Table of contents

This year marks the 125th anniversary of the founding of The Physical Society of London and its Proceedings, which were launched in 1874, from which the Journal of Physics, and later J. Phys. B: At. Mol. Opt. Phys., originated. In contrast, synchrotron radiation was discovered just over 50 years ago [1]. However its first systematic scientific application, which was in the field of atomic and molecular physics, occurred even more recently in 1963 when Madden and Codling performed their pioneering investigations of the absorption spectra of the rare gases in the VUV [2]. Driven in large part by the demands of atomic and molecular science, dedicated storage rings (second generation sources) soon followed, built specifically as light sources to operate from the IR to the x-ray spectral regions. The use of synchrotron radiation has now extended worldwide to over 70 research facilities in every continent of the world - many of them third-generation machines with insertion devices to enhance performance. Although the rapid increase in computational speed is often cited as a dramatic example of modern technological progress, the increase in brightness of synchrotron light sources has occurred even more rapidly [3]. Not surprisingly numerous additional scientific areas ranging from protein crystallography to materials science have also benefited greatly from this progress.

The role of synchrotron radiation in research into atomic, molecular and optical physics continues to expand rapidly as experiments of ever-increasing complexity are combined with increased photon flux and fully characterized polarization. The present special issue of J. Phys. B: At. Mol. Opt. Phys. is intended to bring to the attention of its readers some aspects of current experimental, and theoretically associated, work on synchrotron based studies of molecules that relate to the field of atomic, molecular and optical physics. The research papers on experimental work published in this special issue originate from eight countries and seven synchrotron facilities. The wide range of topics covered includes photoionization processes and dynamics, valence and inner-shell photoelectron spectroscopy, photoabsorption and photofluorescence, photofragmentation and dissociative photoionization and multiple photoionization. The molecules studied range in size from hydrogen, the most abundant and simplest molecule in the Universe, up to DNA, the most important molecule in biology. Some, such as OH, are highly reactive and short-lived. Others, such as fullerene endohedrals, have only recently been discovered. Thus the articles included in this special issue cover a wide variety of exciting topics in the areas of molecular and optical physics. It is hoped, therefore, that this will further stimulate interest and appreciation of the role and importance of synchrotron radiation in the study of molecules.

It is, of course, the contributors and referees who have made this thematic issue on molecular physics with synchrotron radiation possible, and they deserve our thanks and appreciation.

References

[1] Elder F R, Gurewitsch A M, Langmuir R V and Pollock H C 1947 Phys. Rev.71 829

[2] Madden R P and Codling K 1963 Phys. Rev. Lett.10 516

[3] Altarelli M, Schlachter F and Cross J 1998 Sci. Am. December 1998

A helium-like description of photo double ionization of molecular deuterium is extended to derive a dependence of molecular excitation amplitudes on electron energy sharing and dynamical quantum numbers labelling internal modes of excitation of the escaping electron pair. Both linear and circular polarizations are considered, and predictions regarding circular dichroisms in the angular distributions of the electron and ion pairs are given. A detailed comparison of the model with recent (,2e) measurements is also presented.

The angular distribution of the correlated electron pair emitted in single-photon double ionization of the hydrogen molecule is analysed and calculated using a variety of approximations. Attention is directed particularly towards the differences between the molecular angular distribution and that of the corresponding `united atom', i.e. that arising from the double ionization of helium. Qualitative agreement is obtained with recent experiments on photo-double ionization of the hydrogen molecule. The major effects arising from the two-centre nuclear field of the molecule and the orientation of the axis at the moment of photon absorption are exposed in the simpler problem of photoionization of the H₂⁺ ion.

Electron-ion coincidence measurements of carbon monoxide following 1s_C2p (h = 287.4 eV) and 1s_O2p (h = 534.2 eV) resonant photoabsorption and following 1s_C photoionization (h = 350 eV) by synchrotron radiation are presented. Coincidence time-of-flight mass spectra were obtained for several pronounced electron emission channels. Fragmentation is observed and released kinetic energies are estimated. For resonant excitation 1s_C2p branched decay into C⁺+O and C+O⁺ is shown to take place in coincidence with electrons of 255.0 eV. In coincidence with electrons of 249.6 eV after 1s_C2p photoabsorption, fragmentation into C⁺+O⁺ is detected which can only be explained by double ionization in a two-step process.

The photoionization of NO molecules is studied using angle- and spin-resolved photoelectron spectroscopy with circularly polarized synchrotron radiation. The spin-orbit splitting is resolved by preparing only the X ²_1/2 component of the ground state in a helium-seeded supersonic expansion. A pronounced spin polarization in the angular distribution is found for the cold beam when the strong autoionization resonance at 13.82 eV photon energy is excited. The polarization parameters A, and are determined for the v = 2 and 3 vibrational levels of the NO⁺ X ¹⁺ final ionic state. The results lead to the conclusion that resonances with ²⁺ and ² symmetry contribute with similar intensity. The dominant contributions to the dipole transition matrix elements of the emitted electron waves can be derived approximately.

