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Charge stripping effects from highly charged iodine ions formed from Coulomb explosion of CH3I clusters
Chemical Physics 239 Ž1998. 309–315
Charge stripping effects from highly charged iodine ions formed from Coulomb explosion of CH 3 I clusters L. Poth, Q. Zhong, J.V. Ford, S.M. Hurley, A.W. Castleman Jr.
)
152 DaÕey Laboratory, Department of Chemistry, The PennsylÕania State UniÕersity, UniÕersity Park, PA 16802, USA Received 6 April 1998
Abstract Iodine ions of high charge states are observed upon irradiation of methyl iodide clusters with an intense femtosecond laser pulse. All signals from multicharged ions exhibit a peak splitting in the time-of-flight mass spectra, indicating their origin from a Coulomb explosion process. These main peaks are accompanied by smaller peaks attributed to field ionization of highly charged species in the ion optics of the TOF mass spectrometer. It is shown that highly charged atomic ions formed from Coulomb explosion, upon interaction with electric field close to the mesh, can lose another electron leading to the formation of even higher charged species. The observation of this charge stripping process is evidence for the formation of highly excited ions in the course of the Coulomb explosion process, providing new insights into the mechanisms of femtosecond ionization involving multi-electron loss. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In this communication we report on the observation that highly charged iodine species formed from Coulomb explosion of methyl iodide clusters can, upon interaction with meshes of the time-of-flight ŽTOF. apparatus, form even higher charged species according to: I nq™ I Ž nq1.qq ey Iodine ions of high charge states up to I 13q has been observed upon irradiation of methyl iodide clusters with an intense femtosecond laser pulse. All signals from multicharged atomic ions exhibit a peak splitting in the TOFMS spectra indicating their origin from a Coulomb explosion process. )
Corresponding author.
The formation of multicharged iodine and argon ions as highly charged as I 17q and Ar 8q have been reported from our earlier laboratory studies of the ionization of hydrogen iodide clusters, as well as co-clusters of HI and argon, upon interaction with femtosecond laser pulses w1x. Significantly, average kinetic energy releases of several hundred eV were determined in the process of Coulomb explosion of these clusters, with maximum values extending into the several keV energy range. Comparable values of kinetic energy carried by ionic fragments have been measured for a variety of small to medium-size clusters including noble gas clusters seeded with trace amounts of hydrogen iodide w2,3x, acetone clusters w4x and ammonia clusters w5x. The kinetic energy released in the process of Coulomb explosion causes the ion signal to display split peaks, usually comprised of two, and sometimes
0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 8 8 - 2
310
L. Poth et al.r Chemical Physics 239 (1998) 309–315
three, parts. The peak with shorter flight-time arises from ions ejected along the axis of the TOF spectrometer directly towards the detector. Ions ejected in the opposite direction, towards the repeller plate of the ion source, are turned around in the TOF acceleration lens assembly and arrive at the detector at a later time. They produce a peak with a longer flighttime. Energy focusing during the turn-around in the ion source causes the backwards ejected ions to appear as a sharp ion signal, whereas the forward ejected ions directly resemble the kinetic energy distribution of the Coulomb explosion process. The splitting between forward and backward ejected ions, as well as an analysis of the peak shapes, and separately the use of a reflectron as an energy analyzer, allow for independent determinations of the kinetic energy distribution of the formed ions. A detailed analysis of the kinetic energy distributions under different cluster and laser fluence conditions will be subject of a subsequent paper w6x. Herein, we report an accompanying effect observed during studying the Coulomb explosion of methyl iodide clusters, which contributes to a more complete understanding of the overall mechanisms of femtosecond ionization involving multi-electron loss.
2. Experimental The laser based reflectron TOF mass spectrometer used in our studies has been described in detail in previous publications w7,8x. Briefly, neutral clusters comprised of CH 3 I are formed by supersonic expansion of an 8% methyl iodide in argon gas mixture through a pulsed nozzle Ž150 mm diameter. and then ionized by a focused Ž f s 50 cm. laser beam. The laser system is a mode-locked Ti:sapphire femtosecond laser which has an oscillator ŽSpectra Physics, Tsunami 3955. pumped by a 10 W Arq laser ŽSpectra Physics, Beam Lock 2060. and a regenerative amplifier ŽPositive Light. pumped by a 10 Hz Nd:YAG laser ŽSpectra Physics, GCR 150-10.. Following frequency doubling, the final output is typically 1 mJrpulse at 400 nm with a pulse duration around 100 fs. In order to reduce the pump oil background, the ionization region is differentially pumped by a turbomolecular pump. The ionic clusters are accelerated in a Wiley–McLaren type lens
assembly and focused with a group of Einzel lenses. The ion source consists of a solid back plate, which in these experiment is held at a potential of q4000 V, an accelerator mesh Žfirst mesh. held at q3317 V at a distance 1 cm from the back plate, and a second grounded mesh 1 cm behind the accelerator mesh. Both meshes are 20 lines per inch nickel crosswires consisting of oval-shaped wires with a diameter of 0.1 mm in the plane of the mesh and having a diameter of 0.005 mm across. After leaving the ion source chamber, the ions enter a 145-cm-long fieldfree region which terminates at a reflectron where they are turned and travel another 100 cm towards a chevron microchannel plate ŽMCP. used for detection. Varying the potentials on the reflectron allows for a precise determination of the kinetic energy of the ions under investigation.
