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Applying a combined experimental and theoretical approach we demonstrate that doublets of the νs(NO2) band of 4-nitroaniline which have been observed in several environments originate from Fermi resonances. Changes of the line shapes typical for Fermi resonances are reported also for other isotopomers of 4-nitroaniline, however, for each of them in different solvents and solvent mixtures. Simulations of the infrared spectra based on the solvatochromic frequency shifts of the νs(NO2) vibration determined experimentally together with calculated cubic couplings with overtones and combination bands account for the experimental findings. [ABSTRACT FROM AUTHOR]

Ultrafast vibrational excitation and energy redistribution in the electronic ground state after internal conversion of 4-nitroaniline (PNA) and isotope labeled analogs is investigated by picosecond anti-Stokes resonance Raman spectroscopy. In PNA, PNA-di-[sup 15]N and PNA-D[sub 4], anti-Stokes Raman lines of overtones and/or combination bands of out-of-plane vibrations display risetimes close to the decay time of the electronically excited state of about 0.5 ps and pronounced excess populations. Compared to such fast dynamics, the strongly Raman active totally-symmetric modes show a considerably slower picosecond rise time. Our results indicate primary excitation of out-of-plane vibrations by internal conversion and secondary excitation of strongly Raman active vibrations by redistribution of the vibrational energy. [ABSTRACT FROM AUTHOR]

Nonequilibrium vibrational excitations of para-nitroaniline (PNA, 4-nitroaniline) occurring after internal conversion from the photoexcited charge transfer state are studied by picosecond anti-Stokes Raman scattering. Vibrational excess populations with distinctly different picosecond rise and decay times are found for a number of modes with frequencies between 860 and 1510 cm[sup -1], including the overtone of a non-Raman active mode. A nonthermal distribution of vibrational populations exists up to about 6 ps after photoexcitation. The time-resolved experiments are complemented by steady-state infrared and Raman measurements as well as calculations based on density functional theory, providing a detailed analysis of the steady-state vibrational spectra of PNA and two of its isotopomers. A weakly Raman active vibration at about 1510 cm[sup -1] displays the fastest rise time and a pronounced excess population and—thus—represents the main accepting mode. We suggest that an out-of-plane mode giving rise to the overtone Raman band at this frequency acts both as coupling and accepting mode in the internal conversion process. [ABSTRACT FROM AUTHOR]