Search

We present an investigation of the ultrafast dynamics of the polycyclic aromatic hydrocarbon fluorene initiated by an intense femtosecond near-infrared laser pulse (810 nm) and probed by a weak visible pulse (405 nm). Using a multichannel detection scheme (mass spectra, electron and ion velocity-map imaging), we provide a full disentanglement of the complex dynamics of the vibronically excited parent molecule, its excited ionic states, and fragments. We observed various channels resulting from the strong-field ionization regime. In particular, we observed the formation of the unstable tetracation of fluorene, above-threshold ionization features in the photoelectron spectra, and evidence of ubiquitous secondary fragmentation. We produced a global fit of all observed time-dependent photoelectron and photoion channels. This global fit includes four parent ions extracted from the mass spectra, 15 kinetic-energy-resolved ionic fragments extracted from ion velocity map imaging, and five photoelectron channels obtained from electron velocity map imaging. The fit allowed for the extraction of 60 lifetimes of various metastable photoinduced intermediates.

A number of internal rotation codes can deal with the combination of one or two internal rotors with one 14N quadrupole nucleus, but once it comes to two 14N nuclei, no such code is available even for the case of one internal rotor. We present here the extension of our internal rotor program called BELGI-2N using the rho-axis method global approach to deal with compounds containing one methyl rotor and two weakly coupling 14N nuclei. To test our new code, we applied it to the microwave data recorded for N-methylimidazole, 2-methylimidazole, 4-methylimidazole, and 5-methylimidazole using a chirped-pulse Fourier transform microwave spectrometer in the 7.0-18.5 GHz frequency range. Compared to the previously published study, BELGI-2N was able to (i) significantly increase the number of assigned and fitted lines, (ii) fit the complete datasets considering both the internal rotation and the 14N nuclear quadrupole coupling effects simultaneously, and (iii) achieve standard deviations within the measurement accuracy for all methylimidazole isomers., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS 2 , a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np+) and azulene (Az+) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording "molecular cloud in a box" conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np+ ⇌ Az+ quasi-equilibrium. Consequently, dissociation by C2H2-elimination proceeds predominantly through common Az+ decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np+ D0 ← D2 transition, efficiently quenches dissociation for vibrational energies up to ≈1 eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Two isomers of a complex formed between thiazole and two water molecules, thi⋯(H 2 O) 2 , have been identified through Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The complex was generated by the co-expansion of a gas sample containing trace amounts of thiazole and water in an inert buffer gas. For each isomer, rotational constants, A 0 , B 0 , and C 0 ; centrifugal distortion constants, D J , D JK , d 1 , and d 2 ; and nuclear quadrupole coupling constants, χ aa (N) and [χ bb (N) - χ cc (N)], have been determined through fitting of a rotational Hamiltonian to the frequencies of observed transitions. The molecular geometry, energy, and components of the dipole moment of each isomer have been calculated using Density Functional Theory (DFT). The experimental results for four isotopologues of isomer I allow for accurate determinations of atomic coordinates of oxygen atoms by r 0 and r s methods. Isomer II has been assigned as the carrier of an observed spectrum on the basis of very good agreement between DFT-calculated results and a set of spectroscopic parameters (including A 0 , B 0 , and C 0 rotational constants) determined by fitting to measured transition frequencies. Non-covalent interaction and natural bond orbital analyses reveal that two strong hydrogen bonding interactions are present within each of the identified isomers of thi⋯(H 2 O) 2 . The first of these binds H 2 O to the nitrogen of thiazole (OH⋯N), and the second binds the two water molecules (OH⋯O). A third, weaker interaction binds the H 2 O sub-unit to the hydrogen atom that is attached to C2 (for isomer I) or C4 (for isomer II) of the thiazole ring (CH⋯O).

Broadband microwave spectra have been recorded between 7.0 and 18.5 GHz for N -methylimidazole⋯H 2 O and 2-methylimidazole⋯H 2 O complexes. Each complex was generated by co-expansion of low concentrations of methylimidazole and H 2 O in argon buffer gas. The rotational spectra of five isotopologues of each complex have been assigned and analysed to determine rotational constants ( A 0 , B 0 , C 0 ), centrifugal distortion constants ( D J , D JK ) and parameters that describe the internal rotation of the CH 3 group. The results allow the determination of parameters in the ( r 0 ) molecular geometry of each complex. H 2 O is the hydrogen bond donor and the pyridinic nitrogen of imidazole is the hydrogen bond acceptor in each case. The ∠(O-H b ⋯N3) angles are 177(5)° and 166.3(28)° for N -methylimidazole⋯H 2 O and 2-methylimidazole⋯H 2 O respectively. These results are consistent with the presence of a weak electrostatic interaction between the oxygen atom of H 2 O and the hydrogen atom (or CH 3 group) attached to the C2 carbon atom of imidazole, and with the results of density functional theory calculations. The ( V 3 ) barrier to internal rotation of the CH 3 group within N -methylimidazole⋯H 2 O is essentially unchanged from the value of this parameter for the N -methylimidazole monomer. The same parameter is significantly higher for the 2-methylimidazole⋯H 2 O complex than for the 2-methylimidazole monomer as a consequence of the weak electrostatic interaction between the O atom and the CH 3 group of 2-methylimidazole.

Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H 2 O and H 2 O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H 2 O, but these roles are reversed in the H 2 O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H 2 O and three isotopologues of H 2 O···imid have been measured. All spectra have been assigned and fitted to determine rotational ( A 0 , B 0 , C 0 ), centrifugal distortion ( D J , D JK ), and nuclear quadrupole coupling constants ( χ aa (N1), [χ bb (N1) - χ cc (N1)], χ aa (N3), and [χ bb (N3) - χ cc (N3)]). Structural parameters ( r 0 and r s ) have been accurately determined from measured rotational constants for each isomer. The imid···H 2 O complex contains a nonlinear hydrogen bond (∠(O-H b ···N3) = 172.1(26)° in the experimentally determined, r 0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H 2 O. The DFT calculations find that the H 2 O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1-N1) = 174.7°). Two states observed in the spectrum of H 2 O···imid, assigned as 0 - and 0 + states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.

The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A 0 , B 0 , and C 0 , centrifugal distortion constants, D J , D JK , D K , d 1 , and d 2 , and nuclear quadrupole coupling constants of nitrogen atoms, χ aa (N1), χ bb (N1) - χ cc (N1), χ aa (N3), and χ bb (N3) - χ cc (N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13 C or 15 N are used to determine the r s coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH 3 group about its local C 3 axis. The V 3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm -1 , respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH 3 group and π-orbitals on the aromatic ring.

Rotational spectra have been measured and assigned for four conformers of trans-urocanic acid. The acid was transferred into the gas phase through laser vaporisation of a solid sample, mixed with a neon buffer gas and then cooled through supersonic expansion. Molecules and complexes in the expanding gas jet were probed through chirped-pulse, Fourier transform microwave spectroscopy between 2.0 and 18.5 GHz. Rotational constants, A0, B0 and C0; centrifugal distortion constants, ΔJ and ΔJK; and nuclear quadrupole coupling constants of the nitrogen atoms, χaa(N) and χbb(N)-χcc(N), were determined for the various conformers. Data were obtained for ten isotopologues of the conformer that was observed to yield the spectrum of highest intensity. Substitution (rs) coordinates were determined for all carbon atoms and two hydrogen atoms of this conformer. Other observed spectra were assigned to conformers on the basis of excellent agreement between calculated and experimentally-determined rotational constants, and empirical observations of the relative intensities of a- and b-type transitions. The results of DFT calculations imply high barriers to the interconversion of assigned conformers.

Ground-state rotational spectra were observed for ten symmetric-top isotopologues H 3 N⋯S=C=S, H 3 N⋯ 34 S=C=S, H 3 N⋯S=C= 34 S, H 3 N⋯S= 13 C=S, H 3 15 N⋯S=C=S, H 3 15 N⋯ 34 S=C=S, H 3 15 N⋯S=C= 34 S, H 3 15 N⋯S= 13 C=S, H 3 15 N⋯ 33 S=C=S, and H 3 15 N⋯S=C= 33 S, the first five in their natural abundance in a mixture of ammonia and carbon disulphide in argon and the second group with enriched 15 NH 3 . The four asymmetric-rotor isotopomers H 2 DN⋯S=C=S, H 2 DN⋯ 34 S=C=S, H 2 DN⋯S=C= 34 S, and HD 2 N⋯S=C=S were investigated by using a sample composed of ND 3 mixed with CS 2 . Rotational constants, centrifugal distortion constants, and 33 S nuclear quadrupole coupling constants were determined from spectral analyses and were interpreted with the aid of models of the complex to determine its symmetry, geometry, one measure of the strength of the intermolecular binding, and information about the subunit dynamics. The complex has C 3v symmetry, with nuclei in the order H 3 N⋯S=C=S, thereby establishing that the non-covalent interaction is a chalcogen bond involving the non-bonding electron pair of ammonia as the nucleophile and the axial region near one of the S atoms as the electrophile. The small intermolecular stretching force constant k σ = 3.95(5) N m -1 indicates a weak interaction and suggests the assumption of unperturbed component geometries on complex formation. A simple model used to account for the contribution of the subunit angular oscillations to the zero-point motion leads to the intermolecular bond length r(N⋯S) = 3.338(10) Å.

The rotational spectra of H 3 N⋯AgI and H 2 O⋯AgI have been recorded between 6.5 and 18.5 GHz by chirped-pulse Fourier-transform microwave spectroscopy. The complexes were generated through laser vaporisation of a solid target of silver or silver iodide in the presence of an argon gas pulse containing a low concentration of the Lewis base. The gaseous sample subsequently undergoes supersonic expansion which results in cooling of rotational and vibrational motions such that weakly bound complexes can form within the expanding gas jet. Spectroscopic parameters have been determined for eight isotopologues of H 3 N⋯AgI and six isotopologues of H 2 O⋯AgI. Rotational constants, B 0 ; centrifugal distortion constants, D J , D JK or Δ J , Δ JK ; and the nuclear quadrupole coupling constants, χ aa (I) and χ bb (I) - χ cc (I) are reported. H 3 N⋯AgI is shown to adopt a geometry that has C 3v symmetry. The geometry of H 2 O⋯AgI is C s at equilibrium but with a low barrier to inversion such that the vibrational wavefunction for the v = 0 state has C 2v symmetry. Trends in the nuclear quadrupole coupling constant of the iodine nucleus, χ aa (I), of L⋯AgI complexes are examined, where L is varied across the series (L = Ar, H 3 N, H 2 O, H 2 S, H 3 P, or CO). The results of experiments are reported alongside those of ab initio calculations at the CCSD(T)(F12*)/AVXZ level (X = T, Q).