Vacuum Ultraviolet Spectroscopy. Experimental Methods in Physical Sciences

717 Pages·2000·21.422 MB·English

by  J.A.R. Samson and D.L. Ederer (Eds.)

PREFACE
In 1967 the book entitled, Techniques of Vacuum Ultraviolet Spectroscopy was
authored by one of us (JARS). Since then, vacuum ultraviolet (VUV) spec-
troscopy has become so broad a topic that it seemed impossible for one author to
give a complete treatise on the subject. Thus, Vacuum Ultraviolet Spectroscopy,
which was originally published as volumes 13 and 32 in Experimental Methods
ni the Physical Sciences, has come to be a compilation by many authors who are
experts on the various subjects contained within this reference. This edition
brings together the entire contents of those two volumes in a single paperback
book.
The use of synchrotron light sources was in its infancy in 1967 with perhaps a
half dozen sources operating internationally. A few hundred scientists used these
facilities, and at that time most of the vacuum ultraviolet research was conducted
with discharge laboratory sources. The availability of synchrotron radiation has
been the driving force that has produced the enormous growth of scientific
research utilizing the vacuum ultraviolet since the 1960's. The field grew stead-
ily through the next two decades as more and more scientists discovered how
photoelectron spectroscopy, for example, could be used with synchrotron radia-
tion to elucidate the electronic properties of solids, thin films on surfaces, and
gases. The research rapidly extended into the x-ray region of the spectrum as sci-
entists made use of extended x-ray absorption fine structure (EXAFS) to pinpoint
atomic structure and learned how effectively a bright x-ray source can be used to
obtain x-ray diffraction spectra of complex materials, such as biomolecules. Thus
the decade of the eighties became the proving ground for many applications of
synchrotron radiation to research extending over a nine decade spectral range
from the far infrared to the hard x-ray region.
The decade of the nineties has become the decade of the "third generation"
source of exceptional brightness and wavelength range, where experiments can
be performed on diffuse materials, and low efficiency techniques, such as photon
excited fluorescence, can be used to study the electronic properties of complex
materials. Now thirty-two years after the appearance of Techniques of Vacuum
Ultraviolet Spectroscopy, we take synchrotron radiation for granted as the
research efforts of several thousand scientists using this powerful source of radi-
ation has grown into an international effort covering four continents, over a dozen
countries and at last count, over thirty different facilities.
While synchrotron radiation is an important source for scientific research in the
VUV, other sources developments have been included, such as the Electron Beam
xiii
xiv ECAFERP
Ion Trap (EBIT) source, the laser-produced plasma (Chapter 5), and VUV lasers
(Chapter 7). Two chapters have been devoted to the new techniques that are avail-
able for making intensity measurements (Chapter 8, and Chapter 8 V.II).
Along with the development of synchrotron radiation sources, there has been
an extensive improvement in technology related to the VUV, from optical ele-
ments (Chapter 9) to the vacuum chambers that hold them (Chapter 9 V.II). Since
1967 multilayers (Chapter 14) and silicon carbide mirror coatings have been
introduced extensively as materials that have high reflectivity and are well suited
for the VUV spectral region. Furthermore x-ray zone plates (Chapter 15) have
become available for microscopy and for use as monochromators. The extensive
improvement of VUV detectors has been described in Chapters 5-7, V.II, and
new methods for interferometric spectrometers are discussed in Chapter 4, V.II.
Of course to be useful the VUV radiation needs to be monochromatic and tun-
able. The monochromator or spectrometer provides these capabilities. The theory
for monochromator design has been described in Chapters 1-4 V.II for a number
of different optical configurations. This book is focused mainly on the VUV por-
tion of the spectrum, which extends nominally from 01 eV to 1000 ,Ve the range
of spectrometers and monochromators that use diffraction gratings. To give the
reader a sense of the instrumentation that is used at photon energies greater than
1 keV, we have included a chapter dealing with the development of x-ray spec-
trometers (Chapter 19).
