Coherent multidimensional vibrational spectroscopy (CMDVS) is an emerging field that offers new oportunities to probe specifically intramolecular and intermolecular interactions in complex samples with the improved resolution and selectivity expected of a multidimensional method. In particular, it provides structural and dynamical information about correlations between modes. It provides this information on the picosecond time scales characteristic of vibrational dephasing. CMDVS is based on the perturbations that are induced in a mode when other coupled modes are excited. It is the vibrational analogue to multidimensional nucler magnetic resonance (NMR). In this review, we present the background that led to the development of CMDVS, a brief summary of the different methods of non-linear laser spectroscopy and how they can achieve CMDVS, the relationships between CMDVS and multidimensional NMR, and the theory necessary to understand the experimental results. The experimental approaches reviewed fall into ...

Coherent multidimensional vibrational spectroscopy

Efficiency of optical second harmonic generation from pentacene films of different morphology and structure

Multiresonant coherent multidimensional spectroscopy is a frequency-domain method that uses tunable excitation pulses to excite multiple quantum coherences (MQCs) and/or state populations using fully coherent or partially coherent excitation pathways. Pairs of states that are coupled by intra- and intermolecular interactions re-emit light at their frequency differences. The MQCs are coherent and interfere constructively to create phase-matched output beams. Scanning the excitation frequencies with fixed-excitation-pulse time delays creates multidimensional spectra, whereas scanning the time delays with fixed excitation frequencies measures the MQCs’ coherent and incoherent dynamics. Multiresonant methods can excite any combination of vibrational and/or electronic states and use any coherence pathway. Cross-peaks occur between states when the excitation of one perturbs the other. This requirement for coupling acts to eliminate spectral congestion. Spectral resolution is increased because multiresonant meth...

Multiresonant coherent multidimensional spectroscopy.

Recent research has expanded the capabilities of four-wave mixing by providing it with component selectivity, site selectivity, and mode selectivity. The selectivity is achieved by taking advantage of the three resonance enhancements that occur in a four-wave mixing process. New spectral scanning strategies allow one to scan a single resonance while maintaining the other two resonances at constant values. The constant resonances can be used to select a specific component, a specific site within an inhomogeneously broadened envelope of a component, and/or a specific vibrational or vibronic mode of that site. The scanned resonance will then contain enhanced features corresponding to the particular component, site, and/or mode that was chosen by the constant resonances. These component and site selective capabilities of the four-wave mixing complement the single vibronic level fluorescence methods. The relative transition intensities from a specific component or site reflect the mode coupling betwee...

Molecular, multiresonant coherent four-wave mixing spectroscopy

A new family of selective four-wave mixing methods, based on the establishment of vibrational nonlinear polarizations with multiple resonances, is proposed. This family includes double-infrared resonances, vibrationally enhanced Raman resonance, and vibrationally enhanced two-photon resonance. These methods are related to traditional Raman and infrared spectroscopy, but the methods are shown to have the capabilities for component and conformer selectivity, line-narrowing of inhomogeneously broadened vibrational transitions, and mode selection. The theoretical foundations for the methods are developed and directed to possible applications.

Theoretical Foundations for a New Family of Infrared Four-Wave Mixing Spectroscopies

A family of nonlinear four-wave mixing techniques that are capable of site-selective organic spectroscopy are presented. Three lasers are used in the methods in order to achieve fully resonant mixing. Three lasers are shown to provide better sensitivity, selectivity, and versatility in the study of ground and excited electronic state vibrational spectroscopy. New approaches become possible in the establishment of resonances that translate the output signal from the normal Stokes or anti-Stokes side of the lasers to intermediate positions that are free of fluorescence interference. These new methods are divided into Multiply Enhanced Parametric Spectroscopy (MEPS) and Multiply Enhanced Nonparametric Spectroscopy (MENS), depending upon the spectroscopic characteristics for site-selective applications. The characteristics of MEPS and MENS are found to be quite different and depend upon the number and separation of the sites, the power of the lasers, the relative shifts of the levels, and the correlation effects in the inhomogeneous broadening. The feasibility of MENS and the site-selective capability of both CARS and MENS is demonstrated experimentally with the use of the pentacene: p-terphenyl system as a model.

Site-Selective Nonlinear Four-Wave Mixing by Multiply Enhanced Nonparametric and Parametric Spectroscopy

Fully resonant, three laser, coherent Stokes Raman scattering (CSRS) is used to probe the vibrational correlations and couplings associated with the S0−S1 transition of azulene in a low temperature mixed molecular crystal. Unlike incoherent resonant emission, CSRS is not complicated by extra resonances due to relaxed emission. The spectra obtained confirm most of the assignments of the S1 vibronic fundamentals made by incoherent spectroscopies. In addition, however, the vibronic counterpart of the vibrational fundamental at 900 cm−1 is correlated to a previously unobserved vibronic resonance at 842.3 cm−1. Evidence of matrix moderated coupling, including possible site splitting and Fermi resonances involving lattice phonons, is given for the 677, 825, 917, 941, and 900 cm−1 vibrational levels.

Dinh C. Nguyen

Papers

Molecular, multiresonant coherent four-wave mixing spectroscopy

Site-Selective Nonlinear Four-Wave Mixing by Multiply Enhanced Nonparametric and Parametric Spectroscopy

Analysis of vibronic mode coupling in pentacene by fully resonant coherent four‐wave mixing

Analysis of Vibronic Mode Coupling in Pentacene by Fully Resonant Coherent Four-Wave Mixing

Four wave mixing spectroscopy of mixed crystals using three input frequencies