Mapping the Vibronic Structure of a Molecule by Few-Cycle

Jul 31, 2014 - spectroscopy in the IR, visible, and UV have been used to examine the electronic structure of atomic alkali vapors,9 many- body interac...
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Mapping the Vibronic Structure of a Molecule by Few-Cycle Continuum Two-Dimensional Spectroscopy in a Single Pulse Boris Spokoyny and Elad Harel* Northwestern University, Department of Chemistry, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: Accurate mapping of the electronic and vibrational structure of a molecular system is a basic goal of chemistry as it underpins reactivity and function. Experimentally, the challenge is to uncover the intramolecular interactions and ensuing dynamics that define this structure. Multidimensional coherent spectroscopy can map such interactions analogous to the way in which nuclear magnetic resonance provides access to the nuclear spin structure. Here we present two-dimensional coherent spectra measured using few-cycle continuum light. Critically, our approach instantaneously maps the energy landscape of a complex molecular system in a single laser pulse across 350 nm of bandwidth, thereby making it suitable for rapid molecular fingerprinting. We envision few-cycle supercontinuum spectroscopy based on the nonlinear optical response as a powerful tool to examine molecules in the condensed phase at the extremes of time, space, and energy. SECTION: Spectroscopy, Photochemistry, and Excited States

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structure across an unprecedented spectral range, while facilitating its practical implementation. Furthermore, it allows the use of 2D spectroscopy on systems that undergo irreversible chemical or physical changes such as photobleaching or phase transitions, where multiscan approaches may lead to erroneous spectral interpretation. Specifically, we employ ultrashort, few-cycle supercontinuum pulses spanning the visible and near-infrared (NIR) region of the electromagnetic spectrum (500−850 nm) in order to excite all the transition dipoles in the system simultaneously within the source bandwidth, and immediately record the full third-order optical response with single-shot detection by GRadientAssisted Photon Echo Spectroscopy (GRAPES). We call this new method WHITE GRAPES to emphasize its use of whitelight generation. Critically, WHITE GRAPES greatly increases the information content of current multidimensional spectroscopic methods as measured in the spectral surface area despite extremely short acquisition times. Another important feature of WHITE GRAPES is that it may be used under relatively standard laboratory conditions with readily available femtosecond amplified laser sources (∼100 fs), which, with the aforementioned advantages, may pave the way for a new benchtop analytical characterization method based on the thirdorder optical response. We demonstrate WHITE GRAPES on a cyanine dye, which exhibits a broad, featureless absorption band in the visible and NIR regions, but exhibits significant electronic/vibrational structure in the WHITE GRAPE spectrum.

ultidimensional coherent optical spectroscopy is a powerful method to measure the electronic-vibrational structure and dynamics of molecules in the condensed phase at the femtosecond time scale.1−4 Inspired by multidimensional NMR,5 multidimensional optical spectroscopy spreads the response of a system across two or more spectral dimensions. These spectral maps reveal high-order electronic and vibrational correlations that are obscured in linear or onedimensional nonlinear spectra broadened by bath-induced relaxation and congested vibronic transitions. The great advantage of such correlation maps is that they provide insight into the quantum states of the system and their interactions with the fluctuating bath, thereby providing a link between structure and dynamics.6−8 Two and higher-dimension optical spectroscopy in the IR, visible, and UV have been used to examine the electronic structure of atomic alkali vapors,9 manybody interactions in semiconductors,10,11 the dynamics of formation and dissociation of solute−solvent complexes,12 and dynamics of energy transfer in photosynthetic proteins,7,13 to name a few. Yet, despite comparisons to NMR, multidimensional optical spectroscopy remains far from the analytical powerhouse of its nuclear spin cousin, owing to both technical and physical limitations. In the radiofrequency region of the spectrum, NMR14,15 owes its multiplexing advantage to the use of broadband pulses that span the entire chemical-shift range of the nuclei under study. This greatly improves the signal-to-noise and facilitates massively parallel acquisition. In this paper, we demonstrate a means to collect two-dimensional photon echo spectra (2D PES) in the optical regime analogous to 2D NMR by combining supercontinuum generation and single-shot detection. This method greatly advances the analytical utility of the measurement by capturing the electronic and vibrational © 2014 American Chemical Society

