SENSITIZED FLUORESCENCE IN A MIXTURE
OF MERCmiY MD CADMIUM VAPOHS
by
JOSEm SHARON WELLS
B. St, ^uaaas Stat« Collefjo of
Agriculturo and Applied SoleQo«« 1956
A THESIS
submitted in partial fulfillment of the
requirements for the degree
MASTER OF SCIENCE
Department of Physios
KANSAS STATE COLLEGE
OF AGEICULIUHE AND APPLIED SCIENCE
1958
QocuwMiAMS
TABl£ OP CONTE?!TS
INTRODUCTION
1
THEORY
2
•"'^
APPARATUS ASD TECHKiqUBS
15
EXPERIl^NTAL RESULTS
DISCUSSION
QB'
RESUUS
25
S8
CONCLUSIONS
29
ACKNOIfLEDGMBHTS
LITERATURE CITED
......
SO
IHTRODUCTION
Ihen a mixtur* of two different vapors is illuminated by radiation
that oan be absorbed by only one of the vapors, the atoms of the nonThis luminesoenoe is termed
absorbing vapor oan be observed to fluoresce.
sensitized fluoresoenoe*
To better understand this jdienomena, consider a hypothetical mixture
of two different vapors,
A and
B.
Let the atoms of vapor A have an energy
as the first excited level above their normal state, while the
level B
atons of B have as their corresponding level an energy state designated
E^,,
Assume that
E^^
is greater than
E. ,
and further, that the incident
frequency, f^, times Planck's constant is equal to E^,
Then, as atom A
absorbs the incident radiation, it will change to an excited state. A*.
If the half life of the excited state of A» is of lito order of magnitud*
A and
as the time between collisions between atoms
B, it will be probable,
by oollisiona of the second kind, for the energy of A* to be transferred
to atom B.
The energy difference, E
- S
,
will be divided between the
colliding atoms in the form of kinetic energy*
The atom
fl»
oan then emit
energy of a new frequency, t.m
It is also possible for kinetic energy to combine with the energy
of
sua
excited atom to excite a second atom to a higher energy level*
If,
for example, vapor B in the above mentioned mixture were replaced by a
vapor C, that has an atomic energy level, E
it would be possible for the quantum, hf
to excite the atom C to E
here, E
o
- B
,
,
,
,
which is greater than E^,
to combine with thermal energy
its lowest excited state*
The energy difference
would have to be obtained from the relative kinetic energy
a
of the two atoms*
This emission frequency, f_, from atom C* would be
different from either
f^^
or
f, .
Cario and Franck (1) sought to verify these hypotheses experimentally
in 1924.
They reported evidence which substantiated both predictions*
The purpose of this research was to oonfirm their results and to obtain
more quantitative information with the aid of improved techniques and
equipment*
TBEORy
Cario and Franck performed their first experiments using mercury
and thallium vapors*
Their experimental 8a*rangement consisted of two
reservoirs connected to an absorption vessel*
of mercury was deposited.
In one reservoir a sample
The other held thallium.
These reservoirs
were in separate ovens, to enable the experimenter to control the temperature, and hence, the vapor pressure of the materials.
The absorption
vessel was placed in a third oven which was capable of supplying th«
thermal energy necessary tc excite the atoms to the states lying higher
than the quantum energy of incident radiation.
The mixture of the two
vapors in the absorption vessel was irradiated by light from a water
cooled mercury arc lamp*
The principal irradiating line was the 2537 A
resonant line arising from the mercury 6^3
essentieil that this line be unreversed.
- 6*?- transition*
It was
The reradiated light was dis"
persed and detected with the aid of a quarts spectrograph.
Sensitized fluorescence was observed from thallium levels lying both
above and below the 4*86 volt level of the mercury resonant line*
ever, the intensity ratios for some of the lines were not as
expected.
haul
How-
been
This was attributed to the fact that metastable states which
•ri
w
Eh
Oh
•H
oould be excited thermally were present in thallium.
