High intensity, nanosecond, pulsed vacuum ultraviolet light sourc

Weissler, G. L.

English

NASA Technical Reports Server

DEPARTMENT OF PHYSICS 
UNIVERSITY OF SOUTHERN CALIFORNIA 
LOS ANGELZS, CALIFORNIA 90007 
This report covers the period from 
October 1966 to January 1968 
and was issued on 
31 January 1968 
FINAL TECHNICAL REPORT 
on the 
DEVELOPMENT OF A HIGH-INTENSITY, 
NANOSECOND, PULSED VACUUM UV 
LIGHT SOURCE 
under auspices of the 
National Aeronautics and Space Administration 
Ames ReseaFch Center, Contract NkS 2-3846 
Chief Investigator: G. L. Weissler 
Prof e s s or of Physic s 
https://ntrs.nasa.gov/search.jsp?R=19680014969 2020-03-12T10:34:54+00:00Z
4 
31 January 1968 
DEVELOPMEDTT OF A HIGH-INTENSEY, 
NANOSECOND, PULSED VACUUM ULTRAVIOLET LIGHT SOURCE 
I. 
‘ r  
work done under auspices of 
NASA Ames Research Center, Contract NAS 2-3846. 
Contract period f r o m  October 1966 to January 1968 
Chief Investigator: G. L. Weissler 
Department of Physics 
University of Southern California 
Loa Angeles, California 90007 
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, -  
DEVELOPMENT OF A HIGH-INT2NSITY, 
NkNOSECOND, PULSED VACUUM ULTRAVIOLET LIGHT SOURCE 
I. INTRODUCTION 
The subject of this contract was concerned with preliminary trials of a 
high-intensity, pulsed, vacuum spa rk  light source preferably with nanosecond 
pulse duration, which could be used in conjunction with many vacuum ultra- 
violet research problems. 
the optical environment of such a spectrograph must be kept in a vacuum of 
better than one micron. 
such a vacuum spark or plasma source to a vacuum spectrograph, since its 
spark gap needs to  be operated at high plasma densities and temperatures. Two 
methods of operating this source have been attempted and wi l l  be described 
below: 1) a laser-produced metal vapor fireball, either alone or enhanced by 
a condenser discharge, and 2 )  a free-running field emission-triggered vacuum 
spark. 
been accomplished, it is felt that much useful work can be done, for example, 
in connection with pkuton-3nduoad fluorescence in atmospheric and other gases. 
The short half-life of each light pulse might make it possible to measure direct- 
ly the +:-- -6 -iission of certain fluorescence bands excited by this source. 
r.as1S in turn would yield new informatinn curiccrning lifo-times of molecular 
electronic states. 
As the designation of this spectral region implies, 
Therefore, special care must be taken when applying 
Once the capability of this plasma source in this spectral region has 
h this connection, much progress has been made in the last few years 
on the experimental observation of the fluorescence which results when an 
excited molecular iun decays to  a lower state of this ion or to its ground state. 
1 The Present eXPPrill i@n*d lechniqrroc have been described and are concerned 
1. "Wavelength Analysis of Fluorescence from Gases Excited by Vacuum 
Ultraviolet Radiation, 'I D. L. Judge, f;. L. Morse, and G. L. Weisslex, 
Proceedings of the VI International Conference on Ionization Phenomena 
in Gases, Paris, July 8-13, 1963; edited by P. Hubert and E. Zremien- 
*an; vol. IXI, p. 373. See also, "Fluorescence from Excited States of 
N produced by Vacuum Ultraviolet Radiation", D. L. Judge and G. L. 
d i ss le r ,  J. Chem. Phys., to be published in  1968. 
1 
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with the following type of mechanism: 
Monochromatic photons emerging from the exit slit of a Seya vacuum 
monochromator enter into a fluorescence chamber, which is filled with the 
gas under investigation, say N 
photon is short enough, or its energy sufficiently high, say 19 t V ,  then the 
following photo-excitation process occurs. 
If the wavelength of the vacuum ultraviolet 2' 
t* - 
N2 -t kn, (19eV)--..-N 2 t e , 
t* + N2 -----+ N2 t hv (fluorescence). 
