Time-resolved spectroscopic investigations of light-induced chemical reactions with universal detection capitalize recently on single-photon molecular probing using laser pulses in the extreme ultraviolet or X-ray regimes. Direct and simultaneous mappings of the time-evolving populations of ground-state reactants, Franck-Condon (FC) and transition state regions, excited-state intermediates and conical intersections (CI), and photoproducts in photochemical reactions utilize probe pulses that are broadband and energy-tunable. The limits on temporal resolution are set by the transit- or dwell-time of the photoexcited molecules at specific locations on the potential energy surface, typically ranging from a few femtoseconds to several hundred picoseconds. Femtosecond high-harmonic generation (HHG) meets the stringent demands for a universal spectroscopic probe of large regions of the intramolecular phase-space in unimolecular photochemical reactions. Extreme-ultraviolet and soft X-ray pulses generated in this manner with few-femtosecond or sub-femtosecond durations have enormous bandwidths, allowing the probing of many elements simultaneously through excitation or ionization of core-electrons, creating molecular movies that shed light on entire photochemical pathways. At free electron lasers (FELs), powerful investigations are also possible, recognizing their higher flux and tunability but more limited bandwidths. Femtosecond time-resolved X-ray transient absorption spectroscopy, in particular, is a valuable universal probe of reaction pathways that maps changes via the fingerprint core-to-valence resonances. The particular power of this method over valence-ionization probes lies in its unmatched element and chemical-site specificities. The elements carbon, nitrogen, and oxygen constitute the fundamental building blocks of life; photochemical reactions involving these elements are ubiquitous, diverse, and manifold. However, table-top HHG sources in the "water-window" region (280-550 eV), which encompasses the 1s-absorption edges of carbon (284 eV), nitrogen (410 eV), and oxygen (543 eV), are far from abundant or trivial. Recent breakthroughs in the laboratory have embraced this region by using long driving-wavelength optical parametric amplifiers coupled with differentially pumped high-pressure gas source cells. This has opened avenues to study a host of photochemical reactions in organic molecules using femtosecond time-resolved transient absorption at the carbon K-edge. In this Account, we summarize recent efforts to deploy a table-top carbon K-edge source to obtain crucial chemical insights into ultrafast, ultraviolet-induced chemical reactions involving ring-opening, nonadiabatic excited-state relaxation, bond dissociation and radical formation. The X-ray probe provides a direct spectroscopic viewport into the electronic characters and configurations of the valence electronic states through spectroscopic core-level transitions into the frontier molecular orbitals of the photoexcited molecules, laying fertile ground for the real-time mapping of the evolving valence electronic structure. The profound detail and mechanistic insights emerging from the pioneering experiments at the carbon K-edge are outlined here. Comparisons of the experimental methodology with other techniques employed to study similar reactions are drawn, where applicable and relevant. We show that femtosecond time-resolved X-ray transient absorption spectroscopy blazes a new trail in the study of nonadiabatic molecular dynamics. Despite table-top implementations being largely in their infancy, future chemical applications of the technique will set the stage for widely applicable, universal probes of photoinduced molecular dynamics with unprecedented temporal resolution.

Femtosecond extreme ultraviolet (XUV) pulses produced by high harmonic generation are used to probe the transition-state region in the 266 nm photodissociation of CH3I by the real-time evolution of core-to-valence transitions near the iodine N-edge at 45-60 eV. During C-I bond breaking, new core-to-valence electronic states appear in the spectra, which decay concomitantly with the rise of the atomic iodine resonances of I((2)P3/2) and I*((2)P1/2). The short-lived features are assigned to repulsive valence-excited transition-state regions of (3)Q0 and (1)Q1, which can connect to transient core-excited states via promotion of 4d(I) core electrons. A simplified one-electron transition picture is described that accurately predicts the relative energies of the transient states observed. The transition-state resonances reach a maximum at ∼40 fs and decay to complete C-I dissociation in ∼90 fs, representing the shortest-lived chemical transition state observed by core-level, XUV, or X-ray spectroscopy.

Femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy based on a high-harmonic generation source is used to study the 266 nm induced A-band photodissociation dynamics of allyl iodide (CH2 =CHCH2I). The photolysis of the C-I bond at this wavelength produces iodine atoms both in the ground ((2)P3/2, I) and spin-orbit excited ((2)P1/2, I*) states, with the latter as the predominant channel. Using XUV absorption at the iodine N4/5 edge (45-60 eV), the experiments constitute a direct probe of not only the long-lived atomic iodine reaction products but also the fleeting transition state region of the repulsive nIσ(∗) C-I excited states. Specifically, three distinct features are identified in the XUV transient absorption spectrum at 45.3 eV, 47.4 eV, and 48.4 eV (denoted transients A, B, and C, respectively), which arise from the repulsive valence-excited nσ(∗) states and project onto the high-lying core-excited states of the dissociating molecule via excitation of 4d(I) core electrons. Transients A and B originate from 4d(I) → n(I) core-to-valence transitions, whereas transient C is best assigned to a 4d(I) →σ(∗)(C-I) transition. The measured differential absorbance of these new features along with the I/I* branching ratios known from the literature is used to suggest a more definitive assignment, albeit provisional, of the transients to specific dissociative states within the A-band manifold. The transients are found to peak around 55 fs-65 fs and decay completely by 145 fs-185 fs, demonstrating the ability of XUV spectroscopy to map the evolution of reactants into products in real time. The similarity in the energies of transients A and B with analogous features observed in methyl iodide [Attar et al. J. Phys. Chem. Lett. 6, 5072, (2015)] together with the new observation of transient C in the present work provides a more complete picture of the valence electronic structure in the transition state region. The results provide a benchmark for theoretical calculations on the nature of core-excited states in halogenated hydrocarbons, especially in the transition state region along the C-I reaction coordinate.

