Spectral Evaluation of a Sealed Helium Discharge Lamp for Studies in Photoelectron Spectroscopy James A. Kinsinger, William L. Stebbings, Richard A. Valenzi, and James W. Taylor1 Department of Chemistry, Uniuersity of Wisconsin, Madison, W i s . 53706
The construction and spectral properties of a windowed helium microwave discharge lamp for application to studies in photoelectron spectroscopy are presented. The 584-A light flux passing a 1500-A thick aluminum window from a 10-mm i.d. discharge is in excess of 1 x l O I 3 photons/sec. These intensities permit fluxes of 1011-1012photons/sec in a PES source and are sufficient to produce photoelectron spectra of high resolution without spectral interference from impurity lines. Operation of the lamp at lower pressures produced the Hell line at 304 A. The intensity of this line relative to the 584-A line could be obtained routinely at 14% and intermittently up to 50%. Applications of filter techniques are discussed which might increase the 304-A intensity in this type of discharge, or others employed for PES studies.
where hv is the energy of the ionizing radiation. The use of vacuum ultraviolet monochromators can eliminate the spectral impurities (IO, 11) but results in reduced photon flux because of low grating reflectivities at these short wavelengths (5315). It has been known for some time (5,16-19) that thin metal films can function either as windows or filters in the vacuum ultraviolet; and work at the National Bureau of Standards (20-22) provided a striking example of how aluminum windowed rare gas resonance lamps could be used for photoionization studies. The NBS results accelerated work in these laboratories to produce windowed light sources for photoelectron spectroscopy. EXPERIMENTAL
LINE EMISSIONS used for photoelectron spectroscopy are normally generated by microwave ( I ) or dc discharge ( 2 , 3) in a low pressure gas. The most common gas is helium which produces primarily the He1 line at 584.334 A (21.216 eV). The line sources which are shorter in wavelength than 1040 A have usually been operated windowless because no common window materials have been found with transmission at shorter wavelength than the 1040 cutoff for LiF (4). Operation in the windowless mode not only poses problems in differentially pumping the flowing gas stream but also requires that the flowing gas be of high purity. Feed gas impurities (5, 6) can lead to extraneous spectral lines (5-8) which in turn make assignment of the ionization potentials, I,, difficult (9-14). This difficulty occurs because the kinetic energy of the ejected electron, En,is related to In in a simple form by the equation:
A
E,
=
hv
- I,
(1)
Correspondence should be addressed to this author. (1) D. C. Frost, C. A. McDowell, and D. A. Vroom, Proc. Roy. SOC.(London), A296, 566 (1967). (2) M. I. Al-Joboury and D. W. Turner, J . Chem. SOC.,1963, 5141. (3) D. W. Turner, Proc. Roy. SOC.(London), A307, 15 (1968). (4) A. H. Lanfer, A. A. Pirog, and J. R. McNesby, J . Opt. SOC. Amer., 55, 64 (1965). (5) J. A. R. Samson, “Techniques in Vacuum Ultraviolet Spectroscopy,” John Wiley and Sons, New York, N.Y., 1967. ( 6 ) D. W. Turner in “Physical Methods in Advanced Inorganic Chemistry,” M. A. 0. Hill and P. Day, Ed., Interscience, London, 1968, Chapter 3, p 80. (7) H. Okabe, J. Opt. SOC.Amer., 54, 478 (1964). (8) W. L. Stebbings and J. W. Taylor, Int. J . Mass Spectrom. Ion. Phys., 6, 152 (1971). (9) P. Mitchell and M. Wilson, Chem. Phys. Lett., 3, 389 (1969). (10) V. Fuchs and H. Hotop, ibid., 4, 71 (1969). (11) J. A. R. Samson, ibid., p 257. (12) R. B. Cairns, H. Harrison, and R. I. Schoen, Appl. Opr., 9, 605 (1970). (13) D. W. Turner in “Annual Review of Physical Chemistry,” Vol. 21, C. J. Christensen and H. S. Johnson, Ed., Annual Reviews Inc., Palo Alto, Calif., pp 107-128. (14) C. R. Brundle, Appl. Spectrosc., 25, 8 (1971).
