Robert McCanne

Two-dimensional infrared spectroscopy detected by chirped pulse upconversion

A two-dimensional (2D) infrared spectrum of Mn2(CO)10 is measured by using chirped-pulse upconversion (CPU) of the nonlinear signal field plus a reference local oscillator. By converting the spectrum to the visible, a silicon CCD camera can be used. The method offers an attractive alternative to direct IR detection due to the technological maturity of silicon and its greater intrinsic detectivity over HgCdTe. Using CPU, we acquired a rephasing 2D IR spectrum in a few seconds.

Multilevel vibrational coherence transfer and wavepacket dynamics probed with multidimensional IR spectroscopy.

Multidimensional infrared (MDIR) spectroscopy of a strongly coupled multilevel vibrational system Mn(2)(CO)(10) (dimanganese decacarbonyl) in cyclohexane solution reveals fully resolved excited vibrational state coherences that exhibit slow 0.25-0.50 ps(-1) decay constants. Detailed analysis of the waiting-time dependence of certain cross-peak amplitudes shows modulation at multiple frequencies, providing a direct signature of excited vibrational coherences resulting from coherence transfer. A new signature of coherence transfer is observed as temporally modulated cross-peak amplitudes with more than one modulation frequency. The relative importance of different coherence transfer paths is considered in the context of the orientational response of a system which includes two vibrational modes with parallel dipole moments. Since MDIR spectroscopy enables spectral isolation of individual excited vibrational coherences (i.e., coherences between fundamental excitations), these experiments report directly on the frequency-frequency correlation functions of the excited states relative to each other as well as relative to the ground state. These results highlight the rich information contained in fully exploring three-dimensional third-order spectroscopy, particularly regarding chemically relevant slower dynamics and the importance of intramolecular interactions leading to dephasing by optically dark or low-frequency modes of the molecule.

Ultrafast nonequilibrium Fourier-transform two-dimensional infrared spectroscopy

We present what we believe to be the first implementation of nonequilibrium two-dimensional IR spectroscopy (2DIR) combining electronic excitation within the Fourier transform (FT) approach. Nonequilibrium 2DIR spectra of Mn2(CO)10 and its photoproducts are obtained in two modalities: photoexcitation at 400 nm, either before a 2DIR probe or during the waiting time of the FT 2DIR measurement. Extending FT 2DIR to nonequilibrium systems offers insight into complex condensed-phase reaction dynamics.

Structurally Selective Geminate Rebinding Dynamics of Solvent-Caged Radicals Studied with Nonequilibrium Infrared Echo Spectroscopy

Cyclopentadienylmolybdenum(II) tricarbonyl dimer exists in two different equilibrium conformations: trans and gauche. Ultrafast photoexcitation in the ultraviolet cleaves the Mo-Mo bond, permitting observation of the subsequent geminate rebinding reaction (t(rebinding) = 31.6 ps) by monitoring infrared bleach recoveries at frequencies corresponding to the CO stretches of the trans and gauche isomers, the time-resolved measurements revealed that the monomers rebind in the trans configuration only. Further insight into the rebinding reaction was obtained by mapping the full potential energy surface along the reaction coordinate using electronic-structure methods.

Orientational Dynamics of Transient Molecules Measured by Nonequilibrium Two-Dimensional Infrared Spectroscopy

Transient two-dimensional infrared (2DIR) spectroscopy is applied to the photodissociation of Mn2(CO)10 to 2 Mn(CO)5 in cyclohexane solution. By varying both the time delay between the 400 nm phototrigger and the 2DIR probe as well as the waiting time in the 2DIR pulse sequence, we directly determine the orientational relaxation of the vibrationally hot photoproduct. The orientational relaxation slows as the photoproduct cools, providing a measure of the transient temperature decay time of 70 +/- 16 ps. We compare the experimental results with molecular dynamics simulations and find near quantitative agreement for equilibrium orientational diffusion time constants but only qualitative agreement for nonequilibrium thermal relaxation. The simulation also shows that the experiment probes an unusual regime of thermal excitation, where the solute is heated while the solvent remains essentially at room temperature.

