Polarization resolved two-photon resonant ionization spectroscopy of indole and 3-methylindole

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Abstract
Two-photon resonant, four-photon ionization spectra of vapor phase indole and 3-methylindole have been taken using both linearly and circularly polarized excitation. From these measurements, the wavelength dependent polarization ratio Ω has been determined. The value of Ω has been used to identify the onset of absorption to the second electronically excited state La amidst overlapping vibronic transitions to the lower lying Lb excited state. This onset occurs 1000 cm−1 above the Lb origin in indole and 100 cm−1 above the Lb origin in 3-methylindole. Several resolved transitions are also seen in the indole spectrum, roughly 1400 cm−1 above the Lb origin, which are tentatively assigned to the La origin. An analysis of the indole Lb origin rotational contour suggests that the two-photon absorption tensor for this transition is traceless and is dominated by the diagonal elements. The value of Ω for the La transition implies that its two-photon absorption tensor must have a nonzero trace and hence should exhibit a very different rotational contour.