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Superfluid helium nanodroplets are often considered as transparent and chemically inert nanometer-sized cryo-matrices for high-resolution or time-resolved spectroscopy of embedded molecules and clusters. On the other hand, when the helium nanodroplets are resonantly excited with XUV radiation, a multitude of ultrafast processes are initiated, such as relaxation into metastable states, formation of nanoscopic bubbles or excimers, and autoionization channels generating low-energy free electrons. Here, we discuss the full spectrum of ultrafast relaxation processes observed when helium nanodroplets are electronically excited. In particular, we perform an in-depth study of the relaxation dynamics occurring in the lowest 1s2s and 1s2p droplet bands using high resolution, time-resolved photoelectron spectroscopy. The simplified excitation scheme and improved resolution allow us to identify the relaxation into metastable triplet and excimer states even when exciting below the droplets' autoionization threshold, unobserved in previous studies.

Single-site Double-Core Hole (ss-DCH or K

The hitherto unexplored two-photon doubly excited states [Ne^{*}(2p^{-1}3s)]_{2} were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne_{2}^{+} ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping'' to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

High-resolution Auger spectra of doubly excited Be-like neon have been obtained for intermediate collisional velocities (0.89--1.79 a.u.). Ne[sup 8+] was produced at high voltage in the Argonne National Laboratory electron-cyclotron-resonance ion source. A differentially pumped helium gas cell was used to study double electron capture. The [ital LMX] Auger decays of the Be-like neon are observed with a high-resolution 0[degree] tandem electron spectrometer. High-angular-momentum states are observed to be populated during the collisions and their relative amplitudes change dramatically with increasing collisional velocity.

The hyperfine structures of six metastable levels arising from the 3{ital d}{sup 2}4{ital s} and 3{ital d}{sup 3} configurations in {sup 49}Ti II have been measured with high precision using the laser-rf double-resonance technique in a collinear laser--ion-beam geometry. The hfs constants {ital A} and {ital B} were determined to about 4 and 43 kHz, respectively. Less precise results are also reported for the 2{ital p}{sub 1/2} level of the 3{ital d}{sup 3} configuration and for five upper levels in the 3{ital d}{sup 2}4{ital p} configuration. For these states, the optical spectra had Doppler widths of about 40 MHz, leading to {ital A} values determined to about 1 MHz. The results are compared with the presently available Hartree-Fock (HF) calculations. Good agreement is obtained with the HF results only in some cases. Substantial corrections due to many-body interactions and relativistic effects are required for other cases.

The rate coefficients for electron transfer from Ar and H{sub 2} to Ar{sup {ital q}+} ions (3{le}{ital q}{le}6) have been measured using an ion-storage technique in a Penning ion trap. The ions were produced in the trap by {ital K}-shell photoionization of Ar atoms, using broadband synchrotron x-ray radiation. {ital K}-electron removal resulted in vacancy cascading, yielding a distribution of argon-ion charge states peaked near Ar{sup 4+}. The stored ion gas had an initial temperature near 480 K. The basic data determining the rate coefficients {ital k}(Ar{sup {ital q}+}) are the storage time constants of each charge state in the trap, in the presence of a measured pressure of target gas. The results of the measurements (in units of 10{sup {minus}9} cm{sup 3} s{sup {minus}1}) are {ital k}(Ar{sup 3+},H{sub 2})=4.3(0.7), {ital k}(Ar{sup 3+},Ar)=1.6(0.2), {ital k}(Ar{sup 4+},H{sub 2})=5.2(0.6), {ital k}(Ar{sup 4+},Ar)=2.5(0.3), {ital k}(Ar{sup 5+},H{sub 2})=5.9(0.7), {ital k}(Ar{sup 5+},Ar)=2.9(0.3), {ital k}(Ar{sup 6+},H{sub 2})=8.5(1.2), and {ital k}(Ar{sup 6+},Ar)=2.5(0.3).

The molecular-frame angular distributions of resonantly excited CO:C(1s) --pi* Auger electrons were determined using angle-resolved electron-ion coincidence spectroscopy in combination with a novel transformation procedure. Our new methodology yields full three-dimensional electron angular distributions with high energy resolution from the measurement of electrons at only two angles. The experimentally determined distributions are well reproduced by calculations performed in a simple one-center approximation, allowing an unambiguous identification of several overlapping Auger lines.

