Time-resolved imaging of purely valence-electron dynamics during a chemical reaction

Chemical reactions are manifestations of the dynamics of molecular valence electrons and their couplings to atomic motions. Emerging methods in attosecond science can probe purely electronic dynamics in atomic and molecular systems1–6. By contrast, time-resolved structural-dynamics methods such as electron7–10 or X-ray diffraction11 and X-ray absorption12 yield complementary information about the atomic motions. Time-resolved methods that are directly sensitive to both valence-electron dynamics and atomic motions include photoelectron spectroscopy13–15 and high-harmonic generation16,17: in both cases, this sensitivity derives from the ionization-matrix element18,19. Here we demonstrate a time-resolved molecularframe photoelectron-angular-distribution (TRMFPAD) method for imaging the purely valence-electron dynamics during a chemical reaction. Specifically, the TRMFPADs measured during the non-adiabatic photodissociation of carbon disulphide demonstrate how the purely electronic rearrangements of the valence electrons can be projected from inherently coupled electronic–vibrational dynamics. Combined with ongoing efforts in molecular frame alignment20 and orientation21,22, TRMFPADs offer the promise of directly imaging valenceelectron dynamics during molecular processes without involving the use of strong, highly perturbing laser fields23. Figure 1 provides a conceptual overview of our method. Carbon disulphide, CS2, is a molecule that exhibits all the features generic to polyatomic dynamics: vibrational mode coupling, conical intersections, spin conversion and photodissociation. As such, its non-adiabatic photodissociation reaction CS2(X)+ hν(201.2 nm)→ CS2∗(C)→ CS(X)+ S(1D,3P), shown in Fig. 1, provides an excellent test. In this work we combine experimental measurements with theory to demonstrate how the TRMFPAD images the evolution of the valence-electronic structure of an excited-state wavepacket during the complex, coupled electron– nuclear processes inherent to chemical reactions. TRMFPADs probe both nuclear and electronic degrees of freedom through the photoionization matrix elements, d(t )= 〈9+;9e|μ̂ ·E|9i(t )〉. These matrix elements describe how the initial wavepacket, and its subsequent evolution in time (9i(t )), is projected onto the ionization continuum, consisting of the cation (9+) and photoelectron (9e) states, through dipole coupling (μ̂) with the laser field (E; refs 18,24). In the case of polyatomic molecules, 9i(t ) and 9+ are composed of coupled electronic and vibrational components (see Supplementary Information). The significance of the TRMFPAD can be understood by considering the intimate relationship between 9e and the electronic part of 9i(t ). For example, under thewell-knownBorn (plane-wave) approximation,