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HUDSON LABORATORIES
) DOBBS FERRY, N.Y.

Technical Report
o. 73

A Detailed Investigation
Of the Absorption by Water
Of Electromagnetic Radiation

by.
B. P. Fabricand

AUGUST 15, 1957

Contract

N 6 -ONR-27135

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ASTIA No. AD 215630 : CU-93-59-ONR-271-Phys.

Columbia University
Hudson Laboratories
Dobbs Ferry, N.Y.

R. A. Frosch
Director

Technical Report No. 73

A DETAILED INVESTIGATION OF THE ABSORPTION
BY WATER OF ELECTROMAGNETIC RADIATION

by
B. P. Fabricand

August 15, 1957

This report Copy No. “4
consists of‘19) pages of'65, copies

Research Sponsored by
Office of Naval Research
Contract N6-ONR-27135

Further distribution of this report, or of an abstract or
reproductions, may be made only with the approval of Chief
of Naval Research (Code 466). Reproduction in whole or in
part is permitted for any purpose of the United States
government.

ABSTRACT

The various mechanisms for the absorption
and scattering of electromagnetic radiation in sea
water have been examined with the idea of evaluating
the possibilities of windows anywhere in the spectrum.
It is concluded that the chances of finding a window
are negligible except of course for the one in the

visible region which however is rather cloudy.

TABLE OF CONTENTS

Introduction

Absorption Processes at Very High Frequencies

Pair Production and Compton Effect ( > 10r”

The Photoelectric Effect ( > 3x10!°

cycles/sec)

cps)

Absorption Processes in the Visible Region
Scattering of Light by Na’ and Cl ions (Thomson scattering)
Scattering by Small Particles
Molecular Scattering

Absorption and Dispersion for Ultra-Violet andInfrared Frequencies

Absorption at Longer Wavelengths (D. C. to Microwave)

Experimental Results

Discussion

ies)

CoO f£ fF BB

10

13

Introduction

The main purpose of this report is to examine the theoretical and
experimental evidence which exists on the absorption of electromagnetic
radiation by water in the hope of finding windows through which the radi-
ation may propagate with little attenuation.

There were two principal reasons for reopening this subject:

1) In the last two or three years, there have been great advances
made in the detection of radiation, especially in the infrared
region of the spectrum;

2) There does not seem to be any adequate investigation of water
absorption in the infrared using modern techniques.

There have been measurements covering various ranges of frequency, and
these will be reviewed below. Furthermore, the reasons for the known
great absorption in the infrared do not appear to be adequately explained
on a theoretical basis.

The term absorption will also include any scattering process by which
a beam of radiation is attenuated. There are a number of mechanisms of
absorption, and each will be considered in the next sections. Following
this, a review of the experimental work on absorption in water will be pre-
sented. A summary of absorption throughout the whole spectrum will be
given in the next section; and in the final section of this report, a
discussion and recommendations for any future work will be made.

The Absorption Processes at Very High Frequencies

The processes will be considered roughly as a function of the frequency
at which a specific process is most effective, starting with the highest
frequencies.

A. Pair Production and Compton Effect (© 1018 cycles/sec)

These processes are effective only at extremely high frequencies,

in the region of the spectrum usually referred to as gamma rays qao!8 cps
and up). Absorption by pair production arises when a gamma ray disappears
and gives rise to the creation of a positron and an electron. This process
must take place near a nucleus in order to conserve momentum. The Compton
effect arises from the scattering by electrons of the electromagnetic
radiation.

‘
'
‘
aa:
> '
a
} i

The intensity I of a monochromatic beam passing through water
decreases exponentially according to

where Ip is the initial intensity, x the depth of penetration, and a the

absorption coefficient. a is composed additively of a part due to pair
production and a part due to the Compton effects

G= G.-2 Fo
pair compt .

If o represents the cross-section of one atom for one of these processes,
the corresponding a is given by

Or No

where N is the number of atoms per em? .» The cross-sections for these
processes are given in the book by Heitler! and will not be reproduced

here. The expression for Ccompt is considered exact, although errors

arise ino from screening by atomic electrons and use of the Born

pair
approximation. However, these errors are negligible for the present
purpose. Since o (and a) are independent of any molecular and atomic
interactions in the medium, a theoretical calculation of absorption from
these two processes will be extremely accurate.

