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Laser spectroscopy in jets is one of the main sources of structural data from molecular aggregates. Consequently, numerous and sophisticated experimental systems have been developed to extract precise information, which is usually interpreted in the light of quantum mechanical calculations. However, even with the most sophisticated experiments, it is sometimes difficult to interpret the experimental results. We present here the example of water dimer and how after almost 70 years, the assignment of its mass-resolved IR spectrum still generates controversy that extends toward the mechanism of ionization of water aggregates.

The available experimental data provided by ultrafast dynamics studies of pyrrole and its derivatives in excited states of mixed Rydberg/valence 3s/πσ* character are strongly affected by the interaction with the laser pulses. Understanding these data has constituted an endeavor for several groups during the past few years. Here we apply a simple theoretical model that, including the interaction with the laser pulses, allows one to clarify some aspects of the discrepancies between measurements monitoring different experimental observables. New experimental data on pyrrole, 2,4-dimethylpyrrole, and 2,5-dimethylpyrrole are also provided to check the validity of the model and to gain more insight into the excited state dynamics of pyrrole systems.

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.

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 (λ273 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.

The relaxation dynamics of naphthalene (NPH) and 1-aminonaphthalene (AMN) in superexcited (SE) states has been investigated by pump-probe femtosecond ionization. SE states were produced by two-photon resonant absorption via S1 and S2 states and their time evolution probed by ionization with 800 nm light pulses. A fine-tuning of the pump and probe laser intensities permits us to separate the dynamics of the SE states from that of the S1/S2 intermediate states. SE state relaxation pathways were investigated in the channels of the parent and the major fragment ions. The lifetimes of two relaxation processes were derived from the exponential fit and found to be in the femtosecond and the picosecond time scales. The fast component is attributed to internal conversion (IC) from the SE states to the Rydberg states, whereas the slow one, only observed in the fragment transients, is assigned to fragmentation of the relaxed neutral molecules. The study sheds light on the significant role of SE states dynamics in pump-probe ionization experiments.

A detailed investigation on the ion kinetic energy distributions of ions ejected in the nanosecond pulsed laser ablation of aluminum is reported. For laser fluences just over threshold, the emerging ions fit shifted neat Maxwell−Boltzmann-Coulomb (MBC) distributions. For fluences higher than ∼1.3 J/cm2, the Al+ distributions split into two MBC contributions peaked at different energies. It is demonstrated that the observed Al+ ion distribution has two components, one fast, correlated with the direct multiphoton laser ionization, and the other slow, associated with electron−Al0 collisions in the solid. A similar behavior is observed at higher fluences for all Al ion distributions indicating that the electron-impact ionization of Al rate constants is faster than that of recombination and other possible collision channels. In addition, the linear relationship between the Coulomb velocities and the ion charges and the behavior of Coulomb energy of the ions versus the laser fluence suggest the appearance of an...

The adiabatic ionization threshold (AIT) of trans- and gauche-benzocaine has been measured by zero electron kinetic energy-pulsed field ionization (ZEKE-PFI) spectroscopy to be 7.8412+/-0.0008 eV (lasers at 34,134.4 and 29,109.3 cm(-1)) and 7.8421+/-0.0004 eV (34,144.8+29105.7 cm(-1)), respectively. AITs computed at the B3LYP/AUG-cc-p-VDZ level for the two conformers are some approximately 2,500 cm(-1) lower than the experimental; in contrast their energy difference is very close. The trans-benzocaine cation ZEKE spectra has been recorded taking a number of S(1) intermediate vibronic states. The spectra provide an energy threshold for the appearance of intramolecular vibrational redistribution (IVR) around approximately 540 cm(-1) in the S(1) state.

Author Institution: Department of Chemistry, Purdue University; Department of Chemistry and the James Franck Institute, University of Chicago

The electronic and infrared spectra of anthranilic acid in a supersonic jet were measured. The fluorescence excitation spectrum is extremely congested. IR−UV hole-burning measurements indicate that only a single ground-state rotamer contributes to the observed spectrum. Vibrational progressions in 252 and 418 cm-1 modes were observed. Density functional theory calculations indicate that these modes involve the in-plane bending motions of the amino and carboxyl groups of anthranilic acid. The presence of vibrational progressions in these modes suggests that the relative positions of the amino and carboxyl groups are different in the ground and excited electronic states of anthranilic acid. This observation is supported by the fluorescence-dip infrared spectra acquired, which show a shift in the lower frequency NH stretch fundamental from 3394 to 2900 cm-1 upon electronic excitation, suggesting a dramatic strengthening of the intramolecular hydrogen bond in the excited state. The change in the hydrogen-bond...

Binding or dissociation energies of the ground state, S0, the first electronic excited state, S1, and the ion ground state, I0, of the p-methoxyphenetylamine(H2O)1-4 complexes, referred to henceforth as either MPEA(H2O)1-4 or by their stoichiometric ratios 1:1−4, have been analyzed by threshold ion fragmentation. MPEA(H2O)1-4 complexes were prepared by the supersonic expansion of premixed samples containing appropriate amounts of MPEA and water molecules into carrier gas He. A number of isomers, with characteristic transition energies, have been identified: two for MPEA(H2O)1 at 35 676 and 35 689 cm-1, one for MPEA(H2O)2 at 35 687, one for MPEA(H2O)3 at 35 571 and two for MPEA(H2O)4 at 35 419 and 35 422 cm-1 respectively. The ionization energy and fragmentation thresholds, the latter defined as the onset energy of the process MPEA(H2O)n* + hν→ MPEA(H2O)n-1+ + H2O, have been determined, and from them the dissociation energies for the set of MPEA(H2O)1-4 isomers in their ground state, first electronic exci...

A combined wide range of laser spectroscopic techniques and computational quantum chemistry are used to investigate conformer landscapes of large substituted aromatic molecules. The spectroscopic techniques include laser-induced fluorescence (LIF), high-resolution LIF, resonance-enhanced multiphoton ionization (REMPI) with mass detection, two-color resonance ionization with mass detection (R2PI), laser dispersed emission (DE), and ion-dip hole burning (HB). These techniques are balanced with extensive ab initio DFT calculations at the B3LYP/6-31+G* and B3LYP/6-311+G* levels. These methods have been applied to investigate the nine possible conformations of the p−methoxyphenethylamine (MPEA) neurotransmitter molecule. Even when all the spectroscopic techniques play important roles, two color ionization energy (IE) threshold determinations have proven to be especially useful in the study. The gauche and anti conformers exhibit different ionization energy values, and even the amino hydrogen orientation influe...