Characteristics of Vacuum Ultraviolet Emission from a Pulsed Microwave Helium Discharge Carl C. Garber and James W. Taylor" Department of Chemistry, University of Wisconsin, Madison, Wis. 53706
The spectral properties of a pulsed microwave d-ischarge in a windo-sealed helium lamp are presented. Although the atomic lines between 500 and 600 A (especially He 1584.3 A) dominate the emission spectrum, He II 303.8-A emission is observed at pressures less than 1.5 Torr. Maximum He II intensities are obtained at the lowest pressures. The relative intensity of the emission at 303.8 A is strongly dependent on the discharge tube size. Time resolved spectroscopic analysis of He I and He II emission reveals that the He II 303.8-A line, appears before the onset of He I emission. This observation is more pronounced in a 4-mm i.d. discharge tube than in a 10-mm i.d. discharge tube. It is shown that time can be used to discriminate against the He I line emission in applications where isolation of the He II 303.8-A emission is required.
Vacuum ultraviolet emission from rare gas discharges has been produced by dc excitation in both the continuous (1-5) and pulsed modes (6) and by electrodeless high frequency (7), microwave (%IO), and by 0 pinch excitation ( 1 1 ) in either flowing (or windowless) or window-sealed static gas configurations. Application to photoelectron spectroscopy has involved primarily the continuous dc discharge and t o a lesser extent the CW microwave (10, 12) excitation of helium. In both modes, the principal emission line is due to the atomic resonance transition (1lSi 2 lP) at 584.3 A (21.22 eV). Other rare gases have been used less extensively than helium (13) because of their lower photon energy and because the neutral resonance spectra are characterized by doublet emission (14). These doublets add to the complexity of photoelectron spectra. Attempts to utilize the ionic He I1 resonance line (2 2P 1%) a t 303.8 (40.2 eV) have also been reported in the more recent literature, but the scarcity of grazing incidence monochromators has restricted widespread monochromatic application of the He I1 photon emission to photoelectron spectroscopy. In general, it has been demonstrated that high powered discharges a t low pressure have resulted in the greatest He I1 emission intensities (7, 15, 16). The purpose of this work is to investigate the helium discharge under various laboratory experimental conditions with the goal of determining how the He I1 intensity can be maximized with respect to the He I emission and what mechanistic processes influence the observed emissions in helium.
-
-
EXPERIMENTAL The microwave discharge lamp described previously ( I O ) was modified to enable direct pressure measurements of the helium gas (Wallace and Tiernan Model No. 62B-4A-0005, Bourdon type) and to facilitate the interchanging of quartz discharge tubing. The discharge was produced in 4-mm i.d. and 10-mm i.d. quartz tubing 25 to 35 cm in length. A 1500-A thick aluminum window (Luxell Corp., Model TFlOla) was sealed on one end. The opposite end was connected via a Caion Ultra-Torr stainless steel union to a ballast volume containing gettering material. The titanium filaments used previously had the disadvantage of rendering the walls opaque as a titanium film ' was deposited. A flashless, nonevaporating getter (Saes No. S t 171/HI/7-6/150) which operates by physical and chemisorption of 2070
