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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. [ABSTRACT FROM AUTHOR]

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]

Abstract: We present a traveling-wave-type optical parametric amplifier (OPA) pumped at 1.03μm by a Yb:KGW laser that produces tunable high-energy pulses of 6.5–4μJ in the mid-infrared (mid-IR) region from 3.6 to 7μm. Pumping with negatively chirped pulses generates nearly transform-limited (TL) mid-IR pulses of 300–330fs length. Pumping with TL pulses of 200fs not only decreases the output energy by a factor of 1.5, but also decreases the mid-IR pulse-length to 160fs after additional compression. The compact and simple OPA setup is ideal for femtosecond infrared experiments in the fingerprint region. [Copyright &y& Elsevier]

Abstract: Vibrational relaxation after spectrally selective excitation within the NH stretching band of adenine–thymine base pairs in DNA oligomers was studied by subpicosecond infrared-pump/anti-Stokes Raman-probe spectroscopy. The decay of the different NH stretching vibrations populates distinct accepting modes in the NH bending range with a rise time of 0.6ps that is close to the NH stretching decay times. The population of thymine fingerprint modes after excitation of the adenine antisymmetric NH2 stretching mode points to an ultrafast excitation transfer to the thymine NH stretching vibration before relaxation. The nonequilibrium fingerprint populations decay on a time scale of several picoseconds. [Copyright &y& Elsevier]

Abstract: Picosecond time-resolved resonance Raman spectra after excited state intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzoxazole have been investigated. The two most intense resonance Raman lines belonging to the S1-keto tautomer are assigned to vibrations with strong ν(CO) and δ(NH) components, respectively. The intensities of corresponding anti-Stokes resonance Raman spectra indicate that the high-frequency modes are not directly vibrationally excited by the reaction. For modes below 1000cm−1 substantial excess populations were observed. The S1-keto tautomer is already close to thermal equilibrium after 2ps and it cools down within about 8ps. [Copyright &y& Elsevier]

Abstract: The kinetics of Stokes and anti-Stokes resonance Raman intensities of TINUVIN observed after a completed proton transfer cycle is reported. This kinetics of the strongly vibrationally excited molecules can not be described in terms of population numbers of the vibrations alone but demands for considering the influence of time-dependent line shape functions. Based on transform theory, we calculated the dynamics of these functions and corresponding resonance Raman kinetics from the temperature-dependent electronic absorption bands. Using a simple model, these calculations reproduce main characteristics of the measured resonance Raman kinetics. Our results demonstrate how dramatically the time-dependence of the line shape function influences the Stokes as well as the anti-Stokes resonance Raman kinetics of highly vibrationally excited molecules. [Copyright &y& Elsevier]

Abstract: Vibrational excitation and energy redistribution after the ultrafast intramolecular proton transfer cycle of TINUVIN was investigated by picosecond pump–probe resonance Raman spectroscopy. Our studies demonstrate that the line shape function of the resonance Raman cross section dramatically changes due to vibrational excitation. Therefore, instead of determining vibrational populations directly from anti-Stokes resonance Raman kinetics, excitation of the modes has been characterized by their time-dependent ratios of anti-Stokes and Stokes resonance Raman intensities. Using this novel approach it is shown that a low-frequency proton transfer promoting mode serves as a major accepting mode of the proton transfer cycle. [Copyright &y& Elsevier]

The alcoholysis reaction of phenylisocyanate with cyclohexanol ( I ) and of 2,4-toluene-diisocyanate with chloraldhydrate ( II ) is studied by infrared absorption spectroscopy in combination with anharmonic frequency calculations using density functional theory. It is shown that the progress of the reaction can be monitored by measuring infrared marker bands in the isocyanate NCO and alcohol OH stretching regions. Analysis of spectra obtained as a function of time for different temperatures yields a second-order kinetics with an Arrhenius activation energy of 6.7 ± 0.2 and 2.8 ± 0.3 kcal/mol for reaction I and II , respectively. [ABSTRACT FROM AUTHOR]