Apparatus. The microwave generator used in these studies was a Burdick Model MW/200 2.45 Ghz, nominal 125-watt diathermy unit coupled with an Opthos Instrument Company Evenson forced-air cooled tunable cavity (23) which was modified according to the sugestions of McCarroll (24). With an Opthos Model 725.3 reflected power meter, it was possible to adjust the coupling mismatch such that a considerable reduction in heat transmitted to the discharge tube could be achieved with a corresponding reduction in gas impurities introduced from the quartz. The continuum source used in evaluation of the aluminum filter transmission was the University of Wisconsin Physical Sciences Laboratory synchrotron-storage ring. The spectral properties of this source have been described (25). The absolute intensity measurements were made with a triple-plate ionization chamber designed to accept the total beam from a one-meter normal-incidence monochromator. The chamber will be described in a subsequent publication; its design is similar to the double-plate chamber of Samson (26) and the NASA five-plate unit ( 2 7 ) . Wavelength scans were accomplished with a McPherson Model 225 one-meter normal-incidence vacuum ultraviolet monochromator equippFd with a 1200 l/mm gold-overcoated grating blazed for 800 A. The aluminum filter was attached
(15) W. R. Hunter, J. F. Osantowski, and G. Hass, Appl. Opt., 10, 540 (1971). (16) W. C. Walker, 0. P. Rustgi, and G. L. Weissler, J. Opt. SOC. Amer., 49, 471 (1959). (17) R. P. Madden, L. R. Canfield, and G. Hass, ibid., 53, 620 (1963). (18) 0. P. Rustgi, ibid., 55, 630 (1965). (19) W. R. Hunter, D. W. Angel, and R. Tousey, Appl. Opt., 4, 891 (1965). (20) P. Ausloos and S . G. Lias, Radiat. Res. Ren, 1, 75 (1968). (21) R. E. Rebbert and P. Ausloos, J. Amer. Chem. SOC.,90, 7370 (1968). (22) R. Gordon, Jr., R. E. Rebbert, and P. Ausloos, Nat. Bur. Stand. ( U S . ) Tech. Note, 496, 1969. (23) F. C. Fehsenfeld, K. M. Evenson, and H. P. Broida, Rev. Sci. Instrum., 36, 294 (1965). (24) B. McCarroll, ibid., 41, 279 (1970). (25) C. Gahwiller, F. C. Brown, and H. Fujita, ibid., p 1275. (26) J. A. R. Samson, J . Opt. SOC.Amer., 54,6 (1964). (27) H. H. Kim Shardanard, A. J. Caruso, and A. F. Barrington, Rev. Sci. Instrum., 39, 503 (1968). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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Figure 1. Helium discharge lamp (without window attached)
to an off-center shaft t o intercept the dispersed light beam striking a Bendix Model 4503 secondary standard Channeltron multiplier (28). Measurement of the photoelectron spectrum was done with a parallel plate analyzer constructed in these laboratories similar t o that developed by Eland and Danby (29) from a design of Harrower (30). It will he described in a subsequent publication. Materials. The helium used in these studies was Matheson 99.9999% purity. Both windowless and windowed discharge lamps were constructed. The windowless lamp was baked to 150 ‘C and pumped to lo-$ Torr to remove water and other impurities before introduction of the purified helium. Further purification of the helium from the cylinder was accomplished by passing it through a liquid nitrogencooled trap filled with Linde SA molecular sieves (8). In the (28) M. C. Johnson, Reo. Sei. Insfrum., 40,311 (1968). (29) J. D. H. Eland and C. J. Danby, J. Sci. Instrum., Ser. 2,1,406 (1968). (30) G.A. Harrower, Reu. Sci. Insfrum.,26,850 (1955).
-
Figure 2. Discharge spemUm from flowing helium system
.
