Jens Stenger

Excited state structure of 4-(dimethylamino)benzonitrile studied by femtosecond mid-infrared spectroscopy and ab initio calculations

Abstract Combining femtosecond transient vibrational spectroscopy and high-level calculations is a powerful tool in the determination of excited-state structures. Striking differences in the experimental vibrational pattern of the locally excited states of 4-(dimethylamino)benzonitrile (DMABN) and 4-aminobenzonitrile (ABN) are explained on the basis of molecular structures obtained from ab initio complete-active-space self-consistent-field (CASSCF) calculations, giving evidence for a strong sensitivity of the molecular structure on modest changes in the substituents. The 4.0 ps charge-transfer time for DMABN in acetonitrile is resolved for the first time by tracking the downshifted CN stretching mode.

Coherent vibrational ground-state dynamics of an intramolecular hydrogen bond

Abstract We present direct evidence for coherent vibrational motions of low frequency in the v =0 ground state of the high-frequency O–D stretching mode of an intramolecular hydrogen bond. Femtosecond mid-infrared pump–probe studies of hydroxy-deuterated 2-(2 ′ -hydroxyphenyl)benzothiazole in toluene reveal an oscillatory change of O–D stretching mode absorption persisting for several picoseconds after excitation. An infrared resonantly enhanced stimulated Raman process within the bandwidth of the pump pulse creates a vibrational wave packet which is made up of quanta of a 118 cm −1 mode modulating the length of the hydrogen bond and thus the O–D stretching absorption band.

Femtosecond mid-infrared photon echo study of an intramolecular hydrogen bond

Abstract The vibrational dephasing dynamics of O–H stretching excitations in a sterically well-defined intramolecular hydrogen bond (H bond) is studied by femtosecond photon echo spectroscopy in the mid-infrared. The O–H stretching vibration of phthalic acid monomethyl ester in inert solution displays a fast decay of phase coherence on a sub-100 fs time scale. Anharmonic coupling of the stretching motion to an underdamped low-frequency mode of the H bond gives rise to oscillations in the transient grating decay and the 3-pulse photon echo peak shift. Spectral diffusion dynamics on longer time scales is absent in such intramolecular H bonds, as opposed to the weaker intermolecular H bonds in water.

Ultrafast coherent response of hydrogen bonds

Femtosecond pump-probe spectroscopy in the mid-infrared demonstrates the coherent oscillatory modulation of the hydrogen bond distance and its strength in an intramolecular hydrogen bond, giving evidence of anharmonic coupling between high-and low-frequency modes.