Paul R. Winter

The dynamics of conformational isomerization in flexible biomolecules. I. Hole-filling spectroscopy of N-acetyl tryptophan methyl amide and N-acetyl tryptophan amide.

The conformational isomerization dynamics of N-acetyl tryptophan methyl amide (NATMA) and N-acetyl tryptophan amide (NATA) have been studied using the methods of IR-UV hole-filling spectroscopy (HFS) and IR-induced population transfer spectroscopy (IR-PTS), which were developed for this purpose. Single conformations of these molecules were selectively excited in well-defined NH stretch fundamentals. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were recooled into their zero-point levels, partially refilling the hole created in the ground state population of one of the conformers, and creating gains in population in other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion. In HFS, the IR wavelength is fixed and the UV laser tuned in order to determine where the population went following selective infrared excitation. In IR-PTS, the UV is fixed to monitor the population of a given conformation, and the IR is tuned to record the IR-induced changes in the population of the monitored conformer. Besides demonstrating the capability of the experiment to change the downstream conformational population distribution, the IR-PTS scans were used to extract two quantitative results: (i) The fractional populations of the conformers in the absence of the infrared, and (ii) the isomerization quantum yields for each of the six unique amide NH stretch fundamentals (three conformers each with two amide groups). The method for obtaining quantum yields is described in detail. In both NATMA and NATA, the quantum yields show modest conformational specificity, but only a hint of vibrational mode specificity. The prospects for the hole-filling technique for providing insight into energy flow in large molecules are discussed, leaving a more detailed theoretical modeling to the adjoining paper [Evans et al. J. Chem. Phys. 120, 148 (2004)].

The singlet–triplet spectroscopy of 1,3-butadiene using cavity ring-down spectroscopy

The T1←S0 absorption spectrum of gas-phase 1,3-butadiene (C4H6) has been investigated over the region from 20 500 to 23 000 cm−1 using cavity ring-down spectroscopy. Resolved vibrational structure and partially resolved rotational structure have been observed for the first time in the gas phase. The T1←S0 origin transition is located at 20 777 cm−1, with a peak absorption cross section of 2.5×10−26 cm2/molecule. Vibronic bands appear 249, 491, 1166, and 1617 cm−1 above the origin. This structure is observed on top of a rising background whose absolute magnitude and wavelength dependence is quantitatively accounted for as Rayleigh scattering. Using the recent calculations of Brink et al. [J. Phys. Chem. A 102, 6513 (1998)] as a guide, the bands 491, 1166, and 1617 cm−1 above the origin can be assigned as totally symmetric fundamentals, while the band 249 cm−1 above the origin is the first overtone of the bg symmetry CH2 torsion (calculated at 129.6 cm−1) of a planar T1 excited state. The rotational band co...

Photoelectron spectroscopy via the 1 1Δu state of diacetylene

Photoelectron spectra are reported for one-photon resonant, two-photon ionization of jet-cooled diacetylene via a number of vibronic levels of the 1 1Δu state. An improved value for the adiabatic ionization threshold is found to be 82 064±30 cm−1 (10.175±0.004 eV), in good agreement with the earlier result. The photoelectron spectra of different vibronic bands of the 1 1Δu state nearly all show long progressions in what appear to be low frequency bending vibrations. At energies just above the ionization threshold, the observed progressions can be understood in terms of excitation of a single Renner-Teller active mode in the ion, with Renner-Teller parameters similar to those of the ν4+ trans-bending mode in the ground state acetylene cation.