Behlen Laboratory of Physics

'

Total photoionization cross sections of atomic oxygen

from threshold to 44. SA

G.C. Angel and James A.R. Samson

J I (I •

University of Nebraska

Lincoln, NE 68588^0111

ABSTRACT

The relative photoionization cross section of atomic oxygen for the

production of singly charged ions has been remeasured in more detail and

extended to cover the wavelength range 44.3 to 910.5A" by use of synchrotron

radiation. In addition, the contribution of multiple ionization to the

cross sections has been measured allowing total photoionization cross sections to be obtained below 250JL The results have been made absolute by

normalization to our previously measured data. The use of synchrotron

radiation has enabled measurements of the continuum cross section to be

made between the numerous autoionizing resonances that occur near the ionization thresholds. This in turn has allowed a more critical comparison of

the various theoretical estimates of the cross section to be made. The

series of autoionizing resonances leading to the ^P state of the oxygen ion

have been observed for the first time in an ionization type experiment and

their positions have been compared with both theory and previous photographic recordings.

(NASfl-CB- 182992)

CBOSS SECTIONS OF J10EIC GX1GEK FBCB

1EBBSHG1D TO-4«.3Jl (Ketraske Dniv.) 31 p

CSCL 20H Unclas

G3/72 0147025
I. INTRODUCTION

It has long been realized that the photoionization of atomic oxygen

plays an important role in the interpretation of solars-terrestrial effects

and in the study of stellar atmospheres. Experimental difficulties associated with its measurement, however, meant that, for many years, a knowledge

of the magnitude of the photoionization cross section was limited to theoretical estimates. The first calculation was made by Bates et al.1 in 1939

using a Hartree-Fock approximation, but it was not until 1949 that Bates

and Seaton^ reported more detailed calculations that took into account the

effect of exchange. Since then many authors3~l6 have reported calculations

that incorporate increasing degrees of sophistication and complexity associated with the photon-atom interaction. An interesting review on the

progress of our understanding of the photoionization of atomic oxygen since

1939 has been given recently by Seaton.1?

The first absolute measurement of the atomic oxygen photoionization

cross section was made in 1965 by Cairns and Samson

 18 who used an absorption tube technique to measure its total absorption cross section. It was

remeasured by Comes et a_l.1^ using a crossed beam method, but the agreement

was not good. Subsequent measurements by Kohl et al.2^ were made covering

a limited energy range using the dissociation produced in a shock heated

gas. In 1985 Samson and Pareek2^ reported absolute measurements made

using a mass spectrometric technique; they also showed that for photon

energies above - 25eV the atomic cross section was very close to one-half

of the total molecular ionization cross section. Huffman et al_.22 used a

photographic technique to classify the autoionizing resonances leading to

the ^S°,

 2D°, and 2p° thresholds and Dehmer et al.^3 measured relative

photoionization cross sections in the same region. In addition, several

measurements have been made of the branching ratios for the production of
the ^3°,

 2D°, and 2P° states,21*"26 and of the angular distributions of the

ejected photoelectrons.2^!2?

The purpose of the present experiment was to remeasure the relative

photoionization cross section of atomic oxygen, including the effects of

multiple ionization, over as wide an energy range as possible using the

continuum radiation available from a synchrotron storage ring. This would

also allow more detailed measurements at the ionization threshold to be

made. In addition, a clearer picture of the variation of the continuum

cross section between the autoionizing resonances from the ^3° to the

 2P°

ionization thresholds could be obtained, in order to give an indication

which of the presently available theories describes most closely the ionization process in this region. The continuum radiation would also allow

observation of the Rydberg series 2s2p^ (^P)np, the presence of which had

been indicated at only a few discrete wavelengths in our previous work.21

II. EXPERIMENTAL APPROACH AND ANALYSIS OF RESULTS

The experiment was performed at the University of Wisconsin synchrotron

storage ring, Aladdin, using radiation from both a ^ meter normal incidence

and a Mark II Grasshopper grazing incidence monochromator. Figure 1 shows

a schematic diagram of the apparatus used in the experiment. Molecular

oxygen was partially dissociated in a microwave discharge sustained by the

air cooled cavity, C. The discharge, operated at 2450 MHz and with up to

100 watts microwave power, was initiated by a spark from a Tesla coil. The

discharge products flowed along the pyrex tube, T, which was coated with

phosphorus pentoxide to minimize recombination on the walls, and passed,

via a 0.5mm orifice through the center of the collision region, R, and then

to the turbo-molecular pump, PI. The differential pumping region, D, evacuated by the turbomolecular pump, P^, separated the relatively high pres-
sure collision region (-10"^ torr) from the ultra-high vacuum of the

