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A continuous source for production of cold, mass-selected transition metal-cluster ions
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
A CONTINUOUS SOURCE FOR PRODUCTION OF COLD, MASS-SELECTED TRANSITION METAL-CLUSTER SK. LOH, David A. HALES and P.B. ARMENTROUT Chemntry Department,
19 September 1986
IONS
’
Universrty of California, Berkeley, CA 94720, USA
Received 30 June 1986
Characterization of a continuous source of single-charged transition-metal cluster ions is reported. Formation and cooling of the clusters occur via vaporization with an 8 kHz Cu vapor laser in a He flow. Cluster ions are mass-selected by a magnetic sector. Clustering and temporal behavior are examined with respect to variation in the length of the clustering region. Al and Fe show ion intensity decreasing monotonically with cluster size. Introduction of oxygen into the He converts iron cluster ions to iron oxide cluster ions.
1. Introduction Characterization of the physical and chemical properties of gas-phase metal clusters is important to understand the connection between those of the atom and those of the bulk phase. Early studies have concentrated on physical properties, e.g. “magic” numbers in ease of formation [ 1,2], cluster-size dependence of ionization potentials [3], and cluster spectroscopy [4,5]. Increasingly, studies designed to understand the chemistry of transition-metal clusters are being performed. Such studies can elucidate fundamental processes at metal surfaces, such as catalytic reactions at metal imperfections and metal corrosion. The difficulty in making gas-phase transition-metal clusters has been one of the limiting factors in past chemical studies. Metal carbonyls, which can be dissociated into metal clusters, have been one way around this difficulty [6-91. In the past few years, a more universal laser-based vaporization scheme has made more studies possible [lo]. In these, researchers admit a reactant gas into the cluster flow after the clustering region and examine reaction rates and their variation wrth cluster size [11,12]. Another approach uses raregas sputtering [13-l 51; although unless special mea’ NSF Presidential Young Investigator 1984-1989; Alfred P. Sloan Fellow.
0 009.2614/86/$ 03.50 OElsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
sures are taken [ 131, the resultant clusters have very high internal energies. With few exceptions [14,16], these studies work with a distribution of cluster sizes. A natural extension of atomic transition-metal ion reaction studies done in our laboratory [ 17] are studies of the reactivity of size-specific transition-metal cluster ions. Toward this end, we have developed a source of ionic metal clusters designed specifically for these investigations. This source has three important features: (1) the metal cluster ion beam is essentially continuous; (2) clusters are internally cooled by collisions in a carrier gas flow and by supersonic expansion; and (3) the initial cluster distribution is mass-selected to provide a single cluster ion.
2. Experimental The apparatus and source are currently undergoing several modifications, therefore only a brief description will be given here. Metal ions are formed by laser vaporization of a rotating target in a configuration similar to that of Smalley [lo]. Unlike other laser vaporization sources, metal clusters are produced at a high repetition rate, 8 kHz, via a Cu.vapor laser (CVL). The CVL (Plasma Kinetics, Cooper Lasersonics) emits 20-30 ns, 4 mJ pulses of (70%) 510 nm, (3%) 578 nm light, The average laser power to the sample is 527
Volume 129,number6
CHEMICAL PHYSICSLETTERS