The next step was
to replace the thallium by eadmium since it has no such low lyin^ metastable
With the main oven at 400°C, they observed only the cadmium 3261 A
states.
resonant line from the 5^3^ « s'p^ transition.
At 800**C, the triplet emanat-
ing from the 6®S- level of cadmium as well as the 3261 A line was observed
on the spectrograms.
The possibility of exciting this level by stepwise
absorption was ruled out, as the same results were obtained when a mono-
chronator was used to pass only the mercury resonant line as the exciting
radiation.
Referring to Plate
seen that the 6
S^^
1
(Mitchell and Zemansky, 2), It can be
level of cadmium lies at 6»S electron volts.
This
osans that 1.4 electron volts of thermal energy was supplied by the main
oven to excite the atom to this level.
This was cited as evidence support-
ing the contention that thermal and quantum energy can cooperate in
exciting hi^^er levels.
The main improvement in performing the experiment again as described
in this paper was the substitution of a phctomultiplier for photographic
film as the detecting device.
This has the obvious advantage of observing
the results while performing the experiment as well as saving time in
foousiog.
An additional objective was to obtain temperature versus in-
tensi^ graphs for the triplet
of oadmlun.
APPASAIUS AND TECHHIUUE
The source of the 2687
A radiation used in this research was a
Sylvania U-shaped germicidal lamp.
The 2557 A resonant line was pre-
dominant, although the meroury triplet consisting of the 4047, 4358, cmd
5461 A
lines was present in measurable intensity.
A complete spectrum
is listed with the results.
TMs
radiation was focused on the absorption
The lens was supported
vessel by a fused quart* lens of focal length 16,3 em.
by a horitontally mounted rod.
The distance from a reference point on the
rod to the point of foeus for the 25S7 A line was determined by focusing
the same on a fluorescent screen while the viaible li£;ht was filtered out.
This simplified the focusing procedure in the research.
The ootobined
optical system is shown in the center of the photograph on Plate II.
The construction of the ovens was the most time consuming; part of
preparing the experimental equipment, hence ocnaiderable attention has
been given to this item.
The heeter coils were of Chrorael-A wire, closely
would around a 1»6 mm. rod which was mounted in the lathe.
After being
removed from the lathe, the coil was elongated to approximately four tioss
its original length.
The ©oil was first wrapped around an alundum core,
which was helically grooved, then tied in place with a string.
The ooil
was then carefully washed with alcohol to remove any contamination which
might cause oorrcsion upon heating, and inserted into a hollow cylindrical
form which had been poured from refractory concrete.
This sheath served
to hold the coil in position aftwr the strin- had burned away as well as
to ••parate the coils from asbestos.
The sheath, core, and coil were
supported between two plates of tranaito which were bolted together.
bolts also serred as electrodes.
Th«
The system was further insulated therm-
ally by a mixture of asbestos paste v;hioh when dried, lent support to the
oven.
Plate III shows the ovens.
described above.
The two smaller ones were built as
The one on the left contains a quarts observation window
in the end as a means of determining the amount of material in the oven
3
(l4
EXPLAMATIOK OF PIATjS III
Fig. 1*
Photogiraph of front view of ovens and
Fig. 2*
Photograph of diagonal Tieir of OTens and Hg rasorvoir.
Cki
r«serToir.
10
PLATF III
Fig. 1
Fig.-'Z
11
without
reracrrlng
the o-pen.
The main oven in tho center has two quarts
windows in the side through which the incident and the sensitized fluorescence radiations passed.
To help compensate for the heat losses through
the window area, the heating coil was ocmpressed by a factor of three at
the ends and center of the aperture.
Fig. 2 of Plate III shows how tho
thermocouples were rigidly mounted to facilitate oeaeurement of the temperature at the same point on the reservoirs.
not always be In the
ssuae
Since the auxiliary ovens could
position relative to the reservoirs, this was
necessary for reproducible temperature results*
The quartz cell is shown in Plate IV.
absorption vessel.
is the reservoir containing cadmium.