monochroraator output mirror box (-5 x 10~10 torr). . Radiation passed via

the low conductance canals, C-\ and €2, through the middle of the collision

region, R, and was then incident on the photon detector, M. This was either

a sodium salicylate/photomultiplier combination or an aluminum photodiode,

the current from either of which was recorded by a picoammeter. Ions produced in the collision region were extracted and focussed by the electrostatic lens system, L, onto the entrance of the 180° electromagnetic mass

spectrometer, S, where the ions of interest were separated and directed to

the first dynode of the Johnston type MM1 particle multiplier, J, the signal from which was recorded using particle counting techniques. The sig?-

nals from the photon detector and the ion rate meter were simultaneously

recorded on a strip chart recorder.

The experimental investigation was conducted as follows. Previous

investigations in our laboratory have shown that the dissociation fraction

produced by the microwave discharge requires a considerable period in which

to stabalize but that once it has done so the atomic production remains

essentially constant. Consequently, pure C>2 was admitted to the discharge

tube via a needle valve and the microwave discharge struck. This was then

left to stabalize for up to 1 hours. The electromagnet was tuned to select

0

+ ions and the monochromator scanned over the wavelength range of interest. A similar scan was made of the parent molecular ion, the whole process being repeated with the microwave discharge extinguished. Repeat

measurements were made at several discharge gas pressures between 200 and

700 microns and, in the case of the *J meter monochromator, with both the

photomultiplier and photodiode as photon detector.
The number, N+, of ions produced when a flux Io of photons passes

through a gas whose number density/path length product is nS, is given by

the equation

N

+ = I nia, (1)

provided nX,o « 1 , where o is the photoionization cross section. If the

ion extraction, analysis, and detection system has an overall efficiency F

and if the efficiency of the photon detector is G then Eq. (1) becomes

N/F = (ID/G)nio, (2)

where ND and ID are the ion count rates and photon signals, respectively,

actually recorded. It may be seen that for constant n, H, and F we obtain

the expression

o « (N*/ID)G. (3)

Relative values for the efficiency G of the photodiode used in the

experiment could be obtained by running a noble gas (neon or argon) through

the collision region in place of the oxygen and recording the ion count

rate and photo-diode signal simultaneously. Since the noble gas absorption

cross sections 0(Ar, Ne) are known2°»29 Eq. (3) shows that

G cc (ID/N*) a (Ar, Ne).

For the salicylate screen/photomultiplier combination the efficiency is

known to be constant up to photon energies of -60eV;30 this detector was

never used for wavelengths shorter than -400&. Thus, using either photon

?

detector, Eq. (3) may be used to determine the relative variation of the

photoionization cross section for the gases under study.
Data were recorded on the 4 meter monochromator from well before the

ionization threshold to approximately UOO&. Between the atomic ionization

threshold (910.4&) and 662$. the ion signal arose purely from the ionization

of the atom; at wavelengths shorter than this a component caused by disso^-

ciative ionization was also present. At these wavelengths the relative

atomic ionization cross section is given by the expression

-

 a(No

where the second bracket represents the ion to photon ratio with the micro--

wave discharge turned off and a is the fraction of molecular oxygen left

undissociated. It should be noted, however, as observed by Samson and

Pareek21, that Eq. (5) is strictly true only in the absence of excited

states of the oxygen molecule. Several groups1°»2^»33 have shown that the

production of atomic oxygen in a microwave discharge leads to the formation

of molecular oxygen in its a(1A) excited state. Thus, the presence of this

species must be considered. However, it can be shown21 that this has a

negligible effect on the atomic cross section provided that the continuum

photoionization cross section of 03 in its ground X(2£u) states is comparable to 62 in the a(1A) states. From the theoretical calculations of Roche

et a_1.3^ it appears that this is a reasonable assumption.

