We focus on a few highly promising areas of research, selected both because of their scientific interest and because of a general scarcity of previous results.

One of the focus areas for this program is molecular photoionization and photofragmentation dynamics following deep-core-level absorption. In order to unravel the multitude of possible decay paths (radiative, nonradiative, and fragmentation), ion, electron, and x-ray-emission spectroscopies need to be coupled, in coincidence when feasible.

A second focus area is on the effects of non-electric-dipole processes on deep-core-level photoionization, most readily revealed in photoelectron angular distributions. Recent work by Krässig et al.¹ and our own preliminary results have demonstrated the accessibility of this phenomenon with bright SR sources. Aside from intrinsic interest in this area, it is important to determine the extent of these effects on measurements in a variety of fields that rely on the widely used technique of photoelectron spectroscopy.

The third area focusses core-level resonant-Raman spectroscopy, both radiative (x-ray emission) and non-radiative (resonant-Auger emission). Preliminary studies have illustrated both the promise of this technique as well as the extreme difficulty of the measurements due to the need for high flux and brightness.

The fourth area focusses polarized x-ray-emission spectroscopy of molecules. Previous results with a dedicated beamline and endstation at the NSLS showed the great promise of this technique for structural and dynamical studies of molecules.

Finally, the fifth focus area is a first look at transient atomic species and free radicals in the deep-core-level region, building upon our initial measurements at lower energies (i.e., K edge of atomic O). To our knowledge, no such experiments have ever been done.

Theorists experienced in x-ray interactions with atoms and molecules are explicitly included, indicating a serious commitment to rely on theory not just after the fact, but also in planning and performing the experiments. For example, this strong linkage was crucial in determining appropriate design geometries for the electron-TOF system in order to best measure non-dipolar photoelectron angular distributions.

1.) Molecular Photoionization and Photofragmentation Dynamics

For molecular gases, the vast majority of core-level studies have been on shallow core levels.^2,3 Because the primary decay mode for shallow core holes is Auger-electron emission involving two valence electrons, a total molecular charge of 2+ typically is produced, usually leading to dissociation. In shallow core levels, the experimental pursuits of the understanding of ionization and fragmentation dynamics (e.g., stepwise vs. simultaneous dissociation), as well as the possibility of selective fragmentation⁴ have been driving forces behind the development of new SR-based techniques. Of particular importance, beyond single-particle detection such as photoelectron spectroscopy² or ion spectroscopy,³ are coincidence techniques such as photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy.^5,6

In contrast, photoionization and photofragmentation studies of deep core levels of molecules have been rare.⁷ When a molecule absorbs an x-ray via a deep-core-level transition, there are many more decay paths, usually through a cascade of events, by which this excess energy is dissipated. The primary decay process is Auger emission, most commonly involving shallow core electrons on the atom at which the initial absorption took place. This propensity to produce two shallow core holes from a single deep core hole, both of which also can Auger decay, drives the creation of very highly charged molecular species (4+ and higher). For molecules, this degree of ionization immediately leads to dissociation, typically into atomic ions.

In this context, there are two phenomena, first observed in our preliminary measurements on deep core levels,⁸ that we will pursue. The first phenomenon is attributed to the effects of post-collision interaction (PCI). In experiments on HCl and H₂S, and their deuterated counterparts, distinct changes in charge-state (i.e., Clⁿ⁺ and Sⁿ⁺) yields for photon energies at and just above the K-shell thresholds of Cl and S were observed. Increases in yields for the more highly charged ions, attributed to PCI, are accompanied by decreases in yields for the lower-charge ions as the energy is increased above threshold. As far as we know, molecular photodissociation mediated by PCI has never been studied before; indeed, little is known about the effects of PCI on molecular photoionization. It is likely that more detailed measurements of this phenomenon will provide insight into the electron-correlation phenomena inherent to PCI. For example, the asymmetric potential which the outgoing electron experiences in a molecule, as opposed to the spherically symmetrical potential of an atom, could provide an internal "probe" of the details of electron-electron interactions. We will extend PCI studies systematically into the molecular realm, focussing initially on relatively simple systems (e.g., HI, CH₃Cl, COS, SO₂). Undertaking this challenge requires coincidence measurements between ions and low-energy electrons, a capability available with the endstation. We also will perform x-ray photon-ion coincidence measurements in order to "turn off" PCI when an x-ray is emitted instead of an Auger electron.

The second phenomenon is related to our recent observation that selective excitation to antibonding orbitals in HCl and H₂S can lead to dissociation of a neutral hydrogen atom. Also intriguing is that evidence suggests that the neutral H is primarily coupled to production of either a Cl³⁺ or S²⁺ ion. While neutral dissociation is not uncommon following shallow-core-level excitation to antibonding states, we believe this is the first observation of neutrals produced following deep-core-level excitation. This observation suggests the possibility of extremely rapid dissociation in competition with deep-core-hole decay. Alternatively, it is possible that the Cl or S K holes in the initially excited states first undergo resonant-Auger decay (i.e., K-LL) to a singly charged molecular excited state, followed by neutral dissociation in competition with the Auger decay of the (shallow) L core holes. In either case, the dynamics of the process are unusual and not well understood.

