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FLUORESCENT AND SCATTERED SPECTRA : NEAR-THRESHOLD EXCITATION OF ATOMS,
MOLECULES AND SOLIDS
R. Deslattes
To cite this version:
R. Deslattes. FLUORESCENT AND SCATTERED SPECTRA : NEAR-THRESHOLD EXCITA-
TION OF ATOMS, MOLECULES AND SOLIDS. Journal de Physique Colloques, 1987, 48 (C9),
pp.C9-579-C9-590. �10.1051/jphyscol:1987996�. �jpa-00227417�
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Colloque C9, supplément au n°12, Tome 48, décembre 1987 C9-579
FLUORESCENT AND SCATTERED SPECTRA : NEAR-THRESHOLD EXCITATION OF ATOMS, MOLECULES AND SOLIDS
R.D. DESLATTES
National Bureau of Standards, Galthersburg, KD 20899, U.S.A.
Résuné Notre ligne de lumière ( X - 2 4 ) eu NSLS (Brookavhen) est conçue pour la réalisation d'expériences doublement différentielles où les spectres produits en réponse è une excitation lumineuse monochromatique peuvent être analysés 8vec une très haute résolution. L'étude et le diagnostic des transitions satellites, de la contribution des lacunes doubles ou multiples au profil des seuils d'absorption, et de la production de spectres sans satellites constituent autant de motivations importantes pour la mise en œuvre de telles expériences. Alors que les phénomènes précédents sont principalement associés à la notion de seuil de création de lacunes multiples, d'autre processus intéressants se produisent au voisinage du seuil de création d'une lacune simple. Parmi ces derniers nous wons obtenu des résultats nouveaux sur les processus résonants élastiques et Inélastiques (Raman). A très haute résolution, et en présence de une, deux, ou plusieurs résonances en dessous du seuil, les spectres Raman présentent une phénoménologie particulièrement riche. L'addition d'une discrimination de la polarisation simultanément dons le monochromatisation primaire et secondaire donne une nouvelle dimension è la spectroscople des systèmes polyatomlques au voisinage des seuils d'absorption. Les remarquables phénomènes de polarisation que nous avons observés, probablement pour la première fois, dans nos travaux les plus récents, sont une conséquence des symétries moléculaires et de la très courte durée de vie des lacunes en couches profondes. Cette forme de spectroscople de polarisation semble très prometteuse pour l'analyse de systèmes moléculaires non triviaux.
Abstract
Our beamline (X21-A) at the NSLS (Brookhaven) was designed for a class of doubly differential experiments in which spectra produced in response to tunable monochromatic photon excitation could be analyzed with high spectroscopic resolution. An important motivation is associated with satellite diagnostics, the contribution of double (and multiple) vacancy excitation to absorption profiles and the production of satellite-free spectra. While these phenomena are primarily associated with thresholds for double vacancy production, other
interesting processes occur in the single vacancy threshold region. Among these we have obtained new results on resonant eleastic and inelastic (RAMAN)
processes. At high resolution and with one, two or more sub-threshold
resonances, Raman spectra display a particularly rich phenomenology. Addition of polarization discrimination in both primary and secondary monochromatization adds an important new dimension to sub-threshold excitation spectroscopy of polyatomic
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987996
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systems. The remarkable polarization phenomena seen (possibly for the first time) in our recent work are a consequence of molecular symmetry and the extremely short lifetimes of inner-vacancy states. This form of polarization spectroscopy appears to hold significant promise for analysis of non-trivial molecular systems.
1. Introduction.
About 30 years ago Lymann Parratt discussed (in an admittedly speculative vein) a wide variety of unusual electronic configurations which might be produced in the excitation and de-excitation processes involving inner-shell vacancies.' He was led to these considerations by phenomenology encountered in x-ray spectra insofar as they seemed almost perversely to be at odds with expectations which appeared reasonable at the time. The specific pictures which Parratt proposed for
discussion were not easily accessible to calculation at the time and were perhaps richer in complexity than required by the phenomena they were intended to
address.
