- Email: [email protected]
Nascent kinetic energy distributions of the ions produced by electron-impact on the CH3F molecule
22 September 2000
Chemical Physics Letters 328 Ž2000. 135–141 www.elsevier.nlrlocatercplett
Nascent kinetic energy distributions of the ions produced by electron-impact on the CH 3 F molecule ´ b, M.N. Sanchez I. Torres a , R. Martinez Rayo a , F. Castano ´ ˜ a,) a
´ ´ ´ Vasco, Facultad de Ciencias, Apart 644, 48080 Bilbao, Spain Departamento de Quimica Fisica, UniÕersidad del Pais b Facultad de Farmacia, Paseo de la UniÕersidad, 7, 01006 Vitoria, Spain Received 26 April 2000; in final form 6 July 2000
Abstract The nascent translational energy distributions of ions produced by electron-impact on CH 3 F, prepared in a skimmed supersonic expansion, have been investigated by their correlation with the time-of-flight mass-spectrometry band profiles. The electron energies used in the collisions range from the ionisation threshold to 100 eV. CH 3 Fq and other ion fragments close in mass are characterized by low average kinetic energies. Medium-sized mass ion fragments, have kinetic energy distributions with average values of ca. 1 eV, whilst atomic ions have the highest average energies. The set of ion nascent energy distributions produced by electron-impact on CH 3 F, at several electron energies, are reported. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Electron-impact studies on molecules are of crucial interest in the modelling of plasma processes, either industrial ones w1x or those in atmospheric science w2x. Among the molecules studied by electron-impact in the laboratory, the family of fluoromethanes are benchmark species due to their essential role in the development of semiconductor VLSI technology w3x and its relevance in the chemistry of the ionosphere. Electron-impact collisions on molecules yields a plethora of neutral excited states, radicals and ions. The ions may be produced by the direct ionisation process – extraction of one electron either directly from the parent molecule or by dissociative ionisation – or by autoionisation w4x. The ) Corresponding author. Fax: q34-94-464-8500; e-mail: [email protected]
number of dissociative ions produced in collisions with electrons of energies up to 100 eV is usually large, the number of fragments matching any combination of the number of constituent atoms w5–8x. Although the main concern in collision process studies has focused on electronic excitation and ions production,w9,10x it seems obvious that the fragments store energy in other internal degrees of freedom, such as vibrational, rotational and translational. Studies of emission and appearance thresholds usually lead to experimental values higher than those predicted by thermodynamic calculations w5,6x and the excess energy is plausibly stored in the fragment’s internal degrees of freedom. The nascent or the thermal vibrational and rotational energy distributions of a few fragments, sometimes identified by their temperatures w11x have been studied by spontaneous emission w12–15x and laser-induced fluorescence ŽLIF. w16–20x, but the paucity of data does not
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 8 6 1 - 7
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I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
allow one to make predictions on their energy contributions to the whole process. Even fundamental studies of the population inversion following electrical discharges in gases have not attracted enough attention to the urgency of acquiring knowledge of these processes. Nascent ion translational energy distributions have previously been studied by two methods, namely, Doppler broadening w17,21x and time-of-flight mass spectrometry ŽTOF-MS.. The former procedure can be applied, in principle, to any fragment but requires high-resolution spectroscopy in accessible spectral regions, which makes experiments difficult in some cases. The TOF-MS has been applied to samples in the bulk w19,22x, to CO 2 in an effusion beam w23x and to hydrazines and ammonia w24x seeded in a skimmed supersonic beam and further ion deflection detection. In this Letter, a well-characterized skimmed pulsed supersonic beam in conjunction with a commercial TOF-MS is used to determine the nascent translational energy distributions of the parent ion and the dissociative ions of the CH 3 F molecule following the electron-impact in the energy range from threshold to 100 eV.
