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(Abridged) We explore the chemistry of the most abundant C, O, S, and N bearing species in molecular clouds, in the context of the IRAM 30 m Large Programme Gas phase Elemental abundances in Molecular Clouds (GEMS). In this work, we aim to assess the limitations introduced in the observational works when a uniform density is assumed along the line of sight for fitting the observations, developing a very simple numerical model of a turbulent box. We perform a MHD simulation in order to reproduce the turbulent steady-state of a turbulent box with properties typical of a molecular filament before collapse. We post-process the results of the MHD simulation with a chemical code to predict molecular abundances, and then post-process this cube with a radiative transfer code to create synthetic emission maps for a series of rotational transitions observed during the GEMS project. From the chemical point of view, we find that turbulence produces variations on the predicted abundances, but they are more or less critical depending on the chosen transition and the chemical age. When compared to real observations, the results from the turbulent simulation provides a better fit than when assuming a uniform gas distribution along the line of sight. In the view of our results, we conclude that taking into account turbulence when fitting observations might significantly improve the agreement with model predictions. This is especially important for sulfur bearing species that are very sensitive to the variations of density produced by turbulence at early times (0.1 Myr). The abundance of CO is also quite sensitive to turbulence when considering the evolution beyond a few 0.1 Myr., Comment: Published in Astronomy and Astrophysics

We found four series of harmonically related lines in IRC\,+10216 with the Yebes\,40m and IRAM\,30m telescopes. The first series corresponds to a molecule with a rotational constant, $B$, of 1448.5994$\pm$0.0013 MHz and a distortion constant, $D$, of 63.45$\pm$1.15 Hz and covers upper quantum numbers from $J_u$=11 up to 33 (B1449). The second series is fitted with $B$=1446.9380$\pm$0.0098 MHz and $D$=91$\pm$23 Hz and covers upper quantum numbers from $J_u$=11 up to 17 (B1447). The third series is fitted with $B$=598.7495$\pm$0.0011 MHz and D=6.13$\pm$0.43 Hz and covers quantum numbers from $J_u$=26 up to 41 (B599). Finally, the frequencies of the last series of lines can be reproduced with $B$=594.3176$\pm$0.0026 MHz and $D$=4.92$\pm$1.16 Hz (B594). The large values of $D$ point toward four metal-bearing carriers. After exploring all plausible candidates containing Na, Al, Mg, and other metals, our ab initio calculations indicate that the cations MgC$_4$H$^+$, MgC$_3$N$^+$, MgC$_6$H$^+$, and MgC$_5$N$^+$ must be the carriers of B1449, B1447, B599, and B594, respectively. These cations could be formed by the radiative association of Mg$^+$ with C$_4$H, C$_3$N, C$_6$H, and C$_5$N, respectively. We calculated the radiative association rate coefficient of Mg$^+$ with C$_4$H, C$_3$N, C$_6$H, and C$_5$N and incorporated them in our chemical model. The results confirm that the Mg-bearing cations can be formed through these radiative association reactions in the outer layers of IRC\,+10216. This is the first time that cationic metal-bearing species have been found in space. These results provide a new paradigm on the reactivity of ionized metals with abundant radicals and open the door for further characterization of similar species in metal-rich astrophysical environments.

We carried out an observational search for the recently discovered molecule H2NC, and its more stable isomer H2CN, toward eight cold dense clouds (L1544, L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH3, L1498, and L1641N) and two diffuse clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in different types of interstellar regions and shed light on its formation mechanism. We detected H2NC in most of the cold dense clouds targeted, 7 out of 8, while H2CN was only detected in 5 out of 8 clouds. The column densities derived for both H2NC and H2CN are in the range 1e11-1e12 cm-2 and the abundance ratio H2NC/H2CN varies between 0.51 and >2.7. The metastable isomer H2NC is therefore widespread in cold dense clouds where it is present with an abundance similar to that of H2CN. We did not detect either H2NC or H2CN in any of the two diffuse clouds targeted, which does not allow to shed light on how the chemistry of H2NC and H2CN varies between dense and diffuse clouds. We found that the column density of H2NC is correlated with that of NH3, which strongly suggests that these two molecules are chemically linked, most likely ammonia being a precursor of H2NC through the C + NH3 reaction. We performed electronic structure and statistical calculations which show that both H2CN and H2NC can be formed in the C + NH3 reaction through two different channels involving two different transition states which lie very close in energy. The predicted product branching ratio H2NC/H2CN is very method dependent but values between 0.5 and 0.8 are the most likely ones. Therefore, both the astronomical observations and the theoretical calculations support that the reaction C + NH3 is the main source of H2NC in interstellar clouds., Comment: Accepted for publication in A&A