Table I is taken from Heitler and gives a per cm for various
frequencies, these being expressed in units of the rest energy of the electron.

TABLE I, ABSORPTION OF GAMMA RAYS IN H50

Be ito. 7) 220 ei) 50 | 100 "1000 {10000 |
| |

6 x [1.2 x 2.4 x/6 x |i.2 x12 x|t2 x
1019) 102° 10°" 1020 102) 102) 1023 |
043 Lago | ex | 016 \ 015.0165, .017 |

cans ns

It can be seen from Table I that the absorption decreases with

frequency up to at = 100 and then starts increasing again. The fall-off
mc

results from a decrease in Compton scattering with frequency, while the rise

results from the creation of pairs.

ae

The absorption continues to increase with frequency and finally tends to
a constant asymptotic value, The maximum mean free path for the radiation
(the path length needed to decrease the intensity to 1/e of its initial

value) occurs at ng = 100 and equals 1/a = 67 cm. Thus, it can be seen
mc
that there is no hope of finding a window at any frequency interval above

1018 cps. Furthermore, it will be shown in the next subsection on the

photoelectric effect that this lower limit can be extended to about 10'°cps.

B. The Photoelectric Effect (> 3 x 10!° cps)

The photoelectric effect arises when an electron in an atom or
molecule absorbs a photon from incident radiation and makes a transition
to the continuum, becoming a free particle. This effect, then, is effective
at photon energies greater than the ionization energy of the atom or molecule,
and the absorption spectrum is continuous.

For the calculation, the absorption due to water will be approxi-
mated by that of hydrogen. The ionization energy of water is 12.56 volts
and that of hydrogen 13.6 volts, so that the absorption per molecule or atom
is nearly the same. For frequencies much larger than the ionization frequen-
cy of the atom, the absorption coefficient is

Z° fey ee 1c
© (13774 Mt

where N is the number of atoms per ane , Z the nuclear charge, Dp the Thomson

scattering cross-section. For hydrogen in water, Table II gives values
of a asa function of frequency.

TABLE II. ABSORPTION OF GAMMA RAYS BY HYDROGEN
(photoelectric effect)

H
.
|

f

scene SEE TEE SIMI

a a
| if

| .24 | 024 | .0024 | .0012 | .0006
| | :

| } i |
720 180000

| a 5% t
B ERe) * 60

It is seen that the absorption falls off sharply with increasing

frequency (yi/2 from the formula), until it reaches the very small value of

5x10> per cm at Ay = .1. However, Table I shows that in this frequency

mc
range the Compton effect is very appreciable and rising with decreasing
frequency so that there is little penetration by the radiation.

AUSSI a ee mosprcoesecahy

BRC

Furthermore, there will be a great deal more absorption from the other
elements present in water, notably oxygen and nitrogen.

It can be seen, therefore, from this and the preceding section,
that there is no hope of finding a window at any frequency higher than

fe)
the ionization frequency of water (~3x10! eps = 1100 A). At frequencies
below this, the photoelectric effect disappears. The rest of the report
will be concerned with this lower frequency region.

Obsorption Processes in the Visible Region

A. Scattering of light by Na’ and Cl__ions (Thomson scattering)

When light falls on free charged particles, it sets them into
oscillation and causes them to emit radiation. The cross section for this

2
5 2 n (
3 me-
5}

eo Tonie andl ie S08: cus respectively. The

iS
process’ is

+ = -
For Na and Cl ,o is 3.3x10
mean free path of the radiation for this process }\ = ra (where n is the

: ; : : OR.
ion concentration, here the NaCl concentration in the ocean, 3.6x10" )iais

} = —— + ——— 2 10° oo.

3, 6x10! 7(1, 543. 3)x10—

Therefore, any absorption of the incident radiation due to this process is
negligible. If, as is probable, the Na ion forms a complex in water, the
effect will be even less.

B. Scattering by Small Particles
9 The scattering of light by small particles has been discussed by

Mie , Bom® and Stratton’. The mathematics involved is lengthy and laborious,
and only an indication of what goes on will be sketched here. Some experi-
mental results will be presented below.

