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Multiply charged cluster ions of nitrogen
International Journal of Mass Spectrometry and Ion Physics Elsevier Publishing Company, Amsterdam . Printed in the Netherlands
249
MULTIPLY CHARGED CLUSTER IONS OF NITROGEN
W . I-ENKES AND G . ISENBERG Institutfur Kernrerfahrenstechalk der Unicersitdt and des Kernforschungszentrums, 7500 Karlsral,e, Postfach 3640 (Germany)
(Received June 22nd, 1970)
ABSTRACT
Clusters of nitrogen ionized by electron impact are analyzed by a time-offlight method . Stable cluster ions of 22000 nitrogen molecules carrying up to five positive charges have been detected . Evidence for fragmentation as a result of electrostatic repulsion between charges has been obtained .
INTRODUCTION
in a recent paper Falter et aL t investigated the mass-lo-charge ratio of cluster ions produced by electron impact upon clusters of neutral molecules . The variation of the mean specific size N/Z (N = number of molecules, Z = number of elementary charges per cluster ion) with electron energy, was found to be related to the effective ionization cross section per molecule in a cluster . The mean specific size has a minimum that occurs roughly at the maximum of the ionization cross section . A model employing multiple ionization of clusters was used to explain the results . In the meantime we were able to prove directly the existence of multiply charged cluster ions by a time-of-flight method that shows more detail of the distribution function in the high mass region (up to 10 6 a.m .u.) that. previously obtained.
EXPERLMENTAL METHOD AND RESULTS
Cluster beams of nitrogen = are passed through an ion source described previously' . Clusters ionized by electron impact are extracted and the ion current is pulsed by a method similar to that proposed by Wiley and McLaren 4: Two grids define a "storage" section of i cm length (Fig . 1) . The lower grid, G2 , being at positive potential with respect to the anode of the ion source, reflects Int. J. Mass Spectrom. Ion Phys., 5
(1970) 249-254
250
W . HENKES, G . ISENBERG
the ions back into the storage section . A positive puke (peak voltage 0 .8 - 2 kV, rise time < 35 rsec, time constant of exponential decay 12 µsec) is applied to the upper grid, G,, which otherwise is at cathode potential . The positive pulse pushes out all ions in the storage section and blocks it for new ions to enter . The length of the ion pulse thus produced is given by the time necessary for the ions to leave
cluster beam
,
anode Ua ring 5heped t, magnets •, Ue \_- extinction etectrodeG,---_ . c pos. pulse
ja
C
accelera5an electrodes `
Ub
drift space
u - to osalloscape
I
the storage section. It depends on the distance between the grids, the specific size of the cluster ions and the height of the voltage pulse* . After being pulsed the ions are accelerated by a voltage ranging from 5 to 20 kV and enter a drift space of variable length (5 - 40 cm). The current of the ion collector is amplified and displayed by an oscilloscope. The specific size N/Z is related to the time of flight (ToF) t by
N(Z = (t/t,) 2
(1)
where t i is the T0F of the singly charged molecular ion . The distribution function of cluster ion current I leaving the ion source
dI = If(ftlz) d(N/Z) ;
(2)
I.(t) =
(3)
f 0 f(NIz) d(Niz) = 1 a can be shown to be related to the collector current I. by cf(t z tti)(t 2 /t 3 )
' The shape of the voltage pulse is of no particular importance as long as its duration is long compared to the time required by the ions to leave the storage section . Int. S. ldcss Spectrom. Ion Phys., 5 (1970) 249-254
This oscillogram was taken with shorter'Irift space and higher acceleration voltage (lower
MULTIPLY CHARGED CLUSTER IONS OF NITROGEN
251
as follows : the total charge dQ of ions of specific size N/Z stored within the storage section is proportional to the ion density in the beam, that is proportional to ~,/_N/Z d7. Hence - _ J V/Z dI d(N/Z) _ 2f t2 I = do_ dt d(N/Z) dt ti The constant c depends on the geometry and the applied voltages . Equation (3) shows that, because of the factor t22, cluster ions with long TOE are strongly favored in the ToF-spectra . Depending on the length of the drift space a ad the voltages the resolution was of the order of t/dt = 4 to 20, where At is the full width at half maximum of the N+-peak .-peak Fig . 2 shows a typical oscillogram* of the ToF-spectrum of N2-cluster ions . There are five broad, overlapping peaks whose squares of TOE, normalized to that
