The three lowest potential energy surfaces V(R) of the CN molecule. The 2Σ states are located closely on top of each other yielding a high contrast Franck–Condon selection for a v ′ = v ′ ′ transition. Furthermore, the femtosecond laser pulse bandwidths are smaller than the vibrational level separation of ∼2000 wavenumbers yielding a state selective excitation scheme for the CARS setup.
Schematic representation of the creation of the third-order polarization by the three time-delayed laser pulses pump, dump, and probe. Left panel shows the (ro-)vibronic transition path i → n → j → f alternating between the X 2Σ and B 2Σ electronic states. Right panel shows the rotational connection diagram for the six possible routes from the initial level N i (v ′ ′ = 0). P and R represent the two allowed rotational branches while plus and minus signs denote a change in the rotational quantum number N for each of the four transitions.
The time-gated images as obtained by the use of Eq. (15) for two gate widths (a) 500 fs and (b) 3 ps, where thermal distributions correspond to the 30 K case. The wavenumber scale relative to the anti-Stokes origin at 27922 cm−1 separates the R branch emission (positive side) from the P branch (negative). The upper interferogram shows a signal oscillation pattern between the R and P branches (at the zero wavenumber shift) and carries information about the rotational wave packet in the time domain. Panel (b) case approaches the spectrally resolved limit leaving only the next-neighbor polarization beating visible. Panel (c) shows the shift-zero cut S(0, t g ) of the panel (a) signal, and panel (d) shows the FFT power spectrum of the panel (c) trace resolving the interference as separable quantum and intermolecular beats.
The time-integrated frequency-resolved CARS interferogram for the 10 K ensemble of wave packets as obtained by the use of Eq. (16). The contour level increment corresponds to 4% of the intensity maximum. Labeling corresponds to the most intense P and R branch traces. The dashed and dotted lines denote intervals for a global intensity minimum and a maximum, respectively, which relates to a dimmer switch function of the pulse delay.
2D spectrograms for the CARS signal for two values of the control parameter τ32: 890 fs (a,b) and 495 fs (c,d). Images show the light switching functionality between the branches. The interferogram bandwidths are set by the gate durations 500 fs (a,c) and 3 ps (b,d) which demonstrates the temporal versus spectral resolving power.
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