Astrochemical studies at the Cryogenic Storage Ring.

The new Cryogenic Storage Ring at the Max Planck Institute for Nuclear Physics (Heidelberg, Germany) has recently become operational. One of the main research areas foreseen for this unique facility is astrochemical studies with cold molecular ions. The spontaneous radiative cooling of the prototype interstellar molecule CH+ to its lowest rotational states has been demonstrated by photodissociation spectroscopy, paving the way for experiments under true interstellar conditions. To this end, a low-energy electron cooler and a neutral atom beam set-up for merged beams studies have been constructed. These experiments have the potential to provide energy-resolved rate coefficients for fundamental astrochemical processes involving state-selected molecular ions. The main target reactions include some of the key processes of interstellar chemistry, such as the electron recombination of H3+, charge exchange between H2+ and H, or the formation of CH+ in collisions of triatomic hydrogen ions and C atoms. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Radiative Rotational Lifetimes and State-Resolved Relative Detachment Cross Sections from Photodetachment Thermometry of Molecular Anions in a Cryogenic Storage Ring.

Photodetachment thermometry on a beam of OH^{-} in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20 min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J=1 natural lifetime (about 193 s) and determine the OH^{-} rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.

The cryogenic storage ring CSR.

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed, and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 ± 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion), and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm(-3) is derived, equivalent to a room-temperature pressure below 10(-14) mbar. Fast atomic, molecular, and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

Photodissociation of an Internally Cold Beam of CH

We have studied the photodissociation of CH^{+} in the Cryogenic Storage Ring at ambient temperatures below 10 K. Owing to the extremely high vacuum of the cryogenic environment, we were able to store CH^{+} beams with a kinetic energy of ∼60 keV for several minutes. Using a pulsed laser, we observed Feshbach-type near-threshold photodissociation resonances for the rotational levels J=0-2 of CH^{+}, exclusively. In comparison to updated, state-of-the-art calculations, we find excellent agreement in the relative intensities of the resonances for a given J, and we can extract time-dependent level populations. Thus, we can monitor the spontaneous relaxation of CH^{+} to its lowest rotational states and demonstrate the preparation of an internally cold beam of molecular ions.

Structure and stability of the negative hydrogen molecular ion.

We present the results of a Coulomb explosion experiment that allows for the imaging of the rovibrational wave function of the metastable H2- ion. Our measurements confirm the predicted large internuclear separation of 6 a.u., and they show that the ion decays by autodetachment rather than by spontaneous dissociation. Imaging of the resulting H2 products reveals a large angular momentum of J = 25 ± 2, quantifying the rotation that leads to the metastability of this most fundamental molecular anion.

Hot water molecules from dissociative recombination of D3O+ with cold electrons.

Individual product channels in the dissociative recombination of deuterated hydronium ions and cold electrons are studied in an ion storage ring by velocity imaging using spatial and mass-sensitive detection of the neutral reaction fragments. Initial and final molecular excitation are analyzed, finding the outgoing water molecules to carry internal excitation of more than 3 eV in 90% of the recombination events. Initial rotation is found to be substantial and in three-body breakup strongly asymmetric energy repartition among the deuterium products is enhanced for hot parent ions.

Ultraviolet and visible light photodissociation of H3+ in an ion storage ring.

Ultraviolet and visible photodissociation of a vibrationally excited H(3)(+) ion beam, as produced by standard ion sources, was successfully implemented in an ion storage ring with the aim of investigating the decay of the excited molecular levels. A collinear beams configuration was used to measure the photodissociation of H(3)(+) into H(2)(+) + H fragments by transitions into the first excited singlet state with 266 and 532 nm laser beams. A clear signal could be observed up to 5 ms of storage, indicating that enough highly excited rovibrational states survive on the millisecond time scale of the experiment. The decay into H(2)(+) + H shows an effective time constant between about 1 and 1.5 ms. The initial photodissociating states are estimated to lie roughly 1 eV below the dissociation limit of 4.4 eV. The expected low population of these levels gives rise to an effective cross section of several 10(-20) cm(2) for ultraviolet and some 10(-21) cm(2) for visible light. For using multistep resonant dissociation schemes to monitor rotational populations of cold H(3)(+) in low-density environments, these measurements open promising perspectives.