Fig. 2. ToF-spectrum of nitrogen cluster ions . Average size of singly charged ions N a 22000, mean specific size N/Z x 4400_ Time scale 50 ysec l.er division . of the right-hand peak, are 1/5 .2, 1/4.4, 1/3 .3, 1/2_1 and 1 . respectively . Obviously these peaks correspond to cluster ions of approximately 1/5, 1/4, 1/3 and 1/2 the specific size of the largest one with N/Z .: 22000 . This suggests that the peaks arise from clusters of approximately uniform size that are 1, 2, 3, 4, and 5-fold ionized . For the TOF-spectrum of Fig . 3** the experimental conditions of the cluster source were chosen such that smaller clusters were produced . By taking similar spectra with low electron energy and current, that is with a low probability for multiple ionisation, the peak at the right is identified as that of singly ionized clusters of ;z~ 3400 nitrogen molecules . The broad centre peak is attributed to cluster ionscarrying2-3 charges each . In contrast to Fig. 2, the different charge states are
• Figs . 2 and 3 show superpositions of approximately 10 traces each_ ` resolution) compared to Fig. 2 . Int. I Mass Spectrom. Ion Phys., 5 (1970) 249-254
252
W. HENKES, G . ISENBERG
no longer resolved* . At the left there is an indication of a peak at small ToF, corresponding to specific sizes centered around NJZ x 135**. These cluster ions may be the result of fragmentation of larger multiply charged cluster ions due to the electrostatic repulsion between charges .' his peak is not observed at low anode voltage (U ;S 100 V) and low electron current (It S 1 mA), that is when the
Fig. 3 . ToF-spectrum of nitrogen cluster ions . Average size of singly charged ions N ~ 3400, mean specific size N/Z z 1130. Time scale 50 psec per division . probability for multiple ionization is low . The mean specific size derived from the TOF-spectra depends on the electron energy qualitatively in the same manner as observed in ref. 1 . In addition we find that _NiZ decreases nmonotonically with rising electron current .
DISCUSSION Contrary to the conditions chosen in ref. 1, collisions with more than one electron are highly probable due to the increase both in electron current density and in length of the ionization zone_ Thus multiple ionization may be caused not only by one electron of sufficiently high energy but also by the interaction of a cluster with more than one electron . The peaks in Fig_ 2 can be interpreted either as being due to clusters carrying i - 5 charges respective-y or to clusters of 1/2 - 1(5 the original size with .)nly one charge each . The latter assumption, however, would call for a splitting of :<. cluster into fractions of nearly equal size, which is rather improbable on account of the large change in surface energy required . Most likely on energetic grounds v : Duld be the emission of one or more molecular ions carrying along their neighbouring molExcept for the lower resolution this is, at least in part, due to the relative width of the distribution of cluster sizes which is found to be larger for cluster beams of small mean cluster size . ss An evaluation of the distribution function would tend, because of the I/1 2 -correction, to shift the maximum of fragment ions to considerably lower specific sizes .
Int. J. Mass Spectr-m. Ion Phys., 5 (1970) 249-254
MULTIPLY CHARGED CLUSTER IONS OF NITROGEN
253
ecules that are bound to them by polarization forces . This ion field emission may account for the peak of small cluster ions observed in Fig . 3. The results presented in Fig. 2 show that a cluster of 22000 nitrogen molecules is able to support at least five positive charges . If one assumes four of these to be distributed uniformly on the surface of a spherical cluster one arrives at an electrical field of 1 .6 x 10 8 V/m acting upon the fifth . There seem to be no experimental data on field ion emission from nitrogen . Therefore we made a rough estimate of the size limit of a stable, doubly charged nitrogen cluster as follows . A cluster of spherical shape with two elementary charges situated at its poles is assumed to be divided into two fragments, also of spherical shape, carrying one charge each . The size of the fragments is chosen such that the difference between the surface energy required and the energy gained from the electrical field s minimized_ The minimum stable cluster size is found byvarying the cluster size until the energy difference equals zero . Since the temperature of the cluster ions is not known. we assume that their vapour pressure is equal to the background pressure of the vacuum chamber (-- 10 -6 tort) . The temperature determined in this way is 22-23'K for clusters of 150-300 molecules . Lacking data for solid nitrogen, we find the surface tension by linear extrapolation from liquid nitrogen data' . Under this assumption we find the stability limit to be at N = 155 molecules per cluster . The size of the two fragments is found to be N i = 7 and N2 = 148 molecules respectively_ The field strength of one charge acting upon the other equals 3 .3 x 108 V/m*. Thus the estimate yields a limiting field strength about twice as high as the field strength observed in the experiment** . This can be regarded as another piece of evidence that the cluster ions observed in Fig . 2 are in fact multiply charged and stable .
