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TOF-SIMS with primary ions from the spontaneous desorption process
International Journal of Mass Spectrometry and Ion Processes, 96 (1990) 223-227
223
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication TOF-SIMS WITH PRIMARY IONS FROM THE SPONTANEOUS DESORPTION PROCESS
Ch. SCHOPPMANN,
R. SCHMIDT,
B. NEES and H. VOIT
Physikalisches Institut der Universitiit Erlangen-Niirnberg, D-8520 Erlangen (F.R.G.)
(First received 18 July 1989; in final form 15 September 1989)
ABSTRACT It is shown that atomic and cluster ions produced in a spontaneous desorption process can be used as primary ions in TOF-SIMS.
Della Negra et al. [I] and also Nees et al. [2] have shown that it is possible to desorb atomic and molecular ions from a variety of organic and inorganic samples without bombarding the sample with keV or MeV particles or photons from a laser source. All that is needed is a modest electric field maintained between sample and a grid positioned parallel to the sample. Typical field strengths are of the order of 106Vm-‘. Desorption rates obtained so far are considerably smaller than the primary ion rate in time-offlight (TOF) SIMS experiments. The mechanism underlying this desorption, which was named spontaneous desorption (SD) by Della Negra et al. [l], is not yet fully understood (see refs. 1, 3 and 4). It is tempting to investigate whether ions produced in an SD process (called SD ions in the following) can be used as primary ions for TOF-SIMS, particularly molecular ions and atomic cluster ions which are usually not obtainable from a primary ion source. In this brief note we report on the application of SD ions as primary ions for TOF-SIMS. The experimental setup is sketched in Fig. 1. SD ions originate at the primary target (Tl) shown to the left of the figure. The target is positioned on a target ladder together with several other targets which can be positioned in front of the grid (Gl) without breaking the vacuum. The distance between Gl and Tl is 4mm. Here only investigations performed with positive incident SD ions obtained from a primary target consisting of CsI evaporated onto a stainless steel plate are reported. The voltage U, applied to the target was 9.7 kV. The ions are accelerated in the electric field between grid Gl (at ground potential) and Tl, pass the hole in the converter plate (CP) and hit the ni 6x-1 i 76/9nemsn
ft? 1990 Elsevier
Science Publishers
R V
224 Gl
G3
G2
T2
STOP
STAR1
Fig. 1. Diagrams of the experimental setup. Positive SD ions originate at the primary target Tl and sputter secondary ions from the secondary target T2. The voltages U, = 9.7 kV and U, = - 7.0 kV are applied to Tl and T2, respectively. The grids Gl, G2 and G3 are on ground potential. C, CP, CF and TDC stand for channel-plate detector, converter plate, constant fraction discriminator and time digitizer, respectively.
secondary target (T2, also mounted on a target ladder) displayed to the right of Fig. 1. The electrons produced by the SD ion impact are accelerated in the electric field between T2 (U, = - 7 kV) and the grid (G2) in front of T2 and are subsequently deflected by means of a magnetic field onto a channel plate detector (C2). Secondary ions are produced at T2 by the impact of the SD ions. Negatively charged secondary ions are mass analyzed by means of TOF mass analysis. The mass spectrometer consists of the acceleration gap (between T2 and G2), a flight path between G2 and the converter plate and the channel plate detector Cl. The negative secondary ions hit the converter plate (covered with CsI) from the right and produce electrons which are accelerated in the field between CP (biased with U, = - SOOV) and the grid G3 and which are subsequently deflected onto Cl by means of a magnetic field. The output of Cl is connected to the stop input of a multistop time digitizer (TDC); it terminates the TOF measurement which is started by the electrons originating at T2 (due to the impact of the SD ions) and which are detected in C2. It should be noted that the output of C2 is also connected to the stop input of the TDC. This enables both the secondary ion and the SD ion mass spectra to be recorded simultaneously. This is essential since the latter is needed in order to correlate a secondary ion with the particular SD ion responsible for the secondary ion production. The correlation is performed with a special data acquisition program [5]. Primary and secondary ion spectra are well separated since the Cl signal was delayed by 4 ps with respect to the C2 signal.
225
SD spectrum
of
primary
1
20
LO
ions
60
20 0
60
80
m/z
Fig. 2. Upper spectrum: positive ion SD mass spectrum obtained from the CsI target Tl. (Note that the Cs+ peak does not show up in the spectrum since the Cs+ ion is used as TOF trigger. The voltage U, on Tl was 9.7 kV.) Middle and lower spectra: negative secondary ion mass spectra obtained from a valine target (T2) by bombardment with 16.7 keV Cs+ (middle spectrum) and (CsI)Cs+ SD ions (lower spectrum), respectively. (Note that the spectrum obtained with (CsI)Cs+ SD ions (lower spectrum) also contains the Csf induced spectrum, the H- and (M - H)- peaks of which are indexed with Cs’.)