In order to determine the dynamical processes that lead to PCI effects on ion yields and to the production of neutral H following deep-core-level excitation, we will undertake more detailed measurements using coincidence techniques. The need for coincidence measurements is beautifully demonstrated by measurements on N₂O and CO₂⁹ and BF₃.¹⁰ In both experiments, core-level resonant excitation induces a change in the molecular geometry, leading to different dissociation pathways than those observed following core-electron ionization at photon energies above threshold. Such changes in fragmentation pathways as a function of excitation can only he elucidated by PEPIPICO spectroscopy, in which fragment momenta and correlations among those momenta are measured. Details of the many-body fragmentation process are of particular interest because the final momentum partitioning is closely related to the time evolution of the system on the femtosecond time scale; in essence, the bond-breaking process itself is being probed. The ion-TOF portion of the endstation, in which a simple channeltron electron detector or a hemispherical analyzer can be coupled with the ion-TOF analyzer, is well-suited to these studies because it permits selection of specific electronic states of the excited molecule for which subsequent fragmentation can be studied in detail via ion-ion coincidence measurements.

2.) Non-Electric-Dipole Effects in Photoionization

In this focus area, the term "non-electric-dipole" (or simply "non-dipole") is used to signify electric-quadrupole (E2) and magnetic-dipole (M1) effects on photoionization. For interactions of atoms and molecules with x-rays, the most easily observed manifestation of effects beyond the usual electric-dipole (E1) approximation are changes in photoelectron angular distributions. The dipole approximation refers to the assumption that the expansion of exp(ik·r) for the interaction with the radiation field can be set equal to unity if k·r is small. This approximation leads to the well-known angular-distribution equation

where the differential cross section, d (h)/ d, describes the probability for photoelectrons to be emitted into the solid angle d. The quantities(h) and (h) are the total photoionization cross section and the angular-distribution parameter, respectively. Their values are determined by the details of the ionization process as a function of photon energy. The angleis measured between the emission direction of the photoelectron and the polarization vector of the ionizing radiation. For photoionization studies of valence or shallow core levels, where the wavelength of the visible or ultraviolet incident radiation is two to three orders-of-magnitude larger than the dimensions of the absorbing or radiating system, Eq. (1) usually suffices. However, for deep-inner-shell work, where the photon wavelength may be as small as the atom or molecule itself, a more precise approximation is:

For our experimental geometry,is the angle between the propagation vector of the ionizing radiation and the projection of the electron emission vector into the vertical plane containing the x-ray beam. The anglecan vary from 0^o to 180^o. The "non-dipole" parameters,and, arise solely from electric-quadrupole processes.

For rare-gas atoms, theoretical work^11-14 has determined the approximate extent of the expected departures from electric-dipole photoelectron angular distributions for several core and valence levels, as well as the potential for using such measurements to obtain atomic parameters for comparison with theory. These results lead to several conclusions concerning the usefulness of measuring accurate photoelectron angular distributions with x-rays. One key conclusion¹⁴ is that non-dipole corrections to dipolar angular distributions disappear if measurements are taken in a plane perpendicular to the propagation vector of the incident radiation. Theory also indicates that deviations from the dipole approximation may be significant even near ionization thresholds. For ns subshells, the parameter, within the present approximations, is zero, rendering photoionization studies of ns photoelectrons simpler to interpret. This is particularly true at photon energies where relativistic effects also are expected to be small.¹¹ For np subshells, the term comes into play due to core relaxation during the ionization process, providing a unique approach to studying this phenomenon. These indications from theory make it clear that photoelectron angular-distribution measurements can become an increasingly important tool for the assessment of atomic-structure information.

Little experimental work has been done to determine the extent and importance of non-dipole effects on photoionization. Early work¹⁵ on the rare gases, using Al K radiation, was the first to observe non-dipolar angular distributions, but could not provide a comprehensive test of our understanding of this phenomenon. Very recently, a photon-energy-dependent study¹ looked at a few deep core levels in rare gases and found reasonable agreement with theory. Our initial non-dipole measurements¹⁶ at the ALS indicate that the extent and significance of these effects is larger than might be expected. Indeed, we observed that non-dipole effects can be significant not just for core levels but for valence levels as well, even at photon energies well below 1 keV. Determination of non-dipole effects on photoionization can provide an even more detailed handle on electronic structure and dynamics in atoms, just as measurements of photoelectron angular distributions, begun almost three decades ago, gave a first glimpse beyond total and partial cross sections. Furthermore, precise experimental information on these effects is likely to provide an incentive for more detailed theoretical calculations than those achieved to date, which in turn may lead to additional insights into many-body aspects of inner-shell dynamics.

