Tunable resonance hyper-Raman spectroscopy of nonlinear optical chromophores

Two-photon-resonant hyper-Raman spectra have been obtained for several dipolar and octupolar donor-acceptor substituted conjugated organic chromophores that have large first hyperpolarizabilities. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 710 to 950 nm. The hyper-Raman spectra are compared to the linear resonance Raman spectra measured at the laser second harmonic. Excitation into regions that appear to contain more than one electronic transition gives rise to different intensity patterns in the resonance Raman and hyper-Raman spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. Hyper-Raman excitation profiles have been measured on an absolute scale by using the hyper-Rayleigh from acetonitrile as an external standard, and simulated using time-dependent wavepacket propagation techniques to obtain the geometric parameters and one- and two-photon transition strengths for both states. Hyper-Raman is a promising technique for probing electronic transitions that are both one- and two-photon allowed and for examining the origins of molecular first hyperpolarizabilities.