3. Results and discussion Fig. 1 shows a typical mass spectrum obtained from ionization of methyl iodide clusters with a 400 nm femtosecond laser pulse. Signals arising from ions primarily formed in the Coulomb explosion process, e.g. I 2q, exhibit a peak splitting pattern due to the mechanism described in Section 1. These can be easily distinguished from other ions, e.g. the methyl iodide ion, which are not primarily formed in the Coulomb explosion process and therefore do not show a peak splitting pattern. The inset in Fig. 1 is a magnified view of the I 2q signal and shows that the main peak is accompanied by two satellite peaks which exhibit the same peak splitting pattern, indicating their origin from a Coulomb explosion process. Unlike the main peaks in the mass spectrum, these satellite peaks cannot be calibrated to an even mass-to-charge ratio. A detailed analysis of the measured arrival times of these satellite peaks showed that they can be assigned to multiply charged ions which increase their charge state when passing through one of the meshes of our TOF lens. With all distances of the TOF mass spectrometer known, the arrival times of ions can be calculated precisely by solving the TOF equations. Typically these closely match the experimental arrival times; for the lower mass range to within "150 ns. An ion with charge z that is
L. Poth et al.r Chemical Physics 239 (1998) 309–315
311
Fig. 1. A typical mass spectrum showing multiply charged iodine ions obtained by irradiating methyl iodide clusters with a femtosecond laser pulse centered at 400 nm. The inset shows a magnified portion of the mass range around I 2q with the satellite peaks resulting from the interaction of multiply charged ions with meshes in the TOF lens.
generated at the birth potential, U0 , in the repeller region of the TOF lens and changes its charge to z q 1 at the first mesh ŽU2 ., will have a slightly later arrival time than an ion that is directly formed in charge state z q 1, since it experiences a lower acceleration in the first stage of the ion source. On the other hand, an ion generated at the birth potential with charge state z that gains an additional charge at the second grounded mesh or upon interaction with the reflectron mesh will have a slightly earlier arrival time than an ion formed in charge state z that does not gain an additional charge. The earlier arrival time in the later case is caused by the shorter depth of penetration of the ion into the reflectron due to its higher charge state and faster deceleration. Table 1 compares calculated and experimental arrival times for iodine ions of charge states q1 to q4, assuming they change their charge state by q1 on the first or second mesh of the TOF lens. Also tabulated are experimental and calculated arrival times for ions which do not change their charge state on either mesh. For assignment of the experimental arrival time, the center between backward and forward peaks
is used for assigning all ions. The last column of Table 1 lists the deviation between experimental and calculated arrival times and shows that all calculated values are within the error limits of "150 ns. Although the measured arrival times for the satellite peaks strongly indicate their origin from ions interacting with the meshes of the TOF lens, a second procedure as described in what follows has been applied to definitively assign this peaks. Using the reflectron as an energy analyzer, and by dropping the voltage applied to the reflectron, allows a determination of the kinetic energy of the ions by monitoring their disappearance in the mass spectrum whereby they pass through the reflectron instead of being reflected towards the detector. The reflectron voltage, UT , at which this disappearance occurs is called the cut-off voltage of the ion. An ion generated at the birth potential U0 and accelerated out of the TOF lens will have a kinetic energy equal to the product of its charge times this potential U0 . Ions formed from Coulomb explosion contain additional kinetic energy, EKER , gained from the repulsion in the Coulomb explosion process which adds to the
L. Poth et al.r Chemical Physics 239 (1998) 309–315
312
Table 1 Experimental and calculated arrival times for multiply charged ions and signals attributed to ions formed from charge stripping on the meshes of the TOF lens Ion or reaction Iq Iq™ I 2q Iq™ I 2q I 2q I 2q™ I 3q I 2q™ I 3q I 3q I 3q™ I 4q I 3q™ I 4q I 4q I 4q™ I 5q I 4q™ I 5q I 5q
Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh.