2. Experimental The experimental set-up consists of a commercial linear Wiley–McLaren TOF-MS w25x ŽR.M. Jordan Co.. equipped with both a pulsed molecular supersonic and a monoenergetic electron beam, crossed at right angles in an originally electric field-free volume. The vacuum chamber system holding the electron beam and the valve are arranged so that it may be divided into three well-defined regions, commonly referred to as an ionisationrextraction, acceleration and drift tuberMCP detector w26x. In the actual experiment the 180 V negative pulse applied to the extraction grid 300 ns after the electron–molecule collisions take place, aims to drive the ions into the acceleration region, where a biased electric field of 1400 Vrcm drives the ions further into the 86.5 cm long drift tube, where a couple of x–y plates focus the ions onto a three-stage microchannel plate ŽMCP. detector ŽC-0701, f s 18 mm.. In the ionisation region, the repeller plate and the extraction grid are 1.27 cm apart, so that the actual electric field
applied was ca. 146 Vrcm. The electric response generated by the detector is routed to a digital oscilloscope ŽTektronix TDS360. and later to a computer for further analysis and storage. The electron beam is generated by thermoionic emission from an electrically heated W hairpin filament and subsequently accelerated to a predetermined energy, collimated and focused by two electron lenses controlling the beam spread in the ionisation region. The well-focused beam is estimated to be ca. 3 mm in diameter. There is an additional third lens divided into two equal semicircular pieces, in contact with the repeller plate and the extraction grid respectively, so that the same voltage applied to the plates biases the whole excitation region. Electron gun energy was calibrated with the Arq appearance potential Ž15.75 eV. to an accuracy better than "0.7 eV and varied from 0 to 100 eV; its energy spread depends on the filament heating current and is estimated to be of 1 eV at 100 mA electron intensity. Argon noble gas seeded with the target molecule was prepared in an external stainless-steel cylinder at a total pressure of up to 4 barr, as measured with a capacitance manometer ŽMKS-Baratron 750B. to 1% accuracy. The cylinder was directly connected to a pulsed valve ŽGeneral Valve, mod. IOTA ONE. fitted with an 0.8 mm f nozzle, from where the expansion proceeds to the main vacuum chamber Žat - 10y7 mbar., creating a supersonic beam; the valve open switch pulse is used, with 600 ms delay, to trigger the extraction grid voltage that drives the ions towards the acceleration stage. The central region of the beam is skimmed Ž f s 1 mm. at a distance of 5 cm from the nozzle head, in order to obtain an homogeneous beam w27x.
3. Nascent ion kinetic energy distributions The TOF-MS band profile basically depends on both the initial ion position with respect to the extraction plate and the ion kinetic energy w26,28x. For calibration purposes and to understand the physics behind it, Fig. 1 shows the computed TOF Ž14 amu. ions for the Arq Ž40 amu. and CHq 2 modelled for our experimental, as a parametric function of the ion initial distance to the extraction plate
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
Ž . Fig. 1. Simulated TOF of Arq Ž40 amu. and CHq 2 14 amu ions as a function of the ionisation space position with respect to the extraction plate, s. Curves above and below the nascent zero kinetic energy, U0 s 0 eV Žbold line., refer to ions with velocity directed outwards from Ždashed lines. and towards Žsolid lines. the extraction plate, respectively. The energy increase between any two close lines is 0.1 eV.
Ž s, in cm. and the nascent kinetic energy; the bold curves correspond to ions with initial zero kinetic energy and the other curves to ions with velocities directed to Žthin solid lines. and from Ždashed lines. the MCP detector, in kinetic energy steps of 0.1 eV. A comparison of the theoretical and the experimental
137
bandwidth at variable extraction voltages permits one to determine the distance from the electronr molecule beams crossing point to the extraction grid as ( 0.90 " 0.05 cm, a location where the bandwidth is insensitive to the space position ŽFig. 1. and, hence, the ion kinetic energy is the main factor influencing the band profile w26x. However, before the switching of the electric field the ions travel at their own initial velocities during 300 ns following the arrows shown in Fig. 1. Ions travelling away from the detector will not significantly change their TOF, while those moving towards the detector will approach the extraction plate following the solid lines and will reduce the space resolution as they are delayed. As a direct consequence of the variety of initial velocity directions, the TOF-MS bands are no longer symmetric, with a tail on the short-time wing and leaving unperturbed the long-time side, and thus providing reliable KEDs. A decrease of the ionisation delay at the expense of a lower ion yield will reduce the band asymmetry The experimental optimisation of the space focusing condition is achieved by balancing the acceleration and extraction regions electric fields ratio, as observed experimentally by plotting the TOF-MS FWHM peaks as a function of the extraction pulse voltage ŽPV.. The focusing conditions are indepen-
Fig. 2. Mass resolved TOF spectrum effected by 70 eV electron-impact on CH 3 F parent molecule. The mass peaks appear labelled.