A. second monochromator of the Czerny-Turner-type looks at the 
fluorescent gas (in this case N ) in a direction perpendicular t o  the beam of 
vacuum ultraviolet photons shorter than its ionization threshold, the gas under 
investigation can be tested for fluorescence, It has been shown in earlier 
work that intense fluorescence is observed in this fashion. The probability 
of such fluorescence transitions is indicated roughly by the intensity of the 
light emitted, 
2 
1 
If it were possible to use a primary vacuum ultraviolet photon beam 
consisting of very short light pulses, similar to  those provided by the NANO- 
LIGHT developed at A.FCRL by Dr, Heinz Fischer and 
it should be feasible to  observe the time-duration of emission of the fluores- 
cence light pulse as an event, which is substantially longer than the exciting 
light pulse. This measurable decay of the fluorescent light pulse seems to be 
now a distinct possibility with the existence of a laser-produced fireball as a 
light source of nano-second duration. Up to  this time, the types of condenser 
then 
2. H. Fischer, J, 
3. H e  Fischer, J e  
4. H. Fischer and 
5. H. Fischer and 
opt. sot. & n b  51, 543 (1961). 
Opt, SOC, A m .  52, 605 (1962). 
w. B. ~*%.'ppel, Apple W i C S  769 (1964). 
4-b Fritzsche, AppL Optics 3 1235 (1964). 
c 
3 
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6 discharge sources employed in the vacuum ultraviolet region have been 
limited t o  light pulses of a duration of micro-seconds, which a re  a factor of 
several hundreds longer in time than the best capabilities of the laser fireball. 
Under conditions of the present sources of micro-second duration per pulse, 
it is not possible to  make t h e  resolved studies of fluorescent emission. 
/.nother area of research, to which a laser-produced fireball could 
make a significant contribution, is the determination of optical and photo- 
PlccLrir; properties of surfaces and thin films, produced and maintained under 
conditions of ultrahigh vacuum. The reflectance of barium films has been 
measured in this laboratory at about 
pressure pravided excellent agreement with other experiments on electron 
eigen losses and with theory. However, this agreement vanished when such 
-9 experiments were conducted at 10 
other metals the electron eigen losses (or their corresponding plasma 
frequencies) occur at values of 10 eV or more, their optical properties need 
to be evaluated at photon energies above 10 eV or at  wavelengths below 1000 8 .  
This then requires a monochromator - and a light source (a laser fireball o r  
equivalent), both of which can be operated under conditions of ultrahigh 
vacua. (P- low-pressure capillary spark source would clearly introduce 
too much gas into the monochromator and then, through the exit slit, into the 
ultrahigh vacuum reflectometer chamber and on the sample. ) 
7 torr ,  and the results at this low 
to r r  or higher pressures. Since in many 
6 
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6 .  "Some Instrumentation Problems below 1000 1, 'I G. L, Feiss ler ,  I C 0  
Conference, Tokyo and Kyoto, September 2-4, 1964; Japan J. Appl. 
Phys. 4 (Supplement I), 486 (1965). 
Detectgrs for Vacuum UV Radiation between 1000 1 and 100 % . I t ,  G. L. 
Weissler, pp. 229 t o  261; article in book on "Aerospace Measurement 
Technique", ed. Gene G. Manella, NASA-SP-132, U. S. Government 
Printing Office, Washington, D. C. , 1967. 
See also "Light Sources and 
7 .  "Optical Constants of Evaporated Barium in the Vacuum Ultraviolet", 
&I. Fisher, I. Fujita, and G.L. Weissler, J. Opt. Sac, Am. 56, 1560 
(1756). 
- -  
.. 
r 4 
The above two examples of applying a laser-produced fireball or a 
high-vacuum spark as a light source to 1) fluorescence or 2 )  reflectance 
measurements was meant to  serve only as a general indication of the useful- 
ness of such a source in those and undoubtedly in other areas of research, 
where nanosecond pulse duration, high light intensities, and/or high vacua 
a re  of principal importance. Initial attempts to investigate a source with 
these properties were undertaken prior to the initiation of NPSA contract 
no. NAS 2-3846 and showed clearly the Fano profile of the helium absorp- 
tion line at 206 A (due to two-electron excitation), when the emission con- 
tinuum of a laser fireball with platinum as a target was used as  a background 
In the following, the research progress made under the auspices of NASA 
a 
9 
contract no. NAS 2-3846 during the period from October 1966 to  January 1968. 
