Wave packet dynamics in different electronic states investigated by femtosecond time-resolved four-wave-mixing spectroscopy

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The population flow from the first excited singlet state (S 1 ) to the electronic ground state (S 0 ), facilitated through the S 1 -S 0 internal conversion, and the subsequent internal vibrational energy redistribution and vibrational cooling processes are monitored selectively with respect to the different forms of nuclear motion in all-trans-b-carotene. This is realized by exciting the molecule into the second excited singlet state (S 2 ) with an auxiliary pump laser pulse and interrogating the population recovery into the electronic ground state by means of a time-delayed, coherent anti-Stokes Raman (CARS) process. This spectroscopic scheme is referred to as a pump-CARS scheme in analogy to the classical pump-probe scheme in timeresolved spectroscopy. Here, the profound enhancement of the CARS signal intensity in the case of a Raman resonance to a specific vibrational mode is utilized as an intensity filter that amplifies the signal from the vibrational modes of interest, making the contributions from other vibrational modes negligible. This filter allows for the population flow in a specific vibrational mode to be monitored as the radiationless electronic transition between the S 1 and S 0 state takes place. This spectroscopic scheme opens up the possibility of identifying the vibrational motion with a large-amplitude motion in the C C double bond symmetric stretch as the primary acceptor of population from the S 1 state. A mechanism with which the other normal modes are populated is postulated.

Section: Methodsmentioning

confidence: 99%

The role of specific normal modes during non‐Born–Oppenheimer dynamics: the S₁–S₀ internal conversion of β‐carotene interrogated on a femtosecond time‐scale with coherent anti‐Stokes Raman scattering

Siebert

Maksimenka

Materny

et al. 2002

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“…Also, FWM is an ideal method for optimal control studies as it can be used to monitor ground as well as excited state dynamics. 4 The CARS process is initiated with the spatial and temporal overlap of three pulses in the sample. For an electronically resonant excitation, the frequencies w pump and w probe of the pump and probe pulses, respectively, can be tuned to electronic transitions of the molecular system in order to resonantly enhance the signal intensity.…”

Section: Introductionmentioning

confidence: 99%

Selective spectral filtering of molecular modes of β‐carotene in solution using optimal control in four‐wave‐mixing spectroscopy

Konradi

Singh

Scaria

et al. 2006

Femtosecond laser pulses are spectrally broad and therefore coherently excite several molecular modes. While the temporal resolution is high, usually no mode-selective excitation is possible. In this work, we demonstrate that, by phase shaping one of the two pulses, which excite the vibrational modes of bcarotene within a coherent anti-Stokes Raman scattering (CARS) process, specific modes can be enhanced or suppressed. Using the pulse shaper setup in a feedback-controlled closed-loop optimizes this mode selection. Here, the ratio of signal intensities observed in the CARS spectrum serves as a feedback function for an evolutionary algorithm responsible for the optimization. The optimized phase structure of the femtosecond pulse is characterized using the frequency resolved optical gating (FROG) technique to get an insight into the optimization process. Furthermore, it is shown that the temporal resolution after optimization is still high enough to investigate the ultrafast molecular dynamics. The suppression or relative enhancement of vibrational modes persists over the entire coherence time.

“…1,4,6,7 Time-resolution is attained by introducing a variable delay between the simultaneous interaction of a pump and Stokes laser pulse (1 and 2 in Fig. 1(A)) relative to the subsequent-acting probe laser pulse (3 in Fig.…”

Section: Introductionmentioning

confidence: 99%

Ground state vibrational wave‐packet and recovery dynamics studied by time‐resolved CARS and pump‐CARS spectroscopy

Siebert

Schmitt

Gräfe

et al. 2006

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Recent experiments on gaseous iodine and b-carotene are analyzed within a wave-packet description of femtosecond-time resolved four-wave mixing spectroscopy. Employing the same formalism, two different experimental setups, both established in the Kiefer group, are treated. In a transient four-wave-mixing (FWM) experiment, where two pulses interact simultaneously with the system, the interaction with a third time-delayed pulse leads to light emission reflecting ground state vibrational dynamics in I 2 . On the other hand, delaying a FWM-pulse sequence with respect to a single pump pulse interacting with b-carotene yields a signal that is directly proportional to the population of a selected nuclear eigenstate in the electronic ground state. Thus, within this setup, it is possible to monitor ground-state re-population dynamics mode specifically.