The creation and subsequent Auger decay of the 3d^-1 core-hole state in HBr is studied using two-dimensional photoelectron spectroscopy. In this technique the yield of electrons is collected as a function of both photon and electron kinetic energy. The effects of post-collision interaction (PCI) observed in the energy shift of the HBr⁺(3d^-1) photoelectron lines. The energy shifts are compared with previous experimental results on Kr. This comparison suggests that the molecular nature of HBr has little effect on the PCI energy exchange mechanism.

We have recorded threshold photoelectron spectra of HF and DF over the outer valence ionization region under good resolution conditions (3-6 meV) using synchrotron radiation and employing the penetrating-field electron detection technique. The spectra show extensive vibrational structure in the X(²_i) system in the first Franck-Condon gap region that is attributed to resonance autoionization of Rydberg states lying in this energy range. The Rydberg states responsible for these effects are identified as [A(²⁺)]ns¹⁺, with n = 4-6. Analyses of all the vibrational data contained in these spectra for the X(²_i) state using a modified isotopic vibrational Dunham equation have led to greatly improved spectroscopic constants for this state in HF⁺ and DF⁺. The same method was also used to determine improved spectroscopic constants for the A(²⁺) state. Autoionization is found not to be important in the formation of the A(²⁺) state due to the absence of Rydberg states in the energy region of the A state. Based on the observations and interpretations of the present work, combined with literature data, we have constructed a potential-energy diagram of the relevant states of HF and HF⁺. Partially rotationally resolved threshold photoelectron spectra of HF⁺ and DF⁺ over the v⁺ = 0 vibrational band of the X ²_i system show pronounced effects due to autoionization processes.

The near-edge x-ray absorption spectra of dinitrogen oxide and carbon dioxide have been measured with ultra high resolution and high statistical accuracy at the N 1s and O 1s edges, and at the C 1s and O 1s edges in N₂O and CO₂, respectively. The assignment of the peaks in the spectra is achieved via the analysis of the quantum defects of the Rydberg states, the use of the core-equivalent model and a comparison with previous photoabsorption and fragment ion spectra. New high-resolution spectra are provided for the O 1s excitations of CO₂, and several new states are observed at the oxygen edge of N₂O.

The fluorescence processes following vacuum-UV (VUV) photoexcitation of BF₃ and BCl₃ in the energy range 9-22 eV have been studied using synchrotron radiation from the BESSY 1, Berlin source. The fluorescence is dispersed in a secondary monochromator, whereby information on the nature of the emitting species is obtained. For excitation energies in the range 13-17 eV, photodissociation of Rydberg states of BF₃ produces a substantial branching ratio into two, possibly three, excited valence states of the BF₂ free radical, which fluoresce to the ground state. The dispersed spectra are compared with predictions from ab initio calculations of the electronic spectroscopy of this radical. Fluorescence from BCl₃ photoexcited in the range 9-22 eV is more complicated, with emissions having already been observed and mostly assigned in BCl₂, BCl, BCl₃⁺, BCl₂⁺ and B. Using the single-bunch, pulsed mode of the BESSY 1 source, the radiative lifetimes of emitters from BCl₃ lying in the range 3-100 ns have been measured. Our results for BCl₃ are compared with data from other techniques. Where more than one emitter is excited at a particular VUV energy, the dispersion of the induced fluorescence through a monochromator means that the lifetimes of the different emitters can be differentiated.

The absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of ammonia and deuterated ammonia have been measured from the ionization threshold to 25 eV using a double ion chamber and monochromated synchrotron radiation. The photoabsorption spectrum displays extensive vibrational progressions associated with Rydberg series converging onto excited vibrational levels of the ²A₂´´ state. New structure has been observed for ND₃ in the 10.0-11.3 eV range, and vibrational progressions due to transitions into the , and Rydberg states have been recorded with improved resolution. Features have been observed, for the first time, in the photoabsorption spectra of NH₃ and ND₃ due to Rydberg series converging onto the à ²E ionization threshold, and interpretations for some of these features have been proposed based upon the corresponding photoelectron spectra.