about 17 W. Cluster ions are observed with a minimum of al 1 W. With a spot size of about 0.5 mm at the metal target, sufficient numbers of ions are created to obviate the need for an external ionizer. Entrainment of the metal vapor is accomplished with a continuous flow of 200 Torr He. An Ar flow under similar conditions resulted in little or no ions. Metal vapor travels through a 6.4 mm long, 2 mm diameter channel before entering a nozzle. The variable length nozzle, 2 mm diameter closing to 1 mm at the exit, acts as the primary condensation region. All spectra shown use a nozzle length of 5.7 cm. Within this region, a sputtered metal particle will be rapidly thermalized by undergoing approximately 6 X lo8 collisions/s. In a 5.7 cm nozzle, residence times are -170 ps, therefore a particle entering the flow experiences =lO’ collisions with He. Thermalization of clusters which form close to the end of the nozzle may not be as complete. Entrained clusters expand into an initial vacuum chamber which is kept at about 0.1 Torr by both a roots blower (600 cfm) and a mechanical pump (53 cfm). The metal ions are cooled in the He expansion, although we have no direct means of measuring energy distributions at this time. With our parameters, the terminal speed ratio of a free jet expansion is near 15 [18,19], yielding a translational temperature of less than 10 K. In our expansion, cooling is arrested at the skimmer (0.16 cm diameter) which is located approximately 1.9 cm from the nozzle (%2/3 of XM , the distance to the Mach disk) [18]. Ions are extracted in the expansion region by approximately 40 V. This potential may be lowered if it is determined that collisional reheating occurs. Once through the skimmer, the cluster flow enters a differentially pumped region kept at 10m3 Torr by a baffleless diffusion pump (1200 a/s). Cluster ions are focused with an open mesh-type einzel lens through a 0.48 cm channel into the mass spectral region. This chamber is pumped by a liquid-nitrogen baffled diffusion pump (1400 Q/s) and operate$ at 2 X lo-’ Torr. The optics here are high-voltaGe versions of those used elsewhere in our laboratory [20]. Clusters are accelerated to 3 keV for transmission through a magnetic sector for mass analysis. The magnet, which is capable of fields in excess of 9 ICC,has a mass range of 1000 amu. Spectra shown are accumulated for 0.5 s at each magnet position using an electron multiplier detector and standard pulse counting electronics. 528
19 September1986
3. Results and discussion 3.1. Source characterization Effects of varying the clustering nozzle length are shown in fig. 1 for iron cluster ions. A nozzle length (N, in inches) of zero corresponds to the 2 mm diameter 6.4 mm long channel intrinsic to the source. All other values of N are nozzles that close to 1 mm diameter within 1 mm of the exit. The value of N to the immediate right of 0 represents a nozzle consisting of a 0.16 mm plate with a 1 mm diameter bore. Increasing N causes the total flight time of the ions through the instrument to rise from 60 to 230 ps. This increase in ion residence time in the nozzle provides more collisions and enhanced clustering as shown by the corresponding increases in the dimer intensity and the dimer-monomer ratio, fig. 1. At N = 5.7, both of these quantities are still rising indicating that a longer nozzle will move the distribution toward higher clusters and further increase intensities, The reason is two-fold: the metal vapor has more time to cluster and the cluster flow becomes more directional with longer nozzles. Changes in N also affect the temporal distribution of the pulses. The temporal data are taken by monitoring Fe+. Qualitatively similar results are observed for the dimer but are more difficult to quantify due to lower intensity. At the two smallest values of N, there is a sharp main body of pulses lasting 20 I.CS followed immediately by a secondary body lasting 80 ps and a dead time of about 25 /.Is.This bimodal distribution means that some Fe+ blows straight through the nozzle while other ions undergo mixing in the nozzle. By N = 2.5, the pulses merge to form an almost uniform body with a dead time of 60 ps. At larger N, the envelope of pulses becomes even more uniform and the dead time decreases to -10 tits,a duty cycle of 92%. 3.2. Iron clusters Fig. 2 shows a mass spectrum of an iron sample. Observed intensities range from lo7 cps for Fe+ to 2 X lo3 CPS fOrFef6. Cluster ions, n < 10, show intensities of at least one order of magnitude higher than previously reported for transition-metal clusters, although absolute intensities are frequently not report-
Volume 129, number 6
CHEMICAL PHYSICS LETTERS I
12. O-
_
I
I
0 Flight
19 September 1986 I
’
time/20
A Dimer/monomer + Pulse length/l0
(%I Cps>
8. O-
Nozzle
Length
tern)
Fig. 1. Variation of flight time, absolute dimer intensity, ratio of dimer to monomer intensities, and cluster pulse length for Fe; with the length of the nozzle.