The simplest model is that of a small sphere upon which is incident
a plane electromagnetic wave. To describe the scattering, solutions of
Maxwell's equations must be found that add up to give a plane wave at large
distances from the sphere. and also are of the proper form to satisfy boundary
conditions at the surface of the sphere. It turns out that the scattering
cross section can be represented by a series of functions which depend on the
dielectric constant and conductivity of the sphere, and the ratio, a/\, of
sphere radius to wavelength. The first term of the series is seen to be the
radiation emitted by an electric dipole, while the other terms correspond to
emission by higher multipoles.

245

z

by only

for the

|
,
j
ey,
ee)
|
14
H
Hy
i
iy
i {
.05
The (2

with th
pole te
oscilla

| by volu
fe)
6000 A

cross-s

As an example, the back scattering cross-section of a sphere of
dielectric constant ¢ and infinite conductivity for a/\ << 1 is given

the dipole term

5 1.4 (2 | 104
a NOC) _

It is seen that this is the famous Rayleigh scattering which accounts

blue color of the sky.

Fig. 1 shows a plot of this cross-section vs a/\.

EEE a Uae Enea

p
\
/ / \

5
a re et

nmeeed

4
) or Rayleigh scattering is shown by the dotted line. It coincides

e solid curve for a/\ << 1. At about a/\ = .05, higher order multi-
rms become larger and fluctuate in phase and magnitude causing the
tions. As a/\ becomes larger, o approaches the geometrical cross-section.

As a sample calculation, suppose water containing one part per million
me of infinitely conducting spheres scatter radiation of wavelength

° u
(visible region), For radii of 12000 A (~ 10

4 ; : 2 , : :
ection is about geometric, ta . The mean free path for this radius is

cm) the scattering

° 2 : : ; é
given by 1/n 7ra’ where n is the number of scattering particles per cubic

centimeter (2..5x10° in this case), Its value is 130 cm. Experimental results
pertaining to the clarity of ocean water will be discussed below.

25

C. Molecular Scattering

From the microscopic point of view, an electromagnetic wave
incident on a substance distorts the molecules of the substance, thereby
inducing molecular dipolemoments. These induced dipoles can be considered
as vibrating in a vacuum, They radiate 1) a wave which exactly cancels
the incident wave in the substance, 2) a reflected wave, 3) a refracted
wave, and 4) a scattered wave. The details of this process are presented
in the book by Born. It is shown there that the first three processes
depend only on the average induced dipole moment per unit volume, while
the fourth process depends on deviations from this average moment such as
would be produced by density fluctuations.

The intensity of the scattered radiation J is

j= tn 2D" p-(AN)@
ie
4
E C27 ewe
= coe — p- kip K

where p is the induced dipole moment, AN is the fluctuation in the number
of molecules per unit volume from the average value, p the density, K the
compressibility. It depends on the inverse fourth power of the wavelength
as in the case of scattering by particles. However, this scattering occurs
in the purest substances.

The cross: section is obtained by dividing J_ by CE? / arr:

‘A a kTp K,

_ 256° 4
cr ms

since p = a E, where a is the polarizability of the molecule and E the inci-
dent field. For radiation of wavelength 6000 A incident on water at 20°C,

pw AOI K ee aslo .

It

15

The radiation mean free path is aoe 10°~ cm, so that this effect is negli-

gible. se

At the triple point of water, T = .0075°C and p = 4.58 mm, K
becomes extremely large and water becomes practically opaque.

Absorption and Dispersion. for Ultra-Violet and Infrared Frequencies

Up to this point of the report, scattering processes have been con-
sidered (except for the high energy processes of photoelectric absorption
and pair production). This section will be concerned with the absorption
and dispersion(the variation of the index of refraction with frequency) of
radiation. Very general relations exist between the refractive index and
the absorption coefficient that enable one to determine the absorption if

abe

the dependence of the refraction index on the frequency is known through-
out the spectrum from V=0O to V=o and vice versa. These relations
arise from a correlation of these two quantities with the real and amas
nary parts of a complex dielectric constant, as first noted by Kramers” .

The wotlanans’ are as follows if the dielectric constant k=k + ik,:

a v' Ky (v")

os dv
vio- Vv

(v) = -

4 Ir

These relations are very general, assuming only the law of cause
and effect. Their validity is general and irrespective of the model,
which can be either classical or quantum-mechanical. The structural unit
can be an atom, electron, molecule, water drop, etc., provided only that
a considerable number of them be included in a volume unit whose dimensions
are small compared to a wavelength. Qualitatively, the relations state
that any change in refractive index is accompanied by absorption of the
radiation.

