arXiv subjects

Miguel Carvajal

Publications and source records attributed to Miguel Carvajal.

At least 19 recordsLinked to original sources

arXiv2026

Degeneracy beyond the parity-symmetry protection in the Lipkin-Meshkov-Glick model

Degeneracy patterns in quantum mechanics stem from the system symmetries. In particular, the broken-symmetry phase in the well-known Lipkin-Meshkov-Glick (LMG) model is composed of doubly-degenerate states of different parity. In this work, we show that such doublets can exist even if parity is not conserved. For this purpose, our starting point is an anharmonic LMG Hamiltonian with a second-order ground-state quantum phase transition (GSQPT) and a rich spectrum, with two different excited-state quantum phase transitions. The inclusion in the Hamiltonian of a term inducing a first-order GSQPT breaks the parity symmetry but conserving the exponential degeneracy in the energy doublets. We demonstrate that this phenomenon can be traced back to the existence of a $\mathbb{Z}_2$ symmetry (reflection symmetry) in the system's classical limit phase space that leads to an anti-unitary $\mathbb{Z}_2$ symmetry in the quantum system.

quant-ph↗

arXiv2026

High-temperature simulation of the Raman spectra of the isotopologues $^{13}$C$^{16}$O$_2$ and $^{16}$O$^{13}$C$^{18}$O

Recently, experimental Raman spectra of the isotopologues $^{13}$C$^{16}$O$_2$ and $^{16}$O$^{13}$C$^{18}$O at high temperature have been reported. Separately, a reliable approach for obtaining vibrational wave functions for most isotopologues has been proposed. These descriptions, based on the $SU_1(2)\times U(3)\times SU_2(2)$ dynamical group, provide spectroscopic-quality fits to vibrational term values with root-mean-square deviations of $0.06$ and $0.07~\mathrm{cm}^{-1}$, respectively. Using the resulting wave functions, we simulate the Raman spectra of both species by evaluating transition moments of the mean polarizability, represented as an expansion in curvilinear coordinates up to cubic terms within the same algebraic framework. The difference of two independent experimental Raman spectra under comparable temperature conditions is analyzed with the help of the simulations provided by our model. The resulting simulations show excellent agreement with experiment, reinforcing the reliability of the model, while the estimated transition moments are also consistent with experimental values.

physics.chem-ph↗

arXiv2026

Degeneracy beyond the parity-symmetry protection in one-dimensional spinless models: The parity-violating Kerr parametric oscillator

One-dimensional quantum systems that undergo spontaneous symmetry-breaking, having a symmetric (non-degenerate) and a broken-symmetry (doubly-degenerate) phase, have been intensely studied in different branches of physics. In most cases, the spontaneously-broken symmetry is parity. However, it is possible to obtain similar phases in systems without parity symmetry, through an antiunitary symmetry that implies a two-fold symmetry either on momentum or coordinate in the system's classical limit. To illustrate this phenomenon, we use a Kerr parametric oscillator (KPO) with one- and two-photon drives that, despite the breaking of parity symmetry, may have doubly-degenerate levels. Different realizations of squeezed KPOs convey a great deal of attention, as effective Hamiltonians for driven superconducting circuits and the occurrence of degeneracy in such systems could be of practical interest in their application to obtain protected qubits in parity-breaking setups. In addition to this, the reported spectral features strongly indicate the existence of additional symmetries in the system.

quant-ph↗

arXiv2026

Ab Initio Characterization of C2H4N2 Isomers: Structures, electronic energies, spectroscopic parameters and formation pathways

