arXiv subjects

M. Centurion

Publications and source records attributed to M. Centurion.

At least 19 recordsLinked to original sources

arXiv2025

Imaging the Photochemistry of Cyclobutanone using Ultrafast Electron Diffraction: Experimental Results

We investigated the ultrafast structural dynamics of cyclobutanone following photoexcitation at $\lambda=200$ nm using gas-phase megaelectronvolt ultrafast electron diffraction. Our investigation complements the simulation studies of the same process within this special issue. It provides information about both electronic state population and structural dynamics through well-separable inelastic and elastic electron scattering signatures. We observe the depopulation of the photoexcited S$_2$ state of cyclobutanone with n3s Rydberg character through its inelastic electron scattering signature with a time constant of $(0.29 \pm 0.2)$ ps towards the S$_1$ state. The S$_1$ state population undergoes ring-opening via a Norrish Type-I reaction, likely while passing through a conical intersection with S$_0$. The corresponding structural changes can be tracked by elastic electron scattering signatures. These changes appear with a delay of $(0.14 \pm 0.05)$ ps with respect the initial photoexcitation, which is less than the S$_2$ depopulation time constant. This behavior provides evidence for the ballistic nature of the ring-opening once the S$_1$ state is reached. The resulting biradical species react further within $(1.2 \pm 0.2)$ ps via two rival fragmentation channels yielding ketene and ethylene, or propene and carbon monoxide. Our study showcases both the value of gas-phase ultrafast diffraction studies as an experimental benchmark for nonadiabatic dynamics simulation methods and the limits in the interpretation of such experimental data without comparison to such simulations.

physics.chem-ph↗

arXiv2023

Relativistic ultrafast electron diffraction at high repetition rates

The ability to resolve the dynamics of matter on its native temporal and spatial scales constitutes a key challenge and convergent theme across chemistry, biology, and materials science. The last couple of decades have witnessed ultrafast electron diffraction (UED) emerge as one of the forefront techniques with the sensitivity to resolve atomic motions. Increasingly sophisticated UED instruments are being developed that are aimed at increasing the beam brightness in order to observe structural signatures, but so far they have been limited to low average current beams. Here we present the technical design and capabilities of the HiRES (High Repetition Rate Electron Scattering) instrument, which blends relativistic electrons and high repetition rates to achieve orders of magnitude improvement in average beam current compared to the existing state-of-the-art UED instruments. The setup utilizes a novel electron source to deliver femtosecond duration electron pulses at up to MHz repetition rates for UED experiments. We provide example cases of diffraction measurements on solid-state and gas-phase samples, including both micro- and nanodiffraction modes, which showcase the potential of the instrument for novel UED experiments.

physics.ins-det↗

arXiv2021

Conformer-specific Chemistry Imaged in Real Space and Time

Conformational isomers or conformers of molecules play a decisive role in chemistry and biology. However, experimental methods to investigate chemical reaction dynamics are typically not conformer-sensitive. Here, we report on a gas-phase megaelectronvolt ultrafast electron diffraction investigation of {\alpha}-phellandrene undergoing an electrocyclic ring-opening reaction. We directly image the evolution of a specific set of {\alpha}-phellandrene conformers into the product isomer predicted by the Woodward-Hoffmann rules in real space and time. Our experimental results are in quantitative agreement with nonadiabatic quantum molecular dynamics simulations, which provide unprecedented detail of how conformation influences time scale and quantum efficiency of photoinduced ring-opening reactions. Due to the prevalence of large numbers of conformers in organic chemistry, our findings impact our general understanding of reaction dynamics in chemistry and biology.

physics.chem-ph↗

arXiv2018

Imaging the Photochemical Ring-Opening of 1,3-Cyclohexadiene by Ultrafast Electron Diffraction

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Here, we present direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and sub-{\AA}ngstr\"om length scale by megaelectronvolt ultrafast electron diffraction. We follow the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by direct measurement of time-dependent changes in the distribution of interatomic distances. We observe a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the sub-picosecond timescale. Our work demonstrates the potential of megaelectronvolt ultrafast electron diffraction to elucidate photochemical reaction paths in organic chemistry.

physics.chem-ph↗

arXiv2014

The UVES Large Program for testing fundamental physics - III. Constraints on the fine-structure constant from 3 telescopes

