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Photodetachment from anions in a drift cell. Application to SF6− at 337 nm
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
Photodetachment Oddur aInstitutfiir
from anions in a drift cell. Application to SF, at 337nm
Ing61fssona,
Physikalische und Theoretische bHahn-Meitner-Institut
Eugen Illenbergera’*,
Werner-F.
Schmidtb
Chemie der Freien Universitiit Berlin, Takustr. 3, D-14195 Berlin. Germany Berlin, Glienicker Str. 100, D-14109 Berlin, Germany
Received 8 June 1994; accepted 14 August 1994
Abstract A novel technique is introduced for the study of photodetachment from gas phase anions drifting in the electric field between two parallel electrodes. The anions are generated by attachment of photoelectrons to neutral molecules. The electrons are injected from the cathode by an excimer laser pulse. The transient molecular anions formed on free electron attachment are rapidly converted into their ground state by collisions with the buffer gas. Photodetachment from the drifting ground state anions is then effected by a second, delayed pulse from a nitrogen laser. The technique is applied to SF, at 337 nm. Experiments concerning photodetachment from SF; are scarce and rather conflicting. From the results available till now it is not clear whether or not photodetachment occurs for wavelengths X > 310 nm. We demonstrate here that SF; in fact undergoes photodetachment at 337 nm with a cross-section on the lo-” cm2 scale. The implications for the electronic and geometrical structure of SF; are discussed. Keywords:
Ion drift experiment;
Photodetachment;
SF,
1. Introduction
In this paper a novel technique for the study of photoelectron detachment in the gaseous phase from ground state anions drifting in an electric field between two parallel plates is presented. It is based on the much higher mobility of electrons compared to ions which allows the identification of detached electrons on the background of the ion signal. The anions are formed via attachment of photoelectrons produced from an excimer laser pulse at the cathode. *Corresponding author.
Sulphur hexafluoride is probably the most extensively studied system in relation to electron attachment and formation of its molecular anion [l-3]. The molecule possesses one of the largest capture cross-sections for low energy electrons (comparable or even larger than for the fullerenes C60 and CT0 [4-81) and has therefore found extensive application as gaseous insulator in high voltage devices, etc. [ 1,2,9]. The electron attachment properties have been studied in swarm [lo- 121 and beam techniques [13-l 51. In a recent high resolution experiment with a novel laser photoelectron method, electron attachment in the ultralow energy region below 200meV
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et al.lInternational Journal of MassSpectrometry and Ion Processes 139 (1994) 103-110
was studied at a resolution of 0.2 meV [15]. It was shown that the measured cross-section increases with decreasing energy as predicted from different theoretical approaches [ 16,171 and exceeds lo-l3 cm* for electron energies below 4meV. The transient negative ion SF;* formed by free electron attachment is understood as a “nuclear excited Feshbach resonance” where the energy due to the extra electron (comprising its initial energy and the electron affinity of the molecule) is effectively dispersed among the molecular vibrations. Although conflicting data for the autodetachment lifetime of SF; are reported (see for example Ref. [ 181 and references cited therein) it is well known that SF; can be observed by standard mass spectrometric techniques. It is also clear that at pressures beyond single collision conditions (some millibar as in the present experiment) the transient negative ion SF,’ will immediately be stabilised via collisions to give ground state SF,. In the meantime the adiabatic electron affinity is accepted as EA(SF6) = 1.05 f 0.05 derived from electron transfer equilibria and negative ion mass spectrometry [19]. Photodetachment of electrons from SF; and the related problem of the structure of SF, is still an open question. Drzaic and Brauman [18] reported “no observable photodetachment from SF; with visible light” from either broad band arc lamp (with 300nm cut-off filter) or laser sources. Freiser and Beauchamp, as quoted by Hay [20], find a measurable cross-section only at energies above 4eV (X 5 310 nm). Experimental evidence of photodetachment from negative ions of SF6 produced in low pressure discharge was reported by Ishikawa et al. [22]. A recent experiment of Mock and Grimsrud [21] with a “photodetachment modulated electron capture detector” (PDM-ECD) working at atmospheric pressure indicated a shallow onset of the photodetachment signal in the region near 350 nm (g3.54 eV).
