Femtosecond laser pulses lasting only a few optical periods hold the potential for probing and manipulating the electronic degrees of freedom within matter. However, the generation of high-contrast, few-cycle pulses in the high power limit still remains nontrivial. In this Letter, we present the application of ammonium dihydrogen phosphate (ADP) as an optical medium for compensating for the higher-order dispersion of a carrier-envelope stable few-cycle waveform centered at 735 nm. The ADP crystal is capable of removing the residual third-order dispersion present in the spectral phase of an input pulse, resulting in near-transform-limited 2.9 fs pulses lasting only 1.2 optical cycles in duration. By utilizing these high-contrast, few-cycle pulses for high-harmonic generation, we are able to produce nanojoule-scale, isolated attosecond pulses.

The electronic character of photoexcited molecules can abruptly change at avoided crossings and conical intersections. Here, we report direct mapping of the coupled interplay between electrons and nuclei in a prototype molecule, iodine monobromide (IBr), by using attosecond transient absorption spectroscopy. A few-femtosecond visible pulse resonantly excites the [Formula: see text], Y(0⁺), and Z(0⁺) states of IBr, and the photodissociation dynamics are tracked with an attosecond extreme-ultraviolet pulse that simultaneously probes the I-4d and Br-3d core-level absorption edges. Direct comparison with quantum mechanical simulations unambiguously identifies the absorption features associated with adiabatic and diabatic channels at the B/Y avoided crossing and concurrent two-photon dissociation processes that involve the Y/Z avoided crossing. The results show clear evidence for rapid switching of valence-electronic character at the avoided crossing.

Experiments are presented on real-time probing of coherent electron dynamics in xenon initiated by strong-field double ionization. Attosecond transient absorption measurements allow for characterization of electronic coherences as well as relative ionization timings in multiple electronic states of Xe^{+} and Xe^{2+}. A high degree of coherence g=0.4 is observed between ^{3}P_{2}^{0}-^{3}P_{0}^{0} of Xe^{2+}, whereas for other possible pairs of states the coherences are below the detection limits of the experiments. A comparison of the experimental results with numerical simulations based on an uncorrelated electron-emission model shows that the coherences produced by strong-field double ionization are more selective than predicted. Surprisingly short ionization time delays, 0.85 fs, 0.64 fs, and 0.75 fs relative to Xe^{+} formation, are also measured for the ^{3}P_{2}, ^{3}P_{0}, and ^{3}P_{1} states of Xe^{2+}, respectively. Both the unpredicted selectivity in the formation of coherence and the subfemtosecond time delays of specific states provide new insight into correlated electron dynamics in strong-field double ionization.

A method for generating 20 fs UV pump pulses for attosecond transient absorption spectroscopy is presented. An application of the setup to the real-time mapping of conical intersection dynamics and coherent nuclear motion in CH3I is demonstrated.

Extreme ultraviolet (XUV) transient absorption spectroscopy has emerged as a sensitive tool for mapping the real-time structural and electronic evolution of molecules. Here, attosecond XUV transient absorption is used to track dynamics in the A-band of methyl iodide (CH₃I). Gaseous CH₃I molecules are excited to the A-band by a UV pump (277 nm, ∼20 fs) and probed by attosecond XUV pulses targeting iodine I(4d) core-to-valence transitions. Owing to the excellent temporal resolution of the technique, passage through a conical intersection is mapped through spectral signatures of nonadiabatic wave packet bifurcation observed to occur at 15 ± 4 fs following UV photoexcitation. The observed XUV signatures and time dynamics are in agreement with previous simulations [H. Wang, M. Odelius, and D. Prendergast, J. Chem. Phys. 151, 124106 (2019)]. Due to the short duration of the UV pump pulse, coherent vibrational motion in the CH₃I ground state along the C-I stretch mode (538 ± 7 cm^-1) launched by resonant impulsive stimulated Raman scattering and dynamics in multiphoton excited states of CH₃I are also detected.

Attosecond probing of core-level electronic transitions in molecules provides a sensitive tool for real-time observation of chemical dynamics. Here, we employ ultrafast extreme-ultraviolet (XUV) transient-absorption spectroscopy to investigate the excited state electronic and nuclear dynamics in a prototype molecule, I2. A few-femtosecond visible pump pulse is employed to excite the I2 molecule and an attosecond XUV pulse is used to probe the dynamics through iodine-4d core-to-valence transitions. A highly extended vibrational wave packet (ν′=10-50,νmax′=25) is prepared by one-photon absorption in the valence excited B3Π0+u state of I2 and its motion is directly mapped due to the strong shift of the XUV core-level transition with internuclear separation. Through the imaging of this vibrational motion, we directly reconstruct the transition energy between the valence and the core-excited states as a function of internuclear distance. Besides single-photon dynamics, distinct direct dissociation pathways arising from two-photon pump absorption are also revealed.

