Femtosecond hole-burning spectroscopy of the dye DCM in solution: the transition from the locally excited to a charge-transfer state

Transient spectra of the styryl dyM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran) in methanol were studied by the pump-supercontinuum probe technique with 40 fs time resolution. A theory of measurements with a supercontinuum probe is presented. Gain and absorption spectra were measured from 400 to 800 nm with 1.5 nm resolution. Before 70 fs, prominent spectral structure is observed which is mainly due to resonant Raman processes. At longer times the spectrum undergoes a red shift and change of shape (time constant 140 fs) with a well-defined isosbestic point. After 300 fs solvation becomes apparent. The early transient spectrum is assigned to the locally excited state of DCM.