A relatively simple partition of the general inner-shell vacancy problem suffices to frame not only the intervening history but also the problem as it appears even now. We knew then (and still appreciate) that for sufficiently high energy excitation (above a single vacancy threshold) the production of a single well- defined inner vacancy configuration is unlikely. Instead there are population distributions among multiple vacancy configurations (shake-off states) associated with each of which there are multiply excited electronic configurations whose proper characterization remains a problem of daunting complexity even in simple systems. For possibly interesting applications to systems of non-trivial structural complexity, the barriers to progress in either the direct or inverse spectroscopic problem are very high.
The other half of the story is that even a single inner shell vacancy confounds elementary descriptions because of re-arrangements which accompany the production of such a vacancy, eg., shake-up states, environmental polarization and
configurational relaxation. Needless to say, when emission takes place (moving the vacancy to outer shells or to the valence/conduction band), additional complexity ensues. This particular problem area is one in which there has been much progress in recent decades. A considerable gathering of theoretical effort has made this progress possible and, in a general way, understandable, but the work has not proceeded without controversy nor can it fairly be said to be complete even for simple systems at the present time.
To go beyond simple systems, to entertain the possibility of multiple vacancy excitations and at the same time attend well to many-body effects seems beyond the reach of productive effort. The work to which I would like to call attention in this introductory essay took as its initial goal simplification of the
phenomenology by systematic suppression of multiple vacancy processes. In the conceptually clearest circumstance this entails study of (fluorescent) emission spectra excited by monochromatic radiation stepwise tuneable through the
threshold regions for single- and multiple-vacancy production. With such doubly differential spectroscopic capability, important studies beyond multi-vacancy effects also become accessible. These include resonant scattering, both elastic and inelastic, resonant photoemission processes, and photofragmentation near thresholds.
The desirability of such studies as suggested above was evident long before they became practical. In one rather early and difficult effort, a number of
different x-ray targets with and without filters were used to survey the near threshold behavior of x-ray satellites of C1 K@ in KC1.' This particular study 'and others of similar ilk3 depend on fortuitous coincidences between relatively
strong x-ray lines from metallic targets and the absorption edge region of a species whose emission spectra are of interest. Even with such fortuity, experiments are long and tedious, excitation is only stepwise adjustable and never with really good resolution in the excitation channel. Real progress in such work had to await availability of synchrotron radiation sources operated in a mode dedicated to the production of intense radiation. Under such conditions we obtained some good results on K6 satellite production in argon gas several years ago.'
Progress beyond such a simple demonstration of satellite threshold effects to include resonance scattering, the study of molecular families and prototypical solids requires not only a dedicated synchrotron radiation source but also a beamline optimized for such studies and dedicated to their pursuit. During 1986 we began operation of such a beamline at the (U.S.) National Synchrotron Light Source (NSLS) in Brookhaven, Long Island, NY. A number of new results were obtained before the NSLS was turned off in March 1987 for a planned upgrade. The majority of the new results are presented at this Conference as well as being reported elsewhere.
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2. Primary monochromator & secondary spectrometer characteristics
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X24-A.Primary monochromator design for our beamline (X24-A at NSLS, Brookhaven) focussed on obtaining a balance between the need to produce very well-defined primary excitation energies and the need to deliver a high enough photon flux so that secondary spectroscopy becomes not only possible but also practical in reasonable lengths of time. Toward the latter goal, the primary monochromator is designed to accept radiation from nearly 10 milliradians of orbit rotation. A t the same time this divergence is explicitly decoupled from possible pass-band degradation. Finally, monochromatized radiation is brought to a relatively small focus as is needed for optimum illumination of secondary spectrometers. Our efforts to obtain all these beamline monochromator characteristics have led to a system of appreciable complexity whose full functionality is yet to be realized.
General features of the experimental arrangement of primary monochromator and secondary spectrometer as shown in Fig. 1 . Radiation from a bending magnet is incident on mirror MI which has a pre-figured spherical surface (radius -1000 m).