nonrare gases was found to remove impurities quite effectively while a t the same time maintaining the lamp walls free from deposits. Two types of tunable microwave cavities were employed: a McCarroll type cavity as described previously ( I O ) and a cylindrical cavity similar to that of Skogerboe (17). The cavity was connected (Figure 1)by coaxial cable through an isolator (Raytheon Model No. 1,SL 39) with a 5-kW peak power termination and two directional couplers (Narda Model No. 3003-30) to a pulsed microwave power supply (Applied Microwave Model PG5KB). An S band 1N21B mixer diode, which was used as a microwave detector, was mounted on the attenuated output of each directional coupler. The diode output ( 5 0 4 load) was displayed on an oscilloscope (Hewlett-Packard Model 180 A) for observation of the waveforms of the forward and reverse power. Without an isolator, a measurement of the reflected and forwvd power indicated that 35% of the incident power was reflected bbck into the power supply under typical operating conditions. Cable losses and insertion losses of the directional couplers accounted for a 10% attenuation of the applied power. Thus only 55% of the applied power was dissipated by the discharge. Introducing an isolator into the power circuit reduced the reflected power to 2% or less. The maximum rating for the power supply triode oscillator is 4% reflected power, corresponding to a voltage standing wave ratio (VSWR) of 1.50. It was possible to tune either of the cavities to reflect zero power when used with a CW diathermy microwave unit (Burdick Model MW/200). The helium discharge directly illuminated the entrance slit of a 1-m Seya type monochromator which was equipped with a 1440 l./mm rhenium over-coated grating blazed for 600 A in the Seya configuration. A detection system consisting of a sodium salicylate coated window and a water cooled photomultiplier tube (EM1 type 9813B) was positioned a t the exit slit. A reduction in the photomultiplier dark counts of one order magnitude was observed upon reducing the temperature from 25 to 15 "C. The photomultiplier tube output was amplified (Mechtronics Photon Discriminator Model 511) for pulse counting with a Hewlett-Packard 5325B Universal Counter. For time resolved studies, a scanning digital boxcar was constructed (Figure 2). The scanning boxcar circuit consists of two ramp generators with appropriate resets. A differential comparator (bA760DC) fires a monostable (74121) a t the cross-over point of the two ramps on each trigger pulse of the microwave power supply. The monostable subsequently enables a high speed AND gate ( 7 4 H l l ) for the duration of the selectable gate width. The minimum gate is limited by the monostable a t 25 ns. The coincident pulses from the amplifier-discriminator were counted by the pulse counter. The oscilloscope (Hewlett-Packard Model 180A) was also used for gate width adjustment and for time measurements of time resolved spectroscopy. A four-digit digital-to-analog converter (Date1 Model No. 169-160) converted the range selectable seven-digit BCD output of the counter to an analog signal for recording on the X channel of an X-Y recorder (Leeds & Northrup Model No. XL/680). The real time voltage ramp from the scanning digital boxcar drove the Y channel for time resolved studies. The instrumental delay in the detection and counting system which is a constant for all the time resolved measurements is estimated to be 90 ns. With the McCarroll type cavity, the plasma became unstable a t pressures below 1.0 Torr. Under these conditions, irradiation with a Hg lamp (Oriel Optics No. 0-73-16, 17 mA) facilitated breakdown through ionization of trace impurities in the quartz tube. Stable plasmas were achieved in this manner for pressures as low as 0.2 Torr with the 4-mm i.d. quartz tube. With the cylindrical cavity (171, the discharge remained stable for pressures down to 0.35 Torr below which the discharge extinguished. The Hg lamp had no effect on the operation of the discharge with this cavity.
