Photodesorption of CO ices: Rotational and translational energy distributions.

This study investigates the translational and rovibrational energy of vacuum-ultraviolet (VUV) photodesorbed CO molecules from a CO polycrystalline ice (15 K) at ∼8 eV. The electronic excitation was produced by a pulsed VUV laser, and the photodesorption of CO molecules in their ground and first vibrational states was observed using resonance enhanced multiphoton ionization. Time-of-flight and rotationally resolved spectra were measured, and the kinetic and internal energy distribution were obtained.

Flux and fluence effects on the vacuum-UV photodesorption and photoprocessing of CO ices.

CO is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO samples grown at low temperatures ( = 15 K) have been found to be very sensitive to experimental methods and conditions.

Mechanism of Ultraviolet-Induced CO Desorption from CO Ice: Role of Vibrational Relaxation Highlighted.

Although UV photon-induced CO ice desorption is clearly observed in many cold regions of the Universe as well as in the laboratory, the fundamental question of the mechanisms involved at the molecular scale remains debated. In particular, the exact nature of the involved energy transfers in the indirect desorption pathway highlighted in previous experiments is not explained. Using ab initio molecular dynamics simulations, we explore a new indirect desorption mechanism in which a highly vibrationally excited CO (v=40) within an aggregate of 50 CO molecules triggers the desorption of molecules at the surface.

Highlights from : Astrochemistry at high resolution, Baltimore, USA, May 2023.

The 'Astrochemistry at high resolution' was held at the Space Telescope Science Institute, Baltimore, United States, and online from May 31-June 2, 2023. The meeting brought together observers, modellers, and experimentalists at different career stages and from different countries to discuss advancements in astrochemistry resulting from improved spatial resolution, spectral resolution, and sensitivity. This conference report provides highlights of the meeting and summaries of the talks presented.

Vacuum UV photodesorption of organics in the interstellar medium: an experimental study of formic acid HCOOH and methyl formate HCOOCH-containing ices.

Being a potential process that could explain gas phase abundances of so-called complex organic molecules (COMs) in the cold interstellar medium (ISM), the UV photon-induced desorption from organics-containing molecular ices has been experimentally studied. In this work, we focused on the observation of the photodesorbed products and the measurement of the associated photodesorption yields from pure and mixed molecular ices, each containing organic molecules whose detection has been achieved in the gas phase of the cold ISM, namely formic acid HCOOH and methyl formate HCOOCH. Each molecule, in pure ice or in ice mixed with CO or water, was irradiated at 15 K with monochromatic vacuum UV photons in the 7-14 eV range using synchrotron radiation from the SOLEIL synchrotron facility, DESIRS beamline.

Cryogenic Chemistry and Quantitative Non-Thermal Desorption from Pure Methanol Ices: High-Energy Electron versus X-Ray Induced Processes.

X-Ray irradiation of interstellar ice analogues has recently been proven to induce desorption of molecules, thus being a potential source for the still-unexplained presence of gaseous organics in the coldest regions of the interstellar medium, especially in protoplanetary disks. The proposed desorption mechanism involves the Auger decay of excited molecules following soft X-ray absorption, known as X-ray induced electron-stimulated desorption (XESD). Aiming to quantify electron induced desorption in XESD, we irradiated pure methanol (CH OH) ices at 23 K with 505 eV electrons, to simulate the Auger electrons originating from the O 1s core absorption.

Spectroscopic Measurements of Methane Solid-Gas Equilibrium Clapeyron Curve between 40 and 77 K.

The infrared gas-phase absorption spectrum of methane was used to determine its Clapeyron solid-gas equilibrium curve in the 40-77 K temperature range. For comparative purposes and to obtain more reliable results, two different optical experimental setups were used. At higher temperatures (53-77 K), a single pass cryogenically cooled cell was coupled to a standard low-resolution Fourier transform infrared spectrometer.

UV photodesorption of interstellar CO ice analogues: from subsurface excitation to surface desorption.

Carbon monoxide is after H(2) the most abundant molecule identified in the interstellar medium (ISM), and is used as a major tracer for the gas phase physical conditions. Accreted at the surface of water-rich icy grains, CO is considered to be the starting point of a complex organic--presumably prebiotic--chemistry. Non-thermal desorption processes, and especially photodesorption by UV photons, are seen as the main cause that drives the gas-to-ice CO balance in the colder parts of the ISM.

Reactivity of water-electron complexes on crystalline ice surfaces.

The interactions between long-living electrons trapped in defects of crystalline D2O and electronegative molecules have been investigated using two-photon photoemission spectroscopy. When covered by a Xe adlayer, the spectroscopic signature of the trapped electrons vanishes, which provides evidence that the trapping sites are located on the surface of the crystalline ice. The reactive character of these surface-trapped electrons with molecules has been studied.

Low-serotonin levels increase delayed reward discounting in humans.

Previous animal experiments have shown that serotonin is involved in the control of impulsive choice, as characterized by high preference for small immediate rewards over larger delayed rewards. Previous human studies under serotonin manipulation, however, have been either inconclusive on the effect on impulsivity or have shown an effect in the speed of action-reward learning or the optimality of action choice. Here, we manipulated central serotonergic levels of healthy volunteers by dietary tryptophan depletion and loading.

Multiple model-based reinforcement learning explains dopamine neuronal activity.

A number of computational models have explained the behavior of dopamine neurons in terms of temporal difference learning. However, earlier models cannot account for recent results of conditioning experiments; specifically, the behavior of dopamine neurons in case of variation of the interval between a cue stimulus and a reward has not been satisfyingly accounted for. We address this problem by using a modular architecture, in which each module consists of a reward predictor and a value estimator.

Reactivity induced at 25 K by low-energy electron irradiation of condensed NH3-CH3COOD (1 : 1) mixture.

Chemical reactivity is observed following electron irradiation of a binary mixture of ammonia (NH(3)) and acetic acid (CH(3)COOD) at 25 K, without any subsequent thermal activation, as evidenced by vibrational high resolution electron energy loss spectroscopy (HREELS). Analysis of the HREEL spectra and comparison with infrared and Raman data of different molecules are compatible with glycine formation in its zwitterionic form. The onset for electron induced reaction is found to be at about approximately 13 eV.