Exp. arrival time Žms.
Calc. arrival time Žms.
Deviation Žns.
34.760 25.531 34.167 24.599 20.553 24.284 20.035 17.708 19.876 17.425 15.795 17.218 15.572
34.763 25.516 34.335 24.581 20.585 24.418 20.070 17.730 19.994 17.381 15.808 17.345 15.546
q3 q15 y168 q18 y32 y134 y35 y22 y118 q44 y13 y127 q26
kinetic energy gained in the acceleration. The kinetic energy gained from the Coulomb explosion can be determined from the peak splitting observed for multiply charged ions according to: EKER s
Ž U1 y U2 . 8 md 2
UT s
2
q2 Dt2
Ž 1.
where U1 and U2 are the potentials of the repeller plate and the first acceleration mesh, d is the distance between these plates, q is the charge of the ion and D t is the difference in arrival times between forward and backward ejected ions w9x. An ion that is generated at U0 in a certain charge state z and subsequently changes its charge state through interaction with one of the meshes in the TOF lens, will arrive at the reflectron with a kinetic energy specific to its initial charge state and of the mesh at which it gains the additional charge. The cut-off voltage, UT , at the reflectron for an ion that changes its charge state from z to z q 1 on the first mesh in the TOF lens, can be calculated according to: UT s
Ž zU0 q EKER q U2 . zq1
Accordingly, the cut-off voltage for an ion which gains one additional charge through interaction with the second mesh can be calculated using Eq. Ž3..
.
Ž 2.
Here, z is the initial charge state, U0 the birth potential, EKER the initial kinetic energy in eV derived from the peak splitting measurement of the Coulomb explosion peak, and U2 the voltage on the first mesh.
Ž zU0 q EKER . zq1
.
Ž 3.
A reflectron voltage scan for multicharged iodine ions and their satellite peaks is shown in Fig. 2. The reflectron voltage of 3770 V is above the birth potential and all ions are reflected towards the detector. A reflectron voltage of 3700 V is below the birth potential; here the multicharged species formed in the Coulomb explosion penetrate through the reflectron, and their peaks disappear from the mass spectrum. The satellite peaks remain observable for reflectron voltage settings below the birth potential, and exhibit characteristic cut-off voltages. Table 2 is a comparison of cut-off voltages calculated using Eqs. Ž2. and Ž3. with cut-off voltages determined experimentally, assuming the same charge stripping reactions assigned using the arrival times for these ions. The labels in the last column of Table 2 refer to those displayed in Fig. 2 to identify peaks in the reflectron voltage scan. For all peaks, a close match between calculated and experimental voltages is found, again indicating the origin of the satellite peaks from interactions of multiply charged ions with the meshes. For singly charged cluster ions not formed from a Coulomb explosion process, no indication of satellite
L. Poth et al.r Chemical Physics 239 (1998) 309–315
313
Fig. 2. Stack plot of the changes in the mass spectrum observed when the reflectron voltage is dropped. The main peaks and satellite peaks cut off at different reflectron voltage settings. The trace at a reflectron setting of 3770 V showing the main peaks is reduced in size. For the peak assignment see Table 2.
peaks can be found. This strongly points towards a connection between the multi-electron ionization and the loss of an additional electron on the meshes. The observation that highly excited ions can undergo surface induced transitions of the type X qq ) ™ X Ž qq1.qq ey
Ž 4. q
has been first reported for excited Ar ions formed by electron impact ionization of argon atoms w10,11x and the interaction of highly excited Rydberg atoms
with surfaces is a frequently observed phenomenon w12–14x. For interactions of ions with metal surfaces, four types of interactions can be identified: resonance ionization, resonance neutralization, Auger de-excitation, and Auger neutralization. However, only the first of these mechanisms leads to formation of a higher charged ion. Resonance ionization can occur when the work function of the metal is greater than the energy required to bring the highest excited electron of the ion into the vacuum level. Resonance
Table 2 Experimental and calculated cut-off voltages for signals attributed to ions formed from charge stripping on the meshes of the TOF lens Ion reaction Iq™ I 2q Iq™ I 2q I 2q™ I 3q I 2q™ I 3q I 3q™ I 4q I 3q™ I 4q I 4q™ I 5q
Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž2nd mesh.
Exp. cut-off voltage ŽV.
Calc. cut-off voltage ŽV.
Label in Fig. 2
3490 1800 3580 2500 3640 2820 3020
3509 1850 3584 2480 3631 2802 3002
Ža. not included Žc. Žb. Že. Žd. Žf.