This work presents a comprehensive theoretical investigation of key isomers of C2H4N2 using state-of-the-art quantum chemical methods. The objective is to characterize their molecular structures, spectroscopic constants, and electronic energies, and to elucidate plausible formation and destruction pathways, providing data critical for astrochemical and atmospheric detection. High-accuracy ab initio methods were employed, notably CCSD(T)-F12/cc-pVTZ-F12 for optimized geometries. Additional calculations were performed at the CCSD(T)/aug-cc-pVTZ, CCSD(T)/cc-pVTZ, MP2/aug-cc-pVTZ, and CIS levels. Intrinsic reaction coordinate (IRC) calculations were performed at the B3LYP/6-31G(d,p) level to explore reaction pathways. Zero-point energy corrections were determined for all isomers considered. Six low-energy C2H4N2 isomers were identified, all within 1 eV of the global minimum. Among them, methylcyanamide (MCA) exhibits the lowest relative energy (~0.2 eV) and a significant electric dipole moment of 5.00 D, making it a strong candidate for gas-phase detection. The rotational constants for MCA, computed at the CCSD(T)-F12/cc-pVTZ-F12 level, are Ae = 34932.44 MHz, Be = 4995.31 MHz, and Ce = 4520.30 MHz. The V3 torsional barrier was found to be 631.19 cm^{-1}. Centrifugal distortion constants were computed up to sextic order for all isomers. Formation pathways for MCA, such as CH3N + HCN -> CH3NHCN and related isomers, were characterized. The combination of large dipole moments and distinct rotational signatures supports the detectability of methylcyanamide and related C2H4N2 isomers via radioastronomy, infrared, and microwave spectroscopy. Isomerization and reaction pathways involving radical-neutral and neutral-neutral processes were found to be key to their formation in gas-phase environments. These results provide a robust foundation for future observational and modeling efforts.

physics.chem-ph↗

arXiv2025

Theoretical Exploration of the Diene-Transmissive Hetero-Diels-Alder Strategy Toward Boron-Functionalized Octahydroquinolines

A diene-transmissive hetero-Diels-Alder strategy, grounded in previous experimental works and employing boronated dienophiles, is proposed for the synthesis of boron-bearing octahydroquinolines. To assess its feasibility, three representative reactions were investigated, and their thermodynamics were evaluated in toluene and acetonitrile at various temperatures using the WB97X-D level of theory. The peri-, regio-, stereo-, and $π$-facial selectivities were predicted. The reactions mechanisms were elucidated through exploration of the reaction pathways. The predictions are consistent with available experimental work, and show the reactions are feasible with low to moderate polarity. The results also demonstrate that the reactions selectivity can in some cases be tuned by judicious choice of reaction conditions to deliver specific products with high selectivity.

physics.chem-ph↗

arXiv2025

Phenomenological characterization of the isomerization transition state of carbonyl sulfide

Signatures of excited-state quantum phase transitions in the bending degree of freedom of triatomic systems that undergo an isomerization reaction have been recently evinced. In this work, we study the carbonyl sulfide bending motion using an effective Hamiltonian within the two-dimensional limit of the vibron model framework, which has been shown to accurately describe critical phenomena in molecular bending spectra within experimental precision. To estimate the transition state energy barrier, we propose an improvement to a phenomenological formula proposed by Baraban et al.[1] , introducing a new term to capture the anharmonicity change that characterizes quasilinear molecules

physics.chem-ph↗

arXiv2025

Unmasking the physical information inherent to interstellar spectral line profiles with Machine Learning. I. Application of LTE to HCN and HNC transitions

Physical and chemical conditions (kinetic temperature, volume density, molecular composition) of interstellar clouds are inherent in their mm-submm line spectra, making spectral line profiles powerful diagnostics of source conditions. We introduce a novel bottom-up approach employing Machine Learning (ML) algorithms to directly infer physical conditions from line profiles without using radiative transfer equations. We simulated HCN and HNC emission under representative dense molecular cloud and star-forming region conditions across five rotational transitions (J=1-0 to J=5-4) within 30-500 GHz. The generated data cloud was parameterized using line intensities and widths to infer the physical conditions of the analyzed regions. Three ML algorithms were trained, tested, and compared to unravel the excitation conditions of HCN and HNC and their abundance ratio. ML results obtained with two spectral lines, one for each isomer, were compared with a Local Thermodynamic Equilibrium (LTE) analysis for the cold source R CrA IRS 7B, yielding excitation temperatures and relative abundances in agreement with LTE. The optimized pipeline (training, testing, and prediction) can predict interstellar cloud properties from line profile inputs at lower computational cost than traditional methods. This work represents the first mapping of spectral line profiles to physical conditions by charting isomer abundance ratios and excitation temperatures. Our bottom-up approach, based on simulated and semiempirical spectra, offers a new tool to interpret line observations and estimate interstellar conditions using ML methods.

