Near-infrared absorption and radiative cooling of naphthalene dimers (C10H8)2.

The radiative cooling of naphthalene dimer cations, (C10H8)2+ was studied experimentally through action spectroscopy using two different electrostatic ion-beam storage rings, DESIREE in Stockholm and Mini-Ring in Lyon. The spectral characteristics of the charge resonance (CR) band were observed to vary significantly with a storage time of up to 30 seconds in DESIREE. In particular, the position of the CR band shifts to the blue, with specific times (inverse of rates) of 0.64 s and 8.0 s in the 0-5 s and 5-30 s storage time ranges, respectively. These long-time scales indicate that the internal energy distribution of the stored ions evolves by vibrational radiative cooling, which is consistent with the absence of fast radiative cooling via recurrent fluorescence for (C10H8)2+. Density functional based tight binding calculations with local excitations and configuration interactions (DFTB-EXCI) were used to simulate the absorption spectrum for ion temperatures between 10 and 500 K. The evolution of the bandwidth and position with temperature is in qualitative agreement with the experimental findings. Furthermore, these calculations yielded linear temperature dependencies for both the shift and the broadening. Combining the relationship between the CR band position and the ion temperature with the results of the statistical model, we demonstrate that the observed blue shift can be used to determine the radiative cooling rate of (C10H8)2+.

Efficient radiative cooling of tetracene cations C18H12+: absolute recurrent fluorescence rates as a function of internal energy.

We have measured recurrent fluorescence (RF) cooling rates of internally hot tetracene cations, C18H12+, as functions of their storage times and internal energies in two different electrostatic ion-beam storage rings - the cryogenic ring DESIREE with a circumference of 8.6 meters in Stockholm and the much smaller room temperature ring Mini-Ring in Lyon, which has a circumference of 0.71 meters. The RF rates were measured to be as high as 150 to 1000 s-1 for internal energies in the 7 to 9.4 eV energy range, where we have probed the time evolution of the internal energy distribution with nanosecond laser pulses with a 1 kHz repetition rate. These RF rates are found to be significantly higher than those of previously investigated smaller PAHs such as e.g. anthracene and naphthalene, for which the lowest non-forbidden electronic excited state, the D2 state, is populated with a smaller probability by inverse internal conversion. Furthermore, the D2-D0 transition rate is smaller for these smaller molecules than for tetracene. The complementary features of the two storage rings allow for RF rate measurements in a broader internal energy range than has been possible before. The smaller sampling period of about 6 µs in Mini-Ring is ideal to study the cooling dynamics of the hotter ions that decay fast, whereas DESIREE with a sampling period of about 60 µs is better suited to study the colder ions that decay on longer timescales ranging up to hundreds of milliseconds. The excellent agreement between the two series of measurements in the region where they overlap demonstrates the complementarity of the two electrostatic ion-beam storage rings.

Kinetic energy released in the vibrational autodetachment of sulfur hexafluoride anion.

The kinetic energy release distribution (KERD) in the vibrational autodetachment (VAD) from sulfur hexafluoride anion SF6 - has been measured in a velocity map imaging spectrometer for delays in the range of a few tens of microseconds. The experimental KERD is analyzed within the framework of the detailed-balance: first using the standard Langevin model and subsequently using a more refined and realistic model based on the experimental attachment cross section. A discussion on the processes involved in the attachment and the VAD is presented based on an empirical fit of the attachment cross section. The lifetime derived from the model is in good agreement with the experimental time window, strengthening this theoretical approach for this model system.

VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes.

Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium (ISM), yet the branching ratio between ionization and fragmentation is poorly studied. This is crucial for the stability and charge state of PAHs in the ISM in different environments, affecting in turn the chemistry, the energy balance, and the contribution of PAHs to the extinction and emission curves. We studied the interaction of PAH cations with VUV photons in the 7 - 20 eV range from the synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization for a set of eight PAH cations ranging in size from 14 to 24 carbon atoms, with different structures. At photon energies below ~13.6 eV fragmentation dominates for the smaller species, while for larger species ionization is immediately competitive after the second ionization potential (IP). At higher photon energies, all species behave similarly, the ionization yield gradually increases, leveling off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to mainly affect the fragmentation cross section, but not the ionization cross section. We also measured the second IP for all species and the third IP for two of them, all are in good agreement with theoretical ones confirming that PAH cations can be further ionized in the diffuse ISM. Determining actual PAH dication abundances in the ISM will require detailed modeling. Our measured photo-ionization yields for several PAH cations provide a necessary ingredient for such models.

Fragmentation of adenine under energy control.

