The role of water in stabilizing sites of protonation in small gaseous ions is investigated using electrospray ionization (ESI) coupled with infrared photodissociation spectroscopy and computational chemistry. Protonation of p-aminobenzoic acid (PABA) and p-aminobenzoic acid methyl ester (PABAOMe) occurs at the carbonyl oxygen atom both in isolation and when one water molecule is attached. However, protonation occurs at the amine nitrogen atom, which is the most favorable site in aqueous solution, for PABAOMeH(+)·(H(2)O)(3) and for a significant fraction of PABAH(+)·(H(2)O)(6). Fewer water molecules are necessary to stabilize the solution-phase isomer of PABAOMeH(+) (3) than for PABAH(+) (≥6), indicating that the favorable hydrogen bonding in PABAH(+) is a more important factor than the higher gas-phase basicity of PABAOMeH(+) in stabilizing protonation at the carbonyl oxygen atom. Relative Gibbs free energies (133 K) calculated using B3LYP and MP2 with the 6-311++G** basis set were significantly different from each other, and both are in poor agreement with results from the experiments. ωB97X-D/6-311++G**, which includes empirical dispersion corrections, gave results that were most consistent with the experimental data. The relative stabilities of protonating at the carbonyl oxygen atom for PABAH(+)·(H(2)O)(0-6) and PABAOMeH(+)·(H(2)O)(0-2) can be rationalized by resonance delocalization. These findings provide valuable insights into the solvent interactions that stabilize the location of a charge site and the structural transitions that can occur during the ESI desolvation process.

An attosecond pulse is used to create a wavepacket in molecular nitrogen composed of multiple bound and autoionizing electronic states of Rydberg and valence character between 12 and 16.7 eV. A time-delayed, few-femtosecond, near-infrared (NIR) laser pulse is used to couple individual states in the wavepacket to multiple neighboring states, resulting in time-dependent modification of the absorption spectrum and revealing both individual quantum beats of the wavepacket and the energy shifts of the excited states in the presence of the strong NIR field. The broad bandwidth of the attosecond pulse and high energy resolution of the extreme ultraviolet spectrometer allow the simultaneous observation of time-dependent dynamics for many individual vibrational levels in each electronic state. Quantum beating with periods from 1.3 to 12 fs and transient line shape changes are observed among vibrational levels of a progression of electronically autoionizing Rydberg states leading to the excited A (2)Πu N2(+) ion core. Vibrational levels in the valence b (1)Πu state exhibit 50 fs oscillation periods, revealing superpositions between individual vibrational levels within this state. Comparisons are made to previous studies of electronic wavepackets in atoms that highlight similarities to atomic behavior yet illustrate unique contributions of the diatomic molecular structure to the wavepacket, including the influence of different electronic potentials and vibrational-level-specific electronic dynamics.

Electronic wavepackets composed of multiple bound excited states of atomic neon lying between 19.6 and 21.5 eV are launched using an isolated attosecond pulse. Individual quantum beats of the wavepacket are detected by perturbing the induced polarization of the medium with a time-delayed few-femtosecond near-infrared (NIR) pulse via coupling the individual states to multiple neighboring levels. All of the initially excited states are monitored simultaneously in the attosecond transient absorption spectrum, revealing Lorentzian to Fano lineshape spectral changes as well as quantum beats. The most prominent beating of the several that were observed was in the spin-orbit split 3d absorption features, which has a 40 femtosecond period that corresponds to the spin-orbit splitting of 0.1 eV. The few-level models and multilevel calculations confirm that the observed magnitude of oscillation depends strongly on the spectral bandwidth and tuning of the NIR pulse and on the location of possible coupling states.

The decay of highly excited states of xenon after absorption of extreme ultraviolet light is directly tracked via attosecond transient absorption spectroscopy using a time-delayed near-infrared perturbing pulse. The lifetimes of the autoionizing 5s5p 6 6p and 5s5p 6 7p channels are determined to be (21.9 ± 1.3) fs and (48.4 ± 5.0) fs, respectively. The observed values support lifetime estimates obtained by traditional linewidth measurements. The experiment additionally obtains the temporal evolution of the decay as a function of energy detuning from the resonance center, and a quantum mechanical formalism is introduced that correctly accounts for the observed energy dependence.

Coherent narrow band extreme ultraviolet (EUV) light is generated by a near-resonant four wave mixing (FWM) process between attosecond pulse trains and near infrared pulses in neon gas. The near-resonant FWM process involves one vacuum ultraviolet photon and two near-infrared (NIR) photons and produces new higher energy frequency components corresponding to the ns/nd to ground state (2s 2 2p 6) transitions in the neon atom. The EUV emission exhibits small angular divergence (2 mrad) and monotonically increasing intensity over a pressure range of 0.5-16 Torr, suggesting phase matching in the production of the narrow bandwidth coherent EUV light. In addition, time-resolved scans of the NIR nonlinear mixing process reveal the detection of a persistent, ultrafast bound electronic wavepacket based on a coherent superposition initiated by the vacuum ultraviolet (VUV) pulse in the neon atoms. This FWM process using attosecond pulses offers a means for both efficient narrowband EUV source generation and time-resolved investigations of ultrafast dynamics.