We will perform a systematic study of non-dipolar angular distributions, beginning with a complete study of all the rare gas subshells (core and valence) with binding energies up to 5 keV. After this initial effort to solidify the experimental technique and for detailed comparison with theory, we will move on to open-shell atoms and molecules.

Aside from a fundamental interest in non-dipole effects, a wide variety of x-ray-photoionization studies, including gas-phase, surface-science, and materials-science work, may need to account for their influence. However, most such experiments assume that non-dipole processes are unimportant. While this assumption appears to be valid for integrated cross sections¹⁷, our work shows that this assumption may be invalid for differential cross sections even at relatively low photon energies. For example, in the work that reported the first non-dipolar angular distributions¹⁵, theparameter for Ne 2p photoionization was reported near 1-keV photon energy. Our recent results from the ALS disagree with these values, a difference which we attribute to the unaccounted influence of non-dipole effects in the original measurements. The message in these preliminary results is clear; a comprehensive study of non-dipole effects on photoionization, to test the general validity of x-ray-photoionization measurements, is warranted. The endstation has two instruments (the electron-TOF system and the large hemisphere) that are specifically designed to address this issue.

3.) Electron-Correlation Phenomena Studied Via Electron-Ion and Photon-Ion Coincidence

Theoretical prediction of the energy dependence of double ionization of helium by a single photon is a fundamental problem in atomic physics which requires solution of the Coulomb three-body problem. In the independent-particle framework, in which the photon interacts directly with only one electron, double photoionization can proceed only by electron-electron correlation. As a result, photoionization of helium has long been used as a testing ground for understanding correlation phenomena.

Despite the fundamental importance of the problem, available data for double ionization in helium are definitive only for a few eV above the He²⁺ threshold at 79 eV, where the ratio of double-to-single ionization is in good agreement with Wannier theory.¹⁸ At higher energies, theoretical predictions^19-21 vary by as much as a factor of two. Experimental results^22-24 for the He²⁺/He⁺ ratio show that it rises to a maximum of 3-5% in the photon-energy range between 150 and 300 eV, then falls gradually to 0.8-1.7% at photon energies of 10 keV or higher. Reduction of these differences among experiments and between experiment and theory is necessary in order to improve our understanding of electron-correlation phenomena.

For example, even the existence of an asymptotic He²⁺/He⁺ ratio due to photoionization is not yet determined conclusively because of the onset of Compton scattering above 4 keV, before an asymptotic limit in the photoionization ratio is reached. Very recent calculations by Andersson and Burgdörfer²⁰ differ from those of Hino, Bergstrom, and Macek²¹ not only in the asymptotic limit, but in the energy behaviour in the 4-6 keV photon-energy range where both photoionization and Compton scattering contribute significantly to double and single ionization. We perform coincidence measurements to identify the separate photoionization and Compton contributions to the double-to-single ionization ratio. These new measurements will focus on the 4-6 keV photon-energy range, a region for which ALS Beamline 9.3.1 is well-suited.

In single photoionization, the photoelectron receives excess energy, E, according to the Einstein equation, E = h - BE, where BE is the electron's binding energy. In double photoionization, E is shared between two outgoing electrons, producing a distribution expected to be peaked strongly near kinetic energies of 0 and E. In contrast, a peaked distribution is not expected for ionization due to Compton scattering. Thus, photoionization can be distinguished from Compton ionization in an experiment that uses an electron-energy analyzer with a broad pass energy centered near E as the trigger for an ion-TOF analyzer. In previous work²⁴, we measured He²⁺ at about 0.02 Hz at 5 keV photon energy. With an existing cylindrical mirror analyzer available to provide coincidence selection, the coincidence rate should be about 0.0002 Hz, based on geometrical effects, analyzer and detector efficiencies, photoionization and Compton cross sections, and increased x-ray brightness at the ALS. (n.b., We have successfully measured Xe⁸⁺ at this count rate in coincidence with Auger electrons because the coincidence requirement eliminates almost all background.) Thus, two weeks of running at the ALS should provide a ratio for double-to-single photoionization in He at better than the 10% level. Incorporation of a gas jet, already under development, should improve the coincidence rate by an order of magnitude.

Alternatively, in order to determine directly the extent of Compton double ionization, we will develop the capability of x-ray-ion coincidence measurements (see Sect. IV). In these measurements, the inelastically scattered x-ray in the Compton process is used as a trigger for the ion-TOF analyzer; photoionization will be effectively discriminated against by the photon coincidence requirement.

In addition to measurements of electron-correlation effects in helium, the power of coincidence measurements also will be directed to studies of electron correlation manifested in inner-shell-decay processes. For example, we have shown²⁵ that detection of Ar recoil ions produced by inner-shell photoionization, in coincidence with Auger electrons which characterize the K-shell vacancy cascade, results in a much simplified photoion distribution. As a result, the photon-energy dependence is distinct and amenable to interpretation because the ensuing Arⁿ⁺ charge-state spectrum is not superimposed on a background of the same charge states produced by many other processes with varying thresholds and energy dependences.

