V. Kozich

Vibrational excitation and energy redistribution after ultrafast internal conversion in 4-nitroaniline

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−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−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...

Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline

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-15N and PNA-D4, 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.

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 1000 cm(-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.

Interference effects in time-delayed degenerate four-wave mixing with broadband noisy light

Femtosecond beats in time-delayed forward degenerate four-wave mixing (DFWM) with incoherent light in liquid and solid materials have been observed, and their origin is discussed. It is shown that the oscillations are due to the interference between pure DFWM signal and Rayleigh scattering from the excitation beams. It is also shown that, if this effect is not taken into account, incorrect conclusions can be reached.

Stimulated effects in one-photon resonant interferometric four-wave mixing with incoherent light

The interferometric four-wave mixing technique with broadband light is extended to measurements of population relaxation. For what is believed to be the first time, the population relaxation rate in the presence of a pump field is measured. It is shown that stimulated emission can significantly affect the results when one is dealing with resonant transient wave mixing, and, consequently, in most cases stimulated processes should be taken into account.

Correlations between the intensities of pump, depleted pump and Stokes waves in superbroadband stimulated Raman scattering

Abstract The pair correlations between the intensities of input pump laser radiation with spectral bandwidth of 250 cm −1 , which has femtosecond and picosecond noise structures, output depleted pump radiation and Stokes light generated through stimulated Raman scattering (SRS) in compressed hydrogen have been studied experimentally under weak and strong pump depletion. The third order correlation functions were measured by means of time-delayed four-wave mixing in a Kerr-shutter scheme with two noise light beams under investigation. The data obtained show that the Stokes wave is well correlated with the input pump wave up to an energy conversion efficiency of about 30%. The measured functions display the considerable suppression of the femtosecond amplitude fluctuations in the depleted pump beam upon increasing the efficiency of SRS and pump depletion. Such a result shows a possible way for preparing superbroadband light with smooth temporal envelope and predominant phase fluctuations using SRS. The picosecond part of the correlations between the depleted pump and Stokes waves is characterized by the asymmetrical shape that is probably caused by the fact that picosecond peaks in the noise structure of the depleted pump and Stokes waves have opposite temporal profiles.

Tuning intramolecular anharmonic vibrational coupling in 4-nitroaniline by solvent-solute interaction

Applying a combined experimental and theoretical approach we demonstrate that doublets of the nu(s)(NO(2)) 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 nu(s)(NO(2)) vibration determined experimentally together with calculated cubic couplings with overtones and combination bands account for the experimental findings.

Thermal lensing in barium nitrate due to stimulated Raman scattering of nanosecond laser pulses

Barium nitrate crystal are studied using one- and two-beam Z-scan techniques by excitation with the second harmonic radiation of nanosecond Nd:YAG laser and probing with the cw He:Ne laser. For the first time, a thermal lens due to the dissipation of energy of the SRS-excited Ag vibrational mode to the heat is observed and measured.

Time-resolved cross-correlation spectroscopy with incoherent light for studying inter- and intramolecular processes

The application of two-color incoherent cross-correlation spectroscopy to study ultrafast processes in organic liquids and solutions is discussed. This kind of time-resolved spectroscopy is based on using two broadband correlated pulses of long duration with different central frequencies. Its time resolution is determined by the cross-correlation time of the intensities of used radiations. The statistical properties of the radiations have been investigated. The developed technique allowed us to perform kinetic measurements with femtosecond resolution on the ordinary nanosecond laser spectrometer modifying it in a comparatively simple way. The present technique was demonstrated and tested by studying subpicosecond Kerr dynamics in a number of organic liquids and their mixtures. An approach is also proposed to study the population relaxation of electronic and vibronic states of organic molecules including non-luminescent ones.

Vibrational excitation and energy redistribution after ultrafast intramolecular proton transfer of TINUVIN

Vibrational excitation and energy redistribution after ultrafast intramolecular proton transfer of TINUVIN is investigated by picosecond resonance Raman spectroscopy. It is demonstrated that a low-frequency proton transfer promoting mode serves as the major accepting mode.