15 research outputs found
Excited state wavepacket dynamics in NO 2 probed by strong-field ionization
We present an experimental femtosecond time-resolved study of the 399 nm excited state dynamics
of nitrogen dioxide using channel-resolved above threshold ionization (CRATI) as the probe process.
This method relies on photoelectron-photoion coincidence and covariance to correlate the strongfield photoelectron spectrum with ionic fragments, which label the channel. In all ionization channels
observed, we report apparent oscillations in the ion and photoelectron yields as a function of pumpprobe delay. Further, we observe the presence of a persistent, time-invariant above threshold ionization
comb in the photoelectron spectra associated with most ionization channels at long time delays.
These observations are interpreted in terms of single-pump-photon excitation to the first excited
electronic XËś 2A1 state and multi-pump-photon excitations to higher-lying states. The short time
delay (<100 fs) dynamics in the fragment channels show multi-photon pump signatures of higherlying neutral state dynamics, in data sets recorded with higher pump intensities. As expected for
pumping NO2 at 399 nm, non-adiabatic coupling was seen to rapidly re-populate the ground state
following excitation to the first excited electronic state, within 200 fs. Subsequent intramolecular
vibrational energy redistribution results in the spreading of the ground state vibrational wavepacket
into the asymmetric stretch coordinate, allowing the wavepacket to explore nuclear geometries in
the asymptotic region of the ground state potential energy surface. Signatures of the vibrationally
“hot” ground state wavepacket were observed in the CRATI spectra at longer time delays. This study
highlights the complex and sometimes competing phenomena that can arise in strong-field ionization
probing of excited state molecular dynamics
Vacuum ultraviolet excited state dynamics of the smallest ring, cyclopropane. II. Time-resolved photoelectron spectroscopy and ab initio dynamics
The vacuum-ultraviolet photoinduced dynamics of cyclopropane (C3H6) were studied using time-resolved photoelectron spectroscopy (TRPES) in conjunction with ab initio quantum dynamics simulations. Following excitation at 160.8 nm, and subsequent probing via photoionization at 266.45 nm, the initially prepared wave packet is found to exhibit a fast decay (<100 fs) that is attributed to the rapid dissociation of C3H6 to ethylene (C2H4) and methylene (CH2). The photodissociation process proceeds via concerted ring opening and C-C bond cleavage in the excited state. Ab initio multiple spawning simulations indicate that ring-opening occurs prior to dissociation. The dynamics simulations were subsequently employed to simulate a TRPES spectrum, which was found to be in excellent agreement with the experimental result. On the basis of this agreement, the fitted time constants of 35 ± 20 and 57 ± 35 fs were assigned to prompt (i) dissociation on the lowest-lying excited state, prepared directly by the pump pulse, and (ii) non-adiabatic relaxation from higher-lying excited states that lead to delayed dissociation, respectively
Displaced creation
The ionization of atoms and molecules by strong laser fields has become a core technique in modern laser physics. Now, the electrons emerging from ionized molecules are shown to exhibit a memory of the ionization process, resulting in a spatial phase that may influence the interpretation of imaging data