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QC100.U556 V143;1975 C.1NBS.PUB.C1959 NBS MONOGRAPH 143

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Analysis of Optically Excited

Mercury Molecules
NATIONAL BUREAU OF STANDARDS

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^fP 1 4 1975

1-^

Analysis of Optically Excited

Mercury Molecules

R. E. Drullinger, M. M. Hessel,

and E. W. Smith

Quantum Electronics Division

Institute for Basic Standards

l/l / S • National Bureau of Standards

Boulder, Colorado 80302

Supported by:

Los Alamos Scientific Laboratory under

AEC Order No. LS74-4 and the Advanced

Research Projects Agency under ARPA

Contract 981, Amendment No. 9.

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secreiary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued January 1975
Library of Congress Cataloging in Publication Data

Drullinger, R. E. Analysis of Optically Excited Mercury Molecules.

(National Bureau of Standards Monograph; 143) Supt. of Docs. No.: C 13.44:143

1. Laser Spectroscopy. 2. Mercury—Spectra. 3. Excited State Chemistry. I. Hessel, Merrill M., 1933- joint author. 11. Smith

Earl William, 1940- 111. Title. IV. Series: United States. National Bureau of Standards. Monograph; 143. QC100.U556 No. 143 [QD96.L3] 389'.08s t546'663'6| 74-31115

National Bureau of Standards Monograph 143

Nat. Bur. Stand. (U.S.), Monogr. 143, 51 pages (Jan. 1975) CODEN: NBSMA6

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.44:143).

Price $1.10
CONTENTS

I. INTRODUCTION ^

II. EXPERIMENTAL 3

A. Cell and Oven 3

B. Purity Problems and Cell Filling Procedures 4

C. Optical Pumping and Light Sources ^

D. Detection Systems y

III. STEADY STATE DATA g

A. Band Shapes g

B. Band Intensities g

C. Potential Curves

D. The Effective Transition Probability I5

IV. TIME DEPENDENCE

V. SUMMARY jg

VI. ACKNOWLEDGMENTS 19

VII. REFERENCES 20

LIST OF FIGURES

Figure 1.1 Potential curves of Bg^ molecules according to Mrozowski 23

Figure 2.1 Experimental setup 24

Figure 2.2 Mercury ampule preparation system 25

Figure 2.3 Mercury cell distillation system 26

Figure 2.4 Absorption coefficient and integrated band intensity

versus wavelength 2 7

Figure 2.5 Trasmission curve of tri-compartment filter 28

Figure 2.6 Transmission curve of 335 bandpass filter 2 9

Figure 2.7 Transmission curve of 485 bandpass filter 30

Figure 3.1 Spectral distribution of Ug^ fluorescence excited by

Xe Arc 31

Figure 3.2 Spectral distribution of Hg2 fluorescence excited by

257.2 nm laser line 32

Figure 3.3 Spectral distribution of 485 band at different temperatures .. 33

Figure 3.4 Spectral distribution of 335 band at different temperatures .. 34

Figure 3.5 Spectral distribution of 485 band at different vapor

densities 35

iii
Figure 3.6 Spectral distribution of 335 band at different vapor

densities 36

Figure 3.7 Ratio of 485/335 band intensities versus temperature ... 37

Figure 3.8 Ratio of 485/335 band intensities versus density 38

Figure 3.9 Normalized intensities versus temperature in the

485 band 3g

Figure 3.10 Potentials corresponding to the 485 band 40

Figure 3.11 Normalized intensities versus temperature in the

335 band 41

Figure 3.12 Ratio of 485/335 versus temperature 42

Figure 3.13 Potentials corresponding to the 335 band 43

Figure 4.1 Time history of the 485 fluorescence band. (a)lOysec/

division and (b) 2ysec/divlslon 44

Figure 4.2 Long time decay constant versus temperature and

density 45

Figure 4.3 Fluorescence spectrum at various temperatures for a 18 ~3

fixed atomic density of 2 x 10 cm 46

iv
Analysis of Optically Excited Mercury Molecules

R. E. Drullinger, M. M. Hessel, and E. W. Smith

The molecule is representative of a class of molecules which have dissociative ground states

and bound excited states. It can therefore be used as a prototype of this class of molecules which are

of interest as potential new laser candidates. Because of the non-bound ground state, standard absorption spectroscopic techniques cannot be used to obtain the necessary information about the excited

states. We have therefore developed new measurement techniques to obtain potential curves, f-values,

and kinetic behavior for Hg2. These techniques are applicable to this whole class of dissociation

molecular systems.

Key words: Dissociation laser; excimers; excited state kinetics; f-values; mercury molecules; new spectroscopic techniques; optical excitation; potential curves.

I. INTRODUCTION

There is currently much interest in the development of high energy tunable lasers in the visible

and near UV. "Dissociation lasers" operating on electronic transitions in molecules having dissociative ground states give promise of achieving this goal. The molecules of interest as dissociation

laser candidates include pure metals such as Hg2, Zn^, Cd^-^ mixed metals such as HgZn; metal-noble

gas molecules such as HgXe, LiXe; and pure noble gas dimers. Research on this class of laser has

been impeded because most of the interesting systems include metals which are highly reactive at

the high pressures and temperatures of laser interest. Furthermore, the dissociative nature of the

electronic ground state severely inhibits conventional absorption spectroscopy hence, even for such

ubiquitous systems as Hg2, there exists very little basic data (eg. A values, potential energy

curves, energy transfer rates, etc.). At the present time, only pure rare gas systems have been

made to lase, and even these are poorly understood.

The mercury system is currently the most attractive of the metal vapors from the point of view

of developing the measurement techniques and gathering the basic data needed for understanding this

class of lasers. Some, albeit very little, information is available about molecular mercury from

research extending back more than 70 years. The mercury molecule, while somewhat reactive at high

temperatures, can still be studied in conventional glass or quartz cells and discharge excitation

is also relatively easy to obtain. Previous analyses of electronically excited mercury molecules can

be roughly divided into three categories according to the type of excitation employed:

(1) optical excitation via the atomic resonance line^''"'' (253.7)nm at mercury pressures less than 1

2 [2J torr (1 torr = 133 N/m ), (2) low pressure discharge excitation and, more recently, (3) high
[3] energy (Mev) electron beam excitation at pressures of several hundred torr or more . Each of these

techniques has provided useful information and each has its limitations. Discharge excitation can

produce a fairly high density of excimers (electronically excited molecules whose ground state is

dissociative) but the analysis of the states of interest is confused by the presence of charged

particles and highly excited molecular states which can act as metastable energy reservoirs or as

absorbers. Resonance line optical pumping avoids these difficulties (if the resonance lamp is

suitably filtered) , but this technique is restricted to mercury pressures well below the range of

laser interest because, at pressures of 1 torr or more, the exciting radiation is all absorbed in a

very thin sheath at the edge of the vapor and Interactions with the wall dominating the observations.

