Raúl Montero

Ultrafast dynamics of aniline in the 294-234 nm excitation range: The role of the πσ* state

The ultrafast relaxation of jet-cooled aniline was followed by time-resolved ionization, after excitation in the 294-234 interval. The studied range of energy covers the absorption of the two bright ππ∗ excitations, S(1) and S(3), and the almost dark S(2) (πσ∗) state. The employed probe wavelengths permit to identify different ultrafast time constants related with the coupling of the involved electronic surfaces. A τ(1) = 165 ± 30 fs lifetime is attributed to dynamics along the S(2) (πσ∗) repulsive surface. Other relaxation channels as the S(1)→S(0) and S(3)→S(1) internal conversion are also identified and characterized. The work provides a general view of the photophysics of aniline, particularly regarding the role of the πσ∗ state. This state appears as minor dissipation process due to the ineffective coupling with the bright S(1) and S(3) states, being the S(1)→S(0) internal conversion the main non-radiative process in the full studied energy range. Additionally, the influence of the off-resonance adiabatic excitation of higher energy electronic states, particularly S(3), is also observed and discussed.

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 a(1)←π 1a(2)), 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.

Ultrafast Photophysics of the Isolated Indole Molecule

The relaxation dynamics of the isolated indole molecule has been tracked by femtosecond time-resolved ionization. The excitation region explored (283-243 nm) covers three excited states: the two ππ* L(b) and L(a) states, and the dark πσ* state with dissociative character. In the low energy region (λ > 273 nm) the transients collected reflect the absorption of the long living L(b) state. The L(a) state is met 1000-1500 cm(-1) above the L(b) origin, giving rise to an ultrafast lifetime of 40 fs caused by the internal conversion to the lower L(b) minimum through a conical intersection. An additional ~400 fs component, found at excitation wavelengths shorter than 263 nm, is ascribed to dynamics along the πσ* state, which is likely populated through coupling to the photoexcited L(a) state. The study provides a general view of the indole photophysics, which is driven by the interplay between these three excited surfaces and the ground state.

Revisiting the relaxation dynamics of isolated pyrrole

Herein, the interpretation of the femtosecond-scale temporal evolution of the pyrrole ion signal, after excitation in the 267-217 nm interval, recently published by our group [R. Montero, A. Peralta Conde, V. Ovejas, M. Fernández-Fernández, F. Castaño, J. R. Vázquez de Aldana, and A. Longarte, J. Chem. Phys. 137, 064317 (2012)] is re-visited. The observation of a shift in the pyrrole(+) transient respect to zero delay reference, initially attributed to ultrafast dynamics on the πσ* type state (3s a1 ← π 1a2), is demonstrated to be caused by the existence of pump + probe populated states, along the ionization process. The influence of these resonances in pump-prone ionization experiments, when multi-photon probes are used, and the significance of a proper zero-time reference, is discussed. The possibility of preparing the πσ* state by direct excitation is investigated by collecting 1 + 1 photoelectron spectra, at excitation wavelengths ranging from 255 to 219 nm. No conclusive evidences of ionization through this state are found.

Ultrafast evolution of imidazole after electronic excitation.

The ultrafast dynamics of the imidazole chromophore has been tracked after electronic excitation in the 250-217 nm energy region, by time delayed ionization with 800 nm laser pulses. The time-dependent signals collected at the imidazole(+) mass channel show the signature of femtosecond dynamics, originating on the πσ*- and ππ*-type states located in the explored energy region. The fitting of the transients, which due to the appearance of nonresonant coherent adiabatic excitation requires a quantum treatment based in the Bloch equations, yields two lifetimes of 18 ± 4 and 19 ± 4 fs. The first is associated with the πσ* ← ππ* internal conversion, while the second reflects the loss of ionization cross-section as the system evolves along the dissociative πσ* surface. This study provides a comprehensive picture of the photophysics of the molecule that agrees with previous experimental and theoretical findings.

Ultrafast Nonradiative Relaxation Channels of Tryptophan

The nonradiative relaxation channels of gas-phase tryptophan excited along the S1-S4 excited states (287-217 nm) have been tracked by femtosecond time-resolved ionization. In the low-energy region, λ ≥ 240 nm, the measured transient signals reflect nonadiabatic interactions between the two bright La and Lb states of ππ* character and the dark dissociative πσ* state of the indole NH. The observed dynamical behavior is interpreted in terms of the ultrafast conversion of the prepared La state, which simultaneously populates the fluorescent Lb> and the dissociative πσ* states. At higher energies, after excitation of the S4 state, the tryptophan dynamics diverges from that observed in indole, pointing to the opening of a relaxation channel that could involve states of the amino acid part. The work provides a detailed picture of the processes and electronic states involved in the relaxation of the molecule, after photoexcitation in the near part of its UV absorption spectrum.

