A surface ionization source and quadrupole mass filter for photodetachment studies

431 international Journal of Mass Spectrormtry und Ion Physics, 16 (1975) 43 IL441 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

A SURFACE

IONIZATION

SC LJRCE AND

FOR PHOTODETACHMENT

QUADRUPOLE

MASS

FILTER

STUDIES

L. E. LYONS Department

of Chemistry,

Unicersity

of fiueensiand,

St. Lucia, 4067 (Australia)

L. D. PALMER Defence

Standards Laborcrories,

P-0.

Box 50. Ascot

Vale, 3032 (Australia)

(Received 20 August 1974)

ABSTRACT

A surface ionization negative ion source and quadrupole mass flter were used to study photodetachment from tetracyanoethylene (TCNE) negative ions (C6N4-)_ The system gave mass analyzed beam currents of lo- 7 A, with a noise at 400 Hz approximately equal to the shot noise. The variation of ion current with temperature of MO, W, Pt and Re f?laments showed the electron work function of the clean metal was lowered by adsorbed TCNE, up to 2 eV for W, and that (using W) the electron affinity of TCNE was 1.750.3 eV.

INTRODUCTION

Since the first reported experiment in which electrons were photodetached from gaseous negative ions [I 1, photodetachment spectra have been obtained for a number of negative atomic and molecuIar species [Z]. Such experiments give values for electron a!!ties, values which are lacking especially for large molecules by this method. In this paper the surface idnization sonrce. and the quadrupole mass filter used [3] to determine the photodetachment spectrum of negative ions of tetracyanoethylene (TCNE), (NC),C=-C(CN), , are described. In a crossed-beam experiment, the current of photodetached electrons 1, resulting from a negatkre ion current li and photon flux iV(hv) is given by

432 where a(hv) is the photodetachment cross section at photon energy Izv, and Y is the ion velocity. The form factor F rakes account [4] of the degree by which the ion and photon beams overlap at their intersection. For completely overlapping beams with common height z, F ==:z. Electron currents 1, of the order lo-l4 A can be expected from ion currents of lo-* A. The detection of the electron signal current is hindered by a larger current arising from collisions of the negative ions with residual gas molecules, which result in a small fraction of the ion current being collected along with the electrons. At a pressure of 5 - lo-’ torr, ca. 0.01 oA of the ions undergo collisions in the 5-cm path near the electron collector; because &_/Ii = 10M6, the collisional current is several orders larger than that of the electrons_ Chopping the photon beam produces an alternating signal, the detection of which is limited by ion beam noise at the chopping frequency. Ion sources for photodetachment must therefore produce beams with low noise characteristics_ (An alternative approach is to use ultra high vacuum conditions_) Herron et al. IS] reported the formation of parent negative ions of TCNE by surface ionization on a platinum filament. For filament temperatures in the range 1200-1900 K, and with a TCNE pressure of 10m3 torr in the ion source, the maximum current observed was about lo- l1 A. TCNE surface ionization has been -studied also in a magnetron triode by Farragher and Page 141, who derived a value for the electron affinity of TCNE from the temperature variation of the ratio of thermionic electron and (total) negative ion emission from platinum, rhodium and iridium filaments_

ION

!xnJRCE

The principal features of the surface ionization source for (TCNE)photodetachment are shown in Fig. 1. The optimum ion source pressure of TCNE (ca. 10V2 torr) was achieved by heating the sample reservoir and inlet system [7] to about 40 “C. Thin-foil filaments 2-3 mm wide were given a U-shape to increase their effective surface area, and were attached to stainless steel filament support posts either directly by sm;lll screws, or by spot-welding them to nickel wires. Negative ions and electrons_from the filament were injected into the quadrupole mass filter by applying suitable voltages to the ion source electrodes (see Fig. 1). Electrodes D and G were mounted on the sturdy flange El, which supports th: quadrupole assembly on its other side. The use of precision diameter sapphire spheres as insulator-spacers gave good mechanical stability, an essential requirement for low-noise beam production_ The mechanical simplicity facilitated dismantling and reassembly of the source to remove the insuiating films which build up on the electrodes under the relatively high current conditions of photodetachment, and which result in ion beam noise.

