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We have conducted time-resolved experiments (pump–probe and pump–repump–probe) on a model aromatic chromophore, aniline, after excitation in water at 267 nm. In the initial spectra recorded, in addition to the absorption corresponding to the bright ππ* excitation, the fingerprint of a transient state with the electron located on the solvent molecule is identified. We postulate that the latter corresponds to the πσ* state along the N–H bond, whose complete relaxation with a ∼500 ps lifetime results in the formation of the fully solvated electron and cation. This ionization process occurs in parallel with the ππ* photophysical channel that yields the characteristic ∼1 ns fluorescence lifetime. The observed branched pathway is rationalized in terms of the different H-bonds that the water establishes with the amino group. The proposed mechanism could be common for aromatics in water containing N–H or O–H bonds and would allow the formation of separated charges after excitation at the threshold of their electronic absorptions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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, S1 and S3, and the almost dark S2 (πσ*) state. The employed probe wavelengths permit to identify different ultrafast time constants related with the coupling of the involved electronic surfaces. A τ1 = 165 ± 30 fs lifetime is attributed to dynamics along the S2 (πσ*) repulsive surface. Other relaxation channels as the S1→S0 and S3→S1 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 S1 and S3 states, being the S1→S0 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 S3, is also observed and discussed. [ABSTRACT FROM AUTHOR]

Guaiacol (2-methoxyphenol) and eugenol (4-allyl-2-methoxyphenol) molecules are biologically active phenol derivatives with an intramolecular -OH...OCH3 hydrogen bond (H bond). Pulsed supersonic expansions of mixtures of either of the two molecules with He yield weakly bound homodimers as well as other higher-order complexes. A number of complementary and powerful laser spectroscopic techniques, including UV-UV and IR-UV double resonances, have been employed to interrogate the species formed in the expansion in order to get information on their structures and spectroscopic properties. The interpretation of the spectra of eugenol dimer is complex and required a previous investigation on a similar but simpler molecule both to gain insight into the possible structures and support the conclusions. Guaiacol (2-methoxyphenol) has been used for that purpose. The combination of the broad laser study combined with ab initio calculations at the Becke 3 Lee–Yang–Parr/6-31+G(d) level has provided the isomer structures, the potential-energy wells, and shed light on the inter- and intramolecular interactions involved. Guaiacol homodimer has been shown to have a single isomer whereas eugenol dimer has at least two. The comparison between the computed geometries of the dimers, their respective energies, and the vibrational normal modes permits the identification of the spectra. [ABSTRACT FROM AUTHOR]

Eugenol (4-Allyl-2-methoxyphenol), a phenol-derivative with an intramolecular –OH...OCH3 hydrogen bond (H bond), has been studied in a supersonic expansion using a number of complementary laser spectroscopic techniques. The mass-resolved excitation spectrum of eugenol and its water complexes are reported for the first time. The most intense set of bands on the resonantly enhanced multiphoton ionization (REMPI) spectrum of eugenol originate in a conformer whose S1←S0 transition is at 35 202 cm-1 and the ionization threshold at (I0←S0) 62 544±150 cm-1 (7.755±0.019 eV). In addition, two low intensity features redshifted with respect to the 000 transition have been identified as due to a second, less stable conformer. Ab initio calculations show that the potential energy landscape depicts at least three minima associated with one folded and two extended conformers, one of which is the most stable. Clusters of eugenol/water were prepared in a supersonic expansion by seeding eugenol and water in noble gas He and examined by two-color REMPI (R2PI) and IR-UV double resonance spectroscopies. Only one single isomer was observed for both 1:1 and 1:2 complexes, in contrast with the several stable conformers provided by the computations. The dissociation energies of the 1:1 and 1:2 complexes have been determined by the fragmentation threshold method and the results compared with those from ab initio calculations conducted at the B3LYP and MP2 levels with a variety of basis sets. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The conformational isomerization dynamics of melatonin and 5-methoxy N-acctyltryptophan methyl amide (5-methoxy NATMA) have been studied using the methods of IR-UV hole-filling spectroscopy and IR-induced population transfer spectroscopy. Using these techniques, single conformers of melatonin were excited via a well-defined NH stretch fundamental with an IR pump laser. This excess energy was used to drive conthrmational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were recooled into their zero-point levels, partially refilling the hole created in the ground state population of the excited conformer, and creating gains in population of the other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion via an UV probe laser. The isomerization quantum yields for melatonin show some conformation specificity but no hint of vibrational mode specificity. In 5-methoxy NATMA, no isomerization was observed out of the single conformational well populated in the expansion in the absence of the infrared excitation. In order to study the dependence of the isomerization on the cooling rate, the experimental arrangement was modified so that faster cooling conditions could he studied. In this arrangement, the pump and probe lasers were overlapped in space in the high density region of the expansion, and the time dependence of the zero-point level populations of the conformers was probed following selective excitation of a single conformation. The analysis needed to extract isomerization quantum yields from the timing scans was developed and applied to the melatonin timing scans. Comparison between the frequency and time domain isomerization quantum yields under identical experimental conditions produced similar results. Under fast cooling conditions, the product quantum yields were shifted from their values under standard conditions. The results for melatonin are compared with those for N-acetyl tryptophan methyl amide. [ABSTRACT FROM AUTHOR]

The conformational isomerization dynamics of N-acetyl tryptophan methyl amide (NATMA) and N-acetyl tryptophan amide (NATA) have been studied using the methods of IR-UV hole-filling spectroscopy (HFS) and IR-induced population transfer spectroscopy (IR-PTS), which were developed for this purpose. Single conformations of these molecules were selectively excited in well-defined NH stretch fundamentals. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were recooled into their zero-point levels, partially refilling the hole created in the ground state population of one of the conformers, and creating gains in population in other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion. In HFS, the IR wavelength is fixed and the UV laser tuned in order to determine where the population went following selective infrared excitation. In IR-PTS, the UV is fixed to monitor the population of a given conformation, and the IR is tuned to record the IR-induced changes in the population of the monitored conformer. Besides demonstrating the capability of the experiment to change the downstream conformational population distribution, the IR-PTS scans were used to extract two quantitative results: (i) The fractional populations of the conformers in the absence of the infrared, and (ii) the isomerization quantum yields for each of the six unique amide NH stretch fundamentals (three conformers each with two amide groups). The method for obtaining quantum yields is described in detail. In both NATMA and NATA, the quantum yields show modest conformational specificity, but only a hint of vibrational mode specificity. The prospects for the hole-filling technique for providing insight into energy flow in large molecules are discussed, leaving a more detailed theoretical modeling to the adjoining paper [Evans et al. J. Chem. Phys. 120, 148 (2004)]. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The 4-aminobenzonitrile (ABN) molecule has two active sites amenable to hydrogen bonding to methanol (MeOH) and ethanol (EtOH): the amino, –NH[sub 2], and the cyano, –CN, groups. Two-color resonance enhanced multiphoton ionization time-of-flight mass spectroscopy and hole burning spectroscopy in addition to the ionization energies provides sound evidence of the occurrence of two isomers in the ABN(MeOH)[sub 1] complex and one single isomer for the ABN(EtOH)[sub 1] complex. Combining the outcomes from the ABN/methanol and ABN/ethanol experiments with ab initio computations at reliable theory levels one identifies the preferred solvation sites and the optimized geometries. A discussion of the role of the aliphatic chains on the geometry of the isomers and other properties is presented. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The hydride stretch infrared spectra of indole, indole-H[sub 2]O, 3-methyl indole, 3-methyl indole-H[sub 2]O, the main conformer of tryptamine (TRA), two conformers of N-acetyl tryptophan amide (NATA), and three conformers of N-acetyl tryptophan methyl amide (NATMA), have been recorded in the electronically excited singlet states using excited-state fluorescence-dip infrared spectroscopy. NATA and NATMA are methyl-capped dipeptides of tryptophan that have conformational flexibility and exhibit sensitivity in their electronic spectra to the conformation of the dipeptide backbone. In the indole monomer, the indole NH stretch fundamental at the S[sub 1] origin is shifted from its ground-state value (3525 cm[sup -1]) to 3478 cm[sup -1]. The corresponding band in the indole-H[sub 2]O complex appears at 3387 cm[sup -1], shifted by a similar amount from its ground-state position (3436 cm[sup -1]). Higher vibronic levels within 1500 cm[sup -1] of the S[sub 1] origin, which have been identified previously [B. J. Fender et al., Chem. Phys. Lett. 239, 31 (1995)] as being ¹L[sub b], or ¹L[sub a] in