In the coming decades magnetic circular dichroism that depends on the polar-
ization of the VUV radiation (Chapter 12) will be an important technique because
of the importance of developing new materials that may serve as high resolution
substrates in the magnetic recording industry. Spectromicroscopy (Chapter ,11
V.II) has been latent up to the last decade of the Twentieth Century when third
generation high brightness synchrotron sources became available. As the litho-
graphic process (Chapter ,01 V.II) evolves in the x-ray spectral region and new
materials are developed, x-ray microscopy and soft x-ray fluorescence spec-
troscopy (Chapter ,31 V.II) will be used more and more as an analytical and
developmental tool. We hope seuqinhceT of teloivartlU ypocsortcepS will serve
as a useful handbook for interested scientists.
The editors would like to thank all the contributing authors who have labored
hard and long to make this volume a useful guide to the methods of soft x-ray
instrumentation and science. In addition these editors owe a debt of gratitude to
Professor G. L. Weissler, and Professor D. H. Tomboulian who have provided
guidance and leadership in the formative days of research in the VUV so many
years ago.
CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.
E. .T AWAKRA (93), 111 Amherst Lane, Oak Ridge, Tennessee 37830
TRA REIEMDNUARB (93), Department of Physics, Southern Illinois University at
Edwardsville, Edwardsville, Illinois 62026
DRAHKCE RETSROF (401), University of Jena, Institute of Optics and Quantum-
electronics, Max-Wien-Platz ,1 07743, Jena, Germany
CIRE M. NOSKILLUG (257), Center for X-Ray Optics, Materials Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, California
S. L. TREBLUH (1), National Synchrotron Light Source, Brookhaven National
Laboratory, Upton, New York, 11973
.W R. RETNUH (183, 205, 227, 305, 379), AFS Inc. 1401 McCormick Drive,
Largo, Maryland 20774
ERREIP ~ILGEAJ (101), Laboratoire de Spectroscopie Atomique et Ionique, Bat
350, Universite ,duM-siraP 91405 Orsay Cedex, France
LEAHCIM K~HNE (65, 119), Physikalisch-Technische Bundesanstalt, Abbe-
strasse 2-12, D10587 Berlin, Germany
,IAT IHSEZ AKIOMAN (347), 4-2-221, Takamori, Izumi-Ku, Sendai, 981-3203,
Japan
NITRAM NOSDRAHCIR (83), Laser Plasma Laboratory, CREOL/University of
Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32861
SEMAJ R. STREBOR (37), National Institute of Standards and Technology,
Gaithersburg, Maryland 20899
DRAHREBE RELLIPS (271), IBM .T .J Watson Research Center, Yorktown Heights,
New York 10598
SEMAJ H. DOOWREDNU (145), Center for X-Ray Optics, Lawrence Berkeley
Laboratory, One Cyclotron Road, Berkeley, California 94720
~IuY VLADIMmSKu (289), University of Wisconsin-Medicine, CXRL, 3731
Schneider Drive, Stoughton, Wisconsin 53589
NHOJ B. WEST (27), Synchrotron Radiation Department, Daresbury Laboratory,
Daresbury, Warrington, Cheshire, 4AW 4AD, United Kingdom
G. .P SMAILLIW (1), National Synchrotron Light Source, Brookhaven National
Laboratory, Upton, New York 11973
XV
.1 SYNCHROTRON RADIATION SOURCES
S. .L Hulbert and G. R Williams
nevahkoorB lanoitaN yrotarobaL
,notpU weN kroY
1.1 General Description of Synchrotron Radiation
Synchrotron radiation is a very bright, broadband, polarized, pulsed source of
light extending from the infrared to the x-ray region. It is an extremely important
source of vacuum ultraviolet radiation. Brightness is defined as flux per unit area
per unit solid angle and is normally a more important quantity than flux alone
particularly in throughput-limited applications, which include those in which
monochromators are used.