RESULTS AND DISCUSSION The experimentally controllable parameters affecting line emission which are considered in this study include:
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
a. Type of discharge cavity; i.e. mode of coupling of the microwave power to the discharge b. Pulsed or continuous mode of microwave excitation c. Excitation pulse width, pulse repetition frequency, and peak power d. Pressure of helium gas e. Diameter of discharge tube f. Time of emission relative to initiation of the discharge. Typical of the data obtained are the results plotted in Figure 3 for near-optimum excitation parameters compatible with the power supply (3.5 kV, 0.5-ps pulse duration, and 10 000 pulses per second) at extreme pressures of 0.50 and 26.6 Torr in a 4-mm i.d. discharge tube. At helium pressures less than 1.5 Torr, the ionic emission can be observed a t 303.8 A (He 11) or a t 607.6 (2nd order). Because of the blaze efficiency of the grating, a higher intensity was observed in second order. The helium Hopfield continuum is observed under pulsed microwave conditions (18)for pressures as low as 1.7 Torr but is not seen in the CW microwave excitation (6, 10). The observed continuum is attenuated by the transmission of the aluminum window ( 1 0 ) and, above 680 A, the Hopfield continuum is observed only a t very low intensities. A more complete discussion of microwave generation of molecular emission will be presented in a later report. A summary of the observations relating atomic helium emission to the above mentioned variables follows. 1) The two microwave cavities studied herein produced essentially equivalent intensities for the atomic resonance lines. 2) The application of either pulsed or CW microwave energy resulted in similar pressure dependence for the He I 584.3-A line (the a curves in Figure 4 and 5 ) . For the He I 537.0-A, 522.2-A, and 515.6-A lines, a different pressure dependence was observed between the two methods of excitation (curves b, c, and d , respectively, of Figure 4 for the pulsed discharge and Figure 5 for the CW). 3) It was observed that the He I line intensities increased linearly with the applied microwave peak power. In the case of the pulsed power supply, this was also indicated by the plate voltage of the Y599 triode oscillator. This observation covered the range of 1.0 to 4.0 kV. Furthermore, the ratios of the intensities of the upper resonance lines (n = 3,4,5) with respect to the He I 584.3-A line ( n = 2) increased with the applied c
m C1;500pl c2.20pt
RI:
1
A
I
1I
L
M
Figure 1. Experimental arrangement by components A, microwave power supply: B, directional coupler; C, isolator; 13, discharge cavity; E, 1-m Seya type monochromator:F, photomultipliertube; G, PMT high voltage: H, amplifier/discriminator:J, scanning boxcar; K. oscilloscope; L, pulse counter: M, digital-to-analog converter; N, X-Y recorder; and P, microwave
diode
power. For example, in the 4-mm tube, the He I (537.0 A, n = 3) line intensity, as a percentage of the 584.3-A line intensity, varied from 3% at 1.0 kV to 15%at 3.5 kV for a 1.0-ps excitation pulse at 5000 pps and at a helium pressure of 3.6 Torr. By comparison, the 537 AI584 A intensity ratio varied from 2.5% to 4.0% when the CW microwave discharge was changed from 10% to 80% of full power with 3.0 Torr of helium in the tube. These He I intensities were also observed to increase linearly with the excitation pulse duration. In a manner similar to the voltage effect, the ratios of the higher energy resonance lines-relative to the 584.3-A intensity-increased with the pulse duration. The effect of the pulse repetition frequency (prf) on the He I lines was studied over the range from 100 to 16 000 pps, with pressures of 0.45 to 25 Torr, and excitation pulse widths from 0.2 to 2.0 ps. The intensities of the He I lines and the helium continuum were observed to increase according to the nonlinear model (19):intensity a (prf) where m is defined as an intensity enhancement factor. The enhancement factor was found to be independent of pressure but increased with the principal quantum number n for the He I lines (Table I) for pressures above 1.5 Torr. At lower pressures, for example 0.50 Torr, it was observed that the breakdown time decreased from 0.2 to 0.1 ws as the repetition rate increased from 3000 to 10 000 pps. Hence the effective “on” time of the discharge was 33% longer at the higher frequency. The values were also