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I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
dent of the ion considered and its kinetic energy, 185 V being the optimum value for an acceleration biased voltage of 2000 V. Optimised voltages result in narrower and more intense TOF MS peaks and agree very well with the series expansion of an ion TOF formed with zero kinetic energy at a distance s from the extraction plate in the neighbourhood of the excitation region centre, s0 ŽT Ž0, s .. w25x in Wiley– McLaren notation, and also with the calculations conducted on the ‘Simion 3D’ software w29x.
Nascent kinetic energy distributions, nŽUŽ t .., are readily worked out from the TOF band profiles, f Ž t ., according to the relationship w26,28,30x: nŽ UŽ t . . s
2m
Ž qE .
d f Ž t. 2
dt
,
Ž 1.
where E is the electric field in the ionisation region Ž146 Vrcm in our experiments., and m the ion mass with charge q.
Fig. 3. Kinetic energy distributions of Ža. heavy mass ions: CH 3 Fq following electron-impact of 20, 45 and 95 eV; CH 2 Fq Žat 20, 45 and 95 eV.; CHFq Žat 20, 45 and 95 eV. and CFq Žat 20, 45 and 95 eV.. Žb. medium mass ions: CHq 3 following electron-impact of 20, 30, 60 q Ž Ž . and 100 eV, CHq at 25, 45, 55 and 100 eV. and Cq Žat 55 and 100 eV. and c. Fq atomic ion following 2 at 20, 60 and 100 eV , CH electron impact of 20, 30, 60 and 100 eV and Hq atomic ion following electron impact of 20, 50 and 100 eV, respectively. Note the correlation between the average ion velocity and the KEDs broadening with the ion mass.
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
Fig. 3 Ž continued ..
139
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I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
4. Results and discussion The computed nascent KEDs were obtained from the TOF-MS band profiles ŽFig. 2. using Eq. Ž1. and the relevant results for the set of dissociative and parent ions at a few significant electron impact energies are reported in Fig. 3. In most cases the increase of the electron impact energy does not imply a concomitant change in the KEDs, either in intensity or kinetic energy shift, implying processes which are very similar or at least energetically close. The heaviest ions studied including CFq, CHFq, CH 2 Fq and CH 3 Fq show, as a general feature, rather low average KEs – of the order of a few tenths or even hundredths eV – and a systematic increase with the electron energy. It is also observed that the larger the number of C–H bond dissociated by the electron-impact collision, the broader the KED, the higher the averaged translational energy and the lower the ion yield maximum. Heavy fragment velocities perpendicular to the supersonic moleculerelectron beams plane are expected to be small, as has been confirmed experimentally. The estimated errors in the KEDs were obtained by comparing our CF4 experimental KED with that reported by Christophorou w7x, the result being - 20%. CHq 3 ion is accompanied by a C–F bond dissociation and seems to be associated with the appearance of a well-defined maximum at f 1 eV, electron-impact energy independent. Simultaneous dissociation of the C–F bond and of one to three C–H bonds produces a shift of the velocity peak to smaller kinetic energies, a broadening in the KEDs and a decrease in ion yield. The group displays a rich KEDs behaviour, with averaged energies in the 1–5 eV range, roughly one order of magnitude higher than the CFq, CHFq, CH 2 Fq and CH 3 Fq set of ‘heavy’ ions and in some cases higher Ž8–10 eV.. Fq ion KED should be related to the CH 3 , CH 2 and CH radicals’ distributions, which in turn are expected to be similar to those of their corresponding ions. Indeed, the neat feature at 1 eV observed in the Fq KED is noticeably similar to that found in the CHq 3 ion KED. The other maxima observed at higher energies may be related to other companion fragments but the correlation is not as clear as the Fqr CHq 3 couple.