These electron-ion coincidence measurements allowed us to study²⁵ in more detail Rydberg shake-off of np levels, populated by resonant excitation of 1s electrons, and the recapture of the photoelectron through post-collision interaction. We will extend these studies, first to other rare-gas core levels, then to inner shells in small molecules. To augment the powerful electron-ion-coincidence technique, we also will study photoion distributions coincident with x-ray emission. Because x-ray emission significantly reduces vacancy multiplication relative to Auger emission, simplification of charge-state spectra will be even more apparent than that observed in electron-ion coincidence measurements²⁵.

4.) X-Ray Resonant-Raman Spectroscopy and Post-Collision Interaction

In the vicinity of core-level ionization thresholds, atomic photoexcitation and the ensuing Auger-electron or x-ray emission can take place primarily as a single second-order quantum process; the second-order (A·p)² process overwhelms the first-order (A·A) contribution because of resonant enhancement.²⁸ In this picture, it is clear that the width of the emitted x-ray^27-29 or Auger³⁰ line reflects only that of the incident radiation and hence can be narrower than the natural lifetime of the core-hole state. Also, as the incident x-ray energy is tuned through the threshold region, resonant-Auger^31,32 or x-ray-emission^29,33,34 lines display linear dispersion, and their intensities trace out core-hole-state lineshapes. The line-narrowing and linear-dispersion phenomena inherent to resonant photoexcitation and decay processes can be viewed as natural consequences of energy conservation.

More subtle consequences of single-step photoexcitation and decay appear when overlapping intermediate states introduce interference effects into the resonant process.^35-37 Pioneering experimental work focussing on interference effects in x-ray resonant-Raman studies has been difficult because of the need for very high energy resolution for both the incoming photon beam and the emitted electrons or x-rays.

As a general goal of this focus area, we will perform a detailed exploration of interference effects in x-ray resonant-Raman spectra of atoms. We also will use line-narrowing and dispersion effects as tools to probe deep-core-level excited states in molecules for the first time, and study the role resonant-Raman effects play in the evolution of atomic inner-shell dynamics. For the resonant-Auger (electron) measurements stated herein, high energy resolution (<O.5 eV) in both the incident x-rays and resonant-Auger-electron measurements will be required to elucidate interference effects. The endstation's large hemispherical analyzer is capable of performing these measurements at BL 9.3.1.

For resonant x-ray emission, high incident and emitted photon-energy resolution (<O.5 eV) also will be necessary to observe interference effects in resonant decay processes. The endstation's x-ray-emission spectrometer is capable of per-forming these measurements at BL 9.3.1.

Radiative³⁸ and non-radiative^37,39-41 measurements at core-level resonances have been performed on systems that illustrate interference effects as a perturbation to the results expected for "isolated" resonances. To better understand this phenomenon, we will search for systems and conditions for which interference effects are strongly enhanced. We will initiate this search by looking beyond closed-shell rare-gas species to open-shell atoms such as atomic S and Cl, where multiple K edges exist in close proximity, as well as in diatomics (e.g., HCl at the Cl K edge) and triatomics (e.g., H₂S at the S K edge), where the presence of many electronic and vibrational energy levels guarantees significant overlap of excited states. In these exploratory measurements, we will search for anomalies in the intensities of emitted electrons or x-rays as a function of the photon energy of excitation just below the respective core-level thresholds. These anomalies might appear, for example, as non-Rydberg-like intensity variations in atomic excitation series, or as non-Franck-Condon intensity variations in molecular excitations. Clearly, observation of these effects requires high resolution for both the incident x-rays and the analyzer monitoring the emitted particles (i.e., electrons or x-rays).

Using line-narrowing and dispersion effects as a tool, one application is to determine the "anatomy" of x-ray-absorption edges; absorption cross sections can be decomposed into individual components due to electron excitation to valence or Rydberg states and to ionization to the continuum. We will apply these "tools" to core levels in rare gases in which strong Coster-Kronig or super-Coster-Kronig resonant-Auger decay channels induce very rapid decay and thus very large natural linewidths, obscuring spectroscopic details in photoabsorption spectra. Examples include the Ar-L₁, Kr-M_2,3, and Xe-N_2,3 edges at lower energies, and the Kr-L₁ and Xe-L_2,3 and - M_2,3 edges at higher energies. Not only do these edges have large natural linewidths, rendering the line-narrowing in resonant-Raman measurements particularly useful, but the presence of Coster-Kronig decay may lead to unexpected variations in Rydberg and/or continuum intensities.