[4] In this paper, we report results obtained with a type of optical pumping which is applicable

from a few torr to several atmospheres. High power continuum lamps have been used to populate Hg^

excited states directly from the weak Van der Waals well and continuum region of the electronic

ground state. Figure 1.1 shows the postulated Hg^ potential curves''^^ and the location of the absorption band which we have used for our optical pumping (it must be emphasized that very little is

known about these curves and figure 1 may be shown to be incorrect by future research. This absorption band was studied extensively by Kuhn and Freudenberg and Lennuier ''^'^ and has been shown

to extend from the resonance line at 253.7 to beyond 320 nm depending on the pressure and temperature. The band is optically thick in the region of 253.7 nm but we have produced strong volume

excitation even at pressures of 10 atm. The details of this optical pumping technique and the

various experimental procedures are discussed in section 2.

In section 3, we present an analysis of steady state data from the 335 and 485 nm fluorescence

bands. This analysis indicates that the excited electronic states which emit these bands (and

3 1 probably the entire manifold of states which arise from 6 + 6 S^) are in thermal equilibrium

among themselves (but not with the electronic ground state) . This analysis also gives a 6500 cm

activation energy for transitions from the 485 reservoir to the 335 reservoir.

In section 4, we present some preliminary time decay data which shows that the 485 and 335

bands decay with the common lifetime and this lifetime is a strong function of the temperature and

density of the mercury vapor.

2
II. EXPERIMENTAL

In this section, we describe the mercury containment cells, filling procedure, oven and temperature controllers, light sources used to excite the molecular spectra, detection systems, spectrographs, and calibration procedures. Purity of the cells and mercury used in these experiments is

of the utmost importance; some anomalous results that we obtained by filling the cells with laboratory grade triple-distilled mercury and ordinary vacuum techniques will be discussed along with

the mercury handling procedures we have now developed.

In all the experiments described in this monograph, we used a light source to excite mercury

vapor contained in a quartz cell. The pressure of the mercury was determined by a cold finger attached to the cell and the temperature was controlled by heating the cell body. Figure 2.1 is a

typical schematic of our apparatus.

A. Cell and Oven

The cells were made from 2.5 cm dia. quartz tubing in the form of a cross. The four windows

were of UV grade quartz and fused onto the cell body. The optical path length between windows was

3.65 cm. The cell had a small tube extending from its body that acted as a reservoir for the mercur

and as a cold finger so that the mercury vapor pressure could be varied independently of cell temper

ature. Since the cell body was not made out of UV-grade quartz, it fluoresced strongly in a broad

band centered around 400 nm if any of the exciting radiation impinged on it. To help eliminate

this problem a mask of liquid gold was painted around the edge of each window to help prevent incident radiation from directly entering the cell body and to mask the fluorescing edge of the cell

from the detection optics. Cells of this design could not be operated much above atmospheric pres19 -3 sure (densities < 10 cm )

The cell was contained in a two compartment oven constructed of low density fire brick in which

the compartments were heated separately, each with a temperature regulating system. The upper part

of the oven where the cell proper was housed determined the mercury vapor temperature and was equipped with four window ports to coincide with the four windows on the cell, each port was fitted with

a quartz window to prevent heat loss (see fig. 2.1). Inside this oven and surrounding the cell was

a thin-walled stainless steel cylinder with four window ports cut in it. This provided additional

heat and light baffling as well as providing a base on which the heater was wound.

The lower oven housed the mercury reservoir and had no windows, thus providing maximum temperature stability and uniformity. Thermocouples were attached directly to the glass side arm at a poi

where it was full of liquid mercury. The side arm also had a thin layer of asbestos wrapped on it

* A thick organic solution containing gold which when painted on a surface, dried and heated to

> 600°C leaves a thin layer of gold bonded to the surface. The usual coating of "aquadag" will not

work as it burns off in air at temperatures above 500°C.
and was finally wrapped with several layers of heavy aluminum foil to insure temperature uniformity.

The heater for this part of the oven was a coil of nichrome wire wound just on the inside of the fire

brick.

Both oven compartments were heated by direct current through series regulators whereby the current to the heater was controlled by the difference potential between the control thermocouple and a

reference voltage set to the desired temperature.

This system was of low heat capacity and temperature changes as large as 300° C could be accomplished within 15 minutes. However, tight regulation controlled the reservoir temperature to

o

± 0.05 C for periods as long as several days resulting in mercury vapor density control to better

than 0.5%.

B. Purity Problems and Cell Filling Procedures

[7] In the older literature , it had been noted that air and other gases destroyed the fluorescence.

[8] [9] More recently Stupavsky, Drake, and Krause observed an anomalous Hg2 band that Vlkls and LeRoy

have subsequently suggested was due to an impurity of HgCl. In general, mercury is known ^^^'^ to combine with many atomic and molecular species. For these reasons, it is necessary to take great care

to insure purity in preparing sample cells.

In our initial experiment, we filled the cells with laboratory grade triple-distilled mercury.

The cell was pumped down to a pressure of 10 ^ torr and baked to 300°C before the Hg was distilled

into it. We found that when the cells were heated to give a mercury pressure of 2-5 atm and exposed

to any of the blue argon ion laser lines, the characteristic A85 nm blue-green Hg^ fluorescence

band appeared. The first cell failed, however, when the pressure exceeded several atmospheres. With

a new cell and slightly improved vacuum techniques, we tried to reproduce and study this laser induced fluorescence effect under carefully controlled conditions. The fluorescence was weaker and

lasted only a few hours after initial heating. With the improved vacuum techniques for preparing

mercury cells as described below, this laser induced fluorescence completely disappeared, indicating

that it is probably produced by an impurity in some laboratory grade mercury. Although time has not

allowed an investigation of this "sensitized fluorescence", we have used it as an indication of the

cleanliness in the various cells.

The cells used for the experiments described in this monograph were prepared in the following

Prior to the actual filling of a cell, laboratory grade, triple-distilled mercury is distilled

into ampules with the arrangement shown In figure 2.2. The ampule filling apparatus is attached

through a "U" tube trap to a bakeable vacuum station and the entire system is baked at 400 ° C for

-9

24 hours. On cooling, a vacuum of ~ 10 torr is attained. The system is then backfilled with

4
argon to 1 atm pressure and opened at point A. Laboratory grade mercury Is injected into point C and the system closed at point B. The "U" tube trap is then cooled with liquid and the argon

is pumped out of the system. The pressure at this point with the Hg vapor trapped out, is

—8

2 X 10 torr. Using a foam plastic cup fitted over the filling tube (B) and inverted "U" tube (D) and filled with dry ice, a reflux column is set up; C B. A small fraction of the re- fluxing Hg migrates over into the ampules. When an ampule is filled with approximately 5 gm of

mercury, it is tipped off and removed from this system. This procedure results in ampules of pure

mercury sealed under a vacuum of a few times lO"^ torr. To keep the pumping station clean, the

remaining system is again filled with argon and cut between the liquid nitrogen trap and the per- manent part of the station. This removes all glass that has been exposed to mercury.