In any case, there is obviously a large difference between the electron affinity of SF6 and the vertical detachment energy of SF; which, as well as the large kinetic barrier observed in electron transfer from SF, in gas phase ion/molecule reactions strongly [18,191, indicates a large geometry change between neutral and anion. SCF calculations were carried out for the octahedral symmetry (Oh) of the anion by Hay [23] and Tang and Callaway [24] having the configuration SF, (‘Ai,): . . . (1t2U)6(5ti,)6 (lt2s)6 (6ais)’ with the extra electron occupying the (non-degenerate) 6ais MO [23]. This leads to a considerable increase in the S-F bond length on going from the neutral to the anion. Compton and co-workers [13] qualitatively discussed involvement of degenerate MOs of lower symmetry causing Jahn-Teller distortion in the anion which could provide the strong coupling between electronic and nuclear motion necessary to trap the electron for a certain time. In addition to the loosely bound octahedral SF;, Drzaic and Brauman [ 181 also considered SF; existing as an ion/ molecule association complex F--SF5 which would explain lack of a measurable photodetachment signal in their experiment. Following Brauman’s suggestion, Gutsev [25] carried out calculations for a configuration of lower symmetry (e.g. C2, and CA, starting with one S-F bond considerably enlarged) which, however, always resulted in the octahedral anion being the most stable configuration. While the calculations of Hay, and Tang and Callaway resulted in 1.03 eV and 1.19 eV, respectively, for the adiabatic electron affinity (close to the accepted experimental value), Gutsev predicts an unrealistically high value of 3.44eV at the highest level of his calculations.
2. Experimental Fig. 1 shows the experimental
set-up. The
0. Ingdlfson
et al.lInternational
Journal of Mass Spectrometry
Fig. 1. Schematic
representation
cell consists of a stainless steel cylinder equipped with different ports for evacuation, gas supply, pressure measurement and electrical feedthroughs. A quartz window allows transmission of UV light from the lasers. The electrodes are two parallel aluminium discs 20cm in diameter. The separation distance was fixed at 3cm. Photoelectrons are produced by photoemission from the lower plate (cathode) with pulses from an excimer laser (Lambda Physik M6500) operating with KrF at 248 nm (hv M 5 eV) at an average pulse energy of 100 mJ. The excimer light is slightly focused and enters the cell above the anode. The light beam is then deflected by a dielectric mirror, transmitted through a copper mesh onto the cathode. The illuminated area is approximately 4mm x 1Omm in size. The light from the nitrogen laser (Lambda Physik MlOOO, X = 337 nm G 3.78 eV) enters the cell between both plates. Optimal detachment signals were obtained by focusing the beam to the centre of the drift gap, which also avoids the light from striking the metallic electrodes. The vessel was filled with a diluted mixture of SF, in Ar gas.
and Ion Processes 139 (1994) 103-110
of the experimental
105
apparatus.
Taking Q! M 4eV for the work function of Al, the initially produced photoelectrons enter the gas with an energy distribution up to 1 eV. Under the present experimental conditions (Ar buffer gas pressure, lo-150 mbar) the photoelectrons are rapidly thermalised and captured by SF6 (pressure less than 0.02mbar) yielding the transient anion SF,* which itself is rapidly stabilised into its ground state. Ions drifting to the anode are then photodetached by the light from the nitrogen laser (pulse energy M 3 mJ E 4.5 x lOi photons). The detaching pulse is optimised to an appropriate time delay (with respect to the excimer pulse) to establish maximum overlap between the nitrogen laser beam and the drifting ion bunch. The response of the external circuit depends on the number of charges and their drift velocity. Ions and electrons present in the drift gap can be distinguished by their very different drift velocities (see for example Ref. [l]). The ionic current was measured with an electrometer amplifier (Keithley). For the observation of the detached electrons a fast pulse amplifier (rise time 4 ns, fall time 150 ps, amplification
106
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a)
27 mbar Ar
1.0
0.0
b)
0.0
2.5
130 mbar Ar
5.0
Time (ms) Fig. 2. Oscilloscopic trace of the ion current 27mbar Ar; (b) 0.03 mbar SF6, 130mbar Ar.
following
the excimer
x 10, input resistor variable from 10MR to 50 MC?) is used. The input resistor was chosen to yield sufficient sensitivity for the electron signal and a suitable response time. The voltage signal from these preamplifiers is then amplified and monitored by a storage oscilloscope connected to a laboratory computer. Argon from Linde Gas of stated purity of 99.99 vol.% was passed through an Oxisorb cartridge. Sulphur hexafluoride from Linde Gas of stated purity of 99.9 vol.% was used without further purification.
3. Results and discussion Fig. 2 shows the current following an excimer laser pulse. The cell was filled with 0.03mbar SF6 mixed with Ar buffer gas, (a) 27 mbar Ar and (b) 130 mbar Ar, at an electric field strength of 67 V cm-‘. The zero of the time scale corresponds to the triggering of the excimer laser. The sharp peak at zero is due to photoelectrons ejected from the cathode. The electron current leads to an overshoot of the amplifier. The subsequent broad signal is due to negative ions drifting to the anode. Under conditions where the bunch of ions is directly formed at the cathode and is not dispersed during drifting to the anode one would expect a rectangular shape. The
laser pulse at an electric field of 67Vcm-‘:
(a) 0.03 mbar
SF,,
shape of the ion signal observed in the present experiment is mainly due to the effect that SF, ions are initially formed within a more or less extended area within the drift gap. The time and space evolution of an electron swarm in the presence of an electron scavenger (and hence the profile of the initial ion bunch) is a rather complex problem [26,27]. It is further complicated by the fact that in the present experiment the photoejected electrons have an initial energy up to 1 eV. Electron capture by SF6, on the other hand, requires electrons near OeV with a very narrow bandwidth (
0. Ingd(fsson et aLlInternational Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
275
300
325 275
300
325
Time (ps) Fig. 3. Oscilloscopic trace of the signal response in 0.03 mbar SF6/27 mbar Ar gas mixture when the nitrogen laser is triggered with 300 ,USdelay with respect to the excimer laser: (a) the light of both lasers enters the cell (photodetachment occurs); (b) the light of the excimer is blocked (no anions are produced); (c) the light of the nitrogen laser is blocked (anions are produced, but no photodetachment occurs); (d) the recovered photodetachment signal, with both light beams entering the cell.
