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The benefits of high resolution spectroscopy in the VUV
J.-P. Connerade
To cite this version:
J.-P. Connerade. The benefits of high resolution spectroscopy in the VUV. Journal de Physique II, EDP Sciences, 1992, 2 (4), pp.757-772. �10.1051/jp2:1992164�. �jpa-00247670�
Classification Physics Abstracts
32.20J 32.80D 32.60
The benefits of high resolution spectroscopy in the VUV
J.-P. Connerade
The Blackett Laboratory, Imperial College, London SW? 2BZ, G-B-
(Received 30 October 1991, accepted 21 January 1992)
Abstract. Recent experiments in high resolution laser-based spectroscopy extending into the VUV are described. Several motivations are discussed in particular, the study of many-
body effects and interchannel interactions. The phenomenon of autoionisation, especially when
resonances overlap with each other and interactj is singled out, because it provides examples of
several novel effects whose occurrence has only been established through high resolution exper- iments. The theory of these effects is outlined and several examples are given. The techniques involved in the data acquisition involve both thermionic diode detection and magneto-optical
rotation as well as photoabsorption experiments.
1. Introduction.
Developments in optics and in high resolution spectroscopy have always gone hand in hand.
Thus, improvements in interferometry and the invention of the tunable dye laser (to quote but two examples) have found immediate application to the study of atomic and molecular spectra.
Over many years, both themes have been pursued simultaneously at the Laboratoire Aim]
Cotton and also at the Blackett Laboratoryj Imperial College, with new ideas being frequently exchanged between our two institutions. It thus gives me particular pleasure to contribute a
brief review of some recent developments involving the Laser Optics and Spectroscopy group
at Imperial College to this volume in honour of professor Jacquinot, whose leadership and inspiration of the Aim] Cotton Laboratory are much admired by all of
us.
Precisely what one means by high spectral resolution is a matter of definition and depends
rather markedly on the energy range. Through the VUV and soft X-ray rangesj a convenient
experimental working rule is the following [I]: any resolving power higher than
~w 100 times I where I is expressed in I
can be regarded as high resolution. Thus, for example, at 20001,
any resolving power I/Al > 200000 corresponds to high resolution but, at 1001j
even 10 000 is a high experimental resolving powerj and, as one enters into the crystal instrument range,
at about 101, 1000
or more is a high resolving power. The reason for this rule is simple:
the resolution Al of classical dispersive instrumentation throughout this range is limited not
by diffraction but by detector size. Given that the latter is constant and that the maximum
practical dispersion in I
per unit length is also roughly constant
j
then Al is also fixedj and the
resolving power I/Al drops roughly linearly with wavelength. This argument, however, ceases
to appIy to lasers: when one considers the various strategies which can be exploited to achieve short wavelengths (for example up-conversion), clever choices of the atomic transitions can, in principle, transport a high resolution normally associated with optical or near-ultraviolet
frequencies to higher energies. Whatever their virtues, one must remember that lasers are still limited in tunability and coverage, so that advantages of this nature cannot readily be extended to the full energy range.
The highest resolving power is associated with CW laser operation. However, the best performance in the UV and VUV comes from pulsed lasers, and so we will confine our attention to excimer pumped tunable dye systems. As a rough guide to the performance of such lasers, we find that, without any intracavity Fabry-Pdrot dtalon, the bandwidth is typically about 5 GHz,
while with an dtalon, even though several modes may still be present, the bandwidth may be reduced to about 1.5GHz. If several photons are used in a nonlinear up-conversion scheme,
these figures should be increased somewhat, and we believe that the former
one should be about 8GHz in practice. At 15001, this yields a resolving power of 2.5 x 10~, which comfortably
satisfies the above criterion, even without an dtalon.
The sum just given does not actually express the full advantage of the laser, because the
question of detection has not been addressed: in order to achieve really high spectral resolving
power by classical spectroscopy, one often resorts to nonlinear detection by photographic plates.
Thin is for two reasons: first, the grain size of the plate is very small (about 10pm) and,
secondly, with classical radiation sources, one is rapidly starved of photons as the dispersion is increased, so that long accumulation times are convenient. With laser sources, such limitations do not exist. Consequently, a variety of linear detectors can be used, some of which, such as the thermionic diode [8], have specifically been developed for laser spectroscopy and possess
many advantages over the conventional photoabsorption techniques, being not only linear in response but free of opacity effects.