433 0-

5cm

FIN. I. Ion beam apparatus. T = TCNE gas inlet, R = repeller electrode t-340 V), F = ionidng filament (-300 V), G = intensity control electrode (ca. -300 V). D = draw-out electrode (- 180 V). EZ = quadrupole entrance (0 V), Q = quadrupole mass filter rod, EZ = quadrupole exit (0 V), E3 = E4 = univoltage lens electrode (0 V), L = lens electrode (-250 V), P = photon beam axis, S = collecting electrode for photodetached electrons (+I5 V), FC = Faraday cup ion collector, V = oil diffusion vacuum pump.

jiZan2enr.s

Molybdenum

In Fig. 2, the current of (TCNE)ions arriving at the Faraday cup is shown as a function of filament temperature, T,, for two ion source pressures. Ion source and quadrupole voltages were increased from the optimum values shown in Fig. I, so that space charge limitations did not occur. An optical pyrometer was used to measure temperatures above 1000 K, and lower temperatures were estimated by extrapolating the filament temperature-heating current curve. The brightness temperatures have been corrected for the non-biack body emittance 1900

I’[”

1500

TF.

1100

’

1

K

700

1

1

1

10-g -

IO-‘3

1

4

t

8

1

T;’

.

I

12 (K-‘1 X104

I

I

16

Fig. 2. Dependence of (TCNE)ion current from a molybdenu~~filament on the fil%ent_temtorr; (a) ion source TCNE pressure perature. (v) Ion source TCNJZ pressure ca. 3 - iO_’ ca. I.3 - 10mz torr, increasing the filament temperature, and (0) decreasing the lilament temperature at the same pressure.

434 of the filament by assuming a value approximately 0.1 higher than the emissivity [S], which applies to the highly polished metal. The Saha-Langmuir equation N-IN” is often

used

= (constant) exp[- (&J---A&W,] to describe

the temperature

variation

(2) of the negative

ion

emission

a surface of electron work function 4. A, is the electron affinity of the atom or molecule being ionized, and N-/N” i: the ratio of the numbers of negative ions and neutrals leaving the surface. A, $--A, >> kTF for nearly all subshnces ionizing on refractory metals, the neutral particle emission predominates, and is essentially independent of temperature. 4--A, can be regarded as the work function for negative ion emission. The derivation of eqn. (2) assumes that the adsorbed molecules come to thermal equilibrium with the metal surface before they are emitted as either negative ions or neutra! molecules_ The extent to which this occurs is usually expressed in terms of the thermal accommodation coefficient [9], which is unity when the adsorbed molecules assume the surface temperature within their adsorption lifetime. The accommodation coefficient for TCNE on molybdenum and other metals has not been measured. However, the experimental values for various other polyatomic molecules on metal surfaces [9] show that accommodation coefficients are generally m the range 0.7-0.97, and are often signif&mrly less than unity. It is therefore likely that the Saha-Langmuir eqtlation is only an approximate description for (TCNE)- emission from molybdenum. -4nother cause of non-linearity in graphs such as Fig. 2 is the unknown dependence of the work function 4 on filament temperature. For our experiments, the ion source presrure (residual gases: 5 - 10d6 torr, and TCNE: 5 - 10m3 torr) was high enough to produce a considerable surface coverage, and adsorbed gases are known to have pronounced effects on the work function. For example, Page and Goode [lo] reported depressions of the order of 1 eV in the electron work functions of molybdenum carbide, tungsten carbide and tantalum in the presence of TCNE pressures of 10-3-10-5 torr. For two filaments of molybdenum carbide and the same TCNE pressure they report depressions of 0.27 eV and 1.30 eV, indicating that the work function is strongly dependent on tiament history. Assuming that 4 is constant throughout the linear region of Fig. 2, and that the accommodation coefficient is unity, the linear region of Fig. 2 gives a work function for (TCNE)- formation OIL molybdenum of 0.66+_0.1 eV. If the accommodation coefficient is less than unit-f, the effective temperature for eqn. (2) is less than TF, and the observed energy of 0.66 eV represents the upper limit to 4 -A,. From either the Farragher and Page [6] surface ionization result of 2.88 eV