The effect of finite curvatl~re in the sagittal direction is negligible thus the reflected radiation continues to diverge in the orbit plane's extension while perpendicular to this plane the radiation is collimated. Incidentally the range of articulation of the monochromator is such that the mirror, MI can be operated with a multilayer overcoating at its Bragg angle (outside the range of total external reflection). This is an important consideration since the Bragg mirror's operation can considerably reduce the heat generating flux with which the first monochromator crystal must contend.'
Radiation prepared by the first mirror is doubly diffracted by the two-crystals indicated in the ?~sual manner. The basic mechanism providing for the coordinated translation (of the second crystal) and rotation (of both crystals) follows the design of Cowan and Golovchenko6 but in a UHV realization as described by Cowan, et a1.' The rather large (ca 7 cm horizontal extent) parallel and
monochromatized beam is then focussed by the mirror M 2 at a distance of
approximately 10 m downstream. To accomplish this focussing, M2 is a toroid with fixed minor radius of 0.1 m and major radius adjustable by bending in the range
Fig. 1 Overview of the primary monochromator and secondary spectrometer used in all threshold studies described in the text. Primary radiation from the dashed orbit at right is monochromatized in the double mirror, double-crystal monochromator shown. The resulting beam is brought to a focus in the gas cell shown at the center of the experiment chamber. Scattered and/or fluorescent radiation from the target gas is dispersed in the secondary spectrometer shown and a small domain simultaneously registered in the position sensitive proportional counter indicated. Provision is made for the secondary spectrometer to be oriented 90' away from the orientation shown and remounted so as to analyze radiation coming out of the plane of the figure.
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between 700 and mm (numeric focus is obtained for R = 800 m). The resulting focal region of approximately 1x3 mm contains monochromatized photons (about 10'' sec
-'
for a 100 mA beam) in a bandwidth near 0.5 eV at energies near 3 keV.The relatively intense primary beam just described interacts with a windowed gas target as suggested in Fig. 1. Secondary spectra are analyzed by the single crystal imaging spectrometer shown. The imaging detector is a position sensitive proportional counter (PSPC) of the llbackgammonw ~ a r i e t y . ~ By locating the radiation source inside the focal circle, a spectral band amounting to a few percent of the mean energy can be registered simultaneously. A convenient feature of this secondary spectrometer design is the fixed distance from the diffracting crystal to the detector. Aside from the evident mechanical
simplification it tends to alleviate the problem of small linear dispersions at small Bragg angles which afflict customary Rowland circle designs. To obtain the needed focussing action, the crystal's radius has to be variable; one of several procedures for providing this feature was recently described by Henins.'
The target chamber provides for secondary spectrometer mounting so that its entrance' direction lies either parallel to the electric vector of the primary radiation or perpendicular to it. In addition there is the evident possibility to dispose the spectrometer's plane of dispersion either parallel or
perpendicular to the direction of the primary beam. This mounting flexibility is significant for the polarization spectroscopy described below.
3. Double Vacancy effects in emission and absorption
As mentioned in the introduction, while emission spectra produced with general or high energy excitation are appreciably contaminated with satellites, many of these can be extinguished by monochromatic excitation at energies below the threshold(s) for double vacancy production. At the same time there has been evidence available for almost thirty years that features are noticeable in absorption spectra which are plausibly associated with the opening of double vacancy production channel^.'^^^^ Around 1980 we were able to show in a rather pleasant synchrotron radiation exercise at SSRL, that this plausibility was, in fact, quite correct.' Specifically in connection with KB satellites of argon, we
found t h a t e s s e n t i a l l y complete e x t i n c t i o n could be obtained and t h a t a r a t h e r r i c h s t r u c t u r e i n t h e supra-threshold r e g i o n was well connected with t h e onset of t h e s e s a t e l l i t e s .