iwn
C3:200~f C4~xXx)pl C5~002pf C6 -580pl
~6=47on
C7:3000pf C8 :10*1
R7-RI3: 3 9 K - 4 0 0 K n R14-R24:50K-2 OMn
Dl:lN914
CALilKfl PI=I K - I M f l
D2ilN5230
I
5!2!95 R2:IKn R3:IOKO R44Zfl R5- MOfl
-0
r
P2:IK-IOMfl P 3 1K-IOOKfl P 4 2K-50Kn
FAST PAW s4kpLHG - )C L
m
GATE
”m
FUSE
M
PDSlTlVE
L
I
W
NGATIVE W I N
Figure 2. Scanning boxcar schematic (TTL logic component power supply connections are omitted for clarity)
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2071
"1 J
IO'
4 "1 f
Wavelength
I&
Flgure 3. The pulsed microwave vacuum ultraviolet time avemged emission of helium at 0.50 Torr (- -)and 26.6 Torr (-). (The implied wavelength shift is for illustration only)
-
/
Helium P r e q w r e (torr)
/
I
Figure 5. The CW microwave emission of atomic and ionic helium as
a function of pressure in a 10-mm i.d. discharge tube and at 30% power
'Pb \
Flgure 4. The pulsed microwave emlssion of atomic and ionic helium as a function of pressure and discharge tube diameter of 4-mm i.d. (- - -) and IO-mm i.d. (-) a, He 1584.3 A; b, He 1537.0 A; c. He 1522.2 A; d, He 1515.6 A; and e, He II 303.8 A (second Order)
normalized by multiplying by suitable factors to make all effective on-times the same at 0.4 p s , assuming a linear relationship of intensity with pulse width. These values are displayed in the table with their calculated m values. Because the intensities were linear with pulse duration but showed a stronger dependence on the pulse repetition frequency, it was concluded that operation of the discharge under limited duty cycle conditions (pulse duration times pulse repetition frequency) would yield maximum intensities for small pulse duration and high pulse repetition frequency. This 2072
a, He 1584.3 A; b, He 1537.0 A; c, He 1522.2 A; d, He 1515.6 A; and e, He 1 I 303.8 A (second order)
prediction was observed. Optimum conditions were near 0.5-ps pulse duration and 10 000 pulses per second for a duty cycle of 0.005 (the maximum for the PG5KB power supply). 4) The pressure dependent data in Figure 4 were obtained a t these near optimum conditions with the peak power setting at 3.0 kV. Figure 5 indicates the pressure dependence under CW microwave excitation at 30% full power. The He 1584.3-A line was observed at maximum intensity in the region of 1.5 to 2.0 Torr for both types of discharge while the other atomic lines were a maximum at 1.0 Torr (pulsed) and lower pressure (0.35 to 0.5 Torr for the CW). 5) A very strong dependence of the He 1584.3-A intensity was observed on the tube size (Figure 4 and Table 11).The main resonance line intensity was decreased by a factor of 11 at 1.70 Torr as the tube diameter was changed from 10 to 4 mm. The other resonance lines were reduced to a lesser extent; 537.0 A by a factor of 3.0; 522.2 A by 1.6; and 515.6 A by 1.1a t the same pressure in the smaller discharge tube. The minimum pressures for the pulsed discharge were 0.35 Torr for the 4-mm tube and 0.15 Torr for the 10-mm tube while the minimum pressure in the CW operation remained at 0.35 Torr in both tubes. 6) Time resolved analyses were performed on the He I 584.3-A, 537.0-A, 522.2-A, and 515.6-A lines. Typical results are shown in Figure 6 for 537.0 A at 1.70 Torr and 17.0 Torr in the 4-mm discharge tube. The source conditions here were 3.0 kV plate voltage, pulse duration of 0.50 p s and 10 000 pulses per second (pps). In all cases, a sampling gate of 50 ns
ANALYTICAL CHEMISTRY, VOL. 48. NO. 14, DECEMBER 1976
Table I, Value of Enhancement Factor, m,Relating Intensity and Pulse Repetition Frequency
Principal quantum number, n
Emission line
4-mm i.d. tube m m (P2 1.5 Torr) (P= 0.50 Torr)
He 1584.3 8,
2
1.09 f 0.08
He I 537.0 8,
3
1.15 f 0.06
1.28 1.04= 1.50
1.23 10.09
1.30a 1.58
4
He 1522.2 8,
10-mm i.d. tube m m (P2 1.5 Torr) (P= 0.50 Torr) 1.24 f 0.20
1.42 1.14" 1.38 1.10Q
1.25 iz 0.22
...