Finally, the light Hq atomic ion at electron-impact energies well over the ionisation threshold shows a maximum at a kinetic energy of 4 eV, and a possible second maximum centred at 13 eV. These energies are much higher than those found for the heavy and medium mass ions and it can be plausibly stated that the production of lighter ions in systems with electron energies well over the ionisation energy threshold are associated with higher kinetic energies. For electron-impact energies not far from the ion appearance energy thresholds the KEDs do not have well-defined features, due in part to the low ion yield. This fact hampers the determination of the contribution of the kinetic energy to the ionisation process as a whole. Acknowledgements We are grateful to DGES, MEC ŽMadrid. for partial support of this work through Grants-in-Aid PB95-0510 and PB96-1472. Also to GV ŽVitoria. for Co-financial Grants and to the UPV for a three-year Ž1998–2000. Research Group Grant. One of us ŽI.T.. thanks the Gobierno Vasco ŽVitoria. for the award of a fellowship and funds to visit Prof. T.D. Mark’s ¨ laboratory ŽInnsbruck, Austria.. References w1x J. Reece Roth, Industrial Plasma Engeneering, IOP Bristol, 1995. w2x R.P. Wayne, Chemistry of Atmospheres, Oxford University Press, Oxford, 1991. w3x S.J. Moss, A. Ledwith, The Chemistry of the Semiconductor Industry, Blackie, Glasgow, 1987. w4x T.D. Mark, ¨ in: L.G. Christophorou ŽEd.. Electron–Molecule Interactions and their Interactions, Academic Press, Boston, MA, 1984. w5x R. Martinez, F. Castano, ˜ M.N. Sanchez Rayo, J. Phys. B 25 Ž1992. 4951. w6x R. Martinez, F. Castano, ˜ M.N. Sanchez Rayo, R. Pereira, Chem. Phys. 172 Ž1993. 349. ´ w7x R. Martinez, J. Terron, Rayo, F. ´ J.I. Merelas, M.N. Sanchez ´ Castano, ˜ J. Phys. B 31 Ž1998. 1793. ´ w8x I. Torres, R. Martinez, M.N. Sanchez Rayo, J.A. Fernandez, ´ ´ F. Castano, ˜ J. Phys. B 32 Ž1999. 5437. w9x L.G. Christophorou, J.K. Olthoff, M.V.V.S. Rao, J. Phys. Chem. Ref. Data. 25 Ž1996. 1341. w10x L.G. Christophorou, J.K. Olthoff, J. Phys. Chem. Ref. Data. 28 Ž1999. 967.
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141 w11x H. Kawazumi, T. Uchida, T. Ogawa, Bull. Chem. Soc. Jpn. 59 Ž1986. 937. w12x Y. Ito, A. Fujimaki, K. Kobayashi, I. Tokue, Chem. Phys. 105 Ž1886. 417. w13x I. Tokue, H. Shimada, A. Masuda, Y. Ito, J. Chem. Phys. 93 Ž1990. 4812. w14x I. Tokue, M. Kobasyaski, Y. Ito, J. Chem. Phys. 96 Ž1992. 7458. w15x H. Kawazumi, T. Ogawa, Chem. Phys. 114 Ž1987. 149. w16x H. Kawazumi, T. Ogawa, Chem. Lett. Ž1995. 925. w17x P.W. Zetner, M. Darrach, P. Hammond, W.B. Westerveld, R.L. McConkey, J.W. McConkey, Chem. Phys. 124 Ž1988. 483. w18x K. Furuya, A. Matsuo, M. Tokeshi, T. Ogawa, J. Phys. Chem. 99 Ž1995. 13772. w19x M. Darrach, J.W. McConkey, J. Chem. Phys. 95 Ž1991. 754. w20x N. Abramzon, R.B. Siegel, K. Becker, J. Phys. B 32 Ž1999. L247.