We report a clear demonstration of the effect of hyperfine quenching of a forbidden transition by direct comparison of the lifetimes of the 2 {sup 3}{ital P}{sub 0} level in the heliumlike isotopes {sup 61}Ni{sup 26+} and {sup 58}Ni{sup 26+}. We find the quenched lifetime of the 2 {sup 3}{ital P}{sub 0} level in {sup 61}Ni{sup 26+} to be 470(50) ps. From this we deduce the 2 {sup 3}{ital P}{sub 0--}2 {sup 3}{ital P}{sub 1} energy splitting to be 2.33(15) eV. We also report a measurement of the lifetime of the 2 {sup 3}{ital P}{sub 2} level in {sup 58}Ni{sup 26+}, which is found to be 70(3) ps.

High-resolution 0{degree} Auger spectroscopy has been used to study the Ne {ital LMM} and {ital LMN} Auger emission. The doubly excited states in B-like Ne are produced via nonresonant transfer and excitation (NTE) in collisions of 15 to 60-keV/amu Ne{sup 6+} incident on He. Line assignments are given for Auger transitions observed in the energy range between 20 and 100 eV, and capture into a triplet state is observed. The relative cross sections for these rates show a broad ({gt}300 keV full width at half maximum) asymmetrical shape as a function of projectile energy, characteristic of the NTE process.

We describe recent precision experiments in fewelectron ions including measurements of the lifetimes of two-photon-emitting levels in Ni26+ and Ni27+, a measurement of the lifetime of the 23S1 level in Br33+ and measurements of the 23S1 + 23P0,1,2 transition energies in B3+.

We have used spin-resolved electron spectroscopy with circularly polarized synchrotron radiation to delineate the electronic structure of the hydrogen sulfide and carbonyl sulfide molecules and their photoemission dynamics. Specifically, inner-shell spin-polarization measurements of the molecular field split components of the S 2p photolines were carried out to reveal effects of the different molecular environments. The validity of simple atomic models to explain the results is discussed.

The Auger spectrum and the relevant angular distribution and spin polarization parameters have been obtained, both, experimentally and numerically for the resonant Auger decay of the photoexcited Ar*(2p3/2−14s1/2)J=1 state. The experiment has been performed using linearly and circularly polarized light and by means of a time‐of‐flight electron analyzer and a Mott polarimeter. The dynamical as well as the transferred spin polarization of the emitted electrons have been determined. The theoretical data have been obtained within a relativistic distorted wave approximation applying a relaxed orbital method. Our numerical results obtained within an 8 configuration state function calculation are compared to the experimental data for the transferred spin polarization. Good agreement to the experiment has been found. Though single fine structure lines show a large dynamic spin polarization the theoretical data yield almost vanishing dynamic spin polarization for the Auger lines. A detailed analysis in terms of pa...

Detailed analysis of the highest occupied molecular orbital band shape in the photoelectron spectrum of gaseous C60 reveals a dynamic Jahn-Teller effect in the ground state of C60(+). The direct observation of three tunneling states asserts a D(3d) geometry for the isolated cation, originating from a strong vibronic coupling. These results show that the ionic motion plays an important role in the electron-phonon interaction.

High-resolution $K$-shell photodetachment measurements of ${\mathrm{He}}^{\ensuremath{-}}$ giving rise to ${\mathrm{He}}^{+}$ ions have been performed using a merged synchrotron vacuum ultraviolet photon-ion beam technique. The measurements on this fundamental negative ion display dramatic structure differing substantially, qualitatively and quantitatively, from the corresponding process in neutral atoms and positive ions, owing to the dominance of correlation in both initial and final states of ${\mathrm{He}}^{\ensuremath{-}}$. In addition, this experimental investigation provides an unambiguous test of two independent theoretical calculations that report serious discrepancies and shows excellent agreement with one of them.

High-resolution K-shell photodetachment measurements of He- giving rise to He+ ions have been performed using a merged synchrotron vacuum ultraviolet photon-ion beam technique. The measurements on this fundamental negative ion display dramatic structure differing substantially, qualitatively and quantitatively, from the corresponding process in neutral atoms and positive ions, owing to the dominance of correlation in both initial and final states of He-. In addition, this experimental investigation provides an unambiguous test of two independent theoretical calculations that report serious discrepancies and shows excellent agreement with one of them.

