arXiv subjects

A. N. Heays

Publications and source records attributed to A. N. Heays.

13 recordsLinked to original sources

arXiv2023

Ultraviolet photoabsorption in the $B\,{}^3Σ^- - X\,{}^3Σ^-$ and $C\,{}^3Π - X\,{}^3Σ^-$ band systems of SO sulphur isotopologues

High-resolution far-ultraviolet broadband Fourier-transform photoabsorption spectra of ${}^{32}{\rm S}^{16}{\rm O}$, ${}^{33}{\rm S}^{16}{\rm O}$, ${}^{34}{\rm S}^{16}{\rm O}$, and ${}^{36}{\rm S}^{16}{\rm O}$ are recorded in a microwave discharge seeded with SO$_2$ . The $B{}^3Σ^-(v=4-30) \leftarrow X{}^3Σ^-(v=0)$ and $C{}^3Π(v=0-7) \leftarrow X{}^3Σ^-(v=0)$ bands are observed or inferred in the 43000 to 51000 cm$^{-1}$ (196 to 233 nm) spectral range. This is the first experimental detection of a $C{}^3Π(v>2)$ level and of any of these observed bands in an S-substituted isotopologue. Additional measurements of $A{}^3Π(v=1-3) \leftarrow X{}^3Σ^-(v=0)$ provide a calibration of the SO column density. Measured band profiles are fitted to an effective-Hamiltonian model of coupled excited $B{}^3Σ^-$ and $C{}^3Π$ states along with their predissociation linewidths and absorption band strengths. Electronic-state potential-energy curves and transition moments are deduced. The end result is a list of line frequencies, $f$-values, and dissociation widths describing the far-ultraviolet photodissociation spectrum of SO that is accurate enough for computing atmospheric photolytic isotope-fractionation.

physics.atom-ph↗

arXiv2022

Fourier-transform spectroscopy of $^{13}$C$^{17}$O and deperturbation analysis of the A$^1Π$ ($ν$ = 0 - 3) levels

The high-resolution B$^1Σ^+$ - A$^1Π$ (0, 0) and (0, 3) emission bands of the less-abundant $^{13}$C$^{17}$O isotopologue have been investigated by Fourier-transform spectroscopy in the visible region using a Bruker IFS 125HR spectrometer at an accuracy 0.003 cm$^{-1}$. These spectra are combined with high-resolution photoabsorption measurements of the $^{13}$C$^{17}$O B$^1Σ^+$ $\leftarrow$ X$^1Σ^+$ (0, 0), B$^1Σ^+$ $\leftarrow$ X$^1Σ^+$ (1, 0) and C$^1Σ^+$ $\leftarrow$ X$^1Σ^+$ (0, 0) bands recorded with an accuracy of 0.01 cm$^{-1}$ using the vacuum ultraviolet Fourier-transform spectrometer, installed on the DESIRS beamline at the SOLEIL synchrotron. In the studied 17,950 - 22,500 cm$^{-1}$ and 86,800 - 92,100 cm$^{-1}$ regions, 480 transitions have been measured. These new experimental data were combined with data from the C $\to$ A and B $\to$ A systems, previously analyzed in 13C17O. The frequencies of 1003 transitions derived from 12 bands were used to analyze the perturbations between the A$^1Π$ ($ν$ = 0 - 3) levels and rovibrational levels of the d$^3Δ_i$, e$^3Σ^-$, a$'{}^3Σ^+$, I$^1Σ^-$ and D$^1Δ$ states as well as to a preliminary investigation of weak irregularities that appear in the B$^1Σ^+$ ($ν$ = 0) level. Deperturbed molecular constants and term values of the A$^1Π$ state were obtained. The spin-orbit and L-uncoupling interaction parameters as well as isotopologue-independent spin-orbit and rotation-electronic perturbation parameters were derived.

