A coherent photoacoustic approach to excited-state-excited-state absorption spectroscopy: application to the investigation of a near-resonant contribution to ultrasonic diffraction

Abstract

The previously reported coherent photoacoustic method for measuring weak absorptions is extended to excited-state-excited-state spectroscopy for which conventional acoustic and thermal lensing techniques are greatly reduced in sensitivity. The method involves a picosecond pulse sequence with four pulses. The first pulse populates the excited state. Then, a pair of time coincident pulses cross in the sample, making an optical interference pattern and generating an acoustic diffraction grating. The amplitude of the acoustic grating is proportional to the excited-state-excited-state absorption cross section. The amplitude is measured by the Bragg diffraction of the fourth pulse from the acoustic grating. This method can be readily applied to liquids, glasses, or crystals. Excited-state absorption at 532 nm by the SI state of pentacene in p-terphenyl host crystals is examined at room temperature and 1.4 K. The results are used in addressing the mechanism for acoustic diffraction of a probe beam nearly resonant with a strong and narrow ground-state transition, e.g., pentacene in p-terphenyl at 1.4 K. It is demonstrated that there is a density wave induced spectral shift contribution to the acoustic grating which results in greatly increased diffraction efficiency near resonance.