The constrictions seirve to decrease
the rate of diffusion through the tube.
of the main oven.
The bulb in the center is the
At the left end is the mercury reservoirj at the right
They are located within the limits
Since the main oven is maintained at the highest
temperature, this minimizes the possibility of condensation occurring at
the constriction and hence clo^^ging the vessel.
The tube connecting, the vessel tc the reservoir is bent downward to
cause the icercury to flow into the reservoir t^here it can be more readily
observed.
After the quartz vessel had been blown to the proper shape,
it was attached to a vacuum system and evacuated.
The vacuum system con-
sisted of a Welch Duo-seal fore pump and a Wisconsin type mercury diffusion
pump.
A Bayard-Alpert ionization guage was used to measure the pressure.
One liquid nitrogen trap separated the guage from the diffusion pump.
Another trap was placed between the guage and the container being evacuated.
Hiosphorous pentoxide in the systetc absorbed water vapor which might have
been present.
The cell was outgassed under vacuum at 800°C for 24 hoiurs*
13
o
T
1
U3
to
/v
0)
/
.s
s
o
t
i
/\
<s
/
1
'
\
CQ
I
i
fe*
t-
^^
1
1
*'
u
After 0.7 ob
of both a«roury «nd cadmium were distilled into the system,
it w«8 OTacuat«d down to approximately
Z
x 10
mm. of meroury, outgasied
again, and sealed under vacuum.
The light emerging from thlt Teasel was dispersed and its wavelength
measured
with a Bausch and Loab,
SOOube,
grating-type nonoohroniator.
Light traps were used to prevent light froi! the source reaching the monochromator directly.
A nodel 1P28 photomultiplier tube was used as the
light detecting device.
This tube was mounted in the receptor of a Photo-
volt multiplier photometer which is essentially a D.C. amplifier.
The
temperatures were measured with chrome 1-alunel thermocouples in conjunction
with a Leeds and Northrup potentiometer indicator.
The items mentioned
above are shown in Plate II*
One of the more difficult problems in the techniques was the focusing
of the system.
The method found best was to first position the optical
system the premeasured distance from the reference mark to the absorption
vessel.
The monoohromat or was then moved until a naxioum 25S7 A Intensity
was observed.
The optical system and monoohromator wore then alternately
moved until the 25S7 A line could no longer be intensified.
ohronator
wsis
The mono-
then set on the 5261 A line of cadmium and th« procedure
repeated until the cadmium resonant line could not be made more intense.
The system was then assumed to be in focus*
To ascertain that the triplet was not excited by stepwise absorption,
a black wire screen which diminished the intensity by a factor of one- half
was to have been used.
If stepwise absorption wore the responsible
excitation mechanism, the intensity would be reduced by cne half wiien the
screen was placed between the monoohromator and the absorption cell
and
It
by one fourth nhen interposed between the ultra-Tiolet source and th«
absorption cell*
The second order ghost from the 2557 A line was so intense that it
masked the 5086 A line from the cadmium triplet.
This i,host was eliminated,
when Boanriing for this line, by blocking the 2537 A line from the monoohromator with a pyrex glass plate.
£XPKRIM£NTAL
Tables
1
fiJSSULIS
and 2 show a typical source spectrum and the thermal back-
ground from the main oren at 800°C.
It was
not necessary to correct the
mercury lines in the following data for this background, since the mercury
lines in the region where this background was significant were reflected
lines*
However, the cadmium line intensities listed are corrected for
this background*
In Tables 5 through 8,
T^^
represents the average temperature of the
main oven as determined by two thermocouples, one on either side of the
absorption cell.
T
.
and
T.
are the temperatures of the reservoirs for
cadmium and mercury, as determined by a thermocouple in contact with each
reservoir.