To fill a cell, the arrangement shown in figure 2.3 is constructed on the pumping station. It

consists simply of the cell to be filled, an ampule of mercury and a "U" tube for a trap. Since

the cell is to be operated at temperatures up to 900°C, it must be baked at least to this temperature. For this purpose, a small oven made of a heater and fire brick Is placed around the cell

a]

of the vacuum station Is baked at AOO°C for 24-36 hours. After the bake-out, the system is cooled,

the small oven removed, the trap cooled in liquid nitrogen and the mercury ampule opened. At this

point and throughout the rest of the filling procedure, the pressure on the vacuum side of the trap

—8

is less than 10 torr. Some of the mercury is then transferred, again by distillation, into the

cell. Mercury often bolls violently when distilled and mercury droplets entrained in the vapor could

be carried directly into the cell. To prevent this, we boll the mercury gently and make the glass

between the Hg ampule and the cell of sufficient length and slope (fig. 2.3) so that any entrained

mercury condenses on the walls and rolls back toward the ampule. During the tip-off operation, the

cell is heated to cause a large flux of mercury vapor to sweep out past the tip-off point and carry

with it out-gassed products of the tip-off operation.

With the cells prepared in this manner and with appropriate excitation, we have been able to 16 20 —3 observe Hg2 fluorescence over a density range of 10 to 10 cm and vapor temperatures of 175 to

SOCPc. These signals are stable and show no change over 24 hours of operation. With these clean

vapors, we do not observe fluorescence induced by any of the Ar^ laser lines.

5
C. Optical Pumping and Light Sources

Lennuier^^"' and Kuhn and Freudenberg ^^"^ have shown that mercury vapor absorbs radiation in a

broad band from the 253.7 nm resonance line to beyond 320 nm. This type of optical pumping is a

"clean" technique, in which only the bands of interest (335 and A85 nm) are excited and fluorescence

is produced throughout the cell. Figure 2.4 shows Kuhn and Freudenberg ' s relative absorption coefficient a as a function of wavelength where I/l^ = . Also plotted on the same graph Is the

ratio R of integrated fluorescence intensity of the 335 band to the exciting wavelength intensity

[4] as a function of exciting wavelength at a pressure of 600 torr.

These results show that one can couple power into any depth of mercury vapor and at almost any

vapor density and thus obtain volume excitation. To make full use of this pumping scheme for

20 -3 densities up to 10 cm , we have used several types of lamps to maximize the power in the wave16 ""3 length region effective at the particular density region investigated. At low densities < 10 cm

16 18 —3

a germicidal resonance lamp is most effective, for densities from 10 to 10 cm a medium pressure

18 —3 (Osram type) mercury lamp most effectively excites the Hg^ bands. Above 10 cm high pressure

xenon and mercury-xenon compact arc lamps are used. We recently obtained a commercial ADP frequency

doubler which was used to double the 514.5 nm Argon ion laser line thus producing a 10 mW beam of

257.2. nm radiation. This light source produces a small volume, high density excitation.

For the rather preliminary time resolved studies which are presented in section IV, a coaxial

system consisting of a lOJ energy storage capacitor, a switching thyratron and a 2-4 mm air spark

gap was used. During the .5-1 psec discharge time of the circuit, the air spark radiated light

with a spectral distribution roughly corresponding to a 10,000 K blackbody.

Each of these light sources (except for the laser) was used in conjunction with an optical

system which collected, collimated, filtered and refocused the light into the cell. This system

consists of simple, single element f/1.5 quartz lenses and a tri-compar tmented cell which contained

the desired combination of filter solutions, see figure 2.1. The filter used throughout most of this

work is one of NiS0^-H20, CoS0^-H20 and pure Cl^ gas. This combination gives a high transmission

(T > 50%) at 270 nm; the transmission curve is given in figure 2.5. max o a

In addition to the light sources above, we have used the 337.1 nm output of a pulsed 1 MW

nitrogen laser to excite the mercury vapor. The 485 nm band fluorescence was observed through a 19 -3 broad band 480 nm interference filter at densities in excess of 10 cm . A cw He-Cd laser (10

19 -3

mW at 325 nm) also excites the Eg^ band fluorescence at densities > 10 cm . In both cases the

excitation was rather weak and neither technique was used beyond the initial trials.

6
D. Detection Systems

To detect the molecular mercury bands at 335 and 485 nm we have used both S(Q) blue response

and S-20 red response photomultipliers. To observe the molecular bands in the appropriate spectral

region we either used the photomultipliers with filters or a 1/3 meter scanning monochrometer , depending on the type of experiment and light intensity. The transmission curves of the broadband

interference filters used to measure the 335 and 485 band, are shown in figures 2.6 and 2.7.

The spectrometer was calibrated for wavelength, instrument line profile, and (with each of the

photomultipliers) quantum efficiency. The spectrometer has a spectral line width of 0.07 nm (FWHM)

at a slit width of 10 ym and a line width of 0.3 nm at a slit width of 135 ym. The spectrometer

scattered light level beyond 1.5 band widths was found to be < 0.5% in both cases. We generally

used slit widths corresponding to an instrumental bandwidth of 0.3 - 1.0 nm. We used a spectral

irradiance tungsten iodine lamp to calibrate the photomultiplier and spectrometer combinations

for relative quantum efficiency. The lamp was essentially a blackbody source at a specific color

temperature for a fixed current input to the lamp. In our case, the calibration was made at two

color temperatures 3110 ± 15 K and 2785 ± 15 K. The independent calibrations agreed to within ± 1%

in the spectral region 300-700 nm.

