Center for Relativistic Laser Science

TRVS 2019

Two-dimensional electronic spectroscopy (2DES) has been widely used for studying ultrafast energy transfer processes in multi-chromophoric systems. Compared to other time-resolved spectroscopic techniques, the additional excitation-frequency resolvability of 2DES provides far more detailed information on reaction pathway and inhomogeneity of molecular system under investigation. In particular, the femtosecond time-resolution of 2DES enables to observe the electronic and vibrational coherence generated on the excited state, which is of great use to elucidate electronic and nuclear structures of the excited state. However, the 2DES has not been used in studying various photochemical systems involving nanosecond to microsecond processes. Conventional 2DES techniques generate the time-delay between the pump and probe pulses by controlling the optical path length. Thus, it is difficult to generate nanosecond or microsecond time-delay with a mechanical device because it requires meters or kilometers of optical path length difference.

Dual frequency-comb (DFC) consists of two frequency-stabilized lasers with equally-spaced spectral lines. When the repetition rates of the two lasers are slightly detuned as f_r and f_r + Df_r, the time-delay between the two trains of pulses (T) increases as much as DT precisely for every repetition period (1/f_r), where DT can be written as DT = Df_r /f_r². Thus, DFC is capable of automatic scanning of T from zero to 1/f_r, where the latter is tens of ns in general. The scan- and sampling-rates is determined by Df_r, which ranges from tens of Hz to several kHz, and f_r, respectively. This fast and long T-scanning enables DFC to be applied for microscopy, gas-phase spectroscopy and atmospheric analysis [1]. Recently, we have demonstrated DFC based nonlinear spectroscopy of molecules in condensed phases [2], and theoretically described it using time-dependent perturbation theory [3].

In this presentation, a DFC-based 2DES technique (DFC-2DES) covering fs-to-ns time window with the time-resolution of 12.5 fs [4] will be introduced. We further show that DFC-2DES has significant advantages compared to conventional 2DES techniques, which are simple instrumentation, coherent averaging, analytical phase correction, and photochemical damage minimization. From DFC-2DES spectroscopy of molecular dye in solution, we were able to study photophysical processes occurring in fs-to-ns time window, such as vibrational relaxation, solvation, coherent wave packet propagation, electronic relaxation, and even slow local heating process.

References

[1]         N. Picqué, T. W. Hänsch, (2019) Nat. Photon., 13:146-157.

[2]         J. Kim, T. H, Yoon, M. Cho, (2018) J. Phys. Chem. B, 122:9775-9785.

[3]         J. Kim, J. Jeon, T. H, Yoon, M. Cho, (2019) Chem. Phys., 520:122-137.

[4]         J. Kim, J. Jeon, T. H, Yoon, M. Cho, (unpublished)