I n more r e c e n t work a t NSLS t h e e a r l i e r r e s u l t s on argon have been confirmed and new d e t a i l s added.'' We have now extended t h i s type of study t o molecular gases and t o s o l i d s . Our i n t e r e s t s were not only t o produce s a t e l l i t e - f r e e s p e c t r a but a l s o t o s e e how much of t h e absorption f i n e s t r u c t u r e i n t h e range 10-100 eV above threshold might be a s s o c i a t e d with opening of m u l t i p l e vacancy channels i n t h e s e more complex systems. The main r e s u l t s a r e thus f a r very simple: i t was n a t u r a l l y p o s s i b l e t o produce c l e a n , apparently s a t e l l i t e - f r e e emission s p e c t r a ; some of t h e prominent f e a t u r e s i n t h e molecular absorption s p e c t r a were r e a d i l y a s s o c i a t e d with channel openings f o r t h e s a t e l l i t e s . These r e s u l t s concerning emission s p e c t r a need l i t t l e comment i n t h a t t h e i r u t i l i t y w i l l be evident when one undertakes f a i r l y c a r e f u l c a l c u l a t i o n s concerning single-vacancy emission processes. The r e s u l t s on absorption s p e c t r a a r e i n one sense disappointing and i n another r a t h e r hopeful. I had conjectured t h a t a r a t h e r l a r g e r p o r t i o n of t h e observed s t r u c t u r a l r i c h n e s s seen i n molecular absorption s p e c t r a r i s e s from m u l t i p l e vacancy channel openings and t h a t t h i s p o r t i o n needed t o be s t r i p p e d away before t h e r e was a spectrum which could be reasonably described i n terms of s i n g l e vacancy processes.13 My sense of t h e present s i t u a t i o n is t h a t i t s o f a r appears t h a t an adequate d e s c r i p t i o n of phenomenological absorption s p e c t r a should be o b t a i n a b l e within t h e s i n g l e vacancy framework, although account w i l l s u r e l y need t o be taken of ( e l e c t r o n i c ) manyebody e f f e c t s a s well a s p o s s i b l e s t r u c t u r a l d i s t o r t i o n s .
4. The s i n g l e vacancy threshold region
Our i n i t i a l i n t e r e s t i n t h e s i n g l e vacancy threshold and sub-threshold region followed n a t u r a l l y from t h e i n t e r e s t i n g e a r l y work of Sparks1* and Eisenberger, e t a1.15 i n t h i s domain. I n comparison with t h e f a c i l i t i e s a v a i l a b l e i n t h e e a r l i e r work. X24-A o f f e r e d not only higher f l u x and somewhat improved r e s o l u t i o n i n t h e primary beam but a l s o much higher r e s o l u t i o n and/or e f f i c i e n c y i n t h e secondary spectrometer. One p a r t i c u l a r l y t r a n s p a r e n t r e s u l t obtained by
Eisenberger, e t a1.
'
concerned t h e energy evolution of Raman s c a t t e r e d r a d i a t i o n a s t h e primary beam energy was r a i s e d toward t h e threshold and then through i tJOURNAL DE PHYSIQUE
i n t o t h e f r e e e l e c t r o n continuum. The behavior seen was i n most s a t i s f y i n g accord with t h e expectations already found i n H e i t l e r ' s now c l a s s i c t e x t . 1 6 I t w i l l be r e c a l l e d t h a t t h e Raman energy tracked t h e primary energy a t a constant decrement u n t i l t h e bottom of t h e f r e e e l e c t r o n continuum was reached.
Thereafter f u r t h e r i n c r e a s e s i n t h e primary photon energy had no e f f e c t on t h e energy p o s i t i o n of t h e secondary spectrum, i . e . , Raman s c a t t e r i n g had evolved i n t o fluorescence. I n a c l o s e l y r e l a t e d experiment, Briand, e t a1." s t u d i e d t h e case of a s i n g l e well i s o l a t e d resonance (white l i n e ) i n t h e sub-threshold region of Mn K i n KMnO,. Here t h e Raman l i n e continues t o t r a c k t h e primary energy r i g h t through t h e resonance and beyond a t l e a s t u n t i l t h e f r e e e l e c t r o n continuum begins. I n t h i s case t h e Raman "red" l i n e continues u n t i l i t becomes a Raman 11blue81 l i n e .
We have pursued t h i s matter somewhat f u r t h e r with corresponding s t u d i e s on atomic argon1' and sulphur h e x a f l ~ o r i d e ' ~ with i n t e r e s t i n g r e s u l t s . Pre-edge resonances i n atomic argon a r e f a i r l y well described a s Up, 5p
...