314
L. Poth et al.r Chemical Physics 239 (1998) 309–315
ionization requires a direct interaction with the metal surface and is therefore restricted only to ions passing by very close to the nickel wires. Since the meshes in our experimental setup are held at high potentials, another type of interaction can occur caused by the electric field gradient in the vicinity of the wire. Although the electric field between the meshes in a TOF lens is usually assumed to be homogeneous, this is only true for distances larger than the distance between two wires in the mesh. Close to the wire surface, to a first approximation the electrical field is cylindrical with an electric field strength up to several thousand volts per centimeter; this comprises a field strength high enough to further ionize a highly excited Rydberg ion by a field ionization process. Experiments were performed where a third mesh at ground potential was placed after the Einzel lens in the field-free region of the TOFMS. If a surface induced process is operative it should lead to an increase of the ion signal attributed to ions becoming further ionized at the second mesh. No change in the ratio of ionization attributed to first and second mesh could be observed indicating that a field ionization mechanism is operative. Whatever mechanism is operative, the further ionization of these multiply charged ions always requires that these ions are in an excited state before interaction with the electric field provided by the mesh. This fact is also evident from the observation that similar processes cannot be detected for singly charged cluster ions that are not formed from a Coulomb explosion process. The observation that highly excited Rydberg ions are formed in many-electron multiphoton-ionization is another valuable finding in efforts to understand the mechanism of this multi-electron ionization process. The ionization ignition model ŽIIM. w15x, one of the two frequently discussed models to describe the formation of multiply charged ions, is assumed to be driven by the large space charge field within the cluster generated by the high initial density of ions. Whereas during the initial stage ionization occurs via field ionization and barrier suppression ionization w16–18x, formation of highly excited Rydberg ions seems not feasible during the initial events but is possible at a later stage when the cluster is rapidly expanding and the charge density decreases. A sec-
ond model used to describe the multi-electron ionization is called the coherent electron motion model ŽCEMM. w19,20x. CEMM proposes that due to the coherent motion of the field ionized electrons induced by the external driving laser field, excitation of the atoms in the cluster, generation of inner-core vacancies, and concomitant X-ray emission are greatly enhanced, and the minimum threshold laser intensity and corresponding critical cluster size are significantly lowered. Resulting electron collisions, and also X-ray emissions could be possible sources for exciting already formed ions to various Rydberg states. In summary, whether the Coulomb explosion process in clusters is initiated through the CEMM w19,20x, IGM w15x, or related processes proposed by Bandrauk et al. w16,17x, Corkum et al. w18x, or Jortner et al. w21,22x, the interaction of a highly intense femtosecond laser with clusters clearly leads to the enhanced production of multiply charged ions in high Rydberg states that are readily field ionized.
Acknowledgements This research was conducted with financial support by the Air Force Office of Scientific Research, Grant No. F9620-97-1-0183, which is gratefully acknowledged.
References w1x J. Purnell, E.M. Snyder, S. Wei, A.W. Castleman Jr., Chem. Phys. Lett. 229 Ž1994. 333. w2x A.W. Castleman Jr., E.M. Snyder, S.A. Buzza, J. Purnell, S. Wei, Proceedings of the Yamada Conference XLIII on Structures and Dynamics of Clusters, Frontiers Science Series No. 16, Universal Academy Press, Tokyo, Japan. w3x E.M. Snyder, S.A. Buzza, A.W. Castleman Jr., Phys. Rev. Lett. 77 Ž1996. 3347. w4x S.A. Buzza, E.M. Snyder, D.A. Card, D.E. Folmer, A.W. Castleman Jr., J. Chem. Phys. 105 Ž1996. 7425. w5x E.M. Snyder, S. Wei, J. Purnell, S.A. Buzza, A.W. Castleman Jr., Chem. Phys. Lett. 248 Ž1996. 1. w6x J.V. Ford, L. Poth, Q. Zhong, A.W. Castleman, Jr., to be published. w7x Z. Shi, J.V. Ford, S. Wei, A.W. Castleman Jr., J. Chem. Phys. 99 Ž1993. 8009. w8x L. Poth, Z. Shi, Q. Zhong, A.W. Castleman Jr., Int. J. Mass Spectrom. Ion Proc. 154 Ž1996. 35.
