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Topics in Current Chemistry (2018) 376:35
superpositions of molecular (i.e., electronic and vibrational) quantum states. The
quantum evolution of these initial coherent states may be followed spectroscopically, i.e., interrogated by the interaction with another laser pulse, until decoherence (interaction with the bath) occurs on typical time scales of a few femtoseconds (electronic decoherence) up to a few picoseconds (vibrational decoherence)
[1–4]. After electronic decoherence has occurred, the time evolution of the laserinduced populations can be further observed spectroscopically.
Over the last decade, the investigation of coherence in molecular processes
occurring in the condensed phase has become a frontier research topic in molecular
quantum physics [5–11]. In this chapter, we will describe an application of femtosecond coherent multidimensional spectroscopy which engineers vibrational coherence in molecular systems and uses it as a spectroscopic tool. More precisely, the
goal is to follow in time the vibrational coherence imprinted in the electronic excited
states by the non-linear interaction with coherent laser light and exploit the peculiar spectroscopic signatures of such vibrationally coherent molecular states. This
type of spectroscopy can be performed under several distinct experimental implementations, which have been named differently. Here we will summarize all these
implementations under the name multidimensional vibrational coherence spectroscopy, or multi-VCS. This contribution does not deal, however, with other kinds of
vibrational spectroscopies based on so-called rephasing mechanisms like 2D Raman
or 2D infrared spectroscopies. These topics are reviewed in different contributions
in this collection.
We will first shortly describe how VCS results from the third-order interaction
(and higher-order for multi-VCS) of the molecular system with femtosecond light
pulses. This initial description will assist the reader in understanding how the
higher-dimensional versions of VCS are able to report on structural dynamics in
excited states. The main experimental implementations in the time-domain (pumpImpulsive Vibrational Spectroscopy-pump-IVS, pump-Degenerate Four Wave Mixing-pump-DFWM, and Population-controlled IVS) and in the frequency-domain
(Femtosecond Stimulated Raman Scattering-FSRS) will be discussed and compared.
Finally, we will illustrate the success of multi-VCS at revealing a mechanistic understanding of ultrafast photoreactions in a selection of molecular systems: carotenoids
and stilbene derivatives.
2 Introduction to Vibrational Coherence Spectroscopy (VCS)
The principle of vibrational coherence spectroscopy was demonstrated in molecules
as soon as picosecond and then femtosecond laser pulses became available. In particular, laser pulses which are shorter than the period of nuclear motions in a molecule have a spectrum larger than the corresponding vibrational level spacing. With
such a laser pulse, coherent superposition of vibrational levels, also referred to as
vibrational wavepackets, may be produced impulsively in essentially any molecule.
The spectroscopic signature of such vibrational wavepackets allows tracking molecular structural dynamics accompanying ultrafast photoreactions in molecules. Early
209
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
superpositions of molecular (i.e., electronic and vibrational) quantum states. The
quantum evolution of these initial coherent states may be followed spectroscopically, i.e., interrogated by the interaction with another laser pulse, until decoherence (interaction with the bath) occurs on typical time scales of a few femtoseconds (electronic decoherence) up to a few picoseconds (vibrational decoherence)
[1–4]. After electronic decoherence has occurred, the time evolution of the laserinduced populations can be further observed spectroscopically.
Over the last decade, the investigation of coherence in molecular processes
occurring in the condensed phase has become a frontier research topic in molecular
quantum physics [5–11]. In this chapter, we will describe an application of femtosecond coherent multidimensional spectroscopy which engineers vibrational coherence in molecular systems and uses it as a spectroscopic tool. More precisely, the
goal is to follow in time the vibrational coherence imprinted in the electronic excited
states by the non-linear interaction with coherent laser light and exploit the peculiar spectroscopic signatures of such vibrationally coherent molecular states. This
type of spectroscopy can be performed under several distinct experimental implementations, which have been named differently. Here we will summarize all these
implementations under the name multidimensional vibrational coherence spectroscopy, or multi-VCS. This contribution does not deal, however, with other kinds of
vibrational spectroscopies based on so-called rephasing mechanisms like 2D Raman
or 2D infrared spectroscopies. These topics are reviewed in different contributions
in this collection.
We will first shortly describe how VCS results from the third-order interaction
(and higher-order for multi-VCS) of the molecular system with femtosecond light
pulses. This initial description will assist the reader in understanding how the
higher-dimensional versions of VCS are able to report on structural dynamics in
excited states. The main experimental implementations in the time-domain (pumpImpulsive Vibrational Spectroscopy-pump-IVS, pump-Degenerate Four Wave Mixing-pump-DFWM, and Population-controlled IVS) and in the frequency-domain
(Femtosecond Stimulated Raman Scattering-FSRS) will be discussed and compared.
Finally, we will illustrate the success of multi-VCS at revealing a mechanistic understanding of ultrafast photoreactions in a selection of molecular systems: carotenoids
and stilbene derivatives.
2 Introduction to Vibrational Coherence Spectroscopy (VCS)
The principle of vibrational coherence spectroscopy was demonstrated in molecules
as soon as picosecond and then femtosecond laser pulses became available. In particular, laser pulses which are shorter than the period of nuclear motions in a molecule have a spectrum larger than the corresponding vibrational level spacing. With
such a laser pulse, coherent superposition of vibrational levels, also referred to as
vibrational wavepackets, may be produced impulsively in essentially any molecule.
The spectroscopic signature of such vibrational wavepackets allows tracking molecular structural dynamics accompanying ultrafast photoreactions in molecules. Early
209
Reprinted from the journal