The He I excited NH₃⁺(1a₂´´)^-12A₂´´ photoelectron band has been studied experimentally at a resolution of 3 meV and two vibrational progressions, each involving excitation of the ₂⁺ mode, have been observed. The vibrational lines in the main progression show a complex structure associated with rotational excitations. This structure changes gradually in a way that can be explained by the variation of the H-N-H bond angle with the ₂⁺ mode. The effective bond angle has been found to be 120° for v₂⁺ = 0, and similar to that of the neutral ground state near v₂⁺ = 6. The second progression, of weak lines, has been interpreted tentatively as being due to n₂⁺+₄⁺. The ₄⁺ mode is doubly degenerate and the excitation of a single quantum has been explained by vibronic coupling with the à ²E state. In addition, He II excitation has been used to record the entire valence shell photoelectron spectrum.

The ionization and fragmentation of carbon tetrafluoride excited at the C and F 1s thresholds are studied using soft x-ray excitation, threshold electron spectroscopy and state-selective coincidence techniques. The threshold electron spectra reveal indirect low-energy production from pre-edge resonances, and the structure at the thresholds is attributable to post-collision interaction effects, with some evidence of underlying vibrational structure. The coincidence data allow determination of ion and ion pair yield ratios and deduction of the dissociation pathways and final states for the various channels.

Constant initial state (CIS) photoemission spectra of the N₂ 1_u^-1 and 3_g^-1 states have been measured in the photon energy range corresponding to the N 1s * core-to-bound transition, where they acquire intensity from participator decay. The resolution was sufficiently high that the spectra could be recorded individually for the first four vibrational components of the 1_u^-1 state and for the first two vibrational components of the 3_g^-1 state. Differences in the spectra for these two final states are discussed in terms of their potential curves compared to that of the intermediate neutral excited state. We point out the potential usefulness of the CIS method for elucidating the details of the molecular potential.

Relaxation dynamics of CH₃Cl following core-shell photoexcitation in the neighbourhood of the chlorine K-edge ( 2.8 keV) were studied via multi-ion coincidence measurements using a time-of-flight mass spectrometer. The data provide evidence for sequential fragmentation moderated by Coulombic interactions among the fragments. The sequential nature of the fragmentation indicates that chemical forces, especially between the carbon and chlorine atoms, are dominant in determining the kinematics of the fragmentation. This is especially true on resonance where fast dissociation is observed following electron excitations to the 8a₁ antibonding orbital.

Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and oxygen atoms in acetone, creating neutral core-excited resonance states. Electron energy spectra associated with Auger decay of three strong resonances were collected. The character of each resonance state was deduced through analysis of its decay spectrum.

The dissociative photoionization of CH₄ and CD₄ has been studied using synchrotron radiation within the energy range of 12-60 eV. The mass spectra of secondary CH_n⁺/CD_n⁺ ions have been found to display significant isotope effects and to depend on the target gas temperature. The kinetic energy spectra of the fragment protons and deuterons have also been determined and it is shown that, in addition to direct photoionization, intermediate, superexcited states play an important role in the photodissociative ionization process.

Resonant photofragmentation of core-excited methanol has been studied with high-resolution partial ion-yield spectroscopy near both the carbon and oxygen K-edge. Non-coincident and coincident fragmentation channels were monitored simultaneously. A steep increase in the coincidence yield curves above the ionization threshold, where the normal Auger decay produces doubly charged or multiply charged species which yield two or more charged fragments, has been observed. A relevant difference in resonant structure has been observed while comparing the fragmentation at the carbon K-edge and the oxygen K-edge. Evidence for inter-atomic Auger decay is presented for one particular single channel around the oxygen K-edge.

The absolute photoabsorption spectra of ethane (C₂H₆), encompassing the C 1s^-1 3p excitation, the C 1s threshold and the satellite thresholds is measured. Further, photoelectron spectra are taken in the region from the C 1s threshold region up to 380 eV, and the vibrational fine structure and C 1s^-1 partial cross section studied. The complex vibrational fine structure for both ethane and deuterated ethane (C₂D₆) is analysed using a coupled least-squares fit procedure. While we cannot exclude the presence of the C-C stretching mode (₃), the experimental data show that the C 1s photoelectron spectrum is dominated by two vibrational modes, the C-H stretch ₁ and the C-H deformation mode ₂. Compared with their values in the neutral ground state there is a significant increase in the vibrational energies to 407±5 meV and 176±5 meV for ethane and 291±10 meV and 130±10 meV for deuterated ethane, respectively. The lifetime broadening is determined to be 98±10 meV, which is in good agreement with calculations. The investigated shake-up structure closely resembles that of methane. The lowest-lying satellite band gains intensity towards threshold which may be attributed to conjugate shake-up processes.

Integral and backward scattering cross sections are reported for the scattering of electrons from C₆H₅F, C₆H₅Cl, C₆H₅Br and C₆H₅I over the energy range between 10 and 20 meV to 10 eV. The measurements were made in transmission experiments using a synchrotron radiation photoionization apparatus with an energy resolution in the incident electron beam of ~3.5 meV (FWHM). The absolute accuracy of the cross sections is limited by experimental difficulties with respect to strong rotationally inelastic forward scattering, due to the dipolar nature of the target molecules.