It is well known from classical theory of electricity and magnetism that
accelerating charges emit electromagnetic radiation. In the case of synchrotron
radiation, relativistic electrons are accelerated in a circular orbit and emit
electromagnetic radiation in a broad spectral range. The visible portion of this
spectrum was first observed on April 24, 1947, at General Electric's Sche-
nectady, New York facility by Floyd Haber, a machinist working with the
synchrotron team, although the first theoretical predictions were by Lidnard 1
in the latter part of the 1800s. An excellent early history with references was
presented by Blewett 2 and a history covering the development of the utiliza-
tion of synchrotron radiation was presented by Hartman 3.
Synchrotron radiation covers the entire electromagnetic spectrum from the
infrared region through the visible, ultraviolet, and into the x-ray region up to
energies of many tens of kilovolts. If the charged particles are of low mass, such
as electrons, and if they are traveling relativistically, the emitted radiation is
very intense and highly collimated, with opening angles of the order of 1 mrad.
In electron storage rings there are three possible sources of synchrotron radia-
tion: dipole (bending) magnets; wigglers, which act like a sequence of bending
magnets with alternating polarities; and undulators, which are also multiperiod
alternating magnet systems but in which the beam deflections are small, result-
ing in coherent interference of the emitted light.
In typical storage rings used as synchrotron radiation sources, several bunches
of up to ~-- 01 2t electrons circulate in vacuum, guided by magnetic fields. The
bunches are typically several tens of centimeters long, so that the light is pulsed,
being on for a few tens to a few hundreds of picoseconds, and off for several
tens to a few hundreds of nanoseconds depending on the particular machine
and the radio-frequency cavity, which restores the energy lost to synchrotron
2 SYNCHROTRON RADIATION SOURCES
radiation. However, for a ring with a 30-m circumference, the revolution time is
100 ns, so that each bunch of 1012 electrons is seen 10 7 times per second, giving
a current of---1 A.
The most important characteristic of accelerators built specifically as synchro-
tron radiation sources is that they have a magnetic focusing system which is
designed to concentrate the electrons into bunches of very small cross section
and to keep the electron transverse velocities small. The combination of high
intensity with small opening angles and small source dimensions results in very
high brightness.
The first synchrotron radiation sources to be used were operated parasitically
on existing high-energy physics or accelerator development programs. These
were not optimized for brightness and were usually accelerators rather than
storage rings, meaning that the electron beams were constantly being injected,
accelerated, and extracted. Owing to the successful use of these sources for
scientific programs, a second generation of dedicated storage rings was built
starting in the early 1980s. In the mid-1990s, a third generation of sources was
built, this time based largely on special magnetic insertions called undulators
and wigglers. A fourth generation is also under development based on what is
called multiparticle coherent emission, in which coherence along the path of the
electrons, or longitudinal coherence, plays the major role. This is achieved by
microbunching the electrons on a length scale comparable to or smaller than the
scale of the wavelengths emitted. The emission is then proportional to the square
of the number of electrons, N, which, if N is 1012, can be a very large
enhancement. These sources can reach the theoretical diffraction limit of source
emittance (the product of solid angle and area).
2.1 Theory of Synchrotron Radiation Emission
1.2.1 General
The theory describing synchrotron radiation emission is based on classical
electrodynamics and can be found in the works of Tomboulian and Hartman 4
(1956), Schwinger 5 (1949), Jackson 6 (1975), Winick 7 (1980), Hofmann
8 (1980), Krinsky, Perlman, and Watson 9 (1983), and Kim 10 (1989). A
quantum description is presented by Sokolov and Temov 11 (1968).