The fraction a was determined from the molecular ion to photon ratio

obtained from runs with the microwave generator turned on and off. It was

found to be of the order 0.83 ~ 0.93 depending on the pressure in the discharge tube and the condition of the phosphorous pentoxide coating. Values

for the ion count rate and photon signal were measured from the chart recorder traces at appropriate wavelengths, chosen to be at positions not

influenced by the many autoionizing resonances observed. The relative

cross sections were determined using either Eq. (3) or (5) depending on
whether the wavelength was above or below the dissociative ionization

threshold. A further correction to the 0+ production cross section was

necessary between 700 and 725$ because of the production of 0* from the

pair production reaction

hv + 0 ->• 0 -»• 0 ,

which was previously reported by Dehmer and Chupka31 and which was clearly

visible in the 0+ signal trace recorded with the microwave switched off.

No new information could be gained from the width of the autoionizing resonances over that already obtained by Dehmer et ^1.^3 because the 4m mono'-

chromator, although having a designed resolution of .07A for a 100 micron

exit aperture, was found to have an actual resolution of 0.6A that was

largely independent of the monochromator slit width. For this reason, and

because of the number of peaks involved, values for the photoionization

cross section have not been calculated over the autoionizing peaks.

Data on the grazing incidence monochromator were recorded from its long

wavelength limit of 310$ to 44.3$ at which point considerable photon loss

was apparent because of increasing absorption at the carbon K edge caused

by carbon deposited on the mirrors and grating. All the oxygen ion signals, except 03*, taken on this monochromator had dissociative ionization

components associated with them. Thus Eq. (5) was used exclusively to

determine the relative photoionization cross section. Equation (5) also

applies in determining multiple ionization cross sections provided NB+ is

\

substituted for N^', where n is the degree of ionization. The relative

cross section can then be placed on an absolute basis by determining the

constant of proportionality for Eq. (5). This constant is the same for

single or multiple ionization. More details on the multiple ionization of

atomic oxygen are given in a separate paper.35
A major difficulty associated with the use of a continuum as opposed to

a discrete line source is the separation of true signal from that arising

from second and higher order spectra and scattered radiation. Subsidiary

experiments were carried out on both monochromators to try to determine the

extent of the contamination by unwanted radiation. On the 4 meter normal

incidence monochromator using the photomultiplier detector the following

technique was used.

At any particular wavelength it was assumed that the radiation consisted of components of both the first and second orders Io' and Io''.

These in turn would produce ion signals N' and N'1 with cross sections a'

and o'', according to Eq. (2). In the experiment the measured ratio of

ions per photon was given by

(y * y) -

Ratio = .

(V

 + Vf)

Thus,

F/G(I 'nX, o1 + I ' 'nilo")

Ratio =

(V

 + Vf)

Because F, G, L, and the number density/path length product ni are assumed

to be constant this may be reduced to

C [a1 + (I "/I ') o")3

Ratio = ,
where C is a constant. The ratio above could be determined at wavelengths

just above and below the onset of ionization for a particular gas target

yielding two equations for the two unknowns C and (ID*'/ID'), where it is

assumed that this ratio is constant over a restricted wavelength range.

This process was carried out at the argon and neon ionization thresholds,

(787$. and 575&, respectively) and at the threshold for 02"*" production

(10265). dp''/ID') was found never to exceed UJ. Higher order correc^-

tions to data gathered on the 4 meter monochromator thus had only a small

effect on the final result.