We present results on the fragmentation of adenine dication as a function of the excitation energy. The adenine molecule is charged and excited in a single collision with Cl(+) ion at 3 keV and the excitation energy distribution is obtained for each fragmentation channel by measuring the kinetic energy loss of the projectile. This method named collision induced dissociation under energy control is based on the formation of a negative scattered projectile as a result of double electron capture from the target molecule. Comparison between the main dissociation channels of singly and doubly charged adenine shows that fragmentation patterns are very similar consisting mainly of the successive emission of neutral HCN or H(2)CN(+). The energy distributions of the parent adenine dication and the kinetic energy release of the fragments measured for the most abundant fragmentation channels confirms the assumption of successive emission dynamics. A specific fragmentation pathway of the adenine requiring less energy than the usual successive emission of neutral HCN could be identified. It consists of the emission of a charged H(2)CN(+) following on by the emission of a dimer of HCN (precisely HC(2)N(2)). This new channel, measured for a mean excitation energy of 8.4 eV for the adenine dication is very closed to the emission of HCN monomer measured at 7.9 eV. The implications of these results concern the formation of adenine in the sealed-tube reaction of HCN with liquid ammonia as well as the possible formation of the adenine molecule in the interstellar medium. This last point is briefly discussed in relation to astrobiology and exobiology interests.

Control of erythropoiesis after high altitude acclimatization.

Erythropoiesis was studied in 11 subjects submitted to a 4-h hypoxia (HH) in a hypobaric chamber (4,500 m, barometric pressure 58.9 kPa) both before and after a 3-week sojourn in the Andes. On return to sea level, increased red blood cells (+3.27%), packed cell volume (+4.76%), haemoglobin (+6.55%) ( P<0.05), and increased arterial partial pressure of oxygen (+8.56%), arterial oxygen saturation (+7.40%) and arterial oxygen blood content ( C(a)O(2)) (+12.93%) at the end of HH ( P<0.05) attested high altitude acclimatization. Reticulocytes increased during HH after the sojourn only (+36.8% vs +17.9%, P<0.01) indicating a probable higher reticulocyte release and/or production despite decreased serum erythropoietin (EPO) concentrations (-46%, P<0.01). Hormones (thyroid, catecholamines and cortisol), iron status (serum iron, ferritin, transferrin and haptoglobin) and renal function (creatinine, renal, osmolar and free-water clearances) did not significantly vary (except for lower thyroid stimulating hormone at sea level, P<0.01). Levels of 2,3-diphosphoglycerate (2,3-DPG) increased throughout HH on return (+14.7%, P<0.05) and an inverse linear relationship was found between 2,3-DPG and EPO at the end of HH after the sojourn only ( r=-0.66, P<0.03). Inverse linear relationships were also found between C(a)O(2) and EPO at the end of HH before ( r=-0.63, P<0.05) and after the sojourn ( r=-0.60, P=0.05) with identical slopes but different ordinates at the origin, suggesting that the sensitivity but not the gain of the EPO response to hypoxia was modified by altitude acclimatization. Higher 2,3-DPG levels could partly explain this decreased sensitivity of the EPO response to hypoxia. In conclusion, we show that altitude acclimatization modifies the control of erythropoiesis not only at sea level, but also during a subsequent hypoxia.

Declarative memory impairments following a military combat course: parallel neuropsychological and biochemical investigations.

The aim of this study was to investigate the impact on several forms of memory and metabolism of a 5-day combat course including heavy and continuous physical activities and sleep deprivation. Mnemonic performance and biochemical parameters of 21 male soldiers were examined before and at the end of the course. Our results showed that short-term memory (memory span, visual memory, audiovisual association) and long-term memory were significantly impaired, whereas short-term spatial memory and planning tasks were spared. Parallel biochemical analysis showed an adaptation of energy metabolism. The observed decrease in glycaemia may be partly responsible for the long-term memory impairment, whereas the decreases in plasma cholinesterases and choline may be involved in the short-term memory deterioration. However, there are also many other reasons for the observed memory changes, one of them being chronic sleep deprivation.

Behavioural changes after an acute stress: stressor and test types influences.

Behavioural consequences of different acute stressors (30 min of restraint, 20 min of forced swim stress, 15 min of inescapable footshocks) applied at the beginning of the active period were assessed in using two behavioural tests: a 20 min light extinction test 24 h after the stressor exposure in order to explore the psychomotor ability and a 10 min open field session within the dark period 48 h after the stressor exposure to estimate the emotional status and the locomotor activity of the rat. Different behavioural responses were observed depending on the nature of the applied stressor. In the light extinction test, the footshock-stressed rats developed a very low activity independent on light conditions whereas the rats submitted to forced swim and restraint exhibited an activity level depending on the strain. Moreover, restrained rats had a higher transient activity than forced swim rats under light condition. In the open field test, none of the stressed rats did develop differences in behaviour. The efficacy of a 24 h recovery period on the behavioural response to an acute stressor exposure depends on the intensity of the applied stressor and the behavioural demands.