physics.chem-ph↗

arXiv2021

Extraterrestrial Photochemistry: Principles and Applications

Energetic processing of interstellar ice mantles and planetary atmospheres via photochemistry is a critical mechanism in the extraterrestrial synthesis of prebiotic molecules. Photochemistry is defined as chemical processes initiated by photon-induced electronic excitation, not involving ionization. In contrast, photons with energies above the ionization threshold initiate radiation chemistry (radiolysis). Vacuum-ultraviolet (6.2-12.4 eV) light may initiate photochemistry and radiation chemistry because the threshold for producing secondary electrons is lower in the condensed phase than in the gas phase. Approximately half of cosmic-ray induced photons incident on interstellar ices in star-forming regions initiate photochemistry while the rest initiate radiation chemistry. While experimental techniques such as velocity map imaging may be used to extract exquisite details about gas-phase photochemistry, such detailed information cannot be obtained for condensed-phase photochemistry, which involves greater complexity, including the production of excitons, excimers, and exciplexes. Because a primary objective of chemistry is to provide molecular-level mechanistic explanations for macroscopic phenomena, our ultimate goal in this book chapter is to critically evaluate our current understanding of the photochemistry that likely leads to the synthesis of extraterrestrial prebiotic molecules.

astro-ph.GA↗

arXiv2019

The spin-forbidden vacuum-ultraviolet absorption spectrum of 14N15N

Photoabsorption spectra of ${}^{14}{\rm N}{}^{15}{\rm N}$ were recorded at high resolution with a vacuum-ultraviolet Fourier-Transform spectrometer fed by synchrotron radiation in the range 81 to 100 nm. The combination of high column density ($3\times 10^{17}$ cm$^{-2}$) and low temperature (98 K) allowed for the recording of weak spin-forbidden absorption bands exciting levels of triplet character. The triplet states borrow intensity from ${}^1Π_u$ states of Rydberg and valence character while causing their predissociation. New predissociation linewidths and molecular constants are obtained for the states $C\,{}^3Π_u(v=7,8,14,15,16,21)$, $G\,{}^3Π_u(v=0,1,4)$, and $F\,{}^3Π_u(v=0)$. The positions and widths of these levels are shown to be well-predicted by a coupled-Schroedinger equation model with empirical parameters based on experimental data on ${}^{14}{\rm N}_2$ and ${}^{15}{\rm N}_2$ triplet levels.

physics.atom-ph↗

arXiv2018

Precision spectroscopy and comprehensive analysis of perturbations in the $A\,{}^1Π(v=0$) state of ${}^{\rm 13}$C${}^{\rm 18}$O

We have reinvestigated the A$^1Π$$(v=0)$ level of $^{13}$C$^{18}$O\ using new high-resolution spectra obtained via multi-photon laser excitation as well as with synchrotron-based Fourier-transform absorption spectroscopy of the A$^1Π-\textrm{X}^1Σ^+$$(0, 0)$, e$^3Σ^--\textrm{X}^1Σ^+$$(1, 0)$, d$^3Δ-\textrm{X}^1Σ^+$$(4, 0)$, a$'^3Σ^+-\textrm{X}^1Σ^+$$(9, 0)$, and \atX$(11, 0)$ bands. In addition, Fourier-transform emission spectroscopy in the visible range is performed on the ${\rm B}^1Σ^+-{\rm A}^1Π$$(0, 0)$ band. Spectra of the \BX$(0, 0)$ band are measured in order to tie information from the latter emission data to the level structure of A$^1Π$$(v=0)$. The high pressures in the absorption cell at the synchrotron and the high temperatures in the emission discharge permitted monitoring of high rotational quantum levels in A$^1Π$$(v=0)$ up to $J=43$. All information, in total over 900 spectral lines, was included in an effective-Hamiltonian analysis of the A$^1Π(v=0, J)$ levels that are directly perturbed by the e$\,^3Σ^-$$(v=1)$, d$^3\!Δ$$(v=4)$, a$'^3Σ^+$$(v=9)$, d$^3\!Δ$$(v=0)$, I$\,^1Σ^-$$(v=0, 1)$ close-lying levels and the e$\,^3Σ^-$$(v=0,2)$, d$^3\!Δ$$(v=3,5)$, a$'^3Σ^+$$(v=8,10)$ remote levels, as well being indirectly influenced by the a$^3Π$$(v=10, 11)$ state. The influence of nine further perturber levels and their interactions was investigated and are not significant for reproducing the present experimental data. This analysis leads to a much improved description in terms of molecular constants and interaction parameters, compared to previous studies of the same energy region for other CO isotopologues.

