Tracking ultrafast dynamics by sub-20-fs UV pulses generated in the lab open atmosphere

This study describes a simple method to generate sub-20 fs UV-pulses (264 nm) by third-harmonic generation, in an air-plasma filament formed after focusing the fundamental 800 nm beam directly in the lab open-atmosphere. The generated pulses are applied to track the relaxation through the conical intersection that couples the S2 and S1 states, in the benchmark molecule of naphthalene. The transients, with a resolution of about 25 fs, show two differentiate patterns of quantum beats. The assignation of these oscillations to specific modes in the lower S1 state and to electronic coherence between the two coupled states is discussed.All the experiments were carried out at the CLPU facility. The authors acknowledge the assistance from the CLPU technical and administrative personnel. They also thank the financial support from the Spanish MINECO through the grant: PGC2018-098561-B-C21. The work was also funded by the Basque Government (IT1162-19 and IT1491-22). I. Lamas thanks the UPV/EHU for his predoctoral fellowship. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, ESF) is also gratefully acknowledged. The authors greatly appreciate the Open Access funding provided by University of Basque Country

Femtosecond evolution of the pyrrole molecule excited in the near part of its UV spectrum

The evolution of the isolated pyrrole molecule has been followed after excitation in the 265–217 nm range by using femtosecond time delayed ionization. The transients collected in the whole excitation range show the vanishing of the ionization signal in the femtosecond time scale, caused by the relaxation along a πσ* type state (3s a1←π 1a2), which is the lowest excited electronic state of the molecule. This surface is dissociative along the NH bond, yielding a 15 ± 3 fs lifetime that reflects the loss of the ionization cross-section induced by the ultrafast wavepacket motion. Although a weak πσ* absorption is detected, the state is mainly reached through internal conversion of the higher bright ππ* transitions, which occurs with a 19 ± 3 fs lifetime. In addition to its resonant excitation, the intense ππ* absorption extending in the 220–190 nm interval is also out-of-resonance populated at energies far to the red from its absorption onset. This coherent adiabatic excitation of the ππ* transition should follow the excitation pulse (coherent population return effect), but instead the system relaxes toward the lower πσ* surface through a conical intersection during the interaction time, leading to the population of πσ* state at wavelengths as long as 265 nm. According to the observed behavior, the time evolution of the system in the full excitation range studied is modeled by a coherent treatment that provides key insights on the photophysical properties of the molecule.This study was funded by Spanish MICINN (Grant No. CTQ2010-17749) and Consolider Program “Science and Applications of Ultrafast Ultraintense Lasers” (Grant No. CSD2007-00013), and by the Basque Government through the “Ayudas para apoyar las actividades de grupos de investigación del sistema universitario vasco” program. The experiments and theoretical calculations were carried out at the SGIker laser facility, and IZO-SGI of the UPV/EHU, respectively

Dispositivo y método para la caracterización de pulsos ultracortos

Método para caracterización de un pulso ultracorto, que comprende: hacer llegar un pulso de referencia de duración conocida a un primer detector con una banda prohibida menor que la energía por fotón del pulso a caracterizar y a un segundo detector con una banda prohibida mayor que la energía por fotón del pulso a caracterizar y menor que dos veces la energía por fotón del pulso a caracterizar: obtener en dicho primer detector una señal monofotónica de referencia Salref proporcional a la duración del pulso de referencia: obtener en dicho segundo detector una señal bifotónica de referencia beref proporcional a la duración del pulso de referencia; hacer llegar a los detectores un pulso ultracorto a caracterizar; obtener en dicho primer detector una señal monofotónica Sal proporcional a la duración del pulso ultracorto a caracterizar; obtener en dicho segundo detector una señal bifotónica Sbe proporcional a la duración del pulso ultracorto a caracterizar; obtener la duración del pulso a caracterizar a partir de dichas señales monofotónicas, de dichas señales bifotónicas y de la duración conocida del pulso de referencia. Dispositivo para la caracterización de un pulso ultracorto.Solicitud: 201830207 (02.03.2018)Nº Pub. de Solicitud: ES2724110A1 (06.09.2019)Nº de Patente: ES2724110B2 (31.03.2021