[l, 14,281 that attachment of electrons below 2eV yields exclusively SF, and SF? with an intensity ratio of 100:8, respectively, at room temperature. Other fragments such as F-, F,, SF, etc. are observed from resonances at considerably higher energies. The electron drift velocity under the present conditions (E/n z 10-‘7-10-‘6 Vcm2) is uD z
b)
h
300
310
320
Time ().ts)
Fig. 4. Oscilloscopic trace of the signal response due to primary photoelectrons (a) and photodetached electrons (b). The vessel is filled with 0.03mbar SF6 in 27mbar Ar at an electric field strength of 67 V cm-‘. The signal in (b) is amplified by 3 x 104.
107
105-106cms-’ [l] and thus below thermal. Although the resonance maximum for SF5 is above thermal energies [1,14,28], we cannot exclude a small contribution of SF? to the ion signal under these conditions. However, since the electron affinity of SF5 is above the energy of the photons (EA(SF,) = 3.8 + 0.14eV [14], hu = 3.68eV) photodetachment of SF? is not expected. In addition, the actual vertical detachment energy is generally larger than the (adiabatic) electron affinity. Fig. 3 shows different averaged and amplified scans in a time window when the nitrogen laser was triggered with a delay of 300 puswith respect to the excimer pulse. Fig. 3(a) (which can be seen as a magnified section of the ionic curve in Fig. 2(a)) indicates a sharp rise of the current, with a width and shape closely resembling these due to the primary photoelectrons, but much smaller in intensity (see also Fig. 4). To prove that this signal is indeed due to photodetachment (and not due to artefacts like electronic pick-up caused by the laser discharge, effects from scattered light, etc.) we have performed additional checks including blocking of the individual light sources and recording the signal as a function of the delay time of the nitrogen laser pulse. Fig. 3(b) shows the corresponding scan with the excimer light blocked (no photoelectrons). There is still some response visible at 300 ps on the horizontal background (no drifting ions) which may be due to scattered nitrogen laser light striking the cathode. Fig. 3(c) shows the corresponding scan, now with the excimer light on and the nitrogen light blocked (but the laser still in operation). We see the falling background of the anions which are again formed by the photoelectrons from the excimer laser. Fig. 3(d) finally gives the recovered photodetachment signal with both light beams on. The feature is essentially identical to that in Fig. 3(a). In Fig. 5 the intensity of the photodetachment signal (Figs. 3(a) and 3(d)) is shown in
108
0. Ingdlfson
Journal of Mass Spectrometry and Ion Processes I39 (1994) 103-110
et al./International
o
13mbar
Consider
the Lambert-Beer
law
m 27 mbar ??
65 mbar
.
130 mbar
N = No exp(-nal)
(1)
with No and N the number of photons per pulse from the nitrogen laser before and after the photodetaching interaction with the anions, YEthe number density of anions, 0 the photodetachment cross-section and I the interaction length of the photon beam with the anions. With nal < 1 one can write No - N = zo = Nonal
(2)
I
I
400
-
I
800
-
I
1200
Time
’
I
1600
’
I
2000
’
I
2400
(ps)
Fig. 5. The intensity of the photodetachment signal vs. the time delay of the nitrogen laser with respect to the excimer laser. The curves represent four different argon buffer gas pressures (at SF6 , 27mbar; 0, 65mbar; pressure of 0.03mbar): 0, 13mbar; ?? 0, 130mbar.
dependence of the time delay of the nitrogen laser at four different pressures of the buffer gas. One can see that the signal passes a maximum as the centre of the drifting anion bunch passes the profile of the nitrogen laser beam. As expected, the curve broadens as the pressure is raised, reflecting the pressure dependence of the anion drift mobility. The decreasing intensity of the signal with increasing pressure reflects the different electron drift mobility, which is directly proportional to the electric response at the anode. These checks, together with the close similarity between the shape of the photoemission and the photodetachment signal (Fig. 4), provide clear-cut evidence that photodetachment from SF, occurs at 337nm. With the known intensity of the nitrogen laser light, and using the time dependence of the photodetachment signal to estimate the overlap with the drifting anions, one can obtain an estimate of the photodetachment cross-section by comparing the primary electron peak (Fig. 4(a)) with the photodetachment peak (Fig. 4(b)).
