Research

Spectrum of the Week
McCamant Group Home Research People Facilities Contact Publications	Here we have devoted a page to our newest and most exciting data. A glance at the Research page shows that FSRS is an exciting technique that our lab uses and develops to probe many different types of chemical systems. The “McCamant Group Spectrum of the Week” page will highlight the power and flexibility of our method, as well as serve to record McCamant Group progress.
	April 7, 2008



	(top) Contour plot of transient absorption (TA) spectrum of pH 6.8 buffer solution with 0.25 M NaCl and 25 mM phosphate. There are two types of light water interaction appears on this spectrum. One of them is two-photon absorption of water by 266 nm pump pulse and supercontinuum probe, which occurs at 0 ps delay. The other is solvated electron absorption genearted upon two-photon ionization of water, which occurs after 1 ps delay. The peak of solvated electron absorption is between 600 and 700 nm, originally, but there is still a weak absorption observed at the UV region. (middle) Contour plot of TA spectrum of deoxyadenosine monophosphate (dAMP) in phosphate buffer solution. Besides the features observed for the buffer solution, there are two additional prominent signals shown. Ground state population decrease (bleach) of dAMP at about 260 nm and excited state absorption above 270 nm which decays with lifetime of 0.5 ps are measured. (bottom) Contour plot of TA spectrum of dAMP hexamer ((dA)6) in phosphate buffer solution. The same general features are observed with longer decays times for the hexamer.
	February 18, 2008
	First observation of an excited state Raman peak!


	(Top Figure) Stimulated Raman spectra of 4-(Dimethylamino)benzonitrile (DMABN) in methanol solvent. Shown are the solvent (black), Raman spectrum at 100 ps delay between the Actinic pump and Raman probe with solvent subtraction (red), and the excited/charge-transfer Raman spectrum at the same 100 ps delay which has the ground state and solvent peaks subtracted out (blue). Bold vertical lines indicate where ground state peaks are observed either here or in previous spectra not shown. Dashed vertical lines show the expected shift positions for their corresponding excited state peaks. Peak assignments: 2219 cm-1 (C≡N stretching); 1607 cm-1 (ring C=C stretching); 1581 cm-1 (ring C=C stretching excited state); 1178 cm-1 (ring C-H in plane bending); 788 cm-1 (ring breathing) (Bottom Figure) Picture of 4-(Dimethylamino)benzonitrile (DMABN); studied due to its solvent-dependent dual fluorescence and intramolecular electronic donor-acceptor charge-transfer properties. It should be noted that, although the expected excited state peak shift positions are shown for all ground state peaks, the only one which is observed is the ring C=C stretching at 1607 cm-1. As seen in the red spectrum, the ground and excited state shifts are observed in the spectrum as the two overlapping peaks. When the ground state is subtracted out of the spectrum, the resulting spectrum in blue reveals that the higher wavenumber ground state peak vanishes, leaving behind only the down-shifted peak due to the excited charge-transfer state at 1581 cm-1. Also, the peaks that dip negatively illustrate the bleaching of the ground state from the Actinic pump, yet observation of all the corresponding excited state peaks is an on-going process. Experimental conditions such as Raman pump power, Raman pulse duration, limiting cross-phase modulation, and general Continuum probe stability are currently being optimized in order to resolve these peaks and limit over-pumping of the excited-state population from the locally excited S1 electronic level to some higher unobservable Sn level.
	February 4, 2008
	The figures below show our most recent progress on the anharmonic coupling and 2-D Raman project.

	(Top Figure) Stimulated Raman ground state spectrum of CDCl3, commonly referred to as deuterio-chloroform. The peak at 645 cm-1 is the C-Cl stretch and the peak at 2256 cm-1 is the C-D stretch. There are two lower frequency C-Cl bends at 262 cm-1 and 365 cm-1 that are not shown in this spectrum. (Bottom Figure) 2-D Raman Spectrum of CDCl3, showing anharmonic coupling between the bending modes and stretches of CDCl3. In order to obtain this data we: 1) Impulsively drive the low frequency C-Cl bends and the C-Cl stretch into coherent superpositions of their fundamental vibrational states with the prism compressed output of a NOPA (Non-Collinear Optical Paremetric Amplifier). A quick calculation shows this required time resolution of better than 51 fs. 2) Take Stimulated Raman spectra at delays ranging from 400-2400 fs after the impulsive pump. Anharmonic coupling between modes generates small, oscillating sidebands on either side of an ordinary Raman peak. These sidebands are the result of the driven bending modes frequency modulating (like in your car radio) the vibrations of the high frequency modes, and thus, one oscillatory period is equal to one vibrational period of the modulating mode. For example, the spot at (1015 cm-1, 365 cm-1) is evidence of the 365 cm-1 mode modulating the vibration of the 648 cm-1 C-Cl stretch. 3) Fourier transform the spectra taken at positive times, and stacked them in the contour. The x axis is the ordinary wavelength axis, the y axis is the Fourier transformed frequency, and the intensity of the peaks comes out of the page at you. (Middle Figure) Slices of the contour at key Fourier transformed frequencies: namely, those corresponding to the impulsively driven modes.