astro-ph.GA↗

arXiv2025

New routes for PN destruction and formation in the ISM via neutral-neutral gas-phase reactions and an extended database for reactions involving phosphorus

Phosphorus plays an essential role in the chemistry of living organisms, being present in several fundamental biomolecules. The investigation of chemical reactions taking place in different astronomical environments involving phosphorus-containing molecules is essential for understanding how these species are produced and destroyed. Phosphorus monoxide (PO) and phosphorus nitride (PN) are key reservoirs of phosphorus in the Interstellar Medium (ISM). This work presents a computational study of the CPN system to identify viable reaction pathways involving atom-diatom collisions and to explore a potential destruction route for PN in the ISM. We explore the potential energy landscape of the C($\mathrm{^3P}$) + PN($^1Σ^+$), N($\mathrm{^4S}$) + CP($^2Σ^+$) and P($\mathrm{^4S}$) + CN($^2Σ^+$) reactions by performing high-accuracy ab initio calculations and provide their rate coefficients over a wide range of temperatures. The temperature-dependent rate coefficients were fitted to the modified Arrhenius equation: $k(T)=α(T/300)^β\mathrm{exp}(-γ/T)$. An updated chemical network for P-bearing species was used to model the time-dependent abundances and reaction contributions of P, PO, PN, and PH during the chemical evolution of diffuse/translucent and dense clouds. The only neutral-neutral reaction capable of destroying PN without an activation energy seems to be the PN+C one. We have also shown that reactions between CP and N can yield CN and PN barrierless. Chemical models indicate that PO is a crucial species driving the gas-phase formation of PN. Typically, PO/PN ratios exceed 1, though their chemistry is influenced by photon- and cosmic-ray-induced processes. Over time in simulated dense clouds, neutral-neutral reactions such as PO + N, PH + N, P + OH, and PH + O play a significant role in determining the relative abundances of PO and PN.

astro-ph.GA↗

arXiv2024

Insights into the Mechanism, Selectivity, and Substituent Effects in the Diels-Alder Reaction of Azatrienes with Electron-rich Dienophiles: An MEDT Study

The reactivity and mechanistic intricacies of azatrienes in Diels-Alder reactions have been relatively unexplored despite their intriguing potential applications. In this study, we employ Molecular Electron Density Theory to theoretically investigate the hetero-Diels-Alder reaction involving azatrienes with ethyl vinyl ether and allenyl methyl ether. Analysis of Conceptual Density Functional Theory, energetic profiles, and the topological characteristics is conducted to elucidate the reactions. The revealed mechanism manifests as a polar one-step two-stages process under kinetic control. We establish a clear relationship of between the periselectivity, regioselectivity, and stereoselectivity on one hand and the characteristics of the reactions mechanism on the other hand. The influence of weak interactions on reaction activation barriers and bonding evolution are discussed in detail. We demonstrate that substituents enhancing the reverse electron density flux facilitate the feasibility of the reactions. The results lay ground for a meticulous control of the reaction of azatriene in similar synthetic scenarios.

physics.chem-ph↗

arXiv2024

New SiS destruction and formation routes via neutral-neutral reactions and their fundamental role in interstellar clouds at low and high metallicity values