where ZD is the number of detached electrons. In the present experiment, the number density of anions in the area where photodetachment occurs can be expressed as
with Z, the total number of ions in the bunch, A the profile of the laser beam and y the overlap between laser beam and drifting ion bunch (0 5 y < 1). Eq. (2) is thus equivalent to
where IO is the photon density (IO = No/A). The photodetachment cross-section can thus be expressed as zD
1
“=z,lo”i
Under the assumption that all primary photoelectrons are converted into anions (which is true for buffer gas pressures not too low, see Fig. 2) ZD/ZP corresponds to the peak ratio between photodetached and primary electrons respectively. The averaged area of the (focused) laser beam in the interaction region is 0.24 f 0.1 cm* and hence IO = (7.2 f 3)x An estimate of the overlap 1014 photonscm-*. can be obtained in the following way: the approximate ion drift velocities can be determined from Fig. 2 (about 4 x lo3 ems-’ at 27 mbar and about lo3 cm s-l at 130 mbar). This gives the time the ions need to travel
0. Ingdlfsson et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
through the laser beam profile along the drift gap (0.3cm), i.e. an area which can be correlated to the total drift area in Fig. 5. At 130 mbar, for example, the drift time through the profile of the laser beam is 300 ps. This area near the maximum of the intensity distribution in relation to the whole distribution gives y. An evaluation for different buffer gas yielded y = 0.25 f 0.10. With pressures the experimentally obtained ratio Z,/Z,, = (3.5 f 1) x 10-4, we obtain 0 = (1.9 + 5.2/ -1.3) x 10-‘scm’. The present experiment clearly demonstrates that photodetachment of SF; readily occurs at 337nm, thus supporting the recent result of Mock and Grimsrud [21] at atmospheric pressure indicating a threshold near 350nm. Although this is in contrast to the experimental finding of Drzaic and Brauman [I 81 as well as those from Freiser and Beauchamp [20], one should be aware that the detection of a signal is always a question of the detection efficiency of the particular method and instrument. It should be noted that the experiment of Mock and Grimsrud gives a photodetachment crosssection near 2 x lo-” cm2 at 340nm, close to the present value. We are currently involved in extending the experiments to wavelength-dependent photodetachment experiments with a tuneable dye laser. According to the Wigner law [29] the photodetachment cross-section near threshold reflects the angular momentum of the photoelectron and thus the symmetry of the MO in the anion from which it is detached [30]. Although probably shallowed by weak Franck-Condon factors, the wavelength dependence of the cross-section could help to establish the structure of SF;. In conclusion, it can be seen that drift experiments allow photodetachment experiments to be performed from anions formed by attachment of photoelectrons. Owing to collisions with the buffer gas, it is ensured
109
that photodetachment occurs from the ground state of the anion. It is shown that SF; undergoes photodetachment at 337 nm with a cross-section of the order of lo-‘* cm2.
Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft (II 16/9-l) and the Fonds der Chemischen Industrie.
References 111L.G.
Christophorou, Atomic and Molecular Radiation Physics, Wiley Interscience, London, 1971. (Ed.), Electron-Molecule InterPI L.G. Christophorou actions and Their Applications, Vols. I and II, Academic Press, Orlando, FL, 1984. [31 E. Illenberger and J. Momigny, Gaseous Molecular Ions. An Introduction to Elementary Processes Induced by Ionization, Steinkopff, Darmstadt/Springer, New York, 1992. 141 M. Lezius, P. Scheier and T.D. Mark, Chem. Phys. Lett., 203 (1993) 232. [51 D. Smith, P. Spanel and T.D. Mark, Chem. Phys. Lett., 213 (1993) 202. [61 T. Jaffke, E. Illenberger, M. Lezius, S. Matejcik, D. Smith and T.D. Mark, Chem. Phys. Lett., 226 (1994) 213. [71 S. Matejcik, T.D. Mark, P. Spanel, D. Smith, T. Jafie and E. Illenberger, J. Chem. Phys., in press. PI P. Spanel and D. Smith, Chem. Phys. Lett., in press. and M.O. Pace (Eds.), Gaseous [91 L.G. Christophorou Dielectrics V, Pergamon, New York, 1984. [lOI Z. Lj. Petrovic and R.W. Compton, J. Phys. B, 18 (1985) 2177. J. [111 S.R. Hunter, J.G. Carter and L.G. Christophorou, Chem. Phys., 90 (1989) 4879. [121 G.E. Streit, J. Chem. Phys., 77(2) (1982) 826. [I31 J.A.D. Stockdale, R.N. Compton and H.C. Schweinler, J. Chem. Phys., 53 (1970) 1502. v41 M. Fenzlaff, R. Gerhard and E. Illenberger, J. Chem. Phys., 88 (1988) 149. [I51 D. Klar, M.W. Ruf and H. Hotop, Chem. Phys. Lett., 189 (1992) 448. D. Klar, M.W. Ruf and H. Hotop, Aust. J. Phys., 45 (1992) 263. U61 E. Vogt and G.H. Wannier, Phys. Rev., 95 (1954) 1190. 1171 C.E. Klots, Chem. Phys. Lett., 38 (1976) 61. 1181 P.S. Drzaic and J.I. Brauman, J. Am. Chem. Sot., 104 (1982) 13. 1191 E.P. Grimsrud, S. Chowdhury and P. Kebarle, J. Chem. Phys., 83 (1985) 1059.
