Autoionization dynamics of (2P1/2)ns/d states in krypton probed by noncollinear wave mixing with attosecond extreme ultraviolet and few-cycle near infrared pulses.

The autoionization dynamics of the (2P1/2)ns/d Rydberg states in krypton are investigated using spatially isolated wave-mixing signals generated with a short train of subfemtosecond extreme ultraviolet (XUV) pulses and noncollinear, few-cycle near infrared pulses. Despite ubiquitous quantum beat oscillations from XUV-induced coherences within the excited-state manifold, these wave-mixing spectra allow for the simultaneous evaluation of autoionization lifetimes from a series of Rydberg states above the first ionization potential. Experimentally measured lifetimes of 22 ± 8 fs, 33 ± 6 fs, and 49 ± 6 fs for the wave-mixing signals emitting from the (2P1/2)6d/8s, (2P1/2)7d/9s, and (2P1/2)8d/10s resonances compare favorably with lifetimes for the (2P1/2)6d, 7d, and 8d Rydberg states determined from spectral linewidths. Analysis of the quantum beats reveals that the enhancement of wave-mixing pathways that couple the (2P1/2)nd states to themselves leads to individual reporter state-dependent decays in the wave-mixing signals. The results demonstrate the promise of wave-mixing spectroscopies with subfemtosecond XUV pulses to provide valuable insights into processes governed by electronic dynamics.

Nonlinear XUV signal generation probed by transient grating spectroscopy with attosecond pulses.

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.

Multiple pulse coherent dynamics and wave packet control of the N2 a'' 1Σ dark state by attosecond four-wave mixing.

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.

Probing the Dynamics of Rydberg and Valence States of Molecular Nitrogen with Attosecond Transient Absorption Spectroscopy.

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.

Where's the charge? Protonation sites in gaseous ions change with hydration.

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.