The title reaction is studied using a quasi-classical trajectory method for collision energies between 0.1 meV and 10 eV, considering the vibrational excitation of H$_2^+$ reactant. A new potential energy surface is developed based on a Neural Network many body correction of a triatomics-in-molecules potential, which significantly improves the accuracy of the potential up to energies of 17 eV, higher than in other previous fits.The effect of the fit accuracy and the non-adiabatic transitions on the dynamics are analyzed in detail.The reaction cross section for collision energies above 1 eV increases significantly with the increasing of the vibrational excitation of H$_2^+$($v'$), for values up to $v'$=6. The total reaction cross section (including the double fragmentation channel) obtained for $v'$=6 matches the new experimental results obtained by Savic, Schlemmer and Gerlich [Chem. Phys. Chem. 21 (13), 1429.1435(2020)]. The differences among several experimental setups, for collision energies above 1 eV, showing cross sections scattered/dispersed over a rather wide interval, can be explained by the differences in the vibrational excitations obtained in the formation of H$_2^+$ reactants. On the contrary, for collision energies below 1 eV, the cross section is determined by the long range behavior of the potential and do not depend strongly on the vibrational state of H$_2^+$. In addition in this study, the calculated reaction cross sections are used in a plasma model and compared with previous results. We conclude that the efficiency of the formation of H$_3^+$ in the plasma is affected by the potential energy surface used., Comment: 26 pages, 11 figures

Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30m large program aimed at determining the elemental abundances of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical star-forming filaments. In particular, the elemental abundance of S remains uncertain by several orders of magnitude and its determination is one of the most challenging goals of this program. We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO$^+$, HCN, HNC, CS, SO, H$_2$S, OCS, and HCS$^+$ to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions from A$_V$ $\sim$ 3 mag to $>$50 mag, molecular hydrogen densities ranging from a few 10$^3$~cm$^{-3}$ to 3$\times$10$^6$~cm$^{-3}$, and T$_k$ $\sim$ 10$-$35 K. Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry, t=0.1 Myr, $\zeta_{H_2}$$\sim$ (0.5$-$1)$\times$10$^{-16}$ s$^{-1}$, and [S/H]$\sim$1.5$\times$10$^{-6}$. On the contrary, most of the positions in Orion are fitted with t=1~Myr and $\zeta_{H_2}$$\sim$ 10$^{-17}$ s$^{-1}$. Moreover, $\sim$40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H]$\sim$1.5$\times$10$^{-5}$. Our results suggest that sulfur depletion depends on the environment. While the abundances of sulfur-bearing species are consistent with undepleted sulfur in Orion, a depletion factor of $\sim$20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution in the envelopes of the studied clouds might explain these variations. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H] in Orion., Comment: 22 pages, 15 figures, Astronomy and Astrophysics, in press

The H + D_2^+(v=0,1 and 2) charge transfer reaction is studied using an accurate wave packet method, using recently proposed coupled diabatic potential energy surfaces. The state-to-state cross section is obtained for three different channels: non-reactive charge transfer, reactive charge transfer, and exchange reaction. The three processes proceed via the electronic transition from the first excited to the ground electronic state. The cross section for the three processes increases with the initial vibrational excitation. The non-reactive charge transfer process is the dominant channel, whose branching ratio increases with collision energy, and it compares well with experimental measurements at collision energies around 0.5 eV. For lower energies the experimental cross section is considerably higher, suggesting that it corresponds to higher vibrational excitation of D_2^+(v) reactants. Further experimental studies of this reaction and isotopic variants are needed, where conditions are controlled to obtain a better analysis of the vibrational effects of the D_2^+ reagents., Comment: 15 pages, 7 figures