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E
I
Y
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
windowedlamp, the helium was subjected to titanium gettering action for only five minutes before initiating the discharge. The titanium getter wires (Vacuum Instrument Co., New York) were spot welded on a borosilicate glass flare support in a manner similar to the NBS lamp arrangement (22). The filling and pumping flanges, however, differed and Figure 1 shows the arrangement of the components. The aluminum windows, Model TFlO1, were fabricated hy Sigmatron, Ipc., Santa Barbara, Calif., and consisted of a 1500 + 100 A film of aluminum, (0.62 in. in diameter). This film was supported on 70 mesh, 0.0008-in. thick, nickel screen which had 83% transmission. This particular thickness was chosen for freedom from pin-holes and for mechanical strength. The windows were back-lighted and examined under a high power microscope to detect pin-holes before sealing to the 10-mm i.d. quartz tube with Varian Torr-Seal epoxy cement. After allowing the epoxy to dry overnight, the outer part of the window was cut away with a razor blade. Operation of the Windowed Lamp. The lamp volume was determined by measuring the pressure after a known pressure and volume of gas was expanded into it. The lamp preysure could, thereafter, be determined by this procedure. After the lamp was filled at a known pressure, one getter was fired at 8.2 A for approximately 5 minutes, the microwave power generator was turned on, and the discharge initiated with a Tesla coil. The center of the discharge was 13.4 cm from the window and 16.9 cm from the entrance aperture of the PES source. The aluminum window was encircled with a cap which was closely fitted t o the 4-mm entrance aperture of the PES source compartment. This cap allowed only 6% of the total lamp flux t o enter the PES source but permitted the source to operate in the 100-mTorr range while the analyzer and channeltron detector were operated at Torr. Opposite the lamp entrance a closely-fitted light trap was used to reduce the scattered electrons caused by the photons not intercepting sample molecules. RESULTS AND DISCUSSION Early in the photoionization mass spectrometry studies in these laboratories, the problem of spectral purity of the discharge was faced (8). Figure 2 shows the first microwave discharge spectrum obtained from a flowing gas lamp similar t o those described in the literature. The “correct” color was obtained from this discharge (6,221 but the 584-A line was found to be a small fraction of the total line emission. Replacing laboratory grade helium with that of 99.9999Z purity reduced considerably the number of emitting impurities as can he seen
I
Figure 3. Recorder tracing from monochromator with discharge from high purity helium (dotted lines) and from purified high purity helium (solid lines)
1250
1200
I150
I
b
~
1300
1100
1050
1000
I
950
600
Angstroms
from Figure 3 (dotted lines), but the hydrogen, nitrogen, and oxygen impurity emissions remained. By using liquid nitrogen cooled traps filled with molecular sieves, the nitrogen impurity was eliminated and the hydrogen and oxygen impurities were reduced (Figure 3, solid lines). Even more rigorous purifications, however, offered no protection from the possibility that oxygen, nitrogen, and hydrogen might be generated from heating of the tube as the discharge is operated. Flowing fresh gas tends to sweep these impurities, but the excitation process for the traqsfer from helium to the impurity is quite efficient. For example, a lamp specifically designe{ for maximum output of the hydrogen Lyman a line at 1216 A may employ 2 % or less hydrogen in helium (5,31). The use of a monochromator with the line source would assure spectral purity. Indeed, Samson (11) has done this to resolve partially the controversy which has arisen over the orbital assignment in the photoelectron spectrum of benzene. The difficulty, however, lies in the loss in intensity which arises from the poor grating reflectivity at 500-600 A when the grating is operated at normal or near-normal incidence. For example, the McPherson 225 one-meter instrument operating at an angle of 7.50" from the grating normal is only 5-7% efficient at 584 A (5). Higher angles approaching grazing incidence yield higher reflectivities. Hunter et al., for example, report approximately 80 % reflectance at 85" incidence for aluminum mirrors overcoated with MgFz (15). Not all laboratories, however, have access to a normal incidence monochromator, much less to the more expensive grazing incidence instruments. In the absence of these instruments, one must consider a monochromator substitute which is usually a bandpass filter. Figure 4 shows the transmission of one such filter which consists of a 1500 + 100 A film of aluminum supported on 80mesh nickel wire. Plotted on the figure is the per cent transmission obtained from observation of the continuous emission