Ultrafast laser excitation can trigger complex coherent dynamics in molecules. Here, we report attosecond transient-absorption experiments addressing simultaneous probing of electronic and vibrational dynamics in a prototype molecule, deuterium bromide (DBr), following its strong-field ionization. Electronic and vibrational coherences in the ionic XΠ3/22 and XΠ1/22 states are characterized in the Br-3d core-level absorption spectra via quantum beats with 12.6-fs and 19.9-fs periodicities, respectively. Polarization scans reveal that the phase of the electronic quantum beats depends on the probe direction, experimentally showing that the coherent electronic motion corresponds to oscillation of the hole density along the ionization-field direction. The vibrational quantum beats are found to maintain a relatively constant amplitude, whereas the electronic quantum beats exhibit a partial decrease in time. Quantum wave-packet simulations show that decoherence from vibrational motion is insignificant because the XΠ3/22 and XΠ1/22 potentials are nearly parallel. A comparison between the DBr and HBr results suggests that rotational motion is responsible for the decoherence since it leads to initial alignment prepared by the strong-field ionization.

Conical intersections between electronic states often dictate the chemistry of photoexcited molecules. Recently developed sources of ultrashort extreme ultraviolet (XUV) pulses tuned to element-specific transitions in molecules allow for the unambiguous detection of electronic state-switching at a conical intersection. Here, the fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C₃H₇I and t-C₄H₉I) through a conical intersection between ³Q₀/¹Q₁ spin-orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the ³Q₀ state into the ¹Q₁ state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C₃H₇I and t-C₄H₉I directly lead to differences in the photoproduct branching ratio of the two systems.

Attosecond extreme ultraviolet (XUV) and soft x-ray sources provide powerful new tools for studying ultrafast molecular dynamics with atomic, state, and charge specificity. In this report, we employ attosecond transient absorption spectroscopy (ATAS) to follow strong-field-initiated dynamics in vinyl bromide. Probing the Br M edge allows one to assess the competing processes in neutral and ionized molecular species. Using ab initio non-adiabatic molecular dynamics, we simulate the neutral and cationic dynamics resulting from the interaction of the molecule with the strong field. Based on the dynamics results, the corresponding time-dependent XUV transient absorption spectra are calculated by applying high-level multi-reference methods. The state-resolved analysis obtained through the simulated dynamics and related spectral contributions enables a detailed and quantitative comparison with the experimental data. The main outcome of the interaction with the strong field is unambiguously the population of the first three cationic states, D ₁, D ₂, and D ₃. The first two show exclusively vibrational dynamics while the D ₃ state is characterized by an ultrafast dissociation of the molecule via C-Br bond rupture within 100 fs in 50% of the analyzed trajectories. The combination of the three simulated ionic transient absorption spectra is in excellent agreement with the experimental results. This work establishes ATAS in combination with high-level multi-reference simulations as a spectroscopic technique capable of resolving coupled non-adiabatic electronic-nuclear dynamics in photoexcited molecules with sub-femtosecond resolution.

The photodissociation dynamics of alkyl iodides along the C-I bond are captured by attosecond extreme-ultraviolet (XUV) transient absorption spectroscopy employing resonant ∼20 fs UV pump pulses. The methodology of previous experiments on CH₃I [Chang et al., J. Chem. Phys. 154, 234301 (2021)] is extended to the investigation of a C-I bond-breaking reaction in the dissociative A-band of C₂H₅I, i-C₃H₇I, and t-C₄H₉I. Probing iodine 4d core-to-valence transitions in the XUV enables one to map wave packet bifurcation at a conical intersection in the A-band as well as coherent vibrations in the ground state of the parent molecules. Analysis of spectroscopic bifurcation signatures yields conical intersection crossing times of 15 ± 4 fs for CH₃I, 14 ± 5 fs for C₂H₅I, and 24 ± 4 fs for i-C₃H₇I and t-C₄H₉I, respectively. Observations of coherent vibrations, resulting from a projection of A-band structural dynamics onto the ground state by resonant impulsive stimulated Raman scattering, indirectly reveal multimode C-I stretch and CCI bend vibrations in the A-bands of C₂H₅I, i-C₃H₇I, and t-C₄H₉I.