7
III. STEADY STATE DATA

A. Band Shapes

The cw-optlcal pumping schemes described in the previous section have been used to excite mercury

vapor at pressures ranging from about one torr to several atmospheres and temperatures ranging from

175°C to 1000°C. The entire fluorescence spectrum has been scanned under moderate resolution (about

0.3 nm to 1.0 nm) from 230 nm to 700 nm using various spectrometers and photomultipliers. A typical

photoelectric trace is shown in figure 3.1. This trace was taken with a 1/3 meter spectrometer, with

a resolution of about 0.3 nm, using a photomultiplier with an S(Q) response curve. The total response of the instrument was calibrated using a standard lamp, and the trace shown in figure 3.1

includes this correction factor. (The intensities reported in this paper are proportional to the

number of photons per wavelength interval.) Since the correction factor with the S(Q) phototube

becomes rather large for X > 550 nm, another trace was made using the same spectrometer with an

S-20 phototube. This particular curve shows fluorescence excited with the 150 W Xe continuum lamp

filtered to pass only 240 to 290 nm radiation (discussed in Section II). Since this is a very high

intensity source, the radiation scattered by the cell constitutes an appreciable signal which is seen

in figure 3.1 as a continuum between 240 and 290 nm. Measurements of this scattering, taken with the

cell at room temperature, enable us to subtract out its contribution but it doesn't interfere appreciably with the 335 band in any case.

A similar curve, excited with the frequency doubled Argon ion laser, is shown in figure 3.2. In

this case, the doubled laser output was focused into a small volume thus creating a high excimer

13 3 3 density (estimated at 10 /cm ) and some secondary pumping of the 7 S-j^ state of atomic mercury is

observed. This figure also shows scattering of the doubled laser line at 257.2 nm as well as a

small amount of the primary laser line at 514.5 nm which scattered into the cell. More recent

runs using unfocused 257.2 excitation show no atomic emission and with better filtering, the 514.5

scatter has been removed. Comparison of laser excited fluorescence data with lamp excited data

3 shows no difference between the two (other than a weak excitation of the 7 obtained by focusing

the laser into a small volume)

It is interesting to note that the peak of the familiar 485 band occurs at 510 nm in figures 3.1

and 3.2. In our uncorrected traces the peak occurred at 485 but when the data are corrected for the

spectrometer response function, the true maximum appears at 510. The 335 band shape is essentially

unchanged when corrected for the spectrometer response function. Since the band shapes and the location of the maximum intensity are rather important quantities in evaluating potential curves and

transition probabilities, the shift of the apparent maximum from 485 nm to 510 nm emphasizes the

need for well-calibrated detection systems.

8
The shapes of the 335 and 485 bands as functions of density and temperature are shown in figures

3.3 - 3.6. The integrated band intensities are normalized to unity in order to show the shifts of

the populations within the respective bands. As temperature is increased, both bands show a decrease in peak intensity, an overall broadening of the band and a slight shift toward the blue. This

is what one would expect from a Boltzmann distribution of vibrational states; that is, an increase

in temperature reduces the population at the bottom of the well which radiates the intensity maximum,

and correspondingly increases the population of the higher vibration states which radiate in the

wings of the band. The shift to the blue will be discussed later in this section.

Figures 3.5 and 3.6 show that the band shapes are not affected by changes in density. This

is also what one would expect from a thermal distribution of vibrational states since the Boltzmann factor is independent of density. The absence of density dependence of the shape of the 485

band cannot be used to confirm or deny the mechanism of collision induced radiation which is often

proposed for this band.

B. Band Intensities

The intensities of the 485 and 335 bands were measured as functions of temperature and density.

Most of these measurements were made using filters in front of a photomultiplier rather than a spectre

meter in order to increase the signal and thereby extend the range of these measurements. A comparison of the filter functions, figures 2.6 and 2.7, with the observed spectral distribution, figure

3.1, shows that the filters did nor Integrate the entire band. The filters do cover the center of the bands hence the readings are proportional to the total band intensities and the intensity ratios

should be accurate. This assertion was checked by integrating spectrometer tracings, such as figure

3.1 in the temperature-density range where spectrometer measurements could be made; the intensity

ratios obtained with the filters agreed with those obtained from spectrometer tracings to better

than 1%. We certainly do not propose this figure as an estimate of the error in all of our data, but

it does nonetheless indicate excellent agreement between these two measurement techniques at least

as far as intensity ratios are concerned.

In figure 3.7, we have plotted the log of the intensity ratio R = I (485) /I (335) versus 1/kT

where kT is in cm Figure 3.7 shows only six representative curves but several sets of similar

data have been analyzed by computer. A linear least squares program was used to fit the data points

17 -3

for N > 10 and T > 575 K (i.e. 1/kT < 2.5 x 10 ). The slopes in the linear portion of the data,

(solid lines) T >_ 573 K, give an activation energy of 6500 cm with a ± 2% scatter and this scatter

appears random; that is, there is no systematic trend in the calculated slope as a function of densit)

o 17 This linearity shows that, for T ^ 575 K and N > 10 ^ the populations are related by

^335/\85
where e = 6500 cm to within ± 2%. The fact that there is no variation in this slope over nearly

two orders of magnitude in density provides a compelling argument that the two electronic states

(which radiate 485 and 335 respectively) are in thermal equilibrium.

17 The N = 10 run was not analyzed by computer because there are not enough points at high

temperatures to be certain about where thermal equilibrium applies. A straight line was drawn throu

these data with a 6500 cm"-"- slope for comparison with the other data. The N = 5 x lO"""^ run clearly

shows that the two electronic states are not in equilibrium with one another at this density.

The deviation observed by decreasing the temperature at a constant density is not the only means of achieving a deviation from equilibrium. Another technique is to lower the density for a fixed temperature. This is illustrated in figure 3.8 where we plot the Intensity ratio R versus

density for several fixed temperatures lying both above and below the critical value of 575 K discussed above. In addition to our data, we have also plotted some low density data of Matland and

[12] McCoubrey which were obtained with the resonance line excitation discussed in Section I.

It should be noted that the latter were taken in the afterglow (after switching off their resonance

lamp) whereas our data are taken in steady state with a continuum lamp. Since the two electronic

states are not in thermal equilibrium for N < lO"*"^ , this may not be a valid comparison. None of

the data in this figure have been calibrated to account for the band width of the filters or the

relative sensitivity of the phototube in the blue 335 and green 485 regions. These corrections

would only multiply R by a constant, hence the slope of the curves is correct even though the ver -

tical position of the curves is arbitrary. That is, the fact that our data at 470 K joins smoothly

with the Matland-McCoubrey data at 473 K is fortuitous. Furthermore, the 519 K curve lies above

the 470°K only because it was arbitrarily displaced upward one order of magnitude to make the data

easier to read, the actual measurements lie between the 470 K and 623 K curves. The two asjraiptotes

2 R °: N and R « N have been drawn in as dashed lines to aid in the interpretation of the data.

We first consider the region N > lO"*"^ on the 631 K and 623 K curves. The previous discussion

has shown that the states radiating 485 and 335 are in thermal equilibrium for this temperature-density range, yet figure 3.8 shows that the intensity ratio increases linearly with N over at least

two decades in N. Assuming that the 335 nm radiation is due to spontaneous emission, this would

indicate that 485 is radiated either by the collision induced mechanism

* or by stable Hg^:

