, Chem. Phys. Lett., 90 (1982) 51. K. Stephan and T.D. Mark, Proc. Xlth SPIG, Dubrovnik, 1982. K. Stephan, J.H. Futrell, K.I. Peterson, A.W. Castleman, Jr., N. Djuric and T.D. Mark, Proc. 8th lnt. Symp. Molecular Beams, Cannes, 1981, pp. 211-215. K. Stephan, J.H. Futrell, T.D. Mark and A.W. Castleman, Jr., Vat. Technol. Appl. Ion Phys., 33 (1983) 77. K. Stephan, T.D. Mark and A.W. Castleman, Jr., J. Chem. Phys., 78 (1983) 2953. K. Stephan, T.D. Mark, E. Mark, A. Stamatovic, N. Djuric and A.W. Castleman, Jr., Beitr. Plasmaphysik, 23 (1983) 369. K. Stephan, A. Stamatovic and T.D. Mark, Phys. Rev. A, 28 (1983) 3105. T.D. Mark, 4th Symp. Elementary Processes and Chemical Reactions in LTP, Stara Lesna, 1982, pp. 55-73. T.D. Mark, Europhys. Conf. Abstr. 6D, 1982, pp. 29-31. 13 A.J. State and A.K. Shukla, lnt. J. Mass Spectrom. Ion Phys., 36 (1980) 119; Chem. Phys. Lett., 85 (1982) 157. A.J. State and C. Moore, J. Phys. Chem., 86 (1982) 3681; Chem. Phys. Lett., 96 (1982) 80. A.J. State, J. Phys. Chem., 87 (1983) 2286. 14 P.M. Dehmer and S.T. Pratt, J. Chem. Phys., 76 (1982) 843. 15 E.D. Poliakoff, P.M. Dehmer and J.L. Dehmer, J. Chem. Phys., 76 (1982) 5214. 1983. 16 A. Ding, private communication, 17 U. Buch and H. Meyer, Phys. Rev. Lett., 52 (1984) 109. 18 0. Echt, S. Morgan, P.D. Dao, R.J. Stanley and A.W. Castleman, Jr., Ber. Bunsenges. Phys. Chem., 88 (1984) 217. 19 0. Echt, P.D. Dao, S. Morgan and A.W. Castleman, Jr., J. Chem. Phys., in press. 20 0. Echt, D. Kreisle, M. Knapp and E. Recknagel, Chem. Phys. Lett., 108 (1984) 401. and A.W. Castleman, Jr., Chem. Phys., submitted for 21 B.D. Kay, R. Hofmann-Sievert publication. B.D. Kay, Ph.D. Thesis, University of Colorado, 1982. 22 B.D. Kay and A.W. Castleman, Jr., J. Phys. Chem., submitted for publication. 23 P. Holland and A.W. Castleman, Jr., J. Photochem., 16 (1981) 347. New 24 F. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley-Interscience, York, 3rd edn., 1972, p. 443. 25 N. Lee, Ph.D. Thesis, Yale University, 1976. 26 A. Hermann, E. Schumacher and L. Waste, J. Chem. Phys., 68 (1978) 2327. 27 R.L. DeLeon and J.S. Muenter, Atmos. Environ., in press. 28 D. Van den Ende and S. Stolte, Chem. Phys. Lett., 76 (1980) 13.