Size
Cluster IO.
I
12
0
4
o”
b-d
”
’
I
”
’
1
16
”
’
11
+
lFe,
7. 5
x20 x250
$ U 5 s u3 0
5.0
2. 5
~~
0.0
o
L$, LL 100.
OC
300.
400. MASS
500.
600.
700.
800.
900.
(a. m. u. )
Fig. 2. Absolute ion intensities for iron clusters. 529
species other than pure metal clusters can be produced. Fig. 3 shows the effects of introducing 0, to the He flow upstream of the vaporization region (lower trace) and a pure He flow (upper trace). To prevent quenching of cluster growth, the fraction of oxygen is kept to 0.5% of the He. Upon admitting 0, to the flow, the Fe+ and FeO+ intensities are unchanged. This indicates that the FeO+ observed in iron spectra are largely the result of atomic oxygen impurities in the sample. This is consistent with the fact that the reaction of Fe+ with 0, to form FeO+ is endothermic [22]. In contrast, FeOi and FeO; are greatly enhanced upon addition of 0,. These species could result from reactions with Fe+ or larger clusters. Unlike the monomer, addition of 0, depletes the Fe; and Fe,O+ signals and increases larger oxide signals greatly. Fe,O; and Fe20; intensities are particularly prominent, but oxygenation to Fe,Ol is clearly present. If this species were chemically bonded, it would place the iron atoms in a very high oxidation state; however, it is possible that this species is a strongly bound Fe20z weakly bound to an O2 mole-
ed. Ion signals are >105 cps to n = 6 and >104 cps to n = 13, an effective lower liiit for ion-beam reaction studies. The broadening of the n = 16 cluster, due to the natural isotopic abundance of the sample, is approximately 5 amu fwhm. Use of an isotopically pure sample would increase its intensity by about a factor of 3. Fe,O+ and Fe,Oz are observed due to oxygen impurities in either the sample or the He flow. Fe,O$, which in neutral clusters indicate formation in a hot plasma, are not discernable [ 121. The cluster distribution decreases monotonically. With other metals, others have observed an odd-even intensity pattern with “magic” numbers superimposed on a similar semi-exponential fall-off [ 11. Smalley et al. have not observed such variations in Nbi clusters [2 11. Neutral iron clusters have been studied by several groups and the distribution of clusters formed is dependent on specific source configurations. 3.3. Iron/oxygen clusters Another interesting
aspect of this source is that
Cluster 20. 0
19 September 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 6
0
,
2
I
1
MASS
Size
(I-I>
4
6
I
I
Co. m. u. )
Fig. 3.Absolute ion intensities for bare iron clusters (qpper trace) and for oxygenated iron clusters (lower trace).
530
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
19 September 1986
0.1% of Al+, oxides dominate the lower-mass clusters (n Q 4). The sample, stock 6061 Al rod, contains interfering oxygen impurities within the metal and as an external oxide layer. Others have shown that neutral Al clusters can easily be made without problems associated with the oxide layer [IO], therefore a more pure Al sample should alleviate this problem. Fig. 4 also shows a reduction in oxidation for clusters of n > 4. This may be due to a change in reactivity or the lack of oxygen due to removal by the lower clusters.
cule. Collisional activation studies of this species will be useful in determining its structure. Bare metal clusters larger than Fez are completely depleted. Fe30L has a small m = 2 component, but shows much greater m = 3,4 clustering. Fe,OL does not appear until m = 4 and shows strong intensities at the m = 5, 6 clusters. It is clear that larger clusters show more extensive oxygenation, consistent with the preference of iron to retain an oxidation state of between 2 and 3. This indicates that the metal species undergo sufficient collisions with 0, to reach a thermodynamically stable configuration, but not so many that the clustering process is totally disrupted.