Hg^ + Hg -> 3Hg + hv^g^ (3.2)

Hg* + Hg Hg* (3.3)

Hg* - 3Hg + hv^g^ (3.4)

10
It is impossible to distinguish between these two mechanisms with our current data.

17 2 We next consider the data for N < 10 . These data show a strong tendency toward an N de2 pendence at the lower densities. Although none of these data are truly parallel to the N asymptote,

2 they seem to be approaching it more closely as N decreases. The transition from N to N dependence

seems to occur at about lO"*"*^. This change is rather difficult to detect In the 623 K data but there

are clear systematic deviations from linearity for the low density points on this curve. It would

be desirable to have more points at lower N for this temperature because this is one of the cases

which is clearly in thermal equilibrium when N > lO"*"^. It would therefore be instructive to measure

more accurately its deviation from the linear N dependence as N is decreased. Unfortunately, the

485 band becomes very weak for low N and high T and it was possible to go to lower N only for the

lowest temperatures (such as the 519 K curve for example). The latter overlaps the lO"*"^ region

very well; this curve agrees quite well with the low pressure afterglow measurements of Matland-

[12] McCoubrey but at high N it never quite achieves the linear N dependence exhibited by the high

temperature data. This is not surprising since, in view of figure 3.7 we would not expect to be in

thermal equilibrium at this temperature.

In summary then, we have compelling evidence that the states responsible for the 485 and 335

fluorescence band are in thermal equilibrium for high temperatures T > 575 K and densities N >

10'''^.

In the high temperature-density region where thermal equilibrium applies, the linear dependence of

P = 1(485) /I(335) on N would seem to indicate that 485 is emitted by a density dependent process

* such as collision induced radiation, eq. (3.3), or Hg^ formation, eqs. (3.4) and (3.5).

C. Potential Curves

In this section, we present a preHmlnary analysis of the electronic potential curves involved

in the 485 and 335 fluorescence. For similar discussions see Hedges, et. al. '^^^

^ Doyle or

Sando and Dalgarno ^^^^

. The number of photons per unit wavelength emitted in a transition between

a bound excited electronic state "a" and a repulsive ground state "b" ,is

3^ = ^

Z^^^, J

d. 6(hv . E^^j + O

exp( - E ^/kT)S^^, - avJ JJ

I 2 Z(T) ' avJ,beJ' I (3.5)

CN*(T) , exp( - E,,j/kT)Sjj, 2 ^ ^5 vJJ' Z(T) ' avJ,beJ'

where (v,J) are vibration rotation quantum numbers for a, (eJ') are the corresponding quantum numbers

11
for b (e is a continuous nuclear kinetic energy quantum number), D is the transition dipole moment,

it S is the Honl-London factor, the total number of excimers is N (T) , the excimer state population densities are described by a Boltzmann distribution with a partition function Z(T), and C denotes miscellaneous uninteresting constants. The integral over nuclear kinetic energy e removes the delta function and, in the second line, e equals (E^^^-hv)

The dipole matrix element in eq. (3.5) can be expressed in terms of vibrational wavefunctions as

D T 1, T. =

/" D , (R)^-^ (R)4';J'(R)dR (3.6)

avJ,bEJ J ab av b£

where D (R) denotes the dipole matrix element between the electronic states a and b. This integral

ab can be evaluated using WKB wavefunctions ^"""^^ in which case one finds that the integrand is sharply

peaked about a critical radius R^ at which the radial momenta for upper and lower states are equal

(Franck-Condon principle)

or since, e = E --hv,

' avJ

2yR^ 2V\

.T, ^ ^^J(J + 1) „ X ^^J'(J' + 1) hv = V (R, ) ^ - V (R,) 5 ^ ^ 2yR^2

b X 2yR^2

(3.8)

In the last line of eq . (3.8) we have used the fact that the rotational levels with large thermal

populations correspond to large J values, thus the AJ = ±1,0 selection rule Implies that the rotational energies in eq. (3.8) essentially cancel out. That is, for a given vibrational level

and a specific R^, all rotational levels emit photons of essentially the same frequency. There is

some rotational broadening due to the inexact cancellation of the rotational energies, but this is

negligible for our purposes. Since the integrand in Eq. (3.6) is sharply peaked at R^ , the function

D^^ may be factored out as '-'^^,(^;)^) ^^'^ the remaining Franck-Condon overlap integral evaluated by the

method of stationary phase. Mies argues that the WKB phase integral (the integral over the radial

momentum) for the repulsive state b will be small (compared with tt) at the point R^. Equation (3.6)

then reduces to

^avJ.beJ- = Dab^^^C^^) (R ) - V, (R ) dR, ) a ^ b

^ (3.9)

If the WKB phase integral is not small is replaced by a slightly more complicated function of

R-j^. We must therefore emphasize that, for the high temperatures of interest to us (T > 400 K) this

phase integral will be small only if R^ lies near the classical turning point for the repulsive state

12
(within 0.5 X). This will be a good approximation since the WKB continuum wavefunctlon is sharply

peaked about the classical turning point. In fact, one must be somewhat careful about using a WKB

wavefunctlon for '^gy(Rj^) because the latter diverges at the turning point. However, eq. (3.9) can

[17] also be derived using Airy wavefunctions which do not diverge , hence one can readily find well

behaved wavefunctions for use with this expression. It should be noted that eq.(3.9) is quite

[181 similar to the "reflection method" in which the repulsive state wavefunctlon Is replaced by a

delta function at the classical turning point. It should also be noted that eq. (3.9) breaks down

if the derivative of (V - V,) should vanish at R-, ; in such a case one must use the full Airy

a b A function results ^^^^ (i.e. without assuming that the WKB phase Integral for state b is small).

