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The spectra of plasmas produced by focusing the output of a Q-switched ruby laser (output 1 J) on the rare-earth metals have been studied. From samarium (Z = 70), strong quasi-uniform continua are emitted in the wavelength range 40–2000 Å. Line emission from the target elements is absent over most of this wavelength region, particularly below about 600 Å. The use of these continua as simple, reliable background sources for absorption spectroscopy in the vacuum-ultraviolet and soft x-ray region down to 40 Å is demonstrated

Carroll, P.K.

Kennedy, Eugene T.

O'Sullivan, G.

72 OPTICS LETTERS / Vol. 2, No. 3 / March 1978
New continua for absorption spectroscopy from 40 to 2000 A
P. K. Carroll, E. 'T. Kennedy, and G. O'Sullivan
Physics Department, University College, Dublin, Ireland
Received November 7,1977
The spectra of plasmas produced by focusing the output of a Q-switched ruby laser (output I J) on the rare-earth
metals have been studied. From samarium (Z = 62) to ytterbium (Z = 70), strong quasi-uniform continua are
emitted in the wavelength range 40-2000 A. Line emission from the target elements is absent over most of this
wavelength region, particularly below about 600 A. The use of these continua as simple, reliable background
sources for absorption spectroscopy in the vacuum-ultraviolet and soft x-ray region down to 40 A is demonstrated.
Although continuum emission from laser-produced
plasmas has frequently been noted, little effort seems
to have been made to exploit its use in absorption
spectroscopy. This is not surprising because usually
the continuum is overlaid by a much stronger line
spectrum. Thus, in the case of absorption of the ra-
diation from one laser-produced aluminum plasma by
a second aluminum plasma' (also laser-produced), the
execution of the experiment and the interpretation of
the results are both difficult. In the case of laser-pro-
duced plasmas on tungsten targets, however, the work
of Ehler and Weissler2 indicated that in the grazing
incidence region the continuum predominates. Tan-
talum was also studied, more particularly at longer
wavelengths, both by Ehler and Weissler and by Breton
and Papoular,3 the latter workers being primarily in-
terested in obtaining a strong continuum near 1215 A
for the study of Lyman a in various high-power plasma
devices. It was shown in this laboratory4 that the
tungsten continuum could be used to good effect for
absorption studies in the grazing incidence region from
60 to 180 A, and that it was particularly useful when a
pulsed source was required. However, it was found that
the tungsten continuum became strongly contaminated
with lines, both emission and absorption, above about
180 A, thus limiting its usefulness as a background
source. Tantalum was also studied but was found to
be as strongly contaminated with lines as tungsten,
particularly at wavelengths longer than about 150 A.
Following the promising use of the tungsten continuum
for absorption work a study was made of other high-Z
target materials with a view to obtaining, if possible, a
continuum that would be stronger, more extensive in
wavelength coverage, and, in particular, freer from
atomic lines.
In a survey of most of the heavier elements from tel-
lurium to uranium, it was found that the higher rare
earths (Z > 62) emitted strong, and, over wide spectral
regions, relatively line-free continua. The experiments
were carried out from 30 to 500 A with a 2-m grazing
Xl Il l
100
I I I
150
l I
200 A
I I I I I
" - 4d-4f Resonance / 11 d
4d -_6p,7p,-- -
80
I I
_-ii- ________
Fig. 1. (a) Absorption spectrum of xenon from 80 to 200 A. The xenon pressure in the spectrograph was 0.05 Torr, and the
number of laser pulses used was 30. For details of the xenon spectrum in this region see Madden and Codling.5 The unmarked
weak lines near 200 A are due to 0 v. Oxygen present in the target gives rise to some emission lines as well. (b) The ytterbium
continuum from 60 to 100 A. The number of laser shots was 20. As in (a), the spectrum was obtained on a Kodak SC5
plate.
0146-9592/78/0300-0072$0.50/0 © 1978, Optical Society of America
a
LL
4 p - 6s, 7s ---
A1
I
March 1978 / Vol. 2, No. 3 / OPTICS LETTERS 73
48 50
l l
52
L,,,,III * LU
Fig. 2. (a) Terbium continuum from 46 to 54 A obtained
with 30 laser shots. (b) Argon L edge absorption corre-
sponding to the transitions 2p63s23p6 1 S-2p 5 3s23p 6ns,nd.
(See Nakamura et al. 6) The argon pressure was 0.2 Torr, and
30 laser shots were used. As in Fig. 1, the spectra were re-
corded on Kodak SC5 plates.
incidence vacuum spectrograph blazed at 65 A; from 500
to 2800 A the observations were made with a 3-m normal
incidence instrument blazed for 1500 A. In both
spectrographs Bausch & Lomb gratings having 1200
grooves/mm were used. The target was located 4-10 cm
from the spectrograph slit and irradiated with a Q-
switched ruby laser that gave an output of about 1 J in
a pulse that had a FWHM of 25 to 30 nsec. A plano-
convex lens of 5-cm focal length was used to focus the
laser beam onto the target metal. The slit widths used
were typically 10 to 15,im.
The continua emitted by the rare earths were re-
markable for their intensity, relatively slow variation
with wavelength, and, in particular, the absence over
substantial spectral ranges of atomic lines emitted by
the target element. Thus, in the case of ytterbium, an
intense quasi-uniform continuum was emitted from 35
