Browsing by Author "Rebane, Aleks"

Now showing 1 - 2 of 2

Broad-band telescope for focussing femtosecond pulses in two photon absorption spectroscopy experiments
(2013-03) Weas, Tom; Rebane, Aleks
As the intensity of the laser pulses increase, the double photon absorption increases by a power of 2, which causes a problem when trying to measure over a large range of wavelengths. We have built a telescope system that would allow us to focus the laser as a function of wavelength by using two stepper motors that are controlled by a LabVIEW VI. We also address problems that involve diffraction from a gaussian beam vs. geometrical limits.
Two-photon absorption reveals low-energy excited states of a 2,5,8-triamino-heptazine chromophore
(SPIE, 2024-06) Stark, Charles W.; Arak, Johanna; Trummal, Aleksander; Uudsemaa, Merle; Sildoja, Meelis-Mait; Pahapill, Juri; Rebane, Aleks
Triamino-heptazines (TAH's) comprise the fundamental building blocks of graphitic carbon nitride, an alluring material with promising applications in optoelectronics. However, the core D3h molecular symmetry enforces a forbidden lowest-energy excited singlet state, making it a challenge to characterize via conventional spectroscopy. Here, we measure oneand two-photon absorption spectra of an acidic form of triamino-heptazine, 3H-TAH, and use reversible acid/base titration to further probe the symmetry of the low-energy transitions in aqueous solution, which suggests the molecular base structure is dimelem. Two-photon absorption reveals two distinct low-energy transitions in acidic conditions, both of which are one-photon forbidden. The lowest energy state additionally becomes one-photon allowed in basic conditions. Spectroscopic changes can be described according to chromophore symmetry switching, with C3h, D3h, or Cs point group symmetry in respective acidic, neutral, or basic environments.