4. Conclusion 3.4. Aluminum clusters Presented in fig. 4 are results for production of aluminum cluster ions from n = 1 to 12. The monomer is particularly intense, approximately twice that of Fe+. However, clustering is much less efficient, due in large part to oxygenation. With exception of the monomer, where the intensity of AlO+ is less than
Cluster
MASS
Intense continuous beams of cold metal cluster ions can be made through fast laser vaporization and rare-gas condensation. The cold clusters are momentum-analyzed to provide a size-specific metal cluster ion beam. We are nearing completion of an apparatus which will include a reaction region and mass spectrometer for analysis of reaction products. To increase
Size
(a.
6-J
m. u. >
Fig. 4. Absolute ion intensities for aluminum clusters. Bare metal clusters are indicated by asterisks.
531
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
the stagnation pressure, whichwill further enhance cluster growth and beam intensity, an additional differential pumping region is being incorporated. The instrument is designed to measure the energy dependences of absolute reaction cross sections. This will allow detailed characterization of the cluster energy distributions, studies of cluster binding energies and reaction thermochemistry, mechanisms, and dynamics [S]. The ability to create both bare and ligated clusters enables study of a variety of reaction sequences which may be useful in understanding, modeling, and designing catalytic systems.
[7] [S]
[9]
[lo] [ll]
Acknowledgement [12]
This work is supported by the Army Research Office, DAAG2984-0077 and DAAG29-85-K0181 and a National Science Foundation Graduate Fellowship (DAH). We also gratefully acknowledge contributions from Exxon Corporation.
[ 131 [ 14)
[15]
References
[ 161 (171
[l] I. Katakuse, T. Ichihara, Y. Fujita, T. Matsuo, T. Sakurai, and H. Matsuda, Intern. .I. Mass Spectrom. Ion Processes 67 (1985) 229. [2] W.D. Knight, K. Clemenger, W.A. de Heer, W.A. Saunders, M.Y. Chou and M.L. Cohen, Phys. Rev. Letters 52 (1984) 2141. [3] E.A. Rohlfing, D.M. Cox, A. Kaldor and K.H. Johnson, J. Chem. Phys. 81 (1984) 3846. [4] J.B.Hopkins, P.R.R. Langridge-Smith, M.D. Morse and R.E. Smalley, J. Chem. Phys. 78 (1983) 1627. [S] E.A. Rohlfing and J.J. Valentini, Chem. Phys. Letters 126 (1986) 113. [6] D.B. Jacobson and B.S. Freiser, J. Am. Chem. Sot. 108 (1986) 27;
532
[18] [19] [20] [21] [22]
19 September 1986
R.L. Hettig and B.S. Freiser, J. Am. Chem. Sot. 107 (1985) 6222. D.G. Leopold, T.M. Miller and W.C. Lineberger, J. Am. Chem. Sot. 108 (1986) 178. K. Ervin, S.K. Lob, N. Arlstov and P.B. Armentrout, I. Phys. Chem. 87 (1983) 3593; P.B. Armentrout, SK. Lob and K.M. Ervln, J. Am. Chem. Sot. 106 (1984) 1161. V. Vaida, N.J. Cooper, R.J. Hemley and D.G. Leopold, J. Am. Chem. Sot. 103 (1981) 7022; D.G. Leopold and V. Vaida, J. Am. Chem. Sot. 105 (1983) 6809. T.G. Dietz, M.A. Duncan, D.E. Powers and R.E. Smalley, J. Chem. Phys. 74 (1981) 6511. ME. Geusic, M.D. Morse, S.C. O’Brien and R.E. Smalley, Rev. Sci. Instr. 