Received: June 16, 2014 Accepted: July 31, 2014 Published: July 31, 2014 2808

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Figure 1. WHITE GRAPES experimental setup. (A) Two-stage continuum generation. (B) Multiple pulse generation. (C) Single-shot imaging spectrometer. The raw signal after LO subtraction is shown. C.M. = chirped mirror, B.S. = beam splitter, P.S. = periscope, Cyl. M = cylindrical mirror, S.M. = spherical mirror.

Figure 2. (A) 2D PES pulse sequence. (B) Experimentally determined pulse delays as a function of vertical position on camera (Pixel axis). (C) Transient-grating frequency-resolved optical gating (TG-FROG) showing sub-6 fs continuum pulse spanning the 500−850 nm region of the spectrum.

Experiment. To generate high power supercontinuum (SC) light, we use two-stage filamentation similar in design to the setup of Hagemann et al.16 Starting with a 100 fs transform limited pulses at 1 kHz repetition rate, we achieve 25-fold spectral broadening and 18-fold temporal shortening; more details are presented in the Methods section. The resulting pulse spans 500−850 nm with a pulse duration of sub-6 fs. The apparatus for pulse broadening and single-shot multidimensional spectroscopy is depicted in Figure 1. The compressed SC pulse is sent through a series of dispersion-compensated beam splitters to form the four pulses needed for heterodyne detected four-wave mixing. Three pulses serve to excite the sample along a series of coherence pathways while the fourth pulse acts as a

heterodyne pulse for recovering the phase of the generated signal field. The first pulse, k1, generates a coherence in the sample that evolves for a time τ, thereby recording the absorption frequencies in the time domain. A second pulse, k2, then stores this evolved phase to be read out later during detection. After a “waiting time,” T, the third pulse, k3, returns the system to a coherence where an echo forms a time t later. The final coherence is recorded in the frequency domain and the phase evolution that occurred during τ is resolved by heterodyning the emitted signal with the local oscillator (LO) pulse, kLO. The third-order nonlinear polarization induced by the series of pulses then acts as a source for the emitted electric field,17 which is measured as a function of the two delay times, τ 2809

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Figure 3. (A) Molecular structure of IR-144. (B) Linear absorption spectrum and transient absorption spectrum at 300 fs pump−probe delay. (C) Real part of 2D WHITE GRAPE spectrum at T = 300 fs waiting time. Zoomed in 2D spectrum in the 725−800 nm region.

and T, for each emitted frequency, ωt. In point-by-point sampling, each of these delays must be parametrically sampled,2 leading to long acquisition times and data that is highly susceptible to noise during the acquisition window. To achieve single-shot detection, we use a method invented by Harel et al. called GRadient-Assisted Photon Echo Spectroscopy (GRAPES). The details of GRAPES are described elsewhere.18−20 In short, GRAPES maps coherence delays onto a spatial axis of the sample resulting in a parallel acquisition of what would otherwise be individually collected data points (Figure 2). This is achieved by using a geometry that effectively tilts the pulse fronts of the beams in order to create a linear delay along one spatial axis. A major advantage of GRAPES is that by capturing the entire 2D spectrum in one or few shots (e.g. for weak absorbers that requires signal averaging), it greatly reduces indirect noise, while making no sacrifice in signal. While this does come at the cost of more excitation power as focusing only occurs along one dimension, the high power supercontinuum source described here easily fulfills these requirements. Conventional multiscan multidimensional coherent spectroscopies2,21,22 typically cannot incorporate such high power supercontinuum sources due to the inherent nonlinearity and instability of filamentation-based supercontinuum generation.23,24 In multiscan methods, small fluctuations in laser power create noise across the entire spectrum when sampled in the Fourier domain. As the bandwidth of excitation increases, so do the demands placed on phase stability. GRAPES circumvents these limitations by capturing the entire 2D spectrum in a single laser shot on a time scale in which all other mechanical fluctuations are negligible. This insensitivity to phase instability is an important advantage and greatly facilitates the use of the ultrabroadband, few-cycle, high-power continuum source described here.