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w21x T. Ogawa, T. Tomura, K. Nakashima, H. Kawazumi, J. Chem. Phys. 88 Ž1988. 4263. w22x N. Kouchi, M. Ohno, K. Ito, N. Oda, Y. Hatano, Chem. Phys. 67 Ž1982. 287. w23x C. Tian, C.R. Vidal, J. Chem. Phys. 108 Ž1998. 927. w24x J.C. Syage, J. Chem. Phys. 97 Ž1992. 6085. w25x W. C Wiley,, I.H. McLaren, Rev. Sci. Instr. 26 Ž1955. 1150. w26x J.L. Franklin, P.M. Hierl, D.A. Whan, J. Chem. Phys. 47 Ž1967. 3148. w27x J.A. Syage, Chem. Phys. Lett. 143 Ž1988. 19. w28x K. Schafer, D. Gassen, W. Neuwirth, ¨ W.Y. Baek, K. Foster, ¨ Z. Phys. D 21 Ž1991. 137. w29x SIMION-3D, version 3.0. Idaho National Engineering Laboratory, EG&G Idaho Inc., Idoha Falls, ID, 1987. w30x R. Martinez, I. Torres, F. Castano. ˜ 31st Conference of the European Group for Atomic Spectroscopy ŽEGAS., Marseille, France, 1999, pp. 458–459.
Chemical Physics Letters 328 Ž2000. 135–141 www.elsevier.nlrlocatercplett
Nascent kinetic energy distributions of the ions produced by electron-impact on the CH 3 F molecule ´ b, M.N. Sanchez I. Torres a , R. Martinez Rayo a , F. Castano ´ ˜ a,) a
´ ´ ´ Vasco, Facultad de Ciencias, Apart 644, 48080 Bilbao, Spain Departamento de Quimica Fisica, UniÕersidad del Pais b Facultad de Farmacia, Paseo de la UniÕersidad, 7, 01006 Vitoria, Spain Received 26 April 2000; in final form 6 July 2000
Abstract The nascent translational energy distributions of ions produced by electron-impact on CH 3 F, prepared in a skimmed supersonic expansion, have been investigated by their correlation with the time-of-flight mass-spectrometry band profiles. The electron energies used in the collisions range from the ionisation threshold to 100 eV. CH 3 Fq and other ion fragments close in mass are characterized by low average kinetic energies. Medium-sized mass ion fragments, have kinetic energy distributions with average values of ca. 1 eV, whilst atomic ions have the highest average energies. The set of ion nascent energy distributions produced by electron-impact on CH 3 F, at several electron energies, are reported. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Electron-impact studies on molecules are of crucial interest in the modelling of plasma processes, either industrial ones w1x or those in atmospheric science w2x. Among the molecules studied by electron-impact in the laboratory, the family of fluoromethanes are benchmark species due to their essential role in the development of semiconductor VLSI technology w3x and its relevance in the chemistry of the ionosphere. Electron-impact collisions on molecules yields a plethora of neutral excited states, radicals and ions. The ions may be produced by the direct ionisation process – extraction of one electron either directly from the parent molecule or by dissociative ionisation – or by autoionisation w4x. The ) Corresponding author. Fax: q34-94-464-8500; e-mail: [email protected]
number of dissociative ions produced in collisions with electrons of energies up to 100 eV is usually large, the number of fragments matching any combination of the number of constituent atoms w5–8x. Although the main concern in collision process studies has focused on electronic excitation and ions production,w9,10x it seems obvious that the fragments store energy in other internal degrees of freedom, such as vibrational, rotational and translational. Studies of emission and appearance thresholds usually lead to experimental values higher than those predicted by thermodynamic calculations w5,6x and the excess energy is plausibly stored in the fragment’s internal degrees of freedom. The nascent or the thermal vibrational and rotational energy distributions of a few fragments, sometimes identified by their temperatures w11x have been studied by spontaneous emission w12–15x and laser-induced fluorescence ŽLIF. w16–20x, but the paucity of data does not
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 8 6 1 - 7
136
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
allow one to make predictions on their energy contributions to the whole process. Even fundamental studies of the population inversion following electrical discharges in gases have not attracted enough attention to the urgency of acquiring knowledge of these processes. Nascent ion translational energy distributions have previously been studied by two methods, namely, Doppler broadening w17,21x and time-of-flight mass spectrometry ŽTOF-MS.. The former procedure can be applied, in principle, to any fragment but requires high-resolution spectroscopy in accessible spectral regions, which makes experiments difficult in some cases. The TOF-MS has been applied to samples in the bulk w19,22x, to CO 2 in an effusion beam w23x and to hydrazines and ammonia w24x seeded in a skimmed supersonic beam and further ion deflection detection. In this Letter, a well-characterized skimmed pulsed supersonic beam in conjunction with a commercial TOF-MS is used to determine the nascent translational energy distributions of the parent ion and the dissociative ions of the CH 3 F molecule following the electron-impact in the energy range from threshold to 100 eV.
