Siva Umapathy

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the photochemistry of trans-azobenzene in n-hexane. Excitation to the S-1(n pi*) state results in transient absorption bands at ca. 400 nm (strong) and 550 nm (weaker) which decay with a lifetime 2.5 +/- 0.2 ps on excitation at 503 nm, close to the S-1 origin, and with an additional fast component of ca. 0.6 ps on excitation at 390 or 420 nm, both well above the S-1 origin. Excitation to the S-2(pi pi*) state results in transient absorption at 400 nm which decays with a dominant component of ca. 0.9 ps and a weaker component of ca. 15 ps; this 400 nm band itself is observed to rise synchronously as a transient band at 475 nm decays with a lifetime of < 200 fs. These results are discussed in terms of the dual mechanism proposed for azobenzene photoisomerization.

Early time dynamics of trans-azobenzene isomerization in solution from resonance Raman intensity analysis

Resonance Raman spectra have been recorded for trans- azobenzene in carbon tetrachloride using 16 excitation wavelengths in the region from 355–600 nm. It has been observed that for many totally symmetric fundamentals viz. C–N, N5N stretch, etc., the resonance Raman intensities decrease near the maxima of the resonant electronic

Wavepacket dynamical studies on trans-azobenzene: absorption spectrum and resonance Raman excitation profiles of the n−π∗ transition

(2 ^1A_g \leftarrow1 ^1A_g)

Time-Resolved Resonance Raman Spectroscopy: Exploring Reactive Intermediates

transition. This is attributed to interference due to preresonant scattering from the strongly allowed

Mode-dependent dispersion in Raman line shapes: Observation and implications from ultrafast Raman loss spectroscopy

(1 ^1A_u \leftarrow1 ^1A_g)

Dynamic structure of charge carrier in polyaniline by near-infrared excited resonance Raman spectroscopy

electronic transition. The Raman excitation profiles (REPs) for the ten Franck–Condon active fundamentals have been successfully modeled using Heller’s time-dependent approach with the inclusion of interference effect from higher electronic state. The short time isomerization dynamics is then examined from a priori knowledge of ground-state normal mode descriptions to convert the wave packet motion in dimensionless normal coordinates to internal coordinates. It is observed that within 5–30 fs of photoexcitation, the major changes experienced by trans-azobenzene are on N = N and C–N stretching vibrations, while N = N suffers reduction, C–N bond elongates, and with time the ring C atoms distort relatively out of the plane.

Is Chemically Synthesized Graphene ‘Really’ a Unique Substrate for SERS and Fluorescence Quenching?

Abstract Time-dependent wavepacket propagation techniques have been used to calculate the absorption spectrum and the resonance Raman excitation profiles of the n −π ∗ transition in azobenzene. A comparison of both the calculated absorption spectrum and excitation profiles with experiment has been made. From an analysis of the data, it is concluded that the Raman intensities are mainly due to resonance from the n −π ∗ transition and not from the pre-resonance of the π−π ∗ transition, as reported earlier. We find that the isomerization pathway is through the inversion mechanism rather than by rotation. This is the first direct spectroscopic evidence for the isomerization pathway in trans-azobenzene.

Solvatochromic Effect on the Photophysical Properties of Two Coumarins

The study of reaction mechanisms involves systematic investigations of the correlation between structure, reactivity, and time. The challenge is to be able to observe the chemical changes undergone by reactants as they change into products via one or several intermediates such as electronic excited states (singlet and triplet), radicals, radical ions, carbocations, carbanions, carbenes, nitrenes, nitrinium ions, etc. The vast array of intermediates and timescales means there is no single “do-it-all” technique. The simultaneous advances in contemporary time-resolved Raman spectroscopic techniques and computational methods have done much towards visualizing molecular fingerprint snapshots of the reactive intermediates in the microsecond to femtosecond time domain. Raman spectroscopy and its sensitive counterpart resonance Raman spectroscopy have been well proven as means for determining molecular structure, chemical bonding, reactivity, and dynamics of short-lived intermediates in solution phase and are advantageous in comparison to commonly used time-resolved absorption and emission spectroscopy. Today time-resolved Raman spectroscopy is a mature technique; its development owes much to the advent of pulsed tunable lasers, highly efficient spectrometers, and high speed, highly sensitive multichannel detectors able to collect a complete spectrum. This review article will provide a brief chronological development of the experimental setup and demonstrate how experimentalists have conquered numerous challenges to obtain background-free (removing fluorescence), intense, and highly spectrally resolved Raman spectra in the nanosecond to microsecond (ns–μs) and picosecond (ps) time domains and, perhaps surprisingly, laid the foundations for new techniques such as spatially offset Raman spectroscopy.

Partitioning of the total reorganization energy into its vibrational and solvent (inertial and reorientational) contributions using resonance Raman intensities

Ultrafast Raman loss spectroscopy (URLS) enables one to obtain the vibrational structural information of molecular systems including fluorescent materials. URLS, a nonlinear process analog to stimulated Raman gain, involves a narrow bandwidth picosecond Raman pump pulse and a femtosecond broadband white light continuum. Under nonresonant condition, the Raman response appears as a negative (loss) signal, whereas, on resonance with the electronic transition the line shape changes from a negative to a positive through a dispersive form. The intensities observed and thus, the Franck-Condon activity (coordinate dependent), are sensitive to the wavelength of the white light corresponding to a particular Raman frequency with respect to the Raman pump pulse wavelength, i.e., there is a mode-dependent response in URLS.

SIMPLE APPROACH TO DETERMINING ABSOLUTE RAMAN CROSS SECTIONS USING AN OPTICAL PARAMETRIC OSCILLATOR

New vibrational Raman features characteristic to the conductive form of polyaniline have been observed with the near-infrared excitation at 1047 nm. Based on an analogy with the resonance Raman spectrum of Michlers ketone in the lowest excited triplet (T-1) state, we consider these features as due to a dynamic structure of a diimino-1,4-phenylene unit in the polyaniline chain exchanging a positive charge very rapidly. This consideration directly leads to a conducting mechanism in which a positive charge migrates from one nitrogen to the other through the conjugated chain of polyaniline.