FLUORESCENCE DIP INFRARED SPECTROSCOPY OF THE S1S_{1} STATES OF INDOLE AND ITS DERIVATIVES

$^{a}$ A.L. Sobolewski, W. Domcke, Chem. Phys. Lett. 315 (1999) 293.Author Institution: Department of Chemistry, Purdue UniversityFluorescence dip infrared spectroscopy (FDIRS) has been used as the primary technique in supersonic expansions to determine the ground state conformations of indole, 3-methylindole, tryptamine, n-acetyltryptophan amide (NATA) and n-acetyltryptophan methylamide (NATMA). In each of these molecules two close-lying excited states, $^{1}L_{a}$ and $^{1}L_{b}$, are present, and play an important role in the spectroscopy and photophysics of the molecules. The relative separation of the $^{1}L_{a}$ and $^{1}L_{b}$ states is highly sensitive to the type of side chain attached to the C3 position of indole. Recently, a third excited state ($\pi\sigma^{\ast}$ transition localized in the NH bond) that is dissociative along the NH-stretching mode of the indole ring has been predicted to exist in this energy $region^{a}$. We have applied the FDIR technique to the $S_{1}$ state of the above molecules to study this process. The excited state spectra are marked by the absence of the indole-NH stretching mode, suggesting dissociation along this coordinate. The strength of coupling to the dissociative state varies significantly depending on the type of side chain attached to the indole ring. The excited state spectra range from a sharp band in indole, to a broad unresolved continuum in which all of the NH stretches are washed out in NATA and NATMA

INFRARED-INDUCED CONFORMATIONAL HOLE-FILLING SPECTROSCOPY: PROBING THE DYNAMICS OF CONFORMATIONAL ISOMERIZATION

Author Institution: Purdue University; Department of Chemistry, Purdue UniversitySupersonic expansions have been used extensively as a source of vibrationally cold molecules in the study of intramolecular interactions governing conformational selectivity. Through the cooling process, the conformational distributions of molecules present in the pre-expansion gas mixture are often ``frozen'' in to their zero-point levels by the collisions which occur in the supersonic expansion. Down-stream of the collisional region, molecules are free to be interrogated in an collision-free environment. We have developed a new technique which allows us to alter the post-nozzle distribution of molecules via infrared radiation. Specifically, molecules are selectively excited by the idler beam of an IR OPO/OPA source $(2800-3800 cm^{-1})$ less than 1mm from the nozzle. By proper choice of IR frequency, single conformations can be selectively excited at well-defined XH oscillators in the molecule. The point of excitation close to the nozzle orifice insures sufficient collisions following excitation to re-cool the excited molecules. Molecules are then probed downstream via their electronic $S_{1}\longleftarrow S_{0}$ origin by a UV laser. Conformational changes are detected by an increase or decrease of population in the conformation specific origins. N-acetyltryptophan methylamide (NATMA) is used as model system for such studies. Large, conformation-specific changes in conformational population are possible. The dependence on the vibrational mode, bath gas and conformation will be discussed

EXPLORING NEW POSSIBILITIES OF POPULATION TRANSFER METHODS: IR-UV AND UV-UV HOLE-FILLING SPECTROSCOPY OF MELATONIN

Author Institution: Department of Chemistry, Purdue UniversityThe hormone melatonin (Nacetyl-5-methoxytryptamine) is an indole derivative with a flexible peptide-like backbone attached at the C3 position. Using a combination of IR and UV double-resonance methods, the conformational preferences of melatonin in a molecular beam have been determined. Three major trans-amide conformers and two minor cis-amide conformers have been identified in the UV spectrum and characterized with resonant ion-dip infrared spectroscopy and fluorescence dip-infrared spectroscopy. The dynamics of conformational isomerization among these five minima have been investigated using IR-UV hole-filling spectroscopy. Population transfer following resonant IR excitation is efficient within the trans-amide and cis-amide branches of the potential energy surface, respectively, and the quantum yields for these processes have been determined. Population transfer is not observed between the two amide branches due to the larger barrier associated with cis/trans isomerization (15-20 kcal/mol). IR-UV hole-filling spectroscopy has been applied to melatonin-water clusters. The dynamics associated with population transfer on the monomer potential surface following water dissociation with resonant IR excitation will be discussed. Initial results will also be presented for a second type of population transfer spectroscopy, UV-UV hole-filling spectroscopy, where the selective excitation is performed via resonant pumping of a given $S_{0}-S_{1}$ melatonin transition

WATER CLUSTERS OF A MODEL DIPEPTIDE: N-ACETYLTRYPTOPHAN METHYLAMIDE

Author Institution: Purdue University; Department of Chemistry, Purdue UniversityMost molecules of biological relevance have considerable conformational flexibility. Our recent work on the model dipeptide N-acetyltryptophan methylamide, has found that there exist only a few low lying energy structures populated in the pre-nozzle gas mixture. However, this model dipeptide offers numerous hydrogen bonding sites to which water molecules can bind. These water molecules can also influence the conformation of the solute molecule. We present results from a variety of methods including one and two color R2PI, UV-UV Hole-burning and Resonant-Ion Dip Infrared Spectroscopy (RIDIRS), to determine the conformations of water bound clusters of N-Acetyl Tryptophan Methylamide (NATMA) in the gas phase. Quantum chemical calculations are used to calculate the harmonic frequencies of NATMA water clusters, which are compared to the infrared spectra to make assignments of the hydrogen bonding topologies of these clusters. The effects of water and water bridges on the conformational preferences of NATMA will be discussed