L. Poth et al.r Chemical Physics 239 (1998) 309–315 w9x J. Purnell, E.M. Snyder, S. Wei, A.W. Castleman Jr., Chem. Phys. Lett. 229 Ž1994. 333. w10x A.S. Newton, A.F. Sciamanna, R. Clampitt, J. Chem. Phys. 46 Ž1967. 1779. w11x A.S. Newton, A.F. Sciamanna, R. Clampitt, J. Chem. Phys. 47 Ž1967. 4843. w12x C.A. Kocher, G.E. McCown, Science 273 Ž1996. 307. w13x C.A. Kocher, G.E. McCown, Surf. Sci. 244 Ž1991. 321. w14x C. Fabre, M. Gross, J.M. Raimond, S. Haroche, J. Phys.: At. Mol. Phys. 16 Ž1983. L671. w15x C. Rose-Petruck, K.J. Schafer, C.P.J. Barty, Applications of Laser Plasma Radiation II, SPIE 272 Ž1995. 2523. w16x S. Chelkowski, A.D. Bandrauk, J. Phys. B: At. Mol. Opt. Phys. 28 Ž1995. L1.
315
w17x S. Chelkowski, A. Conjusteau, T. Zuo, A.D. Bandrauk, Phys. Rev. A 54 Ž1996. 3235. w18x T. Seideman, M.Y. Ivanov, P.B. Corkum, Phys. Rev. Lett. 75 Ž1995. 2819. w19x K. Boyer, B.D. Thompson, A. McPherson, C.K. Rhodes, J. Phys. B: At. Mol. Opt. Phys. 27 Ž1994. 4373. w20x B.D. Thompson, A. McPherson, K. Boyer, C.K. Rhodes, J. Phys. B: At. Mol. Opt. Phys. 27 Ž1994. 4391. w21x I. Last, I. Schek, J. Jortner, J. Chem. Phys. 107 Ž1997. 6685. w22x I. Last, J. Jortner, Theoretical study of multielectron dissociative ionization of diatomics and clusters in a strong laser field, submitted.
Charge stripping effects from highly charged iodine ions formed from Coulomb explosion of CH 3 I clusters L. Poth, Q. Zhong, J.V. Ford, S.M. Hurley, A.W. Castleman Jr.
)
152 DaÕey Laboratory, Department of Chemistry, The PennsylÕania State UniÕersity, UniÕersity Park, PA 16802, USA Received 6 April 1998
Abstract Iodine ions of high charge states are observed upon irradiation of methyl iodide clusters with an intense femtosecond laser pulse. All signals from multicharged ions exhibit a peak splitting in the time-of-flight mass spectra, indicating their origin from a Coulomb explosion process. These main peaks are accompanied by smaller peaks attributed to field ionization of highly charged species in the ion optics of the TOF mass spectrometer. It is shown that highly charged atomic ions formed from Coulomb explosion, upon interaction with electric field close to the mesh, can lose another electron leading to the formation of even higher charged species. The observation of this charge stripping process is evidence for the formation of highly excited ions in the course of the Coulomb explosion process, providing new insights into the mechanisms of femtosecond ionization involving multi-electron loss. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In this communication we report on the observation that highly charged iodine species formed from Coulomb explosion of methyl iodide clusters can, upon interaction with meshes of the time-of-flight ŽTOF. apparatus, form even higher charged species according to: I nq™ I Ž nq1.qq ey Iodine ions of high charge states up to I 13q has been observed upon irradiation of methyl iodide clusters with an intense femtosecond laser pulse. All signals from multicharged atomic ions exhibit a peak splitting in the TOFMS spectra indicating their origin from a Coulomb explosion process. )
Corresponding author.
The formation of multicharged iodine and argon ions as highly charged as I 17q and Ar 8q have been reported from our earlier laboratory studies of the ionization of hydrogen iodide clusters, as well as co-clusters of HI and argon, upon interaction with femtosecond laser pulses w1x. Significantly, average kinetic energy releases of several hundred eV were determined in the process of Coulomb explosion of these clusters, with maximum values extending into the several keV energy range. Comparable values of kinetic energy carried by ionic fragments have been measured for a variety of small to medium-size clusters including noble gas clusters seeded with trace amounts of hydrogen iodide w2,3x, acetone clusters w4x and ammonia clusters w5x. The kinetic energy released in the process of Coulomb explosion causes the ion signal to display split peaks, usually comprised of two, and sometimes
0301-0104r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 8 8 - 2
310
L. Poth et al.r Chemical Physics 239 (1998) 309–315
three, parts. The peak with shorter flight-time arises from ions ejected along the axis of the TOF spectrometer directly towards the detector. Ions ejected in the opposite direction, towards the repeller plate of the ion source, are turned around in the TOF acceleration lens assembly and arrive at the detector at a later time. They produce a peak with a longer flighttime. Energy focusing during the turn-around in the ion source causes the backwards ejected ions to appear as a sharp ion signal, whereas the forward ejected ions directly resemble the kinetic energy distribution of the Coulomb explosion process. The splitting between forward and backward ejected ions, as well as an analysis of the peak shapes, and separately the use of a reflectron as an energy analyzer, allow for independent determinations of the kinetic energy distribution of the formed ions. A detailed analysis of the kinetic energy distributions under different cluster and laser fluence conditions will be subject of a subsequent paper w6x. Herein, we report an accompanying effect observed during studying the Coulomb explosion of methyl iodide clusters, which contributes to a more complete understanding of the overall mechanisms of femtosecond ionization involving multi-electron loss.