Another potential application of line-narrowing and dispersion effects is the study of extra-atomic influences.^42,43 For example, it is well-known that atomic energy-level shifts caused by metallic environments can open intense radiationless channels that grossly alter hole-state widths;⁴⁴ such effects could be studied in detail, especially near thresholds, and traced to modifications of the wavefunctions in the pertinent Auger matrix elements. We will extend x-ray resonant-Raman spectroscopy into molecular systems, where Rydberg series are complicated by the presence of molecular orbitals, vibrational excitation, and crystal-field splittings.⁴² Even for small molecules such as H₂S, the S K-edge absorption spectrum is not completely understood because of the overlap of the two lowest-lying unoccupied orbitals, both of which are broader than their energy separation. Resonant-Auger and resonant x-ray-emission measurements can provide a means to view beyond the natural width of these states to get at the detailed spectroscopy. Other molecules to be studied include Cl-containing methane and ethane derivatives and sulfur-bearing species such as SF₆, COS, and CS₂.

Intimately related to the subject of x-ray resonant-Raman spectrometry is the question of inner-shell dynamics, i.e. what mechanisms, in different energy regimes, lead to the various deexcitation channels populated following atomic photoabsorption.⁴³ As mentioned above, near-threshold photoexcitation and dercay can be treated as a single quantum process. At incident x-ray energies well above threshold, on the other hand, decay is well-described by a two-step process in which the decay step is separated and insulated from the excitation step by a relaxation phase. For core-hole decays involving Auger emission, these two different dynamical regions are linked by an electron-electron correlation phenomenon known as post-collision interaction (PCI).^46-48 Present capabilities have made it possible to gain information on such PCI phenomena as the "no-passing" effect⁴⁹ and photoelectron recapture;^48,50 but much more remains to be explored. One example is provided by our preliminary work on atomic O at the K edge, where effects due to PCI are clearly observed in partial-ion-yield curves. We will extend these studies, using ion and electron spectroscopy, to a variety of atomic and molecular transient species (see Studies of Transient Atomic and Free-Radical Species) in order to probe the detailed dependence of PCI on valence-shell electronic structure.

5.) Polarized X-Ray-Emission Spectroscopy

Previous work marked the discovery⁵¹ of strongly polarized x-rays emitted by randomly oriented gas-phase molecules (CH₃Cl) following selective near-threshold excitation. These preliminary results illustrated sensitivity to both molecular geometry and molecular-orbital symmetry of occupied and unoccupied valence levels. In conjunction with theory⁵¹, it is also possible to determine relative oscillator strengths of x-ray-absorption and x-ray-emission transitions. Related measurements have shown that complementary results can be obtained with angular-distribution measurements of x-ray emission⁵².

An explanation⁵¹ of the polarized x-ray-emission phenomenon is based on consideration of the time scale for the emission process. Fluorescence lifetimes for deep core levels are a few femtoseconds, orders-of-magnitude shorter than characteristic rotational or vibrational times. As a result, nuclear motion is "frozen" during the excitation/decay process, allowing x-ray emission to effectively take a "snapshot" of the excited, and thus aligned, molecule. The practical result is that molecules in a favorable orientation with respect to the polarization of the incident radiation, as determined by the spectroscopic transition being probed, preferentially absorb x-rays. Thus, if excitation proceeds to a state of well-defined symmetry, only a certain subset of randomly oriented molecules are selected, in effect permitting the study of an ensemble of aligned molecules. Due to the short time scale, this alignment is reflected in the x-ray-emission process. The ultimate result is that participation in x-ray emission by electrons in valence molecular orbitals with different symmetries produces x-rays with different polarizations. X-ray polarization is detected by choosing the Bragg-diffraction angle near 45^o for the analyzing crystal in an x-ray spectrometer (the x-ray '"Brewster" angle). The resulting measurements provide a sensitive probe of both the geometry of the molecule and the symmetry of its valence orbitals.

While the technique of polarized x-ray-emission spectroscopy still remains mostly unexplored, we believe it holds a great deal of promise in many areas of study. For example, because of the atomic-site selectivity inherent to core-level spectroscopy (i.e., core levels for different atoms are well separated in energy), it is feasible to study particular atomic sites in molecules. Our initial efforts will be devoted to revisiting, with our new apparatus, molecular systems similar to those investigated in the pioneering studies, such as Cl₂, Cl-containing methane-derivatives, SF₆, CS₂, etc. As additional experience is gained, larger, less-well-characterized molecules will be studied, with a long-term goal of studying prototypes of molecules of biological interest (i.e., protein and enzyme analogs). Indeed, biologist colleagues who use such prototype species for x-ray-absorption work have indicated interest in this new technique.

Another advantage of using x-rays is penetrability in H₂0, raising the possibility that molecules in aqueous solution can be studied. We plan to be the first group to explore this possibility by developing a liquid-sample cell for use with the endstation's x-ray-emission spectrometer. Emphasis will be placed on exploring liquid-phase interactions and how they affect the spectroscopy and decay dynamics of core-excited states. It may eventually prove feasible to study chemically reactive species in solution during the reaction process, owing to the very short time scale inherent to x-ray emission. This direction holds the possibility of opening up an entirely new area of research with x-rays.