There are two large regions of the spectrum over which the index
of refraction changes appreciably: 1) the region between X-rays and the
visible: and 2) that between the visible and the radio-frequency part.
In these two regions, therefore, great absorption is to be expected. How-
ever, this does not prec lude the existence of windows in these frequency
regions.

The visible region of the spectrum has been discussed above and
Will be considered below from an experimental viewpoint. Obviously, a
window does exist here; the question is how much of one. The region from
d. c, to radio-frequency will also be considered below, although at these
frequencies the conductivity of water contributes to a large absorption.
Ultra-violet and infrared absorption will be considered in the rest of
this section.

In common with other molecules possessing a_dipole moment, water
molecules exhibit an infrared absorption spectrum.’ This spectrum arises
from the rotational and vibrational degrees of freedom of the molecule;

a quantum of radiation induces a transition from one rotational and/or
vibrational state to another, being absorbed in the process. The resulting
spectrum is quite complicated owing to the fact that water is an asymmetric
top molecule, i.e., one that has three different principal moments of inertia.
Quantitative formulae for the_representation of the rotational energy levels

are given in Herzberg’s book i The levels for an asymmetric top molecule

fall somewhere in between the levels for a prolate and oblate symmetric

top molecule, To each value of the total angular momentum J of the
molecule, there are 2J+1 levels which arise because there are components
of the total angular momentum about the figure axis of the molecule. The
rotational spectrum resulting from transitions between the levels is compli-
cated and extends throughout the infrared. It is shown in Herzberg’s book
for water vapor, Although extensive, there are numerous windows in the
vapor. However, the situation in the condensed or liquid state is far

from clear. It would seem that in the liquid, the molecules are not

freely BOL at Big but are in hindered rotational or torsional oscillating
states. Debye?’ bases his theory of non-resonant dispersion in water on

the assumption that the rotations of the molecules are opposed by a viscous

force. Bernal and Fowler’ believe that about 1/6 of the molecules are
able to rotate freely, while PoplelO considers the molecules as under-
going at most torsional oscillations. In any event the rotational ab-
sorption lines will certainly be smeared out in the liquid, although the
smearing should be less the higher the frequency. The question of a window
would have to be decided on experimental grounds because of the uncertain
nature of the liquid. This question will be ‘iscussed further below.

The vibration modes also contribute to infrared absorption, and, to
some extent, absorption in the visible region of the spectrum. Thbre are
three normal modes of vibration possible for a water molecule; and for
each mode there exists a set of vibrational states. Since the spacing
between states decreases with increasing vibrational quantum number, tran-
sitions between states give rise to the well-known absorption bands. For
the water molecule, the three main bands corresponding to the three normal
modes occur at 6.3, 2.66, and 2.74 microns. In addition, other bands arise
because of the anharmonicity of the vibrations--overtone and combination
vibrations occur, which, however, will be much weaker than the fundamentals
since the anharmonicities are usually small. These bands extend well into
the visible spectrum and are tabulated in Herzberg’s book. In the liquid
the fundamental bands occur at about the same frequency as in the vapor.
This is to be expected because of the very short period of the vibrations
compared to any reasonable estimate of collision times in the liquid. There
also appear new bands in the same region of the spectrum which are attri-
buted to molecular association, Absorption graphs will be shown below.

At the high frequency side of the window, absorption results from
the electronic structure of the molecule. Like many molecules water pos-
sesses a continuous absorption band in the blue ultra-violet. These
electronic transitions are not affected very much in the liquid state except
that there is a shift of the band maximum to higher frequencies. It will
be seen below that absorption reaches a minimum in the visible and starts
up sharply on either side.

Absorption at Longer Wavelengths (D.C.

At frequencies lower than infrared, the conductivity of water, most
of which arises from ions of sodium and chlorine, contributes to absorption
of radiation. This can be seen as follows: For a conducting medium, the

to Microwave)

Hee

wave equation is

qd
ica}
|
im
[5
+
IS
a
ica)

which reduces to

kx

: : ‘ ae ‘ i
€ is the dielectric constant, and o the conductivity. Since E~e

it can be seen that the imaginary part of Ke contributes an absorption
term to E . The conductivity o = ney, where n is the ion density

and 4 the ion mobility. For high frequencies (infrared and higher),

the mobility of heavy ions like Nat is approximately zero. At lower
frequencies, the ions can follow the changes in electric field and thus
move, The resulting collisions are taken into account in a macroscopic
way by putting in the free space wave equation a damping term proportional
to E . This is the term which leads to absorption.

