Two Dimensional Fourier-Transform Spectroscopy of Adenine and Uracil Using Shaped Ultrafast Laser Pulses in the Deep UV (original) (raw)
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Probing Ultrafast Dynamics in Adenine With Mid-UV Four-Wave Mixing Spectroscopies
The Journal of Physical Chemistry A, 2011
I. Generation of Short UV pulses in a Hollow-Core Fiber A home-built hollow-core fiber filled with 0.65atm argon gas is used to generate 25fs, 38000 cm-1 (263 nm) laser pulses. The setup is modeled after that developed by Bradforth and co-workers and performs similarly. 1 Briefly, 12660cm-1 and 25320cm-1 pulses with 40 μJ energies are focused into a 75 micron diameter fiber using 25cm focal length lenses. The 12660cm-1 and 25320cm-1 pulses have FWHM bandwidths of 160cm-1. The UV pulses generated inside the fiber have 1.0 μJ energies and FWHM bandwidths of 800-950 cm-1. A fused silica prism compressor with a tip-to-tip prism separation of
Acc. Chem. Res.: Special issue on Coherent Multidimensional Optical Spectroscopy
S pectral line shapes in a condensed phase contain information from various dynamic processes that modulate the transition energy, such as microscopic dynamics, interand intramolecular couplings, and solvent dynamics. Because nonlinear response functions are sensitive to the complex dynamics of chemical processes, multidimensional vibrational spectroscopies can separate these processes. In multidimensional vibrational spectroscopy, the nonlinear response functions of a molecular dipole or polarizability are measured using ultrashort pulses to monitor inter-and intramolecular vibrational motions. Because a complex profile of such signals depends on the many dynamic and structural aspects of a molecular system, researchers would like to have a theoretical understanding of these phenomena. In this Account, we explore and describe the roles of different physical phenomena that arise from the peculiarities of the system-bath coupling in multidimensional spectra. We also present simple analytical expressions for a weakly coupled multimode Brownian system, which we use to analyze the results obtained by the experiments and simulations. To calculate the nonlinear optical response, researchers commonly use a particular form of a system Hamiltonian fit to the experimental results. The optical responses of molecular vibrational motions have been studied in either an oscillator model or a vibration energy state model. In principle, both models should give the same results as long as the energy states are chosen to be the eigenstates of the oscillator model. The energy state model can provide a simple description of nonlinear optical processes because the diagrammatic Liouville space theory that developed in the electronically resonant spectroscopies can easily handle three or four energy states involved in high-frequency vibrations. However, the energy state model breaks down if we include the thermal excitation and relaxation processes in the dynamics to put the system in a thermal equilibrium state. The roles of these excitation and relaxation processes are different and complicated compared with those in the resonant spectroscopy. Observing the effects of such thermal processes is more intuitive with the oscillator model because the bath modes, which cause the fluctuation and dissipation processes, are also described in the coordinate space. This coordinate space system-bath approach complements a realistic full molecular dynamics simulation approach. By comparing the calculated 2D spectra from the coordinate space model and the energy state model, we can examine the role of thermal processes and anharmonic mode-mode couplings in the energy state model. For this purpose, we employed the Brownian oscillator model with the nonlinear system-bath interaction. Using the hierarchy formalism, we could precisely calculate multidimensional spectra for a single and multimode anharmonic system for inter-and intramolecular vibrational modes.
Journal of the American Chemical Society, 1989
Raman cross sections for excitation at wavelengths between 240 and 192 nm have been determined for resonance-enhanced ring modes of phenylalanine (Phe), tyrosine (Tyr), tyrosinate (Tyr-), and tryptophan (Trp) and for the breathing mode of Sod2and the symmetric OH stretching mode of H 2 0 . The S042and H20 bands, which are frequently used for relative intensity measurements, show strong preresonance enhancement at wavelengths below 220 nm, requiring large corrections to convert relative to absolute intensities. The aromatic amino acid intensity measurements were obtained with laser power levels in the linear regime and appear not to be influenced by saturation effects or photochemical transients. The 1210-cm-l ring-C stretching mode of Phe and Tyr and the ring modes u12 (Phe, 1000 cm-') or u1 (Tyr, 850 cm-I) show strong enhancement in resonance with the allowed Ba,b transition and weak enhancement in resonance with the quasi-forbidden La transition. Their excitation profiles (EPs) peak on the red side of the La absorption band, consistent with destructive interference between La and Ba,b contributions to the A-term Raman amplitude. The modes usar vgb (-1610, 1590 cm-I), and u9, (-1180 cm-I), known to be responsible for the vibronic intensity of the La transition, are resonant with the La transition but also with the Ba,b transition. Enhancement of Vgb, which is antisymmetric in the molecular c2, symmetry, requires B-term activity for the Ba,b as well as the La states. The Ba,b enhancement (192-nm excitation) of Vgb, as well as ug, and uga, increases in the order Phe < Tyr < phenolate = Tyr-. This effect is