An upper limit to the useful resolving power is set by the sample. In the VUV, subdoppler spectroscopy does not usually bring any advantage, which sets
an upper bound of the order of
one million or so. Thus, the tunable pulsed laser sources are close to being ideal.
In the work reported here, the resolution of synchrotron data varies between 0.01 and 0.051 while pulsed lasers do somewhat better, of the order of 0.0061, in the VUV without
an
intracavity dtalon, depending on how many photons are involved. Thus, in most of the spectra shown here, the resolution is too small a number to show clearly on the figures.
The benefits of high resolution spectroscopy for the study of atomic and molecular spec-
tra in the vacuum ultraviolet are not immediately apparent. As one crosses the ionisation
and dissociation thresholds into the adjoining continua, the phenomena of autoionisation and
predissociation broadening set another intrinsic limit above which there is not much purpose in increasing the experimental resolving power. As will be shown, this superficial conclusion must be qualified, or at least the useful limit extended to much higher resolving powers than
one might expect. Indeed, it is only with the advent of tunable lasers that sufficiently high resolving power has been reached for many interesting and novel effects to be explored.
Although in principle the capability has existed for a long time to harness synchrotron radia- tion with classical high dispersion spectroscopy, this area received little attention until recently in National Synchrotron Radiation laboratories and few experiments at high resolution were
performed in thi& way. The situation is now changing rapidly: experiments conducted jointly by the University of Bonn and Imperial College demonstrated the value of high resolution
VUV studies of autoionisation in atoms and molecules. Presently, instruments of high resolv-
ing power are being commissioned at Photon Factory in Tsukuba (Japan) and at Surf (USA).
These are probably the most advanced S-R- facilities currently available for high resolution photoabsorption spectroscopy in the VUV range.
Over a much more restricted energy range, tunable coherent sources provide a much higher performance by which I mean both better resolution and a much larger flux than particle
accelerators. Also, they have the attraction that they can be developed and operated within
a more modest laboratory than synchrotron radiation sources. Recently, we have developed
and operated a tunable coherent four-wave mixing source at the Blackett Laboratory, Imperial College, and several examples of its use will be given.
Finally, one should mention the continued development to extend the range of interferometry
into the vacuum ultraviolet. Drs. Anne Thome and Richard Learner have achieved
a notable
breakthrough in the design and operation of a Fourier Transform Spectrometer for the VUV range. I believe that this compact instrument of high light grasp will revolutionise emission spectroscopy in a manner professor Jacquinot will find especially appealing. It will also provide
laser spectroscopists with a new selection of high precision standards in the VUV at the very moment at which they are required.
2. Some motivations for pursuing high resolution atomic spectroscopy.
The study of highly-excited states of free, neutral atoms can be justified from several angles: it h of value as a reference point to molecular, and also to solid state physicists. More importantly,
it provides examples of certain physical effects which can be calculated in more considerable detail than is possible in most branches of physics. At the most fundamental level (but leaving
out nuclear interactions), this involves various kinds of many-body effects or correlations: the free atom is an isolated few-body system governed by the rules of quantum mechanics, and held together by the inverse-square law of force between its constituent parts. This may seem a
very obvious point, but actually many-body effects in atoms deserve more attention than they
have traditionally received. Since most high resolution spectroscopy has been conducted in the
optical range, where photons do not possess enough energy to excite more than one electron at a time, many-body effects are often only probed at the perturbative level. At the opposite
extreme, X-ray spectroscopy ejects electrons of high energy so quickly and the lifetimes of the excited states is so short that the electrons and the vacancies behave essentially as independent particles. It is only in the intermediate range (the VUV and soft X-ray wavelengths) that the
quasi-particle picture breaks down hence the importance of VUV spectroscopy.
To the chenfist, this range is also very interesting, because it involves the possible partici- pation of inner valence electrons, I-e- the first closed subshell below the optical electrons. In
some situations, these electrons remain unmoved by the excitation (we call them spectators) while, in others, they become involved in the excitation process to the extent that it is difficult to give clear nl labels (n is the principal quantum number and I the angular momentum) to
individual electrons. This is a breakdown of the independent particle approximation. Simi- lar problems arise in chenfistry when considering bonds between certain types of atoms, and
experience shows that these usually involve d off electrons.