An idea of the dependence of the amplitude of motion of the ions
on frequency and intensity can be gained from the following. The equation
of motion of a free ion is

eE=mr

and the amplitude of its motion is

Magee
ae) Bas
mo
The following table gives some numbers based on E = .024 e.s.u. (the value

for sunlight).

d\ (Angstroms)

1.3x107-9 5000
5,2x107-9 10000
5,2x107 18 100000

=i 6
52x10 10°(.01 cm)
5,2x10722 108(1 em, 30,000 me)
5.2x107° 1010 (300 me)
5,2x1074 10/2 (3 me)

20"

A rough value of the mean free path of sodium and chlorine ions
in solution may be obtained from the diffusion constant for .2N NaCl

-5 om*
sec
assuming a velocity of a free ion in vacuum at room temperature(2.7x104cm/sec).
Thus it can be seen that there will be no absorption from the ions until the
region between the infrared and microwaves is reached, and even here, the
amount of absorption depends on the amplitude of the electric field. How-
ever, in this intermediate region (about 1 cm) there is great absorption
from other effects. There is a very broad water absorption line at 1.35 cm,
and one due to oxygen (whose concentration in water is about 1/10 that in
air) at 1/2 cm arising from its electronic magnetic moment. For wavelengths
longer than 1 cm, the absorption falls off enough to permit effective use
of radar in air at 10 cm. But in water, the skin effect, which arises from
the ions, becomes of importance and drastically limits propagation.

2
For low frequencies (5-5 >> 1), the absorption factor a = |
Ew
This term is responsible for the well-known skin effect. At 1 mc/sec,
a is about .1 for sea water, giving a mean free path of 10 cm. It is
easily seen that propagation of these frequencies is not feasible except
perhaps at the lowest ones, and here there are immense problems of efficiency
in radiating and receiving equipment. Furthermore, wavelengths in this
region are so long that scattering from any object the size of a submarine
is apt to be extremely small because of the Wie dependence (see above). Low
radio frequencies (\ > 30 cm) show little attenuation and have been used in
practice. The English station Rugby used 16 kc/sec to transmit to a sub-
marine with an antenna 30 feet underwater at ranges up to 3000 miles.

in water which is D = 10 The mean free path 1 = 3D/v = TOMCn

Experimental Results

In this section will be presented a brief survey of experimental
work measuring absorption in the visible and infrared.

Clarke and Backus!) measured a light intensity of 107 watt /em= iu
a depth of 550 meters when there was a surface intensity of .05 watts/cm“.
This experiment took place on July 20, 1955, about 200 miles southeast of
New York City. The measurements correspond to an attenuation of .0125 or
a mean free path of about 80 feet. These measurements agreed with those of

Hulbert? who measured the absorption and scattering of light in distilled
water, Chesapeake Bay, and coastal waters off Hollywood, Florida. Some of
his results are reproduced here:

=10-

Distilled Water Chesapeake Bay

X fe] B nom Epes
4000 A 3.6x107° 4.Ax107°
4500 2.25 1 19 37
5000 1 2.3 17 21
5500 1 3.7 18 14
6000 19 18 25
6500 30 18 34
7000 4 57 18 56

o is the part of the absorption of a light beam arising from scattering
and £ that due to pure absorption. The total absorption is given by

k = o+B where k occurs in the exponential et, Hulbert also exhibits

the following graph:

an a EEE YP Te TN PRE A NA WT ERE RESIN SC LR ET NYP OSES SEER AERTS REC

3

Pkxio°rt 2

rofiresarneem venenatis rt LTO MR ETE EMONC PREECE MLR TENE PEPER

Coastal fe

Se

2 ee ee ee

°
A

4000 5000 6000 7000

==

By comparing Hulbert’s work with that of Burt!°, it may be seen

how very pure ocean water is. Burt measured extinction of light in
Chesapeake Bay caused by suspended particles. He concluded that particle
concentration ranges from 1 to 60 parts per million by volume and that
the average particle radius is 6x10-9 cm. Since coastal water is much
less absorbing than bay water, it may be concluded that it is very dust
free and contains much less than the 1 part per million of suspended
particles than was used in the example of the section, Absorption Pro-
cesses at Very High Frequencies.