Large statistical samples of quasar spectra have previously indicated possible cosmological variations in the fine-structure constant, $\alpha$. A smaller sample of higher signal-to-noise ratio spectra, with dedicated calibration, would allow a detailed test of this evidence. Towards that end, we observed equatorial quasar HS 1549$+$1919 with three telescopes: the Very Large Telescope, Keck and, for the first time in such analyses, Subaru. By directly comparing these spectra to each other, and by `supercalibrating' them using asteroid and iodine-cell tests, we detected and removed long-range distortions of the quasar spectra's wavelength scales which would have caused significant systematic errors in our $\alpha$ measurements. For each telescope we measure the relative deviation in $\alpha$ from the current laboratory value, $\Delta\alpha/\alpha$, in 3 absorption systems at redshifts $z_{\mathrm{abs}}=1.143$, 1.342, and 1.802. The nine measurements of $\Delta\alpha/\alpha$ are all consistent with zero at the 2-$\sigma$ level, with 1-$\sigma$ statistical (systematic) uncertainties 5.6--24 (1.8--7.0) parts per million (ppm). They are also consistent with each other at the 1-$\sigma$ level, allowing us to form a combined value for each telescope and, finally, a single value for this line of sight: $\Delta\alpha/\alpha=-5.4 \pm 3.3_{\mathrm{stat}} \pm 1.5_{\mathrm{sys}}$ ppm, consistent with both zero and previous, large samples. We also average all Large Programme results measuring $\Delta\alpha/\alpha=-0.6 \pm 1.9_{\mathrm{stat}} \pm 0.9_{\mathrm{sys}}$ ppm. Our results demonstrate the robustness and reliability at the 3 ppm level afforded by supercalibration techniques and direct comparison of spectra from different telescopes.

astro-ph.CO↗

arXiv2013

Fundamental constants and high resolution spectroscopy

Absorption-line systems detected in high resolution quasar spectra can be used to compare the value of dimensionless fundamental constants such as the fine-structure constant, alpha, and the proton-to-electron mass ratio, mu = m_p/m_e, as measured in remote regions of the Universe to their value today on Earth. In recent years, some evidence has emerged of small temporal and also spatial variations in alpha on cosmological scales which may reach a fractional level of 10 ppm . We are conducting a Large Programme of observations with VLT UVES to explore these variations. We here provide a general overview of the Large Programme and report on the first results for these two constants, discussed in detail in Molaro et al. and Rahmani et al. A stringent bound for Delta(alpha)/Alpha is obtained for the absorber at_abs = 1.6919 towards HE 2217-2818. The absorption profile is complex with several very narrow features, and is modeled with 32 velocity components. The relative variation in alpha in this system is +1.3+-2.4_{stat}+-1.0_{sys} ppm if Al II lambda 1670AA and three Fe II transitions are used, and +1.1+-2.6_{stat} ppm in a lightly different analysis with only Fe II transitions used. The expectation at this sky position of the recently-reported dipolar variation of alpha is (3.2--5.4)+-1.7 ppm depending on dipole model. This constraint of Delta(alpha)/alpha at face value is not supporting this expectation but is not inconsistent with it at the 3 sigma level. For the proton-to-electron mass ratio the analysis of the H_2 absorption lines of the z_{abs}~2.4018 damped Ly alpha system towards HE 0027- 1836 provides Delta(mu)/mu = (-7.6 +- 8.1_{stat} +- 6.3_{sys}) ppm which is also consistent with a null variation. (abridged)

astro-ph.CO↗

arXiv2013

Limits on the spatial variations of the electron-to-proton mass ratio in the Galactic plane

Aims. To validate the Einstein equivalence principle (local position invariance) by limiting the fractional changes in the electron-to-proton mass ratio, mu = m_e/m_p, measured in Galactic plane objects. Methods. High resolution spectral observations of dark clouds in the inversion line of NH3(1,1) and pure rotational lines of other molecules (the so-called ammonia method) were performed at the Medicina 32-m and the Effelsberg 100-m radio telescopes to measure the radial velocity offsets, Delta RV = V_rot - V_inv, between the rotational and inversion transitions which have different sensitivities to the value of mu. Results. In our previous observations (2008-2010), a mean offset of = 0.027+/-0.010 km/s [3 sigma confidence level (C.L.)] was measured. To test for possible hidden errors, we carried out additional observations of a sample of molecular cores in 2010-2013. As a result, a systematic error in the radial velocities of an amplitude ~0.02 km/s was revealed. The averaged offset between the radial velocities of the rotational transitions of HC3N(2-1), HC5N(9-8), HC7N(16-15), HC7N(21-20), and HC7N(23-22), and the inversion transition of NH3(1,1) = 0.003+/-0.018 km/s (3 sigma C.L.). This value, when interpreted in terms of Delta mu/mu= (mu_obs - mu_lab)/mu_lab, constraints the mu-variation at the level of Delta mu/mu < 2*10^{-8} (3 sigma C.L.), which is the most stringent limit on the fractional changes in mu based on astronomical observations.