Integral scattering cross sections are found to rise to more than 1500 Ų at the lowest energies in C₆H₅F, C₆H₅Cl and C₆H₅Br and to more than 2000 Ų in C₆H₅I due to an additional contribution from electron attachment. At low energy, both integral and backward scattering cross sections are very similar in C₆H₅F, C₆H₅Cl and C₆H₅Br. Data are interpreted in terms of rotationally inelastic scattering, with values of cross sections essentially dictated by the correspondingly similar dipole moments (1.6-1.7 D) of these three species. The Born model for rotationally inelastic scattering consistently underestimates the experimental cross sections and overestimates the degree of forward scattering. Our data illustrate the limitations of the Born model for the estimation of low-energy scattering cross sections for plasma modelling.

We measure the photoabsorption cross sections and photoionization quantum yields of cyclopropane in the photon energy range of 10-24 eV by means of a double ionization chamber equipped with a window and synchrotron radiation as a continuous-wavelength light source. The photoionization cross sections and neutral-dissociation cross sections are derived from them. It is verified that our values are more correct and comprehensive than those reported before. Several superexcited states, valence-excited states and Rydberg states, are found in the curves of cross section against photon energy mentioned above, and it turns out that they are classified into three groups according to the features of the competition between neutral dissociation and autoionization.

Absolute optical (VUV) absorption cross sections for cyclopropane have been measured from 5.0 to 11.2 and 20-40 eV using synchrotron radiation. Also, electron energy-loss (EEL) spectra have been obtained using incident electrons of (a) 150 eV energy scattered through small angles (energy loss 5.0-15 eV) and (b) near-threshold energies scattered through large angles (energy loss 0-10.5 eV). Taken together these confirm that the low-lying excited electronic states of cyclopropane are of Rydberg type and, although spectral bands are diffuse, a known Rydberg series has been extended. Recent computations (Galasso V 1996 Chem. Phys.206 289) appear to give a good account of the experimental spectrum from threshold to about 11 eV, but these must be extended if valence-excited states are to be characterized. Particular attention has been directed at the evaluation of absolute optical cross sections. These are now believed to be established over the energy ranges 5-15 and 20-40 eV. In the gap region (15-20 eV) second-order radiation may affect the optical measurements. From consideration of second-order effects, and comparison of the present studies with earlier measurements, we propose a best-estimate cross section in this energy region also.

A model for the calculation of photoionization cross sections of an atom inside the fullerene cage is proposed. In the frame of this model the M@C₆₀ endohedral is considered as an isolated M atom located in the centre of the spherically symmetrical fullerene shell. The potential of this shell is written as the Dirac delta function in the radial coordinate. On the basis of this model a comparative study of the photoionization cross sections of an isolated M atom and the same atom in M@C₆₀ is carried out. It is shown that near the thresholds of ionization of atomic subshells the resonance peaks appear in the photoeffect cross sections. The positions of these resonances on the scale of photoelectron energy are defined by the fullerene radius and the affinity energy of an electron to C₆₀. The general formulae derived are used to calculate the photoionization cross sections of the inner subshells of the He, Ne and Ca atoms inside the fullerene cage.

Radiation biophysics has sought to understand at a molecular level, the mechanisms through which ionizing radiations damage DNA, and other molecules within living cells. The complexity of lesions produced in the DNA by ionizing radiations is thought to depend on the amount of energy deposited at the site of each lesion. To study the relationship between the energy deposited and the damage produced, we have developed novel techniques for irradiating dry plasmid DNA, partially re-hydrated DNA and DNA in solution using monochromatic vacuum-UV synchrotron radiation. We have used photons in the energy range 7-150 eV, corresponding to the range of energies typically involved in the efficient production of DNA single-strand (SSB), and double-strand breaks (DSB) by ionizing radiation. The data show that both types of breaks are produced at all energies investigated (with, or without water present). Also, the energy dependence for DSB induction follows a similar trend to SSB induction but at a 20-30-fold reduced incidence, suggesting a common precursor for both types of damage. Preliminary studies where DNA has been irradiated in solution indicate a change in the shape of the dose-effect curve (from linear, to linear-quadratic for double-strand break induction) and a large increase in sensitivity due to the presence of water.

In this paper, examples are given of recent measurements made at the Daresbury Synchrotron Radiation Source on four short-lived diatomic molecules. The equipment designed to overcome the difficulties of interfacing reactive species with the ultra-high vacuum environment of a synchrotron radiation source is described. The measurements presented have been selected to show the advantage of using a continuum source of radiation in identifying new structure in the photoionization continuum of these molecules.