The best measurements of infrared absorption seem to be those of

Plyler and Acquista!4 whose results are graphed below. These authors
used a Perkin-Elmer double beam spectrometer and three different sample
thicknesses of water.

100

80

GT 60
40

20

100

%T

eae x

(18 22 26 30 34 36 424

The graphs plot the percentage transmission against wavelength in microns.
The numbers on the graphs give the sample thicknesses. a gives the ap-
proximate absorption coefficient at the specified wavelength in cm-!.

Cartwright !° states that there is a gradual increase in T from 50 to 150
microns.

aOe

The following experiment was carried out in the Hudson Laboratories
by the author to investigate the infrared in a manner more suitable for the
discovery of a window. Infrared absorption experiments invariably use a
beam of narrow frequency range to obtain the highest resolution. Therefore,
it is possible that although there may be some penetration in this narrow
band, it may be too small to detect. Furthermore, the region of wavelength
greater than 50 microns does not seem to be adequately covered.

The experiment was done in such a way that contributions from any
window in the entire infrared, no matter how small, would add up in the
detector to give a response. A “Glo-bar" source furnished a black body
spectrum throughout the infrared. The radiation was then passed into water
contained in an aluminum tray, was reflected from the bottom of the tray
and into an Eppley thermopile which served as a detector. A germanium
filter was used over the detector to cut out any radiation of wavelength
shorter than 1.8 microns, especially the visible which begins at about
.75 micron. The sensitivity of the system was such that a microwatt per
square centimeter of radiation could have been detected. However, 1/2 inch
of water was more than enough to stop any radiation whatsoever from reaching
the detector. The question of whether or not a more sensitive detector
would be worthwhile will be gone into in the next section.

Discussion

Gathering together all the theoretical and experimental results
presented above, the following pertinent facts stand out:

fe}
1) There is no hope of finding a window at wavelengths of 3500 A
and shorter. This result is based on the absorptions due to pair production,
Compton effect, photoelectric effect, and molecular electronic transitions.

2) A window does exist in the visible. However, the radiation
mean free path of about 80 feet precludes any use of these frequencies
for detection at distances greater than a mile, and even this is stretching
the point.

3) There does not seem to be any window in the infrared. This con-
clusion is based mainly on the work done at Hudson Laboratories. As mentioned
above, there remains the question as to whether or not a more sensitive
detector should be employed. This question will be considered below along
with possible reasons for the lack of a window in the infrared.

4) The region from the very far infrared to D. C. is adequately
covered by standard electromagnetic theory. The only possible window is at
very low frequencies where the skin depth approaches distances of interest.
However, equipment difficulties in this region seem insuperable at the present
time. Thus, it would appear that the use of electromagnetic radiation for
detection in sea water is out of the question. As remarked by D. Sternberg!®,
"Evolution is against us."

There remains the question of why there is no window in the infrared
for water when many windows exist for the vapor. Obviously, the reason is
connected with the much closer association of molecules in water. Two
explanations have presented themselves:

SiS

1) With the close association of molecules, there arise clusters
which have a great number of possible modes of vibration. For each mode
there exists a frequency spectrum in a different frequency range. It's
quite possible that there are enough different size clusters to completely
cover the infrared region.

2) As mentioned above, water vapor has a rotational spectrum spread
out through the infrared, In the liquid it is probable that there is no
free rotation for the water molecule, but only hindered rotation. Whether
free or hindered, however, each rotational state will be split by any
electric field present, the amount of splitting depending on the strength
of the field. Because of the fact that water possesses a dipole moment
of 1.87 Debye, rather strong fields exist at each water molecule. An
estimate of this field can be obtained from the formula

-18 3030 5

E Das =ale SON er (2x 10bm ie = 2 oxXlOm easel

r
or about 10° volts/cm. This huge field depends critically on the positions
of neighboring molecules and therefore fluctuates considerably. In effect
the fluctuation smears out each rotational level and could well account
for the lack of a window.

It seems to the author that these two reasons are extremely strong
arguments against the purchase of additional and much more expensive infrared
detectors of greater sensitivity than the Eppey thermopile. Some work
remains to be done in this field dealing with the problems of the effect of
association of molecules on radiation and the measuring of the electric
fields present in water. However, these topics will be covered in a future
report.

