Triply resonant sum frequency (TRSF) and doubly vibrationally enhanced (DOVE) spectroscopies are examples of a recently developed family of coherent multidimensional spectroscopies (CMDS) that are analogous to multidimensional NMR and current analytical spectroscopies. CMDS methods are particularly promising for analytical applications because their inherent selectivity makes them applicable to complex samples. Like NMR, they are based on creating quantum mechanical superposition states that are fully coherent and lack intermediate quantum state populations that cause quenching or other relaxation effects. Instead of the nuclear spin states of NMR, their multidimensional spectral fingerprints result from creating quantum mechanical mixtures of vibrational and electronic states. Vibrational states provide spectral selectivity, and electronic states provide large signal enhancements. This paper presents the first electronically resonant DOVE spectra and demonstrates the capabilities for analytical chemistry applications by comparing electronically resonant TRSF and DOVE spectra with each other and with infrared absorption and resonance Raman spectra using a Styryl 9 M dye as a model system. The methods each use two infrared absorption transitions and a resonant Raman transition to create a coherent output beam, but they differ in how they access the vibrational and electronic states and the frequency of their output signal. Just as FTIR, UV–vis, Raman, and resonance Raman are complementary methods, TRSF and DOVE methods are complementary to coherent Raman methods such as coherent anti-Stokes Raman spectroscopy (CARS).

Hybrid organic–inorganic perovskites demonstrate desirable photophysical behaviors and promising applications from efficient photovoltaics to lasing, but the fundamental nature of excited state species is still under debate. We collected time-resolved photoluminescence of single-crystal nanoplates of methylammonium lead iodide perovskite (MAPbI3) with excitation over a range of fluences and repetition rates to provide a more complete photophysical picture. A fundamentally different way of simulating the photophysics is developed that relies on unnormalized decays, global analysis over a large array of conditions, and inclusion of steady-state behavior; these details are critical to capturing observed behaviors. These additional constraints require inclusion of spatially correlated pairs along with free carriers and traps, demonstrating the importance of our comprehensive analysis. Modeling geminate and nongeminate pathways shows that geminate processes are dominant at high carrier densities and early times and that geminate recombination is catalyzed by free holes. Our combination of data and simulation provides a detailed picture of perovskite photophysics across multiple excitation regimes that was not previously available.

•Complex materials need highly selective spectroscopic methods.•Fully coherent CMDS creates highly selective multidimensional spectral fingerprints.•Fully coherent fingerprints are invariant and independent of relaxation dynamics.•Pump-fully coherent CMDS probe constrains dynamics to pump-probe delay.Spectroscopy is a dominant measurement methodology because it resolves molecular level details over a wide concentration range. Its limitations, however, become challenged when applied to complex materials. Coherent multidimensional spectroscopy (CMDS) is the optical analogue of multidimensional NMR and like NMR, its multidimensionality promises to increase the spectral selectivity of vibrational and electronic spectroscopy. This article explores whether this promise can make CMDS a dominant spectroscopic method throughout the sciences. In order for CMDS to become a dominant methodology, it must create multidimensional spectral fingerprints that provide the selectivity required for probing complex samples. Pump-CMDS probe methods separate the pump’s measurement of dynamics from a multidimensional and selective probe. Fully coherent CMDS methods are ideal multidimensional probes because they avoid relaxation effects, spectrally isolate the output signals, and provide unique and invariant spectral signatures using any combination of vibrational and electronic quantum states.

We report the first coherent multidimensional spectroscopy study of a MoS2 film. A four-layer sample of MoS2 was synthesized on a silica substrate by a simplified sulfidation reaction and characterized by absorption and Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. State-selective coherent multidimensional spectroscopy (CMDS) on the as-prepared MoS2 film resolved the dynamics of a series of diagonal and cross-peak features involving the spin–orbit split A and B excitonic states and continuum states. The spectra are characterized by striped features that are similar to those observed in CMDS studies of quantum wells where the continuum states contribute strongly to the initial excitation of both the diagonal and cross-peak features, while the A and B excitonic states contributed strongly to the final output signal. The strong contribution from the continuum states to the initial excitation shows that the continuum states are coupled to the A and B excitonic states and that fast intraband relaxation is occurring on a sub-70 fs time scale. A comparison of the CMDS excitation signal and the absorption spectrum shows that the relative importance of the continuum states is determined primarily by their absorption strength. Diagonal and cross-peak features decay with a 680 fs time constant characteristic of exciton recombination and/or trapping. The short time dynamics are complicated by coherent and partially coherent pathways that become important when the excitation pulses are temporally overlapped. In this region, the coherent dynamics create diagonal features involving both the excitonic states and continuum states, while the partially coherent pathways contribute to cross-peak features.Keywords: 2D; molybdenum sulfide; multidimensional; nonlinear; transition metal dichalcogenides; ultrafast dynamics;

This work demonstrates the use of triply resonant sum frequency (TRSF) spectroscopy as a “resonance IR” analogue to resonance Raman spectroscopy. TRSF is a four-wave-mixing process where three lasers with independent frequencies interact coherently with a sample to generate an output at their triple summation frequency. The first two lasers are in the infrared and result in two vibrational excitations, while the third laser is visible and induces a two-quantum anti-Stokes resonance Raman transition. The signal intensity grows when the laser frequencies are all in resonance with coupled vibrational and electronic states. The method therefore provides electronic enhancement of IR-active vibrational modes. These modes may be buried beneath solvent in the IR spectrum and also be Raman-inactive and therefore inaccessible by other techniques. The method is presented on the centrosymmetric complex copper phthalocyanine tetrasulfonate. In this study, the two vibrational frequencies were scanned across ring-breathing modes, while the visible frequency was left in resonance with the copper phthalocyanine tetrasulfonate Q band, resulting in a two-dimensional infrared plot that also reveals coupling between vibrational states. TRSF has the potential to be a very useful probe of structurally similar biological motifs such as hemes, as well as synthetic transition-metal complexes.