We next note that the vibrational wavefunctlon depends on J only through the radial kinetic

av J °

2 2 energy E^^j - ^g^^) •^(•J l)/2lJR . Since for not too large values of J the shape of the po2 2 tentlal energy curve V^(R) + -K J(J + l)/2yR does not differ appreciably from that of V^(R), we will

2 2 approximate the rotational energy by^ J(J + l)/2yR^. This approximation ignores a small distortion

[19] produced by the rotational energy but this should be negligible for our purposes . We may therefore replace E^^j in eq. (3.5) by:

_ -fi^J(J + 1) avJ ~

''av "

„ „ 2 (3.10) 2yR^

thus removing all J dependence from the vibrational wavefunctlon. Substituting eqs. (3.7) and (3.10)

into eq, (3.5) and performing the J' sum of S^^, (which yields (2J + 1)), we obtain:

, cAtI, '°ab(^x)l'l\v^>l'

X ^5 vJ (dv/dR^)

(3.11)

(2J + 1) exp

I -(E^y +'fi^J(J + l)/2)JR^^)/kT

|

Z(T)

where W denotes the (rotationless) vibrational wavefunctlon with energy eigenvalue E and the ay ° av derivative °^ ~ V^) was replaced by (dv/dR^) according to eq. (3.8). Next using the identity

E » T (R, ) + V(R, ) where T(R, ) is the nuclear kinetic energy at the point R, and assuming that the av V A A A A rotational spacing is much smaller than kT, we replace the sum over J by an Integral which yields

R,exp {- V (R,)/kTi 2

(dv/dRj^) I ab'

X E (R,)i ^ -p{- yy/'-^

^ CN (T) 2ykT y , ^ ,

. ^5 ^2 (dv/dR,) I ab^ X-" A A (3.12)

^ Z(T)

13
I 1

We have now obtained an expression which relates I-^^ to the dipole strength |D^^(Rt^)| and the

probability of finding the nuclei at the radial separation where the radiative transition takes

2 place. The probability is described by a Boltzmann spatial distribution, ^"^^ exp {-V^(R^) /kT}

multiplied by a momentum distribution which is in the form of a sum over vibrational states. Assuming that the vibrational spacing (estimated to be the order of 150 cm '') is much less than kT (so that

many vibrational levels are populated) , the sum over vibrational states is easily performed using

2 either harmonic oscillator or WKB eigenfunctions and one obtains 2iJkT/'tt . The partition function,

which includes both bound and free states, is dominated by the continuum contribution hence,

Z(T) - (2TTMkT/-ft^)^''^V (3.13)

where V is the volume of the system. Defining the density of excited states by n (T) we finally

obtain (cf. eq. (8) of ref. (13)) 2 -W^'^^Kb^h^^' (3.14)

I = Cn (T)

A (dv/dR^)

If the temperature is less than or comparable to the vibrational spacing It is better to use eq.

(3.12). For our data, eq. (3.14) should be sufficient and it is this expression which we will

use to analyze the data presented in this report. For the purposes of our analysis this result can

be summarized by writing

A

where f(I^^) represents the effective transition probability at the point R-|^.

To use eq. (3.15) effectively, we first consider the ratio of intensities at two different wave-

* lengths in the same electronic band system. The log of this ratio will be (notice that the n (T)

cancels out)

(3.16) log(I,/I^,) =

If we plot this log versus 1/kT, the slope of the linear plot will give V (R, ) - V (R, , ) . By doing

a A a A this for several values of A', we can plot out as a function of wavelength. Of course, we can't

get the absolute energy without using some known energy level as a reference thus, for the present,

we will plot relative to some arbitrarily chosen reference point. For convenience, we will

choose the energy level which is responsible for 500 nm emission since we always have a strong

signal at this wavelength. Figure (3.9) shows a semilog plot of (I^/I^q^) versus 1/kT for several

wavelengths in the 485 band; the number in parentheses gives the slope in cm The excellent

linearity of these plots shows that the vibrational states are in thermal equilibrium as one would

expect. These data, and several more sets, which we have not plotted, have been analyzed by a

least squares computer program to obtain the slopes. The resulting V (R-, ) is plotted in figure (3.10) a A

14
together with the ground state curve obtained from the relation V, (R, ) = V (R, ) - hv. If the 485

D A a A band Is emitted by Hg^, our picture of these potential curves figure (3.10), is somewhat altered

by the fact that V then depends on two radial parameters as well as the angles between the internuclear vectors.

In figure (3.11) the same procedure has been followed for the 335 band choosing the state which

radiates at 336 nm as our energy origin. In figure (3.12), we have plotted ^° ^^^^

potential curves for the 335 band can be plotted relative to the energy of the state which radiates

at 500 nm. These curves are given in figure (3.13).

At this point, we should emphasize that the potential curves plotted in figures (3.10) and (3.13)

were obtained from the slope of the log R versus 1/kT plots in figures (3.9), (3.11) and (3.12). These

potentials are therefore unaffected by temperature independent quantities such as f-values, etc.

Figures (3.10) and (3.13) verify the bound-free nature of the 485 and 335 bands. These figures

also appear to Indicate that the region of the ground state involved in the 485 band lies somewhat

higher on the repulsive wall than the region corresponding to the 335 band and there is some overlap

at the edges of the bands. This is consistent with figure (1.1) but it does not prove that figure

(1.1) is the correct picture because we only have V versus A not V versus R and one cannot say with

certainty that the ground state curves in figures (3.10) and (3.13) actually refer to the same state

(e.g. the ground state of Hg^ is probably similar but not identical to that for Ug^

In conclusion we must emphasize that the potential curves V vs A, presented in this report, are preliminary data and may be somewhat in error, particularly around 350 nm to 400 nm where the

fluorescence bands overlap. We are currently taking more extensive data at various pressures which

will be computer analyzed to provide greater accuracy.

D. The Effective Transition Probability

It is well known that the probability for emission depends on the internuclear spearatlon.

In eq. (3.14) there are several factors which determine this probability and, in eq. (3.15), we have

divided them into two categories according to their temperature dependence. The potential energy

V(R^) and the Boltzmann factor were discussed in the previous section; in this section we will briefly

discuss the temperature independent quantity f(R^).

2 The function f(Rj^) is strongly influenced by the dimensional factor which (for our case)

causes f to increase with decreasing wavelength. Since f is inversely proportional to (dv/dR) <^

(dV /dR) - (dVj^/dR), this factor will tend to make f larger in the region where both and are

2 repulsive (longer wavelengths) thus counteracting the effect of R to some extent. Finally there

Is the dlpole strength D .(Rj) where radial dependence Is unknown.

15
It Is possible to obtain relative values of f as a function of wavelength by comparing the

quantity

= X~^exp { - V^(R^)/kT} (3.17)

with the observed intensity I^ for some fixed temperature. Such a comparison has been made for

both bands and the results are given in tables (3.1) and (3.2). The comparison was made for two

different temperatures in order to give an idea of the scatter in calculated values of f. This

scatter seems to be the order of 10%, but it should be possible to do a bit better when more accurate

potentials are obtained. The functions f(R^) are relatively constant near the centers of the bands

with a rapid increase in the short wavelength (large R^) limit.

At this point, no attempt has been made to estimate an absolute value of f(R^); the present

analysis is intended only to give an idea of the type of gain profiles to be expected for these

bands.

The blue shift of the peaks of both bands, which was observed with increasing temperature,

(figs. 3.3 and 3.4) is now seen to result from a combination of ^(^-y) and the X

^ factor in eq.