A full understanding of the breakdown of the independent particle model does not exist, but certain types of breakdown, which are essentially of a perturbative nature, are well understood.
For example, the simplest kind of many-body coupling, in which an electron in an excited bound state of one channel 'leaks' into the continuum of another channel has been extensively investigated. We begin from the simplest examples of this effect and show how its study has
progressively been refined to include more and more new phenomena, as a result of experiments performed by high resolution spectroscopy.
# 600 WI
400
2346 2351 2356 2361 2366
Wavelength (1) Fig-I- Experimental profile of the 5d5f~Pi doubly excited
resonance in BaI as observed using a
thermionic diode detector and a tunable dye laser [38], at a resolution of1.5 Ghz. The smooth curve is obtained from equation (I). Note that the assumption of a flat continuum background is reasonable
over the width of the resonance. Most of the discrepancy between theory and experiment occurs in the wings of the profile.
3. Autoionisation.
The process just described is, of course, autoionisation broadening. If the excited state is isolated from all others and only one continuum is present, then the simple formula:
~~~~ ~
(q + C)~
l + e2 ~~~
due to Fano [2] provides a fairly good description of the resonances, provided the background
continuum can be regarded as 'flat' in absence of the resonance. The quantity q (the shape index) is a constant for the profile of a given line, but depends both on the initial and the final state, while the width r of the line depends only on the excited state. Figure I gives
an example of how high resolution spectroscopy may reveal departures from the Fano formula
(mainly in the wings of the line) and gives some idea ofthe extent to which the approximation
of a 'flat' bakground may apply in practice.
Of course, even within the independent electron approximation, truly isolated levels do not
occur, and thus our simplest expression for autoionisation should not be [I] above, but should include the fact that the excited level will, in general, belong to a Rydberg series. Formulae for the profiles ofRydberg series broadened by autoionisation have been given by Dubau and
Seaton [3] and, independently, by Giusti-Suzor and Fano [4]. A convenient form of the Dubau- Seaton formula is given by Connerade [5] as:
°~~~ °°
tan~ xv + 2C tan xv + D2 ~~~
Ye 5417s si,~6d
Mg'
3p3d 'Pi 3pGs~i ~~p,,~
~," Type2
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TypeI
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l150 1200 1250 1300 980 990 1000 1010 1020
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Fig.2. Distinction between type I (nearly isolated) and type 2 (overlappipg) profiles, as revealed both by theory (Eq. (2)) and by experimental examples, obtained in photoabsorption investigations
with synchrotron radiation at a resolution of about o.05 A. The theoretical curves are obtained from
equation (2) for different values of the shape parameter C shown on the figure, and fixed values of the parameters D
= 1.2 and (a) B = -o.528, (b) B
= +o.528. The symbols H and )H stand for the Hydrogenic and half-hydrogenic energies (n integral and half-integral) respectively.
where B, C and D are shape indices, assumed constant for the whole series, and v is the energy variable of quantum defect theory.
The form of such profiles is somewhat more general than t§at of equation (I) and includes two types of profiles, the first corresponding to the isolated line limit, where the width of the line is much smaller than the Rydberg spacing and the profile tends towards the Fano shape
and the second corresponding to the overlapping line limit, where the profiles tend towards sinusoidal shape. The distinction between them is illustrated in figure 2.
An expression close to the K matrix form
on which the Dubau-Seaton parametrisation is based was derived early on by Mies [42], who first considered explicitly the problem of over-
lapping autoionising resonances, and demonstrated the need to generalise the standard theory
for isolated resonances, in order to achieve a reliable parametrisation. However, Mies (42) was
unaware of the general rules concerning q-parameter variation within a channel, and therefore
did not obtain the equations given in the present paper.
4. Classical absorption versus laser ionisation spectroscopy.
At this point it is perhaps useful to comment on the experimental methods used to observe line profiles such as the ones in figures I and 2.
Photabsorption spectroscopy is a convenient method of observing short-lived autoionising
states: it is superior to emission spectroscopy, which requires very high electron density for
autoionising transitions to become readily observable, and was therefore favoured by Beutler [6].