2. Experimental The laser based reflectron TOF mass spectrometer used in our studies has been described in detail in previous publications w7,8x. Briefly, neutral clusters comprised of CH 3 I are formed by supersonic expansion of an 8% methyl iodide in argon gas mixture through a pulsed nozzle Ž150 mm diameter. and then ionized by a focused Ž f s 50 cm. laser beam. The laser system is a mode-locked Ti:sapphire femtosecond laser which has an oscillator ŽSpectra Physics, Tsunami 3955. pumped by a 10 W Arq laser ŽSpectra Physics, Beam Lock 2060. and a regenerative amplifier ŽPositive Light. pumped by a 10 Hz Nd:YAG laser ŽSpectra Physics, GCR 150-10.. Following frequency doubling, the final output is typically 1 mJrpulse at 400 nm with a pulse duration around 100 fs. In order to reduce the pump oil background, the ionization region is differentially pumped by a turbomolecular pump. The ionic clusters are accelerated in a Wiley–McLaren type lens
assembly and focused with a group of Einzel lenses. The ion source consists of a solid back plate, which in these experiment is held at a potential of q4000 V, an accelerator mesh Žfirst mesh. held at q3317 V at a distance 1 cm from the back plate, and a second grounded mesh 1 cm behind the accelerator mesh. Both meshes are 20 lines per inch nickel crosswires consisting of oval-shaped wires with a diameter of 0.1 mm in the plane of the mesh and having a diameter of 0.005 mm across. After leaving the ion source chamber, the ions enter a 145-cm-long fieldfree region which terminates at a reflectron where they are turned and travel another 100 cm towards a chevron microchannel plate ŽMCP. used for detection. Varying the potentials on the reflectron allows for a precise determination of the kinetic energy of the ions under investigation.
3. Results and discussion Fig. 1 shows a typical mass spectrum obtained from ionization of methyl iodide clusters with a 400 nm femtosecond laser pulse. Signals arising from ions primarily formed in the Coulomb explosion process, e.g. I 2q, exhibit a peak splitting pattern due to the mechanism described in Section 1. These can be easily distinguished from other ions, e.g. the methyl iodide ion, which are not primarily formed in the Coulomb explosion process and therefore do not show a peak splitting pattern. The inset in Fig. 1 is a magnified view of the I 2q signal and shows that the main peak is accompanied by two satellite peaks which exhibit the same peak splitting pattern, indicating their origin from a Coulomb explosion process. Unlike the main peaks in the mass spectrum, these satellite peaks cannot be calibrated to an even mass-to-charge ratio. A detailed analysis of the measured arrival times of these satellite peaks showed that they can be assigned to multiply charged ions which increase their charge state when passing through one of the meshes of our TOF lens. With all distances of the TOF mass spectrometer known, the arrival times of ions can be calculated precisely by solving the TOF equations. Typically these closely match the experimental arrival times; for the lower mass range to within "150 ns. An ion with charge z that is
L. Poth et al.r Chemical Physics 239 (1998) 309–315
311
Fig. 1. A typical mass spectrum showing multiply charged iodine ions obtained by irradiating methyl iodide clusters with a femtosecond laser pulse centered at 400 nm. The inset shows a magnified portion of the mass range around I 2q with the satellite peaks resulting from the interaction of multiply charged ions with meshes in the TOF lens.
