Matteo Rini

Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer

Abstract Vibrational spectra between 1000 and 1700 cm−1 are studied after ultrafast transfer of a hydrogen atom in the excited state of 2-(2′-hydroxyphenyl)benzothiazole. Femtosecond pump–probe experiments reveal new vibrational bands of the keto-S1 state, including the carbonyl stretching band formed by hydrogen transfer. Such bands display a negligible spectral reshaping but blue-shift by up to 7 cm−1 following biexponential kinetics with time constants of 700 fs and 15 ps. The blue-shift is attributed to the anharmonic coupling of the fingerprint vibrations to Raman-active low-frequency modes that are excited upon electronic excitation and depopulated by intramolecular redistribution and cooling to the solvent.

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...

Femtosecond UV/mid-IR study of photochromism of the spiropyran 1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] in solution

Abstract The ring-opening reaction of 1 ′ ,3 ′ -dihydro-1 ′ ,3 ′ ,3 ′ -trimethyl-6-nitrospiro[2H-1-benzopyran-2,2 ′ -(2H)-indole] is investigated in two solvents, by probing the evolution of the vibrational absorption spectrum with 130 fs time-resolution. Competition between internal conversion and photochemistry is found to depend on the solvent. The internal conversion quantum yield is determined to be 0.63 in perdeuterated acetonitrile and 0.34 in tetrachloroethene. Based on spectral features and biexponential kinetics, the formation of an additional merocyanine isomer in tetrachloroethene is concluded.

Femtosecond laser beam shaping with structured thin-film elements

Spectral interference caused by structured thin-film components has been used for shaping and characterization of few-cycle femtosecond laser beams. Array structures enable spatially resolved measurements of coherence and wavefront. The generation of spatially and temporally localized optical wavepackets with reflective and refractive axicons was demonstrated in theory and experiment.

Bimodal intermolecular proton transfer in acid-base neutralization reactions in water

We present an ultrafast mid-infrared study of excited-state intermolecular proton transfer in photoacid-base pairs in water. Observation of bimodal reaction dynamics demand refinement of the established Eigen-Weller mechanism.