J. Phys. Chem. 1994, 98, 6063-6067
6063
Clusters Formed Directly from Laser Ablation of Ytterbium Oxide: Yb304+ and YbeOg+ John K. Gibson Chemical and Analytical Sciences Division, Oak Ridge National Laboratory. P.O.Box 2008, Oak Ridge, Tennessee 37831 -6375 Received: December 20, 1993%
Excimer laser (A = 308 nm) ablation of solid YbzO3 into vacuum has been found to generate polyatomic ytterbium oxide ions: Ybz+, Yb20+, YbzOz+, Yb304+, and YbaOg+. In contrast, other Ln2O3 (Ln = Sm, Eu, Gd, Dy) formed observable amounts of only the atomic and/or monoxide ions, Ln+ and LnO+. The stoichiometry of Yb&+ suggests mixed-valence YbI1/Il1content. The vapor composition found for vacuum ablation of Yb203 differs notably from both that reported for equilibrium vaporization of Yb2O3 and that for reactive (with 0 2 ) condensation of ablated ytterbium metal.
Introduction Laser ablation has proved a powerful tool for generating novel vapor-phase clusters. Gas-phase nucleation and growth of ablated species in a buffer gas or reactive atmosphere can generate novel clusters, such as fullerenes and their derivatives.' The vapors formed by direct ablation into vacuum are typically dominated by atomic and small molecular species and their ions. Since the chemical composition of these primary products is generally rudimentary, most laser ablation studies of a chemical nature have concentrated on larger gas-phase aggregation products. It has been demonstrated2 that novel and large vapor species can be generated by vacuum ablation, without invoking a buffer gas or postablation supersonic expansion. Furthermore, as has been particularly elaborated for carbon,3 cluster species formed as primary ablation products may exhibit markedly different composition distributions from those generated by gas-phase condensation and can provide unique insights into inherent chemical stabilities and structures, as well as into cluster growth mechanisms. Laser ablation of oxides into vacuum has gained particular attention as a method for obtaining films of high-T, superconductors, especially Y B a 2 C ~ 3 0(YBCO).' ~~ Excimer laser ablation of YBCO has been found to form primarily neutral and singly-charged Y, YO, Ba, BaO, and Cu specie^.^ However, large Y,O,+ clusters (to mass > 10 000 amu) have been identified as direct products in vacuum laser ablation studies of YBC0,6 and smaller Y,O,+ clusters were generated by vacuum ablation of pure Y203.7 Ablation of YBCO using 1060-nm laser light resulted in a significant yield of binary oxide oligomers (e.g., (BaO)3) and smaller intermetallic oxide clusters (e.g., (Ba0)z(YO)&* this work further identified YO, YzO3, and (YO),, (and their ions) as the only binary yttrium oxide species-the series of larger Y,O, clusters was not identified. Although thin films of high-T,lanthanide (Ln) oxides (e.g., L n B a z C u 3 0 ~havebeen ) successfully prepared by laser ablation, studies identifying the important ablation vapor species there are lacking. The dearth of information on the compositions of oxide ablation plumes is particularly significant for those lanthanides-Sm, Eu, and Yb-with the potential for divalence or mixed (Lnll/LnIlI) valence, where novel mechanisms or vapor species may be found. Although relatively few investigationsof laser ablation of binary lanthanide oxides have been reported, the high-temperature equilibrium vaporization mechanisms of these materials are wellestablished.9 The vaporization products of the lanthanide sesquioxides are primarily Ln and LnO, with some LnzO, LnO2, and Ln202 also having been observed as minor vapor constituents. *Abstract published in Advunce ACS Absrracrs, May
15, 1994.
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Figure 1. Schematic representation of the essential components of the
laser ablation mass spectrometer system. Brechignac et al.1° generated larger oxide clusters of Eu, Tm, and Yb by condensation of the laser-ablated metals in an oxygencontaining atmosphere. The distinctions between clustering behavior for these three lanthanides were somewhat subtle, with each forming Ln,O, for all values of m up to at least 10. Also, vacuum Nd:YAG laser ablation of La203 has been found to generate a series of La,O,+ cluster ions to m = 15.11 We have undertaken to determine the vapor species which are generated by excimer laser vacuum ablation of selected lanthanide sesquioxides in the absence of a buffer or reactive atmosphere and without postablation supersonic expansion. The results for Yb203(s) are emphasized as its apparently unique ablation products were of particular interest.
