Search

We discuss how Coulomb explosion imaging (CEI), triggered by intense femtosecond laser pulses and combined with laser-induced alignment and covariance analysis of the angular distributions of the recoiling fragment ions, provides new opportunities for imaging the structures of molecules and molecular complexes. First, focusing on gas phase molecules, we show how the periodic torsional motion of halogenated biphenyl molecules can be measured in real time by timed CEI, and how CEI of one-dimensionally aligned difluoroiodobenzene molecules can uniquely identify four structural isomers. Next, focusing on molecular complexes formed inside He nano-droplets, we show that the conformations of noncovalently bound dimers or trimers, aligned in one or three dimensions, can be determined by CEI. Results presented for homodimers of CS2, OCS, and bromobenzene pave the way for femtosecond time-resolved structure imaging of molecules undergoing bimolecular interactions and ultimately chemical reactions.

Journal of physics / B 55(1), 014001 (2022). doi:10.1088/1361-6455/ac489d, The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH$_2$BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump���XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH$_2$Br radicals produced by C���I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH$_2$BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way., Published by IOP Publ., Bristol

Το αντικείμενο της παρούσας εργασίας αφορά στη μελέτη της αλληλεπίδρασης των μορίων του νερού αλλά και του πλήρως δευτεριομένου ισοτοπολόγου του (βαρύ ύδωρ) με fs παλμούς laser και ασύμμετρα fs ω/2ω πεδία. Η μελέτη διεξήχθη στη φασματική περιοχή του κοντινού υπέρυθρου κάνοντας χρήση ενός οπτικού παραμετρικού ενισχυτή, ο οποίος αντλείται από ένα παλμικό Ti:Sapphire laser με χρονική διάρκεια παλμού ~30 fs. Τα ασύμμετρα πεδία προκύπτουν από τη σύνθεση μίας δέσμης laser του παραμετρικού ενισχυτή με τη δεύτερη αρμονική της.Η ισχύς των πεδίων laser είναι υψηλή (1014 – 1015 W/cm2) και η αλληλεπίδραση με τα μόρια οδηγεί στον πολλαπλό ιονισμό αυτών. Καθώς η μελέτη εστιάζεται στον διπλό ιονισμό των μορίων, ο προσδιορισμός της διαδικασίας ιονισμού απαιτεί ιδιαίτερη προσοχή καθότι το δικατιόν του νερού είναι ουσιαστικά ασταθές και οδηγείται άμεσα σε διάσπαση. Η καταγραφή των αποτελεσμάτων της αλληλεπίδρασης έγινε με την χρήση φασματογράφου μάζας χρόνου πτήσης. Από την συγκριτική μελέτη της εξάρτησης των ιοντικών θραυσμάτων από την πόλωση της δέσμης και αξιολογώντας την γωνιακή κατανομή των θραυσμάτων καταλήξαμε στον συμπέρασμα ότι στον ιονισμό, στην περιοχή του κοντινού υπερύθρου, ενέχεται μια πολυφωτονική διαδικασία. Το συμπέρασμα αυτό βρίσκεται σε συμφωνία με πρόσφατα βιβλιογραφικά δεδομένα για τα 800nm.Η διάσπαση προς δύο ιοντικά θραύσματα του δικατιόντος αποτέλεσε αντικείμενο ιδιαίτερης μελέτης. Το δικατιόν του νερού μέσω δύο διακριτών καναλιών διασπάται προς Η+ και ΟΗ+ καθώς και προς. Η2+ και Ο+. Μελετήθηκε συστηματικά η εξάρτηση των δύο αυτών καναλιών από τις πειραματικές παραμέτρους. Διαπιστώθηκε ότι η διάσπαση από την οποία προκύπτει Η2+/D2+ εξαρτάται δραστικά από το μήκος κύματος του laser. Συγκεκριμένα, όταν το μήκος κύματος του laser οδηγεί σε συντονιστική διέγερση συνδυασμών των κανονικών τρόπων ταλάντωσης των μορίων, αυξάνει η πιθανότητα σχηματισμού των μοριακών αυτών θραυσμάτων περισσότερο από τρείς και έως οκτώ φορές για το νερό και το βαρύ ύδωρ αντίστοιχα. Επιπλέον, ο μεγαλύτερος οπτικός κύκλος ευνοεί επίσης την δημιουργία, ενώ η κυκλική πόλωση μειώνει την πιθανότητα σχηματισμού αυτών των θραυσμάτων. Επιπρόσθετα, μελετήθηκε και προσδιορίστηκε η αυτοïονιζόμενη κατάσταση του κατιόντος, η οποία οδηγείται σε διάσπαση προς Η2+/D2+ και Ο+, παρόλο που βρίσκεται ενεργειακά κάτω από το κατώφλι του διπλού ιόντος. Ο σχηματισμός Η2+/D2+ προϋποθέτει δημιουργία ενός νέου δεσμού πριν την διάσπαση -γεγονός ιδιαίτερα ενδιαφέρον- και προκειμένου να αποκτήσουμε βαθύτερη γνώση, η όλη διαδικασία της διάσπασης του δικατιόντος μελετήθηκε με την χρήση fs ασύμμετρων ω/2ω πεδίων laser. Κάθε μέτρησης προηγήθηκε ο χαρακτηρισμός του ασύμμετρου ω/2ω πεδίου (προσδιορισμός της σχετικής φάσης) με την αξιοποίηση δεδομένων της βιβλιογραφίας για τον ιονισμό του CO. Έπειτα, επικεντρώνοντας στον ιονισμό των ισοτοπολόγων του νερού, ταυτοποιήθηκαν με απόλυτo τρόπο τα διασπαστικά κανάλια του δικατιόντος. Λαμβάνοντας υπόψιν τη φορά του πεδίου, συμπεράναμε ότι το διασπαστικό κανάλι που οδηγεί προς Η2+/D2+ ευνοείται όταν το εξωτερικό ηλεκτρικό πεδίο ωθεί το ηλεκτρονιακό νέφος προς τη μεριά του ΟΗ+. Η μελέτη της