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m Large Program designed to estimate the S, C, N, and O depletions and gas ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus and Orion. Our immediate goal is to build up a complete database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV 3 to 100 mag, n(H2) a few 10$^{3}$ to 10$^6$ cm$^{-3}$, and Tk 10 to 30 K ranges in these clouds, avoiding protostars, HII regions, and outflows. A statistical analysis is carried out to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families: (1) 13CO and C18O; (2) H13CO+, HC18O+, H13CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of 3. The abundances of H13CO+, HC18O+, H13CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law n(H2)$^{-0.8\pm0.2}$. The abundances of S-bearing species also decrease with n(H2) at a rate of (S-bearing/H)gas n(H2)$^{-0.6\pm0.1}$. The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies., Comment: 28 pages, 22 figures. Accepted for publication in A&A

Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood. Motivated by new observations of the Orion Bar PDR - 1'' resolution ALMA images of SH+; IRAM 30m detections of H2S, H2S34, and H2S33; H3S+ (upper limits); and SOFIA observations of SH - we perform a systematic study of the chemistry of S-bearing hydrides. We determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH+ + H2 <-> H2S+ + H and S + H2 <-> SH + H. We find that reactions of UV-pumped H2 (v>1) with S+ explain the presence of SH+ in a high thermal-pressure gas component, P_th~10^8 cm^-3 K, close to the H2 dissociation front. However, subsequent hydrogen abstraction reactions of SH+, H2S+, and S with vibrationally excited H2, fail to ultimately explain the observed H2S column density (~2.5x10^14 cm^-2, with an ortho-to-para ratio of 2.9+/-0.3). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H2S (s-H2S). The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H2S) at warmer dust temperatures and closer to the UV-illuminated edges of molecular clouds. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H2S column density (a few 10^14 cm-^2) and abundance peak (a few 10^-8) nearly independently of n_H and G_0. This agrees with the observed H2S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances., Comment: Accepted for publication in A&A, 25 pages (abridged abstract). This is v2 with minor editorial changes

CS is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. Chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. The CS+O -> CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150-400 K, but the extrapolation to lower temperatures is doubtful. Here we calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. We performed ab initio calculations to obtain the three lowest PES of the CS+O system. These PESs are used to study the reaction dynamics, using several methods to eventually calculate the CS+O thermal reaction rates. We compare the results of our theoretical calculations for 150-400 K with those obtained in the laboratory. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150-400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, which is consistent with the extrapolation of experimental data using the Arrhenius expression. We use the updated chemical network to model the sulfur chemistry in TMC1 based on molecular abundances determined from GEMS project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance., Comment: 11 pages, 10 figures, accepted in Astronomy & Astrophysics

Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium, and the identity of the main sulphur reservoir is still an open question. Our goal is to investigate the H$_{2}$S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Using millimeter observations of CS, SO, H$_{2}$S, and their isotopologues, we determine the physical conditions and H$_{2}$S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model Nautilus is then used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H$_2$S abundance. Our model shows that chemical desorption is the main source of gas-phase H$_2$S in dark cores. The measured H$_{2}$S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when $n_{\rm H}>2\times10^{4}$. This change in the desorption rate is consistent with the formation of thick H$_2$O and CO ice mantles on grain surfaces. The observed SO and H$_2$S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of $5-10$. Along the three cores, atomic S is predicted to be the main sulphur reservoir. We conclude that the gaseous H$_2$S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H$_2$S. The behavior of the observed H$_{2}$S abundance suggests a changing desorption efficiency, which would probe the snowline in these cores. Our model, however, overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10., Comment: 23 pages, 16 figures

