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Femtosecond Multi-dimensional Laser Spectroscopic System
Analytical Service Last update : 2017.08.17 Request for use
RI Category
Analytical Facilities
Keywords
Femtosecond time-resolved spectroscopy, Multi-dimensional optical spectroscopy, Chiroptical measurement, Coherent Raman spectroscopy, Transient absorption spectroscopy
Description
Femtosecond Multidimensional Laser Spectroscopic System (FMLS), which was developed and operated by the Seoul Center since 2009, measures fast molecular events on femtosecond time scale and is being used for investigating ultrafast photochemical reaction dynamics of a variety of molecular systems and nanomaterials in chemistry, biology and material science.
- 2D vibrational and electronic spectroscopy in the infrared and visible frequency ranges (lithium ion, gold nanoparticles, photosynthetic system, etc.)
- Pump-probe transient absorption spectroscopy of molecular systems and materials
- Coherent Raman spectroscopy utilizing nonlinear optical effects (SRS, CARS)
Application Area
1. Time-resolved spectroscopy
- Femtosecond transient absorption (TA)
- Nanosecond time-resolved luminescence measurement
- 2D IR (two-dimensional infrared) spectroscopy
- 2D electronic spectroscopy
- Time-resolved CD (circular dichroism) measurement
2. Real time probing of chemical exchange reactions using 2D IR spectroscopy
3. Study on quantum coherence of molecular aggregates using 2D electronic spectroscopy
4. Development of novel femtosecond chiroptical measurement techniques
Service
Specifications
1. Femtosecond Multi-dimensional Laser Spectroscopic System
- Femtosecond laser light source
· Regenerative amplifier: 1 kHz, 3.5 mJ/pulse, ~50 fs
· Optical parametric amplifier (OPA): tunable wavelength 240nm~2600nm, pulse duration <100fs
- Control of optical pulse sequence (for 2D electronic spectroscopy)
· Programmable acousto-optic pulse shaping
· Modulation speed of optical pulse sequence: 1kHz
· Instantaneous bandwidth: 440 nm
· Spectral resolution: 0.2 nm (at 530 nm)
· Maximum pulse delay time: 8 ps for coherence time (pulse1-pulse2), 1.66 ns for waiting time (pulse2-pulse3)
- Detection part
· 1kHz synchronized CCD detector
· Lock in-amplifier with photodiode

Publication

Ki-Hee Song, Munui Gu, Min-Seok Kim, Hyeok-Jun Kwon, Hanju Rhee, Hogyu Han, and Minhaeng Cho, J. Phys. Chem. Lett., 2015, 6 (21), pp 4314–4318, doi: 10.1021/acs.jpclett.5b02030

The origin of quantum coherence in two-dimensional (2D) electronic spectra of molecular aggregates and light-harvesting complexes still remains an open question. In particular, it could be challenging to distinguish between electronic and vibrational coherences for a coupled system, where both degrees of freedom can be simultaneously excited. In this Letter, we examine quantum beats in the 2D spectra of zinc naphthalocyanine (ZnNc) aggregate and monomer, and compare their characteristic features in terms of the frequency and relative phase of diagonal and off-diagonal amplitude oscillations. The long-lasting oscillating components (>1 ps) at 600–700 cm–1 observed in both the aggregate and monomer are found to be attributed to the vibrational coherence. The wide phase variations of the 2D spectral amplitude oscillations are observed not just in the aggregate but also in the monomer state. This suggests that the unusual 90° phase shift may be attributed to neither quantum population-to-coherence transfer nor vibronic exciton coupling.

Junwoo Baek, Joonyoung F. Joung, Songyi Lee, Hanju Rhee, Myung Hwa Kim, Sungnam Park, and Juyoung Yoon, J. Phys. Chem. Lett., 2016, 7 (2), pp 259–265, doi: 10.1021/acs.jpclett.5b02671

Polydiacetylenes (PDAs) with thermochromic properties undergo colorimetric transitions when the external temperature is varied. This capability has the potential to enable these materials to be used as temperature sensors. These thermochromic properties of PDAs stem from their temperature-dependent optical properties. In this work, we studied the temperature-dependent optical properties of Bis-PDA-Ph, which exhibits reversible thermochromic properties, and PCDA–PDA, which exhibits irreversible thermochromic properties, by UV–visible absorption and femtosecond transient absorption spectroscopy. Our results indicate that the electronic relaxation of PDAs occurs via an intermediate state in cases where the material exhibits reversible thermochromic properties, whereas the excited PDAs relax directly back to the ground state when irreversible thermochromic properties are observed. The existence of this intermediate state in the electronic relaxation of PDAs thus plays an important role in determining their thermochromic properties. These results are very important for both understanding and strategically modulating the thermochromic properties of PDAs.

