Time-resolved Raman spectroscopy is a valuable tool for revealing structural dynamics in ultrafast chemical and biological reactions.1–6
Spontaneous Raman scattering can be used to obtain spectra over a ~1500 cm–1
spectral window, but requires picosecond or longer duration pulses to obtain adequate spectral resolution.7
A significant benefit of Raman scattering is resonance enhancement, which allows observation of the vibrational spectrum of a specific chromophore in a complex system;8
however, electronic resonance is often accompanied by fluorescence backgrounds that can easily overwhelm the spontaneous Raman signal. To overcome these problems of time resolution and spectral quality, we have been developing the technique of femtosecond stimulated Raman spectroscopy (FSRS).9–17
FSRS can obtain Raman spectra over a 1500 cm–1
window free from fluorescence backgrounds, with simultaneous high time and frequency resolution. The system we have developed is unique in its ability to obtain high signal-to-noise spectra in seconds with <100 fs temporal resolution and <10 cm–1
spectral resolution, producing an instrument response product of <1 cm–1
ps, more than an order of magnitude better than the transform limit of spontaneous Raman (15 cm–1
ps) and five or more times better than other FSRS systems, whose products have ranged from 5 to 60 cm–1
Here, we present a detailed technical description of the apparatus and methods we are using to produce FSRS spectra with visible and near-infrared (NIR) laser pulses generated from a femtosecond titanium:Sapphire laser.
A schematic of the FSRS experiment is presented in . The stimulated Raman effect occurs when two coherent optical fields, the Raman pump at ωp
and the Raman probe at ωS
, propagate through a system with a vibrational resonance at ωvib
. Coherent excitation of the vibration produces amplification of the probe beam and attenuation of the pump beam. Stimulated Raman spectroscopy has traditionally been performed by scanning the Stokes frequency, ωS
, and plotting the gain of the probe intensity versus ωp
In femtosecond broadband stimulated Raman spectroscopy, the Raman pump field is provided by a narrow bandwidth picosecond pulse at ~800 nm and the Raman probe by a femtosecond NIR continuum pulse to the red of 800 nm. The broadband probe allows simultaneous observation of a large range of Stokes frequencies. Measurement of the probe spectrum with and without the Raman pump and then calculation of the pump-on:pump-off
ratio generates a Raman gain spectrum in which the amplification of the probe at Raman resonances from the pump frequency is readily apparent . The instrumental resolution of the gain spectrum is determined by the resolution of the spectrograph (~6 cm–1
) and the bandwidth of the Raman pump (3–17 cm–1
). Time resolution is produced by introducing a ~50 fs actinic pump pulse to initiate a photochemical reaction . Depending on the actinic pump–probe time delay, Δt
, the gain spectrum displays the vibrational spectrum of the excited electronic states or photoproducts generated by the actinic pump. The time resolution of the experiment can be <100 fs, and is determined by the ability to resolve the delay between the excitation by the actinic pump and the initiation of the Raman transition by the probe, although the specific dynamics and spectral linewidth observed in an experiment will also depend on the molecule being studied.
FIG. 1. Illustrative diagrams of FSRS. (a) The spectra of the Raman pump and probe pulses for a typical FSRS experiment on cyclohexane. In the presence of the Raman pump pulse, the stimulated Raman effect amplifies (more ...)
The temporal and spectral resolution limits of FSRS can be understood by examining the evolution of the quantum system using the Feynman diagram shown in .21,22
Here, the system evolves temporally from bottom to top and dipole couplings are described by diagonal arrows that produce a transition in either the bra (right-hand side) or ket (left-hand side) part of the wave function. The molecular system initially in the ground state, |g
. is excited by the actinic pump pulse at t1
by the first two ω1
dipole couplings to a vibrational state of the upper electronic surface, |e,n
. After a delay, Δt
, the Raman transition is initiated at t3
by sequential couplings with the Raman pump (ω2
) and the broadband probe (ω3
). The system then propagates in a vibrationally coherent state |e,ne,n
| which decays with a characteristic dephasing time,
. In the homogeneous broadening limit, the dephasing time determines the inherent spectral line shape of the vibrational transition, as
. The Raman transition is completed by a coupling of the ket with the Raman pump field and emission at the Stokes frequency, ω3
, placing the system in the final vibrationally excited state |e,n
. It is important to note that the final emission at ω3
along the probe wave vector k3
can occur long after the probe pulse has left the sample.
FIG. 2. Feynman diagram describing the evolution of the density matrix for FSRS. The time delay, Δt, between the actinic pump (ω1) and probe (ω3) beams determines (more ...)
The time resolution in FSRS is primarily determined by the ability to resolve the time delay between the electronic excitation at t1 and the initiation of the Raman transition at t3 by sequential couplings with ω2 and ω3. The FSRS vibrational peak width is determined by the convolution of the Raman pump pulse with the decaying vibrational coherence. In the long-pulse limit, this convolution is determined solely by the dephasing time of the molecule and the observed peak width will be the minimum possible. However, if a short duration Raman pump pulse is used, the convolution will be truncated by the ω2 pulse envelope, leading to a broader Raman line shape.
Furthermore, since the final detected signal represents an average over the vibrational dephasing time after the initial coupling of the vibrational states, changes in the vibrational frequency or phase during the dephasing time will affect the final FSRS line shape. In a spontaneous Raman experiment, the Raman transition is initiated at any time during the Raman pump pulse by ω2 and the zero-point radiation field at ω3. The resultant frequency and time resolution are both determined by the Raman pump pulse envelope and therefore subject to the transform limit. In contrast, FSRS maintains high time and frequency resolution by "gating" the initiation of the Raman transition with a short probe pulse and then completing the transition with a long Raman pump pulse.
In this article, we present an effective optical configuration for performing FSRS with <10 cm–1 spectral resolution and <100 fs time resolution using a femtosecond titanium:Sapphire laser. The important characteristics of the laser system, detection apparatus, and data collection methods for obtaining stable spectral baselines and shot-noise-limited detection are described. FSRS spectra of rhodamine 6G, chlorophyll a, and 3,3’-diethylthiatricarbocyanine iodide (DTTCI) highlight the unique fluorescence rejection and rapid data acquisition times possible in a variety of resonance conditions. FSRS spectra of bacterial reaction centers obtained using shifted excitation Raman difference spectroscopy (SERDS) illustrate an effective method for removal of large transient absorption backgrounds. Finally, our femtosecond time-resolved spectra of diphenyloctatetraene illustrate the capabilities of FSRS to perform femtosecond time-resolved structural studies of excited states and chemical reaction intermediates.