character, all show similar excited state indole NH stretch absorptions. The corresponding spectra in 3-methyl indole, 3-methyl indole-H[sub 2]O, TRA, and in the C5 conformers of NATA and NATMA all are missing the indole NH stretch absorption. In its place, a broad background absorption appears, spread over the entire 2800-3800 cm[sup -1] region. In these molecules, other CH stretch or amide NH stretch absorptions remain sharp, appearing in their expected frequency ranges. Finally, the C7 conformations of NATA and NATMA, which possess an intramolecular hydrogen bond in the dipeptide backbone, have all infrared transitions washed out, replaced by a stronger broad background absorption. The entire data set can be explained by the presence of an excited ¹πσ[sup *] state which is dissociative along the indole NH stretch coordinate, as recently predicted by... [ABSTRACT FROM AUTHOR]

A combination of methods, including laser-induced fluorescence excitation, fluorescence-dip infrared (FDIR) spectroscopy, and UV-UV hole-burning spectroscopy, have been used to study the infrared and ultraviolet spectra of single conformations of two methyl-capped dipeptides: N-acetyl tryptophan amide (NATA) and N-acetyl tryptophan methyl amide (NATMA). Density functional theory calculations predict that all low-energy conformers of NATA and NATMA belong to one of two conformational families: C5, with its extended dipeptide backbone, or C7[sub eq], in which the dipeptide backbone forms a seven-membered ring joined by a H bond between the ψ-amide NH and the ψ-amide carbonyl groups. In NATA (NATMA), the LIF spectrum has contributions from two (three) conformers. FDIR spectroscopy has been used to record infrared spectra of the individual conformers over the 2800-3600 cm[sup -1] region, free from interference from one another. The NH stretch region provides unequivocal evidence that one of the conformers of NATA is C5, while the other is C7[sub eq]. Similarly, in NATMA, there are two C5 conformers, and one C7[sub eq] structure. Several pieces of evidence are used to assign spectra to particular C5 and C7[sub eq] conformers. NATA(A) and NATMA(B) are both assigned as C5(AP) structures, NATA(B) and NATMA(C) are assigned as C7[sub eq] (ΦP), and NATMA(A) is assigned as C5(AΦ). In both molecules, the C5 structures have sharp vibronic spectra, while the C7[sub eq] conformers are characterized by a dense, highly congested spectrum involving long progressions that extend several hundred wave numbers to the red of the C5 S[sub 1]-S[sub 0] origins. N-acetyl tryptophan ethyl ester (NATE), which can only form C5 conformers, shows only sharp transitions in its LIF spectrum due to four C5 conformers, with no evidence for the broad absorption due to C7[sub eq]. This provides direct experimental evidence for the importance of the peptide backbone conformation in... [ABSTRACT FROM AUTHOR]

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 IR3 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 cyclohexanol's 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. [ABSTRACT FROM AUTHOR]

Weakly bound complexes of phenol (Ph) and fluoromethane (CH[sub 3]F) formed in a supersonic expansion have been identified by one- and two-color mass-resolved and hole burning spectroscopies. Only one isomer has been observed for the 1:1 complex. Threshold fragmentation has been employed to determine the binding energies of the complex in its ground, S[sub 0], and first electronic, S[sub 1], states, as well as in the ion ground state, I[sub 0], yielding the following results: D[sub 0](S[sub 0])=1540±50 cm[sup -1], D[sub 0](S[sub 1])=1713±50 cm[sup -1], and D[sub 0](I[sub 0])=3932±50 cm[sup -1], respectively. In a complementary study, calculations on the complex geometries and binding energies were conducted at the B3LYP/6-31+G[sup *] and the MP2/6-31+G[sup *] levels. It has been shown that the binding energies computed at the MP2/6-31+G[sup *] level are in excellent agreement with the experimental values, whilst those calculated at the B3LYP/6-31+G[sup *] level underestimate them by nearly 30%, probably due to the poor description of the dispersion forces. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Weakly bound hydrogen bonded ethyl-p-aminobenzoate/water complexes, referred to henceforth both as EAB/(H[sub 2]O)[sub n] (n=1-4) or by their stoichiometry 1:n, have been investigated with a combined approach of mass and light detector laser spectroscopic techniques and ab initio calculations. The experimental studies follow explorations with laser induced fluorescence (LIF), and include one-color resonant enhanced multiphoton ionization (REMPI), two-color REMPI (R2PI), pressure dependent R2PI and hole burning (HB) spectroscopies. Calculations were conducted at the B3LYP/6-31+G[sup *] level and for the 1:1 complex led to the existence of six stable isomers, identified as the experimental origin bands at +4, +6, +13, +89, +96, and +108 cm-1 above the bare EAB 0[sub 0][sup 0] transition. It has been shown that three of these bands originate in the EAB trans conformer, while the other three derive from the EAB gauche conformer. None of the experimental methods used lead us to observe the EAB(H[sub 2]O)[sub 2] complex