to 600 A. At this point relatively strong emission lines
appeared, although the underlying continuum was still
present. Again, in the case of samarium, although the
X 850
I
860
intensity was less uniform than in the case of ytterbium,
the continuum extended from 35 to 2000 A with only a
few target lines appearing at longer wavelengths. The
short-wavelength limit of our observations was set by
scattered light from the grating.
It appeared from these results that the continua
would be well suited for absorption work throughout the
whole vacuum region from 2000 A to soft x rays at about
40 A. This proved to be the case, and the results of a
number of experiments are shown in Figs. 1-3. Figure
1 shows the absorption spectrum of xenon from 45-200
A, in which region autoionizing transitions from
inner-shell excitations are prominent. Also shown (at
larger magnification) is part of the ytterbium continuum
itself, from which its cleanliness and relative uniformity
may be seen.
Figure 2 illustrates the argon LII,,,, edge at 50 A ob-
served with the terbium continuum as background.
The LII,III edges of chlorine at 62 A and the K edge of
carbon at 44 A were observed in other experiments.
Although well-established continua are now available
for work above 600 A, the rare-earth sources may be
useful for particular types of experiments. In Fig. 3 is
shown part of the N2 absorption spectrum, again ob-
tained with the terbium continuum. The plates were
taken on the 3-m normal incidence instrument, and the
results indicate that the new continua can be used in
work of intermediate and high resolution.
In the above experiments, as well as in others not
reported here, the absorbing gas was present in the
source as well as in the body of the spectrograph; its
presence, at least up to moderate pressures, does not
appear to inhibit the emission of the continuum. Again,
preliminary experiments indicate that the emission time
of the continuum corresponds fairly closely to the length
of the optical output pulse of the laser-a result similar
to that obtained by Ehler and Weissler2 with tungsten.
This characteristic is of obvious value for experiments
requiring time resolution.
The system is simple to set up and is easy to operate,
and its performance is essentially as reliable and con-
sistent as that of the exciting laser. It is also a relatively
clean source in the sense that little debris from the
laser-produced plasma is thrown onto the slit, especially
880 A
I
870
I
v 21 1520 7119 18 17
R
14
Fig. 3. The absorption spectrum of N2 from 850 to 880 A taken on the 3-m normal incidence vacuum spectrograph with the
terbium continuum. The number of laser shots required was 200, and the nitrogen pressure in the spectrograph was 3.5 X 10-4
Torr. The spectrum was recorded on Kodak SWR plates. Part of the v'-o absorption progression of the b' 12-xll system
is shown, together with a number of Rydberg bands designated R. . For a high-resolution analysis of the N2 spectrum in this
region, see Carroll and Yoshino7 and references therein.
I
I I
74 OPTICS LETTERS / Vol. 2, No. 3 / March 1978
if the plasma plume is directed in a plane parallel to the
slit jaws. Furthermore, the effective intensity of the
continuum, although already adequate for many ex-
periments, could be increased by about an order of
magnitude by the use of suitable condensing optics.8 In
short, the continua described here would seem to have
many attractions for absorption spectroscopy from 40
to 2000 A, particularly in the region below 500 A.
References
1. A. Carillon, P. Jaegle, and P. Dhez, "Extreme ultraviolet
continuum absorption by a laser-generated aluminum
plasma," Phys. Rev. Lett. 25, 140 (1970); A. Carillon, G.
Jamelot, A. Sureau, and P. Jaegle, "Autoionizing series in
the Al3+ ion spectrum of a laser-produced plasma," Phys.
Lett. 38A, 91 (1972); F. P. J. Valero, "Intensity anomalies
in the extreme VUV spectrum of Al3+ obtained in a laser
produced plasma," Appl. Phys. Lett. 25, 64 (1974).
2. A. W. Ehler and G. L. Weissler, "Vacuum ultraviolet ra-
diation from plasmas formed by a laser on metal surfaces,"
Appl. Phys. Lett. 8, 89 (1966).
3. C. Breton and R. Papoular, "Vacuum UV radiation of
laser-produced plasmas," J. Opt. Soc. Am. 63, 1225
(1973).
4. P. K. Carroll and E. T. Kennedy, "Doubly excited au-
toionization resonances in the absorption spectrum of Li+
formed in a laser-produced plasma," Phys. Rev. Lett. 38,
1068 (1977).
5. K. Codling and R. P.Madden, "Newly observed structure
in the photoionization continua of Kr and Xe below 160 A,"
Appl. Opt. 4, 1434 (1965).
6. M. Nakamura, M. Sasanuma, S. Sato, M. Watanabe, H.
Tamashita, Y. Iguchi, A. Egiri, S. Nakai, S. Yamaguchi, T.
Sagawa, Y. Nakai, and T. Oshio, "Absorption structure
near the Lli,iii edge of argon gas," Phys. Rev. Lett. 21,1303
(1968).
7. P. K. Carroll and K. Yoshino, "The c0 lIlu-c'n 1Z+ Rydberg
states of N2; high resolution studies," J. Phys. B 5, 1614
(1972).
8. W. A. Rense and T. Violett, "Method of Increasing the
speed of a grazing-incidence spectrograph," J. Opt. Soc.
Am. 49, 139 (1959).


English

New continua for absorption spectroscopy from 40 to 2000 Å

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New continua for absorption spectroscopy from 40 to 2000 Å

Doubly excited autoionization resonances in the absorption spectrum of Li⁺ formed in a laser-produced plasma

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