Mode-selective O–H stretching relaxation in a hydrogen bond studied by ultrafast vibrational spectroscopy

Abstract

The ultrafast relaxation of the excited O-H stretching vibration is studied by ultrafast infrared-pump/infrared-probe and infrared-pump/Raman-probe spectroscopy. We demonstrate a 200 fs lifetime of the hydrogen-bonded O-H stretching mode in 2-(2'-hydroxy-5'-methyl-phenyl)benzotriazole (TINUVIN P). O-H stretching relaxation occurs through a few major channels that all involve combination and overtone bands of modes with considerable in-plane O-H bending character. In particular, the mode, which contains the largest O-H bending contribution, plays a prominent role for primary processes of intramolecular vibrational energy redistribution. Theoretical calculations of vibrational energy transfer rates based on a Fermi golden rule approach account for the experimental findings.

Mode-selective vibrational redistribution after spectrally selective N–H stretching mode excitation in intermolecular hydrogen bonds

“…Rise and decay times of the antiStokes Raman intensities are fitted by a single-exponential decay with a 2.5 ps relaxation time constant and an instantaneous response due to hyper-Rayleigh scattering. 15,16 Populations of the neighboring modes, e.g., an anti-Stokes signal at 1500 cm −1 for excitation at 1600 cm −1 or other bands in the spectrum, were not observed. Similar dependencies as shown in the inset of Fig.…”

“…15 Subpicosecond pulses with a spectral width of 30-40 cm −1 were generated around 3 m for excitation in the N-H stretching region and around 6 m for pumping fingerprint modes with pulse energies of about 10 and 3 J, respectively. Pulses at 400 and around 350 nm with a spectral width of 20-30 cm −1 were used for anti-Stokes resonance Raman probing.…”

“…Using a simple three level kinetic model enables us to estimate how much energy of an initially excited mode, for instance the N-H stretching mode, is transferred via other modes. 15 The anti-Stokes Raman band at 1600 cm −1 contains two vibrational modes of about the same Raman cross section. From their population kinetics and a portion of 20% of the dimers which were excited by the infrared pulses we estimate that roughly 20% of mode population ͑corresponding to about 10% of the energy of one N-H stretching quantum͒ is transferred through these two modes.…”

“…Kinetics was monitored following infrared excitation at a fixed wavelength in the broad, nearly structureless O -H¯N stretching band applying subpicosecond infrared-pump/antiStokes resonance Raman-probe spectroscopy. 15 In these experiments modes with O-H bending character were identified as primary accepting modes.…”

N–H Stretching Excitations in Adenosine-Thymidine Base Pairs in Solution: Pair Geometries, Infrared Line Shapes, and Ultrafast Vibrational Dynamics

“…This indicates that the vibrational dynamics of the N-H stretching manifold on (sub)picosecond time scales follows vibrational energy flow pathways beyond a simple population decay mechanism for the different N-H stretching modes, consistent with what has been observed before for other hydrogen-bonded systems. 21,58–60 …”