110
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Journal of Mass Spectrometry and Ion Processes I39 (1994) 103-110
E.C.M. Chen, J.R. Wiley, C.F. Batten and W.E. Wentworth, J. Phys. Chem., 98 (1994) 88. [20] B. Freiser and J.L. Beauchamp, personal communication quoted by P.J. Hay, J. Chem. Phys., 76 (1982) 502. [21] R.S. Mock and E.P. Grimsrud, Chem. Phys. Lett., 184 (1991) 99. [22] I. Ishikawa, K. Koike, T. Akitsu, S. Suganomata and H. Matsuzawa, Jpn. J. Appl. Phys., 29 (1990) 767. [23] P.J. Hay, J. Chem. Phys., 76 (1982) 502. [24] R. Tang and J. Callaway, J. Chem. Phys., 84 (1986) 6854.
[25] G.L. Gutsev, Int. J. Mass Spectrom. Ion Processes, 115 (1992) 185. [26] P.G. Datskos, L.G. Christophorou and J.G. Carter, J. Chem. Phys., 98 (1993) 7875. [27] L. Frommhold, Fortschr. Phys., 12 (1964) 597. [28] T. JafIke and E. Illenberger, in preparation. [29] E.P. Wigner, Phys. Rev., 73 (1948) 1002. [30] K.J. Reed, A.H. Zimmerman, H.C. Anderson and J.I. Brauman, J. Chem. Phys., 64 (1976) 1368.
International Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
Photodetachment Oddur aInstitutfiir
from anions in a drift cell. Application to SF, at 337nm
Ing61fssona,
Physikalische und Theoretische bHahn-Meitner-Institut
Eugen Illenbergera’*,
Werner-F.
Schmidtb
Chemie der Freien Universitiit Berlin, Takustr. 3, D-14195 Berlin. Germany Berlin, Glienicker Str. 100, D-14109 Berlin, Germany
Received 8 June 1994; accepted 14 August 1994
Abstract A novel technique is introduced for the study of photodetachment from gas phase anions drifting in the electric field between two parallel electrodes. The anions are generated by attachment of photoelectrons to neutral molecules. The electrons are injected from the cathode by an excimer laser pulse. The transient molecular anions formed on free electron attachment are rapidly converted into their ground state by collisions with the buffer gas. Photodetachment from the drifting ground state anions is then effected by a second, delayed pulse from a nitrogen laser. The technique is applied to SF, at 337 nm. Experiments concerning photodetachment from SF; are scarce and rather conflicting. From the results available till now it is not clear whether or not photodetachment occurs for wavelengths X > 310 nm. We demonstrate here that SF; in fact undergoes photodetachment at 337 nm with a cross-section on the lo-” cm2 scale. The implications for the electronic and geometrical structure of SF; are discussed. Keywords:
Ion drift experiment;
Photodetachment;
SF,
1. Introduction
In this paper a novel technique for the study of photoelectron detachment in the gaseous phase from ground state anions drifting in an electric field between two parallel plates is presented. It is based on the much higher mobility of electrons compared to ions which allows the identification of detached electrons on the background of the ion signal. The anions are formed via attachment of photoelectrons produced from an excimer laser pulse at the cathode. *Corresponding author.
Sulphur hexafluoride is probably the most extensively studied system in relation to electron attachment and formation of its molecular anion [l-3]. The molecule possesses one of the largest capture cross-sections for low energy electrons (comparable or even larger than for the fullerenes C60 and CT0 [4-81) and has therefore found extensive application as gaseous insulator in high voltage devices, etc. [ 1,2,9]. The electron attachment properties have been studied in swarm [lo- 121 and beam techniques [13-l 51. In a recent high resolution experiment with a novel laser photoelectron method, electron attachment in the ultralow energy region below 200meV
016%1176/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168- 1176(94)04066-4
104
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et al.lInternational Journal of MassSpectrometry and Ion Processes 139 (1994) 103-110
was studied at a resolution of 0.2 meV [15]. It was shown that the measured cross-section increases with decreasing energy as predicted from different theoretical approaches [ 16,171 and exceeds lo-l3 cm* for electron energies below 4meV. The transient negative ion SF;* formed by free electron attachment is understood as a “nuclear excited Feshbach resonance” where the energy due to the extra electron (comprising its initial energy and the electron affinity of the molecule) is effectively dispersed among the molecular vibrations. Although conflicting data for the autodetachment lifetime of SF; are reported (see for example Ref. [ 181 and references cited therein) it is well known that SF; can be observed by standard mass spectrometric techniques. It is also clear that at pressures beyond single collision conditions (some millibar as in the present experiment) the transient negative ion SF,’ will immediately be stabilised via collisions to give ground state SF,. In the meantime the adiabatic electron affinity is accepted as EA(SF6) = 1.05 f 0.05 derived from electron transfer equilibria and negative ion mass spectrometry [19]. Photodetachment of electrons from SF; and the related problem of the structure of SF, is still an open question. Drzaic and Brauman [18] reported “no observable photodetachment from SF; with visible light” from either broad band arc lamp (with 300nm cut-off filter) or laser sources. Freiser and Beauchamp, as quoted by Hay [20], find a measurable cross-section only at energies above 4eV (X 5 310 nm). Experimental evidence of photodetachment from negative ions of SF6 produced in low pressure discharge was reported by Ishikawa et al. [22]. A recent experiment of Mock and Grimsrud [21] with a “photodetachment modulated electron capture detector” (PDM-ECD) working at atmospheric pressure indicated a shallow onset of the photodetachment signal in the region near 350 nm (g3.54 eV).