generated at the birth potential, U0 , in the repeller region of the TOF lens and changes its charge to z q 1 at the first mesh ŽU2 ., will have a slightly later arrival time than an ion that is directly formed in charge state z q 1, since it experiences a lower acceleration in the first stage of the ion source. On the other hand, an ion generated at the birth potential with charge state z that gains an additional charge at the second grounded mesh or upon interaction with the reflectron mesh will have a slightly earlier arrival time than an ion formed in charge state z that does not gain an additional charge. The earlier arrival time in the later case is caused by the shorter depth of penetration of the ion into the reflectron due to its higher charge state and faster deceleration. Table 1 compares calculated and experimental arrival times for iodine ions of charge states q1 to q4, assuming they change their charge state by q1 on the first or second mesh of the TOF lens. Also tabulated are experimental and calculated arrival times for ions which do not change their charge state on either mesh. For assignment of the experimental arrival time, the center between backward and forward peaks
is used for assigning all ions. The last column of Table 1 lists the deviation between experimental and calculated arrival times and shows that all calculated values are within the error limits of "150 ns. Although the measured arrival times for the satellite peaks strongly indicate their origin from ions interacting with the meshes of the TOF lens, a second procedure as described in what follows has been applied to definitively assign this peaks. Using the reflectron as an energy analyzer, and by dropping the voltage applied to the reflectron, allows a determination of the kinetic energy of the ions by monitoring their disappearance in the mass spectrum whereby they pass through the reflectron instead of being reflected towards the detector. The reflectron voltage, UT , at which this disappearance occurs is called the cut-off voltage of the ion. An ion generated at the birth potential U0 and accelerated out of the TOF lens will have a kinetic energy equal to the product of its charge times this potential U0 . Ions formed from Coulomb explosion contain additional kinetic energy, EKER , gained from the repulsion in the Coulomb explosion process which adds to the
L. Poth et al.r Chemical Physics 239 (1998) 309–315
312
Table 1 Experimental and calculated arrival times for multiply charged ions and signals attributed to ions formed from charge stripping on the meshes of the TOF lens Ion or reaction Iq Iq™ I 2q Iq™ I 2q I 2q I 2q™ I 3q I 2q™ I 3q I 3q I 3q™ I 4q I 3q™ I 4q I 4q I 4q™ I 5q I 4q™ I 5q I 5q
Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh.
Exp. arrival time Žms.
Calc. arrival time Žms.
Deviation Žns.
34.760 25.531 34.167 24.599 20.553 24.284 20.035 17.708 19.876 17.425 15.795 17.218 15.572
34.763 25.516 34.335 24.581 20.585 24.418 20.070 17.730 19.994 17.381 15.808 17.345 15.546
q3 q15 y168 q18 y32 y134 y35 y22 y118 q44 y13 y127 q26
kinetic energy gained in the acceleration. The kinetic energy gained from the Coulomb explosion can be determined from the peak splitting observed for multiply charged ions according to: EKER s
Ž U1 y U2 . 8 md 2
UT s
2
q2 Dt2
Ž 1.
where U1 and U2 are the potentials of the repeller plate and the first acceleration mesh, d is the distance between these plates, q is the charge of the ion and D t is the difference in arrival times between forward and backward ejected ions w9x. An ion that is generated at U0 in a certain charge state z and subsequently changes its charge state through interaction with one of the meshes in the TOF lens, will arrive at the reflectron with a kinetic energy specific to its initial charge state and of the mesh at which it gains the additional charge. The cut-off voltage, UT , at the reflectron for an ion that changes its charge state from z to z q 1 on the first mesh in the TOF lens, can be calculated according to: UT s
Ž zU0 q EKER q U2 . zq1
Accordingly, the cut-off voltage for an ion which gains one additional charge through interaction with the second mesh can be calculated using Eq. Ž3..
.
Ž 2.
Here, z is the initial charge state, U0 the birth potential, EKER the initial kinetic energy in eV derived from the peak splitting measurement of the Coulomb explosion peak, and U2 the voltage on the first mesh.
Ž zU0 q EKER . zq1
.
Ž 3.
A reflectron voltage scan for multicharged iodine ions and their satellite peaks is shown in Fig. 2. The reflectron voltage of 3770 V is above the birth potential and all ions are reflected towards the detector. A reflectron voltage of 3700 V is below the birth potential; here the multicharged species formed in the Coulomb explosion penetrate through the reflectron, and their peaks disappear from the mass spectrum. The satellite peaks remain observable for reflectron voltage settings below the birth potential, and exhibit characteristic cut-off voltages. Table 2 is a comparison of cut-off voltages calculated using Eqs. Ž2. and Ž3. with cut-off voltages determined experimentally, assuming the same charge stripping reactions assigned using the arrival times for these ions. The labels in the last column of Table 2 refer to those displayed in Fig. 2 to identify peaks in the reflectron voltage scan. For all peaks, a close match between calculated and experimental voltages is found, again indicating the origin of the satellite peaks from interactions of multiply charged ions with the meshes. For singly charged cluster ions not formed from a Coulomb explosion process, no indication of satellite
L. Poth et al.r Chemical Physics 239 (1998) 309–315
313
Fig. 2. Stack plot of the changes in the mass spectrum observed when the reflectron voltage is dropped. The main peaks and satellite peaks cut off at different reflectron voltage settings. The trace at a reflectron setting of 3770 V showing the main peaks is reduced in size. For the peak assignment see Table 2.