Among the silicon bearing species discovered in the interstellar medium, SiS and SiO stand out as key tracers due to their distinct chemistry and abundances in interstellar and circumstellar environments. Our objective is to enhance the network of Si- and S-bearing chemical reactions for a gas-grain model in molecular clouds, encompassing both low and high metallicities. We have calculated the energies and rate coefficients for 6 neutral atom-diatom reactions involved in the SiCS triatomic system, with a special focus on the C+SiS and S+SiC collisions. We employ the coupled cluster method with single and double substitutions and a perturbative treatment of triple substitutions (CCSD(T)) refined at the explicitly correlated CCSD(T)-F12 level. With these computational results in conjunction with data from the literature, we construct an extended network of neutral-neutral chemical reactions. We performed time-dependent models employing the Nautilus gas-grain code, setting the gas temperature to 10 K and the density to 2x10$^4$ cm$^{-3}$. The temperature dependence for the reactions involving SiS were modelled using $k(T)=α\left( T/300 \right)^β \exp{(-γ/T)}$. The high-metallicity models significantly boost the SiS production. Higher initial abundances of C, S, and Si, roughly $\sim$2, 190, and 210 times higher, respectively, contribute to this. Around 10$^3$ yr, destruction mechanisms become relevant. The proposed production reaction S + SiC $\rightarrow$ C + SiS, mitigates these effects. By expanding the gas reaction network using a high metallicity model, we derived estimates for the abundances of interstellar molecules. The inclusion of neutral-neutral mechanisms, particularly via Si+HS and S+SiC channels, played a pivotal role in determining SiS abundance. These mechanisms carry a significance on a par with the well-known and fast ion-neutral reactions.

astro-ph.GA↗

arXiv2023

Observations and chemical modeling of the isotopologues of formaldehyde and the cations of formyl and protonated formaldehyde in the hot molecular core G331.512-0.103

In the interstellar cold gas, the chemistry of formaldehyde (H$_2$CO) can be essential to explain the formation of complex organic molecules. On this matter, the massive and energetic protostellar object G331 is still unexplored and, hence, we carried out a comprehensive study of the isotopologues of H$_2$CO and formyl cation (HCO$^+$), and of protonated formaldehyde (H$_2$COH$^+$) through the APEX observations in the spectral window $\sim$159-356~GHz. We employed observational and theoretical methods to derive the physical properties of the molecular gas combining LTE and non-LTE analyses. Formaldehyde was characterized via 35 lines of H$_2$CO, H$_2^{13}$CO, HDCO and H$_2$C$^{18}$O. The formyl cation was detected via 8 lines of HCO$^+$, H$^{13}$CO$^+$, HC$^{18}$O$^+$ and HC$^{17}$O$^+$. Deuterium was clearly detected via HDCO, whereas DCO$^+$ remained undetected. H$_2$COH$^+$ was detected through 3 clean lines. According to the radiative analysis, formaldehyde appears to be embedded in a bulk gas with a wide range of temperatures ($T\sim$20-90 K), while HCO$^+$ and H$_2$COH$^+$ are primarily associated with a colder gas ($T\lesssim$ 30 K). The reaction H$_2$CO+HCO$^+ \rightarrow$ H$_2$COH$^+$ + CO is crucial for the balance of the three species. We used Nautilus gas-grain code to predict the evolution of their molecular abundances relative to H$_2$ which values at time scales $\sim$10$^3$ yr matched with the observations in G331: [H$_2$CO] = (0.2-2) $\times$10$^{-8}$, [HCO$^+$] = (0.5-4) $\times$10$^{-9}$ and [H$_2$COH$^+$] = (0.2-2) $\times$10$^{-10}$. Based on the molecular evolution of H$_2$CO, HCO$^+$ and H$_2$COH$^+$, we hypothesized about the young lifetime of G331, which is consistent with the active gas-grain chemistry of massive protostellar objects.

astro-ph.GA↗

arXiv2022

The Si+SO$_2$ collision and an extended network of neutral-neutral reactions between silicon and sulphur bearing species

