The generation of coherent phonons in polycrystalline bismuth film excited with femtosecond laser pulse is observed by ultrafast time-resolved electron diffraction. The dynamics of the diffracted intensities from the (110), (202), and (024) lattice planes show pronounced oscillations at 130–150 GHz. The origin of these coherent acoustic phonons is discussed in view of optical phonon decay into two acoustic phonons. Different drop times in the intensity of the diffraction orders are observed and interpreted as anisotropy in the energy transfer rate of coherent optical phonons

The lattice response of 5 nm thick bismuth film to femtosecond laser excitation is probed by ultrafast electron diffraction. The transient decay time after laser excitation is greater for diffraction from (012) lattice planes compared to (110) planes and is reduced for both planes with the increased laser fluence. These results indicate that different energy coupling mechanisms to the lattice occur depending on the crystal direction. The behavior of the diffraction peak width indicates partial disorder of the film upon photoexcitation that increases together with the laser fluence.

The lattice response of bismuth nanoparticles to femtosecond laser excitation is probed by ultrafast electron diffraction. The transient decay time after laser excitation is observed to be longer for diffraction from the (012) lattice planes compared to that from (110). From the position of the (012) diffraction peak, a transient lattice contraction due to hot electron blast force is observed over several picoseconds followed by expansion while the position of the (110) peak shows only expansion. The diffraction peak width indicates partial disorder of the nanoparticles consistent with formation of a liquid shell as the lattice is heated.

The structural properties of bismuth nanoclusters were investigated with transmission high-energy electron diffraction from room temperature up to 525 6 6 K. The Bi nanoclusters were fabricated by thermal evaporation at room temperature on transmission electron microscope grids coated with an ultrathin carbon film, followed by thermal and femtosecond laser annealing. The annealed sample had an average cluster size of $14 nm along the minor axis and $16 nm along the major axis. The Debye temperature of the annealed nanoclusters was found to be 53 6 6 K along the [012] direction and 86 6 9 K along the [110] direction. At T ¼ 464 6 6 K, the diffraction intensity started to deviate from Debye-Waller behavior due to increased lattice anharmonicity. The onset of the melting of the Bi nanoclusters was T $ 500 6 6 K, as measured by the reduction of the nanocluster size through the formation of a liquid shell detected by the width of the diffraction rings. The thermal expansion coefficient of the Bi (012) and (110) planes is positive up to $ 499 6 11 K. However, the expansion coefficient of the Bi (012) planes showed a transition from a positive to a negative value that occurs over the temperature range T c $ 499 6 11 K to 511 6 8 K. For the Bi (110) planes, the thermal expansion coefficient is positive up to their melting point, which is 525 6 6 K. V

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