(3.15). These factors combine to make an effective transition probability which increases strongly

with decreasing wavelength.

16
IV. TIME DEPENDENCE

Using the pulsed discharge continuum lamp described in section II C, we have observed the time

evolution of the 335 and 485 bands throughout a density range of lO'''^ to 10'''^ Hg atoms/cm"^ and a temperature range of 473 to 973 K. Light from the lamp passed through the tri-compartmental filter

(see fig. 2.1) and the resulting fluorescence was observed at right angles through either the 335 or

485 broad band interference filters (fig. 2.6).

18 Figure (4.1) shows a typical time history trace for a density of 10 and a vapor temperature of

523 K. Part "a" shows the overall rise and decay of the 485 band time with a 10 psec/div time

scale while part "b" shows the rise of the fluorescence with an expanded scale of 2 ysec/div. At

17 -3

low pressures (3 x 10 cm ) the rise of the 335 band is the order of 30 ysec and about 25% faster

18 19 -3 than that of the 485 band, whereas at higher pressures (10 -10 cm ), the two bands rise

almost simultaneously with rise times in the range 10 to 1 psec. The observed rise times decrease

linearly with increasing density but show almost no temperature dependence; increasing only slightly

as the temperature increases from 200°C to 350°C and remaining constant after that.

It is interesting to note that while the duration of the flashlamp pulse is 1 usee, the rise

time of the fluorescence bands has been observed to be as long as 40 ysec (at N = 3 x

10'''^) . One

possible explanation for this delay assumes that the postulated merging ^^'^ of the '^l and ^0^ curves

3 (see fig. 1.1) causes the high lying 1^ vibrational states (which are the ones being directly pumped)

to dissociate before they can radiate (the A value is expected to be the order of 10^ - 10^) or before

3 collisions can produce vibrational relaxation. The Pq atomic state would then serve as a reservoir

for this excitation and the slow rise in fluorescence intensity would be determined by the rate

3 at which Pq feeds energy back to the molecules. Initial attempts to test this hypothesis by

3 measuring the time history of the Pq have been hampered by lack of intensity.

-tlx The long time decay fits an exponential, e , from 80% of the maximum intensity out to

about 3 time constants (the limit of our measurement). Both bands decay simultaneously to within

the experiment error of ± 1 ysec at all pressures and temperatures for which we have made the measurement. As shown in figure (4.2) there is a striking temperature dependence; first a sharp increase in

lifetime and then a decrease. The point at which the maximum time constant is observed is where the

dominant emission changes (see fig. 4.3) from 485 nm (low temperature) to 335 nm (at high tempera18 —3 tares). For a density of 2 x 10 cm , this point is at T = 730 K.

The complex behavior of both rise and decay processes indicates the presence of several

competing processes. We made several attempts to model the data but more extensive data will be

needed before any meaningful analysis is possible.

17
V. SUMMARY

This report presents new measurement techniques which have been developed to provide data such

as potential curves and f-values for metal molecules which are of interest as dissociation laser

candidates. Preliminary data and analysis were presented in order to illustrate the measurement

techniques in general and to shed some light on the mercury molecule in particular.

Our results may be briefly summarized as follows:

(1) Purity is extremely important. Normal laboratory grade mercury was not

pure enough and it was necessary to distill the mercury into specially

prepared sample cells.

(2) The two electronic states responsible for the 485 and 335 bands have been

shown to be vibrationally equilibrated at the pressures of interest in laser

applications.

(3) Assuming that these fluorescence bands are emitted by diatomic mercury, the

potential curves for these states have been measured over the regions which

give rise to the fluorescence and are found to be in qualitative agreement

with curves proposed by Mrozowski. Our results do not extend to large internuclear distances so it is not possible to say if they will agree with Mrozowski

in the dissociation limit. However, on the basis of the present data, one cannot

exclude the possibility that the 485 band is radiated by Hg^.

(4) The two electronic states radiating 485 and 335 appear to be in thermal equilibrium with one another, perhaps indicating a curve crossing as proposed by

Mrozowski.

(5) Relative transition probabilities were found to be slowly varying across both

bands with some increase at larger R or smaller wavelengths.

(6) The pressure and temperature dependence of both decay and rise times have

been measured, but no quantitative conclusions have been drawn from these data

as yet.

18
VI. ACKNOWLEDGMENTS

We would like to acknowledge many helpful discussions with Dr. A. V. Phelps and Prof. J. Cooper

of the Joint Institute for Laboratory Astrophysics. We also wish to thank Mme. Denlse Perrin of the

University of Paris VI for providing us with information on the optical pumping of via the continuum absorption band. In addition, we wish to express appreciation to Dr. R. J. Carbone for many

stimulating discussions and for organizing a symposium on mercury at the AEG Los Alamos Laboratories

which provided many new ideas on the kinetics of such systems.

19
VII. REFERENCES

1. A. 0. McCoubrey, Phys. Rev. 93, 1249 (1954); R. A. Phaneuf, J. Skonleczny, and L. Krause,

Phys. Rev. A8, 2980 (1973); J. Skonieczny and L. Krause, Phys. Rev. A9, 1612 (1974) and

references contained therein.

2. A. Atajew, A. Rutscher, and R. Winkler, Beit. Plasma Physik 12, 339 (1972); R. J. Carbone and

M. M. Litvak, J. Appl. Phys, 39, 2413 (1968); H. Takeyama, J. Sci. Hiroshima Univ. 15, 235

(1952). •

3. D. J. Eckstrom, R. M. Hill, D. C. Lorentz, and H. H. Nakano, Chem. Phys. Lett. 23, 112 (1973);

R. M. Hill, D. J. Eckstrom, D. C. Lorentz, and H. H. Nakano, Appl. Phys. Lett. 23, 373 (1973).

4. Subsequent to our work, Mme. D. Perrin referred us to the work of R. Lennuier on non resonance

line pumping of Hg2. (a) R. Lennuier, Compt. Rend. 213 , 169 (1941), (b) R. Lennuier and

Y. Crenn, Compt. Rend. 216 , 486 (1943), and (c) R. Lennuier and Y. Crenn, Compt. Rend. 216,

533 (1943).

5. S. Mrozowski, Rev. Mod. Phys. 1^, 153 (1944).

6. H. Kuhn and K. Freudenberg, z. Physik, 76, 38 (1932). Translated by K-B. Persson and

M. M. Hessel. A copy of this translation is available upon request to the authors of this

report.