By utilizing an entirely reflective, dispersion-free optical design analogous to those used for multiscan acquisition,25 the modified GRAPES design (Figure 1) introduced here is suitable for continuum sources. The beams are arranged in a boxcar geometry before reflecting off a series of four mirrors with adjustable vertical tilt. Beams 2 and 3 reflect off two of these mirrors in parallel, while beams 1 and 2 are directed in opposite vertical directions. After focusing by a common cylindrical mirror in the horizontal direction, all beams align in the vertical and horizontal direction at the geometric focus. This creates a temporal gradient along the plane axis of the cylindrical mirror between pulses 1 and 2/3, as well as between LO and 2/3. The signal is now emitted in the form of a narrow line in which the vertical axis maps uniquely to different 1/2 delay (i.e., τ in the 2D pulse sequence, Figure 2). The signal is then reimaged using a spherical mirror onto the slit of a high-resolution imaging spectrometer, which spectrally resolves the signal in the horizontal direction. A high-speed, cooled CMOS camera is then used to record the signal which, after appropriate probe and scatter subtraction routines (see Experimental Methods), corresponds to S(τ,T,ωt)ELO*(τ,T). The 2D spectrum is then recovered by Fourier transformation of S(τ,T,ωt) along the τ direction to yield S(ωτ,ωt) for each waiting time T, where ωτ is the coherence axis and ωt is the rephasing axis. Single-Shot 2D Spectra of a Cyanine Dye f rom a Continuum Source. The 2D WHITE GRAPE spectrum of the laser dye, anhydro-11-(4-ethoxycarbonyl-1-piperazinyl)-10,12-ethylene3,3,3′,3′-tetramethyl-1,1′-di(3-sulfopropyl)-4,5;4′,5′-dibenzoindotricarbocyanine hydroxide (IR-144) in methanol, at T = 300 fs is shown in Figure 3. We chose IR-144 (Figure 3A) because it was the first molecule studied using two-dimensional electronic spectroscopy by the group of Jonas.26 That work, along with others of IR-144 examined the relatively narrow region from about 780−820 nm.3,20 Here, we use IR-144 to 2810

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observing these details possible. Two salient features are immediately apparent: the separation into multiple bands representing ground-state bleach, stimulated emission, and excited-state absorption in different regions of the 2D spectrum. Many of these bands contain nearly equally spaced peaks along both the coherence and rephasing axis. These features are a result of the vibrational manifold of states on the ground state and on the S1 and higher lying SN electronically excited states of the molecule. These states are not visible in the linear absorption spectrum nor are they well-resolved in TA, which loses all resolution along the coherence frequency axis. In contrast, 2D PES gives cross-peaks that do resolve the underlying structure. In a monomeric system in a dilute solution, these cross peaks arise from vibrational states on the same or different electronic states that share a common ground state. Here, we attempt only to assign features qualitatively, while leaving more specific assignments and analysis to future work, with the aim of showing the richness of detail in the spectrum and highlighting features obscured in TA measurements. The bands, labeled I, in the 725−800 region show a characteristic multipeak pattern found in 2D spectra of other cyanine dyes29more easily seen in the zoomed-in figure. This pattern is a result of Feynman pathways that connect the ground and S1 state, each coupled to one excited vibrational state. Owing to the similar shapes of the ground and S1 potentials, the strong and apparently broadened center bands in this region are in fact from two dipole-allowed transitions with nearly equal energy gaps between potential surfaces. The main peak lies slightly above the diagonal owing to a ∼300 cm−1 Stokes shift from solvent reorganization that lowers the emission frequency. The band (II) is a result of excited-state absorption (675−750 nm), lying above the diagonal and appearing at relatively early waiting times (T > 50 fs). It is broadened along the coherence axis, but shows three distinct bands along the rephasing axis. These bands align with those in region I, indicating that both involve pathways connecting the S1 state, with those in region II ending up in higher (N ≥ 2) electronic states and involving only one quanta of vibrational energy. Note that because these features are relatively weak compared to the strong ground-state bleach features in region I, the projection of the 2D spectrum along λτ effectively obscures them. Therefore, the TA spectrum, which is formally the projection of the 2D spectrum according to the projection-slice theorem, does not resolve such features at this waiting time period. Region III is a result of stimulated emission, again involving the S1 state, but now with emission from another slightly lower lying state, with which it is coupled. These features only arise after about 100 fs waiting time and therefore likely arise after internal conversion to this lower lying state− consistent with other TA measurements that also hypothesized a rapid internal conversion process from S1 in related cyanine molecules.30 Again, the splitting along λτ is obscured in the TA spectrum, but in the 2D spectrum indicate the presence of at least one vibrational quanta contributing to the nonlinear response. The bands in region IV at higher absorption frequency only appear at T > 100 fs waiting time. These bands also arise from stimulated emission and show substantial structure along both λt and λτ. While assignment is difficult by looking at only a few waiting time spectra, these features are also consistent with an internal conversion process, now from a higher lying excited singlet states (SN) to SM (M < N) followed by stimulated emission back to S0. Finally, region V is due to a