Experimental Section The laser ablation mass spectrometer system consists of an excimer laser and a reflection time-of-flight mass spectrometer (RTOFIMS); the essential components are shown schematically in Figure 1. The laser (Lumonics EX744) was operated using XeCl(308 nm), with a typical output of 300 mJ pulse-' (- 15 ns fwhm pulse width) at repetition rates of up to 100 Hz; most experiments used a 10-Hz repetition rate. A 25-cm-focal length quartz lens focused the laser output to a spot of l e 2 cm2 (0.7 X 1.3 mm) at -90° onto the oxide targets. An adjustable iris
0022-3654/94/2098-6063%04.50/00 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
between the laser and focusing lens was used to variably attenuate the ablating beam. Changing the aperture size presumably resulted in variations in the size and spatial profile of the focused laser beam; these variations were not considered in deriving irradiance values. Using the final net ablation crater area, the approximate irradiance on the sample was derived from the measured attenuation of the laser beam by the optical components. The irradiance was varied in the approximate range 5 X 105 W cm-2 (-10 mJ cm-2) to 7 X lo8 W cm-2 (-10 J cm-2). The dual-stageRTOF/MS (Comstock, Inc.) utilizes a gridless, high-transmission reflector in a 1-m-long flight tube and incorporates two dual-microchannel-plate particle detectors, allowing for ion detection in both linear and reflected flight operating modes; the results reported here were obtained using exclusively the higher-resolution reflected mode. The ion source is equipped with an electron gun and is configured for both electron impact ionization (EII) and laser ablation/ionization. No postionization (e.g., EII) was used in thiswork, so that onlypositive ions formed directly in the laser ablation process were analyzed. The mass calibration applied to the laser ablation spectra was confirmed with E11 spectra of C14F24(g). Although the optimal reflectron mass resolution ( m / A m )is better than 1000using EII, the achievable resolution for laser ablation was diminished. The base pressure in the turbomolecular-pumped mass spectrometer was maintained at - 1 t S Pa but could increase to -10-4 Pa during the ablation process. Spectra were recorded with a 500MHz (1 Gsample s-l) digitizing oscilloscope (Tektronix TDS 544A). Although single-shot spectra could be obtained a t the sacrifice of sensitivity, most spectra resulted from averaging of the ion signals from 10 to 100 laser shots (on the same spot). The oxide samples were sintered pieces of Ln2O3 of 99.9% purity as specified by the vendor (Cerac). The specimens were mounted on a sample manipulator for translation and rotation but were not moved during irradiation and were generally maintained stationary for several thousand laser shots. Both the laser beam and the normal to the specimen surface were approximately perpendicular to the flight tube longitudinal axis. The distance from the sample face to the center of the ion source could be varied from 0 to 8 cm, but the most useful results (i.e., minimal background effects relative to the species of interest) were obtained with a distance of -2 cm. The ablation plume freely expanded toward the center of the ion source without passing through any aperture or other restriction. A pulsed extraction-repeller plate (V- = -200 V; rise time = 50 ns) a t the back of the ion source delivered ion bunches from the ablation plume into the -2-kV flight tube through dual grounded 90% transmission grids a t the front of the source. The plume sampling sequence was i. fire laser/ablate sample ii. some plume ions spontaneously leak into flight tube iii. delay 10-1 10 ps after laser shot (td) iv. ion extraction repeller on ( t 0) v. acquire TOF spectrum The spontaneous ion extraction (ii) represents the leakage of some portion of the primary, intense component of the ablation plume into the flight tube without the application of an extractionrepeller pulse. These leakage ions resulted in a relatively lowresolution component of the spectra. The repeller pulse (iv) extracted into the spectrometer some of the small population of free ions present in the ion source after a time delay, t d , of 10-1 10 ks. The two distinct spectra, for leakage and extracted ions, were acquired sesquentially for the same laser pulses, with some intermingling of peaks. The coincident portions of the two types of spectra were deconvoluted by identifying the spontaneously extracted ions as those which were independent of the delay. Repeller extraction provided greater mass resolution and sensitivity for some ions and allowed for the identification of minor species present after the primary ablation plume had largely dissipated.
