Mode-selective vibrational redistribution after spectrally selective N–H stretching mode excitation in intermolecular hydrogen bonds

Mode-selective vibrational redistribution after spectrally selective excitation within the highly structured N–H stretching band of the 7-azaindole dimer was observed by subpicosecond infrared-pump/anti-Stokes Raman-probe spectroscopy. Measurements after relaxation of the N–H stretching vibration indicate ultrafast initial population transfer to vibrations with pronounced N–H bending character. From these modes energy is transferred to modes of frequencies below $1000cm−1$ on a slower time scale of about 3 ps. Tuning the spectrally narrow infrared excitation to the different substructures of the N–H stretching band influences the distribution of populations between the fingerprint modes. Their relative populations are correlated with the contributions of the modes forming the different coupled combination tones of the N–H stretching band. These results provide experimental support to a Fermi resonance model previously used for simulations of the N–H stretching infrared absorption band shape and insight into relaxation from the initially excited combination bands.