One of the main advantages of photoabsorption spectroscopy when pursued with photc- graphic plates, reticons or charge-coupled devices is simultaneous detection at several wave-
lengths (the sc-called parallel advantage) which, in addition to saving time in survey experi- ments, evens out small variations due to fluctuations in the conditions ofthe absorbing column.
Photoabsorption spectroscopy requires a background continuum source, and is therefore usually pursued at electron storage ring or synchrotron sources in the vacuum ultraviolet.
This is not simply a question of wide spectral range: the higher the spectral dispersion of the apparatus, the longer the acquisition time of the experiment becomes unless the input flux is
correspondingly increasgd. Thus the practical limits to resolving power are set just as much by
the brightness of available laser sources as by the nature ofthe spectrographs or spectrometers available.
Further disadvantages of photoabsorption are the insensitivity and poor signal-tc-noise condi- tions of the detection: in photoabsorption experiments, one seeks to detect a small attenuation in a large signal. If the attenuation is too large, opacity effects must be corrected for, and the
low count rate gives poor statistics. If it is smaller than about 2 $l, experience shows it is
undetectable on a high flux backround. One also has best statistics where the cross section to be measured is smallest and worst statistics where it is largest an unfavourable combination.
These problems are all avoided if one measures ionisation signal, particularly if one applies
thermionic diode techniques [7,8] which allow large gain to be achieved. Such detectors provide
a dark background, a linear dynamic range over several decades of intensity, proportionality
to the cross-section in absence of multiple ionisation effects (I.e. below the double ionisation
threshold) and, if properly used, are virtually free of any distortions due to opacity. They
can, however, only be used if illuminated with a single wavelength, and one therefore loses the
parallel advantage.
Very few attempts have been made to combine synchrotron radiation with thermionic diode detectors. An example [9] is shown in figure 3, which demonstrates both the value and the
c~ 3d6p
3eCilOn
Xib 3d5p
So
40
20
to Mop
Energyin cm-' 0
445X 48058 St6tt SSt67 %722 62278 6S833 69389 72944
Fig.3. The 3dnp doubly-excited spectrum of CaI (after Ref. (9)). Note the fluctuation in widths of the autoionising series, which leads to a sharpening of the 3d6p line as a result of perturbation by a 4p5s intruder, I,e. to stabilisation of a resonance in the continuum by a perturbation. The resolution here is only about o-ISA but this experiment uses a thermionic diode detector together
vdth synchrotron radiation.
limitations of this approach. The method is certainly much superior to conventional photoab- sorption spectroscopy for fairly broadband applications, provided enough flux is available. The
signal-tc-noise achieved is excellent, and even the smallest features detected in figure 3 are real
(as later experiments have confirmed). However, the need for very high flux sets a limit on the
resolving power which can be achieved, even with
a very powerful synchrotron radiation source.
The data of figure 3 were actually obtained from
a scanning toroidal monochromator of high light grasp, placed behind the wiggler beamline at the Hasylab laboratory. Most synchrotron
radiation bearnlines would not provide enough brightness for such experiments.
Usually, the thermionic diode detector is combined with a tunable laser source. There are then two ways of achieving energies corresponding to VUV excitation: one may either use two or three-photon excitation or generate a beam of coherent short wavelength light by non-
linear nfixing in some external cell. Both approaches have been used to good effect, and the differences between them are interesting.
For example, inner shell transitions in HgI vapour are well known since the early work of Beutler [6], the great pioneer of VUV photoabsorption spectroscopy: excitation from the 5d subshell gives rise to two types of Rydberg series, namely broad and intense np lines and sharp, weak nf lines. Such, at least, is the appearance of the spectrum when excited by single photon spectroscopy, and it has been reinvestigated several times, by Garton and Connerade [10], by
Mansfield ill], by Baig [12] and by Sommer [13] under progressively improved experimental
conditions. If, however, one excites the spectrum by a two plus one photon process involving a many-body relaxation from a twc-photon excited ns state followed by a single photon transition in which a core electron is excited, then the angular momentum selection rules allow the same final states to be reached, but nevertheless a very different experimental result is obtained: in