-14-

REFERENCES

. The Quantum Theory of Radiation, W. Heitler.

> Mie, Ann. Physik 25, 377 (1908).

» Optik, M. Born.

. Electromagnetic Theory, J. Stratton,

. H. A. Kramers, Como 2, 545 (1927).

» For a detailed discussion, see RL Report No. 735 by J. H. Van Vleck.
. Infrared and Raman Spectra, G. Herzberg.

- Polar Molecules, P. Debye.

. Bernal and Fowler, J. Chem. Phys. 1, 515 (1933).

- J.A. Pople, Proc. Roy. Soc., London, 205A, 163 (1951).
. Clarke and Backus, Deep-Sea Research 4, 1 (1956).

> Hulbert, J. Opt. Soc. 35, 698 (1945).

- Burt, J. of Marine Res. 14, (1955).

. Plyler and Acquista, J. Opt. Soc. Am. 44, 505 (1954).
. Cartwright, Nature, 135 and 136 (1935).

, Private conversation.

JOY AL fSy ab He IE By 20) Ab A 18)! Ju

Office of Naval Research (Code 466)
Navy Department

Washington 25, D. C.

copies 1-2

Commanding Officer and Director

U. S. Navy Underwater Sound Laboratory
Fort Trumbull

New London, Connecticut

copies 3-4

Director

U. S. Naval Ordnance Laboratory
White Oak

Silver Springs 19, Maryland
copies 5-6

Commander

U. S. Naval Air Ordnance Test Station
China Lake, California

copy 7

Director, Marine Physical Laboratory
University of California

Scripps Institute of Oceanography
San Diego, California

copy 8

Director

Bell Telephone Laboratories
Whippany, New Jersey

copies 9-10

Committee on Undersea Warfare
Naval Research Council
National Academy of Sciences
2101 Constitution Avenue
Washington 25, D. C.

copies 11-12

Chief, Bureau of Ships (Code 688)
Navy Department

Washington 25, D. C.

copies 13-14

The Hydrographer

U. S. Navy Hydrographic Office
Washington 25, D. C.

copies 15-16

Director

Naval Research Laboratory
Washington 20, D. C.

Attn: Code 5500, Dr. H. L. Saxton
copies 17-18

Commanding Officer and Director

U. S. Naval Electronics Laboratory
Point Loma

San Diego 52, California

copies 19-20

Commander

U. S. Naval Air Development Center
Johnsville, Pennsylvania

copies 21-22

Dr. F. V. Hunt

Harvard University
Cambridge, Massachusetts
copy 23

Director

Woods Hole Oceanographic Institution
Woods Hole, Massachusetts

copy 24

Director

Lamont Geological Observatory
Torrey Cliff

Palisades, New York

copies 25-26

Chief, Bureau of Aeronautics (AV-43)
Navy Department

Washington 25, D. C.

copies 27-28

Chief, Bureau of Ordnance
Navy Department
Washington 25, D. C.
copies 29-30

Chief of Naval Operations

Navy Department

Washington 25, D. C.
(Gp-001) copy 31
(Op-91) copies 32-33
(Op-312) copies 34-35
(Op-316) copies 36-37

Commanding Officer
ONR Branch Office

Navy No. 100

Fleet Post Office

New York, New York
copies 38-39

British Joint Services Mission
Main Navy Building, Room 4924
Washington 25, D. C.

via

Chief of Naval Operations (Op-316d)
Navy Department

Washington 25, D. C.

copies 40-42

Admiralty Research Laboratory
Teddington-Middlesex, England

via

Chief of Naval Operations (Op-316)
Navy Department

Washington 25, D. C.

copy 43

Commander

Armed Services Technical
Information Agency

Arlington Hall Station

Arlington, Virginia

copies 44-53

Commander

Submarine Development Group II
U. S. Naval Submarine Base
Box 70

New London, Connecticut

copies 94-55

Weapons Systems Evaluation Group
Office of Sec'y of Defense

Room 2E1006- Pentagon Building
Washington 25, D. C.

copy 56

Commanding Officer
ONR Branch Office

346 Broadway

New York 13, New York
copy 57

Canadian Joint Staff

2450 Massachusetts Avenue, N.W.
Washington 25, D. C.

via

Chief of Naval Operations
(Op-316)

Navy Department

Washington 25, D. C.

copies 58-60