6.) Studies of Transient Atomic and Free-Radical Species

The vast majority of gas-phase core-level-spectroscopy studies have been performed on stable atomic and molecular species, particularly the rare gases. There is also a considerable and growing body of work on species such as the alkali metals, the alkaline earths, and transition metals. In contrast, studies of transient atomic or molecular systems from Groups III-VII of the Periodic Table are limited to some results for valence shells⁵³ and a few results on shallow core levels.^54,55 Here, shallow refers to the outermost core level in the absorbing atom, such as Cl L_2,3. Measurements on deeper core levels of such species (e.g., Cl K) have not been performed. The distinction between shallow and deep core levels is significant because the ultimate degree of ionization in the two cases is different, resulting in measurable differences in the decay dynamics of core holes produced by x-ray absorption.

We will initiate deep-core-level studies of transient species by measuring total-ion-yield (pseudo-photoabsorption) curves near selected thresholds, such as the K shells of atomic S and Cl and the L_2,3 subshell of atomic I. For deep-core levels, available results from experiment⁵⁶ and theory⁵⁷ for the Ar-1s shell have combined to describe an unusual energy dependence of the absorption cross section just above threshold, as well as to identify several resonances due to electron correlation at somewhat higher energies. Core-level photoabsorption by open-shell atoms will provide an even more critical test of fundamental theory. Once these initial measurements have been done, extension to ion spectroscopy (i.e., partial ion yields vs. photon energy) and then angle-resolved electron spectroscopy will be made. Results of this type will tax the capabilities of present theoretical approaches because of the open-shell nature of the systems, the multitude of decay pathways available to deep core holes, and the multiply differential modes of measurement. Perhaps even more interesting will be small transient molecular species, such as ClO and HS, for which no core-level information whatsoever is available. At lower photon energies, we also will extend our initial work on atomic O by studying other second-row elements such as C, N, and F at their K edges.

Formation of transient species is accomplished using a microwave discharge and a gas flow tube as described in Ref. 53. This process transforms a fraction of the parent compound to the desired transient, which is then transported to the region of interaction with the x-ray beam. Difference measurements obtained by taking spectra with the discharge on and off produce spectra of the transient alone. The portability of the transient source allows it to be used with both ion- and electron-TOF systems as well as the electrostatic electron analyzer.

References

1. B. Krässig, M. Jung, D.S. Gemmel, E.P. Kanter, T. LeBrun, S.H. Southworth, and L. Young, Phys. Rev. Lett. 75, 4736 (1995).

2. J.L. Dehmer, A.C. Parr, and S.H. Southworth, Handbook on Synchrotron Radiation, Vol. 2, edited by G.V. Marr (North-Holland, Amsterdam, 1987).

3. I. Nenner and J.A. Beswick, ibid.

4. H.C. Schmelz, C. Reynaud, M. Simon, and I. Nenner, J. Chem. Phys. 101, 3742 (1994).

5. J.H.D. Eland, F.S. Wort, and R.N. Royd, J. Electron Spectrosc. 41, 97 (1986).

6. M. Simon, T. LeBrun, P. Morin, M. Lavollee, and J. L. Marechal, Nucl. Instrum. Meth. B62, 167 (1991).

7. See, for example, E. Shigemasa, K. Ueda, Y. Sato, A. Yagishita, H. Maezawa, T. Sasaki, M. Ukai, and T. Hayaishi, Phys. Scr. 41, 67 (1990); F. von Bush, N. Anders, U. Ankerhold, J. Doppelfeld, S. Drees, and B. Esser, in Atomic Physics at High Brilliance Synchrotron Sources (Argonne National Laboratory, Chicago, 1994); M. Simon, M. Lavollee, M. Meyer, and P. Morin, J. Electron Spectrosc. (in press).

8. D.L. Hansen, M. Arrasate, L. Steinbeck, E. Villalobos, D. Johnson, W.L. Manner, D.W. Lindle, J.C. Levin, I.A. Sellin, to be published.

9. P. Morin, M. Lavollee, M. Meyer, and M. Simon, Proceedings of the XVIII ICPEAC, AIP 295, edited by T. Andersen, B. Fastrup, F. Folmann, H. Knudsen, and N. Andersen (1993).

10. M. Simon, P. Morin, P. Lablanquie, M. Lavollee, K. Ueda, N. Kosugi, Chem. Phys. Lett. 238, 42 (1995).

11. A. Bechler and R.H. Pratt, Phys. Rev. A 39, 1774 (1989).

12. J.H. Scofield, Phys. Rev. A 40, 3054 (1989).

13. A. Bechler and R.H. Pratt, Phys. Rev. A 42, 6400 (1990).

14. J.W. Cooper, Phys. Rev. A 42, 6942 (1990).

15. M.O. Krause, Phys. Rev. 177, 151 (1969); F. Wuilleumier and M.O. Krause, Phys. Rev. A 10, 242 (1974).

16. O. Hemmers, G. Fisher, P. Glans, D.L. Hansen, H. Wang, S.B. Whitfield, D.W. Lindle, R. Wehlitz, J.C. Levin, and I-A. Sellin, abstract submitted to The American Physical Society's Division of Atomic, Molecular, and Optical Physics (DAMOP) Annual Meeting (1996); and to be published.

