Hannover 2010 – scientific programme

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MO: Fachverband Molekülphysik

MO 5: Femtosecond Spectroscopy I

MO 5.2: Talk

Tuesday, March 9, 2010, 14:15–14:30, F 102

Ultrafast UV/IR spectroscopy of hydrogen bonded photoacid molecules — •Mirabelle Prémont-Schwarz¹, Dina Pines², Ehud Pines², and Erik T.J. Nibbering¹ — ¹Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany — ²Department of Chemistry, Ben Gurion University of the Negev, P.O.B. 653, Beersheva 84125, Israel

Photoacids are organic molecules which experience a significant decrease in pKa upon electronic excitation, thereby making them much stronger acids in the excited state than in the ground state. This property has been greatly exploited in the last decades permitting dynamical proton transfer and hydrogen bonding research on femtosecond timescales. While their use is now quite well established, the photophysics leading to this drastic change in pKa remain unclear. Transient UV pump/ IR probe spectroscopy is used in order to characterize the photocycle of diverse photoacids in solvents with increasing hydrogen bonding capability. The OH stretching oscillator is a direct probe for hydrogen bond strength. Indeed, the stronger the hydrogen bond, the larger the frequency red-shift. Therefore, by monitoring the hydrogen stretching oscillators in the excited state, insight into the nature of electronic redistibution in the photoacid molecule is obtained. The ultrafast UV/IR pump-probe experiments show a substantial frequency red shift of the OH stretching oscillator upon photoexcitation. It is found that the magnitude of this shift is correlated with the hydrogen bond accepting capabilities of the solvent.