Here we present a phenomenological description in order to highlight the
general concepts involved. Electrons in circular motion radiate in a dipole
pattem as shown schematically in Fig. l a. As the electron energies increase and
the particles start traveling at relativistic velocities, this dipole pattern appears
different to an observer in the rest frame of the laboratory. To find out how this
relativistic dipole pattem appears to the observer at rest, we need only appeal to
THEORY OF SYNCHROTRON RADIATION EMISSION 3
.GIF .1 Conceptual representation of the radiation pattern from a charged particle
undergoing circular acceleration at (a) subrelativistic and (b) relativistic velocities.
standard relativity theory. This tells us that angles 0, in a transmitting object are
related to those in the receiving frame, 0,., by:
sin ,O
tan .,0 = (1)
),(cos 0, - fl)'
with ,7 the ratio of the mass of the electron to its rest mass, being given by
E/moC ,2 E being the electron energy, om the electron rest mass, and c the velocity
of light; fl is the ratio of electron velocity, ,v to the velocity of light, c. Thus for
electrons at relativistic energies, fl ~ 1 so the peak of the dipole emission pattern
in the particle frame, rO = 90 ~ transforms to .,0 ~ tan .,0 -~ 7-~ in the laboratory
frame as shown in Fig. lb. Thus 7-~ is a typical opening angle of the radiation
in the laboratory frame.
Now for an electron viewed in passing by an observer, as shown in Fig. 2, the
duration of the pulse produced by a particle under circular motion of radius p
will be p/Tc in the particle frame, or p/Tc (cid:141) 1/y 2 in the laboratory frame owing
to the time dilation. The Fourier transform of this function will contain fre-
quency components up to the reciprocal of this time interval. For a storage ring
with a radius of 2 m and 7 = 1000, corresponding to a stored electron beam
energy of---500 MeV, the time interval is 10 -17 s, which corresponds to light of
wavelength 30 A.
2.2.1 Bending Magnet Radiation
It is useful to define a few quantities in practical units because these will be
used in the calculations that follow. For an electron storage ring, the relationship
between the electron beam energy E in GeV, bending radius p in meters, and
SYNCHROTRON RADIATION SOURCES
field B in T is
E GeV
p m = . (2)
0.300B T
The ratio :) of the mass of the electron to its rest mass is given by
~,= E/moc 2 = E/0.511 MeV = 1957E GeV, (3)
and s which is defined as the wavelength for which half the power is emitted
above and half below, is
72 = 4rcp/(3y )3 or c2 s = 5.59p m/E 3 GeV 3 = 18.6/(B TE 2 GeVZ).
(4)
The critical frequency and photon energy are
co. = 2rcc/2c = 3c~,3/(2p) or e, eV = hcoc eV = 665.5E 2 GeV 2 B T.
(5)
The angular distribution of synchrotron radiation emitted by electrons moving
through a bending magnet with a circular trajectory in the horizontal plane is
I of,.j~ ~ a(~) ", --y 1 = 1 milliradian
1 p 2
tA = ~ x cr io9 x 3 x ~01
t mc " "~ 1 ", ~mortsgnA03
~q t(,,,)
I0 -Iv secs.
_k
t
.GIF .2 Illustration of the derivation of the spectrum emitted by a charged particle in a
storage ring.
YROEHT FO NORTORHCNYS NOITAIDAR NOISSIME 5
given 9 by
dO d~ er4 2 '~ oc e /cOC\ 1( + ~ K23(~) + 1 + ~ )2r K,,3(~) 2 , (6)
where F is the number of photons per second, 0 the observation angle in the
horizontal plane, ~ the observation angle in the vertical plane, ~o the fine
structure constant (1/137), oc the light frequency, I the beam current, and
= (co/2coc)(1 + ),2~,2)3/z. The subscripted K's are modified Bessel functions
of the second kind. The s1 term represents light linearly polarized parallel to
the electron orbit plane, while the K~/3 term represents light linearly polarized
perpendicular to the orbit plane.