17. A. Ron, I.B. Goldberg, J. Stein, S.T. Manson, R.H. Pratt, and R.Y. Yin, Phys. Rev. A 50, 1312 (1994).

18. H. Kossmann, V. Schmidt, and T. Andersen, Phys. Rev. Lett. 60, 1266 (1988).

19. T. Åberg, Phys. Rev. A 2, 1726 (1970); J.A.R. Samson, C.H. Greene, and R.J. Bartlett, Phys. Rev. Lett. 71, 201 (1993); T. Suric, K. Pisk, B. Logan, and R. Pratt, Phys. Rev. Lett. 73, 790 (1994); S.T. Manson and J.H. McGuire, Phys. Rev. A 51, 400 (1995).

20. L. Andersson and J. Burgdörfer, Phys. Rev. A. 50, R2810 (1994).

21. K. Hino, P. M. Bergstrom, Jr. and J. H. Macek, Phys. Rev. Lett. 72, 1620 (1994).

22. J.A.R. Samson, W.C. Stolte, Y. Lu, J.C. Levin, I.A. Sellin, D.L. Hansen, H. Wang, P. Glans, D.W. Lindle, to be published.

23. R. Wehlitz, F. Heiser, O. Hemmers, B. Langer, A. Menzel, and U. Becker, Phys. Rev. Lett. 67, 3764 (1991); R.J. Bartlett, P.J. Walsh, Z.X. He, Y. Chung, E.-M. Lee, and J.A.R. Samson, Phys. Rev. A 46, 5574 (1992); N. Berrah, F. Heiser, R. Wehlitz, J. Levin, S.B. Whitfield, J. Viefhaus, I.A. Sellin, and U. Becker, Phys. Rev. A 48, R1733 (1993); J.C. Levin, I.A. Sellin, B.M. Johnson, D.W. Lindle, R.D. Miller, N. Berrah, Y.A. Azuma, H.G. Berry, and D.-H. Lee, Phys. Rev. A 47, R16 (1993); M. Sagurton, R.J. Bartlett, J.A.R. Samson, Z.X. He, and D. Morgan, Phys. Rev. A 52, 2829 (1995).

24. J.C. Levin, D.W. Lindle, N. Keller, R.D. Miller, Y. Azuma, N. Berrah Mansour, H.G. Berry, and I.A. Sellin, Phys. Rev. Lett. 67, 968 (1991).

25. J.C. Levin, C. Biedermann, N. Keller, L. Liljeby, C.-S. O, R.T. Short, I.A. Sellin, and D.W. Lindle, Phys. Rev. Lett. 65, 988 (1990).

26. T. Åberg and B. Crasemann, in Resonant Anomalous X-Ray Scattering: Theory and Applications, edited by G. Materlik, C.J. Sparks, and K. Fischer (North-Holland, Amsterdam, 1994) and references therein; B. Crasemann, Röntgen Centennial, Würzburg, 23-27 October 1995.

27. K. Hamalainen, D.P. Siddons, J.B. Hastings, and L.E. Berman, Phys. Rev. Lett. 67, 2850 (1991).

28. Y. Ma, N. Wassdahl, P. Skytt, J. Guo, J. Nordgren, P.D. Johnson, J.-E. Rubensson, T. Böske, W. Eberhardt, and S.D. Kevan, Phys. Rev. Lett. 69, 2598 (1992).

29. M.A. MacDonald, S.H. Southworth, J.C. Levin, A. Henins, R.D. Deslattes, T. LeBrun, Y. Azuma, P.L. Cowan, and B.A. Karlin, Phys. Rev. A 51, 3598 (1995).

30. A. Kivimaki, A. Naves de Brito, S. Aksela, H. Aksela, O.-P. Sairanen, A Ausmees, S.J. Osborne, L.B. Dantas, and S. Svensson, Phys. Rev. Lett. 71, 4307 (1993).