56 (1985) 2123; S.C. Richtsmeier, E.K. Parks, K. Liu, L.G. Pobo and S.J. Riley, J. Chem. Phys. 82 (1985) 3659. R.L.Whetten, D.M. Cox, D.J. Trevor and A. Kaldor. J. Phys. Chem. 89 (1985) 566. L. Hartley and S.L. Anderson, Chem. Phys. Letters 122 (1985) 410. G. Delacretaz, P. Fayet and L. Woste, Ber. Bunsenges. Physik. Chem. 88 (1984) 284; P. Fayet and L. Woste, Surface Sci. 156 (1985) 134. P.B. Freas and J.E. Campana, J. Am. Chem. Sot. 107 (1985) 6202. P.J. Brucat, L.S. Zheng, C.L. Pettiette, S. Yang and R.E. Smalley, J. Chem. Phys. 84 (1986) 3078. J.L. Elklnd and P.B. Armentrout, J. Chem. Phys. 84 (1986) 4862; Inorg. Chem. 25 (1986) 1078; J. Am. Chem. Sot. 108 (1986) 2765; N. Aristov and P.B. Armentrout, J. Am. Chem. Sot. 108 (1986) 1806. R. Campargue, J. Phys. Chem. 88 (1984) 4466. J.P. Toennies and K. Winkelmamt, J. Chem. Phys. 66 (1977) 3965. K.M. Ervin and P.B. Armentrout, J. Chem. Phys. 83 (1985) 166. L.S. Zheng, P.J. Brucat, C.L. Pettiette, S. Yang and R.E. Smalley, J. Chem. Phys. 83 (1985) 4273. P.B. Armentrout, L.F. Halle and J.L. Beauchamp, J. Chem. Phys. 76 (1982) 2449.
CHEMICAL PHYSICS LETTERS
A CONTINUOUS SOURCE FOR PRODUCTION OF COLD, MASS-SELECTED TRANSITION METAL-CLUSTER SK. LOH, David A. HALES and P.B. ARMENTROUT Chemntry Department,
19 September 1986
IONS
’
Universrty of California, Berkeley, CA 94720, USA
Received 30 June 1986
Characterization of a continuous source of single-charged transition-metal cluster ions is reported. Formation and cooling of the clusters occur via vaporization with an 8 kHz Cu vapor laser in a He flow. Cluster ions are mass-selected by a magnetic sector. Clustering and temporal behavior are examined with respect to variation in the length of the clustering region. Al and Fe show ion intensity decreasing monotonically with cluster size. Introduction of oxygen into the He converts iron cluster ions to iron oxide cluster ions.
1. Introduction Characterization of the physical and chemical properties of gas-phase metal clusters is important to understand the connection between those of the atom and those of the bulk phase. Early studies have concentrated on physical properties, e.g. “magic” numbers in ease of formation [ 1,2], cluster-size dependence of ionization potentials [3], and cluster spectroscopy [4,5]. Increasingly, studies designed to understand the chemistry of transition-metal clusters are being performed. Such studies can elucidate fundamental processes at metal surfaces, such as catalytic reactions at metal imperfections and metal corrosion. The difficulty in making gas-phase transition-metal clusters has been one of the limiting factors in past chemical studies. Metal carbonyls, which can be dissociated into metal clusters, have been one way around this difficulty [6-91. In the past few years, a more universal laser-based vaporization scheme has made more studies possible [lo]. In these, researchers admit a reactant gas into the cluster flow after the clustering region and examine reaction rates and their variation wrth cluster size [11,12]. Another approach uses raregas sputtering [13-l 51; although unless special mea’ NSF Presidential Young Investigator 1984-1989; Alfred P. Sloan Fellow.
0 009.2614/86/$ 03.50 OElsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
sures are taken [ 131, the resultant clusters have very high internal energies. With few exceptions [14,16], these studies work with a distribution of cluster sizes. A natural extension of atomic transition-metal ion reaction studies done in our laboratory [ 17] are studies of the reactivity of size-specific transition-metal cluster ions. Toward this end, we have developed a source of ionic metal clusters designed specifically for these investigations. This source has three important features: (1) the metal cluster ion beam is essentially continuous; (2) clusters are internally cooled by collisions in a carrier gas flow and by supersonic expansion; and (3) the initial cluster distribution is mass-selected to provide a single cluster ion.