Using attosecond transient absorption, the dipole response of an argon atom in the vacuum ultraviolet (VUV) region is studied when an external electromagnetic field is present. An isolated attosecond VUV pulse populates Rydberg states lying 15 eV above the argon ground state. A synchronized fewcycle near infrared (NIR) pulse modifies the oscillating dipoles of argon impulsively, leading to alterations in the VUV absorption spectra. As the NIR pulse is delayed with respect to the VUV pulse, multiple features in the absorption profile emerge simultaneously including line broadening, sideband structure, sub-cycle fast modulations, and 5-10 fs slow modulations. These features indicate the coexistence of two general processes of the light-matter interaction: the energy shift of individual atomic levels and coherent population transfer between atomic eigenstates, revealing coherent superpositions. An intuitive formula is derived to treat both effects in a unifying framework, allowing one to identify and quantify the two processes in a single absorption spectrogram.Recently, attosecond transient absorption (ATA) spectroscopy has demonstrated great success in accessing ultrafast dynamics of bound and autoionizing systems in rare gas atoms [1-10] and diatomic molecules [11][12][13]. In this technique, dipole allowed states (bright states) are coherently populated by a broadband attosecond pulse, forming a wavepacket. The coherence between each individual excited state and the initial ground state forms a polarization dipole. Upon the arrival of a delayed short near infrared (NIR) pulse, these dipoles are subject to a complex amplitude change that occurs within a few femtoseconds, leading to novel absorption features in the transmitted spectrum of the attosecond pulse. Various dynamical aspects of the initially launched wavepacket are imprinted in the delay-dependent absorption spectra. Thus far, ATA studies have generally been carried out at photon energies ranging from 20 to 100 eV due to more efficient high harmonic generation above 20 eV. Here, we report ATA experiments on Ar atoms at photon energies around 15 eV, enabling a detailed study of electronic wavepacket dynamics for Rydberg states approaching the first ionization threshold of Ar.There are two major effects of the NIR pulse in ATA experiments. First, it shifts the energy of the individual states, generally known as the ac Stark shift. Secondly, it transfers population between different bright states by a two photon process. How these two effects manifest themselves in an ATA measurement is the essence of understanding the observables in such an experiment. The ac Stark shift corresponds to the energy difference between a dressed energy level and its field-free counterpart. In the time domain, it originates from the extra phase modulation, introduced by the NIR field, on the states of interest. The ac Stark shift manifests itself as reshaping of the absorption line profile in an ATA experiment [14,15]. On the other hand, population transfer between different b...

Nonlinear multidimensional spectroscopy is ubiquitous in the optical and radio frequency regions as a powerful tool to access structure and dynamics. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses holds promise for probing electronic dynamics and correlations with unprecedented time and spatial resolution. In this work, we use noncollinear four-wave mixing of a weak XUV attosecond pulse train (11-17 eV) and few-femtosecond NIR pulses (800 nm) to spectroscopically and dynamically probe the dipole-forbidden double-well potential of the a'' 1∑+g electronic state of nitrogen. The results demonstrate optical coupling of the inner and outer limits of the initial XUV-prepared vibrational wave packet in the valence character b' 1∑+u state to the inner and outer wells, respectively, of the a'' 1∑+g double well state by 800 nm light. Two four-wave mixing schemes with different pulse timing sequences and noncollinear beam geometries are used (one NIR pulse collinear and one NIR pulse noncollinear versus both NIR pulses noncollinear to the XUV beam) to measure the a'' dark state energetic structure and to control the dynamical preparation and motion of a dark state wave packet by selective population of either the inner Rydberg or outer valence-character potential well. Experimental measurements of the a'' 1∑+g outer well vibrational spacing and anharmonicity closely match the values theoretically predicted for this previously unobserved state.

Nonlinear spectroscopies are utilized extensively for selective measurements of chemical dynamics in the optical, infrared, and radio-frequency regimes. The development of these techniques for extreme ultraviolet (XUV) light sources facilitates measurements of electronic dynamics on attosecond timescales. Here, we elucidate the temporal dynamics of nonlinear signal generation by utilizing a transient grating scheme with a subfemtosecond XUV pulse train and two few-cycle near-infrared pulses in atomic helium. Simultaneous detection of multiple diffraction orders reveals delays of ≥1.5 fs in higher-order XUV signal generation, which are reproduced theoretically by solving the coupled Maxwell–Schrödinger equations and with a phase grating model. The delays result in measurable order-dependent differences in the energies of transient light induced states. As nonlinear methods are extended into the attosecond regime, the observed higher-order signal generation delays will significantly impact and aid temporal and spectral measurements of dynamic processes.