peaks can be found. This strongly points towards a connection between the multi-electron ionization and the loss of an additional electron on the meshes. The observation that highly excited ions can undergo surface induced transitions of the type X qq ) ™ X Ž qq1.qq ey
Ž 4. q
has been first reported for excited Ar ions formed by electron impact ionization of argon atoms w10,11x and the interaction of highly excited Rydberg atoms
with surfaces is a frequently observed phenomenon w12–14x. For interactions of ions with metal surfaces, four types of interactions can be identified: resonance ionization, resonance neutralization, Auger de-excitation, and Auger neutralization. However, only the first of these mechanisms leads to formation of a higher charged ion. Resonance ionization can occur when the work function of the metal is greater than the energy required to bring the highest excited electron of the ion into the vacuum level. Resonance
Table 2 Experimental and calculated cut-off voltages for signals attributed to ions formed from charge stripping on the meshes of the TOF lens Ion reaction Iq™ I 2q Iq™ I 2q I 2q™ I 3q I 2q™ I 3q I 3q™ I 4q I 3q™ I 4q I 4q™ I 5q
Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž1st mesh. Ž2nd mesh. Ž2nd mesh.
Exp. cut-off voltage ŽV.
Calc. cut-off voltage ŽV.
Label in Fig. 2
3490 1800 3580 2500 3640 2820 3020
3509 1850 3584 2480 3631 2802 3002
Ža. not included Žc. Žb. Že. Žd. Žf.
314
L. Poth et al.r Chemical Physics 239 (1998) 309–315
ionization requires a direct interaction with the metal surface and is therefore restricted only to ions passing by very close to the nickel wires. Since the meshes in our experimental setup are held at high potentials, another type of interaction can occur caused by the electric field gradient in the vicinity of the wire. Although the electric field between the meshes in a TOF lens is usually assumed to be homogeneous, this is only true for distances larger than the distance between two wires in the mesh. Close to the wire surface, to a first approximation the electrical field is cylindrical with an electric field strength up to several thousand volts per centimeter; this comprises a field strength high enough to further ionize a highly excited Rydberg ion by a field ionization process. Experiments were performed where a third mesh at ground potential was placed after the Einzel lens in the field-free region of the TOFMS. If a surface induced process is operative it should lead to an increase of the ion signal attributed to ions becoming further ionized at the second mesh. No change in the ratio of ionization attributed to first and second mesh could be observed indicating that a field ionization mechanism is operative. Whatever mechanism is operative, the further ionization of these multiply charged ions always requires that these ions are in an excited state before interaction with the electric field provided by the mesh. This fact is also evident from the observation that similar processes cannot be detected for singly charged cluster ions that are not formed from a Coulomb explosion process. The observation that highly excited Rydberg ions are formed in many-electron multiphoton-ionization is another valuable finding in efforts to understand the mechanism of this multi-electron ionization process. The ionization ignition model ŽIIM. w15x, one of the two frequently discussed models to describe the formation of multiply charged ions, is assumed to be driven by the large space charge field within the cluster generated by the high initial density of ions. Whereas during the initial stage ionization occurs via field ionization and barrier suppression ionization w16–18x, formation of highly excited Rydberg ions seems not feasible during the initial events but is possible at a later stage when the cluster is rapidly expanding and the charge density decreases. A sec-
ond model used to describe the multi-electron ionization is called the coherent electron motion model ŽCEMM. w19,20x. CEMM proposes that due to the coherent motion of the field ionized electrons induced by the external driving laser field, excitation of the atoms in the cluster, generation of inner-core vacancies, and concomitant X-ray emission are greatly enhanced, and the minimum threshold laser intensity and corresponding critical cluster size are significantly lowered. Resulting electron collisions, and also X-ray emissions could be possible sources for exciting already formed ions to various Rydberg states. In summary, whether the Coulomb explosion process in clusters is initiated through the CEMM w19,20x, IGM w15x, or related processes proposed by Bandrauk et al. w16,17x, Corkum et al. w18x, or Jortner et al. w21,22x, the interaction of a highly intense femtosecond laser with clusters clearly leads to the enhanced production of multiply charged ions in high Rydberg states that are readily field ionized.
Acknowledgements This research was conducted with financial support by the Air Force Office of Scientific Research, Grant No. F9620-97-1-0183, which is gratefully acknowledged.
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