For the grazing incidence monochromator a different approach was neces--

sary. Because the lowest energy available from the, machine was iJOeV no

threshold for single ionization was available. The thresholds for the

production of doubly, triply, and, in some cases, quadruply charged ions

were accessible however and, by tuning the magnetic mass spectrometer to

the appropriate position, the variation of the production of the multiply

charged ion with respect to the photon energy could be obtained. It was

assumed that any signal at energies below the threshold was primarily

caused by second order radiation. By comparing this signal with the signal

obtained at the appropriate second order wavelengths an estimate of the

higher order fraction present was derived. Using this estimate corrections

to the ion production rate and the photon signals for both noble gas and

molecular oxygen targets were made. The photodiode efficiency was calculated from the noble gas data, and, using this, the relative singly charged

molecular oxygen production cross section was determined. A second esti-^

mate of the higher order percentage was then made that forced these molecular oxygen cross sections to agree, when normalized, with the previously

measured absolute values of Samson et £1.32 This second estimate of the

contamination component was then used to recorrect the ion production rates

and diode signals. The photodiode efficiency was recalculated and the new

9-
value used to redetermine the molecular oxygen cross section. This procedure was repeated until no further change in the higher order component was

necessary. As a check on this higher order subtraction molecular nitrogen

was introduced into the collision region and the cross section for the

production of molecular nitrogen ions was calculated using the correction

factors as determined above. Figure 2 shows both the variation of the

molecular oxygen and nitrogen cross sections as a function of wavelength,

compared to previously measured data.32,36 Although the agreement between

the present data and the previously measured molecular oxygen cross sections is to be expected the good agreement between the two measurements of

the molecular nitrogen cross section acts as a confirmation that the second

order subtraction applied to the data is approximately correct.

Oxygen data, taken on both the 4 meter and grazing incidence monochromator, were normalized to the absolute measurements of Samson and Pareek.21

For the 4 meter data this was done at 584A since Samson and Pareek had

taken considerable care in the laboratory to ensure the accuracy of the

cross section measurement at this wavelength. In addition, although the

ion signals (taken both with the microwave on and off) revealed considerable structure in the long wavelength region, caused by molecular transitions, the region of 584S was almost entirely free from them. For the

grazing incidence measurements a factor was determined that gave a good fit

between the two sets of data over the wavelength range of overlap.

III. RESULTS AND DISCUSSION

The absolute values of the photoionization cross section obtained in

the present work are tabulated in Table I for wavelengths between 911 and

260A1. Because multiple ionization cannot occur in this wavelength range

the tabulated values represent the total photoionization cross sections.

The actual wavelengths used between 911 and 677.5A\ a region of
structure, are tabulated in the first portion of Table I. Towards shorter

wavelengths the data have been averaged with the previous results and

smoothed to give a best fit to the data. Table II lists the specific cross

sections for single, double, and triple photoionization between 254.4 and

44.3&. The final column of Table II gives the total photoionization cross

section.

The present results (not smoothed) are shown in Fig. 3 along with previous experimental results. The present measurements are shown as full

circles while the previous data of Samson and Pareek^l are shown as open

circles. Also shown are the experimental results of Kohl et jil.2^ (inrverted triangles) and those of Comes £t al.19 (triangles). The data at the

oxygen K edge, shown at twice the normal scale, were taken from the compilation of Henke £t al.37

The two points measured by Samson and Pareek^l at 700.3 and 684.6&

appear to be lower than would be expected. However, examination of the

recorder traces reveals that at precisely these wavelengths the ion production rate passes through a minimum because of the influence of the autoionizing resonances leading to the 2p° threshold (see Fig. (5)).

The partial cross sections for producing 0+ in the ^S° ground state and

^0°,

 2P° excited states are also shown in Fig. 3- These results were obtained from the product of the total atomic cross section and the branching

ratios for producing these states. The branching ratios have been measured

by the technique of photoelectron spectroscopy at 584$ by Samson and

Petrosky^ at 3041 by Dehmer and Dehmer25f and between 725 and 580$ by

Hussein et _al.26 The dashed lines show a reasonable extrapolation of the

present data through the partial cross section points at 584 and 304JL The

full line represents a best fit to the present data, excluding the resonance structure leading to the ^P threshold. Representative error bars are

shown at several points; these were determined from the uncertainty intro-
duced on extracting data from the chart recordings. The overall accuracy

of the results depends upon the absolute value of our previous data. An

estimated error of ±9$ was quoted at that time. However, confidence in the

relative and absolute value of the cross sections can be obtained by considering the value of the total oscillator strength for the absorption

process. According to the TRfr-sum rule38 the oscillator strength must add

up to a value of 8, the total number of electrons in the atom. Thus, we

have evaluated the continuum oscillator strength from the ionization

threshold down to 44.3A" using the present total photoionization cross sections and from 44.3A" to 0$ using the cross sections from the compilation by