εξάρτησης της ανισότροπης εκτόξευσης ιοντικών θραυσμάτων (ασυμμετρία ή αλλιώς παράμετρος β) σε σχέση με τη φάση Δφ παρέχει επιπλέον γνώση πάνω στα δύο διασπαστικά κανάλια. Συγκεκριμένα, πραγματοποιήθηκε παραμετρική μελέτη της εξάρτησης των καναλιών διάσπασης από το μήκος κύματος της βασικής δέσμης, τη μορφή του ασύμμετρου πεδίου (παράμετρος γ) και της συνολικής έντασης του πεδίου laser. Διαπιστώθηκε πως οι πειραματικές αυτοί παράμετροι επηρεάζουν σημαντικά το πλάτος της ασυμμετρίας (παράμετρος β) μόνο για το κανάλι διάσπασης προς Η2+/D2+. Αξιολογώντας τα δεδομένα και από τις δύο τεχνικές που χρησιμοποιήθηκαν, αναδεικνύεται με σαφήνεια ότι στην δημιουργία και τελικά την διάσπαση των ισοτοπολόγων του νερού ενέχονται διαδικασίες σύζευξης πυρηνικών και ηλεκτρονιακών κινήσεων που λαμβάνουν χώρα σε fs χρονική κλίμακα. The subject of the present dissertation was the study of the interaction of water isotopologues (water and heavy water) with fs laser pulses and fs asymmetric ω/2ω fields. The ultra-short, near-IR laser pulses are generated by an Optical Parametric Amplifier, seeded by a 30 fs pulsed Ti:Sapphire laser. The asymmetric fields are synthesized by the overlap of the fundamental beam and its second harmonic. Water and heavy water molecules are multiply ionized in the presence of the strong field (Field Intensity: 1014 – 1015 W/cm2). The identification of the ionization process is not trivial since our study is focused on the highly unstable dications. After dissociative ionization, the released ionic fragments are studied by means of mass spectroscopy, using a time-of-flight mass spectrometer. A comparative study of the ionic fragments originated from the dications was performed as a function of the field’s polarization and in addition, their angular distribution was recorded. We concluded that the ionization process, under the present experimental conditions (spectral region and field intensity), involves multiphoton absorption. These results are in good agreement with a recent study performed at 800 nm.Both dissociation channels that lead to two-body fragmentation of the dications were meticulously studied. The first channel leads to the release of H+ and OH+, while the other leads to O+ and H2+ formation. Their dependence on the experimental parameters is systematically explored. It is concluded that, the channel leading to H2+/D2+ production is considerably affected by the laser’s wavelength. It is found that when the laser’s wavelength leads to the resonant excitation of specific overtones and combinations of molecular vibrations, the formation probability of H2+/D2+ is increased by more than three times in the case of water and up to eight times for heavy water. Moreover, longer optical cycles facilitate the creation of H2+/D2+, while circular polarization does the opposite. Additionally, an autoionizing state of the water’s cation, was identified and studied. This state leads to the release of H2+/D2+ and O+, even though it lays lower than the dication’s energy threshold.Formation of H2+/D2+ involves the creation of a new bond before dissociation. In order to gain additional information on the process, the dication’s dissociation was studied using fs asymmetric two-color laser fields (ω/2ω). The first step was to characterize the field’s anisotropy (determination of the fields’ relative phase), which is done by studying the multiple ionization of carbon monoxide by asymmetric fields and comparing the results with previous studies. Subsequently, the dication’s two-body dissociation channels are unambiguously identified by utilizing the asymmetric fields. Then, by taking into account the field’s shape, it is determinated that when the external laser field localizes the electronic cloud towards the OH+ site, the production of H2+/D2+ is favored. The dependence of the ejected ionic fragments’ directionality (asymmetry or β parameter) as a function of the field’s phase can provide additional information on the dissociation channels. More specifically, the two channels’ dependence on the composite field’s fundamental wavelength, shape (γ parameter) and total field intensity was parametrically investigated. It was concluded that as far as the channel that leads to the production of H2+/D2+ is concerned, these experimental parameters have an impact on its asymmetry amplitude (β parameter).The evaluation of the experimental findings leads to the conclusion that the coupling of nuclear and electronic motion in a fs timescale, is involved in the dissociation of the water’s isotopologues. 146 σ.