7. R. W. Wood, Physical Optics (Dover Publications, Inc. New York) 1961 p. 636.

8. M. Stupavsky, G. W. F. Drake and L. Krause, Phys. Lett. 39A, 394 (1972).

9. A. C. Vikis and D. J. LeRoy, Phys. Lett. 44A, 325 (1973).

10. P. Pringsheim, Fluorescence and Phosphorescence (Interscience , New York, 1949).

11. R. Stair, W. E. Schneider, J. K. Jackson, Appl. Optics 1, 1151 (1963).

12. C. G. Matland and A. 0. McCoubrey > unpublished research memo.

13. R. E. M. Hedges, D. L. Drummond, and A. Gallagher, Phys. Rev. 6^, 1519 (1972).

14. Robert 0. Doyle, J. Quant. Spectrosc. and Radiat. Transfer, 8, 1555, (1968).

15. K. M. Sando and A. Dalgarno > Mol. Phys. 20, 103 (1971).

16. F. H. Mies , J. Chem. Phys. 48, 482 (1968).

17. W. H. Miller , J. Chem. Phys. 48, 464 (1968).

18. G. Herzberg, "Spectra of Diatomic Molecules" Van Nostrand Reinhold Co. 1950, p. 392.

20
Table 3.1 for 335 mn band, T = 873K, kT = 605 cm

X -(V^-V33^)/kT

e -5 -(V - V )/kT

(X/336) e ^obs f

300 0.034 .059 .145 2.46

310 .173 .259 .353 1.36

320 .45 .574

.68 1.18

330 .78 .85 .96 1.12

336 1.0 1.0 1.0

340 1.02 .96 .93

.97

350 .88 .72 .545

.76

360 .396

T = 673 K,

.28

kT = 467 cm"''"

.17 .61

300 .013 .022 .066 3.0

310 .10 .15 .21 1.4

320 .36 .46 .53 1.15

330 .726 .79 .91 1.15

336 1.0 1.0 1.0 1.0

340 1.03 .97 .97 1.0

350 .85 .69 .55 .8

360 .30 .21 .26 + 0.1 1.24

21
Table 3.2 for 500 mn band, T = 573 K, kT = 398 cm

-1

-(^-^500)/^^ -5 - ^500^kT

(A/500) e ^ '^^ obs

400 0.00164 .005 .075 11.4

420 .013 .031 .129 4.16

440 .095 .18 .29 1.6

460 .266 .404 .56 1.39

480 .534 .655 .85 1.3

1.0 1.0 1.0 1.0

520 1.386 1.14 .96 .84

540 1.05 .715 .765 1.07

560 .78 .44 .52 1.18

580

\

.405 .193 .32 1.64

i >

T = 773 IC, kT = 536.4 cm"^

400 0.0086 .026 .289 11.1

420 .041 .098 .388 4.0

440 .174 .33 .562 . 17

460 .374 .57 .796 1.4

480 .627 .77 .95 1.23

500 1.0 1.0 1.0 1.0

520 1.27 1.04 .90 .87

540 1.03 .70 .69 .99

560 .83 .47 .51 1.08

580

<

.51 .243 .33 , 1.4

22
23

Figure 2.2 Mercury ampule preparation system.

25
Figure 2.3 Mercury cell distillation system.

26
Figure 2.4 Absorption coefficient and integrated band intensity versus

wavelength.

27
Figure 2.5 Transmission curve of tri-compartment filter.

28

X(nm)

Figure 2.7 Transmission curve of 485 bandpass filter.

30

X (nm

)

Figure 3.2 Spectral distribution of Hg^ fluorescence excited by 257.2 nm

laser line.

32
33
0.40

335 Band Normalized to Unit Area

N= 2 X 10

X in nm

Figure 3.4 Spectral distribution of 335 band at different temperatures.

34
35
T= 763 K

335 Band Shape , N = 9.5XI0'

^N=8,4XI0'

300 310 320 330 340 350 360 370 380

, X { nm

)

Figure 3.6 Spectral distribution of 335 band at different vapor densities.

36
l/kT = l.5XI0"3cm. 2.0 2.5 3.0 T = (960" K) (725 K) (575 K) (485 K)

Figure 3.7 Ratio of 485/335 band intensities versus temperature.

37
ro

I o

c

O)

•H

CO

PS

(U

CO

CO

(U>

CO

(U •H

•H

CO

(Ua

•H

13G

m

CO

enm

00

St

O

•H

00CO

•H

P4

9££j / 98K

-3 Q
o

o

2.0

( I /kT) cm

2.5

520 (130)

530 (50)

540 (20)

560 (100)

580 (360)

480 (250)

460 (527)

440 (938)

420 (1715)

400 (2552)

3.0

Figure 3.9 Normalized intensities versus temperature in the 485 band.

39
X { nm)

Figure 3.10 Potentials corresponding to the 485 band.

40
(0

roro

338 (15) 340 (16) 342 (0)

346 (50)

350 (76)

354 (267)

360 (560)

330 (150)

320 (480)

310 (1060)

1.5 2.0 2.5 3.0

(l/kT) cm

Figure 3.11 Normalized intensities versus temperature in the 335 band.

41
(6542 cm"')

3.

(1/kT) cm

Figure 3.12 Ratio of 485/335 versus temperature.

42
43
-4-l-l-f j 1 1 1 -H-H- II 1 nil 1 1 1

1 11(1

-t-H-H II II

mm'"mm II 1 1 II 1 1

1 1 11 Mil' 1 1 11 1 II 1

1 1 II nil

( b) 2 ytxsec/ Div.

Figure 4.1 Time history of the 485 fluorescence band, (a) lOy sec/division

and (b) 2 ysec/division.

44
45
Figure 4.3 Fluorescence spectrum at various temperatures for a fixed

atomic density of 2 x

10^ ^cm ^.

46
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS MN- 143

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Analysis of Optically Excited Mercury Molecules

5. Publication Date January 1975

6. Performing Organization Code

7. AUTHOR(S)

R. E. Drullinger, M. M. Hessel and E. W. Smith

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2716461

11. Contract/Grant No. 891 & LS74-4

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Advanced Research Projects Agency, 1490 Wilson Blvd.,

Arlington, VA 22209; Los Alamos Scientific Laboratories,

BX1663 Los Alamos, New Mexico 87544

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The Eg^ molecule is representative of a class of molecules which have dissociative ground states and bound excited states. It can therefore be used as a prototype of this class of molecules which are of interest as potential new laser

candidates. Because of the non-bound ground state, standard absorption spectroscopic techniques cannot be used to obtain the necessary information about the

excited states. We have therefore developed new measurement techniques to obtain potential curves, f-values, and kinetic behavior for Hg^, These techniques

are applicable to this whole class of dissociation molecular systems.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Dissociation laser; excimers; excited state kinetics; f-values; mercury molecules; new spectroscopis techniques; optical excitation; potential curves.

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