2. Experimental The apparatus and source are currently undergoing several modifications, therefore only a brief description will be given here. Metal ions are formed by laser vaporization of a rotating target in a configuration similar to that of Smalley [lo]. Unlike other laser vaporization sources, metal clusters are produced at a high repetition rate, 8 kHz, via a Cu.vapor laser (CVL). The CVL (Plasma Kinetics, Cooper Lasersonics) emits 20-30 ns, 4 mJ pulses of (70%) 510 nm, (3%) 578 nm light, The average laser power to the sample is 527
Volume 129,number6
CHEMICAL PHYSICSLETTERS
about 17 W. Cluster ions are observed with a minimum of al 1 W. With a spot size of about 0.5 mm at the metal target, sufficient numbers of ions are created to obviate the need for an external ionizer. Entrainment of the metal vapor is accomplished with a continuous flow of 200 Torr He. An Ar flow under similar conditions resulted in little or no ions. Metal vapor travels through a 6.4 mm long, 2 mm diameter channel before entering a nozzle. The variable length nozzle, 2 mm diameter closing to 1 mm at the exit, acts as the primary condensation region. All spectra shown use a nozzle length of 5.7 cm. Within this region, a sputtered metal particle will be rapidly thermalized by undergoing approximately 6 X lo8 collisions/s. In a 5.7 cm nozzle, residence times are -170 ps, therefore a particle entering the flow experiences =lO’ collisions with He. Thermalization of clusters which form close to the end of the nozzle may not be as complete. Entrained clusters expand into an initial vacuum chamber which is kept at about 0.1 Torr by both a roots blower (600 cfm) and a mechanical pump (53 cfm). The metal ions are cooled in the He expansion, although we have no direct means of measuring energy distributions at this time. With our parameters, the terminal speed ratio of a free jet expansion is near 15 [18,19], yielding a translational temperature of less than 10 K. In our expansion, cooling is arrested at the skimmer (0.16 cm diameter) which is located approximately 1.9 cm from the nozzle (%2/3 of XM , the distance to the Mach disk) [18]. Ions are extracted in the expansion region by approximately 40 V. This potential may be lowered if it is determined that collisional reheating occurs. Once through the skimmer, the cluster flow enters a differentially pumped region kept at 10m3 Torr by a baffleless diffusion pump (1200 a/s). Cluster ions are focused with an open mesh-type einzel lens through a 0.48 cm channel into the mass spectral region. This chamber is pumped by a liquid-nitrogen baffled diffusion pump (1400 Q/s) and operate$ at 2 X lo-’ Torr. The optics here are high-voltaGe versions of those used elsewhere in our laboratory [20]. Clusters are accelerated to 3 keV for transmission through a magnetic sector for mass analysis. The magnet, which is capable of fields in excess of 9 ICC,has a mass range of 1000 amu. Spectra shown are accumulated for 0.5 s at each magnet position using an electron multiplier detector and standard pulse counting electronics. 528
19 September1986
3. Results and discussion 3.1. Source characterization Effects of varying the clustering nozzle length are shown in fig. 1 for iron cluster ions. A nozzle length (N, in inches) of zero corresponds to the 2 mm diameter 6.4 mm long channel intrinsic to the source. All other values of N are nozzles that close to 1 mm diameter within 1 mm of the exit. The value of N to the immediate right of 0 represents a nozzle consisting of a 0.16 mm plate with a 1 mm diameter bore. Increasing N causes the total flight time of the ions through the instrument to rise from 60 to 230 ps. This increase in ion residence time in the nozzle provides more collisions and enhanced clustering as shown by the corresponding increases in the dimer intensity and the dimer-monomer ratio, fig. 1. At N = 5.7, both of these quantities are still rising indicating that a longer nozzle will move the distribution toward higher clusters and further increase intensities, The reason is two-fold: the metal vapor has more time