Rydberg l e v e l s . I n t h e cases where t h e primary energy is tuned c a r e f u l l y t o each of t h e two lower resonances i n t u r n , we s e e very c l e a n n e a r l y i d e n t i c a l K a doublets. Tuning t h e primary energy between these resonances gave a q u i t e d i f f e r e n t p r o f i l e , however.Its remarkably unlovely shape precludes any understanding based on s u p e r p o s i t i o n of t h e doublets seen on resonance with 4p and 5p e x c i t a t i o n . I was wont t o r e f e r t o i t a s having undergone a process of s p e c t r o s c o p i c demolition while Cowan and Brennan saw t h a t t h e empirical p r o f i l e appeared t o be a s u p e r p o s i t i o n of two d o u b l e t s , one s h i f t e d t o lower e n e r g i e s and one s h i f t e d t o higher energies. The underlying physical process is e v i d e n t l y (resonance) Raman s c a t t e r i n g a s
discussed by Tulkki and fiberg." In t h e case a t hand, t h e p r o f i l e seems t o a r i s e from superposing two Raman s p e c t r a , one blue-shifted from i n t e r a c t i o n with t h e 4p resonance and one r e d - s h i f t e d from t h e 5p resonance. The dramatic i n t e n s i t y r e v e r s a l s encountered i n resonance Raman s c a t t e r i n g appear q u a l i t a t i v e l y a b l e t o account f o r t h e observed p r o f i l e ' s complexity.
A s s t r a n g e a s t h e s e argon r e s u l t s seemed a t f i r s t , they d i d l i t t l e t o prepare us f o r what was seen next i n SF6.19 A s is well knownz1 t h e pre-edge s t r u c t u r e of t h e sulphur K region appears dominated by a s i n g l e peak of g r e a t s t r e n g t h . 'This narrow peak l i e s about 4 eV below t h e estimated l o c a t i o n of t h e beginning of a f l f r e e - e l e c t r o n w continuum. On t h i s b a s i s and t h e r e s u l t s presented above, one
could fairly expect this to be a case more favorable than that of Mn K in KMnO, in which a "bluerr Raman line could be tracked for a considerable distance above the isolated resonance. As shown in Ref. 19, the actual spectra show radical changes with slight detuning from the main resonance in some circumstances leading to two completely resolved "arr images separated by about 7 eV. Nothing currently under consideration offers both a satisfactory access to a calculable process (or processes) and a plausible qualitative mechanism for generation of such images.
Due to the very brief period between obtaining effective operation at the NSLS x- ray ring and its shutdown, many avenues are as yet unexplored. We are
particularly interested in studies of other molecular gases (especially ones with lower symmetry) and of insulating solids such as KC1. As for the SF, puzzle, absent some insight yet to appear, we are inclined to see what else is happening when the primary beam energy is moved about near the main resonance. We could, for instance, look at electrons2hnd at molecular ion fragments. The puzzle here is genuine
-
whether or not it leads to a more generally interesting approach to molecular problems remains to be seen.5. Near threshold polarization phenomena in molecules.
This last part of my overview concerns what appears to us to be our most significant new result, namely realization of polarization specific molecular spectroscopy under near threshold e~citation.~" In conformity with what I take to be the entropic principle, namely that good and interesting things happen less often on purpose than management would like to assume, our work in this now fertile area began with certain goals in mind which though quite valid were in the end less exciting than the somewhat unexpected observation which followed.