...
1.35n 5
He 1515.6 8,
1.31 1.07" Enhancement factor after correction to a fixed 0.4-ps excitation period. See text.
He I1 303.8 8, a
...
1.25 f 0.15
2
No emission
No emission
1.26 1.02=
Table 11. Comparison of Typical Intensities of Helium Line Emission=
Pulsed
CW'
0.45 Torr Line He 1584.3 8,
4-mm 1800
1.70 Torr
1.70 Torr,
10-mm
4-mm
10-mm
10-mm
14 000
5 600
60 000
1 100 000
1350
537.0 8, 120 450 455 1350 19 500 522.2 8, 40 100 135 215 4 750 515.6 8, 20 30 65 70 1800 He I1 303.8 8, 185 175 ... ... ... a No corrections are made for transmission or reflectivity coefficients. Pulsed conditions: 3.0 kV, 0.5-ps pulse width, lo4 pulse/s; estimated error, f5%. CW conditions: 30% power; estimated error, 1 2 %
was employed. In general, He I emission was observed throughout the excitation period. In the case of the discharge in the smaller 4-mm tube, the pressure dependent decay rate was observed to be exponential. The rate constants observed for the He 1584.3-A and 537.0-A lines are plotted in Figure 7 as a function of pressure. When the discharge was operated in the larger 10-mm tube, the decay rate was no longer exponential. Approximation of the decay rate as exponential yielded decay rates nearly two times slower than those for the 4-mm case. In addition to the active period emission, atomic emission was also observed very weakly in the afterglow. The results for He I1 303.8-A emission line indicated that the intensity was increased by a factor of five with the cylindrical cavity compared to the McCarroll type cavity. Consequently, the results reported were obtained with the use of the cylindrical cavity. Ionic emission was observed for both the pulsed and CW excitation. However as indicated in Figures 4 and 5 , curve e , the pressure dependence was not the same for the two types of discharge. As with atomic helium emission, the ionic emission increased linearly with the applied power. For short excitation pulse widths, the He I1 intensity showed an initial linear increase with pulse width, but larger widths produced intensities which were invariant with increasing pulse width. This onset of pulse width independence was a function of pressure with invariance occurring at shorter pulse widths as the pressure was increased. The He I1 intensity increased in a nonlinear fashion with increasing pulse repetition (prf). To determine the functionality, the data were applied to the same model as before-intensity a (prf)m.For the He I1 line a t 0.50 Torr with 3.0 kV applied for 0.5 ps over a range of 3 000-10 000 pps, the value of m was determined to be 1.31,
90.
I IPlbi
I
Figure 6. Time resolved spectra of He I 537.0 A at 1.70 Torr (-) 17.0 Torr (- -)
-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
and
2073
2o
IO
1
-I
I
-
I
'.
0
10-
r
I
b
0'
8 -
.-.
Flgure 9. Comparison of time resolved spectra of He I 584.3 A (- -, intensity divided by lo), He I 537.0 A (-), and He II 303.8 A (- -,second order) at 0.50 TLorr
-
1
I
I
I
I
Hilum Picrruri
Flgure 7. Decay rate of He I 584.3 (upper)
r
I
I
100
10
0'1
IO00
(me1
A
(lower) and He I 537.0
A
IOU
I
i
- - _ ... , \ \
\ \
\ \ \
._ - ..
