Ultrafast processes in VO-phthalocyanine film caused by intense excitation

Excited State Dynamics of Phthalocyanine Films

“…Local heating of the material, for instance, is one of the additional effects caused by relaxation which frequently has been ignored. As it was shown in our previous investigations, , this effect largely determines the dynamics of absorption spectra of the VOPc film excited by picosecond light pulses. Femtosecond experiments on H 2 Pc films 1 demonstrate the local heating effect as well, but it is not so strong under these experimental conditions due to the lower excitation energy density.…”

“…Due to the stress created in the material, acoustic waves are generated, causing oscillations in the absorption. Such oscillations were observed in a VOPc film excited by picosecond pulses as well as in some other materials. , Low-amplitude oscillating features are observed in the transient absorption kinetics depicted in Figure at 410 and 880 nm. The period of the oscillation is ∼100 ps; i.e., it is determined by the thickness of the film .…”

“…Therefore, initially after heating of the material it is in a nonstationary state where its density is higher than it should be at this temperature. As it was shown, the absorption of the material in this state is different from that at later times when the expansion has already occurred. Due to the stress created in the material, acoustic waves are generated, causing oscillations in the absorption.…”

“…Similar kinetics (with even stronger wavelength dependence) was observed by investigating VOPc films 4 with picosecond time resolution and has been suggested to be due to the local heating of the material and the generation of acoustic waves. Although the density of the excitation used during the femtosecond measurements was 10−100 times lower than it was in the picosecond experiment, it was sufficiently high to induce local heating effects on the absorption changes. At an excitation intensity of 1.2 mJ/cm 2 the temperature of the VOPc film increases by approximately 15−25 K if all excitation energy is converted into heat and results in the difference absorption spectrum observed at 150 ps (Figure a).…”

“…At high excitation intensities, phthalocyanine (Pc) structures of various molecular arrangements such as thermally evaporated crystalline and amorphous films or phthalocyanines dispersed in polymer matrices demonstrate ultrafast nonlinear annihilation which determines the main relaxation channel on a fast time scale. − Moreover, by means of subpicosecond kinetic measurements of H 2 -phthalocyanine, Greene and Millard 1 showed that the exciton−exciton annihilation rate is time-dependent (∼ t -1/2 ). Later the same time dependence was observed by Terasaki et al in (t-Bu) 1.1 VOPc doped into polystyrene, and we observed it in colloidal particles of AlClPc .…”