ACKNOWLEDGEMENTS
We gratefully acknowledge Prof. E. W. Becker's interest and support throughout the work reported here . Dr. K . Buchheit assisted in setting up the experiments.
* This is lower by two orders of magnitude than the field required for field ion emission from tungsten. *s Even if we assume the cluster temperature to be as high as that of the triple point (vapour pressure 250 torr for a cluster of 300 molecules!) we arrive at a field strength of 2x IOa Vim (N3 = 288, Ni = 13, Ni = 275) . Int. J. Mass Spectrom. Ion Phys., 5 (1970) 249-254
254
W . HENKES, G . ISENBERG
REFERENCES I- H . D . FALTER, O . F . HAGEL1, W litNtcrS AND H. V . WEDEL, Int . J. Mass Spectrum_ Ion Phys., 4 (1970) 145. 2 E, W. BECxER, K . BIER £A cD W . FIENKES, z. Physik, 146 (1956) 333 ; E. W. BECKER. IL KLINGELHOFER ANT) P . LoysE. Z. R'aturforsch_, 17a (1962) 432 . 3 K . BucInnaT AND W. HENxEs. Z. Angew_ Phys . . 24 (1968) 1914 W . C . WILEY AND I . H . McLAREN, Rev. Sci. Instr., 26 (1955) 1150. 5 V. J. ]olnvsoN, Properties of Materials atLow Temperature, (Phase]), Pergamon, Oxford, 1961 . Inc. J. Mass Spectrom. Ion Phys., 5 (1970) 249-254
249
MULTIPLY CHARGED CLUSTER IONS OF NITROGEN
W . I-ENKES AND G . ISENBERG Institutfur Kernrerfahrenstechalk der Unicersitdt and des Kernforschungszentrums, 7500 Karlsral,e, Postfach 3640 (Germany)
(Received June 22nd, 1970)
ABSTRACT
Clusters of nitrogen ionized by electron impact are analyzed by a time-offlight method . Stable cluster ions of 22000 nitrogen molecules carrying up to five positive charges have been detected . Evidence for fragmentation as a result of electrostatic repulsion between charges has been obtained .
INTRODUCTION
in a recent paper Falter et aL t investigated the mass-lo-charge ratio of cluster ions produced by electron impact upon clusters of neutral molecules . The variation of the mean specific size N/Z (N = number of molecules, Z = number of elementary charges per cluster ion) with electron energy, was found to be related to the effective ionization cross section per molecule in a cluster . The mean specific size has a minimum that occurs roughly at the maximum of the ionization cross section . A model employing multiple ionization of clusters was used to explain the results . In the meantime we were able to prove directly the existence of multiply charged cluster ions by a time-of-flight method that shows more detail of the distribution function in the high mass region (up to 10 6 a.m .u.) that. previously obtained.