astro-ph.GA↗

arXiv2013

The UVES Large Program for Testing Fundamental Physics II: Constraints on a Change in {\mu} Towards Quasar HE 0027-1836

We present an accurate analysis of the H2 absorption lines from the zabs ~ 2.4018 damped Ly{\alpha} system towards HE 0027-1836 observed with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT/UVES) as a part of the European Southern Observatory Large Programme "The UVES large programme for testing fundamental physics" to constrain the variation of proton-to-electron mass ratio, {\mu} = mp/me. We perform cross-correlation analysis between 19 individual exposures taken over three years and the combined spectrum to check the wavelength calibration stability. We notice the presence of a possible wavelength dependent velocity drift especially in the data taken in 2012. We use available asteroids spectra taken with UVES close to our observations to confirm and quantify this effect. We consider single and two component Voigt profiles to model the observed H2 absorption profiles. We use both linear regression analysis and Voigt profile fitting where {\Delta}{\mu}/{\mu} is explicitly considered as an additional fitting parameter. The two component model is marginally favored by the statistical indicators and we get {\Delta}{\mu}/{\mu} = (-2.5 +/- 8.1(stat) +/- 6.2(sys)) ppm. When we apply the correction to the wavelength dependent velocity drift we find {\Delta}{\mu}/{\mu} = (-7.6 +/- 8.1(stat) +/- 6.3(sys)) ppm. It will be important to check the extent to which the velocity drift we notice in this study is present in UVES data used for previous {\Delta}{\mu}/{\mu} measurements.

astro-ph.CO↗

arXiv2013

The UVES Large Program for Testing Fundamental Physics: I Bounds on a change in alpha towards quasar HE 2217-2818

Absorption line systems detected in quasar spectra can be used to compare the value of the fine-structure constant, {\alpha}, measured today on Earth with its value in distant galaxies. In recent years, some evidence has emerged of small temporal and also spatial variations of {\alpha} on cosmological scales which may reach a fractional level of ~ 10 ppm (parts per million). To test these claims we are conducting a Large Program with the VLT UVES . We are obtaining high-resolution (R ~ 60000 and high signal-to-noise ratio (S/N ~ 100) UVES spectra calibrated specifically for this purpose. Here we analyse the first complete quasar spectrum from this Program, that of HE 2217-2818. We apply the Many Multiplet method to measure {\alpha} in 5 absorption systems towards this quasar: zabs = 0.7866, 0.9424, 1.5558, 1.6279 and 1.6919. The most precise result is obtained for the absorber at zabs = 1.6919 where 3 Fe II transitions and Al II {\lambda}1670 have high S/N and provide a wide range of sensitivities to {\alpha}. The absorption profile is complex, with several very narrow features, and requires 32 velocity components to be fitted to the data. Our final result for the relative variation in {\alpha} in this system is Delta{\alpha}/{\alpha} = +1.3 +/- 2.4stat +/- 1.0sys ppm. This is one of the tightest current bounds on {\alpha} variation from an individual absorber. The absorbers towards quasar HE 2217-2818 reveal no evidence for variation in {\alpha} at the 3 ppm precision level (1{\sigma} confidence). If the recently reported 10 ppm dipolar variation of {\alpha} across the sky were correct, the expectation at this sky position is (3.2-5.4) +/-1.7 ppm depending on dipole model used . Our constraint of Delta{\alpha}/{\alpha}=+1.3+/-2.4stat +/-1.0sys ppm is not inconsistent with this expectation.

astro-ph.CO↗

arXiv2007

UVES radial velocity accuracy from asteroid observations. Implications for the fine structure constant variability

