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

Abstract

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 systems [1][2][3][4][5][6] . By contrast, time-resolved structural-dynamics methods such as electron 7-10 or X-ray diffraction 11 and X-ray absorption 12 yield complementary information about the atomic motions. Time-resolved methods that are directly sensitive to both valence-electron dynamics and atomic motions include photoelectron spectroscopy 13-15 and high-harmonic generation 16,17 : in both cases, this sensitivity derives from the ionization-matrix element 18,19 . 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 alignment 20 and orientation 21,22 , TRMFPADs offer the promise of directly imaging valenceelectron dynamics during molecular processes without involving the use of strong, highly perturbing laser fields 23 . Figure 1 provides a conceptual overview of our method. Carbon disulphide, CS 2 , 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 CS 2 (X ) 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 electronnuclear processes inherent to chemical reactions. TRMFPADs probe both nuclear and electronic degrees of freedom through the photoionization matrix elements, d(t ) = + ; e |μ ·E| i (t ) . These matrix elements describe how the initial wavepacket, and its subsequent evolution in time ( i (t )), is projected onto the ionization continuum, consisting of the cation ( + ) and photoelectron ( e ) states, through dipole coupling (μ) with the laser field (E; refs 18,24). In the case of polyatomic molecules, i (t ) and + 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 e and the electronic part of i (t ). For example, under the well-known Born (plane-wave) approximation, 1 Steacie Institute for Molecular Sciences, National Research Council, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada, 2 Department of Chemistry, University of Copenhagen, Copenhagen DK-2100, Denmark, 3 Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada. *e-mail: alb...

Effect of electronic angular momentum exchange on photoelectron anisotropy following the two-color ionization of krypton atoms

“…It has been shown that (at least) partial alignment of molecular axes improves the sensitivity of the PAD to electronic character; this alignment can be achieved through photoexcitation, through strong-field alignment methods, or through coincidence imaging. [6][7][8][9][10][11] In the atomic case, alignment of orbital angular momentum serves the same purpose as alignment of molecular axes, and a number of laser-based studies have shown that achieving this through photoexcitation prior to photoionization can lead to a PAD with exquisite sensitivity to the angular momentum composition of the intermediate state. [12][13][14] However, although systematic studies have been performed in which different intermediate electronic states are prepared, generally only a single ionic electronic state has been accessible in each case.…”

“…[1][2][3] This idea has been put to use in studies of photodetachment from anions, 4,5 and elegantly exploited in femtosecond time-resolved photoelectron imaging studies of non-adiabatic dynamics in neutral molecules. [6][7][8][9] Given this interest, there is a need for systematic studies of the variation of PADs with the electronic state of the neutral and the ion. It has been shown that (at least) partial alignment of molecular axes improves the sensitivity of the PAD to electronic character; this alignment can be achieved through photoexcitation, through strong-field alignment methods, or through coincidence imaging.…”

Imaging electron dynamics with time- and angle-resolved photoelectron spectroscopy

“…Therefore, real-time imaging of electron dynamics is one of the most important goals for modern ultrafast science [1][2][3][4][5][6][7][8][9]. In this article, we investigate the opportunities for applying ultrashort extreme ultraviolet (XUV) probe pulses inducing single-photon ionization for imaging coherent electron dynamics in molecules by means of timeand angle-resolved photoelectron spectroscopy (TRARPES), i.e., time-and energy-resolved molecular-frame photoelectron angular distributions.…”

Probing and extracting the structure of vibratingSF6molecules with inner-shell photoelectrons

“…We mention here high-order harmonic-generation (HHG) spectroscopy [8][9][10][11][12] and laser-induced electron diffraction (LIED) [9,[13][14][15][16][17][18]. Time-resolved photoelectron spectroscopy (TRPES) and time-resolved molecular-frame photoelectron angular distribution (TRMFPAD) measurements proposed recently [19,20] are capable of imaging valence-electron dynamics and atom motion during a chemical reaction. In this approach, the temporal resolution is limited to about hundreds of femtoseconds, owing mostly to the vacuum ultraviolet (VUV) laser pulse duration used in the experiments.…”