Molecular triplet states constitute a crucial gateway in the photochemical reactions of organic molecules by serving as a reservoir for the excess electronic energy. Here, we report the remarkable sensitivity of soft X-ray transient absorption spectroscopy for following the intricate electronic structure changes accompanying the non-adiabatic transition of an excited molecule from the singlet to the triplet manifold. Core-level X-ray spectroscopy at the carbon-1s K-edge (284 eV) is applied to identify the role of the triplet state (T₁, ³ππ*) in the ultraviolet-induced photochemistry of pentane-2,4-dione (acetylacetone, AcAc). The excited-state dynamics initiated at 266 nm (¹ππ*, S₂) is investigated with element- and site-specificity using broadband soft X-ray pulses produced by high harmonic generation, in combination with time-dependent density functional theory calculations of the X-ray spectra for the excited electronic singlet and triplet states. The evolution of the core-to-valence resonances at the carbon K-edge establishes an ultrafast population of the T₁ state (³ππ*) in AcAc via intersystem crossing on a 1.5 ± 0.2 ps time scale.

The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a tabletop apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to-valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs.

A fundamental chlorine-containing radical, CH₂Cl, is generated by the ultrafast photodissociation of CH₂ICl at 266 nm and studied at both the carbon K edge (∼284 eV) and chlorine L_2,3 edge (∼200 eV) by femtosecond X-ray transient absorption spectroscopy. The electronic structure of CH₂Cl radical is characterized by a prominent new carbon 1s X-ray absorption feature at lower energy, resulting from a transition to the half-filled frontier carbon 2p orbital (singly occupied molecular orbital of the radical; SOMO). Shifts of other core-to-valence absorption features upon photodissociation of CH₂ICl to yield ·CH₂Cl indicate changes in the energies of core-level transitions of carbon and chlorine to the σ*(C-Cl) valence orbital. When the C-I bond breaks, loss of the electron-withdrawing iodine atom donates electron density back to carbon and shields the carbon 1s core level, resulting in a ∼0.8 eV red shift of the carbon 1s to σ*(C-Cl) transition. Meanwhile, the 2p inner shell of the chlorine atom in the radical is less impacted by the iodine atom removal, as demonstrated by the observation of a ∼0.6 eV blue shift of the transitions at the chlorine L_2,3 edges, mainly due to the stronger C-Cl bond and the increased energy of the σ*(C-Cl) orbital. The results suggest that the shift in the carbon 1s orbital is greater than the shift in the σ*(C-Cl) orbital upon going from the closed-shell molecule to the radical. Ab initio calculations using the equation of motion coupled-cluster theory establish rigorous assignment and positions of the X-ray spectral features in the parent molecule and the location of the SOMO in the CH₂Cl radical.

The ultraviolet-induced photochemistry of five-membered heterocyclic rings often involves ring opening as a prominent excited-state relaxation pathway. The identification of this particular photoinduced mechanism, however, presents a challenge for many experimental methods. We show that femtosecond X-ray transient absorption spectroscopy at the carbon K-edge (∼284 eV) provides core-to-valence spectral fingerprints that enable the unambiguous identification of ring-opened isomers of organic heterocycles. The unique differences in the electronic structure between a carbon atom bonded to the oxygen in the ring versus a carbon atom set free of the oxygen in the ring-opened product are readily apparent in the X-ray spectra. Ultrafast ring opening via C-O bond fission occurs within ∼350 fs in 266-nm photoexcited furfural, as evidenced by fingerprint core (carbon 1s) electronic transitions into a nonbonding orbital of the open-chain carbene intermediate at 283.3 eV. The lack of recovery of the 1sπ* ground-state depletion in furfural at 286.4 eV indicates that internal conversion to the ground state is a minor channel. These experimental results, augmented by recent advances in the generation of isolated attosecond pulses at the carbon K-edge, will pave the way for probing ring-opened conical intersection dynamics in the future.

Disulfide bonds are pivotal for the structure, function, and stability of proteins, and understanding ultraviolet (UV)-induced S-S bond cleavage is highly relevant for elucidating the fundamental mechanisms underlying protein photochemistry. Here, the near-UV photodecomposition mechanisms in gas-phase dimethyl disulfide, a prototype system with a S-S bond, are probed by ultrafast transient X-ray absorption spectroscopy. The evolving electronic structure during and after the dissociation is simultaneously monitored at the sulfur L_1,2,3-edges and the carbon K-edge with 100 fs (FWHM) temporal resolution using the broadband soft X-ray spectrum from a femtosecond high-order harmonics light source. Dissociation products are identified with the help of ADC and RASPT2 electronic-structure calculations. Rapid dissociation into two CH₃S radicals within 120 ± 30 fs is identified as the major relaxation pathway after excitation with 267 nm radiation. Additionally, a 30 ± 10% contribution from asymmetric CH₃S₂ + CH₃ dissociation is indicated by the appearance of CH₃ radicals, which is, however, at least partly the result of multiphoton excitation.