8. U. Fano, Phys. Rev. E, 1866 (1961); see also U. Fano and J. W. 
Cooper, Phys. Rev. 138, A400 (1965). 
"Vacuum Ultraviolet Radiation from Plasmas Produced by a Laser of 
Metal Surfaces", A. V?. Zhler and G. L. Weissler, Physics Letters 
9. 
-i, 8 89 (1966). 
. 
.^ 
5 
IL REVIEW OF LASER FIREBALL VACUUM ULTRAVIOLET 
* -  
EMISSION DURING THE CALENDAR YEAR OF 1967 
The following will be a s u m m a r y  of the research done under the above 
referenced contract during the calendar year of 1967. 
intensive investigation on the most suitable Q- switched laser, produced by 
a commercial firm, was made, and an order was placed with the Santa 
Monica firm of Korad for such an instrument. 
rate of one pulse per second at 100 megawatt power per pulse. 
instrument was delivered at the end of January 1967 and our research began. 
Late in 1966, an 
It was capable of a repetition 
This 
Initially, we repeated earlier investigations using a normal incidence 
vacuum spectrograph capable of both photographic and photoelectric record- 
ing. Many spectra were obtained with target materials from low 2-numbers 
to high Z-numbers. The survey served, primarily, t o  re-establish the 
techniques to  be employed in this research. The main work was to be carried 
out at USC with a 2 meter radius of curvature grating grazing incidence 
vacuum spectrograph. This latter instrument was built by MacPhsrson and 
is capable of photographic recording only. However, it can be modified by 
US for photoelectric work. 
The subsequent discussion will concern itself with the progress made 
on the grazing incidence instrument. 
In order to place the laser-produced fireball as close a s  possible to the 
target, it was necessary to modify the existing entrance slit. This change of 
the instrument has been accomplished and it w a s  feasible to obtain a laser 
€ireball within about 10 mm from t h e  primary slit on the optic axis. During 
the last few months this new setup has become operational, The dispersion 
curve of the spectrograph has been obtained by utilizing a capillary spark 
source which is rich in emission lines down t o  about 70 1. This provides a 
good wavelength calibration of the photographic plate, since the spark source 
.- 
6 
can be mounted on a slit chamber, which contains the target material for 
production of laser fireballs: when the target is pulled upward and out of the 
way of the optic axis, the path is clear for light from the spark source to  
enter the spectrograph for calibration purposes. When the target is placed 
on the optic axis, then spectra from the laser fireball can be obtained. 
In Fig. 1, top, is shown a spectrum taken with the 2 rn grazing incidence 
vacuum spectrograph showing emission lines of a low pressure spark through 
a ceramic (boron nitride) capillary with a 600 grooves per mm grating. 6 
A number of such spectra have been taken at  the time of this writing. The 
results obtained with an aluminum target show a well developed spectrum of 
fireball emission lines with a weak underlying continuum. 
Z-number target is employed, the spectrum is characterized by a continuum. 
More extensive work on more target materials will be done before the end of 
this year. 
required appraximately 50 laser pulses, and each pulse has approximately 
40 megawatts. 
In Fig's. 1 to  4, such laser fireball emission spectra a re  shown with 
When a high 
4.11 the exposures obtained so far on the grazing incidence instrument 
* .  
v . .  
Al, r e ,  Cd, and Pt as target materials, again using a 2 m grazing incidence 
spectrograph with a 600 grooves per millimeter grating. These figures 
contrasts the essentially line emission nature of Al with the continuum emission 
from Pt. The same distinctive differences in spectra are also apparent, when 
the radiation from a fireball is analyzed with a normal incidence spectrograph, 
frrm 500 %, to  3500 h, for low and high Z-number targete as shown in Fig's. 5 
to 7. 
Simultaneously with the laser work on the grazing incidence instrument, 
some research has been done on the previously mentinned normal incidence 
Spectrograph on the subject of the number of photoelectrons emitted per 
laser  pulse in a narrow wavelength region. This was accomplished by 
placing a simple photocell at the Rowland facusing circle with an exit slit of a 
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few millimeters width in front of it. By assuming that our earlier 
measurements on the phetoelectric yield of metals can be applied to this 
9 10 photocell, one obtains a photon-flux of about 10 t o  10 photons per pulse 
for a band width of about 30 -1 at 700 I-. This large number of photons per 
pulse is very encouraging for future development work. 