from

for AG of TCNJZ, or the photodetachment result of 2.i7L-0.2

eV [ll], it can be

435 concluded that the electron work function of molybdenum under our experimental conditions is significantly less than the clean surface value of 4.20 eV 1121. The deviation from linearity shown in Fig. 2 for temperatures above 1500 K is equivalent to a work function increase of the order 0.3 eV. Such a change could result from a decrease in the coverage of TCNE on the filament. Werning 1131 has shown, however, from investigations of the positive surface ionization of barium, strontium, uranium and neodymium on refractory metals, that at temperatures below 2000 K, the deviations from linearity of the type observed in Fig. 2 are caused by chemical reactions at the surface rather than by work function changes resulting from changes in the coverage cf adsorbed molecules. Furthermore the results of Herron et al. [5] for the negative surface ionization mass spectra of a range of molecules show that direct ionization of the parent molecule must usually compete with chemical reactions at the surface_ Our work, primarily designed to produce an intense (TCNEbeam for photodetachment, did not permit an unequivocal interpretation of the high temperature region of Fig. 3. However, it is noted that the second region of increasing ion current at TF > 1800 K eliminates the possibility of space charse limitation, and that for TCNE, parent negative ions account for about 95 % of the total ion current. This indicates that the predomiuant process is direct capture of an electron by TCNE, rather than any bond-breaking chemical reaction. Work function changes due to adsorption effects are therefore likely to be responsible for the departure from linearity in Fig. 2. For photodetachment, molybdenum filament temperatures in the region 1500-1600 K offer a good colmbination of ion beam intensity, filament lifetime and likelihood that the negative ions have vibrational energy close to their ground levels. After optimization of parameters such as the electrode voltages and TCNE pressure, new molybdenum filaments produced currents of 1 - lop7 A for the photodetachment experiments [3]. New filaments are most suitable because after about 10 hours operation at TF z 1500 K, the available current falls to 20 % of its original value.

The temperature dependence of the mass-analysed (TCETE)- current is shown in Fig. 3. The current Zn arriving at the ion source electrode I> (see Fig. 1) is also presented. Estimating the filament temperature for ZD involves some additiona uncertainty because a temperature gradient exists across the filament, and Zn originates at the non-central part of the filament, whereas pyrometer readings are made on the hottest, central part. To allow for this, we have used 90 % of the emissivity-corrected pyrometer readings as the l?lament temperature for the Z, data. Xn Fig. 3, the I,, curve has two distinctly linear regions. In the lower temperatt_re region, Zn is predominantly (TCNE)-, shown by the mass spectral study

436

Fig. 3. Dependence of ion currents from a tungsten filament on filament temperature. (0) Mass-analysed current of (TCNE)measured at the Faraday cup; (0) current arriving at ion source electrode D. Ion source TCNJZ pressure ca. 7 - 10mS tom.

of the concurrent beam reaching the Faraday cup. From the slope, the work function for (TCNE)formation is 0.52-L-0.1 eV_ For the other linear region (T’, > 1300 K), ID is electronic, because the maximum current of (TCNE)- for the region between filament and electrode D is set by space charge considerations at about 1 PA. The electron work function 4 is 2.19 L-O.2 eV. more than 2 eV less than the value for clean tungsten 1121. Farragher and Page 163 used eqn. (3) to derive the electron afEnity of TCNE from surface ionization data. i,/Ii =

(constant) exp( -&J/Z,)

(3)

where i,/li is the ratio of electron and ion emission, and A& is the electron afI?nity of the ionizing molecule, apart from a temperature correction of the order of O-2 eV. Apart from this correction, eqn. (3) is essentially the same as the SahaLangmuir relation, with the electron work function term effectiveIy measured by i,.

With the assumption (supported by the regular shape of the In curve) that the value of + found for the higher temperature region of Fig. 3 also applies in the low temperature linear region, the I, curve and eqn. (3) predict A& = 1.7IfrO.3eV for TCNEL For Laments of platinum, iridium and rhodium, Farragher and Page [6] found A& values of 3.15 eV, 3.01 eV and 3.07 eV respectively. The discrepancy between these values and our result using tungsten shows that a closer investigation of the ionization process is required &fore molecular electron a5ities can be determiped reliably from surface ionization.

437 In Fig. 3 the curve for the mass-analysed (TCNE)current is linear only for a short interval at low temperature_ In this case, the deviation from linearity results from a reduction In the quadrupole mass filter transmission,- caused by space charge_ This is further discussed in the following secilon. PIatinmJihnents The variation with filament tempxature of the (TCNE)current is shown in Fig. 4. The curve shape is quahtatively similar to the tungsten curve (Fig. 3) but the work functicn for ion formation is higher at 1.4&O. 1 eV, a value consistent with the report of Page and Goode [lo] that the electronic work function of platinum is not lowered by the presence of TCNE. Rheniumjihzments Fig. 4 shows also the temperature mriation of the (TCNE)and CN- ion currents, using an Atlas CH4 magnetic mass spectrometer. Filament temperatures were estimated from the filament heating power and the manufacturer’s calibraz300

1500

1900

TF .K

i I-

)_ 4 -

T;'.