EXPERLMENTAL METHOD AND RESULTS
Cluster beams of nitrogen = are passed through an ion source described previously' . Clusters ionized by electron impact are extracted and the ion current is pulsed by a method similar to that proposed by Wiley and McLaren 4: Two grids define a "storage" section of i cm length (Fig . 1) . The lower grid, G2 , being at positive potential with respect to the anode of the ion source, reflects Int. J. Mass Spectrom. Ion Phys., 5
(1970) 249-254
250
W . HENKES, G . ISENBERG
the ions back into the storage section . A positive puke (peak voltage 0 .8 - 2 kV, rise time < 35 rsec, time constant of exponential decay 12 µsec) is applied to the upper grid, G,, which otherwise is at cathode potential . The positive pulse pushes out all ions in the storage section and blocks it for new ions to enter . The length of the ion pulse thus produced is given by the time necessary for the ions to leave
cluster beam
,
anode Ua ring 5heped t, magnets •, Ue \_- extinction etectrodeG,---_ . c pos. pulse
ja
C
accelera5an electrodes `
Ub
drift space
u - to osalloscape
I
the storage section. It depends on the distance between the grids, the specific size of the cluster ions and the height of the voltage pulse* . After being pulsed the ions are accelerated by a voltage ranging from 5 to 20 kV and enter a drift space of variable length (5 - 40 cm). The current of the ion collector is amplified and displayed by an oscilloscope. The specific size N/Z is related to the time of flight (ToF) t by
N(Z = (t/t,) 2
(1)
where t i is the T0F of the singly charged molecular ion . The distribution function of cluster ion current I leaving the ion source
dI = If(ftlz) d(N/Z) ;
(2)
I.(t) =
(3)
f 0 f(NIz) d(Niz) = 1 a can be shown to be related to the collector current I. by cf(t z tti)(t 2 /t 3 )
' The shape of the voltage pulse is of no particular importance as long as its duration is long compared to the time required by the ions to leave the storage section . Int. S. ldcss Spectrom. Ion Phys., 5 (1970) 249-254
This oscillogram was taken with shorter'Irift space and higher acceleration voltage (lower
MULTIPLY CHARGED CLUSTER IONS OF NITROGEN
251
as follows : the total charge dQ of ions of specific size N/Z stored within the storage section is proportional to the ion density in the beam, that is proportional to ~,/_N/Z d7. Hence - _ J V/Z dI d(N/Z) _ 2f t2 I = do_ dt d(N/Z) dt ti The constant c depends on the geometry and the applied voltages . Equation (3) shows that, because of the factor t22, cluster ions with long TOE are strongly favored in the ToF-spectra . Depending on the length of the drift space a ad the voltages the resolution was of the order of t/dt = 4 to 20, where At is the full width at half maximum of the N+-peak .-peak Fig . 2 shows a typical oscillogram* of the ToF-spectrum of N2-cluster ions . There are five broad, overlapping peaks whose squares of TOE, normalized to that
Fig. 2. ToF-spectrum of nitrogen cluster ions . Average size of singly charged ions N a 22000, mean specific size N/Z x 4400_ Time scale 50 ysec l.er division . of the right-hand peak, are 1/5 .2, 1/4.4, 1/3 .3, 1/2_1 and 1 . respectively . Obviously these peaks correspond to cluster ions of approximately 1/5, 1/4, 1/3 and 1/2 the specific size of the largest one with N/Z .: 22000 . This suggests that the peaks arise from clusters of approximately uniform size that are 1, 2, 3, 4, and 5-fold ionized . For the TOF-spectrum of Fig . 3** the experimental conditions of the cluster source were chosen such that smaller clusters were produced . By taking similar spectra with low electron energy and current, that is with a low probability for multiple ionisation, the peak at the right is identified as that of singly ionized clusters of ;z~ 3400 nitrogen molecules . The broad centre peak is attributed to cluster ionscarrying2-3 charges each . In contrast to Fig. 2, the different charge states are
• Figs . 2 and 3 show superpositions of approximately 10 traces each_ ` resolution) compared to Fig. 2 . Int. I Mass Spectrom. Ion Phys., 5 (1970) 249-254
252
W. HENKES, G . ISENBERG
no longer resolved* . At the left there is an indication of a peak at small ToF, corresponding to specific sizes centered around NJZ x 135**. These cluster ions may be the result of fragmentation of larger multiply charged cluster ions due to the electrostatic repulsion between charges .' his peak is not observed at low anode voltage (U ;S 100 V) and low electron current (It S 1 mA), that is when the
Fig. 3 . ToF-spectrum of nitrogen cluster ions . Average size of singly charged ions N ~ 3400, mean specific size N/Z z 1130. Time scale 50 psec per division . probability for multiple ionization is low . The mean specific size derived from the TOF-spectra depends on the electron energy qualitatively in the same manner as observed in ref. 1 . In addition we find that _NiZ decreases nmonotonically with rising electron current .