2. Experimental The experimental set-up consists of a commercial linear Wiley–McLaren TOF-MS w25x ŽR.M. Jordan Co.. equipped with both a pulsed molecular supersonic and a monoenergetic electron beam, crossed at right angles in an originally electric field-free volume. The vacuum chamber system holding the electron beam and the valve are arranged so that it may be divided into three well-defined regions, commonly referred to as an ionisationrextraction, acceleration and drift tuberMCP detector w26x. In the actual experiment the 180 V negative pulse applied to the extraction grid 300 ns after the electron–molecule collisions take place, aims to drive the ions into the acceleration region, where a biased electric field of 1400 Vrcm drives the ions further into the 86.5 cm long drift tube, where a couple of x–y plates focus the ions onto a three-stage microchannel plate ŽMCP. detector ŽC-0701, f s 18 mm.. In the ionisation region, the repeller plate and the extraction grid are 1.27 cm apart, so that the actual electric field
applied was ca. 146 Vrcm. The electric response generated by the detector is routed to a digital oscilloscope ŽTektronix TDS360. and later to a computer for further analysis and storage. The electron beam is generated by thermoionic emission from an electrically heated W hairpin filament and subsequently accelerated to a predetermined energy, collimated and focused by two electron lenses controlling the beam spread in the ionisation region. The well-focused beam is estimated to be ca. 3 mm in diameter. There is an additional third lens divided into two equal semicircular pieces, in contact with the repeller plate and the extraction grid respectively, so that the same voltage applied to the plates biases the whole excitation region. Electron gun energy was calibrated with the Arq appearance potential Ž15.75 eV. to an accuracy better than "0.7 eV and varied from 0 to 100 eV; its energy spread depends on the filament heating current and is estimated to be of 1 eV at 100 mA electron intensity. Argon noble gas seeded with the target molecule was prepared in an external stainless-steel cylinder at a total pressure of up to 4 barr, as measured with a capacitance manometer ŽMKS-Baratron 750B. to 1% accuracy. The cylinder was directly connected to a pulsed valve ŽGeneral Valve, mod. IOTA ONE. fitted with an 0.8 mm f nozzle, from where the expansion proceeds to the main vacuum chamber Žat - 10y7 mbar., creating a supersonic beam; the valve open switch pulse is used, with 600 ms delay, to trigger the extraction grid voltage that drives the ions towards the acceleration stage. The central region of the beam is skimmed Ž f s 1 mm. at a distance of 5 cm from the nozzle head, in order to obtain an homogeneous beam w27x.
3. Nascent ion kinetic energy distributions The TOF-MS band profile basically depends on both the initial ion position with respect to the extraction plate and the ion kinetic energy w26,28x. For calibration purposes and to understand the physics behind it, Fig. 1 shows the computed TOF Ž14 amu. ions for the Arq Ž40 amu. and CHq 2 modelled for our experimental, as a parametric function of the ion initial distance to the extraction plate
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
Ž . Fig. 1. Simulated TOF of Arq Ž40 amu. and CHq 2 14 amu ions as a function of the ionisation space position with respect to the extraction plate, s. Curves above and below the nascent zero kinetic energy, U0 s 0 eV Žbold line., refer to ions with velocity directed outwards from Ždashed lines. and towards Žsolid lines. the extraction plate, respectively. The energy increase between any two close lines is 0.1 eV.
