Two-dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol are used to explore new aspects of optical 2D spectroscopy on a femtosecond timescale. The experiments reported here are pulse sequence and coherence pathway analogs of the two-dimensional magnetic resonance techniques known as COSY (correlated spectroscopy) and NOESY (nuclear Overhauser effect spectroscopy). Noncollinear three pulse scattering allows selection of electronic coherence pathways by choice of phase matching geometry, temporal pulse order, and Fourier transform variables. Signal fields and delays between excitation pulses are measured by spectral interferometry. Separate real (absorptive) and imaginary (dispersive) 2D spectra are generated by measuring the signal field at the sample exit, performing a 2D scan that equally weights rephasing and nonrephasing coherence pathways, and phasing the 2D spectra against spectrally resolved pump–probe signals. A 3D signal propagation function is used to correct the 2D spectra for excitation pulse propagation and signal pulse generation inside the sample. At relaxation times greater than all solvent and vibrational relaxation timescales, the experimental 2D electronic spectra can be predicted from linear spectroscopic measurements without any adjustable parameters. The 2D correlation spectra verify recent computational predictions of a negative region above the diagonal, a displacement of the 2D peak off the diagonal, and a narrowing of the 2D cross-width below the vibrational linewidth. The negative region arises from 4-level four-wave mixing processes with negative transition dipole products, the displacement off the diagonal arises from a dynamic Stokes shift during signal radiation, and the narrow 2D cross-width indicates femtosecond freezing of vibrational motion.

Polarized femtosecond pump-probe spectroscopy is used to observe electronic wavepacket motion for vibrational wavepackets centered on a conical intersection. After excitation of a doubly degenerate electronic state in a square symmetric silicon naphthalocyanine molecule, electronic motions cause a ϳ100 fs drop in the polarization anisotropy that can be quantitatively predicted from vibrational quantum beat modulations of the pump-probe signal. Vibrational symmetries are determined from the polarization anisotropy of the vibrational quantum beats. The polarization anisotropy of the totally symmetric vibrational quantum beats shows that the electronic wavepackets equilibrate via the conical intersection within ϳ200 fs. The relationship used to predict the initial electronic polarization anisotropy decay from the asymmetric vibrational quantum beat amplitudes indicates that the initial width of the vibrational wavepacket determines the initial speed of electronic wavepacket motion. For chemically reactive conical intersections, which can have 1000 times greater stabilization energies than the one observed here, the same theory predicts electronic equilibration within 2 fs. Such electronic movements would be the fastest known chemical processes.

We report measurement and modeling of two-dimensional (2D) electronic spectra of a silicon naphthalocyanine (SiNc) in benzonitrile, a system for which the polarization anisotropy reveals passage through a square-symmetric Jahn-Teller conical intersection in ∼100 fs [D. A. Farrow, W. Qian, E. R. Smith, A. A. Ferro, and D. M. Jonas, J. Chem. Phys. 128, 144510 (2008)]. The measured 2D Fourier transform (FT) spectra indicate loss of electronic coherence on a similar timescale. The 2D spectra arising from femtosecond vibronic dynamics through the conical funnel are modeled by full non-adiabatic treatment of the coupled electronic and vibrational dynamics for a pair of un-damped Jahn-Teller active vibrations responsible for both electronic decoherence and population transfer. Additional damped Jahn-Teller active modes that can cause only decoherence or population transfer are treated with analytical response functions that can be incorporated into the numerical non-adiabatic calculation by exploiting symmetry assignment of degenerate vibronic eigenstates to one of two electronic states. Franck-Condon active totally symmetric modes are incorporated analytically. The calculations reveal that these conical intersection dynamics alone are incapable of destroying the coherence of the initially prepared wavepacket on the experimentally observed timescale and predict an unobserved recurrence in the photon echo slice at ∼200 fs. Agreement with the experimental two-dimensional electronic spectra necessitates a role for totally symmetric vibrational dynamics in causing the echo slice to decay on a ∼100 fs timescale. This extended model also reproduces the ∼100 fs ultrafast electronic anisotropy decay in SiNc when an "asymmetric solvation mode" with a small stabilization energy of ∼2 cm(-1) is included. Although calculations show that inhomogeneities in the energy gap between excited states can broaden the anti-diagonal 2D lineshape, the anti-diagonal width is dominated by totally symmetric vibrational motions in SiNc. For this shallow conical intersection, the non-adiabatic dynamics destroy electronic coherence more slowly than they destroy electronic alignment.

Articles you may be interested inComparison of polarizable continuum model and quantum mechanics/molecular mechanics solute electronic polarization: Study of the optical and magnetic properties of diazines in water Electronic polarization spectroscopy of metal phthalocyanine chloride compounds in superfluid helium droplets Measurements with 26 fs pulses that cover the Q(0 -0) band of silicon 2,3-naphthalocyanine bis͑trihexylsilyloxide͒ yielded an initial anisotropy of 0.40 that decayed to 0.12 over 200 fs. This contradicts theories predicting anisotropy decay from 7/10 to 1/10. Including ground state bleaching and excited state absorption, anisotropy decay from 2/5 to 1/10 is predicted for degenerate electronic reorientation and dephasing.

A complete (both amplitude and phase) measurement of the femtosecond linear free induction decay on a Raman active dye in solution (IR144 in methanol) is demonstrated. For weak femtosecond pulses passing through a homogeneous material, Beer’s Law can be used to predict the spectral amplitude changes, and dispersion relations can be used to calculate the spectral phase change. A modified fast Fourier transform algorithm calculates the phase change by applying dispersion relations to a symmetrical absorption spectrum spanning positive and negative frequencies. A reformulation of the rotating wave approximation in the frequency domain shows that related limits on the accuracy of the rotating wave approximation in nonlinear optics arise from the width of the linear spectrum relative to the center frequency, and not from the excitation pulses.