DISCUSSION Contrary to the conditions chosen in ref. 1, collisions with more than one electron are highly probable due to the increase both in electron current density and in length of the ionization zone_ Thus multiple ionization may be caused not only by one electron of sufficiently high energy but also by the interaction of a cluster with more than one electron . The peaks in Fig_ 2 can be interpreted either as being due to clusters carrying i - 5 charges respective-y or to clusters of 1/2 - 1(5 the original size with .)nly one charge each . The latter assumption, however, would call for a splitting of :<. cluster into fractions of nearly equal size, which is rather improbable on account of the large change in surface energy required . Most likely on energetic grounds v : Duld be the emission of one or more molecular ions carrying along their neighbouring molExcept for the lower resolution this is, at least in part, due to the relative width of the distribution of cluster sizes which is found to be larger for cluster beams of small mean cluster size . ss An evaluation of the distribution function would tend, because of the I/1 2 -correction, to shift the maximum of fragment ions to considerably lower specific sizes .
Int. J. Mass Spectr-m. Ion Phys., 5 (1970) 249-254
MULTIPLY CHARGED CLUSTER IONS OF NITROGEN
253
ecules that are bound to them by polarization forces . This ion field emission may account for the peak of small cluster ions observed in Fig . 3. The results presented in Fig. 2 show that a cluster of 22000 nitrogen molecules is able to support at least five positive charges . If one assumes four of these to be distributed uniformly on the surface of a spherical cluster one arrives at an electrical field of 1 .6 x 10 8 V/m acting upon the fifth . There seem to be no experimental data on field ion emission from nitrogen . Therefore we made a rough estimate of the size limit of a stable, doubly charged nitrogen cluster as follows . A cluster of spherical shape with two elementary charges situated at its poles is assumed to be divided into two fragments, also of spherical shape, carrying one charge each . The size of the fragments is chosen such that the difference between the surface energy required and the energy gained from the electrical field s minimized_ The minimum stable cluster size is found byvarying the cluster size until the energy difference equals zero . Since the temperature of the cluster ions is not known. we assume that their vapour pressure is equal to the background pressure of the vacuum chamber (-- 10 -6 tort) . The temperature determined in this way is 22-23'K for clusters of 150-300 molecules . Lacking data for solid nitrogen, we find the surface tension by linear extrapolation from liquid nitrogen data' . Under this assumption we find the stability limit to be at N = 155 molecules per cluster . The size of the two fragments is found to be N i = 7 and N2 = 148 molecules respectively_ The field strength of one charge acting upon the other equals 3 .3 x 108 V/m*. Thus the estimate yields a limiting field strength about twice as high as the field strength observed in the experiment** . This can be regarded as another piece of evidence that the cluster ions observed in Fig . 2 are in fact multiply charged and stable .
ACKNOWLEDGEMENTS
We gratefully acknowledge Prof. E. W. Becker's interest and support throughout the work reported here . Dr. K . Buchheit assisted in setting up the experiments.
* This is lower by two orders of magnitude than the field required for field ion emission from tungsten. *s Even if we assume the cluster temperature to be as high as that of the triple point (vapour pressure 250 torr for a cluster of 300 molecules!) we arrive at a field strength of 2x IOa Vim (N3 = 288, Ni = 13, Ni = 275) . Int. J. Mass Spectrom. Ion Phys., 5 (1970) 249-254
254
W . HENKES, G . ISENBERG
REFERENCES I- H . D . FALTER, O . F . HAGEL1, W litNtcrS AND H. V . WEDEL, Int . J. Mass Spectrum_ Ion Phys., 4 (1970) 145. 2 E, W. BECxER, K . BIER £A cD W . FIENKES, z. Physik, 146 (1956) 333 ; E. W. BECKER. IL KLINGELHOFER ANT) P . LoysE. Z. R'aturforsch_, 17a (1962) 432 . 3 K . BucInnaT AND W. HENxEs. Z. Angew_ Phys . . 24 (1968) 1914 W . C . WILEY AND I . H . McLAREN, Rev. Sci. Instr., 26 (1955) 1150. 5 V. J. ]olnvsoN, Properties of Materials atLow Temperature, (Phase]), Pergamon, Oxford, 1961 . Inc. J. Mass Spectrom. Ion Phys., 5 (1970) 249-254