THE RELATIVE FLUORESCENT EFFICIENCY OF

SODIUM SALICYLATE BETWEEN 90 AND 800 eV

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

Behlen Laboratory of Physics, University of Nebraska, Lincoln, NE 68588-0111

and

G. Williams

NSLS, Brookhaven National Laboratory, Upton, New York 11973

ABSTRACT

The relative fluorescent quantum efficiency of sodium salicylate was

measured between 90 and 800 eV (138 - 15 A") by the use of synchrotron

radiation. A general increase in efficiency was observed in this spectral

range except for abrupt decreases in efficiency at the carbon and oxygen

K-edges. Beyond the oxygen K-edge (532 eV) the efficiency increased linearly

wit h the incident photon energy to the limit of the present observations.

(NASA-CK-176605) THE RELATIVE fLUOBESCEHT N86-20518

EFFICIENCY OF SODIUM SALICXIA1E BETHEEH 90

AKD 800 eV {Nebraska UDJLV.) 16 p

HC A02/MF A01 CSCL 07D U4clas

G3/2 b U -»oo /
INTRODUCTION

When radiant energy greater than 3.5 eV is incident on sodium salicylate it is partially converted to fluorescent radiation whose spectral distribution, lying between 3500 and 5500 A", has been found to be independent of the

exciting wavelength.1 Previous investigations have shown that the quantum

efficiency, n, of the salicylate, defined as the number of fluorescent photons

produced per incident exciting photon, is constant over an extended wavelength

range (100 - 3500 A").2~6 As a result sodium salicylate in conjunction with

conventional photomultipliers has been widely used as a sensitive detector in

the vacuum ultraviolet (VUV) spectral region. Krokowski,? however, using x-rays

of energy greater than 12 keV, has shown that here the quantum efficiency is no

longer constant but increases linearly as a function of energy with a conversion factor, defined as the ratio of fluorescent to absorbed energy, of 0.04.

By extrapolation of his results Krokowski suggested that a change from an energy independent to an energy dependent efficiency would occur at about 83 eV,

assuming the efficiency to be unity in the energy independent region. This

transition was not observed by Samson and Haddad^ who investigated the relative

fluorescent efficiency of sodium salicylate between 20 and 107 eV and found it

constant to within ± 8%. However, subsequent unpublished results from this

laboratory using a different technique have indicated a possible increase in

efficiency for photon energies greater than 50 eV. In addition Lindle et al^

report an increase in the quantum efficiency of about a factor of 2 between 80

and 270 eV.

Increasingly, synchrotron radiation in the VUV and soft x-ray regions

is being used to study atomic and molecular processes, and in this range sodium
salicylate is a popular detector. When it is used to measure the relative

photon intensity as a function of wavelength it is important that the true

relative variation of the sodium salicylate quantum efficiency be known in

order to interpret accurately the data taken.

With this in mind an experiment was designed to measure the relative

fluorescent efficiency of sodium salicylate over the energy range provided by

the monochromator of beam line U14 at the Brookhaven National Synchrotron Light

Source. We wished particularly to identify the wavelength region where the

transition from an energy dependent to an energy independent efficiency occurs,

and, since both carbon and oxygen form the major components of sodium salicylate (NaCy^C^), to observe the effect on the efficiency of the carbon and

oxygen K-shell absorption.

EXPERIMENTAL APPROACH

The fluorescent efficiency of sodium salicylate n is defined as,

n = If/I0 . (D

where If is the number of fluorescent photons/s and Io is the number of

incident photons/s. If is measured with a sodium salicylate/photomultiplier

combination whose output Ip^ is proportional to If.

The intensity of the incident radiation was determined by measuring tne

ionization produced as it passed through a low pressure ncble gas target.

Because every absorbed photon produces an ion whether singly or multiply

cnarged, the number of ions N collected over a path length I is given by,

N = I0on2, , (2)

where n is the number density of the gas and o is the absorption cross section.

From Eqs. (1 ) and (2} it can be seen that for constant nS, we obtain
n « IPM o/N . (3)

IpM and N are readily measured quantities and o is well known for the rare

gases with an accuracy of better than ± 10*.9 However, the efficiency of

the ion detector will be somewhat different for the various charged states of

the ions. This effect can be minimized by using the detector in the counting

mode.