It should also be noted that only those SD ions can be recorded in this way which originate simultaneously with a lighter particle: this precursor particle has to deliver the start signal for the TOF measurement. H+ ions are usually the light precursors for positive SD ions [2]. If CsI is used as primary target (produced by vacuum evaporation) almost exclusively Cs+ ions, however, are observed as light precursors [2]. The upper spectrum of Fig. 2 shows the positive ion SD spectrum obtained for the CsI target. It clearly exhibits the (CsI),Cs+ cluster ions (n = 1, 2) and an unidentified ion with m/z = 495 u. The Cs+ ion does not show up explicitly since it delivers the start signal for the TOF measurement. The production rate for Cs+, (CsI)C s+ and (CsI),Cs’ observed was roughly 100, 10 and 1 s-‘, respectively. Figure 2 also displays secondary negative ion spectra obtained from a valine target (T2) by bombardment with 16.7 keV Cs+ (middle) and (CsI)Cs+
226
1
100
LOO
1000
m/z
Fig. 3. Negative secondary ion spectra obtained from a CsI target (T2) by bombardment with 16.7 keV Csf, (CsI)Cs+ and (CsI),Cs’ SD primary ions (from top to bottom). The numbers n = l-4 indicate (CsI),I- cluster ions sputtered from the target. The two lower spectra also contain the Csf induced spectrum.
ions (lower spectrum), respectively. These spectra exhibit the deprotonated valine ion besides the usual background ions. Data collection time was 50 and 80 min for the Cs+ and the (CsI)Cs+ incident particle spectrum, respectively. The spectrum obtained with (CsI)Cs+ primary ions also exhibits Csf triggered events since only (CsI)Cs+ SD ions originating simultaneously with Cs+ can be used as primary ions in the present TOF analysis (see above). Figure 3 shows the spectra of negative secondary ions sputtered from a CsI target (T2) by 16.7 keV Cs+, (CsI)Cs+ and (CsI),Cs+ SD ions, respectively (from top to bottom). The spectra clearly exhibit I- and (CsI),I- ions (n = l-4). Th e d a t a collection time was 20, 70 and 220min (from top to bottom), respectively. The present investigations demonstrate that ions originating in an SD process can be used as primary ions for TOF-SIMS, i.e., that an “SD source” can be employed. It should be emphasized that the experimental setup used in the present investigations is by no means optimized with respect to a high primary ion rate. It is to be expected that an increase of the voltage U,, an optimized geometry (distances between the different subunits, hole diameter of the converter, etc.), the use of a focusing system for the primary ions and the application of elevated temperatures at the primary target Tl will increase this rate considerably. An SD source is, in any case, a very promising tool for investigations where
221
molecular ions or atomic cluster ions are needed without employing elaborate and expensive ion sources. A comparable simple source has recently been proposed [6] in which Cf fission fragment induced molecular ions are used as primary ions for TOF-SIMS. The ion rates obtainable by this method (with normal fission fragment sources) are very much the same as the rates obtained with the SD source which has, of course, the advantage that a radioactive source is not needed. Finally it should be noted that an SD source may also allow short data collection times in TOF-SIMS studies (as with conventional pulsed ion sources) if the potential power of a high sputter yield of massive molecular ions is used together with a modest increase (which seems to be feasible, see above) of the rate of these ions produced by SD processes. ACKNOWLEDGMENT
This work was supported by the Deutsche Forschungsgemeinschaft, W. Germany.
Bonn,
REFERENCES 1 S. Della Negra, C. Deprun, Y. LeBeyec, F. Riillgen, K. Standing, B. Monart and G. Bolbach, Int. J. Mass Spectrom. Ion Processes, 75 (1987) 319. 2 B. Nees, R. Schmidt, Ch. Schoppmann and H. Voit, Int. J. Mass Spectrom. Ion Processes, 94 (1989) 305. 3 M. Salehpour and J.E. Hunt, Int. J. Mass Spectrom. Ion Processes, 85 (1988) 99. 4 B. Nees, R. Schmidt, Ch. Schoppmann and H. Voit, Int. J. Mass Spectrom. Ion Processes, 94 (1989) 205. 5 Ch. Schoppmann, Diploma Thesis, Erlangen, 1989, unpublished. 6 M.G. Blain, E.A. Schweikert and E.F. Da Silveira, J. Phys. (Paris) C, 2 (1989) 85.