from the Physical Sciences Laboratory synchrotron-storage ring as dispersed by a one-meter monochromator. Three features are of interest in this figure. The first is that the transmission at 304 A is high (55%). This wavelength corresponds to the He11 line (303.782 A (32), 40.81z eV). The second feature is that the transmission at 584 A is 24.5%. The aluminum film thus provides greater 584 A light intensity by a factor of 3 to 5 than would be expected from a normal incidence monochromator. These figures are in general agreement with the data of others (16-19). The third feature is that beyond 825 A, the film is opaque. Extraneous trace impurities present in the discharge have t&eir most intense emission at wavelengths longer than 950 A, (See Figure 3); this means that their contribution is excluded from the PES source and no extraneous electrons are produced from the impurity radiation. In one experiment to confirm this conclusion and to demonstrate the sensitivity of the 584-A emission to impurities, the lamp discharge was operated at 60% microwave power with 2.5-Torr helium pressure but without firing the getter. The photon flux measured with argon in the triple-plate ionization chamber monitor in this case was 2.9 X lo7photons/sec. After firing the getter for five minutes, however, the measured flux was 1.8 X 1011photons/sec, a gain of l o 4 in photon flux. In similar experiments with the photoelectron spectrometer as the flux monitor and 100-mTorr argon in the source, no electrons of any kinetic energy were detected (above background counts of one/sec), and more than 3000 counts/sec on the argon *P3lzpeak at 15.759 eV at 30-meV resolution were obtained after the getter was fired. The gettering action removes the initial impurities present in the discharge, thereby preventing the excitation transfer from helium t o the impurity. The freshly prepared titanium surface also reacts with any additional gases produced as a result of heating from the discharge and may be expected to be effective for all but the noble gases.
(31) D. Davis and W. Braun, Appl. Opt., 7, 2071 (1968).
(32) B. Edlin, Rep. Progr. Phys., 26, 181 (1963). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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is reduced by a factor two when this thickness of oxide forms. '0°1
quite stable unless the discharge is allowed to come too close to the window. When this occurs or when the window is exposed to laboratory atmospheres containing relatively high humidity, the window will perforate with a multitude of tiny pinholes. With adequate precautions, however, these two conditions can be avoided. The perforation under high humidity conditions is interesting because one method of releasing the films from the vacuum evaporation support involves flotation in water (22). It is possible that a reaction between the support mesh-window cement and water may be occurring or that the stress at the point of attachment causes tearing as the surface hydration changes. These or other explanations require further investigation. From an examination of the PES spectrum of argon, it was possible to determine the contribution of the smaller He1 line which occurs at 537 A (537.030 (32), 23.087 eV). From the counting rates, the intensity of the 537-A line is approximately 2 of the intensity of the 584-A line at a lamp pressure of 3.2 to 2.0 Torr. It is known that the 584-A line is strongly reversed and from the data of Samson (33) we would estimate a line half-width on the order of 10 meV. This is less than the resolution of the PES analyzer and does not contribute significantly to the observed signal width, but the reversal does cause a reduction in sensitivity. The observed ratio of PES count rates, however, does correspond fairly closely to the factor of 60 quoted by Samson ( 5 ) for a dc discharge at 0.16 Torr, This 537-A line cannot be excluded with the aluminum filter, but the spacing of 1.87 eV between the 584-A and 537-A lines can be an aid in the PES interpretation. The emission of other lines in the 500- to 600-A region increased relative to the 584-A line as the lamp pressure was decreased. For example, when Ns was used in the PES source and the lamp pressure was reduced to 0.2-Torr helium, the following line intensities were estiomatedfrom theoelectron counting rate: 584 A-64.8x; 537 A-21.8%; 522 A-8.0Z; 515 A-2.8z; 512 A-1.7%, and 508 A-O.9%. These values assume equivalent cross sections for nitrogen ovcr this wavelength interval. Samson has observed the additional lines: 510.0, 515.7, 522.2, 591.4, and 600.4 A in a dc discharge at 0.16 Torr pressure but these were factors of 1000 to 5000 lower than the 584 