These temperatures can be correlated to equilibrium vapor
pressures with the aid of the graph on Plate V,
(S)
the current symbol, I, refer to the cadmium, mercury
The subscripts aft«r
8uad
main ovens.
The two parts of Table 3 were intensity measurements made when the
constriction between the mercury reservoir and the absorption cell was
about 2 mm* in diameter*
The top set was taken with the monochromator
slits opened to approximately 0.80 mm*, and the bottom set when the slits
were opened to 0.40 mm.. The data in Tables 4 through
8
were taken with
16
mm. in
the oonstriotion mentioned above closed down to approximately 0.6
diameter.
Since the 0.40
mtn.
slit widths were optimum for both intensity
with
and resolution, the data on the pa^es following; Table S were taicer
these apertures.
cadaium
In the earlier work, an attempt to find the 5086 A line of the
second
triplet was not always made, since it would have been masked by the
order appearance of the 2537 A line of mercury.
Later, the previously
entioned technique with the pyrex plate was used to eiiainate this obstacle,
Table 1.
Irradiating spectrum.
Wavelength
in angstroms
«
»
Relative
intensity
2537
25,000
2752
32
2894
62
2967
33
S182
1,100
3541
110
3650
160
4047
3,300
4358
5,300
4916
24
5074
•
5461
2,000
The line appearing, at 5074
was the second order appesirance of the 2537 resonant
line.
Table 2.
Baekrrcund at 800°C.
Wave le n£,th
In anf.stronia
Relative
intensity
3500
3600
3700
3800
5900
4000
4100
4200
4300
4400
0.2
0.4
1.0
1.2
1.8
5.0
4500
4600
4700
4800
4900
4.0
5000
5100
5200
5300
5400
6.8
7.6
10
II
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5500
5600
5700
6800
5900
22.5
25.2
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2S
DISCUSS lOH (W RESULTS
The lines of principal interest for this problem were those of cadmiuiB.
The 5261 A line of cadmium was observed to be moderately intense at tinss.
were
It was found that the optimum initial conditions for its observation
when the mercury and cadmium were together in the reservoir which was to be
hsated.
The runs in Tables 4, 5, and 6 were started by having both cadmium
and mercury in the cadmium reservoir.
the open air.
The mercury reservoir was left in
As the mixture in the cadmium reservoir was heated, both
the mercury and cadmium vapor began to move out.
At a temperature of 250 C,
the cadmium vapor pressure was sufficient for the 5261 A line of cadmium to
be observed.
The results show that the line normally appeared more intense
during a rapid temperature increase, which followed an abrupt increase in
the current.
After the temperature was reasonably stable, the intensity
would decrease.
No positive explanation is given for this action.
it may be that the mercury,
However,
which has a high vapor pressure at these tem-
peratures as indicated by the graph on Plate V, carried small particles of
oadmium into the absorption cell as it rushed from the cadmium reservoir.
These small oadmium particles would then be vaporized and furnish sufficient
concentrations of cadmium vapor for the resonant line to be observed.
As
the cadmium reservoir temperature was further increased, this intensity
pulsing was observed again.
At 400^0, the intensity of the 5261 A line had
diminished to near sero, and remained there even though cadmium remained in
the heated cell and a relatively high mercury vapor pressure was present.
According to the literature, the optimum temperatures for sensitized
C
fluorescence of mercury and cadmium are 400°C for the cadmium and 100
for the mercury.
Further literature research disclosed that the behavior
EXPUHATlOfl OF PLATS V
Pressure ts» temperature ourres for mercury and oadmlim.
A
25
PLATE V
1000
500
100
/
50
mercury
/
10
'a
/
/
/
5-0
/
M
m
1.0
(D
S-.