High resolution observations of the asteroids Iris and Juno have been performed by means of the UVES spectrograph at the ESO VLT to obtain the effective accurac y of the spectrograph's radial velocity. The knowledge of this quantity has impo rtant bearings on studies searching for a variability of the fine structure cons tant carried on with this instrument. Asteroids provide a precise radial velocit y reference at the level of 1 m/s which allows instrumental calibration and the recognition of small instrumental drifts and calibration systematics. In particu lar, radial velocity drifts due to non uniform slit illumination and slit optica l misalignment in the two UVES spectrograph arms can be investigated. The positi on of the solar spectrum reflected by the asteroids are compared with the solar wavelength positions or with that of asteroid observations at other epochs or wi th the twilight to asses UVES instrumental accuracy . Radial velocities offsets in the range 10--50 m/s are generally observed likely due to a non uniform slit illumination. However, no radial velocity patterns with wavelength are detected and the two UVES arms provide consistent radial velocities. These results suggest that the detected alpha variability by Levshakov et al. (2007) deduced from a drift of -180 (+/- 85) m/s at z =1.84, between two sets of FeII lines falling in the two UVES arms may be real or induced by other kinds of systematics than those investigated here. The proposed technique allows real time quality check of the spectrograph and should be followed for very accurate measurements.

astro-ph↗

arXiv2007

A new measure of Delta alpha/alpha at redshift z = 1.84 from very high resolution spectra of Q1101-264

We probe the evolution of the fine-structure constant, alpha, with cosmic time. Accurate positions of the FeII lines 1608, 2382,and 2600 A are measured in the z = 1.84 absorption system from a high-resolution (FWHM = 3.8 km/s) and high signal-to-noise (S/N >= 100) spectrum of the quasar Q1101-264 (z_em = 2.15, V = 16.0), integrated for 15.4 hours. The Single Ion Differential alpha Measurement (SIDAM) procedure and the Delta chi^2 method are used to set constraints on Delta alpha/alpha. We have found a relative radial velocity shift between the 1608 A and 2382,2600 A lines of Delta v = -180 +/- 85 m/s (both random and systematic errors are included), which, if real, would correspond to Delta alpha/alpha = (5.4 +/- 2.5) 10^{-6} (1sigma C.L.). Considering the strong implications of a such variability, additional observations with comparable accuracy at redshift z ~ 1.8 are required to confirm this result.

astro-ph↗

arXiv2006

Extinction and metal column density of HI regions up to redshift z~2

We used the photometric database of the Sloan Digital Sky Survey (SDSS) to estimate the reddening of 13 SDSS quasars selected on the basis of the presence of zinc absorption lines in an intervening Damped Ly alpha (DLA) system. In 5 of these quasars the reddening is detected at ~2 sigma confidence level in two independent color indices of the SDSS ugriz photometric system. A detailed analysis of the data supports an origin of the reddening in the intervening absorbers. We used these rare measurements of extinction in DLA systems to probe the relation between extinction and metal column density in the interval of absorption redshift 0.7 </~ z </~ 2.0. We find that the mean extinction in the V band per atom of iron in the dust is remarkably similar to that found in interstellar clouds of the Milky Way. This result lends support to previous estimates of the dust obscuration effect in DLA systems based on a Milky Way extinction/metal column density relation. We propose a simple mechanism, based on dust grain destruction/accretion properties, which may explain the approximate constancy of the extinction per atom of iron in the dust.

astro-ph↗

arXiv2006

VLT/UVES constraints on the carbon isotope ratio 12C/13C at z=1.15 toward the quasar HE 0515-4414

We analyzed the CI lines associated with the damped Ly-alpha system observed at zabs = 1.15 in the spectrum of HE 0515-4414 to derive the 12C/13C ratio. The spectrum was obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT). The obtained lower limit 12C/13C > 80 (2sigma C.L.) shows for the first time that the abundance of 13C in the extragalactic intervening clouds is very low. This rules out a significant contribution from intermediate-mass stars to the chemical evolution of matter sampled by this line of sight. The estimated low amount of 13C is in agreement with low abundances of nitrogen observed in damped Ly-alpha systems - the element produced in the same nuclear cycles and from about the same stars as 13C.

astro-ph↗

arXiv2005

Most precise single redshift bound to the variability of the fine-structure constant