Figure 1 shows a schematic diagram of the apparatus used in the experiment. Radiation from the grazing incidence monochromator was directed through

two low conductance canals D^ and D2, which provided differential pumping to

maintain the high vacuum conditions necessary for the monochromator and beam

line. The radiation passed through the collision region C and was then incident on the salicylate screen S where the fluorescent radiation was detected by

the photomultiplier tube PM. The signal Ip^ was measured with a picoammeter.

In traversing the collision region the photon beam intersected the collimated

gas flow from the capillary tube L. The gas pressure in the reservoir R was

maintained at a constant pressure, to within approximately 0.2%, by the servoneedle valve S controlled by the capacitance manometer B. This ensured that

that under equilibrium conditions the product nl remained constant. Ions

formed in the collision region were extracted with high efficiency by the extraction and lens system EL and were recorded by an electron multiplier M used

in the single particle counting mode.

Data acquisition and monochromator control was handled by the computer

system provided by the operating staff of Beam line U1H. Photon energies from

< 20 eV to more than 800 eV were provided by the 4 ranges of the monochromator;

target gases helium, neon, and argon were used. For each monochromator range

the photomultiplier current and ion count rates were recorded at a fixed gas

pressure for a series of energies covering the spectral range investigated.

Two salicylate coatings were investigated, each being applied to the glass
window S by spraying a saturated solution of sodium salicylate in methanol onto

the warmed surface. Several applications of light coats were used to build up

a surface with a uniform opaque appearance. No estimate was made of the actual

surface density.

Prior to measurements of the salicylate efficiency the system, which

was pumped by three Balzer turbo molecular pumps, was baked. On cooling, the

base pressures in the differential, collision, and ion detector regions were

2.6 x 10~8, < 1.0 x 10~6, and 1.5 x 10~? torr, respectively. With typical

running conditions of 0.3 torr in the reservoir chamber the target chamber

background pressure rose to 6 x 10~6 torr and the pressure in the differential

region rose to 3 x 10~8 torr. No increase in the monochromator pressure was

apparent. From the measurements of Ip^ and N the relative variation of the

sodium salicylate efficiency could be determined as a function of energy.

ANALYSIS OF THE RESULTS

A major difficulty in analyzing data taken with a continuum, as opposed

to a discrete line source, is separating true signal from that arising from

contamination due to higher order and scattered radiation. An estimate of the

long wavelength scattered radiation was made as follows. Valve V contained a

sapphire window opaque to radiation with X < 1410 A". With this valve closed

the signal from the photomultiplier was recorded as a function of energy for

each of the four monochromator ranges. These scattered radiation spectra, when

normalized to unit electron current circulating in the storage ring, could be

usea to estimate the background scattered signal at the photomultiplier caused

by radiation with \ > 1410A", provided the storage ring current during the

measurement was known. These currents were recorded before and after every

energy scan on each range. The contribution to the signal caused by the long
wavelength scattered radiation and the photomultiplier dark current was subtracted from all photo-multiplier signals. No allowance could be made for scattered radiation with wavelength greater than 1110 X-or for any ion signal

arising from it.

Figure 2(a) shows the data taken on range 2 between 80 and 380 eV. The

continuous curve shows the output from the photomultiplier (with the dark

current and the estimated long wavelength scattered contribution subtracted).

The chain curve shows the Ar ion count rate. Figure 2(b) shows the uncorrected

relative efficiency of the sodium salicylate response calculated using Eq. (3).

The following features in the Figures should be noted.

(a) The increase in the ion count rate between 2^0 and 260 eV in

Fig. 2a corresponds to increased absorption at the argon Lj 3 edge. A similar

increase is observed at approximately 120 eV caused bysecond order 1^3

absorption. Discontinuities at these energies are also observed in the sodium

salicylate efficiency curve. With an exact knowledge of the argon absorption

cross section, the true background signals for both the photomultiplier current

and the ion signal, and an exact calibration of the energy scale these discontinuities would not arise. Since none of these conditions apply such structure

will occur where sharp resonances in the cross section exists. In regions

where the cross section is smoothly varying the effect of these uncertainties

is minimized.

