Two‐dimensional probing of ground‐state vibrational dynamics in porphyrin molecules by fs‐CARS

Abstract

We report on spectrally dispersed femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) for the investigation of the electronic ground-state vibrational dynamics of porphyrin molecules in solution. The two-dimensional experimental data obtained in our measurements provide a detailed mapping of the large number of molecular vibrations in these systems, excited by broadband ultrashort laser pulses, and make a thorough interpretation of the complex signal structure possible. In particular, an analysis of the data by applying Fourier transform methods to the time domain signal allows a clear identification of the excited porphyrin normal modes when compared with results on the mode structure from spectrally resolved (cw) spectroscopy. Altogether the method is capable of yielding detailed information on the dephasing behavior and on the relative spectral positions of the excited vibrations at the same time. Three porphyrin systems, dissolved in dichloromethane, were investigated: magnesium octaethylporphyrin, magnesium tetraphenylporphyrin and the free-base porphyrin octaethylporphyrin. The measurements were performed in the spectral region of the ring vibrations of the porphyrin macrocycle. We also provide a discussion of the expected signals on a theoretical basis by deriving the CARS third-order non-linear polarization for these experiments.

Time- and frequency-dependent model of time-resolved coherent anti-Stokes Raman scattering (CARS) with a picosecond-duration probe pulse

“…In addition to the multiplex nature of this newer probe technique, hybrid fs/ps CARS also provides the additional feature that Raman-active rotational/vibrational frequencies can be directly recovered; 1 they do not need to be deduced from time-dependent coherent oscillations observed in the spectrally dispersed CARS signal beam, as is the case in fs tr-CARS. 20,32,33 We note further that many of the advantages typically ascribed to fs tr-CARS 26,34 are retained in hybrid fs/ps CARS. 1,3 In particular, strong, undesirable contributions from non-resonant (NR) CARS (often referred to as electronic contributions) that generally complicate frequency-domain, nanosecond (ns), and ps tr-CARS probes 35 can be discriminated against in both fs tr-CARS and hybrid fs/ps CARS by precisely varying the timing between the three input CARS pulses.…”

“…[5][6][7][8][9][10][11][12] This probe technique has been developed as a variant of fs time-resolved CARS (tr-CARS), [13][14][15][16][17][18][19][20][21][22][23][24][25] the history of which has been described thoroughly in recent reviews. 26,27 Hybrid fs/ps CARS utilizes the same four-wave-mixing excitation scheme as does fs tr-CARS; however, the use of a time-delayed, narrowband, ps-duration probe pulse-rather than a broadband, fs-duration probe pulse-allows direct frequency-domain detection of Raman-active molecular states.…”

Polarization‐sensitive CARS spectroscopy on free‐base porphyrins: coproporphyrin I tetramethyl ester

“…48 -50 Moreover, the wavenumber-resolved PS CARS data presented in our studies can form a basis to interpret the interference patterns in femtosecond time-resolved CARS spectra of porphyrins; corresponding investigations have already been made on OEP, MgOEP, and MgTPP. 16,18 The wavenumber-resolved data could also be of fundamental use in connection with other modern CARS applications such as femtosecond CARS with ultrabroad multiplex excitation, 51 femtosecond CARS with mode-selective probing, 52 -54 or CARS microscopy with high spatial resolution. 55 -57 …”

Molecular vibrational dynamics in ethanol studied by femtosecond CARS

“…Femtosecond time-resolved CARS is a method for clarifying the origin of ultrafast dynamics associated with molecular motions in liquid [1] in the time domain since 1980s for some decades [2,3]. This technique has been widely used in different samples, such as polydiacetylene [4,5], porphyrin [6,7], toluene [8], methanol-water [9], flame [10][11][12], N 2 [13] and dipicolinic acid [14][15][16]. Recently, there have been some advances in femtosecond time-resolved CARS, such as single-shot [17], hybrid fs/ps [8,[18][19][20], single-beam [21][22][23], spectral focusing [24] and in situ heterodyne detection [25].…”