Atoms

Few-Femtosecond C2H4+ Internal Relaxation Dynamics Accessed by Selective Excitation

“…The belated weak component observed in Figure a at ∼22 fs originates from the fraction of excited states which are populated by H9. Indeed, the ratio between the stronger and weaker component in the fit of the experimental C 2 H 4 + yield is 74:26 and qualitatively agrees with the 65:35 ratio which describes the proportion between the initial population on D0 and on D1–D3, as extracted from the partial cross sections for the case of H9 excitation.…”

“…Since the three harmonics have photon energy comparable with the vertical transitions from the C 2 H 4 ground state to the lowest excited states of the molecular cation (black horizontal marks in Figure c), it is possible to control the initial superposition of molecular states after ionization by changing the selected harmonic. Figure d shows the expected initial state population evaluated by multiplying the experimental cross section reported in ref with the harmonic spectra of Figure c. As it is possible to notice, H9 is expected to enable a very efficient excitation of the cationic ground state X̃, H11 of the first excited state Ã, while H13 predominantly excites the B̃ and C̃ states.…”

“…(c) Spectrum of the three harmonics together with the vertical transition energies (black horizontal lines) and the fragment appearance potentials (colored dotted lines) . (d) Initial state populations calculated with the partial cross sections taken from ref and the harmonic spectra in panel c. (e) Photoelectron spectra obtained by ionizing C 2 H 4 with the harmonics in panel c (same color code). The thinner curves (darker colors) represent the high-resolution spectrum taken at 40 eV from, rigidly shifted to match the actual harmonic energy.…”

“…As a result, most of the ultrafast dynamics initiated by light pulses remain unexplored despite their importance. A remarkable example is the relaxation dynamics of the simplest π-radical system: the ethylene cation C 2 H 4 + . Its dissociation is a typical multistep pathway − in which the exact mechanisms leading to internal energy conversions are not fully known − and recent theoretical investigations proposed different mechanisms which are not completely compatible with one another. , For this reason, it is still unclear how the energy is redistributed among the internal modes and which step is rate-determining.…”

Ultrafast Relaxation Dynamics of the Ethylene Cation C2H4+

“…The additional strong peak at 4 u stems from ionization of He used as buffer gas with a He:C 2 H 4 ratio of 10:1. Starting from the tabulated values for the cross sections of ethylene for monochromatic light, , we estimate the initial state populations for our XUV spectra of Figure b. We find more than 95% of the total population is confined to the cation ground state X̃ 2 B 3u and the first three excited states à 2 B 3g , B̃ 2 A g , and C̃ 2 B 2u (Figure c).…”