suggested to arise from the substituent electronic perturbation, which leads to a B, origin shift along ug, and u9, and vibronic mixing between B, and Bb via ugh. In the La region the EPs of u8,, Vgb, and uga are skewed to the blue side of the L, absorption band, for Tyr more than Phe, suggesting constructive La-Ba,b interference for the B Raman terms. The Phe uga and vgb overtones and combination bands are more strongly enhanced in resonance with Ba,b than are the fundamentals; the 2uga/uga intensity ratio reaches a value of 2.1 at 192 nm. This effect is attributed to force constant changes in the allowed excited states. The 2uga cross section is much lower for Tyr but increases again for Tyr-, although the fundamental band is now much stronger. Evidently the B, force constant change is diminished by the OH substituent, while the increased origin shift for 0-increases the overtone intensity again. The Tyr 2vgb cross sections are about half those of Phe and the 2Vgb/vgb ratio, -0.15, is suggested to reflect the relative magnitude of C and B vibronic terms. Several Trp modes give EPs which track the strong 220-nm absorption band, attributed to Bb These modes have significant five-and six-membered ring involvement, implying delocalized character for the transition. The benzene-like uga and Vgb modes are enhanced in the short-wavelength absorption band (-195 nm), attributed to B,. The v8, mode is also weakly resonant with the Bb, indicating some displacement of the excited state along the vg, coordinate. The Vgb mode, however, is not enhanced in the 220-nm band; this behavior can be understood on the basis of the vgb eigenvector being essentially antisymmetric to the polarization axis and therefore unable to support A-term activity in the Bb transition.
Recent advances in multidimensional ultrafast spectroscopy
Royal Society open science, 2018
Multidimensional ultrafast spectroscopies are one of the premier tools to investigate condensed phase dynamics of biological, chemical and functional nanomaterial systems. As they reach maturity, the variety of frequency domains that can be explored has vastly increased, with experimental techniques capable of correlating excitation and emission frequencies from the terahertz through to the ultraviolet. Some of the most recent innovations also include extreme cross-peak spectroscopies that directly correlate the dynamics of electronic and vibrational states. This review article summarizes the key technological advances that have permitted these recent advances, and the insights gained from new multidimensional spectroscopic probes.
Femtosecond Phase-Coherent Two-Dimensional Spectroscopy
Science, 2003
Femtosecond phase-coherent two-dimensional (2D) spectroscopy has been experimentally demonstrated as the direct optical analog of 2D nuclear magnetic resonance. An acousto-optic pulse shaper created a collinear three-pulse sequence with well-controlled and variable interpulse delays and phases, which interacted with a model atomic system of rubidium vapor. The desired nonlinear polarization was selected by phase cycling (coadding experimental results obtained with different interpulse phases). This method may enhance our ability to probe the femtosecond structural dynamics of macromolecules.
Ultrafast spectroscopy with sub-10 fs deep-ultraviolet pulses
Physical Chemistry Chemical Physics, 2012
Time-resolved transient absorption spectroscopy with sub-9 fs ultrashort laser pulses in the deep-ultraviolet (DUV) region is reported for the first time. Single 8.7 fs DUV pulses with a spectral range of 255-290 nm are generated by a chirped-pulse four-wave mixing technique for use as pump and probe pulses. Electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of thymine over the full spectral range using a 128-channel lock-in detector. Vibrational modes of the electronic ground state and excited states can be observed as well as the decay dynamics of the electronic excited state. Information on the initial phase of the vibrational modes is extracted from the measured difference absorbance trace, which contains oscillatory structures arising from the vibrational modes of the molecule. Along with other techniques such as time-resolved infrared spectroscopy, spectroscopy with sub-9 fs DUV pulses is expected to contribute to a detailed understanding of the photochemical dynamics of biologically significant molecules that absorb in the DUV region such as DNA and amino acids.
Orienting molecules via an ir and uv pulse pair: Implications for coherent Raman spectroscopy
Physical Review A, 2009
Spatial orientation of molecules is a pervasive issue in chemical physics and, by breaking inversion symmetry, has major consequences in nonlinear optics. In this paper, we propose and analyze an approach to molecular orientation. This extracts from an ensemble of aligned diatomic molecules ͑equally AB and BA, relative to the E vector͒ a subensemble that is oriented ͑mostly AB or BA͒. Subjecting an aligned molecule to a tailored infrared ͑ir͒ laser pulse creates a pair of coherent wave packets that correlate vibrational phase with the AB or BA orientation. Subsequent, suitably phased ultraviolet ͑uv͒ or visible pulses dissociate one of these vibrational wave packets, thereby "weeding out" either AB or BA but leaving intact the other orientation. Molecular orientation has significant implications for coherent Raman spectroscopy. In the absence of orientation, coherence between vibrational levels is generated by a pair of laser pulses off which a probe pulse is scattered to produce a signal. Orientation allows direct one-photon ir excitation to achieve ͑in principle͒ maximal Raman coherence.