31. W. Eberhardt, G. Kalkoffen, and C. Kunz, Phys. Rev. Lett. 41, 156 (1978).

32. G.S. Brown, M.H. Chen, B. Crasemann, and G.E. Ice, Phys. Rev. Lett. 45, 1937 (1980).

33. C.J. Sparks, Jr., Phys. Rev. Lett. 33, 262 (1974).

34. P. Eisenberger, P.M. Platzman, and H. Winick, Phys. Rev. Lett. 36, 623 (1976).

35. F.K. Gel'mukhanov, L,N. Mazalov, and A.V. Kondratenko, Chem. Phys. Lett. 46, 133 (1977).

36. L.S. Cederbaum and F. Tarantelli, J. Chem. Phys. 98, 9691 (1993).

37. G.B. Armen, J.C. Levin, and I.A. Sellin, Phys. Rev. A 53, 772 (1996).

38. A. Flores-Riveros, N. Correia, H. Agren, L. Pettersson, M. Backstrom, and J. Nordgren, J. Chem. Phys. 83, 2053 (1985); P. Glans, J. Nordgren, H. Agren, and A. Cesar, J. Phys. B 26, 663 (1993).

39. T.X. Carroll, S.E. Anderson, L. Ungier, and T.D. Thomas, Phys. Rev. Lett. 58, 867 (1987); T.X. Carroll and T.D. Thomas, J. Chem. Phys. 89, 5983 (1988).

40. J.-E. Rubensson, M. Neeb, M. Biermann, and W. Eberhardt, J. Chem. Phys. 99, 1633 (1993).

41. S.J. Osborne, A. Ausmees, S. Svensson, A. Kivimaki, O.-P. Sairanen, A. Naves de Brito, H. Aksela, and S. Aksela, J. Chem. Phys. 102, 7317 (1995).

42. Z.F. Liu, G.M. Bancroft, K.H. Tan, and M. Schachter, Phys. Rev. Lett. 72, 621 (1994).

43. W. Drube, A. Lessmann, and G. Materlik, in Resonant Anomalous X-Ray Scattering: Theory and Applications, edited by G. Materlik, C.J. Sparks, and K. Fischer (North-Holland, Amsterdam, 1994); W. Drube, R. Treusch, and G. Materlik, Phys. Rev. Lett. 74, 42 (1995).

44. L.I. Yin, I. Adler, M.H. Chen, and B. Crasemann, Phys. Rev. A 7, 897 (1973).

45. Atomic Inner-Shell Pkysics, edited by B. Crasemann (Plenum, New York, 1985).

46. B. Crasemann, J. Physique C 9, 389 (1987); Comments At. Mol. Phys. 22, 163 (1989).

47. G.B. Armen, T. Åberg, J.C. Levin, B. Crasemann, M.H. Chen, G.E. Ice, and G.S. Brown, Phys. Rev. Lett. 54, 1142 (1985); J. Tulkki, G.B. Armen, T. Åberg, B. Crasemann, and M.H. Chen, Z. Phys. D 5, 241 (1987).

48. G.B. Armen, J. Tulkki, T. Åberg, and B. Crasemann, Phys. Rev. A 36, 5606 (1987); J. Tulkki, T. Åberg, S.B. Whitfield, and B. Crasemann, Phys. Rev. A 41, 181 (1990).

49. G.B. Armen, S.L. Sorensen, S.B. Whitfield, G.E. Ice, J.C. Levin, G.S. Brown, and B. Crasemann, Phys. Rev. A 35, 3966 (1987).

50. W. Eberhardt, S. Bemstoff, H.W. Jochims, S.B. Whitfield, and B. Crasemann, Phys. Rev. A 38, 3808 (1988).

51. D.W. Lindle, P.L. Cowan, R.E. LaVilla, T. Jach, R.D. Deslattes, B. Karlin, J.A. Sheehy, T.J. Gil, and P.W. Langhoff, Phys. Rev. Lett. 60, 1010 (1988).

52. S.H. Southworth, D.W. Lindle, R. Mayer, and P.L. Cowan, Phys. Rev. Lett. 67, 1098 (1991).

53. See, for example, J.A.R. Samson, Y. Shefer, and G.C. Angel, Phys. Rev. Lett. 56, 2020 (1986); J.A.R. Samson and G.C. Angel, Phys. Rev. A 42, 5328 (1990); P. van der Meulen, M.O. Krause, and C.A. de Lange, Phys. Rev. A 43, 5997 (1991); P. van der Meulen, M.O. Krause, and C.A. de Lange, J. Phys. B 25, 97 (1992); S.J. Schaphorst, M.O. Krause, C.D. Caldwell, H.P. Saha, M. Pahler, and J. Jimenez- Mier, Phys. Rev. A 52, 4656 (1995).

54. C.D. Caldwell and M.O. Krause, Phys. Rev. A 47, R759 (1993).

55. See, for example, J. Tremblay, M. Larzilliere, F. Combet-Famoux, and P. Morin, Phys. Rev. A 38, 3804 (1988); L. Nahon, L. Duffy, P. Morin, F. Combet-Famoux, J. Tremblay, and M. Larzilliere, Phys. Rev. A 41, 4879 (1990).

56. R.D. Deslattes, R.E. LaVilla, P.L. Cowan, and A. Henins, Phys. Rev. A 27, 923 (1983).

57. J. Cooper, Phys. Rev. A 38, 3417 (1988); H.P. Saha, Phys. Rev. A 42, 6507 (1990).

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