223
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication TOF-SIMS WITH PRIMARY IONS FROM THE SPONTANEOUS DESORPTION PROCESS
Ch. SCHOPPMANN,
R. SCHMIDT,
B. NEES and H. VOIT
Physikalisches Institut der Universitiit Erlangen-Niirnberg, D-8520 Erlangen (F.R.G.)
(First received 18 July 1989; in final form 15 September 1989)
ABSTRACT It is shown that atomic and cluster ions produced in a spontaneous desorption process can be used as primary ions in TOF-SIMS.
Della Negra et al. [I] and also Nees et al. [2] have shown that it is possible to desorb atomic and molecular ions from a variety of organic and inorganic samples without bombarding the sample with keV or MeV particles or photons from a laser source. All that is needed is a modest electric field maintained between sample and a grid positioned parallel to the sample. Typical field strengths are of the order of 106Vm-‘. Desorption rates obtained so far are considerably smaller than the primary ion rate in time-offlight (TOF) SIMS experiments. The mechanism underlying this desorption, which was named spontaneous desorption (SD) by Della Negra et al. [l], is not yet fully understood (see refs. 1, 3 and 4). It is tempting to investigate whether ions produced in an SD process (called SD ions in the following) can be used as primary ions for TOF-SIMS, particularly molecular ions and atomic cluster ions which are usually not obtainable from a primary ion source. In this brief note we report on the application of SD ions as primary ions for TOF-SIMS. The experimental setup is sketched in Fig. 1. SD ions originate at the primary target (Tl) shown to the left of the figure. The target is positioned on a target ladder together with several other targets which can be positioned in front of the grid (Gl) without breaking the vacuum. The distance between Gl and Tl is 4mm. Here only investigations performed with positive incident SD ions obtained from a primary target consisting of CsI evaporated onto a stainless steel plate are reported. The voltage U, applied to the target was 9.7 kV. The ions are accelerated in the electric field between grid Gl (at ground potential) and Tl, pass the hole in the converter plate (CP) and hit the ni 6x-1 i 76/9nemsn
ft? 1990 Elsevier
Science Publishers
R V
224 Gl
G3
G2
T2
STOP
STAR1
Fig. 1. Diagrams of the experimental setup. Positive SD ions originate at the primary target Tl and sputter secondary ions from the secondary target T2. The voltages U, = 9.7 kV and U, = - 7.0 kV are applied to Tl and T2, respectively. The grids Gl, G2 and G3 are on ground potential. C, CP, CF and TDC stand for channel-plate detector, converter plate, constant fraction discriminator and time digitizer, respectively.
secondary target (T2, also mounted on a target ladder) displayed to the right of Fig. 1. The electrons produced by the SD ion impact are accelerated in the electric field between T2 (U, = - 7 kV) and the grid (G2) in front of T2 and are subsequently deflected by means of a magnetic field onto a channel plate detector (C2). Secondary ions are produced at T2 by the impact of the SD ions. Negatively charged secondary ions are mass analyzed by means of TOF mass analysis. The mass spectrometer consists of the acceleration gap (between T2 and G2), a flight path between G2 and the converter plate and the channel plate detector Cl. The negative secondary ions hit the converter plate (covered with CsI) from the right and produce electrons which are accelerated in the field between CP (biased with U, = - SOOV) and the grid G3 and which are subsequently deflected onto Cl by means of a magnetic field. The output of Cl is connected to the stop input of a multistop time digitizer (TDC); it terminates the TOF measurement which is started by the electrons originating at T2 (due to the impact of the SD ions) and which are detected in C2. It should be noted that the output of C2 is also connected to the stop input of the TDC. This enables both the secondary ion and the SD ion mass spectra to be recorded simultaneously. This is essential since the latter is needed in order to correlate a secondary ion with the particular SD ion responsible for the secondary ion production. The correlation is performed with a special data acquisition program [5]. Primary and secondary ion spectra are well separated since the Cl signal was delayed by 4 ps with respect to the C2 signal.
225
SD spectrum
of
primary
1
20
LO
ions
60
20 0
60
80
m/z
Fig. 2. Upper spectrum: positive ion SD mass spectrum obtained from the CsI target Tl. (Note that the Cs+ peak does not show up in the spectrum since the Cs+ ion is used as TOF trigger. The voltage U, on Tl was 9.7 kV.) Middle and lower spectra: negative secondary ion mass spectra obtained from a valine target (T2) by bombardment with 16.7 keV Cs+ (middle spectrum) and (CsI)Cs+ SD ions (lower spectrum), respectively. (Note that the spectrum obtained with (CsI)Cs+ SD ions (lower spectrum) also contains the Csf induced spectrum, the H- and (M - H)- peaks of which are indexed with Cs’.)
