Transient absorption of DCM in ethanol

Transient absorption of DCM dye (4-(Dicyanomethylene)–2-methyl-6-(4-dimethylaminostyryl)–4H-pyran) dissolved in ethanol was measured on a time scale of 15 picoseconds with the time step of 100 femtoseconds. Spectrum of the probe pulse ranged from 340nm to 650nm and the wavelength of pump pulses was 267nm. Results in the form of a 2D matrix and the associated spectra are shown in the picture below.

Results show positive signal centered around 450nm corresponding to overlap of higher state absorption and a ground state bleach with higher state absorption being of greater value. Negative band centered around 600nm corresponds to stimulated emission. During the first few hundreds of femtoseconds positive signal at lower wavelengths is present due to transient absorption artefacts (2-photon absorption, cross-phase modulation, etc.).

The figures of associated spectra show time-shift (black arrows) of positive and negative bands with positive band shifting towards blue wavelengths and negative band towards red wavelengths. Time-shift is present because of the solvation (solvent relaxation). With ethanol being polar solvent and DCM molecule having increased dipole moment value in its 1st excited state solvent molecules are reorienting to lower the energy of the 1st photoexcited state, thus increasing the energy required for higher state absorption and lowering the energy of stimulated emission.