^
o
cadmium
/
/
0.5
'
>
/
0.1
.05
.01
—
-I-
-J-
.005
.001
1
100
200
/
300
400
Temperature in degrees centigrade
50
60
ae
Just deaoribod had been observed before by I^iitchell, (4) when studying tim
polariiation of sensitiaed fluorescence in
meror-u-y
and cadmium vapors
.
His
explanation was that the cadmium vapor carried the mercury vapor from th»
absorption cell with it, as it migrated to a colder reservoir.
Due to the
that
high vapor pressure of mercury at 400°C, it is reasonable to assume
little mercury would be left at equilibrium in
the cadmium reservoir.
The
mercury vapor pressure would then be controlled by the reservoir whose
temperature was between 80 and 100°C, where its equilibrium vapor pressure
would be of the order of 1 mm. of mercury, as would be the vapor pressure
of cadmium at 400°C,
The non-equilibrium condition of the cadmium would
cause streaming and a degree of pumping.
Mitchell's explanation
sewaas
consistent with these results.
The constriction between the msroury reservoir and the absorption
cell was reduced to one-fourth of its original diameter in an effort to
decrease this pumping rate.
The data in Tables 4 through 8 are typical
of data taloen after the smaller opening was in use.
Still, at the optimum
temperr.tures, the intensity of the cadmium resonant line was negligible.
8oM
moderate intensities were obtained during the transient stages of
heating.
These intensities might possibly be exceeded if the optimuB
concentrations of the two vapors could be attained under equilibriua
conditions.
Tables 7 and 8 are illustrative of data obtained while trying to find
a mercury temperature which would neutralize this pumping action
after the
cadmium vapor had transported the mercury vapor out of the absorption cell.
Only when the mercury temperature was near SOO°C was the cadmium resonant
line observed again, even then the intensity was low.
In view of this la»
87
intensity and the faot that several mishaps had occurred near this mercury
temperature, it was felt that it would be neither safe nor fruitful to continue at these temperatures*
The primary objective of this experiment was to obtain some quantitative
measurements of the cadmium triplet.
The intensities involved were so
low that they were not discernible above the random fluctuations of the
Considering
measuring device, which were less than one tenth of one unit.
the intensities of the observed oadmiura resonant line, this is not alto£,ether
surprising.
Franok and Carlo calculated that only one part in 5 x 10
of
all collisions have sufficient relative kinetic energy to supply the 1.4 ev
energy difference necessary to excite the cadraium
to the 6
atoia
S.
level.
Since it is energetically possible for all collisions between excited
mercury atoms and cadmium atoms to result in an excited cadmium atom, on*
would expect this ratio or a lower one for the intensities of the resonant
line and the triplet*
The results also indicate some variation in the intensities of the
mercury lines.
These vsriatioas were not in general correlated to the
results for cadmium.
The intensity of the 2557 A line was due not only
to resonance radiation, but also to scattering.
The contribution of each
of these phenomena to the total intensity is not known.
3650 A lines are attributable to scattering.
4S58, and 4047
The 3132 and
The intensities of the 5416,
A lines from the source were moderately high.
It is
possible that these lines could be absorbed by excited mercury atoms in
the 6^P levels, to raise the atom to the 7's. level.
would result.
Resonance radiation
The test for stepwise absorption was meuie.
After correct-
ing for the thermal background, the resultant intensities were the sane
tl
The con-
whather the screen waa placed in the incident or outgoing beam.
clusion was that these were also scattered lines.
The variation in the
First, the
intensity of these lines can be attributed to two factors.
neroury source does not maintain a constant intensity output.
Secondly,
the geometry of the system ohan^;ed slightly due to thermal expansion of
the ovens and the quartz cell.
CONCmSIOKS
Two conclusions can be drawn from this experiment.
Since the optimum
conditions for sensitized fluorescence are to have equal vapor pressures,
this •xperiment should be performed with the mercurj' at 100 C and the
cadmium at 400°C.
Due to the pumping action of the cadmium, this is not
feasible with the present arranf^etnent.