It should also be noted that only those SD ions can be recorded in this way which originate simultaneously with a lighter particle: this precursor particle has to deliver the start signal for the TOF measurement. H+ ions are usually the light precursors for positive SD ions [2]. If CsI is used as primary target (produced by vacuum evaporation) almost exclusively Cs+ ions, however, are observed as light precursors [2]. The upper spectrum of Fig. 2 shows the positive ion SD spectrum obtained for the CsI target. It clearly exhibits the (CsI),Cs+ cluster ions (n = 1, 2) and an unidentified ion with m/z = 495 u. The Cs+ ion does not show up explicitly since it delivers the start signal for the TOF measurement. The production rate for Cs+, (CsI)C s+ and (CsI),Cs’ observed was roughly 100, 10 and 1 s-‘, respectively. Figure 2 also displays secondary negative ion spectra obtained from a valine target (T2) by bombardment with 16.7 keV Cs+ (middle) and (CsI)Cs+
226
1
100
LOO
1000
m/z
Fig. 3. Negative secondary ion spectra obtained from a CsI target (T2) by bombardment with 16.7 keV Csf, (CsI)Cs+ and (CsI),Cs’ SD primary ions (from top to bottom). The numbers n = l-4 indicate (CsI),I- cluster ions sputtered from the target. The two lower spectra also contain the Csf induced spectrum.
ions (lower spectrum), respectively. These spectra exhibit the deprotonated valine ion besides the usual background ions. Data collection time was 50 and 80 min for the Cs+ and the (CsI)Cs+ incident particle spectrum, respectively. The spectrum obtained with (CsI)Cs+ primary ions also exhibits Csf triggered events since only (CsI)Cs+ SD ions originating simultaneously with Cs+ can be used as primary ions in the present TOF analysis (see above). Figure 3 shows the spectra of negative secondary ions sputtered from a CsI target (T2) by 16.7 keV Cs+, (CsI)Cs+ and (CsI),Cs+ SD ions, respectively (from top to bottom). The spectra clearly exhibit I- and (CsI),I- ions (n = l-4). Th e d a t a collection time was 20, 70 and 220min (from top to bottom), respectively. The present investigations demonstrate that ions originating in an SD process can be used as primary ions for TOF-SIMS, i.e., that an “SD source” can be employed. It should be emphasized that the experimental setup used in the present investigations is by no means optimized with respect to a high primary ion rate. It is to be expected that an increase of the voltage U,, an optimized geometry (distances between the different subunits, hole diameter of the converter, etc.), the use of a focusing system for the primary ions and the application of elevated temperatures at the primary target Tl will increase this rate considerably. An SD source is, in any case, a very promising tool for investigations where
221
molecular ions or atomic cluster ions are needed without employing elaborate and expensive ion sources. A comparable simple source has recently been proposed [6] in which Cf fission fragment induced molecular ions are used as primary ions for TOF-SIMS. The ion rates obtainable by this method (with normal fission fragment sources) are very much the same as the rates obtained with the SD source which has, of course, the advantage that a radioactive source is not needed. Finally it should be noted that an SD source may also allow short data collection times in TOF-SIMS studies (as with conventional pulsed ion sources) if the potential power of a high sputter yield of massive molecular ions is used together with a modest increase (which seems to be feasible, see above) of the rate of these ions produced by SD processes. ACKNOWLEDGMENT
This work was supported by the Deutsche Forschungsgemeinschaft, W. Germany.
Bonn,
REFERENCES 1 S. Della Negra, C. Deprun, Y. LeBeyec, F. Riillgen, K. Standing, B. Monart and G. Bolbach, Int. J. Mass Spectrom. Ion Processes, 75 (1987) 319. 2 B. Nees, R. Schmidt, Ch. Schoppmann and H. Voit, Int. J. Mass Spectrom. Ion Processes, 94 (1989) 305. 3 M. Salehpour and J.E. Hunt, Int. J. Mass Spectrom. Ion Processes, 85 (1988) 99. 4 B. Nees, R. Schmidt, Ch. Schoppmann and H. Voit, Int. J. Mass Spectrom. Ion Processes, 94 (1989) 205. 5 Ch. Schoppmann, Diploma Thesis, Erlangen, 1989, unpublished. 6 M.G. Blain, E.A. Schweikert and E.F. Da Silveira, J. Phys. (Paris) C, 2 (1989) 85.