Gibson
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(1 = 0) Figure 2. Laser ablation mass spectrum for Yb203 (irradiance = 2 X IO7 W cm-2; delay = 67 ps). Ion peaks due to the primary plume are dominant, with a small peak due to delay-sampled Yb+.
Results and Discussion Ln+ and LnO+. Laser ablation studies were carried out on the following materials: Sm2O3, Eu203, Gd2O3, Dy2O3, and YbzO3. In Figure 2 is shown a typical spectrum for YbzO3. All of the dominant ion peaks in thecombined spectrum (up to and including the large, broad Yb+ peak near t = 0) were due to spontaneously extracted (leakage) ions from the primary ablation plume. The only peak in this particular spectrum which was due to repellerextracted ions was the second, relatively small Yb+ peak at t = 62.5 ps. The approximate velocity of the spontaneously extracted ions in the leading edge of the plume was estimated from their longer total transit times relative to the repeller extracted ions. The apparent velocity of Yb+ in traversing the -2 cm between the sample and the flight tube entrance was -1 cm ps-l, consistent with velocities determined elsewhere under similar conditions.12 The leakage components of the spectra were dominated by two contributions: (1) ions produced directly from ablation of the sample (e.g., Yb+, Yb2+, and O+; also, some LnO+ may have been obscured by the Ln+ peaks) and (2) secondary ions (e.g., H+, C+, and 0+)from plasma ionization of residual vacuum gases and/or H2O desorbed from the sample. At relatively high irradiances (e.g., lo8 W cm-2) the spectra were dominated by the plasma ionization component (especially H+). At low irradiances the primary ablated ions were dominant; near the ion appearance threshold (- lo6W cm-2), only the Ln+ were evident. Although all of the primary plume ion peaks were broadened relative to the pulsed-extraction ion peaks, the Ln+ peaks (e.g., the Yb+ peak near t = 0 in Figure 2) each exhibited a particularly distinguishing asymmetrical tail extending toward longer flight times. These tails suggested that the ablated Ln+ ions were formed over an extended period and/or volume in the ablation plume. A nonsymmetrical velocity distribution similar to that found for laser-ablated YBCO specied2should apply to theLn+ ions formed here. Various ion energies and formation mechanisms are likely involved throughout the plume, and the velocity/energy distributions of these ions does not result in a more symmetrical peak shape, such as would reflect a Boltzmann-like energy distribution. According to this interpretation, the most energetic species comprised the leading edge of the plume, resulting in a sharp onset of the spontaneously-sampled Ln+ peak; the tail portion of the plume included lower-energy Ln+ (at a lower yield). The shapes of the broadened Ln+ peaks are not as would be expected
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Laser Ablation of Ytterbium Oxide I
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The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6065 I
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Figure 3. Laser ablation mass spectra for Dy203 and GdzO, (irradiance = 4 X lo7 W cm-2; delay = 110 ps).
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Figure 5. Laser ablation mass spectrum for EuzO3 (irradiance= 1 X 108 W cm-z; delay = 60 F.
TABLE 1: Monoxide Dissociation Energies and Observed Ions ~~
Ln Yb
Eu Sm DY
Gd
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PW Figure 4. Laser ablation mass spectrum for Sm203 (irradiance = 3 X lo7 W cm-z; delay = 50 ps).
from metastable ion decomposition in the ~pectrometerl~ but are rather better explained by a distribution of ions in the plume/ source. The delayed-extraction component of the spectra served to characterize the cooler trailing portion of the plume. For each of the Ln2O3 studied, the delayed spectra were dominated by Ln+ and/or LnO+, with very little evidence for secondary ions (e.g., H+and 0+)or more highly ionized lanthanide ions (e.g., Ln2+ and Ln3+). The appearances of the delayed spectra were essentially independent of the laser repetition rate and showed no memory or residual effects upon switchingsamples, supporting that the ions were ablated from the target but were of sufficiently low energy/velocity that some remained in the source (at least) 110ps after the laser pulse. The intensities of the pulse-extracted Ln+/LnO+ peaks were found to decrease with both long delay times (e.g., >-60 ps) and short delay times (e.g.,
