Two-photon absorption in systems with parity permits access to states that cannot be directly prepared by one-photon absorption. Here we investigate ultrafast internal conversion (IC) dynamics of furan by using this strategy in combination with femtosecond time-resolved photoelectron imaging. The dark Rydberg S1 and bright valence S2 states are simultaneously excited by two photons of 405 nm, and then ionized by two photons of 800 nm. The IC from S2 to S1 is clearly observed and extracted from the time dependence of the higher photoelectron kinetic energy (PKE) component. More importantly, the internal conversions to hot S0 from directly-prepared S1 and secondarily-populated S1 are unambiguously identified by the time-dependence of the lower PKE component. The average lifetime of the S2 and S1 states is measured to be 29 fs. The internal conversions of S2 to S1, S1 to hot S0 occur on estimated timescales of 15.4 fs and 38 fs, respectively.

Directly resolving structural changes in material on the atomic scales of time and space is desired in studies of many disciplines. Ultrafast electron diffraction (UED), which combines the temporal resolution of femtosecond-pulse laser and the spatial sensitivity of electron diffraction, is an advancing methodology serving such a goal. Here we present the design of a UED apparatus with multiple operation modes for observation of collective atomic motions in solid material of various morphologies. This multi-mode UED employs a pulsed electron beam with propagation trajectory of parallel and convergent incidences, and diffraction configurations of transmission and reflection, as well utilities of preparation and characterization of cleaned surface and adsorbates. We recorded the process of electron–phonon coupling in single crystal molybdenum ditelluride following excitation of femtosecond laser pulses, and diffraction patterns of polycrystalline graphite thin film under different settings of electron optics, to demonstrate the temporal characteristics and tunable probe spot of the built UED apparatus, respectively.

The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond  time-resolved  mass spectrometry.  The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 380±50 fs was measured from the rising time of the co-fragments of toluene radical (C7H7) and iodine atom (I), which is attributed to the averaged time needed for the C−I bond breaking for the simultaneously excited nσ* and ππ* states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated nσ* and ππ* states. And a rise time of 400±50 fs is extracted from the fitting of time-dependent I+ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.