Anisotropy and molecular rotation in resonant low-energy dissociative recombination.

Angular fragment distributions from the dissociative recombination (DR) of HD(+) were measured with well directed monochromatic low-energy electrons over a dense grid of collision energies from 7 to 35 meV, where pronounced rovibrational Feshbach resonances occur. Significant higher-order anisotropies are found in the distributions, whose size varies along energy in a partial correlation with the relative DR rate from fast-rotating molecules. This may indicate a breakdown of the nonrotation assumption so far applied to predict angular DR fragment distributions.

Occurrence of 3-MCPD fatty acid esters in human breast milk.

A series of twelve breast milk samples were analysed by gas chromatography-mass spectrometry (GC/MS) operated in selected ion monitoring mode for 3-chloropropane-1,2-diol (3-MCPD). Whilst none of the samples contained 3-MCPD above the limit of detection of 3 microg kg(-1) milk, all contained high amounts of 3-MCPD esterified with higher fatty acids. The levels of 3-MCPD released by hydrolysis of these esters (bound 3-MCPD) ranged from the limit of detection (300 microg kg(-1), expressed on a fat basis) to 2195 microg kg(-1); with a mean level of bound 3-MCPD of 1014 microg kg(-1), which corresponded to 35.5 microg kg(-1) milk. The presence of bound 3-MCPD was confirmed using orthogonal gas chromatography coupled with high-speed time-of-flight mass spectrometry analysis for four randomly selected breast milk samples. Six breast milks collected from one of the nursing mothers 14-76 days after childbirth contained bound 3-MCPD within the range of 328-2078 microg kg(-1) fat (mean 930 microg kg(-1) fat). The calculated bound 3-MCPD content of these samples was within the range of 6 and 19 microg kg(-1) milk (mean of 12 microg kg(-1) milk). The major types of 3-MCPD esters were the symmetric diesters with lauric, palmitic, and oleic acids, and asymmetric diesters with palmitic acid/oleic acid among which 3-chloro-1,2-propanediol 1,2-dioleate prevailed.

Recombination of polycyclic aromatic hydrocarbon photoions with electrons in a flowing afterglow plasma.

A new technique, flowing afterglow with photoions (FIAPI), has been developed to measure the rate coefficient for the recombination of complex ions, and, in particular, polycyclic aromatic hydrocarbon (PAH) cations with electrons. The method is based on the flowing afterglow Langmuir probe - mass spectrometer apparatus at the University of Rennes I. A helium plasma is generated by a microwave discharge in a He buffer gas and downstream, a small amount of argon gas is injected to destroy any helium metastables. A very small amount of neutral PAH molecules is added to the afterglow plasma by evaporation from a plate coated with the PAH to be studied. PAH ions are then produced by photoionization of the parent molecule using a pulsed UV laser (157 nm). The laser beam is oriented along the flow tube and so a constant spatial concentration of photoions is obtained. The electron concentration along the flow tube is measured by means of a movable Langmuir probe. Ion concentration decay in time is measured at a fixed position using a quadrupole mass spectrometer which is triggered by the laser pulse. The recombination of anthracene and pyrene cations has been studied using this technique and we have found a recombination rate of (2.4 +/- 0.8) x 10(-6) cm(3) s(-1) for anthracene and (4.1 +/- 1.2) x 10(-6) cm(3) s(-1) for pyrene.

Rate constants and branching ratios for the dissociative recombination of C3D(+)7 and C4D(+)9.

Product branching ratios and thermal rate coefficients for the dissociative recombination of C3D(+)7 and C4D(+)9 have been measured in the ion storage ring CRYRING. The results for C3D(+)7 are believed to be slightly more accurate than those obtained earlier for C3H(+)7. Only the C-C bond breaking channels could be measured for C4D(+)9 and were found to be in excellent agreement with earlier data.