Henke et al.37 A value of 7.29 was obtained. Adding this value to our

previously quoted line oscillator strength of 0.71 gives a total oscillator

strength of 8.00! Obtaining the precise value of the total oscillator is,

of course, fortuitous because the maximum error in the value of the line

oscillator strength (±50$) introduces an error of about ±1.4? in the total

oscillator strength. However, this implies that our value of the continuum

oscillator strength should be accurate to within ±4.4?, which is certainly

in keeping with our estimated value of ±9$. It should be noted that in our

previous work^ we did not take into account the effect of multiple ionization below 250JL

The structure shown in Fig. 3 has not been seen before in photoioniza^

tion experiments and represents the Rydberg triplet series 2s2p^(^P]

np(3o0, 3s°, 3p°). Figure 4 is a trace of the rate meter signal showing

the series in detail compared with the theoretical predictions of

Pradhan^0. Also shown are the peak wavelength predictions of Taylor and

Burke°, which agree to within 1& with those of Pradhan, and the wavelength

assignment of Carton and Drew39 Who have observed the series photographically in an absorption experiment and obtained an identification for the

np(3o°) series. As can be seen the agreement between experiment and theory

is good, although the shape and position of the third peak of the first
theoretical triplet differs from that measured experimentally; no attempt

has been made to allow for peak shifting or broadening of the measured

lines caused by the finite resolution of the monochromator.

\

Figure 5 illustrates the rate meter signal obtained between 760 and

655& showing the autoionizing resonances leading to the

 2D° and 2P° states

of the oxygen ion. The wavelengths marked agree to within 0.6X of the assignments given by Huffmann et jil.22 By comparing the height of the autoionizing peaks with the adjacent continuum a lower limit to the peak cross

sections may be determined. For example, the peak to background ratio at

72^.8$. is 7.1:1; the adjacent continuum cross section is 8.6 Mb from Fig.

3. Thus, the peak cross section is at least 61 Mb.

Figure 6 shows the high energy measurements of the atomic oxygen photoionization cross section (full circles) compared with those of Samson and

Pareek21 (open circles) to which they have been normalized. Representative

error bars are shown and a further 12% caused by systematic error is estimated. The good agreement of the two sets of data in terms of the variation of the cross section with energy also tends to confirm that the higher

order subtraction procedure previously detailed is correct.

Figures 7(a) and 7(b) compare most of the presently available theoretical results with our present data plus those of Samson and Pareek (full

circle data points). The line previously drawn through the experimental

points has been omitted as have the ^P resonances. The individual theoretical data are identified in the Figures by the initials of the authors

as given in parentheses in the following description.

In Fig. 7(a) Bates and Seaton2 (BS) used Hartree^Fock wave functions to

calculate the cross section at the spectral head and then deduced the energy dependence from an approximation given by Bates.1 Dalgarno et al.3

(DL, DV) also used Hartree-Fock wave functions for both the bound and continuum orbitals in their calculations of transitions from the ground state
to specific states of the ion. Their data are shown in both the length (L)

and velocity (V) approximations. Henry (HL, HV) then investigated the

coupling between final state channels, including as many channels as was.

x

technically feasible at the time of computation. McGuire^O (M) used a development of his semi empirical central potential method to calculate the

photoionization cross section of all the elements from helium to xenon,

including that of oxygen, whereas, Kahler^1 (KA) used the scaled Thomas^-

Fermi method of Stewart and Rotenberg.39 Ganas13 (G) used an independent

particle model.