(K-')Xld-

Fig. 4. Dependence of ion currents from rhenium and platinum on filament temperature. (0) (TCNE)from rhenium; (0) CN- from rhenium, both measured by an Atlas CH4 mass spectrometer. (e) (T
438 tion curve. The unknown transmission efficiencies of the Atlas mass spectrometer and the photodetachment apparatus prevent a direct comparison of the currents from platinum and rhenium, although the ratio of the current of the most intense fragment ion CN- and the parent ion current from rhenium is available from Fig. 4. For the ion emission from rhenium, our results give a work function for (TCNE)formation of [email protected] eV. Compared with the temperature variation results found using the photodetachment apparatus, the rhenium curves in Fig. 4 are linear to considerably higher filament temperatures, a consequence of the absence of space charge limitations in the magnetic analyser.

QUADRUPOLE

MA!%

FlLTER

Our adaptation

of a

quadrupole

mass filter is a new approach to the problem

of generating an intense ion beam for photodetachment. The most important requirement is high transmission, with relatively Iow resolution being sufficient. The quadrupole has an advantage over magnetic instruments, in that it can transmit ions which enter within a cone of 30” half-angle [14] into a relatively large entrance aperture 1151. Other attractive features are now described. (1) Mass analysis is performed by electric fields. The low velocity electrons of the signal are very sensitive to magnetic fields. and a magnetic mass filter requires a considerable degree of magnetic shielding. Our quadrupole’s fringing electric field gave no signal detecticn problems, although the electrons were collected only 7 cm beyond the quadrupole exit. (2) The quadrupole operates with good resolution at higher pressures thaii most analysers [15). Ion source sample pressures for photodetachment experiments are usually relatively high in order to produce intense ion beams, and a beam of neutral moleculec accompanies the ion beam into the quadrupoleHowever, strong differential pumping is essential for photodetachmenr, as the negative ions are easily destroyed by collisions with the background gas molecules and the charges produced in such collisions hinder detection of the photodetached electrons. (3) The design and construction of photodetachment equip,ment is simplified by the quadrupole mass filter. In particular, the resolution-transmission properties of the filter can be adjusted electronically. This allows the purity of the ion beam to be readily checked periodically at high resolrltion during the experiment, and the transmission set to the maximum value compatible with ion beam purity. Furthermore, the ion optical elements required for beam production and handling have circular apertures, and axially symmetric systems are easier to design and construct than the slit systems of magnetic filters. The mass filter used for our experiments 13) was based on a quadrupole described by Swingler [16]. The maximum resolution of 50 was adequate for (TCNE)photodetachment, since the molecule contains only carbon and nitrogen

439 atoms. In agreement with the results of Herron et al. [S]. our studies, with both the quadrupole mass filter and the higher resolution Atlas CH4, showed that the parent ion accounted for more than 90 oA of the total negative ion emission. The only other peak of significant intensity was CN- which represented approximately 5 o/0of the total ion current at higher filament temperatures (see Fig. 4). The strongly focusing electric fields of the quadrupole produced a beam emerging from the quadrupole with a semi-angle of 12”, estimated from the mawitude of the transverse electric field which just deflected the ion beam away from the Faraday cup. Consideration of the form factor F in eqn. (1) shows that the ion beam intensity benefits of the quadrupole mass filter are lost if the height z of the ion beam (at the intersection of photon and ion beams) is much larger than a few millimetres. For photodetachment, an ion lens at the quadrupole exit is therefore necessary_ We used the three-element univoltage lens (Fig. 1) to produce an image of the quadrupole exit aperture at P. The focal properties of these Ienses are n-e11 known [IT], and a combination of the lens geometrical parameters which produces the required magnification together with a high angular acceptance can be readily chosen. By observing the fraction of the beam current transmitted through a scanning aperture at P, it was shown that the electrostatic lens gave a 40-fold increase in the axial ion currtnt density. The lens reduced the beam height to approximately 3 mm, a value compatible with the size of the images available from the high intensity xenon arc lamp or the argon ion laser used for photodetachment. Space charge can seriously reduce the ability of a quadrupole mass filter to transmit high ion currents_ Dawson and Whetten [18 J give eqn- (4) as a crude approximation for the effect of the space charge due to the ion beam in the quadrupole. It produces an additional d-c. fieId which is always defocusing. and which produces a change Aa in the Mathieu constant a. Aa = 1.8 Z/fZMi(VQ-VF)*