A line (5). Along wiih the increase of intensity ?f the other lines in the 500- to 600-A region, however, the 304-A intensity also increased from operation of the microwave discharge at low pressure. It was possible, in a few experiments, to obtain counting rates from the 304-A excitation of N2 at 15.58 eV kinetic energy equaling the countingdates from 584-A excitation, but in mostFxperirnents the 304-A excitation was 1-5 % that of the 584-A excitation. The reason for the variation in the 304-A intensity is not presently known, but it appears to be very sensitive to pressure and somewhat sensitive to position of the microwave cavity. Stable operation at lower pressures than 0.2 Torr was precluded by the inability to maintain the discharge. The positional dependence may be explained if the window is reflecting microwave power back into the cavity. Other approaches to the production of the 304-A line which would be useful for PES studies have been described in the literature. At the present time the purity of metal shuttered microwave discharge has not been described by the Japanese workers (34-36) who initially proposed it, although their mass
A
Figure 4. Per cent transmission of 1500-A aluminurn filter
The photon fluxes quoted are those measured from ion currents in an ionization chamber which was designed with apertures to accept the full beam from a one-meter monochromator operated with 400-p maximum slit widths. To approximate intensities from the discharge lamp, it was necessary to assume that the discharge was centered at the microwave cavity and that the flux solid angle was limited by the tube internal diameter where the aluminum window was attached. With an ionization chamber aperture consisting of a tube 1-cm long and 2-mm i.d. located 14 cm from discharge, the ionization chamber accepted only 0.8% of the lamp flux. Using this figure, one can calculate that from the observed 1.8 X 1011photonslsec in the ion chamber, the lamp flux from the discharge is 2 X 10I3 photons/sec at 6 0 z microwave power meter reading and 2.5-Torr helium lamp pressure. This figure is in reasonable agreement with the NBS values where a freshly prepared lamp produced 3 X l O I 4 photons/sec under optimum condition with a 1000-A thick window; values in the range 3-4 X 1013 were obtained at reduced power (22). The NBS intensities were obtained from an ion chamber accepting the whole beam. Our studies confirm the NBS findings that the optimum helium lamp pressure is 1.6-1.8 Torr for maximum 584-A intensity. In Figure 4, there are three minima displayed on the curve which are real and have been explained by Hunter et a1.(19) as an interference effect enhanced by the presence of an oxide layer on the aluminum surface. Indeed, from the tran:mission data of Madden el al. (17) and from the observed 584-A transmission, we conclude that the oxide coating is between 30 and 40 A thick. There is no way to avoid this problem even though the transmission by aluminum of the 584-i%line 776
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(33) J. A. R. Samson, Rec. Sci. Instrum., 40, 1174 (1969). (34) I. Omura and H. Doi, Jap. J . Appl. Phys., 6,275 (1967). (35) I. Omura and H. Doi, Bull. Chem. SOC.Jup., 40, 1090 (1967). (36) I. Omura and H. Doi, US.Patent 3,541,373 (1970).
spectral evidence does indicate the presence of photons of higher energy than expected from the normal pressure discharge. Constricted capillary designs for dc discharges have been mentioned by both Turner (13) and Brundle (14) in their reviews. With the dc discharge designs, it would appear that a windowed system which maintains the helium purity in the discharge should increase the flux obtainable at 304 A. Whatever the discharge conditions, however, it would appear that pure 304-A line production may prove to be d f i cult without discrimination against the lines in the 500- to 600-A region. Two approaches to this problem lie in the use of other met$ filters whose transmission is greater at 304 A than at 584 A and in the use of absorption filters. Several possibilities for the metal filters are apparent from the literature (16-19), and work is in progress in these laboratories to evaluate their applicability to photoelectron spectroscopy. The use of gas absorption filters has been discussed by Poschenrieder and Warnek (37) in a related mass spectrometry study. For application to PES, a gas cell filled with argon appears to offer one excellent filter possibility. The absorption coefficient at 304 8, is 60 cm-l whereas at 584 A, it is approximately 980 cm-l (38). From these ratios, the argon (37) W. P. Poschenrieder and P. Warnek, ANAL.CHEM.,40, 385 (1968). (38) J. A. R. Samson in “Advances in Atomic and Molecular Physics,” D. R. Bates and I. Estermann, Ed., Vol. 11, Academic Press, New York, N.Y., 1966, p 203.