The solution of the problem by
Mitchell indicates that helium equal in pressure to
2 mm. of
mercury
should be added to the cell in future experimsnts of this nature.
the problem of insufficient intensities must be overcome.
system is clearly inadequate in this respect.
Secondly,
The present
If a stronger msreury
light source cannot be obtained, perhaps more than one source could be
used to irradiate the absorption cell.
tf
kcisomEVQvmu
The author wiahea to 9xpr««« his gr«it«ful approciation to Dr.
E* H. MoFarland for the j,uldance, oounsel, and help given by him
during this research and the preparation of the raanusoriptj to Dr.
A. B. Cardwell, Head of the Physics Departmentj to Mr. R. A.
Anderson for his suggestions and his work in the preparation of
the quarts absorption vessels; and especially to the sponsors of
the author's fellowship during the past school term, the Office
of Ordnance Research, United States Army*
M
LITEBATURE CITED
(1)
(2)
Cario« A. and J. Franclc.
irber sensibilisierto Fluorestonz.
1923.
Zoit. i\ Phy. 17i202-212,
Mitohell, A. C. G. and M. W; ZvnaanBlcy.
Resonance radiation and excited atoms.
New Torkt
ItaoDlllan*
1954.
(3)
(4)
Stull, Daniel S.
Vapor pressure of inorganic compounds.
S9t517. 1947.
Uitohell, A. C. G.
Polarization of gensitited fluoreeoence.
209 1 747-756. 1930.
Ind. and Eng. Chem.
Journ. Frackl. Inst.
SENSITIZED FLUORESCENCE IN A MIXTURE
Of MERCURK AInD CADMIUM VAPORS
by
J03EH1 SHARON WELLS
B. S., Kansas State College of
Agriculture and Applied Soienoe, 1956
AH ABSTRACT OF A THESIS
uboitted in partial fulfillment of the
require aents for the degree
MASTER OP SCIENCE
Department of Riysios
KANSAS STATE COLLEGE
OP AGRICULTURE AND APPLIED SCIENCE
1958
Pranok and Carlo have postulated
thsit
atoms can b« excited to highei
energy levels through a cooperative effort between quantum and kinetio
energies during a collision of the second kind.
They cited evidence, which
was obtained through studies of sensitized fluorescence of mercury and cadmium vapors, to substantiate their hypothesis.
The major portion of their
evidence was dependent upon the intensity of a triplet emanating from the
e's
level of cadmium.
This level could be excited only when 1.4 ev of
energy was available in the farm of relative kinetic energy between the
•rcury and
The purpose of this research was to confirm
cadmiuai atoms.
their results and obtain some temperature versus intensity relations with
the aid of improved techniques and equipnant.
In the original work, the sonaitiaed fluorescanoe was a,-jalyzed with
the aid of a quart* speotrograp*!.
The intensities on the spectroj^ram were
weak and only a two point correlation was obtained between the temperatures
and the intensities of the cadmium in question.
intensity was tero.
to a greater degree.
One of these values of
In the present research, temperatures were controllable
The desired lines were selected for msaaureoient by a
Bftusoh and Lomb moncohromator.
A photo multiplier and D.C. amplifier system
of high sensitivity were used to measure the relative intensities.
system had the advantages
of
This
speed in focusing, continuous monitoring of
intensities, and a relative scale for the value of the intensities.
The problem of weak intensities again prevented the desired quanti-
tative information from being obtained.
The intensities of the triplet
were less than the random fluctuations of the measuring device.
Possibly
this low intensity was partially due to the behavior of the materials in
the absorption well.
The recommended concent rati ens of the iBroury and
cadmium vapors could not be obtained under equilibrium conditions due to
the pumping action of the cadmium at the optiinum temperature.
The addition
of 2 mm. of mercury pressure of helium to the absorption cell to prevent
pumping and the acquisition of a stronger mercury light source were concluded to be necessary for future experiments of this nature.