(4)

where I is the beam current (mA) of ions of mass number M,j‘(MHz) is the quadrupole frequency, and Yo- V, is the axial ion voltage in the quadruI;ole, measured (in kV) relative to the ionizing filament. At moderate resolution. a change in a of lo- 3 produces a noticeable change in the quadrupole transmission. On this basis, the space charge of the (TCNE)beam reduces the transmission efficiency when I is of the order of lo-’ A and V,-- V, = 6 V_ The curve for transmit&d (TCNE)in Fig. 3 was obtained with VQ- V, = 6 V. The maximum current observed is an order of magnitude smaller than eqn. (4) predicts. This is not surprising in view of the approximate nature of eqn. (4), and the fact that the quedrupole transmission also depends strongly on the fringing field at its entrance_ At V,- V, = 60 V, the maximum beam current increased to 3.5 nA, approximately obeying the square root dependence on V,- VF required

by eqn. (4). The filament temperature at which the maximum current was found also increased as V,-- V, increased, showing that space charge limitations in the quadrupole are at least partly responsible for the departure from linearity at high filament temperatures. Because of their lower mass, electrons make a smaller space charge contribution per particle than the (TCNE) - ions_ Equation (4) predicts that the space charge effect of the electron emission becomes comparable with that of the ion

beam currenr beam when i,/Z, z 500. On this basis, the decreasing (TCNE)temperatures above 1400 K is attributed to the space shown in Fig. 3 for fiIament charge of electrons near the quadrupole entrance.

ION BEAM

NOISE

The ion beam has an associated shot noise whose root mean square value is given by LS &IS = (2e Ii Af )*

(5)

where e is the electronic charge and Af is the frequency interval. Fluctuations in addition to the inherent shot noise can be caused by variations in any of the parameters on which ion beam intensity depends. These include filament temperature, ion beam electrode position, TCNE pressure, and the electrode voltages. The ion beam noise at 400 Hz, the frequency used. was measured with an amplifier-phase sensitive detector-chart recorder system connected to the Faraday cup. A beam current of 13 nA had associated noise of l+_ 1 - lo-l4 A in a bandwidth of 0.03 Hz. This is close to the shot noise limit of 1.2 - lo-l4 A. Thus the surface ionization source offers significant advantages over discharges. For example, Fite [I9 3was unable to reduce ion beam noise below 1000 times the shot noise level.

ACKNOWLEDGEhlENTS

We thank Mr. D. L. Swingler for advice on the construction of the quadrupole; Assoc. Prof. I. Lauder for use of the Atlas mass spectrometer; Mr. M. G. Browr; for help with electronics: the Australian Departments of Supply and Education for financial assistance (to L.D.P.); and the Australian Research Grants Committee for substantial support.

REFERENCES

1 L. M. Braascomband W. L. Fite, Phys. Rev., A, 93 (1954) 651. 2 B. Steiner, in E. W. McDaniel and M. R. C. McDowell (Ed%), Case Studies in Atomic Collision Physics, Vol. 2. North-Holland, Amsterdam, 1972, p_484.

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and Conlrol

in Science and Industr>,

Temperarare:

Vol. 3, Reinhold, New York,

515-52.2. Kaminsky, Atomic and Ionic Impact Phenomena at Metal Surfaces, Springer-Verlag, New York, 1965, pp. 56-93. F. M. Page and G. C. Goode, Negative Ions and the Magnetron, Wiley-Interscience, London, 1969, pp- 6-5. Derived from results presented in Ref. 3. R. C. Weast (Ed.), Handbook of Chemistry and Physics, Chemical Rubber Co., Cleveland, 54th edn., 197311974. J. R. Wcrning, U.S. At. Energy Comm. Report No. UCRL8455, 1958. W. M. Brubaker, 5th International Instruments and Measurements Conference, Stockholm. 1960, Vol. 1, Academic Press, London, 1961, p_ 305. W_ M. Brubaker and J. Tuul, Reo. Sci. Instram., 35 (1964) 1007. D. L. Swingler, Yacrrrrm,18 (1968) 669. K. J. Hanszen and R. Lauer, in A. Septier (Ed.), Focusing of Charged Particles, VoL 1,

9 M.

10 11 12 13 14 15 16 17

Its

1962, pp-

Academic Press, London, 1967, pp. X1-308. 18 P. H. Dawson and N. R. Whetten, Adcan. Elecrron. Electron Ph~s., 27 (1969) 114. 19 W. L. Flte, Phys. Reb., 89 (1953) 411.