Excited-State Molecular Vibration Observed for a Probe Pulse Preceding the Pump Pulse by Real-Time Optical Spectroscopy

It is shown experimentally that the absorbance change observed in the “negative” time range, where probe pulse precedes pump pulse in real-time vibrational spectroscopy is induced only by the excited-state wave-packet motion as theoretically expected. Coherent molecular vibration of a polymer in the excited state was observed in the real-time trace without the effect of wave-packet motion in the ground state, which usually makes it difficult to ascribe the signal either to the ground state or to the excited state.

Figure 1

The vibrational real-time spectra of difference absorption in the whole pump-probe delay time range from $-200\mathrm{fs}$ to 1800 fs at four typical probe photon energies of 2.08, 2.01, 1.98, and 1.92 eV. The dotted lines show zero absorbance change ($\Delta A$). The inset in (a) is the molecular structure of PHTDMABQ, and that in (b) is sample stationary absorption spectrum (dotted line) and laser spectrum (solid line).

Figure 2

The expanded real-time traces of corresponding near zero-delay time (in both negative and positive probe delay times) in Fig. 1. The solid lines show the original graphs. The dotted curves were obtained by inverting the real-time traces in the positive time region to the negative time with respect to the origin of the graph at point [$t=0$, $\Delta A(t=0)$] [except the curve in graph (a), which was obtained by mirror imaging at $t=0$].

Figure 3

The FT power spectra of real-time vibrational spectrum at 2.01 eV (618 nm).

Figure 4

The contour map of the two-dimension Fourier power spectra of vibrational components obtained by spectrogram calculation at 2.01 eV (618 nm). The solid line denotes the instantaneous Fourier power peak at each delay time.