Photophysics of 1-Aminonaphthalene: A Theoretical and Time-Resolved Experimental Study

The photophysics of 1-aminonaphthalene (1-napthylamine, AMN) has been investigated on the basis of a constructive experimental-theoretical interplay derived from time-resolved measurements and high-level quantum-chemical ab initio CASPT2//CASSCF calculations. Transient ionization signals at femtosecond resolution were collected for AMN cold isolated molecules following excitation from the vibrationless ground level to a number of vibrational states (within the pump resolution) in the lowest accessible excited state and further multiphoton ionization probing at 500, 800, and 1300 nm. Theory predicts two pipi* states, (1)L(b) and (1)L(a), as the lowest singlet electronic excitations, with adiabatic transitions from S(0) at 3.50 and 3.69 eV, respectively. Since the associated oscillator strength for the lowest transition is exceedingly small, the (1)L(b) state is not expected to become populated significantly and the (1)L(a) state appears as the main protagonist of the AMN photophysics. Though calculations foresee a surface crossing between (1)L(a) and the lower (1)L(b) states, no dynamical signature of it is observed in the time-dependent measurements. In the relaxation of (1)L(a), the radiant emission competes with the intersystem crossing and internal conversion channels. The rates of these mechanisms have been determined at different excitation energies. The internal conversion is mediated by a (1)L(a)/S(0) conical intersection located 0.7 eV above the (1)L(a) minimum. The relaxation of a higher-lying singlet excited state, observed above 40 000 cm(-1) (4.96 eV) and calculated at 5.18 eV, has been also explored.

Mass-resolved infrared spectroscopy of complexes without chromophore by nonresonant femtosecond ionization detection.

Mass-resolved excitation spectroscopic techniques are usually limited to systems with a chromophore, that is, a functional group with electronic transitions in the Vis/UV, with lifetimes from hundreds of picoseconds to some microseconds. In this paper, we expand such techniques to any system, by using a combination of nanosecond IR pulses with nonresonant ionization with 800 nm femtosecond laser pulses. Furthermore, we demonstrate that the technique can achieve conformational specificity introducing an additional nanosecond IR laser. As a proof-of-principle, we apply the technique to the study of phenol(H(2)O)(1), propofol(H(2)O)(1) γ-butyrolactone(H(2)O)(n), n = 1-3, and (H(2)O)(2) complexes. While monohydrated phenol and propofol clusters permit a direct comparison with a well-studied system including an aromatic chromophore, γ-butyrolactone is a cyclic nonaromatic molecule, whose mass-resolved spectroscopy cannot be tackled by conventional techniques. Finally, we further demonstrate the potential of the technique by obtaining the first mass-resolved IR spectrum of the neutral water dimer, a nice example of a system whose ionization-based detection had not been possible to date.

Tracking the Relaxation of 2,5-Dimethylpyrrole by Femtosecond Time-Resolved Photoelectron and Photoion Detection

The relaxation of 2,5-dimethylpyrrole after excitation in the 290-239 nm range, which covers the weak absorption of the S1 (1)A2 πσ* state, dissociative along the N-H bond, and the stronger band mostly attributed to the (1)B2 ππ* state, has been investigated by time-resolved ion and photoelectron techniques. The measurements yield an invariant lifetime of ∼55 fs for the (1)πσ* state, after preparation in its Franck-Condon region with increasing vibrational content. This ultrafast rate indicates that, contrary to the observations made in pyrrole (Roberts et al. Faraday Discuss. 2013, 163, 95-116), the molecule reaches the dissociative part of the potential without any barrier effect, although calculations predict the latter to be higher than in the pyrrole case. The results are rationalized in terms of a barrier free multidimensional pathway that very likely involves out-of-plane vibrations. Additionally, a lifetime of ∼100 fs is found after excitation along the higher (1)B2 ππ* ← S0 transition. The relaxation of this state by coupling to a very short living S1 (1)πσ* state, or by alternative routes, is discussed in the light of the collected photoelectron measurements.

IR mass-resolved spectroscopy of complexes without chromophore: Cyclohexanol·(H2O)n, n = 1–3 and cyclohexanol dimer

Mass-resolved IR spectra of cyclohexanol-water clusters and cyclohexanol dimer in supersonic expansions are presented for the first time. A combination of ns and fs IR lasers made possible recording such spectra without inclusion of a chromophore or a messenger atom. Furthermore, employment of the recently developed IR(3) technique [I. León, R. Montero, F. Castaño, A. Longarte, and J. A. Fernández, J. Phys. Chem. A 116, 6798 (2012)] allowed us to discriminate between the contribution of different species to the IR spectrum. Comparison of the experimental spectra with the predictions at the M06-2X/6-311++G(d,p) calculation level confirmed the assignment of the spectrum of cyclohexanol·(H2O)1 to a structure in which water is accepting a proton from cyclohexanols OH group, and those of cyclohexanol·(H2O)(2,3) to structures with cyclic hydrogen bond networks. A comparative analysis of the results obtained with those reported on other aromatic alcohols is also offered.