Verification of theoretical predictions of an oscillating behavior of the fine-structure constant, alpha, with cosmic time requires high precision measurements at individual redshifts, while in earlier studies the mean Delta alpha/alpha values averaged over wide redshift intervals were usually reported. This requirement can be met via the Single Ion Differential alpha Measurement (SIDAM) procedure. We apply SIDAM to the FeII lines associated with the damped Ly-alpha system observed at z=1.15 in the spectrum of HE0515-4414. The weighted mean calculated on base of carefully selected 34 FeII pairs is =(-0.07+/-0.84)10^{-6}. The precision of this estimate represents the absolute improvement with respect to what has been done in the measurements of Delta alpha/alpha.

astro-ph↗

arXiv2005

Most precise single redshift bound to Delta alpha/alpha

Verification of theoretical predictions of an oscillating behavior of the fine-structure constant alpha with cosmic time requires high precision Delta alpha/alpha measurements at individual redshifts, while in earlier studies the mean Delta alpha/alpha values averaged over wide redshift intervals were usually reported. This requirement can be met via the single ion differential alpha measurement (SIDAM) procedure proposed in Levshakov et al. (2005). We apply the SIDAM to the FeII lines associated with the damped Ly-alpha system observed at z=1.15 in the spectrum of HE0515-4414. The weighted mean calculated on base of carefully selected 34 FeII pairs {1608,X} (X = 2344, 2374, and 2586 A) is = (-0.07+/-0.84) 10^{-6} (1sigma C.L.). The precision of this estimate improves by a factor 2 the previous one reported for the same system by Quast et al. (2004). The obtained result represents an absolute improvement with respect to what has been done in the measurements of Delta alpha/alpha.

astro-ph↗

arXiv2005

Spectral energy distribution of the metagalactic ionizing radiation field from QSO absorption spectra

A computational procedure is presented to estimate the spectral shape of the ionizing background between 1 and 10 Ryd by analyzing optically thin absorption systems in the spectra of high redshift quasars. The procedure is based on the response surface methodology from the theory of experimental design. The shape of the recovered UV background at z~3 shows a significant intensity decrease between 3 and 4 Ryd compared to the metagalactic spectrum of Haardt & Madau (1996). This decrease is interpreted as produced by HeII Gunn-Peterson effect. There are no features indicating a contribution from galaxies to the UV background which is therefore dominated by QSOs at z~3.

astro-ph↗

arXiv2005

VLT/UVES shows no cosmological variability of alpha

The cosmological variability of alpha is probed from individual observations of pairs of FeII lines. This procedure allows a better control of the systematics and avoids the influence of the spectral shifts due to ionization inhomogeneities in the absorbers and/or non-zero offsets between different exposures. Applied to the FeII lines of the metal absorption systems at zabs = 1.839 in Q1101--264 and at zabs = 1.15 in HE0515--4414 observed by means of UVES at the ESO-VLT, it provides da/a = 0.4 (+/- 1.5 stat)x10^{-6}. The result is shifted with respect to the Keck/HIRES mean da/a = -5.7(+/- 1.1 stat})x10^{-6} (Murphy et al. 2004) at a high confidence level (95%). Full details of this work are given in Levshakov et al (2005)

astro-ph↗

arXiv2005

VLT/UVES constraints on the cosmological variability of the fine-structure constant

We propose a new methodology for probing the cosmological variability of alpha from pairs of FeII lines (SIDAM, single ion differential alpha measurement) observed in individual exposures from a high resolution spectrograph. By this we avoid the influence of the spectral shifts due to (i) ionization inhomogeneities in the absorbers and (ii) non-zero offsets between different exposures. Applied to the FeII lines of the metal absorption line system at zabs = 1.839 in the spectrum of Q1101-264 obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT), SIDAM provides da/a = (2.4+/-3.8) 10^{-6}. The zabs = 1.15 FeII system toward HE0515-4414 has been re-analyzed by this method thus obtaining for the combined sample da/a = (0.4+/-1.5) 10^{-6}. These values are shifted with respect to the Keck/HIRES mean da/a = (-5.7+/-1.1) 10^{-6} (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the da/a measurement with the VLT/UVES is discussed to figure the prospects for future observations. It is suggested that with a spectrograph of 10 times the UVES dispersion coupled to a 100m class telescope the present Oklo level (da/a >= 4.5 10^{-8}) can be achieved along cosmological distances with differential measurements of da/a.

astro-ph↗