The Si+SO$_2$ reaction is investigated to verify its impact on the abundances of molecules with astrochemical interest, such as SiS, SiO, SO and others. According to our results Si($^3$P) and SO$_2$ react barrierlessly yielding only the monoxides SO and SiO as products. No favourable pathway has been found leading to other products, and this reaction should not contribute to SiS abundance. Furthermore, it is predicted that SiS is stable in collisions with O$_2$, and that S($^3$P)+SiO$_2$ and O($^3$P)+OSiS will also produce SO+SiO. Using these results and gathering further experimental and computational data from the literature, we provide an extended network of neutral-neutral reactions involving Si- and S-bearing molecules. The effects of these reactions were examined in a protostellar shock model, using the Nautilus gas-grain code. This consisted in simulating the physicochemical conditions of a shocked gas evolving from $i.$ primeval cold core, $ii.$ the shock region itself, $iii.$ and finally the gas bulk conditions after the passage of the shock. Emphasising on the cloud ages and including systematically these chemical reactions, we found that [SiS/H$_2$] can be of the order of $\sim$ 10$^{-8}$ in shocks that evolves from clouds of $t=1\times 10^6$ yr, whose values are mostly affected by the SiS+O $\longrightarrow$SiO+S reaction. Perspectives on further models along with observations are discussed in the context of sources harbouring molecular outflows.

astro-ph.GA↗

arXiv2022

Anharmonicity-induced excited-state quantum phase transition in the symmetric phase of the two-dimensional limit of the vibron model

In most cases, excited state quantum phase transitions can be associated with the existence of critical points (local extrema or saddle points) in a system's classical limit energy functional. However, an excited-state quantum phase transition might also stem from the lowering of the asymptotic energy of the corresponding energy functional. One such example occurs in the 2D limit of the vibron model, once an anharmonic term in the form of a quadratic bosonic number operator is added to the Hamiltonian. The study of this case in the broken-symmetry phase was presented in Phys. Rev. A. 81 050101 (2010). In the present work, we delve further into the nature of this excited-state quantum phase transition and we characterize it in the, previously overlooked, symmetric phase of the model making use of quantities such as the effective frequency, the expected value of the quantum number operator, the participation ratio, the density of states, and the quantum fidelity susceptibility. In addition to this, we extend the usage of the quasilinearity parameter, introduced in molecular physics, to characterize the phases in the spectrum of the anharmonic 2D limit of the vibron model and a down-to-earth analysis has been included with the characterization of the critical energies for the linear isomers HCN/HNC.

quant-ph↗

arXiv2021

Isocyanic acid (HNCO) in the Hot Molecular Core G331.512-0.103: Observations and Chemical Modelling

Isocyanic acid (HNCO) is a simple molecule with a potential to form prebiotic and complex organic species. Using a spectral survey collected with the Atacama Pathfinder EXperiment (APEX), in this work we report the detection of 42 transitions of HNCO in the hot molecular core/outflow G331.512-0.103 (hereafter G331). The spectral lines were observed in the frequency interval $\sim$ 160 - 355 GHz. By means of Local Thermodynamic Equilibrium (LTE) analyses, applying the rotational diagram method, we studied the excitation conditions of HNCO. The excitation temperature and column density are estimated to be $T_{ex}$ = 58.8 $\pm$ 2.7 K and $N$ = (3.7 $\pm$ 0.5) $\times$ 10$^{15}$ cm$^{-2}$, considering beam dilution effects. The derived relative abundance is between (3.8 $\pm$ 0.5) $\times $10$^{-9}$ and (1.4 $\pm$ 0.2) $\times $10$^{-8}$. In comparison with other hot molecular cores, our column densities and abundances are in agreement. An update of the internal partition functions of the four CHNO isomers: HNCO; cyanic acid, HOCN; fulminic acid, HCNO; and isofulminic acid, HONC is provided. We also used the astrochemical code Nautilus to model and discuss HNCO abundances. The simulations could reproduce the abundances with a simple zero-dimensional model at a temperature of 60 K and for a chemical age of $\sim$ 10$^5$ years, which is larger than the estimated dynamical age for G331. This result could suggest the need for a more robust model and even the revision of chemical reactions associated with HNCO.

astro-ph.GA↗

arXiv2020

Excited state quantum phase transitions in the bending spectra of molecules

We present an extension of the Hamiltonian of the two dimensional limit of the vibron model encompassing all possible interactions up to four-body operators. We apply this Hamiltonian to the modeling of the experimental bending spectrum of fourteen molecules. The bending degrees of freedom of the selected molecular species include all possible situations: linear, bent, and nonrigid equilibrium structures; demonstrating the flexibility of the algebraic approach, that allows for the consideration of utterly different physical cases with a general formalism and a single Hamiltonian. For each case, we compute predicted term values used to depict the quantum monodromy diagram, the Birge-Sponer plot, the participation ratio. We also show the bending energy functional obtained using the coherent --or intrinsic-- state formalism.