In this paper we present a new multiresonant coherent multidimensional spectroscopy (CMDS) technique employing a pathway that is both fully coherent and necessarily unique. This technique is based on a Triple Sum Frequency (TSF) coherence pathway with three excitation pulses having frequencies ω1, ω2, and ω3 and the phase matching condition k⃗1 + k⃗2 + k⃗3. Two-dimensional spectra are created by independently tuning the ω1 and ω2 pulses across vibrational resonances while monitoring the intensity of a visible output beam created by a Raman transition induced by the ω3 pulse. Two-dimensional plots of the coherent dynamics are created by independently scanning the τ21 and τ31 delay times between the different frequency excitation pulses over all time orderings. TSF CMDS separates fundamental and overtone/combination band states uniquely onto the ω1 and ω2 axes when τ21 ≠ 0. TSF is valuable in its ability to probe states of complementary parity to those seen in Doubly Vibrationally Enhanced Four-Wave Mixing (DOVE-FWM), the other fully coherent mixed electronic/vibrational CMDS method. This capability is demonstrated through the use of neat benzene as a model system, where the center of inversion imposes strict parity selection rules.

Nuclear magnetic resonance spectroscopy relies on using multiple excitation pulses to create multiple quantum coherences that provide great specificity for chemical measurements. Coherent multidimensional spectroscopy (CMDS) is the optical analogue of NMR. Current CMDS methods use three excitation pulses and phase matching to create zero, single, and double quantum coherences. In order to create higher order multiple quantum coherences, the number of interactions must be increased by raising the excitation intensities high enough to create Rabi frequencies that are comparable to the dephasing rates of vibrational coherences. The higher Rabi frequencies create multiple, odd-order coherence pathways. The coherence pathways that involve intermediate populations are partially coherent and are sensitive to population relaxation effects. Pathways that are fully coherent involve only coherences and measure the direct coupling between excited quantum states. The fully coherent pathways are related to the multiple quantum coherences created in multiple pulse NMR methods such as heteronuclear multiple quantum coherence (HMQC) spectroscopy with the important difference that HMQC NMR methods have a defined number of interactions and avoid dynamic Stark effects whereas the multiply enhanced odd-order wave-mixing pathways do not. The difference arises because CMDS methods use phase matching to define the interactions and at high intensities, multiple pathways obey the same phase matching conditions. The multiple pathways correspond to the pathways created by dynamic Stark effects. This paper uses rhodium dicarbonyl chelate (RDC) as a model to demonstrate the characteristics of multiply enhanced odd-order wave-mixing (MEOW) methods. Dynamic Stark effects excite vibrational ladders on the symmetric and asymmetric CO stretch modes and create a series of multiple quantum coherences and populations using partially and fully coherent pathways. Vibrational quantum states up to v = 6 are excited. A series of spectra provides different two-dimensional cross sections through the multidimensional parameter space involving two excitation frequencies, the frequency of the output coherence, and the excitation pulse time delays. The spectra allow the identification of 18 different overtone and combination band states. Comparison with a local mode model with two anharmonic Morse oscillators with interbond coupling shows excellent agreement.

Extending current coherent multidimensional spectroscopy (CMDS) methods to higher order multiwave mixing requires excitation intensities where dynamic Stark effects become important. This paper examines the dynamic Stark effects that occur in mixed frequency/time domain CMDS methods at high excitation intensities in a model system with an isolated vibrational state. The phase-matching restrictions in CMDS define the excitation beams that interact by nonlinear mixing while the dynamic Stark effects create vibrational ladders of increasingly more energetic overtone and combination band states. The excited quantum states form coherences that reemit the output beams. This paper uses the phase-matching conditions k⃗out = k⃗1 − k⃗2 + k⃗2′ and k⃗out =− k⃗1 + k⃗2 + k⃗2′, where the subscripts denote the excitation frequencies of each excitation pulse and the output pulse. The phase-matching condition constrains each pulse to have an odd number of interactions so the overall mixing process that creates the output coherence must also involve an odd number of interactions. Tuning the excitation frequencies and spectrally resolving the output intensity creates three-dimensional spectra that resolve the individual overtone states. Changing the excitation pulse time delays measures the dynamics of the coherences and populations created by the multiple excitations. The multidimensional spectra probe the highly excited states of a molecular potential energy surface. This paper uses tungsten hexacarbonyl (W(CO)6) as a model for observing how dynamic Stark effects change the multidimensional spectra of a simple system. The simplicity of the W(CO)6 system provides the experimental data required to develop the nonperturbative theoretical methods that will be necessary to model this new approach to CMDS.

A new family of coherent multidimensional vibrational spectroscopies has been developed which provide opportunities for establishing the correlations between vibrational modes that result from intra- and intermolecular interactions. The multidimensional spectroscopies contain cross-peaks between the modes because of coupling where the oscillations from one mode affect the optical properties of other modes. This paper presents an experimental overview of representative methods for coherent multidimensional vibrational spectroscopy.