Primary radiation is highly polarized both naturally and because Bragg
reflections near 4 5 ~ u p p r e s s radiation whose polarization lies in the plane.of instrumental dispersion. Our secondary spectrometer, when operated near a Bragg angle of 45' is likewise an efficient polarization filter. Since Rayleigh scattering preserves polarization, one is evidently permitted to dispose the secondary spectrometer toward suppression of elastically scattered radiation by polarization discrimination. When this is done, tuning through the pre-edge resonance region gives resonantly scattered signals rising from a "zerou
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background. This expectation was easily verified by re-mounting the secondary spectrometer so as to provide the needed polarization suppression in the
appropriate reference frame." For purposes of obtaining accurate yield values, fluorescent spectra were obtained with the spectrometer transparent to elastic scattering while principal spectroscopic data were obtained in the other
orientation since this gave a more favorable rate. Though-we had planned to look for polarization effects in molecular emission spectra, these were considered more to be matters of detail than to be of potentially very high interest. The changes in fact seen in relative intensities among molecular emission components were, in fact, so large as to suggest at first that previous data had been mis- measured or that the spectrometer was mal-functioning. Neither alternative was correct
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we had, in fact seen strongly polarized molecular emission which, as discussed in the next paragraphs appears to open a rather rich new area of study.To fix ideas, consider a polyatomic molecule whose pre-edge region has distinct resonance features associated with promotion of an inner-shell electron to unoccupied levels each of which has known symmetry character. One can associate with each tpansition a dipole matrix element (transition moment) m, which is a vector in the body-fixed frame of the molecule. Among a collection of randomly oriented molecules in a ga3 phase target those whose transition moments are parallel to the incoming radiation's electric vector will be preferentially excited according to E a r n
'.
This process thus selects from the initially random distribution of orientation, a sub-population preferentially aligned with the polarization vector. Subsequent transitions by which the inner-shell vacancy is re-filled by electrons originating in occupied valence orbitals similarly have transition moment vectors with well defined relationships to the molecular body- fixed frame. These are naturally polarized in the body fixed frame and hence also in the laboratory frame owing to the alignment established in the excitation step.Processes such as that just described are well-known at longer w a ~ e l e n g t h s . ~ ~ Two rather important aspects of this process in the x-ray domain commend it to further study. Firstly since inner vacancy lifetimes are very short (-10 fs), very little in the way of molecular re-orientation either spontaneol~s or collisionally driven can take place. Thus whatever alignment is initially produced will be carried into the radiative decay process without loss.
Secondly, i n a general polyatomic molecule atom-specific t h r e s h o l d s permit populating t h e unoccupied o r b i t a l s from d i f f e r e n t s i t e s . That i s t o say, t h e same MO ( o r a t l e a s t members of the same manifold) can be addressed with d i f f e r e n t l y o r i e n t e d t r a n s i t i o n moments, hence i n i t i a l o r i e n t a t i o n s can be s e l e c t e d not only by choice of which unoccupied l e v e l is t o be populated but a l s o by t h e s i t e from which t h e excited e l e c t r o n is promoted. For each p o s s i b l e i n i t i a l alignment, one can observe p o l a r i z a t i o n parameters f o r each p o s s i b l e valence MO t o inner vacancy t r a n s i t i o n . I t would appear t h a t c o l l e c t e d
s y s t e m a t i c a l l y t h e a r r a y of d i r e c t i o n cosines t h u s obtained might be useful i n a geometrical r e c o n s t r u c t i o n of t h e moment a r r a y . T h i s , i n t u r n may be s u f f i c i e n t when combined with stoichiometry and a b i t of chemical i n t u i t i o n t o c o n s t r u c t one o r more p l a u s i b l e molecular geometries. Should such a procedure u l t i m a t e l y be developed one could ( o p t i m i s t i c a l l y , of course) hope t h a t i t might prove u s e f u l f o r n o n - t r i v i a l cases.
I n t h e meantime t o s e e whether t h i s general p i c t u r e is c o r r e c t , we have taken experimental d a t a on C H , C l and t h e f r e o n s (C1 K edge) and compared observed i n t e n s i t y r a t i o changes with those obtained by ab i n i t i o calculation^.^^ The r e s u l t s a r e t h u s f a r i n good agreement. I n passing I note t h a t a p p r e c i a b l e p o l a r i z a t i o n sometimes p e r s i s t s above t h e bottom of t h e f r e e - e l e c t r o n continuum.
This is r a t h e r i n t e r e s t i n g and appears t o o f f e r a way t o gain i n s i g h t s i n t o t h e n a t u r e of s t r u c t u r e s i n t h e region above i o n i z a t i o n t h r e s h o l d s .
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