In any case, there is obviously a large difference between the electron affinity of SF6 and the vertical detachment energy of SF; which, as well as the large kinetic barrier observed in electron transfer from SF, in gas phase ion/molecule reactions strongly [18,191, indicates a large geometry change between neutral and anion. SCF calculations were carried out for the octahedral symmetry (Oh) of the anion by Hay [23] and Tang and Callaway [24] having the configuration SF, (‘Ai,): . . . (1t2U)6(5ti,)6 (lt2s)6 (6ais)’ with the extra electron occupying the (non-degenerate) 6ais MO [23]. This leads to a considerable increase in the S-F bond length on going from the neutral to the anion. Compton and co-workers [13] qualitatively discussed involvement of degenerate MOs of lower symmetry causing Jahn-Teller distortion in the anion which could provide the strong coupling between electronic and nuclear motion necessary to trap the electron for a certain time. In addition to the loosely bound octahedral SF;, Drzaic and Brauman [ 181 also considered SF; existing as an ion/ molecule association complex F--SF5 which would explain lack of a measurable photodetachment signal in their experiment. Following Brauman’s suggestion, Gutsev [25] carried out calculations for a configuration of lower symmetry (e.g. C2, and CA, starting with one S-F bond considerably enlarged) which, however, always resulted in the octahedral anion being the most stable configuration. While the calculations of Hay, and Tang and Callaway resulted in 1.03 eV and 1.19 eV, respectively, for the adiabatic electron affinity (close to the accepted experimental value), Gutsev predicts an unrealistically high value of 3.44eV at the highest level of his calculations.
2. Experimental Fig. 1 shows the experimental
set-up. The
0. Ingdlfson
et al.lInternational
Journal of Mass Spectrometry
Fig. 1. Schematic
representation
cell consists of a stainless steel cylinder equipped with different ports for evacuation, gas supply, pressure measurement and electrical feedthroughs. A quartz window allows transmission of UV light from the lasers. The electrodes are two parallel aluminium discs 20cm in diameter. The separation distance was fixed at 3cm. Photoelectrons are produced by photoemission from the lower plate (cathode) with pulses from an excimer laser (Lambda Physik M6500) operating with KrF at 248 nm (hv M 5 eV) at an average pulse energy of 100 mJ. The excimer light is slightly focused and enters the cell above the anode. The light beam is then deflected by a dielectric mirror, transmitted through a copper mesh onto the cathode. The illuminated area is approximately 4mm x 1Omm in size. The light from the nitrogen laser (Lambda Physik MlOOO, X = 337 nm G 3.78 eV) enters the cell between both plates. Optimal detachment signals were obtained by focusing the beam to the centre of the drift gap, which also avoids the light from striking the metallic electrodes. The vessel was filled with a diluted mixture of SF, in Ar gas.
and Ion Processes 139 (1994) 103-110
of the experimental
105
apparatus.
Taking Q! M 4eV for the work function of Al, the initially produced photoelectrons enter the gas with an energy distribution up to 1 eV. Under the present experimental conditions (Ar buffer gas pressure, lo-150 mbar) the photoelectrons are rapidly thermalised and captured by SF6 (pressure less than 0.02mbar) yielding the transient anion SF,* which itself is rapidly stabilised into its ground state. Ions drifting to the anode are then photodetached by the light from the nitrogen laser (pulse energy M 3 mJ E 4.5 x lOi photons). The detaching pulse is optimised to an appropriate time delay (with respect to the excimer pulse) to establish maximum overlap between the nitrogen laser beam and the drifting ion bunch. The response of the external circuit depends on the number of charges and their drift velocity. Ions and electrons present in the drift gap can be distinguished by their very different drift velocities (see for example Ref. [l]). The ionic current was measured with an electrometer amplifier (Keithley). For the observation of the detached electrons a fast pulse amplifier (rise time 4 ns, fall time 150 ps, amplification
106
0. Ingdlfsson et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
a)
27 mbar Ar
1.0
0.0
b)
0.0
2.5
130 mbar Ar
5.0
Time (ms) Fig. 2. Oscilloscopic trace of the ion current 27mbar Ar; (b) 0.03 mbar SF6, 130mbar Ar.
