Multidimensional resonance raman spectroscopy by six-wave mixing in the deep UV

Abstract

Two-dimensional (2D) resonance Raman spectroscopies hold great potential for uncovering photoinduced relaxation processes in molecules but are not yet widely applied because of technical challenges. Here, we describe a newly developed 2D resonance Raman experiment operational at the third-harmonic of a Titanium-Sapphire laser. High-sensitivity and rapid data acquisition are achieved by combining spectral interferometry with a background-free (six-pulse) laser beam geometry. The third-harmonic laser pulses are generated in a filament produced by the fundamental and second-harmonic pulses in neon gas at pressures up to 35 atm. The capabilities of the setup are demonstrated by probing ground-state wavepacket motions in triiodide. The information provided by the experiment is explored with two different representations of the signal. In one representation, Fourier transforms are carried out with respect to the two experimentally controlled delay times to obtain a 2D Raman spectrum. Further insights are derived in a second representation by dispersing the signal pulse in a spectrometer. It is shown that, as in traditional pump-probe experiments, the six-wave mixing signal spectrum encodes the wavepacket's position by way of the (time-evolving) emission frequency. Anharmonicity additionally induces dynamics in the vibrational resonance frequency. In all cases, the experimental signals are compared to model calculations based on a cumulant expansion approach. This study suggests that multi-dimensional resonance Raman spectroscopies conducted on systems with Franck-Condon active modes are fairly immune to many of the technical issues that challenge off-resonant 2D Raman spectroscopies (e.g., third-order cascades) and photon-echo experiments in the deep UV (e.g., coherence spikes). The development of higher-order nonlinear spectroscopies operational in the deep UV is motivated by studies of biological systems and elementary organic photochemistries.

Perspective: Two-dimensional resonance Raman spectroscopy

“…45,46 In principle, 2D data sets for triiodide can be generated with less than 1000 points as the Nyquist criterion is satisfied, although at least 1600 points were sampled in our work in an effort to detect overtones. The point-by-point, all-broadband approach loses viability in systems with higher-frequency modes because data acquisition times become impractical.…”

“…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.…”

“…[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.…”

“…One option is to obtain the coherent Raman spectrum with a sequence of short laser pulses. 29,34,38,45,46 However, Mathies and co-workers have developed a less time-consuming method, referred to as J. Chem. Phys.…”