Ž s, in cm. and the nascent kinetic energy; the bold curves correspond to ions with initial zero kinetic energy and the other curves to ions with velocities directed to Žthin solid lines. and from Ždashed lines. the MCP detector, in kinetic energy steps of 0.1 eV. A comparison of the theoretical and the experimental
137
bandwidth at variable extraction voltages permits one to determine the distance from the electronr molecule beams crossing point to the extraction grid as ( 0.90 " 0.05 cm, a location where the bandwidth is insensitive to the space position ŽFig. 1. and, hence, the ion kinetic energy is the main factor influencing the band profile w26x. However, before the switching of the electric field the ions travel at their own initial velocities during 300 ns following the arrows shown in Fig. 1. Ions travelling away from the detector will not significantly change their TOF, while those moving towards the detector will approach the extraction plate following the solid lines and will reduce the space resolution as they are delayed. As a direct consequence of the variety of initial velocity directions, the TOF-MS bands are no longer symmetric, with a tail on the short-time wing and leaving unperturbed the long-time side, and thus providing reliable KEDs. A decrease of the ionisation delay at the expense of a lower ion yield will reduce the band asymmetry The experimental optimisation of the space focusing condition is achieved by balancing the acceleration and extraction regions electric fields ratio, as observed experimentally by plotting the TOF-MS FWHM peaks as a function of the extraction pulse voltage ŽPV.. The focusing conditions are indepen-
Fig. 2. Mass resolved TOF spectrum effected by 70 eV electron-impact on CH 3 F parent molecule. The mass peaks appear labelled.
138
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
dent of the ion considered and its kinetic energy, 185 V being the optimum value for an acceleration biased voltage of 2000 V. Optimised voltages result in narrower and more intense TOF MS peaks and agree very well with the series expansion of an ion TOF formed with zero kinetic energy at a distance s from the extraction plate in the neighbourhood of the excitation region centre, s0 ŽT Ž0, s .. w25x in Wiley– McLaren notation, and also with the calculations conducted on the ‘Simion 3D’ software w29x.
Nascent kinetic energy distributions, nŽUŽ t .., are readily worked out from the TOF band profiles, f Ž t ., according to the relationship w26,28,30x: nŽ UŽ t . . s
2m
Ž qE .
d f Ž t. 2
dt
,
Ž 1.
where E is the electric field in the ionisation region Ž146 Vrcm in our experiments., and m the ion mass with charge q.
Fig. 3. Kinetic energy distributions of Ža. heavy mass ions: CH 3 Fq following electron-impact of 20, 45 and 95 eV; CH 2 Fq Žat 20, 45 and 95 eV.; CHFq Žat 20, 45 and 95 eV. and CFq Žat 20, 45 and 95 eV.. Žb. medium mass ions: CHq 3 following electron-impact of 20, 30, 60 q Ž Ž . and 100 eV, CHq at 25, 45, 55 and 100 eV. and Cq Žat 55 and 100 eV. and c. Fq atomic ion following 2 at 20, 60 and 100 eV , CH electron impact of 20, 30, 60 and 100 eV and Hq atomic ion following electron impact of 20, 50 and 100 eV, respectively. Note the correlation between the average ion velocity and the KEDs broadening with the ion mass.
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
Fig. 3 Ž continued ..
