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A detailed mechanistic investigationof the early stages of thePaternò–Büchi reaction following 267 nm excitationof benzaldehyde in cyclohexene has been completed using ultrafast,broadband transient UV–visible and IR absorption spectroscopies.Absorption due to electronically excited triplet state benzaldehydedecays on a 80 ps time scale via reaction with cyclohexene. The growthand subsequent decay of the biradical intermediate produced followingC–O bond formation is followed by transient vibrational spectroscopy.The biradical decays by ring closure to an oxetane or by dissociating,reforming the ground state reactants. Detailed kinetic analysis allowedderivation of quantum yields and rate constants for these competingbiradical decay processes, ϕoxetane= 0.53, ϕdiss= 0.47, koxetane= 0.27 ±0.09 ns–1and kdiss=0.24 ± 0.09 ns–1. This study provides a strikingillustration of the ways in which contemporary ultrafast transientabsorption spectroscopy methods can be used to dissect the mechanismand kinetics of a classic photoreaction. [ABSTRACT FROM AUTHOR]

In J-aggregates of cyanine dyes,closely packed molecules formmesoscopic tubes with nanometer-diameter and micrometer-length. Theirefficient energy transfer pathways make them suitable candidates forartificial light harvesting systems. This great potential calls foran in-depth spectroscopic analysis of the underlying energy deactivationnetwork and coherence dynamics. We use two-dimensional electronicspectroscopy with sub-10 fs laser pulses in combination with two-dimensionaldecay-associated spectra analysis to describe the population flowwithin the aggregate. Based on the analysis of Fourier-transform amplitudemaps, we distinguish between vibrational or vibronic coherence dynamicsas the origin of pronounced oscillations in our two-dimensional electronicspectra. [ABSTRACT FROM AUTHOR]

Transient infrared absorption spectroscopymonitors condensed-phasephotodissociation dynamics of 30 mM CHBr3and 50 mM CHI3in liquid CCl4. The experiments have picosecondtime resolution and monitor the C–H stretch region of boththe parent polyhalomethanes and their photolytically generated isomers.The C–H stretching transitions of these isomers, in which theemergent halogen atom returns to form a C–X–X bondingmotif, appear about 9 ps after photolysis for iso-CHBr2–Br and in about 46 ps for iso-CHI2–I. These time scales are consistent with,but differ from, the time evolution of the transient electronic absorptionspectra of the same samples, highlighting the subtle differences betweenmonitoring the vibrational and electronic chromophores. The specificityof using vibrational transitions to track condensed-phase reactiondynamics permits reassessment of the transient electronic spectrumof photolysis in neat CHBr3, which has an additional promptfeature near 400 nm. Calculations show that this feature, which arisesfrom a precursor to the isomer, is a charge-transfer transition ofa contact pair between the nascent Br fragment and a nearby CHBr3molecule. Dilution and solvent studies show that transitionis independent of the solvent. The iso-CHBr2–Br transition wavelength, however, shifts over the rangeof 400 to 510 nm depending on the solvent. Time-dependent densityfunctional calculations faithfully reproduce these trends. [ABSTRACT FROM AUTHOR]

The electronic spectra of Co+(H2O), Co+(HOD), and Co+(D2O) havebeen measuredfrom 13 500 to 18 400 cm–1using photodissociation spectroscopy. Transitionsto four excited electronic states with vibrational and partially resolvedrotational structure are observed. Each electronic transition hasan extended progression in the metal–ligand stretch, v3, and the absolute vibrational quantum numbering is assignedby comparing isotopic shifts between Co+(H216O) and Co+(H218O). For thelow-lying excited electronic states, the first observed transitionis to v3′ = 1. This allows the Co+–(H2O) binding energy to be determined as D0(0 K)(Co+–H2O) = 13730 ±90 cm–1(164.2 ± 1.1 kJ/mol). The photodissociationspectrum shows a well-resolved Kaband structure due to rotation about the Co–O axis.This permits determination of the spin rotation constants ϵaa″= −6 cm–1and ϵaa′= 4cm–1. However, the Karotational structure depends on v3′.These perturbations in the spectrum make the rotational constantsunreliable. From the nuclear spin statistics of the rotational structure,the ground state is assigned as 3B1. The electronictransitions observed are from the Co+(H2O) groundstate, which correlates to the cobalt ion’s 3F,3d8ground state, to excited states which correlate tothe 3F, 3d74s and 3P, 3d8excited states of Co+. These excited states of Co+interact less strongly with water than the ground state.As a result, the excited states are less tightly bound and have longermetal–ligand bonds. Calculations at the CCSD(T)/aug-cc-pVTZlevel also predict that binding to Co+increases the H–O–Hangle in water from 104.1° to 106.8°, as the metal removeselectron density from the oxygen lone pairs. The O–H stretchingfrequencies of the ground electronic state of Co+(H2O) and Co+(HOD) have been measured by combiningIR excitation with visible photodissociation in a double resonanceexperiment. In Co+(H2O) the O–H symmetricstretch is ν1″ = 3609.7 ± 1 cm–1. The antisymmetric stretch is ν5″ = 3679.5± 2 cm–1. These values are 47 and 76 cm–1, respectively, lower than those in bare H2O. In Co+(HOD) the O–H stretch is observed at 3650cm–1, a red shift of 57 cm–1relativeto bare HOD. [ABSTRACT FROM AUTHOR]

In this work we analyze how nuclear coherences modulate diagonal and off-diagonal peaks in two-dimensional electronic spectroscopy. 2D electronic spectra of pinacyanol chloride are measured with 8 fs pulses, which allows coherent excitation of the 1300 cm−1 vibrational mode. The 2D spectrum reveals both diagonal and off-diagonal peaks related to the vibrational mode. On early time scales, up to 30 fs, coherent dynamics give rise to oscillations in the amplitudes, positions, and shapes of the peaks in the 2D spectrum. We find an anticorrelation between the amplitude and the diagonal width of the two diagonal peaks. The measured data are reproduced with a model incorporating a high frequency mode coupled to an electronic two-level-system. Our results show that these anticorrelated oscillations occur for vibrational wavepackets and not exclusively for electronic coherences as has been assumed previously. [ABSTRACT FROM AUTHOR]