physics.chem-ph↗

arXiv2019

Calculation of Transition State Energies in the HCN-HNC Isomerization with an Algebraic Model

Recent works have shown that the spectroscopic access to highly-excited states provides enough information to characterize transition states in isomerization reactions. Here, we show that the transition state of the bond breaking HCN-HNC isomerization reaction can also be achieved with the two-dimensional limit of the algebraic vibron model. We describe the system's bending vibration with the algebraic Hamiltonian and use its classical limit to characterize the transition state. Using either the coherent state formalism or a recently proposed approach by Baraban et al. [ Science 2015 , 350 , 1338], we obtain an accurate description of the isomerization transition state. In addition, we show that the energy level dynamics and the transition state wave function structure indicate that the spectrum in the vicinity of the isomerization saddle point can be understood in terms of the formalism for excited state quantum phase transitions.

physics.chem-ph↗

arXiv2019

Impact of nonconvergence and various approximations of the partition function on the molecular column densities in the interstellar medium

We emphasize that the completeness of the partition function, that is, the use of a converged partition function at the typical temperature range of the survey, is very important to decrease the uncertainty on this quantity and thus to derive reliable interstellar molecular densities. In that context, we show how the use of different approximations for the rovibrational partition function together with some interpolation and/or extrapolation procedures may affect the estimate of the interstellar molecular column density. For that purpose, we apply the partition function calculations to astronomical observations performed with the IRAM-30m telescope towards the NGC7538-IRS1 source of two N-bearing molecules: isocyanic acid (HNCO, a quasilinear molecule) and methyl cyanide (CH$_3$CN, a symmetric top molecule). The case of methyl formate (HCOOCH$_3$), which is an asymmetric top O-bearing molecule containing an internal rotor is also discussed. Our analysis shows that the use of different partition function approximations leads to relative differences in the resulting column densities in the range 9 to 43\%. Thus, we expect this work to be relevant for surveys of sources with temperatures higher than 300~K and to observations in the infrared.

astro-ph.GA↗

arXiv2019

New spectral characterization of dimethyl ether isotopologues CH$_3$OCH$_3$ and $^{13}$CH$_3$OCH$_3$ in the THz region

The torsional Raman spectra of two astrophysically detected isotopologues of dimethyl-ether, ($^{12}$CH$_3$O$^{12}$CH$_3$ and $^{13}$CH$_3$O$^{12}$CH$_3$), have been recorded at room temperature and cooled in supersonic jet, and interpreted with the help of highly correlated ab initio calculations. Dimethyl-ether displays excited torsional and vibrational levels at low energy that can be populated at the temperatures of the star forming regions, obliging to extend the analysis of the rotational spectrum over the ground state. Its spectrum in the THz region is rather complex due to the coupling of the torsional overtones $2ν_{11}$ and $2ν_{15}$ with the COC bending mode, and the presence of many hot bands. The torsional overtones are set here at $2ν_{11}=385.2$~cm$^{-1}$ and $2ν_{15}=482.0$~cm$^{-1}$ for $^{12}$CH$_3$O$^{12}$CH$_3$, and $2ν_{11}=385.0$~cm$^{-1}$ and $2ν_{15}=481.1$~cm$^{-1}$ for $^{13}$CH$_3$O$^{12}$CH$_3$. The new assignment of $2ν_{11}$ is downshifted around $\sim 10$~cm$^{-1}$ with respect to the literature. All the other (hot) bands have been re-assigned consistently. In addition, the infrared-forbidden torsional fundamental band $ν_{11}$ is observed here at 197.8~cm$^{-1}$. The new spectral characterization in the THz region reported here provides improved values of the Hamiltonian parameters, to be used in the analysis of the rotational spectra of DME isotopologues for further astrophysical detections.

astro-ph.GA↗