Finally, following the original design, a plane-parallel plate condenser 
was constructed in our shop and tested in the laboratory. 
earlier design prwed troublesome in a number of aspects: it was awkward 
with this s m r c e  to observe the laser fireball alone, separate from the fire- 
ball enhanced by the condenser discharge, secondly, the earlier design mage 
the distance between the plasma and the primary slit larger than desirable 
for a very narrow plasma source, of the order of lmrn, thirdly, the 
geometrical arrangement did not allow for good evacuation of the discharge 
gap. For this reason, the condenssr source has been redesigned as shorn 
schematically in Fig. 8. 
angle of about 30" with the optic axis of the spectrograph. The adaptor 
is shown on the right. 
the spectrograph via this adapter. The central rod of the short cylindrical 
transmission line to the discharge gap can be moved along its axis in order 
to  change the gap length. The laser light can be focused on the fixed portion 
of the short cylindrical transmission line. It is clear that this transmission 
line of the spark gap at its termination end brings both the laser fireball and 
condenser discharge much closer to the primary slit than was previously 
possible. 
separate spectra for either the fireball alone or the condenser discharge. 
It is, at this time, our plan to provide NASA Ames with two such condenser 
light sources at the termination of our present contract with you, some time 
soon after January 8, 1968. 
However, this 
In this figure, the condenesr source is shown on the left making an 
It will allow the condenser source to  be fastened to 
In this configuration, there will be no difficulty in obtaining 
. 
' 15 
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In the following Fig's. 9 to 12 are  shown a number of vacuum 
ultraviolet spectra obtained with the 50 cm radius of curvature grating 
(600 grooves per mm) mounted at normal incidence. These figures demon- 
strate the enhancement of light intensity, when the plasma from the laser- 
produced fireball is used to trigger a vacuum spark, which discharges a 
commercial (Bosch) capacitor of 0.5 p F  at 12.5 kV. It is to  be noted, 
that the vacuum gap for convience was made of Fe electrodes. La addition 
the nature of the vacuum spark condenser-enhanced spectra showed a 
predominance of line emission. The continuum emission characteristic of 
the Fe fireball alone seemed to  have been largely suppressed. This was 
probably due to  a gradually widened gap (after each discharge), which in 
turn gave rise to  less confinement of the vacuum spark with the concomitant 
decrease in plasma density and continuum emission. 
pointed out, that by virtue of attaching (with copper straps of relatively 
large distributed inductance) a 0.5 p F  commercial capacitor to the source 
shown in Fig. 8, the pulse duration increased materially from tens of 
nanoseconds for the laser-produced fireball alone to  several microseconds 
for the condenser discharge. 
It should also be 
Finally, the enhancement of light from the laser fireball by the 
low-capacity plane parallel plate condenser alone (about 0,005 pF), shown 
in Fig. 8, was observed in a qualitative fashion: the light pulses were very 
short, of nanosecond duration, and roughly of twice the intensity of the 
laser  -produced fireball alone. Without laser triggering, the plane parallel 
CondPnoor o-*.--n -Le-- :n V G a -  Q UOUU bo u ~ e ~ u t x d  in €r.ce-ruming mode, 
probably triggered by field emission, if a very small gap of the order of 0.1 
~ I X I  was used. Then the light output per pulse was roughly equivalent to 
9 
~ 1 1 ~ t  of he fireball alone, of the order of 10 photons per pulse at 700 A 
with a 30 A band-pass for the exit slit on the Rowland focusing circle, 
However, these free-running vacuum sparks exhibited a somewhat erratic 
4 behavior, undoubtedly due to small changes in the configuration of the . 
vacuum gap. 
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It is our feeling at this time, that much more extensive work is 
required, in order to make the low-capacity plane parallel condenser 
source into a routinely operated spectroscopic tool. There is no doubt 
in our collective opinion here (including Dr, G. Hersberg's at the occasion 
of his visit t o  USC in January of 1968), that such a light source will be of 
considerable use to  spectroscopists and is well worth the forthcoming 
development effort, 
The financial assistance provided for this prelimiary development 
work by the NASA Ames Research Center is hereby gratefully acknowledged. 


Development of a high-intensity, nanosecond, pulsed vacuum ultraviolet light source Final technical report, Oct. 1966 - Jan. 1968

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