absorption should discriminate against the 584-A line, and others in the 500- to 600-A region, by a factor of 16 to 1. Coupled with the aluminum transmission efficiency from two windows necessary to contain the gas, the total discrimination should approach a factor of approximately 60. To employ this combination, however, an extremely intense source will be necessary to overcome the light losses from divergence of the photon beam. This approach, however, does offer one other possibility for laboratory production of 304-A light for PES studies. We are presently investigating this combination. ACKNOWLEDGMENT
We thank Pierre Ausloos for his most helpful information on the construction of the NBS lamps. We also thank David Lynch of Iowa State University for the use of the McPherson monochromator; the Physical Sciences Laboratory personnel for the operation of the synchrotron-storage ring under AFSOR contract. RECEIVED for review November 2,1971. Accepted December 13, 1971. A portion of this work was presented at the International Conference on Electron Spectroscopy, Pacific Grove, Calif., Sept. 1971. J. A. K.was a NSF Trainee, 1967-1969; NSF Fellow, 1969-1971. This research is supported by the Air Force Office of Scientific Research under AFOSR 69-1725 and by the Wisconsin Alumni Research Foundation.
Pulse Overlap Effects on Linearity and Signal-to-Noise Ratio in Photon Counting Systems J. D. Ingle, Jr.,l and S . R. Crouch Department of Chemistry, Michigan State University, East Lansing, Mich. 48823
At moderately high light levels found in many spectrometric applications, pulse overlap in photon counting systems can cause nonlinearity. Under these conditions, the effect of the photon counting circuit components on the readout, the readout variance, and the signal-to-noise ratio are discussed. Three limiting types of photon counting circuits are considered. A mathematical treatment reveals that the relationship between the observed and true pulse rate is dependent on the type of counting circuitry, the dead time, and the discriminator level. A new treatment of dead time compensation reveals that the linear range can be extended over an order of magnitude. Under conditions of significant pulse overlap, the variance is lower and the signal-to-noise ratio is higher than predicted by the Poisson distribution.
PHOTON COUNTING in spectrometric systems has been discussed and compared to other photon measurement systems by many authors (1-14). The inherent advantages of photon Present address, Department of Chemistry, Oregon State University, Corvallis, Ore. 97331, (1) R. G. Tull, Appl. Opt., 7 , 2023 (1968). (2) M. L. Franklin, G. Horlick, and H. V. Malmstadt, ANAL. CHEM., 41, 2 (1969). ( 3 ) F. Robben, Appl. Opt., 10,776 (1971). (4) J. K. Nakamura and S . E. Schwatz, ibid., 7 , 1073 (1968). ( 5 ) J. Rolfe and S. E. Moore, ibid., 9 , 63 (1970). (6) A. T. Young, ibid., 8, 2431 (1969).
counting, such as direct digital processing of discrete spectral information, discrimination against dark current not originating at the photocathode, elimination of reading error, and system stability against drift, have made this technique attractive for many applications. Particularly at low light levels, where the signal-to-noise ratio (S/N) approaches unity, the inherent S/N for photon counting has been shown to be better than obtained from other detection techniques (4, 8). Because of the better S/N at lower light levels, precision is increased and spectral resolution can be improved since the slit width can be reduced while still retaining a favorable S/N. For many analytical atomic and molecular spectrometric applications, the light levels are high enough that, in addition to the S/N, it is important to know the factors influencing the (7) J. A. Topp, H. W. Schrotter, H. Hacker, and J. Brandmuller, Reo. Sci. Instrum., 49, 1164 (1969). (8) R. R. Alfano and N. Ockman, J. Opt. SOC.Amer., 58,90 (1968). (9) E. H. Piepmeier, D. E. Braun, and R. R. Rhodes, ANAL. CHEM.,40, 1667 (1968). (10) K. C. Ash and E. H. Piepmeier, ibid., 43, 26 (1971). (11) R. Foord, R. Jones, C. J. Oliver, and E. R. Pike, Appl. Opt., 8, 1975 (1969). (12) R. H. Eather and P. L. Reasoner, ibid., 8, 227 (1969). (13) C. J. Oliver and E. R. Pike, Brit. J. Appl. Phys., 1, 1459 (1968). (14) G. A. Morton, Appl. Opt., 7 , (1968). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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