physics.atom-ph↗

arXiv2017

High-resolution one-photon absorption spectroscopy of the $D\,{}^2Σ^- \leftarrow X\,{}^2Π$ system of radical OH and OD

Vacuum-ultraviolet (VUV) photoabsorption spectra were recorded of the $A\,{}^2Σ^+(v'=0)\leftarrow{}X\,{}^2Π(v''=0)$, $D\,{}^2Σ^-(v'=0)\leftarrow{}X\,{}^2Π(v''=0)$ and $D\,{}^2Σ^-(v'=1)\leftarrow{}X\,{}^2Π(v''=0)$ bands of the OH and OD radicals generated in a plasma-discharge source using synchrotron radiation as a background continuum coupled with the VUV Fourier-transform spectrometer on the DESIRS beamline of synchrotron SOLEIL. High-resolution spectra permitted the quantification of transition frequencies, relative $f$-values, and natural line broadening. The $f$-values were absolutely calibrated with respect to a previous measurement of $A\,{}^2Σ^+(v'=0)\leftarrow{}X\,{}^2Π(v''=0)$ ([wang1979]). Lifetime broadening of the excited $D\,{}^2Σ^-(v=0)$ and $D\,{}^2Σ^-(v=1)$ levels is measured for the first time and compared with previous experimental limits, and implies a lifetime 5 times shorter than a theoretical prediction ([van_der_loo2005]). A local perturbation of the $D\,{}^2Σ^-(v=0)$ level in OH was found.

physics.atom-ph↗

arXiv2017

Non-thermal photons and direct photodissociation of H2, HD and HeH+ in the chemistry of the primordial Universe

Non-thermal photons deriving from radiative transitions among the internal ladder of atoms and molecules are an important source of photons in addition to thermal and stellar sources in many astrophysical environments. In the present work the calculation of reaction rates for the direct photodissociation of some molecules relevant in early Universe chemistry is presented; in particular, the calculations include non-thermal photons deriving from the recombination of primordial hydrogen and helium atoms for the cases of H2, HD and HeH+. New effects on the fractional abundances of chemical species are investigated and the fits for the HeH+ photodissociation rates by thermal photons are provided.

astro-ph.GA↗

arXiv2017

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of shielding types depends on the species in question and the dust optical properties. The new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of rates to variation of temperature and isotope, and cross section uncertainties, are tested. Tests were conducted with an interstellar-cloud chemical model, and find general agreement (within a factor of two) with the previous iteration of the Leiden database for the ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

astro-ph.SR↗

arXiv2016

Spirals in protoplanetary disks from photon travel time

Spiral structures are a common feature in scattered-light images of protoplanetary disks, and of great interest as possible tracers of the presence of planets. However, other mechanisms have been put foward to explain them, including self-gravity, disk-envelope interactions, and dead zone boundaries. These mechanisms explain many spirals very well, but are unable to easily account for very loosely wound spirals and single spiral arms. We study the effect of light travel time on the shape of a shadow cast by a clump orbiting close (within ${\sim}1\,$au) of the central star, where there can be significant orbital motion during the light travel time from the clump to the outer disk and then to the sky plane. This delay in light rays reaching the sky plane gives rise to a variety of spiral- and arc-shaped shadows, which we describe with a general fitting formula for a flared, inclined disk.

astro-ph.EP↗

arXiv2016

Spectroscopy and perturbation analysis of the A$^1Π$(v=0) state of $^{13}$C$^{16}$O

