21 research outputs found
Imaging the ring opening reaction of 1,3-cyclohexadiene with MeV ultrafast electron diffraction
We resolve the structural dynamics of the ultrafast photoinduced ring opening reaction of 1,3-cyclohexadiene in space and time employing megaelectronvolt gas phase ultrafast electron diffraction. We, furthermore, observe coherent large amplitude motions of the photoproduct
Determining Orientations of Optical Transition Dipole Moments Using Ultrafast X-ray Scattering
Identification
of the initially prepared, optically active state
remains a challenging problem in many studies of ultrafast photoinduced
processes. We show that the initially excited electronic state can
be determined using the anisotropic component of ultrafast time-resolved
X-ray scattering signals. The concept is demonstrated using the time-dependent
X-ray scattering of <i>N</i>-methyl morpholine in the gas
phase upon excitation by a 200 nm linearly polarized optical pulse.
Analysis of the angular dependence of the scattering signal near time
zero renders the orientation of the transition dipole moment in the
molecular frame and identifies the initially excited state as the
3p<sub><i>z</i></sub> Rydberg state, thus bypassing the
need for further experimental studies to determine the starting point
of the photoinduced dynamics and clarifying inconsistent computational
results
Photoacoustic transients generated by laser irradiation of thin films
Irradiation of an optically thin layer immersed in a transparent fluid with pulsed laser radiation can generate photoacoustic waves through two mechanisms. The first of these is the conventional optical heating of the layer followed by thermal expansion, in which the mechanical motion of the expansion launches a pair of oppositely directed sound waves. A second, recently reported mechanism, is operative when heat is conducted to the transparent medium raising its temperature, while at the same time reducing the temperature in the absorbing body. The latter mechanism has been shown to result in compressive transients at the leading edges of the photoacoustic waveforms. Here the photoacoustic effect produced by irradiating thin metal films which undergo negligible thermal expansion under optical irradiation, but which generate sound solely by the heat transfer mechanism is investigated. Solution to the wave equation for the photoacoustic effect from the heat transfer mechanism is given and compared with the results of experiments using nanosecond laser pulses to irradiate thin metal films