Ultrafast equilibrium and non-equilibrium chemical reaction dynamics probed with multidimensional infrared spectroscopy

Two-dimensional infrared (2D-IR) spectroscopy provides powerful tools to investigate chemical reaction dynamics in the condensed phase. Correlating excitation and detection frequencies grants access to structural and dynamical information that is hidden in a linear absorption spectrum. Low-barrier reactions naturally can occur on the picosecond time scale, and although they are too rapid to study using nuclear magnetic resonance spectroscopy, the intrinsic ultrafast time resolution of coherent 2D-IR spectroscopy enables direct tracking of equilibrium reactive barrier crossings. 2D-IR chemical exchange spectroscopy can monitor the picosecond dynamics of non-triggered chemical reactions by correlating excited reactant frequencies with detected product frequencies. Solvent and temperature-dependent variations enable comparisons with microscopic rate theories at an unprecedented level of detail. 2D-IR spectroscopy is also emerging as a powerful probe of non-equilibrium light-driven chemical transformations. Transient 2D-IR spectroscopy is able to follow nascent photoproducts caused by electronic excitation or by a temperature jump. Soon it will be possible to study transient species with the full range of 2D observables, such as line shapes and waiting-time dynamics that have motivated the wide adoption of equilibrium 2D-IR spectroscopy. This review summarises the general progress in using 2D-IR spectroscopy to study chemical reactions in solution, focusing on our investigations into reactions of isomerisation of CO2(CO)8, photodissociation of Mn2(CO)10, geminate rebinding in [CpMo(CO)3]2 and charge transfer in betaine-30 as viewed from the first solvation shell.

Transient Vibrational Echo versus Transient Absorption Spectroscopy: A Direct Experimental and Theoretical Comparison

Transient dispersed vibrational echo (DVE) spectroscopy is a practical alternative to transient-absorption spectroscopy because it affords increased sensitivity as well as greater signal-to-noise ratio without the need to detect a reference spectrum. However, as a third-order nonlinear probe, the extraction of kinetic information from transient-DVE is somewhat cumbersome compared to transient absorption. This article provides a direct experimental and theoretical comparison between transient-absorption and transient-DVE measurements and presents a framework for analyzing kinetic measurements while exploring the implications of making some simplifying assumptions in the data analysis. The equations for computing the signal-to-noise ratios under different experimental conditions are derived and used in the analysis of the experimental data. The results, obtained under the same experimental conditions, show that for a relatively strong terminal carbonyl stretching mode, signal-to-noise ratios in transient-DVE spectroscopy are approximately 2.5 times greater than transient absorption. The experimental results along with the theoretical models indicate that transient-DVE could become an attractive alternative to transient-absorption spectroscopy for measuring the kinetics of light-induced processes.

Watching Chemical Reactions and Dynamics with Ultrafast Multidimensional Infrared Spectroscopy

Using equilibrium and non-equilibrium multidimensional infrared spectroscopy, we can track chemical reactions with exquisite detail. Examples we will discuss include: ultrafast chemical exchange, bimolecular recombination, charge transfer and solvation dynamics.

Triggered-exchange Two-dimensional Infrared Spectroscopy of Metal Carbonyl Photodissociation Dynamics

We present an ultrafast study of the dissociation dynamics of a metal carbonyl complex using transient Fourier transform 2DIR spectroscopy.

Vibrational Coherence Decay in Metal Carbonyls: Solvent Dependence of Coherence Lifetimes Studied with MDIR

Multidimensional infrared spectra of a metal carbonyl in different solvents are presented as a function of the waiting time, t 2. The evolution of each peak is related to excitedstate coherences and the relative correlation between excited state vibrational energies.