spectrum and the inspection of the EAB(H[sub 2]O)[sub 3] REMPI and R2PI spectra has been shown to be a fragmentation from the EAB(H[sub 2]O)[sub 4] complex. The structures and identification of the set of isomers are reported and a comparison with the results on the family complexes methyl-p-aminobenzoate/water is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Methyl-p-aminobenzoate(NH[sub 3])[sub 1] complex, henceforth MAB(NH[sub 3])[sub 1], prepared in a pulsed supersonic expansion, has been examined by laser mass-selective spectroscopies and density functional theory calculations, aiming to ascertain its isomer number, structures, identification, ionization energies, and vibrational assignments. Resonance enhanced multiphoton ionization and hole burning spectra of the species in supersonic beams show two 0[sub 0][sup 0] transitions redshifted by -715 and -709 cm-1 from that of bare MAB band origin and are plausibly associated with two different isomers, whereas ab initio calculations indicate the likely existence of five stable isomer structures. Identification of the experimental isomer spectra with the calculated structures is reported and, in particular, several isomer vibrational bands are identified by contrast with the calculated modes. Properties and features of the MAB(NH[sub 3])[sub 1] are compared with those of the MAB/water complexes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A complementary laser spectroscopy and computational study of the MAB(NH[sub 3])[sub 2-4] complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of -1160 cm-1 from the bare MAB 0[sub 0][sup 0] transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced two-color photo ionization (R2PI) studies. "Hole burning" spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the B3LYP/6-31+G[sup *] level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the 1:4 with a four ammonia chain coordinated to the NH[sub 2] group. The good agreement between calculated and experimental vibrational frequencies confirms the ammonia ring structure and allows us to assign a number of MAB(NH[sub 3])[sub 4] inter- and intramolecular vibrational bands. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A combined computational and experimental study of the methyl-p-aminobenzoate(H[sub 2]O)[sub n], (n=2,3,4) complexes [MAB(H[sub 2]O)[sub n]] is reported. Complexes potential energy surfaces were explored by ab initio density functional theory (DFT) methods, at the B3LYP/6-31G level, and the stable isomer structures and vibrational modes further computed at the B3LYP/6-31+G[sup *] level. A set of self-contained experimental techniques, including laser induced fluorescence (LIF), resonance enhanced multiphoton ionization mass-resolved spectroscopy (REMPI), two-color resonance enhanced multiphoton ionization mass-resolved spectroscopy (R2PI), "hole burning" spectroscopy (HB), and two-color ionization thresholds were used to study the spectra and other physical features of the complexes. Of the three title complexes only MAB(H[sub 2]O)[sub 4] has been observed with our experimental methods, while the MAB(H[sub 2]O)[sub 3] was formed by evaporation and MAB(H[sub 2]O)[sub 2] was not detected at all. It has been shown that the observed MAB(H[sub 2]O)[sub 4] complex has only one isomer with a hydrogen bonded water ring structure attached to the amino hydrogens and its low vibrational modes (up to 165 cm-1) have been assigned. A discussion of the results, including structures of stable isomers, isomer energies, ionization thresholds, and the difficulties in observing some solvated complexes is presented. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Methyl-p-aminobenzoate(H[sub 2]O)[sub 1] complex, henceforth MAB(H[sub 2]O)[sub 1], prepared by pulsed supersonic expansion, has been examined by a broad range of laser based spectroscopic, mass and isomer selective techniques and density functional theory (DFT) calculations, in order to identify its isomer structures, ionization energies, and vibrational frequencies. The experimental techniques used include laser induced fluorescence (LIF), mass resolved excitation spectroscopy (MRES) either with one (REMPI) or two laser colors (R2PI), laser excited dispersed emission (DE), high resolution MRES, pressure controlled R2PI, hole burning (HB) spectroscopy, and photoion fragmentation threshold (PIFT). Experimental results have been interpreted, rationalized and extended with density functional theory (DFT) computations at the B3LYP/6-31G and B3LYP/6-31+G* levels. Although bare MAB molecule have four possible solvation sites, prone to yielding hydrogen bonds with the water molecule, LIF, R2PI, and HB spectroscopy of MAB(H[sub 2]O)[sub 1] only pick out the presence of three blue shifted isomers, each accompanied by a number of vibrational features extending to approx. 500 cm-1. The high intensity bands have been demonstrated to originate in three isomers and their ionization energies, dispersed emission, vibrational spectra, and photoion fragmentation threshold have been measured and characterized. Isomer shifts and structures are discussed in the light of experimental and theoretical results. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]