Hanju Rhee, Young-Gun June, Jang-Soo Lee, Kyung-Koo Lee, Jeong-Hyon Ha, Zee Hwan Kim, Seung-Joon Jeon & Minhaeng Cho, Nature 458, 310-313 (19 March 2009) | doi:10.1038/nature07846

Optical activity is the result of chiral molecules interacting differently with left versus right circularly polarized light. Because of this intrinsic link to molecular structure, the determination of optical activity through circular dichroism (CD) spectroscopy has long served as a routine method for obtaining structural information about chemical and biological systems in condensed phases. A recent development is time-resolved CD spectroscopy, which can in principle map the structural changes associated with biomolecular function7 and thus lead to mechanistic insights into fundamental biological processes. But implementing time-resolved CD measurements is experimentally challenging because CD is a notoriously weak effect (a factor of 10-4–10-6 smaller than absorption). In fact, this problem has so far prevented time-resolved vibrational CD experiments. Here we show that vibrational CD spectroscopy with femtosecond time resolution can be realized when using heterodyned spectral interferometry to detect the phase and amplitude of the infrared optical activity free-induction-decay field in time (much like in a pulsed NMR experiment). We show that we can detect extremely weak signals in the presence of large achiral background contributions, by simultaneously measuring with a femtosecond laser pulse the vibrational CD and optical rotatory dispersion spectra of dissolved chiral limonene molecules. We have so far only targeted molecules in equilibrium, but it would be straightforward to extend the method for the observation of ultrafast structural changes such as those occurring during protein folding or asymmetric chemical reactions. That is, we should now be in a position to produce 'molecular motion pictures'11 of fundamental molecular processes from a chiral perspective.

Hanju Rhee, Jun-Ho Choi, and Minhaeng Cho, Acc. Chem. Res., 2010, 43 (12), pp 1527–1536, doi: 10.1021/ar100090q

Vibrational circular dichroism (VCD) spectroscopy provides detailed information about the absolute configurations of chiral molecules including biomolecules and synthetic drugs. This method is the infrared (IR) analogue of the more popular electronic CD spectroscopy that uses the ultraviolet and visible ranges of the electromagnetic spectrum. Because conventional electronic CD spectroscopy measures the difference in signal intensity, problems such as weak signal and low time-resolution can limit its utility. To overcome the difficulties associated with that approach, we have recently developed femtosecond IR optical activity (IOA) spectrometry, which directly measures the IOA free-induction-decay (FID), the impulsive chiroptical IR response that occurs over time. In this Account, we review the time-domain electric field measurement and calculation methods used to simultaneously characterize VCD and related vibrational optical rotatory dispersion (VORD) spectra. Although conventional methods measure the electric field intensity, this vibrational technique is based on a direct phase-and-amplitude measurement of the electric field of the chiroptical signal over time. This method uses a cross-polarization analyzer to carry out heterodyned spectral interferometry. The cross-polarization scheme enables us to selectively remove the achiral background signal, which is the dominant noise component present in differential intensity measurement techniques. Because we can detect the IOA FID signal in a phase-amplitude-sensitive manner, we can directly characterize the time-dependent electric dipole/magnetic dipole response function and the complex chiral susceptibility that contain information about the angular oscillations of charged particles. These parameters yield information about the VCD and VORD spectra. In parallel with such experimental developments, we have also calculated the IOA FID signal and the resulting VCD spectrum. These simulations use a quantum mechanical/molecular mechanical molecular dynamics (QM/MM MD) method and calculate the electric dipole/magnetic dipole cross-correlation function in the time domain. Although many quantum chemistry calculation approaches can only consider a limited number of geometry-optimized conformations of chiral molecules in a gas phase, this computational method includes the solute−solvent interactions and the inhomogeneous distributions of solute conformers in condensed phases. A subsequent Fourier transformation of the chiral response function produced a theoretical VCD spectrum in the entire mid-IR frequency range. Directly comparing theory and experiment, we demonstrate quantitative agreement between frequency-tunable femtosecond IOA measurements and QM/MM MD simulations of (1S)-β- pinene in CCl4 solution. We anticipate that these direct IOA measurement and calculation methods will be applied to the studies of equilibrium chiroptical properties and structure determinations. These methods provide tools to investigate ultrafast structural dynamics of chiral systems with unprecedented time resolution.

Hanju Rhee, Intae Eom, Sung-Hyun Ahna and Minhaeng Cho, Chemical Society Reviews Issue 12, 4381-4584 (2012), doi: 10.1039/c2cs15336j

Intrinsic handedness encountered in molecular sciences plays an essential role in diverse physical, chemical and biological processes. Optical activity spectroscopy has enabled one to characterize such molecular handedness (chirality) and demonstrated its unique ability to provide stereo-specific structural insight into chiral molecular systems including biopolymers, chiral drugs, and superchiral materials. However, more extended applications including time-resolved studies have often been hindered by inherent limitations of conventional differential methods utilizing both left- and right-handed radiations. The latest methodological advance is heterodyned detection methods measuring wave interferences between signal and reference fields, which allowed direct characterizations of coherent chiroptical signals in a flash. With its ultimate sensitivity, the heterodyned chiroptical method promises to open new possibilities of transient electronic or vibrational optical activity measurements in the ultrafast time domain.

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