to cluster and the cluster flow becomes more directional with longer nozzles. Changes in N also affect the temporal distribution of the pulses. The temporal data are taken by monitoring Fe+. Qualitatively similar results are observed for the dimer but are more difficult to quantify due to lower intensity. At the two smallest values of N, there is a sharp main body of pulses lasting 20 I.CS followed immediately by a secondary body lasting 80 ps and a dead time of about 25 /.Is.This bimodal distribution means that some Fe+ blows straight through the nozzle while other ions undergo mixing in the nozzle. By N = 2.5, the pulses merge to form an almost uniform body with a dead time of 60 ps. At larger N, the envelope of pulses becomes even more uniform and the dead time decreases to -10 tits,a duty cycle of 92%. 3.2. Iron clusters Fig. 2 shows a mass spectrum of an iron sample. Observed intensities range from lo7 cps for Fe+ to 2 X lo3 CPS fOrFef6. Cluster ions, n < 10, show intensities of at least one order of magnitude higher than previously reported for transition-metal clusters, although absolute intensities are frequently not report-
Volume 129, number 6
CHEMICAL PHYSICS LETTERS I
12. O-
_
I
I
0 Flight
19 September 1986 I
’
time/20
A Dimer/monomer + Pulse length/l0
(%I Cps>
8. O-
Nozzle
Length
tern)
Fig. 1. Variation of flight time, absolute dimer intensity, ratio of dimer to monomer intensities, and cluster pulse length for Fe; with the length of the nozzle.
Size
Cluster IO.
I
12
0
4
o”
b-d
”
’
I
”
’
1
16
”
’
11
+
lFe,
7. 5
x20 x250
$ U 5 s u3 0
5.0
2. 5
~~
0.0
o
L$, LL 100.
OC
300.
400. MASS
500.
600.
700.
800.
900.
(a. m. u. )
Fig. 2. Absolute ion intensities for iron clusters. 529
species other than pure metal clusters can be produced. Fig. 3 shows the effects of introducing 0, to the He flow upstream of the vaporization region (lower trace) and a pure He flow (upper trace). To prevent quenching of cluster growth, the fraction of oxygen is kept to 0.5% of the He. Upon admitting 0, to the flow, the Fe+ and FeO+ intensities are unchanged. This indicates that the FeO+ observed in iron spectra are largely the result of atomic oxygen impurities in the sample. This is consistent with the fact that the reaction of Fe+ with 0, to form FeO+ is endothermic [22]. In contrast, FeOi and FeO; are greatly enhanced upon addition of 0,. These species could result from reactions with Fe+ or larger clusters. Unlike the monomer, addition of 0, depletes the Fe; and Fe,O+ signals and increases larger oxide signals greatly. Fe,O; and Fe20; intensities are particularly prominent, but oxygenation to Fe,Ol is clearly present. If this species were chemically bonded, it would place the iron atoms in a very high oxidation state; however, it is possible that this species is a strongly bound Fe20z weakly bound to an O2 mole-
ed. Ion signals are >105 cps to n = 6 and >104 cps to n = 13, an effective lower liiit for ion-beam reaction studies. The broadening of the n = 16 cluster, due to the natural isotopic abundance of the sample, is approximately 5 amu fwhm. Use of an isotopically pure sample would increase its intensity by about a factor of 3. Fe,O+ and Fe,Oz are observed due to oxygen impurities in either the sample or the He flow. Fe,O$, which in neutral clusters indicate formation in a hot plasma, are not discernable [ 121. The cluster distribution decreases monotonically. With other metals, others have observed an odd-even intensity pattern with “magic” numbers superimposed on a similar semi-exponential fall-off [ 11. Smalley et al. have not observed such variations in Nbi clusters [2 11. Neutral iron clusters have been studied by several groups and the distribution of clusters formed is dependent on specific source configurations. 3.3. Iron/oxygen clusters Another interesting
aspect of this source is that
Cluster 20. 0
19 September 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 6
0
,
2
I
1
MASS
Size
(I-I>
4
6
I
I
Co. m. u. )
Fig. 3.Absolute ion intensities for bare iron clusters (qpper trace) and for oxygenated iron clusters (lower trace).