following
the excimer
x 10, input resistor variable from 10MR to 50 MC?) is used. The input resistor was chosen to yield sufficient sensitivity for the electron signal and a suitable response time. The voltage signal from these preamplifiers is then amplified and monitored by a storage oscilloscope connected to a laboratory computer. Argon from Linde Gas of stated purity of 99.99 vol.% was passed through an Oxisorb cartridge. Sulphur hexafluoride from Linde Gas of stated purity of 99.9 vol.% was used without further purification.
3. Results and discussion Fig. 2 shows the current following an excimer laser pulse. The cell was filled with 0.03mbar SF6 mixed with Ar buffer gas, (a) 27 mbar Ar and (b) 130 mbar Ar, at an electric field strength of 67 V cm-‘. The zero of the time scale corresponds to the triggering of the excimer laser. The sharp peak at zero is due to photoelectrons ejected from the cathode. The electron current leads to an overshoot of the amplifier. The subsequent broad signal is due to negative ions drifting to the anode. Under conditions where the bunch of ions is directly formed at the cathode and is not dispersed during drifting to the anode one would expect a rectangular shape. The
laser pulse at an electric field of 67Vcm-‘:
(a) 0.03 mbar
SF,,
shape of the ion signal observed in the present experiment is mainly due to the effect that SF, ions are initially formed within a more or less extended area within the drift gap. The time and space evolution of an electron swarm in the presence of an electron scavenger (and hence the profile of the initial ion bunch) is a rather complex problem [26,27]. It is further complicated by the fact that in the present experiment the photoejected electrons have an initial energy up to 1 eV. Electron capture by SF6, on the other hand, requires electrons near OeV with a very narrow bandwidth (
0. Ingd(fsson et aLlInternational Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
275
300
325 275
300
325
Time (ps) Fig. 3. Oscilloscopic trace of the signal response in 0.03 mbar SF6/27 mbar Ar gas mixture when the nitrogen laser is triggered with 300 ,USdelay with respect to the excimer laser: (a) the light of both lasers enters the cell (photodetachment occurs); (b) the light of the excimer is blocked (no anions are produced); (c) the light of the nitrogen laser is blocked (anions are produced, but no photodetachment occurs); (d) the recovered photodetachment signal, with both light beams entering the cell.
[l, 14,281 that attachment of electrons below 2eV yields exclusively SF, and SF? with an intensity ratio of 100:8, respectively, at room temperature. Other fragments such as F-, F,, SF, etc. are observed from resonances at considerably higher energies. The electron drift velocity under the present conditions (E/n z 10-‘7-10-‘6 Vcm2) is uD z
b)
h
300
310
320
Time ().ts)
Fig. 4. Oscilloscopic trace of the signal response due to primary photoelectrons (a) and photodetached electrons (b). The vessel is filled with 0.03mbar SF6 in 27mbar Ar at an electric field strength of 67 V cm-‘. The signal in (b) is amplified by 3 x 104.
107
105-106cms-’ [l] and thus below thermal. Although the resonance maximum for SF5 is above thermal energies [1,14,28], we cannot exclude a small contribution of SF? to the ion signal under these conditions. However, since the electron affinity of SF5 is above the energy of the photons (EA(SF,) = 3.8 + 0.14eV [14], hu = 3.68eV) photodetachment of SF? is not expected. In addition, the actual vertical detachment energy is generally larger than the (adiabatic) electron affinity. Fig. 3 shows different averaged and amplified scans in a time window when the nitrogen laser was triggered with a delay of 300 puswith respect to the excimer pulse. Fig. 3(a) (which can be seen as a magnified section of the ionic curve in Fig. 2(a)) indicates a sharp rise of the current, with a width and shape closely resembling these due to the primary photoelectrons, but much smaller in intensity (see also Fig. 4). To prove that this signal is indeed due to photodetachment (and not due to artefacts like electronic pick-up caused by the laser discharge, effects from scattered light, etc.) we have performed additional checks including blocking of the individual light sources and recording the signal as a function of the delay time of the nitrogen laser pulse. Fig. 3(b) shows the corresponding scan with the excimer light blocked (no photoelectrons). There is still some response visible at 300 ps on the horizontal background (no drifting ions) which may be due to scattered nitrogen laser light striking the cathode. Fig. 3(c) shows the corresponding scan, now with the excimer light on and the nitrogen light blocked (but the laser still in operation). We see the falling background of the anions which are again formed by the photoelectrons from the excimer laser. Fig. 3(d) finally gives the recovered photodetachment signal with both light beams on. The feature is essentially identical to that in Fig. 3(a). In Fig. 5 the intensity of the photodetachment signal (Figs. 3(a) and 3(d)) is shown in
108
0. Ingdlfson
Journal of Mass Spectrometry and Ion Processes I39 (1994) 103-110
et al./International
o
13mbar
Consider
the Lambert-Beer
law
m 27 mbar ??