demonstrate the power of WHITE GRAPES by utilizing its enormous spectral coverage to map a large portion of the electronic and vibrational structure of the molecule. The spectrum, shown in Figure 3, shows features in an area that spans 500−810 nm along the emission axis (λt) and 600−815 nm along the absorption axis (λt). The entire 2D spectrum was acquired in one laser shot, corresponding to less than 100 μs of acquisition time−limited by the camera integration time and the laser repetition rate, and not by the signal-to-noise. Other systems with weaker absorbers may require longer acquisitions but still achieve a minimum savings of about two-orders of magnitude in sampling time compared to methods that sample τ at optical frequencies2 and about an order of magnitude savings compared to rotating-frame, pulse shaping methods.21,27 Conventional 2D PES has two major advantages over linear spectroscopy and one-dimensional nonlinear spectroscopy. Because 2D PES measures the full third-order optical response of system, it can reveal correlated statesthose connected by a common ground state. By correlating the dipole oscillations during the first period, τ, with that during the final period, t, 2D PES records diagonal and off-diagonal peaks that provide information on population relaxation and electronic or vibrational couplings, respectively. A second major advantage is that two-dimensional spectroscopy eliminates inhomogeneous broadening along the direct t-domain, revealing the homogeneous line width. This makes it a particularly powerful probe for resolving states that are otherwise hidden in the linear absorption spectrum. For WHITE GRAPES, there are two additional major advantages: (1) the ability to examine the entire energy range of transition dipoles in the molecule covered by the continuum spectrum without a trade-off in spectraltemporal resolution of one-dimensional nonlinear spectroscopies such as transient absorption (TA) and (2) by recording the signal in a single laser shot, WHITE GRAPES greatly enhances the signal-to-noise while reducing the possibility of photodamage, bleaching, or other laser-induced changes in the sample that would lead to artifacts in the 2D spectrum. The advantages of broad spectral coverage and high signalto-noise arising from single-shot acquisition are readily apparent in the real-valued 2D WHITE GRAPES spectrum of IR-144 (Figure 3C). The linear absorption spectrum shows few resolvable features, a dominant broad peak centered at 765 nm and very weak features at 553 and 589 nm (Figure 3B). The main features in the linear spectrum are approximately reproduced (minus the effects of excited-state absorption) by comparison to a cut through the diagonal of the 2D spectrum at early (T < 20 fs) waiting times (not shown). Previous TA measurements28 on IR-144 show more structure−positive absorption bands at 450 and 580 nm and negative band at 720 and 840 nm, upon excitation at 532 nm. The band at 720 nm is a ground-state bleach, and the 840 nm band is stimulated emission by the broadband probe. A TA spectrum at 300 fs with continuum excitation and detection using the same WHITE GRAPES setup (see Experimental Methods) is shown in Figure 3B, exhibiting qualitatively similar features to the previous study. This spectrum was used to phase the 2D WHITE GRAPE spectrum and obtain its real component (see Experimental Methods). As shown in Figure 3C, the WHITE GRAPE spectrum shows rich detail, which maps directly to the electronic and vibrational structure of the molecule. The large spectral surface area afforded by WHITE GRAPES and the narrowed line widths inherent in the photon echo makes 2811