Electronic coherences in molecules are ultrafast charge oscillations on the Molecular Frame MF and their direct observation and separation from electronic population dynamics is challenging. Here we present a valence shell Lab Frame LF scattering method suited to probing electronic coherences in isolated systems. MF electronic coherences lead to LF electronic anisotropies observable by ultrafast angle resolved scattering. Moment analysis of the LF anisotropy completely separates electronic coherences from population dynamics, demonstrated in excited state NH3 using ultrafast time energy angle resolved photoelectron spectroscopy. This general approach applies equally to attosecond femtosecond electronic coherences in isolated systems

Journal of physics / B Atomic, molecular and optical physics 53(16), 164006 (2020). doi:10.1088/1361-6455/ab9658, Attosecond pulses can be used to generate coherent superpositions of cationic electronic states in molecules through photoionisation. These can drive coherent electronic dynamics, which may decay within a few femtoseconds due to nuclear motion. In this work, we study the impact of the photoelectron on decoherence in the valence electron system of molecules following attosecond photoionisation. To this end, we include the photoelectron as a classical point charge in a quantum–classical simulation of light-induced ultrafast molecular dynamics and consider ionisation by sub-femtosecond pulses with distinct qualities. By disentangling the contributions of photoelectron and nuclei to the overall electronic decoherence, we find that the photoelectron causes partial decoherence within the first 50 attoseconds. This timescale is noticed to be independent of the ionising pulse. Full electronic decoherence is only seen when the spatial extension of the nuclear wave packet is considered., Published by IOP Publ., Bristol

X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Ångstrom, and HHG provides unprecedented time resolution (~50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ~280 eV (44 Ångstroms) and the bond length in methane of ~1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science. ISSN:1361-6455 ISSN:0368-3508 ISSN:0953-4075 ISSN:0022-3700

The possibility to produce ultrashort light pulses with pulse durations in the femtosecond and attosecond time scale (currently even down to a pulse length of 43as [1]) has triggered a worldwide eort of research on time-resolved measurements of electronic dynamics [2, 3, 4] in atoms, molecules [5] and, since very recently, even in solids [6]. For molecules, restructuring of the electronic system induces nuclear dynamics such as the breakage of a chemical bond, which is the primary process of any chemical reaction. In order to study processes like photoionization or photodissociation, measurement techniques needed to be developed, giving us more insight into those processes. One such technique is Photoelectron-Photoion Coincidence Spectroscopy (PEPICO) where the energies of electrons and the mass-to-charge ratio of ions emerging from the interaction of a molecule with an ultrashort light pulse are measured in coincidence. This approach adopted in a time-resolved manner to a pump-probe scheme has already shown great results like direct observation of photochemical activation energy in acetone [7], discovering electronic shake-up in molecular fragmentation processes [8] or the detection of Feshbach resonances [9]. The motivation for this diploma thesis is the eort to implement and test the possibility for PEPICO measurements in an attosecond XUV beamline equipped with a single time-of- ight mass spectrometer. The implementation was enabled by developing a fast high-voltage electrical switch, which permits switching from 0 to 2kV within a sub-100 nanosecond duration synchronously with the kHz repetition rate of the laser pulse used for PEPICO measurements, and by developing a suitable software for triggering, data read-out and data visualization during the experiment.

Ce document, dans cette mise à jour, accompagne un cours de l'INRS-EMT en interaction laser-matière. Il présente un survol des progrès réalisés depuis une dizaine d'années dans la compréhension du comportement dynamique de systèmes moléculaires complexes excités par des impulsions lumineuses ultra-brèves et observés sur des échelles de temps infiniment rapides. Ce texte loin d'être exhaustif se veut une illustration de la richesse du domaine et du potentiel des méthodes et outils développés récemment dans ce domaine. Ce texte est en accès ouvert. Pour le citer : J.C. Kieffer, Imagerie dynamique, INRS-NRG9207 (2017)

X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Ångstrom, and HHG provides unprecedented time resolution (~50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ~280 eV (44 Ångstroms) and the bond length in methane of ~1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science., Journal of Physics B: Atomic, Molecular and Optical Physics, 51 (3), ISSN:1361-6455, ISSN:0368-3508, ISSN:0953-4075, ISSN:0022-3700