High-resolution absolute experimental measurements and two independent theoretical calculations were performed for photoionization of O+ ions from the 2P(o) and 2D(o) metastable levels and from the 4S(o) ground state in the photon energy range 30-35.5 eV. This is believed to be the first comparison of experiment and theory to be reported for photoionization from metastable states of ions. While there is correspondence between the predicted and measured positions and relative strengths of the resonances, the cross-section magnitudes and fine structure are sensitive to the choice of basis states.

An experimental K-shell photodetachment study of Li(-) giving rise to doubly photoionized Li(+) ions has been carried out at the Advanced Light Source, using a collinear photon-ion beam apparatus. The experiment reveals dramatic structure, differing substantially both qualitatively and quantitatively from the corresponding processes above the 1s ionization threshold in Li and Li(+), as predicted by our enhanced R-matrix calculation. The experimental/theoretical comparison shows good agreement over some of the photon energy range, and also reveals some puzzling discrepancies.

Two decades ago, it was predicted [Y. S. Kim et al., Phys. Rev. Lett. 46, 1326 (1981)] that relativistic effects should alter the dynamics of the photoionization process in the vicinity of Cooper minima. The present experimental and theoretical study of the angular distributions of Xe 4d(3/2) and 4d(5/2) photoelectrons demonstrates this effect for the first time. The results clearly imply that relativistic effects are likely to be important for intermediate- Z atoms at most energies.

The angular distribution and spin polarization of the Xe N4,5O2,3O2,3 Auger electrons was measured with linearly and circularly polarized synchrotron radiation in a wide photon energy range, covering the Cooper minimum of the 4d photoionization cross section. Using the framework of the two-step model of Auger decay the photoion alignment and orientation were determined.

A day-long retreat was held January 15, 1999 to chart the future directions for chemical dynamics studies at the Advanced Light Source. This represents an important period for the Chemical Dynamics Beamline, as the hardware is well-developed, most of the initial experimental objectives have been realized and the mission is now to identify the future scientific priorities for the beamline and attract users of the highest caliber. To this end, we have developed a detailed scientific program for the near term; identified and prioritized the long range scientific opportunities, identified essential new hardware, and outlined an aggressive outreach program to involve the chemical physics community.

We have applied the laser-rf-laser double-resonance method to a molecular ion. Fifty-six hyperfine components of fine-structure transitions (\ensuremath{\Delta}J=\ifmmode\pm\else\textpm\fi{}1) were measured in rotational levels from N=1 to 27 of the \ensuremath{\nu}''=1 vibrational level of the ${\mathit{X}}^{2}$ ${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}^{+}$ ground electronic state of $^{14}\mathrm{N}_{2}^{+}$. In order to fit the data, we required seven molecular constants, corresponding to the following interactions: electron spin-rotation fine structure and its centrifugal distortion, Fermi-contact hyperfine structure (hfs), dipolar hfs and its centrifugal distortion, and electric quadrupole and nuclear spin-rotation hfs. The results are in reasonable agreement with theoretical calculations based upon a Hartree-Fock-Roothaan wave function.

We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He{sup ++}/He{sup +}=1.6{plus minus}0.3% at {ital h}{nu}=2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Aberg, who predict 1.7%.

We have made precision wavelength measurements on the 1s2s $^{3}$${\mathit{S}}_{1}$--1s2p $^{3}$${\mathit{P}}_{0,1,2}$\ifmmode^\circ\else\textdegree\fi{} transitions in heliumlike boron using fast-beam laser excitation. These measurements are accurate to 2 parts in ${10}^{7}$ of the transition energy and test the most recent calculations at a level of 0.1% of the Lamb shift. The relatively high Z of boron, in comparison to helium, makes the transition energies sensitive to higher powers of \ensuremath{\alpha}Z in the calculations, and our measurements show discrepancies which may be due to these uncalculated higher-order terms.

Stimulated Raman spectroscopy has been used in a fast ion beam to measure the hyperfine structure in the 4{ital d}{sup 2} configuration of Y II. A simple theory is provided for the line shape of the coherent two-photon transition which gives a transit-time limited linewidth of 0.9/{ital T}, comparable to the resolution of the laser--radio-frequency double-resonance technique. The hyperfine structure is compared with a multiconfiguration Dirac-Fock calculation and with that of the homologous Sc II ion. This comparison confirms the role of core polarization as a significant factor in the hyperfine structure of these ions.