Femtosecond stimulated Raman spectroscopy by six-wave mixing

Abstract

Femtosecond Stimulated Raman Spectroscopy (FSRS) is motivated by the knowledge of the molecular geometry changes that accompany sub-picosecond chemical reactions. The detection of vibrational resonances throughout the entire fingerprint region of the spectrum with sub-100-fs delay precision is fairly straightforward to accomplish with the FSRS technique. Despite its utility, FSRS must contend with substantial technical challenges that stem from a large background of residual laser light and lower-order nonlinearities when all laser pulses are electronically resonant with the equilibrium system. In this work, a geometry based on five incident laser beams is used to eliminate much of this undesired background in experiments conducted on metmyoglobin. Compared to a three-beam FSRS geometry with all electronically resonant laser pulses, the five-beam approach described here offers major improvements in the data acquisition rate, sensitivity, and background suppression. The susceptibility of the five-beam geometry to experimental artifacts is investigated using control experiments and model calculations. Of particular concern are undesired cascades of third-order nonlinearities, which are known to challenge FSRS measurements carried out on electronically off-resonant systems. It is generally understood that "forbidden" steps in the desired nonlinear optical processes are the origin of the problems encountered under off-resonant conditions. In contrast, the present experiments are carried out under electronically resonant conditions, where such unfortunate selection rules do not apply. Nonetheless, control experiments based on spectroscopic line shapes, signal phases, and sample concentrations are conducted to rule out significant contributions from cascades of third-order processes. Theoretical calculations are further used to estimate the relative intensities of the direct and cascaded responses. Overall, the control experiments and model calculations presented in this work suggest promise for multidimensional resonance Raman investigations of heme proteins.

Perspective: Two-dimensional resonance Raman spectroscopy

“…Such groundstate vibrational resonances were most prominent in our applications to triiodide and myoglobin because of short excited-state lifetimes. [45][46][47][48] In addition, we qualitatively discuss the information provided by 2DRR spectroscopy for systems in which ultrafast chemical reactions take place.…”

“…We have explored a variety of 2DRR laser beam geometries in earlier works and consider this to be a flexible technical issue. [45][46][47][48] In this section, we focus on two ways that sequences of laser pulses can be configured (in the time domain) in order to define the two dimensions of the spectrum. The two approaches are referred to as "all-broadband" and "FSRS-like" methods.…”

“…47,48 Fourier windows up to 1400 cm −1 were readily achieved in 15-min experiments, where the delay, τ 1 , was scanned 30 times to optimize the signal-to-noise ratio. 48 Signal averaging is critical in systems with modest Franck-Condon activity because the 2DRR response may only be a few percent of the total fifth-order signal.…”

“…[45][46][47][48]51 Here, the goal is to draw connections between components of the response function and the resonances observed in experiments. For the purpose of illustration, we consider 2 of the 16 polarization components associated with the response of a system with one electronic resonance and a single harmonic vibrational mode.…”

“…These two approaches possess strengths and weaknesses that have been exploited in our development of 2D resonance Raman (2DRR) spectroscopies. [45][46][47][48] The motivations for 2DRR experiments are largely system-dependent as the technique is quite versatile. To begin, like spontaneous resonance Raman spectroscopy, 2DRR may leverage electronic resonance enhancement to probe specific components of complex systems.…”