139
140
I. Torres et al.r Chemical Physics Letters 328 (2000) 135–141
4. Results and discussion The computed nascent KEDs were obtained from the TOF-MS band profiles ŽFig. 2. using Eq. Ž1. and the relevant results for the set of dissociative and parent ions at a few significant electron impact energies are reported in Fig. 3. In most cases the increase of the electron impact energy does not imply a concomitant change in the KEDs, either in intensity or kinetic energy shift, implying processes which are very similar or at least energetically close. The heaviest ions studied including CFq, CHFq, CH 2 Fq and CH 3 Fq show, as a general feature, rather low average KEs – of the order of a few tenths or even hundredths eV – and a systematic increase with the electron energy. It is also observed that the larger the number of C–H bond dissociated by the electron-impact collision, the broader the KED, the higher the averaged translational energy and the lower the ion yield maximum. Heavy fragment velocities perpendicular to the supersonic moleculerelectron beams plane are expected to be small, as has been confirmed experimentally. The estimated errors in the KEDs were obtained by comparing our CF4 experimental KED with that reported by Christophorou w7x, the result being - 20%. CHq 3 ion is accompanied by a C–F bond dissociation and seems to be associated with the appearance of a well-defined maximum at f 1 eV, electron-impact energy independent. Simultaneous dissociation of the C–F bond and of one to three C–H bonds produces a shift of the velocity peak to smaller kinetic energies, a broadening in the KEDs and a decrease in ion yield. The group displays a rich KEDs behaviour, with averaged energies in the 1–5 eV range, roughly one order of magnitude higher than the CFq, CHFq, CH 2 Fq and CH 3 Fq set of ‘heavy’ ions and in some cases higher Ž8–10 eV.. Fq ion KED should be related to the CH 3 , CH 2 and CH radicals’ distributions, which in turn are expected to be similar to those of their corresponding ions. Indeed, the neat feature at 1 eV observed in the Fq KED is noticeably similar to that found in the CHq 3 ion KED. The other maxima observed at higher energies may be related to other companion fragments but the correlation is not as clear as the Fqr CHq 3 couple.
Finally, the light Hq atomic ion at electron-impact energies well over the ionisation threshold shows a maximum at a kinetic energy of 4 eV, and a possible second maximum centred at 13 eV. These energies are much higher than those found for the heavy and medium mass ions and it can be plausibly stated that the production of lighter ions in systems with electron energies well over the ionisation energy threshold are associated with higher kinetic energies. For electron-impact energies not far from the ion appearance energy thresholds the KEDs do not have well-defined features, due in part to the low ion yield. This fact hampers the determination of the contribution of the kinetic energy to the ionisation process as a whole. Acknowledgements We are grateful to DGES, MEC ŽMadrid. for partial support of this work through Grants-in-Aid PB95-0510 and PB96-1472. Also to GV ŽVitoria. for Co-financial Grants and to the UPV for a three-year Ž1998–2000. Research Group Grant. One of us ŽI.T.. thanks the Gobierno Vasco ŽVitoria. for the award of a fellowship and funds to visit Prof. T.D. Mark’s ¨ laboratory ŽInnsbruck, Austria.. References w1x J. Reece Roth, Industrial Plasma Engeneering, IOP Bristol, 1995. w2x R.P. Wayne, Chemistry of Atmospheres, Oxford University Press, Oxford, 1991. w3x S.J. Moss, A. Ledwith, The Chemistry of the Semiconductor Industry, Blackie, Glasgow, 1987. w4x T.D. Mark, ¨ in: L.G. Christophorou ŽEd.. Electron–Molecule Interactions and their Interactions, Academic Press, Boston, MA, 1984. w5x R. Martinez, F. Castano, ˜ M.N. Sanchez Rayo, J. Phys. B 25 Ž1992. 4951. w6x R. Martinez, F. Castano, ˜ M.N. Sanchez Rayo, R. Pereira, Chem. Phys. 172 Ž1993. 349. ´ w7x R. Martinez, J. Terron, Rayo, F. ´ J.I. Merelas, M.N. Sanchez ´ Castano, ˜ J. Phys. B 31 Ž1998. 1793. ´ w8x I. Torres, R. Martinez, M.N. Sanchez Rayo, J.A. Fernandez, ´ ´ F. Castano, ˜ J. Phys. B 32 Ž1999. 5437. w9x L.G. Christophorou, J.K. Olthoff, M.V.V.S. Rao, J. Phys. Chem. Ref. Data. 25 Ž1996. 1341. w10x L.G. Christophorou, J.K. Olthoff, J. Phys. Chem. Ref. Data. 28 Ž1999. 967.
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