The lowest $v=0$ level of the A$^1Π$, state of the $^{13}$C$^{16}$O isotopologue of carbon monoxide has been reinvestigated with a variety of high resolution spectroscopic techniques. The A$^1Π-$X$^1Σ^+(0,0)$ band has been studied by vacuum-ultraviolet Fourier-transform absorption spectroscopy, using the SOLEIL synchrotron as a radiation source. Spectra were obtained under quasi-static gas conditions at liquid-nitrogen temperature, room temperature and at an elevated temperature of 900 K, with absolute accuracies of 0.01$-$0.03 cm$^{-1}$. Two-photon Doppler-free laser spectroscopy has been applied to a limited number of transitions in the A$^1Π-$X$^1Σ^+(0,0)$ band, under collision-free circumstances of a molecular beam, yielding an absolute accuracy of 0.002 cm$^{-1}$. The third technique is high-resolution Fourier-transform emission spectroscopy in the visible region applied to the B$^1Σ^+-$A$^1Π(0,0)$ band in a gas discharge, at an absolute accuracy of up to 0.003 cm$^{-1}$. With these methods rotational levels of A$^1Π(0)$ could be studied in both parity components up to a rotational quantum number of $J=46$. The frequencies of 397 transitions were used to analyse the perturbations between the A$^1Π(0)$ level by vibrational levels of the D$^1\!Δ$, e$^3Σ^-$, d$^3\!Δ$, and a$'^3Σ^+$ states.

physics.atom-ph↗

arXiv2015

Spectroscopy and perturbation analysis of the CO A$^1Π-$X$^1Σ^+$ (2,0), (3,0) and (4,0) bands

The (2,0) (3,0) and (4,0) bands of the A$^1Π-$X$^1Σ^+$ system of $^{12}$C$^{16}$O have been re-investigated by high-resolution vacuum ultraviolet absorption spectroscopy. A VUV Fourier-transform spectrometer, illuminated by synchrotron radiation, was applied to record a jet-cooled spectrum, a room temperature static gas spectrum and a high temperature (900 K) quasi-static gas spectrum, resulting in absolute accuracies of 0.01$-$0.02 cm$^{-1}$ for the rotational line frequencies. Precise laser-based data were included in the analysis allowing for a highly accurate determination of band origins. Rotational levels up to $J=52$ were observed. The data were used to perform an improved analysis of the perturbations in the A$^1Π$, $v=2$, $v=3$, and $v=4$ levels by vibrational levels of the D$^1Δ$, I$^1Σ^-$, e$^3Σ^-$, d$^3Δ$, and a$'^3Σ^+$ states.

physics.atom-ph↗

arXiv2014

Precision spectroscopy of the $X\,^{1}Σ_{g}^{+}, v=0\rightarrow 1$ ($J=0-2$) rovibrational splittings in H$_{2}$, HD and D$_{2}$

Accurate experimental values for the vibrational ground tone or fundamental vibrational energy splitting of H$_2$, HD, and D$_2$ are presented. Absolute accuracies of $2\times10^{-4}$ cm$^{-1}$ are obtained from Doppler-free laser spectroscopy applied in a collisionless environment. The vibrational splitting frequencies are derived from the combination difference between separate electronic excitations from the $X^{1}Σ_{g}^{+}, v=0, J$ and $v=1, J$ vibrational states to a common $EF^{1}Σ^{+}_{g}, v=0, J$ state. The present work on rotational quantum states $J=1,2$ extends the results reported by Dickenson et al. on $J=0$ [Phys. Rev. Lett. 110 (2013) 193601]. The experimental procedures leading to this high accuracy are discussed in detail. A comparison is made with full \emph{ab initio} calculations encompassing Born-Oppenheimer energies, adiabatic and non-adiabatic corrections, as well as relativistic corrections and QED-contributions. The present agreement between the experimental results and the calculations provides a stringent test on the application of quantum electrodynamics in molecules. Furthermore, the combined experimental-theoretical uncertainty can be interpreted to provide bounds to new interactions beyond the Standard Model of Physics or fifth forces between hadrons.

physics.atom-ph↗

arXiv2012

High-resolution Fourier-transform XUV photoabsorption spectroscopy of 14N15N

The first comprehensive high-resolution photoabsorption spectrum of 14N15N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet (XUV) and span 100,000-109,000 cm-1 (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N2 that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schroedinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

physics.atm-clus↗