(b) The rapid decrease in the photomultiplier signal above 280 eV

corresponds to the absorption of the incident radiation by carbon in sodium

salicylate at the carbon K-edge (288 eV). This is reflected in the sharp

decrease in the sodium salicylate efficiency shown in Fig. 2(b).

The structure in the photomulciplier signal at approximately 1 HO eV is

caused by second order carbon K edge absorption. Similar structure at - 95 eV

shows third order contamination. All the structure observed in the sodium

salicylate response curve between 80 and 160 eV is a result of higher order

6
contamination. From the nature of the monochromator design10*

11 each of its

ranges had limits within which reflection filtering had theoretically reduced

higher order contamination to zero. However, while in theory photon energies

from 20 to 800 eV could be provided free of higher orders, it was found in

practice that the two lowest energy monochromator ranges, 3 and i<, had either

significant higher order contamination, or insufficient radiant output over the

wavelength range of interest, so that the acquisition of reliable data was

precluded. In order to obtain information on the variation of the salicylate

efficiency over as wide an energy range as possible an attempt was made to

remove the effects of higher order contamination from the data taken with the

range 2 mirror at energies below 150 eV. Figure 3 shows a detailed plot of the

photomultiplier output between 80 and 160 eV; the effect of the 2nd order carbon K edge at 1 HO eV is clearly seen. Beneath the measured curve (full line)

is an estimate of the true variation of the first order radiation (chain

curve). By subtraction the contribution due to second order radiation at any

energy E could be found and expressed as a percentage of the radiation at

energy 2E. This percentage then gave the contribution of the ion signal at

energy E caused by ions formed by radiation of energy 2 E, and this contribution could be subtracted from the total ion count rate. The corrected photomultiplier currents and ion count rates could then be used to calculate

corrected salicylate efficiencies. A similar procedure was used to correct the

data taken with the range 1 mirror; the correction was only appreciable between

400 and H50 eV.

DISCUSSION

Figure 4 shows a plot of the corrected relative fluorescent quantum

efficiency of sodium salicylate as a function of the incident photon energy

from 90 to 800 eV. The solid data points represent the results taken with a

single salicylate coating with argon as the target gas. The open data points
represent the results taken with neon as the target gas. The same overall

relative variation was shown by a second coating and other target gases. The

difference in the Ar and Ne data between 90 and 120 eV demonstrates the problem

of correcting for second and higher order spectra in this region.

The results in Fig. 4 were normalized to the data of Krokowski in the

region above 550 eV where it was assumed that 4? of the incident energy was

converted to fluorescent photons as he reported. The incident photon energy

Elnc and mean fluorescent energy Ef are then related by the expression

N Ef

= 0.04 , (4)

Einc

where N is the number of fluorescent photons emitted. The scale of the

ordinate was adjusted so that a least square fit to our data points, passing

through the origin, showed the appropriate slope of .0144 where we have assumed

Ef = 2.78 eV. The dashed horizontal line represents a constant quantum efficiency of unity and intersects the extrapolated slope at 70 eV. However, the

absolute value of the constant quantum efficiency portion has been variously

reported in the literature to be in the range 0.5 to 1.0.12~17 Thus, the ordinate scale represents a maximum efficiency for our present results as shown in

Fig. 4. This is because by normalizing the constant quantum efficiency to the

tail of the Ar or Ne data would require lowering the entire curve to allow the

constant efficiency portion to be less than or equal to unity.

The distinctive features above 160 eV are independent of the absolute

ordinate scale. Thus the steep increase in the response preceding the fall at

the carbon absorption edge, the less steep increase preceding the discontinuity

at the oxygen edge followed by the uniform increase to 800 eV shows the relative variation of the quantum efficiency of sodium salicylate as measured in

the experiment. There is insufficient data to say whether the apparent change

in slope at 400 eV is significant. Figure 5 shows a detailed plot of the

8
salicylate efficiency in the region of the carbon K edge. The point of inflexion at - 290 eV was a repeatable feature independent of target gas for both

salicylate coatings. Norman18 in his review of surface EXAFS and XANES has

shown that absorption spectra in the neighborhood of absorption edges are rich

in structure which may frquently be associated with specific interatomic bonds.