GEMS is an IRAM 30m Large Program whose aim is determining the elemental depletions and the ionization fraction in a set of prototypical star-forming regions. This paper presents the first results from the prototypical dark cloud TMC 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3mm and 2mm) and the 40m Yebes telescope (1.3cm and 7mm) to determine the fractional abundances of CO, HCO+, HCN, CS, SO, HCS+, and N2H+ in three cuts which intersect the dense filament at the well-known positions TMC 1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from Av~3 to ~20mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of (1-5)x10$^3$ cm$^{-3}$; and ii) the dense phase, located at Av>10mag, with molecular hydrogen densities >10$^4$ cm$^{-3}$. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C+/C/CO transition zone where C/H~8x10$^{-5}$ and C/O~0.8-1, until the beginning of the dense phase at Av~10 mag. This is consistent with the grain temperatures being below the CO evaporation temperature in this region. In the case of sulfur, a strong depletion should occur before the translucent phase where we estimate a S/H~(0.4 - 2.2) x10$^{-6}$, an abundance ~7-40 times lower than the solar value. A second strong depletion must be present during the formation of the thick icy mantles to achieve the values of S/H measured in the dense cold cores (S/H~8x10$^{-8}$). Based on our chemical modeling, we constrain the value of $\zeta_{\rm H_2}$ to ~(0.5 - 1.8) x10$^{-16}$ s$^{-1}$ in the translucent cloud., Comment: 26 pages, 20 figures, A&A in press

We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction between C$^+(^2P)$ and H$_2(\nu_{\rm H_2}=1, 2)$, while the collisional (de)excitation and destruction of CH$^+$ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000~K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH$^+$ with H atoms at kinetic temperatures below 50~K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H$_2$ has a substantial effect on the excitation of CH$^+$ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH$^+$ toward the Orion Bar and the planetary nebula NGC~7027. Our results further suggest that the population of $\nu_{\rm H_2}=2$ might be significant in the photon-dominated region of NGC~7027., Comment: 10 pages, 8 figures, accepted 2017 April 7

As many organic molecules, formic acid (HCOOH) has two conformers (trans and cis). The energy barrier to internal conversion from trans to cis is much higher than the thermal energy available in molecular clouds. Thus, only the most stable conformer (trans) is expected to exist in detectable amounts. We report the first interstellar detection of cis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a low trans-to-cis abundance ratio of 2.8+-1.0, supports a photoswitching mechanism: a given conformer absorbs a stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we specifically study with ab initio quantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation., Comment: Manuscript accepted for publication in A&A

Barnard B1b has revealed as one of the most interesting globules from the chemical and dynamical point of view. It presents a rich molecular chemistry characterized by large abundances of deuterated and complex molecules. Furthermore, it hosts an extremely young Class 0 object and one candidate to First Hydrostatic Core (FHSC). Our aim was to determine the cosmic ray ionization rate and the depletion factors in this extremely young star forming region. We carried out a spectral survey towards Barnard 1b as part of the IRAM Large program ASAI using the IRAM 30-m telescope at Pico Veleta (Spain). This provided a very complete inventory of neutral and ionic C-, N- and S- bearing species with, up to our knowledge, the first secure detections of the deuterated ions DCS+ and DOCO+. We used a state-of-the-art pseudo-time-dependent gas-phase chemical model to determine the value of the cosmic ray ionization rate and the depletion factors. The observational data were well fitted with $\zeta_{H_2}$ between 3E-17 s$^{-1}$ and 1E-16 s$^{-1}$. Elemental depletions were estimated to be ~10 for C and O, ~1 for N and ~25 for S. Barnard B1b presents similar depletions of C and O than those measured in pre-stellar cores. The depletion of sulfur is higher than that of C and O but not as extreme as in cold cores. In fact, it is similar to the values found in some bipolar outflows, hot cores and photon-dominated regions. Several scenarios are discussed to account for these peculiar abundances. We propose that it is the consequence of the initial conditions (important outflows and enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that permits to maintain most S- and N-bearing species in gas phase to great optical depths. The interaction of the compact outflow associated with B1b-S with the surrounding material could enhance the abundances of S-bearing molecules, as well., Comment: Paper accepted in Astronomy and Astrophysics; 28 pags, 21 figures