,
I
'\
\
\
00
01
04
06
OB
10
' \
I2
14
T3m. ipr.r.1
Flgure 8. Time resolved spectra of He II 303.8 A (second order) at 1.O Torr (-), 0.55 Torr (- -), and 0.30 Torr (- - -)
-
-
Table I. However, by correcting for the change in effective pulse width, a linear relationship between He I1 intensity and the pulse repetition frequency was obtained. From the relationships between the He I1 intensity and the excitation pulse width and pulse repetition frequency, it was expected that the optimum excitation conditions would be a small excitation pulse width and high pulse repetition frequency. This was observed. For example, at 0.35 Torr the optimum conditions were 0.60 ps and 8333 pps. As the pres2074
sure was increased, the optimum conditions changed to shorter excitation pulse width and higher pulse repetition frequency. Greater insight into the interrelationship of pressure and excitation pulse width on the He I1 intensity was indicated in the time resolved spectra in Figure 8 for an exciting pulse of 1.0-ps duration, 3.0 kV, and 5000 pps at helium pressures of 0.30, 0.55, and 1.00 Torr in a 4-mm discharge tube. A 50-11s sampling gate was set on the scanning boxcar. The intensity was observed to decay at two different time intervals for the high pressure cases. The early or premature decay in intensity is attributable to processes in the discharge and will be discussed later. The second decay of He I1 emission corresponded to the cut-off time of the excitation pulse (40 to 80 ns for the 10% to 90% fall time). Slight variations in cut-off time (and decay time) were directly related to the cavity tuning as evidenced by the shape of the forward and reverse waveforms displayed on the oscilloscope. As the excitation pulse duration was extended beyond the time interval of the earlier decay (Figure 8 ) , the He I1 intensity became almost independent of pulse width. A comparison of the time dependence of He I and He I1 is presented in Figure 9 a t excitation conditions of 3.0 kV,0.5-ps pulse width, and 10 000 pulses per second in a 4-mm tube. Not only was the ionic emission observed earlier in time, but the rate of increase in intensity was much greater than for the atomic emission. For a 10-mm tube under similar conditions, the difference in the time for emission onset was considerably less. If the ionic emission commences earlier than the atomic emission (see Figure 9), and the ionic emission decay rate is determined by the cut-off time of the excitation pulse (40 to 80 ns) while the He I (584.3 A) decays over a time span of microseconds, then it should be possible to discriminate against some of the He I (584.3 A) photons by reducing the detector gate width to that of the excitation pulse or less. Table I11 demonstrates how the variation of the detector gate width can be used to modify the intensity ratio of the He I1 303.8-A line to that of the He I 584.3-A line. The discharge was generated in both 4- and 10-mm i.d. tubing with the pulsed microwave power supply at 3.8 kV, an excitation pulse of 0.3 ,us at 16 000 pps and 0.40 Torr of helium. The He I1 303.8-A and He I 584.3-A intensities are only representative of their total intensities actually produced in the discharge. Factors which affect the observed intensities are window transmission, grating efficiency, and the division of intensity into the other diffraction orders. To make appropriate comparisons of their
* ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
describe and contrast various mechanisms which may be operative in the anticipation that an improvement in the knowledge of these processes will lead to better control of the radiation desired for a given vacuum ultraviolet experiment. He I Emission Analysis. The pressure dependence of the He I 584.3-8, line generated by the pulsed microwave discharge (Figure 4) is quite similar to the results for CW microwave excitation (Figure 5 and also Ref. 9). This suggests similar processes of excitation and de-excitation for the two systems. However, the pressure dependence of the higher energy lines (537.0 A, 522.2 %., and 515.6 A) shows significant differences between the two modes of excitation. In contrast to the monotonically decreasing intensity of these lines with increasing pressure in the CW microwave discharge, their emission intensity from the pulsed discharge reaches a maximum in the region of 1.5 Torr, similar to the pressure dependence of the 584.3-w line. The decrease in 537.0-w intensity has been observed in dc discharges (14) and is believed to be the result of the rapid associative ionization reaction
Table 111. Relative Change in the He I1 303.8-Aa to He I 584.3-A Intensity Ratio with Respect to Detector Gate Width
a. 4-mm discharge tube, He = 0.40 Torr Gate width, !Js
5.0 0.35 0.30
,
0.20
Diathermy (ungated)
He 11, counts 270 195 118
55 2 200
He I 584.3 A, counts 1150 190 33 5 5 500
He II/He I' 1.00 4.3 15 47 1.70
b. 10-mm discharge tube, He = 0.40 Torr 5.0 0.50 0.40 0.30
315 295 285 205
0.20
68 3 100
Diathermy (ungated)
15 500 3 975 2 350 790 97 380 000
0.086 0.32 0.51 1.1
3.0 0.035
He*
c. 10-mm discharge tube, He = 0.20 torr
5.0 315 3 100 0.43 0.50 305 1060 1.2 0.40 310 560 2.4 0.30 185 175 4.5 0.20 20 10 8.5 a 303.8-A intensity measured in second order at 607.6 A. Discharge conditions: 3.8 kV;0.3-ps pulse width, 16 000-pps rate. Ratio is normalized to 1.00 for gate width of 5.0 ps.