530
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
19 September 1986
0.1% of Al+, oxides dominate the lower-mass clusters (n Q 4). The sample, stock 6061 Al rod, contains interfering oxygen impurities within the metal and as an external oxide layer. Others have shown that neutral Al clusters can easily be made without problems associated with the oxide layer [IO], therefore a more pure Al sample should alleviate this problem. Fig. 4 also shows a reduction in oxidation for clusters of n > 4. This may be due to a change in reactivity or the lack of oxygen due to removal by the lower clusters.
cule. Collisional activation studies of this species will be useful in determining its structure. Bare metal clusters larger than Fez are completely depleted. Fe30L has a small m = 2 component, but shows much greater m = 3,4 clustering. Fe,OL does not appear until m = 4 and shows strong intensities at the m = 5, 6 clusters. It is clear that larger clusters show more extensive oxygenation, consistent with the preference of iron to retain an oxidation state of between 2 and 3. This indicates that the metal species undergo sufficient collisions with 0, to reach a thermodynamically stable configuration, but not so many that the clustering process is totally disrupted.
4. Conclusion 3.4. Aluminum clusters Presented in fig. 4 are results for production of aluminum cluster ions from n = 1 to 12. The monomer is particularly intense, approximately twice that of Fe+. However, clustering is much less efficient, due in large part to oxygenation. With exception of the monomer, where the intensity of AlO+ is less than
Cluster
MASS
Intense continuous beams of cold metal cluster ions can be made through fast laser vaporization and rare-gas condensation. The cold clusters are momentum-analyzed to provide a size-specific metal cluster ion beam. We are nearing completion of an apparatus which will include a reaction region and mass spectrometer for analysis of reaction products. To increase
Size
(a.
6-J
m. u. >
Fig. 4. Absolute ion intensities for aluminum clusters. Bare metal clusters are indicated by asterisks.
531
Volume 129, number 6
CHEMICAL PHYSICS LETTERS
the stagnation pressure, whichwill further enhance cluster growth and beam intensity, an additional differential pumping region is being incorporated. The instrument is designed to measure the energy dependences of absolute reaction cross sections. This will allow detailed characterization of the cluster energy distributions, studies of cluster binding energies and reaction thermochemistry, mechanisms, and dynamics [S]. The ability to create both bare and ligated clusters enables study of a variety of reaction sequences which may be useful in understanding, modeling, and designing catalytic systems.
[7] [S]
[9]
[lo] [ll]
Acknowledgement [12]
This work is supported by the Army Research Office, DAAG2984-0077 and DAAG29-85-K0181 and a National Science Foundation Graduate Fellowship (DAH). We also gratefully acknowledge contributions from Exxon Corporation.
[ 131 [ 14)
[15]
References
[ 161 (171
[l] I. Katakuse, T. Ichihara, Y. Fujita, T. Matsuo, T. Sakurai, and H. Matsuda, Intern. .I. Mass Spectrom. Ion Processes 67 (1985) 229. [2] W.D. Knight, K. Clemenger, W.A. de Heer, W.A. Saunders, M.Y. Chou and M.L. Cohen, Phys. Rev. Letters 52 (1984) 2141. [3] E.A. Rohlfing, D.M. Cox, A. Kaldor and K.H. Johnson, J. Chem. Phys. 81 (1984) 3846. [4] J.B.Hopkins, P.R.R. Langridge-Smith, M.D. Morse and R.E. Smalley, J. Chem. Phys. 78 (1983) 1627. [S] E.A. Rohlfing and J.J. Valentini, Chem. Phys. Letters 126 (1986) 113. [6] D.B. Jacobson and B.S. Freiser, J. Am. Chem. Sot. 108 (1986) 27;
532
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