If one integrates over all vertical angles, then the total intensity per
radian is
dF, .... (co) .- . 3-J . y~c . .Ao I co Ks/3(y) dy. (7)
dO 2= oc e coc .,.,/,,
In practical units these formulas become:
d 2 Fh,,,(co)
-- 1.326 (cid:141) 10'3E 2 GeV 2 I A(1 + )L((2)2//~2~ 2
dO d~ \ oc ., /
2 ~ (8)
(cid:141) K2~3(~) + 1 +Yy~2 2Kl,'3(c,) 2 ~
in units of photons/s/mrad2/0.1% bandwidth, and
dfz .... (co) = 2.457 x 10~3E aeV I A -- Ks/3(y) dy (9)
dO coc ,~,,~,
photons per second per milliradian per 0.1% bandwidth.
The Bessel functions can be computed easily using the algorithms of
Kostroun 12:
X-e_/ ~ e-X )hr(hsoc }
K,,(x) = hi-x-- + 1=.,~ cosh(vrh) ( 1 )O
and
(cid:127)f
{e-' ~ )h-,(hsoc,.- c~ l
Kv(q) d# - h -7- + ~=-, e ~ J (11)
for all x and for any fractional order ,v where h is some suitable interval such as
0.5. In evaluating the series, the sum is terminated when the rth term is small,
< 01 .5 for example.
SYNCHROTRON RADIATION SOURCES
01
i ' ' I ..... x .... ~ " I ' .... ~ " I ...... ' '
r
1.0
C.9
0.01
0.001
I L L 1 , , 1
0.001 0.01 1.0 1 10
- Xc/X
.GIF .3 Universal synchrotron radiation output curve.
In Fig. 3 we plot the universal function
O6 OC
G I --- ~
(-Oc .~o&),
from Eq. (7) or (9), so that the photon energy dependence of the flux from a
given ring can be calculated readily. It is found that the emission falls off
exponentially as e -)J~ for wavelengths shorter than 2c, but only as 2 -~'3 at
longer wavelengths.
The vertical angular distribution is more complicated. For a given ring and
wavelength, there is a characteristic natural opening angle for the emitted light.
The opening angle increases with increasing wavelength. If we define ,~ as the
vertical angle relative to the orbital plane, and if the vertical angular distribution
of the emitted flux is assumed to be Gaussian in shape, then the rms divergence
,yrc is calculated by taking the ratio of Eqs. (7)/(6) evaluated at ~/= 0:
/~1( ~ ) - ~ 671~~f )y(a/SK dy
"
In reality, the distribution is not Gaussian, especially in view of the fact that the
distribution for the vertically polarized component vanishes in the horizontal
plane (~/= 0). However, ,yrc defined by Eq. (12) is still a simple and useful
THEORY OF SYNCHROTRON RADIATION EMISSION 7
measure of the angular divergence. Equation (12) is of the form:
1
cry, = - C(co/coc), (13)
7
and the function C(co/eac) 10 is plotted in Fig. 4. At oc = ,cOC ,~7( ~ 0.64/7. The
asymptotic values of ,~7( can ) be 107( obtained from the asymptotic values of the
Bessel functions and are
o'~,~~ ; co~COc (14)
7
and
,~-o --- ') ; oc >~ coc. (15)
In Fig. 5 we show examples of the normalized vertical angular distributions of
both parallel and perpendicularly polarized synchrotron radiation for a selection
of wavelengths.
1.2.3 Circular Polarization and Aperturing for Magnetic Circular
Dichroism
Circularly polarized radiation is a valuable tool for the study of the electronic,
magnetic, and geometric structures of a wide variety of materials. The dichroic
response in the soft x-ray spectral region (100 to 1500 eV) is especially
001
I . . . . 1 1 r
01
~_. c(1) - 0.8
0 ,~~o~ 0.32/y
-
_
1.0
1 1 1 ,.
0.0oi 10.0 1.0 I 01
y - c/~: c
.GIF .4 Plot of the function C(y) defined in .qE (13).