relative intensities, which are observed in the different experiments of this study, the absolute values of the relative intensities are not necessary; whereas, the changes in relative intensities are important. Consequently, the data ha+e been normalized for comparison by setting to 1.0 the unattenuated intensity ratio obtained in the 4-mm tube. In the 4-mm tube, the relative intensity was observed to increase by a factor of 47 upon reducing the detector gate from 5 to as short as 0.2 p s and by a factor of 35 in the 10-mm tube. In addition, the He II/He I intensity ratio was more than 15 times greater for the smaller 4-mm tube than for the more commonly used 10-mm i.d. tube. When the helium pressure was reduced to 0.2 Torr in the larger tube, the relative intensity was still 6 times less than that obtained in the 4-mm tube a t 0.4 Torr. Similar data for the CW excitation at 90% full power is also presented in Table 111. The He II/He I relative intensity ratio in the 4-mm tube is 49 times greater than in the 10-mm tube. Also the relative intensity in the 4-mm tube is greater for continuous excitation than for pulsed excitation with a 5.0-ps detection gate. This is due to two factors. First, the data in this table were not obtained under optimum pulsed excitation conditions for excitation pulse duration and pulse repetition frequency. More importantly, a significant proportion of the total He 1584.3-A emission intensity appears after the cessation of the excitation pulse. If this is due to processes related to the cooling of the plasma, then this effect will not be observed in the continuous excitation mode. The results summarized here can be used to select the correct experimental conditions for discharge operation to maximize either the He I or He I1 radiation. Complications from other atomic lines (and even the second ionic resonance line at 256.3 A-Figure 3) are also indicated in the data presented. The reasons for the effects observed under these various conditions are not all clear, but we can attempt to
+ He(1 IS)
-
Hez+
+e
where the energy of the excited atomic species must be greater than the appearance potential of Hez+ (20, 21). Thus, this process occurs for all the n IP levels of atomic helium except for the He 584.3 8, ( n = 2) line. If both the electric field and the electron mean energy increase a t reduced pressures, as pointed out by Maksimov (22),then it would be expected that this effect will also contribute to the observed intensities of the higher energy lines. Because all the atomic lines have similar pressure dependences under pulsed microwave conditions, some effect common to all these lines would be expected to dominate over the associative ionization and change in electron energy effects. The increasing difficulty in breakdown and slower buildup of electron density may provide an explanation for the intensity dependence a t lower pressures. Information on the pressure dependence of the atomic line intensities can also be gained from analysis of the intensity decay rates (compare Figures 4 and 6). The increased decay rates at high pressures support competitive collisional deexcitation of the excited atomic,state (23, 24). At very low pressures, the reduced intensity and faster decay rates are indicative of diffusion losses. Over the pressure range of this study, the observed rate constants are considerably slower than the natural decay rate (1.8 X lo9 sec-' for He 2 IP, Ref. 25). Two factors which contribute to this reduction in decay rate from the natural decay value involve resonace imprisonment or trapping (26) and excitation of atomic metastables to the n IP levels in the early afterglow by low energy electrons. Quantitative analysis of the decay rate must include not only the effect of metastable excitation but quenching or collisional loss processes before the actual imprisonment effect can be retrieved. Qualitatively its effect is to delay radiative de-excitation to allow collisional excitation transfer and molecular formation processes. The relative changes in the intensities of the various He I lines as a function of discharge tube diameter appear to suggest a radial distribution of excitation (27). The most energetic atomic line analyzed, He 1515.6 A, changed intensity insignificantly, while the least energetic line (He 1584.3 A) changed by a factor of 11 when the discharge tube diameter changed by a factor of 2.5-from 4-mm to 10-mm i.d. Complicating the spatial distribution explanation for the excited atomic species, however, is the possibility that the mean electron energy increases as a function of tube diameter (22). If the electric field and electron energies shift to higher values, then the higher energy atomic levels are expected to be pop-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
