Two-dimensional Fourier transform electronic spectroscopy

TLDR

The study uses two‑dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol to investigate new aspects of optical 2D spectroscopy on the femtosecond timescale. The authors perform noncollinear three‑pulse scattering experiments analogous to COSY and NOESY, selecting coherence pathways via phase‑matching geometry, temporal pulse order, and Fourier variables, measuring signal fields and delays by spectral interferometry, generating real and imaginary 2D spectra through a balanced rephasing/nonrephasing scan and phasing against pump–probe signals, and correcting the spectra with a 3D signal‑propagation function. At relaxation times exceeding all solvent and vibrational timescales, the measured 2D electronic spectra agree with linear spectroscopic predictions, revealing a negative region above the diagonal, an off‑diagonal peak displacement, and a narrowed cross‑width that confirm computational models and indicate femtosecond freezing of vibrational motion.

Abstract

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.