In Fig. 7(b) Taylor and Burke8 (TB) and Pradhan1° (P) both introduced

close coupling expansions for the initial and final states, but they used

different numerical techniques for the solutions, Taylor and Burke employ<-

ing the R^-matrix method while Pradhan used the method of reduction to

linear equations; they obtained similar results. Saxon, £t jal.1^ (SA),

also using the R-matrix technique, calculated the photoionization cross

section of ground state atoms leading to ground state ions as a preliminary

to calculations the cross section for excited oxygen atoms. Their results

agree closely with the work of both Taylor and Burke, and Pradhan. Thomas

and Helliwell1^ (TH) developed an approximate central potential model for

the calculation of the cross section. Koppel12 (KO) used the scaled

Thomas-Fermi method of Stewart and Rotenberg^1 , as did Kahler (KA) in Fig.

7(a). The results obtained by the two authors differ considerably.

Starace _et alJ1* (ST) used Herman-Skillman wave functions in their calculation of the cross section.

Most of the theoretical data lie within ±^0% of the experimental data.

However, it is interesting to note that at the ^S° ionization threshold the

more sophisticated close-coupling and R^matrix calculations show a larger

disagreement with experiment than many of the other calculations. The

theory that most closely describes the variation of the atomic oxygen ion--
ization cross section is that due to Kahler1 ^ (see Fig. 7(a)). Many of the

others accurately predict the variation of the cross section between ionization onset and the 2p° threshold though agreement is generally poor between the

 2D° and 2P° onsets. None of the theories accurately predict the

shape of the cross section between the

 2P° and **P thresholds where the ex-^

perimental results show a more pronounced peak in the cross section. At

higher energies all the theoretical calculations tend to converge towards

the experimental results.

ACKNOWLEDGMENTS

This material is based upon work supported by the National Science

PHY -

Foundation under Grant No. ATM-86l7670J\and by the National Aeronautics and

Space Administration under Grant No. NGR 28t-OOMc021 . It is with pleasure

that we acknowledge the help of the personnel at the Stoughton Synchrotron

Radiation Center, which is supported by NSF Grant No. DMR^8601
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TABLE I. Absolute photoionization cross sections of atomic oxygen

between 911 and 26oX. Units are in megabarns (1 Mb = 10~18Cm2)

X a)

911.00

910.50

910.00

909.80

907.50

905.00

900.00

895.00

890.00

885.00

870.00

865.00

860.00

855.00

850.00

845.00

Cross

Section (Mb)

0.15

0.70

1.15

2.45

2.75

2.75

2.85

2.75

2.70

2.85

3.10

3.10

3.10

3.10

3.20

3.25

X U)

760.0

752.3

732.2

730.6

729.0

727.5

722.5

720.9

720.0

719.2

717.6

715.7

714.1

712.5

710.8

710.0

Cross

Section (Mb)

3.90

4.15

8.03

8.40

9.17

8.90

9.02

8.91

9.00

9.00

8.79

9.24

9.18

9.26

9.30

9.15

X (A)

690.6

690.0

683.9

683.0

682.8

677.5

665.3

660

650

640

630

620

610

600

590

580

Cross

Section (Mb)

9.47

9.80

9.25

9.65

9.69

10.0

12.0

12.6

13.0

13.3

13.4

13.4

13.4

13.3

13.2

13.0

Cross

X (i) Section (Mb)

490 12.0

structure

430 11.5

420 11.3

410 11.0

400 11.8

390 10.5

380 10.2

370 9.90

360 9.57

350 9.25

340 8.92

330 8.60

320 8.25

310 7.90

304 7.70*

*Absolute value used for normalization (from ref. 21).

1 9-
TABLE.I. Continued.

x U)

8i<0.00

835.00

830.00

825.00

785.00

780.00

765.00

760.2

CrossSection (Mb)

3.30

3.30

3.30

3.30

3.70

3.70

4.00

4.06

X (A)

709.2

707.6

705.9

705.0

695.3

693-8

692.5

692.2

Cross

Section (Mb)

9.71

9.88

9.71

9.60

9.06

9.00

9.60

9.51

X (A)

570

560

550

540

530

520

510

500

Cross

Section (Mb)

12.9

12.7

12.5

12.3

12.1

12.0

11.9

11.9

X (A)

300

290

280

270

260

Cross

Section (Mb)

7.55

7.40

6.85

6.50

6.15

30
TABLE II. Absolute photoionization cross section for single, double,

and triple photoionization from 44 to 254X

hv(eV)