No investigation of sodium salicylate has been reported but it seems not unreasonable that the structure in the efficiency curve arises from processes similar to those revealed by EXAFS.

The present results disagree with Lindles1 reported measurement of

an increase of a factor 2 between 80 and 270 eV.8 we find an increase of 3.6

between 150 and 270 eV, a region in which we have applied no corrections to the

data.

It is unfortunate that data taken with the low energy mirrors could

not be adequately analyzed to provide reliable information on the interesting

transition region from 50 to 150 eV. It is hoped to study this range in more

detail in subsequent experiments.

ACKNOWLEDGEMENTS

This material is based upon work supported by the National Science

Foundation under Grant Nos. PHY-821^172 and ATM-8H12820 and the National

Aeronautics and Space Administration Grant No. NC-R 28-002-01 .

9.
REFERENCES

1. D.H. Thurnau, J. Opt. Soc. Am. 16, 346 (1956).

2. F.S. Johnson, K. Watanabe, and R. Tousey, J. Opt. Soc. Am. 41, 702

(1951).

3. K. Watanabe and E.C.Y. Inn, J. Opt. Soc. Am. £3, 32 (1953).

4. J.A.R. Samson, J. Opt. Soc. Am. 5v1, 6 (1961).

5. J.A.R. Samson and G.N. Haddad, J. Opt. Soc. Am. £1, 1316 (1971)

6. R.A. Knapp and A.M. Smith, Appl. Optics 3, 637 (1961).~

7. E. Krokowski, Naturwiss. _15, 509 (1958).

8. D.W. Lindle, P.A. Heimann, T.A. Ferrett, P.H. Kobrin, C.M.

Truesdale, U. Becker, H.G. Kerkhoff, and D.A. Shirley, Phys. Rev.

A 33, 319 (1986).

9. G.V. Marr and J.B. West, At. Data Nucl. Data Tables J_8, 197 (1976).

10. M.R. Howells, Nucl. Instr. Methods, 177, 127 (1980).

11. G.P. Williams, M.R. Howells, N. Lucas, and P.Z. Takacs, Nucl.

Instr. Methods, 222, 99 (1981).

12. R. Allison, J. Burns, and A.J. Tuzzolino, J. Opt. Soc. Am. 51, 717

and 1381 (1961).

13- E.G. Bruner, Jr., J. Opt. Soc. Am. j>9, 201 (1969).

11. K.J. Nygaard, Brit. J. Appl. Phys. JJ5, 597 (1961).

15. F.J. Studer, J. Opt. Soc. Am. _55, 613 (1965).

16. H. Inokuchi, Y. Harada, and T. Kondow, J. Opt. Soc. Am. Si, 8^2

(1961).

17. N. Kristianpoller. J. Opt. Soc. Am. j>1, 1285 (1961).

18. D. Norman. J. Phys. C. (to be published).

10
I

(

FIGURE CAPTIONS

Fig. 1. Schematic diagram of the apparatus, (a) seen from the side and

(b) viewed end on.

Fig. 2. (a) Photomultiplier signal (full curve) and ion count rate

(chain curve) as a function of photon energy, (b) Salicylate

efficiency as calculated using Eq. (3) from the data shown in

Fig. 2a.

Fig. 3. Photomultiplier output (full curve) between 80 and 160 eV and

the estimated true first order signal (chain curve) used to

correct for second order contamination (see text).

Fig. H. The variation of sodium salicylate quantum efficiency as a

function of photon energy. The full circles are data taken

with Argon as the target gas, the open circles were taken with

neon. Both the argon and neon data are corrected for higher

order contamination below 160 eV; the horizontal dashed line

represents the unit efficiency of previous measurements. The

full line represents the 4J energy conversion factor predicted

by Krokowski. The positions of the carbon and oxygen K absorption edges are marked.

Fig. 5. The sodium salicylate efficiency as a function of photon energy in the region of the carbon K edge. The line is drawn as

a guide to the eye only.

11
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