First observation of four-body breakup in electron recombination: C2D+5.

We report the first observation of four-body breakup in electron dissociative recombination of a molecular ion: C2D+5. In an ion storage ring experiment, the branching ratio for the process C2D+5 + e(-)-->C2D2 + D + D + D was determined to be 13%. This means that three covalent chemical bonds are broken as a result of the action of a single electron. This is the first time a four-body breakup of chemical bonds has been observed in a low-energy binary reaction.

An enhanced cosmic-ray flux towards zeta Persei inferred from a laboratory study of the H3+-e- recombination rate.

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

Conservative treatment versus surgery in spondylotic cervical myelopathy: a prospective randomised study.

A prospective randomised 2-year study was performed to compare the conservative and operative treatment of mild and moderate forms of spondylotic cervical myelopathy (SCM). Forty-eight patients presenting with the clinical syndrome of SCM, with a modified Japanese Orthopaedic Association (mJOA) score of 12 points or more, were randomised into two groups. Group A, treated conservatively, consisted of 27 patients, mean age 55.6 +/- 8.6 years, while group B was treated surgically (21 patients, mean age 52.7 +/- 8.1 years). The clinical outcome was measured by the mJOA score, recovery rate (RR), timed 10 m walk, score of daily activities (recorded by video and evaluated by two observers blinded to the therapy), and by the subjective assessment of the patients at 6, 12, and 24 months of the follow-up. There was, on average, no significant deterioration in mJOA score, recovery ratio, or timed 10 m walk within either group during the 2 years of follow-up. In the surgery group there was a slight decline in the scores for daily activities and subjective evaluation. A comparison of the two groups showed no significant differences in changes over time in mJOA score or quantified gait, but there were significant differences in the score of daily activities recorded by video at 24 months, which was a little lower in the surgical group, and also in RR and subjective evaluation, which were both worse in the surgical group at months 12 and 24. However, at month 6, this last parameter was significantly better in the surgical than in conservative group. Surgical treatment of mild and moderate forms of SCM in the present study design, comprising the patients with no or very slow, insidious progression and a relatively long duration of symptoms, did not show better results than conservative treatment over the 2-year follow-up.

The value of somatosensory and motor evoked evoked potentials in pre-clinical spondylotic cervical cord compression.

Previous studies have yielded conflicting data concerning the value of evoked potential parameters in the assessment of clinical relevance of cervical cord compression in clinically "silent" cases. The aim of this study was to assess the value of somatosensory (SEP) and motor evoked potentials (MEP) in the evaluation and prediction of the clinical course, by means of a 2-year follow-up prospective electrophysiological and clinical study performed in patients with clinically "silent" spondylotic cervical cord compression. Thirty patients with MR signs of spondylotic cervical cord compression but without clinical signs of myelopathy were evaluated clinically and using SEPs and MEPs during a 2-year period. The results of the study showed that SEPs and MEPs documented subclinical involvement of cervical cord in 50% of patients with clinically "silent" spondylotic cervical cord compression. During the 2-year period clinical signs of cervical myelopathy were observed in one-third of patients with entry EP abnormality in comparison with no patients with normal EP tests. Combined SEPs and MEPs proved to be a valuable tool in the assessment of the functional relevance of subclinical spondylotic cervical cord compression. Normal EP findings predict a favourable 2-year clinical outcome.

Value of hyperventilation in pattern-reversal visual evoked potentials.

The effects of a standard 3 minutes' hyperventilation on the full-field pattern-reversal visual evoked potential (VEP) were studied in 33 normal subjects, 30 definite multiple sclerosis patients and in twenty-five patients with abnormal VEPs due to either tumourous compression of the anterior visual pathways or optic atrophy of other origin. Significantly greater reductions in P100 latency occurred in multiple sclerosis patients in comparison with controls (p less than 0.05). This change appeared to be specific for demyelinative type of lesion, for it was not found in cases with other types of pathology. Hyperventilation also increased the sensitivity of visual pathway impairment detection in multiple sclerosis.