The rate constants required to model the OH$^+$ observations in different regions of the interstellar medium have been determined using state of the art quantum methods. First, state-to-state rate constants for the H$_2(v=0,J=0,1)$+ O$^+$($^4S$) $\rightarrow$ H + OH$^+(X ^3\Sigma^-, v', N)$ reaction have been obtained using a quantum wave packet method. The calculations have been compared with time-independent results to asses the accuracy of reaction probabilities at collision energies of about 1 meV. The good agreement between the simulations and the existing experimental cross sections in the $0.01-$1 eV energy range shows the quality of the results. The calculated state-to-state rate constants have been fitted to an analytical form. Second, the Einstein coefficients of OH$^+$ have been obtained for all astronomically significant ro-vibrational bands involving the $X^3\Sigma^-$ and/or $A^3\Pi$ electronic states. For this purpose the potential energy curves and electric dipole transition moments for seven electronic states of OH$^+$ are calculated with {\it ab initio} methods at the highest level and including spin-orbit terms, and the rovibrational levels have been calculated including the empirical spin-rotation and spin-spin terms. Third, the state-to-state rate constants for inelastic collisions between He and OH$^+(X ^3\Sigma^-)$ have been calculated using a time-independent close coupling method on a new potential energy surface. All these rates have been implemented in detailed chemical and radiative transfer models. Applications of these models to various astronomical sources show that inelastic collisions dominate the excitation of the rotational levels of OH$^+$. In the models considered the excitation resulting from the chemical formation of OH$^+$ increases the line fluxes by about 10 % or less depending on the density of the gas.

The behavior displayed by a quantum system when it is perturbed by a series of von Neumann measurements along time is analyzed. Because of the similarity between this general process with giving a deck of playing cards a shuffle, here it is referred to as quantum shuffling, showing that the quantum Zeno and anti-Zeno effects emerge naturally as two time limits. Within this framework, a connection between the gradual transition from anti-Zeno to Zeno behavior and the appearance of an underlying Markovian dynamics is found. Accordingly, although a priori it might result counterintuitive, the quantum Zeno effect corresponds to a dynamical regime where any trace of knowledge on how the unperturbed system should evolve initially is wiped out (very rapid shuffling). This would explain why the system apparently does not evolve or decay for a relatively long time, although it eventually undergoes an exponential decay. By means of a simple working model, conditions characterizing the shuffling dynamics have been determined, which can be of help to understand and to devise quantum control mechanisms in a number of processes from the atomic, molecular and optical physics., Comment: 12 pages, 2 figures

The possibility to observe quantum Zeno and anti-Zeno scenarios for atom-diatom reactive collisions is investigated for two diferent processes (F+HD and H+O_2) by means of time-dependent wave packet propagations. A novel approach is proposed in which the survival probabilities investigated are those obtained when the initial state is redefined after time intervals tau at which measurements are performed on the system. The comparison with the actual probability for the unperturbed system reveals the existence of a regime in which the decay from the initial state appears to be inhibited (the quantum Zeno effect) or accelerated (the anti-Zeno effect). In contrast with preceding interpretations, given that the time-evolving wave packet is not affected at any time ("flying arrow"), the present procedure does not require to invoke to counterintuitive hindered evolutions ("stopping arrow") to explain the results. On the contrary the consecutive update of the reference state of the system solves the apparent modern version of Zeno's paradox.

A quantum control mechanism is proposed for molecular fragmentation processes within a scenario grounded on the quantum Zeno effect. In particular, we focus on the van der Waals Ne-Br$_2$ complex, which displays two competing dissociation channels via vibrational and electronic predissociation. Accordingly, realistic three dimensional wave packet simulations are carried out by using ab initio interaction potentials recently obtained to reproduce available experimental data. Two numerical models to simulate the repeated measurements are reported and analyzed. It is found that the otherwise fast vibrational predissociation is slowed down in favor of the slow electronic (double fragmentation) predissociation, which is enhanced by several orders of magnitude. Based on these theoretical predictions, some hints to experimentalists to confirm their validity are also proposed., Comment: 4 pages, 3 figures