Abstract

The dephasing times T₂ of vibrational transitions were determined by interferometric coherent anti-Stokes Raman spectroscopy (ICARS).¹ The broadband incoherent pump laser (centered at 510 nm, matching to the long wavelength absorption edge of the dyes) has a coherence time of 40 fs, which defines the time resolution for ICARS. The broadband laser beam is split by an interferometer in two twin beams, which join in the sample with a narrowband Stokes laser. A typical oscillation of the ICARS intensity as a function of delay between the two twin beams is depicted in Fig. 1 for narrow-band detection. The sample was bis-(di-methylaminojheptamethine iodide in ethanol (10⁻³ M). The oscillation frequency is the difference between the detection frequency and the twice Raman transformed Stokes laser frequency (Rabi detuning oscillation). This allows to determine the vibrational frequency(ies) of the excited vibration(s). The damping is given by 2T₂. Corresponding results are listed in Table 1. The data obtained are in good agreement with those calculated from lineshapes and frequencies observed in multiplex-CARS experiments. This indicates a predominantly homogeneous broadening of the Raman lines. The two different fits in Fig. 1 demonstrate the high accuracy obtainable with ICARS. The accuracy is mainly limited by errors in the calibration of the monochromator applied for narrowband detection.

© 1994 IEEE