Soroush Khosravi Dehaghi,
Department of Physics,
University of Connecticut

Femtosecond Laser Transient Spectroscopy of Carotenoids and Carbon Nanotubes

Carotenoids and semiconducting single-wall carbon nanotubes, which are both nanoscale carbon-based quantum mechanical systems, were studied using different optical spectroscopy techniques.

The lifetime of the S\(_2\) excited state of the carotenoids were estimated using a model-based lifetime analysis routine. It was found that the addition of a conjugated carbonyl group to the carotenoid has several crucial effects on the optical response due to the alteration of the delocalized conjugated \(\pi\) molecular orbital of the molecule. These effects include the shortening of the lifetimes of the excited states, the change in the decay mechanism from the S\(_2\) excited state to the S\(_1\) excited state, and the unresolved vibrational peaks in the steady-state absorption spectrum.

The decay of the E\(_{22}\) excited state of (6,5) semiconducting carbon nanotube dispersed in flavin mononucleotide with water was observed using transient grating spectroscopy. The lifetime of this excited state was estimated to be 450 ± 50 fs. A very strong single-frequency oscillatory component was observed in the transient grating decay profile.

Astro interview talk (TBD)

Prof. Mark Johnson, Department of Chemistry, Yale University

Molecular aspects of proton and electron accommodation by water with cryogenic, size-selected cluster spectroscopy

When water becomes electrically charged by accepting either an “extra” proton or electron, it accommodates that particle by distorting the local hydrogen bonded water network as well as nearby water molecules. These structural motifs can be measured experimentally by monitoring the vibrational and electronic spectra of the ionic clusters (H\(_2\)O)\(_n^¯\) and H\(^+\)(H\(_2\)O)\(_n\). These measurements are carried out with custom built, hybrid instruments that combine optical spectroscopy with mass spectrometry in a mode where cluster ions are generated using a cryogenic radiofrequency ion trap.

The negatively charged (H\(_2\)O)\(\_n^¯\) clusters are microscopic analogues of the “hydrated electron,” a key species that mediates radiation damage in biological systems. The excess electron is accommodated outside the valence electrons in the water molecules, and is trapped by long-range attraction to the electric dipole induced in the supporting water network. The physical size of the extra electron’s wavefunction can be measured using spectral moment analysis, and is found to be strongly dependent on the number of water molecules in the cluster. At small sizes, the electron is so weakly bound that the OH stretching vibrational levels lie above the electron binding energy. This situation leads to strong Fano resonances in the electron photodetachment continuum, and analysis of these lineshapes reveals the intrinsic coupling between the diffuse electron cloud and molecular vibrations.

Proton accommodation within the H\(^+\)(H\(_2\)O)\(_n\) clusters is similarly size-dependent, this time involving direct incorporation into a water molecule with formation of the H\(_3\)O\(^+\) hydronium ion expected from freshman chemistry. But that motif quickly disappears as the structures evolve dramatically through the first half dozen water molecules. In this progression, the degree of charge delocalization oscillates between being distributed over the three protons in an H\(_3\)O\(^+\) molecular ion core to being localized between the oxygen atoms on two nearby water molecules to form the “Zundel ion”, H\(_2\)O-H\(^+\)-OH\(_2\). The spectral signatures associated with these structures can be understood by considering the shape of the trapping potential energy surface and its response to the topology of the surrounding water network.

Prof. Lee Roberts, Boston University

Astro interview talk (TBD)