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the supercontinuum was measured to be ∼300 μJ/pulse in the 500−850 nm range with ∼25 μJ/pulse of energy in the 700− 550 nm region, although only a small portion of the total available power was used for the GRAPES experiment. Pulse Characterization. A home-built Transient Grating Frequency Resolved Optical Gating (TG-FROG)32 apparatus was used to perform all pulse characterization. TG-FROG pulse characterization is particularly well suited for measuring fewcycle pulses in GRAPES since it enables unambiguous characterization of temporal and spatial chirp as well as wavelength dependent pulse-front tilt.33 The white light supercontinuum was expanded by a telescope and then passed through a metallic pinhole array, which generated three beams in a 5 mm square boxcar geometry. Fused silica windows were introduced into the beam path to exactly match the amount of dispersion experienced by the beams in the WHITE GRAPES experiment. The first two beams of the TG-FROG sequence impinged on a common, stationary mirror that ensured their temporal overlap. The third beam mirror was translated by means of a nanometer-resolution linear stage (Aerotech, Soloist). All three beams were then focused with a concave mirror into a 0.2 mm thick fused-silica window. The generated TG signal was isolated by an iris followed by collimation and refocusing by two off-axis parabolic mirrors into a 150 μm core diameter fiber. The signal was then spectrally resolved by a high-resolution USB spectrometer (Ocean Optics, USB2000+). The TG-FROG trace was generated by collecting a series of signal spectra as a function of beam 3 delay. Timing Procedure. The temporal gradient between beams 1,2 and 3,LO are measured in a single shot by spectral interferometry.34 The 1,2 gradient was measured to be 270 ± 4 attosecond/pixel and the 3,LO gradient was −276 ± 8 attosecond/pixel. Both gradient values are needed to retrieve the correct frequency axes of the 2D spectrum. 2D Pulse Sequence and Data Collection. The white light supercontinuum beam generated by two-stage filamentation was split into four pulses by means of 0.5 mm thick ultrabroadband beam splitters (Layertec). The relative delays between the beams were manipulated by passing beams 2, 3, and 4 off retroreflectors mounted on motorized linear stages (Newport, XPS). All four beams were then focused into a 400 μm, quartz flow-cell using a concave cylindrical mirror which produced a maximum ∼200 fs of coherence time gradient across the sample. Beams 1−3 had a pulse energy of 400 nJ/ pulse or energy density of 60 μJ/cm2, while the LO was attenuated by 10−2 by reflection off a beam sampler. The signal, emitted as a narrow line from the focal plane of the cylindrical mirror was reimaged onto the slit of a high resolution imaging spectrometer (Horiba, iHR320). During data collection, only the signal and the collinear LO were allowed into the spectrometer. The detection was carried out by using a thermoelectrically cooled, sCMOS camera (Andor Neo) with an exposure of 100 μs. Scattering of beams 1, 2, and 3 in the direction of the local oscillator was subtracted from the data. Transient Absorption. TA spectra were measured on the same setup as the GRAPES experiment. Beam 1 acted as the pump and the LO beam acted as the probe. The timing between 1 and LO were measured at each vertical position on the camera by spectral interferometry. Beam 1 was then set to a particular pump−probe time delay, and images of the probe were recorded with the pump on and off. The transient absorption spectrum was then recovered by subtracting pump-on minus pump-off probe spectra.