65 mbar
.
130 mbar
N = No exp(-nal)
(1)
with No and N the number of photons per pulse from the nitrogen laser before and after the photodetaching interaction with the anions, YEthe number density of anions, 0 the photodetachment cross-section and I the interaction length of the photon beam with the anions. With nal < 1 one can write No - N = zo = Nonal
(2)
I
I
400
-
I
800
-
I
1200
Time
’
I
1600
’
I
2000
’
I
2400
(ps)
Fig. 5. The intensity of the photodetachment signal vs. the time delay of the nitrogen laser with respect to the excimer laser. The curves represent four different argon buffer gas pressures (at SF6 , 27mbar; 0, 65mbar; pressure of 0.03mbar): 0, 13mbar; ?? 0, 130mbar.
dependence of the time delay of the nitrogen laser at four different pressures of the buffer gas. One can see that the signal passes a maximum as the centre of the drifting anion bunch passes the profile of the nitrogen laser beam. As expected, the curve broadens as the pressure is raised, reflecting the pressure dependence of the anion drift mobility. The decreasing intensity of the signal with increasing pressure reflects the different electron drift mobility, which is directly proportional to the electric response at the anode. These checks, together with the close similarity between the shape of the photoemission and the photodetachment signal (Fig. 4), provide clear-cut evidence that photodetachment from SF, occurs at 337nm. With the known intensity of the nitrogen laser light, and using the time dependence of the photodetachment signal to estimate the overlap with the drifting anions, one can obtain an estimate of the photodetachment cross-section by comparing the primary electron peak (Fig. 4(a)) with the photodetachment peak (Fig. 4(b)).
where ZD is the number of detached electrons. In the present experiment, the number density of anions in the area where photodetachment occurs can be expressed as
with Z, the total number of ions in the bunch, A the profile of the laser beam and y the overlap between laser beam and drifting ion bunch (0 5 y < 1). Eq. (2) is thus equivalent to
where IO is the photon density (IO = No/A). The photodetachment cross-section can thus be expressed as zD
1
“=z,lo”i
Under the assumption that all primary photoelectrons are converted into anions (which is true for buffer gas pressures not too low, see Fig. 2) ZD/ZP corresponds to the peak ratio between photodetached and primary electrons respectively. The averaged area of the (focused) laser beam in the interaction region is 0.24 f 0.1 cm* and hence IO = (7.2 f 3)x An estimate of the overlap 1014 photonscm-*. can be obtained in the following way: the approximate ion drift velocities can be determined from Fig. 2 (about 4 x lo3 ems-’ at 27 mbar and about lo3 cm s-l at 130 mbar). This gives the time the ions need to travel
0. Ingdlfsson et al./International Journal of Mass Spectrometry and Ion Processes 139 (1994) 103-110
through the laser beam profile along the drift gap (0.3cm), i.e. an area which can be correlated to the total drift area in Fig. 5. At 130 mbar, for example, the drift time through the profile of the laser beam is 300 ps. This area near the maximum of the intensity distribution in relation to the whole distribution gives y. An evaluation for different buffer gas yielded y = 0.25 f 0.10. With pressures the experimentally obtained ratio Z,/Z,, = (3.5 f 1) x 10-4, we obtain 0 = (1.9 + 5.2/ -1.3) x 10-‘scm’. The present experiment clearly demonstrates that photodetachment of SF; readily occurs at 337nm, thus supporting the recent result of Mock and Grimsrud [21] at atmospheric pressure indicating a threshold near 350nm. Although this is in contrast to the experimental finding of Drzaic and Brauman [I 81 as well as those from Freiser and Beauchamp [20], one should be aware that the detection of a signal is always a question of the detection efficiency of the particular method and instrument. It should be noted that the experiment of Mock and Grimsrud gives a photodetachment crosssection near 2 x lo-” cm2 at 340nm, close to the present value. We are currently involved in extending the experiments to wavelength-dependent photodetachment experiments with a tuneable dye laser. According to the Wigner law [29] the photodetachment cross-section near threshold reflects the angular momentum of the photoelectron and thus the symmetry of the MO in the anion from which it is detached [30]. Although probably shallowed by weak Franck-Condon factors, the wavelength dependence of the cross-section could help to establish the structure of SF;. In conclusion, it can be seen that drift experiments allow photodetachment experiments to be performed from anions formed by attachment of photoelectrons. Owing to collisions with the buffer gas, it is ensured
109
that photodetachment occurs from the ground state of the anion. It is shown that SF; undergoes photodetachment at 337 nm with a cross-section of the order of lo-‘* cm2.
Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft (II 16/9-l) and the Fonds der Chemischen Industrie.
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