48.736

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

us,

254.402

248.0

206.6

177.1

155.0

137.8

124.0

112.7

103-3

95.4

88.6

82.7

77.5

72.9

68.9

65.3

62.0

59.0

56.4

53.9

51.7

49.6

47.7

45.9

44.3

0+

(Mb)

6.00

5.75

4.25

3.20

2.37

1.84

1.44

1 .20

1.01

0.86

0.72

0.62

0.52

0.45

0.39

0.33

0.286

0.250

0.220

0.197

0.180

0.168

0.161

0.154

0.150

(10"2Mb)

0.0

2.0

18.7

22.5

23.0

22.1

20.5

18.5

. 16.4

14.2

12.1

10.3

8.4

6.9

5.8

4.8

3.9

3.2

2.55

2.05

1.75

1.50

1 .30

1 .20

1.05

(10~3Mb)

*..u

hc-r:

*~

r-.— .

^

^-

0.28

1.01

1 .65

2.02

2.20

2.12

1.95

1.75

1 .51

1.28

1 .07

0.87

0.69

0.54

0.44

0.37

0.32

0.27

° total

(Mb)

6.00

5.77

4.44

3.43

2.60

2.06

1 .65

1.39

1.17

1 .00

0.84

0.73

0.61

0.52

0.45

0.38

0.33

0.28

0.25

0.22

0.20

0.18

0.17

0.17

0.16
Figure Captions

1. Schematic diagram of the apparatus showing the side view (upper diagram) and end view (lower diagram) as seen from M looking towards D

(for symbols see text).

2. The photoionization cross section for production of $2*

 from

 °2 (left

ordinate) and N2+ from Ng (right ordinate). 0, present data; 0, (C>2

 + ) ,

Samson e± al. (ref. 32); A, (N2+), Samson £t. al. (ref. 36).

3. The photoionization cross section of atomic oxygen for the production

of singly charged ions as a function of wavelength. 0, present data;

0, previous data, Samson and Pareek (ref. 21); V, Kohl et al.

(ref. 20); A, Comes _et al. (ref. 19). Partial cross sections; ,

Hussein £t al. (ref. 26); , Dehmer and Dehmer (ref. 25); solid

circles with error bars at 58MA, Samson and Petrosky (ref 24).

4. The autoionizing resonances leading to the ^P threshold; upper curve,

the rate meter signal from the present experiment; lower curve, the

theoretical predictions of Pradhan (ref. 10). The upper wavelength

assignments are those derived from the present work and the photo^

graphic measurements of Carton and Drew (ref. 39), the lower are those

reported by Pradhan (ref. 10), and Taylor and Burke (ref. 8).

5. The ratemeter signal in the present experiment showing the autoionizing

resonances leading to the

 2D° and 2p° ion thresholds. The wavelength

assignments are derived from the present work. The dashed line represents the decrease in the stored ring current over the period that the

scan was made and is thus proportional to the photon flux.

6. The high energy region of the photoionization cross section of atomic

oxygen leading to the production of singly charged ions. 0, present

data; 0, previous data of Samson and Pareek (ref. 21).
7. A comparison of the experimental and theoretical photoionization cross

sections of atomic oxygen. In both 7(a) and 7(b) the full circles

represent the combined data of the present experiment and the previous

measurements of Samson and Pareek (ref. 21). Theoretical estimates:

7(a):

BS, Bates and Seaton, ref. 2

DL, ."* * * « Dalgarno et al. (length formulation), ref. 3

DV, -x.*-x Dalgarno et al. (velocity formulation)

HL, '-•>-• Henry (length formulation), refs. 5 and 6

HV, .-..v.. Henry (velocity formulation)

M, VVV McGuire, ref. 40

G, -xx^xx Ganas, ref. 13

KA, 000 Kahler, ref. 11

7(b):

TB, >- --- Taylor and Burke (length formulation), ref. 8

P, -x-x Pradhan, ref. 10

TH, *-••-•• Thomas and Helliwell, ref. 15

KO, •-•-• Koppel, ref. 12

ST, Starace et al., ref. 14

SA, -xx-xx Saxon et al., ref. 16
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