broad excited-state absorption from either the S1 state or the lower lying coupled state involved in region III, to the SN states. After the initial absorption, vibrational wavepacket propagation and relaxation on the S1 surface occurs before excited state absorption into the SN manifold a time T later. The vibrational wave packet on the S1 state is thought to survive for hundreds of femtoseconds to picoseconds after internal conversion, supporting this assignment.30 Most of the features in this band are captured by the TA spectrum, which is not surprising given the featureless absorption features along λτ. The increased broadening compared to bands in region I are a result of much faster decoherence times within the higher lying electronically excited states (S2,...,SN). More specific assignments can be discerned by examining the 2D spectrum as a function of the waiting time, T. Pathways that are static or exhibit quantum coherence provide a more direct link between the signal and the specific Feynman pathway where it originated.29 We also note that from the large bandwidth of excitation, we expect to see simultaneous electronic and vibrational coherences in these monomeric systems, perhaps lending insight into the debate over the nature of coherence in photosynthetic systems.7,13,31 These inquiries, as well as more specific spectral assignments, will be the topic of future work. The results presented in this work are intended to demonstrate the power of WHITE GRAPES to examine the vibrational and electronic structure of a complex system in its entirety. It represents the most advanced third-order optical spectroscopy ever demonstrated to date in terms of bandwidth, time resolution, and acquisition time. In the field of ultrafast science, WHITE GRAPES may be used to study a host of processes on the subpicosecond time scale with sub-6 fs temporal resolution−approaching the ultimate temporal resolution and bandwidth possible in the visible-NIR region of the spectrum. Outside of ultrafast science, WHITE GRAPES may be a powerful, and, general analytical technique to identify systems by their optical response in two dimensions instead of one, thereby providing specificity not possible with traditional UV/vis or Raman spectroscopic methods. The single-shot nature of the technique, coupled to its use with relatively readily available 100 fs light sources, eliminates the stringent stability requirements of other two-dimensional coherent spectroscopies. We envision that this advance will help proliferate multidimensional optical spectroscopy to a much wider user base.

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EXPERIMENTAL METHODS Supercontinuum Generation. The white light supercontinuum used in the above WHITE GRAPES experiments was generated using two sequential filamentation stages similar to the setup of Hagemann et al.16 The output of a regenerative Ti:Saph amplifier (Coherent, Legend Elite) was focused into a pipe filled with 1 bar of Argon, capped with 1 mm thick fused-silica windows. The initial laser spectrum had a 15 nm full-width at half-maximum (fwhm) bandwidth centered at 800 nm, ∼100 fs pulse duration, and was attenuated to a power of ∼1 mJ/pulse. The resulting pulse out of the pressurized pipe was spectrally broadened to 65 nm fwhm, collimated with a concave mirror and precompressed to 20 fs with five double-bounces off chirped mirrors (Layertec). The beam was then focused into air at ambient pressure. The resulting white light supercontinuum was collimated with a concave mirror and finally compressed to a fwhm duration of ∼6 fs with three double-bounces on a second set of chirped mirrors (Layertec). The total power of 2812

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“Phasing” the 2D Spectrum. In general, the detected signal, while phase resolved through heterodyne detection, has an unknown phase term that arises from uncertainty in the phase of the excitation and local oscillator pulses.3,22 To properly “phase” the spectrum, we apply the projection-slice theorem,5,35 which states that the projection of the 2D spectrum along the coherence axis is equal to the spectrally resolved pump probe. The pump−probe signal is acquired as described above for each waiting time and then a phase term up to second order in the rephasing frequency is applied to the signal and matched through a nonlinear least-squares procedure. Sample Preparation. The laser dye IR-144 (Exciton) was dissolved in methanol to create a 0.5 mM solution with an optical density of approximately 0.4 at 780 nm in a 400 μm fused quartz flow cell (Starna). Solutions were prepared immediately prior to the experiment to avoid sample degradation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Army Research Office (W911NF-13-1-0290), the Air Force Office of Scientific Research (FA9550-14-1-0005), and the Packard Foundation (2013-39272) in part.

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