* 2075
ulated to a greater extent in the smaller tube. We are unable at this time to determine the relative contribution of these two effects to the total observed phenomena. The tube diameter was also observed to affect the pressure dependence of atomic emission. Both the minimum pressure for operation and the pressure of highest intensity are shifted to higher values for the smaller discharge tube. This observation is consistent with the importance of a loss process involving collision of an energetic species with the tube walls. Compared to the heavier and less mobile atoms and ions, it might be expected that the energetic electrons would diffuse most rapidly to the walls (27, 28). The effects of the applied power and excitation pulse duration indicate an excitation process related to the electron density and energy. If the atomic emission is directly related to the total electron concentration (i.e., current), then a linear dependence of intensity on applied power is expected. This was experimentally observed. However, since the higher energy lines increased to a greater extent relative to the He I 584.3-A line, this suggests that the mean electron energy increases with the applied power. Another possibility for this effect involves excitation of atomic metastables by low energy electrons. The contribution of this process to the overall excitation is dependent on the metastable population ( 40.8 eV)
-
+
Hef(2 2P) e
1 H e + ( l 2S)
+ hv(303.8 A)
(5)
It is evident that the generation of He I1 emission in the helium discharge is dependent on the populations of high energy electrons ( 2 40.8 eV) and that of the helium ions as well as the electron impact excitation cross-section. If overall electroneutrality of the plasma is assumed, then the electron density, N e , is equal to the ionic density
Ne
[He+]
+ [Hez']
(6)
At the low pressures for He I1 excitation [He+] >> [Hez+] so that
N e = [He+]
(7)
Consequently the limiting factor for the generation of He+ (2 2P)will be the percentage of sufficiently energetic electrons for the excitation process. Maxwellian and Druyvesteyn distributions have been utilized as models for the energy distribution of electrons in discharges. However, large deviations have been reported (29-33) especially for energies above the first excitation potential (19.8eV for helium). Because Maxwellian distribution theory does not account for inelastic collisions, such as excitation and ionization, the predicted populations for electrons in the inelastic region are significantly too large. Approximate solutions to the Boltzmann transport equation have been obtained for the distribution of electrons with terms included for inelastic losses. For example, Reder and Brown (32) used this approach to calculate the distribution function of electrons in a continuous microwave discharge of helium. Their results, presented graphically, indicated a strong dependence of the high energy electron population on the ratio of electric field to gas pressure. Thus, a maximum in the He I1 emission should occur under conditions of low pressure and large electric fields. The observed premature loss of He I1 emission before the end of the excitation pulse (Figure 8) may be attributed to a change in the electric field-for any given pressure-as a function of time. Initially, in order to achieve breakdown for each pulse, an over-potential must be applied to the insulating gas. As the electron density, i.e., current, increases after breakdown, the electric field reduces. This is due not only to the large reduction in the IR drop as the gas changes from an insulator to a conductor, but also due to the loading of the power supply. Electron-electron coulombic relaxation also reduces the population of high energy electrons as the electron density
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
